Factors affecting the spontaneous release of (3H) gamma-aminobutyric acid from the frog retina in vitro

Glycine stimulates calcium-independent release of 3H-GABA from isolated retinas of Xenopus laevis

A perfusion system was used to monitor the release of [3H]-GABA from isolated retinas of Xenopus laevis. Measurable release was stimulated by glycine at concentrations as low as 200 microM. Glycine-stimulated release was blocked by strychnine, and was not reduced in "calcium-free" Ringer's solution (0 Ca2+/20 mM Mg2+). Glutamate also stimulated calcium-independent release, using concentrations as low as 100 microM. In contrast, release stimulated by 25 mM potassium was reduced by 80% in calcium-free medium. In most experiments, agonists were applied in six consecutive 4-min pulses separated by 10-min washes with Ringer's solution. Under these conditions, the release stimulated by 0.5 mM glutamate or 25 mM potassium decreased by at least 50% from the first to the second pulse, and then gradually decreased with successive applications. In contrast, the response to 0.5 mM glycine at first increased and then only gradually decreased with successive pulses. These patterns of response to different agonists were similar in calcium-free medium. Somatostatin (-14 or -28) also stimulated release, and this effect was inhibited by AOAA, an inhibitor of GABA degradation. In the presence of AOAA, somatostatin had little effect, except at high concentrations of somatostatin (5 microM), which increased both basal and glycine-stimulated release. In contrast to somatostatin, glycine-stimulated release was much larger in the presence of AOAA. Autoradiography was used to investigate which cell types released [3H]-GABA under our conditions. Autoradiograms showed that horizontal cells and a population of apparent "off" bipolar cells were well-labeled by [3H]-GABA high-affinity uptake. In addition, light labeling was seen over numerous amacrine cells. After application of glycine, glutamate, or potassium, there was a decrease in label density over horizontal cells.

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.

Study of the mechanism of release of [3H]GABA from a teleost retina in vitro

gamma-Aminobutyric acid (GABA) is thought to be a neurotransmitter in the vetebrate retina. We studied the voltage and Ca2+ dependency of the process of release of [3H]GABA from the retina of the teleost Eugerres plumieri, using a microsuperfusion technique. Two depolarizing agents, veratridine and high potassium, produced a concentration-dependent release of [3H]GABA. The veratridine effect was inhibited in Na+-free solution, but was not affected by 1 microM tetrodotoxin. A substantial inhibition (about 75%) of the veratridine- and potassium-stimulated release of [3H]GABA occurred in Ca2+-free medium. Inhibitors of the Ca2+ channel, such as Mg2+ (20 mM), La3+ (0.1 mM), and methoxy-verapamil (4 microM-0.4 mM), inhibited the veratridine- and K+-stimulated release. However, Co2+ and Cd2+ caused a potentiation and no change of the K+- and veratridine-stimulated release, respectively. This release process is apparently specific, since both depolarizing agents were unable to release [3H]methionine, a nontransmitter amino acid, under the same experimental conditions. Autoradiographic studies with [3H]GABA, using the same incubation conditions as for the release experiments, showed a high density of silver grains over the horizontal cells with almost no accumulation by amacrine cells and Müller cells. beta-Alanine and nipecotic acid were used as two relative specific inhibitors of the glial and neuronal GABA uptake mechanisms, respectively. Only a small heteroexchange with [3H]GABA was found with beta-alanine, and no inhibition of the subsequent veratridine-stimulated release. On the other hand, nipecotic acid produced a strong heteroexchange with [3H]GABA and lacked the capacity to induce the veratridine-stimulated release of [3H]GABA. These results suggest a voltage- and Ca2+-dependent neuronal release of [3H]GABA from retina.

Stimulation of GABA release from retinal horizontal cells by potassium and acidic amino acid agonists

The release of [3H]GABA from horizontal cells of goldfish retina was studied by biochemical analysis of perfused isolated retina. Retinas were incubated for 15 min in 0.72 microM [3H]GABA, rinsed for 30 min and then perfused with 1 min pulses of increasing concentrations of K+ and acidic amino acid agonists under a variety of conditions. Radioactivity in the perfusate was determined by liquid scintillation spectroscopy. The main findings are: (1) virtually all of the [3H]GABA released by L-glutamate (L-Glu) and L-aspartate (L-Asp) and 50% of the K+-evoked release, is calcium independent; (2) K+-evoked [3H]GABA release is only 10% of that released by L-Glu; (3) threshold [3H]GABA release occurs with 320 microM L-Glu, 1175 microM L-Asp, 4 microM quisqualic acid (QA), 4 microM kainic acid (KA) and 53 microM N-methyl-DL-aspartate (NMDLA); (4) the quisqualate antagonist glutamic acid diethyl ester (GDEE), has no specific inhibitory action on any of the agonists, whereas D-alpha-aminoadipic acid (D alpha AA), an NMDA antagonist, potently inhibits the action of NMDLA and L-Asp; (5) the presence of Mg2+, even at 1 mM, totally inhibits NMDLA and also inhibits the action of L-Glu and L-Asp below 1 mM; (6) D-Asp potentiates the action of L-Glu by 0.6-0.8 log units and completely inhibits the action of L-Asp; (7) L-Asp at a ratio of 3:1 potentiates the effect of L-Glu. From these and other results one concludes that: (a) [3H]GABA release from H1 cells is calcium independent and depends on factors other than passive depolarization, probably sodium; (2) the likely transmitter of red cones is L-Glu acting on quisqualate or kainate receptors, and (3) L-Asp acts predominantly on NMDA receptors and may provide a modulatory role in the outer retina by potentiating the action of L-Glu.

Carrier-mediated release of GABA from retinal horizontal cells

H1 horizontal cells in goldfish retina are probably GABAergic and receive excitatory synaptic input from red cones. This input should affect the synaptic release of GABA from H1 cells. We studied the uptake and release of [3H]GABA from the isolated goldfish retina by use of autoradiography. When retinas were incubated in the light for 15 min in 0.72 microM [3H]GABA, heavy label was found over the somata (HS) and axon terminals (HAT) of H1 horizontal cells, and over pyriform amacrine cell bodies and their processes in sublamina b of the IPL. Postincubation of retinas, preloaded with [3H]GABA, in 0.5-10 mM L-glutamate or 0.1-10 mM L-aspartate, resulted in a dose-dependent and selective loss of [3H]GABA from HS and very little loss from HAT. This loss was not due to an efflux of metabolites of [3H]GABA or to any calcium-dependent vesicular release of [3H]GABA from HS. The glutamate-evoked release of [3H]GABA by H1 cells was sodium dependent, sensitive to substitution of lithium for sodium, and inhibited by nipecotic acid. In addition, [3H]GABA was released from HS by 0.1 mM ouabain but not by 50 mM potassium chloride. Our results suggest that the chemically evoked release of [3H]GABA from HS is mediated by a sodium-dependent transport carrier which may be responsible for the high affinity uptake of [3H]GABA by H1 cells as well. Since synaptic vesicles are not found at presumed synaptic release sites in H1 cells, we suggest that the GABA which is released synaptically from H1 cells may derive from a cytoplasmic pool of GABA and is released by means of a transport carrier. This carrier appears to depend primarily on the sodium concentration gradient across the H1 cell membrane rather than on the membrane potential of the H1 cell for its action. The relevance of the carrier-mediated release of GABA from HS in regard to the synaptic function of H1 cells is discussed.

Calcium-independent release of GABA from isolated horizontal cells of the toad retina

1. When toad retinae were incubated first with veratrine, then with antibodies that reacted with the outer segments of photoreceptors, and finally with complement, horizontal cells survived and most other neurones died. This preparation of 'isolated' horizontal cells accumulated radioactive GABA from the incubation medium. The subsequent release of radioactive GABA could then be measured. 2. The efflux of GABA was increased by exposure to an elevated potassium concentration or added glutamate. Both procedures are known to depolarize horizontal cells. 3. GABA in the external medium also increased the efflux of GABA. 4. The increase in GABA efflux produced by an elevated potassium concentration was unaffected with calcium in the external medium was replaced with cobalt and when sodium was replaced with choline or lithium. 5. The increase in GABA efflux produced by glutamate was unaffected when calcium was replaced with cobalt and when sodium was replaced with lithium, but was inhibited when sodium was replaced with choline. 6. The increase in GABA efflux produced by external GABA was unaffected when calcium was replaced with cobalt but required sodium. Neither choline nor lithium would substitute for sodium. 7. An increase in GABA efflux was accompanied by an increase in sodium efflux. 8. After a high concentration of GABA (2-20 mM) had produced a maximal increase in GABA efflux, the addition of glutamate produced no further effect. Conversely, after a high concentration of glutamate (2-20 mM) had produced a maximal increase in efflux, the addition of external GABA produced only a small further increase. These and the preceding results could occur if GABA release were mediated by a carrier system which could be activated by either depolarization or homoexchange.

GABA-mediated control of transient signals in the inner retina

The effects of chemicals on visual activity was investigated by recording the proximal negative response (PNR) in the perfused eyecup of frogs. Various amino acids reversibly decrease or abolish the PNR over an equivalent concentration range but differ in this depressive action in the presence of convulsant alkaloids. Depression produced by GABA was antagonized by picrotoxin or bicuculline; while glycine-produced depression was selectively antagonized by strychnine. In addition to selective antagonism of amino acids, the convulsants produced differential enhancement of sustained and transient phases of the PNR. In this respect, the effects of picrotoxin and bicuculline are similar to those of semicarbazide and chloride-free solutions which may also inactivate GABA pathways. The effects of these chemicals on the time course and input-output relation of the PNR suggest that processes involving GABA may regulate the transformation from sustained to transient signals which occurs in the inner retina.

Photic release of radioactivity from rabbit retina preloaded with [3H]GABA

The in vitro release of radioactivity by light from rabbit retina was studied after intravitreal injection of [3H]GABA (gamma-aminobutyric acid). The site of uptake of [3H]GABA into the retina after intravitreal injection was checked by autoradiography and was localized at 2 h after the injection mainly in glia but at 4 h in amacrine cells and some ganglion cells. If the retina loaded with [3H]GABA was stimulated by light flashes the efflux of radioactivity increased significantly. Chromatographic analysis of the superfusate demonstrated that [3H]GABA was released by light stimulation. However, the efflux was not elicited by constant light, and was abolished in a medium containing 20 mM Mg++ and 0.2 mM++. When the metabolism of GABA was inhibited by amino-oxyacetic acid, light flashes no longer increased the efflux of radioactivity significantly. No light-evoked release of radioactivity could be demonstrated from glia. Pentobarbitone inhibited the spontaneous efflux and prior anaesthetization with pentobarbitone abolished the light-evoked release. These results support the view that GABA is a retinal neurotransmitter in the rabbit.

Efflux of phenylalanine and troptophan from cerebral cortex slices of adult and 7-day-old rats

An equation of two exponential terms was found to adequately describe the efflux of phenylalanine and tryptophan from cerebral cortex slices of adult and 7-day-old rats. These exponential terms described two forms (components) of efflux taking place at different rates. The fast component of efflux was more prominent in the slices from young rats than in those from adults. In both age groups the contribution of the fast component increased with an increasing amino acid concentration in the superfusate, but there were no changes in the rate constants. The rate constants of the fast components were equal in all experimental conditions in both age groups. The rate constants for the slow components of both amino acids were lower in the slices from adult rats than in those from young rats. The reason for efflux occurring at two different rates is discussed. The fast component probably consisted of the amino acid originating from the extracellular space of the slices and of the intracellular amino acid released by exchange. The slow component consisted of the amino acid released from the cells by other mechanisms, e.g. by diffusion through the membranes or by some active efflux processes. The cerebral cortex slices from adult rats have greater ability to concentrate aromatic amino acids than the slices from young rats. This may partly be dependent on the more effective influx and partly on the slower efflux of amino acids in the slices from adults.

Factors affecting the spontaneous release of (3H)glycine from rabbit retina

The efflux of [3H]glycine was studied in superfused rabbit retina in the presence of various amino acids, ouabain, or high K+ or low Ca2+ concentrations in the superfusion medium. Unlabelled glycine evoked an accelerated efflux as did the structurally similar neutral alpha-amino acids. beta-alanine and GABA were ineffective. The results demonstrate a homoexchange of glycine, and a heteroexchange with the neutral alpha-amino acids. A low concentration of glutamic acid (10(-5) M) will release glycine from the retina. This is an ATPase dependent process which is partially blocked by a high Mg2+/Ca2+ ratio and which may be related to a retinal transmitter function of glutamic acid. A high concentration of K+ or the presence of ouabain in the superfusing medium greatly increases the rate at which glycine is lost from the retina.

Light evoked release of radioactivity from rabbit retinas preloaded with (3H)-GABA

Light flashes evoke an increased release of radioactivity in vitro from rabbit retinas preloaded with (3H)-GABA in vivo. Constant light does not affect the release. No light evoked release can be demonstrated from the glia. Pentobarbitone and AOAA depress the evoked release. The results are consistent with GABA being a retinal neuro-transmitter, most likely in a class of amacrines.

Stimulated release of [3H]beta-alanine from the rabbit retina

The efflux of [3H]beta-alanine from rabbit retina after intravitreal injection has been studied. The site of uptake of [3H]beta-alanine into retina was checked by autoradiography and was found mainly in the inner plexiform layer and in cells with the position of amacrines and in some ganglion cells. When the preloaded retina was stimulated by light flashes and release of radioactivity increased significantly. Chromatography of the superfusate demonstrated a single radioactive spot which cochromatographed with authentic beta-alanine. The efflux of [3H]beta-alanine was affected by raising the K+ concentration. The rate of efflux was also immediately increased when unlabelled beta-alanine or GABA was added to the superfusion medium. Glycine was much less effective. The present study shows that light stimulation releases [3H]beta-alanine from the retina and that beta-alanine may use the same transport system as GABA. This further supports the suggestion that beta-alanine may act as a "false transmitter" replacing GABA in the retina.

Potassium-stimulated gamma-aminobutyric acid release from neurons and glia

Potassium-stimulated [3H]GABA release has been studied with fractions of glial cells, neuronal perikarya and synaptosomes using a superfusion technique. A monotonic increase in the [3H]GABA release was observed when external K+ concentration was raised from 5 to 15 mM. However, there was no further increase in the [3H]GABA release when K+ was raised above 15 mM. The KCl-stimulated release was saturable at the level of 60-100% stimulation. Potassium-stimulated [3H]GABA release from glial cells was unchanged in the absence of calcium while release from synaptosomes and neuronal perikarya demonstrated Ca2+-dependence. The potassium-stimulated [3H]GABA release was enhanced when superfusion was performed in the presence of 10(-5) M ouabain.

Gamma-aminobutyric acid release from neurons and glia

Release of 3H-GABA into the medium from bulk-isolated glial cells, neuronal perikarya and synaptosomes has been studied, using a continuous perfusion system. The fractions were incubated with 3H-GABA and spontaneous efflux of label in the presence and absence of aminooxyacetic acid (AOAA) was recorded. Unlabelled GABA at micromolar concentrations markedly stimulated 3H-GABA efflux from glia, synaptosomes and neurons. 3H-GABA from all fractions was also markedly sensitive to stimulation by ouabain. Glutamate was effective in stimulating the spontaneous efflux in the presence of AOAA, while superfusion with a calcium-deprived medium only slightly stimulated the 3H-GABA efflux from neurons and glia.

The distribution of free amino acids in subdivision of rat and frog retinae obtained by a new technique

A technique is described for splitting intact rat and frog retinae into laminae. These laminae have been examined histologically and the distribution of free amino acids in the laminae has been determined by the dansylation technique. In both rat and frog retinae, taurine was selectively concentrated in the photoreceptor cells and their processes. In the rat, glycine, glutamine and GABA were especially abundant in a lamina which contained the ganglion, inner plexiform and inner neuronal layers. High concentrations of GABA were also present in the outer plexiform layer. These results are discussed in terms of a possible transmitter function for these amino acids in the retina.

The release of [3-H]gamma-aminobutyric acid and neurophysin from the isolated rat posterior pituitary

The efflux of [3-H]GABA from glial cells in the rat posterior pituitary was followed in isolated glands incubated in the presence of 10-minus 5 M aminooxyacetic acid which is known to inhibit GABA metabolism. Electrical stimulation of the pituitary stalk evoked an increase in the rate of efflux of [3-H]GABA as did elevation of the extracellular potassium concentration. The release of neurophysin from nerve terminals in the gland was also increased by electrical stimulation. The increase in [3-H]GABA efflux appeared to be independent of frequency at 2, 5, or 25 HZ if the number of pulses delivered was kept constant, although stimulation at 10 HZ was more effective than either 2 or 25 HZ. The efflux of [3-H]GABA evoked by 56 mM K+ was inhibited by 50% when calcium was removed from the washing fluid and 3 mM EGTA added, while the response evoked by electrical stimulation was unaffected by this procedure. The electrically induced efflux of [3-H]GABA was inhibited by 50% when choline chloride was substituted for sodium chloride in the washing medium, although it was unaffected by tetrodotoxin (0.8 times 10-minus 6 g/ml). The release of exogenous GABA from the pituicyte glia is compared with that of neurophysin from the nerve terminals in the posterior pituitary, and the results are discussed with reference to possible mechanisms of the glial release process.