THE DOPAMINERGIC SYSTEM MODULATES THE ENDOGENOUS OPIOID SYSTEM IN GUINEA-PIG ISOLATED ILEAL LONGITUDINAL MUSCLE

The effects of the dopamine antagonists haloperidol and sultopride wer e 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 twit ch inhibition) and by application of opioids. Both haloperidol and sul topride concentration-dependently inhibited the twitch response, with threshold concentrations of 2 and 50 mu M, respectively, and could als o shift the concentration-response curve for ACh-contraction to the ri ght in a non-competitive manner. Haloperidol (1 mu M) and sultopride ( 20 mu M) increased post-tetanic twitch inhibition and this could be pr evented by naloxone (100 nM). Twitch inhibition induced by morphine an d dynorphin 1-13 was not affected by haloperidol (1 mu M) or sultoprid e (20 mu M). Prazosin (1 mu M) and yohimbine (2 mu M) did not affect e ither the twitch response or the post-tetanic twitch inhibition. These results suggest that dopamine receptors are involved in the modulatio n of the ileal opioid system, in such a manner as to diminish the rele ase of endogenous opioids by tetanic stimulation.