Although a great deal is known about the effects of day length on seasonal changes in circulating levels and gonadal function in mammals, much less is known about the effects of day length on the neural mechanisms which mediate seasonal changes in the reproductive system. Recently it has been demonstrated that male Siberian hamsters (Phodopus sungorus) transferred from short to long days exhibit an increase in the number of hypothalamic neurons containing detectable levels of gonadotrophin-releasing hormone (GnRH) mRNA after exposure to the first long day. Subsequently, the number of GnRH mRNA-containing cells returns to baseline (short-day) levels. This rapid and transient change in gene expression defines a window of time in which to elucidate the early cellular, neurochemical, and molecular changes that occur in the brain during the initial transition from a photically inhibited neuroendocrine system to a photostimulated state. This proposal describes three sets of experiments designed to characterize and elucidate these mechanisms. The specific aims of this project are to test the following hypotheses: 1) photostimulation causes a rapid release of GnRH from the hypothalamus and synthesis is subsequently stimulated to return hypothalamic GnRH content to baseline levels, 2) the expression the proto-oncogene c-fos can be used as a marker of GnRH neurons involved in the early stages of photo- induced stimulation of the hypothalamo-pituitary-gonadal (HPG) axis, and may be related to increases in hypothalamic cells containing GnRH mRNA, an 3) rapid increases in GnRH gene expression are dependent upon changes in patterns of melatonin secretion from the pineal gland. The results of these studies are expected to shed new light on the physiological, cellular, and molecular mechanisms underlying seasonal changes in the HPG axis. Moreover, the results may provide important insight into the basic mechanisms mediating plasticity in neuroendocrine systems in animals, including humans.