Neuroprotective action of agonists and modulators of A 1 adenosine receptors upon hyperexcitation: mechanism of the antiepileptic activity and role of neuron-glial interaction

Hyperexcitation of neuronal networks is believed to be the main reason for the excitotoxic death of neurons in different central nervous system pathologies, including epilepsy, ischemic stroke, and traumatic brain injury. G i-coupled receptors can be considered as promising targets for the development of new neuroprotectors. Here, we studied the anticonvulsant activity of the agonists and positive allosteric modulators (PAM) of A 1 adenosine receptors (A 1 Rs). Our experiments demonstrate that A 1 R agonists, CCPA and N 6-cyclohexyladenosine (N 6-CHA), suppress hyperexcitation in three different in vitro models , including acute glutamate excitotoxicity, NH 4 Cl-and bicuculline-induced epileptiform activity. We have found that the inhibitory action of the agonists is mediated by the activation of not only the neuronal A 1 Rs but also the astrocytic receptors. In astrocytes, A 1 R agonists enhance GABA release, possibly via induction of calcium transients. Using inhibitory analysis, we have demonstrated that Gβγ-mediated activation of phospholipase C and subsequent Ca 2+ mobilization from internal stores are essential for generating calcium transients in astrocytes following N 6-CHA application. We have shown first that Ca 2+-dependent activation of protein kinase C, which is involved in the mechanism of GABA release by astrocytes, is a pivotal step in the realization of the antiepileptic action of A 1 R agonists. Moreover, using the model of epileptiform activity induced by GABA A R blockade, we have shown that PAMs, PD81723 and VCP171, also suppress hyperexcitation. Furthermore, using the picrotoxin-induced epilepsy model in mice, we demonstrated that A 1 R agonists exhibit significant anticonvulsant effects and improve animal survival. The PAMs PD81723 and VCP171, when administered one hour before seizure induction, did not significantly affect seizure severity or survival rates. However, chronic administration of VCP171 produced a pronounced anticonvulsant effect and significantly increased survival.. Importantly, PAMs provided therapeutic benefits without significantly affecting overall activity levels in mice. Thus, our study demonstrates that both agonists and PAMs of A 1 R can be considered as potential therapeutic agents with antiepileptic and neuroprotective activity.