Photoperiod Alters Duration and Intensity of Non–Rapid Eye Movement Sleep Following Immune Challenge in Siberian Hamsters (Phodopus sungorus)

Sleep is regulated by circadian and homeostatic processes, but can be altered by infectious disease. During infection or exposure to inflammatory stimuli, such as bacterial lipopolysaccharide (LPS), the duration and intensity of non–rapid eye movement sleep (NREMS), as measured by electoencephalogram (EEG) delta waves (.5–4?Hz), increase. These sleep alterations are hypothesized to conserve or redirect energy for immune system activation. Many vertebrates exhibit seasonal changes in immune function and sleep-wake cycle, and photoperiod (day length) serves as a reliable environmental cue. For example, winter is energetically demanding for most animals, and Siberian hamsters (Phodopus sungorus) adapted to short winter day lengths display reduced fever after LPS administration to presumably conserve energy. We hypothesized that short days increase the duration and intensity of NREMS after LPS challenge to create additional energy savings, despite evidence to the contrary that high fever is associated with increased NREMS. Male hamsters were housed under long (16?h light (L):8?h dark (D)) or short (8L:16D) day lengths, and chronically implanted with transmitters that recorded EEG and electromyogram (EMG) biopotentials simultaneously or core body temperature. After >10 wks in photoperiod conditions, hamsters received an i.p. injection of LPS or saline (control), and vigilance states (duration and distribution of NREMS, rapid eye movement sleep (REMS), and wakefulness) and EEG delta power spectra (NREMS intensity) were assessed. As expected, LPS treatment increased the duration and intensity of NREMS compared to controls. Hamsters adapted to short photoperiods exhibited cumulatively larger increases in NREMS duration and EEG delta wave amplitude 0–8?h after LPS injection compared to long-day LPS-treated hamsters despite short-day attenuation of fever. These results suggest a seasonal decoupling of LPS-induced fever with sleep to promote energy conservation during predictable energy shortages. Ultimately, the combination of increased sleep and reduced fever could represent a suite of physiological adaptations that increase the probability of surviving winter. (Author correspondence: Noah.Ashley@osumc.edu)     

Time-of-day modulation of homeostatic and allostatic sleep responses to chronic sleep restriction in rats

To study sleep responses to chronic sleep restriction (CSR) and time-of-day influences on these responses, we developed a rat model of CSR that takes into account the polyphasic sleep patterns in rats. Adult male rats underwent cycles of 3 h of sleep deprivation (SD) and 1 h of sleep opportunity (SO) continuously for 4 days, beginning at the onset of the 12-h light phase ("3/1" protocol). Electroencephalogram (EEG) and electromyogram (EMG) recordings were made before, during, and after CSR. During CSR, total sleep time was reduced by ～60% from baseline levels. Both rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) during SO periods increased initially relative to baseline and remained elevated for the rest of the CSR period. In contrast, NREMS EEG delta power (a measure of sleep intensity) increased initially, but then declined gradually, in parallel with increases in high-frequency power in the NREMS EEG. The amplitude of daily rhythms in NREMS and REMS amounts was maintained during SO periods, whereas that of NREMS delta power was reduced. Compensatory responses during the 2-day post-CSR recovery period were either modest or negative and gated by time of day. NREMS, REMS, and EEG delta power lost during CSR were not recovered by the end of the second recovery day. Thus the "3/1" CSR protocol triggered both homeostatic responses (increased sleep amounts and intensity during SOs) and allostatic responses (gradual decline in sleep intensity during SOs and muted or negative post-CSR sleep recovery), and both responses were modulated by time of day.     

Short day lengths attenuate the symptoms of infection in Siberian hamsters

Symptoms of infection, such as fever, anorexia and lethargy, are ubiquitous among vertebrates. Rather than nonspecific manifestations of illness, these responses are organized, adaptive strategies that are often critical to host survival. During times of energetic shortage such as winter, however, it may be detrimental for individuals to prolong energetically demanding symptoms such as fever. Individuals may adjust their immune responses prior to winter by using day length to anticipate energetically-demanding conditions. If the expression of sickness behaviours is constrained by energy availability, then cytokine production, fever, and anorexia should be attenuated in infected Siberian hamsters housed under simulated winter photoperiods. We housed hamsters in either long (14 L : 10 D) or short (10 L : 14 D) day lengths and assessed cytokines, anorexia and fever following injections of lipopolysaccharide (LPS). Short days attenuated the response to lipopolysaccharide, by decreasing the production of interleukin (IL)-6 and IL-1beta, and diminishing the duration of fever and anorexia. Short-day exposure in hamsters also decreased the ingestion of dietary iron, a nutrient vital to bacterial replication. Taken together, short day lengths attenuated the symptoms of infection, presumably to optimize energy expenditure and survival outcome.     

Alterations in EEG activity and sleep after influenza viral infection in GHRH receptor-deficient mice.

Viral infections induce excess non-rapid eye movement sleep (NREMS) in mice. Growth hormone-releasing hormone receptor (GHRH receptor) was previously identified as a candidate gene responsible for NREMS responses to influenza challenge in mice. The dwarf lit/lit mouse with a nonfunctional GHRH receptor was used to assess the role of the GHRH receptor in viral-induced NREMS. After influenza A virus infection the duration and intensity [electroencephalogram (EEG) delta power] of NREMS increased in heterozygous mice with the normal phenotype, whereas NREMS and EEG delta power decreased in homozygous lit/lit mice. Lit/lit mice developed a pathological state with EEG slow waves and enhanced muscle tone. Other influenza-induced responses (decreases in rapid eye movement sleep, changes in the EEG high-frequency bands during the various stages of vigilance, hypothermia, and decreased motor activity) did not differ between the heterozygous and lit/lit mice. GH replacement failed to normalize the NREMS responses in the lit/lit mice after influenza inoculation. Decreases in NREMS paralleled hypothermia in the lit/lit mice. Lung virus levels were similar in the two mouse strains. Lit/lit mice had a higher death rate after influenza challenge than the heterozygotes. In conclusion, GHRH signaling is involved in the NREMS response to influenza infection.     

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.     

State-dependent effects of light-dark cycle on somatosensory and visual cortex EEG in rats

Somatosensory (SSctx) and visual cortex (Vctx) EEG were evaluated in rats under a 12:12-h light-dark (LD) cycle and under constant light (LL) or constant dark (DD) in each sleep or wake state. Under LD conditions during light period, relative Vctx EEG slow-wave activity (SWA) was higher than that of the SSctx, whereas during dark period, relative Vctx EEG SWA was lower than in the SSctx. These effects were state specific, occurring only during non-rapid eye movement sleep (NREMS). Under LL conditions, the duration of REMS and NREMS during the period that would have been dark if the LD cycle had continued (subjective dark period) was greater than under LD conditions. DD conditions had little effect on the duration of NREMS and REMS. SSctx and Vctx EEG SWA were suppressed by LL during the subjective dark period; however, the degree of Vctx SWA suppression was smaller than that of the SSctx. DD conditions during the subjective light period enhanced SSctx SWA, whereas Vctx SWA was suppressed. Under LL conditions during the subjective dark period, Vctx EEG power was higher than that of the SSctx across a broad frequency range during NREMS, REMS, and wakefulness. During DD, SSctx EEG power during NREMS was higher than that of the Vctx in the delta wave band, whereas SSctx power during REMS and wakefulness was higher than that of the Vctx in frequencies higher than 8 Hz. We concluded that the SSctx and Vctx EEGs are differentially affected by light during subsequent sleep. Results provide support for the notion that regional sleep intensity is dependent on prior regional afferent input.     

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

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

Photoperiodic influences on ultradian rhythms of male siberian hamsters

Seasonal changes in mammalian physiology and behavior are proximately controlled by the annual variation in day length. Long summer and short winter day lengths markedly alter the amplitude of endogenous circadian rhythms and may affect ultradian oscillations, but the threshold photoperiods for inducing these changes are not known. We assessed the effects of short and intermediate day lengths and changes in reproductive physiology on circadian and ultradian rhythms of locomotor activity in Siberian hamsters. Males were maintained in a long photoperiod from birth (15 h light/day; 15 L) and transferred in adulthood to 1 of 7 experimental photoperiods ranging from 14 L to 9 L. Decreases in circadian rhythm (CR) robustness, mesor and amplitude were evident in photoperiods ≤14 L, as were delays in the timing of CR acrophase and expansion of nocturnal activity duration. Nocturnal ultradian rhythms (URs) were comparably prevalent in all day lengths, but 15 L markedly inhibited the expression of light-phase URs. The period (τ'), amplitude and complexity of URs increased in day lengths ≤13 L. Among hamsters that failed to undergo gonadal regression in short day lengths (nonresponders), τ' of the dark-phase UR was longer than in photoresponsive hamsters; in 13 L the incidence and amplitude of light-phase URs were greater in hamsters that did not undergo testicular regression. Day lengths as long as 14 L were sufficient to trigger changes in the waveform of CRs without affecting UR waveform. The transition from a long- to a short-day ultradian phenotype occurred for most UR components at day lengths of 12 L-13 L, thereby establishing different thresholds for CR and UR responses to day length. At the UR-threshold photoperiod of 13 L, differences in gonadal status were largely without effect on most UR parameters.     

Growth hormone-releasing hormone: cerebral cortical sleep-related EEG actions and expression

Growth hormone-releasing hormone (GHRH), its receptor (GHRHR), and other members of the somatotropic axis are involved in non-rapid eye movement sleep (NREMS) regulation. Previously, studies established the involvement of hypothalamic GHRHergic mechanisms in NREMS regulation, but cerebral cortical GHRH mechanisms in sleep regulation remained uninvestigated. Here, we show that unilateral application of low doses of GHRH to the surface of the rat somatosensory cortex ipsilaterally decreased EEG delta wave power, while higher doses enhanced delta power. These actions of GHRH on EEG delta wave power occurred during NREMS but not during rapid eye movement sleep. Further, the cortical forms of GHRH and GHRHR were identical to those found in the hypothalamus and pituitary, respectively. Cortical GHRHR mRNA and protein levels did not vary across the day-night cycle, whereas cortical GHRH mRNA increased with sleep deprivation. These results suggest that cortical GHRH and GHRHR have a role in the regulation of localized EEG delta power that is state dependent, as well as in their more classic hypothalamic role in NREMS regulation.     

MicroRNA 132 alters sleep and varies with time in brain

MicroRNA (miRNA) levels in brain are altered by sleep deprivation; however, the direct effects of any miRNA on sleep have not heretofore been described. We report herein that intracerebroventricular application of a miRNA-132 mimetic (preMIR-132) decreased duration of non-rapid-eye-movement sleep (NREMS) while simultaneously increasing duration of rapid eye movement sleep (REMS) during the light phase. Further, preMIR-132 decreased electroencephalographic (EEG) slow-wave activity (SWA) during NREMS, an index of sleep intensity. In separate experiments unilateral supracortical application of preMIR-132 ipsilaterally decreased EEG SWA during NREMS but did not alter global sleep duration. In addition, after ventricular or supracortical injections of preMIR-132, the mimetic-induced effects were state specific, occurring only during NREMS. After local supracortical injections of the mimetic, cortical miRNA-132 levels were higher at the time sleep-related EEG effects were manifest. We also report that spontaneous cortical levels of miRNA-132 were lower at the end of the sleep-dominant light period compared with at the end of the dark period in rats. Results suggest that miRNAs play a regulatory role in sleep and provide a new tool for investigating sleep regulation.     

Leptin, but not immune function, is linked to reproductive responsiveness to photoperiod

Energetic demands are high while energy availability is minimum during winter. To cope with this energetic bottleneck, animals exhibit numerous energy-conserving adaptations during winter, including changes in immune and reproductive functions. A majority of individual rodents within a population inhibits reproductive function (responders) as winter approaches. A substantial proportion of small rodents within a species, however, fails to inhibit reproduction (nonresponders) during winter in the field or in the laboratory when maintained in winter-simulated day lengths. In contrast, immune function is bolstered by short day lengths in some species. The specific mechanisms that link reproductive and immune functions remain unspecified. Leptin is a hormone produced by adipose tissue, and several studies suggest that leptin modulates reproductive and immune functions. The present study sought to determine if photoperiodic alterations in reproductive function and leptin concentrations are linked to photoperiod-modulated changes in immune function. Siberian hamsters (Phodopus sungorus) were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 9 wk. After 9 wk, blood samples were collected during the middle of the light and dark phase to assess leptin concentrations. One week later, animals were injected with keyhole limpet hemocyanin to evaluate humoral immunity. Body mass, body fat content, and serum leptin concentrations were correlated with reproductive responsiveness to photoperiod; short-day animals with regressed gonads exhibited a reduction in these measures, whereas short-day nonresponders resembled long-day animals. In contrast, immune function was influenced by photoperiod but not reproductive status. Taken together, these data suggest that humoral immune function in Siberian hamsters is independent of photoperiod-mediated changes in leptin concentrations.     

Development of the nocturnal sleep electroencephalogram in human infants

The development of nocturnal sleep and the sleep electroencephalogram (EEG) was investigated in a longitudinal study during infancy. All-night polysomnographic recordings were obtained at home at 2 wk and at 2, 4, 6, and 9 mo after birth (analysis of 7 infants). Total sleep time and the percentage of quiet sleep or non-rapid eye movement sleep (QS/NREMS) increased with age, whereas the percentage of active sleep or rapid eye movement sleep (AS/REMS) decreased. Spectral power of the sleep EEG was higher in QS/NREMS than in AS/REMS over a large part of the 0.75- to 25-Hz frequency range. In both QS/NREMS and AS/REMS, EEG power increased with age in the frequency range <10 Hz and >17 Hz. The largest rise occurred between 2 and 6 mo. A salient feature of the QS/NREMS spectrum was the emergence of a peak in the sigma band (12-14 Hz) at 2 mo that corresponded to the appearance of sleep spindles. Between 2 and 9 mo, low-frequency delta activity (0.75-1.75 Hz) showed an alternating pattern with a high level occurring in every other QS/NREMS episode. At 6 mo, sigma activity showed a similar pattern. In contrast, theta activity (6.5-9 Hz) exhibited a monotonic decline over consecutive QS/NREMS episodes, a trend that at 9 mo could be closely approximated by an exponential function. The results suggest that 1) EEG markers of sleep homeostasis appear in the first postnatal months, and 2) sleep homeostasis goes through a period of maturation. Theta activity and not delta activity seems to reflect the dissipation of sleep propensity during infancy.     

Shortening of the photoperiod affects sleep distribution, EEG and cortical temperature in the Djungarian hamster

To asses the influence of photoperiod on sleep regulation EEG, EMG, and cortical temperature were continuously recorded for two baseline days and after 4 h sleep deprivation in Djungarian hamsters (Phodopus sungorus) adapted to a short photoperiod (light dark 816). Comparison to previous data collected in a long photoperiod (lightdark 168) showed several major effects of photoperiod: 1. A prominent change in the 24-h distribution, duration and number of vigilance state episodes, whereas the total amount of sleep and waking was unchanged; 2. Cortical temperature was 0.7°C lower in the short photoperiod; 3. There was a significant negative correlation between cortical temperature and the frequency of REM sleep episodes; and 4. Absolute EEG power density showed a marked reduction in the short photoperiod. After sleep deprivation EEG slow-wave activity (mean power density 0.75–4.0 Hz) in NREM sleep showed a remarkably similar increase in both photoperiods demonstrating the robustness of the homeostatic regulation of sleep. Cortical temperature remained above baseline values after sleep deprivation in the short photoperiod whereas a negative rebound was present in the long photoperiod. Our results support the hypothesis that cortical temperature has a strong influence on REM sleep propensity and indicate the possibility of an optimum cortical temperature for recovery sleep after sleep deprivation. The lower EEG power density in the short photoperiod may contribute to energy conservation.Abbreviations LP long photoperiod - NREM non-rapid-eye-movement - REM rapid-eye-movement - SCN suprachiasmatic nucleus - SD sleep deprivation - SP short photoperiod - SWA slow-wave activity - T _CRT cortical temperature     

The thalamic intergeniculate leaflet mediates locomotor activity-induced reversal of phenotype in photoperiod nonresponsive Siberian hamsters

The role of the intergeniculate leaflet of the thalamus (IGL) in photoperiod responsiveness was examined in a laboratory-selected line of photoperiod nonresponsive (NR) Siberian hamsters. NR hamsters fail to exhibit typical winter-type responses (i.e., gonadal regression and development of winter-type pelage) when exposed to short day lengths (e.g., 10 h of light/day). Earlier studies revealed that NR hamsters will exhibit winter-type responses when exposed to short photoperiod if they are given free access to a running wheel. The present study tested the hypothesis that this locomotor activity-induced reversal of phenotype is dependent on the IGL. Male NR hamsters underwent destruction of the IGL prior to being housed in short day lengths in cages equipped with running wheels. Activity rhythms were monitored for 8 weeks, after which time pelage response and paired testes weights were obtained. In contrast to sham-operated NR animals given access to running wheels, IGL-ablated animals showed no increase in the duration of nocturnal running wheel activity and became active later in the night than sham-lesioned animals. Lesioned animals also failed to exhibit the typical short photoperiod-induced gonadal regression and pelage molt. The results implicate the IGL in the mechanism by which running wheel activity can influence photoperiodic responses.     

A plastic interval timer synchronizes pubertal development of summer- and fall-born hamsters

Summer and fall decreases in day length induce reproductive regression in adult hamsters and delay reproductive maturation of their young. The following year pubertal development is triggered by an interval timer (IT) that renders animals refractory to inhibitory short day lengths after approximately 25 wk. Timing of gonadal and somatic development was examined among offspring born to Siberian hamsters in early-August vs. late-September day lengths. Pubertal maturation was delayed in both groups until late winter. Gonadal growth occurred at significantly later ages among August- vs. September-born males as did late-winter spurts in ponderal growth of both sexes. Timing of reproductive and somatic development depended on postnatal rather than prenatal photoperiod exposure and was unrelated to the circadian entrainment status of dams. When developmental patterns were assessed in relation to time of year, group differences were largely eliminated. Because the IT triggers these developmental events, its duration must be plastic. This plasticity facilitates a relative synchronization or entrainment of developmental milestones in hamsters born into different late-summer/early-fall photoperiods.     

Low ambient temperature accelerates short-day responses in Siberian hamsters by altering responsiveness to melatonin

Exposure to low ambient temperatures (Ta) accelerates appearance of the winter phenotype in Siberian hamsters transferred from long to short day lengths. Because melatonin transduces the effects of day length on the neuroendocrine axis, the authors assessed whether low Ta promotes the transition to winterlike traits by accelerating the onset of increased nocturnal melatonin secretion or by enhancing responsiveness to melatonin in short day lengths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h light/day) photoperiods and held at 5 degrees C or 22 degrees C. Locomotor activity was recorded continuously, and body mass, testis size, and pelage color were determined biweekly for 8 weeks. The duration of nocturnal locomotion (alpha), a reliable indicator of the duration of nocturnal melatonin secretion, lengthened significantly earlier in hamsters exposed to a Ta of 5 degrees C than 22 degrees C. Cold exposure increased the proportion of hamsters that were photoresponsive: gonadal regression in short days increased from 44% at 22 degrees C to 81% at 5 degrees C (p < 0.05); low Ta did not, however, accelerate testicular regression in animals that were photoresponsive. Nonphotoresponsive animals at 5 degrees C temporarily had longer alphas during the first 4 weeks in short days and significant decreases in body mass and testicular size that were reversed during the ensuing weeks when alpha decreased. In a 2nd experiment, pinealectomized male hamsters infused for 10 h/day with melatonin for 2 weeks had significantly lower body and testes masses when maintained at 5 degrees C but not 22 degrees C. Low-ambient temperature appears to accelerate the appearance of the winter phenotype primarily by increasing target tissue responsiveness to melatonin and to a lesser extent by augmenting the rate at which the duration of nocturnal melatonin secretion increases in short day lengths.     

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.     

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.     

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.     

Short day lengths affect perinatal development of the male reproductive system in the Siberian hamster, Phodopus sungorus.

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, seasonal breeding can be controlled by photoperiod, which affects the duration of nightly melatonin secretion. Winterlike, short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo pubertal gonadal development later than animals born into long days. However, to date there have been no reports of gestational photoperiod affecting fetal development of reproductive systems. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), compose a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8 h light, 16 h dark; 8L:16D) versus long-day (16L:8D) conditions. On the day of birth, and at postnatal (PN) days 2 and 18, the BC/LA muscles of hamsters gestated and raised in the short photoperiod were significantly reduced relative to those of their long-day counterparts. Testes weights were not significantly different between groups until day 18. Thus, photoperiod exposure during gestation and after birth affects perinatal development of the SNB system in this species, and these effects can be seen as early as the day of birth. Because photoperiod did not significantly affect testes weights until PN18, these results suggest that either perinatal photoperiod affects fetal androgen production without affecting testes weight or it influences BC/LA development independently from androgen.