Single pulse electrical field stimulation (EFS, 0.5 ms pulse width, 60 V at a frequency of 0.05 Hz) induced twitch contractions of mucosa-free circular muscle strips from the guinea-pig proximal colon which were abolished by atropine (0.3 microM), tetrodotoxin (0.3 microM) or omega-conotoxin GVIA (0.1 microM). 2. Various opioid receptor agonist concentration-dependently inhibited twitches with the following rank order of potency (EC50 values in brackets): U 50488 (0.31 nM) > dermorphin (4.3 nM) = dynorphin A (1-13) (6.2 nM) > [D-Ala2, N-MePhe4, Gly5-ol]-enkephalin (DAMGO, 33.5 nM) = [D-Ala2, D-Leu5]-enkephalin (DADLE, 60 nM) > [D-Pen2, D-Pen2, D-Pen5]-enkepahlin (DPDPE, 1144 nM). 3. Peptidase inhibitors (captopril, thiorphan and bestatin, 1 microM each) did not modify the amplitude of twitches. In the presence of peptidase inhibitors the concentration-response curve to dynorphin A (1-13) was displaced to the left to yield an EC50 of 0.35 nM, comparable to that of the selective kappa receptor agonist, U50488. The curves to the other opioid receptor agonist were unaffected by peptidase inhibitors. 4. DPDPE, DADLE, dermorphin and DAMGO consistently induced a concentration-unrelated transient increase in basal tone and a small and transient facilitation of twitches before development of their inhibitory effect. These transient excitatory effects were not observed upon application of dynorphin A (1-13) or U 50488. The contraction produced by DPDPE (30 nM) was largely inhibited (> 80%) by 1 microM atropine. 5. Twitches suppression induced by dynorphin A (1-13) (30 nM) was partly reversed (46 +/- 8%, n = 6) by naloxone (0.3 microM). The potent and selective kappa opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 3-100 nM)) did not affect the amplitude of twitches and potently antagonized (pKB 9.83 +/- 0.09, n = 10) the inhibitory effect of dynorphin. 6. Naloxone (1-300 nM) concentration-dependently depressed the cholinergic twitches: this depressant effect was largely counteracted in the presence of apamin (0.1 microM) and NG-nitro-L-arginine (30 microM) which potentiated cholinergic twitches on their own. 7. Dynorphin A (1-13) (10 nM, n = 6) did not affect the contractile response to exogenous acetylcholine (1 microM), indicating that depression of evoked twitches occurs prejunctionally. 8. We conclude that multiple opioid receptors modulate cholinergic twitches in the circular muscle of guinea-pig proximal colon. While mu and delta opioid receptor agonists produced mixed excitatory and inhibitory effects, kappa opioid receptors, activated by sub-nanomolar concentrations of dynorphin A (1-13), mediate a powerful and pure prejunctional inhibition of acetylcholine release.
