Seasonal regulation of neuroendocrine activity in male Turkish hamsters (Mesocricetus brandti): role of the hypothalamic paraventricular nucleus.

Male Turkish hamsters received horizontal knife cuts within the hypothalamus to investigate the role of afferent and efferent projections of the paraventricular nucleus (PVN) in seasonal regulation of endocrine function. Following surgery, the animals were exposed to either a long (16 h light/8 h dark) or a short (8 h light/16 h dark) photoperiod. Similar to pinealectomy, knife cuts placed ventral to the PVN, so as to disrupt transfer of photic information to the pineal gland, resulted in a rapid decline in circulating prolactin (PRL) and follicle-stimulating hormone (FSH) levels, as well as a reduction in testicular size, under both long- and short-photoperiod conditions. In contrast, knife cuts placed just dorsal to the PVN, 50 as to leave connections to the pineal gland intact, did not prevent the effects of short-day exposure on PRL secretion; however, short-day induced declines in FSH levels and testicular size were largely prevented by these lesions. The present results are consistent with past findings that the integrity of the neural pathway to the pineal gland is necessary for the maintenance of appropriate reproductive responses to photoperiod in seasonal mammals. They further indicate a dissociation between the role of hypothalamic connections in the photoperiod-dependent secretion of two anterior pituitary hormones, i.e., PRL and FSH, and suggest that release of these hormones may depend upon different neural pathways for their expression.