Action of avermectin B1a on the leg muscles and the nervous system of the American cockroach

Abstract The action of avermectin was studied in the leg muscle and the central nervous system of the American cockroach, Periplanata americana L. Avermectin at a low concentration (10 −7 M ) causes a failure of the leg muscles to respond to external stimuli within 30 min without affecting the magnitude of contraction. Avermectin was found to stimulate Cl − uptake by the leg muscles within 4 min at 10 −7 M . The threshold concentration to cause such stimulation was on the order of 10 −8 M . This stimulatory action could be antagonized by picrotoxinin (10 −4 M ) and to a lesser extent by bicuculline methiodide (10 −4 M ). The phenomenon is observable under both Na + -free and K + -free conditions. It was concluded that the action of avermectin is to open the chloride channel on the plasma membrane. This action of avermectin does not seem to be mediated through GABA, GABA receptors, diazepine receptors, or picrotoxinin receptor in this insect species, and therefore suggests that avermectin directly attacks the chloride channel proper both in the central nervous and the neuromuscular systems.