Nicotinic mechanisms involved in the dopamine activating and reinforcing properties of ethanol

Ethanol shares with all major dependence producing drugs the ability to activate brain mesocorticolimbic dopamine neurons, an important part of the brain reward systems. This dopamine activation may be involved in mediating the positive reinforcing effects of ethanol. The mechanisms of action of ethanol in its activation of this dopamine system remain, however, to be elucidated. A selective pharmacological interference with these mechanisms may offer a possibility to reduce the reinforcing properties of ethanol without simultaneously interfering with the reinforcing properties of natural rewards. Ethanol has been shown to directly influence the function of various ligand-gated ion-channels. Several of these are located on or nearby mesocorticolimbic dopamine neurons. One such receptor is the nicotinic acetylcholine receptor (nAChR). The present article reviews a series of investigations aimed at investigating whether nAChRs are involved in the dopamine activating and reinforcing properties of ethanol. To this end acute and chronic behavioral and neurochemical experiments were performed in mice and rats. The results obtained indicate that central nAChRs in the ventral tegmental area are involved in mediating the mesolimbic dopamine activating and reinforcing effects of ethanol. Furthermore, the ethanol-induced activation of these receptors is probably indirect, subsequent to a primary interference of ethanol in the nucleus accumbens. Moreover, subchronic nicotine treatment enhances the reinforcing and dopamine activating properties of ethanol. This long-term effect may, however, derive from autonomic adaptations in response to intermittent blockade of peripheral nAChRs (rather than from intermittent stimulation of central receptors), and appears to be associated with development of a disinhibitory behavior that could involve also other neurotransmittors, e.g. serotonin. Taken together, these findings could provide a neurobiological explanation to the often observed co-abuse of nicotine and ethanol in man. Furthermore, since the behavioral models applied previously have predicted therapeutic drug effects in the clinic, the results suggest that selective blockade of the ventral tegmental nAChRs that are involved in the above effects may provide a new pharmacological alternative in the treatment of alcoholism.