Receptors and Actions of Corticotropin-Releasing Hormone in the Primate Pituitary Gland

Abstract

Receptors for CRH were identified in the pituitary gland of several primate species, and their binding characteristics were compared to the ability of CRH to elicit ACTH and cAMP responses in vitro. Autoradiographic analysis of the binding of [125I]Tyr-ovine CRH to frozen pituitary sections revealed CRH receptors in the intermediate and anterior lobes of human, marmoset, and cynomolgus monkey pituitaries. In the cynomolgus monkey, a high density of CRH receptors was present throughout the anterior and intermediate lobes. In the human pituitary, binding was concentrated in the anteromedial portion of the gland, whereas in the marmoset, binding was dense in the intermediate lobe and scattered as clusters throughout the anterior lobe. In membrane-rich fractions from the cynomolgus pituitary binding of [125I]Tyr-ovine CRH was time and temperature dependent, and was specific for CRH-related peptides; specific binding was increased by divalent cations and inhibited by guanyl nucleotides. Scatchard analyses of the binding data revealed a single class of high affinity sites [Kd, 1.93 +/- 0.23 (+/- SEM) nM], with a receptor concentration of 605 +/- 121 fmol/mg. In marmoset pituitary membranes, there were fewer receptors (200 +/- 15 fmol/mg), in agreement with the lower autoradiographic density of CRH binding. In anterior pituitary cell cultures from cynomolgus monkeys, CRH caused a dose-dependent stimulation of cAMP production and ACTH release, with half-maximum effective concentrations in the range of the CRH receptor affinity. Vasopressin and norepinephrine stimulated ACTH release to a much lesser extent, but both potentiated the stimulatory effect of CRH. Angiotensin II had no effect alone, but it also potentiated the effect of CRH. These data demonstrate the presence of CRH receptors in the primate pituitary, with characteristics similar to those in other species in their binding properties, coupling to adenylate cyclase, and functional interactions with other regulators of ACTH secretion that mediate the stimulatory effect of the peptide in the corticotroph.