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At least 19 recordsLinked to original sources
PubMed2000

Inhibition of reproductive maturation and somatic growth of Fischer 344 rats by photoperiods shorter than L14:D10 and by gradually decreasing photoperiod.

Photoperiod is the major regulator of reproduction in temperate-zone mammals. Laboratory rats are generally considered to be nonphotoresponsive, but young male Fischer 344 (F344) rats have a uniquely robust response to short photoperiods of 8 h of light. Rats transferred at weaning from a photoperiod of 16 h to photoperiods of < 14 h of light slowed in both reproductive development and somatic growth rate. Those in photoperiods < 13 h of light underwent the strongest responses. The critical photoperiod of F344 rats can be defined as 13.5 h of light, but photoperiods of </= 12.5 h are required to fully suppress reproduction and somatic growth. This demonstrates that the 12-h photoperiod that is standard in some laboratory colonies would have significant effects on reproductive maturation and growth rate of this common rat strain. Young F344 rats in decreasing photoperiods that mimic natural change experienced delayed reproductive development and decreased growth rate to a greater extent and for a longer duration than those transferred at birth to a short photoperiod. The effects of gradual changes in photoperiod persisted for at least 12 wk after weaning. This indicates that young male F344 rats possess responses to photoperiod that would result in functional photoperiodism in a wild mammal.

Animals↗
PubMed2000

[Influence of genetic background of photoperiodic response on expression of short photoperiod sensitive male sterility in rice].

The photoperiodic response of F1 and its parents was analyzed, and the photoperiodic response and short photoperiod sensitive male sterility of F2 progenies were studied in this paper. The results showed that the strong response was dominant over the weak response to photoperiod. The photoperiodic response of plants with short photoperiod sensitive male sterility in F2 was above medial degree(the rate of heading acceleration > 15%). Photoperiodic response showed a positive correlation with short photoperiod sensitive male sterility. The main sterility genes of Yid1s and Yid2s were allelism. It can be inferred that the short photoperiod sensitive male sterile gene might be expressed upon the presence of the photoperiodic response genes, and that the short photoperiod sensitive male sterility of plants with the same genes could differ in their reproductive expressions, not because of the changes of the genes, but the difference in genetic backgrounds of photoperiodic response.

Fertility↗
PubMed1997

Influence of previous photoperiodic exposure on the reproductive response to a specific photoperiod signal in ewes.

Two experiments were carried out to determine whether the reproductive response of ewes to a specific photoperiodic signal depends on the time of year that the signal is given, and, if so, whether this dependence can be attributed to the photoperiodic history of the ewes. The aim of experiment 1 was to expand upon previous findings that the reproductive response to a specific photoperiodic challenge in ewes previously maintained on natural photoperiod varies with time of year. Ewes were transferred at one of three times of year from natural photoperiod to photochambers and were immediately exposed to 35 long days (18L:6D) followed by continuous exposure to short days (8.5L: 15.5D); this treatment is referred to as LD-->SD. The three times of year when long days started corresponded to the beginning of the breeding season, the mid-breeding season, and early anestrus (September 21, December 21, March 21, respectively). In ewes exposed to LD-->SD beginning in September, the breeding season and subsequent anestrous season was not altered. In ewes exposed to LD-->SD beginning in December, the transition to anestrus was advanced (p < 0.05) relative to that in controls maintained in simulated natural photoperiod. Subsequently, half of these ewes resumed reproductive activity within 180 days; this occurred 131 +/- 8 days after transfer to short days. In contrast, all ewes exposed to LD-->SD beginning in March resumed reproductive activity; this began 100 +/- 3 days after transfer to short days (p < 0.05 versus December group). The purpose of experiment 2 was to assess the extent to which the difference in response to a photoperiodic challenge can be attributed to photoperiodic history. Ewes were maintained on short days from the winter solstice interrupted with 35 long days from March 21, June 21, September 21, or December 21. The majority of ewes exhibited an onset of reproductive activity after exposure to LD-->SD at the different times of year, and there was no group difference in latency to onset of reproductive activity. The duration of reproductive activity, however, was longer (p < 0.05) in ewes exposed to LD-->SD beginning in June than in the other groups. Thus we conclude that the seasonal difference in the ability of the photoperiodic challenge of long followed by short days to induce reproductive activity in ewes previously maintained outdoors can be attributed, in large measure, to photoperiodic history. Other factors, such as phase of the endogenous rhythm, however, may influence the duration of reproductive activity resulting from this photoperiodic challenge.

Analysis of Variance↗
PubMed1988

Photoperiod and heat stress influence on lactating sow performance and photoperiod effects on nursery pig performance.

Three experiments were conducted to evaluate sow and piglet productivity under extended photoperiod. In Exp. 1, 98 crossbred, lactating sows were housed in one of four treatments: thermoneutral air temperature (23.6 degrees C) in either (h of light:dark) 1:23 or 16:8 photoperiods, or heat stress (30.4 degrees C) in either 1:23 or 16:8 photoperiods. Heat stress reduced (P less than .05) sow feed intake, piglet mortality and piglet weaning weight and increased (P less than .01) sow lactation weight loss. Number of pigs weaned per litter was increased (P less than .01) when sows were heat-stressed. Extended photoperiod reduced (P greater than .05) time for sows to rebreed postweaning by .4 d. The interaction between air temperature and photoperiod was significant only for sow lactation weight loss. Heat stress increased sow lactation weight loss, but this effect was more severe in the 1:23 than in the 16:8 photoperiod. Experiments 2 and 3 examined the effects of 1:23 or 16:8 photoperiods on nursery pig performance when pigs were weaned from sows experiencing 1:23 (Exp. 3) or 16:8 (Exp. 2) photoperiods. In both nursery studies, photoperiod did not influence (P greater than .10) postweaning pig mortality, feed intake, weight gain or gain:feed ratio. In conclusion, extended photoperiod reduced days to return to estrus and reduced sow lactation weight loss, especially during heat stress. No benefits in preweaning or postweaning piglet weight or survival were observed by use of extended photoperiod.

Animals↗
PubMed1986

Photoperiod and body weight in female Syrian hamsters: skeleton photoperiods, response magnitude, and development of photorefractoriness.

Two experiments examined the effects of various photoperiods on body weight and reproductive function in Syrian hamsters (Mesocricetus auratus). In Experiment 1 female hamsters were exposed to symmetrical skeleton photoperiods in which dawn and dusk were mimicked by 1-hr light pulses. A skeleton long (LD 16:8) photoperiod had no effect on body weight or estrous cyclicity (compared with animals in a complete LD 16:8 photoperiod), but exposure to a skeleton short (LD 10:14) photoperiod increased body weight and interrupted estrous cycles. Thus, body weight appears to respond to the relative timing of the two light pulses (a circadian mechanism) rather than to the absolute amount of light or darkness (an hourglass mechanism), just as does reproduction. In Experiment 2 female hamsters were exposed to a long photoperiod (LD 16:8) or to one of two short photoperiods (LD 10:14 and LD 8:16). Both short photoperiods increased body weight and interrupted estrous cycles, but the LD 8:16 photoperiod was substantially more effective at increasing body weight than was the LD 10:14. Thus, hamster body weight appears to be capable of a graded response to photoperiod, with shorter days producing a greater obesity. With prolonged exposure to the two short photoperiods (greater than 30 weeks), body weights spontaneously returned to the same level as the long-day controls, and estrous cycles resumed. When these hamsters were treated with the pineal gland hormone, melatonin, only those housed in long days exhibited the characteristic body weight gains, growth of brown and white adipose tissues, and decreases in uterine weight. Therefore, with prolonged exposure to short days, energy balance develops a photorefractoriness and an insensitivity to melatonin treatment, just as with reproductive function.

Adipose Tissue, Brown↗
PubMed2006

Differential response of type 2 deiodinase gene expression to photoperiod between photoperiodic Fischer 344 and nonphotoperiodic Wistar rats.

The molecular basis of seasonal or nonseasonal breeding remains unknown. Although laboratory rats are generally regarded as photoperiod-insensitive species, the testicular weight of the Fischer 344 (F344) strain responds to photoperiod. Recently, it was clarified that photoperiodic regulation of type 2 iodothyronine deiodinase (Dio2) in the mediobasal hypothalamus (MBH) is critical in photoperiodic gonadal regulation. Strain-dependent differences in photoperiod sensitivity may now provide the opportunity to address the regulatory mechanism of seasonality by studying Dio2 expression. Therefore, in the present study, we examined the effect of photoperiod on Dio2 expression in photoperiod-sensitive F344 and photoperiod-insensitive Wistar rats. A statistically significant difference was observed between short and long days in terms of testicular weight and Dio2 expression in the F344 strain, while no difference was observed in the Wistar strain. These results suggest that differential responses of the Dio2 gene to photoperiod may determine the strain-dependent differences in photoperiod sensitivity in laboratory rats.

Animals↗
PubMed2001

Genetic variation in photoperiodism among naturally photoperiodic rat strains.

Rattus norvegicus has been considered nonphotoperiodic, but Fischer 344 (F344) rats are inhibited in growth and reproductive development by short photoperiod (SD). We tested photoresponsiveness of the genetically divergent Brown Norway (BN) strain of rats. Peripubertal males were tested in long photoperiod or SD, with or without 30% food reduction. Young males were photoresponsive, with reductions in testis size, body mass, and food intake in SD and with enhanced responses to SD when food restricted. Photoperiods < or =11 h of light inhibited reproductive maturation and somatic growth, whereas photoperiods of 12 h or more produced little or no response. F344/BN hybrids differ from both parent strains in the timing, amplitude, and critical photoperiod of photoperiodic responses, indicating genetic differences in photoperiodism between these strains. This is consistent with the hypothesis that ancestors of laboratory rats were genetically variable for photoperiodism and that different combinations of alleles for photoperiodism have been fixed in different strains of rats.

Animals↗
PubMed1989

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.

Aging↗
PubMed1986

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)

Animals↗
PubMed1982

Photoperiodic Control of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy : The Effect of Skeleton and Continuous Light Photoperiods.

The control of night-break timing was studied in dark-grown seedlings of Pharbitis nil (Choisy cv. Violet) following a single continuous or skeleton photoperiod. There was a rhythmic response to a red (R) interruption of an inductive dark period, and the phasing of the rhythm was influenced by the preceding light treatment.Following a continuous white light photoperiod of 6 hours or less, the points of maximum inhibition of flowering were constant in real time. Following a continuous photoperiod of more than 6 hours, maximum inhibition occurred at 9 and 32.5 hours after the end of the light period. The amplitude of the rhythm during the second circadian cycle was much reduced following prolonged photoperiods.Following a skeleton photoperiod, the time of maximum sensitivity to a R interruption was always related to the second pulse of the skeleton, R(2), with the first point of maximum inhibition of flowering occurring after 12 to 18 hours and the second after 39 hours. Without a second R pulse, the time of maximum sensitivity to a R interruption was related to the initial R(1) pulse. A ;light-off' or dusk signal was not mimicked by a R pulse ending a skeleton photoperiod; such a pulse only generated a ;light-on' signal and initiated a new rhythm.It is concluded that the timing of sensitivity to a R interruption of an inductive dark period in Pharbitis nil is controlled by a single circadian rhythm initiated by a light-on signal. After 6 hours in continuous white light, the phase of this rhythm is determined by the transition to darkness. Following an extended photoperiod, the timing characteristics were those of an hourglass; this seemed to be due to an effect on the coupling or expression of a single circadian timer during the second and subsequent cycles, rather than to the operation of a different timing mechanism.In addition to the effects on timing, the photoperiod affected the magnitude of the flowering response.

Journal Article↗
PubMed1986

Photoperiodic effects in the Djungarian hamster. Rate of testicular regression and extension of pineal melatonin pattern depend on the way of change from long to short photoperiods.

In the Djungarian hamster, short photoperiods induce regression of testes and accessory glands. There is evidence that the length of time melatonin levels are elevated is the signal conveying the photoperiodic effects to the neuroendocrine axis. When the temporal course of decompression of the pattern of pineal melatonin content was followed after a change from long to short photoperiods by symmetrical extension of the dark time, extension proceeded first into the morning hours. The present study investigated whether, after transition from long to short photoperiods by either extending the dark period into the morning hours, or starting the dark time at noon, differences in the gonadal reaction and in the rate of extension of the pineal pattern ensue. Three and 7 weeks after transition from long to short photoperiods highly significant differences were found in the rate of involution of testes and accessory glands. Regression was more advanced after extension of the dark period into the morning hours. The length of time that pineal melatonin content was elevated also differed markedly between the two groups. After 3 weeks in short photoperiods, the length of the melatonin peak was more than 9 h when the dark period was extended into the morning, but only about 7 h if it was extended into the afternoon. It is suggested that the different rate of gonadal regression after different ways of transition into the same photoperiod is due to the different rate of decompression of the melatonin pattern.

Animals↗
PubMed1975

Photoperiodic control of LH release in the ram. I. Influence of increasing and decreasing light photoperiods.

The influence of variations in the duration of light photoperiod on LH release has been studied in intact and castrated rams submitted to a 6 months "annual" light rhythm. Blood was collected once a month and the plasma LH levels were determined by radioimmunoassay. In intact rams, the LH level is decreased when the light photoperiod is increased, but as soon as this photoperiod begins to decrease, a sharp increase (about 80%) in the LH level is observed. Thereafter a slow decline in plasma LH occurs until the photoperiod reaches a minimum. In the castrated animals, the plasma LH level is three to four times higher than in the intact animals but the pattern of the variations with regard to the light photoperiod duration, is essentially the same. Thus an increase of 59 and 43% in plasma LH occurs in the two breeds used when the light photoperiod begins to decrease. These results indicate that hypothalamo-hypophyseal activity is modulated by gradual changes in the duration of light photoperiods either in the presence or absence of androgens.

Animals↗
PubMed2001

Mammalian photoperiodic system: formal properties and neuroendocrine mechanisms of photoperiodic time measurement.

Photoperiodism is a process whereby organisms are able to use both absolute measures of day length and the direction of day length change as a basis for regulating seasonal changes in physiology and behavior. The use of day length cues allows organisms to essentially track time-of-year and to "anticipate" relatively predictable annual variations in important environmental parameters. Thus, adaptive types of seasonal biological changes can be molded through evolution to fit annual environmental cycles. Studies of the formal properties of photoperiodic mechanisms have revealed that most organisms use circadian oscillators to measure day length. Two types of paradigms, designated as the external and internal coincidence models, have been proposed to account for photoperiodic time measurement by a circadian mechanism. Both models postulate that the timing of light exposure, rather than the total amount of light, is critical to the organism's perception of day length. In mammals, a circadian oscillator(s) in the suprachiasmatic nucleus of the hypothalamus receives photic stimuli via the retinohypothalamic tract. The circadian system regulates the rhythmic secretion of the pineal hormone, melatonin. Melatonin is secreted at night, and the duration of secretion varies in inverse relation to day length; thus, photoperiod information is "encoded" in the melatonin signal. The melatonin signal is presumably "decoded" in melatonin target tissues that are involved in the regulation of a variety of seasonal responses. Variations in photoperiodic response are seen not only between species but also between breeding populations within a species and between individuals within single breeding populations. Sometimes these variations appear to be the result of differences in responsiveness to melatonin; in other cases, variations in photoperiod responsiveness may depend on differences in patterns of melatonin secretion related to circadian variation. Sites of action for melatonin in mammals are not yet well characterized, but potential targets of particular interest include the pars tuberalis of the pituitary gland and the suprachiasmatic nuclei. Both these sites exhibit uptake of radiolabeled melatonin in various species, and there is some evidence for direct action of melatonin at these sites. However, it appears that there are species differences with respect to the importance and specific functions of various melatonin target sites.

Animals↗
PubMed1997

Alteration of testicular response to long photoperiod by transient exposure to short photoperiod in collared lemmings (Dicrostonyx groenlandicus).

The reproductive response of collared lemmings (Dicrostonyx groenlandicus) to photoperiod is unique for rodents. Whereas most reproductively photoresponsive rodents show maximal gonadal growth when exposed to long photoperiod (long day), collared lemmings show delayed maturation when born and maintained under this condition. However, transfer of lemmings from short photoperiod (short day) to long day results in maximal gonadal growth, indicating that the response to long day depends upon photoperiod history. We hypothesized that the slowing of maturation observed in animals born and maintained on long day reflects an inability to respond fully to long day, resulting from the absence of previous exposure to short day. To determine whether young lemmings born in long day are capable of being stimulated by long day, we exposed them at weaning (19 days of age) to 1, 6 or 10 weeks of short day, and then challenged them with a second exposure to long day. Relative to animals transferred permanently to short day at weaning, lemmings exposed to 6 weeks of short day showed accelerated gonadal growth after both 5 and 10 weeks of subsequent exposure to long day, and those exposed to 10 weeks of short day had larger testes after 6 weeks of long day. Thus, during transient exposure to short day, the animals acquired sensitivity to the stimulatory effects of long day. The responses of body mass, bifid claw width and pelage colour to the photoperiod manipulations did not parallel that of the gonads, indicating independent regulation of somatic and reproductive parameters. The unique way in which the reproductive system of collared lemmings responds to photoperiod may reflect evolution in an environment where the production of offspring during periods of unchanging long day (for example, the Arctic summer) is not selectively advantageous.

Animals↗
PubMed1997

Description of the photoperiodic control of larval burrowing in the blowfly Lucilia cuprina: a novel index for photoperiodic research.

The photoperiodic control of larval burrowing depth in the Australian sheep blowfly Lucilia cuprina (Wiedemann) (Diptera: Calliphoridae) was investigated by measuring burrowing depth under controlled laboratory photoperiods. The results demonstrated that larvae exposed to long photoperiods (LD 18 : 6) burrowed to deeper depths than those in shorter photoperiods (LD 12 : 12), and that this behavior was induced during the third instar stage. The ecological significance of this behavior is discussed, as are the ways in which daylength is measured and depth assessed. The use of burrowing depth could prove to be a novel index of a photoperiodic response and provide a far simpler approach to the study of photoperiodism in certain insect species.

Animals↗
PubMed1990

Maternal transfer of photoperiodic information in Siberian hamsters. IV. Peripubertal reproductive development in the absence of maternal photoperiodic signals during gestation.

Peripubertal reproductive development of Siberian hamsters is influenced by photoperiodic information received during gestation; the maternal pineal is important for this process. We observed that in the absence of the maternal pineal, the fetus appears to receive no information about gestational photoperiods. This is not the equivalent of receipt of a long-day signal by the fetus. Pinealectomized and sham-operated pregnant females were exposed to photoperiods of 12L:12D, 14L:10D, 16L:8D, or constant light (LL); young were reared from birth to 28 days of age in LL or 14L:10D. Regardless of the gestational photoperiod, LL-reared male young born to pinealectomized dams had smaller testes than LL-reared young of pineal-intact dams exposed to 16L:8D while pregnant. Thus, pinealectomy did not result in transmission of a long-day signal, nor did young born of pinealectomized dams receive short- or intermediate-day signals. Unlike young of pineal-intact females exposed to 12L:12D or 14L:10D while pregnant, young born of pinealectomized dams had small testes when reared in 14L:10D, irrespective of gestational photoperiod. Uterine weights of female young presented similar patterns of responses. In a second experiment, adult females were entrained to 12L:12D, 14L:10D, or 16L:8D for 3 wk prior to pinealectomy to determine if the effect of maternal pinealectomy would be altered. Entrainment to the new photoperiod prior to surgery did not alter the effects of maternal pinealectomy.

Animals↗
PubMed1989

Effect of pinealectomy, eye enucleation, and melatonin treatment on ovarian activity and vitellogenin levels in the catfish exposed to short photoperiod or long photoperiod.

To investigate the relative importance of pineal, and eyes, and melatonin treatment on ovarian activity, catfish. Heteropneustes fossilis subjected to pinealectomy, eye enucleation, or both were exposed to short (LD 9:15) or long (LD 14:10) photoperiod during the different stages of the annual reproductive cycle. During the preparatory period, pinealectomy accelerated ovarian recrudescence under long photoperiod, but no effects of pinealectomy were observed under short photoperiod. Pineal has no influence on ovarian activity during the prespawning and spawning periods of the reproductive cycle. In the late postspawning period, ovarian recrudescence is accelerated after pinealectomy in catfish maintained under short photoperiod (LD 9:15 at 25 degrees C). But under long photoperiod (LD 14:10 at 25 degrees C), pinealectomy delayed ovarian recrudescence. An investigation into the role of the eyes revealed that eye enucleation nullifies the inhibitory effects of LD 9:15 and represses the stimulatory effects of LD 14:10. Combined surgery inhibited ovarian development under both the regimes. It is significant that pinealectomy and/or blinding neither counteracted nor delayed the postspawning ovarian regression. The findings suggest that the role of the pineal in catfish reproduction is variable and depends upon the photoperiod to which they are exposed as well as on the time of the year and the stage of the reproductive cycle. It is concluded that extraretinal and extrapineal photoreceptors also are involved in influencing seasonal reproduction. Further, no effects of melatonin treatment on ovarian activity or on vitellogenin levels during the preparatory or prespawning periods were observed, indicating that the pineal effects on gonadal activity in catfish may not be mediated through the secretion of melatonin.

Animals↗
PubMed1992

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)

Animals↗