Comparison of morphine-induced effects on thermal nociception, mechanoreception, and hind limb flexion in chronic spinal rats.

The effect of systemic morphine on 3 behaviors in the same group of chronic spinal rats was examined: the tail-flick (TF) reflex to a noxious thermal stimulus, limb withdrawal (LW) to mechanoreceptor (von Frey hair) stimulation, and hind limb flexion (flexor reflex [FR]) elicited by innocuous electrical stimulation of the toes. Compared with intact rats, the potency of morphine on both the TF and the hind paw (but not the forepaw) LW response was significantly reduced. Morphine's effect on the FR depended on the dose. The lowest dose (1.0 mg/kg) produced no change, 4.0 mg/kg decreased response magnitude by approximately 50% (indicating an antispastic effect), and 8.0 mg/kg increased flexor magnitude by 100%. The concurrent TF and FR assays revealed a dissociation of morphine's effects in that the highest dose (8.0 mg/kg) significantly inhibited the nociceptive TF response but facilitated the FR in the same chronic spinal rats. This outcome may be relevant to the phenomenon of "opioid-related myoclonus" recently described in cancer patients, which "was highly associated with nerve dysfunction due to spinal cord lesions" (S. Mercadante, 1998, p. 6).