Developmental changes in hypothalamic melatonin levels of male rats

Circadian neuroendocrine role in age-related changes in body fat stores and insulin sensitivity of the male Sprague-Dawley rat

A role for circadian neuroendocrine rhythms in the age-related development of obesity and insulin resistance was investigated in the male Sprague-Dawley rat. The phases and amplitudes of the plasma rhythms of several metabolic hormones (i.e., corticosterone, prolactin, insulin, and triiodothyronine) differed in lean, insulin-sensitive (3-week-old rats), insulin-resistant (8-week-old rats) and obese, insulin-resistant (44-week-old rats) animals. Simulation of the daily rhythms of endogenous corticosterone and prolactin by daily injections of the hormones at times corresponding to the peak levels found in 3-week-old rats reversed age-related increases in insulin resistance and body fat in older (5-6-month-old) rats. Ten such daily injections of corticosterone and prolactin in 12-14-week-old rats produced long-term reductions in body fat stores (30%), plasma insulin concentration (40%), and insulin resistance (60%) (determined by a glucose tolerance test) measured 11-14 weeks after the treatment. Alterations in circadian neuroendocrine rhythms may account for age-related changes in carbohydrate and lipid metabolism in the male Sprague-Dawley rat, and resetting of these rhythms by appropriately timed daily injections of corticosterone and prolactin may help maintain metabolism characteristic of younger animals.

Effects of thyroid hormone on light/dark melatonin synthesis and release by young and maturing rat pineal glands in vitro

Synthesis and release of melatonin were studied in pineal explants from 14- (young) and 60-day-old ('maturing') male Long-Evans rats with or without added thyroid hormone, triiodothyronine (T3), at or near physiological levels and under light and dark conditions. Incubation for 6 hr (1200-1800) was in a synthetic medium; melatonin was measured by radioimmunoassay (RIA). In light, T3 increased melatonin levels in pineal and medium of cultures from either young or maturing animals. In dark, T3 decreased melatonin levels in the pineals of either age, but was without significant effect on levels in the medium. Since it is known from other work that 14-day-old rat pineal glands do not yet have a complete sympathetic innervation system, it is here doubly evident that T3 can modulate directly pineal synthesis and release of melatonin, and may not depend upon a mature sympathetic innervation. Light in the studied conditions was permissive from the stimulatory action of T3 on pineal synthesis and release of melatonin in vitro.