ANGIOTENSIN BINDING AND PRESSOR ACTIVITY IN THE RAT VENTRICULAR SYSTEM AND MIDBRAIN

Specific angiotensin II (AII) binding capacity was investigated in regions of rat brain thought to be involved in the central pressor effects of AII. In the midbrain specific angiotensin II binding capacity was principally localised in the superior colliculi. When the superior colliculi was divided into rostral and caudal portions, the caudal portion had a higher binding capacity than the rostral portion. In the hypothalamus AII binding capacity was only a third of that in the superior colliculi and was localised principally in the lateral hypothalamus. The medial hypothalamus and the preoptic recess each had about a quarter of the binding capacity of the lateral hypothalamus. To investigate whether AII binding capacity could be correlated with pressor activity AII was applied locally (local) to the surface of the superior colliculi (5 μg/5 μl saline) of rats with femoral artery cannulas and anaesthetised with pentobarbitone. These responses were compared with responses to the same dose given intraventricularly (IVT). When the surface of the midbrain was intact there was a pressor response to the local application not significantly different in magnitude and latency from the response to IVT AII. The midbrain was aspirated at the site of AII application and pressor responses to AII evoked during aspiration. As the midbrain lesion was extended the latency of the local pressor response was prolonged (P < 0.05) but its magnitude did not change. Both local and IVT AII became ineffective after extensive aspiration in 5 out of 9 rats. Histological examination of the brains of these 9 rats showed that the central pressor responses to AII depended on the integrity of a discrete region of the midbrain and periaqueductal gray which included the caudal region of the superior colliculi. © 1979.