Frequency spectrum and amplitude analysis of dark- and light-adapted oscillatory potentials in albino mouse, rat and rabbit

Hilbert transform analysis of the mouse scotopic electroretinogram reveals two distinct bursts of oscillatory potentials with progressively dimmer flashes

PURPOSE

Study the scotopic oscillatory potentials (OPs) in mice over a wide range of flash luminance levels using the Hilbert transform (HT) to extract new features of the high frequency components of the electroretinogram (ERG).

METHODS

Scotopic ERGs [Intensity: - 6.3 to 0.9 log cd∙s∙m-2; 12 h of dark-adaptation] were obtained from adult mice (C57BL/6; n = 7). The Hilbert transform (HT) was obtained within 3 consecutive frequency bands (65-90 Hz, 90-115 Hz and 115-140 Hz), with OPs being denoised, automatically identified and analyzed. Measurements included: number of OPs, duration of the OP response, surface-under-the-curve (SUC) of the HT envelopes, implicit times, and instantaneous frequency at the HT envelope peak, mean peak time differences (PTD) between the envelopes of each frequency band (measuring their synchrony), correlation coefficient and lag between consecutive HT envelopes, as well as the number of peaks on the HT envelopes.

RESULTS

The OP response duration, number of OPs and PTD all peaked for flashes between the level corresponding to the RodVmax (maximal b-wave amplitude of the rod ERG; i.e., the first asymptote of the scotopic luminance-response curve) and K (the flash luminance at which the amplitude of the b-wave is half of that of the RodVmax;), i.e., between -3.9 and -2.4 log cd∙s∙m-2. The correlation between consecutive envelopes is close to 1 at flashes > -1.2 log cd∙s∙m-2, with small lags (min. = 1.93 ± 0.45 ms at - 1.2 log cd∙s∙m-2), then gradually drops to 0.81 ± 0.02 at the dimmest flash intensity (with a max. lag = 14.76 ± 8.92 ms at - 5.1 log cd∙s∙m-2). Finally, we found that the single OP burst (i.e., a single HT envelope peak) seen at flash intensities >  - 1.2 log cd∙s∙m-2 progressively divided in two (or more) OP bursts (i.e., multiple HT envelope peaks) with gradually dimmer flashes.

CONCLUSIONS

Our HT method enabled the analysis of the OP response without the subjective interpretation of the experimenter. Analysis of the scotopic OPs at dim flashes with the HT revealed a novel feature of the OP response not yet reported elsewhere, namely: a split of the OP response into two (or more) distinct bursts. Furthermore, the synchrony peak (measured with the PTD) matched the peak in OP response duration between K and RodVmax, suggesting a disorganization (or dephasing) of the retinal signal in ERGs evoked for weaker flashes. The increased synchronization and correlation of the single burst observed for the strongest flashes could suggest an optimization or saturation of the retinal response. We believe that these novel features of the OP components of the ERG went unnoticed given that previous studies did not use weak enough flashes and failed to recognize the added value that time and frequency domain analysis of the ERG (such as what is achieved with the HT) brings to the interpretation (and our understanding) of the retinal response.

Anesthetic effects on electrophysiological responses across the visual pathway

Anesthetics are widely used in electrophysiological tests to assess retinal and visual system functions to avoid experimental errors caused by movement and stress in experimental animals. To determine the most suitable anesthetic for visual electrophysiological tests, excluding ketamine and chloral hydrate due to regulatory and side effect concerns, this study investigated the effects of ethyl carbamate (EC), avertin (AR), and pentobarbital sodium (PS) on visual signal conduction in the retina and primary visual cortex. Assessments included flash electroretinogram (FERG), pattern electroretinogram (PERG), pattern visual evoked potentials (PVEP), and flash visual evoked potentials (FVEP), FERG and FVEP were used to evaluate the responses of the retina and visual cortex to flash stimuli, respectively, while PERG and PVEP assessed responses to pattern stimuli. The research showed that AR demonstrates the least disruption to the visual signal pathway, as evidenced by consistently high characteristic peaks in the AR group across various tests. In contrast, mice given EC exhibited the lowest peak values in both FERG and FVEP, while subjects anesthetized with PS showed suppressed oscillatory potentials and PERG responses. Notably, substantial PVEP characteristic peaks were observed only in mice anesthetized with AR. Consequently, among the three anesthetics tested, AR is the most suitable for visual electrophysiological studies.

Electroretinogram responses in myopia: a review

The stretching of a myopic eye is associated with several structural and functional changes in the retina and posterior segment of the eye. Recent research highlights the role of retinal signaling in ocular growth. Evidence from studies conducted on animal models and humans suggests that visual mechanisms regulating refractive development are primarily localized at the retina and that the visual signals from the retinal periphery are also critical for visually guided eye growth. Therefore, it is important to study the structural and functional changes in the retina in relation to refractive errors. This review will specifically focus on electroretinogram (ERG) changes in myopia and their implications in understanding the nature of retinal functioning in myopic eyes. Based on the available literature, we will discuss the fundamentals of retinal neurophysiology in the regulation of vision-dependent ocular growth, findings from various studies that investigated global and localized retinal functions in myopia using various types of ERGs.

Oscillatory Potentials in Achromatopsia as a Tool for Understanding Cone Retinal Functions

Achromatopsia (ACHM) is an inherited autosomal recessive disease lacking cone photoreceptors functions. In this study, we characterize the time-frequency representation of the full-field electroretinogram (ffERG) component oscillatory potentials (OPs), to investigate the connections between photoreceptors and the inner retinal network using ACHM as a model. Time-frequency characterization of OPs was extracted from 52 controls and 41 achromat individuals. The stimulation via ffERG was delivered under dark-adaptation (DA, 3.0 and 10.0 cd·s·m-2) to assess mixed rod-cone responses. The ffERG signal was subsequently analyzed using a continuous complex Morlet transform. Time-frequency maps of both DA conditions show the characterization of OPs, disclosing in both groups two distinct time-frequency windows (~70-100 Hz and >100 Hz) within 50 ms. Our main result indicates a significant cluster (p < 0.05) in both conditions of reduced relative power (dB) in ACHM people compared to controls, mainly at the time-frequency window >100 Hz. These results suggest that the strongly reduced but not absent activity of OPs above 100 Hz is mostly driven by cones and only in small part by rods. Thus, the lack of cone modulation of OPs gives important insights into interactions between photoreceptors and the inner retinal network and can be used as a biomarker for monitoring cone connection to the inner retina.

Increase in electroretinogram rod-driven peak frequency of oscillatory potentials and dark-adapted responses in a cohort of myopia patients

PURPOSE

To study whether rod- and cone-driven electroretinogram (ERG) responses are altered in myopia patients.

METHODS

Dark- and light-adapted ERGs were recorded from 57 myopic eyes of 32 patients aged 22-30 and 19 emmetropic eyes of 10 age-matched normal subjects. The myopic eyes were divided into 3 groups according to spherical equivalent (SE) of manifest refraction: 18 low myopia eyes (≤ - 3.00 diopter (D), 23 moderate myopia eyes (- 3.25 to - 6.00 D), and 16 high myopia eyes (> - 6.25 D). The amplitudes of the dark- and light-adapted ERG a- and b-waves, as well as the frequency spectra of the cone-driven and rod-driven oscillatory potentials (OPs), were analyzed by fast Fourier transform. The peak frequency, implicit time, and total power of the OPs were determined. The axial length was measured with an IOL Master. The ERG parameters including those of the cone- and rod-driven OPs were compared among three groups.

RESULTS

The amplitudes of the a-wave and b-wave of the dark-adapted ERGs were increased with refractive power (P < 0.05). Interestingly, the average peak frequency of the rod-driven OPs showed a significant positive correlation with refractive power (P < 0.001): 123.41 ± 9.13 Hz in emmetropic controls, 129.12 ± 10.28 Hz in low myopia, 133.90 ± 9.13 Hz in moderate myopia, and 139.51 ± 5.78 Hz in high myopia. However, the parameters of the light-adapted ERGs and the cone-driven OPs in myopic eyes were within normal ranges.

CONCLUSION

We found significant positive correlation between the peak frequency of rod-driven OPs, as well as the amplitudes of rod-driven ERG a- and b-waves, and the refractive power. The results suggest that the rod system function was changing during the progress of myopia, while the cone system function appeared unaffected. The peak frequency of OPs appeared as a novel ERG parameter for myopia, a common ocular condition.

Contribution of GABAa, GABAc and glycine receptors to rat dark-adapted oscillatory potentials in the time and frequency domain

Retinal oscillatory potentials (OPs) consist of a series of relatively high-frequency rhythmic wavelets, superimposed onto the ascending phase of the b-wave of the electroretinogram (ERG). However, the origin of OPs is uncertain and methods of measurement of OPs are diverse. In this study, we first isolated OPs from the rat ERG and fitted them with Gabor functions and found that the envelope of the OP contained information about maximum amplitude and time-to-peak to enable satisfactory quantification of the later OPs. And the OP/b-wave ratio should be evaluated to exclude an effect of the b-wave on the OPs. Next, we recorded OPs after intravitreal injection of 2-amino-4-phosphonobutyric acid (APB), tetrodotoxin (TTX), γ-aminobutyric acid (GABA), strychnine (STR), SR95531 (SR), isoguvacine (ISO), (1,2,5,6-tetrahydropyridin-4-yl) methylphosphinic acid (TPMPA) and GABA+TPMPA. We showed that GABA and APB only removed the later OPs, when compared to control eyes. TTX delayed the peak time, and STR, SR and ISO reduced the amplitude of OPs. TPMPA delayed the peak time but increased the ratio of OPs to b-wave. Furthermore, administration of combined GABA and TPMPA caused the later OPs to increase in amplitude with time, compared with those after delivery of GABA alone. Finally, we observed that GABAc and glycine receptors contributed to a low-frequency component of the OPs, while GABAa contributed to both components. These results suggest that the early components of the OPs are mainly generated by the photoreceptors, whilst the later components are mainly regulated by GABAa, GABAc and glycine receptors.

A safety study of high concentration and high frequency intravitreal injection of conbercept in rabbits

The novel anti-VEGF drug conbercept has been used in the treatment of several retinal neovascular diseases. Owning to the alteration of the structure, the newest drug is capable of combining more molecular targets and present higher affinity to the angiogenesis promoting factors. However, it is unknown whether it will cause any unwanted effects like other anti-VEGF agents. We studied the short-term safety of high concentration and high frequency intravitreal injection of conbercept in rabbits. Intraocular pressure, fundus-photography, ERGs were applied. Retinal morphology, the amount of apoptotic cells and protein levels of IL-6, IL-8 and TNF-α in the aqueous humor were determined. Retinal proteomics was detected using tandem mass tags (TMTs) quantitative mass spectrometry. The difference of IOP, ERGs, protein levels of inflammatory factors among rabbits received conbercept and PBS was not significant (P > 0.05). Fundus photographs and retinal morphology of animals in the conbercept-injected groups mimic those observed in the PBS-injected groups. No TUNEL-positive cell was seen in the retinal ganglion cell layer in the conbercept-injected groups. Proteomics did not show significant changes of inflammation or apoptosis associated proteins in the conbercept-injected eyes. We conclude that intravitreal injection of high concentration and high frequency conbercept is well tolerated at least in a short-term in rabbits.

Contribution of Nav1.8 sodium channels to retinal function

We examined the contribution of the sodium channel isoform Na_v1.8 to retinal function using the specific blocker A803467. We found that A803467 has little influence on the electroretinogram (ERG) a- and b-waves, but significantly reduces the oscillatory potentials (OPs) to 40-60% of their original amplitude, with significant changes in implicit time in the rod-driven range. To date, only two cell types were found in mouse to express Na_v1.8; the starburst amacrine cells (SBACs), and a subtype of retinal ganglion cells (RGCs). When we recorded light responses from ganglion cells using a multielectrode array we found significant and opposing changes in two physiological groups of RGCs. ON-sustained cells showed significant decreases while transient ON-OFF cells showed significant increases. The effects on ON-OFF transient cells but not ON-sustained cells disappeared in the presence of an inhibitory cocktail. We have previously shown that RGCs have only a minor contribution to the OPs (Smith et al., 2014), therefore suggesting that SBACs might be a significant contributor to this ERG component. Targeting SBACs with the cholinergic neurotoxin ethylcholine mustard aziridinium (AF64A) caused a reduction in the amplitude of the OPs similar to A803467. Our results, both using the ERG and MEA recordings from RGCs, suggest that Na_v1.8 plays a role in modulating specific aspects of the retinal physiology and that SBACs are a fundamental cellular contributor to the OPs in mice, a clear demonstration of the dichotomy between ERG b-wave and OPs.

P2X7R modulation of visually evoked synaptic responses in the retina

P2X₇Rs are distributed throughout all layers of the retina, and thus, their localisation on various cell types puts into question their specific site(s) of action. Using a dark-adapted, ex vivo mouse retinal whole mount preparation, the present study aimed to characterise the effect of P2X₇R activation on light-evoked, excitatory RGC ON-field excitatory post-synaptic potentials (fEPSPs) and on outer retinal electroretinogram (ERG) responses under comparable conditions. The pharmacologically isolated NMDA receptor-mediated RGC ON-fEPSP was reduced in the presence of BzATP, an effect which was significantly attenuated by A438079 and other selective P2X₇R antagonists A804598 or AF27139. In physiological Krebs medium, BzATP induced a significant potentiation of the ERG a-wave, with a concomitant reduction in the b-wave and the power of the oscillatory potentials. Conversely, in the pharmacologically modified Mg²⁺-free perfusate, BzATP reduced both the a-wave and b-wave. The effects of BzATP on the ERG components were suppressed by A438079. A role for P2X₇R function in visual processing in both the inner and outer retina under physiological conditions remains controversial. The ON-fEPSP was significantly reduced in the presence of A804598 but not by A438079 or AF27139. Furthermore, A438079 did not have any effect on the ERG components in physiological Krebs but potentiated and reduced the a-wave and b-wave, respectively, when applied to the pharmacologically modified medium. Therefore, activation of P2X₇Rs affects the function in the retinal ON pathway. The presence of a high concentration of extracellular ATP would most likely contribute to the modulation of visual transmission in the retina in the pathophysiological microenvironment.

Whole-eye electrical stimulation therapy preserves visual function and structure in P23H-1 rats

Low-level electrical stimulation to the eye has been shown to be neuroprotective against retinal degeneration in both human and animal subjects, using approaches such as subretinal implants and transcorneal electrical stimulation. In this study, we investigated the benefits of whole-eye electrical stimulation (WES) in a rodent model of retinitis pigmentosa. Transgenic rats with a P23H-1 rhodopsin mutation were treated with 30 min of low-level electrical stimulation (4 μA at 5 Hz; n = 10) or sham stimulation (Sham group; n = 15), twice per week, from 4 to 24 weeks of age. Retinal and visual functions were assessed every 4 weeks using electroretinography and optokinetic tracking, respectively. At the final time point, eyes were enucleated and processed for histology. Separate cohorts were stimulated once for 30 min, and retinal tissue harvested at 1 h and 24 h post-stimulation for real-time PCR detection of growth factors and inflammatory and apoptotic markers. At all time-points after treatment, WES-treated rat eyes exhibited significantly higher spatial frequency thresholds than untreated eyes. Inner retinal function, as measured by ERG oscillatory potentials (OPs), showed significantly improved OP amplitudes at 8 and 12 weeks post-WES compared to Sham eyes. Additionally, while photoreceptor segment and nuclei thicknesses in P23H-1 rats did not change between treatment groups, WES-treated eyes had significantly greater numbers of retinal ganglion cell nuclei than Sham eyes at 20 weeks post-WES. Gene expression levels of brain-derived neurotrophic factor (BDNF), caspase 3, fibroblast growth factor 2 (FGF2), and glutamine synthetase (GS) were significantly higher at 1 h, but not 24 h after WES treatment. Our findings suggest that WES has a beneficial effect on visual function in a rat model of retinal degeneration and that post-receptoral neurons may be particularly responsive to electrical stimulation therapy.

Early adaptive response of the retina to a pro-diabetogenic diet: Impairment of cone response and gene expression changes in high-fructose fed rats

The lack of plasticity of neurons to respond to dietary changes, such as high fat and high fructose diets, by modulating gene and protein expression has been associated with functional and behavioral impairments that can have detrimental consequences. The inhibition of high fat-induced rewiring of hypothalamic neurons induced obesity. Feeding rodents with high fructose is a recognized and widely used model to trigger obesity and metabolic syndrome. However the adaptive response of the retina to short term feeding with high fructose is poorly documented. We therefore aimed to characterize both the functional and gene expression changes in the neurosensory retina of Brown Norway rats fed during 3 and 8 days with a 60%-rich fructose diet (n = 16 per diet and per time point). Glucose, insulin, leptin, triacylglycerols, total cholesterol, HDL-cholesterol, LDL-cholesterol and fructosamine were quantified in plasma (n = 8 in each group). Functionality of the inner retina was studied using scotopic single flash electroretinography (n = 8 in each group) and the individual response of rod and cone photoreceptors was determined using 8.02 Hz Flicker electroretinography (n = 8 in each group). Analysis of gene expression in the neurosensory retina was performed by Affymetrix genechips, and confirmed by RT-qPCR (n = 6 in each group). Elevated glycemia (+13%), insulinemia (+83%), and leptinemia (+172%) was observed after 8 days of fructose feeding. The cone photoreceptor response was altered at day 8 in high fructose fed rats (Δ = 0.5 log unit of light stimulus intensity). Affymetrix analysis of gene expression highlighted significant modulation of the pathways of eIF2 signaling and endoplasmic reticulum stress, regulation of eIF4 and p70S6K signaling, as well as mTOR signaling and mitochondrial dysfunction. RT-qPCR analysis confirmed the down regulation of Crystallins, Npy, Nid1 and Optc genes after 3 days of fructose feeding, and up regulation of End2. Meanwhile, a trend towards an increased expression of αA- and αB-crystallin proteins was observed at day 8. Our results are consistent with early alterations of the functioning and gene expression in the retina in a pro diabetogenic environment.

Contribution of N-methyl-DL-aspartic acid (NMDA)-sensitive neurons to generating oscillatory potentials in Royal College of Surgeons rats

PURPOSE

We investigated how the N-methyl-DL-aspartic acid (NMDA) receptor contributes to generating oscillatory potentials (OPs) of the electroretinogram (ERG) in the Royal College of Surgeons (RCS) rat.

METHODS

Scotopic ERGs were recorded from dystrophic and wild-type congenic (WT) RCS rats (n = 20 of each) at 25, 30, 35, and 40 days of age. The stimulus intensity was increased from -2.82 to 0.71 log cd-s/m(2) to obtain intensity-response function. NMDA was injected into the vitreous cavity of the right eyes. The left eyes were injected with saline as controls. The P3 obtained by a-wave fitting was digitally subtracted from the scotopic ERG to isolate the P2. For the OPs, the P2 was digitally filtered between 65 and 500 Hz. The amplitudes of OP1, OP2, OP3, and OP4 were then measured and summed and designated as ΣOPs. The implicit times of OP1, OP2, and OP3 were also measured. The frequency spectra of the OPs were analyzed using fast Fourier transform (FFT).

RESULTS

The maximum ERG a- and b-waves as well as ΣOPs amplitudes reduced with age in dystrophic rats. Compared with intravitreal saline injection, administration of NMDA decreased ΣOPs amplitudes from 30 days of age in dystrophic rats, while it did not attenuate ΣOPs amplitudes in WT rats. The implicit times of the OPs of the maximum ERG were prolonged by NMDA injections in WT and dystrophic rats. NMDA/saline ratios of ΣOPs amplitudes area under the FFT curves were significantly lower in dystrophic rats from 30 days of age than that in WT rats.

CONCLUSION

In the early stage of photoreceptor degeneration, intravitreal NMDA injection attenuated OPs amplitudes in dystrophic rats. This indicates that NMDA receptors play a significant role in generating OPs amplitudes with advancing photoreceptor degeneration.

Rod-driven OFF pathway responses in the distal retina: dark-adapted flicker electroretinogram in mouse

Purpose

The rodent retina does not exhibit a positive OFF-response in the electroretinogram (ERG), which makes it difficult to evaluate its OFF-pathway functions in vivo. We studied the rod-driven OFF pathway responses by using a dark-adapted 10-Hz flicker ERG procedure in mouse.

Materials and Methods

Conventional ERGs and 10-Hz dark-adapted flicker ERGs were obtained in wild-type mice (C57BL/6), in mice with pure rod (cpfl1) or pure cone (rho^−/−) function, and in nob1 mice which have a selective ON-pathway defect. To isolate the response from ON or OFF pathway, glutamate analogs 2-amino-4-phosphobutyric acid (APB, an ON pathway blocker) and cis-2, 3-piperidine-dicarboxylic acid (PDA, an OFF pathway blocker), were injected intravitreally.

Results

The amplitude-intensity profile of the dark-adapted 10-Hz flicker ERG in the wild-type mice exhibits two peaks at middle and high light intensities. The two peaks represent rod- and cone-driven responses respectively. In APB-treated C57BL/6 mice and in nob1 mice, the dark-adapted ERG b-waves were absent. However, both rod- and cone-driven OFF pathway responses were evident with flicker ERG recording. At middle light intensities that activate only rod system, the flicker ERG responses in saline-injected nob1 mice were similar to those in APB-injected cpfl1 mice and wild-type mice. These responses are sensitive to PDA. The amplitudes of these rod-driven OFF pathway responses were approximately 20% of the total rod-driven flicker ERG responses.

Conclusion

We demonstrate that the rod-OFF bipolar cell pathway is functional in the outer retina. The dark-adapted flicker ERG is practical for the evaluation of rod- and cone-driven responses, and the residual OFF pathway signals in subjects with ON pathway defects.

Signal processing techniques for oscillatory potential extraction in the electroretinogram: automated highpass cutoff frequency estimation

Oscillatory potentials (OPs) are typically isolated from the electroretinogram (ERG) via linear, time-invariant, bandpass filtering. The use of a highpass cutoff frequency that is too low results in a- and b-wave contamination of the OP signal, while a cutoff frequency that is too high removes significant OP signal energy. Two methods for automated highpass cutoff frequency estimation were developed and evaluated. An OP amplitude analysis method exploited a trend in variation of maximum OP amplitude with cutoff frequency. A second method fit a time-varying exponential model to the rising edge of the b-wave and selected a cutoff frequency based on minimizing the error between the residual ERG signal (the signal formed by subtracting the OP signal from the original ERG signal) and the exponential fit. The performance of each method was evaluated at 11 luminances (0.001-100 scot cd · s/m(2)) in ten wild-type adult mice by comparing the automated selections to expert-selected highpass cutoff frequencies. It was noted that cutoff frequency selection was not critical at the lower luminance levels, but strongly influenced the OP signal shape for higher luminances. At the highest luminance, errors between the OP amplitude and exponential model versus expert selection were -6.3 ± 13 and -8.2 ± 7.3 Hz, respectively. ANOVAs showed that estimations made by the OP amplitude analysis method were generally statistically indistinguishable from the expert identifications. Furthermore, both OP amplitude analysis and exponential fitting error analysis provided excellent fits to the manual selections for the four highest stimulus luminance values.