Although the efficacy of morphine in a neuropathic pain state is somewhat controversial, spinally administered morphine reversed the tactile allodynia in a previous animal study. Using a behaviorial test, we examined the involvement of serotonergic and adenosine receptors in the mechanism of the antiallodynic action of the intrathecal morphine by injection of serotonergic and adenosine antagonist in a rat model of neuropathic pain induced by a spinal nerve ligation.

Male Sprague-Dawley rats were prepared with a tight ligation of the left lumbar 5th and 6th spinal nerve and a chronic lumbar intrathecal catheter implantation. Morphine 1 microgram was administered intrathecally to attenuate the tactile allodynia. Methysergide 10 microgram and 30 microgram, theophylline 20 microgram was administered intrathecally before and after the injection of morphine in order to investigate the reversal of an increased allodynic threshold by morphine. The allodynic thresholds for the left hindpaw withdrawl to von Frey hairs were assessed and converted to %MPE.

The tactile allodynic threshold was significantly increased by 1 microgram of intrathecal morphine (P < 0.05). Methysergide 10 microgram and 30 microgram, but not theophylline 20 microgram, reversed significantly the antiallodynic effect of intrathecal morphine in pre- and post-treatment (P < 0.05).

The results suggested that the mechanism of tactile antiallodynia induced by intrathecal morphine appears to be mediated by serotonin receptor system at the spinal level in the rat model of spinal nerve ligation.