Studies indicate that approximately 30% of Americans suffer from chronic pain conditions annually resulting in >100 billion dollars in health-care related costs. Currently prescribed narcotics used to treat severe pain have significant side effects and are prone to physical addiction. In addition, these drugs are ineffective in treating neuropathic pain resulting from lesion or dysfunction of the nervous system. The unmyelinated C fibers of peripheral nerve transmit painful sensation (nociception) from the sensory nerve terminals located in tissues to the central nervous system. These slowly conducting nerve fibers are the initial link in the nociception pathway and are therefore important targets of pain therapy. Nociceptors express a combination of ligand- and voltage-gated ion channels that enable these neurons to respond to peripheral tissue damage and nerve injury. The cell bodies of nociceptors are located in the dorsal root ganglion (DRG) and express a unique Na current that is resistant to tetrodotoxin (TTX), a prototypical inhibitor of sodium channels. This TTX-resistant Na current has been the focus of intense research because of its purported role in chronic and neuropathic pain syndromes. Two Na channels (Nav1.7, Nav1.8) have been cloned from peripheral nerve that display properties similar to the TTX-sensitive and TTX-resistant currents expressed in DRG nociceptors. The proposed studies will characterize the biophysical properties of the Nav1.7 and Nav1.8 channels heterologously expressed in a mammalian cell line. In vivo, these Na channels are associated with one or more accessory beta subunits that regulate the expression and voltage-dependent gating of the channels. The proposed studies will use single-cell RT-PCR to identify the beta subunits that are expressed in the pain-sensing nociceptors of the DRG. The effects of the identified beta subunits on the kinetics and voltage-dependent gating of heterologously expressed peripheral nerve Na channels (Nav1.7, Nav1.8) will be investigated. Overall, our proposed studies will provide new insights into the Na channels and regulatory subunits that govern the electrical excitability of nociceptive neurons. ? ? ?
