Investigations of nerve populations influencing ion transport that can be stimulated electrically, by serotonin and by a nicotinic agonist

Summary

It is known that the majority of the mucosal nerve fibres in the guinea-pig small intestine arise from submucous ganglia. There are a number of neurochemically distinct populations of nerve cells in these ganglia, approximately half of them being cholinergic. In these studies we have stimulated isolated preparations of mucosa and submucosa with electrical field stimulation (EFS), 5-hydroxytryptamine (5-HT) and the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) and monitored changes in ion transport.

Segments of intestine were dissected free of external muscle and myenteric plexus and mounted in Ussing chambers. Short-circuit current (I _sc) was measured as an indication of net ion transport across the tissue. EFS consisted of passing bipolar rectangular stimulus pulses through two platinum wires, one placed on each of the mucosal and submucosal sides of the tissue.

EFS, 5-HT and DMPP each caused a transient increase inI _sc. Tetrodotoxin (TTX) abolished all of the EFS response and the majority of the response observed with 5-HT or DMPP, suggesting that the action of these stimuli on the mucosa is primarily nerve-mediated. The TTX-sensitive responses to 5-HT (>5×10⁻⁷ M) and DMPP consisted of two components, appearing with different latencies. The response to EFS also consisted of two components. Hyoscine abolished the first component of each of these responses and significantly reduced the amplitude of the second, by 40% (for EFS and 5-HT) and 84% (for DMPP). At lower 5-HT concentrations, only the later component was seen, and this was unaffected by hyoscine. These results suggest that the early component of each response is due to the release of acetylcholine from cholinergic nerves. The hyoscine-resistant responses to EFS and DMPP were reduced by a substance P antagonist (d-Arg¹,d-Pro²,d-Trp^7,9, Leu¹¹), suggesting that these responses involve activation of substance P receptors in the mucosa.

The studies suggest that EFS and 5-HT (>5×10⁻⁷ M) stimulate both cholinergic and non-cholinergic nerves effectively, that 5-HT (10⁻⁸–5×10⁻⁷ M) preferentially stimulates non-cholinergic nerves and that DMPP preferentially stimulates cholinergic nerves.