Actions of dihydroavermectin B1α on insect muscle

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Abstract
1 Muscle bundle 33 of the locust (Schistocerca gregaria) extensor tibiae muscle, which is sensitive to γ-aminobutyric acid (GABA) and receives inhibitory innervation, exhibited both reversible and irreversible responses to dihydroavermectin B1α (DHAVM). These responses involved increases in Cl− permeability. 2 DHAVM (0.000075-0.0075 μg ml−1) induced reversible dose-dependent increases in Cl− permeability and partially blocked GABA-induced Cl− conductance. These effects appear to be due to an interaction of DHAVM with the GABA receptor-Cl− ion channel complex. 3 DHAVM (0.01-1.0 μg ml−1) induced an irreversible increase in Cl− conductance which continued to rise after DHAVM application was stopped. At these concentrations DHAVM potentiated GABA-induced Cl− conductances which were in turn reduced by microperfusion of DHAVM (0.01-1.0 μg ml−1) during bath application of GABA. 4 DHAVM (0.0001-1.0 μg ml−1) induced only irreversible increases in Cl− conductance when applied to fast muscle bundles (21–26) of the locust extensor tibiae muscle, which are GABA-insensitive and have no inhibitory innervation. 5 The actions of DHAVM on locust muscle appear to involve more than one site. Reversible actions of DHAVM appear to be related to GABA sensitivity and may involve the GABA receptor-ionophore complex. This is unlikely to be the site of action for the irreversible increases in Cl− conductance caused by DHAVM.