Analysis of contributions of acetylcholine and tachykinins to neuro‐neuronal transmission in motility reflexes in the guinea‐pig ileum

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Abstract
1 The roles of acetylcholine (ACh) and tachykinins in neuro-neuronal transmission during ascending excitatory and descending inhibitory reflexes were studied by recording intracellular reflex responses of the circular muscle to physiological stimuli. Experiments were carried out in opened segments of guinea-pig ileum in an organ bath that was partitioned so that three regions could be independently exposed to drugs. 2 Ascending excitatory reflexes evoked by either distension from the serosal side or compression of the mucosa were depressed by 55% and 85%, respectively, in the presence of hexamethonium (200 μm) and by 30% and 45%, respectively, by a desensitizing concentration of the selective NK3 receptor agonist, senktide (1 μm), in the chamber in which reflexes were initiated. Together, hexamethonium and senktide abolished reponses to compression. A residual response to distension persisted. This was abolished by hyoscine (1 μm). 3 Hexamethonium (200 μm) abolished ascending reflexes when applied to the region between the stimulus and the recording sites, or to the recording chamber. 4 Descending reflex responses were reduced by 35% by synaptic blockade in the stimulus chamber with physiological saline containing 0.1 mM Ca2+ plus 10 mM Mg2+. Senktide (1 μm) in the stimulus chamber reduced distension reflexes to the same extent as synaptic blockade, whereas hexamethonium (200 μm) and hyoscine (1 μm) depressed responses by less than 20%. Responses to compression were reduced by 40% by senktide alone, while senktide and hexamethonium together reduced responses by 60%, an effect similar to synaptic blockade. Under these conditions, hyoscine in the stimulus chamber restored reflexes evoked by distension, but did not alter those evoked by mucosal compression. 5 Total synaptic blockade in the intermediate chamber, between stimulus and recording sites, reduced descending reflex responses by more than 90%. In contrast, hexamethonium (200 μm) had no effect and hyoscine (1 μm) reduced only the responses to distension (by 30%). Senktide (1 μm) depressed responses to both stimuli by approximately 80%. 6 Application of hexamethonium (200 μm) to the recording chamber depressed descending reflex responses to distension applied in the near stimulation chamber by 15%, but had no effect on responses to compression in the near chamber or to either stimulus applied in the far chamber. 7 Descending reflexes evoked by near chamber stimuli were unaffected by hyoscine (1 μm) or senktide (1 μm) applied to the recording chamber; hyoscine enhanced reflexes evoked by compression in the far chamber by 50%. 8 For the ascending excitatory reflex pathway, it is concluded that transmission from sensory neurones is mediated by ACh acting via both nicotinic and muscarinic receptors, and by tachykinins acting at NK3 receptors. Transmission from ascending interneurones appears to be predominantly via nicotinic receptors. The descending inhibitory pathways are more complex, and while transmission from sensory neurones involves nicotinic, muscarinic and NK3 receptor-dependent components, transmission from descending interneurones to inhibitory motor neurones is neither cholinergic nor due to tachykinins acting via NK3 receptors.

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