Anion selectivity of gamma-aminobutyric acid (GABA) 22,23-dihydroavermectin B1a (DHAVM)-induced conductance changes on locust muscle

Input conductance was recorded from extensor tibia muscle fibres of the locust Schistocerca gregaria. gamma-Aminobutyric acid (GABA; 2.5 X 10(-5) M to 5 X 10(-3) M) and 22,23-dihydroavermectin B1a (DHAVM; 0.01 micrograms/ml, 1.2 X 10(-8) M to 1.0 microgram/ml, 1.2 X 10(-6) M) induced dose-dependent increases in chloride (Cl-) ion permeability on distal muscle fibres. Changing the anion content (Br-, or I- substituted for Cl-) of the saline perfusing the locust muscle fibres induced additional conductance and small hyperpolarizations of the membrane potential. The predominant anion permeability sequences for conductance increases induced by GABA and DHAVM were I- greater than Br- greater than Cl- (n = 5) and Cl- greater than Br- greater than I- (n = 12), respectively. These findings suggest that the anion channel activated by GABA has low selectivity. The irreversible increase in conductance induced by DHAVM appears to involve an ion channel with greater selectivity for Cl-. However, DHAVM-induced changes in filtering properties of a single type of Cl- channel are not precluded. This is consistent with DHAVM action not being dependent on GABA sensitivity of the locust muscle fibres.