Angiotensin II receptor subtype distribution in the rabbit brain

Previous work has reported that the distribution of AT(1) binding sites in the rabbit brain is similar to that in the rat, but AT(2) binding sites are confined to the septum and cerebellum of the rabbit brain. This receptor autoradiographic study was designed to enhance the detection of angiotensin II binding sites by using greater radioligand concentrations, and to survey the midbrain in more detail than in previous studies. Tissue sections from five rabbit forebrains, three midbrains, and three hindbrains were incubated with 520 pM (125)I-sar(1)ile(8) angiotensin II. The results confirm abundant AT(1) binding in regions involved in cardiovascular and drinking regulation: the nucleus of the solitary tract, ventrolateral medulla, subfornical organ, organum vasculosum of the lamina terminalis, median eminence, and several hypothalamic structures. Novel AT(1) binding sites were discovered in the pituitary, retrorubral field, periolivary region, dorsolateral nucleus of the lateral lemniscus, dorsal raphe, and laterodorsal tegmental nuclei. The distribution of AT(1) binding was similar to the distribution of monoaminergic neurons. AT(2) binding was moderately dense and well visualized in the cerebellum. In contrast to the rat, AT(2) binding was not detected in the inferior olive of the rabbit, but lobe 9 of the cerebellum exhibited a banding pattern of AT(2) binding reminiscent of the pattern of neuronal projections from the inferior olive. It is possible that AT(2) protein is observed at different stages of axonal transport between the inferior olive and the cerebellum in the two species. Our results did identify new AT(2) binding sites in the superior colliculus and cerebral cortex, but it is clear that AT(2) binding in the rabbit brain is weak and is not as widely distributed as in the rat.