Oxygen-sensitive interneurons exhibit increased activity and GABA release during ROS scavenging in the cerebral cortex of the western painted turtle

Abstract

The western painted turtle ( Chrysemys picta bellii) has the unique ability of surviving several months in the absence of oxygen, which is termed anoxia. One major protective strategy that the turtle employs during anoxia is a reduction in neuronal electrical activity, which may result from a natural reduction in reactive oxygen species (ROS). We previously linked a reduction in ROS levels to an increase in γ-amino butyric acid (GABA) receptor currents. The purpose of this study is to understand how fast-spiking, GABA-releasing neurons respond to reductions in ROS and how this affects GABA release. Using a fluorescence-coupled enzymatic microplate assay for GABA, we found that anoxia, the ROS scavenger N-(2-mercaptopriopionyl)glycine (MPG), or the mitochondria-specific ROS scavenger MitoTEMPO resulted in a 2.5-, 2.0-, and 2.5-fold increase in extracellular GABA concentration, respectively. This phenomenon could be blocked with TTX, indicating that it is activity dependent. Using whole cell patch-clamping techniques, we found that fast-spiking, burst-firing GABAergic turtle neurons increase the duration and number of action potentials per burst by 26% and 42%, respectively, in response to ROS scavenging via MPG. These results suggest that the reduction in mitochondrially produced ROS that occurs during anoxia leads to increased GABA release, which promotes postsynaptic inhibitory activity through activation of GABA receptors. NEW & NOTEWORTHY This is a novel study examining the response of cerebral cortical stellate interneurons to anoxia and mitochondrial reactive oxygen species (ROS) scavenging with MitoTEMPO. Under both conditions burst firing increases in these cells, and we show that extracellular GABA release increases in the presence of the ROS scavenger. We conclude that in the anoxia-tolerant painted turtle brain, a decrease in ROS levels is an important low oxygen signal.

GABA receptor inhibition and severe hypoxia induce a paroxysmal depolarization shift in goldfish neurons

“…With severe hypoxia, ROS production and intracellular [ROS] are suppressed. Scavenging ROS during anoxia increases the whole-cell NMDA currents by 100% and also promotes direct GABA release from inhibitory interneurons in the anoxia-tolerant turtle, which does not happen normally (55,56). This suggests that ROS exposure might be a key regulator for anoxic protection in addition to GABAergic signaling.…”

Scavenging of reactive oxygen species mimics the anoxic response in goldfish pyramidal neurons

“…These results were mirrored by application of ROS scavengers and oxidants (H 2 O 2 ), indicating that ROS may be another important part of the mechanism (Hogg et al, 2015). However, the effect on pyramidal neurons seems to result from increased stellate neuron activity and GABA release that causes V m depolarization in the pyramidal neuron, which appears to be mediated by a decrease in stellate neuron mitochondrial [ROS] (Hawrysh and Buck, 2019).…”

Cytoskeletal Arrest: An Anoxia Tolerance Mechanism

“…In the basal forebrain of hibernating Syrian hamsters, cholinergic neurons express phosphorylated tau, while tau phosphorylation is largely absent in γ-amino butyric acid (GABA)ergic neurons [ 112 ]. In painted turtles [ 44 , 71 ] and goldfish [ 113 ], GABAergic inhibition of pyramidal cells by less abundant stellate cells [ 114 , 115 ] is a key mechanism of anoxia tolerance. If selective phosphorylation of tau occurs in anoxic turtles and fish, as it does in hamsters, then excitatory synapses could be inhibited while relatively less abundant inhibitory synapses are maintained.…”