Surangin B: insecticidal properties and mechanism underlying its transmitter-releasing action in nerve terminal fractions isolated from mammalian brain

Surangin B is insecticidal to Acheta domesticus by both injection (LD50 = 0.3 micrograms) and topical application (approximate LD50 = 2 micrograms). Exposure of pure synaptosomal fractions isolated from mouse brain to low concentrations of surangin B results in release of the neurotransmitters gamma-aminobutyric acid (GABA) and L-glutamate. Efflux of these neurotransmitters is accompanied by extensive depolarization of the nerve terminal plasma membrane and a rise in intraterminal free calcium ion concentration. These events are not blocked by micromolar concentrations of tetrodotoxin, and moderate reductions in the ability of surangin B to depolarize the plasma membrane and release L-glutamate are observed when sodium ions are excluded from the external saline. Ca2+-free saline produces a partial reduction in the rise in free [Ca2+] following exposure of synaptosomes to surangin B, suggesting that an important component of this compound's action is to liberate Ca2+ from an intracellular pool. Experiments to examine potential effects of surangin B on energy metabolism have demonstrated that it inhibits basal oxygen consumption by isolated nerve endings and blocks the stimulation in oxygen consumption induced by the uncoupling agent carbonyl cyanide chlorophenylhydrazone. In addition, surangin B produces a marked and rapid depolarization of the intraterminal mitochondrial membrane at low nanomolar concentrations. The effects we observe with surangin B on synaptosomal mitochondrial function would be expected to (1) limit the supply of ATP to the Na+/K+ ATPase in the plasma membrane, resulting in a failure to maintain the resting potential and (2) elicit displacement of calcium ions from the mitochondrial matrix into the cytoplasm both of which would activate release of GABA and L-glutamate. We conclude that depolarization of the plasma membrane, the rise in intraterminal free ionic calcium, and transmitter release observed when isolated nerve endings are exposed to surangin B are mainly consequences of its direct interference with mitochondrial function.