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The antiepileptic drug carbamazepine is effective in a subpopulation of trigeminal neuralgia patients, in particular those suffering from pain associated with vascular compression of a trigeminal nerve. However, carbamazepine has failed in clinical trials for other neuropathic pain conditions, even those associated with peripheral nerve compression injuries. Carbamazepine is both a GABAA receptor (GAR) agonist and a blocker of voltage-gated Na+ channels (VGSC), and our recent data suggest that differences between trigeminal and somatic nerves with respect to the VGSC present are not sufficient to account for the therapeutic selectivity of carbamazepine. Therefore, we hypothesized that these observations reflected a differential involvement of GARs in somatic and trigeminal nerves. To test this, we employed a combination of electrophysiological and behavioral experiments in rat models of somatic (sciatic nerve, SN) and trigeminal (infraorbital nerve, ION) nerve compression (CCI). We observed that the potency of carbamazepine-induced compound action potential (CAP) block was increased by CCI of the ION but not the SN, and this increase in potency was reduced by the GAR blocker picrotoxin. The potency of the GAR agonist muscimol-induced block of the ION was greater than the SN, and CCI produced a further increase in efficacy. The GABAA-rho subunit mRNA and protein was increased by CCI of the ION, but not SN. The GABAA-rho receptor selective antagonist TPMPA reduced carbamazepine-induced CAP block of the ION. CCI was also associated with an increase in the potency of the GABAA-rho receptor preferring agonist TACA-induced block of the ION, but not SN. Finally, local administration of TACA to the ION but not SN, dose-dependently reversed CCI-induced mechanical sensitivity. Taken altogether, our results suggest GABAA-rho receptors are present and functional in ION, but not SN. This differential distribution of GABAA receptors could account for the therapeutic selectivity of carbamazepine for the treatment of TN. The work was supported by generous donations from TN patients and family members as well as NIH grant R01NS064988.
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