Lesions of dorsolateral funiculi (DLF) do not affect the depressive effects of systemic morphine upon dorsal horn convergent neuronal activities related to pain in the rat.

It has been shown that morphine could interact with supraspinal inhibitory controls which modulate the transmission of nociceptive messages at the spinal level. However, the way in which such interactions occur is still a subject of controversy. Based on behavioural experiments, it has been proposed that systemic morphine increases descending inhibitory controls travelling via the dorsolateral funiculus (DLF). To directly test this hypothesis from an electrophysiological standpoint, we have investigated the effects of morphine upon C-fibre responses of dorsal horn convergent neurones in rats with bilateral lesions of the DLF. Previous studies by our group have shown that 6 mg/kg morphine chlorhydrate was the intravenous mean effective dose for depressing by 50% (ED50) the C-fibre evoked responses of convergent neurones recorded at the lumbar level. In the present work, the effects of 6 mg/kg morphine were investigated under identical experimental conditions, except that a bilateral destruction of the dorsolateral funiculus (DLF) was performed previously. These lesions did not change the mean C-fibre evoked responses. Following morphine administration, the C-fibre evoked responses were depressed by 47.1 +/- 10.1% (n = 13) and reversal of these effects by naloxone was always observed. The A-fibre evoked responses, concurrently recorded, were not modified by the drug. As the depressive effects observed with this dose of morphine appear to be essentially of the same magnitude as those previously found in the intact or spinal preparations, we conclude that the DLF is not involved in the depressive effects of systemic morphine on dorsal horn convergent neurones.