The waveform and scotopic CFF of the sheep electroretinogram

Flicker cone function in normal and day blind sheep: a large animal model for human achromatopsia caused by CNGA3 mutation

PURPOSE

Recently we reported on day blindness in sheep caused by a mutation in the CNGA3 gene, thus making affected sheep a naturally occurring large animal model for therapeutic intervention in CNGA3 achromatopsia patients. The purpose of this study was to characterize flicker cone function in normal and day blind sheep, with the aim of generating a normative data base for ongoing gene therapy studies.

METHODS

Electoretinographic (ERG) cone responses were evoked with full-field conditions in 10 normal, 6 heterozygous carriers and 36 day blind sheep. Following light adaptation (10 min, 30 cd/m(2)), responses were recorded at four increasing light intensities (1, 2.5, 5 and 10 cd s/m(2)). At each of these intensities, a single photopic flash response followed by 8 cone flicker responses (10-80 Hz) was recorded. Results were used to generate a normative data base for the three groups. Differences between day blind and normal control animals were tested in two age-matched groups (n = 10 per group).

RESULTS

The normal sheep cone ERG wave is bipartite in nature, with critical flicker fusion frequency (CFF) >80 Hz. In all four flash intensities, the single photopic flash a-wave and b-wave amplitudes were significantly lower (p < 0.005), and implicit times significantly delayed (p < 0.0001), in day blind animals. In all four flash intensities, CFF values were significantly lower (p < 0.0001) in day blind sheep.

CONCLUSIONS

Cone function is severely depressed in day blind sheep. Our results will provide a normative data base for ongoing gene therapy studies.

Clinical, electroretinographic and histomorphometric evaluation of the retina in sheep with natural scrapie

Background

The retina is part of the diencephalon in a peripheral location and may be involved in prion diseases. Retinal function and structural changes were assessed in naturally scrapie-affected red face Manech ewes presenting the classical signs of the disease, and clinically healthy age-matched subjects for controls. Ophthalmic examination was done prior to electroretinography (ERG), which was carried out under conditions that allowed photopic and scotopic activities to be assessed. Histomorphometry of the inner and outer retinal layers was performed post-mortem, and retinas were also examined for evidence of abnormal prion protein (PrP^Sc) accumulation and glial fibrillary acidic protein (GFAP) upregulation as a marker of gliosis. Scrapie status was determined by examination of brain tissue

Results

Ocular reflexes and ophthalmoscopy did not reveal any difference between scrapie affected and control animals. Although the light-and dark-adapted ERG responses of both rod-and cone-mediated functions had a similar waveform in scrapie-affected and control sheep, a significant reduction in the amplitude of the ERG a-and b-waves was observed in affected animals compared to controls. These functional alterations were correlated with a substantial loss of cells in the outer nuclear layer (ONL), lengthening and disorganization in photoreceptor segments, and substantial reduction in cellularity and thickness of the inner nuclear layer (INL). The degenerative changes in the INL and ONL were most marked in the central and paracentral areas of the scrapie retinas, and were accompanied in all scrapie retinas by PrP^Sc deposition in the ganglion cell and synaptic layers. GFAP immunoreactivity was mainly increased in the ganglion cell and inner plexiform layers.

Conclusions

No appreciable fundoscopic changes were observed in the scrapie-affected ewes although reproducible changes in retinal function as measured by ERG were observed in these animals. The alterations in the receptoral and post-receptoral pathways corresponded to the degenerative lesions observed in the ONL and INL of the scrapie retinas. The retinal degeneration was associated with prion protein infectivity which presumably spread via the optic nerve.

Topography of ganglion cells in human retina

We quantified the spatial distribution of presumed ganglion cells and displaced amacrine cells in unstained whole mounts of six young normal human retinas whose photoreceptor distributions had previously been characterized. Cells with large somata compared to their nuclei were considered ganglion cells; cells with small somata relative to their nuclei were considered displaced amacrine cells. Within the central area, ganglion cell densities reach 32,000-38,000 cells/mm2 in a horizontally oriented elliptical ring 0.4-2.0 mm from the foveal center. In peripheral retina, densities in nasal retina exceed those at corresponding eccentricities in temporal retina by more than 300%; superior exceeds inferior by 60%. Displaced amacrine cells represented 3% of the total cells in central retina and nearly 80% in the far periphery. A twofold range in the total number of ganglion cells (0.7 to 1.5 million) was largely explained by a similar range in ganglion cell density in different eyes. Cone and ganglion cell number were not correlated, and the overall cone:ganglion cell ratio ranged from 2.9 to 7.5 in different eyes. Peripheral cones and ganglion cells have different topographies, thus suggesting meridianal differences in convergence onto individual ganglion cells. Low convergence of foveal cones onto individual ganglion cells is an important mechanism for preserving high resolution at later stages of neural processing. Our improved estimates for the density of central ganglion cells allowed us to ask whether there are enough ganglion cells for each cone at the foveal center to have a direct line to the brain. Our calculations indicate that 1) there are so many ganglion cells relative to cones that a ratio of only one ganglion cell per foveal cone would require fibers of Henle radiating toward rather than away from the foveal center; and 2) like the macaque, the human retina may have enough ganglion cells to transmit the information afforded by closely spaced foveal cones to both ON- and OFF-channels. Comparison of ganglion cell topography with the visual field representation in V1 reveals similarities consistent with the idea that cortical magnification is proportional to ganglion cell density throughout the visual field.

The effect of lithium on pupillary response to pulses of light in sheep

Administration of lithium carbonate to sheep in the dose range of 600-1800 mg/day resulted in a linearly described rise in lithium levels in plasma and red blood cells. In contrast to the rodent model but in agreement with the human condition, plasma lithium levels exceeded those of red blood cells. Polydipsia and body weight changes were not evident. At plasma levels of 0.70-0.8 mM/l, lithium attenuated the ability of the pupil to constrict in response to 30-sec pulses of light in the 25-150 microW/cm2 intensity range but not the ability to dilate in the dark. Thus, similar to observations in normal human volunteers and bipolar patients, lithium reduces sensitivity to light. The sheep is proposed as a useful model for studying the actions of lithium.

Glycine inhibition of pupillary responses to pulses of light in conscious sheep

The use of a 1.5% glycine solution as a bladder irrigant during surgical removal of the prostate has been associated with transient visual impairment. Glycine is thought to be an inhibitory retinal transmitter. Adult female sheep were infused with a 1.5% glycine solution to provide a dose of 0, 7.5, 15, 30, or 60 g of glycine. The volume control was a solution of dextrose and saline. The degree of constriction of the pupil in response to 30 seconds of bright light following dilation in the dark was used as an index of visual response. Observations were made before and 2, 4, 6, 12, 24, 48, 96, and 192 hours after a single infusion. Significant inhibition of pupil response to light but not to the dark was apparent following systemically administered glycine resulting in plasma levels over 5000 mumol/L. Inhibition of pupil response was paralleled by behavioral indices of visual impairment but not by changes in plasma sodium, potassium, chloride or osmolality. The duration of the effect was dose dependent with visual impairment following a single high dose of glycine being detectable five days later and long after glycine levels had returned to normal. However, the relationship between dose, time, and effect appears to be complex. It is important to note that the present observations occurred at plasma glycine levels frequently experienced in routine surgical practice. The finding that significant long term detrimental effects of glycine could be detected should evoke a re-evaluation of the use of glycine as an irrigating solution for surgical procedures and stimulate further investigation of the effects of glycine on retinal function and the ability to perceive light.

Maturational sequence of the visual system: serial measurements of visual evoked potential and electroretinogram in the healthy neonatal lamb

The value of measurements of visual evoked potential (VEP) for neurological assessment of the adult is widely accepted. Its use for neonatal evaluation following birth asphyxia has been limited by our knowledge of developmental changes occurring in the newborn brain. VEPs and electroretinograms (ERGs) were simultaneously recorded from five healthy newborn lambs from birth until 30 days of life. Newborn age was then compared with amplitude (height of the signal) and latency (time from stimulus to a specific wave deflection) measurements from these neuroelectrical signals. Latencies to A wave (photoreceptor activity) and B wave (bipolar and ganglion cells) of the ERG declined in a small but significant manner, while the latency interval from A to B wave did not change with age. Latency to wave N1 of the VEP remained stable while latency to P2 declined from birth to 30 days. The ERG and VEP signals exhibited linear increases in amplitude as the lambs became older. We conclude from these observations that ganglion and bipolar cells (B wave of the ERG) within the retina and primary nerve tracts (N1 of the VEP) along the visual system are mature at birth. In contrast, photoreceptor activity within the retina (A wave of the ERG) and nerve conduction through the visual cortex (P2 of the VEP) exhibit changes which are consistent with ongoing maturation of these more specialized areas through the early newborn period.