Role of plasma membrane estrogen receptors in mediating the estrogen induction of progesterone receptors in hypothalamic ventromedial neurons.

Progesterone is well known for its role in the modulation of sexual behavior. In the ventromedial nucleus (VMN), a part of the mediobasal hypothalamus that regulates sexual behavior in female rodents, estrogens induce the expression of progesterone receptors (PRs). This effect is known to be dependent on the activation of nuclear estrogen receptors (ERs). However, recent studies have documented estrogen activation of genomic transcription triggered by protein-protein phosphorylation cascades initiated at membrane receptors. The aim of this study was to examine if membrane-initiated estradiol (E2 ) stimulation is able to induce PR expression in the VMN or, at least, to modulate nuclear ER action. To achieve this goal, 2-month-old ovariectomized Wistar rats were injected bilaterally, in the vicinity of VMN, with free E2 and with E2 conjugated with bovine serum albumin (E2 BSA), alone or in sequence, by using a two-pulse injection paradigm. Stereological methods and western blot analysis were used to estimate the total number of PR-immunoreactive neurons in the VMN and the PR protein content of the VMN, respectively. The results showed that the administration of E2 BSA alone increases the number of PR-immunoreactive neurons and the expression level of PR protein to values similar to those resulting from E2 administration. They also showed that the sequential administration of E2 and E2 BSA potentiates the effects resulting from the injection of E2 or E2 BSA alone. These data provide the first evidence that membrane-initiated E2 stimulation is able to induce and to potentiate the genomic activation of PR expression in the VMN.