An immunohistochemically distinct zone was identified in the superficial aspect of trigeminal nucleus caudalis of the New World owl monkey that is not immunoreactive for substance P or serotonin, in stark contrast to the dense staining present in the surrounding laminae I and II. Thionin-stained sections in different planes showed that this is a subregion of lamina I containing clusters of neurons that appear to have pyramidal or polygonal somata. Extracellular microelectrode recordings in this region revealed clusters of thermoreceptive-specific (COLD) cells with nasal or labial receptive fields, whereas nociceptive neurons were found in the adjacent portions of lamina I. Anterograde tracer injections in this region produced trigeminothalamic terminal labeling in the site homologous to the lamina I spino-thalamo-cortical relay nucleus identified previously in the Old World macaque monkey and in humans. Retrograde tracer injections involving this thalamic site, where recordings of trigeminal COLD-like neurons were obtained, produced clusters of retrogradely labeled trigeminothalamic neurons in this immunohistochemically distinct subregion of lamina I, nearly all of which are pyramidal neurons. We conclude that the nocturnal owl monkey has a specialized perinasal thermoreceptive trigeminothalamic sensory pathway that is probably of behavioral significance during olfactory sniffing. In addition, these observations corroborate other findings that have indicated that lamina I COLD cells are pyramidal neurons and are not physiologically modulated by substance P or serotonin, in contrast to nociceptive neurons.
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