In whole segments of rabbit distal colon with mucosa removed, descending reflex relaxations of the circular muscle (descending inhibition) elicited by inflating (0.1-1 ml) an intraluminal balloon, were partially antagonized by 100 μM hexamethonium and the 5-HT3 receptor antagonist, ondansetron (3 μM), and abolished by 1 μM tetrodotoxin. The inhibitory effects of hexamethonium and ondansetron were additive. Conversely, hexamethonium (100 μM) and ondansetron (3 μM) failed to reduce electrically induced non-adrenergic non-cholinergic (NANC) relaxations of colonic circular muscle. It is concluded that interneuronally released acetylcholine and 5-hydroxytryptamine (5-HT) activate descending inhibitory pathways supplying the circular coat, via nicotinic and 5-HT3 receptors, respectively. This evidence suggests a functional involvement of 5-hydroxytryptaminergic transmission in the descending inhibition of rabbit colon.
- 5-HT (5-hydroxytryptamine, serotonin)
- 5-HT receptor antagonism
- Descending inhibition
- Neuro-neuronal transmission
ASJC Scopus subject areas
- Cellular and Molecular Neuroscience