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.