In the raphe nuclei the vast majority of serotonergic neurons express strong α3 subunit immunoreactivity but are devoid of α1 subunit staining. In contrast, both the α1 and the α3 subunits are expressed in GABAergic neurons (Gao and Fritschy, 1993). In other studies it was demonstrated that 84–95% of the cholinergic neurons in the basal forebrain expressed the α3 subunit but not the α1 subunit. In contrast, parvalbumin-positive GABAergic neurons in these brain regions were frequently co-stained with the α1-subunit and to a lesser extent with the α3 subunit antibody (Gao et al., 1995). The α3 subunit, however, not only is associated with serotonergic or cholinergic neurons, but also with noradrenergic and dopaminergic neurons in the brainstem (Gao et al., 1995). In addition, data indicating an overlapping distribution of α3, θ, and ε GABAA receptor subunits in the dorsal raphe and the locus coeruleus (Moragues et al., 2000; Moragues et al., 2002), suggest that novel GABAA receptor subtypes that so far have not been studied in detail, may regulate neuroendocrine and modulatory systems in the brain.
