Ethanol counteraction of clonidine-evoked inhibition of norepinephrine release in rostral ventrolateral medulla of rats.

Previous studies from our laboratory demonstrated an antagonistic hemodynamic interaction between ethanol and clonidine in conscious and in urethane-anesthetized rats. The present study tested the hypothesis that ethanol produces its effect by counteracting clonidine-evoked inhibition of norepinephrine (NE) release at its major site of action, the rostral ventrolateral medulla (RVLM). In vivo electrochemical measurement of real-time changes in NE level in the RVLM of urethane-anesthetized Sprague-Dawley rats was made along with blood pressure and heart rate. Clonidine (30 microg/kg, i.v.) produced significant decreases (p < 0.05) in NE electrochemical signal and blood pressure. Ethanol (1 g/kg, i.v.) administered 10 min after clonidine significantly (p < 0.05) increased NE signal and counteracted clonidine-evoked hypotension. Equal volume of saline had no effect on NE signal in the RVLM nor on the hypotensive response to clonidine. Pretreatment with the same dose of ethanol (1 g/kg) caused slight increases in RVLM NE level and in blood pressure, but did not influence the electrochemical and blood pressure responses to clonidine; clonidine (30 microg/kg) administration 10 min after ethanol resulted in significant (p < 0.05) decreases in NE signal and blood pressure. These findings suggest that: (i) ethanol counteraction of the hypotensive action of clonidine involves, at least in part, opposite effects on central pathways that use NE as a neurotransmitter; (ii) the RVLM represents a possible site for the adverse hemodynamic interaction between ethanol and clonidine; and (iii) ethanol-evoked increase in RVLM NE, which correlates with its pressor effect, is much enhanced when RVLM NE level is reduced by clonidine.