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M J Neal

Publications and source records attributed to M J Neal.

At least 109 records · Page 6Linked to original sources
PubMed1976

The uptake of L-glutamate by the retina.

The accumulation of L[14C]glutamate by the isolated rat retina has been studied. When retinae were incubated at 37 degrees C in a medium containing L-[14C]glutamate, tissue/medium ratios of about 40:1 were achieved after 60 min. The labelled L-glutamate was rapidly metabolised and after 10 min about 50% of the radioactivity in the tissue amino acids was present as glutamine, aspartate, and 4-aminobutyrate (GABA). The process responsible for L-glutamate uptake showed many of the properties of an active uptake system: it was temperature sensitive, sodium dependent, inhibited by metabolic inhibitors and showed saturation kinetics. The saturable uptake process could be resolved into two components; a 'high' affinity process (apparent Km = 21 muM, Vmax = 35 nmoles/min/g tissue) and a 'low' affinity process (Km = 630 muM, Vmax = 881 nmoles/min/g tissue). The 'high' affinity and 'low' affinity uptake processes for L-glutamate appeared to have identical properties in the retina. The uptake of L-glutamate was not specific and was inhibited by other acidic amino acids including D-glutamate but not by neutral or basic amino acids. The retinal uptake of L-glutamate is not likely to be due to a homoexchange phenomenon because the retina was capable of achieving a large net uptake of glutamate and the efflux of L-[14C]glutamate from the tissue was not increased by the addition of non-radioactive L-glutamate to the incubation medium. Autoradiographic studies indicated that the sites for glutamate uptake are largely in the neuroglial Muller cells.

4-Chloromercuribenzenesulfonate↗
PubMed1973

The uptake of 3Hp -aminobutyric acid by the retina.

1. The accumulation of (3)H-gamma-aminobutyric acid (GABA) by the isolated rat retina has been measured.2. When retinae were incubated at 37 degrees C in a medium containing (3)H-GABA, tissue:medium ratios of about 25:1 were attained after a 30 min incubation.3. After incubations of 40 min at 37 degrees C, almost all (98%) the radioactivity in the tissue was present as unchanged (3)H-GABA.4. The process responsible for (3)H-GABA uptake showed many of the properties of an active uptake system: it was temperature-sensitive, required the presence of sodium ions in the external medium, was inhibited by anoxia, dinitrophenol and ouabain, and showed saturation kinetics.5. The estimated Km value of GABA was 4.0 x 10(-5)M, and V(max) was 0.167 (mumoles/min)/g retina.6. The uptake of (3)H-GABA was not affected by the presence of large molar excesses of glycine, L-glutamate, L-aspartate, L-alanine, L-proline, or L-histidine, but was inhibited by DL-gamma-amino-beta-hydroxybutyrate, beta-guanidinopropionate, and L-2,4-diaminobutyrate.7. The retina was capable of achieving a large net uptake of GABA, indicating that the accumulation of (3)H-GABA by the tissue was not due only to an exchange process with the endogenous GABA pool.8. The uptake of (3)H-GABA occurred only in tissue from the central nervous system. Thus, retina and cerebral cortex rapidly accumulated radioactivity, but slices of cornea, posterior wall of the eye, and liver achieved tissue: medium ratios of approximately one.9. There was a rapid efflux of radioactivity from retinae placed in fresh medium and after 60 min, 90% of the radioactivity was lost from the tissue. The radioactivity released into the medium was present largely as (3)H-acidic and neutral metabolites. When the metabolism of GABA was inhibited by the presence of amino-oxyacetic acid in the medium, only about 10% of the radio-activity was lost from the tissue during a similar 60 min incubation, and the radioactivity released was present largely as unchanged (3)H-GABA.10. It is suggested that the GABA uptake process may represent a possible mechanism for the inactivation of GABA if this amino acid is released at inhibitory synapses in the retina.

Acetates↗
PubMed1973

Effect of centrally acting drugs on the uptake of gamma-aminobutyric acid (GABA) by slices of rat cerebral cortex.

1. The effects of centrally acting drugs on the uptake of (3)H-gamma-aminobutyric acid (GABA) by slices of rat cerebral cortex have been studied.2. Many centrally acting drugs at concentrations of 0.1-1.0 mM significantly inhibited the uptake of (3)H-GABA by cortical slices, but the only classes of drugs in which all members consistently produced inhibition of uptake were the phenothiazines, tricyclic antidepressants, and butyrophenones.3. The receptor blocking drugs; phentolamine, propranolol, thymoxamine, mepyramine, and diphenhydramine at concentrations of 0.5-1 mM also significantly reduced the uptake of (3)H-GABA. However, atropine, hexamethonium and (+)-tubocurarine had little effect on the uptake of (3)H-GABA by cortical slices.4. Centrally acting drugs, which did not significantly inhibit (3)H-GABA uptake, included barbiturates, local anaesthetics, hallucinogens, monoamine oxidase inhibitors, anticonvulsants, and convulsants (except picrotoxin).5. Chlorpromazine, prochlorperazine, L-2,4,diaminobutyric acid, desmethylimipramine, and iprindole inhibited the uptake of (3)H-GABA by 50% (IC50) at concentrations of 30-100 muM. The most potent inhibitor of (3)H-GABA uptake was p-chloromercuriphenylsulphonate (IC50 = 18 muM).6. With the exception of L-2,4,diaminobutyric acid, an outstanding characteristic of these drugs was their complete lack of specificity. Thus at the IC50 for GABA, p-chloromercuriphenylsulphonate, chlorpromazine, prochlorperazine, iprindole, desmethylimipramine, apomorphine and diphenylhydramine also inhibited the uptake of radioactive glycine, alanine, noradrenaline, and 5-hydroxytryptamine. The uptake of the latter two compounds was often inhibited to a greater extent than GABA, glycine and alanine.7. Kinetic analysis indicated that the inhibition of (3)H-GABA by p-chloromercuriphenylsulphonate, chlorpromazine, and desmethylimipramine was noncompetitive. L-2,4,Diaminobutyric acid reduced the uptake of (3)H-GABA by a ;mixed' type of inhibition.8. The present results do not support the suggestion that some centrally acting drugs may produce their effects by reducing the uptake of GABA in the brain after its release from inhibitory nerve terminals. Conceivably, the design of compounds which interfere effectively with the mechanisms of GABA operated synapses may lead to the introduction of whole new groups of centrally acting drugs.

Alanine↗
PubMed1973

Effect of inhibitors of -aminobutyrate aminotransferase on the accumulation of 3H- -aminobutyric acid by the retina.

1. Rat retinae pre-incubated and incubated at 37 degrees C in media containing amino-oxyacetic acid (AOAA) (0.1 muM to 1 mM) accumulated more (3)H-gamma-aminobutyric acid ((3)H-GABA) than control retinae incubated in the absence of AOAA. This increased accumulation of (3)H-GABA by tissue exposed to AOAA was not apparent at short incubation times (0-20 min), but became significant after incubations of 30 min, and maximal after incubation for 60 minutes.2. At a concentration of 10 muM, AOAA did not alter the apparent K(m) for (3)H-GABA uptake or V(max) for either the low or the high affinity GABA uptake systems present in retina.3. The potentiation of (3)H-GABA accumulation produced by AOAA appeared to parallel the inhibitory effect of this compound on 2-oxoglutarate-4-aminobutyrate aminotransferase (GABA-T). Similarly, hydrazinopropionic acid inhibited retinal GABA-T and potentiated the accumulation of (3)H-GABA, but hydroxylamine and thiosemicarbazide which did not affect GABA-T, were also without effect on the retinal accumulation of (3)H-GABA.4. In vitro incubation with AOAA did not increase the endogenous levels of GABA or other amino acids in the retina.5. AOAA did not significantly increase the retinal accumulation of radioactive L-glutamate, L-glutamine, taurine, glycine, alpha-aminoisobutyrate or dopamine: the accumulation of L-aspartate was increased by approximately 30%.6. The inhibition of retinal GABA-T by AOAA was time-dependent and was not reversed by pyridoxal-5'-phosphate or by repeated washing of the tissue with fresh medium.7. AOAA also inhibited glutamate decarboxylase (GAD) in retinae incubated in vitro. This inhibitory effect was partially reversed by pyridoxal-5'-phosphate.8. Efflux of radioactivity from the retina was strikingly reduced in the presence of AOAA at concentrations sufficient to inhibit GABA-T by 100%.9. These findings suggest that AOAA potentiates the accumulation of (3)H-GABA by isolated retina, not by increasing the exchange of (3)H-GABA with the endogenous GABA pools, but by reducing the metabolism of the amino acid and hence reducing the loss of radioactivity from the tissue in the form of tritiated metabolites.

Acetates↗