Abstract
One important function of GABAB receptors is the control of neuronal activity to prevent overexcitation and thereby excitotoxic death, which is a hallmark of cerebral ischemia. Consequently, sustained activation of GABAB receptors with the selective agonist baclofen provides neuroprotection in in vitro and in vivo models of cerebral ischemia. However, excitotoxic conditions severely downregulate the receptors, which would compromise the neuroprotective effectiveness of baclofen. On the other hand, recent work suggests that sustained activation of GABAB receptors stabilizes receptor expression. Therefore, we addressed the question whether sustained activation of GABAB receptors reduces downregulation of the receptor under excitotoxic conditions and thereby preserves GABAB receptor-mediated inhibition. In cultured neurons subjected to oxygen and glucose deprivation (OGD), to mimic cerebral ischemia, GABAB receptors were severely downregulated. Treatment of the cultures with baclofen after OGD restored GABAB receptor expression and reduced loss of neurons. Restoration of GABAB receptors was due to enhanced fast recycling of the receptors, which reduced OGD-induced sorting of the receptors to lysosomal degradation. Utilizing the middle cerebral artery occlusion (MCAO) mouse model of cerebral ischemia, we verified the severe downregulation of GABAB receptors in the affected cortex and a partial restoration of the receptors after systemic injection of baclofen. Restored receptor expression recovered GABAB receptor-mediated currents, normalized the enhanced neuronal excitability observed after MCAO and limited progressive loss of neurons. These results suggest that baclofen-induced restoration of GABAB receptors provides the basis for the neuroprotective activity of baclofen after an ischemic insult. Since GABAB receptors regulate multiple beneficial pathways, they are promising targets for a neuroprotective strategy in acute cerebral ischemia.
Highlights
The main cause for progressing neuronal death in cerebral ischemia is excitotoxicity
We show that sustained activation of GABAB receptors after an ischemic insult restores downregulated GABAB receptor expression and function, which limited excitotoxic neuronal death
This study is based on previous work demonstrating that GABAB receptors are severely downregulated under excitotoxic conditions (Guetg et al, 2010; Maier et al, 2010, 2014; Terunuma et al, 2010; Kim et al, 2011; Kantamneni et al, 2014; Zhu et al, 2015; Huang et al, 2017) and indications from work on GABAB receptors recombinantly expressed in HEK 293 cells that sustained activation of the receptors with baclofen stabilized receptor expression on the cell surface (Zhang et al, 2015)
Summary
The main cause for progressing neuronal death in cerebral ischemia is excitotoxicity. Activation of GABAB receptors by the neurotransmitter GABA provides long lasting tonic inhibition mainly by suppression of voltage gated Ca2+ channel activity and activation of inwardly rectifying K+ channels. This inhibits neurotransmitter release via GABAB receptors located at presynaptic sites and reduces neuronal excitability via postsynaptic receptors (Gassmann and Bettler, 2012). Under the strong excitotoxic conditions caused by cerebral ischemia, GABAB receptors are downregulated (Kim et al, 2011; Zhu et al, 2015; Huang et al, 2017) and are apparently no longer able to protect neurons from overexcitation. Studies on cultured neurons revealed that downregulation of GABAB receptors is primarily due to aberrant sorting of the receptors to lysosomal degradation on the expense of recycling them to the cell surface (Guetg et al, 2010; Maier et al, 2010; Terunuma et al, 2010; Kantamneni et al, 2014)
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