Unmasking the Progesterone Receptor in the Preoptic Area and Hypothalamus of the Ewe: No Colocalization with Gonadotropin-Releasing Neurons

Progesterone powerfully inhibits GnRH secretion in ewes, as in other species, but the neural mechanisms underlying this effect remain poorly understood. Visualization of the neural ovine progesterone receptor has proved elusive but, using a high temperature antigen unmasking technique, the progesterone receptor was revealed in the ewe brain. Progesterone receptors were located in the preoptic-hypothalamic continuum, especially in the preoptic area, ventrolateral region of the ventromedial nucleus and the arcuate nucleus. This study also suggests that the inhibitory action of progesterone on GnRH release is not transduced directly through the GnRH neurons as a single GnRH perikaryon of 732 was immunoreactive for the progesterone receptor.