Neurophysins, rather than Receptors, are Involved in [³H]Oxytocin and [³H]Vasopressin Binding Detected by Autoradiography in the Hypothalamo‐Neurohypophyseal System

The goal of the present experiments was to analyse the binding of oxytocin (OT) and vasopressin (VP) in the hypothalamo‐neurohypophyseal system to determine whether [³H]OT and [³H]VP binding in this system involved interaction with receptor sites or with neurophysins. Using quantitative autoradiography, several experiments were performed to compare [³H]OT‐ and [³H]VP‐binding characteristics in this system and in brain areas containing identified receptor sites. Saturation experiments indicated much lower affinity of [³H]OT and [³H]VP binding in the magnocellular nuclei and neural lobe than on brain receptors. Competition experiments using selective ligands indicated interaction with neurophysins rather than with receptors in the hypothalamo‐neurohypophyseal system. This system was never labelled in the presence of a [¹²⁵I]OT antagonist, a selective OT receptor ligand. In contrast with receptors elsewhere in the brain, the magnocellular nuclei were labelled by [³H]OT and [³H]VP in the absence of MgCI₂. In the pituitary neural lobe, density of binding sites was moreover obviously related to the amount of neurosecretory granules, as seen in acutely dehydrated rats. Taken together, these data strongly suggest that in the hypothalamo‐neurohypophyseal system [³H]OT and [³H]VP bind to neurophysins rather than to specific receptors.