The effect of light and potassium depolarization on the release of endogenous amino acids from the isolated rat retina

2-amino-4-phosphonobutyric acid exerts a light-dependent effect on post-gabaculine levels of retinal gamma-aminobutyric acid (GABA): evidence that ON synaptic pathways regulate retinal GABAergic transmission

The effects of light, 2-amino-4-phosphonobutyric acid (APB), and kainic acid on rat retinal gamma-aminobutyric acid (GABA)-ergic transmission were studied by measuring levels of retinal GABA following subcutaneous injection of gabaculine, an irreversible inhibitor of GABA-transaminase. Post-gabaculine levels of retinal GABA in light-exposed rats were significantly greater than those in rats held in darkness. The synaptic mechanism of this effect of light was examined by measuring post-gabaculine levels of retinal GABA in rats placed into either lighted or darkened conditions after receiving unilateral intravitreal injections of APB, a glutamate analogue that selectively decreases the activity of ON synaptic pathways in the retina. APB attenuated the post-gabaculine accumulation of GABA in rats held in the light, but not in those placed into darkness. Furthermore, the light-dependent increment in post-gabaculine accumulation of retinal GABA was entirely APB sensitive, and the effect of APB was entirely light dependent. In contrast to APB, kainic acid stimulated the post-gabaculine accumulation of retinal GABA in vivo. Our findings suggest that APB and kainic acid influence GABAergic transmission at different sites in the retina and that some retinal GABAergic neurons are either ON or ON-OFF amacrine cells.

Free amino acids in the pre-retinal vitreous space. Effect of high potassium and nipecotic acid

The pre-retinal vitreous space of the eye of albino rabbits was perfused in vivo via a dialysis probe in order to measure the concentration of endogenous amino acids. Studies of isotope concentrations in the vitreal effluent following intracarotid injections of 3HOH and [14C]mannitol demonstrated that entrance of 3HOH into the dialysis probe occurred within a few minutes whereas [14C]mannitol was almost completely excluded. Among the amino acids, analysed with high-pressure liquid chromatography (HPLC), glutamine had a concentration similar to that in plasma and cerebrospinal fluid (CSF). Taurine was four times higher than in the CSF. Perfusion with 125 mM KCl media increased particularly the taurine and phosphoethanolamine content. Nipecotic acid, a potent inhibitor of gamma-aminobutyric acid (GABA) uptake, increased GABA concentration over 60 times. In addition, it increased taurine levels by almost 10 times.

Autoradiographic localization of high affinity GABA, benzodiazepine, dopaminergic, adrenergic and muscarinic cholinergic receptors in the rat, monkey and human retina

High affinity gamma-aminobutyric acid, benzodiazepine, strychnine (glycine), dopamine, spirodecanone, alpha 1-adrenergic, alpha 2-adrenergic, beta-adrenergic and muscarinic cholinergic binding sites were localized by semiquantitative autoradiography in rat and, in some instances, in monkey and human retinae using [3H]muscimol, [3H]flunitrazepam, [3H]strychnine, [3H]spiperone, [3H]prazosin, [3H]para-aminoclonidine, [3H]dihydroalprenolol and [3H]quinuclidinyl benzylate, respectively. In nearly every case, the inner plexiform layer (IP) contained a high receptor density. The distribution of alpha 1 sites was unusual in that binding was concentrated in the outer plexiform layer (OP). Dopaminergic and, to a lesser extent, beta-adrenergic binding was diffusely distributed in the outer nuclear layer, the OP, the inner nuclear layer and the IP. The ganglion cell layer displayed significant benzodiazepine binding. The intraretinal distribution of pre- and postsynaptic markers of these neurotransmitters is discussed.

Action and localization of glycine and taurine in the cat retina

The effects on retinal ganglion cells of iontophoretically applied glycine, taurine and strychnine were studied in the optically intact eye of the cat. Glycine and taurine suppressed the light-evoked discharge of all on-centre and off-centre brisk ganglion cells, regardless of the visual stimulus used. Strychnine blocked the action of externally applied glycine and taurine. The light-evoked response of all ganglion cells was raised by strychnine. The tonic discharge of the light response was suppressed or raised by the drugs more than the phasic response. A population of amacrine cells, which was heavily labelled by [3H]glycine, did not take up [3H]taurine. [3H]taurine was only weakly accumulated by inner nuclear layer neurones and was predominantly located in the outer retina.

Retinal neurotransmission

The mammalian retina is classically divided into ten layers which contain the neuronal elements identified as photoreceptors, horizontal cells, bipolar cells, amacrine cells and ganglion cells. Using various neuroscientific techniques possible neurotransmitter substances have been assigned to each of these cell types. Thus the localization of transmitter synthesizing enzymes and storage vesicles, the demonstration of release of transmitter in response to specific stimuli, the observation of post-synaptic events mimicked or blocked by the iontophoretic application of exogenous transmitter/agonist or antagonist drug respectively, and the identification of efficient transmitter inactivation mechanisms synaptically add evidence for the association of certain proposed transmitter substances with specific neuronal elements. The evidence for the proposal that the excitatory amino acids glutamate and aspartate are transmitters of photoreceptors, that gamma-aminobutyric acid (GABA) is the inhibitory transmitter of horizontal and amacrine cells, that acetylcholine is associated with the functioning of bipolar cells, and that taurine, glycine and dopamine may all also play neurotransmitter or neuromodulatory roles at amacrine cell synapses is discussed.

Excitatory amino acid analogs evoke release of endogenous amino acids and acetyl choline from chick retina in vitro

There is mounting evidence that excitatory amino acids may play a role in retinal synaptic neurotransmission. In this study, we demonstrate the release of endogenous amino acids and acetylcholine from isolated chick retina in vitro evoked by three excitatory amino acid analogs, kainic acid (KA), quisqualic acid (Quis), and N-methyl-D,L-aspartic acid (NMDA). The release is dose-dependent and involves putative transmitters from both inner and outer retina. Release from the inner retina is partially Ca2+-dependent, while release from the outer retina is Ca2+-independent and Na+-dependent. Release experiments carried out in the presence of specific excitatory amino acid blocking agents suggest that the release is mediated by two receptors, the kainate receptor and the NMDA receptor. These results are supportive of a role for excitatory amino acids in synaptic neurotransmission in both inner and outer retina.

The dissociation of evoked release of [3H]-GABA and of endogenous GABA from chick retina in vitro

The release of prelabeled [3H]-GABA and endogenous GABA evoked by KCl and excitatory amino acid analogs was compared in intact chick retina and retina previously lesioned by the neurotoxins, kainic acid and N-methyl-D,L-aspartic acid. Significant discrepancies were found in the results obtained by the two techniques; and while changes in endogenous GABA release after neurotoxin lesions correlated with retinal glutamate decarboxylase activity, a marker for GABAergic neurons, the release of [3H]-GABA did not. These results call into question the validity of the prelabeling technique for studying GABA release from retina.

Interactions of dopamine and the release of [3H]-taurine and [3H]-glycine from the isolated retina of the rat

1 The dose-related, calcium-dependent, potassium-stimulated release of preloaded [(3)H]-dopamine from the superfused rat retina has been demonstrated.2 A high-affinity uptake system for dopamine exists in rat retina in vitro; K(m) value was calculated as 1.89 muM, V(max) value as 1.4 nmol g(-1) tissue h(-1).3 Dopamine (0.8 and 4 mM) inhibited the spontaneous release of [(3)H]-glycine from retina, and in the case of 0.8 mM dopamine this inhibitory effect was antagonized by 10 muM (+)-butaclamol but not by 10 muM (-)-butaclamol.4 The potassium-evoked (25 mM) release of [(3)H]-glycine from rat retina was similarly inhibited by dopamine (0.4-4 mM) in a dose-related manner when added to the superfusate with the potassium. The effect of 0.8 mM dopamine was antagonized by 10 muM (+)-butaclamol but not by 10 muM (-)-butaclamol.5 Dopamine (4 mM) significantly reduced the spontaneous release of [(3)H]-taurine from rat retina.6 The potassium-stimulated (25 mM) release of [(3)H]-taurine occurred after the cessation of the depolarizing stimulus. This delayed release of [(3)H]-taurine was unaffected if dopamine was applied to the superfusate at the same time as the potassium, but it was significantly reduced if dopamine (0.8 and 4 mM) was applied after the depolarizing stimulus had been removed and during the actual amino acid release phase.7 The inhibition of K(+)-stimulated (25 mM) delayed release of [(3)H]-taurine by applying dopamine (0.8 mM) after the depolarizing stimulus was blocked by 10 muM (+)-butaclamol but not by 10 muM (-)-butaclamol.8 The results are discussed with respect to the possible neurotransmitter role for dopamine within the rat retina, and its possible interaction with glycine and taurine.

Potassium-stimulated release of radiolabelled taurine and glycine from the isolated rat retina

The release of preloaded [3H]glycine and [3H]taurine in response to a depolarising stimulus (12.5-50 mM KCl) has been studied in the superfused rat retina. High external potassium concentration immediately increased the spontaneous efflux of [3H]glycine, the effect of 50 mM K+ apparently being abolished by omitting calcium from the superfusing medium. In contrast, although high potassium concentrations increased the spontaneous efflux of [3H]taurine from the superfused rat retina, this release was not evident until the depolarising stimulus was removed from the superfusing medium. The magnitude of this "late" release of [3H]taurine was dependent on external K+ concentrations, and appeared immediately after cessation of the stimulus irrespective of whether it was applied for 4, 8, or 12 min. Potassium (50 mM)-induced release of taurine appeared partially calcium-dependent, being significantly reduced (p less than 0.01) but not abolished by replacing calcium with 1 mM EDTA in the superfusate. High-affinity uptake systems for both [3H]glycine and [3H]taurine were demonstrated in the rat retina in vitro (Km values, 1.67 microM and 2.97 microM; Vmax values, 19.3 and 23.1 nmol/g wet weight tissue/h, respectively). The results are discussed with respect to the possible neurotransmitter roles of both amino acids in the rat retina.

Cells accumulating 3h-glycine in the goldfish retina

Glycine accumulating neurons occur among amacrine cells of the goldfish retina with processes distributed in the inner plexiform layer. Similar cells also occur, although much more rarely, among the horizontal cells and in the outer plexiform layer. The latter cells are readily observable only after heavy labelling. They may emit processes ending close to the photoreceptor terminals.

The release of endogenous amino acids into the vitreous of the intact eye of the albino rat: effect of light, potassium, and ouabain

The vitreal space of the intact eye of albino rats was perfused in vivo. The concentration of several endogenous amino acids in the vitreal effluent was measured by the [3H]microdansylation procedure. GABA was never detected despite a sensitivity of the method of 0.5 pmol. In contrast to previous results obtained in pigmented rats, photic stimulation with flashing white light did not alter the release of glycine or any of the other amino acids. Potassium (60 mM) and ouabain (0.1 mM) evoked a specific release of glycine. The potassium-evoked release was blocked by magnesium suggesting a neuronal site of origin of glycine. Ouabain-evoked release was not blocked by magnesium. The results were contrasted with experiments on radiolabeled amino acid release from retinas preloaded and superfused in vitro, a condition in which glial localization of exogenous amino acids predominates.

GABA antagonists enhance dopamine turnover in the rat retina in vivo

Previous results provided evidence for an inhibitory GABAergic influence on the dopamine neurons of the rat retina, without proving that endogenous GABA physiologically regulates the activity of these cells. We injected picrotoxinin intraocularly to dark-adapted rats and measured retinal dopamine turnover. Dopamine was analyzed radioenzymatically, and the decline in dopamine after alpha-methyl-p-tyrosine was used as an index of dopamine turnover. Picrotoxinin significantly stimulated dopamine turnover (P less than 0.05). In similar experiments with light-exposed rats picrotoxinin slightly enhanced dopamine turnover beyond that produced by light alone. Intraocular bicuculline methiodide produced similar results in dark-adapted and also in light-exposed rats. These data suggest that endogenous GABA tonically inhibits the activity of the retinal dopamine neurons in the dark and that there may be some remaining GABA tone on these cells in the light-exposed rat. Atropine pretreatment did not affect the picrotoxinin-induced activation of retinal dopamine turnover which negates the involvement of a cholinergic interneuron.

The synthesis of neuroactive amino acids from radioactive glucose and glutamine in the rat retina: effects of light stimulation

The effect of light stimulation in vitro on the labelling of neuroactive amino acids derived from [14C]glucose or [14C]glutamine in the rat retina has been studied. [14C]Glutamine, at 700 microM, provided about 50% of the tissue pools of glutamate, aspartate, and GABA; and the labelling of these decreased on light stimulation, both in the photoreceptor cells (glu and asp) and in the inner retina (glu, asp, and GABA). In contrast, there were no significant changes in the entry of label derived from [14C]glucose, although similar trends were apparent in the data obtained for the photoreceptor cell layer. The pools may, therefore, be separate. Other results support the contention that glucose is the principal energy source for the retina, its entry into non-amino acid derivates being decreased on light stimulation.

Inhibition of aspartate release from the retina of the anaesthetised rabbit by stimulation with light flashes

Rabbits were anaesthetised and the cornea, iris, lens and vitreous were removed from eyes to form an eye-cup which was filled with Krebs-bicarbonate Ringer. The medium in the eye-cup was replaced at 10-min intervals and the amino acids in each resulting sample were assayed. Exposure of the dark-adapted retina to flashes of light (3 Hz) did not alter the efflux of glutamate glutamine, alanine, glycine or GABA. The efflux of taurine and ACh was greatly increased by light flashes, but in contrast, the release of aspartate was reduced by more than 50%, a result consistent with the suggestion that aspartate may be a photoreceptor transmitter substance.