Functional studies of nerve projections in the canine intestine

An in vivo model has been developed to study nerve connections in the canine intestine, using spread of field stimulated contractions recorded proximally and distally with strain gauges and local intra-arterial injections of drugs. Excitation spread orally for several centimetres, more effectively at lower frequencies of field stimulation. This excitation was blocked by local hexamethonium or by a combination of atropine and naloxone (each of which reduced the contractions). Distal excitation occurred after a longer delay than oral excitation; during the delay there was frequently an initial relaxation response. Distal excitation was greater at higher frequencies of field stimulation, but like oral excitation it was blocked by hexamethonium or by a combination of atropine and naloxone. Distal relaxation responses were unaffected by atropine or naloxone, but were abolished by hexamethonium. "Off" contractions, those that followed cessation of field stimulation, occurred at higher frequencies of field stimulation proximally and distally near the site of field stimulation and were blocked by atropine but not by naloxone or hexamethonium. The effects of all agents given locally extended beyond the sites of injection. These results suggest that a chain of cholinergic nerves with nicotinic synapses transmit excitation orally and distally to circular muscle; these effects seem to be facilitated proximally and distally by opioid nerves and to be inhibited initially distally by a noncholinergic mechanism. Explanations of these findings are proposed.