Short-day Hamsters Research Articles

Short Days and Exogenous Melatonin Increase Aggression of Male Syrian Hamsters ( Mesocricetus auratus)

Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate increased aggression in short, “winter-like” compared with long “summer-like” day lengths. The precise neuroendocrine mechanisms mediating changes in aggression, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggression in male Syrian hamsters ( Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous melatonin injections (15 μg/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short-day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident-intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters underwent gonadal regression and displayed increased aggression compared with long-day animals. In Experiment 2, melatonin treatment also increased aggression compared with control hamsters without affecting circulating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggression in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression provide an important, ecologically relevant model with which to study the neuroendocrine mechanisms underlying aggression in rodents.

Sympathoadrenal system differentially affects photoperiodic changes in humoral immunity of Siberian hamsters (Phodopus sungorus).

Siberian hamsters (Phodopus sungorus) rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behaviour. Among these seasonal changes is reduced immune function in short 'winter-like' days, compared to long 'summer-like' days. Previous evidence suggests that immune function is regulated, in part, by the sympathoadrenal system. The precise role of the sympathoadrenal system in regulating photoperiodic changes in immune function, however, remains unspecified. The goal of the present study was to examine the differential contributions of direct sympathetic innervation of immune target tissue, as well as adrenal medullary catecholamines, to photoperiodic changes in immune function in male Siberian hamsters. In Experiment 1, hamsters underwent either bilateral surgical removal of the adrenal medulla (ADMEDx), or sham surgeries, and were maintained in long (LD 16 : 8) or short days (LD 8 : 16). In Experiment 2, hamsters received either surgical denervation of the spleen, or sham surgeries, and were then housed in long or short days. Serum anti-KLH IgG concentrations and splenic norepinephrine (NE) content were determined in both experiments. Short-day hamsters had reduced humoral immunity compared to long-day hamsters. ADMEDx reduced immune function, but only in long-day hamsters. In contrast, splenic denervation reduced humoral immunity, but only in short-day hamsters. Splenic NE content was increased in short days and by ADMEDx. NE content was markedly reduced in denervated hamsters compared to sham-operated hamsters. Collectively, these results suggest that the sympathoadrenal system is associated with photoperiodic changes in immune function.

Role of photoperiod and the pineal gland in T cell-dependent humoral immune reactivity in the Siberian hamster.

The present study tested the hypothesis that antibody production in response to xenoantigen is modulated by daylength and dependent upon the pineal gland. Alter injection of sheep erythrocytes (SRBC), serum immunoglobulin (Ig) concentrations were 5-fold lower in hamsters in short versus long days. Pinealectomy (Pinx) abolished the nocturnal melatonin rhythm, blocked short-day-mediated testis regression, and eliminated the short-day reduction in Ig production after SRBC treatment. Antibody titers in response to SRBC were equivalently augmented in short-day Pinx and long-day sham hamsters. The results indicate that photoperiodic effects on T cell-dependent humoral immunity are dependent upon the pineal gland. These findings raise the possibility that day length-associated changes in some immune system functions are mediated by the pineal melatonin rhythm.

Influence of photoperiod on immune cell functions in the male Siberian hamster.

The present study tested the hypothesis that immune cell function is influenced by ambient photoperiod. The male Siberian hamster served as the experimental model because day length regulates a variety of seasonal adaptations in physiology. Adult hamsters were in long days (16 h of light daily), which sustains gonadal function, or transferred to short days (8 h) for >4 wk to induce testes regression. Blood was drawn from the ocular sinus or splenocytes obtained to assess basal indexes of immune cell function. In hamsters in short days, natural killer cell cytolytic capacity, as well as spontaneous blastogenesis in both whole blood and isolated lymphocytes, were enhanced compared with that in hamsters in long days. By contrast, phagocytosis and oxidative burst activity by both granulocytes and monocytes were suppressed in hamsters by exposure to short days versus long days. Selective changes in immune cell function coincided with short-day-induced gonadal atrophy. These findings raise the hypothesis that photoperiod regulation of physiological adaptations, including distinct immune cell functions, may help individuals anticipate seasonal challenges posed by opportunistic diseases or climate to facilitate survival.

Seasonal Neuroendocrine Rhythms in the Male Siberian Hamster Persist After Monosodium Glutamate‐Induced Lesions of the Arcuate Nucleus in the Neonatal Period

The aim of these experiments was to examine the role of the arcuate nucleus in the control of seasonal cycles of body weight, feed intake, moulting and reproduction in the Siberian hamster. The arcuate nucleus has previously been implicated as a central site where systemic feedback signals (e.g. leptin) might act to regulate feed intake and body weight, so it was predicted that hamsters with lesions of this structure would be unable to display the inhibitory effects of short days on these parameters. In the first series of studies, lesions that destroyed approximately 80% of the cells in the arcuate nucleus were produced by treating hamsters neonatally with monosodium glutamate (MSG; 4 mg/g body weight sc), and vehicle- and MSG-treated males were raised from birth in long days (LD) or short days (SD). In hamsters raised in LD, the initial gain in body weight and testicular growth were significantly reduced by MSG treatment, however, growth rate and testis weight were still significantly greater than in vehicle- or MSG-treated hamsters raised in SD. In the second study, hamsters treated neonatally with vehicle or MSG were raised in LD for 8 weeks and, subsequently, approximately half in each group were transferred to SD for 18 weeks. As expected, vehicle-treated hamsters showed a characteristic decline in body weight when exposed to SD, while those remaining in LD continued to increase body weight. Feed intake decreased in parallel with the decline in body weight in SD, a complete moult to the white winter pelage occurred by 16 weeks in SD, and testicular regression occurred. Responses to SD also occurred in the MSG-treated hamsters: body weight decreased in SD but increased in their lesioned litter mates remaining in LD, and feed intake paralleled body weight changes in these groups. The moult to winter pelage was significantly retarded in MSG-treated hamsters transferred to SD. The testes were completely regressed in sham- and MSG-treated hamsters exposed to SD, whereas testes weights in MSG-treated hamsters maintained in LD were intermediate between those in vehicle-treated hamsters in SD and LD. Thus, despite initial effects on growth, the MSG-treated hamsters bearing substantial lesions of the arcuate nucleus were able to show appropriate responses to photoperiod, although not always of the same magnitude as the unlesioned controls. We conclude that feedback mechanisms operating via the arcuate nucleus are not the major regulators of seasonal cycles of body weight, feed intake, pelage and reproduction.

Short day lengths delay development of the SNB neuromuscular system in the Siberian hamster, Phodopus sungorus

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, the seasonal breeding can be controlled by photoperiod, which affects the durations of nightly melatonin secretions. Winterlike short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo gonadal development much later than animals born into long days. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), comprise 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 (8:16 h light/dark cycle) versus long-day (16:8 h light/dark cycle) conditions. At 40–47 days of age, development of three components of the SNB neuromuscular system were all significantly delayed in hamsters raised in the short photoperiod: BC/LA muscle weight, the size of SNB motoneuronal somata, and the area of the neuromuscular junctions at the BC/LA muscles of short-day hamsters were each significantly reduced relative to those of long-day counterparts. Thus, development of the SNB reproductive system is delayed under short day lengths in this species. © 1998 John Wiley & Sons, Inc. J Neurobiol 35: 355–360, 1998

Short day lengths delay development of the SNB neuromuscular system in the Siberian hamster,Phodopus sungorus

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, the seasonal breeding can be controlled by photoperiod, which affects the durations of nightly melatonin secretions. Winterlike short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo gonadal development much later than animals born into long days. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), comprise a sexually dimorphic, androgensensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8:16 h light/dark cycle) versus long-day (16:8 h light/dark cycle) conditions. At 40-47 days of age, development of three components of the SNB neuromuscular system were all significantly delayed in hamsters raised in the short photoperiod: BC/LA muscle weight, the size of SNB motoneuronal somata, and the area of the neuromuscular junctions at the BC/LA muscles of short-day hamsters were each significantly reduced relative to those of longday counterparts. Thus, development of the SNB reproductive system is delayed under short day lengths in this species.

Melatonin rhythm onset in the adult siberian hamster: influence of photoperiod but not 60-Hz magnetic field exposure on melatonin content in the pineal gland and in circulation.

To determine the relationship between pineal melatonin production and its appearance in circulation, the rising phase of the pineal and serum melatonin rhythm was studied in the adult Siberian hamster. Melatonin concentrations increased in the pineal gland and in serum at 1.50 and 1.75 h, respectively, relative to lights off in long days (16 h of light/day) and at 2.00 and 2.75 h, respectively, in short days (10 h of light/day). Thus, a photoperiod-dependent melatonin rise in circulation lagged production by the pineal gland by 0.50 h--a delay of 0.75 h in short-day hamsters versus 0.25 h in long-day hamsters. Following initiation of this rise, concentrations that were typical of the nighttime peak were achieved within 2 h of melatonin rhythm onset, regardless of photoperiod. To determine whether clock control of the rising phase of the melatonin rhythm, in the absence of photoperiod cues, may be disrupted by perturbations in the ambient magnetic field, hamsters in constant darkness were acutely exposed to a 1-Gauss, 60-Hz magnetic field for 15 min or were daily exposed to this treatment for 14 or 21 days. Neither the melatonin rise in pineal content or circulation during subjective night was affected by acute or chronic magnetic field exposures; testes regression similarly occurred in sham and daily magnetic field-exposed hamsters in constant darkness. These findings indicate that magnetic field exposures are unlikely to serve as a zeitgeber for the circadian mechanism that controls onset of the melatonin rhythm; rather, photoperiod is a predominant cue that may differentially regulate the rising phase of melatonin production in the pineal gland and concentration in circulation.

Gonadotropin-releasing hormone neural projections to the systemic vasculature during sexual maturation and delayed puberty in the male Djungarian hamster.

To study GnRH neuron projections to the systemic circulation during sexual maturation and in photoperiod-induced delayed puberty, we injected fluorogold (i.p.) into long- or short-day-reared male Djungarian hamsters. Five days after fluorogold treatment, at 15, 25, or 40 days of age, most GnRH neurons were found to access the systemic vasculature from terminal fields in the median eminence and organum vasculosum of the lamina terminalis. More than 80% of GnRH cell bodies in the medial preoptic area and diagonal band of Broca contained fluorogold, the locus for the majority of the GnRH neuron system in this species. GnRH cell numbers (approximately 180) and percentages with fluorogold were the same in 15- and 25-day-old males in long days, respectively pre-and peripubertal, and were not different from that in short-day hamsters with delayed puberty. Neither the number of fluorogold-labeled GnRH somata nor intensity of the fluorogold per cell was altered in hamsters during the critical period for initiation of reproductive maturation. Only later, at age 40 days, was the percentage of GnRH somata with fluorogold increased in long-day hamsters or reduced in 40-day-old males in short days relative to that in males at 15 days of age (same day length). The findings indicate that GnRH neuron projections to regions outside the blood-brain barrier are established before puberty and are independent of photoperiod control of reproductive development.

Photoperiod Control of the Melatonin Rhythm and Reproductive Maturation in the Juvenile Djungarian Hamster: 60-Hz Magnetic Field Exposure Effects1

Photoperiodic regulation of the melatonin rhythm was studied in juvenile Djungarian hamsters to test the hypothesis that magnetic field (MF) exposures disrupt sexual maturation. Juveniles in long or short days postweaning were exposed each day to a 1-gauss 60-Hz MF (15 min, 2 h before lights-off). At age 25 days, nighttime duration of the melatonin rise in the pineal gland and circulation of sham-treated controls (adjacent coil system but without current) was < 8 h in long-day juveniles and nearly 13 h in short-day hamsters. This is the first study in juveniles to demonstrate that the melatonin rhythm duration is regulated by photoperiod and that reproductive development was not disrupted by daily or acute MF exposures; puberty was initiated in long days but arrested by short days, irrespective of MF treatment. In a replicate study, MF exposures had no effect on the duration of increased melatonin. In the initial and the replicate study, amplitude differences in the nighttime pineal or serum melatonin rise possibly reflected inherent variability in repetitive melatonin rhythms. In addition, effects of daily melatonin injections to arrest puberty were not blocked by daily MF exposures. The findings support the conclusion that MF exposures fail to disrupt photoperiodic time measurement or the neuroendocrine mechanism regulating reproductive maturation in the juvenile Djungarian hamster.

60-Hz magnetic field exposure effects on the melatonin rhythm and photoperiod control of reproduction.

Adult Djungarian hamsters in long (16 h of light) or short (10 h of light) days for 6 wk were acutely exposed to a 1-G 60-Hz magnetic field (MF) for 15 min 2 h before dark. The nighttime rise in melatonin was delayed and duration reduced in MF-exposed hamsters in both photoperiods compared with sham controls. In a second replicate experiment, MF effects on melatonin rhythm duration were repeated in hamsters in short but not long days, and amplitudes at some clock times differed between the same treatment groups in the two studies. To test the hypothesis that daily MF abbreviates melatonin rhythm duration and induces a long-day reproductive response, adults in shot days were exposed daily to MF. After 3 wk, pineal and serum melatonin rhythms were the same in MF and sham groups; reproduction remained suppressed. Irrespective of acute MF exposure effects on the melatonin rhythm, daily MF treatment does not alter photoperiodic time measurement or the clock mechanism controlling reproduction. Adaption to environmental MF exposures may be part of a normal physiological mechanism that maintains photoperiodic responsiveness in individuals and a seasonal pattern of reproduction.

Neuropeptide Y: distribution of immunoreactivity and quantitative analysis in diencephalic structures and cerebral cortex of dwarf hamsters under different photoperiods.

The distribution of neuropeptide Y-like immunoreactivity (NPY-LI) was investigated by immunohistochemistry and radioimmunoassay (RIA) in the brain of the Djungarian hamster (Phodopus sungorus) held under either long or short photoperiods. In the diencephalic and telencephalic structures studied, distinct patterns of NPY-LI were basically consistent in male and female animals of both groups. NPY levels detected by RIA from tissue samples taken at six time points throughout the 24-hour cycle were in the range of 15-60 pmol/mg protein in the diencephalon or below 5 pmol/mg protein in cerebral cortex. In the diencephalon, immunoreactive structures were seen in the preoptic, peri- and paraventricular, supraoptic, anterior, lateral, dorso- and ventromedial hypothalamic nuclei and in the median eminence. The suprachiasmatic nuclei exhibited a dense innervation by NPY-LI terminals mainly in its ventrolateral subdivision. NPY levels in the suprachiasmatic nucleus were nocturnally augmented under long-day, but not under short-day conditions. The quantification of NPY in the paraventricular nucleus revealed a decrease at night in long-day animals and a small nocturnal augmentation in short-day hamsters. In the pineal gland and habenular nuclei, varicose fibers were observed which appeared mainly perivascular in location (pineal) or formed a dense plexus (habenular nuclei). Pineal NPY contents fell during the night in long-day animals and were relatively constant in short-day hamsters. NPY-LI structures were also observed in the metathalamic intergeniculate leaflet and in a variety of telencephalic structures including the cerebral cortex, caudate nucleus putamen, lateral septal nucleus, bed nucleus of the stria terminalis and amygdala.

Photoperiod regulates the LH response to central glutamatergic stimulation in the male Syrian hamster.

This study investigated central glutamatergic function in relation to photoperiodically-induced changes in the secretion of luteinizing hormone (LH). The experimental approach was to compare the central effects of glutamate agonists on LH secretion in reproductively active hamsters kept in long days (LD) with those in photoinhibited hamsters kept in short days (SD) for 6 weeks and having regressed testes. Agonists were delivered via a cannula into the III ventricle of freely moving hamsters, and blood samples collected 10 to 15 min after the start of the infusion. A high dose (3.0 nmole) of N-methyl-D-L-aspartate (NMDA) induced significant (P<0.01) increases in serum concentrations of LH in hamsters in both photoperiods, though the NMDA-induced increase relative to endogenous LH concentrations was greater in SD than in LD. However, a lower dose of NMDA (0.3 nmole revealed a difference in sensitivity. This dose significantly increased serum LH (P<0.05) in hamsters in SD but had no effect in those in LD. The seasonal difference in response to NMDA was compared with the response to an equimolar dose of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a non-NMDA agonist. This dose of AMPA (0.3 nmole) induced a two-fold increase (P<0.05) in serum concentrations of LH in hamsters in both photoperiods, relative to vehicle-treated controls. In a third experiment the dose-response effects of central AMPA on LH secretion were examined more closely. The sensitivity of LH secretion to stimulation with AMPA did not differ between SD- and LD-housed hamsters. Thus the photoperiod-related difference in sensitivity to stimulation with glutamate agonists is specific for NMDA receptor-mediated activation, rather than a passive reflection of differences in the capacity to secrete GnRH/LH in SD and LD photoperiods. To investigate the site of action of NMDA, the expression of the c-fos immediate-early gene, as assessed by immunocytochemistry for its protein product Fos, was used as a marker of neuronal activation, because previous studies in rodents indicate that a high proportion of GnRH neurons express c-fos at the time of the mid-cycle LH surge. NMDA induced widespread expression of c-fos in many periventricular regions including the medial preoptic area (POA) and ventromedial hypothalamic nucleus. However, dual ICC revealed that in neither photoperiod was Fos present in GnRH-positive neurons 1 h after infusion of 3 nmole of NMDA, despite the increases in LH secretion induced by the infusion. AMPA injected icv at doses which released LH did not enhance expression of c-fos in the hypothalamus. Thus, in the male, enhanced expression of c-fos cannot be detected in GnRH neurons at the time of increased secretion of this hormone induced by glutamate agonists. In conclusion, these results show that both NMDA and non-NMDA glutamatergic pathways potentially regulated LH secretion in the Syrian hamster. The increased sensitivity to NMDA but unaltered sensitivity to AMPA in photoinhibited hamsters in SD is consistent with the view that changes in photoperiod might induce specific alterations in NMDA-mediated pathways that ultimately regulate GnRH neurosecretory activity.

Photoperiodic modulation of luteinizing hormone secretion in orchidectomized Syrian hamsters and the influence of excitatory amino acids.

The effect of N-methyl-D,L-aspartate (NMA) on LH secretion was investigated in Syrian hamsters of the LSH/Ss Lak strain, maintained under either long days (14 h of light, 10 h of darkness) or short days (6 h of light, 18 h of darkness). After 6 weeks of photoperiodic treatment, the animals were orchidectomized. Ten days later, 10-min blood samples were remotely collected from each animal, using surgically implanted intraatrial catheters, and individual pulsatile LH release profiles were subsequently determined by RIA. At the end of the 4-h sampling period, NMA was administered iv (30 mg/kg BW), and the LH response was determined in the next three plasma samples. Hamsters maintained under long days had very high mean plasma LH levels (453 +/- 46 ng/ml) and displayed episodic release patterns characterized by high amplitude pulses. In marked contrast, hamsters that were maintained under short days had significantly (P less than 0.001) lower mean plasma LH levels (90 +/- 12 ng/ml) and showed a significantly (P less than 0.001) lower mean pulse amplitude. The interpulse interval was similar in both of the groups, with LH peaks occurring, on the average, once every 65 min. When challenged with NMA, the long-day hamsters showed only a 37% increase in mean plasma LH levels, which was not statistically significant. In contrast, the short-day hamsters showed a highly significant (P less than 0.01) increase of 294%. Interestingly, the mean plasma LH levels after NMA administration were the same in the two photoperiodic groups despite a marked difference in the plasma levels that preceded the administration. These findings demonstrate that short days can inhibit the neuroendocrine activity of the hamster's reproductive axis independently of gonadal influences. They also suggest that the LHRH neurons have an intrinsic capacity to secrete very high levels of the neuropeptide, regardless of the photoperiod. Taken together, the results support the hypothesis that the breeding season in the hamster is regulated by a gonad-independent mechanism involving a photoperiodic modulation of neuroexcitatory inputs to the LHRH neurons.

Photoperiodic modulation of luteinizing hormone secretion in orchidectomized Syrian hamsters and the influence of excitatory amino acids

The effect of N-methyl-D,L-aspartate (NMA) on LH secretion was investigated in Syrian hamsters of the LSH/Ss Lak strain, maintained under either long days (14 h of light, 10 h of darkness) or short days (6 h of light, 18 h of darkness). After 6 weeks of photoperiodic treatment, the animals were orchidectomized. Ten days later, 10-min blood samples were remotely collected from each animal, using surgically implanted intraatrial catheters, and individual pulsatile LH release profiles were subsequently determined by RIA. At the end of the 4-h sampling period, NMA was administered iv (30 mg/kg BW), and the LH response was determined in the next three plasma samples. Hamsters maintained under long days had very high mean plasma LH levels (453 +/- 46 ng/ml) and displayed episodic release patterns characterized by high amplitude pulses. In marked contrast, hamsters that were maintained under short days had significantly (P less than 0.001) lower mean plasma LH levels (90 +/- 12 ng/ml) and showed a significantly (P less than 0.001) lower mean pulse amplitude. The interpulse interval was similar in both of the groups, with LH peaks occurring, on the average, once every 65 min. When challenged with NMA, the long-day hamsters showed only a 37% increase in mean plasma LH levels, which was not statistically significant. In contrast, the short-day hamsters showed a highly significant (P less than 0.01) increase of 294%. Interestingly, the mean plasma LH levels after NMA administration were the same in the two photoperiodic groups despite a marked difference in the plasma levels that preceded the administration. These findings demonstrate that short days can inhibit the neuroendocrine activity of the hamster's reproductive axis independently of gonadal influences. They also suggest that the LHRH neurons have an intrinsic capacity to secrete very high levels of the neuropeptide, regardless of the photoperiod. Taken together, the results support the hypothesis that the breeding season in the hamster is regulated by a gonad-independent mechanism involving a photoperiodic modulation of neuroexcitatory inputs to the LHRH neurons.

Photoperiodic regulation of glutamatergic stimulation of secretion of luteinizing hormone in male Syrian hamsters.

It has been suggested that changes in endogenous glutamatergic stimulation of secretion of luteinizing hormone (LH) induced by photoperiod play a role in regulating seasonal cycles of reproductive activity. The aim of this study was to test the hypothesis that the glutamatergic control of the secretion of LH in the male Syrian hamster is sensitive to photoperiod, by determining whether the glutamate agonist N-methyl-D-aspartate (NMDA) could stimulate LH secretion in this species and, if so, to determine whether the response varied among animals exposed to different daylengths. In the first experiment, adult male hamsters were housed in either short day (8 h light: 16 h dark) for 6 weeks to induce testicular regression, or long days (16 h light: 8 h dark) to maintain testicular function, and the effects of systemic administration of NMDA on serum LH concentrations were determined. In the short-day hamsters, all s.c. doses of NMDA (25-75 mg kg-1 body weight) produced a robust rise in serum LH concentrations within 15 min. In the long-day hamsters, basal LH concentrations were higher than in short-day hamsters, but only the highest dose of NMDA produced a significant increase in LH concentrations, and the magnitude of this increment was less than those observed in short days. In hamsters in long days, the low doses of NMDA that did not significantly alter LH concentrations nevertheless significantly suppressed serum prolactin concentrations, demonstrating the efficacy of the drug. In hamsters in short days, serum prolactin concentrations were at the limit of detection of the assay, so no inhibitory effect of NMDA on prolactin secretion could be determined on this photoperiod. In the second experiment, the effects of a fixed dose of NMDA (50 mg kg-1 body weight) was tested at intervals in hamsters exposed to short days for a prolonged period such that their testes initially regressed, but then became scotorefractory and testicular recrudescence occurred. After 6 and 12 weeks in short days, NMDA stimulated LH secretion. However, after 24 weeks in short days when testicular recrudescence was complete, the response to NMDA was lost. A third experiment determined whether the reduced response to NMDA in hamsters on long days relative to those in short days might result from higher concentrations of circulating testosterone. Hamsters in long days were castrated to remove the influence of gonadal feedback, and the response to NMDA tested 3 weeks later when endogenous LH concentrations had risen to levels characteristic of the chronically castrated condition.(ABSTRACT TRUNCATED AT 400 WORDS)

Daily torpor in the absence of the suprachiasmatic nucleus in Siberian hamsters.

Siberian hamsters express torpor spontaneously after several weeks of exposure to short days. In long days, torpor is expressed only when food intake is restricted. Hamsters maintained in a long photoperiod (16 h light/day) at 15 degrees C expressed daily torpor during food restriction both before and after bilateral ablation of the suprachiasmatic nucleus (SCN). Hamsters housed in short days (8 h light/day, ambient temperature 15 degrees C) and fed ad libitum displayed torpor before, but not after, ablation of the SCN (SCNX). Torpor was reinstated in all short-day SCNX hamsters during postoperative food restriction and persisted in several animals even after ad libitum feeding was reinstated. Torpor was entrained to the light-dark cycle in both long- and short-day hamsters preoperatively but appeared to occur in a temporally random fashion in SCNX animals. SCNX hamsters, unlike control animals, displayed multiple torpor bouts per 24 h. The SCN is not essential for the expression of torpor but plays a crucial role in its temporal organization.

Photoperiodic regulation of pulsatile luteinizing hormone secretion and adenohypophyseal gene expression in female golden hamsters.

LH concentrations were measured in serum collected at 10-min intervals from chronically ovariectomized female Syrian hamsters that had been maintained for 9 wk in stimulatory (long) or inhibitory (short) photoperiods. Short days reduced the number of detectable LH pulses during both the morning and the afternoon. Most short-day hamsters experienced a gradual afternoon rise in serum LH concentrations; this rise was not composed of multiple pulses. In separate groups of similarly treated hamsters, pituitary LH-beta mRNA abundance was significantly reduced by short-day exposure at both times of day even though serum LH concentrations rose in the afternoon. Estradiol treatment induced an afternoon surge of serum LH in both photoperiods, and eliminated the effect of photoperiod on LH-beta mRNA abundance in the afternoon. Serum prolactin (PRL) concentrations were not consistently influenced by day length in castrated hamsters with or without estrogen treatment, but PRL mRNA abundance was significantly suppressed by short-day exposure in all groups. The results indicate that day length exerts profound steroid-independent effects upon hypophyseal gene expression, and that the regulation of LH-beta mRNA abundance may be due to photoperiodic control of the neural GnRH pulse generator.

Pulsatile Luteinizing Hormone Responses to Intermittent iV-Methyl-D,L- Aspartate Administration in Hamsters Exposed to Long- and Short-Day Photoperiods*

Hamsters exposed to short-day photoperiods (6-h light, 18-h darkness) develop a reversible inhibition of the reproductive axis which includes decreased LH pulse frequency, decreased FSH and testosterone levels, testicular regression, and presumably, decreased luteinizing hormone-releasing hormone (LHRH) release. The decrease in LHRH release could reflect a decrease in the ability of LHRH neurons to release the decapeptide in response to intermittent neuronal excitation. To analyze this possibility, the LHRH secretagogue, N-methyl-D-aspartate (NMA) was used to estimate relative releasability of endogenous LHRH pulses in hamsters exposed to long- and short-day photoperiods. Long-day (LD) or short-day (SD) hamsters were fitted with indwelling atrial catheters 2 days before experimentation. During 5-h sampling sessions (1300-1800 h), blood was withdrawn at 10-min intervals. In initial experiments, a NMA dose-response analysis revealed that 10 and 20 mg/kg NMA but not 2.5 and 5 mg/kg NMA, produced significant, dose-related LH responses in LD hamsters. Treatments with LHRH antagonist 4 h prior to NMA treatment completely blocked LH responses. The 10 mg/kg NMA dose was then used to directly compare LHRH responsiveness in LD and SD hamsters. NMA injections were administered to groups of LD and SD hamsters after hours 1, 2, 3, and 4 of sampling. In LD hamsters, LH responses to the first NMA pulse (2.65 +/- 0.09) were followed by diminishing responses to the second, third, and fourth NMA pulses (1.22 +/- 0.50, 1.22 +/- 0.43, and 1.15 +/- 0.42). By contrast, initial LH responses in SD hamsters (1.61 +/- 0.31) were followed by even more robust LH responses to the second, third, and fourth NMA challenges (2.89 +/- 0.34, 3.08 +/- 0.59, 2.65 +/- 0.32). Although LH responses to the first NMA pulse were slightly less in SD vs. LD animals, responses to all subsequent injections were actually greater in SD hamsters vs. their LD counterparts. Injections of 20 ng/kg LHRH in SD hamsters produced LH responses similar to those evoked by NMA. The same LHRH dose also produced continued, robust LH responses in LD hamsters, indicating that decrements in response to NMA were not due to diminished pituitary responsiveness. These results suggest that 1) the stimulatory actions of NMA are mediated by LHRH, 2) exposure to short days does not decrease the responsiveness of LHRH neurons to NMA, and 3) successive NMA challenges produce diminishing LH responses in LD hamsters, but not in SD hamsters.(ABSTRACT TRUNCATED AT 400 WORDS)

A Role forN-Methyl-D-Aspartate Receptors in the Control of Seasonal Breeding*

The neuroendocrine basis for short-day induced testicular regression was studied in Syrian hamsters of the LSH/Ss Lak strain. Adult animals were maintained either under long or short days (14:10D or 6L:18D, respectively) and given single, daily ip injections of N-methyl-D-aspartate (NMDA) (25 mg/kg BW); control animals received injections of saline. As expected, the testes of the short-day controls had completely regressed to a prepubertal condition within 6 weeks, a change that was associated with significantly reduced mean plasma gonadotropin levels. In contrast, the NMDA-treated hamsters from both the long-day and short-day groups, as well as the long-day controls, all maintained large testes and elevated plasma gonadotropin levels, although plasma luteinizing hormone (LH) was partially suppressed in the short-day group. In a second experiment which lasted 2 weeks, short-day hamsters with completely regressed testes were either transferred to long days, maintained further on short days, or maintained on short days and given a daily ip injection of NMDA (25 mg/kg BW). The short-term exposure to long days caused an expected increase in plasma and pituitary concentrations of follicle-stimulating hormone, pituitary LH, and testicular weight. Similar, but even more marked, changes were observed in the short-day hamsters that were treated with NMDA, including significant increases in plasma LH and serum testosterone concentrations. Moreover, histological examination revealed that the recrudescing testes from this latter group already contained mature spermatocytes and in some individuals even spermatozoa. These results demonstrate that NMDA receptors may play a pivotal role in both the termination and onset of the breeding season in photoperiodic species.