Robert A. Linsenmeier

Receptive field properties of X and Y cells in the cat retina derived from contrast sensitivity measurements

The contrast sensitivity to gratings drifting at 2.0 Hz has been measured for X and Y type retinal ganglion cells, and these data have been used to characterize the sizes and peak sensitivities of centers and surrounds. The assumption of Gaussian sensitivity distributions is adequate for both types of cells, but allows a better description of X than of Y cells. The size and peak sensitivity can be specified more precisely, in general, for the center than for the surround. The data also show that for both types of cells (1) center radius increases with eccentricity, but is two to three times larger than Y cells than for X cells at a given eccentricity, (2) spatial resolution is an excellent predictor of center size, (3) the larger the center or surround, the lower its small spot sensitivity at a specific mean lumminance and (4) the surround is nearly as strong as the center for large or diffuse stimuli. X cell surrounds are relatively weaker in the middle of the receptive field than Y cell surrounds, but X cell surrounds are larger relative to their centers.

Photoreceptor inner segments in monkey and human retina: Mitochondrial density, optics, and regional variation

The present work quantifies aspects of photoreceptor structure related to mitochondria, inner segment dimensions, and optical properties, as a basis for furthering our understanding of rod and cone function. Electron-microscopic analyses were performed on the retina of one stumptail macaque (Macaca arctoides) to obtain stereological measurements of ellipsoid mitochondrial density, and sizes and shapes of outer and inner segments. In addition, the distribution of mitochondria and the optical properties of human foveal cones were examined with electron microscopy and Nomarski differential interference contrast (NDIC) imaging. Mitochondria comprised 74-85% of cone ellipsoids and 54-66% of rod ellipsoids in macaque. Ellipsoid volume increased with eccentricity by 2.4-fold for rods and more than 6-fold for cones over eccentricities to 12.75 mm, while the volume of the outer segment supported by the ellipsoid was essentially constant for both rods and cones. Per unit volume of outer segment, cones contained ten times as much mitochondria as rods. In human fovea, as in the rest of the retina, most cone mitochondria were located in the distal inner segment. In the foveal center, however, there are also mitochondria in the myoid, as well as in the outer fiber, proximal to the external limiting membrane (ELM). Analyses of the optical aperture of human foveal cones, the point at which their refractive index clearly differs from the extrareceptoral space, showed that it correlated well with the location of mitochondria, except in the foveal center, where the aperture appeared proximal to the ELM. While mitochondria have an important metabolic function, we suggest that the striking differences between rods and cones in mitochondrial content are unlikely to be determined by metabolic demand alone. The numerous cone mitochondria may enhance the waveguide properties of cones, particularly in the periphery.

Chapter 2 Retinal pigment epithelial cell contributions to the electroretinogram and electrooculogram

2. Circui t o f the E R G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.1. Vol tages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2. Resis tances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Mathematical models of the spatial distribution of retinal oxygen tension and consumption, including changes upon illumination

To better understand oxygen utilization by the retina, a mathematical model of oxygen diffusion and consumption in the cat outer, avascular retina was developed by analyzing previously recorded profiles of oxygen tension (PO2) as a function of retinal depth. Simple diffusion modelling of the oxygen distribution through the outer retina is possible because the PO2 depends only on diffusion from the choroidal and retinal circulations and on consumption within the tissue. Several different models were evaluated in order to determine the best one from the standpoints of their ability to represent the data and to agree with physiological reality. For the steady state one-dimensional diffusion model adopted (the special three-layer diffusion model), oxygen consumption was constant through the middle layer and zero in the layers near the choroid and near the inner retina. On the average, the oxygen consuming layer, as found by nonlinear regression for each profile, extended from about 75% to 85% of the retinal depth from the vitreous. This is a narrow band through the mid-region of the photoreceptors. Oxygen consumption of the entire avascular retina, determined from fitting eight PO2 profiles measured in light-adapted retinas, averaged 2.7 ml O2(STP)/(100 g tissue · min), while the value determined from fitting thirty-two PO2 profiles measured in dark-adapted retinas averaged 4.4 ml O2(STP)/(100 g tissue · min). Consumption in the light was thus only 60% of that in the dark. This suggests that the outer retina is at greater risk of hypoxic injury in the dark than in the light, a fuinding of considerable clinical significance.

Origin and sensitivity of the light peak in the intact cat eye

1. The light peak is a large light‐induced change in the DC potential across the eye (standing potential) that reaches its maximum in 5‐13 min in mammals. The light peak of the intact cat eye was studied in order to define its cellular origin and stimulus—response characteristics. Direct‐coupled recordings were made with a vitreal electrode and also with intraretinal and intracellular micro‐electrodes. Light peaks were generally evoked with 300 sec periods of diffuse white illumination.

Three light-evoked responses of the retinal pigment epithelium

This paper summarizes our findings on light-evoked changes in retinal pigment epithelial cell (RPE) membrane potentials. Experiments were performed on the eye of the anesthetized or decerebrate cat and on isolated tissues from the eyes of a lizard, Gekko gekko, and a frog, Rana catesbeiana. In cat, as was previously shown, the RPE apical membrane potential responds to changes in [K+]0 in the subretinal space. At the onset of illumination it hyperpolarizes to a peak at 4.0 sec as [K+]0 decreases. The next RPE response is a hyperpolarization of the basal membrane that peaks at 20 sec and is also dependent on the decrease in subretinal [K+]0. The last and slowest response is a depolarization of the basal membrane that peaks at 300 sec, and is not obviously associated with K+ changes. The same responses also appear in gecko at a slower time-course, but only the apical-membrane K+-response is present in frog. The three responses also are associated with changes of the opposite polarity at the offset of illumination. These changes in membrane potential are the origin, respectively, of the RPE component of the ERG c-wave, the fast oscillation, and the light peak (slow oscillation).

Electrophysiological consequences of retinal hypoxia

Experiments on cats show that electrical activity of the inner (proximal) retina is unaffected during systemic hypoxia as long as arterial oxygen tension (PaO2) is above 40 mm Hg. This is due to effective regulation of inner retinal tissue PO2 by the retinal circulation. In contrast, some electrical signals generated in the outer (distal) retina begin to change when PaO2 falls below 70–80 mmHg. The outer retinal responses are generated by the retinal pigment epithelium, but their susceptibility to hypoxia results primarily from their dependence on photoreceptors. Photoreceptor metabolism is sensitive to hypoxia because of the high oxygen consumption of photoreceptors and their reliance on the choroidal circulation, which cannot regulate PO2 in the outer retina. Retinal electrophysiology and oxygen distribution are altered by acutely elevated intraocular pressure just as by hypoxia. These results raise the question as to how inner retinal function can be preserved when outer retinal function is altered. The explanations proposed relate to (1) differences in conditions of light adaptation in different studies, (2) the possible inappropriateness of the previous measurements in the inner retina for revealing photoreceptor dysfunction, and (3) a possible preservation of photoreceptor electrical responses when their metabolism is altered. Comparison of cat and human studies suggests that the human retina is affected in much the same way during hypoxia as the cat retina, but further experiments are required for an understanding of the role of hypoxia in human disease.

Estimation of retinal oxygen transients from measurements made in the vitreous humor

Measurements of the oxygen tension in the vitreous humor close to the retina were used to estimate the values at the retinal surface in the cat. The retina was modelled as an infinite plane in a semi-infinite medium and the analytical solution was obtained for the transient following a change in breathing gas from room air to 95% O2/5% CO2. The solution was, compared with experimental measurements. Our measurements indicate that at steady state, breathing room air, the oxygen tension at the surface of the retina is between 15 and 20 mmHg and the gradient from it averages −2·8 mmHg/mm. Transient oxygen tension measurements at several locations within 1·7 mm of the retinal surface agreed well with our mathematical model which could then be used to predict the transient at the retinal surface. In this way, our model can be used to estimate retinal transients from measurements in the preretinal vitreous humor and the distortion introduced by not measuring on the retinal surface itself can be eliminated. The model predicts that the distortion would be relatively small if the electrode were less than about 100 μm from the retina, provided the retinal transient had a time constant greater than a minute. In the steady state, the error would be less than 0·3 mmHg at this distance. These results are not only important for oxygen but apply, with minor adjustments, to all small molecules and ions which passively diffuse from the retina into the vitreous humor.

Visible light optical coherence tomography measures retinal oxygen metabolic response to systemic oxygenation

The lack of capability to quantify oxygen metabolism noninvasively impedes both fundamental investigation and clinical diagnosis of a wide spectrum of diseases including all the major blinding diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. Using visible light optical coherence tomography (vis-OCT), we demonstrated accurate and robust measurement of retinal oxygen metabolic rate (rMRO2) noninvasively in rat eyes. We continuously monitored the regulatory response of oxygen consumption to a progressive hypoxic challenge. We found that both oxygen delivery, and rMRO2 increased from the highly regulated retinal circulation (RC) under hypoxia, by 0.28 ± 0.08 μL min−1 (p < 0.001), and 0.20 ± 0.04 μL min−1 (p < 0.001) per 100 mmHg systemic pO2 reduction, respectively. The increased oxygen extraction compensated for the deficient oxygen supply from the poorly regulated choroidal circulation. Results from an oxygen diffusion model based on previous oxygen electrode measurements corroborated our in vivo observations. We believe that vis-OCT has the potential to reveal the fundamental role of oxygen metabolism in various retinal diseases.

The contrast sensitivity of cat retinal ganglion cells at reduced oxygen tensions.

1. These experiments were done to investigate the effect of various degrees of hypoxia on the function of retinal ganglion cells (recorded in the optic tract) and on retinal oxygen tension. 2. The contrast sensitivity of the centre of X and Y cells, the surround of X cells and the non‐linear subunits of Y cells were measured separately by choosing appropriate spatial and temporal parameters of a sinusoidal grating pattern. 3. Retinal oxygen tension was measured with a bipolar polarographic oxygen electrode positioned in the vitreous humor close to the retina. 4. The time course of changes in ganglion cell sensitivity and retinal oxygen tension was similar. However, oxygen tension frequently overshot the prehypoxic value at the end of hypoxia, while sensitivity did not. 5. The cat retina was rather resistant to hypoxia. Contrast sensitivity and mean firing rate did not change provided the arterial oxygen tension was above about 35 mmHg, but usually dropped precipitously at lower arterial values. 6. The apparent reason for this resistance is that retinal oxygen tension was well regulated, falling only 0.14 mmHg per mmHg of arterial oxygen tension for arterial values above about 35 mmHg, which corresponds to a retinal oxygen tension of about 10 mmHg. Retinal oxygen tension decreased more sharply (0.62 mmHg per mmHg) at lower values of arterial oxygen tension, where sensitivity also decreased. 7. The centre, surround and subunits reacted similarly to hypoxia. This suggests that lateral pathways (i.e. surround) and pathways which might be expected to use more synapses than the centre (i.e. surround and subunits) are not more susceptible to hypoxia.