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Abstract

Monoiodination of the V₁ vasopressin antagonist [Mca¹,Sar⁷]AVp did not alter its high-affinity binding to liver plasma membranes. Monoradioiodinated [Mca¹,¹²⁵I-Tyr²,Sar⁷]AVP was therefore used to label V₁-specific binding sites in the rat brain and kidney. The accumbens nucleus, the septal nucleus, the central amygdala, the bed nucleus of the stria terminalis, the stigmoid hypothalamic nucleus and the nucleus of the solitary tract exhibited specific labeling with both the radioiodinated V₁ antagonist and tritiated AVP. Of the circumventricular structures only the choroid plexi and the area postrema showed V₁-specific binding sites. The subfornical organ and hypothalamic loci of AVP synthesis such as the paraventricular nucleus, the supraoptic nucleus and the suprachiasmatic nucleus were not marked by the V₁ antagonist while bearing [³H]AVP binding sites. As demonstrated by HPLC and binding to liver plasma membranes, the radiolabeled antagonist remained intact during tissue incubation. In addition to renal cortical and medullary [³H]AVP binding sites, medullary tubular and vascular structures could be labeled with the V₁ antagonist, indicating the presence of both V₁ and V₂ AVP receptor subtypes in the rat kidney.