Androgenic activity in 15-day-old male rats: role of the maternal pineal gland.

Female Sprague-Dawley rats, exposed to a long (18L:6D) or a short (6L:18D) photoperiod from 21 days of age, were mated when they reached 55 days of age. On Day 2 of gestation, dams were pinealectomized or sham-operated. Pre- and postnatal photoperiods were identical, and offspring were killed at 15 days of age. Maternal pinealectomy had no effect when rats were kept on 18L:6D. Rats born to sham-operated mothers and kept on 6L:18D had higher testicular testosterone and androstenedione content than offspring raised on the long photoperiod. This stimulatory effect of the short photoperiod was blocked by maternal pinealectomy and was not dependent on the offspring's own pineal since it was observed in both sham-operated and neonatally (on Day 5 after birth) pinealectomized rats. When sham-operated mothers housed on 18L:6D were treated daily during pregnancy and lactation by s.c. melatonin injection, there was an increase in the testicular testosterone content of offspring. It was concluded that when rats are maintained on a 6L:18D cycle the maternal pineal gland enhances the testicular testosterone and androstenedione content in 15-day-old offspring. This effect is probably mediated by maternally derived melatonin. At 15 days of age, the pineal of the offspring had no influence on testicular function.     

Maternal transfer of photoperiodic information in Siberian hamsters. III. Melatonin injections program postnatal reproductive development expressed in constant light

In Siberian hamsters, transference of photoperiodic information from dam to fetus influences pubertal testicular development of the young when reared either in constant light (LL) or postnatal photoperiods of intermediate length (i.e. 14L:10D). The effects of short photoperiods during gestation can be mimicked by administering melatonin to pregnant females. This experiment examined whether there exists a daily pattern of sensitivity to melatonin when it is administered to pineal-intact pregnant females housed on a long photoperiod. Groups of pregnant and lactating females received melatonin at each hour of the day. The young were not treated with exogenous melatonin. At the approximate time of maturation of their endogenous pineal melatonin rhythm (Day 15), the young were placed in LL to suppress pineal melatonin secretion. Young males were killed at 28 days of age. Afternoon (1200 h-2000 h) and late night (0400 h) injections of melatonin into females caused their male young to develop as though gestation occurred on a short photoperiod. Melatonin injections at other times were ineffective. The daily pattern of effectiveness of exogenous melatonin administration to pregnant females resembles that observed in adult males of this and other hamster species and is consistent with the hypothesis that a daily rhythm in sensitivity to melatonin is involved in the transduction of photoperiodic signals.     

Influence of the pineal gland on testicular function in offspring of pinealectomized rats

Female Sprague-Dawley rats exposed to a short (6L:18D) photoperiod from 21 days of age were mated when they reached 55 days of age. On Day 2 of gestation animals were pinealectomized or sham-operated. On Day 5 after birth male pups of the two groups of dams were either pinealectomized or sham-operated. They were killed at 42 and 49 days of age. In offspring born to sham-operated dams and in those born to pinealectomized mothers, neonatal pineal ablation resulted in increased testicular testosterone and androstenedione content. In sham-operated and neonatally pinealectomized rats removal of the maternal pineal gland induced a decrease in testicular testosterone and androstenedione content. In contrast, after maternal pinealectomy there was a decrease in plasma testosterone and dihydrotestosterone values and testicular dihydrotestosterone content in sham-operated rats but not in those neonatally pinealectomized. We conclude that (1) the pineal glands of the mother and offspring are required to maintain normal testicular testosterone and androstenedione content in the rat, and (2) the pineal of the offspring influences the inhibitory effects of maternal pinealectomy on testicular dihydrotestosterone content and on plasma testosterone and dihydrotestosterone concentration in the offspring.     

Maternal transfer of photoperiodic information in Siberian hamsters. II. The nature of the maternal signal, time of signal transfer, and the effect of the maternal signal on peripubertal reproductive development in the absence of photoperiodic input

Peripubertal reproductive development of Siberian hamsters is controlled by maternally derived photoperiodic information and the ambient photoperiod present after weaning. Previous experiments suggested that the maternally derived information is transferred during gestation, not during lactation. Development was examined in several photoperiods following manipulation of gestational and lactational photoperiods; development was influenced by the gestational, but not lactational, photoperiod. Second, effects of the gestational photoperiod were observed in young reared in constant light (LL) from Day 15. Depriving the young of ambient photoperiodic information after Day 15 allows a more direct assessment of the signal received from their dams. Finally, melatonin injections to long-day dams, at certain times of day, caused transmission of a short-day signal to young, as evidenced by their development in LL and light-dark cycles. Thus, a maternally derived signal that is dependent on melatonin influences reproductive development of the young during gestation; the maternally directed pattern of development can subsequently be modified by the youngs' own response to ambient photoperiods after weaning.     

Maternal transfer of photoperiodic information in Siberian hamsters. V. Effects of melatonin implants are dependent on photoperiod.

Photoperiodic information is transferred from female Siberian hamsters to their fetuses during gestation. Although maternal melatonin is known to be essential for the transfer of prenatal photoperiodic information, its specific role is not well defined. The duration of the daily melatonin signal, expressed as an elevation of serum melatonin levels in the maternal circulation, has been hypothesized to convey day length information to the fetus. If this hypothesis is valid, it predicts that identical maternal melatonin signals should affect the fetuses identically, regardless of the prenatal photoperiod. To test this hypothesis, adult females received melatonin in beeswax or beeswax alone. They were paired with males and housed in photoperiods of 12L:12D or 16L:8D. On the day of parturition, mother and young were transferred to constant light (LL). Young males were killed on Day 28 of life, and weights of testes were determined. Prenatal treatment with beeswax alone did not affect the nature of the signal transferred from mother to fetus; young gestated in 12L:12D and reared in LL developed small testes, while those gestated in 16L:8D had large testes. On the other hand, the effect of the prenatal melatonin treatment on postnatal testicular development in LL was inversely dependent on the prenatal photoperiod: testicular growth was stimulated in young gestated in 12L:12D, but inhibited in young gestated in 16L:8D. To verify that the melatonin pellets produced equivalent serum melatonin levels in adult females in 12L:12D and 16L:8D, unmated adult females were killed 6-10 wk after receiving melatonin pellets. Serum levels were elevated in both groups throughout the day and night.(ABSTRACT TRUNCATED AT 250 WORDS)     

Maternal transfer of photoperiodic information influences the photoperiodic response of prepubertal Djungarian hamsters (Phodopus sungorus sungorus)

Daylengths during the spring are repeated in reverse order in the autumn. For some photoperiodic species, a given photoperiod may be stimulatory for reproduction in the spring and inhibitory in the autumn. The mechanisms regulating this type of seasonal response have, until recently, remained a mystery. Horton (1984a) showed in Microtus montanus that the photoperiod experienced by the mother influences the gonadal development of her young after weaning. To determine if this phenomenon is characteristic of other photoperiodic rodents, adult Djungarian hamsters were paired on 16L:8D, 14L:10D, or 12L:12D. Young males born from these pairings were killed at 15, 28, and 34 days of age to assess gonadal development (testes weight). At 15 days testicular development was identical in all groups; by 28 days, however, males raised in 16L:8D or 14L:10D exhibited a greater degree of testicular development than those raised in 12L:12D. Next, females maintained on each of the three photoperiods throughout gestation were transferred, with their offspring, to the other two photoperiods at birth. Postnatal exposure to 14L:10D or 12L:12D inhibited testicular development in young that had been gestated on 16L:8D. Both 16L:8D and 14L:10D stimulated testicular growth in animals that had been gestated on 12L:12D or 14L:10D. Therefore, a) 16L:8D stimulates testicular growth in all animals, b) 12L:12D inhibits testicular growth in all animals, and c) the testicular response to 14L:10D depends on the photoperiod experienced by the mother during pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reception of photoperiodic information by fetal Siberian hamsters: role of the mother's pineal gland

The rate of reproductive development in juvenile male Siberian hamsters is strongly influenced by daylength (photoperiod). Recent studies indicate that reception of photoperiodic cues begins during fetal life. The present experiments yielded a further demonstration that developing male Siberian hamsters receive information about the photoperiod to which their mother is exposed during pregnancy. The possibility that photoperiodic information is transmitted from mother to young after birth was investigated by cross-fostering young gestated on 12L and 16L to mothers from the other photoperiod. Litters were cross-fostered on the day of birth and then were transferred, along with their foster mothers, to 14L. We found no influence of the mother after birth, indicating that transmission of photoperiodic information from mother to young must occur during gestation. To determine if the pineal gland of the mother is required for this response, adult females were pinealectomized or sham-operated and paired with intact males in 12L, 14L, or 16L. After parturition parents and offspring were exposed to 14L. The influence of prenatal photoperiod on postnatal testicular development in 14L was blocked by pinealectomy of the mother. Postnatal testicular development was retarded in offspring that experienced a photoperiod transfer from either 15L to 14L or 8L to 12L at birth. In contrast, the inhibitory effect of a transfer from 16L to 14L at birth was abolished when juvenile males were exposed to a single long photoperiod (16.3 h light) at age 17-21 days and then were returned to 14L.     

Effect of maternal pinealectomy and reverse photoperiod on the circadian melatonin rhythm in the sheep and fetus during the last trimester of pregnancy

The present study tested the hypothesis that the nocturnal melatonin rhythm in the fetal sheep results from transfer across the placenta of melatonin from maternal circulation. Pregnant ewes were exposed to an artificial reverse photoperiod at about 100 days gestation (n = 6; lights on 10 h, 2200-0800 h PST). This treatment tested for entrainment in the ewe and its fetus of the 24-h pattern of melatonin production from the pineal gland. Other ewes were pinealectomized at 55 days post-breeding (n = 6), and similarly treated. Catheters were implanted and blood samples were collected between 117 and 142 days gestation at two 48-h periods, about every 0.5-4 h, to assess the pattern of melatonin in maternal and fetal circulations. In pineal-intact ewes and their fetuses, melatonin rhythms conformed to the reverse photoperiod, i.e. plasma melatonin concentrations were relatively low during the light period and significantly increased for the duration of darkness. In contrast, maternal pinealectomy abolished the melatonin rhythms in both the ewe and fetus; melatonin concentrations remained at or below the limits of detection. Pineal-intact sheep gave birth about 139 +/- 2 days (mean +/- SE, n = 4) at 1915 +/- 0.7 h and pinealectomized ewes (n = 5 of 6) lambed at 149 +/- 2 days at 0424 +/- 0.5 h. Finally, in lambs (n = 3) born to pinealectomized ewes, typical melatonin rhythms were present within the first week of life. The findings indicate that the maternal pineal gland is responsible for the 24-h pattern of melatonin in the ewe and its fetus during the last trimester of pregnancy.     

Effects of pinealectomy on pituitary gonadotrophs, pituitary gonadotropin potency and hypothalamic gonadotropin releasing activity in Notemigonus crysoleucas

The effects of pinealectomy on pituitary gonadotrophs, pituitary gonadotropin potency and hypothalamic gonadotropin releasing activity were examined in the cyprinid teleost, Notemigonus crysoleucas, exposed to various photoperiod-temperature regimes. In fish exposed to a long photoperiod-warm temperature regime, pinealectomy resulted in a decrease in gonadal activity, in hypothalamic gonadotropin releasing activity and an increase in pituitary gonadotropin potency. Fewer gonadotrophs were present in the pituitary of sham operated fish than in the pituitary of pinealectomized fish. Ovarian development was more rapid in sham operated than in pinealectomized fish exposed to a long photoperiod–low temperature regime. Pituitary gonadotropin activity was also greater in shams than in pinealectomized fish. A short photoperiod-warm temperature regime retarded ovarian development in N. crysoleucas. Pinealectomy reversed this trend. Gonadotrophs made up a greater area of the pituitary in pinealectomized fish than in shams under these conditions. Gonadotropin potency of the pituitary and hypothalamic gonadotropin releasing activity were also greater in pinealectomized fish than in shams. The area of the pituitary occupied by gonadotrophs was greater in pinealectomized than in sham operated animals maintained on a short photoperiod-low temperature regime. Pituitary gonadotropin activity was also greater in pinealectomized fish as compared to shams. Pituitary gonadotropin potency varies diurnally in animals maintained on both short and long photoperiods; the rhythm of variation differs depending on photoperiod. Pinealectomy alters the diurnal rhythm of pituitary gonadotropin potency in animals exposed to both long and short photoperiods. It is concluded that pinealectomy has a pronounced effect on reproductive activity in N. crysoleucas. The effects of pinealectomy on reproduction vary with photoperiod, but are mediated via the hypothalamus and pituitary. In fish exposed to long daylengths the pineal favours reproductive activity, but the epiphysis retards reproductive processes in animals maintained on short photoperiods.     

Photoperiodic responses during larval development and diapause of two geographic ecotypes of the rice stem maggot, Chlorops oryzae

Chlorops oryzae is bivoltine in northern Japan but trivoltine in the southern part of the country. In the bivoltine strain, both the egg and larval stages were found to be sensitive to photoperiod. When the egg stage was exposed to a long-day photoperiod (16L:8D), larval development showed a short-day type response, and mature third-instar larvae entered a summer diapause under a long-day photoperiod (15L:9D). When eggs experienced short days, the first-instar larvae entered a winter diapause under short-day conditions, and the critical photoperiod in the larval stage ranged from about 14L:10D to about 12L:12D as the photoperiod experienced by the eggs increased from 12L:12D to 14L:10D. However, the development of the larvae after overwintering was not influenced by the photoperiod. In the trivoltine strain, larval development was retarded under a 14L:10D photoperiod but not under either shorter or longer photoperiods, when larvae had spent the egg stage under a 16L:8D photoperiod. The critical photoperiod of the larval stage for the induction of a winter diapause in the first instar was about 12L:12D, though it varied to some extent with the photoperiod during the egg stage. Thus, Chlorops oryzae was able to adapt itself to the local climatic conditions by the development of variable and complicated photoperiodic responses.     

Synchronization of the circannual reproductive rhythm of the ewe by discrete photoperiodic signals

Although many species display endogenous circannual rhythms of biological activity that are synchronized by day length, the specific photoperiodic requirements for synchronizing such rhythms are not established for any species. We tested the hypothesis that the circannual reproductive rhythm of sheep can be synchronized by exposure to just one or two discrete blocks of photoperiodic information each year. Ewes were pinealectomized to prevent their ability to transduce photoperiodic information into altered reproductive neuroendocrine activity. During the 53/4 yr following pinealectomy, specific photoperiodic signals were restored for discrete periods of time via replacement of 24-h patterns of melatonin, the pineal hormone that transmits photic information to the reproductive neuroendocrine axis. The ewes were kept in a 12-mo photoycycle that alternated between short (8L:16D) and long (16L:8D) days every 6 mo and that was 6 mo out of phase with the geophysical year. Pineal-intact control ewes exhibited synchronous annual reproductive cycles. Noninfused pinealectomized control ewes did not exhibit synchronous cycles. Pinealectomized ewes infused with alternating 70-day blocks of short- and long-day patterns of melatonin every 6 mo for the first 21/2 yr of the experiment exhibited synchronous annual reproductive cycles that were 6 mo out of phase with those of ewes maintained outdoors. This synchrony persisted when the frequency of the melatonin treatment was reduced to just one 70-day block of a long-day pattern of melatonin each 365 days. Cycle period was 368 +/- 3 days; standard deviation of the date of onset of reproductive induction averaged only 3 days. Our study provides the first direct evidence that a single block of photoperiodic information a year can synchronize a circannual rhythm.     

Geographic differences for delay of sexual maturation in Peromyscus leucopus: effects of photoperiod, pinealectomy, and melatonin

Effects of short-day photoperiod, pinealectomy, and melatonin on sexual maturation were tested in Peromyscus leucopus from either Connecticut (CT) or Georgia (GA). Laboratory reared-stocks from CT and GA were exposed to short daylength (photoperiod) from birth or 25 days of age. At 12 wk of age, delay in sexual maturation was indicated in most CT mice by decreased testis length, combined testes weight, and seminal vesicle weight. Conversely, GA animals did not delay sexual maturation when exposed to short-day photoperiod from either birth or 25 days of age. These results indicate that responses to short daylengths differ for juvenile CT and GA populations. In a second experiment, pinealectomized or sham-operated CT males were exposed to short-day (9L:15D) or long-day (16L:8D) photoperiod from birth. Pinealectomy blocked the effect of short daylength on reproduction. Therefore, the pineal must be involved in the delay of sexual maturation observed for short-day CT mice. The effects of melatonin, a pineal gland hormone, were tested with chronic s.c. implants or daily injections. In CT mice given either melatonin implants or afternoon injections, sexual maturation was delayed. GA mice were insensitive to all melatonin treatments. Further, no differences in circadian organization (phase angle, duration of activity, period under constant dark) between GA and CT animals were apparent. Collectively, these studies indicate that melatonin is involved in the mechanism responsible for delay of sexual maturation in CT mice. Short-day insensitivity of GA Peromyscus leucopus probably results from a deficiency in the melatonin effector pathway and is not due to a disruption of circadian organization.     

Pineal-dependent and -independent effects of photoperiod on immune function in Siberian hamsters (Phodopus sungorus)

Siberian hamsters (Phodopus sungorus) exhibit reproductive and immunological responses to photoperiod. Short (<10-h light/day) days induce gonadal atrophy, increase leukocyte concentrations, and attenuate thermoregulatory and behavioral responses to infection. Whereas hamster reproductive responses to photoperiod are dependent on pineal melatonin secretion, the role of the pineal in short-day induced changes in immune function is not fully understood. To examine this, adult hamsters were pinealectomized (PINx) or sham-PINx, and transferred to short days (9-h light/day; SD) or kept in their natal long-day (15-h light/day; LD) photoperiod. Intact and PINx hamsters housed in LD maintained large testes over the next 12 weeks; sham-PINx hamsters exhibited gonadal regression in SD, and PINx abolished this effect. Among pineal-intact hamsters, blood samples revealed increases in leukocyte, lymphocyte, CD62L+ lymphocyte, and T cell counts in SD relative to LD; PINx did not affect leukocyte numbers in LD hamsters, but abolished the SD increase in these measures. Hamsters were then treated with bacterial lipopolysaccharide (LPS), which induced thermoregulatory (fever), behavioral (anorexia, reductions in nest building), and somatic (weight loss) sickness responses in all groups. Among pineal-intact hamsters, febrile and behavioral responses to LPS were attenuated in SD relative to LD. PINx did not affect sickness responses to LPS in LD hamsters, but abolished the ameliorating effects of SD on behavioral responses to LPS. Surprisingly, PINx failed to abolish the effect of SD on fever. In common with the reproductive system, PINx induces the LD phenotype in most aspects of the immune system. The pineal gland is required for photoperiodic regulation of circulating leukocytes and neural-immune interactions that mediate select aspects of sickness behaviors.     

Plasma melatonin rhythms in euthermic marmots (Marmota flaviventris)

Plasma melatonin concentrations were measured in marmots (Marmota flaviventris) maintained under three short-day (4L:20D; 8L:10D; 10L:14D) and one long-day (14L:10D) photoperiod(s). Each animal had a daily rhythm of plasma melatonin with elevated plasma melatonin levels occurring during the dark period of the lighting cycle. There were no significant differences between any peak values during the night. The mean duration of elevated night melatonin concentrations was significantly different between long-day (16L:8D) and 8L:16D or 4L:20D animals (P less than 0.01). Daytime plasma melatonin levels were not significantly different among the photoperiods. These results characterize plasma melatonin rhythms in a sciurid rodent and demonstrate that this rhythm is modified by photoperiod. Therefore, the plasma melatonin profile could convey information about day length to the animal or, alternatively, the rhythm may be acting as a time-keeping mechanism for other physiological functions.     

Seasonal effects of pinealectomy on testicular recrudescence and secretory activity of seminal vesicles in the catfish Heteropneustes fossilis (Bloch)

The effects of pinealectomy on testicular activity and secretory activity of seminal vesicles were examined in the catfish Heteropneustes fossilis under various combinations of photoperiod and temperature during different periods of the annual reproductive cycle. Pinealectomy had no effect on gonadal activity during the preparatory, prespawning and spawning periods of the reproductive cycle. However, during the postspawning period, under long (LD 14:10) or short (LD 9:15) photoperiod at 25° C or at gradually increasing ambient temperature, pinealectomy accelerated testicular recrudescence and secretory activity of the seminal vesicles. Nevertheless. during this period the presence of the pineal facilitated the recrudescence of testes and seminal vesicles in catfish exposed to continuous light (LL), continuous darkness (DD) and 12 hL:l2 hD (LD) at 25° C. These findings suggest that the role of the pineal in catfish reproduction is variable and depends upon the photoperiod and temperature regimes to which the fish are exposed, as well as on the time of the year and the state of the reproductive cycle. The results also suggest that the effects of pinealectomy in catfish are mediated through an influence on the hypothalamo-hypophyseal gonadal axis.     

Pineal, Photoperiod and Gonadal Function in the Indian Palm Squirrel, Funambulus pennanti

Effects of morning and evening injections of pineal 5-methoxyindoles (MI), melatonin (aMT) and 5-methoxytryptamine (MT), for 60 continuous days, were observed on the testes of sham-operated (SO) and pinealectomized (Px) Indian palm squirrel, Funambulus pennanti maintained under different photoperiods during the gonad active phase. Long photoperiod (LP) of 14L:10D appeared stimulatory to the testes and caused a significant increase in the weight and seminiferous tubule diameter of both SO and Px animals, as compared to the animals under natural day-length (NDL). Short photoperiod (SP) of 10L:14D had an inhibitory influence and reduced the testes weight and its tubule diameter. aMT and MT injections during evening hours significantly reduced testes weight and tubule diameter of SO and Px animals under NDL, LP and SP. However morning injections, under all conditions, were without any significant effect. The results suggest an inhibitory effect of aMT and MT, under above photoperiodic conditions, on the testes of this tropical mammal. The time of administration of the MI is important in the expression of the effect.     

Inheritance of the photoperiodic response controlling larval diapause in the blow fly,Calliphora vicina

Larvae of the blow fly Calliphora vicina R-D. (Diptera: Calliphoridae) display a diapause in response to the exposure of their parents to short photoperiods. Due to geographic variation in photoperiodic response, flies from a southern, English population show a long-day response to the fixed photoperiod of L:D 15.5:8.5 whilst flies from a northern population from Finland show a short-day response to the same photoperiod. Crosses between these strains have shown previously that diapause incidence is a maternal characteristic; here we demonstrate that the hybrid female offspring of such crosses are not intermediate between the two parental strains but show a photoperiodic response biased towards their maternal line. Thus not only are males unable to influence directly the diapause incidence among their offspring but the indirect effects of inheritance down the male line are weaker than down the female. Diapause duration, in contrast, is influenced by each parent in a similar manner. Diapause lasts longer in larvae with a greater admixture of northern genes regardless of whether they were maternal or paternal.     

Short day lengths delay reproductive aging

Caloric restriction and hormone treatment delay reproductive senescence in female mammals, but a natural model of decelerated reproductive aging does not presently exist. In addition to describing such a model, this study shows that an abiotic signal (photoperiod) can induce physiological changes that slow senescence. Relative to animals born in April, rodents born in September delay their first reproductive effort by up to 7 mo, at which age reduced fertility is expected. We tested the hypothesis that the shorter day lengths experienced by late-born Siberian hamsters ameliorate the reproductive decline associated with advancing age. Short-day females (10L:14D) achieved puberty at a much later age than long-day animals (14L:10D) and had twice as many ovarian primordial follicles. At 10 mo of age, 86% of females previously maintained in short day lengths produced litters, compared with 58% of their long day counterparts. Changes in pineal gland production of melatonin appear to mediate the effects of day length on reproductive aging; only 30% of pinealectomized females housed in short days produced litters. Exposure to short days induces substantial decreases in voluntary food intake and body mass, reduced ovarian estradiol secretion, and enhanced production of melatonin. One or more of these changes may account for the protective effect of short day lengths on female reproduction. In delaying reproductive senescence, the decrease in day length after the summer solstice is of presumed adaptive significance for offspring born late in the breeding season that first breed at an advanced chronological age.     

Functional maturation of the gonads of Turkish hamsters under various photoperiods

The golden hamster, Mesocricetus auratus, is the only photoperiodic rodent to date that has been shown to fail to respond to inhibitory (i.e., short, less than 12.5 h/day) photoperiods until after pubertal onset. In other photoperiodic hamsters, mice, and voles, short photoperiods greatly retard gonadal maturation. The Turkish hamster, Mesocricetus brandti, is a photoperiodic rodent that as an adult is reproductively competent only on photoperiods of 15-17 h of light per day; photoperiods of less than 15 or greater than 17 h of light promote gonadal regression. In this report we addressed two questions: a) are prepubertal M. brandti photoperiodic, and b) if so, is gonadal maturation enhanced or suppressed by exposure to photoperiods of greater than 17 h of light per day? Turkish hamsters were raised on photoperiods of 12, 16, 20, or 24 (= LL) h of light per day. Testicular growth was retarded for 16 wk by 12L:12D. Very long days, 20L:4D, or LL did not retard testicular development. In females, pubertal onset, as indicated by first vaginal estrus, was delayed in young raised on 12L:12D and in 2 of 18 and 4 of 19 young raised on 20L:4D and LL, respectively. These results demonstrate that prepubertal Turkish hamsters are photoperiodic, but respond differently from adults to photoperiods greater than 17 h of light per day.