GRP neurones in the rat small intestine issue long anal projections

Gastrin releasing peptide (GRP) immunoreactive nerve fibres are numerous in the gut wall. Nerve cell bodies containing GRP are regularly found in the myenteric ganglia. The projections of GRP neurones in the rat small intestine were studied after myectomy or transection of the gut wall. Operated rats were left for 8-10 days or 5 weeks. Specimens were studied by immunocytochemistry, immunochemistry and in vitro for motor activity. GRP fibres were absent and GRP was markedly reduced in the gut wall underlying the area of myectomy and 10 mm anally to the myectomy or site of transection. Further anally, GRP and the GRP fibres gradually returned and were back to normal 25-30 mm from the lesion. Myenteric GRP neurones in the rat small intestine thus project anally over a distance of approximately 20-25 mm. A series of experiments was performed in order to test the idea that GRP is directly involved in intestinal motor functions. The results did not support this view. Strips of longitudinal smooth muscle with adherent myenteric ganglia were taken orally and anally to the myectomy and the motor activity of the specimens was compared. Electrical stimulation evoked a contractile response in the oral segment that was 6 times greater than that of the anal segment. However, GRP (10(-9)-10(-5) M) did not evoke contraction and the electrically induced contractile response was unaffected by GRP but could be blocked by atropine. The reduced contractile response in the 'denervated' anal segment is thus probably not due to a shortage of GRP fibres.