Josephine Lai - US grants

DESCRIPTION: (adapted from applicant's abstract) Pain associated with nerve injury (i.e., neuropathic pain) has been hypothesized to be related to increased afferent discharge associated with ectopic foci occurring in the injured nerve. One mechanism which may induce abnormal spontaneous afferent activity is the expression of a TTX-resistant sodium channel (called PN3 or SNS) which has recently been cloned and localized selectively to cells in the dorsal root ganglia (DRG). The applicant proposes to test the hypothesis that a) expression of PN3/SNS is a primary event that underlies the consequences of nerve-injury, including tactile allodynia and hyperalgesia, and b) that strategies aimed at interfering with expression or function of PN3/SNS may lead to a breakthrough for the treatment of neuropathic pain. This hypothesis will be tested by four specific aims: 1) localization of the PN3/SNS protein in the DRG as well as in the injured nerve fibers, using immunocytochemistry; 2) measuring the temporal expression of PN3/SNS (at transcriptional and translational levels) after peripheral nerve injury and correlation with behavioral consequences of onset and offset of nerve injury; 3) characterizing the time-response, reversibility, dose-effects, uptake and specificity of PN3/SNS antisense oligodeoxynucleotides (ODN) on the expression of PN3/SNS protein; and 4) determining the effects of pre- or post-treatment with antisense ODN on the behavioral consequences of nerve-injury and whether such changes in behavior correlate with changes in PN3/SNS protein. These studies may have great practical significance towards the development of an effective therapy for neuropathic pain with few, or no, side effects.

DESCRIPTION (provided by applicant): Recent evidence suggests that processes that initiate neuropathic pain may differ from those that maintain such pain. That persistent neuropathic pain states require descending facilitation arising from the rostral ventromedial medulla (RVM) is suggested by the finding that lidocaine injected into the RVM effectively blocks experimental neuropathic pain at post-in jury day 6 (and later), but not at post-injury day 3. Onset of this descending facilitation may arise from activity of RVM cells that express mu opioid receptors, since either a selective lesion of these cells with a saporin-mu opioid conjugate or surgical lesion of the dorsolateral funiculus (DLF), a descending fiber pathway to the dorsal horn, prevents and reverses the expression of neuropathic pain. These findings implicate a time-dependent activation of descending facilitatory processes in the RVM following injury to maintain, rather than initiate, abnormal pain. The nature of the RVM neuroplasticity that drives such pain is unknown. One clue to this mechanism is our observation that RVM microinjection of a CCK 2 receptor antagonist produces a reversible blockade of established neuropathic pain. In addition, microinjection of CCK-8(S) into the RVM of uninjured rats produces tactile and thermal hypersensitivity reminiscent of states of nerve-injury induced pain. These effects are prevented by lesions of the DLF, or of RVM cells expressing mu opioid receptors, suggesting the possibility that RVM CCK may promote injury- induced pain by activating RVM neurons that are phenotypically defined by the expression of mu opioid receptors. For these reasons, we hypothesize that nerve-injury results in a time-related increase in RVM CCK activity to drive descending facilitation and to maintain nerve-injury induced pain. This hypothesis will be tested by the following specific aims. Aim 1 will characterize (a) the basal CCK activity in the RVM, and changes in CCK activity in response to mechanical and thermal cutaneous inputs in naive rats, and (b) the time-dependent nature of CCK activity in the RVM following nerve injury. Specifically, basal CCK release, and CCK release in response to mechanical and thermal cutaneous inputs in the RVM will be measured at early and late time points after spinal nerve ligation injury. Aim 2 will identify the CCK receptor(s) at which RVM CCK may act to drive descending facilitation. The functional roles of CCK and CCK2 receptor types in the RVM pre- and post-injury will be evaluated pharmacologically by microinjection of selective CCK receptor antagonists. The spatial distribution of the CCK receptors will be analyzed by semi-quantitative in situ hybridization and receptor autoradiography. Aim 2 will also examine the possibility that CCK receptors may co-localize with mu opioid receptors in RVM neurons. The proposed experiments will reveal some features of the RVM plasticity that may be critical in maintaining neuropathic pain. As patients who suffer from neuropathic pain are likely to seek treatment long after the precipitating injury has occurred, understanding the mechanisms that maintain the neuropathic state will be essential for the development of rational therapeutic interventions. In this regard, the proposed experiments may reveal an important role for CCK receptor antagonists as therapy for neuropathic pain.