Saxitoxin binding to sodium channels in head extracts from wild-type and tetrodotoxin-sensitive strains of Drosophila melanogaster.

Extracts prepared from heads of Drosophila melanogaster show high-affinity binding (KD = 1.9 nM) of [3H]saxitonin, a compound known to bind to and block voltage-sensitive sodium channels in other organisms. The interaction between saxitoxin and the Drosophila saxitoxin receptor is non-cooperative and reversible with a half-life of 18.3 s for binding at 4 degrees C. The saturable binding is specifically inhibited by tetrodotoxin with a K1 = 0.30 nM. The number of saturable binding sites in the extract is 97 fmol/mg protein. Since approx. 50% of the binding activity is recovered in the extract, the number of binding sites in the head is estimated to be 6.4 fmol/mg head. Nerve conduction in Drosophila larvae is completely blocked after 20 min in a bathing solution containing 200 nM tetrodotoxin. A comparison between the binding and the electrophysiological studies in Drosophila and other organisms suggests that the Drosophila saxitoxin receptor is part of the voltage-sensitive sodium channel involved in the propagation of action potentials. A mutant (ttxs), which is abnormally sensitive to dietary tetrodotoxin, is shown to be indistinguishable from wild type with respect to [3H]saxitonin-binding properties and physiological sensitivity to tetrodotoxin. These studies provide techniques which can be used to identify mutants with defects in the saxitoxin-binding component of the sodium channel.