Novel aspect of ketone action: β-hydroxybutyrate increases brain synthesis of kynurenic acid in vitro.

Ketone bodies formed during ketogenic diet or non-treated diabetes mellitus may exert neuroprotective and antiepileptic effects. Here, we assessed the influence of ketone body, β-hydroxybutyrate (BHB) on the brain synthesis of kynurenic acid (KYNA), an endogenous antagonist of glutamatergic and α7-nicotinic receptors. In brain cortical slices and in primary glial cultures, BHB enhanced KYNA production. KT 5270, an inhibitor of protein kinase A, has prevented this action. At hypoglycemia, under pH 7.0 and 7.4, profound (15 mM BHB), but not mild (3 mM) ketosis increased synthesis of KYNA. In paradigm resembling diabetic ketoacidosis in vitro (30 mM glucose, pH 7.0), neither mild nor profound ketosis influenced the production of KYNA. At pH 7.4 and in 30 mM glucose though, both mild and severe ketonemia evoked an increase of KYNA production. The activity of KYNA biosynthetic enzymes, KAT I and KAT II, in cortical homogenate was not altered by BHB (0.05-10.0 mM). However, in cultured glial cells exposed to BHB (10 mM), the activity of KATs increased. This effect was reversed by the co-incubation of cells with KT 5270. Presented data reveal a novel mechanism of action of BHB. Increased synthesis of KYNA in the presence of BHB is most probably mediated by protein kinase A-dependent stimulation of KATs expression/activity leading to an increase of KYNA formation. Ensuing attenuation of the excessive excitatory glutamate-mediated neurotransmission may, at least in part, explain the neuroprotective actions of BHB.