The Use of Inhibitors of GABA‐Transaminase for the Determination of GABA Turnover in Mouse Brain Regions: An Evaluation of Aminooxyacetic Acid and Gabaculine

Abstract

The accumulation of γ‐aminobutyric acid (GABA) after inhibition of GABA‐T (4‐aminobutyrate: 2‐oxoglutamate aminotransferase, EC 2.6.1.19) by various doses of aminooxyacetic acid (AOAA) and gabaculine was studied in four different regions of the mouse brain. The dose‐response curve for GABA accumulation after treatment with AOAA was linear up to 10 mg/kg i.p., and then leveled off. The increase in GABA accumulation after gabaculine treatment was linear up to 100 mg/kg i.p. No further increase was observed with doses up to 300 mg/kg i.p. The selectivity of both GABA‐T inhibitors was assessed by measuring their effects on the content of free amino acids in mouse brain. Apart from the substantial increase in the GABA concentration, there were significant decreases in the content of glutamic acid, aspartic acid, alanine and glutamine, and an increase in ornithine content after administration of gabaculine. The same changes in amino acid content were observed after treatment with AOAA, but the level of lysine was also increased and the change in alanine level was biphasic. All these changes, however, were very small compared with the large increase in GABA level. A method for estimating the rate of the GABA turnover in vivo by measuring the initial rate of GABA accumulation after administration of AOAA or gabaculine is proposed, and the validity of the two techniques is discussed. The effect of diazepam on GABA levels and on the gabaculine‐induced accumulation of GABA was studied. The results obtained with diazepam show that this method can provide valuable insight into the effects of drugs on GABAergic mechanisms in vivo.

Inhibition of the rate of GABA synthesis in regions of rat brain following a convulsion

“…This approach has been used by several other groups (e.g. Bernasconi et al, 1982) and in the current studies has been found to produce comparable data to that obtained using the technique of gas chromatography -mass spectrometry (mass fragmentography) coupled with infusion of ["Cl-glucose (Bertilsson & Costa, 1976). Some of the findings have been reported in preliminary form to the British Pharmacological Society (Green et al, 1985).…”

“…Nevertheless the technique of GABA transaminase inhibition does give a good indication of changes in GABA synthesis rate following drug administration since GABA transaminase is the major degradation pathway for the transmitter (Loscher, 1980). Bernasconi et al (1982) demonstrated that amino-oxyacetic acid (AOAA) was arguably the best drug to use, being a more rapid inhibitor than gabuculline and further suggested, in contrast to the data of Walters et al (1978) Bernasconi et al, 1982) Roberts & Simonsen (1963). Brain regions were rapidly removed from rats killed by thoracic stun and decapitation and homogenised at a concentration of 10mg ml-' in the following mixture (hippocampi were pooled from two animals): potassium phosphate buffer (50mM), pH 6.4 containing 0.025% w/v EDTA; 0.…”

Effect of Increased Brain GABA Concentrations on Breathing in Unanesthetized Newborn Rabbits

“…At the end of HX-III, animals were injected with 3-MPA (IP-3-mercaptopropionic acid, an inhibitor of glutamic acid decarboxylase, a GABA-synthesizing enzyme; 100 mg/kg i.p.) to prevent postmortem formation of GABA [17,19]. Two minutes later they were sacrificed by decapitation.…”

“…Since AOAA inhibits GABA-T, the amount of GABA accumulated after AOAA is assumed to represent the amount of GABA synthesized [17]. Accumulation of GABA after AOAA is linear for 2 h after the injection [19].…”

“…Therefore, the first aim of this study was to evaluate effects of endogenous GABA on ventilation in unanesthetized newborns. To increase the GABA concentrations, we inhibited the major GABA catabolic pathway by injecting intraperitoneally aminooxyacetic acid (AOAA), an inhibitor of GABA tranaminase [17,18].…”