Effect of long-term ethanol feeding on brainstem α2-receptor binding in Wistar–Kyoto and spontaneously hypertensive rats

Our previous studies have shown that ethanol attenuates baroreflex function in Wistar–Kyoto (WKY) but not in spontaneously hypertensive rats (SHRs). The present study determined the effects of chronic ethanol administration on α₂-binding sites in brainstem areas that modulate baroreflexes. In vitro autoradiography was utilized to evaluate the effect of a 3-month ethanol feeding on the density (B_max) and affinity (K_D) of α₂-adrenoceptors in the middle (mNTS) and rostral (rNTS) portions of the nucleus tractus solitarius of SHRs and WKY rats. Autoradiographic examination of brainstem sections preincubated with [¹²⁵I]p-iodoclonidine revealed no inter-strain differences in α₂-binding in control rats. Ethanol feeding caused strain-dependent changes in α₂-binding activity, which comprised significant (P<0.05) decreases in the density of α₂-binding sites in both areas of the NTS in SHRs versus no effect in WKY rats. These findings do not favor a role for brainstem α₂-adrenoceptors in ethanol-induced attenuation of baroreflexes. Interestingly, the ethanol-evoked reduction in the NTS α₂-receptor density in SHRs may explain reported findings that ethanol abolishes the hypotensive effect of the α₂-adrenoceptor agonist clonidine in this rat model.