RAPID PHOTOPERIOD-INDUCED INCREASE IN DETECTABLE GNRH MESSENGER-RNA-CONTAINING CELLS IN SIBERIAN HAMSTER

To determine whether changes in gonadotropin-releasing hormone (GnRH) neurons are early indicators of photostimulation, Siberian hamsters we re placed in short days (6:18-h Light-dark) at 3 (experiment 1) or 6 ( experiment 2) wk of age where they were held for 3 (experiment 1) or 4 (experiment 2) wk. Hamsters were then moved to long photoperiod (16:8 -h light-dark.). In experiment I, brains were collected 1-21 days afte r transfer from short to long days. In experiment 2, brains were colle cted only on the second morning of long day exposure. Long and short d ay controls were included in both experiments. Cells containing GnRH m RNA, as visualized by in situ hybridization, were counted. As expected , there were no differences in the number of detectable GnRH mRNA-cont aining cells among animals chronically exposed to long or short photop eriods. However, on the second morning after transfer from short to lo ng photoperiod, a positive shift in the distribution of GnRH mRNA-cont aining cells occurred relative to the respective controls in the two e xperiments. Increases in follicle-stimulating hormone secretion and go nadal growth occurred days later. In conclusion, a rapid but transient increase in the distribution of detectable GnRH mRNA-containing cells is an early step in the photostimulation of the hypothalamic-pituitar y-gonadal axis.