Alpha-7 nicotinic receptor expression by two distinct cell types in the dorsal raphe nucleus and locus coeruleus of rat

The alpha7 nicotinic acetylcholine receptor (nAChR) subunit can be assembled to form a homomeric-pentamer with high permeability to calcium. Although the expression of the alpha7-nAChR has been demonstrated throughout the CNS, the neurochemical phenotype of neurons expressing alpha7 remains to a large extent unknown. Using an antibody against the alpha7 nAChR subunit, immunohistochemical staining was observed in rat dorsal raphe nucleus (DRN) and locus coeruleus (LC), serotonergic and noradrenergic brainstem nuclei, respectively. In both the DRN and LC, there appeared to be two histologically distinct alpha7-expressing cell types as distinguished by size, i.e. large versus small diameter. In rats treated with either a serotonergic (5,7-dihydroxytryptamine) or noradrenergic (anti-dopamine-beta-hydroxylase saporin neurotoxin, tryptophan hydroxylase and tyrosine hydroxylase immunostaining was abolished, respectively. Similarly, the alpha7-positive large-diameter cells were no longer detectable, suggesting that these cells were serotonergic DRN and noradrenergic LC neurons. Indeed, double-labeling experiments revealed in the large cell types coexpression of alpha7 with tryptophan hydroxylase in the DRN and with tyrosine hydroxylase in the LC of saline-treated rats. In contrast to the large-diameter cells, the alpha7-positive small-diameter cells were neither serotonergic nor adrenergic, and were still detected in both the DRN and LC of lesioned rats. Moreover, cell counts revealed an increase number of these cells in lesioned rats with expression of alpha7 in somal processes not seen in non-lesioned controls. Double labeling revealed coexpression of alpha7 and GABA within the majority, but not all, of the toxin-resistant cells. The results of these studies suggest that both serotonergic and noradrenergic neurons express alpha7 nAChRs. In addition, there appears to be a small-diameter cell-type in both the DRN and LC, possibly a GABAergic interneuron, expressing alpha7 that may be regulated by neurotoxic injury. (C) 2002 Elsevier Science B.V. All rights reserved.