Effects of various gabergic anthelmintics on the guanidine-induced twitch responses in isolated frog rectus preparations were examined. All gabergic anthelmintics such as milbemycin oxime, milbemycin D, avermectin B1a, ivermectin, and diethylcarbamazine (DEC) showed stimulatory effects on the guanidine-induced twitch responses at their higher concentrations. Only piperazine caused inhibitory effects on the twitch responses, even at higher concentrations. The stimulation of the twitch responses by the gabergic anthelmintics was antagonized with tetrodotoxin, hemicholinium-3, d-tubocurarine, and strychnine. These results suggest that all gabergic anthelmintics except piperazine stimulate the release of acetylcholine from the nerve endings and that all of them, including piperazine, have different effects on the gabergic mechanism at lower concentrations and on the cholinergic mechanism at higher concentrations.
