Photoperiodic regulation of POMC systems in the golden hamster.

Abstract

In this dissertation, I investigated proopiomelanocortin (POMC) systems in the hypothalamus, testes, and anterior pituitary that are altered by switching the male golden hamster (Mesocricetus auratus) from long photoperiods (LP) to short photoperiods (SP). I tested the following hypotheses: (1) SP stimulate POMC peptide secretion from the mediobasal hypothalamus (MBH); (2) SP inhibit POMC peptide secretion from the testes; (3) SP inhibit POMC peptide secretion from the anterior pituitary (AP); and (4) SP and agonistic encounters (i.e., social isolation followed by fighting) stimulate POMC peptide secretion from the AP. After exposure to SP (i.e., less than 12.5 hours of daily light), the hamster exhibits changes in the secretion of hormones regulating reproduction, metabolism, and social behavior, which are thought to be critical for winter survival. Although the underlying mechanisms are not completely understood, the POMC precursor, which encodes beta-endorphin ($\beta$-END) and adrenocorticotropic hormone (ACTH), may play a key role. The results show that the MBH $\beta$-END content, as assessed by radioimmunoassay (RIA), increases prior to the loss of testicular activity during SP. MBH $\beta$-END is thought to inhibit gonadal activity. In addition, increases followed by decreases in the testicular $\beta$-END content and POMC mRNA levels, as assessed by RIA and Northern analysis, suggest that testicular POMC is regulated by the photoperiod During SP, decreases in plasma ACTH and glucocorticoid levels, as assessed by RIA, are not temporally associated with decreases in the body fat content, even though the glucocorticoids are known to stimulate fat catabolism. In hamsters subjected to agonistic encounters and SP, increases in plasma ACTH and glucocorticoid levels are associated with pronounced increases in agonistic behavior. ACTH, via its stimulation of glucocorticoid secretion, is known to increase agonistic behavior. I conclude from these results that POMC peptides are involved in the photoperiodic regulation of gonadal and adrenal activities and possibly in the seasonal regulation of reproduction and social behavior, but not fat metabolism.