The Hypoxia Mimetic Protocatechuic Acid Ethyl Ester Inhibits Synaptic Signaling and Plasticity in the Rat Hippocampus

Abstract

During hypoxia a number of physiological changes occur within neurons including the stabilization of hypoxia-inducible factors (HIFs). The activity of these proteins is regulated by O, Fe, 2-OG and ascorbate-dependant hydroxylases which contain prolyl-4-hydroxylase domains (PHDs). PHD inhibitors have been widely used and have been shown to have a preconditioning and protective effect against a later and more severe hypoxic insult. In this study we have investigated the neuroprotective effects of the PHD inhibitor, protocatechuic acid ethyl ester (ethyl 3,4, dihydroxybenzoate: EDHB), as well as its effects on synaptic transmission and plasticity in the rat hippocampus using electrophysiological techniques. We report for the first time, an acute concentration-dependent and reversible inhibitory effect of EDHB (10-100 μM) on synaptic transmission in the dentate gyrus but not Cornu Ammonis 1 (CA1) region which does not affect cell viability. This effect was attenuated through the application of the NMDA or GABA receptor antagonists, AP-5 and picrotoxin in the dentate gyrus. There were no changes in the ratio of paired responses after EDHB application suggesting a post-synaptic mechanism of action. EDHB (100 μM), was found to inhibit synaptic plasticity in both the dentate gyrus and CA1 regions. Application of exogenous Fe (100 μM) or digoxin (100 nM) did not reverse EDHB's inhibitory effect on synaptic transmission or plasticity in both regions, suggesting that its effects may be HIF-independent. These results highlight a novel modulatory role for the PHD inhibitor EDHB in hippocampal synaptic transmission and plasticity. A novel post-synaptic mechanism of action may be involved, possibly involving NMDA and GABA receptor activation.

Acute hypoxic exposure and prolyl-hydroxylase inhibition improves synaptic transmission recovery time from a subsequent hypoxic insult in rat hippocampus

“…We were also interested in the effect of PHD inhibition on the rate of synaptic transmission recovery during superfusion of 95%N2/5%CO2 and reoxygenation. We have previously demonstrated acute effects of PHD inhibitors on synaptic signalling and synaptic plasticity (LTP) in the rat hippocampus Lanigan et al, 2018), whereby DMOG, DFO and EDHB can have depressive effects on synaptic signalling in the hippocampus possibly acting through NMDA receptors. In the current experiments DMOG also resulted in a 10% decrease in synaptic transmission.…”

“…In vitro, various types of PHD inhibitors have been shown to stabilize HIF-1α, either by 2-OG antagonism (Noxalylglycine, DMOG, 3,4-dihydroxybenzoate, DHB), iron chelation (deferoxamine, DFO) or heavy metal substitution of iron (CoCl) (Epstein et al, 2001;Huang et al, 2003;Siddiq et al, 2005). We have recently demonstrated fast acting (within minutes) mechanisms of action of PHD inhibition on synaptic signalling and plasticity (Corocoran et al, 2013;Lanigan & O'Connor, 2018). In the following studies we have set out to investigate the effects of an initial hypoxic (95% N2/5% CO2) exposure or pre-treatment with the PHD inhibitor, DMOG, on the rate of recovery of synaptic transmission from a subsequent hypoxic exposure (see methods).…”

An Electrophysiological and Proteomic Analysis of the Effects of the Superoxide Dismutase Mimetic, MnTMPyP, on Synaptic Signalling Post-Ischemia in Isolated Rat Hippocampal Slices

“…Re-oxygenation was obtained by switching the gas from 95% N 2 /5% CO 2 back to 95% O 2 /5% CO 2 . Previous reports in our laboratory have reported O 2 brain slice levels that occur from this procedure [ 21 , 22 ]. OGD slices were perfused with glucose-free aCSF supplemented with equimolar sucrose and gassed with nitrogen for 10 min (95% N 2 /5% CO 2 ).…”