Steady state analysis of hypothalamic GnRH mRNA levels in male Syrian hamsters: influences of photoperiod and androgen.

An in situ hybridization assay, utilizing a free floating technique was used to estimate the steady state levels of hypothalamic luteinizing hormone-releasing hormone (GnRH) mRNA levels in the brains of male golden hamsters maintained in different photoperiods. In situ histochemistry was performed using a 32P-labelled 66-nucleotide long oligomer complementary to the sequence of the human GnRH mRNA coding region. The oligonucleotide hybridized specifically to mRNA encoding the GnRH precursor as suggested by the distribution of labelled neurons and as shown by an RNAse protection assay on septal and preoptic-hypothalamic mRNA from gonadally regressed hamsters. To test the hypothesis that short-day photoperiods reduce GnRH synthesis, intact male hamsters or castrated males bearing subcutaneously inserted testosterone implants were exposed to long-day (14 h light:10 h dark) or short-day (10 h light 14 h dark) photoperiods for 4 weeks. Exposure to short day lengths never caused a decrease in GnRH expressing neurons and actually was associated with an increase in the number of radiolabelled cells specifically in the diagonal band of Broca/medial septum in the gonadally intact group. The mean number of grains per labelled cell for the short day animals similarly was not reduced from that seen in long day animals. The results are consistent with previous studies on photoperiod and GnRH content in the same brain regions and support the notion that the suppression of the synthesis of GnRH does not accompany the low levels of LH secretion observed during the early stages of reproductive quiescence in this species.