Ethanol Suppresses Fast Potentiation of Glycine Currents by Glutamate

Abstract

Excitatory (glutamate) and inhibitory (GABA_Aand glycine) receptor/channels coexist in many neurons. To assess effects of ethanol on the interaction of glutamate and glycine receptors, glycine-induced current (I_Gly) was recorded by a whole-cell patch-clamp technique from neurons freshly dissociated from the ventral tegmental area of rats. A conditioning prepulse of glutamate (1–3 s, 1 mM) significantly and reversibly potentiated responses to a pulse of glycine. This potentiation was increased when extracellular calcium was raised to 12 mM and reduced by including 10 mM 1,2-bis-(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid in the internal recording medium. It was not affected by 5 μM 1-N,O-bis-(5-isoquinolinesulfonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62), a selective inhibitor of calcium/calmodulin-dependent protein kinase II. In a concentration-response analysis, a conditioning pulse of glutamate significantly lowered the EC₅₀ for glycine and increased the maximal I_Gly. Kinetic analysis of the currents indicated that glutamate slowed deactivation of glycine-gated chloride channels; therefore, glutamate may increase the affinity of glycine receptors for glycine. When coapplied with glycine, ethanol (10 mM) potentiated I_Gly in 35% of neurons from the ventral tegmental area. In contrast, when coapplied with glutamate and glycine, ethanol suppressed the glutamate-induced potentiation of I_Gly in these neurons. This suppression was also observed when ethanol and glycine were coapplied after a glutamate prepulse. A similar effect was observed when ethanol alone did not potentiate I_Gly. These findings suggest that glutamate-induced calcium influx modulates glycine receptors by a mechanism that can be blocked by ethanol.