Hormone release is tied to changes in cell size in the osmoreceptive prolactin cell of a euryhaline teleost fish, the tilapia, Oreochromis mossambicus.

Prolactin (PRL) cells from a teleost fish, the tilapia, Oreochromis mossambicus, facilitate the direct study of osmoreception. The release of two prolactins, PRL(188) and PRL(177), which act in freshwater osmoregulation in teleost fish, rises in vitro within 5 min after extracellular osmolality falls. An increase in cell size accompanied this rise. Cell size and PRL release also increased, albeit more slowly, following the partial replacement of medium NaCl (55 mOsmolal) with an equivalent concentration of urea, a membrane-permeant molecule. Similar replacement using mannitol, which is membrane-impermeant, elicits no response. These findings suggest that osmoreception is linked to changes in cell volume rather than to extracellular osmolality per se.

Influence of near-ultraviolet radiation on reproductive and immunological development in juvenile male Siberian hamsters.

The aim of this study was to characterize the lenticular ultraviolet transmission of the Siberian hamster (Phodopus sungorus) and to probe the range of near-ultraviolet (UV-A, 315-400 nm) and visible wavelengths (400-760 nm) for modulating the photoperiodic regulation of its reproductive and immune systems. Ocular lenses from adult hamsters were found to transmit UV-A wavelengths at similar levels to visible wavelengths, with a short-wavelength cut-off of 300 nm. Five separate studies compared the responses of juvenile male hamsters to long photoperiods (16 h:8 h L:D), short photoperiods (10 h:14 h L:D) and short photoperiods interrupted by an equal photon pulse of monochromatic light of 320, 340, 360, 500 or 725 nm during the night. The results show that UV-A wavelengths at 320, 340 and 360 nm can regulate both reproductive and immune short-photoperiod responses as effectively as visible monochromatic light at 500 nm. In contrast, long-wavelength visible light at 725 nm did not block the short-photoperiod responses. These results suggest that both wavelengths in the visible spectrum, together with UV-A wavelengths, contribute to hamster photoperiodism in natural habitats.

Hormonal control of osmoregulation in the channel catfish Ictalurus punctatus.

Prolactin (PRL) is an important hormone for freshwater adaptation in many teleost species. In some euryhaline fishes, growth hormone (GH) and cortisol are involved in seawater adaptation by stimulating ion extrusion. When channel catfish (Ictalurus punctatus) were transferred from fresh water to dilute seawater (300-400 mOsm), their plasma osmolality was always higher than the environmental salinity. In correlation with the increase in plasma osmolality, significant increases in plasma cortisol were observed. However, no effect of ovine GH or cortisol was seen in plasma osmolality or gill Na, K-ATPase activity when the hormones were given during the course of acclimation to dilute seawater. When catfish in fresh water were hypophysectomized, plasma osmolality was significantly decreased by 24 h, reaching a minimum level after 2 days. When they were transferred to dilute seawater, the plasma osmolality of the sham-operated fish was consistently higher than that of environmental water, whereas the osmolality of the hypophysectomized fish was equivalent to the environmental salinity. Ovine PRL restored the plasma osmolality of the hypophysectomized fish in fresh water to the level of sham-operated fish. Cortisol was also effective, but the effect was less pronounced than the effect of PRL. Injection of PRL in combination with cortisol resulted in a marked additive increase in plasma osmolality to a level even above that of the sham-operated fish. Ovine GH was without effect. These treatments in hypophysectomized fish transferred to dilute seawater produced essentially the same results as those in fish in fresh water. Plasma osmolality was also increased after PRL treatment of the intact fish in fresh water. There was a synergistic effect between PRL and cortisol in hypophysectomized fish in dilute seawater as well as in intact fish in fresh water. PRL did not stimulate cortisol secretion either in hypophysectomized fish or in intact fish. In the stenohaline catfish, both PRL and cortisol seem to be involved importantly in ion uptake from the environment not only in fresh water but also in brackish water.

A test of the coincidence and duration models of melatonin action in Siberian hamsters: the effects of 1-hr melatonin infusions on testicular development in intact and pinealectomized prepubertal Phodopus sungorus.

The pineal hormone melatonin is known to play an important role in mediating photoperiodic messages to the reproductive system in seasonal breeding animals. Our goal was to test, in a single experimental paradigm, two hypotheses that have been forwarded to describe how the circadian rhythm of pineal melatonin transmits photoperiodic information to the reproductive system: 1) induction, i.e., a short-day effect, occurs when secreted melatonin and a circadian rhythm of sensitivity to melatonin coincide in time; 2) induction occurs following exposure to elevated circulating melatonin levels for a prescribed duration. In order to determine the relative validity of these hypotheses, we investigated the testicular maturation response to 1-hr daily infusions of 10, 25, and 50 ng of melatonin in pinealectomized intact and prepubertal Siberian hamsters (Phodopus sungorus). Animals received, beginning on day 15 of life, programmed subcutaneous infusions of melatonin or vehicle at one of five time points (19:00-20:00, 20:00-21:00, 21:00-22:00, 24:00-01:00, and 03:00-04:00 hr) for 15 days. In animals gestated and raised in a long photoperiod (LD16:8 = 16L, where L is the duration of light in hours, and D that of dark), melatonin infusion right after lights off (20:00-21:00 hr) significantly retarded gonadal maturation; this dose was ineffective at other times tested. Doses of 10 and 25 ng melatonin were ineffective at all time points. Identical results were obtained in prepubertal hamsters gestated in a short photoperiod (LD10:14 = 10L) and raised in 16L; these results were independent of the presence or absence of the pineal gland. In animals gestated and raised in 10L, melatonin infusions failed to suppress testicular development beyond that induced by the photoperiod; testicular development was maximally suppressed in all groups. The results of these investigations are best explained under the experimental conditions employed here: 1) the photoperiodic gonadal response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered melatonin with an intrinsic rhythm of sensitivity to melatonin, which, under the constraints imposed by our experimental design, occurred at 20:00-21:00 hr; and 2) the duration of the melatonin signal alone, equal in all groups, cannot explain the results.

Effects of age and photoperiod on reproduction and the spleen in the marsh rice rat (Oryzomys palustris).

To examine the interactions between age and photoperiod on reproduction and spleen weights, we exposed adult male and female rice rats of various ages to photoperiods of 16:8-h light-dark photoperiods (16L:8D) or 12L:12D. After 10 wk, animals were killed and the following data were recorded: weights of testes, seminal vesicles, uterus, ovaries, body, and spleen and, in addition, vaginal patency. Young adult males displayed a greater degree of testicular and seminal vesicle regression in short photoperiods than did older males; the testes of most older males did not regress in response to short photoperiods. Spleen weight was unresponsive to short photoperiods in all males, but was affected by age. Females, however, exhibited reproductive organ regression and decreased vaginal patency in response to short photoperiods at all ages examined. Body weights were affected by photoperiod in young females, and, as in males, photoperiod had no effect on spleen weights. These data suggest that the reproductive response to photoperiod in adult male rice rats declines with age, whereas in adult females it does not.

A test of the coincidence and duration models of melatonin action in Siberian hamsters. II. The effects of 4- and 8-hr melatonin infusions on testicular development of pinealectomized juvenile Siberian hamsters (Phodopus sungorus).

In a previous paper we demonstrated that properly timed 1-hr infusions of 50 ng melatonin effectively suppressed testicular development in juvenile Siberian hamsters. Only melatonin infused between 20:00 and 21:00 hr was effective in animals exposed to 16L (lights off 20:00 hr). In this paper we further investigate the importance of the coincidence and duration hypotheses of daily exposure of melatonin. Prepubertal Siberian hamsters received either 4- or 8-hr melatonin infusions at various times either on long photoperiod (LD 16:8 = 16L) or on short photoperiod (LD 10:14 = 10L). Daily 8-hr melatonin infusions suppressed testicular development in both photoperiods. Daily 4-hr, 50 ng/hr, melatonin infusions at 17:00-21:00 hr inhibited testicular growth in 16L and daily 4-hr melatonin infusions (either 50 ng/h or 50 ng/day) inhibited testicular growth at 17:00-21:00 hr in 10L. We also tested the efficacy of an interrupted melatonin infusion of long duration (8 hr). Pinealectomized prepubertal male Siberian hamsters, born on 16L, were infused with two signals of 4 hr separated by an interval of 2 hr. Melatonin-infused groups had significantly inhibited testicular growth compared to vehicle-infused animals. Testicular development was maximally inhibited only in those groups in which the period of melatonin sensitivity identified in the previous paper (20:00-21:00 hr) overlapped or immediately followed a period of melatonin infusion. Considering the restrictions of the experimental design employed in these studies, the results are best explained by the hypothesis that the photoperiodic gonadal response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered melatonin with an intrinsic rhythm of sensitivity to melatonin, which occurred at 20:00-21:00 hr. The duration of the melatonin signal alone can not explain the results.

Properties of the melatonin-generating system of the sailfin molly, Poecilia velifera.

The properties of the melatonin-generating system of a tropical teleost, the sailfin molly (Poecilia velifera), were investigated in vitro in a series of experiments using static or perifusion culture techniques. The properties examined included photic entrainment, circadian rhythmicity under continuous light (LL) and continuous darkness (DD), functionality of the melatonin-generating system at birth, and presence of multiple circadian oscillators in the molly pineal. Pineal glands or skull caps with the pineal gland firmly attached were dissected from adult and new-born fishes, respectively, and placed into static or perifusion culture at constant temperature (27 degrees C) depending upon the experiment. Melatonin release in samples was quantified by RIA. Rhythmic melatonin release was observed from isolated adult pineals under 12L:12D and 14L:10D, with low amounts of melatonin released during the light and high amounts during the dark. Melatonin release was inhibited by LL. However, under DD, melatonin release was robust and rhythmic with a circadian period (Tau) that ranged between 21.3 and 27.0 h (n = 21). Pineals from new-born (1-day old) mollies released melatonin rhythmically under a light:dark cycle and DD in both static and perifusion culture. Melatonin release from half and quarter pineals of adult mollies under DD was robust and rhythmic with circadian periods that ranged between 22.5 and 29.0 h (n = 19). Taken together, these data show that the molly pineal is photosensitive, fully functional from birth, and contains multiple circadian oscillators (at least four) regulating melatonin production.

Presence of an intrapineal circadian oscillator in the teleostean family Poeciliidae.

In most fish, rhythmic melatonin production is controlled by circadian oscillators located within the pineal (=pineal clocks) that are reset daily by the ambient light:dark (LD) cycle. However, one question that has yet to be addressed concerns the phylogenetic distribution of the pineal clock within fish families. We tested whether a pineal clock identified in the sailfin molly (Poecilia velifera) in an earlier study is also present in some other representatives of the teleostean family Poeciliidae. Isolated pineals from adults belonging to the genus Poecilia (P. velifera albino, P. reticulata, and P. sphenops), genus Xiphophorus (X. helleri and X. maculatus), and genus Limia (L. vittata) were obtained and cultured under LD and/or continuous darkness (DD) at constant temperature (27 degrees C). With one exception, free-running rhythms in melatonin release with circadian periodicities ranging from 19.5 to 27.4 h (n = 26) were detected in isolated pineals from all poeciliid representatives tested under DD exposure. In addition, rhythmic melatonin production was also observed in isolated pineals of some representatives tested from all three genera under LD exposure, suggesting the property of direct photosensitivity. Taken together, these data suggest that a circadian oscillator residing in the pineal of the sailfin molly also appears to be present in all of the poeciliid representatives tested in our system, supporting the notion that the presence of a pineal clock occurs at the family level of taxonomic organization.

Spontaneous testicular regression in collared lemmings (Dicrostonyx groenlandicus) exposed to prolonged long photoperiod.

Reproductive maturation and attainment of maximal gonadal size in collared lemmings (Dicrostonyx groenlandicus) is facilitated by transfer from a short photoperiod (8 h light: 16 h dark) to a long photoperiod (22 h light:2 h dark). However, reproductive maturation in lemmings born in 22 h light:2 h dark is impeded by exposure to the natal photoperiod of 22 h light:2 h dark. Data from adult lemmings suggest that prolonged exposure to a long photoperiod of 20 h light:4 h dark results in gonadal regression. The present experiments were conducted to test the hypothesis that adult lemmings exposed to a prolonged long photoperiod undergo testicular regression. Male collared lemmings were transferred at weaning (19 days of age) from the natal photoperiod of 8 h light:16 h dark to one of two long photoperiods: 22 h light:2 h dark or 24 h light:0 h dark (constant light). Gonadal mass was maximal in both groups 10 weeks after weaning. However, 20 weeks after weaning, gonadal mass in both groups was low (relative to that at 10 weeks), suggesting that regression had occurred. When testes mass was maximal 10 weeks after weaning, animals were transferred in groups from 22 h light:2 h dark to one of several shorter photoperiods: 20 h light:4 h dark, 18 h light:6 h dark, 16 h light:8 h dark, or 8 h light:16 h dark. Testes mass 20 weeks after weaning was greatest, that is, not different from maximal, in animals from the 20 h light:4 h dark and 18 h light:6 h dark groups, intermediate in animals from the 16 h light:8 h dark group, and low in animals from the 8 h light:16 h dark group. These observations indicate that transfer from 22 h light:2 h dark to a slightly shorter photoperiod permits maintenance of gonadal size, but transfer to a short photoperiod causes testicular regression. Body weight, bifid claw width and pelage colour did not change during spontaneous gonadal regression in animals housed in a long photoperiod for 20 weeks. These results suggest that prolonged exposure to an unchanging long photoperiod causes spontaneous testicular regression, and that photoperiod regulates reproductive and somatic parameters independently.

The impact of photoperiods and melatonin on gonadal development in juvenile Turkish hamsters (Mesocricetus brandti).

The reproductive response of both intact adult and juvenile Turkish hamsters has been thoroughly studied and shown to be similar, unlike the golden hamster where juveniles remain aphotoperiodic until approximately 8 weeks of age. Unstudied to date, however, is the role of the pineal and its hormone melatonin in generating the testicular response to photoperiod in juvenile Turkish hamsters. Therefore, in this study we examined the reproductive response of prepubertal male Turkish hamsters, subjected to four different photoperiods (8L:16D, 16L:8D, 20L:4D, and 24L:0D) with altered pineal gland function. At 15 days of age, long-day-born (16L:8D) hamsters were either pinealectomized, received melatonin implants, or remained untreated. Testes sizes were measured every 2 weeks. Testicular growth occurred only in untreated and beeswax implanted groups in 16L:8D. Exposure to other photoperiods inhibited testicular development in untreated and beeswax implanted animals. Removal of the pineal gland, masking of the daily melatonin rhythm with constant release subcutaneous melatonin implants, or eliminating the daily rhythm of melatonin by continuous light exposure resulted in inhibition of gonadal development. These results demonstrate that juvenile Turkish hamsters respond similarly to adults on all photoperiods and under all conditions of pineal function tested.

The neuroendocrine control of clock-timed gonadotropin release in the female Syrian hamster: role of serotonin.

We hypothesized that rhythms in hypothalamic serotonergic activity were permissive to daily and estrous cycle-related rhythms of LH, FSH and prolactin (PRL). In the Syrian hamster, proestrus (PRO) is characterized by a surge of LH, FSH and PRL; diestrus (DIE) by low LH and FSH and a small surge of PRL, while in photoperiod-induced anestrous (PIA) animals there is a surge of LH and FSH and low PRL. Turnover rates of serotonin (5HT) in four brain areas were determined for the three reproductive states at 2-h intervals. Turnover in the preoptic area and arcuate nuclei did not change, indicating that 5HT projections to these regions probably do not control LH, FSH or PRL release. Serotonin turnover in the median eminence (ME) was elevated at 0600 h in PIA females, at 0600 h, 0800 h, and 1400 h on DIE and at 0600 h and 2200 h on PRO. Since the pattern of 5HT turnover in the ME is different during each of the three reproductive states, 5HT in this area is likely not crucial to the control of LH, FSH and PRL. Turnover of 5HT also did not change in the suprachiasmatic nuclei (SCN) of PRO or PIA animals. However, 5HT turnover rates in the SCN were elevated at 1200 h, 2000 h, and 2400 h on DIE. The correlation of high 5HT turnover in the SCN of DIE but not PRO and PIA animals suggested that elevated serotonergic activity in the SCN is part of the mechanism by which the gonadotropin surge is prevented on DIE. To test this, PRO and DIE hamsters were injected with 5HT receptor ligands. Administration of a 5HT agonist attenuated the PRO surge of LH and blocked the surge of PRL. In contrast, administration of two 5HT antagonists failed to elicit a surge of LH in DIE and phenobarbital-blocked PRO females, an indication that other mechanisms also contribute to inhibition of gonadotropin and PRL surges.

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.

Dissociation of spontaneous testicular recrudescence and photorefractoriness in the Syrian hamster.

In male Syrian hamsters, short days induce regression of the reproductive system, but eventually spontaneous recrudescence occurs ensuing from refractoriness to the inhibitory photoperiod. Although the photoperiod of 12 L:12 D (12 h light:12 h dark) may act like a short day by inducing the testicular cycle outlined above, it may fail to evoke the increase of circulating concentrations of prolactin that accompanies testicular recrudescence. This photoperiod may fail to induce photorefractoriness, as indicated by the prolonged low concentrations of prolactin in the blood. Herein, hamsters were exposed to either 8 L:16 D or 12 L:12 D from weaning and by 28 weeks exposure to either photoperiod, the hamsters had large testes following recrudescence from a photoperiod induced-regression). Transfer to 8 L:16 D from 12 L:12 D at 28 weeks resulted in a second testicular regression and recrudescence. In a second experiment, the testes of hamsters moved to 8 L:16 D from 12 L:12 D after 29 weeks exposure to the latter photoperiod similarly regressed and then regrew. Serum concentrations of prolactin decreased in these males after transfer to the shorter daylength but also decreased in hamsters kept under 12 L:12 D both groups were usually below those of hamsters moved to 14 L:10 D. These results show that exposure for 28 or 29 weeks to 12 L:12 D was insufficient to induce photorefractoriness, despite the recrudescence of the testes following involution.

Influence of photoperiod, time, and sex on hormone concentrations in collared lemmings (Dicrostonyx groenlandicus).

Collared lemmings (Dicrostonyx groenlandicus) of both sexes show seasonal changes in body mass and body composition. Previous studies using single-point sampling indicated that, in young males, these photoperiod-mediated changes are associated with changes in circulating growth hormone (GH), corticosterone (B), and thyroid hormones. The present study was conducted to (1) examine daily fluctuations in serum levels of GH, B, and thyroxine (T4) in animals exposed to long (22L:2D, "LD"), intermediate (16L:8D, "ID"), and short (8L:16D, "SD") photoperiods, (2) confirm that conclusions based on single-point sampling are valid when photoperiod-related differences in hormone concentration are examined over 24 hr, (3) examine the effect of photoperiod on hormone concentrations in adults of both sexes, and (4) characterize the daily pineal melatonin rhythm in this species. Adult male and female collared lemmings housed in SD had higher levels of GH, and lower levels of B and T4, even when the diurnal variations in serum concentrations of these hormones were taken into account. A significant effect of time was observed on serum B (ID animals only) and serum T4. ID lemmings had B levels that were similar to those of SD animals, but concentrations of GH that more closely resembled those of LD animals. Females had lower GH and T4 than males. Pineal melatonin concentration closely tracked the dark phase of the day in each of the three photoperiods. Photoperiod-mediated changes in melatonin synthesis may mediate observed day length-related differences in serum concentrations of metabolic hormones, which in turn may contribute to the seasonal changes in body composition observed in collared lemmings.

Release of amino acids and related compounds from the adrenal equivalent and caudal vein of the eel in vitro.

The release of catecholamines and cortisol from the perifused adrenal region and caudal vein of the eel (Anguilla rostrata) was compared with the release of 39 amino acids and related compounds. Dopamine, norepinephrine and epinephrine were present in all perifusates of the adrenal region. Dopamine release from the caudal vein exceeded that from the adrenal region, and norepinephrine and epinephrine were not detected. Cortisol was present in the perifusate of the adrenal region but virtually absent in caudal vein perifusate. Of the six substances with known or suspected neurotransmitter function, taurine, aspartate, glutamate, glycine and alanine were present in all or almost all samples from both the adrenal equivalent and the caudal vein. gamma-aminobutyric acid (GABA) was detected in a few samples from either preparation. The release of taurine and phosphoethanolamine may be linked to that of norepinephrine and epinephrine. Adrenocorticotropic hormone (ACTH) enhanced the release of cortisol, aspartate, valine, leucine and ornithine from the adrenal region, but the release appears to be from differing sources or cellular pools. Overall, the study revealed that both the adrenal region and caudal vein release a large number of amino acids and related substances. The caudal vein, and possibly other blood vessels as well, may be a major source of circulating dopamine.

Pineal gland and melatonin affect testicular status in the adult marsh rice rat (Oryzomys palustris).

The effects of the presence or absence of the pineal gland and of melatonin administration were examined on testicular maintenance, regression, and recrudescence in adult male rice rats. Pinealectomy at the beginning of the study caused significant testicular regression in animals housed on both long (16L:8D) and short (12L:12D) photoperiods (Experiment 1). Sham-operated males maintained testicular weight on 16L:8D and underwent testicular regression on 12L:12D. The magnitude of testicular regression in pinealectomized animals was reduced relative to that of sham-operated animals housed on a short photoperiod and occurred on a different time course. Animals pinealectomized after testicular regression had occurred on 12L:12D demonstrated delayed photostimulated testicular recrudescence when housed on 16L:8D, while spontaneous testicular recrudescence was unaffected in animals maintained on 12L:12D (Experiment 2). Administration of melatonin via subcutaneous implants at the beginning of the study (Experiment 3) produced similar effects on testicular function attributed to pinealectomy in Experiment 1. Finally, melatonin implants administered after the testes regressed on 12L:12D delayed both photostimulated and spontaneous testicular recrudescence in animals housed on long or short photoperiods, respectively (Experiment 4). These results suggest a role for both the pineal gland and melatonin at various stages of the annual reproductive cycle of the male rice rat.

Effects of prenatal and postnatal photoperiods and of the pineal gland on early testicular development in the marsh rice rat (Oryzomys palustris).

Prepubertal male rice rats are responsive reproductively to photoperiod early in postnatal development. Testicular growth was inhibited in animals gestated and reared to 28 days of age on photoperiods ranging from 8 to 13 h of light per day, and stimulated in animals reared on 14 h or more of light per day, suggesting a critical photoperiod between 13 and 14 h for testicular growth. In addition, postnatal photoperiod markedly affected testicular development in animals exposed to various prenatal and postnatal photoperiods. Postnatal exposure to constant light unmasked effects of the prenatal photoperiod on testicular development. Exposure of rice rats to short or long photoperiods during the period of lactation (Days 1-14 of life) affected testicular maturation. In addition, alteration of the photoperiod experienced after Day 14 of life (post-lactational) markedly affected testicular development and was the primary factor determining whether maturation would occur. Finally, pinealectomy had little to no effect on the magnitude of testicular development in animals maintained on the presurgical photoperiod, but did prevent animals from demonstrating the appropriate testicular response to a new photoperiod after surgery. These data suggest that early testicular development in rice rats is affected by the prenatal and postnatal photoperiod and that the pineal gland is involved in the transduction of this environmental information.

Effects of photoperiod on pineal melatonin in the marsh rice rat (Oryzomys palustris).

Pineal melatonin content was examined under four different photoperiods (10L:14D, 12L:12D, 14L:10D, and 16L:8D) in adult female rice rats (Experiment 1). Pineal melatonin was basal during the light and increased beginning 1 hr after lights off. Within 2 hr after lights off, melatonin increased to levels that were maintained throughout the dark period. In all but one photoperiod (10L:14D), melatonin remained elevated prior to light onset and decreased markedly within one hour after lights on. In addition, the duration of pineal melatonin was inversely related to the length of the photoperiod. In Experiment 2, the time course of pineal melatonin content on 16L:8D was examined every 20 min during the first hour after lights off and the first hour after lights on. Melatonin content increased gradually during the first hour and decreased markedly within 20 min after lights on. These data show that pineal melatonin in female rice rats is regulated by photoperiod.

Effects of photoperiod, pinealectomy, and melatonin implants on testicular development in juvenile Siberian hamsters (Phodopus sungorus).

When transferred from a long to short photoperiod, Siberian hamsters (Phodopus sungorus) undergo a number of physiological and morphological changes including suppression of gonadal activity, a change in pelage color, a decrease in body weight, and, in response to a simultaneous decrease in ambient temperature, physiological changes associated with the induction of daily torpor. All these functions can be affected by photoperiod and melatonin treatment. To investigate the interactive effects of photoperiod, pinealectomy, and melatonin on gonadal development, two experiments were performed using juvenile Siberian hamsters. In experiment 1, animals born in a long photoperiod (16L:8D) either remained in a long photoperiod or were transferred to a short photoperiod (8L:16D) from 15 days of age, when surgeries (pinealectomy and/or melatonin implantation) were performed. Testicular development was inhibited in all animals bearing melatonin implants irrespective of the presence or absence of the pineal gland. Pinealectomy blocked the inhibitory effect of short photoperiod on maturation of the reproductive system. Therefore, the pineal gland must be involved in the short photoperiod- induced inhibition of testicular maturation of juvenile Siberian hamsters. In experiment 2, a similar experimental design was employed except that the hamsters were born and raised to 15 days of age in 8L:16D. Exogenous melatonin, pinealectomy, or both retarded gonadal development in hamsters born in 8L:16D and transferred on Day 15 of age to 16L:8D. All hamsters maintained in a short photoperiod had small testes irrespective of the presence of absence of the pineal gland or of melatonin implants. Hamsters transferred to a long photoperiod after pinealectomy and/or melatonin implantation had small testes compared to those of the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of collared lemmings to melatonin: I. Implants and photoperiod.

We examined the effect of constant-release melatonin capsules on the physiology and morphology of female collared lemmings exposed to either chronic long (22L:2D) or short (8L:16D) photoperiod, or to a change in photoperiod. When animals were maintained on unchanging long or short photoperiod, subcutaneous melatonin implants were without effect. However, when animals were reared on either 22L:2D or 8L:16D and transferred to the alternate photoperiod at weaning, melatonin (implanted at weaning) prevented most photoperiod-related responses. At sacrifice (after 8 weeks of treatment), melatonin-implanted animals exposed to a change in photoperiod did not differ from animals remaining in the original photoperiod with respect to pelage color, bifid claw size, uterine mass, or serum prolactin (PRL). In contrast, regardless of treatment, animals exposed to a photoperiod transfer developed a body mass that partially or fully reflected that characteristic of the secondary photoperiod; i.e., both control- and melatonin-implanted animals transferred from long to short photoperiod developed a large body mass. These results indicate that masking the endogenous melatonin rhythm via constant-release melatonin implants renders collared lemmings unable to respond to a change in photoperiod with respect to most physiological parameters. However, the striking seasonal change in body mass experienced by collared lemmings appears to be at least partially independent of a melatonin signal.