Pontomedullary distribution of 5-HT2A receptor-like protein in the rat

Serotonin (5-HT) exerts excitatory effects in many brainstem regions involved in autonomic, somatic, motor, and sensory functions, and in control of vigilance. To determine the potential role of 5-HT2A receptors in these effects, we immunohistochemically mapped the distribution of 5-HT2A receptor-like protein in the rat pontomedullary brainstem. Areas containing the densest labeling included the trigeminal, facial, hypoglossal, dorsal vagal motor nuclei, medullary linear nucleus, and the inferior olive. In the nucleus ambiguus, labeled cells were located in the areas containing pharyngeal and laryngeal motoneurons. Intensely labeled cells were loosely scattered in the reticular formation adjacent to the raphe magnus and obscurus nuclei, in the gigantocellular region, in the caudal pedunculopontine and laterodorsal tegmental nuclei, dorsomedial pontine reticular formation, and nucleus subcoeruleus. In the nucleus prepositus hypoglossi, all vestibular, abducens, cuneate, and lateral reticular nuclei, labeled neurons commingled with unlabeled ones. Few labeled neurons were located in the rostral and caudal ventrolateral medulla and parvicellular reticular formation. In the nucleus of the solitary tract, two patches of diffuse labeling not associated with cellular profiles were present: one in the medial, and the other in the interstitial subnucleus. Similar diffuse labeling was present in the lateral parabrachial region and the lateral rim of the caudal spinal trigeminal sensory nucleus. No labeled cells were found in the locus coeruleus, dorsal raphe, superior olive, or area postrema. The distinct pontomedullary distribution of 5-HT2A receptors, combined with the known arousal-dependent activity of serotonergic neurons, show that these receptors may mediate post- and presynaptic effects in the motor, selected somatic and visceral sensory, oculo-vestibulo-precerebellar, and sleep-related regions.