Molecular Mechanisms Underlying Noradrenergic α_2A Receptor-mediated Inhibition Of Chronic Inflammatory Pain

Norepinephrine (NE) released from descending inhibitory system activatesα2-noradrenergic receptors in spinal dorsal horn to exert a critical role in painful information processing. Intrathecal injection of exogeneousα2 receptor agonists has also been shown to alleviate chronic pathological pain induced by peripheral tissue or nerve injury. Of an array ofα2 receptor subtypes, A-typeα2 receptor (α2A receptor) in spinal dorsal horn is especially important for producing analgesic action. However, the molecular mechanisms underlyingα2A receptor-mediated analgesia are still poorly defined.Previous studies have demonstrated that ionotropic NMDA-type glutamate receptors (NMDA receptors) play an essential role in the initiation and development of pathological pain. To investigate the correlation ofα2A receptor-mediated analgesia with NMDA receptors, the present study combined behavioral tests with molecular biology to deeply characterize the anti-nociception produced byα2A receptors against Complete Freund’s Adjuvant (CFA)-induced inflammatory pain.The resultant data showed that:(1) Intrathecal injection ofα2 receptor agonist clonidine or specificα2A receptor against guanfacine simultaneously repressed the increase in the synaptic expression of NMDA receptor NR1/NR2B subunits in spinal dorsal horn and alleviated inflammatory pain. Clonidine and guanfacine, however, generated little effects on the synaptic content of NR2A subunit, suggesting that activation of spinalα2A receptor depressed the over-expression of NR2B-containing NMDA receptors at synapses; (2) Activation ofα2 receptor had no effects on the total expression of each NMDA receptor subunit in spinal dorsal horn, suggesting thatα2 receptor inhibited inflammatory pain not by regulating gene transcription and protein translation; (3) Fyn phosphorylation of NR2B at tyrosine residue 1472 (Y1472) has been established to account for the synaptic accumulation of NR2B in spinal dorsal horn of inflamed animals; Interestingly, activation ofα2 receptor could completely abolished CFA-induced NR2B-Y1472 phosphorylation, suggesting that Fyn might be involved inα2 receptor-mediated analgesia; (4) Immunoprecipitation experiments showed that activation ofα2 receptor could significantly depress Fyn phosphorylation at tyrosine 418 (Y418) to inhibit Fyn catalytic activity; (5) Moreover, intrathecal injection of SFKs inhibitors completely occluded the inhibitory effects ofα2 receptor on NR2B-Y1472 phosphorylation, confirming thatα2 receptor might inactivate Fyn to reverse the expression of NR2B at synapses; (6) Activation of spinalα2 receptor redistributed C-terminal Src kinase (CSK) at synapses, suggesting thatα2 receptor might enhance the function of CSK to inactivate Fyn.Together, our data suggested that spinalα2 receptor might inhibit Fyn activity via CSK signaling to reduce the synaptic content of NR2B receptors, thus producing potent analgesic action against inflammatory pain.