THE RAT ELECTRORETINOGRAM. I. CONTRASTING EFFECTS OF ADAPTATION ON THE AMPLITUDE AND LATENCY OF THE B-WAVE

Quantitative Analysis of Retinal Structure and Function in Two Chromosomally Altered Mouse Models of Down Syndrome

Purpose

Ophthalmic disorders are among the most prevalent Down syndrome (DS) comorbidities. Therefore, when studying mouse models of DS, ignoring how vision is affected can lead to misinterpretation of results from assessments dependent on the integrity of the visual system. Here, we used imaging and electroretinography (ERG) to study eye structure and function in two important mouse models of DS: Ts65Dn and Dp(16)1Yey/+.

Methods

Cornea and anterior segment were examined with a slit-lamp. Thickness of retinal layers was quantified by optical coherence tomography (OCT). Eye and lens dimensions were measured by magnetic resonance imaging (MRI). Retinal vasculature parameters were assessed by bright field and fluorescent imaging, and by retinal flat-mount preparations. Ganzfeld ERG responses to flash stimuli were used to assess retinal function in adult mice.

Results

Total retinal thickness is significantly increased in Ts65Dn and Dp(16)1Yey/+ compared with control mice, because of increased thickness of inner retinal layers, including the inner nuclear layer (INL). Increased retinal vessel caliber was found in both chromosomally altered mice when compared with controls. ERG responses in Ts65Dn and Dp(16)1Yey/+ mice showed subtle alterations compared with controls. These, however, seemed to be unrelated to the thickness of the INL, but instead dependent on the anesthetic agent used (ketamine, tribromoethanol, or urethane).

Conclusions

We provide evidence of retinal alterations in Ts65Dn and Dp(16)1Yey/+ mice that are similar to those reported in persons with DS. Our ERG results are also a reminder that consideration should be given to the choice of anesthetic agents in such experiments.

The Shift of ERG B-Wave Induced by Hours' Dark Exposure in Rodents

Purpose

Dark adaptation can induce a rapid functional shift in the retina, and after that, the retinal function is believed to remain stable during the continuous dark exposure. However, we found that electroretinograms (ERG) b-waves gradually shifted during 24 hours’ dark exposure in rodents. Detailed experiments were designed to explore this non-classical dark adaptation.

Methods

In vivo ERG recording in adult and developing rodents after light manipulations.

Results

We revealed a five-fold decrease in ERG b-waves in adult rats that were dark exposed for 24 hours. The ERG b-waves significantly increased within the first hour’s dark exposure, but after that decreased continuously and finally attained steady state after 1 day’s dark exposure. After 3 repetitive, 10 minutes’ light exposure, the dark exposed rats fully recovered. This recovery effect was eye-specific, and light exposure to one eye could not restore the ERGs in the non-exposed eye. The prolonged dark exposure-induced functional shift was also reflected in the down-regulation on the amplitude of intensity-ERG response curve, but the dynamic range of the responsive light intensity remained largely stable. Furthermore, the ERG b-wave shifts occurred in and beyond classical critical period, and in both rats and mice. Importantly, when ERG b-wave greatly shifted, the amplitude of ERG a-wave did not change significantly after the prolonged dark exposure.

Conclusions

This rapid age-independent ERG change demonstrates a generally existing functional shift in the retina, which is at the entry level of visual system.

Functional segregation of retinal ganglion cell projections to the optic tectum of rainbow trout

The interpretation of visual information relies on precise maps of retinal representation in the brain coupled with local circuitry that encodes specific features of the visual scenery. In nonmammalian vertebrates, the main target of ganglion cell projections is the optic tectum. Although the topography of retinotectal projections has been documented for several species, the spatiotemporal patterns of activity and how these depend on background adaptation have not been explored. In this study, we used a combination of electrical and optical recordings to reveal a retinotectal map of ganglion cell projections to the optic tectum of rainbow trout and characterized the spatial and chromatic distribution of ganglion cell fibers coding for increments (ON) and decrements (OFF) of light. Recordings of optic nerve activity under various adapting light backgrounds, which isolated the input of different cone mechanisms, yielded dynamic patterns of ON and OFF input characterized by segregation of these two fiber types. Chromatic adaptation decreased the sensitivity and response latency of affected cone mechanisms, revealing their variable contributions to the ON and OFF responses. Our experiments further demonstrated restricted input from a UV cone mechanism to the anterolateral optic tectum, in accordance with the limited presence of UV cones in the dorsotemporal retina of juvenile rainbow trout. Together, our findings show that retinal inputs to the optic tectum of this species are not homogeneous, exhibit highly dynamic activity patterns, and are likely determined by a combination of biased projections and specific retinal cell distributions and their activity states.

The electroretinogram and visual evoked potential of freely moving rats

The vascularised rat retina could be one of the most useful experimental objects in visual neuroscience to understand human visual physiological and pathological processes. We report here on a new method of implantation for studying the visual system of freely moving rats that provides a rat model for simultaneous recording at corneal and cortical level and is stable enough to record for months. We implanted light emitting diodes onto the skull behind the eyeball to stimulate the eye with flashes and to light adapt the retina with constant light levels. A multistrand, stainless steel, flexible fine wire electrode placed on the eyeball was used for electroretinogram recording and screw electrodes (left/right visual and parietal cortical) were used to record the visual evoked potential and the electroencephalogram. In the present report we focus on the new method of implantation for recording the corneal flash electroretinogram of normal, freely moving rats simultaneously with the visual evoked cortical potential showing examples in various visual experiments. We also introduce a program for retinogram and visual evoked potential analysis, which defines various measures (latencies, areas, amplitudes, and durations) and draw attention to the benefits of this method for those involved in visual, functional genomic, pharmacological, and human ophthalmologic research.

RAT ELECTRORETINOGRAM: EVIDENCE FOR SEPARATE PROCESSES GOVERNING B-WAVE LATENCY AND AMPLITUDE

Studies of the simultaneous changes in the latency and amplitude of the b-wave of the rat electroretinogram (ERG) under three different conditions of adaptation show that the latency is primarily a function of the absolute stimulus intensity, being only slightly affected by conditions that strongly reduce the amplitude. This implies that the latency and the amplitude are determined by two independent processes, with the latency-process more closely linked to the initial photochemical events. Furthermore, the different adaptation conditions have surprisingly similar effects on the shape and amplitude of the electroretinogram, which suggests that they all produce only one type of variation in the amplitudedetermining process.

Dose-response relationship between light irradiance and the suppression of plasma melatonin in human volunteers

This study tested the capacity of different irradiances of monochromatic light to reduce plasma melatonin in normal humans. Six healthy male volunteers, 24-34 years old, were exposed to 0.01, 0.3, 1.6, 5, or 13 microW/cm2 of 509 nm monochromatic light for 1 h during the night on separate occasions. Light irradiance depressed plasma melatonin in a dose-response pattern. The data indicate that the mean threshold irradiance for suppressing melatonin is between 1.6 and 5 microW/cm2. Individual variations in threshold responses to monochromatic light were observed among the volunteers.

Temporal properties of the short-wavelength cone mechanism: comparison of receptor and postreceptor signals in the ground squirrel

Temporal properties of the short-wavelength cone mechanism of the California ground squirrel (Spermophilus beecheyi), a dichromat, were explored with single light pulses and pulse trains. Both the electroretinogram (ERG) and the isolated PIII component of the retinal gross response were recorded under chromatic adaptation conditions that favored the detection of test lights by either the middle-wavelength mechanism or the short-wavelength mechanism. The ERG b-wave generated by the short-wavelength mechanism was significantly slower than that of the middle-wavelength mechanism, but there was no such difference in the PIII components of the two mechanisms. Analysis of PIII revealed that both cone types respond equally well to flicker at rates at least as high as 46 Hz. Differences in the temporal response properties of these two cone mechanisms must arise largely from post-receptoral interactions.

The variable interdependence of amplitude and implicit-time in PIII, b-wave and optic-nerve responses of the cat

Amplitude and implicit-times of responses representing different retinal layers (PIII, b-wave and optic-nerve response) were measured in electrophysiological recordings from isolated, arterially perfused cat eyes. The amplitude of these potentials was found to saturate at lower stimulus irradiances than the implicit-time in dark-adapted eyes. Light adaptation had a strong effect on the amplitudes, whereas the implicit-times were altered only slightly. Similar results were obtained in double-flash experiments. The injection of phosphodiesterase inhibiting drugs had different effects on amplitude, latency and implicit-time.

Temporal modulation sensitivity in cone dark adaptation

The temporal modulation sensitivity of cone vision was studied using sinusoidally modulated lights. We found that the effects of bright flash bleaches mirror the effects of light adaptation and raise low frequency threshold most. Prolonged pre-exposures, however, raise high frequency threshold most. Two processes are therefore required to describe the mechanisms of recovery of sensitivity following bleaching exposures, one which acts like a background light and selectively attenuates low frequencies, and another process with a long integration time which selectively attenuates high frequencies.

Relationship between the amplitudes of the b wave and the a wave as a useful index for evaluating the electroretinogram

The present methods for the assessment of the electroretinogram (ERG) make the exchange of data between laboratories difficult owing to differences in photostimulation and recording techniques. In this study it is suggested that in addition to present techniques the b wave to a wave relationship may be a useful index for evaluating the ERG. The ERG responses evoked by relatively bright flashes are recorded in the dark-adapted state. A plot of the b wave amplitude as a function of the a wave amplitude describes the functional integrity of the retina. A normally functioning retina should elicit an ERG response that fits the normal curve regardless of the apparatus used for ERG recording. The only constraint is the use of full-field illumination. This method is based on physiological considerations regarding the origin of the ERG components. Such a method will facilitate data exchange between centres and can also be used for clinical assessment of the retina.

Desensitization of skate photoreceptors by bleaching and background light

Through extracellular measurements of photoreceptor responses to flashed stimuli, we examined how the bleaching of rhodopsin affects increment receptor threshold in the isolated retina of the skate (Raja oscellata and R. erinacea). Both initially unbleached and previously bleached photoreceptors, when exposed to full-field luminous backgrounds of fixed intensity, attain approximately stable levels of increment threshold that vary with the intensity of the background light. Values of stabilized increment thresholds measured after various extents of bleaching (less than approximately 50%), when plotted against background intensity in log-log coordinates, tend to converge with increasing intensity of the background; this relationship of the increment threshold functions resembles that which Blakemore and Rushton (1965b) found to describe the transient effect of bleaching on psychophysical increment threshold for the human rod mechanism. Our data are consistent with the possibility that related photochemical processes govern the stabilized levels of receptor sensitivity exhibited by the isolated retina (a) during steady illumination and (b) long after substantial bleaching.

Two retinal processes displayed in the cat electroretinogram

Local stimulation of dark adapted cat's retina with spots of light of various intensities elicits a complex b-wave in which four deflections can be consistently identified. An analysis of these wavelets recorded with the low frequency cut off at 80 Hz is made in an attempt to infer their origin. The experimental conditions under which these wavelets were studied were standardized by stimulation with a spot of light at different intensities, frequencies and positions in the visual field. The changes in amplitude and latency by those different conditions of stimulation and by the intravenous (i.v.) injection of additional doses of nembutal, suggests that these peaks are triggered by two retinal excitatory processes related to the scotopic system. It is also pointed out that components 1-2 are mainly determined by the activation of retinal cells which correspond to the area centralis and that components 3-4 represent the activity of retinal cells from the lateral regions (nasal and temporal). The possible clinical use of our results is discussed.

The damaging effects of light on the retina. Empirical findings, theoretical and practical implications

Light well below the intensity which causes thermal burns physiologically damages the retina. This damage is primarily localized in the receptors. The outer segments are most sensitive and slow recovery is possible if damage does not proceed to destruction of the inner segment. Many variables affect the extent and severity of light damage. Damage is correlated with continuity of source, light intensity, elevated body temperature, nocturnality, and albinism. Light damage has been considered only minimally in visual research with light preferences, reinforcement and discrimination, or in clinical settings. Based on the available evidence, it is suggested that retinal damage may be produced by such common light sources as room lighting, phototherapy techniques, ophthalmoscopes and fundus cameras. Further studies are recommended.

A Ganzfeld contact lens electrode

A small piece of Ping-Pong ball material fitted to a Burian-Allen contact lens electrode provided a large field of evenly distributed light on the retina that resulted in larger electroretinographic amplitudes and more accurate measurement of b-wave latencies.

Intracellular recordings of rod responses during dark-adaptation

1. Dark-adaptation of rod photoreceptors has been studied in the isolated axolotl (Ambystoma mexicanum) retina by intracellular recordings. Rod responsiveness was greatly reduced immediately after a 30 sec partial bleach, but partially recovered with time in the dark. 2. In parallel spectrophotometric measurements using isolated retinas, regeneration of the rod pigment could not be detected after a 30 sec bleach. 3. During rod dark-adaptation, the response of a rod to a given stimulus increased in amplitude, duration, and rate of rise but did not recover completely to the dark-adapted values. Response latency was lengthened immediately after a bleach but ultimately returned to the dark-adapted level. 4. The time courses of dark-adaptation determined on the basis of the intensity of a stimulus needed to evoke a response having a criterion amplitude, a criterion duration, or a criterion rate of rise were similar. On the other hand changes in latency of the response and magnitude of the saturated amplitude followed different time courses. Change in log threshold was found to be related to change in saturated amplitude by an exponential function during dark-adaptation. 5. After bleaching 10% or less of the rod pigment, the kinetics of both recovery of log threshold and decrease in absorbance at 400 nm (metarhodopsin II+free retinal) could be described by two concurrent first-order processes having similar time constants. However, after bleaching more than 10% of the rod pigment, changes in sensitivity and absorbance did not follow parallel time courses. 6. Metarhodopsin III cannot be solely responsible for setting the axolotl rod sensitivity since rod thresholds decrease monotonically during dark-adaptation whereas meta III concentration reaches a peak 3 min after the bleach and decreases thereafter.

The site of visual adaptation

In response to background illumination, the adaptation properties of the b-wave are similar to those observed in the human eye with psychophysical methods. With increasing background luminance the b-wave sensitivity is diminished; except at the lowest background intensity the elevation of the log threshold is linearly related to the increase of background intensity, the relation having a slope of almost 1. The a-wave, however, behaves quite differently. At low background luminances it shows little adaptation. With higher background luminances the awave saturates, and no a-wave potential can be elicited with any stimulus intensity. The L-type S-potentials respond to background light in much the same way as the a-wave does. Thus, the b-wave is the first of the known responses in the visual system to show typical adaptation properties. This suggests that the site of visual adaptation may be in the bi-polarcell layer, the presumed locus of b-wave generation. Recent electron microscopic studies have demonstrated reciprocal synapses between the bipolar terminals and amacrine processes, and it is suggested that such a synaptic arrangement could account for visual adaptation by a mechanism of inhibitory feedback on the bipolar cells.