5-HT2A receptor-like protein is present in small neurons located in rat mesopontine cholinergic nuclei, but absent from cholinergic neurons

The cholinergic neurons of the pedunculopontine and laterodorsal tegmental nuclei (PPN and LDN) increase their activity during wakefulness and REM sleep and interact with brainstem neurons containing serotonin (5-HT) and other amines. To determine whether mesopontine neurons that contain nitric oxide synthase (NOS), a marker for cholinergic cells, express 5-HT2A receptors, dual immunostaining for 5-HT2A receptor-like protein and NOS was employed with either peroxidase or fluorescent secondary probes. Within the PPN and LDN, different cells expressed 5-HT2A receptors and NOS. In addition to the lack of co-localization, the 5-HT2A receptor-expressing cells were smaller and less numerous than the adjacent NOS neurons. We propose that 5-HT2A receptor-expressing cells are local inhibitory interneurons whose one function is to ensure the reciprocal patterns of activity in subpopulations of mesopontine cholinergic and aminergic neurons. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.