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

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Abstract
9a) 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 μm 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°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-β 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....