Guillermo Ceballos Reyes

Modulation of brain glutamate dehydrogenase as a tool for controlling seizures.

Abstract Glutamate (Glu) is a major excitatory neurotransmitter involved in epilepsy. Glu is synthesized by glutamate dehydrogenase (GDH, E.C. 1.4.1.3) and dysfunction of the enzymatic activity of GDH is associated with brain pathologies. The main goal of this work is to establish the role of GDH in the effects of antiepileptic drugs (AEDs) such as valproate (VALP), diazepam (DIAZ) and diphenylhydantoin (DPH) and its repercussions on oxygen consumption. Oxidative deamination of Glu and reductive amination of aketoglutarate (αK) in mice brain were investigated. Our results show that AEDs decrease GDH activity and oxygen consumption in vitro. In ex vivo experiments, AEDs increased GDH activity but decreased oxygen consumption during Glu oxidative deamination. VALP and DPH reversed the increase in reductive amination of αK caused by the chemoconvulsant pentylenetetrazol. These results suggest that AEDs act by modulating brain GDH activity, which in turn decreased oxygen consumption. GDH represents an important regulation point of neuronal excitability, and modulation of its activity represents a potential target for metabolic treatment of epilepsy and for the development of new AEDs.

Anticonvulsant Drugs, Brain Glutamate Dehydrogenase Activity and Oxygen Consumption

Glutamate dehydrogenase (GDH, E.C. 1.4.1.3.) is a key enzyme for the biosynthesis and modulation of glutamate (GLU) metabolism and an indirect γ-aminobutyric acid (GABA) source, here we studied the effect of anticonvulsants such as pyridoxal phosphate (PPAL), aminooxyacetic acid (AAOA), and hydroxylamine (OHAMINE) on GDH activity in mouse brain. Moreover, since GLU is a glucogenic molecule and anoxia is a primary cause of convulsions, we explore the effect of these drugs on oxygen consumption. Experiments were performed in vitro as well as in vivo for both oxidative deamination of GLU and reductive amination of α-ketoglutarate (αK). Results in vitro showed that PPAL decreased oxidative deamination of GLU and oxygen consumption, whereas AAOA and OHAMINE inhibited GDH activity competitively and also inhibited oxygen consumption when αK reductive amination was carried out. In contrast, results showed that in vivo, all anticonvulsants enhanced GLU utilization by GDH and also decreased oxygen consumption. Together, results suggest that GDH activity has repercussions on oxygen consumption, which may indicate that the enzyme activity is highly regulated by energy requirements for metabolic activity. Besides, GDH may participate in regulation of GLU and, indirectly GABA levels, hence in neuronal excitability, becoming a key enzyme in seizures mechanism.

Role of nitric oxide synthase on brain GABA transaminase activity and GABA levels

Abstract In an attempt to clarify the controversial role of nitric oxide (NO) in seizures, the effects of NO on brain GABA transaminase (GABA-T) activity and GABA levels were investigated. To this aim, the effects of the substrate (l-arginine) and inhibitors (Nω-nitro-l-arginine methyl ester, 7-nitroindazole) of NO synthase (NOS) on GABA-T activity and GABA levels in vitro and ex vivo were analyzed. In vitro NO diminished GABA-T activity and increased GABA. Ex vivo NO modified GABA-T activity and GABA levels biphasically. Inhibition of endothelial and neuronal NOS (eNOS and nNOS) had opposite effects on GABA-T activity and GABA levels, even during seizures induced by pentylenetetrazole. Different effects of NO on GABA-T activity and on GABA levels, depending on the NOS isoform involved, may explain its contradictory role in seizures, the endothelial NOS acting as an anticonvulsant and the neuronal NOS as a proconvulsant. nNOS inhibitors may represent a new generation of antiepileptics.