Regulation of transmitter synthesis and release in mesolimbic dopaminergic nerve terminals: Effect of ethanol

Slices from rat olfactory tubercle were incubated in freshly oxygenated Krebs-Ringer phosphate (KRP) and in the presence of l-tyrosine [¹⁴C-U] as dopamine (DA) precursor. Thereafter, the newly synthesized [¹⁴C]DA and the [¹⁴C]DA released into the incubation media were isolated by Alumina column, and ion-exchange, chromatography. The presence of K⁺ depolarizing concentrations (25–70 mM) in the incubation media markedly increased the formation of [¹⁴C]DA from [¹⁴C]tyrosine, following a rather complex and biphasic pattern. Dibutyryl cyclic AMP (dB-cAMP) and theophylline also increased the formation of newly synthesized [¹⁴C]DA. Ethanol (0.2 to 0.4%. w/v significantly blocked the stimulation of [¹⁴C]DA biosynthesis that was induced by low K⁺ depolarizing concentrations (25 mM) and by dB-cAMP (5 × 10^?4M) or theophylline (1 × 10³M). In contrast, only higher ethanol concentrations (0.8 to 1.1%, w/v) blocked the increase in DA formation induced by high K⁺ depolarizing concentrations (40 and 55 mM). Potassium depolarization (40 mM) markedly evoked the release of newly synthesized [³H]DA or [³H]DA previously taken up by the slices. The release was shown to be dependent upon the presence of Ca²⁺ and inhibited by an excess of Mg²⁺ (12mM). Ethanol (0.8 to 1.1%, w/v) produced no effect on K⁺-induced release of [³H]DA. The model described in this paper can be used as a simple experimental tool to study neurotransmitter synthesis and release from nerve terminals belonging to the mesolimbic dopaminergic system. The results reported suggest the existence of at least two mechanisms by which neuronal depolarization increases transmitter formation in mesolimbic dopaminergic terminals. Ethanol, at relatively low concentrations, seems to produce a specific inhibitory effect upon the mechanism that predominates under low depolarizing conditions. The possibility is raised that the effects described for ethanol may play a role in the neuropharmacological responses induced by this agent in vivo.