Receptors for Cor ticotropin-Releasing Hormone in Human Pituitary: Binding Characteristics and Autoradiographic Localization to Immunocytochemically Defined Proopiomelanocortin Cells

Using autoradiography combined with immunocytochemistry, we demonstrated that the target cells of CRH in the human pituitary were proopiomelanocortin cells. Scatchard analysis of [I-125]Tyr0-oCRH saturation binding revealed the presence of one class of saturable, high affinity sites on pituitary tissue homogenate. The equilibrium dissociation constant (K(d)) for [I-125]Tyr0-oCRH ranged from 1.1-1.6 nM, and the receptor density was between 200-350 fmol/mg protein. Fixation of cryostat sections with 4% paraformaldehyde before tracer incubation improved both tissue preservation and localization of the CRH receptor at the cellular level. Additional postfixation with 1% glutaraldehyde inhibited tracer diffusion during subsequent immunocytochemistry and autoradiography. [I-125]Tyr0-oCRH was found in cytoplasmic inclusions or at the cell periphery of ACTH/beta-endorphin cells in the anterior pituitary. Single cells of the posterior pituitary were also CRH receptor positive. Cells staining for PRL or GH were CRH receptor negative. We conclude that CRH binds only to high affinity receptors on ACTH/beta-endorphin cells in the human pituitary.