A comparison of intrathecally administered narcotic and nonnarcotic analgesics for experimental chronic neuropathic pain

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Abstract
The antinociceptive actions of morphine and tizanidine (an alpha 2-adrenergic agonist) administered intrathecally in a rat model of mononeuropathic pain were investigated. Tizanidine increased to normal levels the intensity of a noxious pressure stimulus required to induce paw withdrawal (p < 0.01) and decreased the duration of limb withdrawal from both normal-temperature and cooled floors in a dose-dependent manner (p < 0.01). Tizanidine had virtually no effect on the latency of paw withdrawal from a noxious heat stimulus. In comparison, morphine significantly decreased, in a dose-dependent manner, limb withdrawal from the normal-temperature and cooled floors and increased to cutoff values the withdrawal latencies of both noxious heat and pressure stimuli (p < 0.01). The effect of tizanidine was limited to the hyperalgesic limb and served to normalize reactive latencies, whereas morphine affected both hindlimbs and increased latencies to supranormal cutoff values. These data suggest that intrathecal tizanidine may be more specific than morphine in reversing the allodynia and hyperpathia associated with neuropathic pain states and may be of value in the management of patients with these clinical syndromes.