Measurement of Electroretinograms and Visually Evoked Potentials in Awake Moving Mice

Regulatory role of the p38 MAPK/ATF2 signaling pathway in visual function and visual cortical plasticity in mice with monocular deprivation

BACKGROUND

This study aimed to examine the role of the p38 mitogen-activated protein kinase (MAPK)/ activating transcription factor 2 (ATF2) signaling in visual function impairment and visual cortical plasticity in mice with monocular deprivation (MD).

METHODS

Visual behavioral tests, including visual water task, visual cliff test, and flash visual evoked potential, were performed on each group. We studied the density of dendritic spines and the synaptic ultrastructure by Golgi staining and transmission electron microscope. We performed Western blot and immunohistochemistry and detected the expression of ATF2, PSD-95, p38 MAPK, and phosphor-p38 MAPK in the left visual cortex.

RESULTS

In the MD + SB group, the visual acuity in deprived eyes substantially improved, the impairment of visual depth perception was alleviated, and the P wave amplitude and C/I ratio increased. The density of dendritic spines and the numerical density of synapses increased significantly, the width of the synaptic cleft decreased significantly, and the length of the active synaptic zone and the thickness of post-synaptic density (PSD) increased substantially. The protein expression of phosphor-p38 MAPK decreased, whereas that of PSD-95 and ATF2 increased significantly.

CONCLUSIONS

Inhibiting the phosphorylation of p38 MAPK and negative feedback upregulated ATF2 expression, alleviated damage to visual function, and protected against synaptic plasticity in mice with MD.

Visual evoked potentials waveform analysis to measure intracortical damage in a preclinical model of multiple sclerosis

Introduction

Visual evoked potentials (VEPs) are a non-invasive technique routinely used in clinical and preclinical practice. Discussion about inclusion of VEPs in McDonald criteria, used for Multiple Sclerosis (MS) diagnosis, increased the importance of VEP in MS preclinical models. While the interpretation of the N1 peak is recognized, less is known about the first and second positive VEP peaks, P1 and P2, and the implicit time of the different segments. Our hypothesis is that P2 latency delay describes intracortical neurophysiological dysfunction from the visual cortex to the other cortical areas.

Methods

In this work, we analyzed VEP traces that were included in our two recently published papers on Experimental Autoimmune Encephalomyelitis (EAE) mouse model. Compared with these previous publications other VEP peaks, P1 and P2, and the implicit time of components P1-N1, N1-P2 and P1-P2, were analyzed in blind.

Results

Latencies of P2, P1-P2, P1-N1 and N1-P2 were increased in all EAE mice, including group without N1 latency change delay at early time points. In particular, at 7 dpi the P2 latency delay change was significantly higher compared with N1 latency change delay. Moreover, new analysis of these VEP components under the influence of neurostimulation revealed a decrease in P2 delay in stimulated animals.

Discussion

P2 latency delay, P1-P2, P1-N1, and N1-P2 latency changes which reflect intracortical dysfunction, were consistently detected across all EAE groups before N1 change. Results underline the importance of analyzing all VEP components for a complete overview of the neurophysiological visual pathway dysfunction and treatment efficacy.

X-Linked Retinitis Pigmentosa Gene Therapy: Preclinical Aspects

The most common inherited eye disease is retinitis pigmentosa (RP). X-linked RP (XLRP) is one of the most severe types of RP, with a considerable disease burden. Patients with XLRP experience a decrease in their vision and become blind in their 4th decade of life, causing much morbidity after starting a rather normal life. Treatment of XLRP remains challenging, and current treatments are not effective enough in restoring vision. Gene therapy of XLRP, capable of restoring the functional RPGR gene, showed promising results in preclinical studies and clinical trials; however, to date, no approved product has entered the market. The development of a gene therapy product needs through preliminary assessment of the drug in animal models before administration to humans. In this article, we reviewed the genetic pathology of XLRP, along with the preclinical aspects of the XLRP gene therapy, animal models, associated assessments, and future challenges and directions.

Differences in the Effects of Pentobarbital Anesthetic and Combination of Medetomidine Hydrochloride, Midazolam, and Butorphanol Tartrate Anesthetic on Electroretinogram in Spontaneously Diabetic Torii Fatty Rats

Purpose

The aim of this study was to investigate the effects of different anesthetic agents on electroretinograms (ERGs) in Spontaneously Diabetic Torii fatty rats (SDT fatty rats).

Methods

The ERG recordings were measured under general anesthesia using pentobarbital or a combination of medetomidine hydrochloride, midazolam, and butorphanol (MMB) tartrate anesthesia in 12 9-week-old normal Sprague-Dawley rats (Jcl:SD rats) and 16 SDT fatty rats. Each animal model was divided into 2 groups, the pentobarbital group and MMB group. The amplitudes and peak times of the a- and b-waves and oscillatory potentials (OPs) were measured from 0.0001 candela per square meter (cd.s/m²) to 10.0 cd.s/m².

Results

The amplitude of the a-wave was significantly higher in the MMB group of Jcl:SD rats, but there was no significant difference in amplitude between the two groups of SDT fatty rats. There was no significant difference in the OP1 amplitude between both groups of Jcl:SD rats, but the OP1 amplitude was significantly higher in the MMB group of SDT fatty rats. The OP2 amplitude was significantly higher in the pentobarbital group in both the Jcl:SD rats and SDT fatty rats. There was no significant difference in the OP3 amplitude between the Jcl:SD and SDT fatty rat groups. The amplitude of the OP4 waves was significantly higher in the MMB group for both Jcl:SD and SDT fatty rats. There was no significant difference in the sums of the OP1 to OP4 (ΣOPs) amplitudes between the Jcl:SD and SDT fatty rat groups. There was no significant difference in the b-wave amplitude between the Jcl:SD rat groups, but the b-wave amplitude was significantly higher in the SDT fatty rats that received pentobarbital. The peak times for a-wave, OP1, OP2, OP3, OP4, and ΣOPs were significantly longer in the pentobarbital group of SD rats. The peak time of the b-wave was significantly longer in the MMB group of Jcl:SD rats, but the same result was obtained in the SDT fatty rats except that there was no significant difference in the a-wave.

Conclusion

The overall ERG results vary depending on the anesthetic agent used. The OPs can be observed in detail when using MMB. Since the SDT fatty rat is a diabetic model animal, we recommend MMB as the anesthesia of choice when studying the OP waves in detail.

Non-ablative doses of focal ionizing radiation alters function of central neural circuits

BACKGROUND

Modulation of pathological neural circuit activity in the brain with a minimum of complications is an area of intense interest.

OBJECTIVE

The goal of the study was to alter neurons' physiological states without apparent damage of cellular integrity using stereotactic radiosurgery (SRS).

METHODS

We treated a 7.5 mm-diameter target on the visual cortex of Göttingen minipigs with doses of 40, 60, 80, and 100 Gy. Six months post-irradiation, the pigs were implanted with a 9 mm-wide, eight-shank multi-electrode probe, which spanned the radiation focus as well as the low-exposure neighboring areas.

RESULTS

Doses of 40 Gy led to an increase of spontaneous firing rate, six months post-irradiation, while doses of 60 Gy and greater were associated with a decrease. Subjecting the animals to visual stimuli resulted in typical visual evoked potentials (VEP). At 40 Gy, a significant reduction of the P1 peak time, indicative of higher network excitability was observed. At 80 Gy, P1 peak time was not affected, while a minor reduction at 60 Gy was seen. No distance-dependent effects on spontaneous firing rate, or on VEP were observed. Post-mortem histology revealed no evidence of necrosis at doses below 60 Gy. In an in vitro assay comprising of iPS-derived human neuron-astrocyte co-cultures, we found a higher vulnerability of inhibitory neurons than excitatory neurons with respect to radiation, which might provide the cellular mechanism of the disinhibitory effect observed in vivo.

CONCLUSION

We provide initial evidence for a rather circuit-wide, long-lasting disinhibitory effect of low sub-ablative doses of SRS.

Failed remyelination of the nonhuman primate optic nerve leads to axon degeneration, retinal damages, and visual dysfunction

Promotion of remyelination has become a new therapeutic avenue to prevent neuronal degeneration and promote recovery in white matter diseases, such as multiple sclerosis (MS). To date most of these strategies have been developed in short-lived rodent models of demyelination, which spontaneously repair. Well-defined nonhuman primate models closer to man would allow us to efficiently advance therapeutic approaches. Here we present a nonhuman primate model of optic nerve demyelination that recapitulates several features of MS lesions. The model leads to failed remyelination, associated with progressive axonal degeneration and visual dysfunction, thus providing the missing link to translate emerging preclinical therapies to the clinic for myelin disorders such as MS.

White matter disorders of the central nervous system (CNS), such as multiple sclerosis (MS), lead to failure of nerve conduction and long-lasting neurological disabilities affecting a variety of sensory and motor systems, including vision. While most disease-modifying therapies target the immune and inflammatory response, the promotion of remyelination has become a new therapeutic avenue to prevent neuronal degeneration and promote recovery. Most of these strategies have been developed in short-lived rodent models of demyelination, which spontaneously repair and do not reflect the size, organization, and biology of the human CNS. Thus, well-defined nonhuman primate models are required to efficiently advance therapeutic approaches for patients. Here, we followed the consequence of long-term toxin-induced demyelination of the macaque optic nerve on remyelination and axon preservation, as well as its impact on visual functions. Findings from oculomotor behavior, ophthalmic examination, electrophysiology, and retinal imaging indicate visual impairment involving the optic nerve and retina. These visual dysfunctions fully correlated at the anatomical level, with sustained optic nerve demyelination, axonal degeneration, and alterations of the inner retinal layers. This nonhuman primate model of chronic optic nerve demyelination associated with axonal degeneration and visual dysfunction, recapitulates several key features of MS lesions and should be instrumental in providing the missing link to translate emerging repair promyelinating/neuroprotective therapies to the clinic for myelin disorders, such as MS.

An optimized procedure to record visual evoked potential in mice

Visual evoked potential (VEP) is commonly used to evaluate visual acuity in both clinical and basic studies. Subdermal needle electrodes or skull pre-implanted screw electrodes are usually used to record VEP in rodents. However, the VEP amplitudes recorded by the former are small while the latter may damage the brain. In this study, we established a new invasive procedure for VEP recording, and made a series of comparisons of VEP parameters recorded from different electrode locations, different times of day (day and night) and bilateral eyes, to evaluate the influence of these factors on VEP in mice. Our data reveal that our invasive method is reliable and can record VEP with good waveforms and large amplitudes. The comparison data show that VEP is greatly influenced by active electrode locations and difference between day and night. In C57 or CD1 ONC (optic nerve crush) models and Brn3b^AP/AP mice, which are featured by loss of retinal ganglion cells (RGCs), amplitudes of VEP N1 and P1 waves are drastically reduced. The newly established VEP procedure is very reliable and stable, and is particularly useful for detecting losses of RGC quantities, functions or connections to the brain. Our analyses of various recording conditions also provide useful references for future studies.

Effects of top-down influence suppression on behavioral and V1 neuronal contrast sensitivity functions in cats

To explore the relative contributions of higher-order and primary visual cortex (V1) to visual perception, we compared cats' behavioral and V1 neuronal contrast sensitivity functions (CSF) and threshold versus external noise contrast (TvC) functions before and after top-down influence of area 7 (A7) was modulated with transcranial direct current stimulation (tDCS). We found that suppressing top-down influence of A7 with cathode-tDCS, but not sham-tDCS, reduced behavioral and neuronal contrast sensitivity in the same range of spatial frequencies and increased behavioral and neuronal contrast thresholds in the same range of external noise levels. The neuronal CSF and TvC functions were highly correlated with their behavioral counterparts both before and after the top-down suppression. Analysis of TvC functions using the Perceptual Template Model (PTM) indicated that top-down influence of A7 increased both behavioral and V1 neuronal contrast sensitivity by reducing internal additive noise and the impact of external noise.

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Top-down suppression lowers both behavioral and V1 neuronal CSF functions

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Top-down suppression raises both behavioral and V1 neuronal TvC functions

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The neuronal CSFs and TvCs are highly correlated with their behavioral counterparts

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Top-down influence lowers internal additive noise and impact of external noise in V1

Comparisons of α2-Adrenergic Agents, Medetomidine and Xylazine, with Pentobarbital for Anesthesia: Important Pitfalls in Diabetic and Nondiabetic Rats

Purpose: Anesthesia is necessary to conduct rodent electroretinograms (ERGs). We evaluated utility of the α2-agonist medetomidine versus xylazine for ERG studies in nondiabetic and diabetic rats. Pentobarbital was included as a comparator. Methods: Male Sprague-Dawley rats, with and without streptozotocin (STZ)-induced diabetes, were anesthetized with medetomidine (1 mg/kg), xylazine (10 mg/kg) (both with ketamine 75 mg/kg), or pentobarbital (70 mg/kg). The depth of anesthesia was assessed, and if adequate, scotopic ERGs were recorded. Blood glucose was monitored. Results: In nondiabetic rats, all three agents induced satisfactory anesthesia, but with differing durations: medetomidine > pentobarbital > xylazine. ERG responses were similar under medetomidine and xylazine, but relatively reduced under pentobarbital. Both α2-agonists (but not pentobarbital) elicited marked hyperglycemia (peak values 316.1 ± 42.6 and 300.3 ± 29.5 mg/dL, respectively), persisting for 12 h. In diabetic rats, elevated blood glucose concentrations were not affected by any of the agents, but the depth of anesthesia under medetomidine and xylazine was inadequate for ERG recording. Conclusions: In nondiabetic rats, medetomidine and xylazine elicited comparable effects on ERGs that differ from pentobarbital, but both perturbed glucose metabolism, potentially confounding experimental outcomes. In STZ-diabetic rats, neither α2-agent provided adequate anesthesia, while pentobarbital did so. Problems with α2-anesthetic agents, including medetomidine, must be recognized to ensure meaningful interpretation of experimental results.

Non-invasive visual evoked potentials under sevoflurane versus ketamine-xylazine in rats

Background

Visual Evoked Potential (VEP) quantifies electrical signals produced in visual cortex in response to visual stimuli. VEP elicited by light flashes is a useful biomarker to evaluate visual function in preclinical models and it can be recorded in awake or anaesthetised state. Different types of anaesthesia influence VEP properties, such as latency, which measures the propagation speed along nerve fibers, and amplitude that quantifies the power of electrical signal.

Aim

The goal of this work is to compare VEPs elicited in Dark Agouti rats under two types of anaesthesia: volatile sevoflurane or injectable ketamine-xylazine.

Methods

VEP latency, amplitude, signal-to-noise ratio and recording duration were measured in Dark Agouti rats randomly assigned to two groups, the first subjected to volatile sevoflurane and the second to injectable ketamine-xylazine. Taking advantage of non-invasive flash-VEP recording through epidermal cup electrodes, three time points of VEP recordings were assessed in two weeks intervals.

Results

VEP recorded under ketamine-xylazine showed longer latency and higher amplitude compared with sevoflurane, with analogous repeatability over time. However, sevoflurane tended to suppress electrical signals from visual cortex, resulting in a lower signal-to-noise ratio. Moreover, VEP procedure duration lasted longer in rats anaesthetised with sevoflurane than ketamine-xylazine.

Conclusions

In Dark Agouti rats, the use of different anaesthesia can influence VEP components in terms of latency and amplitude. Notably, sevoflurane and ketamine-xylazine revealed satisfying repeatability over time, which is critical to perform reliable follow-up studies. Ketamine-xylazine allowed to obtain more clearly discernible VEP components and less background noise, together with a quicker recording procedure and a consequently improved animal safety and welfare.

Visual Dysfunction after Repetitive Mild Traumatic Brain Injury in a Mouse Model and Ramifications on Behavioral Metrics

Mild traumatic brain injury (mTBI) is a major cause of morbidity and mortality with a poorly understood pathophysiology. Animal models have been increasingly utilized to better understand mTBI and recent research has identified visual deficits in these models that correspond to human literature. While visual impairment is being further characterized within TBI, the implications of impaired vision on behavioral tasks commonly utilized in animal models has not been well described thus far. Visual deficits may well confound behavioral tests that are believed to be isolated to cognitive functioning such as learning and memory. We utilized a mouse model of repetitive mTBI (rmTBI) to further characterize visual deficits using an optomotor task, electroretinogram, and visually evoked potential, and located likely areas of damage to the visual pathway. Mice were tested on multiple behavioral metrics, including a touchscreen conditional learning task to better identify the contribution of visual dysfunction to behavioral alterations. We found that rmTBI caused visual dysfunction resulting from damage distal to the retina that likely involves pathology within the optic nerve. Moreover, loss of vision led to poorer performance of rmTBI animals on classic behavioral tests such as the Morris water maze that would otherwise be attributed solely to learning and memory deficits. The touchscreen conditional learning task was able to differentiate rmTBI induced learning and memory dysfunction from visual impairment and is a valuable tool for elucidating subtle changes resulting from TBI.

Transplantation of reprogrammed peripheral blood cells differentiates into retinal ganglion cells in the mouse eye with NMDA-induced injury

The generation of patient-specific induced pluripotent stem cells (iPSCs) holds significant implications for replacement therapy in treating optic neuropathies such as glaucoma. Stem-cell-based therapy targeted at replacing and replenishing retinal ganglion cells is progressing at a fast pace. However, clinical application necessitates an efficient and robust approach for cell manufacturing. Here, we examine whether the embryo body derived from human peripheral blood-derived iPSC can localize into the host retina and differentiate into retinal ganglion cells after transplantation into a glaucoma injury model. Human peripheral blood T cells were isolated and reprogrammed into an induced pluripotent stem cell (TiPSC) line using Sendai virus transduction carrying transcription factors Sox2, Klf4, c-Myc, and Oct4. TiPSCs were differentiated into RGC using neural basal culture. For in vivo studies, embryo bodies derived from TiPSCs (TiPSC-EB) were injected into the vitreous cavity of N-Methyl-d-aspartic acid (NMDA)-treated mice 2 weeks before sacrifice and retinal dissection. Induced pluripotent stem cells generated from human peripheral blood T cells display stem cell morphology and pluripotency markers. Furthermore, RGC-like cells differentiated from TiPSC exhibit extending axons and RGC marker TUJ1. When transplanted intravitreally into NMDA-treated mice, embryo bodies derived from TiPSC survived, migrated, and incorporated into the retina's GCL layer. In addition, TiPSC-EB transplants were able to differentiate into TUJ1 positive RGC-like cells. Retinal ganglion cells can be differentiated using human peripheral blood cells derived iPSC. Transplantation of embryo body derived from TiPSCs into a glaucoma mouse model could incorporate into host GCL and differentiate into RGC-like cells.

Responsivity to light in familial hemiplegic migraine type 1 mutant mice reveals frequency-dependent enhancement of visual network excitability

Migraine patients often report (inter)ictal hypersensitivity to light, but the underlying mechanisms remain an enigma. Both hypo- and hyperresponsivity of the visual network have been reported, which may reflect either intra-individual dynamics of the network or large inter-individual variation in the measurement of human visual evoked potential data. Therefore, we studied visual system responsivity in freely behaving mice using combined epidural electroencephalography and intracortical multi-unit activity to reduce variation in recordings and gain insight into visual cortex dynamics. For better clinical translation, we investigated transgenic mice that carry the human pathogenic R192Q missense mutation in the α_1A subunit of voltage-gated Ca_V 2.1 Ca²⁺ channels leading to enhanced neurotransmission and familial hemiplegic migraine type 1 in patients. Visual evoked potentials were studied in response to visual stimulation paradigms with flashes of light. Following intensity-dependent visual stimulation, FHM1 mutant mice displayed faster visual evoked potential responses, with lower initial amplitude, followed by less pronounced neuronal suppression compared to wild-type mice. Similar to what was reported for migraine patients, frequency-dependent stimulation in mutant mice revealed enhanced photic drive in the EEG beta-gamma band. The frequency-dependent increases in visual network responses in mutant mice may reflect the context-dependent enhancement of visual cortex excitability, which could contribute to our understanding of sensory hypersensitivity in migraine.

Anodal and cathodal tDCS modulate neural activity and selectively affect GABA and glutamate syntheses in the visual cortex of cats

KEY POINTS

The present study showed that anodal and cathodal transcranial direct current stimulation (tDCS) can respectively increase and decrease the amplitude of visually evoked field potentials in the stimulated visual cortex of cats, with the effect lasting for ∼60-70 min. We directly measured tDCS-induced changes in the concentration of inhibitory and excitatory neurotransmitters in the visual cortex using the enzyme-linked immunosorbent assay method and showed that anodal and cathodal tDCS can selectively decrease the concentration of GABA and glutamate in the stimulated cortical area. Anodal and cathodal tDCS can selectively inhibit the synthesis of GABA and glutamate by suppressing the expression of GABA- and glutamate-synthesizing enzymes, respectively.

ABSTRACT

Transcranial direct current stimulation (tDCS) evokes long-lasting neuronal excitability in the target brain region. The underlying neural mechanisms remain poorly understood. The present study examined tDCS-induced alterations in neuronal activities, as well as the concentration and synthesis of GABA and glutamate (GLU), in area 21a (A21a) of cat visual cortex. Our analysis showed that anodal and cathodal tDCS respectively enhanced and suppressed neuronal activities in A21a, as indicated by a significantly increased and decreased amplitude of visually evoked field potentials (VEPs). The tDCS-induced effect lasted for ∼60-70 min. By contrast, sham tDCS had no significant impact on the VEPs in A21a. On the other hand, the concentration of GABA, but not that of GLU, in A21a significantly decreased after anodal tDCS relative to sham tDCS, whereas the concentration of GLU, but not that of GABA, in A21a significantly decreased after cathodal tDCS relative to sham tDCS. Furthermore, the expression of GABA-synthesizing enzymes GAD65 and GAD67 in A21a significantly decreased in terms of both mRNA and protein concentrations after anodal tDCS relative to sham tDCS, whereas that of GLU-synthesizing enzyme glutaminase (GLS) did not change significantly after anodal tDCS. By contrast, both mRNA and protein concentrations of GLS in A21a significantly decreased after cathodal tDCS relative to sham tDCS, whereas those of GAD65/GAD67 showed no significant change after cathodal tDCS. Taken together, these results indicate that anodal and cathodal tDCS may selectively reduce GABA and GLU syntheses and thus respectively enhance and suppress neuronal excitability in the stimulated brain area.

A new electrophysiological non-invasive method to assess retinocortical conduction time in the Dark Agouti rat through the simultaneous recording of electroretinogram and visual evoked potential

PURPOSE

To develop a non-invasive method exploiting simultaneous recording of epidermal visual evoked potential (VEP) and epicorneal electroretinogram (ERG) to study retinocortical function and to evaluate its reliability and repeatability over time.

METHODS

Female wild-type DA rats were anesthetized with ketamine/xylazine (40/5 mg/kg). Epidermal VEP (Ag/AgCl cup electrode on scalp) and epicorneal ERG (gold ring electrode on eye surface) were recorded simultaneously in response to flash stimulation.

RESULTS

ANOVA for repeated measures showed that peak times of ERG b-wave and of VEP N1 and P2 were stable across 6 weekly time-points, as well as the corresponding amplitudes. Mean retinocortical time from b-wave to N1 (RCT1) was 7.6 ms and remained comparable across the 6 time-points. Mean retinocortical time from b-wave to P2 (RCT2) was 28.7 ms and did not show significant variations over time. Coefficient of variation (CoV%) and CoV% adjusted for sample size, namely relative standard error (RSE%), were calculated as indexes of repeatability. Good RSE% over time was obtained (< 5% for b-wave, N1 and P2 peak times; < 20% and < 7% for RCT1 and RCT2, respectively).

CONCLUSIONS

Simultaneous recording of ERG and VEP has been previously achieved through invasive methods requiring surgery. Here, we present a new non-invasive method, which allowed to obtain peak and retinocortical times that were constant across a long period and had a good repeatability over time. This method will ensure not only a gain in animal welfare, but will also avoid stress and eye or brain lesions which can interfere with experimental variables.

Single AAV-mediated mutation replacement genome editing in limited number of photoreceptors restores vision in mice

Supplementing wildtype copies of functionally defective genes with adeno-associated virus (AAV) is a strategy being explored clinically for various retinal dystrophies. However, the low cargo limit of this vector allows its use in only a fraction of patients with mutations in relatively small pathogenic genes. To overcome this issue, we developed a single AAV platform that allows local replacement of a mutated sequence with its wildtype counterpart, based on combined CRISPR-Cas9 and micro-homology-mediated end-joining (MMEJ). In blind mice, the mutation replacement rescued approximately 10% of photoreceptors, resulting in an improvement in light sensitivity and an increase in visual acuity. These effects were comparable to restoration mediated by gene supplementation, which targets a greater number of photoreceptors. This strategy may be applied for the treatment of inherited disorders caused by mutations in larger genes, for which conventional gene supplementation therapy is not currently feasible.

Replacing mutant genes with wildtype copies using adeno-associated virus (AAV) has been explored for the treatment of inherited retinopathies, but the low cargo limit restricts its use. Here the authors describe a single AAV platform that allows local replacement of a mutated sequence with its wildtype counterpart, based on combined CRISPR-Cas9 and micro-homology-mediated end joining.

In vivo imaging of the light response in mouse retinal ganglion cells based on a neuronal activity-dependent promoter

Diseases of the retinal ganglion cells (RGCs) are an important cause of blindness, yet the light response of individual RGCs is difficult to assess in vivo, particularly in mammals, due to a lack of effective methods. We report a simple in vivo platform for imaging the light response of mouse RGCs based on a fluorescent reporter-tagged enhanced synaptic activity-responsive element (E-SARE) that mediates neuronal activity-dependent gene transcription. When E-SARE-driven d2Venus, packaged into an AAV vector, was injected intravitreally, light-responsive retinal neurons expressing d2Venus were visible at single-cell resolution using confocal ophthalmoscopy. Immunohistological assessment identified the majority of these cells as RGCs. In a murine model of RGC injury, the number of d2Venus-positive cells was correlated with the amplitude of light-induced responses and with visual acuity, measured electrophysiologically at the visual cortex, indicating that the vector can be used as a tool to assess visual function in RGCs. The platform described herein allows a simple in vivo assessment of RGC function, which should help basic research into the mechanisms of RGC death and the development of treatments for diseases involving the RGCs.

Effect of midazolam, medetomidine, and butorphanol tartrate combination anesthetic on electroretinograms of mice

PURPOSE

To evaluate electroretinogram (ERG) responses under anesthesia with midazolam, medetomidine, and butorphanol tartrate (MMB) combination compared with pentobarbital sodium and ketamine/xylazine (KX).

METHODS

Six-week-old male C57BL/6J mice were divided into MMB-, pentobarbital sodium-, and KX-administered groups. Following overnight dark adaptation, an ERG was performed. The parameters sensitivity (S), log maximum amplitude (R_max ), R_max , and time delay to the onset (T_d ) of the ERG a-waves were computed based on the Lamb and Pugh model. The parameters light intensity at half maximum amplitude (K), R_max , and n of the ERG b-waves were computed based on the Naka-Rushton equation. The amplitude and the implicit time of oscillatory potentials (OPs) were quantified.

RESULTS

The T_d of the dark-adapted a-waves was statistically significantly larger under anesthesia with the MMB combination and pentobarbital sodium compared to KX. The K of the dark-adapted b-waves was statistically significantly larger under anesthesia with pentobarbital sodium compared to the MMB combination. The amplitude of the dark-adapted OPs was statistically significantly larger under anesthesia with the MMB combination compared to pentobarbital sodium. The implicit time of the dark-adapted OPs was statistically significantly smaller under anesthesia with the KX combination compared to pentobarbital sodium.

CONCLUSIONS

The results suggested that ERG responses, especially in OPs, are greatly affected by the type of anesthetic. It is important to consider the sensitive responses influenced by the selection of anesthetics when ERG is performed.

The microphthalmia-associated transcription factor (Mitf) gene and its role in regulating eye function

Mutations in the microphthalmia-associated transcription factor (Mitf) gene can cause retinal pigment epithelium (RPE) and retinal dysfunction and degeneration. We examined retinal and RPE structure and function in 3 month old mice homo- or heterozygous or compound heterozygous for different Mitf mutations (Mitf^mi-vga9/+, Mitf^{mi-enu22(398)}/Mitf^{mi-enu22(398)}, Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi) which all have normal eye size with apparently normal eye pigmentation. Here we show that their vision and retinal structures are differentially affected. Hypopigmentation was evident in all the mutants while bright-field fundus images showed yellow spots with non-pigmented areas in the Mitf^mi-vga9/+ mice. Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi mice showed large non-pigmented areas. Fluorescent angiography (FA) of all mutants except Mitf^mi-vga9/+ mice showed hyperfluorescent areas, whereas FA from both Mitf^-Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi mice showed reduced capillary network as well as hyperfluorescent areas. Electroretinogram (ERG) recordings show that Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi mice are severely impaired functionally whereas the scotopic and photopic ERG responses of Mitf^mi-vga9/+ and Mitf^{mi-enu22(398)}/Mitf^{mi-enu22(398)} mice were not significantly different from wild type mice. Histological sections demonstrated that the outer retinal layers were absent from the Mitf^Mi-Wh/+ and Mitf^Mi-Wh/Mitf^mi blind mutants. Our results show that Mitf mutations affect eye function, even in the heterozygous condition and that the alleles studied can be arranged in an allelic series in this respect.

Therapeutic Effects of Intravitreously Administered Bacteriophage in a Mouse Model of Endophthalmitis Caused by Vancomycin-Sensitive or -Resistant Enterococcus faecalis

Endophthalmitis due to infection with Enterococcus spp. progresses rapidly and often results in substantial and irreversible vision loss. Given that the frequency of this condition caused by vancomycin-resistant Enterococcus faecalis has been increasing, the development of novel therapeutics is urgently required. We have demonstrated the therapeutic potential of bacteriophage ΦEF24C-P2 in a mouse model of endophthalmitis caused by vancomycin-sensitive (EF24) or vancomycin-resistant (VRE2) strains of E. faecalis Phage ΦEF24C-P2 induced rapid and pronounced bacterial lysis in turbidity reduction assays with EF24, VRE2, and clinical isolates derived from patients with E. faecalis-related postoperative endophthalmitis. Endophthalmitis was induced in mice by injection of EF24 or VRE2 (1 × 10⁴ cells) into the vitreous. The number of viable bacteria in the eye increased to >1 × 10⁷ CFU, and neutrophil infiltration into the eye was detected as an increase in myeloperoxidase activity at 24 h after infection. A clinical score based on loss of visibility of the fundus as well as the number of viable bacteria and the level of myeloperoxidase activity in the eye were all significantly decreased by intravitreous injection of ΦEF24C-P2 6 h after injection of EF24 or VRE2. Whereas histopathologic analysis revealed massive infiltration of inflammatory cells and retinal detachment in vehicle-treated eyes, the number of these cells was greatly reduced and retinal structural integrity was preserved in phage-treated eyes. Our results thus suggest that intravitreous phage therapy is a potential treatment for endophthalmitis caused by vancomycin-sensitive or -resistant strains of E. faecalis.

32-channel mouse EEG: Visual evoked potentials

BACKGROUND

Measuring visual evoked potentials (VEP) by means of EEG allows the quasi non-invasive assessment of visual function in mice. Such sensory phenotyping is important to screen for genetic or aging effects on vision in preclinical mouse models. Thus, a standardized EEG-like approach for the assessment of sensory evoked potentials in mice is desirable.

NEW METHOD

We describe a method to obtain the topographical distribution of flash evoked VEPs with 32-channel thin-film EEG electrode arrays in anesthetized mice. Further, we provide suggestions for the optimal choice of adequate digital filtering, referencing, and stimulus parameters for fast and reliable assessment of VEP parameters and distribution.

RESULTS

32-channel thin-film electrodes provided clear information on the VEP topography across the skull. Re-referencing, such as bipolar, common average, and local average montages could be used to further refine the information on VEP topography. A balanced choice of digital high-pass filter, signal averaging and stimulus rate allowed to minimize measurement duration and at the same time assured good VEP signal-to-noise ratio.

COMPARISON WITH EXISTING METHODS

Subdermal electrodes or single skull screws provide only limited topographical information of the VEP. Assessment of VEPs with 32-channel thin-film electrodes can provide comparable signal quality with superior spatial resolution and standardized topographical and hemispheric information of VEP distribution.

CONCLUSIONS

EEG-like thin-film electrodes are an efficient tool for fast, comprehensive sensory phenotyping with topographical information in mice. This is a step towards the use of standardized mouse EEG to characterize EEG biomarkers in mouse models of human diseases.

Reliable detection of low visual acuity in mice with pattern visually evoked potentials

Measuring the optokinetic response (OKR) to rotating sinusoidal gratings is becoming an increasingly common method to determine visual function thresholds in mice. This is possible also through direct electrophysiological recording of the response of the neurons in the visual cortex to the presentation of reversing patterned stimuli, i.e. the pattern visually evoked potential (pVEP). Herein, we optimized the conditions for recording pVEPs in wild-type mice: we investigated the optimal depth (1, 2, or 3 mm) of the inserted electrode and the optimal stimulus pattern (vertical, horizontal, or oblique black and white stripes, or a checkerboard pattern). Visual acuity was higher when measured with the optimal pVEP recording conditions, i.e., with the electrode at 2 mm and a vertical-stripe stimulus (0.530 ± 0.021 cycle/degree), than with OKR (0.455 ± 0.006 cycle/degree). Moreover, in murine eyes with optic nerve crush-induced low vision, OKR could not measure any visual acuity, while pVEPs allowed the reliable quantification of residual vision (0.064 ± 0.004 cycle/degree). Our results show that pVEPs allow more sensitive measurement of visual function than the OKR-based method. This technique should be particularly useful in mouse models of ocular disease and low vision.

Retained Plasticity and Substantial Recovery of Rod-Mediated Visual Acuity at the Visual Cortex in Blind Adult Mice with Retinal Dystrophy

In patients born blind with retinal dystrophies, understanding the critical periods of cortical plasticity is important for successful visual restoration. In this study, we sought to model childhood blindness and investigate the plasticity of visual pathways. To this end, we generated double-mutant (Pde6c^cpfl1/cpfl1Gnat1^IRD2/IRD2) mice with absent rod and cone photoreceptor function, and we evaluated their response for restoring rod (GNAT1) function through gene therapy. Despite the limited effectiveness of gene therapy in restoring visual acuity in patients with retinal dystrophy, visual acuity was, unexpectedly, successfully restored in the mice at the level of the primary visual cortex in this study. This success in visual restoration, defined by changes in the quantified optokinetic response and pattern visually evoked potential, was achieved regardless of the age at treatment (up to 16 months). In the contralateral visual cortex, cortical plasticity, tagged with light-triggered transcription of Arc, was also restored after the treatment in blind mice carrying an Arc promoter-driven reporter gene, dVenus. Our results demonstrate the remarkable plasticity of visual circuits for one of the two photoreceptor mechanisms in older as well as younger mice with congenital blindness due to retinal dystrophies.

The neuroprotective effect of hesperidin in NMDA-induced retinal injury acts by suppressing oxidative stress and excessive calpain activation

We found that hesperidin, a plant-derived bioflavonoid, may be a candidate agent for neuroprotective treatment in the retina, after screening 41 materials for anti-oxidative properties in a primary retinal cell culture under oxidative stress. We found that the intravitreal injection of hesperidin in mice prevented reductions in markers of the retinal ganglion cells (RGCs) and RGC death after N-methyl-D-aspartate (NMDA)-induced excitotoxicity. Hesperidin treatment also reduced calpain activation, reactive oxygen species generation and TNF-α gene expression. Finally, hesperidin treatment improved electrophysiological function, measured with visual evoked potential, and visual function, measured with optomotry. Thus, we found that hesperidin suppressed a number of cytotoxic factors associated with NMDA-induced cell death signaling, such as oxidative stress, over-activation of calpain, and inflammation, thereby protecting the RGCs in mice. Therefore, hesperidin may have potential as a therapeutic supplement for protecting the retina against the damage associated with excitotoxic injury, such as occurs in glaucoma and diabetic retinopathy.

Vision in laboratory rodents-Tools to measure it and implications for behavioral research

Mice and rats are nocturnal mammals and their vision is specialized for detection of motion and contrast in dim light conditions. These species possess a large proportion of UV-sensitive cones in their retinas and the majority of their optic nerve axons target superior colliculus rather than visual cortex. Therefore, it was a widely held belief that laboratory rodents hardly utilize vision during day-time behavior. This dogma is being questioned as accumulating evidence suggests that laboratory rodents are able to perform complex visual functions, such as perceiving subjective contours, and that declined vision may affect their performance in many behavioral tasks. For instance, genetic engineering may have unexpected consequences on vision as mouse models of Alzheimer's and Huntington's diseases have declined visual function. Rodent vision can be tested in numerous ways using operant training or reflex-based behavioral tasks, or alternatively using electrophysiological recordings. In this article, we will first provide a summary of visual system and explain its characteristics unique to rodents. Then, we present well-established techniques to test rodent vision, with an emphasis on pattern vision: visual water test, optomotor reflex test, pattern electroretinography and pattern visual evoked potentials. Finally, we highlight the importance of visual phenotyping in rodents. As the number of genetically engineered rodent models and volume of behavioral testing increase simultaneously, the possibility of visual dysfunctions needs to be addressed. Neglect in this matter potentially leads to crude biases in the field of neuroscience and beyond.

Time-Dependent Decline in Multifocal Electroretinogram Requires Faster Recording Procedures in Anesthetized Pigs

Purpose

The time-dependent effect of anesthetics on the retinal function is debated. We hypothesize that in anesthetized animals there is a time-dependent decline that requires optimized multifocal electroretinogram (mfERG) recording procedures.

Methods

Conventional and four-frame global-flash mfERG recordings were obtained approximately 15, 60, and 150 minutes after the induction of propofol anesthesia (20 pigs) and isoflurane anesthesia (nine pigs). In six of the propofol-anesthetized pigs, the mfERG recordings were split in 3-minute segments. Two to 4 weeks after initial recordings, an intraocular injection of tetrodotoxin (TTX) was given and the mfERG was rerecorded as described above. Data were analyzed using mixed models in SAS statistical software.

Results

Propofol significantly decreases the conventional and global-flash amplitudes over time. The only significant effect of isoflurane is a decrease in the global-flash amplitudes. At 15 minutes after TTX injection several of the mfERG amplitudes are significantly decreased. There is a linear correlation between the conventional P1 and the global-flash DR mfERG-amplitude (R² = 0.82, slope = 0.72, P < 0.0001). There is no significant difference between the 3-minute and the prolonged mfERG recordings for conventional amplitudes and the global-flash direct response. The global flash–induced component significantly decreases with prolonged mfERG recordings.

Conclusions

A 3-minute mfERG recording and a single stimulation protocol is sufficient in anesthetized pigs. Recordings should be obtained immediately after the induction of anesthesia. The effect of TTX is significant 15 minutes after injection, but is contaminated by the effect of anesthesia 90 minutes after injection. Therefore, the quality of mfERG recordings can be further improved by determining the necessary time-of-delay from intraocular injection of a drug to full effect.

Translational Relevance

General anesthesia is a possible source of error in mfERG recordings. Therefore, it is important to investigate the translational relevance of the results to mfERG recordings in children in general anesthesia.