Lesions of the dorsolateral funiculus block supraspinal opioid delta receptor mediated antinociception in the rat

Previous experiments have demonstrated that [D-Ala²,Glu⁴]deltorphin (DELT) produces δ-receptor mediated antinociceptive effects when microinjected into the rat lateral ventricle and ventral medial medullary reticular formation (MRF), but not in the periaqueductal grey region (PAG). The present experiments were undertaken to further characterize the role of δ opioid agonists microinjected into the MRF and to explore the possibility of a descending pain modulatory system which might be linked to supraspinal δ opioid receptors. Rats received formalin into the dorsum of the right hindpaw and flinching responses were recorded. DELT given intracerebroventricularly (i.c.v.), intrathecally (i.th.) or into the MRF before formalin produced a dose-dependent and δ opioid receptor-mediated attenuation of both the first and second phases of the formalin-induced foot flinch response. DELT given i.c.v., i.th., or into the MRF also blocked formalin-induced increase in Fos-like immunoreactivity (FLI) in the dorsal horn of lumbar spinal cord ipsilateral to the formalin injection. Unilateral lesioning of the ipsilateral dorsolateral funiculus (DLF) did not alter nociceptive responses to formalin alone, but blocked the antinociceptive effect of DELT administered into the MRF; DELT was fully active in sham-DLF lesioned rats. Additionally, rats with DLF lesions did not show decreases in formalin-induced FLI in the ipsilateral lumbar spinal cord after injection of DELT into the MRF. These data suggest that δ opioid receptors in the MRF may be involved in activation of a descending inhibitory pain pathway projecting through the DLF to modulate tonic nociceptive input at the spinal level. Copyright (C) 1999 International Association for the Study of Pain. Published by Elsevier Science B.V.