Thyroid hormone receptor (alpha) distribution in hamster and sheep brain: colocalization in gonadotropin-releasing hormone and other identified neurons.

Abstract

Thyroid hormones appear to play an important role in the seasonal reproductive transitions of a number of mammalian and avian species. These seasonal transitions as well as the effects of thyroid hormones on the reproductive neuroendocrine axis are mediated by the GnRH system. How thyroid hormones affect the GnRH system is unclear. Double label immunocytochemistry was used to examine GnRH- and other neurotransmitter/neuropeptide-containing neurons for thyroid hormone receptor (alphaTHR) colocalization in two seasonal breeders, the golden hamster and the sheep. AlphaTHR was identified in hamster and sheep brain by Western blot analysis. Furthermore, alphaTHR immunoreactivity was widely distributed in brain and was colocalized in identified populations: GnRH neurons (hamster, 28%; sheep, 46%); dopaminergic neurons of the A14 (hypothalamic) and A16 (olfactory bulb) cell groups, but not in the hypothalamic A13 cell group; and neurophysin-immunoreactive neurons of the supraoptic and paraventricular nuclei. The finding of alphaTHR in GnRH and A14 dopamine neurons provides an anatomical substrate for direct thyroid hormone action on the reproductive neuroendocrine system of these two seasonally breeding species. It remains to be determined whether the GnRH gene itself or the gene of another constituent within the same GnRH neuron is responsive to thyroid hormones.