some forebrain connections of the gustatory system in the goldfishcarassius auratus visualized by...

Some Forebrain Connections of theGustatory System in the GoldfishCarassius Auratus Visualized bySeparate DiI Application to the

Hypothalamic Inferior Lobe and theTorus Lateralis

ELKE RINK AND MARIO F. WULLIMANN*Brain Research Institute and Center for Cognition Research, University of Bremen, D-28334

Bremen, Germany

ABSTRACTThe neuroanatomical connections of the diencephalic torus lateralis and inferior lobe of the

goldfish (Carassius auratus) were studied by retrograde and anterograde labeling with thecarbocyanine dye DiI. Both structures have afferents originating in the central zone of the dorsaltelencephalic area as well as in the supracommissural nucleus of the ventral telencephalic area, andin the secondary gustatory, tertiary gustatory, and posterior thalamic nuclei. Both structuresinvestigated have efferents to the tertiary gustatory and posterior thalamic nuclei, as well as to thedorsal hypothalamus (dorsal hypothalamic neuropil) and superior reticular formation.

The torus lateralis receives additional afferents from the secondary general visceralnucleus and, sparsely, from the dorsal tegmental nucleus. The inferior lobe receives additionalafferents from the medial zone of the dorsal telencephalic area, as well as from thesuprachiasmatic, posterior pretectal, central posterior thalamic, caudal preglomerular, twotegmental nuclei (T1 and T2), corpus mamillare, and, sparsely, from the cerebellar valvula.The inferior lobe has additional efferents to the dorsal and ventral thalamus and subglomeru-lar nucleus. The lateral torus and inferior lobe are also mutually interconnected.

The lateral torus and inferior lobe map topographically onto the vagal-related (intraoral) oronto the facial-related (extraoral) portions, respectively, of both the secondary and tertiary gustatorynuclei. Because the posterior thalamic nucleus is reciprocally connected with the lateral torus andinferior lobe and is further known to project in turn to the area doralis telencephali, it likelyrepresents a quaternary gustatory projection nucleus to the telencephalon in cyprinids.

Whereas the lateral torus seems to be exclusively involved with gustatory and generalvisceral systems, the inferior lobe has inputs from additional sensory (e.g., octavolateralis,visual) systems, and, thus, likely represents a multisensory integration center. J. Comp.Neurol. 394:152–170, 1998. r 1998 Wiley-Liss, Inc.

Indexing terms: gustation; hypothalamus; posterior tuberculum; teleost visual system; topography

The gustatory sense plays a dominant role in the lives ofmany cyprinid teleosts such as carp and goldfish (Cyprini-formes, Ostariophysi; Finger, 1983; Kanwal and Finger,1992; Marui and Caprio, 1992). This finding has beendocumented in cyprinids at the level of behavior (Sibbingand Uribe, 1985; Sibbing et al., 1986; Lamb and Finger,1995) and at the level of the sensory (distribution of tastebuds and their innervation) and motor (palatal organ)periphery. Also, the gustatory representation in the pri-mary sensory medullary centers (Luiten, 1975; Morita and

Finger, 1985b, 1987; Puzdrowski 1987; Goehler and Fin-ger, 1992), as well as at higher order central nervous levels(Morita et al., 1980, 1983) is well established. The three

Grant sponsor: Deutsche Forschungsgemeinschaft (DFG); Grant num-ber: Wu 211/1-2.

*Correspondence to: Mario F. Wullimann, Brain Research Institute andCenter for Cognition Research, University of Bremen, FB 2, P.O. Box 33 0440, D-28334 Bremen, Germany. E-mail: [email protected]

Received 22 May 1997; Revised 7 November 1997;Accepted 7 December 1997

THE JOURNAL OF COMPARATIVE NEUROLOGY 394:152–170 (1998)

r 1998 WILEY-LISS, INC.

nerves subserving gustation, i.e., facial, glossopharyngeal,and vagal nerves, project to three separate primary sen-sory lobes in the goldfish. The two lobes receiving theextraoral gustatory information (facial lobe) and the intra-oral gustatory information (vagal lobe; compare Fig. 1) areespecially greatly hypertrophied. Moreover, hypertrophyis apparent along the entire neuraxis in the goldfishgustatory system (Rupp et al., 1996), e.g., in the secondary

gustatory nucleus located in the isthmic region and indiencephalic tertiary gustatory centers, especially in thepreglomerular tertiary gustatory nucleus (formerly‘‘nucleus glomerulosus’’ of cyprinids).

The ascending gustatory connections to the diencepha-lon have been established with neuronal tracing in thecrucian carp Carassius carassius, the non-domesticatedform of the goldfish (Morita et al., 1980, 1983; Morita and

Abbreviations

A anterior thalamic nucleusACo anterior commissureBO olfactory bulbCC cerebellar crestCe, CCe corpus cerebelliCM corpus mamillareCP central posterior thalamic nucleusCpop postoptic commissureDc central nucleus of dorsal telencephalic areaDHN dorsal hypothalamic neuropilDl lateral nucleus of dorsal telencephalic areaDm medial nucleus of dorsal telencephalic areaEG granular eminenceFLM medial longitudinal fascicleFLo facial lobeFR habenulo-interpeduncular tract (fasciculus retroflexus)G glossopharyngeal lobeGC central grayHC horizontal commissureIL inferior lobe of hypothalamusIN interpeduncular nucleusLCe caudal cerebellar lobeLFB lateral forebrain bundleLL lateral line nerves (Fig. 1), lateral lemniscus (Fig. 3)M molecular layer of valvula cerebelliMCA anterior mesencephalocerebellar tractMO medulla oblongataMS medulla spinalisNPLI nucleus periventricularis recessus lateralis of the ILNTP posterior thalamic nucleusON optic nerveONvl ventrolateral optic tractPE nucleus praeeminentialisPG preglomerular areaPGa anterior preglomerular nucleus

PGc caudal preglomerular nucleusPGl lateral preglomerular nucleusPO posterior pretectal nucleusPsp parvocellular superficial pretectal nucleusSC suprachiasmatic nucleusSG subglomerular nucleusSGN secondary gustatory nucleusSGT secondary gustatory tractSR superior rapheSRF superior reticular formationSTN isthmic sensory trigeminal nucleusSVN secondary general visceral nucleusT1 teltegmental nucleus 1T2 tegmental nucleus 2Tel telencephalonTeO optic tectumTGN preglomerular tertiary gustatory nucleusTGT tertiary gustatory tractTH tuberal hypothalamusTla lateral torusTolo longitudinal torusVa valvula cerebelliVLo vagal lobeVM ventromedial thalamic nucleusVs supracommissural nucleus of ventral telencephalic areaI olfactory nerveII optic nerve (ON)III oculomotor nerveIV trochlear nerveV trigeminal nerveVIIs sensory root of the facial nerveVIII octaval nerveIX glossopharyngeal nerveX vagal nerve

Fig. 1. Brain of Carassius auratus in lateral view. Asterisksdesignate the two investigated structures, torus lateralis (Tla) andinferior lobe (IL), respectively. Numbers at the top indicate sectionlevels of chartings shown in Figure 2 (Tla applications) and numbers

at the bottom indicate section levels of chartings shown in Figure 3 (ILapplications). Rostral is to the left. For abbreviations, see list. Scalebar 5 0.4 mm.

GOLDFISH GUSTATORY FOREBRAIN 153

Finger, 1985b). However, none of the tertiary gustatorycenters, i.e., the preglomerular tertiary gustatory nucleus,the hypothalamic inferior lobe, and the torus lateralis,appear to project to the pallial area dorsalis of the telen-cephalon in the goldfish (Wullimann and Meyer, 1993).This is in contrast to what has been found in siluroids(catfishes) which are also ostariophysan teleosts. In thechannel catfish Ictalurus punctatus, a tertiary gustatorynucleus, the lobobulbar nucleus, projects to the medial andcentral zones of the dorsal telencephalic area and anothertertiary gustatory center, the central nucleus of the infe-rior lobe, projects to the central zone of the dorsal telence-phalic area (Kanwal et al., 1988; Lamb and Caprio, 1993a,1993b). In cyprinids, however, it is not known whethergustation is relayed to the telencephalon and, if so, whichnuclei in the diencephalon and target areas in the telen-cephalon are involved. Furthermore, there is no informa-tion regarding the gustatory circuitry within the cypriniddiencephalon. To gain information on the higher ordergustatory circuitry in the forebrain of the goldfish, we haveapplied the fluorescent tracer DiI alternatively to twodiencephalic tertiary gustatory centers, i.e., the hypotha-lamic inferior lobe and the lateral torus.

This study is the first report of the full set of connectionsof the lateral torus and the inferior lobe in a teleost. Ourfindings in the goldfish indicate that both structures arepart of a complex intradiencephalic gustatory network inwhich the topography of the extraoral facial and intraoralvagal systems is maintained. Whereas the torus lateralisseems to be almost solely involved with gustatory circuitry,the inferior lobe has various additional connections thatreveal its multisensory input.

Part of this study represents the diploma work of thefirst author (Rink, 1996). An abstract reporting prelimi-nary results has also been published (Rink and Wulli-mann, 1996).

MATERIALS AND METHODS

A total of 31 specimens of the goldfish Carassius auratus(Cyprinidae) used in the present study were purchasedfrom commercial pet shops. The total lengths varied from 7to 10 cm. The animals were anesthetized with a solution oftricaine methanesulfonate (MS 222, Sigma, Deisenhofen,Germany) at a concentration of 0.05%. They were thenperfused transcardially with cold Sorensen-phosphate-buffer (PB, pH 7.38) followed by 4% paraformaldehyde inPB. The animals were decapitated, and the heads werepostfixed for 3 days. After brain removal from the skull,crystals of the carbocyanine dye DiI (1,1’,dioctadecyl-3,3,3’,3’-tetra-methylindo-carbocyanineperchlorate, Mo-lecular Probes, Eugene, OR) were applied either to theinferior lobe or to the lateral torus, and covered with warmagar-agar (4%). Eleven animals received applications inthe torus lateralis, five of which were located in the caudalpart, three in the intermediate part, and three in therostral part of the torus lateralis. In another 20 animals,the inferior lobe received a DiI application, 8 of which inthe caudal part, 5 in the intermediate part, and 7 in therostral part of the inferior lobe. After 4 to 10 weeks ofincubation in paraformaldehyde at 40°C, the brains wereembedded in agar-agar (4%) and cut transversely on aVibratome at 50 µm. Sections were observed with afluorescence microscope equipped with a rhodamine filterset to visualize the DiI fluorescence. The same sections

were then observed under phase contrast for histologicinterpretation.

The pooled results of tracer applications to lateral torusand inferior lobe, respectively, were charted onto appropri-ate drawings (levels are indicated in Fig. 1). A set ofBodian-stained transverse sections of an adult goldfishwas available for comparison. Representative sections ofthe tracing experiments were photographed on KodakTMAX 400 ASA film (Eastman Kodak, Rochester, NY). Theterminology of nuclei in the goldfish diencephalon andtelencephalon is based on Braford and Northcutt (1983),Wullimann and Meyer (1990), and Striedter (1992).

This study was carried out on fixed brain tissue, i.e., thework did not involve animal experiments in the sense ofthe Deutsche Tierschutzgesetz and is furthermore in agree-ment with the guidelines of the Veterinaramt of the Senatof the City of Bremen (Germany).

RESULTS

We first report the connections shared by the toruslateralis (Tla) and the inferior lobe (IL) followed by para-graphs on the additional connections specific to Tla and IL,respectively. A general word of caution seems appropriatewith respect to the herein reported terminal fields. Theirsuspected origins in the Tla and the IL need retrogradeconfirmation with additional tracer applications into theseterminal fields themselves. An exception to this are theterminals in Tla and IL, because they are retrogradelyconfirmed in this study.

Connections shared by the torus lateralisand the inferior lobe

Secondary gustatory nucleus. This dumb-bell shapednucleus in the isthmic region of the rhombencephalon liesat the basis of the corpus cerebelli. Our tracing data clearlyshow that there is a distinct partition in the secondarygustatory nucleus (SGN), between the lateral vagal gusta-tory area, which projects to the lateral torus and themedial facial gustatory area projecting to the inferior lobe.After tracer application to the Tla, retrogradely labeledcell bodies appear only in the lateral part of the ipsilateralsecondary gustatory nucleus (Figs. 2, 4A,B). After tracerapplication to the IL, retrogradely labeled cell bodies arelocated in the medial part of this nucleus ipsilaterally(Figs. 3, 4C,D). There was no detectable difference in theextent of label after application of tracer into three rostro-caudally separate sites in the Tla. Similarly, no differencewas seen in the extent of label in the secondary gustatorynucleus after intermediate and caudal DiI applications tothe IL. However, applications into the rostral IL yieldedlabeled cell bodies concentrated in the center of thesecondary gustatory nucleus. These neurons may corre-spond to the glossopharyngeal division (see Discussionsection).

Preglomerular tertiary gustatory nucleus. The pre-glomerular tertiary gustatory nucleus (TGN) is positionedwithin the medial preglomerular area of the diencephalonand was previously misidentified as ‘‘nucleus glomerulo-sus’’ in cyprinids (see Wullimann et al., 1996, for discus-sion). In this diencephalic nucleus, we observed mostclearly differences in the location of label within thenucleus after Tla and after IL applications, and also after

154 E. RINK AND M.F. WULLIMANN

different application sites rostrocaudally within the Tla orthe IL. After rostral and intermediate applications (seeMaterials and Methods section) into the Tla, anterogradeand retrograde label appears in the rostral portion of thetertiary gustatory nucleus, whereas after caudal Tla appli-cations, the label is in the middle portion of the tertiarygustatory nucleus. Corresponding results are seen after IL

applications. After intermediate and caudal applicationsto the IL, the anterograde and retrograde label appears inthe caudal tertiary gustatory nucleus, whereas after ros-tral IL applications, the label occurs in the middle part ofthe tertiary gustatory nucleus. In summary, it can be saidthat labeled cell bodies and terminals are found in therostral tertiary gustatory nucleus after Tla applications

Fig. 2. Chartings of transverse sections at levels from rostral (1) tocaudal (5) after DiI applications (solid black area) into torus lateralisof Carassius auratus (compare Fig. 1). Note that orthogradely labeledterminals (small dots) and retrogradely labeled cell bodies (large dots)appear only in the rostral tertiary gustatory nucleus (TGN), and

labeled cells of the secondary gustatory nucleus (SGN) are only in itslateral part. In contrast to IL applications (compare Fig. 3), thesecondary general visceral nucleus (SVN) contains labeled cell bodiesafter Tla applications. For abbreviations, see list. Scale bar 5 1 mm.


Fig. 3. Chartings of transverse sections at levels from rostral (1) tocaudal (10) after DiI applications (solid black areas) into inferior lobeof Carassius auratus (compare Fig. 1). Note that orthogradely labeledterminals (small dots) and retrogradely labeled cell bodies (large dots)appear only in the caudal part of the tertiary gustatory nucleus (TGN).Labeled TGN cells form a dorsocaudal cap upon the nucleus preglo-merulosus caudalis (PGc; panel 7), which is also labeled, as is thecorpus mamillare (CM; compare Fig. 7). The retrogradely labeled cell

bodies of the secondary gustatory nucleus (SGN) are located only in itsmedial part (panel No. 10). The labeled cell bodies of the posteriorthalamic nucleus (NTP) appear to be more concentrated in the medialaspect. The dorsal hypothalamic neuropil (DHN) exhibits the sameintensity and extent as in the torus lateralis (Tla) cases. The labeledtertiary gustatory tract (TGT) gives off fibers to the superior reticularformation (SRF), but clear terminals are not visible. For abbrevia-tions, see list. Scale bar 5 1 mm.

Fig

.4.

Ph

otom

icro

grap

hs

oftr

ansv

erse

sect

ion

ssh

owin

gre

trog

rade

lyla

bele

dce

llbo

dies

inth

ese

con

dary

gust

ator

yn

ucl

eus

(SG

N).

A:A

fter

DiI

appl

icat

ion

into

the

toru

sla

tera

lis,

labe

led

cell

bodi

eson

lyoc

cur

inth

ela

tera

lpar

toft

he

SG

Nan

dfu

rth

erin

the

seco

nda

ryge

ner

alvi

scer

aln

ucl

eus

(SV

N).

B:S

ame

sect

ion

asin

Abu

tu

nde

rph

ase

con

tras

t.C

:Th

ela

beli

ssh

arpl

yli

mit

ed

toth

em

edia

lpar

tof

the

seco

nda

rygu

stat

ory

nu

cleu

saf

ter

DiI

appl

icat

ion

into

the

infe

rior

lobe

.D

:Sam

ese

ctio

nas

inC

but

un

der

phas

eco

ntr

ast.

For

abbr

evia

tion

s,se

eli

st.L

ater

alis

toth

eri

ght.

Sca

leba

rs5

0.1

mm

(app

lies

toA

–D).

(Figs. 2, 5A,B) and in the caudal tertiary gustatory nucleusafter IL applications (Figs. 3, 5C,D).

Posterior thalamic nucleus. The posterior thalamicnucleus (NTP) is also located in the preglomerular regionof the diencephalon. Labeled cell bodies and terminals inthis nucleus are found after Tla as well as after ILapplications (Figs. 2, 3, 6A,B). There is topography in therostral part of the nucleus. After Tla applications, thelabeled cell bodies and terminals tend to occur in therostrolateral part, whereas after IL applications the labelappears in the rostromedial part of the posterior thalamicnucleus (compare Figs. 2, 3). There were no obviousdifferences in the position of label in the nucleus after thethree rostrocaudally different DiI applications to the Tla orthe IL.

Dorsal hypothalamic neuropil. In all Tla and ILcases, a confined and dense terminal field was observed inthe dorsal hypothalamus, between the periventricularnucleus of the lateral hypothalamic recess and the preglo-merular tertiary gustatory nucleus (Figs. 2, 3, 6C,D). Thisterminal field has not been described previously and iscalled here the dorsal hypothalamic neuropil (DHN). Insome (2 of 11) Tla cases, but never in IL cases, also a fewcell bodies are labeled within the dorsal hypothalamicneuropil (compare Figs. 2, 6C).

Telencephalon. After Tla and IL applications, labeledfibers are present in the lateral forebrain bundle coursingto the ipsilateral telencephalon (Tel; Fig. 9C,D). A fewlabeled cell bodies are present in the central zone of areadorsalis and even fewer cells in the supracommissuralnucleus of area ventralis (Figs. 2, 3). Labeled telencephaliccells do not appear in six Tla cases. However, in these sixcases, the time of incubation was considerably shorter. Inno cases were terminal fields observed in the telencepha-lon.

Superior reticular formation. After both Tla and ILapplications, a labeled tract is observed to branch off fromthe axons coursing to their somata in the secondarygustatory nucleus. This tract remains ventrally in thebrainstem for some distance and gives off fibers into thevery rostral superior reticular formation before it fades out(depicted only in the inferior lobe chartings in Fig. 3). Itappears more likely that these fibers originate in the ILand the Tla than in the secondary gustatory nucleus,because the latter does not project to the reticular forma-tion in C. carassius (Morita et al., 1983). However, theprojection to the superior reticular formation may repre-sent collateral labeling originating in other retrogradelylabeled cells.

Additional connections of the torus lateralis

Inferior lobe. Labeled cell bodies and terminals aredistributed throughout the lateral part of the diffusenucleus and the central nucleus of the inferior lobe (IL)after Tla applications. Only the most rostral part of theinferior lobe is free of label (Fig. 2).

Secondary general visceral nucleus. Immediatelyventral to the lateral part of the secondary gustatorynucleus and dorsolateral to the tertiary gustatory tract liesthe secondary general visceral nucleus (SVN; Finger andKanwal, 1992). It projects to the torus lateralis, becauseretrogradely labeled cell bodies were found in the second-ary general visceral nucleus after Tla applications (Figs. 2,4A,B).

Dorsal tegmental nucleus. After Tla applications,very few labeled cell bodies (and only in 2 of 11 cases) arefound in the dorsal tegmental nucleus (not charted).

Additional connections of the inferior lobe

Torus lateralis. After IL applications, clearly recogniz-able cell bodies and terminals are found in those parts ofthe torus lateralis (Tla) remote from the IL applicationsite. Because of the proximity of the Tla to the applicationsite, it is not definite whether label is present in theportion of the Tla close to the IL in our slide preparations.However, tracer was applied specifically to the inferior lobeunder visual control by using the dissecting scope anddefinitely was not displaced or misplaced also after visualcontrol of the application site a few days after startingincubation.

Nucleus subglomerulosus. This preglomerular nucleus,the nucleus subglomerulosus (SG), lies ventral and caudal tothe tertiary gustatory nucleus and contains dense anterogradelabeled terminals after IL applications (Figs. 3, 5C,D).

Nucleus preglomerulosus caudalis and corpus mamil-

lare. As already mentioned, the tertiary gustatorynucleus contains labeled cell bodies only in its caudal partafter IL applications. In its most caudal extent, the ter-tiary gustatory nucleus forms a cap of labeled cell bodiesabove the nucleus preglomerulosus caudalis (Pgc) in whichlabeled cell bodies also appear (Fig. 3). More caudally andmedially, these two nuclei appear to fuse (Figs. 3, 7A,B).Still further mediocaudally the nucleus preglomerulosuscaudalis is replaced by the corpus mamillare (CM), whichalso contains labeled cell bodies throughout its wholeextent up to its most caudal boundary (Figs. 3, 7C,D). Nofunctional segregation of the mammillary body into dorsaland ventral halves as indicated by Yoshimoto and Ito(1993) and Ito et al. (1997) was apparent with respect to itsprojection to the inferior lobe.

Dorsal and ventral thalamus. Many labeled somataare present in the dorsal thalamic central posterior nucleus(CP; Figs. 3, 8). Furthermore, a terminal field appearsbetween the habenulo-interpeduncular tract (FR) and theanterior thalamic nucleus of the dorsal thalamus. Some ofthese terminals lie within the anterior thalamic nucleus(Figs. 3, 9A,B). Another terminal field area appears in theventral thalamus just mediodorsally to, and also partlywithin the ventromedial thalamic nucleus (VM; Figs. 3,9A,B).

Tegmental nuclei. Additional label after IL applica-tions was seen as two bands of labeled cell bodies that, incomparison with Bodian-stained material, extend throughmost of the mesencephalic tegmentum. However, cytoarchi-tectonically clearly delineated nuclei corresponding tothese two labeled cell populations could not be identifiedand a match with previously described tegmental nuclei isnot possible at present (but see Discussion section). Thus,we identify the two bands of label tentatively as tegmentalnuclei T1 and T2 (Fig. 3). Labeled cells of T1 can befollowed along the horizontal commissure in rostral direc-tion up to the level of the central posterior thalamicnucleus (CP; Fig. 8A,B). Throughout its rostrocaudal ex-tent, the second tegmental nucleus (T2) contains perilem-niscal neurons, i.e., neurons closely associated with thelateral lemniscus (Fig. 3).

Posterior pretectal nucleus. Scattered labeled cell bod-ies within the diencephalic posterior pretectal nucleus (PO)


Fig

.5.

Ph

otom

icro

grap

hs

oftr

ansv

erse

sect

ion

ssh

owin

gre

trog

rade

and

ante

rogr

ade

labe

laf

ter

DiI

appl

icat

ion

into

toru

sla

tera

lis

(A,B

)or

infe

rior

lobe

(C,D

).A

:App

lica

tion

site

into

rus

late

rali

san

dla

bele

dce

llbo

dies

and

term

inal

sin

the

rost

ral

part

ofth

ete

rtia

rygu

stat

ory

nu

cleu

s.L

abel

edso

mat

aar

eso

mew

hat

obsc

ure

dby

the

brig

ht

flu

ores

cen

ce,

but

the

nar

row

con

nec

tin

gtr

act

betw

een

the

toru

sla

tera

lis

(wh

ich

rece

ived

aD

iap

plic

atio

n)

and

the

tert

iary

gust

ator

yn

ucl

eus

iscl

earl

yvi

sibl

e.T

he

dors

alh

ypot

hal

amic

neu

ropi

l(D

HN

)ap

pear

son

this

sect

ion

just

atth

ebo

rder

ofit

sro

stra

lex

ten

t.M

ore

exte

nsi

vela

bel

ofth

em

ore

cau

dal

DH

Nis

show

nin

Fig

ure

6C.B

:Sam

ese

ctio

nas

inA

but

un

der

phas

eco

ntr

ast.

C:L

abel

edce

llbo

dies

and

term

inal

sin

cau

dalp

art

ofth

ete

rtia

rygu

stat

ory

nu

cleu

s(T

GN

)an

dde

nse

term

inal

fiel

din

the

subg

lom

eru

lar

nu

cleu

s(S

G).

Som

ela

bele

dfi

bers

and

term

inal

sar

evi

sibl

ew

ith

inth

epo

ster

ior

thal

amic

nu

cleu

s(N

TP

).D

:Sam

ese

ctio

nas

inC

but

un

der

phas

eco

ntr

ast.

For

abbr

evia

tion

s,se

eli

st.L

ater

alis

toth

eri

ght.

Sca

leba

rs5

0.1

mm

(app

lies

toA

–D).

Fig

.6.

Ph

otom

icro

grap

hs

oftr

ansv

erse

sect

ion

ssh

owin

gre

trog

rade

and

ante

rogr

ade

labe

laf

ter

DiI

appl

icat

ion

into

toru

sla

tera

lis.

A:L

abel

edte

rmin

als

and

cell

bodi

esin

the

post

erio

rth

alam

icn

ucl

eus

(NT

P).

Not

eth

atth

eca

uda

lpa

rtof

the

tert

iary

gust

ator

yn

ucl

eus

and

the

subg

lom

eru

lar

nu

cleu

s(S

G)a

refr

eeof

labe

l.B

:Sam

ese

ctio

nas

inA

but

un

der

phas

eco

ntr

ast.

C:T

he

dors

alh

ypot

hal

amic

neu

ropi

l(D

HN

)is

labe

led

imm

edia

tely

dors

alto

nu

cleu

spe

rive

n-

tric

ula

ris

ofth

ela

tera

lrec

ess

(NP

LI)

and

ven

tral

toth

ete

rtia

rygu

stat

ory

nu

cleu

s(T

GN

),w

hic

hal

soco

nta

ins

labe

led

cell

bodi

esan

dte

rmin

als.

Als

ovi

sibl

eis

the

clea

rbo

un

dary

betw

een

toru

sla

tera

lis

and

infe

rior

lobe

.Lab

eled

fibe

rsen

ter

the

infe

rior

lobe

com

ing

from

the

toru

sla

tera

lis.

D:S

ame

sect

ion

asin

Cbu

tu

nde

rph

ase

con

tras

t.F

orab

brev

iati

ons,

see

list

.Lat

eral

isto

the

righ

t.S

cale

bars

50.

1m

m(a

ppli

esto

A–D

).

Fig

.7.

Pho

tom

icro

grap

hsof

tran

sver

sese

ctio

nssh

owin

gre

trog

rade

and

ante

rogr

ade

labe

laft

erD

iIap

plic

atio

nin

toin

feri

orlo

be.A

:Lab

eled

cell

bodi

esin

the

mos

tcau

dalp

arto

fthe

tert

iary

gust

ator

ynu

cleu

s(T

GN

)and

inth

eca

udal

preg

lom

erul

arnu

cleu

s(P

Gc)

.In

this

case

,lab

elin

corp

usm

amill

are

(CM

)ap

pear

son

lyin

the

mor

eca

udal

part

ofth

enu

cleu

s.B

:Sam

ese

ctio

nas

inA

but

unde

rph

ase

cont

rast

.C:H

ighe

rpo

wer

phot

ogra

phof

labe

led

cell

bodi

esin

the

CM

.Not

eth

atno

labe

led

term

inal

sar

epr

esen

tin

CM

.Als

o,so

me

labe

led

cell

bodi

esar

evi

sibl

ein

the

caud

alin

feri

orlo

bead

jace

ntto

the

high

lyflu

ores

cent

appl

icat

ion

site

.D:S

ame

sect

ion

asin

Cbu

tun

der

phas

eco

ntra

st.F

orab

brev

ia-

tion

s,se

elis

t.L

ater

alis

toth

eri

ght.

Scal

eba

rs5

0.1

mm

(app

lies

toA

–D).

are located at the periphery of the nucleus throughout itsextent. Fine labeled fibers extending into the center of thenucleus probably represent dendrites of retrogradely la-beled posterior pretectal neurons. These dendrites demar-cate very sharply the complete rostrocaudal extent of theposterior pretectal nucleus in its position ventral to thenucleus pretectalis superficialis pars parvocellularis (PSp),which itself is free of label (Figs. 3, 9A,B).

Suprachiasmatic nucleus. The suprachiasmaticnucleus (SC) is the only labeled nucleus that lies in thepreoptic region. The suprachiasmatic nucleus contains

some labeled cell bodies (Fig. 3) also only after DiI applica-tions to IL.

Telencephalon. In addition to the above-mentionedlabeled cells in the central zone of area dorsalis and thesupracommissural nucleus of the area ventralis, we foundlabeled cell bodies in the medial zone of area dorsalis onlyafter IL applications (Fig. 3).

Cerebellum (Valvula). In 2 (of 20) inferior lobe cases,a few retrogradely labeled cell bodies were found in thevalvula cerebelli (not charted). Judged from their positionin the ganglionic layer immediately adjacent to the granular

Fig. 8. Photomicrographs of transverse sections showing retro-grade label after DiI application to inferior lobe. A: Higher powerphotograph taken from framed area of section shown under phasecontrast in B. Note labeled cell bodies in the central posterior thalamic

nucleus (CP) and a few labeled cell bodies belonging to the most rostralend of T1 (arrow). For abbreviations, see list. Lateral is to the right.Scale bar 5 0.1 mm (applies to A,B).


Fig. 9. Photomicrographs of transverse sections showing retro-grade and anterograde label after DiI application to inferior lobe.A: Labeled cell bodies in the posterior pretectal nucleus (PO) andanterogradely labeled terminals positioned on the one hand betweenthe habenulo-interpeduncular tract (FR) and anterior thalamic nucleusand on the other hand lateral to the ventromedial thalamic nucleus(VM; compare Fig. 3, panel 3). Further labeled fibers occur in thelateral forebrain bundle (LFB). B: Same section as in A but under

phase contrast. C: Labeled cell bodies in medial zone of dorsaltelencephalic area (Dm). A few labeled fibers are present in thesupracommissural nucleus of ventral telencephalic area (Vs) as well asin the central nucleus of dorsal telencephalic area (Dc); the respectivelabeled cell bodies are not present at this level (compare Fig. 3).D: Same section as in C but under phase contrast. For abbreviations,see list. Lateral is to the right. Scale bars 5 0.1 mm (applies to A–D).

layer, these labeled cells most likely represent euryden-droid (basal) cells recognized by Nieuwenhuys and Nichol-son (1969), which are the efferent cells of the teleosteancerebellum (Finger, 1978).

DISCUSSION

The discussion considers in sequence the forebraingustatory connections in cyprinids, the multisensory func-tions of the inferior lobe in cyprinids, and comparisons ofgustatory and inferior lobe circuitry among all teleosts.

Gustatory circuitry in cyprinids

The connections shared by the lateral torus (Tla) and theinferior lobe (IL) of the goldfish (i.e., afferents from thesecondary gustatory, tertiary gustatory, and posterior tha-lamic nuclei and efferents to the latter two; Figs. 10, 11)reveal their common involvement in the gustatory system.

Topography. Morita et al. (1983) described in C.carassius the topographical distributions of ascendinggustatory fibers within the secondary and tertiary gusta-tory nucleus (formerly ‘‘nucleus glomerulosus’’) with re-spect to their vagal and facial nature, respectively. Of theprimary gustatory centers, the vagal lobe projects ipsilater-ally to the lateral third of the secondary gustatory nucleus,the facial lobe fibers terminate ipsilaterally and contralat-erally in its medial part, and glossopharyngeal lobe effer-ents project in between these parts (Morita et al., 1983;Morita and Finger, 1985b). The tertiary gustatory nucleusreceives gustatory input from the secondary gustatorynucleus. Morita et al. (1983) described a separation be-

tween the vagal and facial gustatory systems up to thisdiencephalic nucleus, because the lateral secondary gusta-tory nucleus projects to the rostral tertiary gustatorynucleus and the medial secondary gustatory nucleusprojects to the caudal tertiary gustatory nucleus (Fig. 10).Our present results indicate a similar maintenance of thisvagal and facial separation in additional tertiary gusta-tory centers in the diencephalon. The Tla is connected withvagal system related nuclear subdivisions, i.e., the lateralsecondary gustatory nucleus and the rostral tertiary gusta-tory nucleus. The IL is connected with facial system-related nuclear subdivisions, i.e., the medial secondarygustatory nucleus, and the caudal tertiary gustatorynucleus.

These results are confirmed in detail when the topo-graphical representation of label in the tertiary gustatorynucleus is analyzed for three rostrocaudally adjacentapplication sites in Tla and in IL. The rostrocaudal se-quence of applications to the Tla and to the IL reveals thetotal topographical representation of the vagal and facialsystems in the tertiary gustatory nucleus (Fig. 12). The Tlamaps in a rostral to caudal sequence onto the rostral half ofthe tertiary gustatory nucleus, and the IL maps also in arostral to caudal fashion onto the adjacent caudal half ofthe tertiary gustatory nucleus. The detailed hodologic data(see Results section) indicate that there may be an area ofoverlap of interconnections between facial and vagal sys-tems in the middle portion of the tertiary gustatorynucleus. However, this middle region more likely repre-sents a distinct domain for glossopharyngeal gustatoryinformation processing as seen in the secondary gustatory

Fig. 10. Schematic diagram of ipsilateral central gustatory connec-tions of cyprinids after DiI-applications into (A) torus lateralis (Tla) or(B) inferior lobe (IL). Solid lines/filled arrowheads represent thisstudy’s connections, those indicated differently were established previ-ously by Morita et al. (1980,1983), Morita and Finger (1985b), andWullimann and Meyer (1993). Circuitry and rostrocaudal arrange-ment of gustatory nuclei in medulla oblongata (VLo, FLo), isthmic

region (SGN), diencephalon (IL, Tla, TGN, NTP), and telencephalon(Tel) is shown. The circuitry shown in A is referred to as intraoralbecause the lateral secondary gustatory nucleus receives input fromthe intraorally related vagal lobe. The circuitry shown in B is referredto as extraoral because the medial secondary gustatory nucleusreceives input from the extraorally related facial lobe.


nucleus (see below). Morita et al. (1983) described thetopographical relationships of the vagal and facial systemsbetween the secondary gustatory nucleus and the IL andthe Tla (their rostral part of the IL) in a HRP studyidentical to our results.

More specific statements about the glossopharyngealgustatory system and its representation at diencephaliclevels are difficult to make. As mentioned earlier, theglossopharyngeal domain in the secondary gustatorynucleus is sandwiched between the lateral (vagal) and

Fig. 11. Schematic cyprinid brain in parasagittal view summarizing known connections of thegustatory system (see text for references). Rostral is to the left. For abbreviations, see list.

Fig. 12. Schematic topographical relationships of gustatory centers inbrainstem, including primary regions such as vagal lobe (VLo), facial lobe(FLo), and glossopharyngeal lobe (G), and in diencephalon, includingtertiary regions such lateral torus (Tla) and inferior lobe (IL), all shown inlateral view in A. The tertiary gustatory nucleus (TGN) and rhombence-

phalic secondary gustatory nucleus (SGN) are shown in dorsal view in B.Green and blue zones show vagally and facially related gustatory regions,respectively. Red zone in caudal Tla and rostral IL is associated with redportions of the secondary and tertiary gustatory nuclei and may processglossopharyngeal information (see text). Scale bar 5 1 mm.


medial (facial) portions of this nucleus, but without a clearhistologic boundary. Therefore, it is possible that the mostlateral and most medial cell bodies labeled after IL and Tlaapplications, respectively, in the secondary gustatorynucleus belong to the glossopharyngeal domain. Thiswould mean that the glossopharyngeal gustatory informa-tion is represented in the diencephalon in two distinctareas, i.e., in the caudal Tla and in the rostral IL (red zonein Fig. 12). This hypothesis is supported by the fact thatafter rostral IL applications labeled cell bodies appearselectively in the central part of the secondary gustatorynucleus. However, a comparable selective central label inthe secondary gustatory nucleus is not seen after caudalTla applications as would be expected. A possible explana-tion for this is that, after caudal Tla applications, axonsoriginating in the secondary gustatory nucleus and enter-ing the more rostral parts of the Tla were interrupted,such that most cells in the central and lateral portions ofthe secondary gustatory nucleus were labeled. Thus, aresulting selective central label in the secondary gustatorynucleus might be impossible to achieve with this method.Alternatively, the glossopharyngeal gustatory informationmay ascend exclusively to the rostral diencephalic inferiorlobe.

The dorsal hypothalamic neuropil is clearly nontopo-graphically organized with respect to facial and vagalsystems and represents an additional possible area forinteractions between the intraoral and extraoral systems.

Telencephalic-diencephalic interrelationships. Theforebrain gustatory connections reveal functional anatomi-cal links between the facial and vagal gustatory systems.Both the vagal (intraoral) and the facial (extraoral) gusta-tory systems are represented in four diencephalic nuclei:the tertiary gustatory nucleus, the Tla, the IL, and theposterior thalamic nucleus. Although the Tla is primarilyassociated with the intraoral system, it also has an indi-rect link to the extraoral (facial) system by means of itsinterconnections with the inferior lobe (Fig. 10). Thefunctional reciprocity of these connections also appliesbecause the extraoral system is primarily represented inthe IL.

Whereas the posterior thalamic nucleus has no inputfrom the secondary gustatory nucleus, it likely receives, bymeans of its interconnections with two tertiary gustatorycenters, vagal information from the Tla and facial informa-tion from the IL. The posterior thalamic nucleus, in turn,has efferents to the telencephalon (Wullimann and Meyer,1993; Figs. 10, 11), and it is therefore suggested that thisnucleus acts as a quaternary gustatory relay nucleus tothe telencephalon. The posterior thalamic nucleus is alsoknown to have descending connections to the primarygustatory centers (Figs. 10, 11), and the cell bodies of thesedescending fibers are arranged in a topographical mannerin the posterior thalamic nucleus with respect to vagal andfacial lobe (Morita et al., 1983). Interestingly, our observa-tions regarding the distribution of labeled cell bodies andterminals within the posterior thalamic nucleus (at leastin its rostral part) after Tla and IL applications seem to becoincident with the vagal lobe and facial lobe relateddescending portions of this nucleus.

A further similarity between Tla and IL exists withrespect to their telencephalic inputs originating in bothcases in the central zone of the area dorsalis telencephali.The dorsal telencephalic area in general is a major targetof ascending thalamic (Kanwal et al., 1988) and preglo-

merular projections, such as the (lateral line-related)lateral preglomerular nucleus (Striedter, 1992; McCor-mick and Hernandez, 1996). However, the preglomerulartertiary gustatory nucleus does not project to the areadorsalis telencephali, but the posterior thalamic nucleusdoes (Wullimann and Meyer, 1993). This finding is indirect support of the above-mentioned hypothesis aboutthe quaternary gustatory function of the posterior tha-lamic nucleus, because it alone among all identified gusta-tory nuclei in the diencephalon of cyprinids is known toproject to the telencephalon.

The available neuroanatomical data suggest that thefacial and vagal systems are topographically separate tothe diencephalic level in cyprinids. We cannot discern inour data whether such a separation is maintained in theconnections between gustatory centers of the diencephalonand the telencephalon. A topography is, for example, notevident in the descending connections of the central zoneof the area dorsalis telencephali to Tla and IL. Roberts andSavage (1978) showed in a behavioral study that thetelencephalon had almost no regulative effect in foodintake and growth rate. Yet, electrical stimulation of thediencephalon evokes complex feeding behavior in variousteleosts, e.g., in the goldfish and sunfish (Demski et al.,1975; Demski, 1983). The area around the lateral lobes ofthe goldfish hypothalamus was therefore sometimes consid-ered to represent ‘‘the highest area of the brain’’ related tofeeding behavior (Roberts and Savage, 1978). This findingmight suggest, in accordance with the data in the presentstudy, that the distinct transmission of gustatory informa-tion with respect to peripheral topography is only function-ally necessary up to the level of the diencephalon. How-ever, the significance of the telencephalon in gustationmay be linked to long-term effects of learning and memory,a notion that is supported by recent ablation studies (Salaset al., 1996a,b).

In summary, the lateral torus in C. auratus is almostexclusively associated with the gustatory system, whereasthe inferior lobe has additional connections to severalother nuclei related to other functional systems (seebelow). The Tla has only one other nongustatory associa-tion, the connection from the secondary general visceralnucleus, but the IL appears not to be involved in thegeneral visceral system. The Tla has predominant connec-tions with the vagal gustatory portions of the secondaryand tertiary gustatory nuclei and the IL is interconnectedwith the facial gustatory portions of the said nuclei. Inaddition, both Tla and IL have efferent connections to adorsal hypothalamic neuropil, they both receive fibersfrom the telencephalon, and they have efferent as well asafferent connections to the posterior thalamic nucleus. Theposterior thalamic nucleus is suggested as a quaternarygustatory nucleus, because it is the only known projectionnucleus to the telencephalon, which has reciprocal connec-tions with tertiary gustatory centers, i.e., the Tla and theIL. Whether it has interconnections also with the tertiarygustatory nucleus remains to be determined.

Inferior lobe receives multisensoryinformation in cyprinids

In contrast to the torus lateralis (see above), the hypotha-lamic inferior lobe shows a variety of connections inaddition to its strong gustatory involvement. Whereassome of these additional connections (i.e., input from twotegmental nuclei T1 and T2) cannot be related to a specific


sensory or other function at present, the remaining affer-ents reveal a multisensory input to the goldfish inferiorlobe.

Octavolateralis connections. The teleostean centralposterior thalamic nucleus represents the dorsal thalamicrelay nucleus for the auditory system and also maybe forpart of the lateral line mechanosensory system (Echteler,1984, 1985; Ito et al., 1986; Murakami et al., 1986a,b;Striedter, 1991; Lu and Fay, 1995). The central posteriorthalamic nucleus has a strong projection to the inferiorlobe as revealed by our experiments (Fig. 8). This indicatesthat auditory and eventually mechanosensory lateral lineinformation reaches the inferior lobe in addition to gusta-tion (see above) and vision (see below).

Visual connections. The conspicuous posterior pretec-tal nucleus seen in most outgroups of ostariophysanteleosts (Butler et al., 1991) and the even more differenti-ated, evolutionarily derived nucleus glomerulosus of acan-thomorph teleosts (Ito and Kishida, 1975) were demon-strated to be homologous based on acetylcholinesterasehistochemistry (Wullimann and Meyer, 1990) and hodol-ogy (Wullimann et al., 1991). Both the posterior pretectalnucleus and the nucleus glomerulosus receive higher-order retinal information, and the efferents of the twonuclei reach the central nucleus of the hypothalamicinferior lobe, i.e., these homologous nuclei are part of aretino-pretecto-hypothalamic pathway (Sakamoto and Ito,1982; Murakami et al., 1986c; Striedter and Northcutt,1989; Wullimann et al., 1991). Acetylcholinesterase histo-chemistry, together with the fact that a large posteriorpretectal nucleus is present in teleosts plesiomorphically,led Wullimann and Meyer (1990) to postulate that cau-dally adjacent to the tectorecipient magnocellular superfi-cial pretectal nucleus of the goldfish, and of cyprinids ingeneral, is a highly reduced homologue of the ancestrallylarge posterior pretectal nucleus. An alternative hypoth-esis is that the area delimited by acetylcholinesterase inthe goldfish represents exclusively one nucleus, i.e., themagnocellular superficial pretectal nucleus (Yoshimotoand Ito, 1993). The latter nucleus receives a tectal inputand has efferents to the corpus mamillare of the hypothala-mus and to the nucleus lateralis valvulae in cyprinids(Northcutt and Braford, 1984; Ito and Yoshimoto, 1990;Yoshimoto and Ito, 1993). Critical to the recognition of aposterior pretectal nucleus separate from the magnocellu-lar superficial pretectal nucleus therefore is a connectionfrom the posterior pretectal nucleus to the central nucleusof the inferior lobe in cyprinids.

Our DiI applications to the inferior lobe included thediffuse as well as the central nucleus in the goldfish andrevealed labeled cell bodies in the posterior pretectalnucleus (Figs. 3, 9A,B). The possibility that we interruptedwith our DiI applications the efferent fibers of the magno-cellular superficial pretectal nucleus can be excluded,because terminals were seen neither in the nucleus latera-lis valvulae nor in the corpus mamillare in our experi-ments, although the latter showed many labeled cellbodies (Figs. 3, 7C,D). These results demonstrate thatthere are neurons in the posterior pretectal nucleus of thegoldfish that project to the central (and/or the diffuse)nucleus of the inferior lobe and, in contrast to the neuronsof the magnocellular superficial pretectal nucleus, not tothe corpus mamillare and the nucleus lateralis valvulae.We present herewith a crucial piece of hodologic informa-tion to support the existence of a morphologically reduced

posterior pretectal nucleus in the goldfish and, thus,further substantiate the existence of a morphologicallysimplified pattern of visual retino-pretecto-hypothalamicpathways in cyprinids as proposed earlier (Wullimann andMeyer, 1990).

The suprachiasmatic nucleus represents an additionalvisual input to the inferior lobe of the goldfish. Unlike thevisual input from the posterior pretectal nucleus, whichprovides spatial information on moving objects (likelyrelated to predation at least in percomorph teleosts, seeWullimann, 1997, for discussion), the visual projectionfrom the suprachiasmatic nucleus to the inferior lobe islikely related to the light/dark cycle of the environmentallight. In conclusion, it can be stated based on the hereinpresented hodologic information that the inferior lobe ofthe goldfish represents a multisensory integration center.

Comparison of cyprinid forebrain circuitrywith other teleosts

First, the gustatory circuitry in other teleosts will becompared with that in cyprinids, followed by a similarcomparison of the inferior lobe connectivity in teleosts.

Gustatory circuitry. Both siluroids and cyprinids arehighly gustatory-guided animals. However, the emphasiswith respect to extraoral versus intraoral portions isopposed. Whereas cyprinids rely relatively more on theintraoral gustatory system, especially the sophisticatedsensorimotor palatal organ in the roof of the oropharynx,the reverse is the case for siluroids, which display amultitude of taste buds on their extraoral barbels (Kanwaland Finger, 1992). Although the ascending gustatory infor-mation is relayed very similarly in the central nervoussystems of these two teleost groups, i.e. from the facial andvagal lobes by means of a isthmic secondary gustatorynucleus to several diencephalic tertiary gustatory centers,specific differences also exist (Wullimann, 1997).

A first difference relates to the number and connectivityof tertiary gustatory centers in the diencephalon. Whereasthe central and diffuse nuclei of the inferior lobe arecommon to both groups, an (intraorally related) lateraltorus is only present in cyprinids (Fig. 1). Although incyprinids the secondary gustatory nucleus does not projectto the posterior thalamic nucleus, we identify the latterhere as the homologue of the nucleus lobobulbaris ofsiluroids based on (1) anatomical position and large neuro-nal soma size, and on the common presence of (2) anafferent projection from the inferior lobe (Lamb and Cap-rio, 1993a; this study), (3) an efferent projection to thecentral zone of area dorsalis telencephali (Wullimann andMeyer, 1993; Kanwal et al., 1988), and (4) descendingprojections to the vagal and facial lobes (Morita et al.,1983; Morita and Finger, 1985a). Interestingly, Striedter(1990b) also concluded to name the caudal (telencephalo-projecting) part of the lobobulbar nucleus of I. punctatus‘‘posterior thalamic nucleus.’’ Furthermore, the nucleus ofthe lateral thalamus of siluroids is recognized here as thehomologue of the preglomerular tertiary gustatory nucleusof cyprinids based on (1) its anatomical position and smallneuronal soma size and the common presence of (2) atertiary gustatory input from the secondary gustatorynucleus (Lamb and Caprio, 1993a; Morita et al., 1983), and(3) an afferent projection from the inferior lobe (Lamb andCaprio, 1993a; this study).

A second difference between siluroids and cyprinids isthat siluroids have direct (secondary gustatory) projec-


tions from the (extraorally related) facial lobe to thediencephalon, i.e., the nucleus lobobulbaris, in addition tothe latter nucleus’ tertiary gustatory input (Lamb andCaprio, 1993a). In contrast, all efferents of the primarygustatory lobes in cyprinids project exclusively to thesecondary gustatory nucleus (Morita et al., 1983). A thirddifference is that the telencephalon is reciprocally intercon-nected with the inferior lobe in siluroids (Lamb andCaprio, 1993a), whereas in cyprinids only descendingtelencephalic projections to the inferior lobe have beendemonstrated (Wullimann and Meyer, 1993; this study).

Many if not all of these differences in the complexforebrain gustatory networks between siluroids and cy-prinids may be related to the relative predominance ofextraoral (facial) versus intraoral (vagal) gustatory compo-nents. Whereas the separation of the facial and vagalsystem is anatomically evident in both groups at the levelof the secondary gustatory nucleus, at forebrain levels aclear anatomical segregation is only obvious in cyprinids(Morita et al., 1980, 1983; Morita and Finger, 1985a; thisstudy). The relatively larger size of the cyprinid dience-phalic gustatory-related centers compared with siluroidsmay facilitate the recognition of topographical relation-ships. Alternatively, facial and vagal information mayconverge to a greater extent in diencephalic nuclei thatmay therefore be smaller in siluroids compared withcyprinids. Anatomically, the lateral torus seems to beexclusively related to the intraoral gustatory system incyprinids (this study). Therefore, it is not surprising thatin the extraorally dominated siluroids a lateral torus canneither be identified morphologically nor in terms ofascending higher order vagal lobe projections. Physiologicstudies in siluroids further demonstrate the predominanceof the extraoral system and the absence of a maintenanceof extraoral/intraoral topography at the physiologic levelin the gustatory forebrain neuronal network (Kanwal etal., 1988; Caprio et al., 1993; Lamb and Caprio, 1993b).

Both siluroids and cyprinids are gustatory specialists,and the ancestral condition of the teleostean gustatorysystem is likely to have been less elaborate. Additionalinformation on ascending gustatory projections to thediencephalon in teleosts is available for the green sunfish,Lepomis cyanellus (Wullimann, 1988). This percomorphspecies is far more visually oriented, and its gustatorypathways may represent a more plesiomorphic teleosteancondition. Interestingly, also L. cyanellus displays primarygustatory centers projecting to a isthmic secondary gusta-tory nucleus which, in turn, projects to the preglomerulartertiary gustatory nucleus, the lateral torus, and theinferior lobe.

Connections of the inferior lobe. The hypothalamicinferior lobe is a large and highly differentiated brain partin teleosts comparable to other major brain parts such asthe cerebellum, optic tectum, or telencephalon, and itincludes several histologically distinct areas, such as thecentral nucleus, the periventricular nucleus of the lateralrecess, and various divisions of the diffuse nucleus (Bra-ford and Northcutt, 1983). Whereas we cannot offer newinformation on how these subdivisions of the inferior lobeare intrinsically interconnected, the major afferents andefferents of the inferior lobe of the goldfish are reportedhere (Fig. 3). They reveal, in contrast to the lateral torus, amultisensory input (see above), suggesting an integrativefunction of the inferior lobe.

Less-completely documented reports on inferior lobeconnections in other teleost species are in good agreementwith our results. A connectivity pattern very similar tothat of the goldfish was reported in the osteoglossomorphteleost Osteoglossum bicirrhosum. In this species, inferiorlobe inputs arise in the suprachiasmatic nucleus, theposterior pretectal nucleus, a lateral tegmental nucleus(maybe comparable to the goldfish T2), and in the second-ary gustatory nucleus, and a comparable efferent tractexits the inferior lobe and terminates in the superiorreticular formation (Wullimann and Northcutt, 1990; Wul-limann et al., 1991). Furthermore, labeled fibers can befollowed in O. bicirrhosum into the central zone of thedorsal telencephalic area, but it could not be determinedwhether they represent an afferent and/or efferent connec-tion (Wullimann and Northcutt, 1990).

Lamb and Caprio (1993a) have traced selectively theconnections of the central nucleus of the inferior lobe of thechannel catfish I. punctatus. Remarkably similar to ourresults is that efferents of the inferior lobe reach two otherdiencephalic tertiary gustatory centers, i.e., the lobobulbarnucleus (homologous to the posterior thalamic nucleus incyprinids, see above) and the nucleus of the lateral thala-mus (homologous to the preglomerular tertiary gustatorynucleus in cyprinids, see above). In the goldfish, however,these connections are reciprocal (compare Figs. 10, 11).Also comparable to our results are the inferior lobe effer-ents of I. punctatus to the subglomerular nucleus and to anarea in the vicinity of the periventricular nucleus of thelateral recess (the latter probably corresponding to ourdorsal hypothalamic neuropil). However, the reciprocity ofthose two connections in I. punctatus would be interpretedas a specialization of siluroids, as are the additionalefferent projections of the inferior lobe to a medial preglo-merular nucleus and to the central zone of the dorsaltelencephalic area in the channel catfish (Lamb and Cap-rio, 1993a). The input from the latter to the inferior lobe isagain shared by siluroids and cyprinids. Surprisingly, noinput to the central nucleus of the inferior lobe wasreported to originate in the pretectal area in I. punctatus.Thus, there is no candidate for a siluroid posterior pretec-tal nucleus. This conclusion accords with Striedter’s (1990a)finding that the entire superficial pretectum is absent in I.punctatus.

Further information on the connections of the inferiorlobe is available for a member of the sistergroup ofsiluroids, the gymnotoid (weakly electric) fish Eigenman-nia virescens (Keller et al., 1990). The strongest input tothe inferior lobe in E. virescens was reported from thenucleus electrosensorius, a forebrain target of the ascend-ing electrosensory pathway, and from the closely associ-ated pretectal nucleus (Keller et al., 1990). The latter verylikely represents the posterior pretectal nucleus of otherteleosts and the inferior lobe connection of the nucleuselectrosensorius might indicate that this nucleus alsoevolved from the pretectal region. However, the electrosen-sory nucleus of the channel catfish I. punctatus (Striedter,1990a) surprisingly was not reported to project to theinferior lobe (Lamb and Caprio, 1993a). Also similar to ourresults in the goldfish, Keller et al. (1990) further report inE. virescens an input to the inferior lobe from the cerebel-lum and from the ‘‘subvalvular area,’’ the latter likelyrepresenting the secondary gustatory nucleus. Keller etal.’s globular terminal field in their ‘‘glomerular nucleus’’probably represents our dense terminal field over the


subglomerular nucleus. Thus, several sensory modalities,i.e., gustation, vision, and electroreception, the latterrepresenting a newly evolved sensory modality for someteleost groups (Bullock et al., 1983), appear to convergealso in the inferior lobe of gymnotoids.

In conclusion, the available information on inferior lobeconnections in teleosts is consistent with a hypothesis thatthis brain part serves as a multisensory integration center.Although it would be premature to analyze the scatteredpieces of hodologic information in a phylogenetic context,the following connections are common to at least someostariophysan teleosts and to one osteoglossomorph (O.bicirrhosum) and may represent the plesiomorphic connec-tivity pattern of the teleostean inferior lobe. This patternappears to consist of inferior lobe inputs from the second-ary gustatory nucleus, the posterior pretectal nucleus, thesuprachiasmatic nucleus, one tegmental nucleus, and fromthe central zone of area dorsalis telencephali, whereas thedescending output of the inferior lobe is to the rostralbrainstem reticular formation. Other connections dis-cussed above are more likely to represent specializations ofparticular teleost groups. Except for the strong inputarising in the posterior pretectal nucleus, the remainder ofthe mentioned plesiomorphic connectivity pattern of theinferior lobe is also present in a cartilaginous fish (theclearnose skate; Smeets and Boord, 1985), a fact thatfurther substantiates the plesiomorphic nature of thispattern.

ACKNOWLEDGMENTS

We thank Prof. Dr. Gerhard Roth for making it possibleto carry out this study in his laboratory and Carolin Pfaufor help with the photographic printing.

LITERATURE CITED

Braford, M.R. Jr. and R.G. Northcutt (1983) Organization of the diencepha-lon and pretectum of the ray-finned fishes. In R.E. Davis and R.G.Northcutt (eds): Fish Neurobiology, Vol. 2, Higher Brain Areas andFunctions. Ann Arbor: University of Michigan Press, pp. 117–140.

Bullock, T.H., D.A. Bodznick, and R.G. Northcutt (1983) The phylogeneticdistribution of electroreception: Evidence for convergent evolution of aprimitive vertebrate sense modality. Brain Res. Rev. 6:25–46.

Butler, A.B., M.F. Wullimann, and R.G. Northcutt (1991) Comparativecytoarchitectonic analysis of some visual pretectal nuclei in teleosts.Brain Behav. Evol. 38:92–114.

Caprio, J., J.G. Brand, J.H. Teeter, T. Valentinic, D.L. Kalinoski, J.Kohbara, T. Kumazawa, and S. Wegert (1993) The taste system of thechannel catfish: From biophysics to behavior. TINS 16:192–197.

Demski, L.S. (1983) Behavioral effects of electrical stimulation of the brain.In R.E. Davis and R.G. Northcutt (eds): Fish Neurobiology, Vol. 2,Higher Brain Areas and Functions. Ann Arbor: University of MichiganPress, pp. 317–359.

Demski, L.S., A.P. Evan, and L.C. Saland (1975) The structure of theinferior lobe of the teleost hypothalamus. J. Comp. Neurol. 161:483–498.

Echteler, S.M. (1984) Connections of the auditory midbrain in a teleost fish,Cyprinus carpio. J. Comp. Neurol. 230:536–551.

Echteler, S. M.(1985) Organization of central auditory pathways in a teleostfish, Cyprinus carpio. J. Comp. Physiol. A 156:267–280.

Finger, T.E. (1978) Efferent neurons of the teleost cerebellum. Brain Res.153:608–614.

Finger, T.E. (1983) The gustatory system in teleost fish. In R.G. Northcuttand R.E. Davis (eds): Fish Neurobiology, Vol. 1, Brain Stem and SenseOrgans. Ann Arbor: University of Michigan Press, pp. 285–301.

Finger, T.E. and J.S. Kanwal (1992) Ascending general visceral pathwayswithin the brainstem of two teleost fishes: Ictalurus punctatus andCarassius auratus. J. Comp. Neurol. 320:509–520.

Goehler, L. and T.E. Finger (1992) Functional organization of vagal reflexsystems in the brainstem of the goldfish, Carassius auratus. J. Comp.Neurol. 319:463–478.

Ito, H. and R. Kishida (1975) Organization of the teleostean nucleusrotundus. J. Morphol. 147:89–108.

Ito, H. and M. Yoshimoto (1990) Cytoarchitecture and fiber connections ofthe nucleus lateralis valvulae in the carp (Cyprinus carpio). J. Comp.Neurol. 298:385–399.

Ito, H., T. Murakami, T. Fukuoka, and R. Kishida (1986) Thalamic fiberconnections in a teleost (Sebastiscus marmoratus): Visual, somatosen-sory, octavolateral, and cerebellar relay region to the telencephalon. J.Comp. Neurol. 250:215–227.

Ito, H., M. Yoshimoto, J.S. Albert, Y. Yamane, N. Yamamoto, N. Sawai, andA. Kaur (1997) Terminal morphology of two branches arising from asingle stem-axon of pretectal (PSm) neurons in the common carp. J.Comp. Neurol. 378:379–388.

Kanwal, J.S. and T.E. Finger (1992) Central representation and projectionsof gustatory systems. In T.J. Hara (ed): Fish Chemoreception. London:Chapman & Hall, pp. 79–102.

Kanwal, J.S., T.E. Finger, and J. Caprio (1988) Forebrain connections of thegustatory system in ictalurid fishes. J. Comp. Neurol. 278:353–376.

Keller, C.H., L. Maler, W. Heiligenberg (1990) Structural and functionalorganization of a diencephalic sensory-motor interface in the gymnoti-form fish, Eigenmannia. J. Comp. Neurol. 293:347–376.

Lamb, C.F. and J. Caprio (1993a) Diencephalic gustatory connections in thechannel catfish. J. Comp. Neurol. 337:400–418.

Lamb, C.F. and J. Caprio (1993b) Taste and tactile responsiveness ofneurons in the posterior diencephalon of the channel catfish. J. Comp.Neurol. 337:419–430.

Lamb, C.F. and T.E. Finger (1995) Gustatory control of feeding behavior ingoldfish. Physiol. Behav. 57:483–488.

Lu, Z. and R.R. Fay (1995) Acoustic response properties of single neurons inthe central posterior nucleus of the thalamus of the goldfish, Carassiusauratus. J. Comp. Physiol. A 176:747–760.

Luiten, P.G.M. (1975) The central projections of the trigeminal, facial andanterior lateral line nerves in the carp (Cyprinus carpio L.) J. Comp.Neurol. 160:399–417.

Marui, T. and J. Caprio (1992) Teleost gustation. In T.J. Hara (ed): FishChemoreception. London: Chapman & Hall, pp. 171–198.

McCormick, C.A. and D.V. Hernandez (1996) Connections of the octaval andlateral line nuclei of the medulla in the goldfish, including the cytoarchi-tecture of the secondary octaval population in goldfish and catfish.Brain Behav. Evol. 47:113–138.

Morita, Y. and T.E. Finger (1985a) Reflex connections of the facial and vagalgustatory systems in the brainstem of the bullhead catfish, Ictalurusnebulosus. J. Comp. Neurol. 231:547–558.

Morita, Y. and T.E. Finger (1985b) Topographic and laminar organization ofthe vagal gustatory system in the goldfish, Carassius auratus. J. Comp.Neurol. 238:187–201.

Morita, Y. and T.E. Finger (1987) Topographic representation of the sensoryand motor roots of the vagus nerve in the medulla of goldfish, Carassiusauratus. J. Comp. Neurol. 264:231–249.

Morita, Y., H. Ito, and H. Masai (1980) Central gustatory paths in thecrucian carp, Carassius carassius. J. Comp. Neurol. 191:119–132.

Morita, Y., T. Murakami, and H. Ito (1983) Cytoarchitecture and topo-graphic projections of the gustatory centers in a teleost, Carassiuscarassius. J. Comp. Neurol. 218:378–394.

Murakami, T., T. Fukuoka, and H. Ito (1986a) Telencephalic ascendingacousticolateral system in a teleost, Sebastiscus marmoratus, withspecial reference to fiber connections of the nucleus preglomerulosus. J.Comp. Neurol. 247:383–397.

Murakami, T., H. Ito, and Y. Morita (1986b) Telencephalic afferent nuclei inthe carp diencephalon, with special reference to fiber connections of thenucleus praeglomerulosus pars lateralis. Brain Res. 382:97–103.

Murakami, T., Y. Morita, and H. Ito (1986c) Cytoarchitecture and fiberconnections of the superficial pretectum in a teleost, Navadon modes-tus. Brain Res. 373:213–221.

Nieuwenhuys, R. and C. Nicholson (1969) Aspects of the histology of thecerebellum of mormyrid fishes. In R. Llinas (ed): Neurobiology ofCerebellar Evolution and Development. Chicago: American MedicalAssociation, pp. 135–169.

Northcutt, R.G. and M.R. Braford Jr. (1984) Some efferent connections ofthe superficial pretectum in the goldfish. Brain Res. 296:181–184.


Puzdrowski, R.L. (1987) The peripheral distribution and central projectionsof the sensory rami of the facial sensory nerve in goldfish, Carassiusauratus. J. Comp. Neurol. 259:382–392.

Rink, E. (1996) Zentralnervose Konnektivitatsstudien des gustatorischenSystems bei Karpfenfischen (Cyprinidae). Diploma thesis, University ofBremen, Germany.

Rink, E. and M.F. Wullimann (1996) Forebrain gustatory pathways in thegoldfish, Carassius auratus. In N. Elsner and H.-U. Schnitzler (eds):Brain and Evolution, Proceedings of the 24th Gıttingen NeurobiologyConference, Vol. II. Stuttgart: Thieme, pp. 287.

Roberts, M.G. and G.E. Savage (1978) Effects of hypothalamic lesions onthe food intake of the goldfish (Carassius auratus). Brain Behav. Evol.15:150–164.

Rupp, B., M.F. Wullimann, and H. Reichert (1996) The zebrafish brain: Aneuroanatomical comparison with the goldfish.Anat. Embryol. 194:187–203.

Sakamoto, N. and H. Ito (1982) Fiber connections of the corpus glomerulo-sus in a teleost, Navodon modestus. J. Comp. Neurol. 205:291–298.

Salas, C., C. Broglio, F. Rodriguez, J.C. Lopez, M. Portavella, and B. Torres(1996a) Telencephalic ablation in goldfish impairs performance in a‘‘spatial constancy’’ problem but not in a cued one. Behav. Brain Res.79:193–200.

Salas, C., F. Rodriguez, J.P. Vargas, E. Duran, and B. Torres (1996b) Spatiallearning and memory deficits after telencephalic ablation in goldfishtrained in place and turn maze procedures. Behav. Neurosci. 110:965–980.

Sibbing, F.A. and R. Uribe (1985) Regional specializations in the oropharyn-geal wall and food processing in the carp (Cyprinus carpio L.). Neth. J.Zool. 35:377–422.

Sibbing, F.A., I.W.M. Osse, and A. Terlouw (1986) Food handling in the carp(Cyprinus carpio): Its movement patterns, mechanisms and limitations.J. Zool. (Lond.) 210:161–203.

Smeets, W.J.A.J. and R.L. Boord (1985) Connections of the lobus inferiorhypothalami of the clearnose skate Raja eglanteria. J. Comp. Neurol.234:380–392.

Striedter, G.F. (1990a) The diencephalon of the channel catfish, Ictaluruspunctatus: I. Nuclear organization. Brain Behav. Evol. 36:329–354.

Striedter, G.F. (1990b) The diencephalon of the channel catfish, Ictaluruspunctatus: II. Retinal, tectal, cerebellar and telencephalic connections.Brain Behav. Evol. 36:355–377.

Striedter, G.F. (1991) Auditory, electrosensory and mechanosensory lateralline pathways through the forebrain in channel catfishes. J. Comp.Neurol. 312:311–331.

Striedter, G.F. (1992) Phylogenetic changes in the connection of the lateralpreglomerular nucleus in ostariophysan teleosts: A pluralistic view ofbrain evolution. Brain Behav. Evol. 39:329–357.

Striedter, G.F. and R.G. Northcutt (1989) Two distinct visual pathwaysthrough the superficial pretectum in a percomorph fish. J. Comp.Neurol. 283:342–354.

Wullimann, M.F. (1988) The tertiary gustatory center in sunfishes is notnucleus glomerulosus. Neurosci. Lett. 86:6–10.

Wullimann, M.F. (1997) The central nervous system. In D.H. Evans (ed):The Physiology of Fishes. Boca Raton: CRC Press, pp. 245–282.

Wullimann, M.F. and D.L. Meyer (1990) Phylogeny of putative cholinergicvisual pathways through the pretectum to the hypothalamus in teleostfish. Brain Behav. Evol. 36:14–29.

Wullimann, M.F. and D.L. Meyer (1993) Possible multiple evolution ofindirect telencephalo-cerebellar pathways in teleosts: Studies in Caras-sius auratus and Pantodon buchholzi. Cell. Tissue Res. 274:447–455.

Wullimann, M.F. and R.G. Northcutt (1990) Connections of the hypotha-lamic inferior lobes in the teleost fish Osteoglossum bicirrhosum. Eur. J.Neurosci. Suppl. 3:188.

Wullimann, M.F., D.L. Meyer, and R.G. Northcutt (1991) The visuallyrelated posterior pretectal nucleus in the non-percomorph teleostOsteoglossum bicirrhosum projects to the hypothalamus: A Dil study. J.Comp. Neurol. 312:415–435.

Wullimann, M.F., B. Rupp, and H. Reichert (1996) Neuroanatomy of theZebrafish Brain. A Topological Atlas. Basel: Birkhauser.

Yoshimoto, M. and H. Ito (1993) Cytoarchitecture, fiber connections andultrastructure of the nucleus pretectalis superficialis pars magnocellu-laris (PSm) in carp. J. Comp. Neurol. 336:433–446.


some forebrain connections of the gustatory system in the goldfishcarassius auratus visualized by...

Documents