Hypothalamic luteinizing-hormone-releasing hormone (GnRH) and gonadotropin-releasing-hormone-associated peptide (GAP) biosynthesis and storage were estimated by immunocytochemistry in male golden hamsters maintained in different photopenods. Intact or castrated male hamsters with subcutaneously inserted testosterone implants were exposed to long-day (14:10) or short-day photopenods (10:14) for 4-8 weeks. Exposure to short photoperiod for 4 weeks, an interval characterized by a suppression of gonadotropin secretion but not gonadal regression, was associated with an increase in the number of GnRH- and GAP-immunoreactive cells in the diagonal band of Broca/medial septum. Furthermore, morphometric analysis revealed that these animals displayed significantly more GnRH but not GAP immunoreactivity in the median eminence as opposed to hamsters exposed to long-day photopenods. In additional studies, gonadally regressed hamsters exposed to short day lengths for 8 weeks had equal numbers of GnRH cells as did the long-day controls. These patterns suggest that reproductive quiescence in golden hamsters is not the result of depletions of neuronal GnRH stores available for secretion.

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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.

In an effort to understand the potential neuroendocrine mechanisms underlying photoperiodic control of fertility in seasonally breeding species, we monitored the intracellular processing and nuclear uptake of [1α,2α-³H]testosterone (³H-T) within the brain-pituitary complex as well as the patterns of episodic luteinizing hormone (LH) secretion in male golden hamsters exposed to long day (14 h light: 10 h dark) and short day (10 h light: 14 h dark) photoperiods. Target tissue specific patterns of nuclear ³H-androgens and estrogens were observed in castrated, T-replaced hamsters exposed to long and short days for 7 weeks or longer. Significantly, ³H-T metabolism or receptor-mediated nuclear uptake in the hamsters in short days was not influenced in any manner that would explain their increased responsiveness to androgen feedback suppression of LH release. Comparable patterns of episodic LH secretion were observed in acutely catheterized hamsters castrated for 7 weeks prior to exposure to 8 weeks of long or short days. Similar patterns were also observed in animals maintained in long days and castrated 1 or 2 weeks prior to blood collection. However, such a pattern was not seen in acutely castrated hamsters maintained in the short-day photoperiod. The data suggest that steroid-independent mechanisms play an important role in suppressing gonadotropin release in short days in this species. However, such mechanisms appear to be most effective when the animals are or have recently been exposed to circulating androgens.

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