Differentiation of Brain Angiotensin Type 1a and 1b Receptor mRNAs

The objective was to examine the effect of dehydration on the expression of the angiotensin type 1 (AT(1)) receptor subtype mRNAs in mice by using an in situ hybridization method. The method used free-floating brain sections with (35)S-labeled probes specific for the untranslated 5' (AT(1a)) and 3' (AT(1b)) regions. AT(1a) and AT(1b) mRNA levels in the subfornical organ (SFO) and anterior third ventricle (AV3V) were quantified by using a phosphor-imaging system. Emulsion autoradiography with cresyl violet counterstaining was used to show cellular expression. Adult male C57BL mice (25 to 30 g) were given water ad libitum or were deprived of water for 48 hours. Dehydration produced increases in plasma osmolality (349+/-6 versus 314+/-4 mOsm/kg) and hematocrit (58+/-2% versus 47+/-1%). In situ hybridization showed that there was expression of AT(1a) and of AT(1b) mRNA in SFO and AV3V. Dehydration produced an increase in AT(1a) mRNA in both regions, with no changes noted for AT(1b). AT(1a) mRNA was increased in the AV3V region from 0.3+/-0.2 to 0.7+/-0.2 muCi/g and in the SFO from 0.6+/-0.3 to 1.0+/-0.2 muCi/g. These results provide information regarding the localization and physiological importance of a subset of angiotensin receptors that are important in volume and blood pressure regulation. AT(1a) and AT(1b) mRNAs showed a similar pattern of expression in rostral forebrain osmosensitive regions. However, osmotic/volume stimulation with dehydration produced specific activation of AT(1a) receptors. This verifies the role of AT(1a) receptors in volume regulation but raises a question concerning the physiological role of the AT(1b) subtype.