Changes in preoptic and hypothalamic levels of progesterone receptor mRNA across the oestrous cycle of the ewe

We measured the levels of progesterone receptor (PR) mRNA in the hypothalam us and preoptic area (POA) of the ewe across the oestrous cycle. Perfusion- fixed hypothalamic tissue was collected from sheep killed during the luteal and follicular phases and during behavioural oestrus, Blood samples taken at the time of tissue collection verified that the oestrous ewes were under going a preovulatory luteinizing hormone (LH) surge, Matched sections were taken from the POA, periventricular nucleus (PeVN), ventromedial nucleus (V MN) and arcuate nucleus of each animal, In situ hybridization was performed using a sheep specific, S-35-labelled riboprobe for PR and semiquantitativ e image analysis was conducted on emulsion-dipped slides. The number of sil ver grains per cell was greater in the VMN and arcuate nucleus of oestrous ewes than in luteal and follicular phase ewes; there was no cyclic variatio n in the level of PR mRNA expression in the cells of the POA and PeVN. The number of labelled cells per mm(2) in the VMN was higher in the oestrous ew es than in luteal phase and follicular phase ewes, The number of labelled c ells in the PeVN was also higher in the oestrous ewes than in follicular ph ase ewes, but there was no cyclic variation in the POA and arcuate nucleus. In the ewe, the onset of behavioural oestrus corresponds to the onset of t he preovulatory LH surge and increased PR mRNA expression at this time is l ikely to be due to the high concentrations of circulating oestrogen that pr ecede this period.