Regulation of intermediate pituitary corticotropin-releasing hormone receptors by dopamine.

Abstract

It has been shown that chronic cold exposure results in selective CRH receptor up-regulation in the intermediate pituitary. Since the intermediate pituitary is under dopaminergic control, the participation of a dopaminergic mechanism in the effect of cold stress was studied in rats treated with dopaminergic agonists and antagonists. CRH receptors were measured by the binding of radioiodinated Tyr-ovine (o) CRH to neurointermediate pituitary membranes of slide-mounted sections. Cold exposure for 60 h caused the expected increase in CRH binding in neurointermediate lobe membranes. Administration of the dopaminergic agonist bromocriptine did not prevent the effect of cold stress, but increased CRH binding in control rats. The dopaminergic antagonist metoclopramide decreased intermediate pituitary CRH binding in control and cold-exposed rats. Bromocriptine administration for 1-8 days caused a progressive increase in the binding of [125I]Tyr-oCRH in neurointermediate pituitary membranes, despite atrophy of the intermediate zone. Scatchard analysis of the binding data indicated that the changes were due to variations in receptor concentration, without changes in affinity. No changes in anterior pituitary CRH receptors were observed with agonist or antagonist treatment. Autoradiographic analysis of CRH binding after 3 days of treatment with bromocriptine or haloperidol confirmed the results observed in membranes and demonstrated that changes in binding were confined to the intermediate lobe. The functional consequences of the changes in CRH binding were studied by analysis of adenylate cyclase activity in cells and homogenates of intermediate pituitaries of rats treated with bromocriptine. In 18-h cultured intermediate pituitary cells from rats treated with bromocriptine for 3 days, CRH-stimulated cAMP production, measured in the presence of phosphodiesterase inhibitors, was increased to levels only slightly higher than those in cells from control rats. Likewise, CRH-stimulated adenylate cyclase, measured by conversion of [32P]ATP to [32P] cAMP, was not significantly different in homogenates from microdissected intermediate lobes from control and bromocriptine-treated rats. The lack of parallel changes in adenylate cyclase responsiveness suggests only partial receptor coupling, probably reflecting an inhibitory effect of dopamine on components of the adenylate cyclase. This study demonstrates that in contrast to the recognized inhibitory effect on cell division and POMC mRNA expression, dopamine causes up-regulation of CRH receptors in the intermediate pituitary. The qualitatively similar and nonadditive effects of cold stress and dopaminergic agonists suggest that a dopaminergic mechanism may be involved in intermediate pituitary CRH receptor regulation during chronic cold stress.