Role of spinal σ1 and σ2 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 σ1 and σ2 opioid receptors to the antinociception produced by microinjection of L-glutamate in either the nucleus raphe magnus (NRM) or the nucleus reticularis gigantocellularis pars a (NGCpα) of the rat. Intrathecal (i.t.) pretreatment with 1 µg of 7-benzylidinenaltrexone (BNTX), a σ1 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 NOCpα. In contrast, i.t. pretreatment with 3 µg of naltriben (NTB), a σ2 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 NGCpα. These results suggest that the antinociception produced by activation of these bulbospinal pathways is predominantly mediated by spinal σ2 opioid receptors and that there is little, if any, contribution by spinal 61 opioid receptors.