Consistent with their roles in mediating neuronal inhibition, deficits in GABAB receptor function play significant roles in both neurological and psychiatric disorders that include ischemia, epilepsy, schizophrenia, addiction, and nociception. Given the clear significance of GABAB receptors in neurotransmission it is of fundamental importance to understand the endogenous mechanisms neurons use to regulate their activity under both control and pathological conditions. Previous studies from our laboratory have illustrated that GABAB receptors are phosphorylated by intimately-associated AMP-dependent protein kinase (AMPK) a process that enhances their activity, together with the strength of neuronal inhibition. AMPK is a key enzyme in regulating cellular levels of ATP and in mediating cellular responses to metabolic stressors that include ischemia and anoxia. Transient anoxia also increases AMPK-phosphorylation of GABAB receptors and neuronal survival after this trauma. In concordance with our results it is well documented that the GABAB agonist baclofen is neuroprotective in animal models of ischemia and is used """"""""off-label"""""""" to aid recovery of patients after stroke. Given the established role of glutamate receptors in ischemia-induced excitotoxicity we have begun to explore the link between their activation and the regulation of AMPK-dependent phosphorylation of GABAB receptors. These preliminary studies have led us to formulate our central hypothesis: Prolonged exposure of neurons to glutamate via the activation of NMDA receptors inhibits AMPK-dependent phosphorylation of GABAB receptors. This deficit in phosphorylation enhances GABAB receptor ubiquitination, promoting their endocytosis from the plasma membrane and thereby reducing the efficacy of neuronal inhibition mediated by these critical GPCRS. Our experiments will focus on four distinct but related specific aims. 1. We will test the hypothesis that glutamate receptors modulate GABAB receptor phosphorylation and functional coupling. 2. We will test the hypothesis that glutamate receptor activation modulates the endocytosis and endocytic sorting of GABAB receptors. 3. We will test the hypothesis that inhibiting PP2A and proteasome activity regulates the effects of glutamate receptor activation on GABAB receptor cell surface stability. 4. We will test the hypothesis that blocking GABAB receptor dephosphorylation promotes neuronal survival after anoxia.
GABAB receptors are G-protein coupled receptors (GPCRs) that mediate slow and prolonged synaptic inhibition in the brain. Results of the experiments in this proposal will provide insights into the biochemical regulation of neuronal GABAB receptors under control and after anoxic insult similar to that occurring following ischemic brain injury. Given the critical roles that GABAB receptors play in regulating neuronal inhibition our studies will have the potential to make contributions to the development of novel therapeutics to enhance neuronal survival after ischemic injury and to alleviate other debilitating disorders including epilepsy, depression, schizophrenia, and pain.
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