Cotinine selectively activates a Subpopulation of α3/α6β2*nicotinic receptors in monkey striatum

The nicotine metabolite cotinine is an abundant long-lived bioactive compound that may contribute to the overall physiological effects of tobacco use. Although its mechanism of action in the central nervous system has not been extensively investigated, cotinine is known to evoke dopamine release in the nigrostriatal pathway through an interaction at nicotinic receptors (nAChRs). Because considerable evidence now demonstrates the presence of multiple nAChRs in the striatum, the present experiments were done to determine the subtypes through which cotinine exerts its effects in monkeys, a species that expresses similar densities of striatal alpha 4 beta 2* (nAChR containing the alpha 4 and beta 2 subunits, but not alpha 3 or beta 6) and alpha 3/alpha 6 beta 2* (nAChR composed of the alpha 3 or alpha 6 subunits and beta 2) nAChRs. Competition binding studies showed that cotinine interacts with both alpha 4 beta 2* and alpha 3/alpha 6 beta 2* nAChR subtypes in the caudate, with cotinine IC50 values for inhibition of 5-[I-125]iodo-3-[2(S)-azetinylmethoxy]pyridine-2HCl([I-125]A-85380) and I-125-alpha-conotoxinMII binding in the micromolar range. This interaction at the receptor level is of functional significance because cotinine stimulated both alpha 4 beta 2* and alpha 3/alpha 6 beta 2* nAChR [H-3] dopamine release from caudate synaptosomes. Our results unexpectedly showed that nicotine evokes [H-3] dopamine release from two alpha 3/alpha 6 beta 2* nAChR populations, one of which was sensitive to cotinine and the other was not. This cotinine-insensitive subtype was only present in the medial caudate and was preferentially lost with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced nigrostriatal damage. In contrast, cotinine and nicotine elicited equivalent levels of alpha 4 beta 2* nAChR-mediated dopamine release. These data demonstrate that cotinine functionally discriminates between two alpha 3/alpha 6 beta 2* nAChRs in monkey striatum, with the cotinine-insensitive alpha 3/alpha 6 beta 2* nAChR preferentially vulnerable to nigrostriatal damage.