Transport systems for GABA and for other amino acids in incubated chick brain tissue during development

Developing concepts of cerebral amino acid uptake 1950-1970

This issue of the journal honors Professor Henry McIlwain for his contributions to our knowledge of neurochemistry, as a pioneer (an important contributor already in the 1950s), as a scientist, and as a teacher of great influence and help to the next generation of neurochemists. It is fitting that in his semi-retirement he turns his interest to the history and background of our discipline and demonstrates to us that there is a great deal to learn from the past. In today's explosion of knowledge and new approaches, and the consequent rush to do the work, we tend to forget not only the important past accomplishments but also the past mistakes not to be repeated. It is worthwhile from time to time to take stock, to look back at the path that led to the present. This paper is an attempt to explore this retrospection by a discussion of some of the background of research on cerebral amino acid transport. Emphasis for the purpose is on illustration, with arbitrarily selected examples rather than an exhaustive review of the subject.

Acidic amino acid transport in animal cells and tissues

1. The occurrence and characterization of acidic amino acid transport in the plasma membrane of a variety of cells and tissues of a number of organisms is reviewed. 2. Several cell types, especially in brain, possess both high- and low-affinity transport systems for acidic amino acids. 3. High-affinity systems in brain may function to remove neurotransmitter amino acid from the extracellular environment. 4. Many cell systems for acidic amino acid transport are energized by an inwardly directed Na+ gradient. Moreover, certain cell types, such as rat brain neurons, human placental trophoblast and rabbit and rat kidney cortex epithelium, respond to an outwardly directed K+ gradient as an additional source of energization. This simultaneous action may account for the high accumulation ratios seen with acidic amino acids. 5. Rabbit kidney has been found to have a glutamate-H+ co-transport system which is subject to stimulation by protons in the medium. 6. Acidic amino acid transport in rat brain neurons occurs with a stoichiometric coupling of 1 mol of amino acid to 2 mol of Na+. For rabbit intestine, one Na+ is predicted to migrate for each mol of amino acid. 7. Uptake in rat kidney cortex and in high-K+ dog erythrocytes is electrogenic. However, uptake in rabbit and newt kidney and in rat and rabbit intestine is electroneutral. 8. Na+-independent acidic amino acid transport systems have been described in the mouse lymphocyte, the human fibroblast, the mouse Ehrlich cell and in rat hepatoma cells. 9. In a number of cell systems, D-acidic amino acids have substantial affinity for transport; D-glutamate, in a number of systems, however, appears to have little reactivity. 10. Acidic amino acid transport in some cell systems appears to occur via the "classical" routes (Christensen, Adv. Enzymol. Relat. Areas Mol. Biol. 49, 41-101, 1979). For example, uptake in the Ehrlich cell is partitioned between the Na+-dependent A system (which transports a wide spectrum of neutral amino acids), the Na+-dependent ASC system (which transports alanine, serine, threonine, homoserine, etc.), and the Na+-independent L system (which shows reactivity centering around neutral amino acids such as leucine and phenylalanine). Also, a minor component of uptake in mouse lymphocytes occurs by a route resembling the A system. 11. Human fibroblasts possess a Na+-independent adaptive transport system for cystine and glutamate that is enhanced in activity by cystine starvation.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogeny of GABAergic neurons in chick brain: studies in vivo and in vitro

The ontogeny of presynaptic elements of GABAergic neurons has been studied in the cerebrum of the chick embryo both in vivo and in vitro. The specific activity of glutamate decarboxylase (GAD) in tissue extracts followed a rising curve and approached a plateau value after 28 days in vivo. One-half of the adult levels of GAD were achieved by day 20. The specific activity of Na+-gamma-aminobutyric acid (GABA) cotransport in membrane vesicles followed a similar pattern and reached maximal levels by 28 days in vivo. One-half of the adult levels of GABA uptake were observed at day 17. The development of these markers was also studied in cultured neurons prepared from the cerebrum of 8-day-old chick embryos. The GAD activity in neuronal extracts increased linearly with time in culture up to 14 days. At this point the specific activity had reached 20% of that observed for the adult cerebrum. The specific activity of GABA uptake by intact neurons followed a pattern similar to that for GAD from days 2 to 9 in culture. Both activities increased 4-5-fold during this period, but the level of GABA transport declined thereafter. In order to compare GABA uptake values for cultured cells with those for embryonic and adult brain, membrane vesicles were prepared from cultures. At the maximal level (9-10 days in culture) the vesicular GABA uptake represented 33% of that in the 18-day embryo and 20% of adult levels. Thus the presynaptic GABAergic components developed according to similar schedules both in vivo and in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell membrane amino acid transport processes in the domestic fowl (Gallus domesticus)

Intestinal absorption of amino acids in the chicken occurs by way of processes which are concentrative, Na+-dependent and dependent upon metabolic energy in the form of ATP. Intestinal transport is carrier-mediated, subject to exchange transport (trans-membrane effects) and is inhibitable by sugars, reagents which inactivate sulfhydryl groups, potassium ion, and by deoxpyridoxine, an anti-vitamin B6 agent. It is stimulated by phlorizin, a potent inhibitor of sugar transport, and in Na+-leached tissue by modifiers of tissue cyclic AMP levels, e.g. theophylline, histamine, carbachol and secretin. Separate transport sites with broad, overlapping specificities function in the intestinal absorption of the various classes of common amino acids. A simple model for these sites includes one for leucine and other neutral amino acids, one for proline, beta-alanine and related imino and amino acids, one for basic amino acids, and one for acidic amino acids. Absorption of amino acids appears to be widespread in occurrence in the digestive tract of the domestic fowl; transport has been reported to be present in the crop, gizzard, proventriculus, small intestine and in the colon. By the end of the first week of life post-hatch, the caecum loses its ability to transport. Similarly, the yolk sac loses its ability by the second day post-hatch. Intestinal transport was noted before hatch and was found to be maximal immediately post-hatch. A requirement for Ca2+ appears to be lost after the first week of life post-hatch. The cationic amino acids appear to be reabsorbed by a common mechanism in the kidney. Transport rates of leucine measured in the intestine or in the erythrocyte were found to cluster about discrete values when many individual chickens were surveyed; such patterns may be an expression of gene differences between individuals. Two lines of chickens have been developed, one high and the other low uptake, through selective breeding based on the ability of individual birds to absorb leucine in erythrocytes. High leucine absorbing chickens were found to be more effective in absorbing lysine and glycine, were more effectively stimulated by Na+, had greater erythrocyte Na+, K+-ATPase activity, and their erythrocytes contained about 20% less Na+ than low line erythrocytes. The underlying genetic difference between these lines may reside at the level of the Na+, K+-ATPase and (or) with a regulatory gene determining carrier copies. Amino acid transport in erythrocytes was noted to be highest in pre-hatch chicks and to diminish during post-hatch development.(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental changes in amino acid transport in the chicken erythrocyte

1. Influx of leucine, lysine and glycine was found to be highest in prehatch (day -1) chicken red blood cells and to diminish during posthatch development when tested at two and four weeks of age. 2. The greatest decline in transport rate during development was seen with leucine; lysine showed a substantial age-related decline only at substrate concentrations greater than Km, the apparent Michaelis constant of transport. 3. Vmax, the maximal transport influx, of each amino acid tested declined during development. 4. Km of glycine and leucine appeared to increase slightly over the test period. 5. In contrast, a 7-fold decrease in Km for lysine transport was seen over the same period. 6. These results are discussed in context of changes in kinetic parameters of amino acid transport during development reported for various animal organs or tissues.

Postnatal changes of GABAergic and glutamatergic parameters

In the rat cortex and striatum, glutamate decarboxylase, a marker for GABAergic nerve terminals, increased almost linearly for the first postnatal month, in agreement with previous reports. High affinity GABA and glutamate transport appeared to develop earlier, reaching, respectively, 170-190% and 72-80% in adult rates by the fifteenth postnatal day. Glial and neuronal uptakes of GABA in infant and adult tissues were investigated using beta-alanine and cis-3-aminocyclohexane-carboxylic acid. The relatively high striatal uptake of GABA observed in 2-day-old rats was found to be mainly due to early development of neuronal rather than glial transport in this region. Both neuronal and glial uptake, however, contributed equally to the enhanced uptake of GABA obtained in both regions at day 15. Neuron/glia ratios were estimated to increase by more than 12-fold in the cortex, and about 4-fold in the striatum from newborns to adults. The present results also indicate that there may exist in the immature striatum some glioblasts which might accumulate beta-alanine but not GABA. The 2-fold developmental increase in ornithine aminotransferase activity is consistent with the hypothesis that this enzyme may be enriched in glutamatergic neurons.

Differential binding of antidepressants to noradrenaline and serotonin transport sites in central nerve endings

Imipramine and mianserin are equipotent inhibitors of noradrenaline (NA) uptake in synaptosomes. However, after in vivo administration, NA uptake was inhibited only in synaptosomes from imipramine-treated rats, suggesting that imipramine, or its metabolite desipramine, binds to the NA carrier in a manner outlasting the preparation of synaptosomes, whereas mianserin is washed away. To evaluate binding to the NA carrier, synaptosomes prelabeled with 3H-NA were pretreated with an antidepressant and the release of 3H-NA was then stimulated with unlabeled NA. Any reduction of release was taken as an indication of binding. Pretreatment with desipramine, but not with imipramine or mianserin, reduced 3H-NA release suggesting that desipramine is responsible for NA uptake inhibition in synaptosomes from imipramine-treated rats. Transformation of tertiary into secondary amines seems to be crucial for binding to the NA carrier, as confirmed by the stronger binding of nortriptyline and chlordesipramine compared to amitryptiline and chlorimipramine, respectively. In contrast, tertiary amines bound more strongly than secondary amines to the serotonin carrier. Adult and 8-day old synaptosomes showed similar binding properties towards imipramine and desipraine.

High-affinity GABA and glutamate transport in developing nerve ending particles

The high-affinity transport of [3H]GABA and [14C]glutamate was measured in discrete nerve ending fractions isolated from the developing rat cortex, beginning on day 7 postpartum and prepared using discontinuous Ficoll-isotonic sucrose gradients. On day 7 the material sedimenting in the lightest gradient fractions contained the highest density of particles capable of high-affinity transport. With increasing age, the transport sites became progressively associated with more dense particles in the gradient. On day 14, the Vmax values for both [3H]GABA and [14C-A1glutamate transport in the most dense nerve ending fraction were significantly increased over the adult value. This transient increase in the Vmax values disappeared by day 21. While the Km values for both transport systems were constant in all fractions for the adult animal, this was not the case in the developing pup. The Km values in the lightest fractions were significantly greater than those in the more dense fractions, suggesting that different transport systems may be present during development. Electron micrographs of the various fractions were prepared from 7-day and adult animals. These data illustrate that the lightest fractions of the '7-day' gradient are enriched in profiles similar to developing nerve endings. With development, these light fractions become infiltrated with myelin, the immature nerve ending profiles disappear, and the bulk of the nerve endings are found in more dense fractions.

Influence of membrane potential on the sodium-dependent uptake of gamma-aminobutyric acid by presynaptic nerve terminals: experimental observations and theoretical considerations

Sodium, potassium and veratridine were tested for their effects on the uptake of gamma-aminobutyric acid (GABA) by pinched-off presynaptic nerve terminals (synaptosomes). As noted by previous investigators, the uptake from media containing 1 mum GABA ("high-affinity" uptake) is markedly Na-dependent; the uptake averaged 65 pmoles/mg synaptosome protein x min, with [Na]0=145mm and [K]0=5mm, and declined by about 90 percent when the external Na concentration ([Na]0) was reduced to 13mm (Na replaced by Li). The relationship between [Na]0 was GABA uptake was sigmoid, suggesting that two or more Na+ ions may be required to activate the uptake of one GABA molecule. Thermodynamic considerations indicate that with a Na+/GABA stoichiometry of 2:1, the Na electrochemical gradient, alone, could provide sufficient energy to maintain a maximum steady-state GABA gradient ([GABA]i/[GABA]0) of about 104 across the plasma membrane of GABA-nergic terminals. In Ca-free media with constant [Na]0, GABA uptake was inhibited, without delay, by increasing [K]0 or by introducing 75mum veratridine; the effect of veratridine was blocked by 200 nm tetrodotoxin. The rapid onset (within 10 sec) of the veratridine and elevated-K effects implies that alterations in intra-terminal ion concentrations are not responsible for the inhibition. The uptake of GABA was inversely proportional to log [K]0. These observations are consistent with the idea that the inhibitory effects of both veratridine and elevated [K]0 may be a consequence of their depolarizing action. The data are discussed in terms of a barrier model (Hall, J.E., Mead, C.A., Szabo, G. 1973. J. Membrane Biol. 11:75) which relates carrier-mediated ionic flux to membrane potential.

Alpha-aminoisobutyric acid transport into human glia and glioma cells in culture

The AIB transport into human glia and glioma cells in culture has been studied. Because of the high affinity of AIB to the plastic culture dishes, a special washing technique had to be developed. With this technique, it was possible to perform transport experiments in a single plate containing about one million cells. The cells were viable, intact and adhered to the supporting medium throughout the experiment. The AIB transport into both types of cells was Na+-dependent and showed saturation kinetics when the small component of the transport due to diffusion had been subtracted. The AIB transport capacity of neoplastic glioma cells was 3.6 times higher than that of glia cells. This difference was related to the Vmax-values for the two types of cells. The apparent Km-values were the same. Inhibition experiments with other amino acids support the view that AIB is transported via System A in both glia and glioma cells. Sulfhydryl reagents (ethacrynic acid and NEM) and cytochalasin B clearly inhibited the AIB transport into glia cells whereas the effect on glioma cells was minimal.

Neurochemical aspects of the ontogenesis of GABAnergic neurons in the rat brain

Examination of various biochemical characteristics of the GABAnergic nervous system in the rat brain was made between 15 days of gestation and adulthood. At birth, the concentration of GABA in whole brain and most regions is approximately 50% of adult levels, whereas the medulla-pons has achieved adult levels by this time. Compared to GABA levels, there is a marked lag in the development of the activity of glutamic acid decarboxylase in all areas studied; however, the activity of the enzyme increases in a linear fashion from birth to adulthood. The development of the uptake of GABA into particulate fractions prepared from whole brain and regions differs markedly from that of GABA and glutamic acid decarboxylase, with uptake near adult levels by birth, peaking considerably above that of the adult between one to two weeks after birth and then declining toward adult activity by 4 weeks after birth. Examination of the kinetics of GABA uptake into resuspended P2 fractions demonstrates that the developmental changes in the uptake reflects differences in the Vmax whereas the Kt remains constant; Studies on the development of the apparent postsynaptic receptor for GABA reveals that in all regions binding is approximately 25% of adult up to 8 days after birth, at which time it increases dramatically, approximating adult levels by 4 weeks after birth. The rise in the density of the apparent postsynaptic GABA receptor after 8 days postpartum correlates best with the increase in the activity of glutamic acid decarboxylase, a presynaptic marker.

Glutamate uptake by chick retina

The uptake of glutamate was found to be via a single high-affinity transport mechanism with Km values of 35 and 95 mum for chick-embryo and mature chick retina respectively. These data contrast with the uptake of gamma-aminobutyratewhich in the same tissue has previously been shown to display two kinetically distinct mechanisms in the embryo, but a single low-affinity process in the mature retina.

Amino acid uptake in vivo by the mouse brain and by various regions of the rabbit brain after drug-induced convulsions

The levels of the free amino acids of the mouse brain were studied after convulsions induced by Metrazol; a decrease of level of taurine and a significant increase of level of alanine, phenylalanine and isoleucine were found. The net uptake of L-histidine by the mouse brain in vivo was similar under normal conditions and after Metrazol-induced generalized convulsions; that of L-methionine was much higher after convulsions and there was no uptake of L-aspartic acid, either under physiological conditions or after convulsions. The net uptake of L-histidine by various regions of the rabbit brain in vivo after convulsions was significantly higher than control values in the cortical areas, while that of L-methionine was significantly higher in the subcortical ones; there was no uptake of L-aspartic acid by the rabbit brain in normal condition, whereas after convulsions a small entry seemed to occur in the subcortical areas. These results indicate that cerebral permeability processes of amino acids are somewhat altered during convulsive phenomena, as already described elsewhere for ions andproteins.

Uptake of (3H)glycine and (14C)glutamate by cultures of chick spinal cord

1. Spinal cord explants from chick embryos, grown in culture for up to 16 days, rapidly accumulated [(3)H]glycine and [(14)C]glutamate when incubated at 25 degrees C in a medium containing either 2 x 10(-10)M glycine or 4.8 x 10(-8)M glutamate.2. After 90 min incubation, a tissue/medium ratio of 60:1 and 20:1 was attained for [(14)C]glutamate and [(3)H]glycine respectively.3. The uptake systems, in addition to requiring sodium ions in the medium, were temperature sensitive, showed saturation kinetics, and were inhibited by ouabain.4. For the glutamate and glycine accumulation the K(m) value was 4.3 x 10(-5) and 4.1 x 10(-5)M respectively, indicating that a high affinity uptake process is involved.5. The rate of accumulation of both glutamate and glycine increased in cultures between the ages of 3 and 10 days thus matching their morphological development.6. In light of previous evidence, the demonstration of an active transport mechanism for both glutamate and glycine in spinal-cord-cultures that also shows a relationship with morphological maturity, suggests that these two amino acids may play a major role in spinal cord function.