A Complex Neuroprotective Effect of Alpha-2-Adrenergic Receptor Agonists in a Model of Cerebral Ischemia–Reoxygenation In Vitro

Abstract

The mechanism of the neuroprotective action of α2-adrenergic receptor agonists in a model of oxygen-glucose deprivation (OGD) in vitro was investigated. Using fluorescent microscopy, immunostaining, inhibitor analysis, and real-time PCR analysis, we demonstrated that OGD evokes a biphasic [Ca2+]i increase in all cells. Initial Ca2+ impulse in astrocytes and Ca2+ oscillations in neurons were followed by a slower global increase of [Ca2+]i in both cell populations. Accumulation of pro-inflammatory factors, such as IL1b and TNFα, was observed during 40-min OGD. It was established that reoxygenation is followed by hyperexcitation of neurons, caspase-3 activation, and subsequent cell death. We showed that a 24-h pretreatment of cell cultures with selective α2-adrenergic receptor agonists guanfacine and UK-14,304 abolished the global [Ca2+]i increase in astrocytes and neurons but did not suppress the first phase of the OGD-induced Ca2+ impulse in astrocytes. In addition, the number of dead cells after OGD was decreased in cell cultures pretreated with the α2-agonists. Guanfacine inhibited caspase-3 activation and suppressed apoptosis in our experiments. In particular, the expression of antiapoptotic genes Stat3 and Bcl-2 was enhanced after the pretreatment with guanfacine. On the contrary, the expression of proapoptotic genes (Socs-3, p53, fas, and Ikk) was decreased. Moreover, application of guanfacine evoked Ca2+ response in astrocytes and led to Ca2+-mediated ATP release and this way suppressed hyperexcitation of the neurons. Thus, activation of astrocytes and Ca2+-mediated ATP release possibly contribute to the complex neuroprotective effects of the α2-adrenergic receptor agonists.