Functional tetrodotoxin-resistant Na+ channels are expressed presynaptically in rat dorsal root ganglia neurons

The tetrodotoxin-resistant (TTX-R) voltage-gated Na+ channels Nav1.8 and Nav1.9 are expressed by a subset of primary sensory neurons and have been implicated in various pain states. Although recent studies suggest involvement of TTX-R Na+ channels in sensory synaptic transmission and spinal pain processing, it remains unknown whether TTX-R Na+ channels are expressed and function presynaptically. We examined expression of TTX-R channels at sensory synapses formed between rat dorsal root ganglion (DRG) and spinal cord (SC) neurons in a DRG/SC co-culture system. Immunostaining showed extensive labeling of presynaptic axonal boutons with Nav1.8- and Nav1.9-specific antibodies. Measurements using the fluorescent Na+ indicator SBFI demonstrated action potential–induced presynaptic Na+ entry that was resistant to tetrodotoxin (TTX) but was blocked by lidocaine. Furthermore, presynaptic [Ca2+]i elevation in response to a single action potential was not affected by TTX in TTX-resistant DRG neurons. Finally, glutamatergic synaptic transmission was not inhibited by TTX in more than 50% of synaptic pairs examined; subsequent treatment with lidocaine completely blocked these TTX-resistant excitatory postsynaptic currents. Taken together, these results provide evidence for presynaptic expression of functional TTX-R Na+ channels that may be important for shaping presynaptic action potentials and regulating transmitter release at the first sensory synapse.