The effects of the dopamine antagonists haloperidol and sultopride were investigated on the twitch response, evoked by 0.1 Hz stimulation of guinea-pig isolated ileal longitudinal muscle, and on the inhibition of the twitch response induced by 10 Hz stimulation (post-tetanic twitch inhibition) and by application of opioids. Both haloperidol and sultopride concentration-dependently inhibited the twitch response, with threshold concentrations of 2 and 50 μM, respectively, and could also shift the concentration-response curve for ACh-contraction to the right in a non-competitive manner. Haloperidol (1 μM) and sultopride (20 μM) increased post-tetanic twitch inhibition and this could be prevented by naloxone (100 nM). Twitch inhibition induced by morphine and dynorphin 1–13 was not affected by haloperidol (1 μM) or sultopride (20 μM). Prazosin (1 μM) and yohimbine (2 μM) did not affect either the twitch response or the post-tetanic twitch inhibition. These results suggest that dopamine receptors are involved in the modulation of the ileal opioid system, in such a manner as to diminish the release of endogenous opioids by tetanic stimulation.
Neuropharmacology – Elsevier
Published: May 1, 1995
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