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Acute Tolerance to Ethanol Inhibition of NMDA-Induced Responses in RatRostral Ventrolateral Medulla Neurons

The present study was performed to examine the effects of acute ethanol exposure on N-methyl-D-aspartate (NMDA)-induced responses and the development of acute tolerance in rat rostral ventrolateral medulla (RVLM) in vivo and in vitro. Repeated microinjections of NMDA (0.14 nmol) into the RVLM every 30 min caused reproducible increases in mean arterial pressure in urethane-anesthetized rats weighing 325–350 g. Intravenous injections of ethanol (0.16 or 0.32 g, 1 ml) inhibited NMDA-induced pressor effects in a blood-concentration-dependent and reversible manner. The inhibitory effect of ethanol was reduced over time during continuous infusion of ethanol or on the second injection 3.5 h after prior injection of a higher dose of ethanol (0.32 g). A high dose of ethanol (0.32 g) had no significant effects on α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, γ-aminobutyric acid and glycine-induced changes in blood pressure. In vitro studies showed that ethanol (10– 100 mM) dose-dependently inhibited inward currents elicited by pressure ejection of NMDA (10 mM) in RVLM neurons of neonatal brainstem slice preparations. When the superfusion time of ethanol (100 mM) was increased to 50 min, its inhibitory effect decreased gradually after 30–40 min in 60% of RVLM neurons examined. These data suggested that ethanol inhibition and subsequent tolerance development is associated with changed sensitivity to NMDA in the RVLM, which may play important roles in the ethanol regulation of cardiovascular function.