Role of spinal delta(1) and delta(2) opioid receptors in the antinociception produced by microinjection of L-glutamate in the ventromedial medulla of the rat

This study examined the contribution of spinal delta(1) and delta(2) opioid receptors to the antinociception produced by microinjection of L-glutamate in either the nucleus raphe magnus (NRM) or the nucleus reticularis gigantocellularis pars alpha (NGCp alpha) of the rat. Intrathecal (i.t.) pretreatment with 1 mu g of 7-benzylidinenaltrexone (BNTX), a delta(1) opioid receptor antagonist, did not antagonize the increase in tail flick latency (TFL) produced by microinjection of L-glutamate in either the NRM or the NGCp alpha. In contrast, i.t. pretreatment with 3 mu g of naltriben (NTB), a delta(2) opioid receptor antagonist, completely antagonized the increase in TFL evoked by microinjection of L-glutamate in the NRM, but did not antagonize the increase in TFL evoked from the NGCp alpha. These results suggest that the antinociception produced by activation of these bulbospinal pathways is predominantly mediated by spinal delta(2) opioid receptors and that there is little, if any,contribution by spinal delta(1) opioid receptors. (C) 1997 Elsevier Science B.V.