Ethanol potentiation of glycine-induced responses in dissociated neurons of rat ventral tegmental area

The potentiation of glycine-induced responses by ethanol (EtOH) was studied in neurons freshly dissociated from the ventral tegmental area (VTA) of 5- to 14-day-old postnatal rats using whole-cell and gramicidin-perforated patch-clamp techniques. Under current-clamp conditions, EtOH increased glycine-induced membrane depolarization and action potential firing. Under voltage-clamp conditions, EtOH (0.1-40 mM) alone did not elicit a current. When coapplied with glycine, EtOH enhanced the glycine-induced current in 35% (180 of 474) of the neurons. The EtOH-induced enhancement of glycine current was independent of membrane potential (between -60 and +60 mV); the reversal potential was not changed. Concentration-response analysis showed that in the presence of EtOH (10 mM), the EC₅₀ for glycine decreased from 25 ± 4 to 14 ± 3 μM; the Hill coefficient increased from 1.5 ± 0.2 to 1.9 ± 0.3. Kinetic analysis of glycine currents indicated that EtOH decreased the time constant of activation and increased the time constant of deactivation of glycine-gated chloride channels. EtOH may accelerate glycine association with its receptor at the agonist binding site and increase the apparent agonist affinity. Our observations suggest that, at pharmacologically relevant concentrations, EtOH alters the function of glycine receptors and thus the excitability of neonatal VTA neurons. This action of EtOH may contribute to the neurobehavioral disturbances associated with fetal alcohol syndrome.