Interactions of testosterone and short-photoperiod exposure on the neuroendocrine axis of the male Syrian hamster

Exposure of adult male golden hamsters to short days (less than 12.5 h light/day) leads to suppression of gonadal function which is secondary to reductions in gonadotropin and prolactin (PRL) secretion. Short-day (SD) exposure also leads to a reduction in hypothalamic norepinephrine (NE) and dopamine (DA) metabolism and an increase in hypothalamic LHRH content which appears to be related to a decrease in LHRH release. To determine whether SD-induced changes in NE and DA metabolism are dependent or independent of changes in circulating testosterone (T) levels and thus possible mediators of photoperiod effects on gonadotropin secretion, the effects of castration and steroid replacement on hypothalamic amine metabolism were studied in male hamsters maintained under long or short photoperiod conditions. The presence of Silastic T-implants resulted in a greater suppression of LH and FSH in SD than in long-day (LD) hamster, but increased median eminence (ME) LHRH content in both groups. Exposure of castrate hamsters to short days led to a reduction of NE turnover in the ME and medial preoptic-suprachiasmatic area (MPOA) and a decrease in serum FSH levels. LH levels tended to be lower, but not significantly so. The decrease in ME NE turnover was potentiated by T replacement, but in the MPOA-SCN, T-implants reversed the effects of short days. NE turnover in the MBH was reduced by T in both LD and SD animals, but the effect was much greater in the SD animals. SD exposure also caused a decrease in ME DA metabolism that was reversed by T replacement.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulatory effect of RFRP-3 on the gonadotrophic axis in the male Syrian hamster: the exception proves the rule

In seasonal mammals, a distinct photoneuroendocrine circuit that involves the pineal hormone melatonin tightly synchronizes reproduction with seasons. In the Syrian hamster, a seasonal model in which sexual activity is inhibited by short days, we have previously shown that the potent GnRH stimulator, kisspeptin, is crucial to convey melatonin's message; however, the precise mechanisms through which melatonin affects kisspeptin remain unclear. Interestingly, rfrp gene expression in the neurons of the dorsomedial hypothalamic nucleus, a brain region in which melatonin receptors are present in the Syrian hamster, is strongly down-regulated by melatonin in short days. Because a large body of evidence now indicates that RFamide-related peptide (RFRP)-3, the product of the rfrp gene, is an inhibitor of gonadotropin secretion in various mammalian species, we sought to investigate its effect on the gonadotrophic axis in the Syrian hamster. We show that acute central injection of RFRP-3 induces c-Fos expression in GnRH neurons and increases LH, FSH, and testosterone secretion. Moreover, chronic central administration of RFRP-3 restores testicular activity and Kiss1 levels in the arcuate nucleus of hamsters despite persisting photoinhibitory conditions. By contrast RFRP-3 does not have a hypophysiotrophic effect. Overall, these findings demonstrate that, in the male Syrian hamster, RFRP-3 exerts a stimulatory effect on the reproductive axis, most likely via hypothalamic targets. This places RFRP-3 in a decisive position between the melatonergic message and Kiss1 seasonal regulation. Additionally, our data suggest for the first time that the function of this peptide depends on the species and the physiological status of the animal model.     

Brain aromatization of testosterone in the male Syrian hamster: effects of androgen and photoperiod

Estrogen formed by aromatization of testosterone (T) is involved in the activation of sexual behavior and control of the neuroendocrine system in the male Syrian hamster. Our study examined whether daylength influences formation of estrogen in the preoptic area (POA) and other neuroendocrine areas (anterior hypothalamus, AHT, and medial amygdala, MA) which are targets for T in behaviorally active males. Estrogen formation in individual brain samples was assayed in vitro using the stereospecific production of 3H2O from (1 beta-3H) T as a measurement of aromatase activity. Serum levels of PRL, LH, FSH and T were compared with brain aromatase activity. Groups of intact, castrated and T-treated (chronic silastic T implants) male hamsters, previously selected on behavioral criteria as being sexually active, were maintained on long (16:8LD) or short (8:16LD) daylength for 16 weeks. Two further groups of males either intact or castrated and T-treated were shifted after 7 weeks from the long photoperiod to 12:12LD. POA, AHT and MA areas of sexually active males contained active aromatase systems which converted 3H-T to estrogens. Enzyme activity differed between the areas (POA, MA greater than AHT). Aromatase activity was low in medial septum and cerebral samples. Castration, which reduced serum T to undetectable levels and elevated LH and FSH, had no effect on preoptic aromatase activity. Although estrogen formation in POA did not differ between 8:16LD and 16:8LD males, castration reduced aromatase activity in AHT of both short- and long-day groups. Aromatase activity in AHT was also significantly reduced in photo-inhibited 12:12LD intact males. There was no analogous decrease in 5 alpha-reductase or 17 beta-hydroxysteroid dehydrogenase (HSD) activity, indicating a specific effect on the aromatase. The effect of photoperiod on aromatase activity was not reversed by T treatment. Therefore, photoinhibition acts in part through the effects of reduced T levels on the anterior hypothalamus, but it also acts independently of circulating T. Our results suggest that both androgen and photoperiod may regulate the AHT aromatase system and that this occurs by different mechanisms. The more active aromatase system in POA is insensitive to both castration and photoperiod. Behavioral deficits in short-day males are not due to changes in the preoptic aromatase system, but may be related to changes in aromatase activity within AHT. We conclude that there is a difference in the regulation of two locally active aromatase systems within the preoptic-anterior hypothalamic complex of the male hamster.     

The hypothalamus and photoperiodic control of FSH secretion by melatonin in the male Syrian hamster

In the photoinhibited castrated male Syrian hamster, removal of the pineal gland or transfer to long photoperiods was followed by a rapid increase in the serum concentration of FSH. Levels were significantly above those of controls within 10 days. Central passive immunization of pineal-intact photoinhibited castrated animals against melatonin had a stimulatory effect on serum FSH levels, comparable with that observed following pinealectomy or transfer to short days. The effects of pinealectomy were blocked by programmed s.c. infusions of melatonin in a time-dependent manner. Serum FSH levels remained low in animals receiving 100 ng melatonin delivered over 10 h but the same mass of melatonin delivered over 4 h had no effect on the response to pinealectomy. Lesions of the anterior hypothalamus had no effect on the pinealectomy-induced increase of serum FSH in animals receiving saline infusions. However, in lesioned animals, programmed infusions of melatonin were no longer able to suppress the rise in FSH following pinealectomy. These results demonstrated that pineal melatonin is the mediator of central photoperiodic control of FSH secretion. The duration of the melatonin signal determines its effectiveness and an intact anterior hypothalamus is necessary for the signal to be read and/or the appropriate neuroendocrine response expressed.     

Harderian gland N-acetyltransferase activity in the male Syrian hamster: effects of gonadectomy, short photoperiod exposure, or subcutaneous melatonin implants

The activities of NAT and HIOMT and the melatonin concentration in the Harderian glands of intact, gonadectomized, and gonadally-regressed male Syrian hamsters were studied. To produce gonadal regression, hamsters were exposed to either artificial or naturally short photoperiods. NAT activity of castrated and gonadally-regressed hamsters was always less in comparison to that of animals with intact gonads. Castrated hamsters exposed to long days showed higher NAT activity than that of castrated animals exposed to short photoperiods indicating that light may have some influence on Harderian NAT independent of the gonadal status. Also, gonadally-regressed hamsters exposed to long photoperiods exhibited higher NAT activity in comparison to gonadally-regressed animals exposed to short days. The HIOMT activity and melatonin content of Harderian glands in all these groups of male Syrian hamster were very low.     

The effects of short photoperiod, pinealectomy, and melatonin treatment on oxytocin synthesis and release in the male syrian hamster

The pineal gland has been shown to affect plasma oxytocin (OT) levels, but the mechanism of this action is not apparent. In the present study, the ability of the photoperiod to affect plasma OT levels, neurointermediate lobe (NIL) OT content, and hypothalamic OT mRNA levels was studied in male Syrian hamsters. In addition, the ability of pinealectomy to prevent and melatonin (MEL) to mimic the short photoperiod-induced changes were also determined. Exposure to short days (SD) led to the expected decrease in testes weight and plasma PRL levels, but plasma OT levels were unchanged. However, NIL OT content was increased in the SD-exposed animals. Hypothalamic OT mRNA levels were not significantly altered by SD exposure. Pinealectomy blocked the effects of SD on testes weight, whereas afternoon MEL injections mimicked the effects of SD. In long day (LD)-exposed hamsters, pinealectomy induced a decrease in NIL OT content without altering hypothalamic OT mRNA levels. In SD-exposed animals, NIL OT content was not affected by pinealectomy. Melatonin injections had no significant effect on NIL OT content or hypothalamic OT mRNA levels. The data from the present study suggest that exposure of male Syrian hamsters to short photoperiods influences some aspects of OT synthesis and/or transport to produce its increased accumulation in the NIL, but does not affect OT release. These changes are apparently not the result of SD-induced changes in MEL secretion, but conceivably could be related to the previously documented effects of SD on hypothalamic catecholamine turnover.     

Naloxone-induced secretion of LH in the male Syrian hamster: modulation by photoperiod and gonadal steroids

The role of endogenous opiates in the regulation of photoperiodically induced testicular regression was studied in the male Syrian hamster. In reproductively active hamsters exposed to a long photoperiod (LD; 16 h light: 8 h darkness) or to short days (SD; 8 h light: 16 h darkness) for 20 weeks or to SD after pinealectomy, administration of naloxone, a competitive opiate receptor antagonist, at doses of 2.5-20 mg/kg, significantly increased serum LH concentrations. In marked contrast, these doses of naloxone did not produce any change in LH levels in reproductively quiescent hamsters exposed to SD for 8 weeks. The influence of gonadal steroids on the LH response to naloxone was studied in hamsters castrated or castrated and implanted s.c with a capsule containing testosterone. Naloxone did not induce LH release in castrated hamsters maintained in LD or in SD, but this response was restored in LD but not SD when serum testosterone concentrations were maintained at levels similar to those observed in intact reproductively active hamsters. These results show that inhibition of reproduction by the photoperiod prevents naloxone-induced LH release in the male hamster. This lack of response to naloxone is not due, however, to the lower testosterone titres present in these animals compared with reproductively active animals. Responsiveness to naloxone can be restored when the animal is rendered insensitive to the inhibitory photoperiod either by removal of the pineal gland or by induction of photorefractoriness by extended exposure to SD.     

Daily and photoperiodic 2- 125I-melatonin binding changes in the pars tuberalis of the Syrian hamster (Mesocricetus auratus): Effect of constant light exposure and pinealectomy

Abstract: Using quantitative autoradiography, 2¹²⁵I-melatonin binding was investigated throughout the light/dark cycle in the pars tuberalis (PT) of the pituitary of adult Syrian hamsters kept for 8 weeks either in long or short photoperiod (LP or SP, respectively). Melatonin receptor density in the PT displayed photoperiod dependent daily variations (maximal values in LP). Indeed, in LP, melatonin receptor density underwent strong daily variations with maximal values during the first half of the light period and minimal values at the end of the night. These variations depended on changes in the maximal binding (B_max) without differences in the dissociation constant (K_d). In contrast, PT melatonin receptor density was constant and at a very low level throughout the 1ight:dark cycle in SP exposed animals. Daily PT melatonin receptor density variations of LP exposed animals were abolished by pinealectomy or continuous light exposure. These results show clearly that both at the daily and at the seasonal level the regulation of PT melatonin receptors is strongly dependent on circulating melatonin concentrations in the Syrian hamster, but that other regulatory factors, yet unclarified, might also play a role.     

Prolactin cell activity in female and male Syrian hamsters: an apparent sexually dimorphic response to light deprivation and pinealectomy

In order to determine the role of the pineal gland in mediating the effects of long-term light deprivation on prolactin (PRL) cell activity in a highly photosensitive species, PRL synthesis, storage and release were determined in both female and male Syrian hamsters that were either blind, blind and pinealectomized or left intact for 14 weeks. PRL release was determined in vivo by measuring the amount of immunoreactive (RIA) PRL in the serum of the animals in each group with a heterologous RIA for hamster PRL. Binding resulted in a 98 and 88% reduction in serum PRL levels in female and male hamsters, respectively. Pinealectomy largely prevented the suppressive effects of blinding on PRL release; however, PRL levels in blind pinealectomized animals were intermediate between those in intact and blind animals. PRL synthesis was evaluated by assessing the amount of 3H-leucine incorporated into PRL by anterior pituitaries in vitro. In female hamsters 14 weeks of light deprivation resulted in an 87% decrease in the incorporation of 3H-leucine into newly synthesized PRL whereas in males only a 40% reduction occurred. While pinealectomy completely prevented the inhibitory effects of blinding on PRL synthesis in males, it was less effective in female hamsters inasmuch as PRL synthesis was still nearly 50% lower in blind pinealectomized animals than in controls. Stored PRL, as represented by the total amount of RIA-PRL in vitro, was 96% lower in blind female hamsters as compared with intact controls; in blind male hamsters, stored PRL was reduced by 77%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pinealectomy on the undernutrition-induced suppression of the reproductive axis in rats

Food restriction is known to sensitize the reproductive axis of rats to the inhibitory action of the pineal gland and its hormone, melatonin. The present study investigated the effect of pinealectomy on the undernutrition-related reproductive suppression. Three groups of animals (control, sham pinealectomy or pinealectomy) were subjected to either 3 weeks of 50% food restriction or ad libitum feeding under a lighting regimen of 14 h light and 10 h dark. At the end of the treatment period, body weight, testes weight, accessory organ weights, serum LH and serum testosterone were determined. In the pinealectomized animals, accessory organ weights and serum testosterone levels increased significantly. As compared to the controls, underfed pinealectomized animals had a partial recovery of the reduced accessory organ weights and normalization of the serum testosterone levels. These findings suggests that pinealectomy has a protective effect on the gonads of both control and underfed animals and that underfed animals are more sensitive to this protective mechanism.     

Gonadal effects of pinealectomy and immunization against N-acetylindolealkylamines in the hamster.

In order to determine whether circulating melatonin (MEL) is necessary for the gonadal regression seen in hamsters exposed to a short photoperiod, animals were immunized with N-acetylserotonin (NAS) conjugated to protein. Antibodies formed against this conjugate bind MEL and NAS equally. When exposed to a short photoperiod (L:D, 1:23), a dramatic regression was seen in gonadal weight with no effect on body, Harderian gland or adrenal weight. Plasma testosterone levels also dropped. Immunization against NAS did not modify these effects, suggesting that circulating MEL does not mediate the antigonadal effect of the short photoperiod.     

Pineal melatonin concentrations in the Syrian hamster

Pineal melatonin concentrations exhibited a marked diurnal rhythmicity in gold hamsters maintained in a light-dark cycle of 15 h of light and 10 h of darkness. When tissue was collected at 3-h intervals throughout a 24-h period, daytime levels of 95--232 pg/pineal gland rose to concentrations of 760--1335 pg/pineal gland (P greater than 0.001 vs. all other values) at 0400 h, 8 h after the onset of darkness. When tissue was collected sequentially during the dark phase, pineal melatonin concentrations remained significantly elevated from 0200--0500 h (P less than 0.01 and P less than 0.001 vs. daytime values, respectively). Superior cervical ganglionectomy abolished the rhythm of pineal melatonin concentrations, and the concentrations were maintained at 60--105 pg/pineal gland throughout a 24-hr period.     

Elevated ambient temperature retards the atrophic response of the neuroendocrine-reproductive axis of male Syrian hamsters to either daily afternoon melatonin injections or to short photoperiod exposure

Adult male Syrian (golden) hamsters, maintained under either 22 +/- 2 or 32 +/- 2 degrees C, were treated with 8 or 11 weeks of exposure to either long photoperiod (14:10), short photoperiod (8:16), or to long photoperiod with a daily afternoon melatonin injection. By 8 weeks, the animals kept at 22 degrees C and treated with daily afternoon melatonin injection exhibited a dramatic reduction in testicular and accessory sex organ weight, but the animals kept at 32 degrees C and treated in the same way exhibited only slight decreases in testicular and accessory organ weights. Short photoperiod caused a slight decrease in testicular and accessory organ weights of hamster kept at 22 degrees C, while it had no significant effects on reproductive organ weights of the animals maintained under 32 degrees C. By 11 weeks, the daily afternoon melatonin injection elicited further reduction in testicular and accessory organ weights of the animals maintained under both 22 and 32 degrees C. However, the reduction in animals kept at 32 degrees C was not as great as that in animals kept at 22 degrees C. Although short photoperiod caused an obvious decline in reproductive organ weights of the animals at 22 degrees C, only a slight decrease was seen in hamsters at 32 degrees C. As with reproductive organ weights, testosterone levels were depressed more rapidly and completely in animals maintained at 22 degrees C. These results indicate that elevated ambient temperature changes the rate at which the gonads of hamsters regress in response to daily afternoon melatonin injections or short photoperiod.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulation of hamster pulmonary neuroendocrine cells and associated peptides by repeated exposure to cigarette smoke

Although the normal function of pulmonary neuroendocrine (PNE) cells containing bioactive peptides is poorly understood, various pulmonary diseases are associated with hyperplasia of these cells, and they also may be progenitors for small cell lung cancer in humans. In this study we have investigated the effects of subchronic cigarette smoke exposure in the hamster on the PNE cells and their peptide content. Daily exposure to standard research cigarette smoke for as long as 90 days led to progressively higher levels of serum calcitonin (iCT) as well as higher lung tissue iCT and the gastrin releasing-like peptides or mammalian bombesin (MB). Subsequent to a 30-day period during which there was no further exposure to smoke, serum levels returned to control levels, but the lung levels of both iCT and MB remained higher than control levels. Also, after the 90 days of exposure, immunocytochemistry revealed an increase in the number of iCT-containing PNE cells. This increase in the number of PNE cells correlated well with the increased iCT content of the lung tissue. We conclude that subchronic cigarette smoke exposure causes an increase in pulmonary levels of iCT and MB, which may be linked to the observed proliferative response of the PNE cells.     

Effect of light irradiance and wavelength on the Syrian hamster reproductive system

The purpose of this study was to examine how the physical characteristics of light influence the short photoperiod-induced atrophy of the reproductive system. In the first experiment, groups of Syrian hamsters were exposed to a 14-h light, 10-h dark (14:10) photoperiod, a 11:13 photoperiod or a 11:13 photoperiod extended by 3 h of light at an irradiance of 20, 2, 0.2, or 0.002 microW/cm2. Animals in the 11:13 photoperiod and the 11:13 photoperiod extended by 0.02 and 0.002 microW/cm2 underwent reproductive regression, as evidenced by decreased testicular and accessory sex organ weight and pituitary PRL levels. In the second study, groups of hamsters were exposed to a 14:10, 11:13, or 11:13 photoperiod extended by 3 h of red, yellow, green, blue, or near-UV light at 0.2 microW/cm2. Animals in the 11:13 and the 11:13 photoperiod extended by red or yellow light underwent reproductive atrophy. These results demonstrate that light irradiance and wavelength are important determinants of the photoperiodic regulation of the Syrian hamster reproductive system.     

Effects of continuous light exposure on antioxidant enzymes,porphyric enzymes and cellular damage in the Harderian gland of the Syrian hamster

The Syrian hamster Harderian gland (HG), an organ present in the male two secretory cell types (type-I and type-II cells), is physiologically exposed to high oxidative stress because of high concentrations of porphyrins and their precursor, 5-aminolevulinic acid. Because of its juxtaorbital location, the HG is accessible to light, and subject to phototoxic effects of these substances. After having previously demonstrated circadian rhythms in antioxidant enzymes, porphyric enzymes and oxidative damage of proteins and lipids, as well as influences of melatonin on these parameters, we have now studied the effects of continuous light (LL), which suppresses melatonin secretion by the pineal gland. Measurements were performed in two different circadian phases, in order to detect the presence or absence of day/night differences. In LL, no differences between circadian phases of subjective day and subjective night were demonstrable for 5-aminolevulinate synthase, 5-aminolevulinate dehydratase, porphobilinogen deaminase, or superoxide dismutase; temporal differences in glutathione reductase and catalase were markedly diminished, whereas all these parameters showed marked day/night differences in the rats exposed to a light/dark cycle of 14:10. In LL, oxidative damage to lipids was minimally effected, while protein damage was enhanced. LL also caused a reduction in the percentage of type-II cells. Therefore, cell differentiation in the HG does not seem to be controlled only by the androgen, but, unexpectedly, also by melatonin.