The dopaminergic system modulates the endogenous opioid system in guinea-pig isolated ileal longitudinal muscle.

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 microM, respectively, and could also shift the concentration-response curve for ACh-contraction to the right in a non-competitive manner. Haloperidol (1 microM) and sultopride (20 microM) 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 microM) or sultopride (20 microM). Prazosin (1 microM) and yohimbine (2 microM) 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.