Regulation of GABA Metabolism in Discrete Rabbit Brain Regions under Methoxypyridoxine—Regional Differences in Cofactor Saturation and the Preictal Activation of Glutamate Decarboxylase Activity¹

Abstract

The activities of the enzymes of the GABA system, glutamate decarboxylase (GAD) and GABA‐transaminase, were measured in discrete regions of the rabbit brain before the onset and during the course of sustained epileptiform seizures induced by the vitamin B6, analogue methoxypyridoxine (MP). GAD activities were measured in a reaction mixture alternatively containing the cofactor pyridoxal‐5′‐phosphate (PLP) in excess or containing no PLP (holoenzyme of GAD). A comparison between these two estimations showed that the apoenzyme of GAD is only partially saturated with cofactor and that the degree of saturation varied from brain area to brain area, being highest in cerebellar cortex and lowest in substantia nigra. Holoenzyme activity fell steeply after administration of 100 mg/kg MP. The regional degree of enzyme inhibition by MP was a function of the saturation of the apoenzyme with cofactor; i.e., a low rate of saturation resulted in a high degree of inhibition, and vice versa. That GAD from the regio inferior of the hippocampus did not fit into the scheme (strong inhibition is present although the degree of saturation is high) is discussed in view of the role of the hippocampus in seizure genesis and generalization. Inhibition of GAD activity by MP was completely reversible in vitro by excess PLP. Before the onset of seizures but not during their course, apoenzyme activity surpassed control levels. This preictal activation is significant in regio inferior of hippocampus, in superior colliculus, and in cerebellar cortex. GABA‐transaminase activities were not significantly altered. The present study demonstrates that only investigation during the preictal period and in regional brain areas can reveal changes specific for the drug and perhaps representing the cause for seizure development, without being masked by additional alterations resulting from the severe functional and metabolic derangement during the ictal events. Thereby, it was disclosed that a decrease in vivo in the level of the enzyme product, GABA, is able to activate GAD.

Different distributions of GAD65 and GAD67 mRNAS suggest that the two glutamate decarboxylases play distinctive functional roles

“…Indeed, regional differences in the fraction of GAD that is bound to PLP led to the initial suggestions that two forms of GAD exist (Nitsch, 1980;Itoh and Uchimura, 1981;Denner and Wu, 1985).…”

Large dense-core vesicle exocytosis and membrane recycling in the mossy fibre synapses of the rabbit hippocampus during epileptiform seizures

“…Our studies have shown that MP, probably in its phosphorylated form, competitively displaces pyridoxal phosphate from its binding sites at the protein component of the GABA-synthesizing enzyme glutamate decarboxylase (Nitsch, 1980). Glutamate decarboxylase inhibition reaches its peak before the onset of the seizures (Nitsch, 1980), resulting in a substantial decrease in the content of the inhibitory transmitter GABA , especially in GABA-rich brain centres (Nitsch & Okada, 1976). The already preictally significant drop in GABA content can be interpreted as at least one of the factors giving rise to the seizure (Nitsch & Okada, 1976).…”

Alterations in the Content of Amino Acid Neurotransmitters Before the Onset and During the Course of Methoxypyridoxine‐Induced Seizures in Individual Rabbit Brain Regions

“…The antagonistic preictal behavior of taurine and GABA is discussed with respect to synthesis, uptake from the blood, and antiepileptic properties. Key Words: Epileptic Since methoxypyridoxine, as a vitamin B6 analogue, was expected to be a potent inhibitor of the glutamate decarboxylase (GAD) activity, previous biochemical studies have focused on the dramatic increase in y-aminobutyric acid (GABA) content taking place before the appearance of generalized seizures (Nitsch and Okada, 1976) and the activity of related enzymes (Nitsch, 1980). Apart from a preliminary communication on the preictal increase in taurine level , other relevant amino acids have been virtually neglected.…”