The Effect of Anteroventral Third Ventricular Lesions on the Changes in Cholecystokinin Receptor Density in the Rat Supraoptic Nucleus Following Saline Drinking

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Abstract
Autoradiography and computerized image analysis were used to study the density of Cholecystokinin binding sites in the supraoptic nucleus of sham‐lesioned and anteroventral third ventricle (AV3V)‐lesioned animals in which the magnocellular system had been activated by salt‐loading with 2% saline for 48 h. Rats were maintained in metabolic cages for 5 to 7 days prior to a sham‐ or AV3V‐lesioning procedure, and the ratio of sodium intake:urinary sodium output used as a measure of sodium excretion. Following the sham or lesion procedure half of the rats had their drinking water replaced with 2% saline and the other half were maintained on normal drinking water. Neurohypophysial hormone levels were measured by specific radioimmunoassay in trunk blood samples taken 48 h after the saline or water treatment. The AV3V‐lesioned group of animals were characterized by an inability to excrete the excess sodium load and by a failure to increase secretion of both oxytocin and vasopressin into the general circulation in response to the salt‐stimulus. Despite this inappropriate response, [125 l]cholecystokinin octapeptide binding in the oxytocin‐rich dorsal portion of the supraoptic nucleus was similarly elevated in both sham‐ and AV3V‐lesioned rats following 2 days of saline treatment. These results suggest that the magnocellular oxytocin system is capable of responding to an osmotic stimulus even when the release of hormone has been severely impaired.