How does altered metabolism lead to seizure control? Partially filling the knowledge gap

Ketogenic diets and thermal pain: dissociation of hypoalgesia, elevated ketones, and lowered glucose in rats

Ketogenic diets (KDs) are high-fat, low-carbohydrate formulations effective in treating medically refractory epilepsy, and recently we demonstrated lowered sensitivity to thermal pain in rats fed a KD for 3 to 4 weeks. Regarding anticonvulsant and hypoalgesic mechanisms, theories are divided as to direct effects of increased ketones and/or decreased glucose, metabolic hallmarks of these diets. To address this point, we characterized the time course of KD-induced thermal hypoalgesia, ketosis, and lowered glucose in young male rats fed ad libitum on normal chow or KDs. A strict 6.6:1 (fat:[carbohydrates + protein], by weight) KD increased blood ketones and reduced blood glucose by 2 days of feeding, but thermal hypoalgesia did not appear until 10 days. Thus, ketosis and decreased glucose are not sufficient for hypoalgesia. After feeding a 6.6:1 KD for 19 days, decreased thermal pain sensitivity and changes in blood chemistry reversed 1 day after return to normal chow. Effects were consistent between 2 different diet formulations: a more moderate and clinically relevant KD formula (3.0:1) produced hypoalgesia and similar changes in blood chemistry as the 6.6:1 diet, thus increasing translational potential. Furthermore, feeding the 3.0:1 diet throughout an extended protocol (10-11 weeks) revealed that significant hypoalgesia and increased ketones persisted whereas low glucose did not, demonstrating that KD-induced hypoalgesia does not depend on reduced glucose. In separate experiments we determined that effects on thermal pain responses were not secondary to motor or cognitive changes. Together, these findings dissociate diet-related changes in nociception from direct actions of elevated ketones or decreased glucose, and suggest mechanisms with a slower onset in this paradigm. Overall, our data indicate that metabolic approaches can relieve pain.

PERSPECTIVE

Chronic pain is a common and debilitating condition. We show that a KD, a high-fat, very low carbohydrate diet well known for treating epilepsy, lowers sensitivity to thermal pain in rats. This reduced pain is not temporally correlated with hallmark diet-induced changes in blood glucose and ketones.

Purines and neuronal excitability: links to the ketogenic diet

ATP and adenosine are purines that play dual roles in cell metabolism and neuronal signaling. Acting at the A(1) receptor (A(1)R) subtype, adenosine acts directly on neurons to inhibit excitability and is a powerful endogenous neuroprotective and anticonvulsant molecule. Previous research showed an increase in ATP and other cell energy parameters when an animal is administered a ketogenic diet, an established metabolic therapy to reduce epileptic seizures, but the relationship among purines, neuronal excitability and the ketogenic diet was unclear. Recent work in vivo and in vitro tested the specific hypothesis that adenosine acting at A(1)Rs is a key mechanism underlying the success of ketogenic diet therapy and yielded direct evidence linking A(1)Rs to the antiepileptic effects of a ketogenic diet. Specifically, an in vitro mimic of a ketogenic diet revealed an A(1)R-dependent metabolic autocrine hyperpolarization of hippocampal neurons. In parallel, applying the ketogenic diet in vivo to transgenic mouse models with spontaneous electrographic seizures revealed that intact A(1)Rs are necessary for the seizure-suppressing effects of the diet. This is the first direct in vivo evidence linking A(1)Rs to the antiepileptic effects of a ketogenic diet. Other predictions of the relationship between purines and the ketogenic diet are discussed. Taken together, recent research on the role of purines may offer new opportunities for metabolic therapy and insight into its underlying mechanisms.