Activation of Tetrodotoxin-Resistant Sodium Channel NaV1.9 in Rat Primary Sensory Neurons Contributes to Melittin-Induced Pain Behavior

Tetrodotoxin-resistant (TTX-R) sodium channels Na_V1.8 and Na_V1.9 in dorsal root ganglion (DRG) neurons play important roles in pathological pain. We recently reported that melittin, the major toxin of whole bee venom, induced action potential firings in DRG neurons even in the presence of a high concentration (500 nM) of TTX, indicating the contribution of TTX-R sodium channels. This hypothesis is fully investigated in the present study. After subcutaneous injection of melittin, Na_V1.8 and Na_V1.9 significantly upregulate mRNA and protein expressions, and related sodium currents also increase. Double immunohistochemical results show that Na_V1.8-positive neurons are mainly medium- and small-sized, whereas Na_V1.9-positive ones are only small-sized. Antisense oligodeoxynucleotides (AS ODNs) targeting Na_V1.8 and Na_V1.9 are used to evaluate functional significance of the increased expressions of TTX-R sodium channels. Behavioral tests demonstrate that AS ODN targeting Na_V1.9, but not Na_V1.8, reverses melittin-induced heat hypersensitivity. Neither Na_V1.8 AS ODN nor Na_V1.9 AS ODN affects melittin-induced mechanical hypersensitivity. These results provide previously unknown evidence that upregulation of Na_V1.9, but not Na_V1.8, in small-sized DRG neurons contributes to melittin-induced heat hypersensitivity. Furthermore, melittin-induced biological effect indicates a potential strategy to study properties of TTX-R sodium channels.