GABA Receptor and KCa Channel-mediated Electrical Suppression in Anoxic Cortical Pyramidal Neurons of the Painted Turtle (Chrysemys picta bellii)

The high energetic cost of neuronal communication requires large amounts of ATP to maintain function. In mammals, oxidative phosphorylation is required to meet these energy demands and without sufficient oxygen supply neuronal hyperexcitability and excitotoxic cell death occurs. This catastrophic cascade of events does not happen in the freshwater painted turtle Chrysemys picta bellii, instead there is a coordinated downregulation of cellular ATP consuming processes to match the lower anaerobic ATP supply. To reduce the energetic burden during anoxia turtles have evolved a sophisticated network of neuroprotective mechanisms including channel arrest and spike arrest which combine to prevent membrane depolarization and activation of excessive electrical activity. The aim of my research was to identify and elucidate cellular mechanisms responsible for suppression of electrical activity in anoxic turtle dorsal cortical brain pyramidal neurons. Using electrophysiological and fluorescent imaging techniques I demonstrate for the first time that: 1) in turtle dorsal cortex a-aminobutyric acid (GABA) release increases during anoxia and enhances a unique GABAA receptor-mediated giant postsynaptic current, and shifts membrane potential to the GABA reversal potential (E GABA), reducing electrical excitability; 2) endogenous GABAergic mechanisms responsible for anoxia-tolerance also protect against a debilitating ischemic solution that mimics the cerebral fluid in the penumbral area that surrounds the infarct core; 3) in pyramidal neurons the open probability of Ca 2+-activated K+ channels decreases during anoxia and this is prevented by inhibition of protein kinase C; and 4) anoxia-mediated decreases in mitochondrial reactive oxygen species (ROS) production are sufficient to initiate a redox-sensitive inhibitory GABA signaling cascade that suppresses electrical activity. Together this research significantly contributes to our understanding of the turtle's natural anoxia-tolerant strategy in brain and highlights the integral role of channel arrest and GABAergic spike arrest in prevention of anoxic or ischemic cell damage.