Photoperiodic control of body weight and energy metabolism in Syrian hamsters (Mesocricetus auratus): role of pineal gland, melatonin, gonads, and diet.

The effects of photoperiod and the pineal hormone melatonin on the regulation of body weight and energy metabolism were examined in Syrian hamsters, Mesocricetus auratus. Short photoperiod-housed female and male hamsters showed increases in body weight gain, feed efficiency, carcass lipid, brown adipose tissue mass, and thermogenic capacity. These effects of short photoperiods were mimicked by afternoon melatonin injections to hamsters in long-day photoperiods and were exaggerated in hamsters fed high fat diets. To determine the role of the gonads in these effects, ovariectomized hamsters were treated similarly and found to exhibit changes in body weight and energy metabolism that were 80-90% of those in gonadally intact hamsters. The role of the pineal gland in short photoperiod-induced body weight gain was examined in sham-pinealectomized and pinealectomized hamsters. Short photoperiod-induced increases in body weight were seen in both pinealectomized and sham-pinealectomized hamsters. Thus, pinealectomy blocks the effects of short photoperiods on reproductive function and thyroid activity, but not on body weight regulation. These results suggest that the effects of short photoperiods on body weight and energy metabolism are mediated by multiple, redundant mechanisms involving decreases in gonadal hormone secretion, changes in melatonin secretion, and gonad- and pineal-independent changes. All of the effects of short photoperiods are exaggerated in hamsters fed a high fat diet. These changes may represent adaptive preparatory responses that enhance winter survival in Syrian hamsters.