Neuronal activity in the ventral tegmental area (VTA) is proposed to be involved in alcohol preference and reward mechanisms by regulating mesolimbic dopamine (DA) release. We are studying the excitability mechanisms in freshly isolated VTA neurons from rat brain using the whole-cell patch-clamp recording method at a membrane holding potential of -60 mV. On the basis of their electrical properties, VTA neurons were classified into 3 cell types. Neurons with high input resistance, hyperpolarization-activated inward current (Ih), and an outward tail-current were classified as Type A neurons. On the basis of previous studies, these are thought to be dopaminergic neurons. Neurons in which hyperperpolarizing steps elicited no Ih current but did elicit an outward tail-current were classified as Type B neurons, and neurons in which hyperperpolarizing steps elicited neither an Ih current nor a tail-current were classified as Type C neurons; these may be non-dopaminergic neurons. The neurons differed in their pattern of action potential generation. In all 3 cell types, spontaneous inhibitory postsynaptic currents (sIPSCs) were recorded in the presence of the glutmate and glycine antagonists, AP5, CNQX and strychnine. In addition, the sIPSCs were blocked by the GABAa receptor antagonist, bicuculline, indicating that the sIPSCs are mediated by the spontaneous release of GABA acting on postsynaptic GABAa receptors. In 46% of the Type A neurons, 5-HT and the specific 5-HT3 receptor agonist, mCPBG, evoked fast, desensitizing, inward current without affecting sIPSC amplitude or frequency. Similar results were obtained from 41% of Type B neurons and 36% of Type C neurons. In 12% of Type B neurons, 5-HT and mCPBG elicited presynaptic facilitation of GABA release without a postsynaptic effect. No neurons were observed with both pre- and postsynaptic 5-HT3 receptor-mediated responses. The results suggest that for dopaminergic neurons, 5-HT3 receptors, when present, are located on the soma-dendritic region. On the other hand, for non-dopaminergic neurons, 5-HT3 receptors, when present, may be postsynaptic or presynaptic on GABAergic terminals.
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