The distribution of progesterone receptor immunoreactivity and mRNA in thepreoptic area and hypothalamus of the ewe: Upregulation of progesterone receptor mRNA in the mediobasal hypothalamus by oestrogen

The distribution of progesterone receptors (PR) was mapped in the hypothala mus of the ewe using immunocytochemistry, These results were confirmed usin g in situ hybridization with a sheep-specific S-35-labelled riboprobe. In a ddition, the effect of oestrogen on the level of PR mRNA in the hypothalamu s was examined in ovariectomized (OVX) ewes following treatment with an oes trogen implant or without treatment. PR immunoreactive (-ir) cells were rea dily detected in OVX animals. Labelled cells were observed in four main hyp othalamic regions: the preoptic area (POA), including the organum vasculosu m of the lamina terminalis, periventricular nucleus (PeVN), ventromedial nu cleus (VMN) and the arcuate nucleus (ARC) (including the region Ventral to the mamillary recess). In addition, lightly stained PR-ir cells were observ ed in the supraoptic nucleus and a few PR-ir cells were also found in the d iagonal band of Broca. No PR-ir cells were found in the brainstem. PR mRNA- containing cells were found in the same hypothalamic regions as the PR-ir c ells. Image analysis of emulsion-dipped slides following in situ hybridizat ion indicated that oestrogen treatment increased (P < 0.01) the mean number of silver grains/cell and the density of labelled cells in the VMN and ARC but had no effect on the level of PR mRNA expression in the POA or PeN. Th e distribution of PR-containing cells in the hypothalamus is similar to tha t described in other species and all cells were located in nuclei that cont ain large populations of oestrogen receptor-containing cells. These include regions implicated in the regulation of reproductive neuroendocrine functi on, and reproductive behaviour. Oestrogen and progesterone synergize to inh ibit GnRH secretion and the present results suggest that these functions ma y involve cells of the VMN and ARC, with oestrogen acting to upregulate PR.