Electrophysiological analysis of taurine and glycine action on neurons of the mudpuppy retina. II. ERG, PNR and Müller cell recordings

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Reciprocal regulation between taurine and glutamate response via Ca2+-dependent pathways in retinal third-order neurons

Although taurine and glutamate are the most abundant amino acids conducting neural signals in the central nervous system, the communication between these two neurotransmitters is largely unknown. This study explores the interaction of taurine and glutamate in the retinal third-order neurons. Using specific antibodies, both taurine and taurine transporters were localized in photoreceptors and Off-bipolar cells, glutamatergic neurons in retinas. It is possible that Off-bipolar cells release juxtaposed glutamate and taurine to activate the third-order neurons in retina. The interaction of taurine and glutamate was studied in acutely dissociated third-order neurons in whole-cell patch-clamp recording and Ca²⁺ imaging. We find that taurine effectively reduces glutamate-induced Ca²⁺ influx via ionotropic glutamate receptors and voltage-dependent Ca²⁺ channels in the neurons, and the effect of taurine was selectively inhibited by strychnine and picrotoxin, but not GABA receptor antagonists, although GABA receptors are present in the neurons. A CaMKII inhibitor partially reversed the effect of taurine, suggesting that a Ca²⁺/calmodulin-dependent pathway is involved in taurine regulation. On the other hand, a rapid influx of Ca²⁺ through ionotropic glutamate receptors could inhibit the amplitude and kinetics of taurine-elicited currents in the third-order neurons, which could be controlled with intracellular application of BAPTA a fast Ca²⁺ chelator. This study indicates that taurine is a potential neuromodulator in glutamate transmission. The reciprocal inhibition between taurine and glutamate in the postsynaptic neurons contributes to computation of visual signals in the retinal neurons.

Amine and amino acid transmitters in the eye of the mollusc Bulla gouldiana: an immunocytochemical study

We identified putative transmitters of the photoreceptors and circadian pacemaker neurons and found candidates for efferent control in the eye of the marine mollusc Bulla gouldiana. Established antisera against octopamine, dopamine, serotonin, histamine, glutamate, gamma-aminobutyric acid (GABA), and taurine were used, and central ganglia were processed in parallel to evaluate general staining quality. Photoreceptors and circadian pacemaker cells both expressed immunoreactivity for glutamate and taurine. The eye and its sheath were devoid of GABA-like immunoreactive material, and none of the antisera directed against biogenic amines labelled cells or processes in the nervous tissue of the eye. However, dopamine and octopamine antisera stained large spherical granules (diameter 2-3 microm) contained in granular cells that are located in the connective tissue encapsulating the eye and the optic nerve. The serotonin antiserum revealed a sparse distribution of varicose axon fibers in the optic nerve and eye sheath. No histamine-immunoreactive processes were revealed in the eye. The functional significance of these findings for the molluscan eye and its circadian clock is discussed.

Modification of the Xenopus electroretinogram by actions of glycine in the proximal retina

The electroretinogram (ERG) was recorded from the Xenopus retina, to examine the effects of glycine and strychnine on these responses and to determine the origins of these changes. Glycine at concentrations between 0.1 and 10 mM reduced the b- and d-waves of the ERG in a dose-dependent manner, while strychnine increased their amplitude. 2-Amino-4-phosphonobutyric acid (APB) reduced the b-wave and blocked the effect of glycine, but not strychnine, on the d-wave. When the d-wave had first been blocked by kynurenic acid (KYN) or reduced by (+/-)cis-2,3-piperidine dicarboxylic acid (PDA) the b-wave was enhanced by glycine, but not by strychnine. N-methyl-DL-aspartate (NMDLA), which alters responses in the proximal retina only, blocked the effects of glycine and strychnine on the ERG. This suggests that the glycinergic effects on the ERG are at least partly mediated by processes in the proximal retina. The results further support the suggestion that inhibitory neurotransmitters in the proximal retina may modulate both the b- and d-waves of the Xenopus ERG.

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.

Oscillatory potentials in the retina: what do they reveal

This chapter is an overview of current knowledge on the oscillatory potentials (OPs) of the retina. The first section describes the characteristics of the OPs. The basic, adaptational, pharmacological and developmental characteristics of the OPs are different from the a- and b-waves, the major components of the electroretinogram (ERG). The OPs are most easily recorded in mesopic adaptational conditions and reflect rapid changes of adaptation. They represent photopic and scotopic processes, probably an interaction between cone and rod activity in the retina. The OPs are sensitive to disruption of inhibitory (dopamine, GABA-, and glycine-mediated) neuronal pathways and are not selectively affected by excitatory amino acids. The earlier OPs are associated with the on-components and the late OPs with the off-components in response to a brief stimulus of light. The postnatal appearance of the first oscillatory activity is preceded by the a- and b-waves. The earlier OPs appear postnatally prior to, and mature differently from, the later ones. The second section deals with present views on the origin of the OPs. These views are developed from experimental studies with the vertebrate retina including the primate retina and clinical studies. Findings favor the conclusion that the OPs reflect neuronal synaptic activity in inhibitory feedback pathways initiated by the amacrines in the inner retina. The bipolar (or the interplexiform) cells are the probable generators of the OPs. Dopaminergic neurons, probably amacrines (or interplexiform cells), are involved in the generation of the OPs. The earlier OPs are generated in neurons related to the on-pathway of the retina and the later ones to the off-channel system. Peptidergic neurons may be indirectly involved as modulators. The individual OPs seem to represent the activation of several retinal generators. The earlier OPs are more dependent on an intact rod function and the later ones on an intact cone system. Thus, the OPs are good indicators of neuronal adaptive mechanisms in the retina and are probably the only post-synaptic neuronal components that can be recorded in the ERG except when structured stimuli are used. The last section describes the usefulness of the oscillatory response as an instrument to study the postnatal development of neuronal adaptation of the retina. In this section clinical examples of of the sensitivity of the OPs for revealing early disturbance in neuronal function in different retinal diseases such as pediatric, vascular and degenerative retinopathies are also given.

Effects of 2-amino-4-phosphonobutyric acid (APB) and glycine on the oscillatory potentials of the rat electroretinogram

Oscillatory potentials of the electroretinogram were monitored in dark-adapted rats following intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), a glutamate analog that preferentially blocks the light response of depolarizing bipolar cells, or glycine, a known endogenous inhibitory neurotransmitter that suppresses the light response of cells in the inner retina postsynaptic to glycinergic neurons. Oscillatory potentials were abolished in conjunction with the b-wave with APB and selectively reduced or eliminated by glycine: neither agent attenuated the a-wave. The results are compatible with the idea that light-induced depolarizing bipolar cell, hyperpolarizing bipolar cell, and glycinergic amacrine cell responses are all necessary for the generation of oscillatory potentials in the rat. The results also suggest that hyperpolarizing bipolar cells do not contribute to b-wave generation.

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.

Distribution of glycine, gamma-aminobutyric acid, glutamate decarboxylase, and gamma-aminobutyric acid transaminase in rabbit and mudpuppy retinas

The distributions of glycine, gamma-aminobutyric acid (GABA), glutamate decarboxylase (EC 4.1.1.15), and GABA transaminase (EC 2.6.1.19) were determined in rabbit and mudpuppy retinas. In both species, peak levels of the amino acids and the enzymes occurred in the inner plexiform layer. Glutamate decarboxylase was almost entirely confined to the inner plexiform layer. Determinations were also made of the GABA content of 107 individual putative amacrine cell somas from mudpuppy retina. About 30% of those somas were found to have high endogenous GABA levels.

Effect of gamma-aminobutyric acid agonists, glycine, taurine and neuropeptides on acetylcholine release from the rabbit retina

The light-evoked release of [3H]acetylcholine (ACh) from the rabbit retina in vivo was measured and taken as an index of cholinergic amacrine cell activity. The light-evoked release of [3H]ACh was reduced by locally applied gamma-aminobutyric acid (GABA), muscimol and 3-aminopropanesulphonic acid (3-APS). The concentrations of these drugs which reduced the light-evoked release of [3H]ACh by 50% (EC50) were 900, 0.3 and 5 microM respectively. In contrast, (-)-baclofen (5 mM), but not (+)-baclofen, significantly increased the light-evoked release of [3H]ACh. The GABA antagonist, bicuculline increased the resting release of [3H]ACh but abolished the inhibitory action of muscimol on the light-evoked release of [3H]ACh. Glycine and taurine also reduced the light-evoked release of [3H]ACh from the retina, their EC50 values being 1.5 and 0.3 mM respectively. This action was blocked by strychnine, but not by bicuculline. In contrast to the GABA antagonist, strychnine did not affect the spontaneous resting release of [3H]ACh. Retinal [3H]ACh release was not affected by dopamine, 5-hydroxytryptamine (5-HT) morphine, substance P, somatostatin, cholecystokinin sulphate, thyrotropin releasing hormone, luteinizing hormone releasing hormone or angiotensin. Electroretinographic changes produced by amino acids and GABA agonists involved mainly the b-wave and were not correlated with their effects on ACh release. Thus, GABA increased the b-wave amplitude, 3-APS had no effect, whilst muscimol, taurine and glycine either had no effect, or reduced the b-wave amplitude. No obvious changes in the e.r.g. were produced by baclofen, dopamine, 5-HT, morphine or any of the peptides studied with the exception of somatostatin, which reduced the amplitude of the b-wave. It is concluded that cholinergic amacrine cell activity in the rabbit retina may be affected by inputs from other amacrines using GABA or glycine (taurine) as their transmitters, but probably not by inputs from peptidergic or dopaminergic amacrine cells. Our experiments do not provide evidence on the sites of action of GABA, glycine or taurine but the action of bicuculline on the resting release of ACh implies that the activity of the cholinergic amacrine cells is affected by a tonically active GABAergic input.

Further studies of the chemical sensitivity of the oscillatory potentials of the electroretinogram (ERG). III. Some omega amino acids and ethanol

The action of some omega amino acids and the influence of ethanol on the oscillatory potentials (OPs) of the ERG were tested. Low doses of beta-alanine, an inhibitory amino acid, selectively suppressed the amplitude of the OPs without affecting the a- and b-waves. The earlier OPs (O1 O2) appeared somewhat more sensitive to the drug than the later ones (O3-O5). Higher doses extinguished all the OPs but also affected the maximum amplitude of the a- and b-wave. There was no noticeable change of the threshold sensitivity of the a- and b-waves. Valine in corresponding concentrations did not affect the OPs. Taurine did not change the OPs in most experiments but in some experiments a concurrent decrease of the OPs, a- and b-waves occurred. Thus, these findings support the view that only inhibitory feed-back circuits initiated by the amacrines give rise to the OPs. Ethanol had a differential and selective effect on the OPs. The later OPs (O3-O5) were sensitive to ethanol. In conclusion, these results support the notion that the individual oscillatory peaks are likely to have different origins and that the later OPs may perhaps be related to the off-components in response to brief stimulus light.