Stimulation of β-adrenoceptors activates astrocytes and provides neuroprotection

Abstract

Our previous studies established that induction of growth factor synthesis and neuroprotection by the β 2-adrenoceptor agonist clenbuterol in vitro and in vivo was associated with the activation of astrocytes, the major source of trophic factors in the brain. In the present study, we further investigated the specificity of β 2-adrenoceptor-mediated effects on astrocyte activation and neuroprotection. In mixed hippocampal cultures neuroprotection against glutamate-induced cell death by clenbuterol (1 μM) was blocked by the β 1/2-adrenoceptor antagonist propranolol and the specific β 2-adrenoceptor antagonists 1-[2,3-(Dihydro-7-methyl-1 H-inden-4-yl)-oxy]-3-[(1-methylethyl)-amino]-2-butanol (ICI 118,551, 10 μM) and butoxamine (10 μM), while the β 1-adrenoceptor-selective antagonist metoprolol (10 μM) showed no effect. The β 2-adrenoceptor agonists clenbuterol (1–100 μM) and salmeterol (0.01–1 μM) induced profound morphological changes of cultured astrocytes which transformed into activated astroglia with pronounced dendrite-like processes. This phenomenon was blocked by butoxamine (1 mM) and propranolol (10 μM), but not by metoprolol (10 μM). However, similar morphological changes in astrocytes were also observed after stimulation of β 1-adrenoceptors by dobutamine (1–10 μM) and norepinephrine (1–10 μM). This effect was blocked by propranolol (10 μM) and metoprolol (10 μM) but not by butoxamine (1 mM), suggesting that stimulation of either β 1- or β 2-adrenoceptors was sufficient to induce activation of astrocytes. In addition, β 1-adrenoceptor stimulation by dobutamine (1–10 μM) protected hippocampal neurons against glutamate toxicity. In a model of focal cerebral ischemia in mice the cerebroprotective effect of clenbuterol (0.3 mg/kg) was blocked by propranolol (5 mg/kg) and butoxamine (5 mg/kg). Interestingly, the infarct size was reduced after co-treatment with clenbuterol (0.3 mg/kg) and metoprolol (5 mg/kg) as compared to clenbuterol treatment (0.3 mg/kg) alone. In conclusion, activation of astrocytes and neuroprotection can be achieved by stimulation of either β 1- or β 2-adrenoceptors in vitro, whereas in vivo neuroprotection is preferentially mediated through β 2-adrenoceptors.