An eyecup slice preparation for intracellular recording in vertebrate retinas

Dynamic regulation of D-serine release in the vertebrate retina

KEY POINTS

Activation of NMDA receptors (NMDARs) is essential for encoding visual stimuli into signals for the brain, although their over-activation can cause cell death. The recruitment of NMDARs is important for encoding light intensity in retinal ganglion cells. D-serine binding is essential for proper activation of NMDARs, although its role in signal processing and the mechanisms that underlie its availability are not well understood. In these light-evoked experiments, the addition of exogenous D-serine had a large effect on low contrast and low intensity NMDAR responses that decreased as the intensity was increased. The degradation of endogenous D-serine decreased the responses more at higher intensities. The results provide compelling evidence favouring a new interpretation of NMDAR recruitment in which light-evoked D-serine release serves an important regulatory control over the recruitment of NMDARs.

ABSTRACT

The present study aimed to investigate the functional properties of NMDA receptor coagonist release and to specifically evaluate whether light-evoked release mechanisms contribute to the availability of the coagonist D-serine. Two different methods were involved in our approach: (i) whole-cell recordings from identified retinal ganglion cells in the tiger salamander were used to study light adaptation with positive and negative contrast stimuli over a range of ± 1 log unit against a steady background illumination and (ii) the mechanisms for intensity encoding to a range of light intensities covering 6 log10 units were investigated. This latter study employed extracellular recordings of the proximal negative field potential, pharmacologically manipulated to generate a pure NMDA mediated response. For the adaptation study, we examined the light-evoked responses under control conditions, followed by light stimuli presented in the presence of D-serine, followed by light stimulation in the presence of dichlorokynurenic acid to block the coagonist site of NMDA receptors. For the brightness encoding studies, we examined the action of D-serine on each intensity used and then applied the enzyme D-serine deaminase to remove significant levels of D-serine. These studies provided new insights into the mechanisms that regulate coagonist availability in the vertebrate retina. Our results strongly support the idea that light-evoked coagonist release, a major component of which is D-serine, is needed to provide the full range of coagonist availability for optimal activation of NMDA receptors.

A developmental time line in a retinal slice from rainbow trout

The retina in teleost fish continues to grow throughout much of the life of the animal, in part by the continuing differentiation of new tissue at the retinal margin, an area termed the peripheral growth zone (PGZ) (Lyall, Q J Micros Sci, 1957:98:101-110). We have developed a retinal slice preparation--including the PGZ--from juvenile rainbow trout (Onchorynchus mykiss), a species in which retinal growth is rapid and the PGZ is correspondingly pronounced. The PGZ slice preparation contains a time line of retinal development, with cells at different stages of maturation present side by side. We present evidence that the birth sequence of the various retinal cell types in the PGZ recapitulates the sequence during embryonic development. We also report data on the rate of growth of the PGZ in juvenile trout in vivo. Finally, we have used the PGZ slice preparation to make whole-cell voltage clamp recordings from individual retinal GCs at both early and late stages of maturation. We report that the amplitude of delayed rectifier and A-type potassium currents increases during GC maturation.

The influence of center-surround antagonism on light adaptation in cones in the retina of the turtle

The influence of center-surround antagonism on light adaptation in cone photoreceptors was investigated by intracellular recording from red-sensitive cones in the retina of the turtle, Pseudemys scripta elegans. Test flashes of 0.15-mm diameter were applied at the center of background fields of 0.25-mm or 2.2-mm diameter. Immediately upon expanding the background from 0.25 to 2.2 mm, the membrane potential depolarized by about 1-4 mV. The test flash response was enhanced if the depolarization was primarily due to synaptic feedback from horizontal cells, whereas the response was attenuated if the prolonged depolarization, an intrinsic response of the cone, was the dominant source of the depolarization. After several seconds, however, only the synaptic depolarization was maintained so maintained illumination of the large background field produced an enhancement of the cone's incremental sensitivity. The enhancement was examined in detail in steady-state conditions by obtaining amplitude-intensity measurements for centered test flashes on steady background fields over a large range of intensity. The effect of the large background field at any fixed intensity was fairly well described as a vertical (upward) shift of the amplitude-intensity curve obtained on the small field. This operation constitutes a quasi-subtractive mechanism of light adaptation and might provide a basis for the sort of subtractive mechanisms inferred from psychophysical studies of human vision. The enhancement was quantified by measuring the incremental sensitivity over four decades of background illumination. The magnitude of the enhancement increased with background intensity and then tended to stabilize at higher background intensities. The maximum difference in incremental sensitivity obtained on the large vs. small background field averaged 0.46 log unit (+/- 0.12 S.D.). At higher background intensities, incremental sensitivity conformed to Weber's Law behavior about equally well for flashes applied on either small or large background fields. In sum, the present results provide evidence for an additional mechanism of light adaptation in cone photoreceptors by showing that the incremental light sensitivity, initially set by mechanisms in the outer segment, can be modulated some three-fold by synaptic feedback at the inner segment of the cone.

Synaptic feedback, depolarization, and color opponency in cone photoreceptors

For some 20 years, synaptic feedback from horizontal cells to cones has often been invoked, more or less convincingly, in discussions of retinal action and vision. However, feedback in cones has proved to be rather complex and difficult to study experimentally. The mechanisms and consequences of feedback are therefore still only partly understood. This review attempts to assess the knowns and unknowns. The limitations of the evidence for feedback are reviewed to support the position that unequivocal evidence still largely rests on intracellular recording from cones. Of the three distinct types of depolarization observed in cones, the graded depolarization is taken as the fundamental manifestation of feedback. The evidence for the hypothesis that GABA is the neurotransmitter for feedback appears reasonably strong but several complications will have to be resolved to make the hypothesis more secure. There is evidence that feedback contributes to aspects of light adaptation and spatiotemporal processing of visual information. The contributions seem modest in magnitude. The role of feedback in shaping the color-opponent responses of retinal neurons is evaluated with particular emphasis on pharmacological studies, spatial and temporal aspects of the response of chromatic horizontal cells, and the enigmatic nature of depolarizations in blue- and green-sensitive cones. On this and other evidence, it is suggested that feedback may impress some detectable wavelength dependency in some cones but the dominant mechanisms for color opponency probably reside beyond the photoreceptors.

Light-evoked changes in near-infrared transmission by the ON and OFF channels of the anuran retina

We have developed an optical method to monitor the activity of the ON and OFF channels in the anuran retina. The change in the fraction of near infrared that is transmitted transversely through the retina in an eyecup slice is monitored during stimulation by visible, green light. Near-infrared transmission increases both at the onset and at the termination of a step stimulus. This "ON/OFF" response is maximal in the neural retina. Sodium L-aspartate, which blocks the light-evoked activity of post-photoreceptor neurons, abolishes the "ON/OFF" response. L-AP4, used as a selective blocker of the ON channel, reduces the "ON" component and has little or no effect on the "OFF" component. The "ON" and "OFF" processes observed optically are distinct from those that generate the b- and d-waves of the electroretinogram, and the "ON" and "OFF" components may be superior to the b- and d-waves as indicators of ON and OFF channel activity. The optical method is almost as simple as electroretinography and has the advantages that responses can be spatially localized with ease.

Prolonged depolarization in rods in situ

Intracellular recordings were made from rods in the superfused retina of the marine toad (Bufo marinus). It was found that injection of a brief depolarizing current pulse (0.04-1 nA) evoked a distinctive, long-lasting response, here called "the prolonged depolarization." The response appears to be regenerative, has a stereotypical waveform, is typically about 6 mV in amplitude and 3 s in duration, and has a relatively long recovery period (10-60 s). As a rule, the response cannot be directly evoked by light but the current-evoked response is significantly enhanced in the presence of steady illumination. The light-evoked hyperpolarization and the depolarizing spikes of the rod are both attenuated in the presence of the prolonged depolarization. The prolonged depolarization is not an altered manifestation of the depolarizing spikes of toad rods since both can be recorded simultaneously and steady illumination suppresses the spikes while enhancing the prolonged depolarization. The response is enhanced in chloride-free superfusate and also appears to be enhanced by the use of electrodes containing chloride. The response is markedly shortened in superfusates that lack calcium or contain 1-5 mM cobalt. On this and other evidence, it is suggested that the response may be generated by the sequential action of calcium channels and calcium-activated chloride channels. Although rarely evoked by light, the prolonged depolarization of toad rods is otherwise remarkably similar to the prolonged depolarization of turtle cones. It is proposed that the prolonged depolarization, in contrast to the feedback depolarization of cones, arises from mechanisms common to both rods and cones.

Thin slices of teleost retina continue to grow in culture

Thin slices of differentiated fish retinas were maintained up to 5 days in culture conditions where they exhibited properties essentially identical to those found in retinas of intact animals. Retinal slices were prepared by embedding eyecups from young fish in agarose and sectioning them on a vibratome. Phenotypic integrity of specific cell types was maintained, as demonstrated by specific antibody staining patterns. Stem cells in the retinal margin and presumptive rod progenitor cells in the outer nuclear layer continued to proliferate in vitro, just as they do in vivo. Some of these cells differentiated in vitro as demonstrated by labelling both cell division and cell phenotype. After several days in culture, some regeneration-like responses were observed, such as growth of neurites and swelling of cell bodies in the ganglion cell layer. This retinal slice preparation appears to offer a unique opportunity for studying the interactions among developing retinal cells.

Effects of synaptic blocking agents on the depolarizing responses of turtle cones evoked by surround illumination

The effects of synaptic blocking agents on the antagonistic surround of the receptive field of cone photoreceptors were studied by intracellular recording in the retina of the turtle (Pseudemys scripta elegans). Illumination of a cone's receptive-field surround typically evoked a hybrid depolarizing response composed of two components: (1) the graded synaptic feedback depolarization and (2) the prolonged depolarization, a distinctive, intrinsic response of the cone. The locus of action of synaptic blocking agents was analyzed by comparing their effects on the light-evoked response of horizontal cells, the hybrid cone depolarization evoked by surround illumination, and the pure prolonged depolarization evoked by intracellular current injection. The excitatory amino-acid antagonists, d-O-phosphoserine (DOS) and kynurenic acid (KynA), suppressed the light responses of horizontal cells and eliminated the surround-evoked, hybrid cone depolarization without affecting the prolonged depolarization evoked by current injection. Cobalt at 5-10 mM suppressed horizontal cell responses and thereby eliminated surround-evoked cone depolarizations. Cobalt (5-10 mM) also blocked the current-evoked prolonged depolarization, suggesting that the intrinsic cone mechanisms responsible for the prolonged depolarization are likely to be calcium-dependent. Various GABA agonists and antagonists were found to have no effect on the surround-evoked depolarizations of cones. In contrast, a very low concentration of cobalt (0.5 mM) selectively suppressed the light-evoked feedback depolarization of cones without affecting horizontal cell responses or the current-evoked prolonged depolarization. Cobalt at 0.5 mM thus blocks the light-evoked action of the cone feedback synapse while sparing feedforward synaptic transmission from cones to horizontal cells. The implications of the present work for the possible neurotransmitters used at these synapses is discussed.