Sympathetic inhibition of ascending and descending interneurones during the peristaltic reflex in the isolated guinea‐pig distal colon

1 We investigated the effects of sympathetic nerve stimulation within ascending and descending reflex pathways underlying the peristaltic reflex in the guinea‐pig distal colon. 2 A three‐chambered partitioned bath was used to divide a segment of distal colon into stimulation, recording and intermediate regions. The effects of lumbar colonic nerves (LCN) could be localized to the intermediate region by surgical lesions of the mesentery and by application of guanethidine (3 μm) to the stimulation and recording chambers. 3 Brush stroking the mucosa in the anal and oral stimulation chambers elicited a synchronous contraction of the longitudinal muscle (LM) and circular muscle (CM) oral to, and transient relaxation of the LM and CM anal to, the stimulus, respectively. 4 After Nω‐nitro‐L‐arginine (L‐NA; 100 μm) in the oral and intermediate chambers, mucosal stimulation in the oral chamber elicited a prolonged descending inhibitory and excitatory complex in both the LM and CM in the anal recording chamber. This was blocked by hexamethonium (300 μm), which did not affect the transient relaxation response recorded in control conditions. 5 Stimulation of the LCN (1200 pulses, 20 Hz), delivered to the intermediate region, abolished the oral contraction and the L‐NA‐induced anal complex in both the LM and CM, but was without effect on the transient hexamethonium‐resistant anal relaxation. These effects of LCN stimulation were reversed by phentolamine (3 μm) or yohimbine (100 nM), but not propranolol (10 μm), when added to the intermediate chamber. 6 LCN stimuli (2‐20 Hz, 600 μs pulses) directed to the recording chamber elicited synchronous relaxations in the LM and CM that were unaffected by hexamethonium (300 μm), but were reduced by yohimbine and usually blocked by the further addition of propranolol (10 μm). 7 In conclusion, sympathetic nerve stimulation inhibits orally and anally projecting cholinergic interneurones underlying the peristaltic reflex in the distal colon. In addition, the LM and CM relax synchronously following release of sympathetic neurotransmitter, over a range of stimulus frequencies.