Ethanol Inhibition of Glycine-Activated Responses in Neurons of Ventral Tegmental Area of Neonatal Rats

Abstract

The brain is particularly sensitive to alcohol during the period of its rapid growth. To better understand the mechanism(s) involved, we studied ethanol effects on glycine-activated responses of ventral tegmental area (VTA) neurons isolated from the newborn rat, using whole cell and gramicidin perforated patch-clamp techniques. Previously we reported that 0.1-40 mM ethanol enhances glycine-induced responses of 35% of VTA neurons. We now direct our attention to the inhibitory effects of ethanol observed in 45% (312 of 694) of neonatal VTA neurons. Under current-clamp conditions, 1 mM ethanol had no effect on the membrane potential of these cells, but it decreased glycine-induced membrane depolarization and the frequency of spontaneous action potentials. Under voltage-clamp conditions, 0.1-10 mM ethanol did not elicit a current but depressed the glycine-induced currents. The ethanol-induced inhibition of glycine current was independent of membrane potential (between -60 and +60 mV). Likewise, ethanol did not alter the reversal potential of the glycine-activated currents. Ethanol-mediated inhibition of glycine current depended on the glycine concentration. While ethanol strongly depressed currents activated by 30 microM glycine, it had no appreciable effect on maximal currents activated by 1 mM glycine. In the presence of ethanol (1 mM), the EC(50) for glycine increased from 32 +/- 5 to 60 +/- 3 microM. Thus ethanol may decrease the agonist affinity of glycine receptors. A kinetic analysis indicated that ethanol shortens the time constant of glycine current deactivation but has no effect on activation. In conclusion, by altering VTA neuronal function, ethanol-induced changes in glycine receptors may contribute to neurobehavioral manifestations of the fetal alcohol syndrome.

What is in that Drink: The Biological Actions of Ethanol, Acetaldehyde, and Salsolinol

“…EtOH excitation in dopaminergic neurons of the rat has been shown to be blocked by quinidine, but not by blockers of h-current (Appel et al, 2003). Furthermore, EtOH excites individual VTA neurons dissociated from brain slices (Brodie et al, 1999;Ye et al, 2001), a preparation that would be expected to reduce the ability of synthesized ACD to act on these independent VTA neurons, and the concentration-response for EtOH excitation of dissociated dopaminergic VTA neurons (Brodie et al, 1999) is similar to that observed in brain slices (Brodie et al, 1990).…”

“…In general, the data reflect the complexity of EtOH within the CNS. There is evidence that EtOH can directly act at receptors and to stimulate VTA DA neurons (Brodie et al, 1999;Ye et al, 2001;Lovinger and White, 1991). Convergent evidence that both ACD and SAL have distinct actions within the mesolimbic dopamine system has recently been reported.…”

Single-Channel Analysis of Ethanol Enhancement of Glycine Receptor Function

“…Furthermore, concentrations of EtOH that enhance GlyR function in mouse spinal cord neurons (10 -200 mM) have no effects on membrane lipid order (Tapia et al, 1998), suggesting a protein site of action. Studies using dissociated ventral tegmental area neurons also demonstrate EtOH enhancement of glycine receptor function (Ye et al, 2001). Finally, EtOH increases glycine-mediated chloride uptake into rat brain synaptoneurosomes (Engblom and Akerman, 1991).…”

Synaptic Effects Induced by Alcohol

“…The most common EtOH action is to potentiate channel opening in the presence of a low concentration of agonist by increasing the probability of channel opening (Zhou et al 1998), and/or increasing agonist affinity (Tonner and Miller 1995;Welsh et al 2009). This potentiating effect can influence both synaptic and extrasynaptic receptors (Sebe et al 2003;Ye et al 2001;Eggers and Berger 2004;Ziskind-Conhaim et al 2003) (Fig. 1).…”

“…Inclusion of the β subunit along with α2 eliminates potentiation . Potentiation has also been observed in neurons from the brain and spinal cord, particularly in regions where the α1 subunit is expressed (Aguayo et al 1996;Ye et al 2001). Potentiation of the function of GABA A and glycine receptors is thought to increase inhibition of neurons.…”