Progesterone receptors as neuroendocrine integrators

Intracellular progesterone receptors (PRs) are ligand-inducible transcripti on factors that mediate the majority of the effects of progesterone (P) on neuroendocrine functions. During the past decade, evidence has accumulated which suggest that PRs can also be activated independently of P, by signals propagated through membrane-bound receptors to the interior of cells. The activation of PRs by this type of "cross-talk" mechanism has been implicate d in the physiological regulation of several important neuroendocrine proce sses, including estrous behavior and periovulatory hormone secretions. We r eview evidence that both ligand-dependent and ligand-independent activation of PRs occurs in central neurons and in anterior pituitary cells and that the convergence and summation of these signals at the PR serves to integrat e neural and endocrine signals which direct several critically important ne uroendocrine processes. An integrative function for PRs is reviewed in seve ral physiological contexts, including the display of lordosis behavior in f emale rodents, the neurosecretion of gonadotropin-releasing hormone surges, secretion of preovulatory gonadotropin surges, and release of periovulator y follicle stimulating hormone surges. The weight of evidence indicates tha t cross talk at the intracellular PR is an essential component of the integ rative mechanisms that direct each of these neuroendocrine events. The recu rrence of PR's integrative actions in several different physiological conte xts suggests that other intracellular steroid receptors similarly function as integrators of neural and endocrine signals in other neuroendocrine proc esses.