Charlene B. Y. Kim

Effects of aging on the densities, numbers, and sizes of retinal ganglion cells in rhesus monkey

We used sterological procedures that yield unbiased estimates to quantify the densities, numbers, and soma sizes of retinal ganglion cells in seven young adult and six old rhesus monkeys. The retinae were flat mounted so that we could determine whether there are different aging-related losses in different retinal regions. The mean (+/-standard deviation) total number of ganglion cells was 1,529,039 +/- 115,260 in young-adult retinae and 1,556,698 +/- 165,056 in old retinae, a difference that was not statistically significant. There also were no significant differences between young and old retinae in the densities or total numbers of ganglion cells in the four retinal quadrants, in four concentric retinal zones from fovea to peripheral retina, or in smaller hemiretinal regions of the concentric zones. Ganglion-cell soma sizes also did not differ significantly between young and old animals. Moreover, counts of the largest ganglion cells, which probably correspond to P alpha ganglion cells, revealed no selective loss of these cells with aging. These results are consistent with our previous anatomical and physiological studies of the LGN. Together they suggest that the retino-geniculate pathways are relatively unaffected by aging in the rhesus monkey.

Effects of aging on numbers and sizes of neurons in histochemically defined subregions of monkey striate cortex.

In addition to its horizontal layers, primate striate cortex has a vertical modular organization. Among the vertical modules are histochemically defined areas of high and low cytochrome oxidase labeling in the supragranular layers, referred to, respectively, as blobs and interblobs. Cytochrome c oxidase (CO) blobs and interblobs differ in their inputs from the magnocellular and parvocellular visual pathways, their physiological properties, and many aspects of their neurochemistry. The present study investigated whether aging differentially affects neuron numbers or sizes in the supragranular blobs or interblobs.

Functional and Anatomic Consequences of Subretinal Dosing in the Cynomolgus Macaque

OBJECTIVE To characterize functional and anatomic sequelae of a bleb induced by subretinal injection. METHODS Subretinal injections (100 μL) of balanced salt solution were placed in the superotemporal macula of 1 eye in 3 cynomolgus macaques. Fellow eyes received intravitreal injections (100 μL) of balanced salt solution. Fundus photography, ocular coherence tomography, and multifocal electroretinography were performed before and immediately after injection and again at intervals up to 3 months postinjection. Histopathologic analyses included transmission electron microscopy and immunohistochemistry for glial fibrillary acidic protein, rhodopsin, M/L-cone opsin, and S-cone opsin. RESULTS Retinas were reattached by 2 days postinjection (seen by ocular coherence tomography). Multifocal electroretinography waveforms were suppressed post-subretinal injection within the subretinal injection bleb and, surprisingly, also in regions far peripheral to this area. Multifocal electroretinography amplitudes were nearly completely recovered by 90 days. The spectral-domain ocular coherence tomography inner segment-outer segment line had decreased reflectivity at 92 days. Glial fibrillary acidic protein and S-cone opsin staining were unaffected. Rhodopsin and M/L-cone opsins were partially displaced into the inner segments. Transmission electron microscopy revealed disorganization of the outer segment rod (but not cone) discs. At all postinjection intervals, eyes with intravitreal injection were similar to baseline. CONCLUSIONS Subretinal injection is a promising route for drug delivery to the eye. Three months post-subretinal injection, retinal function was nearly recovered, although reorganization of the outer segment rod disc remained disrupted. Understanding the functional and anatomic effects of subretinal injection is important for interpretation of the effects of compounds delivered to the subretinal space. CLINICAL RELEVANCE Subretinal injection is a new potential route for drug delivery to the eye. Separating drug effects from the procedural effects is critical.

VEP and PERG acuity in anesthetized young adult rhesus monkeys.

This study used the swept spatial-frequency method to compare retinal and cortical acuity in anesthetized young adult rhesus monkeys. Visual evoked potentials (VEPs) and pattern electroretinographic responses (PERGs) were recorded from 25 monkeys (age range: 4-12 years) anesthetized with a continuous infusion of propofol. The stimuli were temporally countermodulated sine-wave gratings that increased in spatial frequency within a 10.24-s period. All animals were refracted using acuity estimated from the zero micro-volt intercept of the linear regression of evoked potential amplitude on spatial frequency. Average sweep acuities were 23.7 cycles/deg +/- 1.5 S.E.M. and 23.1 cycles/deg +/- 1.8 S.E.M. for the PERG and VEP, respectively. VEP and PERG acuities were within the range expected based on acuities estimated from behavioral studies in macaques. PERG and VEP acuities were highly correlated (r = 0.90) and equally sensitive to spherical blur. On a subset of animals, test-retest reliability of animals, and interocular correlations, were high (r = 0.87 and r = 0.83, respectively). Increasing propofol dosage 8-fold did not degrade PERG or VEP acuity. This study demonstrates that high spatial-frequency acuities can be rapidly obtained from young adult rhesus monkeys under a wide dose range of propofol anesthesia using the swept spatial-frequency method.

Binocular processing in the cat's dorsal lateral geniculate nucleus III. Spatial frequency, orientation, and direction sensitivity of nondominant-eye influences

SummaryThe present experiments examined the extent to which binocular processing in the cats dorsal lateral geniculate nucleus (LGN) depends upon the spatial frequency, orientation, and direction of movement of stimuli presented to the nondominant eye. In Experiment 1, we tested the effects of varying these stimulus parameters on the responses of LGN cells to nondominant-eye stimulation. Sixteen of 34 cells tested had statistically significant responses to the nondominant eye and, in agreement with a previous study (Guido et al. 1989), the responsive cells were spatial-frequency sensitive. However, there was little evidence for orientation or direction sensitivity in responses to the nondominant eye: changes in discharge with changes in stimulus orientation and direction were small and were statistically significant in only nine of the cells. In Experiment 2, we tested the effects of varying spatial frequency, orientation, and direction of movement of stimuli presented to the nondominant eye on its ability to influence responses to the dominant eye (i.e., on binocular interactions). The dominant eye was stimulated with the optimal spatial frequency for the cell being tested. For 22 of 45 cells tested, nondominant-eye stimulation had a statistically significant effect on the response to the dominant eye. Fourteen of these cells showed band-pass spatial-frequency sensitivity in the nondominant-eye influence, and eight showed low-pass spatial-frequency sensitivity. However, only 11 of the cells had statistically significant variations in their binocular interactions that depended on the orientation or direction of stimuli presented to the nondominant eye. Furthermore, even for those cells, the effect of varying orientation and direction was only about half as strong as the effect of varying spatial frequency. We conclude that binocular processing in the LGN, including responses to the nondominant eye and nondominant-eye influences on responses to the dominant eye, are affected significantly by the spatial frequency of the nondominant-eye stimulus and relatively little by stimulus orientation or direction of movement. The significance of these findings for understanding the functions of LGN binocular processing is discussed.

Influence of the superior colliculus on responses of lateral geniculate neurons in the cat

The superior colliculus (SC) projects to all layers of the cats lateral geniculate nucleus (LGN) and thus is in a position to influence information transmission through the LGN. We investigated the function of the tecto-geniculate pathway by studying the responses of cat LGN neurons before, during, and after inactivating the SC with microinjections of lidocaine. The LGN cells were stimulated with drifting sine-wave gratings that varied in spatial frequency and contrast. Among 71 LGN neurons that were studied, 53 showed a statistically significant change in response during SC inactivation. Control experiments with mock injections indicated that some changes could be attributed to slow waxing and waning of responsiveness over time. However, this could not account for all of the effects of SC inactivation that were observed. Forty cells showed changes that were attributed to the removal of tecto-geniculate influences. About equal numbers of cells showed increases (22 cells) and decreases (18 cells) in some aspect of their response to visual stimuli during SC inactivation. The proportion of cells that showed tecto-geniculate influences was somewhat higher in the C layers (68% of the cells) than in the A layers (44% of the cells). In addition, among cells that showed a significant change in maximal response to visual stimulation, the change was larger for cells in the C layers (64% average change) than in the A layers (26% average change) and it was larger for W cells (61% average change) than for X and Y cells (29% average change). Nearly all of the X cells that showed changes had an increase in response, and nearly all of the Y cells had a decrease in response. In addition, across all cell classes, 80% of the cells with receptive fields < 15 deg from the area centralis had an increase in response, and 80% of the cells with receptive fields > or = 15 deg from the area centralis had a decrease in response. None of the LGN cells had significant changes in spatial resolution, and only three cells had changes in optimal spatial frequency. Ten cells had a change in contrast threshold, 25 cells had a change in contrast gain, and 29 cells had a change in the maximal response to a high-contrast stimulus. Thus, our results suggest that the tecto-geniculate pathway has little or no effect on spatial processing by LGN neurons. Rather, the major influence is on maximal response levels and the relationship between response and stimulus contrast.(ABSTRACT TRUNCATED AT 400 WORDS)

Multifocal Visual Evoked Potentials in the Anesthetized Non-human Primate

Purpose: To evaluate monkey multifocal visual evoked cortical potentials (mfVEPs) recorded from central and peripheral fields for reliability and isolation from electroretinographic (ERG) activity. Methods: The mfVEP stimulus consisted of a 7-element hexagonal array that subtended 80 degrees of the central visual field. Recordings were made under intravenous pentobarbital sodium (15 mg/kg) anesthesia. Two monkeys with absent optic nerve and ganglion cell function after combined unilateral optic nerve transection and experimental ocular hypertension (ONT/OHT) were followed longitudinally. In a second study, 16 ophthalmologically normal monkeys were tested once. Results: Testing of the non-transected eye in two transected animals revealed robust first- and second-order kernel, first slice (K1 and K2.1) mfVEPs. Stimulation of the transected eye revealed no contamination of the mfVEP from the concurrently recorded multifocal ERGs. There was complete separation of the root-mean-square (RMS) mfVEP amplitudes from the transected and the fellow eyes tested repeatedly across a 4- to 17- month period. The largest amplitude mfVEP was generated by the central element; however, mfVEPs were recorded from outside the central 20 degrees element. The 16 normal animals showed waveforms similar to the normal eyes of the ONT/OHT animals both in shape and distribution throughout the visual field. A scalar-product measure showed both K1 and K2.1 mfVEPs from central and some peripheral elements were statistically distinct from noise. Conclusions: mfVEPs can be reliably recorded from non-human primates anesthetized with pentobarbital. Under the recording conditions described, mfVEPs are not contaminated by ERG activity. mfVEPs may be useful in animal models of diseases that differentially affect macular and peripheral visual field responsiveness.

Functional plasticity in extrastriate visual cortex following neonatal visual cortex damage and monocular enucleation

Neonatal lesions of primary visual cortex (areas 17, 18 and 19; VC) in cats lead to significant changes in the organization of visual pathways, including severe retrograde degeneration of retinal ganglion cells of the X/beta class. Cells in posteromedial lateral suprasylvian (PMLS) cortex display plasticity in that they develop normal receptive-field properties despite these changes, but they do not acquire the response properties of striate neurons that were damaged (e.g., high spatial-frequency tuning, low contrast threshold). One possibility is that the loss of X-pathway information, which is thought to underlie striate cortical properties in normal animals, precludes the acquisition of these responses by cells in remaining brain areas following neonatal VC damage. Previously, we have shown that monocular enucleation at the time of VC lesion prevents the X-/beta-cell loss in the remaining eye. The purpose of the present study was to determine whether this sparing of retinal X-cells leads to the development of striate-like response properties in PMLS cortex. We recorded the responses of PMLS neurons to visual stimuli to assess spatial-frequency tuning, spatial resolution, and contrast threshold. Results indicated that some PMLS cells in animals with a neonatal VC lesion and monocular enucleation displayed a preference for higher spatial frequencies, had higher spatial resolution, and had lower contrast thresholds than PMLS cells in cats with VC lesion alone. Taken together, these results suggest that preserving X-pathway input during this critical period leads to the addition of some X-like properties to PMLS visual responses.

Regional Choroidal Blood Flow and Multifocal Electroretinography in Experimental Glaucoma in Rhesus Macaques

PURPOSE To test a hypothesis of regional variation in the effect of experimental glaucoma on choroidal blood flow (ChBF) and retinal function. METHODS Five rhesus macaques underwent laser trabecular destruction (LTD) to induce elevated intraocular pressure (IOP). Intraocular pressures were elevated for 56 to 57 weeks. Multifocal electroretinographic (mfERG) and multifocal visual evoked cortical potential (mfVEP) testing were performed at regular intervals before and during the period of IOP elevation. At euthanasia, the IOP was manometrically controlled at 35 (experimentally glaucomatous eye) and 15 (fellow control eye) mm Hg. Fluorescent microspheres were injected into the left ventricle. Regional ChBF was determined. RESULTS All of the experimentally glaucomatous eyes exhibited supranormal first-order kernel (K1) root mean square (RMS) early portions of the mfERG waveforms and decreased amplitudes of the late waveforms. The supranormality was somewhat greater in the central macula. Second-order kernel, first slice (K2.1) RMS mfVEP response was inversely correlated (R(2) = 0.97) with axonal loss. Total ChBF was reduced in the experimentally glaucomatous eyes. The mean blood flow was 893 ± 123 and 481 ± 37 μL/min in the control and glaucomatous eyes, respectively. The ChBF showed regional variability with the greatest proportional decrement most often found in the central macula. CONCLUSIONS This is the first demonstration of globally reduced ChBF in chronic experimental glaucoma in the nonhuman primate. Both the alteration of mfERG waveform components associated with outer retinal function and the reduction in ChBF were greatest in the macula, suggesting that there may be a spatial colocalization between ChBF and some outer retinal effects in glaucoma.