Autoradiographic localization and characterization of circumventricular angiotensin II receptors in duck brain

Binding of [125I-5Val]angiotensin II and [125I-1Sar, 8Ile]angiotensin II to target sites in the hypothalamus of the Pekin duck was determined by quantitative receptor autoradiography and conventional membrane binding techniques. Circumventricular areas involved in body fluid homeostasis like the subfornical organ (SFO), median eminence and the anterior-ventral region of the third ventricle showed highest labeling density. Binding sites for angiotensin II were also found in the paraventricular and supraoptic nuclei. The physiological relevance of the labeled SFO angiotensin II receptor is indicated by similarities between rank orders of potency of angiotensin II analogues in displacing radiolabeled ligands and their physiological osmoregulatory potencies. Receptor density in the SFO of saltwater-acclimated ducks was increased 3-fold compared to non-acclimated freshwater ducks, indicating an up-regulation of the angiotensinergic system in ducks under conditions of dehydration or high salt intake.