ERK phosphorylation in intact, adult brain by α2-adrenergic transactivation of EGF receptors

Our previous work demonstrated dexmedetomidine-activated phosphorylation of extracellular regulated kinases 1 and 2 (ERK1/2) in primary cultures of mouse astrocytes and showed that it is evoked by α2-adrenoceptor-mediated transactivation of epidermal growth factor (EGF) receptors, a known response to activation of Gi/o- or Gq-coupled receptors [Li, B., Du, T., Li, H., Gu, L., Zhang, H., Huang, J., Hertz, L., Peng, L., 2008a. Signaling pathways for transactivation by dexmedetomidine of epidermal growth factor receptors in astrocytes and its paracrine effect on neurons. Br. J. Pharmacol. 154, 191–203]. Like most studies of transactivation, that study used cultured cells, raising the question whether a similar effect can be demonstrated in intact brain tissue and the brain in vivo. In the present study we have shown that (i) dexmedetomidine-mediated ERK1/2 phosphorylation occurs in mouse brain slices with a similar concentration dependence as in cultured astrocytes (near-maximum effect at 50 nM); (ii) intraperitoneal injection of dexmedetomidine (3 μg/kg) in adult mice causes rapid phosphorylation of the EGF receptor (at Y845 and Y992) and of ERK1/2 in the brain; (iii) both EGF receptor and ERK1/2 phosphorylation are inhibited by intraventricular administration of (a) AG 1478, a specific inhibitor of the receptor-tyrosine kinase of the EGF receptor; (b) GM 6001, an inhibitor of metalloproteinase(s) required for release of EGF receptor agonists from membrane-bound precursors; or (c) heparin, neutralizing heparin-binding EGF (HB-EGF). Thus, in intact brain HB-EGF, known to be expressed in brain, may be the major EGF agonist released in response to stimulation of α2-adrenoceptors, the released agonist(s) activate(s) EGF receptors, and ERK1/2 is phosphorylated as a conventional response to EGF receptor activation. Our previous paper (see above) showed that dexmedetomidine evokes no ERK1/2 phosphorylation in cultured neurons, but neurons respond to astrocyte-conditioned medium (and to EGF) with ERK1/2 phosphorylation. The present findings therefore suggest that EGF receptor transactivation in astrocytes in the mature brain in vivo is an important process in response to α2-adrenoceptor stimulation and may lead to phosphorylation of ERK1/2 both in astrocytes themselves and in adjacent neurons.