Sampling and interpolation of the a-wave of the electroretinogram

Origin of Retinal Oscillatory Potentials in the Mouse, a Tool to Specifically Locate Retinal Damage

To determine the origin of oscillatory potentials (OPs), binocular electroretinogram (ERG) recordings were performed under light and dark adaptation on adult healthy C57BL/6J mice. In the experimental group, 1 μL of PBS was injected into the left eye, while the right eye was injected with 1 μL of PBS containing different agents: APB, GABA, Bicuculline, TPMPA, Glutamate, DNQX, Glycine, Strychnine, or HEPES. The OP response depends on the type of photoreceptors involved, showing their maximum response amplitude in the ERG induced by mixed rod/cone stimulation. The oscillatory components of the OPs were affected by the injected agents, with some drugs inducing the complete abolition of oscillations (APB, GABA, Glutamate, or DNQX), whereas other drugs merely reduced the oscillatory amplitudes (Bicuculline, Glycine, Strychnine, or HEPES) or did not even affect the oscillations (TPMPA). Assuming that rod bipolar cells (RBC) express metabotropic Glutamate receptors, GABA_A, GABA_C, and Glycine receptors and that they release glutamate mainly on Glycinergic AII amacrine cells and GABAergic A17 amacrine cells, which are differently affected by the mentioned drugs, we propose that RBC-AII/A17 reciprocal synapses are responsible for the OP generation in the ERG recordings in the mice. We conclude that the reciprocal synapses between RBC and AII/A17 are the basis of the ERG OP oscillations of the light response, and this fact must be taken into consideration in any ERG test that shows a decrease in the OPs' amplitude.

Retinal Function in X-Linked Juvenile Retinoschisis

Purpose

To assess retinal function in young patients with X-linked juvenile retinoschisis (XLRS), a disorder that is known to alter ERG postreceptor retinal components and also possibly photoreceptor components.

Methods

ERG responses to full-field stimuli were recorded under scotopic and photopic conditions in 12 XLRS patients aged 1 to 15 (median 8) years. A- and b-wave amplitudes and implicit times were examined over a range of stimulus intensities. Rod and cone photoreceptor (SROD, RROD, SCONE, RCONE) and rod-driven postreceptor (log σ, VMAX) response parameters were calculated from the a- and b-waves. Data from XLRS patients were evaluated for significant change with age.

Results

A- and b-wave amplitudes were smaller in XLRS patients compared with controls under both scotopic and photopic conditions. Saturated photoresponse amplitude (RROD), postreceptor b-wave (log σ), and saturated b-wave amplitude (VMAX) were significantly lower in XLRS patients than in controls; SROD did not differ between the two groups. SCONE and RCONE values were normal. In XLRS patients, neither a- and b-wave amplitudes nor calculated parameters (SROD, RROD, log σ, VMAX,SCONE, and RCONE) changed with age.

Conclusions

In these young XLRS patients, RROD and a-wave amplitudes were significantly smaller than in controls. Thus, in addition to XLRS causing postreceptor dysfunction, an effect of XLRS on rod photoreceptors cannot be ignored.

Homeostatic Plasticity Mediated by Rod-Cone Gap Junction Coupling in Retinal Degenerative Dystrophic RCS Rats

Rod-cone gap junctions open at night to allow rod signals to pass to cones and activate the cone-bipolar pathway. This enhances the ability to detect large, dim objects at night. This electrical synaptic switch is governed by the circadian clock and represents a novel form of homeostatic plasticity that regulates retinal excitability according to network activity. We used tracer labeling and ERG recording in the retinae of control and retinal degenerative dystrophic RCS rats. We found that in the control animals, rod-cone gap junction coupling was regulated by the circadian clock via the modulation of the phosphorylation of the melatonin synthetic enzyme arylalkylamine N-acetyltransferase (AANAT). However, in dystrophic RCS rats, AANAT was constitutively phosphorylated, causing rod-cone gap junctions to remain open. A further b/a-wave ratio analysis revealed that dystrophic RCS rats had stronger synaptic strength between photoreceptors and bipolar cells, possibly because rod-cone gap junctions remained open. This was despite the fact that a decrease was observed in the amplitude of both a- and b-waves as a result of the progressive loss of rods during early degenerative stages. These results suggest that electric synaptic strength is increased during the day to allow cone signals to pass to the remaining rods and to be propagated to rod bipolar cells, thereby partially compensating for the weak visual input caused by the loss of rods.

A comparison among different techniques for human ERG signals processing and classification

Feature detection in biomedical signals is crucial for deepening our knowledge about the involved physiological processes. To achieve this aim, many analytic approaches can be applied but only few are able to deal with signals whose time dependent features provide useful clinical information. Among the biomedical signals, the electroretinogram (ERG), that records the retinal response to a light flash, can improve our comprehension of the complex photoreceptoral activities. The present study is focused on the analysis of the early response of the photoreceptoral human system, known as a-wave ERG-component. This wave reflects the functional integrity of the photoreceptors, rods and cones, whose activation dynamics are not yet completely understood. Moreover, since in incipient photoreceptoral pathologies eventual anomalies in a-wave are not always detectable with a "naked eye" analysis of the traces, the possibility to discriminate pathologic from healthy traces, by means of appropriate analytical techniques, could help in clinical diagnosis. In the present paper, we discuss and compare the efficiency of various techniques of signal processing, such as Fourier analysis (FA), Principal Component Analysis (PCA), Wavelet Analysis (WA) in recognising pathological traces from the healthy ones. The investigated retinal pathologies are Achromatopsia, a cone disease and Congenital Stationary Night Blindness, affecting the photoreceptoral signal transmission. Our findings prove that both PCA and FA of conventional ERGs, don't add clinical information useful for the diagnosis of ocular pathologies, whereas the use of a more sophisticated analysis, based on the wavelet transform, provides a powerful tool for routine clinical examinations of patients.

Test of the paired-flash electroretinographic method in mice lacking b-waves

Previous studies of rod photoreceptors in vivo have employed a paired-flash electroretinographic (ERG) technique to determine rod response properties. To test whether absence versus presence of the ERG b-wave affects the photoreceptor response derived by the paired-flash method, we examined paired-flash-derived responses obtained from nob mice, a mutant strain with a defect in signal transduction between photoreceptors and ON bipolar cells that causes a lack of the b-wave. Normal littermates of the nob mice served as controls. The normalized amplitude-intensity relation of the derived response determined in nob mice at the near-peak time of 86 ms was similar to that determined for the controls. The full time course of the derived rod response was obtained for test flash strengths ranging from 0.11 to 17.38 scotopic cd s m(-2) (sc cd s m(-2)). Time-course data obtained from nob and control mice exhibited significant but generally modest differences. With saturating test flash strengths, half-recovery times for the derived response of nob versus control mice differed by approximately 60 ms or less about the combined (nob and control) average respective values. Time course data also were obtained before versus after intravitreal injection of L-2-amino-4-phosphonobutyrate (APB) (which blocks transmission from photoreceptors to depolarizing bipolar cells) and of cis 2,3-piperidine dicarboxylic acid (PDA) (which blocks transmission to OFF bipolar cells, and to horizontal, amacrine and ganglion cells). Neither APB nor PDA substantially affected derived responses obtained from nob or control mice. The results provide quantitative information on the effect of b-wave removal on the paired-flash-derived response in mouse. They argue against a substantial skewing effect of the b-wave on the paired-flash-derived response obtained in normal mice and are consistent with the notion that, to good approximation, this derived response represents the isolated flash response of the photoreceptors in both nob and normal mice.

Analysis of the human a-wave ERG component

The a-wave is one of the main issues of research in the field of ocular electrophysiology, since it is strictly connected with early photoreceptoral activities. The present study proposes mathematical methods that analyse this component in human subjects, and supports experimental evidence relating to possible correlations among the responses of photoreceptoral units under a light stimulus. The investigation is organized in two parts: the first part concerns the onset and the initial slope, up to the first minimum (about 10-15 ms), the second part deals with the main portion of the wave, up to about 30 ms. In both cases, the a-waves, recorded at various levels of luminance, have been fitted with a set of appropriate functions representing possible models of physiological behaviour which would take place in the early stages of phototransduction. The statistical nature of the underlying processes is also discussed. The results indicate that correlations occur in the early stages, whereas random processes are set up later.

In vivo studies of signaling in rod pathways of the mouse using the electroretinogram

PURPOSE

(a) To examine the possibility that there is a threshold in the synaptic mechanism linking rods to rod bipolar cells that can reduce the transmission of continuous noise from the rods without blocking the transmission of any significant proportion of single-photon responses. (b) To estimate the level of this threshold and the amplitude of the continuous noise which it can serve to reduce. (c) To identify the location of the threshold mechanism in the rod to rod bipolar cell pathway.

METHODS

Corneal electroretinogram recordings were made from dark-adapted mice anesthetized with ketamine/xylazine after inner-retinal components had been suppressed to isolate PII, the response of depolarizing bipolar cells. Suppression was achieved by intravitreal injections of GABA, TTX, or in Cx36 KO animals by crushing the optic nerve and waiting for ganglion cells to degenerate.

RESULTS

All energy-scaled records of isolated PII obtained with ganzfeld stimuli that gave rise to much less than one photoisomerization (R*) per rod (0.01-0.2 R*/rod), had an essentially identical waveform. Stronger stimuli caused a reduction in the peak amplitude of energy-scaled records (saturation) and stimuli strong enough to produce multiple isomerizations in individual rods resulted in a shortening of the response latency and an increase of the energy-scaled amplitude at early times (supralinearity). The shape of the rising edge of isolated PII changed with flash energy in a way that was consistent with the existence of a synaptic threshold whose level was less than one tenth of the amplitude of single-photon signals and a continuous noise whose rms amplitude was even less than this. However, when measured at the time of the peak, the amplitude of PII increased linearly in proportion to stimulus energy from the very lowest levels up to the point where there was, on average, 0.2 R*/rod.

CONCLUSIONS

There is a threshold nonlinearity operating at the output of the rod to rod bipolar cell synapse that can usefully reduce the transmission of continuous rod noise without significantly affecting the transmission of single-photon signals. This nonlinearity does not affect the overall linear function of the rod pathway at levels at which it is effectively operating in a photon-counting mode.