Binding assay of (125)I-BmK IT2, a depressant insect-selective scorpion toxin showed two non-interacting binding sites on insect neuronal membranes: a high affinity (K(d(1))=0.65+/-0.20 nM) and low capacity (B(max(1))=0.46+/-0.13 pmol/mg protein) binding site, as well as a low-affinity (K(d(2))=78.7+/-16.4 nM) and high capacity (B(max(2))=33.1+/-8.5 pmol/mg protein) binding site. BmK IT2 could associate with and dissociate from its binding sites on insect neuronal membranes in quick manner (k(1)=5.4 x 10(5) S(-1) M(-1) and k(2)=3.2 x 10(4) S(-1) M(-1); k(-1)=7.4 x 10(-4) S(-1) and k(-2)=5.3 x 10(-3) S(-1)). The binding of (125)I-BmK IT2 to insect synaptosomes could be significantly inhibited by native BmK IT2, BmK AS and BmK AS-1 in a dose-dependent manner, and partially by BmK I, but not modified by depolarization of membrane potential and veratridine, In addition, specific binding of (125)I-BmK IT2 seem to be undetectable on rat brain synaptosomes even at high concentration. Whole cell patch-clamping recording found that BmK IT2 could partially inhibit total sodium currents of rat DRG neurons, the inhibitory effects were reversible. The results suggest that the receptor binding site of BmK IT2 on insect sodium channels might be similar to that on sodium channels of mammal peripheral nervous system, but different from that of mammal central nervous system.