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Despite profound photoperiodic differences in circulating gonadotropin levels, consistent differences in the GnRH system have not been observed in Siberian hamsters (Phodopus sungorus) housed chronically in short or long days. During the transition from short to long days, however, male hamsters exhibit a transient increase in the number of cells expressing prepro-GnRH mRNA on the morning of the second long day. Here, we present two experiments designed to examine whether or not this change in mRNA level is associated with changes in GnRH protein synthesis. In the first experiment, we used RIA to measure GnRH content in preoptic area-mediobasal hypothalamic homogenates. We observed a significant increase in GnRH protein levels on the morning of the second long day relative to short- and long-day controls. The latter two groups did not differ from one another. In the second experiment, we used immunocytochemistry to quantify GnRH cell number in the various treatment groups. GnRH-immunoreactive (-ir) cell number did not increase significantly after long-day transfer relative to that in short-day controls; however, both of these groups had significantly more GnRH-ir neurons than long-day controls. We hypothesize that during the transition from short to long days, male Siberian hamsters experience a transient increase in GnRH production in a stable population of neurons. When GnRH secretion subsequently increases on long days, peptide content within neuronal cell bodies declines, leading to a decrease in the number of immunoreactive neurons detected. The rapid response of the hypothalamo-pituitary-gonadal axis in Siberian hamsters to a change in day length defines a narrow temporal window in which to identify the physiological, cellular, and molecular mechanisms mediating the photoperiodic regulation of reproduction.

Photoperiodic Effects on Gonadotropin-Releasing Hormone (GnRH) Content and the GnRH-Immunoreactive Neuronal System of Male Siberian Hamsters1