Activation of the spinal sigma-1 receptor enhances NMDA-induced pain via PKC- and PKA-dependent phosphorylation of the NR1 subunit in mice

Background and purpose: Previously we demonstrated that the spinal sigma-1 receptor (Sig-1 R) plays an important role in pain transmission, although the exact mechanism is still unclear. It has been suggested that Sig-1 R agonists increase glutamate-induced calcium influx through N-methyl-D-aspartate (NMDA) receptors. Despite data suggesting a link between Sig-1 Rs and NMDA receptors, there are no studies addressing whether Sig-1 R activation directly affects NMDA receptor sensitivity. Experimental approach: We studied the effect of intrathecal (i.t.) administration of Sig-1 R agonists on protein kinase C (PKC) and protein kinase A (PKA) dependent phosphorylation of the NMDA receptor subunit NR1 (pNR1) as a marker of NMDA receptor sensitization. In addition, we examined whether this Sig-1 R mediated phosphorylation of NR1 plays an important role in sensory function using a model of NMDA-induced pain. Key results: Both Western blot assays and image analysis of pNR1 immunohistochemical staining in the spinal cord indicated that i.t. injection of the Sig-1 R agonists, PRE-084 or carbetapentane dose dependently enhanced pNR1 expression in the murine dorsal horn. This increased pNR1 expression was significantly reduced by pretreatment with the specific Sig-1 R antagonist, BD-1047. In another set of experiments Sig-1 R agonists further potentiated NMDA-induced pain behaviour and pNR1 immunoreactivity and this was also reversed with BD-1047. Conclusions and implications: The results of this study suggest that the activation of spinal Sig-1 R enhances NMDA-induced pain via PKC- and PKA-dependent phosphorylation of the NMDA receptor NR1 subunit.