Angiotensin II binding sites in the hamster brain: localization and subtype distribution

This study was designed to characterize the distribution of angiotensin II (AII) binding sites in the hamster brain. Brain sections were incubated with [¹²⁵I][sar¹, ile⁸]-angiotensin II in the absence and presence of angiotensin II receptor subtype selective compounds, losartan (AAT, subtype) and PD123177 (AT₂ subtype). Binding was quantified by densoritometric autoradiograms and localized by comparison with adjacent thionin stained sections. The distribution of AII binding sites was similar to that found in the rat, with some exceptions. [¹²⁵I][sar¹, ile⁸]-angiotensin II binding was not evident in the subthalamic nucleus and thalamic regions, inferior olive, suprachiasmatic nucleus, and piriform cortex of the hamster, regions of prominent binding in the rat brain. However, intense binding was observed in the interpeduncular nucleus and the medial habenula of the hamster, nuclei void of binding in the rat brain. Competition with receptor subtype selective compounds revealed a similar AII receptor subtype profile in brain regions where binding is evident in both species. One notable exception is the medial geniculate nucleus, predominately AT₁ binding sites in the hamster but AT₂ in the rat. Generally, the AII binding site distribution in the hamster brain parallels that of the other species studied, particularly in brain regions associated with cardiovascular and dipsogenic functions. Functional correlates for AII binding sites have not been elucidated in the majority of brain regions and species mismatches might provide clues in this regard.