Substance P-immunoreactive processes on 5HT/SP-immunoreactive medullary cells.

Two-color immunoperoxidase staining has been used to localize substance P (SP)-immunoreactive processes on neurons in the caudal medulla that exhibited both serotonin (5HT)- and SP-immunoreactivity (5HTI/SPI cells). The punctate SP-immunoreactive processes were present on 5HTI/SPI cells in the raphe nuclei and ventral reticular formation. This close association suggests that activity in bulbospinal 5HT/SP pathways, which can influence sympathetic preganglionic neurons, may be affected by the release of SP in the brainstem.