The Glycolytic Metabolite, Fructose-1,6-bisphosphate, Blocks Epileptiform Bursts by Attenuating Voltage-Activated Calcium Currents in Hippocampal Slices.

Li-Rong Shao,Carl E Stafstrom,Guangxin Wang

doi:10.3389/fncel.2018.00168

Abstract

Manipulation of metabolic pathways (e.g., ketogenic diet (KD), glycolytic inhibition) alters neural excitability and represents a novel strategy for treatment of drug-refractory seizures. We have previously shown that inhibition of glycolysis suppresses epileptiform activity in hippocampal slices. In the present study, we aimed to examine the role of a “branching” metabolic pathway stemming off glycolysis (i.e., the pentose-phosphate pathway, PPP) in regulating seizure activity, by using a potent PPP stimulator and glycolytic intermediate, fructose-1,6-bisphosphate (F1,6BP). Employing electrophysiological approaches, we investigated the action of F1,6BP on epileptiform population bursts, intrinsic neuronal firing, glutamatergic and GABAergic synaptic transmission and voltage-activated calcium currents (ICa) in the CA3 area of hippocampal slices. Bath application of F1,6BP (2.5–5 mM) blocked epileptiform population bursts induced in Mg2+-free medium containing 4-aminopyridine, in ~2/3 of the slices. The blockade occurred relatively rapidly (~4 min), suggesting an extracellular mechanism. However, F1,6BP did not block spontaneous intrinsic firing of the CA3 neurons (when synaptic transmission was eliminated with DNQX, AP-5 and SR95531), nor did it significantly reduce AMPA or NMDA receptor-mediated excitatory postsynaptic currents (EPSCAMPA and EPSCNMDA). In contrast, F1,6BP caused moderate reduction (~50%) in GABAA receptor-mediated current, suggesting it affects excitatory and inhibitory synapses differently. Finally and unexpectedly, F1,6BP consistently attenuated ICa by ~40% without altering channel activation or inactivation kinetics, which may explain its anticonvulsant action, at least in this in vitro seizure model. Consistent with these results, epileptiform population bursts in CA3 were readily blocked by the nonspecific Ca2+ channel blocker, CdCl2 (20 μM), suggesting that these bursts are calcium dependent. Altogether, these data demonstrate that the glycolytic metabolite, F1,6BP, blocks epileptiform activity via a previously unrecognized extracellular effect on ICa, which provides new insight into the metabolic control of neural excitability.

Highlights

Accumulating evidence suggests that neuronal activity is tightly coupled to energy metabolism (Magistretti and Pellerin, 1996; Ames, 2000), which is not surprising given that the brain consumes ∼20% of the body’s energy
The rapid time to blockade with F1,6BP (4.2 ± 0.7 min, Figures 2A,B) was surprising, because it occurred faster than expected for an action mediated via an intracellular metabolic pathway
The main findings of the present study are: (1) the glucose metabolite and phosphate pathway (PPP) stimulator, F1,6BP, effectively blocks epileptiform activity in hippocampal slices; (2) F1,6BP does not stop intrinsic neuronal firing or significantly reduce AMPAR or NMDAR-mediated synaptic currents; (3) F1,6BP causes a reduction of GABAAR-mediated synaptic currents; (4) F1,6BP consistently attenuates Inhibits Voltage-Activated Calcium Currents (ICa); and (5) epileptiform bursts in this model are Ca2+ dependent. This is the first examination of the role of PPP in seizure reduction and the anticonvulsant action of F1,6BP in brain slices, focusing on underlying cellular mechanisms

Summary

Introduction

Accumulating evidence suggests that neuronal activity is tightly coupled to energy metabolism (Magistretti and Pellerin, 1996; Ames, 2000), which is not surprising given that the brain consumes ∼20% of the body’s energy. Metabolic manipulation provides a unique strategy to regulate excessive neuronal activity (such as seizures), which is important given that at least 1/3 of patients with epilepsy are resistant to drug treatment. Partial inhibition of glycolysis with 2-DG blocks seizure activity in brain slices and in several animal models of seizures and epileptogenesis (Garriga-Canut et al, 2006; Stafstrom et al, 2009; Forte et al, 2016; Shao and Stafstrom, 2017). Stimulation of the PPP has the potential to control excessive neuronal activity and seizures. A previous study reported that administration (i.p.) of the PPP stimulator F1,6BP is associated with anticonvulsant effects in animal models (Lian et al, 2007), supporting the hypothesis that PPP may play an important role in regulating seizures. How PPP alters neural excitability and seizure activity is largely elusive

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