Chapter 10 Kainic acid as a tool in retinal research

Functional impairment of the rat superior colliculus after kainic acid intraocular injection: A 2-Deoxyglucose study

Long Evans rats monocularly injected with the kainic acid (KA), were exposed to "tonic" (diffuse steady light, stationary pattern, total darkness) and "phasic" (flashing, moving pattern) stimulations. By means of the autoradiographic 2-deoxyglucose (2DG) technique we assessed the functional activity of the Superior Colliculus (SC) contralateral to the injected eye as compared to the normal eye SC. In the control SC all "tonic" stimulations determined low 2DG uptake not modified by the intraocular KA injection. On the contrary, "phasic" stimulations elicited a strong 2DG consumption in the normal SC, with a peculiar pattern of distribution depending on the kind of stimulus. Considering the total 2DG uptake as the added intrinsic and afferent metabolism, KA was able to affect only the latter, decreasing two-fold that expected for the afferent input loss. These findings can suggest a possible KA effect on off-line ganglion cells and, on the other side, they confirm the role of the SC in discriminating "phasic" and sudden phenomena from "tonic" and continuous ones.

Inhibitory modulation of photoreceptor melatonin synthesis via a nitric oxide-mediated mechanism

Nitric oxide (NO) has been suggested to have many physiological functions in the vertebrate retina, including a role in light-adaptive processes. The aim of this study was to examine the influence of the NO-donor sodium nitroprusside (SNP) on the activity of arylalkylamine-N-acetyltransferase (AA-NAT; EC. 2.3.1.87), the activity of which responds to light and reflects the changes in retinal melatonin synthesis--a key feature of light-adaptive responses in photoreceptors. Incubation of dark-adapted and dark-maintained retinas with SNP lead to the NO-specific suppression of AA-NAT activity, with NO suppressing AA-NAT activity to a level similar to that seen in the presence of dopaminergic agonists or light. Increased levels of cGMP appeared to be causally involved in the suppression of AA-NAT activity by SNP, as non-hydrolysable analogues of cGMP and the cGMP-specific phosphodiesterase (PDE) inhibitor zaprinast also significantly suppressed AA-NAT activity, while an inhibitor of soluble guanylate cyclase blocked the effect of SNP. While this chain of events may not be part of the normal physiology of the retina, it could be important in pathological circumstances that are associated with marked increase in levels of cGMP, as is found to be the case in certain forms photoreceptor degeneration, which are produced by defects in cGMP phosphodiesterase activity.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Comparative study of glutamate mediated gamma-aminobutyric acid release from nitric oxide synthase and tyrosine hydroxylase immunoreactive cells of the Cebus apella retina

The effects of excitatory amino acids (EAAs) upon transporter-mediated gamma-aminobutyric acid (GABA) release were investigated in cells containing tyrosine hydroxylase (TH) or nitric oxide synthase (NOS) in retina of the primate Cebus apella. Retinas were treated in vitro with 50 microM Kainate (KA) or 5 mM L-Glutamate (L-Glu), for 30 min at 37 degrees C, in an Mg2+-free Locke's solution with or without Ca2+. The effects of EAAs were measured immunocytochemically by determining the GABA content in TH or NOS-immunoreactive cells in the inner retina, after stimulation. L-Glu and KA induced a Ca2+-independent GABA release from most GABA-immunoreactive cells of the inner retina. Double label experiments indicated that this release occurs in NOS+/GABA+ cells, but not in TH+/GABA+ cells suggesting that these cell subpopulations may be differentiated in some functional aspects.

6,7-Dinitroquinoxaline-2,3-dione but not MK-801 exerts a protective effect against kainic acid neurotoxicity in the goldfish retina

Recent findings indicated that the excitotoxicity of glutamate analogues was prevented in the mammalian nervous system by N-methyl-D-aspartate (NMDA) antagonists. The neurodegenerative effects of kainic acid, and the putative protection of MK-801 and 6,7-dinitroquinoxaline-2,3-dione (DNQX), were investigated by morphological studies showing the toxicity of kainic acid to the neurons of the inner nuclear layer, and measuring choline acetyltransferase and glutamate decarboxylase activities in the retina. In addition, the proliferation of Müller retinal cells was assumed as an index of neuronal degeneration and was quantified by counting glial fibrillary acidic protein immunopositive cells. Our observations suggest that the non-NMDA receptor antagonist DNQX exerted a protective effect on goldfish retinal neurons, while MK-801 did not prevent the neurotoxicity induced by kainic acid in the goldfish retina. This finding is in agreement with previous work on kainic acid toxicity in the goldfish optic tectum.

Melatonin synthesis in chicken retina: effect of kainic acid-induced lesions on the diurnal rhythm and D2-dopamine receptor-mediated regulation of serotonin N-acetyltransferase activity

The effect of kainic acid (KA)-induced lesions of retinal neurons on regulation of serotonin N-acetyltransferase (NAT) activity in chicken retina was investigated. Although NAT activity was higher in KA-lesioned retinas than in controls, the pattern of diurnal variation of enzyme activity throughout 36 h of constant darkness was similar for both tissues. Quinpirole, a selective D2-dopamine receptor agonist, inhibited the nocturnal increase of NAT activity in both control and KA-treated retinas. Quinpirole was significantly more potent in KA-treated retinas than in controls; the ED50 value for quinpirole was 3 times lower in KA-treated retinas than in control tissues. KA treatment markedly reduced retinal levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). We conclude that: (1) NAT activity in retina is localized primarily to KA-insensitive cells, presumably photoreceptors; (2) KA-sensitive inner retinal neurons are not essential to the maintenance of the circadian rhythm of NAT activity; and (3) KA-induced lesions of retinal cells result in supersensitivity of D2-dopamine receptors regulating NAT activity in a mechanism that involves adaptive changes following a decline in retinal dopamine neurotransmission.

Excitatory amino acid neurotoxicity in cultured retinal neurons: involvement of N-methyl-D-aspartate (NMDA) and non-NMDA receptors and effect of ganglioside GM1

Cultures of chicken day 8 embryo retinal cells, essentially free of contaminating non-neuronal elements, were used to examine the neurotoxicity of various excitatory amino acid transmitter receptor agonists. At 7 days in vitro, N-methyl-D-aspartate (NMDA), following 24 hr exposure to 0.1-1.0 mM, destroyed 60-70% of the multipolar neurons, but apparently spared photoreceptors. The cytotoxic effect of NMDA was prevented by extracellular Mg2+ or phencyclidine, suggesting a role for the NMDA ion channel; competitive NMDA antagonists were also neuroprotective. The mixed excitatory amino acid receptor agonist glutamate (0.1-1.0 mM) was also neurotoxic (approximately 70% loss of multipolar neurons) and strongly blocked by NMDA (but weakly by non-NMDA) antagonists and Mg2+, indicating a major action at NMDA receptors. As with NMDA, glutamate did not appear to affect photoreceptors. The neurotoxic action of kainate against multipolar retinal neurons, as reported by others, was confirmed here. Kainate neuronal injury was sensitive to the quinoxalinedione non-NMDA antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 6-cyanoquinoxaline-2,3-dione (CNQX), but not to Mg2+ or phencyclidine. Ibotenate and quisqualate, even at millimolar concentrations, were not neurotoxic. The monosialoganglioside GM1 was also effective in reducing NMDA and non-NMDA agonist neurotoxicity to retinal neurons. Maximal ganglioside benefit required 1-2 hr of pretreatment with 100-200 microM GM1. The percentage of multipolar neurons remaining after the neurotoxin insult approximately doubled with GM1 treatment. Gangliosides may thus have a therapeutic potential in excitatory amino acid-initiated neuropathologies.

A quantitative analysis of the effects of excitatory neurotoxins on retinal ganglion cells in the chick

The present study examines the differential effects of three excitotoxins, kainic acid (KA), N-methyl-D-aspartate (NMDA), and alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) on neurons within the ganglion cell layer (GCL) of the chick retina. Two-day-old chicks were given a single, 5 microliters, intravitreal injection of KA, NMDA, or AMPA at a range of doses. Following treatment with 40 nmol KA, there was a 21% loss of neurons in the GCL. At 200 nmol KA, the loss increased to 46%. Exposure to KA eliminated mainly small neurons of soma area 5-15 microns2, and medium-sized ganglion cells of soma area 15-25 microns2. Large ganglion cells (greater than 25 microns2) remained unaffected. The vast majority of small cells were probably displaced amacrine cells. Exposure to 400 nmol NMDA resulted in a 14% loss of neurons, predominantly involving the large ganglion cells. At a dose of 3000 nmol NMDA, no further loss of cells was evident. Exposure to 200 nmol AMPA resulted in a 30% loss of large and some medium-sized ganglion cells. In a further series of experiments, exposure to excitotoxin was followed by a retinal scratch, which eliminated retinal ganglion cells within the axotomized region. The results indicate that only a small proportion of displaced amacrine cells are destroyed by NMDA and AMPA, whereas virtually all displaced amacrine cells are sensitive to KA. The findings of this study indicate the existence of subclasses of ganglion cells with specificity towards different types of excitatory amino acids (EAA).

MK801-induced antagonism of NMDA-preferring excitatory amino acid receptors in horizontal cells of the turtle retina

Intracellular recordings were made from axon terminals of L-type horizontal cells in the turtle (Pseudemys scripta elegans) retina. Superfusion with Ringer's solution containing 3.0 mM N-methyl-D-aspartate (NMDA) or 0.2 mM kainic acid (KA) induced depolarization and reduction in the hyperpolarizing light responses of horizontal cells, consistent with an agonist effect of these excitatory amino acid (EAA) analogs on postsynaptic receptors. Delivery of 0.1 mM MK801, a selective blocker of NMDA-type EAA receptors, had no apparent effect on membrane potential or photoresponses, nor did it change the KA depolarization. Exposure of the retina to 3.0 mM NMDA following 0.1 mM MK801 always caused hyperpolarization of the horizontal cell and loss of light responses. Because MK801 is specific for NMDA-preferring receptors, we suggest that the reversal of the NMDA response to one of antagonism following MK801 is strong evidence for the presence of NMDA-preferring EAA receptors in turtle horizontal cells.

Differential effects of excitatory amino acids on photoreceptors of the chick retina: an electron-microscopical study using the zinc-iodide-osmium technique

Although excitotoxins derived from acidic amino acids are known to damage neurons in the inner nuclear and ganglion cell layers of the retina, little is known about their effects on photoreceptors. This study examines the acute and long-term effects of excitotoxins on photoreceptors of the chick retina. The zinc-iodide-osmium (ZIO) technique, which darkly labels a substantial subpopulation of synaptic vesicles in normal photoreceptor terminals, was used to supplement routine electron microscopy. Two-day-old chicks received a single intraocular injection of either 10, 50, or 200 nmoles kainic acid (KA), 200 nmoles N-methyl-D-aspartic acid (NMDA), or 200 nmoles quisqualic acid (QUIS), and were allowed to survive for either 6 h, 7 d, or 21 d. At 6 h, following exposure to 10, 50, and 200 nmoles KA, there was swelling and disruption of photoreceptor lamellae of the outer segments. At 7- and 21-d survival, 50 and 200 nmoles KA resulted in rounded, condensed synaptic terminals, which contained a high density of synaptic vesicles. However, there was complete loss of ZIO-positive vesicles within these photoreceptors. Outer segments were still disrupted, although small patches of lamellae were evident, suggestive of regeneration. Following exposure to QUIS, there was extensive swelling of outer segment lamellae at 6 h survival. Synaptic ribbons in terminals were also swollen. At longer survival periods, QUIS exposure resulted in a reduction of ZIO-positive vesicles, as well as swollen lamellae in outer segments. NMDA exposure, at either short or long-term survival, did not alter photoreceptor morphology, including the pattern of ZIO stain. The prolonged effects of KA, and to a lesser extent QUIS, on photoreceptors suggests that these drugs have a long-term effect on photoreceptor function. The ZIO technique provides a novel and potentially useful approach for identification of damaged photoreceptors.

Spatiotemporal gradients of kainate-sensitivity in the developing chicken retina

We have studied the age-dependence of the effects of kainate (KA) on the chick retina as a prelude to the accompanying paper on the effects of target-removal on the isthmo-optic nucleus. KA was injected into the eyes of chick embryos and chicks at different ages, and the retinas were fixed a few hours or several days later. The former group of retinas was scanned for pyknotic cells. The earliest age at which KA caused pyknosis was embryonic day 10 (E10), when pyknotic cells appeared in a ventrotemporal patch in the amacrine sublayer near the fundus. Over the next two days the sensitive region expanded tangentially, reaching the periphery first temporally, then nasally. Only after E12 did the KA cause pyknotic cells to occur also in the bipolar sublayer, where the sensitivity spread in the same spatiotemporal sequence as the initial wave, but two days later. Cell loss was examined in embryos that survived a week or more after the KA injection. Substantial cell depletion was found in both the inner nuclear and ganglion cell layers, but only when the injection had been made after E12. With progressively later injections, the depleted zone expanded in the same spatiotemporal sequence as described above, until at E15 the injections caused depletion throughout the entire extent of the retina. The reasons for the lack of cell depletion after KA injections made before E12 are discussed. Cell counts in the ganglion cell layer and studies of anterograde transport of intravitreally injected peroxidase along the retinofugal fibers showed that about half the ganglion cells (including the displaced ganglion cells) pass through a period of vulnerability to the KA injections, to which they subsequently become sensitive.

Nature and nurture in the differentiation of retinal photoreceptors and neurons

This article reviews recent studies using a novel experimental system in which undifferentiated precursor cells from the 8-day chick embryo retina are grown in low density, clump-free, dissociated cell culture. The cultures initially consist of a morphologically homogeneous population of isolated process-free, round cells. Analysis of the cultures by phase contrast light microscopy, scanning and transmission electron microscopy, immunocytochemistry and autoradiography, shows that during the first week in vitro some precursor cells acquire a well differentiated photoreceptor phenotype, while others develop as neurons. Given that these divergent differentiation pathways are followed by cells developing in a homogeneous microenvironment in the absence of intercellular contacts, the evidence suggests that precursor cells present in the 8-day chick embryo retina are already preprogrammed to undergo an extensive series of chemical and structural modifications necessary to differentiate as either neurons or photoreceptors.

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: I. Description of effects

The neurotoxic action of kainic acid (KA) was investigated by histological methods in the isolated retina of toads and goldfish. Particular attention was paid to the earliest and most sensitive response to KA in the outer plexiform layer (OPL). KA caused vacuolization of proximal and distal segments of horizontal cell dendrites in the OPL as well as perikaryal vacuolization and/or chromatin clumping in selected classes of neurons in the inner nuclear layer. Further, KA caused vacuolization and swelling in the inner plexiform layer. These effects were very similar in the retinae of goldfish and toad. The extent of vacuolization in the OPL was graded with KA concentration and with length of incubation. For 15-minute incubations, half-maximal vacuolization was found at 10-20 microM KA. At 25 microM KA, OPL vacuolization was evident within 1-2 minutes of application of KA. In goldfish, but not in toad, rod-connecting dendrites were less sensitive to KA than cone-connecting dendrites.

Visual deficits following intraocular treatment of chicks with glutamate or kainic acid

After intraocular treatment with kainate or glutamate, chicks were tested for visual discrimination performance, pecking aim, bead detection, optokinetic response and pupillary response. Kainate at an amount of 6 nmol per eye impaired performance in all of the tests except pupillary response. No effects were seen after 0.6 nmol of kainic acid. Glutamate at 6000 nmol impaired performance on all tests but the optokinetic and pupillary response. Although glutamate caused less severe deficits in visual behaviour than the high dose of kainate, cellular organization of the retina, at least in some regions, was more disrupted in these animals. Glutamate at 600 nmol caused a late onset defect in visual discrimination performance but no effects on optokinetic response.

Off-pathway synaptic transmission in the outer retina of the axolotl is mediated by a kainic acid-preferring receptor

Intracellular recordings were made from OFF-centre bipolar cells and horizontal cells in the superfused axolotl retina eyecup preparation. Bath-applied (+/-)cis-2,3-piperidine dicarboxylic acid (PDA), gamma-D-glutamylglycine (DGG), L-glutamic acid diethyl ester (GDEE), (+/-)2-amino-5-phosphonovaleric acid (2-APV) and magnesium ions were assessed as antagonists of the actions of the photoreceptor transmitter. The rank order of antagonist efficacy was PDA greater than DGG greater than greater than 2-APV = GDEE = Mg2+. The results indicate that transmission at OFF-pathway synapses in the outer retina of the axolotl is mediated by a kainic acid-preferring receptor.