ROLE OF GONADAL-STEROIDS AND INHIBITORY PHOTOPERIOD IN REGULATING BODY-WEIGHT AND FOOD-INTAKE IN DEER MICE (PEROMYSCUS-MANICULATUS)

We investigated the role of declining daylength and gonadal steroids o n body weight and food intake in male deer mice (Peromyscus maniculatu s). This species was chosen for study because individual males display different reproductive responses to inhibitory daylength. About one-t hird of all mice exposed to short days undergo testicular regression a nd exhibit reduced circulating levels of luteinizing hormone and testo sterone (reproductive responsive males). In contrast, testicular funct ion and circulating levels of both these hormones remain unaffected in an equal number of mice (reproductive nonresponsive males). Previous studies have shown that each phenotype exhibits a distinct set of meta bolic responses to short days, including adjustments in body weight. T hese characteristics make deer mice a useful animal model in which to study the interactive effects of gonadal steroids and photoperiod on n eural substrates regulating body weight. A gonadectomy/steroid replace ment experimental model was used to test the role of testosterone in r egulating body weight and food intake in short day-housed male deer mi ce. Among gonad-intact males, short daylength caused a decline in body weight in both reproductive responsive and nonresponsive individuals. However, reproductive responsive mice lost significantly more body we ight than did nonresponsive mice. Furthermore, while the weight loss w as accompanied by a significant reduction in food intake in responsive mice, the relatively minor weight loss in nonresponsive mice was not accompanied by a change in food intake. Because changes in body weight and food intake (data not shown) occurred nearly simultaneously durin g the 8-week exposure to short daylength, results suggest that modific ations in body weight are not responsible for the decline in food cons umed, and vice versa. Gonadectomized reproductive responsive mice lost the same amount of weight as intact responsive mice but ate significa ntly more food. Among nonresponsive males, gonadectomy led to signific antly greater weight loss, relative to intact mice, but caused an incr ease in food intake per gram body weight. Steroid replacement prevente d weight loss and increased food intake in both gonadal phenotypes. De spite the observations that food intake was steroid dependent and the magnitude of the effect differed between reproductive phenotypes, chan ges in food consumption do not fully explain the inhibitory effects of short days on body weight in either phenotype. Taken together with pr evious studies, these results suggest that reproductive quiescence con fers significant metabolic benefits to individual deer mice by reducin g the amount of daily energy requirements via a reduction in body weig ht. Conversely, maintenance of reproductive function during the nonbre eding winter season carries greater metabolic costs; these costs lead to increased amounts of food required to maintain body weight.