Influence of aminooxyacetic acid on the potassium-evoked release of [3H]gamma-aminobutyric acid from slices of rat cerebral cortex

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Release of endogenous taurine and gamma-aminobutyric acid from brain slices from the adult and developing mouse

The spontaneous and potassium-stimulated release of endogenous taurine and gamma-aminobutyric acid (GABA) from cerebral cortex and cerebellum slices from adult and developing mice was studied in a superfusion system. The spontaneous release of GABA was of the same magnitude in slices from adult and developing mice, but the spontaneous release of taurine was considerably greater in the adults. The potassium-stimulated release of GABA from cerebral cortex slices was about five times greater in adult than in 3-day-old mice, but the potassium-stimulated release of taurine was more than six times greater in 3-day-old than in adult mice. In cerebellar slices from 7-day-old mice, potassium stimulation also evoked a massive release of taurine, whereas the evoked release from slices from adult mice was rather negligible. Also in cerebellar slices the potassium-stimulated release of GABA exhibited the opposite quantitative pattern. The stimulated release of both GABA and taurine was partially calcium dependent. The results suggest that taurine may be an important regulator of excitability in the developing brain.

The mechanism by which monoamine oxidase inhibitors give rise to a non-calcium-dependent component in the depolarization-induced release of 5-HT from rat brain synaptosomes

1. The effects of the monoamine oxidase inhibitors pargyline and nialamide on the Ca2+-dependency of [3H]-5-hydroxytryptamine release from superfused rat brain synaptosomes has been studied in order to evaluate the discrepancies that have occasionally been observed in studying transmitter release by in vivo and in vitro techniques. 2. The application of K+ pulses of low concentration (12.5-20 mM) caused an essentially Ca2+-dependent release of [3H]-5-HT. However, at K+ concentrations above 30 mM, a small non-Ca2+-dependent component appeared. 3. At high concentrations of K+ (30-55 mM), nialamide (18 microM) or pargyline (7 microM) increased the amount of [3H]-5-HT released which could be accounted for by an increase in the non-Ca2+-dependent component of release. 4. The elevation of the non-Ca2+-dependent component of release caused by the monoamine oxidase inhibitors was totally abolished by the inhibitors of the plasma membrane 5-HT carrier, chlomipramine (500 nM), citalopram (50 nM) and fluoxetine (1 microM). 5. The results suggest that the non-Ca2+-dependent component of release seen with high depolarizing concentrations of K+, particularly in the presence of monoamine oxidase inhibitors, is caused by the efflux of [3H]-5-HT through the plasma membrane carrier which seems to be activated during depolarization. 6. The significance of these findings to the physiological in vivo situation, and to the use of in vitro preparations in the study of transmitter release is discussed.

Effect of aminooxyacetic acid on the release of preloaded [3H]GABA and radioactive metabolites from slices of developing mouse brain

The release of [3H]gamma-aminobutyric acid (GABA) and its radioactive metabolites from slices of the cerebral cortex, cerebellum, striatum and brain stem of developing and adult mice was studied. The slices were incubated and superfused in the absence and presence of the GABA aminotransferase (GABA-T) inhibitor aminooxyacetic acid (AOAA). Exposure to 100 microM AOAA totally inhibited GABA-T and all radioactivity released from slices was in authentic GABA. In studies on developing brain the 10-microM concentration was also effective enough, except in cerebellar slices. In the absence of AOAA the major part of radioactivity spontaneously released from slices of adult cerebral cortex and cerebellum was tritiated water and still about one third part in the presence of 10 microM AOAA. Potassium stimulation induced only the release of radioactive GABA but not labeled metabolites in both presence and absence of AOAA. AOAA reduced the stimulation-induced release of GABA. It is recommended that the use of GABA-T inhibitors should be discontinued in release experiments. Then labeled GABA must be separated in the effluents from its radioactive breakdown products.

Aminooxyacetic acid inhibits the malate-aspartate shuttle in isolated nerve terminals and prevents the mitochondria from utilizing glycolytic substrates

Aminooxyacetate, an inhibitor of pyridoxal-dependent enzymes, is routinely used to inhibit gamma-aminobutyrate metabolism. The bioenergetic effects of the inhibitor on guinea-pig cerebral cortical synaptosomes are investigated. It prevents the reoxidation of cytosolic NADH by the mitochondria by inhibiting the malate-aspartate shuttle, causing a 26 mV negative shift in the cytosolic NAD+/NADH redox potential, an increase in the lactate/pyruvate ratio and an inhibition of the ability of the mitochondria to utilize glycolytic pyruvate. The 3-hydroxybutyrate/acetoacetate ratio decreased significantly, indicating oxidation of the mitochondrial NAD+/NADH couple. The results are consistent with a predominant role of the malate-aspartate shuttle in the reoxidation of cytosolic NADH in isolated nerve terminals. Aminooxyacetate limits respiratory capacity and lowers mitochondrial membrane potential and synaptosomal ATP/ADP ratios to an extent similar to glucose deprivation. Thus, the inhibitor induces a functional 'hypoglycaemia' in nerve terminals and should be used with caution.

Effects of the gamma-aminobutyrate transaminase inhibitors gabaculine and gamma-vinyl GABA on gamma-aminobutyric acid release from slices of rat cerebral cortex

The release of [3H]gamma-aminobutyric acid (GABA) from pre-loaded slices of rat cerebral cortex was investigated in the presence and absence of the GABA-transaminase inhibitors gabaculine and gamma-vinyl GABA. In the experiments carried out without an inhibitor, an ion-exchange column chromatographic technique was used to separate [3H]GABA from tritiated metabolites released with it into the superfusate. The presence of gabaculine (5 microM) substantially reduced the Ca2+-dependence of the release of [3H]GABA evoked by a 4 min 30 mM K+ pulse, whereas this was not appreciably reduced by the presence of gamma-vinyl GABA (2 mM or 10 mM). Nevertheless, the characteristics of [3H]GABA release were not identical in the presence and absence of either inhibitor.

Effect of aminooxyacetic acid on gamma-aminobutyric acid release from frog retina

The Ca2+-dependence of the release of [3H]gamma-aminobutyric acid (GABA) from the pre-loaded, superfused frog retina was investigated in the presence and absence of the GABA-transaminase inhibitor, aminooxyacetic acid (AOAA). In the latter case, an ion-exchange column chromatographic technique was used to separate [3H]GABA from tritiated metabolites released with it into the superfusate. In the presence of aminooxyacetic acid, a 2 min 30 mM K+ pulse released similar amounts of [3H]GABA in normal and Ca2+-free Ringer solutions. However, when aminooxyacetic acid was absent, the K+-evoked release of [3H]GABA was substantially reduced under Ca2+-free conditions. It is concluded that the presence of aminooxyacetic acid can reduce the Ca2+-dependence of K+-evoked release of GABA from the frog retina. This is in accordance with its effect on cerebral cortex of the rat, reported previously.