.~I~:(:TION IV. - ANTIDEPRESSANT DRUGS

KFFECTS OF LONG-TERM DRUG TREATMENTS ON THE SENSITMTY OF PRESYNAPTIC ItECEPTORS REGULATING NEUROTRANSMITTER RELEASE

\ I . ltAITERI, A. PITTALUGA, 1%. LEARDI, G. MAURA ani1 M. MARCHI

I~isbitute of Phar?nacology aitd Phnrrnacqnosy, University of Cenon, Italy

Sniiiinnry. - Tltc r r l ~ a s ~ rif ii~i~rotri~nnrnilt~'r,i from l m y n a p t i c nerzie rndingn eumL be rpgulated tlrrouglt

/ / I < > actjt:at&on of reccptors Incdizcd on //te rztwnal ,rce~r~hrmr~e of fhe w m e eailings (pres?lnmptic rmeptrm). T/ce release o/ a given transrnitter can he inhibitpd by 1/11? t~ran~miltcr itself tlwouyh thr: u,cl%~rrrtion o/ receptors L n m a (6s presyriaptic uuto,.crrptovs or c m be mmiuln~t~il h!/ another transmitter tl~rough the mt%va,tion o/ receptms /i.rmed presynaptic kterorcrepfors. A number o/ auto- rcnd heteroreceptors have been characterized in onr iaboratoy using rat 6rtzin synaptosomes. In spap to - somes prepared from rats chronically pretrrated urith rlrngs actiziating the different presynaptic nutorrcrptors, (5 6 v e n concen,tration of exogenous NA, A C h m 5 H T inhibited the release o/ the corresponding raclioactioc transmitter lrss efictively than in synaptosomes pre- pared from rats receiving a sin,gle dose of the a,qonist. The opposite ( e . higher in,hihition) u n s observd Jollowing chronic rdministration of nutnrrceptor anta- qonists. l a contrast, the sensitiwity o/ the muscarinic prcsyn,aptic heterorpceptors mediatinq potrntiation of 311-DA rrlease in striatal synaptosomes r ema in~d wachanged dlcrin,g chronic admin,istrntion of paraoxon or scopolamine. S,imilarly, chronic administration of (+)mianserin or (-)minnserin &id wot wwdify rhe extent of inhihition of 3H;IHT relense liy e x o p o u n N A in cortical nerve endings. Tlce /rcct that the diffe- rences betzaeen pres?/naptic au~toreeeyitnrs, und /he corresponding prrsynnptic I~eterorecey~tors, ,in tenns of sensihility wr.odzdatiow, brcorr~ rcjopent folloning rhronic drug tr~f&$n,enla winy hrrsr i~nlurrsting clinirnl implirn~tions.

Riassunto (EfTetti <li un trnttaiiicnto proluiigah siilla sensibiliti dei recettori prrsinaptici che regolano la liheraziime del neiirotrnsmrttitore). - IA librmzione dei ne t~rorarnet t i to i dalle terwcinrmioni nerziose p6 rrsere r~golnla ~lrcll'a~ltiaazione d i ,reeellori localizmti nrlle nwmbrrr,n~ ~ s t w n e delle twminwioni nrrtrnse stesse

It is widely accepted that t,he ueiisit,iv~ty of neliro- transniitter receptors c m he iiiodified during long- terin activntion or hlockade of thesc rcceptors Ly specilic ilriigs. hlthoiigh scnttererl exaiiiples of recep-

tors wliosc sensit,ivity was ririt altcrell b~ tlic a.liouo ~iiii,ni]nilati«iis 1i;~rc Iiern r ~ p o r t a l , tlir iiif<irmat,ioii availalde ivoultl noi, allow tlir spcc~ilatiou whet,hrr certain hypes of reonptors lindergo sensitivity cìiaiiges more easily than others. Receptors nic.diat,iiig regu- lation of neurotrans~iiit,ter releasc exist on nerve teriniiials of tlie mamninliaii 1)raiii (prrrynaptic recep- t,ors). Tlie relcase of a givcii trniisniittcr can Iie regii- laterl Iiy tlic transiiiittci itself through tlie activatioii of presynaptic aut,orecc]~tors. as well as hy otlier t,ransmit,ters tlirungh tlic activation of p~e,?ynaptic het,erorcceptoru. Prcsynalrtic roceptorn leiid t,heiii- selver; to functioiial stndies of sensitivity niodulat,ion since neurotransiiiit~t~er reicase cali ])C nicnured easiiy in brain tissue froni aiiiinals pretreated with recrptor agonists or antaponisti;. A nunil~er of presynalrtic aiit,o- and heterorecrptors liave Iieen characterizeil i11 onr lahoratiiry iisiiig mrve eiiiiiiigs isolated froiii variuiis arcas nf tlie innniiiialiaii hrain and snnic rif tlic recriit rcsiilt,s liavi, Iieeii siiiiiiiiariz~~I iii t1ii.s chapter.

The data ~iwsentcd siig~cst that prc~s,yna]it,ic auto- recepbors nlay 111. iiiorr? niisceptibìr tìian ~lresyiinptir. bcterorecept,ors to c,hanges of seiisitivity elicited by long-term in vivo adininistrat,ion of agoriist or anta- gonist drngs.

E.EE:DIIACI< INIIIHITION OF TRANSMWTER RELICASI.:

MEDIATEn BY PRESYNAPTIC AUTORECEPTORS.

A nuinber of criteria sliould Ire mct Iiefo~c assuiiiiiifi the existeiice of aiitoreceptors (or lieterorecept,ors) rnediating repulatio~i of transmitter release ou tlic memlrane snrface of preyiiaj~tic nerve t,erinina,ls: rr) tho release of a given t,ransinit,t,er shoiild be regu- lat,ed hy the transmit,trr itself (or by other trnns- niitt,ern) acting at the cxternal presynaptic memliranr; ò) thc replat ion of rclease shonld be detect,able iii isolated iierve terniinals; c) the regulatory action of the transinitter should be countemcted l ~ y sclect~iw receptor aiitagoiiists.

Direct evideiice tbat acetylc,holine (ACh) cali inliibit, it,s own release in cholinergic nerve tcrniinnls nas obt.ained using synaptosomes prepared froin rat, h i p p o c m n p u ~ l ] or cerebral cortex [2], prelahelcd wit,li 'H-choline and depolariecd n i th 1.5 mM IKX in a superfusioii systeni 131. As showii in Fig. 1, 10 pM ACh depressed by 50 76 tlie depolarizaticu- evoked CazLdependent release of 3H-ACh from rat' hippocampal ~ynnptosoriics. Atropine (l pM) con- plrtely counteraot,ed the sction of ACh. Various muscarinic agonists niimicked ACh as inhihitors of 'H-ACh release., &her in hrain slices [4, 51 or in syiiap- tosomes [ l , 2, 61 and their action was antagoiiized

Fic. 1 . - Rhcarinir, alplis2-ndreiierpic and t;erotoniiiergir ~iiboreeeptora on nei\-e endings of mt brain. Syninaptoaoriiea frani hipporampiis, cortex or liypotlialamus werr Isbrled reslieot,ivrly witli iH-clialine (11.1 @f: 4 min), 3H-norarln>- natine (0.114 pilI; I O mi») or 'H-5-~draxyt~yptarninr (O.(W pM; I O oiin) and sul>wfusrd ar previoudy dereribed [3). Uepolarization was obtnincd aith 15 mX KCI. The frartiona collectrd iluring ~iiporfimion were anzlyzed far tlieir roiitont, nf' ]H-aniiiw or 'H-aretylrlioliiic aiid fraotionnl rclonw PWVPS obtninril aa dwi:ribcrl prcvioiisly [ l 1 , 151. Tlie data i.aport,erl rrpwsent tlir p w c ~ i i t of tlic I<++vokr~rl rolirxae

st tlie prd< ioi in( a i tlio relcuse <,iirve%

hy ntroliini: or scq~daiiiiiii:. Striatal nnrve riidiiigs sceiii tu dilfrr froiii liip~>ucainpal or cortical nrrvc rndings; in fact,, n~uscaririic aiit,oreceptors irihiliiting t,lie Ki-cvoked 'H-ACIi releasc were niiicli lrss effectivc in ra t striatal synaptosomes [2].

I'resyiiaptic aizt~orrccptors of t,lie aljiliaz-adicnmgic t y p havc Ixmi rcported t,o exist l i d i in tlie ]ieriplie- rd arid in tlie ccnt,ral noradrciiergic aystein [7, 81. Rclease stiidics carried out a i t l i isolat,ed hrain nerve tcrininals [Y, 101 have yielded res~ilts support~iiig the existencc of alplia,-adrencrgic aiitorecept,ors on t,lic exteriinl nienilirann of centra1 noradrenergic iierve eridirigs. Exogenous noradrenalinc (NA), in thc presmce of dcsipraniine addrd t o prevent NA intra- terininal p t~~e t ra t ion t,lirough the NA uptakn carrier. inhibited in a conccnt,~,at,ion-dependent way tlie release of 'H-NA evoked by 15 iiiM liC1 froni ra t cortex synaptosomes [Il]. Tbe inliibit,ory actioii of NA (0.1 @M) was nntagonizcd hy yoliiinl~inc ( l M ) , a selective alpha2-adrrnorel,tor blocker. biit not hy the alpha,-aiitagonist prazosin (Fig. 1).

Also the depolarization-evoked release of tritiate11 5-hydroxytryptamine (5HT), previously taken ii],

into hypothalaniic syiiaptosomes [l'L] or cerel~rnl cort,ex sliccs [13], w s iriliihit,eil lry cxogciious 5HT. in th r prcseiicc of c,liloriniipraii~irir: added to preveiii the intrat,rrniiiial penetration of 5HT throiigli it- uptake carrier. The actiori of 5HT was antagonizril by methiot,hepin, l u t not hy other known 5HT r i , ~ ceptur antagonist,~, like cyproheptadine, methysw gide or mianscrin 1121 (Fig. l), suggesting tha t 5H'I' presyna,ptie autoreoe~~tors differ pharniaeologically from 5HT postsyna,ptic receptors.

I i i concliision, t,he (lata obtaiiied vit,Ii isolate11 iiwrv,. rndings niid iiininarized iii Fig. 1 imlicnte that i s r i n i c c o l i n e r i c alplia,-adrenrrgic and srro- !<\ilin aiitorrcrl>tors nre locilterl on presyiinpt,ic nervc a~ii~liiig"iii v;xriotis meas nf tho i a t 11rnin.

~ . ' / id im~qic rquIr~,t~iow o/ d r ~ p w i m r d m v 1,11~ro~1~11 ,,iiisc~irirric /zrtero,.eeeplors. - It mas fniiiiil that tlic I>;i,<al release of dopainine (DA), newly syiit~hrsizrrl I'riiiii t,yrosiiie in strintal ilices, wns potcnt,intrd hy .\CI: tlirniigh thc act,ivnt,ioii r~f iiiiiscnriiiic reccptors I I,4]. We reel,ort,ed rccently [l51 th:d iniiscariiiic i-iwptors locnteil oii strintal DA terrninals (iiiiiscn,rinic Iivtei-orecept,ors) caii iiicdinte potent,iatioii of DA d e n s e not oiily iinder resting cnnilitions, 11nt alsti iuheii '' fliinsi-l~liysi«logicnl " ilepolnrieing stiiiiuli ( i .r . lfi niK KC11) nre iiserl. Fig. 2 sliown thnt tlir releasc of 'H-DA iiidiicerl by 1.5 iiiM KCI froiii sitprrftised mt xtriatnl syiinptosn~iirs, ~ireviously labellecl wit,li tlie ~ldioact ive ;~iiiiiie, was iiicrtwc~l Iiy esoyeiioiis hC1i. Atropine (0.1 [LM) tiitnlly :mtagoiiizeil t,lie .stir~iiil~~tory act,ion of 10 pM 4Cili. Tlie poteiitiatiim Ily ACIi of tlie I<'--evokcd IH-DA relense decreaseil witli incrrasing I<' ci~iiceiitrntions and totally <lis;y>- pcnxil wlieii syiinpt~osnines wcre depolnrize<l wil:li 55 inX KCL (not shnwn). Miiscnrinic rccclit,ors 1iirili;i- ting ~ ~ i h ~ ~ t , i ; ~ t , i r ~ i i of ~lect,rically evol<e<l 'l[. ] ) I \

rclease werr rcport<vl to exist d s o i r i r : ~ t <:;iiiil;ili' sliccs [16].

1 1 i o 1 of .sri~rilowìn rrlmse Lh'r(~w/A, u I p h ~ ~ - rrdr~nr>.r+c lrr~lrror~~reptors. - .i.; repnrt~cl nlirwt~, iioi.;i- ilrriiergic nxon trrniiiials ;ire c n d i i ~ r d t i >iiit,ii- wcrptorx o€ t,lie nl11h;l-arlreiiergic typr. Htit<lirv witli rnt hippocninpnl 1171 and «ccipit,t~l ciirtc:~ 1181

slic<:n iiitli<i:ate tlint ;ilso th r ,Iepolnrjs,atir>n-cvi>lceil re lene o€ )H-SHT caii 110 iiiliihited 1iy exiisriioiis NA tliroiigh t,lie nct,ivnt,ioii of all>ha,-a11rcnoceptoi.s. Experiiiiiwt,~ cnrrird oict witli crrel~rni cort,es sy11n1)- t,osoiries 1.19) Iiavc ~~rovi i l rd ilircct cvideirce for tlie i&teiice of ]~rrs';iinpt,ic 1~llilin~ai1rciiei~;ic Iietero- recrlhirs o11 tlii? rstcrii;i,l n i i i i ~ r n ~ i e of ser»t,r~niii iierve cniliiiys. Tlir resiilts nf tlirse expcriiiients arr siitniiiiirizcil in Fig. 3 sliowing t h t tlie iiiliil~itoy ;~ctiilii ~ i f cxogi~noiis iVA oti the T<.~-evukrd 'H-fiHT rclu;tsr Frinii siipcrftisril coitical syiinptosomrs wis roiint,er;lctril liy tlic iiiixed tylie ;~lplia,-a,ll~lin~ ;ulwi~rrgic aiit~z~gotiist ~ ~ I i ~ ~ i ~ t ~ f ~ I ~ i ~ ~ i i i ~ i ~ niid 11y yol i i i~~- lini,. liiit iiot Iiy pr:iziisiii. Also iniiii~srrin, riti anti- rlt:psess~u~t r ciiilowe(1 witli nll~li;i,-a,lrt~nripio ;~iit;iy~,iiistic act,ivity LZOI con111 ri.\vrs,: t , I i~ iiiliil,itiun I J ~ XA 01' T - 5 l I T i.i.Ii,;isi~.

H b a s a i r e l e a s e H 15 mM K C I w + 10 PM ACh H 8, + ,, + 0.1 VM ATROPINE

time o f siiperfiision ( i i i tn , l

Stiidics o€ rrrrptor s~nuitivity Iiaw! I i e m cnrrieri oiit cxscntinlly Iiy thi. rn~lioliyand I~iiidiiig tecliiiique. FJowcvrr, witli t,liin t,r~cliniqiie i t is nfteii ilificult tri cstalilisli \\-lirt,lier tlic chnnges in Iiiiiilin$ sites, iiimsiircil €or iiistaiirn nft,er Iiiiig-tccrni driig trcat- iiicrit,~, Iinvc iiiileed occiire~l a t tlic ~irexynaptic level. \V'? t,liouglit t,liiit t,ransziiitter rclense fsoin ,superfuse11 synal~tosoriics coiilil rclireseiit n gor~d functioiial [~;~'.'wwtrr to inve~tigntr C I I I L I I ~ ~ of c n ~ i t i v i t y of

t i/

--P - 18 19 20 21 22 2 3 24

time of superfusion (mi r i . )

prcsyiiapt,ir n i ~ t o - aiid Iint~rri>i.ec~elitol.us iiirolred iii

trniisiiiit,t,c~i. r<rlrvsc regiililt,ioii. Jl;it,s wrrc t,rrat,ed wit,li t,hc difftwnt npniiists nrid

~itit:ipiwist.s tini1 t,lir neiir«t~raiisii~itt<.i. relrase wi.;

st;iirlied in syriiq~t~rromrs ~>rclal~clril witli ZII-NA, 211-51W, IR-DA or )H-CIi. Clinngcs i11 seiisit,ivit,y to exogriioils N.4 or ACIi ivrrcJ dct~eririiiirrl Jiy coiiiliarii~g t,lir cffects of snmc agonists o11 t,lie KCI (15 111hl)- evokril traiiiiiitt,cr rrlcasr in syiialit~osoiiies piqinrcrl froin driig-trcated uiirl nnliiic-trcatrd rnts.

or aciitcly (une siiigle i . p iiijrctirm, 10 rrig/kg). t~iiil syiiapt,osoiiien wero prcpnrril from tlie Iiypot~li;i.laiiiiis 1 rlayn after the Iast inject,ion.

Exogenoiis NA decrenseil the rclease of q - N A stiniiilateil by 15 mM KCI more effect,ively in t,hr ncrve tcrminals prepzrcrl froni tlie a,irimals chroiiic;i,lly t'rentrd witli (~+iiinrisrrin t,linn iii synapt,nsoitics ~>htaineii friiiii rnt,s givcii ;L singlc ilr~sr of tlir rlriig 1,211 (Fig. 5) .

The possiliility tlint cliolini-rpic miiacnrinir n,iito- recept,orn iintlcrgo athptive chisngcs fullowing loi~g- term nctivntion or liloclzatle wns investigatrd in synii~ptosomrs ol~tniiird from rnts wliich lini1 1 w n prrtreateil with t,hc ncrtylcholinesternsc inliihitor paraoxon (0.15 mg/kg on rlny 1 nnil 2, fiillowcil Iiy 0.07 i n g / k ~ froni ilay 3 t,o 11; s.c.), or ivitli tlic iiiiisci~- rinic ant<agonist, w,oliolamiiic (10 ing/kg; 8.e. for I I ilays), or with d i n e (fnr 11 clays). Groiips of rnts receivcil a single iiijection of paraoxoii (0.15 ingllfp; s .~ . ) , sccipol:~inine ( l0 ing/l<g, s.c.) or saline. FIippo- cnn~pal or rortical syiinpt~osoriies ivere ~ircp;i,rcd 2 clays nftcr t,hc last inject,ion. lirelnliclrcl with 3H- I o l i i i r n l i ~ l n r i i l in I I I ~ ~ I I S ~ ~ I I i l I5 i1111 KCI.

As sliown in Pia. 6, in synnptosrmii~s froiii tlic i~iiimnls chronic,nlly trrntrcl witli pn,rai~xoii or scopo- l in ine , exogenrous ACIi W ~ S respcctivcly less mrl i i i i m ciììcient t,hnii in synnpt,osoiiicm friiiii :mitrly-

,\LI t,i>g<b,thr. t,lii. r<wiltx ~wwir twl i i i Ilii* wctiiiii iitiliriiti? th;~t. tlir :iiittiriwlitrir nwiliiiti~~l tiiwliiiiiisii~~ Sor t,Iic r?giiIi~t,ioii (11' t , l i ~ ! rvlvnw hll'i', NA , N : \ l% rrow i ~ ~ ~ i ~ t ~ r ; ~ l I I I > Y V , : w l i ~ i p rnii I W V O I I ~ V s i ~ I ~ w ~ ~ i ~ i l , i w ~ 11r s i i l~~mmxit ivi~ I ' d l i i w i ~ i ~ I i i i i ~ tt<-riii :u:tiv:itiim { ~ r Iil~ick:iili! witli ;igoni.it or ;iiit;t,goni~t i l r i i g ~ I t w;is

rqmrtivl rr~cr~iitly p!:1I t,liat :ilsn tlici [)i\ iccnl~titrs involveil i r i t,lic L t i 1 ; t i n of '11-DA rclwuc From iahliit ca,iiilate slicw ~o i i ld lieciiii~u siipcreii- sitive fulh~wing long-tecriri ;iiliniiiist~rnt~i<iii of tlie L ) A r w r l ~ t o r nntngrmist linliiperiilol.

If onc msiiiiics t,liii.t riciirotrni~siiiitti~r rrccpt,or siipcrwnsit~ivity origiiiatrs iliiriiig rL long-trrni ilepri- v:ltioii o r i~ i i i~ i i~ i~ i io~ i s t,i;insiiiit,tr3r, tìie <I;ltn piweiiteil in tlii.; m!t~inii siip& t,lint ;L t,iiiiic ;ict,i\rntioii of aiitoiecey~t~ors OCCII~S i11 t,lic livii~g 1,r:~in.

to c,ompnrc aiit,o- aiid Iirterorcc~ptors iii t,eriiis rir

siiscept~ibility to t,lir arlalitivc clianges iii~liireil liy chronic drug treatiileiits.

Fig. 5 shows that clironic miariserin racriiinf~ elicit,erl alplia,, niitoreceptor siipersrnsitivit,y iii nil, Iiypot~hnlaiiiiis synnptosoiiies (= increase of tlic

Table 2. - C h y r s of ~sr~ii.v~ti~~i/y of prr.y~n.ptic ~ I ~ I I - miì 1 ~ r t ~ v o ~ r q ~ t o ~ s 7?(f?lht~;7!!, ~ r r ~ t , ~ o ~ r < , ~ ~ r i r ; , i / t , ~ r w I ~ , I I s ~ , jr137ih rnntr11.1 mwir rndinp.

GHT nut,orrrnptor

5HT ~iitoreoopt,or

NA nulornooptnr ACIi niitnrw<.ptor

A(% nii1,orwiptor

114 nut,orrcrptor

N A Iiot,crorecnptnr

NA hot,erorrrcptor

NA he1,ororeraptor

NA Iietrorereptor

ACIi liotororeieptor

ACIi Ireterorweploi.

DA Iieterori:repto~

(30: T:LI& l )

(Fig. 41

(21; Fig. 6 )

(!!% Fig. ( i )

(!!2; Fig. f i )

(23)

(26: Fig. 8)

(26; Vig. 8)

(?S. 31;)

(09)

(Zii; Fig. ui (3: Fig. li)

(23)

--

(a) Synaptolornoa. (b] Slirrs.

, i f I r e s y n q t i c nutoreocptors, biit iiot of presynilptic Iivt,i~roreceptors of the same type.

i assuxning that receptor siipersensitivity rc.llccts a long-lnsting deprivation of endogerious iigi~nist, i t is tempting to speculate that a tonic iictivation of presynaptic hcteroreceptors does not occur in the living braiil. This would explaiii the liick of siipersensitivity of the presynaptic hetero- rweptors so iar exairiincrl after clironic administration 01' aiitagonists. However, tlie exi~tence oP a &?e- rivitial plasticity Iietwcen presynaptic auto- a d Iii+xoreceptors oannot be excliided and may reprc- :;i,iit a n alternative explanation.

I n nny case, the fact that the differences bet,wcrii rrmyuaptic autoreceptors and the correspomling lmsynaptic heteroreccptors, in terms of sensitivity iiii,ilulatioii. heci7rne apparent folluwing long-temi (lrog treatmeiits niay hnve importrtiit cliiiical inipli- cations.

Froni the resdts preseiitprl, the following conelusions coiild be clrawn:

a) autoreceptors are present iin serotoliin, iiora- drenaline and acet,ylcholino nerve terniinals, in various areas of the ra t h a i n :

b) muscariiiic rcceptors are prcsent oli dopaniine nerve terminalu in the corpiis rtriatizm; they mediate potentiation of thc release of dopamine hy acetyl- clioline;

c ) alpha,-aclrenergic heteroreccptors are present on scrotonin ncrve endings in the cercbral cortex; they nicdiate inhibition by noradrenaline of the clopolarizstiori-evokcd serotoniii relcase;

d ) long-torni drug treatments can indiice chmgcs of serisitivity of presynnptic autoreceptors. The presynaptio Iieteroreccptors so far exainined appcsr to l x refractory towarils t,hc adalitive changes evoked Iiy the treatments: affecting autoreceptor senaitivity.

oenous I n ysrticiilar, lung-terrn deprivation of endo, agonirt leads tu siiperscnsitivity of autoreceptors biit not of heteroreceptms. The difference in siiscepti- bility betweeii auto- nnd heterorecept,orn, with respect to changes of sensitivity, may reprcnent n further criterion t« rliscriininato hetween reccptor siibtypcu.

Tlie work roportod in t l i i n h y i t o r wtim *IIII[XI~I<HI Ijy ( h n t s fmni t h u Ltdinn (!.,\l.lt. (l'mgiittn Pinnlinrnto (:liitnl<:a Vine o Sowmdurii~: CT H0.00648.(W, (T Hl.Oci:l25.lU nnd CT 88.1P2052.M) anù from tlio Iti<liiin Miniatry a l iCducstion.

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from rnt brain. Ilr. J . Phmmcol. 40: 4G6-417.

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