Effects of diabetes and insulin on photoperiodic responses in Siberian hamsters.

Short day pineal melatonin secretion patterns cause gonadal regression in Siberian hamsters, an effect due to alterations in gonadotropin secretion that ultimately affects the gonads. An analogous endocrine response sequence may underlie short day (melatonin)-induced decrease in body weight (fat) in this species. A potential mediating hormone for this effect of melatonin may be insulin. The purpose of the present experiments was to determine whether alterations in insulin status might affect the photoperiodic control of body weight (fat) and reproductive condition. Insulin was controlled by inducing experimental diabetes mellitus via injections of streptozotocin across several days, followed by treatment with one of two doses of long-lasting insulin. Both insulin doses normalized urinary glucose, but not body weight in long days. Seven weeks of short day exposure caused gonadal regression in all groups regardless of insulin status. The magnitude of the short day-induced decrease in body weight and carcass lipid was directly related to long day baseline body weights; however, the body weight nadir reached was similar among the groups. A 'regulation' of seasonally appropriate body weight (fat) is implied by these findings.