Noradrenergic α2 receptor attenuated inflammatory pain through STEP61/ERK signalling

Activation of noradrenergic α2 receptor in spinal dorsal horn effectively alleviates the pathological pain. However, the precise mechanisms underlying noradrenergic pain suppression are not fully understood. Convincing evidence has indicated that extracellular signal-regulated kinases 1 and 2 (ERK1/2) play a key role in spinal sensitization. The present study investigated the potential influence of noradrenergic α2 receptor agonist clonidine on ERK1/2 activity.

Clonidine was intrathecally given after intraplantar injection of complete Freund's adjuvant (CFA) in mice. The possible changes of ERK1/2 signalling were detected by Western blot, immunohistochemistry, co-immunoprecipitation and behavioural tests.

CFA significantly enhanced ERK1/2 activity in spinal dorsal horn, which was, however, greatly attenuated by clonidine application. Pretreatment with pertussis toxin abolished the inhibitory effect of clonidine on ERK1/2, suggesting the involvement of Giα subunit (Gi protein). Noradrenergic α2 receptor/Gi protein might repress ERK1/2 through cAMP-dependent protein kinase (PKA) pathway, because direct ERK1/2 activation by PKA agonist forskolin was also suppressed by clonidine. We found that 61 kD isoform of striatal-enriched protein phosphatase (STEP61) was a key intermediary for α2 receptor/Gi protein/PKA signalling to manipulate ERK1/2 activity. By reducing PKA-mediated phosphorylation of STEP61 at Ser221, clonidine significantly resumed the inhibition conferred by STEP61 on ERK1/2. Direct expression of STEP61 mutant devoid of Ser221 phosphorylation mimicked clonidine by inhibiting ERK1/2 and pain sensitization in CFA-injected mice.

The analgesic action produced by noradrenergic α2 receptor agonist clonidine involved the reversal of ERK1/2 hyperactivity in spinal dorsal horn of inflamed mice.