[Absence of the effect of opioid peptides on muscarine receptors in the frog vestibular apparatus].

We studied the effects of opioid peptide leu-enkephaline, a specific antagonist of acetylcholine receptors atropine, and non-selective opiate antagonist naloxone on synaptic transmission and responses evoked by acetylcholine in semicircular organs of the frog. A decrease in frequency of acetylcholine (0.1-5.0 microM) responses under leu-enkephaline (10 nM) id not differ from the frequency decline induced by leu-enkephaline alone. Atropine (1 microM) left the response to leu-enkephaline intact while blocking the excitatory effect of acetylcholine. No modification of the acetylcholine response under leu-enkephaline was observed in the presence of naloxone (1 microM). The findings suggest that no interaction exists between the acetylcholine-mediated excitatory action on resting activity in the isolated semicircular canal preparation and the suppressive action of leu-enkephaline.