Gonadal hormone-dependent and -independent regulation of immune function by photoperiod in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit changes in reproductive and immune function in response to seasonal variations in day length. Exposure to short days induces gonadal regression and inhibits testosterone secretion. In parallel, short days enhance immune function: increasing leukocyte numbers and attenuating cytokine and behavioral responses to infection. We examined whether photoperiodic changes in leukocyte phenotypes and sickness behaviors are dependent on concurrent photoperiodic changes in gonadal function. Male hamsters were gonadectomized or sham-gonadectomized and either exposed to short days (9 h light/day; SD) or kept in their natal long-day (15 h light/day; LD) photoperiod for 10-13 wk. Blood samples were obtained for leukocyte enumeration, and hamsters were challenged with bacterial LPS, which induced behavioral (anorexia, reductions in nest building) and somatic (weight loss) sickness responses. Among gonad-intact hamsters, exposure to SD increased total and CD62L+ lymphocytes and CD3+ T lymphocytes in blood and significantly attenuated LPS-induced sickness responses. Independent of photoperiod, castration alone increased total and CD62L+ lymphocyte and CD3+ T lymphocyte numbers and attenuated somatic and anorexic sickness responses. Among castrated hamsters, SD exposure increased lymphocyte numbers and suppressed sickness behaviors. In castrated hamsters, the magnitude of most immunological effects of SD were diminished relative to those evident in gonad-intact hamsters. The SD phenotype in several measures of immunity can be instated via elimination of gonadal hormones alone; however, photoperiodic effects on immune function persist even in castrated hamsters. Thus, photoperiod affects the immune system and neural-immune interactions underlying sickness behaviors via gonadal hormone-dependent and -independent mechanisms.     

Photoperiodic regulation of circulating leukocytes in juvenile Siberian hamsters: mediation by melatonin and testosterone

The reproductive system of Siberian hamsters (Phodopus sungorus) undergoes rapid phenotypic responses to changes in day length that occur around the time of weaning. The present experiments tested whether the immune system of Siberian hamsters is similarly photoperiodic early in life and whether photoperiodic changes in melatonin or gonadal hormone secretions mediate any such responses to day length. Circulating blood leukocyte concentrations (WBC) were measured in juvenile male Siberian hamsters that were gestated in long-days (LD), transferred to short-days (SD) on the day of birth, and subsequently either remained in SD or were transferred from SD to LD at 18 days of age (day 18). WBC values were comparable between LD and SD hamsters on day 18. Between day 18 and day 32, SD hamsters exhibited a 3-fold increase in WBC, whereas LD hamsters failed to undergo a significant increase in WBC during this interval. WBC of LD hamsters was significantly lower than that of SD hamsters on day 25 and on day 32. In LD housed males, peripheral injections of melatonin delivered so as to extend the nocturnal duration of elevated endogenous melatonin secretion (i.e., provided in late afternoon) on days 18-31 increased WBC as measured on day 32. Peripubertal (day 17) gonadectomy abolished the immunosuppressive effect of LD exposure on WBC, and treatment with silastic implants containing testosterone suppressed WBC independent of photoperiod treatment. These data indicate that juvenile Siberian hamsters are immunologically responsive to photoperiod and that the leukocyte responses to day length are the result of melatonin-mediated effects of photoperiod on testicular hormone secretion.     

Peripubertal immune challenges attenuate reproductive development in male Siberian hamsters (Phodopus sungorus)

Differential allocation of energy to reproduction versus host defense is assumed to drive the seasonal antiphase relation between peak reproductive function and immunocompetence; however, evidence supporting this assumption is only correlational. These experiments tested whether photoperiod affects immune responses to antigens in peripubertal Siberian hamsters, whether such activation of the immune system exacts energetic and reproductive costs, and whether such costs vary seasonally. Male Siberian hamsters were raised from birth in long (LD) or short days (SD), which respectively initiate or inhibit the onset of puberty. To elicit a specific immune response, hamsters were injected with a novel antigen (keyhole limpet hemocyanin [KLH]) as juveniles. Reproductive development was attenuated and body temperature was elevated in LD hamsters relative to saline-injected control animals. In contrast, KLH treatments affected neither thermoregulation nor reproductive development in photoinhibited SD hamsters. In experiment 2, juvenile male hamsters were challenged with bacterial lipopolysaccharide (LPS) in order to elicit an innate immune response. Febrile and anorexic responses to LPS were greater in reproductively stimulated LD hamsters relative to reproductively inhibited SD hamsters. LPS treatments attenuated somatic and testicular development in LD hamsters, but did not significantly affect circulating testosterone concentrations. In contrast, LPS treatments were without effect on somatic and reproductive development in SD hamsters. These experiments indicate that photoperiod affects antigen-specific antibody production, febrile responses to LPS, and sickness behaviors in juvenile Siberian hamsters, and that peripubertal activation of the immune system exacts energetic and metabolic costs that can diminish the magnitude of somatic and reproductive maturation in LD. The data also underscore the importance of seasonally dependent life history factors in assessing physiological tradeoffs.     

Pineal-independent regulation of photo-nonresponsiveness in the Siberian hamster (Phodopus sungorus)

The pineal hormone melatonin influences circadian rhythms and also mediates reproductive responses to photoperiod. The authors tested whether pinealectomy influences circadian oscillators responsible for induction of nonresponsiveness to short day lengths by preventing normal short-day patterns of circadian entrainment. Adult male Siberian hamsters were pinealectomized or sham operated, maintained in either 18 h light per day (18L) or 15L for 10 weeks, and then tested for responsiveness to 10L. Because pinealectomized hamsters do not show gonadal regression in short day lengths, responsiveness was assessed by measuring phase angle of entrainment and the length of the nightly activity period following transfer to 10L. The incidence of nonresponsiveness was significantly higher in 18L hamsters than in 15L hamsters but was unaffected by pineal status. Fully 88% of 18L hamsters failed to entrain to 10L in the normal short-day manner; the duration of nightly activity remained compressed, and the phase angle of entrainment was large and negative relative to lights off. The 15L hamsters entrained normally to 10L. Exposure to constant light after 10L treatment was equally effective in inducing arrhythmicity in pinealectomized and intact hamsters. Changes in the period of morning and evening circadian oscillators subsequent to 18L treatment did not predict circadian responsiveness to short photoperiod. Long-day induction of photo-nonresponsiveness, which prevents winter responses to short day lengths, occurs independently of pineal melatonin feedback on the circadian system.     

Kisspeptin mediates the photoperiodic control of reproduction in hamsters

The KiSS-1 gene encodes kisspeptin, the endogenous ligand of the G-protein-coupled receptor GPR54. Recent data indicate that the KiSS-1/GPR54 system is critical for the regulation of reproduction and is required for puberty onset. In seasonal breeders, reproduction is tightly controlled by photoperiod (i.e., day length). The Syrian hamster is a seasonal model in which reproductive activity is promoted by long summer days (LD) and inhibited by short winter days (SD). Using in situ hybridization and immunohistochemistry, we show that KiSS-1 is expressed in the arcuate nucleus of LD hamsters. Importantly, the KiSS-1 mRNA level was lower in SD animals but not in SD-refractory animals, which spontaneously reactivated their sexual activity after several months in SD. These changes of expression are not secondary to the photoperiodic variations of gonadal steroids. In contrast, melatonin appears to be necessary for these seasonal changes because pineal-gland ablation prevented the SD-induced downregulation of KiSS-1 expression. Remarkably, a chronic administration of kisspeptin-10 restored the testicular activity of SD hamsters despite persisting photoinhibitory conditions. Overall, these findings are consistent with a role of KiSS-1/GPR54 in the seasonal control of reproduction. We propose that photoperiod, via melatonin, modulates KiSS-1 signaling to drive the reproductive axis.     

Excitatory amino acid receptors may mediate the effects of light on the reproductive system of the golden hamster.

Seasonal regulation of reproduction occurs in golden hamsters (Mesocricetus auratus) in response to changes in day length (photoperiod). We report here the results of experiments designed to evaluate whether excitatory amino acid neurotransmission is involved in mediating the effects of light on reproduction. We found that MK-801, a specific antagonist of the N-methyl-D-aspartate subclass of excitatory amino acid receptors, blocked the stimulatory action of short pulses of light on gonadal function in hamsters maintained in short days. Furthermore, MK-801 also inhibited the light-induced decline in pineal melatonin content. These findings suggest that excitatory amino acids may mediate the effects of light on the reproductive system.     

Photoperiodic regulation of the orexigenic effects of ghrelin in Siberian hamsters

Animals living in temperate climates with predictable seasonal changes in food availability may use seasonal information to engage different metabolic strategies. Siberian hamsters decrease costs of thermoregulation during winter by reducing food intake and body mass in response to decreasing or short-day lengths (SD). These experiments examined whether SD reduction in food intake in hamsters is driven, at least in part, by altered behavioral responses to ghrelin, a gut-derived orexigenic peptide which induces food intake via NPY-dependent mechanisms. Relative to hamsters housed in long-day (LD) photoperiods, SD hamsters consumed less food in response to i.p. treatment with ghrelin across a range of doses from 0.03 to 3 mg/kg. To determine whether changes in photoperiod alter behavioral responses to ghrelin-induced activation of NPY neurons, c-Fos and NPY expression were quantified in the arcuate nucleus (ARC) via double-label fluorescent immunocytochemistry following i.p. treatment with 0.3 mg/kg ghrelin or saline. Ghrelin induced c-Fos immunoreactivity (-ir) in a greater proportion of NPY-ir neurons of LD relative to SD hamsters. In addition, following ghrelin treatment, a greater proportion of ARC c-Fos-ir neurons were identifiable as NPY-ir in LD relative to SD hamsters. Changes in day length markedly alter the behavioral response to ghrelin. The data also identify photoperiod-induced changes in the ability of ghrelin to activate ARC NPY neurons as a possible mechanism by which changes in day length alter food intake.     

Hypothalamic Ventricular Ependymal Thyroid Hormone Deiodinases Are an Important Element of Circannual Timing in the Siberian Hamster (Phodopus sungorus)

Exposure to short days (SD) induces profound changes in the physiology and behaviour of Siberian hamsters, including gonadal regression and up to 30% loss in body weight. In a continuous SD environment after approximately 20 weeks, Siberian hamsters spontaneously revert to a long day (LD) phenotype, a phenomenon referred to as the photorefractory response. Previously we have identified a number of genes that are regulated by short photoperiod in the neuropil and ventricular ependymal (VE) cells of the hypothalamus, although their importance and contribution to photoperiod induced physiology is unclear. In this refractory model we hypothesised that the return to LD physiology involves reversal of SD expression levels of key hypothalamic genes to their LD values and thereby implicate genes required for LD physiology. Male Siberian hamsters were kept in either LD or SD for up to 39 weeks during which time SD hamster body weight decreased before increasing, after more than 20 weeks, back to LD values. Brain tissue was collected between 14 and 39 weeks for in situ hybridization to determine hypothalamic gene expression. In VE cells lining the third ventricle, expression of nestin, vimentin, Crbp1 and Gpr50 were down-regulated at 18 weeks in SD photoperiod, but expression was not restored to the LD level in photorefractory hamsters. Dio2, Mct8 and Tsh-r expression were altered by SD photoperiod and were fully restored, or even exceeded values found in LD hamsters in the refractory state. In hypothalamic nuclei, expression of Srif and Mc3r mRNAs was altered at 18 weeks in SD, but were similar to LD expression values in photorefractory hamsters. We conclude that in refractory hamsters not all VE cell functions are required to establish LD physiology. However, thyroid hormone signalling from ependymal cells and reversal of neuronal gene expression appear to be essential for the SD refractory response.     

Perinatal photoperiod organizes adult immune responses in Siberian hamsters (Phodopus sungorus)

Individuals of many nontropical rodent species display reproductive, immunological, and somatic responses to day length. In general, short day (SD) lengths inhibit reproduction and enhance immune function in the laboratory when all other conditions are held constant. Most studies to date have focused on seasonal variation in immune function in adulthood. However, perinatal photoperiods also communicate critical day length information and serve to establish a developmental trajectory appropriate for the time of year. Nontropical rodents born early in the breeding season undergo rapid reproductive development, presumably to promote mating success during their first reproductive season. Rodents born late in the breeding season suspend somatic growth and puberty until the following vernal breeding season. We tested the hypothesis that perinatal day lengths have similar enduring effects on the immune system of rodents. Siberian hamsters (Phodopus sungorus) were maintained prenatally and until weaning (21 days) in either SDs (8 h light:16 h dark) or long days (LD) (16 h light:8 h dark), then they were weaned into either the opposite photoperiod or maintained in their natal photoperiod, forming four groups (LD-LD, LD-SD, SD-LD, and SD-SD). After 8-wk in these conditions, cell-mediated immune activity was compared among groups. SD-SD hamsters of both sexes enhanced immune function relative to all other groups. The reproductive effects of perinatal photoperiod were not evident by the end of the experiment; circulating testosterone and cortisol sampled at the end of the experiment reflected the postweaning, but not the perinatal photoperiod. This experiment demonstrates long-lasting organizational effects of perinatal photoperiod on the rodent immune system and indicates that photoperiod-induced changes in the immune system are dissociable from changes in the reproductive system.     

Photoperiod modulates pubertal shifts in behavioral responsiveness to testosterone.

This study examined the effect of photoperiod on pubertal maturation of steroid-dependent reproductive behaviors in male European ferrets (Mustela putorius furo). In the first experiment, levels of neck gripping, mounting, and pelvic thrusting in gonadally intact prepubertal (PRE) ferrets were compared with those of adults that had undergone puberty either while housed in short days (8 hr light/16 hr darkness per day; SD), or after transfer from SD to long days (18 hr light/6 hr darkness per day; LD) at 12 weeks of age. Both LD and SD adults demonstrated significantly greater amounts of neck gripping and mounting than PRE males. In addition, a significantly greater proportion of adults in both SD and LD displayed at least one incidence of the three behaviors compared to PRE ferrets. There were no statistically significant differences in behavior of the gonadally intact LD and SD adults. In the second experiment, dose-response curves for behavioral responses to subcutaneous injections of 0, 0.5, 1.25, 2.5, 5, and 10 mg/kg testosterone propionate (TP) in oil were generated in castrated PRE, SD, and LD males. The lowest dose of TP elicited significantly greater amounts of all three behaviors in LD adults than in PRE ferrets. In addition, levels of mounting and thrusting elicited by the lowest dose of TP were significantly greater in LD adults than in SD adults. These data indicate that pubertal activation of male sexual behavior in male ferrets is accompanied by a pubertal increase in responsiveness to the behavioral effects of testosterone. Furthermore, the degree of behavioral responsiveness of adult ferrets to testosterone is modulated by environmental photoperiod experienced during reproductive maturation.     

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

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

Photoperiod controls the induction,retention, and retrieval of antigen-specific immunological memory

Changes in day length affect several measures of immunity in seasonally breeding mammals. In Siberian hamsters (Phodopus sungorus), short day lengths suppress specific secondary antibody responses to the keyhole limpet hemocyanin (KLH) antigen and enhance cutaneous delayed-type hypersensitivity (DTH) responses to dinitrofluorobenzene (DNFB). These experiments tested whether day length affects secondary antibody and DTH responses by altering immune function solely during the interval after the initial exposure to each antigen, solely during the interval after the second exposure, or during both stages of the respective immune responses. Adult male Siberian hamsters were exposed to either a long (16 h light/day; LD) or a short (8 h light/day; SD) photoperiod for 7.5 wk before receiving an initial exposure to each antigen (KLH injection, cutaneous DNFB treatment; separate groups of animals for each antigen). A subset of LD hamsters was transferred to the SD photo-period, and a subset of SD hamsters was transferred to the LD photoperiod. Other hamsters remained in LD or SD. Eight weeks later, all hamsters were challenged with a second subcutaneous injection of KLH or a second application of DNFB to the ear, and immune responses were measured. Exposure to SD during the primary antibody response did not affect secondary IgG responses, but SD exposure during the secondary response significantly suppressed IgG production independent of day length during the initial KLH treatment. In contrast, exposure to SD during the DNFB challenge enhanced the ensuing DTH response, but this enhancement depended on the photoperiod prevailing during the initial exposure. Exposure to SD during the sensitization stage did not enhance DTH in hamsters subsequently exposed to LD. The data suggest that short photoperiods have enduring effects on immune responsiveness and on the establishment and retention of immunological memory.     

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.     

Cue hierarchies and testicular development: is food a more potent stimulus than day length in an opportunistic breeder (Taeniopygia g. guttata)?

Opportunistic breeders inhabit areas with unpredictable changes in environmental conditions. In such places favorable breeding conditions can occur during any time of year, and one prediction is that individuals should attend to photoperiod less than to more immediate cues to time reproduction. This study tests whether zebra finches utilize photoperiod independently of other proximate cues, specifically food availability. We transferred semi-domesticated male Lesser Sundas zebra finches (Taeniopygia guttata guttata) from 8 h light, 16 h dark per day (8L:16D) with ad libitum food availability to 20L:4D with ad libitum food (LD ad lib group) or food restriction (LD restricted group). A third group remained on 8L:16D with ad libitum food availability (SD ad lib group). Testis volume in LD ad lib males increased and was larger than other groups within 30 and 60 days of photostimulation. By contrast, LD restricted males and SD ad lib males did not exhibit significant gonadal growth. However, both LD groups increased mass irrespective of food availability. Surprisingly, at the end of the experiment the SD ad lib group sang the most undirected song. Our data demonstrate that long days alone are not sufficient to drive reproductive development in this opportunistically breeding species. Rather, it appears that reproductive development is stimulated by extended feeding times or increased food abundance during long days, and not by changes in day length per se. These data lend support to the proposition that photoperiod acts as a supplementary cue or permissive factor in this system, and thus represents the possibility of a reversal in the hierarchy of cue sensitivity.     

Sustained reproductive responses in Djungarian hamsters (Phodopus sungorus) exposed to a single long day

Male and female Djungarian hamsters maintained from birth in a short photoperiod (8 h light per day; 8L:16D) showed substantial testicular and uterine growth in response to a single long photoperiod or a 15-min light pulse that interrupted the 16-h dark period at 18 days of age. These light regimens resulted in heavier testes and uteri at 30 and 35 days of age when compared with those of control animals. Similar results were obtained in hamsters maintained from birth to Day 18 in a long photoperiod (16L:8D), given a single longer day (20L:4D) or constant light on Day 18 and then transferred to a short photoperiod (8L:16D) on Day 19. At 35 days of age animals that received extended light treatment on Day 18 had significantly more developed reproductive structures than did control hamsters. The marked effects of brief light treatment in producing long-term changes in the reproductive axis provide a convenient mammalian model system in which to study neuroendocrine events that underlie photoperiodism.     

Housing condition alters immunological and reproductive responses to day length in Siberian hamsters (Phodopus sungorus)

During winter, increased thermoregulatory demands coincide with limited food availability necessitating physiological tradeoffs among expensive physiological processes resulting in seasonal breeding among small mammals. In the laboratory, short winter-like day lengths induce regression of the reproductive tract, but also enhance many aspects of immune function. It remains unspecified the extent to which bolstered immune responses in short days represent enhanced immune function per se compared to long days or represents energetic disinhibition mediated by the regression of the reproductive tract. Cohabitation of male Siberian hamsters with intact female conspecifics can block short-day reproductive regression. We sought to determine whether female cohabitation could also block the enhanced immune function associated with short days. Adult male Siberian hamsters were housed in long or short day lengths in one of three housing conditions: (1) single-housed, (2) housed with a same sex littermate, or (3) housed with an ovariectomized female. Delayed-type hypersensitivity (DTH) responses were assessed after 8 weeks of photoperiod treatment. Housing with an ovariectomized female was not sufficient to block short-day reproductive regression, but prevented short-day enhancement of DTH responses. Housing with a male littermate did not alter reproductive or immune responses in either photoperiod. These data suggest that short day enhancement of immune function is independent of photoperiod-mediated changes in the reproductive system.     

Circadian regulation of pineal melatonin and reproduction in the Djungarian hamster

Gonadal state, pineal melatonin rhythms, and locomotor activity rhythms were examined in juvenile male Djungarian hamsters exposed to non-24-hr light cycles ("T-cycles") or to full photoperiods. At the end of 1 month, hamsters exposed to a 1-hr pulse of light every 24.33 hr (T 24.33) exhibited small testes, whereas those receiving the same amount of light every 24.78 hr (T 24.78) displayed stimulated gonads, ten-fold larger in size. Accompanying the nonstimulatory effect of the T 24.33 cycle were nocturnal peaks in both pineal melatonin content and serum melatonin concentration which were longer by approximately 4 hr than those observed on the photostimulatory T 24.78 cycle. Exposure to an intermediate-length T-cycle (T 24.53) resulted in a mixed gonadal response and in pineal and serum melatonin peaks of intermediate duration. Wheel-running activity was entrained to the T-cycles such that light was present only near the beginning of the subjective night, its phase (relative to activity onset) differing only slightly among T-cycle groups. Hence the durational differences observed in the melatonin peaks were apparently not due to the acute suppressive or phase-advancing effects of morning light on melatonin biosynthesis, but were rather the result of differences in the endogenous control of pineal activity by the circadian pacemaker system. While no strong correlation was detected between gonadal state and the phase of locomotor activity onset relative to the light pulse, a significant correlation was observed between gonadal state and the duration of daily locomotor activity (alpha). These data were compared to similar measures obtained from hamsters exposed to long-versus short-day full photoperiods (LD 16:8 vs. LD 10:14). In summary, the results of this study indicate involvement of the circadian pacemaker system of Djungarian hamsters in the control of pineal melatonin synthesis and secretion, and in photoperiodic time measurement. Furthermore, these data strengthen the hypothesis that it is the duration of nocturnal pineal melatonin secretion that is the critical feature of this neuroendocrine gland's photoperiodic signal.     

Lipolytic and antilipolytic responses of the Siberian hamster (Phodopus sungorus sungorus) white adipocytes after weight loss induced by short photoperiod exposure

Short day photoperiod promotes thermogenesis and extensive weight loss in Siberian hamsters (Phodopus sungorus sungorus). To determine whether a change in hormone-sensitive lipolysis occurs after short-photoperiod exposure, some lipolytic responses were measured on white adipocytes isolated from animals exposed in warm conditions to short or Long daylight photoperiod. The body mass of male Siberian hamsters exposed during 11 weeks to short days (SD; light: dark, 6:18 hr) reached only 50% of those kept in long days (LD; 16: 8 hr). In SD-hamsters, adipose depot mass also represented approximately 50% of the LD group. A lower DNA content was observed in intra-abdominal fat pads of SD-hamsters. Lipolytic responses to noradrenaline, adrenaline, isoproterenol and ACTH were unchanged. However, sensitivity to the beta-3 adrenergic agonist, BRL 37344, was moderately increased. The major component of the adrenergic control of lipolysis was mediated by beta-3 adrenoceptors in both LD- and SD-Siberian hamsters. The limited antilipolytic effect of alpha-2 adrenergic agonists, PYY or insulin was rather surprising in Siberian hamsters since these inhibitory systems are efficient in hibernants and other photoperiod-sensitive rodents. Our results show that, after short photoperiod exposure, white adipose tissue mass and DNA content are reduced, especially in the epididymal fat pad, with only minor changes in the adipocyte sensitivity to lipolytic hormones.     

Changes in the testicular binding of luteinizing hormone and plasma testosterone concentrations in the Djungarian hamster subjected to different photoperiods and temperatures and effects of long-term testosterone treatment on the binding

The effects of artificial photoperiod, temperature, and long-term testosterone treatment on testicular luteinizing hormone (LH) binding were studied in adult male Djungarian hamsters. In hamsters transferred to long-day (LD; 16 hr light, 8 hr dark) photoperiod 8 weeks after adaptation in short-day (SD; 8 hr light, 16 hr dark) photoperiod of 25 degrees C, testicular growth was associated with an increase in the total LH binding per two testes and a decrease in LH binding per unit testicular weight. Plasma testosterone levels reached a peak 47 days after transfer to LD and tended to decrease thereafter, while the testes continued growing. In contrast, when hamsters reared under LD conditions at 25 degrees C for 12 weeks were transferred to SD, testicular regression was associated with a decrease in plasma testosterone and the total LH binding per two testes and an increase in LH binding per unit testicular weight. A significant decrease in LH binding per unit weight compared to SD controls was observed in those hamsters exposed to SD with continuous testosterone treatment. The testosterone treatment tended to induce decrease in the total LH binding. Scatchard plot analyses of the binding suggested that changes in LH binding were due to changes in the number of binding sites. When sexually mature male hamsters were subjected for 8 weeks to two different ambient temperatures (7 degrees C and 25 degrees C) and photoperiods (LD and SD), the difference between the two temperature groups was statistically not significant regarding the weights of testes, epididymides, and prostates; plasma testosterone levels; and LH binding in either LD or SD group. These results suggest that photoperiod is a more important environmental factor than temperature for the regulation of testicular activity and LH receptors and that testosterone reduces the number of LH receptors per unit testicular weight in adult male Djungarian hamsters.     

Photorefractoriness and energy availability interact to permit facultative timing of spring breeding

In seasonally breeding mammals, vernal reproductive development is not directly triggered by increases in day length, rather, an endogenous program of photorefractoriness to short winter days initiates spontaneous development in advance of spring. The transition to the reproductive phenotype is energetically demanding. How food availability in late winter and early spring impacts the onset and expression of photorefractoriness is not known. In this study, male Siberian hamsters were born into a simulated natural photoperiod, and at the winter solstice, they were subjected to a restricted feeding protocol in which a daily food ration was provided in an amount equal to ad libitum (AL) intake during the weeks preceding the solstice. Over the next several months, AL-fed control hamsters exhibited spontaneous recrudescence or spontaneous development. In contrast, vernal reproductive development was abolished in most food-rationed hamsters. In food-rationed hamsters that did exhibit recrudescence, conspicuous delays in the onset of gonadal development and decreases in the magnitude of growth were evident. In all hamsters, the termination of food rationing triggered rapid gonadal development. The data indicate that late winter/early spring increases in environmental food availability are required for the normal manifestation of photorefractoriness-induced reproductive development and suggest that a function of photorefractoriness may be merely to disinhibit the reproductive axis from photoperiodic suppression. Vernal gonadal development or recrudescence appears to be strongly affected by proximate energy availability.