Morphine attenuates cholinergic nerve activity in human isolated colonic muscle

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Abstract
1 The action of morphine on cholinergic nerves in human sigmoid taenia coli muscle strips (taenia) was investigated using a radiolabelling technique. 2 Basal release of tritiated material from taenia was increased by electrical field stimulation (EFS). This increase was tetrodotoxin (3.14 μM)-sensitive and calcium-dependent. Analysis of basal and stimulated release of tritiated material indicated that evoked release (i.e. stimulated minus basal) is almost entirely due to an increase in [3H]-acetylcholine ([3H]-ACh) output. 3 Evoked release of [3H]-ACh was dependent on the current strength and could be greatly reduced by exposing taenia to hemicholinium (34.8, 87.0 μM) before and during incubation with [3H]-choline (4 μCi ml−1, 15 Ci mmol−1). 4 Spontaneous activity, muscle tone and the motor response of taenia to EFS were unaffected by morphine. 5 Evoked, but not basal, release of tritiated material was inhibited by morphine (1.32-13.20 μM) in a concentration-dependent manner. The inhibition of release was frequency-dependent and naloxone (0.28 μM)-sensitive. 6 The possible relationship between the effects of morphine on cholinergic nerves in taenia muscle and its actions in vivo are discussed.