Effects of gamma-aminobutyric acid (GABA) and sodium L-glutamate (glutamate) on the visual system and EEG of chicks

Gamma-aminobutyric acid- and picrotoxin-induced changes in c-wave and light peak of retinal potentials in the chick

Retinal potentials were recorded from the eyes of anesthetized and immobilized chicks by a standard direct current method. The amplitude of the electroretinogram (ERG) c-wave was measured 2 and 5 sec after the onset of the light stimulus, as indexes of the fast-rise c-wave (cF-wave) and the slow-rise c-wave (cS-wave), respectively. An intravitreal injection of gamma-aminobutyric acid (GABA) at an estimated intravitreal concentration of 10(-9)-10(-7) M resulted in an increase of the amplitude of the cS-wave, a less remarkable change in the a- and cF-waves, and a slight decrease in the b-wave. The light peak of the retinal standing potential increased in amplitude following GABA administration (10(-7)-10(-4) M). Following an intravitreal injection of picrotoxin (10(-5)-10(-3) M), the polarity of the cS-wave changed from positive to negative and a significant decrease and deformation in the light peak was observed. The amplitude of the a-wave, however, increased in the range of the higher dose, while that of the b- and cF-waves decreased markedly but no polarity reversal of the cF-wave was found. The results may suggest that the GABA-ergic synapse plays a significant role in production of the cS-wave and the light peak, along with that of the pigment epithelium.

Alteration of GABA system in frog retina following short light and dark adaptations - a quantitative comparison with retinal taurine

Effect of short light and dark adaptations on retinal GABA and taurine was studied using bull frog (Rana catesbiana). The retinal GABA was increased significantly in light-adapted state, and this increase was accompanied by the increases of L-glutamate decarboxylase (GAD) activity and [3H]-GABA release. The activation of retinal GABA-transaminase succinic semialdehyde dehydrogenase (GABA-T:SSADH) was also observed after a lag period of several hours. Under the same experimental conditions, however, no significant changes were noted in retinal taurine content and cysteine sulfinate decarboxylase (CSD) activity. These findings suggest that a short light adaptation induces differential effects on retinal GABA and taurine, and the activation of GABAergic neurons in the retina may be involved in the process of short light adaptation.

Effects of some amino acids (GABA, glycine, taurine) and of their antagonists (picrotoxin, strychnine) on spatial and temporal features of frog retinal ganglion cell responses

When intravitreally injected in the frog, GABA reduced the receptive field area of transient retinal ganglion cells, and it decreased the response duration and the number of spikes both at ON and at OFF. Conversely, its antagonist picrotoxin provoked an increase in the duration of both ON and OFF discharges as well as a marked increase in the number of spikes. Furthermore, picrotoxin provoked a marked increase in the size of the receptive field of both sustained and ON-OFF cells by abolishing the inhibition exerted by the surround upon the centre of the field. Glycine and taurine did not affect the size of the receptive field of these ganglion cells. They had no effect on the responses of sustained ganglion cells, while they totally suppressed OFF discharges of transient ganglion cells, without modifying their ON discharges. Conversely, their antagonist strychnine totally suppressed the ON dishcarges while the OFF discharges were still recorded, though with a reduced number of spikes and an increased latency, An histoautoradiographic study, carried out in parallel, showed that GABA is taken up by both horizontal cells and amacrine cells, while glycine and taurine are taken up by the amacrine cells only.

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.

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.

Pharmacological properties of amino-oxyacetic acid in the chicken

1. The effects of amino-oxyacetic acid (AOAA) on the central nervous system and on skeletal muscle have been examined in the chicken.2. AOAA had both anticonvulsant and convulsant effects, depending on the dose, as in other species.3. The convulsant effect, accompanied by EEG spiking, decreased rapidly with increase in age of young chicks.4. The convulsant effect was exerted primarily through supraspinal centres.5. Of control depressants tested, only troxidone and small doses of AOAA afforded significant protection against AOAA seizures.