The ketogenic diet: metabolic influences on brain excitability and epilepsy

Abstract

A dietary therapy for pediatric epilepsy known as the ketogenic diet has seen a revival in its clinical use during the past decade. Although the underlying mechanism of the diet remains unknown, modern scientific approaches, such as the genetic disruption of glucose metabolism, are allowing for more detailed questions to be addressed. Recent work indicates that several mechanisms may exist for the ketogenic diet, including disruption of glutamatergic synaptic transmission, inhibition of glycolysis, and activation of ATP-sensitive potassium channels. Here, we describe on-going work in these areas that is providing a better understanding of metabolic influences on brain excitability and epilepsy.

Figure 1. Ketone body inhibition of vesicular glutamate transport

(A) Transport of glutamate into synaptic vesicles occurs via vesicular glutamate transporters (VGLUT). Acetoacetate (ACA), a ketone body whose level is elevated in patients on the ketogenic diet, was shown to be an inhibitor of VGLUT, competing with chloride for the site of allosteric regulation [14]. (B) When ACA was applied to hippocampal brain slices, glutamatergic synaptic transmission onto CA1 pyramidal cells was significantly reduced [14]. This reduced glutamatergic signaling may reduce brain excitability and potentially contribute to the mechanism of the ketogenic diet. Abbreviations: mEPSC, miniature excitatory postsynaptic current, **, P< 0.01. Reproduced, with permission, from [14].

Figure 2. K_ATP channels mediate the seizure resistance of BAD mutant mice

BAD regulates mitochondrial metabolism of glucose and ketone bodies. In BAD knockout or mutant animals, glucose utilization is reduced while ketone body metabolism is elevated [48]. This metabolic switch results in increased K_ATP channel activity, as demonstrated by cell-attached recordings of K_ATP channels in dentate granule neurons in brain slices from BAD knockout and mutant brains [49]. BADdeficient mice are more resistant to seizures induced by injection of kainic acid (bottom left panel) and show reduced cortical seizure activity as recorded by electroencephalogram (EEG) (bottom right panel) [49]. Reproduced, with permission, from [49] (middle and bottom panels).

Figure 3. Potential mechanisms of the ketogenic diet

(A) Schematic diagram illustrating the key metabolic pathways and diet targets. 1) Ketone bodies enter neurons via the monocarboxylate transporter (MCT) and are directly metabolized by mitochondria. 2) Glucose levels are reduced during the ketogenic diet as ketone bodies become the major fuel in the brain. Reduction of glucose metabolism in BAD mutant mice, or inhibition of glycolysis using 2-deoxyglucose, results in seizure protection [49, 56]. 3) Ion pumps maintain ion homeostasis and consume intracellular ATP. Neuronal function is energetically demanding [81], which allows for changes in metabolism to modulate neuronal function. 4) K_ATP channels hyperpolarize neurons and may link changes in metabolism to neuronal excitability [35, 36]. Decreases in cytosolic ATP, possibly via increased pump consumption of ATP, lead to increased K_ATP channel activity [26, 27]. 5) Extracellular adenosine can signal through A₁ receptors and G-proteins to activate hyperpolarizing conductances like the K_ATP channel [39, 40]. Release of adenine nucleotides through pannexin hemichannels or by exocytosis can modulate extracellular concentrations of adenosine. 6) Antioxidant capacity can be increased by shifts of glucose metabolism into the pentose phosphate pathway (PPP). This improves cellular handling of ROS and can act to protect neurons; such neuroprotection may be important for antiepileptogenesis [– 65]. 7) Gene expression can respond to changes in cellular metabolism, with downstream effects on excitability. 2-deoxyglucose inhibits glycolysis, which results in decreased in BDNF expression [56]. Other changes in gene expression are likely to occur during the ketogenic diet. (B-D) Hypotheses for how metabolic changes could result in reduced hyperexcitability. B. Reduced Glycolysis – reduced BDNF expression. Reduced glycolysis (for instance, by application of 2-deoxyglucose) leads to a decrease in cytosolic NADH, which in turn represses BDNF expression [56]. BDNF and its receptor, TrkB, have been implicated in epileptogenesis (ie. in the development of spontaneous seizures after status epilepticus) [56, 57]. C. Increased adenosine – Increased K_ATP. In conditions of low glucose, extracellular adenosine levels may be elevated via release of adenosine or ATP from neurons through pannexin hemichannels [40]. Activation of adenosine A₁ receptors could then increase K_ATP channel activity and reduce neuronal excitability. D. Metabolic switch – Increased K_ATP. A switch from glucose to ketone body metabolism reduces the activity of glycolysis [4]. The reduced glycolytic production of ATP, combined with pump consumption of ATP during neuronal activity, could lead to reduced ATP levels near the plasma membrane. This in turn would increase K_ATP channel activity and reduce the excitability of neurons. The metabolic switch can be induced either by increased availability of ketone bodies (supplied by the liver via blood vessels, when on the ketogenic diet) or by mutation of BAD (white outlines) [49]. Ketone bodies may also be produced from fatty acids by neighboring astrocytes [71].