Daylength influences pelage and plasma prolactin concentrations but not reproduction in the prairie vole, Microtus ochrogaster

Short daylengths did not affect testes weight or spermatogenic index in male voles or uterine weight in female voles. Short daylengths did stimulate the growth of a winter pelage in both sexes; short-day voles had longer underhairs and guard hairs and a thicker, more dense pelage than did long-day voles. Plasma prolactin concentrations were five times higher in long-day than in short-day females and 25% higher in long-day males than in short-day males. The effect of short daylength on pelage was prevented by pinealectomy. We suggest that the growth of a winter coat is an obligate adaptation for winter survival, stimulated by exposure to short daylengths, but that changes in breeding activity are facultative and dependent to a greater extent on other cues for seasonal synchronization.     

Hormonal regulation of the annual pelage color cycle in the Djungarian hamster, Phodopus sungorus. I. Role of the gonads and pituitary

The Djungarian hamster exhibits an agouti pelage in the summer and a predominantly white pelage in the winter. This pelage color cycle is known to be regulated by the length of the daily photoperiod probably acting through the pineal gland, as is the seasonal cycle of reproductive function with which it is closely correlated ( Figala et al., '73; Hoffmann, ' 78b ). The possibility of a causal relationship between the decline in gonadal hormone secretion and the coat color change occurring in short photoperiod was examined. Gonadectomized and intact male and female hamsters were exposed to either long (16L:8D) or short ( 10L : 14D ) photoperiod for several months. Gonadectomy neither induced the change to the winter pelage color in long photoperiod-housed animals, nor prevented either the change to the winter pelage or the spontaneous return to summer pelage color in short photoperiod-housed animals. Chronic implants of testosterone in castrated males delayed and attenuated the short photoperiod-induced coat color change. Administration of ovine prolactin (100 micrograms/day) stimulated pigmentation in hamsters with the winter pelage, whereas administration of a alpha MSH (30 micrograms/day) was without effect. These results suggest that changes in pelage color may be regulated largely by changes in pituitary prolactin secretion and modified to some extent by changes in gonadal steroid hormone secretion.     

Interaction of daylength and lactation in the control of pelage development and nest-building in female meadow voles (Microtus pennsylvanicus)

Pregnancy and lactation inhibited moult into winter pelage in voles maintained in short daylengths; development of a winter pelage was, however, greatly accelerated once the short-day dams weaned their litters. The presumed elevation of prolactin titres during lactation appears to mask full development and expression of pelage changes induced by short daylengths. Nest-building behaviour, by contrast, was increased in response to short photoperiods and was further augmented during lactation and may thereby facilitate thermoregulation in short-day dams that do not develop a winter pelage.     

Hormonal regulation of the annual pelage color cycle in the Djungarian hamster, Phodopus sungorus. II. Role of prolactin

This study investigated whether photoperiod-induced changes in circulating prolactin levels, which have been observed in the Djungarian hamster ( Yellon and Goldman, '83; Duncan and Goldman, ' 83a ), might be involved in seasonal pelage color changes in this species. Injection of ovine prolactin (100 micrograms/day) inhibited the short photoperiod-induced winter molt. This finding indicated that the suppression of endogenous prolactin levels normally occurring in short photoperiod-housed hamsters (Duncan and Goldman, ' 83a ) may induce the winter molt. Suppression of prolactin secretion with bromoergocryptine (200 micrograms/day) strongly inhibited the spring molt, while concomitant treatment with ovine prolactin (100 micrograms/day) overcame this effect of bromoergocryptine. Injection of bromoergocryptine (200 micrograms/day) stimulated the winter molt in castrated hamsters housed in long photoperiod; concomitant injection of prolactin (100 micrograms/day) reversed this effect as well. These findings strongly suggested that an increase in endogenous prolactin levels may be necessary for the development and maintenance of the summer pelage.     

Seasonal changes in body mass and thermogenesis in tree shrews (Tupaia belangeri): The roles of photoperiod and cold

Environmental factors play an important role in the seasonal adaptation of body mass and thermogenesis in small, wild mammals. To determine the contributions of photoperiod and cold on seasonal changes in energy metabolism and body mass, the resting metabolic rates (RMR), nonshivering thermogenesis (NST), energy intake and gut morphology of the tree shrews were determined in winter and summer and in laboratory acclimated animals. Body mass, RMR and NST increased in winter, and these changes were mimicked by exposing animals to short-day photoperiod or cold in the animal house. Energy intake and digested energy also increased significantly in winter, and also during exposure of housed animals to both short-day photoperiod and cold. The lengths and weights of small intestine increased in winter. These results indicated that Tupaia belangeri overcomes winter thermoregulatory challenges by increasing energy intake and thermogenesis, and adjusted gut morphology to balance the total energy requirements. Short-day photoperiod and cold can serve as environmental cues during seasonal acclimatization.     

Early photoperiod history and short-day responsiveness in Siberian hamsters

Siberian hamsters exhibit seasonal, photoperiod influenced cycles of reproductive activity, body size, pelage characteristics, and thermoregulatory behavior. Laboratory populations generally exhibit inter-individual variability in expression of photoperiod responsiveness, with a subset of individuals that fail to show the species typical responses to short photoperiod. This variability is partly explained by a genetic component, as it has been possible to increase the number of short-day nonresponders by artificial selection. Responsiveness to short photoperiod is also substantially influenced by photoperiod history in this species; hamsters that have been raised under long (16L) or very long (18L) day lengths are less likely to exhibit winter-type responses to short days as compared to hamsters raised under an intermediate (14L) day length. In the present experiment, we examined effects of age and early photoperiod history in a strain of Siberian hamsters that had been selected for short-day nonresponsiveness. Hamsters transferred into short photoperiod on the day of birth were uniform in exhibiting winter-type responses. However, hamsters raised until 25 days of age in either continuous illumination or in 16L exhibited variation in responsiveness when subsequently moved into short photoperiod. We conclude that virtually all hamsters of the short-day nonresponsive strain are born responsive to short days. Subsequent development of resistance to potential short day effects is dependent on age and/or photoperiod history.     

Correlation with changes in horns and pelage, but not reproduction, of seasonal cycles in the secretion of prolactin in rams of wild, feral and domesticated breeds of sheep

Seasonal cycles were monitored in groups of wild (mouflon), feral (Soay) and domesticated breeds of sheep (Shetland, Blackface, Herdwick, Norfolk, Wiltshire, Portland, Merino, Soay x Portland and Soay x Merino) living outdoors near Edinburgh (56 degrees N). Changes in the blood plasma concentrations of prolactin and FSH, and growth of the horns and pelage were measured every half calendar month from 1 to 3 years of age. In all breeds there was a clearly defined seasonal cycle in the plasma concentration of prolactin with an 18-66-fold increase in mean values from the nadir in November and December to the peak in May and June. The seasonal increase in prolactin was closely correlated with the seasonal increase in the growth of the horns, both within and between breeds (e.g. time of peak prolactin vs horn growth for 11 breeds, R = 0.62, P less than 0.05). In the mouflon, Soay and some of the domesticated breeds of sheep (Wiltshire, Herdwick and Shetland), the seasonal increase in prolactin was also temporally correlated with the resurgence of growth of the pelage in spring and a conspicuous moult. In the other breeds developed to produce fine wool (e.g. Norfolk, Portland and Merino), there was no clear seasonal change in the pelage and growth continued throughout the year. Comparison between breeds indicated that continuous growth of the pelage was associated with higher plasma prolactin concentrations in winter. The times of the seasonal changes in plasma concentrations of prolactin were not significantly correlated with the corresponding changes in the plasma concentrations of FSH. The overall results are consistent with a role for prolactin related to the growth of the horns and pelage rather than the seasonal cycle in reproduction. The differences between the wild-type and the domesticated breeds in the pelage represent the effect of selective breeding to produce a long fine fleece which has involved changes in both the seasonal pattern of prolactin secretion and the growth characteristics of the hair fibres.     

Molting in the Djungarian hamster (Phodopus sungorus Pallas): seasonal or continuous process?

After transfer into a short daylight regimen, the brownish summer pelage of the Djungarian hamster (Phodopus sungorus) changes into the whitish winter phenotype. Although changes in serum prolactin levels are identified as the initiating hormonal signal, morphological data about molting in that species are sparse. The aim of this study was to characterize in detail the summer and winter pelage of the Djungarian hamster and to analyze the alterations in the skin and pelage induced by photoperiodic changes. The main difference between summer and winter hair types is the pattern of pigmentation. In contrast to other mammalian species showing seasonal changes, the winter coat of the Djungarian hamster is not characterized by an increase in hair density. Molting patches were observed at all times, even in the winter coat, showing that the light regimen does not control the process of molting itself but the pattern of pigmentation and eventually the loss of hair during the single molting wave.     

Genetic and environmental influences on short-day responsiveness in Siberian hamsters (Phodopus sungorus)

Siberian hamsters are photoperiodic rodents that typically exhibit several physiological changes when exposed to a short-day photoperiod. However, development of the winter phenotype in short days is largely conditional on prior photoperiod history: Hamsters that have been reared in an exceptionally long day length (18 L) do not usually exhibit the winter phenotype after transfer to short days, whereas animals reared under "moderately" long days (16 L) are more variable in responsiveness to subsequent short-day exposure, with 20% to 30% generally failing to exhibit winter-type responses. Hamsters reared exclusively in an "intermediate" day length (14 L) are almost uniformly responsive to short photoperiod. In the present study, the authors examine the influence of photoperiod history on short-day responsiveness in a breeding line of hamsters that has been subjected to artificial selection for resistance to the effects of short days. The results demonstrate that photoperiod history is an important determinant of short-day responsiveness in both random-bred (UNS) hamsters and animals artificially selected and bred for nonresponsiveness to short photoperiod (PNR). The PNR hamsters have a reduced requirement for long-day exposure to evoke a state of unresponsiveness to short days. The results are discussed in relation to possible significance for the origin of population and species differences in photoperiod responsiveness.     

Genetic variation for seasonal adaptation in Peromyscus leucopus: nonreciprocal breakdown in a population cross

The genetic basis for adaptations to the diverse environments encountered by a wide-ranging species should be reflected in the phenotypes of hybrids between differentiated populations. We crossed mice from two ecologically different populations to determine whether adaptations to seasonality in Peromyscus leucopus, the white-footed mouse, display directional dominance or whether they are affected by specific interactions between genes. Connecticut mice (C) have many adaptations to seasonality that are reduced or absent in Georgia mice (G) and that affect both reproductive and thermoregulatory traits. Because these adaptations are cued by photoperiod, temperature, or both, parental and hybrid mice were acclimated to 13 degrees C in either long-day or short-day photoperiod, and several morphological, reproductive, and thermoregulatory traits were measured. Several traits, especially those involved in response to the environment, such as nest size at 13 degrees C, reproductive regression under short-day photoperiod, and molt to winter pelage, showed a non-Mendelian pattern of inheritance. However, there was a pronounced difference between maternal lineages in the F2 generation in that C-maternal F2 mice were less responsive to short-day photoperiod and cold than were the G-maternal F2 mice. Because these two classes of F2 mice are genetically equivalent, this breakdown cannot be explained as a disruption of epistatic gene interactions unless recombination rates are higher in the C-maternal lineage.     

Role of photoperiod in reproductive maturation and peripubertal hormone concentrations in male collared lemmings (Dicrostonyx groenlandicus).

Onset of sexual maturation was determined in weanling male collared lemmings exposed to one of three experimental regimens of different photoperiods before and after weaning. Animals gestated in photoperiods of either 16 h light:8 h dark or 8 h light:16 h dark. Those from 16 h light:8 h dark were transferred at 19 days of age to either 20 h light:4 h dark or 8 h light:16 h dark; those gestated under 8 h light: 16 h dark remained in that photoperiod throughout the experiment. After exposure for 15, 20, 25 or 30 days to the postweaning photoperiod, animals were killed and the following parameters assessed: body weight, testes weight, seminal vesicle weight, the presence or absence of epididymal spermatozoa and serum concentrations of prolactin, testosterone and corticosterone. All parameters except serum testosterone were significantly influenced by photoperiod. Animals housed under 8 h light:16 h dark had significantly greater body weights than those housed under 20 h light:4 h dark, a response that differs from that reported for other arvicoline rodents. The group gestated on 16 h light:8 h dark and transferred on day 19 to 8 h light:16 h dark had lower testes and seminal vesicle weights than the other two groups, and mature spermatozoa in the epididymides appeared 5 days later than in the 20 h light:4 h dark group. Serum prolactin was largely undetectable in animals from both 8 h light:16 h dark groups, but all males housed in 20 h light:4 h dark had 2.0-15.0 ng prolactin ml-1. Concentration of serum corticosterone was higher in animals weaned into long photoperiod, and decreased with age. These data indicate that weanling male D. groenlandicus are reproductively photoresponsive, but use a decrease in photoperiod, not static short-photoperiod exposure, to alter the rate of development. Prolactin was largely undetectable in animals exposed to short photoperiod, indicating that high concentrations of this hormone are not important for maturation. Low prolactin concentrations in animals in short photoperiods may mediate the annual moult to white pelage. The short-photoperiod-mediated decrease in corticosterone may play a role in seasonal changes in body weight and composition.     

Regulation of seasonal reproductive activity in the stallion, ram and hamster

This review considers seasonal reproduction in male animals with emphasis on the stallion, ram and hamster. The pineal hormone melatonin is the common link between photoperiod and reproduction. An increase in the daily diurnal period of melatonin secretion is associated with a decrease in GnRH release in long-day breeders, but an increase in GnRH release in short-day breeders. Melatonin influences GnRH release within or close to the mediobasal hypothalamus in rams; whereas melatonin receptors have not been found in the hypothalamus of horses. Prolactin release is positively correlated with daylength. Prolactin concentrations are consequently low during the breeding season of sheep and high during the breeding season of horses and hamsters. Prolactin stimulates testicular function in rams. Seasonal changes in GnRH release in the horse are regulated by changes in a GnRH-inhibitory opioidergic tone. Opioids are at least, in part, responsible for the decrease in testicular function during winter. An opioidergic inhibition of LH release is present during the breeding season in rams; but dopaminergic pathways inhibit LH release during long daylight hours. A dopaminergic inhibition of LH release does not exist in stallions.     

Photoperiod regulates corticosterone rhythms by altered adrenal sensitivity via melatonin-independent mechanisms in Fischer 344 rats and C57BL/6J mice

Most species living in temperate zones adapt their physiology and behavior to seasonal changes in the environment by using the photoperiod as a primary cue. The mechanisms underlying photoperiodic regulation of stress-related functions are not well understood. In this study, we analyzed the effects of photoperiod on the hypothalamic-pituitary-adrenal axis in photoperiod-sensitive Fischer 344 rats. We first examined how photoperiod affects diurnal variations in plasma concentrations of adrenocorticotropic hormone (ACTH) and corticosterone. ACTH levels did not exhibit diurnal variations under long- and short-day conditions. On the other hand, corticosterone levels exhibited a clear rhythm under short-day condition with a peak during dark phase. This peak was not observed under long-day condition in which a significant rhythm was not detected. To analyze the mechanisms responsible for the photoperiodic regulation of corticosterone rhythms, ACTH was intraperitoneally injected at the onset of the light or dark phase in dexamethasone-treated rats maintained under long- and short-day conditions. ACTH induced higher corticosterone levels in rats examined at dark onset under short-day condition than those maintained under long-day condition. Next, we asked whether melatonin signals are involved in photoperiodic regulation of corticosterone rhythms, and rats were intraperitoneally injected with melatonin at late afternoon under long-day condition for 3 weeks. However, melatonin injections did not affect the corticosterone rhythms. In addition, photoperiodic changes in the amplitude of corticosterone rhythms were also observed in melatonin-deficient C57BL/6J mice, in which expression profiles of several clock genes and steroidgenesis genes in adrenal gland were modified by the photoperiod. Our data suggest that photoperiod regulates corticosterone rhythms by altered adrenal sensitivity through melatonin-independent mechanisms that may involve the adrenal clock.     

Seasonal changes of thermoregulatory efficiency in Djungarian hamsters

In their natural habitat, Djungarian hamsters are faced with dramatic seasonal changes. This requires various morphological and physiological adaptations allowing cope with harsh climate and food shortage, particularly in winter. These seasonal changes are controlled by the photoperiod and can be observed also in the laboratory at room temperature. The aim of the present study was to investigate if the efficiency of thermoregulation also depends on the photoperiod. For this reason, Djungarian hamsters were transferred to short-day conditions (SDC) with 8 h light and 16 h darkness. Two-thirds of the animals were classified as responders showing the typical seasonal changes – decrease of body mass, fur change, testes regression, vagina closing. The total activity per day did not change but, the nocturnal activity was spread over the longer dark time. The body temperature decreased, and the animals showed regular daily torpor. To investigate the thermoregulatory efficiency, body temperatures were correlated with motor activity. The obtained regression coefficients describe formally the effect of motor activity on body temperature, a measure for the efficiency of thermoregulation. In SDC, the coefficients were elevated, both during rest and activity, i.e. the same amount of activity did produce a larger increase in body temperature. Under field conditions, this might be an additional mechanism to compensate the bigger in winter heat loss. Also, the high coefficients may support the increase in body temperature at the end of a torpor phase by a bout of motor activity. The results show that, seasonal changes of thermoregulatory efficiency are an effective accessory way to cope with different temperatures in hamsters’ natural environment.     

Seasonal adjustments in body mass and thermogenesis in Mongolian gerbils (<Emphasis Type="Italic">Meriones unguiculatus</Emphasis>): the roles of short photoperiod and cold

Seasonal adjustments in body mass and thermogenesis are important for the survival of small mammals during acclimatization in the temperate zone. To determine the contributions of short photoperiod and cold temperatures to seasonal changes in thermogenesis and body mass in Mongolian gerbils (Meriones unguiculatus), body mass, basal metabolic rate (BMR), nonshivering thermogenesis (NST), energy intake and energy digestibility were determined in seasonally acclimatized and laboratory acclimated animals. Body mass showed significant seasonal changes and decreased to a minimum in winter. Both BMR and NST increased in winter, and these changes were mimicked by exposing animals to short photoperiod or cold temperatures in the animal house. Digestible energy intake also increased significantly in winter, and also during exposure of housed animals to both short photoperiod and cold. These results suggest that Mongolian gerbils overcome winter thermoregulatory challenges by increasing energy intake and thermogenesis, and decreasing body mass to reduce total energy requirements. Short photoperiod and cold can serve as effective environmental cues during seasonal acclimatization.     

Are the short-photoperiod-induced decreases in serum prolactin responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters?

Serum prolactin (PRL) decreases in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus sungorus) hamsters following short-photoperiod exposure. Both species also exhibit short-photoperiod-induced changes in body and lipid mass, but in opposite directions; Syrian hamsters increase and Siberian hamsters decrease their body weight, changes reflected nearly exclusively in their carcass lipid content. The purpose of these experiments was to determine whether the photoperiod-induced changes in PRL were responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters by using the strategy of experimentally producing serum PRL levels opposite to those normally associated with the photoperiod in which the animals were housed. In long photoperiods serum PRL was reduced to short-day levels by subcutaneous (s.c.) CB-154 (bromoergocryptine, a dopamine agonist) injections. In short photoperiods, serum PRL was elevated to long-day levels in Syrian hamsters by ectopic pituitary explants, and in Siberian hamsters, serum PRL was elevated by chronic s.c. infusions of ovine PRL (oPRL). In both species, manipulations of serum PRL did not affect food intake, carcass composition, or the wet weight of various white and brown adipose tissue pads (WAT and BAT, respectively). Body weight increased in CB-154-treated Syrian hamsters and decreased in Siberian hamsters, an effect partially reversed by coadministration of oPRL in Syrian, but not Siberian, hamsters. Thus, lowering serum PRL to short-day levels in long-day-housed hamsters of both species mimicked the directional change in body weight appropriate for each species when they are exposed to short days. This effect of CB-154 on body weight may be a result of some as yet unidentified effect of dopaminergic stimulation on overall growth since 1) these changes in body weight were not reflected as changes in lipid mass, as occurs naturally following short-day exposure for each species, and 2) neither species exhibited a reciprocal change in body weight when serum PRL was experimentally elevated in short days. Alternatively, it may be that once the energetic responses to short-day exposure have been fully expressed, the ability of PRL to stimulate the target sites of action for PRL for these responses may be decreased. BAT protein content, cytochrome oxidase activity (measures of metabolic growth of this tissue), and retroperitoneal total and specific lipoprotein lipase (LPL) activities were increased by CB-154 treatment in Syrian hamsters.(ABSTRACT TRUNCATED AT 400 WORDS)     

Endogenous circannual cycles of ovarian activity and changes in prolactin and melatonin secretion in wild and domestic female sheep maintained under a long-day photoperiod

The present study examines the ovulatory activity of wild and domesticated ewes subjected to either a constant photoperiod of long days (16L:8D) or natural changes in daily photoperiod for 16 mo. The aim was to determine whether an endogenous reproductive rhythm controls seasonal reproductive activity in these sheep, and how the photoperiod might affect this. The effects of long-day photoperiods on long-term changes in prolactin and melatonin secretion were also evaluated. The two species showed changes in reproductive activity under the constant photoperiod conditions, suggesting the existence of an endogenous rhythm of reproduction. This rhythm was differently expressed in the two types of ewe (P < 0.05), with the domestic animals exhibiting much greater sensitivity to the effects of long days. A circannual rhythm of plasma prolactin concentration was also seen in both species and under both photoperiod conditions, although in both species the amplitude was always lower in the long-day animals (P < 0.01). The duration of the nocturnal melatonin plasma concentrations reflected the duration of darkness in both species and treatments. The peak melatonin concentration did not differ between seasons either under natural or long-day photoperiods.     

Melatonin implants prevent the onset of seasonal quiescence and suppress the release of prolactin in response to a dopamine antagonist in the Bennett's wallaby (Macropus rufogriseus rufogriseus)

Three groups of adult female wallabies were maintained out of doors under conditions of natural photoperiod and temperature from late December to mid-August. One group (M1; N = 6) received Silastic elastomer melatonin implants on 14 December, a second group (M2; N = 5) were given implants on 16 February and a third group (C; N = 7) were unimplanted controls. Group C animals had all ceased cycling by 15 March and the subsequent breeding season commenced on 5 July +/- 6.9 days. Group M1 wallabies continued to cycle throughout the experimental period and did not exhibit ovarian quiescence. In Group M2, 2/5 animals continued to undergo repeated oestrous cycles and 3/5 ceased cycling between 14 December and 27 January and began again after the insertion of melatonin implants on 16 February. The prolactin response 30 min after s.c. administration of the dopamine antagonist domperidone was determined approximately every 4 weeks. In Group C, peak responses were high during the period of seasonal quiescence (January-June; mean range 14.2-19.6 ng/ml) and fell significantly (P less than 0.02) at the beginning of the breeding season in early July to 7.4 +/- 3.1 ng/ml. In Group M1, prolactin levels remained low (2.8-8.2 ng/ml) throughout the course of the experiment while in Group M2, response to domperidone fell following the insertion of the implants and subsequently remained at levels similar to those in Group M1. Our data support the hypothesis that photoperiod-induced changes in the secretion of melatonin after the winter solstice drive this species into seasonal quiescence by influencing the dopaminergic control of prolactin secretion.     

Separate neural systems mediate the steroid-dependent and steroid-independent suppression of tonic luteinizing hormone secretion in the anestrous ewe

In the ewe, two types of seasonal fluctuations in secretion of tonic luteinizing hormone (LH) have been described: a steroid-dependent change whereby estradiol gains the capacity to suppress LH pulse frequency in anestrus, and a steroid-independent decrease in pulse frequency in ovariectomized animals during anestrus. We have proposed that the former reflects activation, in anestrus, of estradiol-sensitive catecholaminergic neurons that inhibit gonadotropin-releasing hormone (GnRH). Three results reported here support this hypothesis: dopaminergic (pimozide) and alpha-adrenergic (phenoxybenzamine) antagonists increased LH in intact anestrous ewes without altering pituitary responses to GnRH; other dopaminergic (fluphenazine) and alpha-adrenergic (dibenamine) antagonists also increased LH in anestrus; agonists for dopaminergic (apomorphine) and alpha-adrenergic (clonidine) receptors suppressed LH secretion in both seasons, suggesting that the appropriate receptors are present in breeding-season ewes. In contrast, catecholamines do not appear to mediate the steroid-independent suppression of pulse frequency; neither pimozide nor phenoxybenzamine increased LH pulse frequency in ovariectomized ewes during anestrus. When antagonists for 6 other neurotransmitter receptors (muscarinic and nicotinic cholinergic, GABAnergic, serotonergic, opioid, and beta-adrenergic) were tested in anestrus, only cyproheptadine, the serotonergic antagonist, increased pulse frequency in ovariectomized ewes. Cyproheptadine had no effect on frequency during the breeding season. On the basis of these results, we propose that the steroid-dependent and -independent actions of anestrous photoperiod occur via catecholaminergic and serotonergic neurons, respectively.     

Photoperiod and temperature affect reproductive and nonreproductive functions in male prairie voles (Microtus ochrogaster)

Adult male prairie voles (Microtus ochrogaster) were housed for 10 wk and exposed to long (16L:8D) or short (8L:16D) photoperiods at 21 degrees or 5 degrees C. Maintenance in short day lengths reduced testicular, epididymal, and seminal vesicle mass and also significantly depressed spermatogenic activity. Cold ambient temperature further suppressed gonadal size in voles exposed to short days. Several pelage characteristics were affected by photoperiod, but not by temperature. Increased fur density, fur depth, and length of guard hair and underhair were observed in voles exposed to short days. Intrascapular brown fat and gonadal fat pad mass as well as body mass were significantly less in voles housed in cold temperatures than in voles exposed to warm ambient temperatures; photoperiod did not affect these parameters. Approximately 30% of the male voles exposed to short days maintained their reproductive systems, yet they clearly processed photoperiodic information; all short-day males, regardless of reproductive condition, had comparable winter pelage development. Our results suggest that in prairie voles, photoperiod may be a predictive cue for reproductive function in nature; however, it appears that pelage development is a more obligatory response to photoperiod than is reproduction.