Alpha 4 nicotinic acetylcholine receptor subunit links cholinergic to brainstem monoaminergic neurotransmission

Agonists of nicotinic receptors containing the α4-subunit produce antinociception accompanied by several adverse side effects. The purpose of this study was to determine the distribution of the α4-subunit of nicotinic acetylcholine receptors (nAChR) in brainstem monoaminergic nuclei that may contribute to these effects using dual labeling immunofluorescence methods. The α4-subunit immunoreactivity was enriched in serotonergic (nucleus raphe magnus, pallidus, obscurus, and dorsalis) and noradrenergic (A5, locus coeruleus (LC), A7) areas associated with antinociception, where it was commonly colocalized with serotonin (5-HT) or tyrosine hydroxylase (TH) immunoreactivity. However, it was also noted that α4 was present in all other brainstem monoaminergic nuclei examined (adrenergic C1-C 3, noradrenergic A1-α4, dopamine A9 and A1O, nucleus raphe medianus). To determine if α4 agonists could impact neural activity in brainstem, monoaminergic nuclei that are associated with antinociception, the expression of c-Fos in response to the systemic administration of epibatidine (2.5, 5, or 10 μg/kg) was examined. Epibatidine produced a robust (2-5-fold) increase in c-Fos expression, which was not dose dependent, in all of these areas examined except the nucleus raphe magnus. These results suggest that the α4 subunit is positioned to mediate the effects of acetylcholine widely across many, if not all, monoaminergic neurons in the brainstem. These observations emphasize the potential involvement of noradrenergic, as well as serotonergic mechanisms in epibatidine's analgesic effects, and they also suggest that even selective α4 ligand may have widespread effects on brain monoamine neurotransmission.