Background: The mesolimbic/mesocortical dopamine pathway mediates the rewarding effects of ethanol and other drugs of abuse like cocaine and opiates. Dopaminergic neurons of the ventral tegmental area (VTA) are the cells of origin of the mesolimbic/mesocortical dopamine pathway. Ethanol's rewarding properties result from its ability to excite dopaminergic cell bodies in the VTA which results in increased dopamine release in the nucleus accumbens. Many recent papers have speculated that ethanol excitation of dopaminergic VTA neurons is indirect, either that ethanol acts on GABAergic or other interneurons, which in turn modulate the activity of dopaminergic VTA neurons, or that ethanol modulates the action of neurotransmitter-gated ion channels in the VTA.
Methods: VTA neurons were acutely dissociated and plated onto a cover slip in an electrophysiological recording chamber. These neurons generated spontaneous action potentials which could be measured with cell attached loose patch recording. The dissociation procedure truncated the dendritic trees, severed synaptic contacts and widely dispersed these neurons. Dopamine (10-50 nM) and ethanol (20-120 mM) were bath applied and their effects on firing rate were measured. After some experiments, plated cells were fixed and processed for immunostaining of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis.
Results: All neurons met electrophysiological criteria previously established for dopaminergic VTA neurons. Dopamine inhibited all VTA neurons tested, indicating the presence of dopamine autoreceptors. All neurons identified as dopaminergic by these electrophysiological and pharmacological criteria, and that were processed for immunohistochemistry, stained positive for tyrosine hydroxylase immunoreactivity. All acutely dissociated VTA neurons, identified as dopaminergic by electrophysiological, pharmacological and immunohistochemical criteria, were robustly excited by behaviorally relevant concentrations of ethanol. The ethanol-induced excitation was concentration-dependent.
Conclusions: These data provide strong evidence that ethanol directly excites dopaminergic VTA neurons, as this excitation still occurs in the absence of input from surrounding neurons.