Effects of photoperiod and androgen on proopiomelanocortin gene expression in the arcuate nucleus of golden hamsters.

In golden hamsters, seasonal changes in day length act via a pineal-dependent mechanism to regulate feedback and behavioral effects of androgen. Endogenous opiates participate in photoperiodically regulated neuroendocrine functions, but the effects of androgen on expression of the gene encoding POMC, the precursor of beta-endorphin, have been controversial. We used quantitative in situ hybridization to examine regulation of POMC messenger RNA (mRNA) by testosterone and to test the hypothesis that short day lengths act through the pineal gland to amplify POMC mRNA expression. We studied intact hamsters and castrates with or without androgen treatment held in long (14 h of light, 10 h of darkness) or short (5 h of light, 19 h of darkness) days for 10 weeks. POMC gene expression differed with rostral-caudal plane, photoperiod, and surgical treatment (castration and testosterone administration). Testosterone increased the number of silver grains in labeled cells throughout the arcuate nucleus, and short day castrates given androgen consistently had more silver grains per labeled cell than did their long day counterparts. Testosterone exerted an inhibitory effect, however, on the number of POMC mRNA-positive cells, and more POMC mRNA-labeled cells were found in the arcuate nucleus of long than short day castrates treated with testosterone. Photoperiod had no significant influence in castrates not receiving androgen. Testosterone treatment had generally similar effects whether it was begun at the time of castration or 5 weeks later. Pinealectomy blocked the influence of photoperiod on both the mean number of silver grains per labeled cell and the number of labeled cells. The results indicate that day length regulates POMC gene expression when androgen levels are held constant, but that androgen is necessary for photoperiod effects to be expressed.

FOS immunoreactivity after exposure to conspecific or heterospecific urine: where are chemosensory cues sorted?

Female prairie voles (Microtus ochrogaster) typically require an intact vomeronasal system and exposure to a chemical signal found in urine from male prairie voles to induce uterine growth necessary for reproduction. Urine from male mice (Mus musculus) does not contain an effective cue for activation of female vole reproduction: after 4 days of exposure to stimuli, voles exposed to urine from female or water had still uteri whereas voles exposed to urine from male voles had large uteri. The initial response to urine from male voles included neuronal activity in the vomeronasal system as indicated by FOS immunocytochemistry. Stimuli (urine from a male vole or a male mouse, or water) were painted on the nose of naive female voles 1 h before sacrifice. Female voles exposed to urine from male voles had more FOS-immunoreactive cells in the accessory olfactory bulb than voles exposed to mouse urine or to water. We conclude that exposure to urine from male voles stimulates the vomeronasal pathway (as measured by FOS immunoreactivity) and induces uterine growth in female voles, whereas exposure to urine from male mice (or water) does not. This suggests that some degree of functional specificity of the clinical cue is determined at or before the accessory olfactory bulb, perhaps in the expression of specific receptors within the vomeronasal organ, rather than entirely within the central nervous system.

Short days increase sensitivity to methadone inhibition of male copulatory behavior.

Exposure to short, winter-like daylengths produces deficits in male golden hamster copulatory behavior, even when exogenous testosterone is administered to replicate serum concentrations typical of hamsters housed in long days. Daylength also regulates opiate receptor concentrations in limbic brain regions which control sexual behavior, and the response of gonadotropin secretion to opiate receptor antagonists is modulated by daylength. This study tests the hypothesis that short days amplify the opiatergic inhibition of copulatory behavior. Male golden hamsters were castrated and given testosterone implants before transfer to short days. Hamsters were tested for copulatory behavior after injections of saline or various doses of methadone, an opiate agonist believed to be specific for mu receptors. Locomotor activity was also measured. Hamsters housed in short days showed copulatory deficits in response to lower doses of methadone than hamsters housed in long days. Short days enhanced copulatory deficits after methadone at doses which did not affect general activity.

Steroidal and photoperiodic regulation of opiate binding in male golden hamsters.

This study was designed to investigate interactions between daylength and testosterone (T) in the regulation of 3H-naloxone binding which may contribute to seasonal changes in the negative feedback and behavioral effects of androgens in the golden hamster. Photoperiod influenced opiate binding in hamsters with intact gonads only in the medial amygdala. Castration elevated specific 3H-naloxone binding in the medial amygdala, medial preoptic nucleus, bed nucleus of the stria terminalis, and in the basolateral amygdaloid nucleus of hamsters exposed to either long (14L:10D) or short (5L:19D) days. Exposure to SD renders hamsters less sensitive to T maintenance in reversing these effects. Delay of T replacement until 5 weeks after castration eliminated the ability of this androgen to reverse the influence of castration upon opiate receptors in the medial amygdala. Pinealectomy markedly increased 3H-naloxone binding in short days in several brain areas. The data demonstrate that androgens and photoperiod interact to regulate 3H-naloxone binding, particularly in the medial amygdala. These effects may play a functionally relevant role in seasonal changes in the expression of sexual behavior and/or gonadotropin secretion.

Photoperiodic changes in opiate binding and their functional implications in golden hamsters.

Daylength modulates gonadotropin secretion, gonadal steroid hormone feedback, sexual behavior and body weight in male golden hamsters. Endogenous opiates regulate each of these phenomena, and the ability of opiate receptor blockade to elevate serum LH secretion is photoperiod-dependent. We used in vitro autoradiography to localize and quantify effects of daylength in golden hamsters. Hamsters were exposed to stimulatory (14 h light: 10 h dark) or inhibitory (10 h light: 14 h dark) photoperiods for 10 weeks before specific [3H]naloxone binding was assessed. Short days significantly decreased binding in medial amygdala and the intercalated amygdaloid nucleus. This effect was reversed by superior cervical ganglionectomy. No significant effects of daylength were observed in other amygdaloid, hypothalamic or preoptic areas. Lesions of the medial amygdala decreased copulatory behavior, short day-induced weight loss, and anogenital chemoinvestigation but did not affect gonadal regression or other forms of chemoinvestigation. These lesions facilitated testosterone's negative feedback on luteinizing hormone in long days but did not interfere with the potentiation of negative feedback by short days.