Oxytocin Excites Neurons Located in the Ventromedial Nucleus of the Guinea‐Pig Hypothalamus

The area of the ventromedial nucleus of the hypothalamus in the guinea-pig was shown in autoradiographs to contain high affinity binding sites for oxytocin. In order to ascertain whether these sites may represent neuronal receptors, single-cell extracellular recordings were obtained from ventromedial neurons in coronal slices of the hypothalamus of adult guinea-pigs. Oxytocin applied in the nanomolar range excited about half of the neurons tested; none were inhibited. The response to the peptide was reversible and concentration-dependent. It was exerted directly since it persisted under the condition of synaptic isolation. Moreover, the effect was specific since it could be mimicked by a selective oxytocin agonist and since vasopressin was usually at least 10-fold weaker than oxytocin. These findings suggest that the binding sites for oxytocin detected by light microscopic autoradiography in the guinea-pig hypothalamic ventromedial nucleus represent functional receptors.