5-HT2 receptor immunoreactivity on cholinergic neurons of the pontomesencephalic tegmentum shown by double immunofluorescence.

The serotonin-2 (5-HT2) receptor subtype is implicated in several behavioral and physiological processes, and may be the site of action of hallucinogens and certain psychotherapeutic drugs. To better understand the function and regulation of 5-HT2 receptors, it is necessary to determine the specific brain regions and cell types expressing them. By double immunofluorescence using a polyclonal antibody raised against the rat 5-HT2 receptor in conjunction with an antibody against choline acetyltransferase (ChAT), the synthetic enzyme for acetylcholine, we have shown that cholinergic neurons in the rat laterodorsal and pedunculopontine tegmental nuclei express 5-HT2 receptors. In contrast, there was little co-localization of 5-HT2 and ChAT immunoreactivity in neurons of the basal forebrain or striatum, even though the 5-HT2- and ChAT-positive cells in these regions overlapped extensively. These findings are discussed in relation to the potential interaction between cholinergic and serotonergic systems in sleep regulation, hallucinogenesis and the pathophysiology of neuropsychiatric disorders.