Intravenous morphine does not modify dorsal horn touch-evoked allodynia in the mononeuropathic rat: a Fos study.

In a model of mononeuropathic pain (chronic constriction injury of the sciatic nerve, CCI), we have demonstrated that light touch stimuli (stroking) to the paw induced Fos-like immunoreactivity (Fos-LI) in the superficial and deep dorsal horn of the rat spinal cord (Catheline et al., Pain 80 (1999a) 347). The efficacy of opioids in neuropathic pain being controversial, we have tested the effects of morphine (0.3, 1 and 3 mg/kg intravenous, i.v.) on this spinal Fos-LI evoked by light tactile stimuli, which could be related to mechanical allodynia. Morphine did not change the level of spinal Fos-LI observed following light touch stimuli in the CCI rats (43 +/- 3, 38 +/- 7, and 37 +/- 4 Fos-LI neurones/40 microm L4-L5 section, respectively, for the three doses versus 32 +/- 4 in the control group). In contrast, the administration of 3 mg/kg of i.v. morphine reduced by 30% the number of Fos-LI neurones induced by heat stimulation (52 degrees C, 15 s duration) in CCI rats (P < 0.05) as in sham-operated rats. These effects were reversed by the systemic administration of naloxone. The lack of effect of morphine on touch-evoked Fos-LI in the superficial dorsal horn reinforces the assertion that dynamic mechanical allodynia is related to information transmitted by A-beta fibres, since opioid receptors are mainly located on thin primary afferent fibres. Our results provide a basis for a certain form of allodynia that is insensitive to morphine.