Dexmedetomidine Increases Hippocampal Phosphorylated Extracellular Signal-regulated Protein Kinase 1 and 2 Content by an α2-Adrenoceptor-independent Mechanism : Evidence for the Involvement of Imidazoline I1 Receptors

Background: Dexmedetomidine is a potent and selective α 2 -adrenoceptor (α 2 AR) agonist that exhibits a broad pattern of actions, including sedation, analgesia, and neuroprotection. Recent studies have emphasized the role of phosphorylated extracellular signal-regulated protein kinases (pERK1 and 2) in coupling rapid events such as neurotransmitter release and receptor stimulation long-lasting changes in synaptic plasticity and cell survival. Here, the authors hypothesized that dexmedetomidine increases pERK1 and 2 content and examined the mechanisms involved in this effect. Methods: The effects of dexmedetomidine and their sensitivity to various pharmacologic agents on expression of pERK1 and 2 were studied by Western blots in hippocampal slices obtained from rats, wild-type mice, and mice carrying targeted deletions of the α 2 AR subtypes. Results: Dexmedetomidine induced a concentration-related increase in the expression of pERK1 and 2 in rat hippocampal slices (EC 50 [95% confidence interval] for pERK1, 0.97 [0.68-1.37] μM; for pERK2, 1.15 [0.62-2.14] μM). This effect was insensitive to the inhibitors of the α 2 AR-mediated signaling pathway, to prazosin, and to PP2, an inhibitor of the focal adhesion kinase-Src kinases. In contrast, it was still present in mice deleted for each of the α 2 AR subtypes and was markedly decreased by the antagonist of the Il-imidazoline receptors efaroxan, by phospholipase C and protein kinase C inhibitors, and by PD 098059, a direct inhibitor of ERK1 and 2 phosphorylation. Conclusion: Dexmedetomidine increases the expression of pERKI and 2 via mechanisms independent of α 2 AR activation. The Il-imidazoline receptors likely contribute to these effects. The results may be relevant to some long-term effects (e.g., neuroprotective) of dexmedetomidine in the brain.