Hydroxycarboxylic Acid Receptor 2 Mediates β‐hydroxybutyrate's Antiseizure Effect in Mice

Objective The ketogenic diet, a high‐fat, low‐carbohydrate regimen, is often used to treat drug‐resistant seizures and is being studied for Alzheimer's disease and other neuropsychiatric disorders. However, its mechanism of action remains unclear. β‐hydroxybutyrate, a primary circulating ketone body produced by the ketogenic diet, may mediate its effects on seizures by binding to a recently identified Gi‐coupled receptor: hydrocarboxylic acid receptor 2 (HCAR2). Methods RNAscope in situ hybridization assay and real‐time quantitative polymerase chain reaction were used to assess HCAR2 expression in the mouse brain. We generated HCAR2 − / − using the CRISPR‐Cas technique on an S129 mouse background. Whole‐cell current‐clamp was performed to measure the passive and active membrane properties of hippocampal dentate granule cells. The voltage‐clamp was performed to record synaptic currents. Two complementary in vivo mouse models—continuous hippocampal stimulation to induce status epilepticus (SE) and kindling—were used to induce seizures. Results HCAR2 was localized in dentate granule cells and microglia. In mice with HCAR2, β‐hydroxybutyrate reduced neuronal excitability by hyperpolarizing the resting membrane potential, raising the action potential threshold, and reducing the firing frequency of dentate granule cells. β‐hydroxybutyrate suppressed excitatory synaptic transmission. These effects were nullified in HCAR2 − / − mice. HCAR2 − / − mice showed no cognitive impairment. Moreover, β‐hydroxybutyrate did not affect seizures in HCAR2 − / − mice. However, it diminished both the duration and severity of seizures in HCAR2 + / + mice. Interpretation These findings demonstrate that HCAR2 mediates β‐hydroxybutyrate's antiseizure effects by regulating neuronal excitability and synaptic transmission. These studies propose a new mechanism for the antiseizure action of the ketogenic diet. ANN NEUROL 2026;99:809–824