Mechanisms Underlying Midazolam-Induced Peripheral Nerve Block and Neurotoxicity

Background and Objectives: The benzodiazepine midazolam has been reported to facilitate the actions of spinally administrated local anesthetics. Interestingly, despite the lack of convincing evidence for the presence of gamma-aminobutyric acid type A (GABA(A)) receptors along peripheral nerve axons, midazolam also has been shown to have analgesic efficacy when applied alone to peripheral nerves. These observations suggest midazolam-induced nerve block is due to another site of action. Furthermore, because of evidence indicating that midazolam has equal potency at the benzodiazepine site on the GABA(A) receptor and the 18-kd translocator protein (TSPO), it is possible that at least the nerve-blocking actions of midazolam are mediated by this alternative site of action. Methods: We used the benzodiazepine receptor antagonist flumazenil, and the TSPO antagonist PK11195, with midazolam on rat sciatic nerves and isolated sensory neurons to determine if either receptor mediates midazolam-induced nerve block and/or neurotoxicity. Results: Midazolam (300 mu M)-induced block of nerve conduction was reversed by PK11195 (3 mu M), but not flumazenil (30 mu M). Midazolam-induced neurotoxicity was blocked by neither PK11195 nor flumazenil. Midazolam also causes the release of Ca2+ from internal stores in sensory neurons, and there was a small but significant attenuation of midazolam-induced neurotoxicity by the Ca2+ chelator, BAPTA. BAPTA (30 mu M) significantly attenuated midazolam-induced nerve block. Conclusions: Our results indicate that processes underlying midazolam-induced nerve block and neurotoxicity are separable, and suggest that selective activation of TSPO may facilitate modality-selective nerve block while minimizing the potential for neurotoxicity.