Developmental changes in male Siberian hamsters (Phodopussungorus) exposed to different gestational and postnatalphotoperiods

Introduction

Siberian hamsters (Phodopus sungorus) are long-day breed-ers and photoperiod plays a major role in the regulation ofseasonal cycles of both reproductive and nonreproductive

traits in this species. In adult males, photoperiods longerthan 14L (14L:10D; 14 hr light:10 hr dark) result in themaintenance of reproductive competence, whereas photo-

periods <14L result in testicular regression [1]. Day lengthalso affects reproductive development in juvenile malehamsters; long days promote testis growth, whereas shortphotoperiods delay reproductive maturation [2, 3]. How-

ever, short days do not delay gonadal maturation indefin-itely. Siberian hamsters raised in 8L exhibited testicularmaturation, increases in body weight, and changes in pelage

after approximately 130 days, similar to the �spontaneousrecrudescence� that is observed in adult photoperiodicrodents held for several months in short days [2]. The

pineal gland, via its rhythmic secretion of melatonin, is animportant component of the mammalian photoperiodicmechanism [4], and Siberian hamsters have been extensively

studied in this regard [5].

Photoperiodic requirements for reproductive parametersmay be different from those for other traits. For example,the critical day length for maintenance of testis function

(14L) is greater than the day length required for maintainingthe summer pelage in adult male Siberian hamsters [1]. Inboth juvenile and adult males, short-day induced inhibition

of testis function is associated with decreased secretion offollicle-stimulating (FSH) and luteinizing (LH) hormones [6,7]. Development of winter pelage in short days is associated

with a marked decrease in secretion of prolactin (PRL) [8].In addition to the absolute ambient day length, photo-

period history is also important in determining seasonalresponses of both adult and juvenile hamsters. In adult

males, a photoperiod of 14L was inhibitory to the repro-ductive axis if the animals had previously been exposed to alonger (16L) day length; the same 14L day length was

stimulatory to testis growth in males that had been exposedpreviously to a shorter (8L) photoperiod [9]. Also, malehamsters whose mothers had been exposed to 10L or 12L

during gestation showed relatively rapid testicular growthwhen raised postnatally in 14L, as compared with malesborn to mothers exposed to 16L during gestation and

reared postnatally in 14L [10, 11].

Abstract: In Siberian hamsters, juvenile somatic and reproductive

development is influenced by the photoperiods experienced both during

gestation and after birth. On the day of parturition, parents and young were

transferred from either 16L (16 hr of light and 8 hr of darkness/day) or 10L

to one of the three photoperiods (14L, 12L, and 10L), and on postnatal day

27 male juveniles were either pinealectomized or sham-operated. At various

intervals from postnatal days 27–330, the following parameters were

determined: body weight, testis size, pelage type, serum concentrations of

follicle-stimulating hormone (FSH) and prolactin (PRL). A postnatal

photoperiod <14L was required to initiate delayed pubertal development

followed by an eventual �spontaneous� achievement of body weight, testis

size, pelage, and serum FSH and PRL levels characteristic of adult, long-day

males. The data suggest that serum FSH �surges� in the pineal-intact hamsters

are associated with spontaneous testicular development regardless of

gestation photoperiod. The results also indicate that gestational photoperiod

affects the timing of the molt to winter-type pelage and its eventual

spontaneous development in pineal-intact hamsters that are exposed to short

photoperiod following birth. Finally, our observations suggest that the

interval timer that operates during prolonged short-day exposure to

ultimately trigger a transition to the summer-type physiology may begin to

function before birth in the offspring of females exposed to short

photoperiod during gestation.

Donald Shaw1 and Bruce D.Goldman2

1Department of Biology, Augustana College,

Rock Island, IL; 2Department of Ecology and

Evolutionary Biology, University of

Connecticut, Storrs, CT, USA

Key words: follicle-stimulating hormone,

pelage, photoperiods, prolactin, puberty,

Siberian hamster, testis

Address reprint requests to Bruce D. Goldman,

Department of Ecology and Evolutionary Bio-

logy, University of Connecticut, Storrs, CT

06269-3033, USA.

E-mail: bruce.goldman@uconn.edu

Received December 11, 2006;

accepted February 20, 2007.

J. Pineal Res. 2007; 43:25–34Doi:10.1111/j.1600-079X.2007.00439.x

� 2007 The AuthorsJournal compilation � 2007 Blackwell Munksgaard

Journal of Pineal Research

25

The goal of the present investigation was to determinewhether juvenile male hamsters exposed to different shortday lengths would exhibit different temporal patterns of

reproductive maturation, changes in body weight, pelagedevelopment, and associated changes in serum concentra-tions of FSH and PRL. The present report also extendsearlier observations [12] that indicated impacts of the

gestation photoperiod on the postnatal photoperiodicinfluence on pituitary FSH and PRL secretion. Effects ofpinealectomy on reproductive development were also

investigated.

Materials and methods

Animals and housing

All animal procedures in the present studies were

approved by the Institutional Animal Care and UseCommittee, University of Connecticut. The breedingcolony of hamsters used to generate animals for these

experiments was derived from animals obtained fromDr Klaus Hoffmann. Breeding pairs were housed on Sani-Chip bedding (P.J. Murphy Forest Products Corp.,

Montville, NJ, USA) in plastic cages measuring29 · 18 · 13 cm. Breeding pairs received water and food(Agway Prolab Chow 3500; Agway Inc., Syracuse, NY,

USA) ad libitum with a weekly supplement of sunflowerseeds. Following weaning, hamsters were fed Prolab Chowwith occasional supplements of sunflower seeds. Roomtemperature was 22 ± 1�C. In 16L, lights were on at

02:00–18:00 hr. In all photoperiods, the time of lights-offwas adjusted so that the midpoint of the dark phasealways occurred at 22:00 hr. The illumination intensity

was 200 –400 lx at cage level.

Correlation between testis width and testis weight

Male Siberian hamsters maintained on either long or shortphotoperiods for various lengths of time were used toestablish the relation between testis width and testis

weight. Testes width was determined to the nearest0.1 mm, and the testes were then removed and weighed.Testis weight was highly correlated (r ¼ 0.97) with testis

width (Fig. 1).

Experiment 1: Developmental changes in bodyweight, testis size, pelage, and serum hormoneconcentrations in males reared from birth in differentphotoperiods

The objective of this experiment was to examine the effectsof different photoperiods and the pineal gland on thedevelopmental patterns of body growth, reproductive

maturation, and pelage. Adult hamsters were paired forbreeding in 16L. Pairs were checked each afternoon forbirth of litters. On the day of birth (D0; day 0) the parents

and their litters were transferred to 14L, 12L, or 10L. Thus,each experimental group consisted of two different photo-period exposures: gestational (G) and postnatal (P). For

example, the abbreviation G16-P14 would indicate thatyoung were gestated in 16L and were raised from birth

onwards in 14L. On D27, the hamsters in 14L were eitherpinealectomized (Pinx) or sham-operated (Sham). Allhamsters in the other two photoperiod groups weresham-operated at the same age. Thus, there were four

treatment groups: (1) G16-P14-Pinx, (2) G16-P14-Sham, (3)G16-P12-Sham, and (4) G16-P10-Sham. At various inter-vals from D27 to D330 blood samples were obtained for

hormone determinations, and body weight, testis width andpelage condition were determined.

Experiment 2: Developmental changes in bodyweight, testis size, pelage, and serum hormoneconcentrations in males that were derived fromdifferent gestation photoperiods and reared frombirth in 10L

The goal of this experiment was to determine the effects of

long versus short photoperiods during gestation on devel-opmental patterns in male hamsters raised from birth inshort days (10L). Adults from 16L were paired for breeding

and randomly assigned to 16L or 10L on the day of pairing.Pairs were checked daily for birth of litters. On the day ofbirth, parents and their young were either left in 10L or

transferred from 16L to a 10L room. Litters were weanedon D14. The juveniles were group housed by sex and onlymales were used for this study. On D27, the G10-P10 males

were either pinealectomized or sham-operated; all theG16-P10 males were pinealectomized. Thus, there werethree treatment groups, designated as follows: (1) G16-P10-Pinx, (2) G10-P10-Pinx, and (3) G10-P10-Sham. At various

intervals from D27 to D330, blood samples were obtainedfor hormone determinations; body weight, testis width andpelage condition were determined. This experiment was run

at approximately the same time as the preceding study, andthe G16-P10-Sham group was a single group of males thatserved for both studies.

450

360

270

180

Test

is w

eigh

t (m

g)

Testis width (mm)

90

02 4 6 8 10

Fig. 1. Correlation (r ¼ 0.97) between testis weights (mg) andtestis widths (mm) of male Siberian hamsters maintained on eitherstimulatory or inhibitory light cycles for various lengths of time.Each circle represents one testis. Actual testis weight was deter-mined after sacrifice and is expressed as a function of testis width.Sample size is 409.

Shaw and Goldman

26

Pinealectomy

Pinealectomies were performed when hamsters were

27 days old, using a modification of a published technique[13, 14]. Animals were anesthetized with Equithesin(0.3 mL/100 g, administered i.p.). The top of the headwas shaved and swabbed with 70% alcohol, and the animal

was placed in a Kopf stereotaxic apparatus (David KopfInstruments, Tujunga, CA, USA). The cranium wasexposed through a midline skin incision and a dental drill

was used to remove a bone disc (approximately 5 mm indiameter) just above the pineal. The gland was observedwith the aid of a surgical microscope and was removed with

watchmaker forceps. The wound was covered with a pieceof Gelfoam absorbable gelatin sponge (Upjohn, Kalama-zoo, MI, USA) and the skin incision was closed with 9-mmwound clips. Merthiolate was applied over the surgical area

to prevent infection. Sham operations were performedidentically to pinealectomy except that the pineal was notremoved.

Blood samples

Blood samples were collected during the light phase byretroorbital puncture without anesthesia [15]. The tip of amicrocapillary tube (250 lL; Natelson, Pittsburgh, PA,

USA) was maneuvered behind the eyeball to burst thevenous plexus in back of the orbit. Blood was transferredinto a test tube (6 · 30 mm) and allowed to clot overnightin a refrigerator. Samples were centrifuged for 15 min, and

serum was harvested into plastic vials for storage at )50�Cuntil radioimmunoassays (RIA) were performed.

Testis width

Animals were briefly anesthetized with methoxyflurane

(Schering-Plough Animal Health, Union, NJ, USA) and anincision was made through the scrotum. One testis wasexposed and its width was measured using a hand-heldcalipers. The testis was then placed back into the scrotum,

the opening was closed with surgical sutures, and Merth-iolate was applied to the skin in the area of the incision.This method of evaluation of testis condition has the

advantage of allowing for repeated measurements withminimal surgical trauma.

Pelage

Pelage scorings were based on a published method [16].

Following transfer to short day lengths, Siberian hamstersmolt the agouti summer fur and develop a mostly whitewinter pelage. Hamsters raised from birth in short days firstdevelop a summer coat, and this is followed by a molt to the

winter condition. Hamsters were inspected without anesthe-sia and the pelage was rated on the following scale, based oncolor: Stage 1 represents the summer pelage, with agouti fur

on the dorsal surface and light gray fur on the lateral surface.Stage 2 represents the earliest stage ofmolt to thewinter coat,with white fur on the posterior dorsum and around the eyes

and ears. Stage 3 is a more advanced winter molt with whitefur extending to the mid-dorsal and lateral trunk. Stage 4

represents full winter pelagewithwhite fur covering the entirebody except for a mid-dorsal dark stripe.

Hormone assays

Serum concentrations of PRL and FSH were determined bydouble antibody RIA. For PRL assays the standard was

purified Syrian hamster PRL (AFP10302E), provided byDr Albert Parlow, UCLA-Harbor Medical Center. The firstantibody was raised against Syrian hamster PRL and was

provided by Dr Katerina Borer. The antibody was valid-ated for RIA of PRL in both Syrian [17] and Siberianhamsters [18] and has 4% cross-reactivity with Syrian

hamster growth hormone. Purified Syrian hamster PRL,provided by Dr Frank Talamantes, was used to prepareradiolabeled PRL. The second antibody was anti-rabbitgamma globulin. Samples that fell outside the 15–85%

binding limits were assigned the value appropriate for either15% or 85% binding. The range of detectable values was1.2–130.0 ng PRL/mL serum.

Serum FSH was measured by the use of the NIAMDDrat FSH RIA kit, with rFSH-RP-2 as the standard andanti-rat FSH-S-10 as the first antibody. The second

antibody was anti-rabbit gamma globulin. This methodhas been validated for use in the measurement of Siberianhamster FSH [3]. The range of detectable values was

2.5–31.9 ng FSH/mL serum. All hormone measurementswere carried out in a single assay for each hormone.

Statistics

A correlation coefficient for testis weight versus testis widthwas obtained by using the MS Excel program. A one-way

analysis of variance was used to compare treatment groupswith respect to body weights, testis widths, pelage stage, andserum hormone concentrations. When significant F values

were found, Tukey’s HSD tests were conducted to test forsignificance of differences between groups. Within eachtreatment group, the Student t-tests were used to test fordifferences between measurements made at pairs of sequen-

tial ages. For all analyses, differences were consideredstatistically significant when the probability level was<0.05.

Results

Hamsters from the G16-P10-Sham group tended to have

the lowest body weights among the four treatment groupsin Experiment 1 at most ages from 27 to 210 days (Fig. 2).However, the differences reached statistical significance

only at 27, 42, and 52 days. Specifically, G16-P10-Shammales had lower body weights compared with the G16-P12-Sham animals at 27 and 42 days; the G16-P10-Shamanimals were smaller than the G16-P14-Sham males at

42 and 52 days. Pinealectomy had little effect on the patternof somatic growth in theG16-P14males as compared to theirsham-operated controls. Specifically, G16-P14 Pinx animals

had significantly lower body weights only at 42 days.During the first two months of life, all groups in

Experiment 1 exhibited significant increases in body weight

(Fig. 2). This increase in body size continued rather steadilyand gradually for the G16-P14-Pinx animals until they were

Photoperiod during pre- and postnatal development

27

autopsied at D240. In the other three groups, body weightappeared to remain fairly constant for a period after D62.However, there was a further significant increase in bodyweight in the G16-P12-Sham males between 90 and

120 days, and there was a significant increase in theG16-P10-Sham animals between 120 and 150 days, fol-lowed by more gradual increases thereafter.

Testis growth was relatively rapid in the G16-P14 Shamand Pinx groups until D62 (Fig. 3). After that time, the G16-P14-Pinx group continued to show testis growth, achieving

maximum testis size by D120. Testis growth in the G16-P14-Sham hamsters stopped after D62 and resumed only afterD180, and maximum testis size was reached by D270. Testis

growth was markedly delayed in the G16-P12 and G16-P10groups. Rapid testis growth occurred only after D120 in boththese groups; maximum testis size appeared to be achievedin the G16-P12 hamsters by D180, but no later measurements

were obtained for this group. In the G16-P10 animals,maximum testis size was attained by D240. By 240–270 daysof age, the testis sizes of the two groups raised in 14L (P14)

and the one remaining short-day group were equivalent.Pelage initially developed in the summer phase in all

treatment groups (Fig. 4). However, at 2–3 months of age

the G16-P12 and G16-P10 groups initiated a molt towinter pelage. The study was terminated for the G16-P12group at D180, when the animals were still in winter-typepelage. The G16-P10 hamsters returned to summer-type

pelage at D210.

Serum PRL concentrations were consistently low in theG16-P10 hamsters until D210, with a modest elevationthereafter; however, serum PRL concentrations of thesehamsters were not significantly different from other sham-

operated animals or within the same group (Fig. 5). PRLconcentrations were higher in the G16-P14-Sham and G16-P14-Pinx hamsters, and there were no significant differences

between these two groups.Serum FSH concentrations were low in the G16-P12 and

G16-P10 males from 27 to 90 days of age (Fig. 6). FSH

concentrations were increased significantly in the G16-P10males at D120 and continued to increase thereafter. TheG16-P12 males showed a trend to increased serum FSH at

D120, but this did not reach statistical significance and nofurther measurements were obtained for this group. TheG16-P14 Sham and Pinx groups showed FSH peaks at D42

when these values were significantly higher than those of

G16-P12 and G16-P10 males; the FSH elevation at thistime was particularly marked in the pinealectomizedanimals. Subsequently, FSH concentrations declined in

both groups.Although only three experimental groups were included

in Experiment 2, the data from the G16-P10-Sham males of

Experiment 1 are replotted here for ease of comparison withthe G16-P10-Pinx animals. Body weight increased some-what more slowly in the G16-P10-Sham males as comparedwith males from the other treatment groups (Fig. 7).

Specifically, body weights of G16-P10-Sham males were

48

45

42

39

36

33

30

27

24

21

1827 42 52 62 75 90 120 150 180 210 240 270 300 330

Bod

y w

eigh

t (g)

G16-P14-Pinx

G16-P14-Sham

G16-P12-Sham

G16-P10-Sham

Age (days)

BB

B

BB

A

A

AB

AB

A

A

AB

Fig. 2. Developmental changes in body weights in male Siberian hamsters. The animals were gestated under 16L (G16) and raisedpostnatally under three photoperiods – 14L (P14), 12L (P12), or 10L (P10) – from the day of birth. At 27 days of age, hamsters receivedeither pinealectomy (Pinx) or sham-pinealectomy (Sham). Body weights in males from 27 to 330 days of age have been plotted. Each pointrepresents the mean ± S.E.M. Where S.E.M. bars are not present, the variance was encompassed by the area of the symbol. Comparisonsbetween treatment groups at different ages are indicated with uppercase letters. At any age, groups marked with any same letter are notstatistically different (P > 0.05). Within each treatment group, asterisks indicate significant differences (P < 0.05) when compared withtheir immediate preceding values by t-tests. The age (in days) of hamsters are expressed in the figure caption as Dn where n specifies the exactage in number. For example, D27 represents 27 days of age. Sample sizes: (1) G16-P14-Pinx: D27 ¼ 26, D42 ¼ 24, D52 ¼ 21, D62 ¼ 11,D75 ¼ 11, D90 ¼ 10, D120 ¼ 7, D150 ¼ 2, D180 ¼ 2, D240 ¼ 4; (2) G16-P14-Sham: D27 ¼ 15, D42 ¼ 11, D52 ¼ 18, D62 ¼ 12, D75 ¼ 13,D90 ¼ 11, D120 ¼ 9, D150 ¼ 6, D180 ¼ 5, D270 ¼ 3, D300 ¼ 5; (3) G16-P12-Sham: D27 ¼ 5, D42 ¼ 10, D52 ¼ 8, D62 ¼ 7, D75 ¼ 6, D90 ¼ 5,D120 ¼ 2, D180 ¼ 1; (4) G16-P10-Sham: D27 ¼ 6, D42 ¼ 6, D52 ¼ 5, D62 ¼ 13, D90 ¼ 10, D120 ¼ 9, D150 ¼ 5, D180 ¼ 5, D210 ¼ 4, D240 ¼6, D270 ¼ 2, D330 ¼ 3.

Shaw and Goldman

28

significantly lower at 90, 120, and 150 days as comparedwith those of pinx males. By 180–210 days, there were no

significant differences in body size between the groups.Testis growth was delayed until D120 in the G16-P10

and G10-P10 sham-operated males (Fig. 8). Testis growthwas significantly rapid at 120–150 days within these

groups and maximum testis sizes were achieved by 180–240 days. The two pinealectomized groups showed earliertestis growth. That is at 90 and 120 days testis sizes were

significant larger in these two pinx groups as comparedwith their corresponding sham males. There was a trendtoward more rapid testis growth in the G16-P10-Pinx

animals as compared with the G10-P10-Pinx males;however, testis sizes were not significantly differentbetween these two groups at any of the sampling times.

All groups initially developed a summer pelage, and boththe pinealectomized groups retained summer pelagethroughout the experiment (Fig. 9). The G10-P10-Sham

animals began to molt to winter pelage by 52 days andwere in complete winter pelage by 90–120 days. The

G16-P10-Sham males molted to winter pelage between62 and 90 days. The G10-P10-Sham males showed asignificant return to summer pelage when the study wasterminated for that group at 180 days. The G16-P10-Sham

animals returned significantly to summer pelage atD210.There were no clear differences between the treatmentgroups with respect to patterns of serum PRL, but PRL

concentrations tended to be lower in the two sham-operated groups until about D180 (Fig. 10).Serum FSH concentrations were low in the sham-

operated groups through D90 and began to increasethereafter; especially at D120 serum FSH concentrationsof G16-P10-Sham males significantly increased from those

of D90 (Fig. 11). Serum FSH levels were low in bothpinealectomized groups at D27, but increased by D62 andremained elevated throughout the remainder of the study.

9

8

7

6

5

4

3

2

27 42 52 62 75 90 120 150 180 210 240 270 300 330

Test

is w

idth

(m

m)

G16-P14-Pinx

G16-P14-Sham

G16-P12-Sham

G16-P10-Sham

Age (days)

B BB

B

B

BBBB

BB

B

C

CC

A

A

A

A A

A AA

A

A

A

A

BCFig. 3. Developmental changes in testicu-lar width of male Siberian hamstersexposed from birth to three differentpostnatal photoperiods. Refer to Fig. 2caption for details. Sample sizes were thesame as for Fig. 2 except (1) G16-P14-Pinx: D27 ¼ 14, D42 ¼ 12, D52 ¼ 11; (2)G16-P14-Sham: D52 ¼ 17; (4) G16-P10-Sham: D27 ¼ 16.

Ave

rage

pel

age

colo

r st

age

(1=

sum

mer

, 4 =

win

ter)

G16-P14-Pinx

G16-P12-Sham

G16-P14-Sham

G16-P10-Sham

Age (days)

B

B

B

B

B B B

BBC

C

A

AA

A

ABA

A

1

2

3

4

27 42 52 62 75 90 120 150 180 210 240 270 330300

Fig. 4. Pelage colors in male Siberianhamsters exposed postnatally from birthto three different photoperiods. Pelagecolor stages ranged from 1 (summerpelage) to 4 (complete winter pelage).Refer to Fig. 2 caption for details.Sample sizes were the same as those inFig. 3 except G16-P10-Sham: D27 ¼ 6,D42 ¼ 6, D52 ¼ 5, D62 ¼ 4, D90 ¼ 10,D120 ¼ 9, D150 ¼ 5, D180 ¼ 5, D210 ¼4, D240 ¼ 6, D270 ¼ 2, D330 ¼ 3.

Photoperiod during pre- and postnatal development

29

Discussion

Our results confirm earlier reports showing that maleSiberian hamsters reared from birth in short photoperiodsexhibit delayed achievement of maximum body size and

testicular maturation [2, 19]. Body mass is also influenced byphotoperiod in adult Siberian hamsters [20]. The effects ofphotoperiod and pinealectomy on body weight are at least

partly mediated by changes in testis hormone(s), sincecastration (like exposure to short days) results in decreasedbody mass in long-day males. There appears to be an

additional gonad-independent effect of photoperiod on bodyweight, and this effect is mediated by the pineal gland [21].The present data demonstrate that a postnatal photope-

riod shorter than 14L is required to delay testicularmaturation for approximately 3 months; in males raisedin 10L or 12L, testicular maturation starts at 120–150 daysof age as compared with about 40–50 days in males reared

under 14L. The profiles of delayed onsets of testicular

maturation in these hamsters are parallel with the timelineof further significant increase in body weights. These results

are in agreement with previous reports that puberty isdelayed by several months when male Siberian hamsters areraised in short photoperiods; full testicular developmentdoes not occur until about 5–6 months of age under these

conditions [2], but interruption of short days by treatmentsthat alter pineal activity may disrupt the typical course ofthe interval timer that appears to determine the time of

puberty in short day males [22–24]. Similarly, in adultSiberian hamsters the testes begin recrudescence after about112–126 days of complete testicular regression [23, 25]. In

the juvenile males of the present study, pinealectomyadvanced testicular maturation in that full testiculardevelopment was reached by 90–120 days of age in the

G16-P14-Pinx group, at least 60 days earlier than forhamsters in the G16-P14-Sham group.Short-day exposed Siberian hamsters undergo a change

in pelage from agouti (summer-type) to white (winter-type);

27 42 52 62 75 90 120

BBB

AB AB

A

A

A

AB ABB

150 180 210 240 270 300 330

Age (days)

0

15

30

45

60

75S

erum

PR

L (n

g/m

L)G16-P14-Pinx

G16-P12-Sham

G10-P14-Sham

G16-P10-Sham

Fig. 5. Serum prolactin concentrationsfrom 27 to 330 days of age in male Sibe-rian hamsters reared in three differentphotoperiods from birth. See Fig. 2 fordetails. Sample sizes were the same asthose in Fig. 2 except (1) G16-P14-Pinx:D27 ¼ 14; (2) G16-P12-Sham: D90 ¼ 4;(3) G16-P10-Sham: D120 ¼ 11, D150 ¼ 7,D240 ¼ 7, D330 ¼ 2.

27 42 52 62 75 90 120

B

B

AB

A

BC

C

B

B A

A

B

150 180 210 240 270 300 330

Age (days)

2

3

4

5

6

7

8

9

10

11

Ser

um F

SH

(ng

/mL)

G16-P14-Pinx

G16-P12-Sham

G10-P14-Sham

G16-P10-Sham

Fig. 6. Serum follicle-stimulating hor-mone concentrations from 27 to 330 daysof age in male Siberian hamsters rearedunder three different photoperiods frombirth. See Fig. 2 for details. Sample sizeswere the same as those in Fig. 2 except (1)G16-P14-Pinx: D27 ¼ 14; (2) G16-P12-Sham: D42 ¼ 9, D75 ¼ 5, D90 ¼ 4; (3)G16-P10-Sham: D120 ¼ 11, D150 ¼ 7,D240 ¼ 5, D330 ¼ 3.

Shaw and Goldman

30

the agouti fur returns after 5 months in short days [8].Juvenile Siberian hamsters initially develop a summer-type

pelage in all photoperiods, but males housed under 8Lchanged into the winter pelage starting between about40–80 days of age; the winter pelage started to molt backtoward the summer coat between about 110–150 days of

age [2]. The results presented here confirm that a photope-riod <14L is required to initiate molting into the winter

coat in pineal-intact hamsters and that pinealectomy has noeffect on pelage in 14L as hamsters in both G16-P14 Pinx

and Sham groups had summer-type pelage throughout theexperiment. In addition, our results provide evidence thatthere is a temporally different response of pelage whenhamsters are reared in different short days. Specifically, at

90 days of age, G16-P12-Sham males had less advancedwinter-pelage stages compared to those of G16-P10-Sham

39

36

33

30

27

24

21

18

48

45

42

Bod

y w

eigh

t (g)

27 42 52 62 75 90 120 150 180 210 240 270 300 330Age (days)

G16-P10-Pinx

G16-P10-Sham

G10-P10-Pinx

G10-P10-Sham

*

**

*

*

*

*

*

A

A

AB

AB

AB

AA

BC

BC

C

C

B

Fig. 7. Developmental changes in body weights in male Siberian hamsters. The animals were gestated under either 16L (G16) or 10L (G10)and raised postnatally under 10L (P10) from the day of birth. At 27 days of age, hamsters received either pinealectomy (Pinx) or sham-pinealectomy (Sham). Body weights in males from 27 to 330 days of age have been plotted. Each point represents the mean ± S.E.M.Where S.E.M. bars are not present, the variance was encompassed by the area of the symbol. Comparisons between treatment groups atdifferent ages are indicated with uppercase letters. At any age, groups marked with any same letter are not statistically different (P > 0.05).Within each treatment group, asterisks indicate significant differences (P < 0.05) when compared with their immediate preceding values byt-tests. The age (in days) of hamsters are expressed in the figure caption as Dn where n specifies the exact age in number. For example, D27

represents 27 days of age. Sample sizes: (1) G16-P10-Pinx: D27 ¼ 11, D62 ¼ 11, D75 ¼ 10, D90 ¼ 10, D120 ¼ 9, D150 ¼ 8, D180 ¼ 7,D210 ¼ 6, D240 ¼ 3, D330 ¼ 2; (2) G16-P10-Sham: D27 ¼ 6, D42 ¼ 6, D52 ¼ 5, D62 ¼ 13, D90 ¼ 10, D120 ¼ 9, D150 ¼ 5, D180 ¼ 5, D210 ¼4, D240 ¼ 6, D270 ¼ 2, D330 ¼ 3; (3) G10-P10-Pinx: D27 ¼ 14, D62 ¼ 11, D90 ¼ 9, D120 ¼ 7, D150 ¼ 6, D180 ¼ 6, D210 ¼ 6, D240 ¼ 5,D330 ¼ 4; (4) G10-P10-Sham: D27 ¼ 4, D42 ¼ 4, D52 ¼ 2, D62 ¼ 11, D90 ¼ 8, D120 ¼ 5, D150 ¼ 5, D180 ¼ 2.

9

8

7

6

5

4

3

2

Test

is W

idth

(m

m)

27 42 52 62 75 90 120 150 180 210 240 270 300 330

Age (days)

G16-P10-Pinx

G16-P10-Sham

G10-P10-Pinx

G10-P10-Sham

A A

A

A

A

AB

B

B BBBB

*

**

*

*

*

Fig. 8. Developmental changes in testicu-lar width of male Siberian hamsters. Referto Fig. 7 caption for details. Sample sizeswere the same as those in Fig. 7 except (1)G16-P10-Sham: D27 ¼ 16, D270 ¼ 2,D300 ¼ 3, D330 ¼ 0; (2) G10-P10-Pinx:D27 ¼ 4.

Photoperiod during pre- and postnatal development

31

males. Subsequently, a pelage molt back toward the

summer coat was first observed in G16-P10-Sham malesat D210; data are not available to determine the age of moltback to the summer coat for G16-P12-Sham males.

Seasonal changes in pelage type are regulated by the levelsof circulating PRL, with summer pelage produced whenserum PRL is elevated and winter pelage developing whenthe PRL concentration is low [8]. Photoperiod has a marked

effect on PRL secretion in Siberian hamsters. Serum PRLconcentrationsweremuch lower in 10L as comparedwith theconcentrations in hamsters exposed to long days [1, 3]. In the

current study, pineal-intact males in the 10L postnatalphotoperiod consistently had the lowest serum PRL levelsamong the various treatment groups, but the values for these

animals were not statistically different from those of otherpineal-intact groups. Nevertheless, our results demonstratethat pineal-intact hamsters reared in either 10L or 12Lpostnatal photoperiods had lower PRL levels through the

first 120 days of life as comparedwith pinealectomizedmales

or males reared in 14L. Together with our results in the 10L-or 12L-reared hamsters, these observations are consistentwith earlier findings demonstrating that decreased serum

PRL is necessary for the development of the white winterpelage in this species [22]. Previous observations indicatedthat in juvenile male Siberian hamsters, serum FSH levels,between 20 and 60 days of age, were significantly reduced in

10L-reared hamsters compared with those in males rearedunder long days. In addition, short days blocked peak FSHsecretion between 20 and 30 days of age [3]. Likewise, in

adult male Siberian and Syrian hamsters, serum FSHconcentrations are reduced after several weeks of exposureto short days [1, 6, 26].

The results presented here concur with and extend earlierreports in that postnatal photoperiods of 12L or 10Lresulted in reduced serum FSH concentration from 27 to90 days of age in pineal-intact male hamsters [12]. In

1

2

3

4

27 42 52 62 75 90 120 150 180 210 240 270 300 330

Ave

rage

pel

age

colo

r st

age

(1=

sum

mer

,4=

win

ter)

Age (days)

B B

B

B

B

B B

B

B

B

B

B

A

A

A

AA

A

A

A

AA

G16-P10-Pinx

G16-P10-Sham

G10-P10-Pinx

G10-P10-ShamFig. 9. Pelage colors in male Siberianhamsters. Pelage color stages ranged from1 (summer pelage) to 4 (complete winterpelage). Refer to Fig. 7 caption for details.Sample sizes were the same as those inFig. 7 except (1) G16-P10-Pinx: D27 ¼ 0;(2) G16-P10-Sham: D62 ¼ 4; (3) G10-P10-Pinx: D27 ¼ 4; (4) G10-P10-Sham: D27 ¼2, D62 ¼ 4.

27

0

4

8

12

16

20

24

28

42 52 62 75 90 120 150 180 210 240 270 300 330

Age (days)

Ser

um P

RL

(ng/

mL)

B

A

G16-P10-Pinx

G16-P10-Sham

G10-P10-Pinx

G10-P10-Sham

Fig. 10. Serum prolactin concentrationsfrom 27 to 330 days of age in male Sibe-rian hamsters. Refer to Fig. 7 caption fordetails. Sample sizes are the same as thosein Fig. 7 except (1) G16-P10-Pinx: D27 ¼10, D62 ¼ 9; (2) G16-P10-Sham: D120 ¼11, D150 ¼ 7, D240 ¼ 7, D330 ¼ 2; (3)G10-P10-Pinx: D27 ¼ 4; (4) G10-P10-Sham: D27 ¼ 2, D150 ¼ 4.

Shaw and Goldman

32

addition, males of G16-P14-Sham and G16-P14-Pinxgroups showed FSH peaks that correlated with theirstimulated testicular status during first two months of life.

Furthermore, the results presented here show that FSHconcentrations increase by D120 in 10L- and 12L-rearedmales, suggesting direct involvement of FSH in the rapid

testicular development that occurs after D120 in thesehamsters.

Our data indicate that a postnatal photoperiod <14L is

required to both delay juvenile/pubertal patterns ofdevelopment of body weights, testis widths, pelage color,and serum FSH levels as well as to result in the subsequentinitiation of �spontaneous� development of these four

parameters. Onsets of spontaneous development in theseparameters are closely associated with each other in atemporal manner. Specifically peak serum FSH levels

appear to shortly precede achievement of full testisdevelopment, body weights increase in association withincreases in testis size, and increased serum PRL levels

precede the pelage molt back to the summer coat.Previous observations revealed that males and females

complete a molt into the winter pelage after about

10–14 wk (i.e. 70–98 days) in 10L [8]. In juvenile hamsters,males molt into the winter pelage after about 40–80 days in8L [2]. However, it was unknown how gestational photo-period would impact the pelage. The results of the present

study indicate that the photoperiod during gestationinfluences the timing of molt to winter pelage in malehamsters raised from birth in short days. Specifically,

G10-P10 intact males began molting to winter pelage about3–4 wk earlier than G16-P10 males (Fig. 9). The G10-P10males also began the spontaneous molt back to summer fur

about 2–4 wk earlier than the G16-P10 hamsters. It is likelythat timing of the initial molt to winter pelage begins whenthe animals are first exposed to short days. This occursduring fetal life for animals whose mothers were maintained

in 10L during gestation. For pups whose mothers were in16L during gestation, exposure to short days began on theday of birth. However, earlier studies suggest that devel-

oping hamsters receive a photoperiod cue from the motherduring late gestation and are then unresponsive to furtherday length information until about 2 wk postpartum, when

the pups� pineal melatonin rhythm first develops [27]. Thus,the offspring of G16 mothers may not have actuallyrecorded short day cues until 2–3 wk after the initial

(prenatal) reception of such cues by the offspring of G10dams. This could account for the approximately 2–4 wkdifference in timing of pelage molts by these two groups.

The present data, taken in conjunction with previousreports [27], suggest that all components of the melatoninresponse system, including the interval timer that isoperative in short days, may be established before birth.

If so, only the development of an endogenous, light-entrainable pineal melatonin rhythm remains to completethe establishment of the photoperiodic mechanism at about

2 wk of age.At least some of the various parameters of early

photoperiodic responses, including effects of the photope-

riod during gestation, are likely to be adaptations thatenable juvenile hamsters to rapidly adopt a developmentalprofile appropriate to the season of birth. Because of their

relatively rapid responses to photoperiod and because ofthe interesting interactions between effects of gestationaland postpartum photoperiods, developing Siberian ham-sters have proved a useful subject for studies of photoperi-

odism [5, 27].

Acknowledgment

We wish to thank Dr David King for analyzing the data inthe current studies and Dr David Freeman for assisting

with the hormone assays.

References

1. Duncan MJ, Goldman BD, Di Pinto MN et al. Testicular

function and pelage color have different critical daylengths in

272

3

4

5

6

7

8

9

42 52 62 75 90 120 150 180 210 240 270 300 330

Age (days)

Ser

um F

SH

(ng

/mL)

B

BB

AB

AB

AB

AB

A A

A

A

G16-P10-Pinx G16-P10-Sham

G10-P10-Pinx G10-P10-Sham

Fig. 11. Serum follicle-stimulating hor-mone concentrations from 27 to 330 daysof age in male Siberian hamsters. Refer toFig. 7 caption for details. Sample sizes arethe same as those in Fig. 7 except (1) G16-P10-Pinx: D27 ¼ 10; (2) G16-P10-Sham:D120 ¼ 11, D150 ¼ 7, D240 ¼ 5, D330 ¼ 0;(3) G10-P10-Pinx: D27 ¼ 4; (4) G10-P10-Sham: D27 ¼ 2.

Photoperiod during pre- and postnatal development

33

the Djungarian hamster, Phodopus sungorus sungorus. Endo-

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