Pattern ERG in rats following section of the optic nerve

TwinF interface inhibitor FP802 prevents retinal ganglion cell loss in a mouse model of amyotrophic lateral sclerosis

Motor neuron loss is well recognized in amyotrophic lateral sclerosis (ALS), but research on retinal ganglion cells (RGCs) is limited. Ocular symptoms are generally not considered classic ALS symptoms, although RGCs and spinal motor neurons share certain cell pathologies, including hallmark signs of glutamate neurotoxicity, which may be triggered by activation of extrasynaptic NMDA receptors (NMDARs). To explore potential novel strategies to prevent ALS-associated death of RGCs, we utilized inhibition of the TwinF interface, a new pharmacological principle that detoxifies extrasynaptic NMDARs by disrupting the NMDAR/TRPM4 death signaling complex. Using the ALS mouse model SOD1G93A, we found that the small molecule TwinF interface inhibitor FP802 prevents the loss of RGCs, improves pattern electroretinogram (pERG) performance, increases the retinal expression of Bdnf, and restores the retinal expression of the immediate early genes, Inhibin beta A and Npas4. Thus, FP802 not only prevents, as recently described, death of spinal motor neurons in SOD1G93A mice, but it also mitigates ALS-associated retinal damage. TwinF interface inhibitors have great potential for alleviating neuro-ophthalmologic symptoms in ALS patients and offer a promising new avenue for therapeutic intervention.

Correlations between Steady-State Pattern Electroretinogram and Humphrey Visual Field Analyzer Global Indices and Their Associations with Retinal Ganglion Cell Layer-Inner Plexiform Layer Thickness in Glaucoma Suspects

Purpose

The purpose of this study was to investigate the utility of steady state pattern electroretinogram (ss-PERG) in detecting retinal ganglion cell (RGC) dysfunction in glaucoma suspects (GS) who had normal 24-2 Humphrey Visual Fields (HFA).

Materials and Methods

This was a prospective cohort study of GS patients who were identified based on optic disc appearance with normal HFAs. Patients received a complete eye examination, standard automated perimetry (SAP), optical coherence tomography (OCT), and ss-PERG measurements. The ss-PERG parameters, Magnitude (Mag), Magnitude D (MagD), and MagD/Mag ratio, were examined, along with their relationships between HFA and OCT measurements.

Results

Twenty-five patients were included in this study, with a total of 49 eyes. Fifteen eyes had abnormal ss-PERG parameters and when compared to GS eyes with normal ss-PERG parameters, there were significant differences in HFA 24-2, retinal nerve fiber layer (RNFL) thickness, and ganglion cell layer and inner plexiform layer (GCL + IPL) thickness. All ss-PERG parameters were significantly correlated with 24-2 VF mean deviation (MD) and visual field index (VFI), as well as 10-2 VF MD after controlling for age, sex, intraocular pressure, central corneal thickness, and spherical equivalent. When controlled for age, spherical equivalent, and IOP, MagD/Mag ratio significantly contributed to the variance in average GCL + IPL thicknesses, whereas 24-2 VF MD and 10-2 VF MD did not. MagD/Mag ratio also significantly accounted for variance in all macular GCL + IPL sectors, while 10-2 VF MD did not.

Conclusions

ss-PERG has significant correlations with HFA global indices and was predictive of GCL + IPL thickness in GS patients. Clinical Significance. ss-PERG may serve as a useful functional tool for detecting and measuring RGC dysfunction in GS. It appears to be more sensitive than HFA in the detection of early changes in GCL + IPL thicknesses and may be helpful to use in conjunction with current diagnostic studies to improve the ability of monitoring GS progression.

Visual function in guinea pigs: behavior and electrophysiology

CLINICAL RELEVANCE

Guinea pig visual function is characterised based on behavioural and electrophysiological measures and retinal ganglion cell density is examined to further develop the guinea pig as a model of human ocular conditions.

BACKGROUND

Guinea pigs are an important model of human ocular conditions. Here, guinea pig spatial frequency discrimination, pattern and full-field photopic electroretinography (ERG), and retinal ganglion cell distribution were investigated.

METHODS

Adult guinea pigs (n = 6) were included. Optomotor responses to square-wave gratings from 0.3 to 2.4 cycles per degree (cpd) were assessed. Pattern ERG responses were recorded using square-wave gratings from 0.025 to 0.25 cpd at 100% contrast, alternating at a temporal frequency of 1.05 Hz. Full-field ERG responses were recorded using a 10.0 cd.s/m² flash. Ganglion cell density was determined histologically from retinal whole mounts.

RESULTS

Maximum spatial frequency discrimination was 1.65 ± 0.49 cpd for stimuli rotating temporally to nasally and 0.75 ± 0.16 cpd for stimuli rotating nasally to temporally. For pattern ERG, a maximum amplitude of 3.50 ± 1.16 µV for the first negative to positive peak (N1P1) was elicited with a 0.025 cpd grating, and 2.5 ± 0.1 µV for the positive to second negative peak (P1N2) was elicited with a 0.05 cpd grating. For full-field ERG, a-wave amplitude was 19.2 ± 4.24 µV, b-wave amplitude was 33.6 ± 8.22 µV, and the PhNR was 24.0 ± 5.72 µV. Peak retinal ganglion cell density was 1621 ± 129 cells/mm², located 1-2 mm superior to the optic nerve head.

CONCLUSION

Guinea pigs show directional selectivity for stimuli moving in the temporal to the nasal visual field. Guinea pigs demonstrate a quantifiable PhNR in the full-field ERG and negative and positive waveforms in the pattern ERG. The visual streak is located in the superior retina.

Ovariectomy worsens visual function after mild optic nerve crush in rodents

Glaucoma is the leading cause of irreversible blindness worldwide, and women represent roughly 60% of the affected population. Early menopause and estrogen signaling defects are risk factors for glaucoma. Recently, we found that surgical menopause exacerbated visual dysfunction in an ocular hypertension model of glaucoma. Here, we investigated if surgical menopause exacerbated visual dysfunction in a model of direct retinal ganglion cell (RGC) damage via optic nerve crush (ONC). Female Long Evans rats (n = 12) underwent ovariectomy (OVX) to induce surgical menopause or Sham surgery. Eight weeks post-surgery, baseline visual function was assessed via optomotor response. Afterwards, rats underwent monocular ONC. Visual function was assessed at 4, 8, and 12 weeks post-ONC. At 12 weeks, retinal function via electroretinography and retinal nerve fiber layer (RNFL) thickness via optical coherence tomography were measured. Visual acuity was reduced after ONC (p < 0.001), with surgical menopausal animals having 31.7% lower visual acuity than Sham animals at 12 weeks (p = 0.01). RNFL thinning (p < 0.0001) and decreased RGC function (p = 0.0016) occurred at 12 weeks in ONC groups. Surgical menopause worsens visual acuity after direct RGC damage using an ONC model. This demonstrates that surgical menopause plays a role in visual function after injury.

Retinal ganglion cell ablation in guinea pigs

Guinea pigs are a common model of human ocular conditions; however, their visual function has not been fully characterized. The purpose of this study was to determine the contributions of retinal ganglion cells to structural and functional measures in guinea pigs. Healthy adult guinea pigs (n = 12) underwent unilateral optic nerve crush. Retinal structure was assessed with spectral domain optical coherence tomography (OCT), and thickness of the ganglion cell/nerve fiber layer (GC/NFL) was determined. Visual function was assessed with optomotor tracking of a drifting grating and light adapted electroretinograms (ERGs). From flash ERGs, a-wave, b-wave, oscillatory potentials (OPs), and photopic negative response (PhNR) were analyzed. From pattern ERGs, N1P1 and P1N2 were analyzed. Histological studies were done at various time points for ganglion cell quantification. Optomotor tracking was absent in optic nerve crush eyes following optic nerve crush. Significant thinning of the GC/NFL was evident four weeks following the crush. Flash ERGs revealed a significant reduction in the OP1 amplitude two weeks following crush (P < 0.01) and in the PhNR amplitude six weeks following crush (P < 0.01). There were no significant changes in a-wave, b-wave, or pattern ERG responses (P > 0.05 for all). In vivo OCT imaging showed progressive thinning of inner retinal layers. Ganglion cell density, quantified histologically, was significantly reduced by 75% in the optic nerve crush eye compared to the control eye at four weeks following crush. These findings indicate that retinal ganglion cells contribute to the PhNR and OP1 components of the full field flash ERG, but not significantly to the pattern ERG in guinea pigs. This study demonstrates that OCT imaging and full field flash ERGs are valuable in assessing retinal ganglion cell loss in vivo in guinea pigs and will help to further establish the guinea pig as a model of human ocular pathologies.

Inducible rodent models of glaucoma

Glaucoma is one of the leading causes of vision impairment worldwide. In order to further understand the molecular pathobiology of this disease and to develop better therapies, clinically relevant animal models are necessary. In recent years, both the rat and mouse have become popular models in glaucoma research. Key reasons are: many important biological similarities shared among rodent eyes and the human eye; development of improved methods to induce glaucoma and to evaluate glaucomatous damage; availability of genetic tools in the mouse; as well as the relatively low cost of rodent studies. Commonly studied rat and mouse glaucoma models include intraocular pressure (IOP)-dependent and pressure-independent models. The pressure-dependent models address the most important risk factor of elevated IOP, whereas the pressure-independent models assess "normal tension" glaucoma and other "non-IOP" related factors associated with glaucomatous damage. The current article provides descriptions of these models, their characterizations, specific techniques to induce glaucoma, mechanisms of injury, advantages, and limitations.

The pattern ERG in chicks - Stimulus dependence and optic nerve section

The chick is widely used in studies of eye growth regulation and myopia. The aim of this study was to explore the utility of pattern (p)ERG as a tool to assess retinal function in such studies. Effects of optical defocus and diffusing blur, manipulations used to alter eye growth experimentally, were evaluated. PERGs were recorded from White-Leghorn chickens, using a checkerboard pattern, including 8 spatial frequencies (0.05-2.2c/d SF), 13 contrast levels (1-100%), and 8 temporal reversal frequencies (0.5-20Hz). The acute effects of defocus and diffusing blur were examined. Flash- and pERGs were also recorded from chicks that underwent monocular optic nerve section (ONS), to explore the contribution of retinal ganglion cells (RGCs). Measurements were made up to 6weeks post-ONS, complemented with SD-OCT imaging. In normal chicks, the response to 1Hz, 100% contrast stimuli showed positive- and negative-going waveforms at 43ms (P1) and 75ms (N95), respectively, with 0.06-0.1c/d SF eliciting the largest P1 amplitudes of 21.9±2.5μV. Contrast levels above 5% yielded measurable P1 responses. Responses were transient and monophasic for 0.5-5Hzreversal rates, with higher temporal frequencies yielding steady state responses. Defocus and diffusing blur decreased pERG amplitude across all SFs. pERG responses remained normal after ONS, despite the loss of RGCs. In conclusion, chicks show robust pERG responses, which are attenuated by defocus and diffusing blur. The pERG response is not affected by ONS, suggesting that RGCs do not contribute to the chick pERG.

[Protective effects of Ginkgo biloba extract on morphology and function of retinal ganglion cells after optic nerve transection in guinea pigs]

OBJECTIVE

To investigate the effects of Ginkgo biloba extract (EGb 761) on the morphology and function of retinal ganglion cells (RGC) in guinea pigs with optic nerve transection.

METHODS

Seventy-five albino guinea pigs were randomly divided into five groups: normal control group, sham-operated group, untreated group, normal saline group and EGb 761 group. No operation was performed in the normal control group. Optic nerve was merely exposed in the sham-operated group, but transected at 1.0 mm from posterior pole of the eye ball in the untreated, normal saline and EGb 761 groups. Guinea pigs in the EGb 761 group or the normal saline group received daily intraperitoneal injection of EGb 761 (100 mg/kg) or corresponding volume of normal saline from 7 days before experiment to 28 days after experiment. Three guinea pigs in each group were sacrificed for apoptosis assay (TUNEL method) of RGC. Pattern electoretinograms (PERGs) were recorded 14 and 28 days after transection, respectively. At the end of the examination, six guinea pigs were killed for histological examination and RGC count.

RESULTS

No TUNEL-positive cells were observed in the normal control, sham-operated and EGb 761 groups, but there were TUNEL-positive cells in the untreated group and the normal saline group. The numbers of RGCs in the untreated and normal saline groups were less than those in the normal control and sham-operated groups at 14 days or 28 days (P<0.05). Although the number of RGCs in the EGb 761 group was less than those in the normal control and sham-operated groups (P<0.05), it was more than those in the untreated and normal saline groups (P<0.05). N(95) amplitude in EGb 761 group was higher than those in the untreated and normal saline groups (P<0.05) and close to those in the normal control and sham-operated groups (P>0.05) at 14 days or 28 days. The number of RGCs was positive correlated to N(95) amplitude (r=0.859, P=0.001 5).

CONCLUSION

EGb 761 can inhibit the apoptosis of RGCs in guinea pigs after optic nerve transection, thus protect the morphology and function of RGCs.

Electrophysiological assessment of retinal ganglion cell function

The function of retinal ganglion cells (RGCs) can be non-invasively assessed in experimental and genetic models of glaucoma by means of variants of the ERG technique that emphasize the activity of inner retina neurons. The best understood technique is the Pattern Electroretinogram (PERG) in response to contrast-reversing gratings or checkerboards, which selectively depends on the presence of functional RGCs. In glaucoma models, the PERG can be altered before histological loss of RGCs; PERG alterations may be either reversed with moderate IOP lowering or exacerbated with moderate IOP elevation. Under particular luminance-stimulus conditions, the Flash-ERG displays components that may reflect electrical activity originating in the proximal retina and be altered in some experimental glaucoma models (positive Scotopic Threshold response, pSTR; negative Scotopic Threshold Response, nSTR; Photopic Negative Response, PhNR; Oscillatory Potentials, OPs; multifocal ERG, mfERG). It is not yet known which of these components is most sensitive to glaucomatous damage. Electrophysiological assessment of RGC function appears to be a necessary outcome measure in experimental glaucoma models, which complements structural assessment and may even predict it. Neuroprotective strategies could be tested based on enhancement of baseline electrophysiological function that results in improved RGC survival. The use of electrophysiology in glaucoma models may be facilitated by specifically designed instruments that allow high throughput, robust assessment of electrophysiological function.

The bioelectric field of the pattern electroretinogram in the mouse

PURPOSE

To compare the bioelectric field associated with the pattern electroretinogram (PERG) with that of the flash electroretinogram (FERG) in the mouse.

METHODS

PERGs and FERGs were recorded from each eye in 32 C57BL/6J mice using corneal silver loops referenced to a subcutaneous needle on the back of the head. PERG stimuli were horizontal gratings of 0.05 cycles per degree and 98% contrast reversing 2 times per second. Light-adapted FERG stimuli were bright strobe flashes. Stimuli were presented either monocularly or binocularly. In some experiments, TTX was injected in one eye and saline in the contralateral eye.

RESULTS

The PERG recorded from the contralateral, occluded eye had slightly larger amplitude (1.14 ×, P < 0.01) and longer latency (+1.57 ms, P < 0.01) compared with the ipsilateral eye. Under binocular stimulation, the PERG amplitude was much larger (1.67 ×, P < 0.01) than the monocular amplitude. TTX injected in the stimulated eye drastically reduced the PERG in both eyes. Monocular FERGs were recordable from the stimulated eye only and were moderately reduced by TTX. Binocular and monocular FERGs had similar amplitudes.

CONCLUSIONS

PERG and FERG generate different bioelectric fields in the mouse. The PERG bioelectric field is consistent with a dipole model whose axis is orthogonal to the eye axis, whereas the standard dipole model for the FERG is coaxial. Possible sources of the PERG bioelectric field are unmyelinated optic nerve axons adjacent to the sclera. Results provide new insights on the generators of the PERG signal and its alterations in mouse models of glaucoma and optic nerve diseases.

Retinal pathway origins of the pattern electroretinogram (PERG)

PURPOSE

To determine retinal pathway origins of pattern electroretinogram (PERG) in macaque monkeys using pharmacologic dissections, uniform-field flashes, and PERG simulations.

METHODS

Transient (2 Hz, 4 reversals/s) and steady state (8.3 Hz, 16.6 reversals/s) PERGs and uniform-field ERGs were recorded before and after intravitreal injections of L-AP4 (not APB) (2-amino-4-phosphonobutyric acid, 1.6-2.0 mM), to prevent ON pathway responses; PDA (cis-2,3-piperidinedicarboxylic acid, 3.3-3.8 mM), to block activity of hyperpolarizing second- and all third-order retinal neurons; and TTX (tetrodotoxin, 6 μM), to block Na+-dependent spiking. PERGs were also recorded from macaques with advanced unilateral experimental glaucoma, and were simulated by averaging ON and OFF responses to uniform-field flashes.

RESULTS

For 2-Hz stimulation, L-AP4 reduced both negative- and positive-going (N95 and P50) amplitudes in transient PERGs, and their counterparts, N2 and P1 in simulations, to half-amplitude. PDA eliminated N95 and N2, but increased P50 and P1 amplitudes, in that it enhanced b-waves. As previously reported, severe experimental glaucoma or TTX eliminated photopic negative responses, N95, and N2; glaucoma eliminated P50 and reduced P1 amplitude; TTX reduced P50 and hardly altered P1. For 8.3-Hz stimulation, L-AP4 eliminated the steady state PERG and reduced simulated PERG amplitude, whereas PDA enhanced both responses. TTX reduced PERG amplitude to less than half; simulations were less reduced. Blockade of all postreceptoral activity eliminated transient and steady state PERGs, but left small residual P1 in simulations.

CONCLUSIONS

Transient PERG receives nearly equal amplitude contributions from ON and OFF pathways. N95 reflects spiking activity of ganglion cells; P50 reflects nonspiking activity as well. Steady state PERG, in contrast, reflects mainly spike-related ON pathway activity.

Head-up tilt lowers IOP and improves RGC dysfunction in glaucomatous DBA/2J mice

The inbred DBA/2J (D2) mouse strain is a well established model of spontaneously elevated intraocular pressure (IOP), progressive glaucomatous loss of retinal ganglion cells (RGCs), and early damage of RGC axons at the level of optic nerve head. Pattern electroretinogram (PERG) studies have shown that surviving RGCs in mice 6-12-month-old may be dysfunctional. RGC dysfunction seems to be IOP-dependent, since it may be exacerbated by means of acute IOP elevation with head-down body tilt. Here we test the hypothesis that head-up body posture lowers IOP, resulting in improvement of PERG amplitude in aged D2 mice with glaucoma. We show that head-up body tilt induces age-independent IOP lowering whose magnitude increases with the angle of tilt. For a fixed angle (-60 degrees ) of head-up tilt, IOP progressively decreases with a time constant of about 5 min and stabilizes at a value lower by about 5-6 mm Hg compared to the baseline. Head-up tilt also results in an improvement of PERG amplitude in older D2 mice with glaucoma but not in younger D2 mice without glaucoma. Improvement of PERG amplitude in aged D2 mice upon head-up-induced IOP lowering is consistent with the idea that RGCs undergo a stage of IOP-dependent, reversible dysfunction before death. The head-up IOP/PERG protocol may represent a non-invasive way to probe the potential for recovery of RGC dysfunction in D2 mice.

Setting the pace for retinal development: environmental enrichment acts through insulin-like growth factor 1 and brain-derived neurotrophic factor

Environmental enrichment strongly affects visual system maturation both at retinal and cortical levels. Which molecular pathways are activated by an enriched environment (EE) to regulate visual system development has not been clarified. Here, we show that early [postnatal day 1 (P1) to P7] insulin-like growth factor 1 (IGF-1) injections in the eyes of non-EE rat pups mimic EE effects both in increasing BDNF levels in the retinal ganglion cell layer at P10 and in determining a more adult-like retinal acuity, assessed with pattern electroretinogram at P25. Blocking IGF-1 action in EE animals during the same early postnatal time window by injecting the IGF-1 receptor antagonist JB1 prevents EE effects both on BDNF expression and on retinal acuity maturation. Reducing BDNF expression in the retina of IGF-1-treated rats prevents IGF-1 effects on retinal acuity development. Finally, we show that tyrosine hydroxylase (TH) expression is increased in the retina of P10 EE and IGF-1-treated rats and that blocking TH expression in EE animals prevents EE from affecting retinal acuity development. Thus, early levels of IGF-1 play a key role in mediating EE effects on retinal development through an action that requires BDNF and involves dopaminergic amacrine cell network.

Prolonged illumination up-regulates arrestin and two guanylate cyclase activating proteins: a novel mechanism for light adaptation

Light adaptation in vertebrate photoreceptors is mediated by multiple mechanisms, one of which could involve nuclear feedback and changes in gene expression. Therefore, we have investigated light adaptation-associated changes in gene expression using microarrays and real-time PCR in isolated photoreceptors, in cultured isolated retinas and in acutely isolated retinas. In all three preparations after 2 h of an exposure to a bright light, we observed an up-regulation of almost 100% of three genes, Sag, Guca1a and Guca1b, coding for proteins known to play a major role in phototransduction: arrestin, GCAP1 and GCAP2. No detectable up-regulation occurred for light exposures of less than 1 h. Functional in vivo electroretinographic tests show that a partial recovery of the dark current occurred 1-2 h after prolonged illumination with a steady light that initially caused a substantial suppression of the photoresponse. These observations demonstrate that prolonged illumination results in the up-regulation of genes coding for proteins involved in the phototransduction signalling cascade, possibly underlying a novel component of light adaptation occurring 1-2 h after the onset of a steady bright light.

Retinal pathway origins of the pattern ERG of the mouse

This study investigated contributions from the retinal On and Off pathways, and the spiking and nonspiking activity of neurons in those pathways to the pattern ERG of the mouse. Light-adapted pattern and ganzfeld ERGs were recorded from anesthetized C57BL/6 mice 3-4 months of age. Recordings were made before and after intravitreal injections of PDA (cis-2,3-piperidine-dicarboxylic acid) to block transmission to hyperpolarizing 2nd order and all 3rd order neurons, TTX (tetrodotoxin) to block Na(+)-dependent spiking, APB (2-amino-4-phosphonobutyric acid) to block synapses between photoreceptors and ON-bipolar cells, and APB + TTX and PDA + TTX cocktails. The pattern stimuli consisted of 0.05 cy/deg gratings reversing in contrast at 1 Hz, presented at various contrasts (50-90%) and a rod saturating mean luminance. For flash ERGs, brief green ganzfeld flashes were presented on a rod-suppressing green background. Recordings were made 39-42 days after unilateral optic nerve crush (ONC) in a subset of animals in which ganglion cell degeneration was subsequently confirmed in retinal sections. Pattern ERGs were similar in waveform for all contrasts, with a positive wave (P1) peak for 90% contrast around 60 ms on average and maximum trough for a negative wave (N2) around 132 ms after each contrast reversal; amplitudes were greatest for 90% contrast which became the standard stimulus. ONC eliminated or nearly eliminated the pattern ERG but did not affect the major waves of the flash ERG. PDA and TTX both delayed P1 and N2 waves of the pattern ERG, and reduced their amplitudes, with effects of PDA on N2 greater than those of TTX. In the flash ERG, PDA reduced a-wave amplitudes, removed OPs but hardly affected b-wave amplitudes. In contrast, TTX reduced b-wave amplitudes substantially, as previously observed in rat. APB removed P1 of the pattern ERG, but left a negative wave of similar timing and amplitude to N2. In the flash ERG, APB removed the b-wave, producing a negative ERG. Addition of TTX to the APB injection removed most of N2 of the pattern ERG, while other waves of the pattern and flash ERG resembled those after APB alone. Addition of TTX to the PDA injection had little effect on the pattern ERG beyond that of PDA alone, but it prolonged the b-wave of the flash ERG. In conclusion, this study confirmed that a selective lesion of ganglion cells will practically eliminate the pattern ERG. The study also showed that P1 of the mouse pattern ERG is dominated by contributions, mainly spiking, from ON pathway neurons, whereas N2 reflects substantial spiking activity from the OFF pathway as well as nonspiking contributions from both pathways.

Longitudinal evaluation of retinal ganglion cell function and IOP in the DBA/2J mouse model of glaucoma

PURPOSE

To characterize progressive changes of retinal ganglion cell (RGC) function and intraocular pressure (IOP) in the DBA/2J mouse model of spontaneous glaucoma.

METHODS

Serial pattern electroretinograms (PERGs) and IOPs measures were obtained from both eyes of 32 anesthetized DBA/2J mice over an age range of 2 to 12 months at 1-month intervals. Cone-driven flash-ERGs (FERGs) were also recorded. The endpoint was defined as the age at which the PERG amplitude reached the noise level in at least one eye. At that point, both eyes were histologically processed to evaluate the thickness of the retinal fiber layer (RNFL).

RESULTS

IOP increased moderately between 2 and 6 months ( approximately 14-17 mm Hg) and then more steeply, until it leveled off at approximately 28 mm Hg by 9 to 11 months. The mean PERG amplitude decreased progressively after 3 months of age to reach the noise level (85% reduction of normal amplitude) at approximately 9 to 12 months in different animals. When the PERG was at noise level, the RNFL showed a relatively smaller reduction (40%) in normal thickness. The FERG displayed minor changes throughout the observation period. IOP and PERG changes were highly correlated (r(2) = 0.51, P < 0.001).

CONCLUSIONS

Results indicate that inner retina function in DBA/2J mice progressively decreases after 3 months of age, and it is nearly abolished by 10 to 11 months, whereas outer retina function shows little change and the RNFL thickness is relatively spared. This result suggests that surviving RGCs may not be functional. Progression of inner retinal dysfunction is strongly associated with increased IOP.

Rodent models for glaucoma retinopathy and optic neuropathy

Animal models are useful to elucidate the etiology and pathology of glaucoma and to develop novel and more effective therapies for the disease. Because of the substantial similarities between the rodent and primate eyes, and the advances of relevant study techniques, rat and mouse models of glaucoma have recently become popular as research tools. This review surveys research techniques used in the measurement of rodent intraocular pressure, and also the evaluation of pertinent morphologic, biochemical, and functional changes in the retina, optic nerve head, and optic nerve. This review further describes in detail the individual rodent models, some of which serve as surrogate models and do not entail ocular hypertension, whereas others involve transient or chronic increases of intraocular pressure. The technical considerations and theoretical concerns of these models, their advantages, and limitations, are also discussed.

The mouse pattern electroretinogram

Mouse models of optic nerve disease such as glaucoma, optic neuritis, ischemic optic neuropathy, and mitochondrial optic neuropathy are being developed at increasing rate to investigate specific pathophysiological mechanisms and the effect of neuroprotective treatments. The use of these models may be greatly enhanced by the availability of non-invasive methods able to monitor retinal ganglion cell (RGC) function longitudinally such as the Pattern Electroretinogram (PERG). While the use of the PERG as a tool to probe inner retina function in mammals is known since 25 years, relatively less information is available for the mouse. Here, the PERG technique and the main applications in the mouse are reviewed.

Retinal functional development is sensitive to environmental enrichment: a role for BDNF

Retina has long been considered less plastic than cortex or hippocampus, the very sites of experience-dependent plasticity. Now, we show that retinal development is responsive to the experience provided by an enriched environment (EE): the maturation of retinal acuity, which is a sensitive index of retinal circuitry development, is strongly accelerated in EE rats. This effect is present also in rats exposed to EE up to P10, that is before eye opening, suggesting that factors sufficient to trigger retinal acuity development are affected by EE during the first days of life. Brain derived neurotrophic factor (BDNF) is precociously expressed in the ganglion cell layer of EE with respect to non-EE rats and reduction of BDNF expression in EE animals counteracts EE effects on retinal acuity. Thus, EE controls the development of retinal circuitry, and this action depends on retinal BDNF expression.

Presence of calpain-induced proteolysis in retinal degeneration and dysfunction in a rat model of acute ocular hypertension

The purpose of this study was to determine if calpain-induced proteolysis was associated with retinal degeneration or dysfunction in the rat acute ocular hypertensive model. Acute glaucoma was produced by elevation of IOP to 120 mm Hg for 1 hr. Retinal degeneration was evaluated by H&E staining and apoptosis was determined by TUNEL staining in histologic sections of retina. Electroretinogram (ERG) was carried out to evaluate changes in functionality. Activation of calpains was determined by casein zymography and immunoblotting. Total calcium in retina was measured by atomic absorption spectrophotometry. Proteolysis of alpha-spectrin, tau, cdk5, and p35 (a regulator of cdk5) were evaluated by immunoblotting. The thickness of inner plexiform layer (IPL) and inner nuclear layer (INL), and the number of cells in the ganglion cell layer (GCL) decreased after ocular hypertension. Numerous cells in the INL stained positive for TUNEL and some cells in the outer nuclear layer (ONL) showed TUNEL staining. The a-wave in ERG was temporarily decreased after ocular hypertension and then recovered to normal. In contrast, the b-wave was completely lost. Calpains were activated after ocular hypertension. Activation of calpains was associated with increased calcium in retina. Calpain-dependent proteolysis of alpha-spectrin, tau, and p35 were observed in retina after ocular hypertension. The results suggested that increased calcium and subsequent proteolysis by activated calpains was associated with the death of inner retinal cells due to acute ocular hypertension in the rat model. Calpain inhibitors may be candidate drugs for treatment of retinal degeneration and dysfunction resulting from glaucoma.

Development of inner retinal function, evidenced by the pattern electroretinogram, in the rat

Though the rat is increasingly used as an animal model in ophthalmic research, including the study of glaucoma, little is known about age-related changes in its inner retinal function. The aim of this study was to evaluate these changes in the rat during the first 18 weeks of life. The pattern electroretinogram (PERG) was used to monitor inner retinal activity in 16 developing rats. In each animal, recordings were conducted at ages 3, 5, 7, 11, 14 and 18 weeks to assess age-related changes in function. Signals were evoked by five stimuli of progressively increasing check width (subtending 82-1312 arc minutes of visual angle) that were projected directly onto the fundus through a specially modified ophthalmoscope which allowed visual and manual control of stimulus quality. Poor signal:noise ratio prevented signal analysis at age 3 weeks. Subsequently, PERG amplitude increased significantly, up to 242% (depending on stimulus check width), during weeks 5-11. After peaking at 11 weeks, signal amplitude declined moderately. Signal latency mirrored that of amplitude, decreasing during the first 11 weeks, and then increasing steadily. Latency was not affected by stimulus check width. Age was highly correlated with P1 latency (R(2)=0.80) and moderately correlated with N2 latency (R(2)=0.52). Therefore, we propose that studies of inner retinal diseases (such as glaucoma) in the rat model should use age-matched controls, as electrophysiological results may be confounded by age-related changes. The rat PERG undergoes many of the age-related changes that have been reported in humans, and thus may serve as an animal model to study development of inner retinal function.

Spatial contrast sensitivity of birds

Contrast sensitivity (CS) is the ability of the observer to discriminate between adjacent stimuli on the basis of their differences in relative luminosity (contrast) rather than their absolute luminances. In previous studies, using a narrow range of species, birds have been reported to have low contrast detection thresholds relative to mammals and fishes. This was an unexpected finding because birds had been traditionally reported to have excellent visual acuity and color vision. This study reports CS in six species of birds that represent a range of visual adaptations to varying environments. The species studied were American kestrels (Falco sparverius), barn owls (Tyto alba), Japanese quail (Coturnix coturnix japonica), white Carneaux pigeons (Columba livia), starlings (Sturnus vulgaris), and red-bellied woodpeckers (Melanerpes carolinus). Contrast sensitivity functions (CSFs) were obtained from these birds using the pattern electroretinogram and compared with CSFs from the literature when possible. All of these species exhibited low CS relative to humans and most mammals, which suggests that low CS is a general characteristic of birds. Their low maximum CS may represent a trade-off of contrast detection for some other ecologically vital capacity such as UV detection or other aspects of their unique color vision.

Retinal ganglion cell activity from the multifocal electroretinogram in pig: optic nerve section, anaesthesia and intravitreal tetrodotoxin

Non-invasive recordings of the retinal activity have an important role to play in the diagnosis of retinal pathologies. The detection of diseases that involve retinal ganglion cells (RGCs), such as optic atrophy and glaucoma, may be improved by isolating the RGC contribution from the multifocal electroretinogram (mfERG). In this study, mfERGs were performed on 20 pigs, 1-6 weeks following unilateral retrobulbar optic nerve section (ONS). The stimuli were 103 non-scaled high-contrast hexagons from which summed and individual mfERG responses were obtained in experimental and control fellow eyes under conditions of ketamine (n = 11) or isoflurane anaesthesia (n = 9). The effect of intravitreal injection of tetrodotoxin (TTX; n = 6) was also investigated. The summed mfERG responses showed a first positive peak (P1) with a short latency (21 ms) followed by two smaller peaks (P2 and P3) of longer latency (46 and 65 ms, respectively). While P2 and P3 amplitude were highly correlated with the time post-optic nerve section (ONS) (P2: r(2) = 0.669; P = 0.007; P3: r(2) = 0.651; P = 0.005), P1 was not (r(2) = 0.193; P = 0.38). P1 and P2 showed no implicit time variation as a function of retinal location, while P3 implicit time varied along the axis of the visual streak, generating a naso-temporal asymmetry. However, the P3 implicit time did not vary consistently with distance away from the optic nerve head. Intravitreal injections of TTX reduced P2 and P3 in the control eyes, consistent with the effect of ONS, and also induced a series of regular oscillations lasting up to 200 ms post stimulus. Under isoflurane anaesthesia, all components of the mfERG ifn experimental and control eyes were, at all time points post-ONS, of similar amplitude and without naso-temporal asymmetry, suggesting a reduced participation of RGCs under these anaesthesic conditions. These data clearly demonstrate that it is possible to isolate the RGC contribution from non-invasive multifocal electroretinography.

Pattern electroretinography in a rat model of ocular hypertension: functional evidence for early detection of inner retinal damage

With the increasing use of the rat as an animal model for glaucoma and for the evaluation of neuroprotective treatments, there is a need for a sensitive test of retinal ganglion cell (RGC) function in this species. The aims of this study were to detect functional abnormalities of the inner retina in a rat model of high intraocular pressure (IOP) using the pattern electroretinogram (PERG), and to correlate them with morphometric analysis of RGC survival and the functional integrity of the inner retina. Unilateral ocular hypertension was induced in 17 Lewis rats through laser photocoagulation. Pattern ERGs were recorded prior to lasering and 3 weeks later, using a series of shifting patterns of decreasing spatial frequency projected directly onto the animals' fundus. IOP was measured at the same intervals, and the number of surviving RGCs estimated. Low amplitude PERG signals could be recorded in response to a narrow grating of 0.368 cycles per degree (cpd), and increased with stimulus size. Lasering caused mean (+/-s.d.) IOP to increase significantly from 18.3+/-4.5 (baseline) to 29.8+/-8.8 mmHg within 3 weeks (p<0.0001). At this time, PERG amplitudes were significantly reduced (p<0.05), declining an average of 45% compared to the normotensive, control eyes. No outer retinal damage was observed, but the mean number of RGCs decreased significantly (p<0.001), from 2 525.0+/-372.4 to 1 542.8+/-333.8 cells per mm2. This decrease in RGC number was significantly (p=0.03) correlated the decrease in PERG amplitude. The correlation between functional integrity of the inner retina and the rat PERG was further demonstrated by intravitreal tetrodotoxin injections, which temporarily abolished the PERG but did not affect outer retinal activity, reflected in the flash ERG. The evidence for early functional deficits, combined with tonometry and documentation of correlated ganglion cells loss, confirms the sensitivity of this diagnostic tool and the validity and importance of this animal model in glaucoma research.

T-cell-based vaccination for morphological and functional neuroprotection in a rat model of chronically elevated intraocular pressure

Acute or chronic glaucoma is often associated with an increase in intraocular pressure (IOP). In many patients, however, therapeutic pressure reduction does not halt disease progression. Neuroprotection has been proposed as a complementary therapeutic approach. We previously demonstrated effective T-cell-based neuroprotection in experimental animals vaccinated with the synthetic copolymer glatiramer acetate (copolymer-1, Cop-1), a weak agonist of self-antigens. This study was undertaken to test different routes and modes of vaccination with Cop-1 as treatment modalities for protection against retinal ganglion cell (RGC) death caused by chronic elevation of IOP in rats, and to determine whether anatomical neuroprotection is accompanied by functional neuroprotection. In a chronic model of unilaterally high IOP, Cop-1 vaccination, with or without an adjuvant, protected rats against IOP-induced loss of RGCs by eliciting a systemic T-cell-mediated response capable of cross-reacting with self-antigens residing in the eye. In rats deprived of T cells, Cop-1 (unlike treatment with alpha2-adrenoreceptor agonists) was not protective of RGCs, substantiating the contention that its beneficial effect is not conferred directly but is T-cell-mediated. Pattern electroretinography provided evidence of functional protection. Thus, vaccination with adjuvant-free Cop-1 can protect RGCs from the consequences of elevated IOP in rats. This protection is manifested both morphologically and functionally. These findings can be readily implemented for the development of a therapeutic vaccination to arrest the progression of glaucoma.

Habituation of retinal ganglion cell activity in response to steady state pattern visual stimuli in normal subjects

PURPOSE

To evaluate autoregulatory changes of retinal ganglion cell (RGC) activity, as measured by the pattern electroretinogram (PERG), when the eye is exposed to a steady state presentation of stimuli that maximize PERG amplitude and blood flow.

METHODS

The PERG was recorded from both eyes of 14 normal subjects in response to steady state presentation (4 minutes) of contrast-reversing (16.28/s) gratings (1.6 cyc/deg) with different contrast (12%-99%) and mean luminance (40-1.3 cd/m(2)). One temporal period of the stimulus (122.8 ms) was sampled and averaged in packets of 50 sweeps ( approximately 15 seconds each). PERG amplitude and phase were evaluated by Discrete Fourier Transform and displayed as a function of time. Data were fitted with an exponential decay function to evaluate PERG changes with time.

RESULTS

For patterns of 99% contrast, the PERG amplitude progressively decreased with time until reaching a plateau approximately 30% lower than the initial amplitude after approximately 2 minutes (habituation). The ratio between initial and plateau amplitude did not change by reducing the stimulus luminance by 1 log unit. However, reducing contrast decreased amplitude habituation. The habituation was abolished at 25% contrast.

CONCLUSIONS

Decrease of PERG amplitude with time is consistent with a slow adaptive change of RGC activity in response to high-contrast, steady state stimuli. The authors propose that the initial amplitude represents an index of RGC activity, and the plateau amplitude represents a dynamic equilibrium between RGC activity and the available energy supply. These results are relevant for a better understanding of glaucomatous optic neuropathy.

Restoration of retinal ganglion cell function in early glaucoma after intraocular pressure reduction: a pilot study

PURPOSE

To investigate the changes of pattern electroretinogram (PERG) after intraocular pressure lowering in glaucoma patients and normal controls.

DESIGN

Interventional retrospective cross-sectional study.

PARTICIPANTS

Twenty-five patients (49 eyes) with ocular hypertension or glaucoma undergoing topical treatment to lower IOP served as a study group; 22 patients (44 eyes) with ocular hypertension or glaucoma observed without treatment served as a control group for treated glaucoma patients; 9 normal subjects (18 eyes) receiving a 250-mg acetazolamide tablet served as a second study group; and 17 normal subjects (34 eyes) from a previous study served as a second control group for treated normal subjects.

METHOD

Pattern electroretinograms were recorded simultaneously from both eyes using skin electrodes and automated analysis. Visual field (VF) analyses were performed with white-on-white standard automated perimetry (SAP). Intraocular pressure was measured with Goldmann applanation tonometry; central corneal thickness was measured with pachymetry.

MAIN OUTCOME MEASURES

Pattern electroretinogram amplitude (microvolts), phase (pi rads), and test-retest variability (test 2-to-test 1 ratio, in decibels), SAP mean deviation (decibels), and IOP (millimeters of mercury).

RESULTS

In 56% of right eyes and 21% of left eyes of the treated glaucoma subgroup, the PERG amplitude and/or phase improved beyond the 95% confidence intervals of the test-retest variability of the untreated glaucoma control group. Pattern electroretinogram improvement with IOP lowering occurred in both high- and low-tension glaucoma eyes. Eyes with severely impaired VFs showed little improvement in PERG; however, eyes of normal subjects treated with acetazolamide did not show significant PERG changes relative to the test-retest variability of normal controls.

CONCLUSIONS

Retinal ganglion cell function can be at least partially restored after IOP reduction in glaucomatous eyes with early VF impairment.

Electroretinogram changes associated with retinal upregulation of trophic factors: observations following optic nerve section

The purpose of the present work was to assess whether upregulation of trophic factors and protection from damage induced in the retina by optic nerve section are associated with changes in the flash electroretinogram (ERG). We have examined the ERG in adult pigmented rat at different survival times over a period of 3 months following section of the optic nerve. The a-wave was analyzed using the Lamb-Pugh model and the parameters of best fit were estimated in control animals and at successive survival times. The amplitudes of the a- and b-waves were reduced over the first 7 days after nerve section. The a-wave recovered its relative amplitude by 21 days, but the b-wave remained depressed 5 weeks following nerve section. Analysis of the a-wave indicated a 20-30% reduction in the dark current of sectioned eyes at 7 days survival. A significant reduction of the amplification constant was observed in both nerve-sectioned and nerve-intact eyes, relative to normal and sham-operated controls. This reduction persisted to the longest survival time examined. The reduction of the a-wave at 7 days after nerve section coincides with a period of upregulation of ciliary nerve trophic factor. The amplification factor is influenced over a longer time course, which corresponds with a period of up-regulation of basic fibroblast growth factor. These changes in growth factor expression and ERG parameters are in turn associated with protection of photoreceptors against light damage. Present results suggest that the sensitivity of the retina to light may be regulated by mechanisms which protect photoreceptors against stress.

Ganglion cell contributions to the rat full-field electroretinogram

The purpose of this study was to determine what contributions are made to the rat full-field electroretinogram (ERG) by ganglion cells (GCs). To that end, the ERG was assessed longitudinally following optic nerve transection (ONTx). Additional studies were conducted using intravitreal injections of pharmacologically active substances. The ERG was recorded simultaneously from both eyes of anaesthetized adult Brown-Norway rats (ketamine: xylazine: acepromazine, 55: 5: 1 mg kg(-1)) using custom silver chloride electrodes. Stimuli were brief, white xenon discharges delivered via a Ganzfeld under dark-adapted and light-adapted conditions (150 cd m(-2)). ERGs were obtained 1, 2, 3, 4 and 9 weeks after ONTx (n = 8) or sham (n = 8) operations. ONTx reduced both positive and negative components of the scotopic threshold response (pSTR and nSTR). Scotopic ERG responses to brighter flashes, including a-waves, b-waves and oscillatory potentials (OPs) were unaffected by ONTx. ONTx reduced the photopic b-wave and OPs. TTX (6 microM) reduced the pSTR and nSTR, but not the scotopic a-wave, b-wave or OPs. TTX had dramatic effects on the photopic ERG, surpassing the effects of ONTx. TTX application 9 weeks post-ONTx had little additional effect on the STR. Inhibition of inner retinal responses using GABA (10 mM) or NMDA (0.8 mM) reduced the nSTR substantially. Similar results were obtained with antagonists of AMPA/KA ionotropic glutamate receptors 6-cyano-7-nitroquinoxaline-2,3(1H,4H)-dione (CNQX, 0.2 mM) or cis-2,3-piperidinedicarboxylic acid (PDA, 5 mm); however, both also reduced the scotopic b-wave by approximately 40 %. By contrast, the NMDA receptor antagonist D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7, 0.2 mM) had no effect alone, but the combination of D-AP7 and CNQX completely abolished the STR. The results of this study indicate that: (1) both pSTR and nSTR components in the rat depend directly upon intact GC responses, and that amacrine cell contributions to these components are relatively small; (2) scotopic ERG response components to brighter flashes receive little influence from GCs; (3) the rat photopic ERG also reflects GC signals and may serve as an additional useful test of GC function; (4) TTX had dramatic effects on the rat photopic ERG that were not attributable to GC currents, but rather to voltage-gated sodium currents in amacrine or interplexiform cells; (5) a small residual negative STR persisted after ONTx that was likely to be generated by graded responses of third-order retinal cells, most likely amacrine cells.

Long-term effect of retinal ganglion cell axotomy on the histomorphometry of other cells in the porcine retina

PURPOSE

To determine the effect of retinal ganglion cell axotomy on the thickness of inner plexiform, inner nuclear, and outer plexiform layers, as well as the densities of short- and middle-to-long-wavelength cones, in the porcine retina.

METHODS

Unilateral retinal ganglion cell axotomy was performed in seven domestic pigs by either surgical optic nerve section or peripapillary argon laser photocoagulation. Damage to the retinal vasculature was ruled out with fluorescein angiography. Histologic examination of the retinal tissue was performed nine months later. Cone densities were determined immunohistochemically with the anti-visual pigment antibodies COS-1 and OS-2. Image analysis of semithin retinal cross sections was used to measure the thickness of the retinal layers. The effect of axotomy was quantified by optic nerve axon counts and estimations of retinal ganglion cell counts. The data were compared between the eyes with axotomy and the contralateral normal eye using the nonparametric Wilcoxon rank sum test.

RESULTS

Treatment of the peripapillary retina with the argon laser resulted in a median decrease in axon counts and retinal ganglion cell density estimates of 31%. No optic nerve axons and cells resembling retinal ganglion cells were found in the eyes with transected optic nerves. There was no significant difference in either the thickness of any retinal layers or cone densities between axotomized and normal control eyes.

CONCLUSION

No signs of retrograde transsynaptic degeneration were observed in porcine retinas nine months after retinal ganglion cell axotomy.

Transient and steady state focal and pattern electroretinogram nerve section losses in cats with unilateral optic

This study shows the ketamine/xylazine anaesthetised cat is a useful model for the effect of unilateral optic nerve section on pattern electroretinograms (PERGs), especially if stimuli extending to previously untested low spatial frequencies and preferably down to the focal ERG (FERG) are included. The transient reversal rate, seldom used in animals,has advantages over steady state recording. Transient PERGs had signs of true spatial tuning, a higher amplitude and signal noise ratio and showed the effect of optic atrophy at low spatial frequencies more rapidly.

Down-regulation of glutamate-induced conductances of retinal horizontal cells after ganglion cell axotomy

After a complete optic nerve section (ONS), retinal neurons may display retrograde transneuronal modifications in synaptic structure and function related to the retinal disconnection from the brain. The molecular and physiological basis of these changes is not yet fully understood. Immunoreactivity for calbindin was used to specifically immunolabel the horizontal cells (HC) in order to study any morphologic changes in the outer plexiform layer (OPL) after axotomy-induced degeneration of retinal ganglion cells (RGC) in the rabbit retina. Glutamate-gated conductance expressed by HC enzymatically dissociated from the rabbit retina were studied at 12 and 21 days after ONS by using the whole-cell voltage-clamp technique. The amplitudes of glutamate-induced currents on HC were significantly reduced 3 weeks after axotomy. However, no morphologic changes within the OPL were detected coincident with the progressive loss of glutamatergic responses; similarly, HC dissociated from the axotomized retinal tissue did not differ in morphology or appearance from control retinas. The main finding in this study is that the HC experiment a retrograde transneuronal down-regulation of their ionotropic glutamate-induced conductance following axotomy-induced degeneration of RGC.

Contrast sensitivity in pigeons: a comparison of behavioral and pattern ERG methods

Contrast sensitivity (CS) is often used to assess spatial and temporal vision in animals. Conventional behavioral psychophysical techniques are both time and labor intensive, whereas measurement of CS functions by means of the pattern electroretinogram (PERG) is considerably more rapid and efficient. Are the two methods comparable, however? To answer this question, contrast-sensitivity functions were obtained using both the PERG and behavioral psychophysics in the same subjects, which were White Carneaux pigeons. The stimuli, in both methods, were phase-reversing, contrast-modulated sweeps of sinusoidal gratings. The PERG-CS functions were recorded via corneal electrodes and the behavioral data were collected using a modified staircase method that used moderate food deprivation and food reward. The results indicated that the PERG-CS functions had comparable bandwidth and peak spatial frequency to the behavioral CS functions. The PERG-CS functions, however, were lower on average than the behavioral curves by about 54%. The visual acuity of the two methods, as estimated from the high-frequency cutoff of the CS functions, differed by 37%. Both of these values are roughly consistent with the square root of 2 advantage of binocular viewing (behavioral method) over monocular viewing (PERG method). In addition, the peak spatial frequency showed a decrease of 0.125 c/deg with the PERG method and bandwidth was reduced by approximately 0.5 octave. These findings suggest that the PERG is an acceptable alternative to behavioral measurement of CS functions, especially in animal psychophysics, if one takes into account the underestimation of CS by the PERG method and the small changes in peak spatial frequency and bandwidth.

Acute neuronal apoptosis in a rat model of multiple sclerosis

Demyelination caused by inflammation of the CNS has been considered to be a major hallmark of multiple sclerosis (MS). Using experimental autoimmune encephalomyelitis, a model of MS, we demonstrate that an immune-mediated attack of the optic nerve is accompanied by an early degeneration of retinal ganglion cells (RGCs). The decrease of neuronal cell density was correlated with functional disabilities as assessed by visual evoked cortical potentials and electroretinogram. Visual acuity was significantly reduced. DNA degradation and activation of caspase-3 in RGCs indicate that cell death of RGCs is apoptotic. These findings show for the first time that an inflammatory attack against myelin components can lead to acute neuronal cell loss by apoptosis.

Evaluation of short-term increased intraocular pressure on flash- and pattern-generated electroretinograms of dogs

OBJECTIVE

To determine the electrodiagnostic and histologic response of short-term increases of intraocular pressure (IOP) on transient pattern electroretinograms (PERG) and flash electroretinograms (FERG) in the eyes of dogs.

ANIMALS

8 healthy mixed-breed dogs.

PROCEDURE

Transient PERG and FERG waveforms were recorded from dogs (while anesthetized) as IOP was increased from baseline (7 to 19 mm Hg) to 90 mm Hg. One hundred mean PERG responses and a single FERG response were recorded at each step during 3 recording sessions. Globes of each dog were enucleated after euthanasia on posttreatment day 7 and evaluated by a pathologist.

RESULTS

Increases in spatial frequency resulted in decreased amplitudes of N2 (second negative PERG peak). Increases in IOP resulted in decreases in all 3 PERG waveforms and the FERG waveform. All values began to return to baseline after short-term increases in IOP on day 0, and waveforms were not significantly different on posttreatment days 3 and 7

CONCLUSIONS

Data suggest that short-term increases in IOP affect PERG and FERG waveforms, and PERG waveforms are more sensitive to increases in IOP Differences were not detected between treated and control eyes on histologic examination. Further studies are necessary to determine at what IOP permanent damage to ganglion and photoreceptor cells will develop and whether PERG is a reliable clinical diagnostic technique for use in dogs to reveal retinal damage that is secondary to increased IOP prior to changes in waveforms generated by FERG in dogs.

Retinal ganglion cells regenerating through the peripheral nerve graft retain their electroretinographic responses and mediate light-induced behavior

To assess the light-induced electrical activity of rodent retinal ganglion cells (RGCs) regenerating into a peripheral nerve (PN) graft we used non-invasive recording of electroretinographic responses to the contrast-reversal of sinusoidal gratings (p-ERG). On comparing the retinas that received a PN graft and retinas with only optic nerve (ON) transection, p-ERG responses were present in grafted retinas as late as 20 months after the surgery while they completely disappeared in non-transplanted controls within 4 months of ON transection. Next, the ability of regenerating RGCs to form functional connections with their targets in the superior colliculus (SC) was tested by a light-escape task. While the bilaterally blinded animals did not improve during the test, unilaterally grafted animals (with the contralateral eye blinded) reached 26% success in the last quartile of the light-escape task. This performance was significantly better than that of blind animals (ANOVA and Student-Newman-Keuls test; p<0.05), but did not reach the level of intact rats (87%). The transplanted rats, therefore, were capable of light perception, but at a sub-normal ability. In addition, we were also able to correlate the amplitude of the p-ERG response with the visual behavioral performance for each transplanted animal. This finding indicates that there is a direct link between the RGC electrophysiological activity and the functional capacity of the regenerated visual pathway. In conclusion, the above results indicate that (a) PN grafts help to preserve the normal electroretinographic activity of injured and regenerating RGCs (b) the regenerated visual pathway is functional and capable of mediating simple visual behavior and that (c) there is a correlation between the light-evoked RGC electrical activity and visual behavior and, finally, that (d) the effect of PN graft on the electrophysiological and functional restoration of the visual pathway is long-lasting or even permanent.

Mid-peripheral pattern electrical retinal responses in normals, glaucoma suspects, and glaucoma patients

AIMS

Reliance on intraocular pressure, optic nerve cupping changes, nerve fibre layer integrity, and visual field changes may delay treatment of glaucoma since irreversible changes may have already occurred at the time of diagnosis. Abnormal pattern electrical retinal responses (PERR or PERG) have been demonstrated in patients with ocular hypertension (no visual field changes) and glaucoma when visual stimulation was presented to the central field. Since glaucomatous visual field changes tend to occur first in the mid-periphery, the use of PERR outside of the central field may offer an earlier indication of glaucomatous involvement.

METHODS

Glaucoma suspects and glaucoma patients were derived from a university practice. Normal subjects were recruited from non-patient volunteers. Alternating bar gratings were presented in the supranasal, supratemporal, infratemporal, and infranasal visual field. Six spatial frequencies, from 0.25 to 6.0 cycles per degree, were used for normal volunteers; three spatial frequencies, from 0.38 to 1.5 cycles per degree, were presented to suspects and glaucoma patients. Time of onset of the first negative (N35) and first positive peak (P50) and the amplitude consisting of the absolute difference between the first negative peak and first positive peak (P50 amplitude) are reported. Age corrected values were determined for normals, suspects, and glaucoma patients for each spatial frequency and for each quadrant in the visual field.

RESULTS

Mean P50 amplitudes from normal subjects showed spatial tuning in all quadrants with reduced low frequency attenuation. Normals demonstrated a small decline in amplitude with age. Glaucoma patients demonstrated an age corrected reduction in amplitude and early implicit times. Glaucoma suspects had values between those of normal and glaucoma subjects. P50 amplitudes were weakly correlated with increasing cup to disc diameter ratio. A glaucoma patient with asymmetric visual field loss demonstrated significant diminution of the PERR bilaterally.

CONCLUSION

The PERR, using mid-peripheral stimulation, may be a sensitive tool for the early detection of glaucoma. Further refinements can speed clinical data acquisition and enhance signal to noise ratio.

Does pattern electroretinogram spatial tuning alteration in Parkinson's disease depend on motor disturbances or retinal dopaminergic loss?

Systemic decrease of dopaminergic cells, such as in Parkinson's disease may produce visual alterations in humans. In order to show possible pattern electroretinogram (PERG) spatial tuning function (STF) alterations due to impaired dopaminergic transmission in humans, we studied a group of Parkinson's disease patients before and during treatment with the dopamine precursor, levodopa, and compared their performances with those of an age-matched control group. Moreover, in order to exclude the possible involvement of motor disabilities to produce PERG alterations, we also investigated PERG responses in post-traumatic parkinsonian patients who exhibited motor abnormalities as a consequence of focal lesions of basal ganglia, in the absence of systemic dopaminergic degeneration. Our results showed a clear decrease of PERG responses in Parkinson's disease patients particularly at medium spatial frequency range (2.7-4.0 cycles/degree) with a substantial preservation of responses at low frequencies. Levodopa therapy reversed these alterations in Parkinson's disease patients, resulting in the recovery of a normal tuning function shape. In contrast to Parkinson's disease, the tuning function appeared to be preserved in post-traumatic parkinsonian patients. Our results clearly establish a relationship between retinal alteration in PD patients and dopaminergic retinal function.

The visual response of retinal ganglion cells is not altered by optic nerve transection in transgenic mice overexpressing Bcl-2

Attempts to rescue retinal ganglion cells from retrograde degeneration have had limited success, and the residual function of surviving neurons is not known. Recently, it has been found that axotomized retinal ganglion cells die by apoptotic mechanisms. We have used adult transgenic mice overexpressing the Bcl-2 protein, a powerful inhibitor of apoptosis, as a model for preventing injury-induced cell death in vivo. Several months after axotomy, the majority of retinal ganglion cells survived and exhibited normal visual responses. In control wild-type mice, the vast majority of axotomized retinal ganglion cells degenerated, and the physiological responses were abolished. These results suggest that strategies aimed at increasing Bcl-2 expression, or mimicking its function, might effectively counteract trauma-induced cell death in the central nervous system. Neuronal survival is a necessary condition in the challenge for promoting regeneration and eventually restoring neuronal function.

Flash and pattern electroretinogram changes with optic atrophy and glaucoma

We investigated recent reports that, contrary to common belief, glaucoma can affect flash as well as pattern electroretinograms. An extensive flash and pattern electroretinogram test protocol was used in a large sample of glaucoma patients and age-matched controls who were either visually normal or had other optic nerve diseases. All electroretinogram parameters were reduced and delayed in normal people > 55 years of age. The effect did not increase in later decades. In patients aged < or = 55 years, flash electroretinograms showed mild reductions and delays from optic atrophy alone. Glaucomatous ERG changes were larger and increased with disease severity. Pattern electroretinograms and oscillatory potentials were almost equally reduced in optic atrophy and all degrees of glaucoma. Mildly affected patients > 55 years of age had similar electroretinogram change to age-matched normals in most conditions. Advanced glaucoma patients showed similar differences from normal irrespective of age. This suggests that direct diagnostic application of these results to older patients will be difficult, that the ERG changes in glaucoma cannot be attributed simply to optic atrophy and that additional widespread outer retinal damage occurs in glaucoma.

Receptive-field properties of displaced starburst amacrine cells change following axotomy-induced degeneration of ganglion cells

Starburst amacrine cells in the rabbit retina were labeled following an intraocular injection of the fluorescent dye, 4,6,diamidino-2-phenylindole (DAPI). From each eye a strip of retina was removed, mounted on a platform beneath an epifluorescence microscope, and superfused with a physiological solution. The tip of a tungsten microelectrode (for extracellular recording) was visually positioned near the cell body of a DAPI-labeled starburst amacrine cell that was located in the ganglion cell layer. Light-evoked responses from the displaced starburst amacrine cells were studied in normal retinas and in retinas that had received a small electrolytic lesion near the optic disk 5-9 months beforehand. In normal retinas, a small spot of light centered over the receptive field of a displaced starburst amacrine cell in nearly all cases evoked a brief burst of spikes only at light onset. When stimulated with a large spot or an annulus of light, many cells gave a small burst of spikes at light offset. In lesioned retinas, the light responses of displaced starburst amacrine cells were recorded in areas of the retina where ganglion cells had degenerated. All cells responded with a large burst of spikes at the onset and offset of a small, centered spot of light. Large spots and annuli of light also evoked robust ON/OFF responses from these cells. The results from this study show that the receptive-field properties of displaced starburst amacrine cells change following axotomy-induced degeneration of ganglion cells. This finding indicates that changes in either synaptic transmission or the membrane properties of neurons occur in the retina following degeneration of ganglion cells.

Presence and further development of retinal dysfunction after 3-year follow up in IDDM patients without angiographically documented vasculopathy

Abnormalities in neuroretinal function may play a role in the development of diabetic retinopathy. The natural course of diabetic retinal dysfunction in a group of subjects with insulin-dependent diabetes mellitus and with no apparent microvascular alterations in the retina was followed-up with fluorescein angiography and a sensitive electrophysiological technique, i.e., steady-state focal electroretinogram at the macula, for 3 years. Before the beginning and throughout our study, strict glycaemic control was maintained by three or four daily insulin injections under careful monitoring. Analysis of macular electroretinogram provided information from different neural layers. At the first examination, functional activities of postreceptoral neurons were significantly decreased with respect to those of age-matched control subjects. Diabetic patients showed a functional loss of both ganglion cell (0.53 +/- 0.09 vs 0.42 +/- 0.11 microV; t = 5; p = 0.0001) and preganglion cell (0.51 +/- 0.13 vs 0.42 +/- 0.14 microV; t = 2.8; p = 0.007) layers. Diabetes did not alter photoreceptor activity. After 3 years, dysfunction was significantly greater in the preganglion cell layer (0.28 +/- 0.11 microV; t = 6.3; p = 0.0001). Although in some patients further impairment of ganglion cell function was shown, no significant difference was found in 3 years. Photoreceptor function remained unaltered. No vascular abnormalities in the retina were noted after 3 years in this group of patients. Metabolic control was not correlated to functional changes. Our findings suggest that the middle retinal layer is the most sensitive physiological locus of progressive diabetes-induced dysfunction in the absence of angiographically documented abnormalities.

Electrophysiological methods in early glaucoma detection

Electrophysiological methods like pattern electroretinogram and pattern visual evoked potentials may be the best approach for early glaucoma detection, as they have demonstrated to be sensitive to the ganglion cells functional changes. Reported values of both P50 and N95 pattern electroretinogram components differ widely, a fact perhaps related to a heterogeneous grouping of patients suffering from ocular hypertension and glaucoma, rather than to the type of technique utilized. This study is based on a total of 42 subjects: 14 normals, 16 subjects suffering from ocular hypertension, and 12 patients with glaucoma, with 79 eyes examined. Pattern electroretinogram and pattern visual evoked potentials were used as successive techniques. Setting condition for pattern electroretinogram, such as low temporal frequency (2 Hz), 30 min check size, high contrast (99%) and luminance (93 cd/m2), identified the N95 component as the best index for early glaucoma detection. In the control group N95 mean amplitude was 1.62 muV +/- 0.59 SD and showed almost significant difference with ocular hypertension (p = 0.07) and highly significant difference with the glaucoma group (p < 0.01), with decrement of 58.6% in the glaucoma group. P50 mean amplitude, on the contrary, did not show significant differences among the groups (Newman-Keul test), its reduction in glaucoma being 28%. The mean pattern visual evoked potentials latency was alos highly significant between glaucoma and control groups, but not between ocular hypertension and control groups; the mean amplitude did not show significant differences. A proportion of 26.6% abnormal pattern electroretinogram was found in the group suspected of having glaucoma where conventional methods had proved normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies to nerve growth factor affect the postnatal development of the visual system

Exogenous supply of nerve growth factor (NGF) prevents the effects of monocular deprivation. This suggests that visual afferents may be competing for an endogenous neurotrophic factor, related to NGF, whose production by postsynaptic cells depends on the activity of afferent fibers. To test the hypothesis that endogenous NGF may play a role in the functional and anatomical development of the rat geniculo cortical system, the physiological action of NGF in the rat visual system was antagonized by using two independent monoclonal antibodies which neutralize NGF (alpha D11 and 4C8). To provide a continuous supply of antibodies during the period of visual cortical plasticity, alpha D11 or 4C8 antibody-producing hybridoma cells were implanted in the lateral ventricle of rats at postnatal day 15. This resulted in dramatic alterations of two of the most important parameters characterizing the functional development of the visual system, namely, visual acuity and binocularity of cortical neurons and in shrinkage of cells in the lateral geniculate nucleus. This demonstrates that the action of endogenous NGF is necessary for the normal functional and anatomical development of the geniculocortical system.