Coexistence of immunoreactive substance P and serotonin in neurones of the gut.

Serotonin (5-HT) and substance P (SP) were visualized by indirect immunofluorescence using two fluorochromes applied sequentially on the same preparation of myenteric plexus from the guinea pig gut. Three groups of labelled neurones were found: (i) neurones with 5-HT-like immunoreactivity only; (ii) neurones with SP-like immunoreactivity only; (iii) neurones with both 5-HT- and SP-like immunoreactivity, which represented approximately one-third of the 5-HT-like immunoreactive neurones.