Persistent neuropathic pain is produced by spinal cord injury (SCI) in a majority of patients. Like other forms of central neuropathic pain, SCI pain is often debilitating and quite resistant to clinical treatment. Most research on mechanisms of SCI pain has focused on increases in the responsiveness and spontaneous electrical activity of central neurons within pain pathways, especially second-order neurons in the dorsal horn near the injury site. Hyperexcitability of dorsal horn neurons after SCI appears to involve many plausible causes, but one that has received little attention is an enhancement of spontaneous activity (SA) and excitability in the sensory neurons (especially nociceptors) that normally excite dorsal horn neurons. Indeed, surprisingly little is known about how sensory neurons in the dorsal root ganglion (DRG) respond to SCI. The proposed studies are based on the novel hypothesis that SCI triggers a chronic hyperfunctional state in nociceptors which results in the generation of SA within their somata in DRGs, and that this continuing SA excites central pain pathways, driving spontaneous pain, allodynia, and hyperalgesia. This hypothesis will be tested by 1) examining the effects of SCI on SA and electrophysiological properties of DRG neurons that are subsequently dissociated and tested in depth, 2) examining SA in DRG neurons in vivo after SCI, and 3) by attempting to block SA (and associated SCI pain) by disconnecting the DRG from the spinal cord (dorsal rhizotomy prior to the contusion) or by knocking down a voltage-gated Na+ channel that is necessary for the generation of SA by nociceptors. SCI will be produced in a standard contusion injury model, with the impact at spinal level T10. Behavioral and electrophysiological tests will be conducted 3 days, 1 month, and 3 months after injury. The behavioral tests will assess motor loss and possible recovery, and (at 1 and 3 months) spontaneous pain, allodynia, and hyperalgesia at, above, and below the injury level. In vitro electrophysiological tests will be conducted with whole-cell current clamp methods on DRG neurons dissociated from T9, T11, and L4 levels. In addition to recording SA, a complex test protocol will define intrinsic passive and active membrane properties at resting membrane potential and at holding potentials of -80 mV (where little inactivation of voltage-gated sodium channels occurs) and -50 mV (where many of these channels are inactivated). In vivo electrophysiological tests will use extracellular recording from filaments teased from the dorsal root to see how much SA is present before and after disconnecting the DRG from the periphery. Four predictions of the hyperfunctional nociceptor hypothesis will be tested: first, SCI should enhance SA of putative nociceptive DRG neurons, initially at and below the level of injury, but later above the injury as well. Second, that enhanced SA in vitro and in vivo, and hyperexcitability in vitro, should be correlated with enhanced behavioral signs of pain, allodynia, and hyperalgesia. Third, if SCI pain depends in part upon SA in nociceptors, SCI pain should be reduced by selectively suppressing nociceptor SA in vivo. This will be tested by delivering antisense oligonucleotides intrathecally to knock down the expression of a Na+ channel, Nav1.8, that is expressed selectively in nociceptive sensory neurons and is necessary for generating SA in these neurons. Fourth, the assumption that retrograde signals to nociceptor somata from central processes of these neurons are necessary for triggering the SA will be tested by performing a dorsal rhizotomy immediately before the SCI. These exploratory studies will test a novel hypothesis about mechanisms important for SCI pain, define intrinsic electrophysiological alterations in DRG neurons linked to neuropathic pain, and begin to test an intervention that appears potentially useful for treating SCI pain.
Spinal cord injury patients often suffer debilitating pain that is highly resistant to clinical treatments. Although most investigations of this problem have focused on alterations in central neurons within pain pathways, preliminary data suggest that alterations of sensory neurons that normally convey pain information from peripheral tissues may play an important role. The proposed studies will test the hypothesis that chronic pain caused by spinal cord injury is produced in part by spontaneous electrical activity in sensory neurons, and that pain may be reduced by blocking this activity.
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