Antinociceptive Effects of Mirtazapine, Pregabalin, and Gabapentin After Chronic Constriction Injury of the Infraorbital Nerve in Rats

Aims: To clarify the antiallodynic effects of the alpha 2-adrenergic receptor antagonist mirtazapine compared with those of gabapentin and pregabalin in a rat model of orofacial neuropathic pain. Methods: Mirtazapine (10, 30, and 100 mu g), gabapentin (10, 30, and 100 mu g), and pregabalin (3, 10, and 30 mu g) were administered intrathecally to eight male Sprague-Dawley rats with orofacial neuropathic pain induced by chronic constriction injury of the infraorbital nerve that had been carried out 2 weeks previously. Stimulation using von Frey filaments (1.0 to 15.0 g) applied to skin innervated by the injured infraorbital nerve enabled the measurement of mechanical thresholds 0 to 180 minutes after drug injection. Time-course data for the dose-response effects were analyzed using two-way analysis of variance and the posthoc Tukey-Kramer multiple-comparison test. Results: Intrathecal administration of not only gabapentin and pregabalin but also mirtazapine reversed the lowered mechanical nociceptive thresholds produced by the nerve injury. The ED50 (95% confidence interval) was (in mu g) 49.00 (39.71-58.29) for mirtazapine, 54.84 (46.12-63.56) for gabapentin, and 13.47 (11.24-15.69) for pregabalin. Conclusion: Intraspinal administration of either mirtazapine, gabapentin, or pregabalin reverses the lowered facial mechanical thresholds produced in a rat model of trigeminal neuropathic pain.