Temperate zone mammals often exhibit dramatic seasonal fluctuations in a number of physiological processes, including reproduction, body weight and lipid storage, thermoregulation, and metabolic and endocrine functions. These changes allow the animals to adjust their activities, especially production of offspring, and anticipate seasonal variations in food supplies and metabolic demands. The objective of this research program is to elucidate the neural and endocrine mechanisms controlling seasonal cycles in body weight and fat content, primarily in Syrian (Mesocrietcus auratus) and Djungarian (Phodopus sungorus sungorus) hamsters. The principal environmental cue for seasonal body weight change in hamsters is photoperiod, the number of hours of light per 24-hr period. When photoperiod falls below a critical value, as in the autumn, Syrian hamsters gain weight and fatten, whereas Djungarian hamsters lose weight, primarily adipose tissue. The effects of photoperiod on body weight are mediated by the pineal gland and its hormone, melatonin, Pinealectomized (PINX) hamsters typically do not respond to changes in photoperiod, and properly timed melatonin injections can induced winter-like responses in hamsters housed in long (summer-like) photoperiods. Virtually nothing is known about the neural circuits mediating the effects of photoperiod on seasonal body weight cycles. One group of experiments will use selective electrolytic lesions, axon- sparing neurotoxin lesions, and knife cuts in various brain regions to see whether responsiveness to photoperiod is affected. Intracerebral infusions of melationin will be used in an attempt to identify melatonin-sensitive loci. The pineal secretes melatonin almost solely at night, and the duration of noctual melatonin secretion is inversely related to day length. However, it is not clear just what aspect of this melatonin rhythm (phase, duration, amplitude, total amount) signals the changes in body weight in Syrian hamsters. This questions will be addressed by giving PINX hamsters systemic infusions of melatonin in different doses and temporal patterns. Other questions of interest include: (1) What are the effects of photoperiod and melatonin treatments on ingestive behaviors (e.g., diet selection)? (2) Do body weight and reproductive function respond similarly to manipulations of photoperiod? (3) What role, if any, do other endocrine glands play in seasonal body weight cycles?