The angiotensin receptor subtype AT1A predominates in rat forebrain areas involved in blood pressure, body fluid homeostasis and neuroendocrine control.

Subtypes of the angiotensin II (Ang II) type-1 (AT1) receptor are probably involved in distinct actions of the peptide, since their distribution in peripheral organs and regulation of their gene expression are different. We investigated the distribution of AT1A and AT1B receptor subtype mRNAs in the rat forebrain and pituitary using sensitive cRNA probes for in situ hybridization. High level of AT1A receptor mRNA expression is observed in the subfornical organ (SFO) and in the anterior hypothalamus, particularly the periventricular tissue surrounding the anterior portion of the 3rd ventricle (AV3V), which contains the organum vasculosum of the lamina terminalis (OVLT), the median preoptic nucleus and the preoptic periventricular nucleus as well as in the hypothalamic periventricular nucleus and in the parvocellular part of the paraventricular nucleus (PVN). Moderate to strong AT1A labeling was found in the anterior olfactory nucleus, the piriform cortex and the nucleus of the lateral olfactory tract. Very low AT1B receptor mRNA expression was found in the SFO and the PVN. In contrast, strong AT1B receptor mRNA expression coincided with low AT1A receptor mRNA expression in the anterior pituitary. Labeling was cytoplasmic at the light microscopic level. We thus suggest that the AT1A receptor is responsible for the central actions of Ang II in the rat forebrain whereas direct actions of Ang II on the anterior pituitary are mediated by the AT1B receptor subtype.