Dehydration reduces kappa-opiate receptor binding in the neurohypophysis of the rat.

Quantitative autoradiography was used to examine the effects of dehydration on kappa-opiate receptor binding in the neural lobe of the pituitary in rats. Dehydration was produced by 5 days of water deprivation or ingestion of 2% saline. Brattleboro rats homozygous for diabetes insipidus were used as a model of chronically disturbed water balance. Slide-mounted pituitary sections were incubated with [3H]bremazocine using kappa-receptor-selective assay conditions. Binding in the neural lobe was quantified by densitometry of film autoradiographs. Specific [3H]bremazocine binding in neural lobe sections of control rats was 45.4 fmol/mg wet weight. In contrast, binding in neural lobe sections of water-deprived, saline-treated, and water-sated homozygous Brattleboro rats was lower by 50%, 35% and 37%, respectively. We suggest that chronic dehydration elevates levels of endogenous neurohypophyseal dynorphin and produces a down-regulation of kappa-opiate receptors in the neurohypophysis, a change in affinity of the receptors for kappa-agonists, or both.