Sodium dependence of GABA transport in rat hypothalamic synaptosomes

Role of sialic acid in synaptosomal transport of amino acid transmitters

Active, high-affinity, sodium-dependent uptake of gamma-aminobutyric acid and of the acidic amino acid D-aspartate was inhibited by pretreatment of synaptosomes with neuraminidase from Vibrio cholerae. Inhibition was of a noncompetitive type and was related to the amount of sialic acid released. The maximum accumulation ratios of both amino acids (intracellular [amino acid]/extracellular [amino acid]) remained largely unaltered. Treatment with neuraminidase affected neither the synaptosomal energy levels nor the concentration of internal potassium. It is suggested that the gamma-aminobutyric acid and acidic amino acid transporters are glycosylated and that sialic acid is involved in the operation of the carrier proteins directly and not through modification of driving forces responsible for amino acid uptake.

Endogenous GABA concentration in cortical synaptosomes from young and aged rats

The objectives of this study are threefold: to compare the gamma-aminobutyric acid (GABA) concentration of a preparation of cortical synaptosomes from young male rats to that of aged rats; to compare the GABA concentration in synaptosomes retained by Millipore filtration for young and aged rats; and to compare the GABA concentration in the synaptosomal preparation to that in synaptosomes retained by Millipore filtration for young and aged rats. The GABA concentration is measured in cortical synaptosomes from Long-Evans rats at 2 months and 30 months of age. Concentration of the synaptosomal preparation declined significantly from 2.97 mM in 2-month animals to 2.68 mM in 30-month animals. In order to compare the results with previous studies of GABA transport, in which Millipore filters are used to separate synaptosomes from incubation medium, the GABA concentration is also measured following placement of synaptosomes on 0.65 mu pore size Millipore filters. For both young and aged animals, this population of synaptosomes is found to contain GABA at a concentration more than triple that in the overall population. This is the expected result if the filtration process reduces the non-synaptosomal component of the total preparation, since GABA uptake is known to be a function of the nerve-ending component. Comparison of 2-month and 30-month synaptosomes reveals that the GABA concentration of those synaptosomes retained by the filtration process declined to a greater extent in the aged group than did the content of the overall population (from 9.33 mM to 8.37 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

gamma-Hydroxybutyrate uptake by rat brain striatal slices

gamma-Hydroxybutyrate uptake by rat brain striatal slices was studied. The uptake was saturable with a Km of 702 +/- 107.10(-6) M. gamma-Hydroxybutyrate uptake was sodium dependent and inhibited by the omission of potassium. In addition, the effect of ouabain suggests that the transport is dependent on a cation gradient. Several analogues of gamma-hydroxybutyrate inhibit the transport system. GABA has no significant effect. This energy and cation dependent transport system is in favor of a transmitter or modulator role of gamma-hydroxybutyrate in the rat brain striatum.

The effect of membrane potential on initial velocity of GABA uptake and steady state distribution ratio in rat cortical synaptosomes

Initial velocity of synaptosomal GABA uptake and steady state distribution ratio (DR) have been measured as a function of medium potassium concentration [( K]o) and in the presence of veratridine. Previous studies of the effect of [K]o and veratridine on membrane potential have been utilized to relate GABA uptake to membrane potential. Both initial velocity of uptake and steady state DR are functions of membrane potential; initial velocity and in DR are linearly related to membrane potential. Final DR is determined by the electrochemical potential gradient for the transported ions. Previous studies of the relation of [Na]o to uptake have shown that either one or two sodium ions may be translocated with each GABA molecule. Present results are consistent with this assumption if it is also assumed that an anion is co-transported with each GABA molecule.

Effect of hyperbaric oxygen on GABA transport in rat brain synaptosomes

Initial velocities of uptake of GABA have been measured in rat brain synaptosomes from animals which had been exposed to oxygen at high pressure (OHP) and compared to similar measurements in normobaric controls. For hypothalamus, no changes in GABA uptake occurred subsequent to exposure to OHP. For cortical synaptosomes, however, exposure to OHP resulted in a decreased velocity of GABA uptake at all combinations of [Na] and [GABA] used. The OHP data were found to fit the same transport model as found previously for control data. Thus, OHP exposure did not alter the basic mechanism by which sodium and GABA interact with the carrier in the process of transport. However, the constants which quantitate the model were changed by OHP exposure. As a consequence, the several kinetic parameters which are calculated from the model change in the OHP animals. These kinetic parameters are compared to similar calculations for both normobaric control animals and normobaric aged animals. Although the effects of OHP do not precisely parallel the effects of aging, the alterations in kinetic parameters are in several ways similar in the aged and OHP animals.

Critical assessment of noradrenaline uptake in synaptosomal preparations

Synaptosomes and other subcellular organelles were prepared from rat brain using a vertical rotor. The preparation time was reduced by up to 60% compared to conventional techniques. Uptake of [3H]-(-)-noradrenaline into subcellular fractions was characterized. The characteristics of this uptake were dependent on the subcellular composition and anatomical origin. Various methods of correction for energy independent processes were compared, but only sodium ion removal from the medium selectively inhibited the energy dependent uptake mechanism. Kinetic analysis of data revealed that high and low affinity uptake systems were dependent on the fraction under analysis. Noradrenaline uptake was not exclusively localised in noradrenergic terminals. Selective inhibitors of the noradrenaline uptake process (tricyclic antidepressants) inhibited energy dependent uptake completely only in purified synaptosomes prepared from cortex. In whole brain synaptosomal fractions, noradrenaline was partially accumulated into dopaminergic neurones; this uptake process was not inhibited by tricyclic antidepressants.

beta-Alanine uptake by mouse brain slices

beta-[3H]Alanine uptake by mouse brain slices was studied in Krebs-Ringer-HEPES-glucose medium (pH 7.4) under O2. The uptake was temperature-sensitive and consisted of two saturable transport components, high- and low-affinity, with kinetic parameters comparable to those of amino acid neurotransmitter candidates. beta-Alanine uptake was strictly sodium-dependent and also inhibited by the omission of potassium and presence of ouabain, suggesting that the transport is mainly fuelled by cation gradients. Sodium ions showed positive cooperative effects in beta-alanine uptake, indicating the association of at least two sodium ions in the transfer of one molecule of beta-alanine. The uptake was strongly inhibited by gamma-aminobutyrate and hypotaurine, the high-affinity uptake component completely disappearing in the presence of hypotaurine. Taurine had no measurable effect. The results suggest that the high-affinity transports of beta-alanine and hypotaurine may be mediated by the same system.

Aging of the high affinity GABA transporter in synaptosomes from the hypothalamus of the rat

Initial velocity of GABA uptake in hypothalamic synaptosomes has been measured in aged animals as a function of both sodium and GABA concentration and compared to previous studies in young animals. The data give good fit to the model found previously to give minimal best fit to control data. Thus it appears that aging has not resulted in a change in fundamental mechanism by which carrier, sodium, and GABA interact in the process of transport. However, changes were found in the constants which quantitate the model. As a consequence, there is a reduction in transport capability in the aged animals which is particularly apparent at the higher sodium concentrations. The best fit constants were used along with the rate equation from the model to calculate several kinetic parameters which are useful in comparing transport mechanisms. In the physiological range of sodium concentration, those kinetic parameters related to maximal transport capabilities declined with age, while apparent carrier affinities increased with age. Similar results were found in previous studies of GABA (Wheeler, 1982) and glutamic acid (Wheeler, 1980a) transport in cortical synaptosomes.

Effects of cations on taurine, hypotaurine, and GABA uptake in mouse brain slices

The effects of cations on taurine, hypotaurine and GABA uptake were studied in mouse brain slices under identical experimental conditions. The uptakes were all strictly sodium-dependent. The omission or excess of K+ inhibited similarly taurine, hypotaurine and GABA uptake. The effects of omission of Ca2+ or Mg2+ were less pronounced. In both normal-sodium and low-sodium media all uptakes were saturable, consisting of both low- and high-affinity transport components. The Km constants for both low- and high-affinity transport components of hypotaurine and GABA increased in low-sodium medium, suggesting that sodium ions are necessary for their attachment to possible carrier sites in plasma membranes. In the case of taurine, however, the translation rate rather than the affinity of carrier sites was affected in Na+-free media. More than two sodium ions may be involved in the transport of one hypotaurine and one GABA molecule, whereas the coupling ratio between sodium and taurine was at least three. In its cation dependence hypotaurine uptake thus resembled more GABA uptake than taurine uptake.

Effect of castration on the high affinity glutamate transporter in rat hypothalamic and cortical synaptosomes

High affinity transport of glutamic acid has been studied in cortical and hypothalamic synaptosomes from castrated male rats and compared to normal controls. For hypothalamic synaptosomes, both initial velocity of uptake of Va (apparent maximal velocity) were found to be about one-third lower in the castrated animals. Kt (glutamate concentration giving Va/2), however, was reduced by only 5%. Initial velocity of uptake in cortical synaptosomes was measured as a function of both sodium and glutamate concentration. Reductions in uptake subsequent to castration were found to be much less for cortical synaptosomes (2-15%) than for hypothalamic synaptosomes. Fit of these data to various models for the sodium dependence of transport resulted in the same minimal best fit model as that found for control animals. Thus castration does not alter the fundamental nature of the mechanism by which carrier, sodium and glutamate interact in the process of transport. However, quantitative changes were found to occur, as reflected in the best fit constants. These constants were used along with the rate equation for the minimal best fit model to calculate certain parameters which were then used to delineate the quantitative changes in the transporter following castration. A neuroregulatory role for glutamate in gonadotropin secretion has been recently proposed; the present study now provides additional information on the relationship between reproductive function and one aspect of glutamatergic synaptic function, namely, the high affinity transport system.

Aging of membrane transport mechanisms in the central nervous system. GABA transport in rat cortical synaptosomes

A kinetic study of sodium dependent GABA transport has been made in synaptosomes from 30-month old Long-Evans rats and compared to results from 2-month old animals. Initial velocity of uptake was measured as a function of both sodium and GABA concentration, and these data were than fitted to the model which was found to give best fit for the 2-month data. An excellent fit was also obtained for the 30-month data. Thus there has been no change with age in the fundamental mechanism by which carrier, sodium, and GABA interact in the process of transport. However, quantitative changes were found to occur with age both in initial velocity of uptake and in those constants which quantitate the model. The rate equation for the model was utilized along with the best fit constants to define and calculate certain parameters which were then used to quantitatively compare the transport mechanism in young and aged animals.