Implications of retinal effects observed in chronic toxicity studies on the clinical development of a CNS-active drug candidate.

The development path described for JNJ-26489112 provides perspectives on interpretation of retinal effects observed in nonclinical studies and their implications for clinical development. JNJ-26489112 is a CNS-active investigational drug that has potential as a novel treatment for treatment-resistant and bipolar depression, epilepsy, and neuropathic/inflammatory pain. In a 6-month toxicity study in albino rats, retinal atrophy was observed at supratherapeutic exposures to JNJ-26489112. The histopathological changes and topography of the lesions were characteristic of light-induced damage specific to albino rats. The species/strain specificity is supported by an absence of any ocular effects in dogs and in pigmented and albino rats, housed under standard and reduced lighting, respectively. To further evaluate its potential to cause ocular effects, in vivo functional and structural ocular analyses were included in a 9-month monkey toxicity study. Reductions in rod- and cone-mediated electroretinograms were observed at supratherapeutic exposures but without any histopathologic changes. These data suggested that the effects of JNJ-26489112 in monkeys were neuromodulatory and not neurotoxic. Taken together, data related to the light-induced atrophy in albino rats and reversible neuromodulatory effects in monkeys, supported the safe evaluation of JNJ-26489112 in a clinical proof-of-concept study that included comprehensive functional and structural ocular monitoring.

Effect of H-7 and Lat-B on retinal physiology.

PURPOSE: To investigate the effects of H-7 and Latrunculin B (Lat-B) on retinal vascular permeability and electrophysiology at concentrations that increase outflow facility in monkeys. METHODS: One eye of 1 rhesus and 22 cynomolgus monkeys received an intravitreal bolus injection of H-7 or Lat-B; the opposite eye received vehicle. Multifocal electroretinograms (mfERGs), and photopic and scotopic full-field electroretinograms (ffERGs, sERGs) were recorded in subsets of monkeys at baseline and at multiple time-points post-H-7 or Lat-B. Vitreous fluorophotometry (VF) and fluorescein angiography (FA) were also performed. RESULTS: No differences between the H-7 or Lat-B treated and control eyes were found in ffERGs, mfERGs, sERGs, or in FAs in any monkey. No significant difference was found in vitreous fluorescein levels between H-7 treated or Lat-B treated vs. control eyes. CONCLUSIONS: No effect on retinal vascular permeability or retinal electrophysiology was apparent after intravitreal administration of H-7 or Lat-B at doses that increase outflow facility and lower IOP when given intracamerally.

Selective loss of S-cones in diabetic retinopathy.

OBJECTIVE: To determine whether selective cone loss could explain the acquired tritan-like color confusion found in diabetic retinopathy. METHODS: Terminal deoxynucleotidyl transferase-mediated biotin-deoxyuridine triphosphate nick end labeling (TUNEL) was employed on paraffin sections of retinas from 5 donors with diabetic retinopathy. For quantitative analysis, postmortem retinas were obtained from 13 human donors; 7 from patients with various durations and stages of diabetic retinopathy (4 background, 3 proliferative) and 6 controls. Enzyme histochemical analysis for carbonic anhydrase (CA) was used to distinguish L/M-cones (positive for CA) from S-cones (negative for CA). Cone topography was determined by sampling 360 degrees from 0.1 to 1.5 mm of foveal eccentricity and along the horizontal meridians from 1.5 to 15.0 mm. RESULTS: Rare cells in both the inner and outer nuclear layers of the diabetic eyes were positively labeled with the TUNEL method. The CA staining revealed incomplete and patchy losses of S-cones that were limited to the diabetic retinas. Statistically significant reduction in the density of S-cones was found at nearly all foveal eccentricities from 0.1 mm to 15.0 mm. This was not the case for the L/M-cones. On average, for all locations, the percentage of S-cones compared with L/M-cones was decreased by 21.0% +/- 3.4% with respect to the controls. CONCLUSION: The S-cones selectively die in diabetic retinopathy. CLINICAL RELEVANCE: Selective loss of S-cones may contribute to the tritan-like color vision deficit seen in patients with diabetic retinopathy.

Protection of ganglion cells in experimental glaucoma by retinal laser photocoagulation.

OBJECTIVE: To test a hypothesis of photoreceptor involvement in retinal ganglion cell (RGC) death in chronic glaucoma. METHODS: Laser spots were applied to 6 eyes of 3 rhesus monkeys, causing focal destruction of the outer retina, including the photoreceptors. After 3 to 4 weeks, experimental glaucoma was induced in the right eyes of each monkey using argon laser trabecular destruction (ALTD). The intraocular pressures in these eyes were elevated for 3 to 7 months. As a control, 1 additional monkey underwent retinal laser photocoagulation followed by optic nerve transection instead of ALTD. Following enucleation, the retinas were embedded and sectioned for histologic evaluation. RESULTS: There was extensive loss of RGCs in the eyes with ALTD except over the large retinal laser spots, where there was an increased survival of RGCs. The RGC protection was not observed in the monkey that had undergone optic nerve transection. CONCLUSION: Photocoagulation of the outer retina that completely destroys the photoreceptors results in survival of the overlying RGCs in experimental glaucoma in monkey eyes. CLINICAL RELEVANCE: Although this is an experimental model and not a therapeutic option, these results suggest that treatments other than lowering intraocular pressure may be potential therapies for preventing RGC death in glaucomatous eyes. Arch Ophthalmol. 2000;118:1242-1250

Swelling and loss of photoreceptors in chronic human and experimental glaucomas.

OBJECTIVE: To determine whether outer retinal changes occur in chronic, presumed primary open-angle glaucoma (POAG). METHODS: The outer retinas from 128 human eyes with a diagnosis of chronic glaucoma (presumably POAG in most cases) and 90 control eyes were examined histologically by 3 masked observers for photoreceptor swelling and loss. Retinas from 9 rhesus monkeys with glaucoma induced experimentally by laser trabecular destruction were compared with 7 fellow (control) eyes. The mean pressure elevations in the eyes with laser trabecular destruction ranged from 26.6 to 53.6 mm Hg with durations varying from 7 to 33 weeks. RESULTS: Swelling of the red- and green-sensitive cones was observed in a statistically significantly greater proportion of human eyes with presumed POAG compared with the control eyes. Patchy loss of red/green cones and rods was also found in some of the glaucomatous retinas. In a subset of the human eyes with end-stage disease, cone swelling was a variable finding. Although no photoreceptor loss was found in the 9 monkey eyes with experimental glaucoma, 8 had swelling of their red/green cones that was remarkably similar to that seen in the human eyes. Swelling was not present in any of the control monkey eyes. CONCLUSIONS: The photoreceptors are affected by chronically elevated intraocular pressure. CLINICAL RELEVANCE: These findings may explain some of the abnormalities of color vision and the electrophysiological effects that have been observed in patients with POAG.

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.

The artifact of retinoscopy revisited: comparison of refractive error measured by retinoscopy and visual evoked potential in the rat.

PURPOSE: The validity of retinoscopy in small eyes has not been clearly established due to uncertainty regarding the source of the ocular reflections assessed during this procedure. A widely cited model which proposes that their origin is the inner limiting membrane of the retina was evaluated in the rat by comparing refractive errors measured by retinoscopy to those measured by visual evoked potentials (VEPs). METHODS: Ten rats were refracted both by cycloplegic streak retinoscopy and by VEP while viewing 0.05 to 0.15 cpd square-wave gratings-reversed at 1.875 Hz. Spherical aberration of the rat eye was assessed as a potentially confounding variable in VEP refraction by sequential retinoscopic refractions across the rat's natural pupil through a 1.5-mm pinhole. RESULTS: All animals were moderately to highly hyperopic by both methods (range = +4.5 to +18.5 D). Spherical aberration was minimal (median = 3.5 D of overcorrected aberration). The median difference between retinoscopic refractions and those by VEP was not significant (+1.94 D more hyperopia by retinoscopy; p = 0.062, Wilcoxon signed rank) but was significantly less than the +9.64 D difference predicted by an inner limiting membrane model (upper 95% limit = +3.76 D). CONCLUSION: This suggests that the origin of the retinoscopic reflex is located in the outer retina rather than at the inner limiting membrane. Correction factors for retinoscopy in small eyes may be smaller than previously assumed.

A sweep VEP test for color vision deficits in infants and young children.

PURPOSE: Color vision testing in young children typically is precluded by the motor and cognitive skills required by standard tests; yet this information can be useful for diagnosis and counseling in many conditions. The purpose of this study is to evaluate a visual evoked potential (VEP) method for assessing red-green color vision anomalies in pediatric patients. METHOD: The relative chromatic luminance (C = R/R + G) of a rapidly reversing red-green checkerboard was varied across a wide range within a short viewing period (10 sec). Swept-parameter VEP methods were used to measure the cortical response to the range of C presented. RESULTS: Individuals with normal color vision exhibit a VEP response that exceeds noise levels across all values of C, often with an amplitude minima near the photopic equiluminant point (C = 0.5). Results from children with established protan and deutan color vision anomalies show loss of VEP amplitude and phase at values of C consistent with the respective color defect. A patient with achromatopsia showed a generalized depression of VEP response across all values of C tested. CONCLUSION: Color sweep VEP techniques appear promising for the clinical assessment of color status in pediatric patients.

Binocular cortical interactions in the monofixation syndrome.

This study examines the relationship between clinically assessed binocular sensory status and binocular visual evoked potentials (BVEPs) in 11 patients with monofixation syndrome (MFS) and in 12 patients with absent binocular fusion (ABF). BVEPs were elicited using a two-sinusoid "beat" technique. The luminances of red/green anaglyphic circular fields were modulated independently at temporal rates of 6 and 8 Hz. This stimulus elicits a BVEP at 14 Hz in children with normal stereoscopic vision. In this study, we found that this BVEP also is present in patients with MFS, whereas it is near background noise levels in patients with ABF. These results demonstrate the presence of functional binocular cortical mechanisms in patients with MFS. The findings are consistent with the hypothesis that elicitation of this class of BVEPs requires gross sensory fusion, but not fine stereoscopic vision.

VECP evidence for binocular function in infantile esotropia.

Since commonly used clinical methods of measuring binocular function require subjective responses, this testing has been limited to more cooperative children, usually older than 2 years of age. Recently, we have begun using a clinically practical, visually evoked cortical potential (VECP) method to detect the presence of binocular neurons in infants and young children. We studied 14 children, ages 4 to 44 months, with infantile esotropia. Nine had surgical correction for esotropia by the age of 2 years. Twenty-five normal infants ages 6 weeks to 22 months served as controls. Most normal infants showed the development of the "beat" by 2 months of age. The "sum" VECP was not consistently present until age 6 months. Four of the five esotropic infants less than 1 year of age, demonstrated neither a "sum" nor "beat" response. All nine patients with corrective surgery performed before 2 years of age developed a significant sum response and three developed a beat. Five patients had not had surgery until after 2 years of age. Two developed a sum and one a beat, but none had both beat and sum responses. The results suggest that there is a loss of nonlinear binocular response in esotropic children not corrected before the age of 2 years and that these responses can be restored after early treatment even if not present at the time of surgery.

Effect of orientation on spatiotemporal contrast sensitivity in multiple sclerosis.

Spatiotemporal contrast sensitivity at three orientations, vertical, horizontal and oblique, was studied in 18 patients with clinically definite and laboratory-confirmed definite multiple sclerosis (MS). Nineteen age-matched control subjects were also studied under identical experimental conditions. Contrast thresholds for detecting steady and counterphase modulated (5 Hz) gratings ranging in spatial frequency from 0.5 to 12 c/deg were measured by a modified psychophysical method of limits. With the exception of two patients (three eyes) whose Snellen acuity scores were 20/70, all observers had acuity scores of 20/30 or better. All subjects were corrected for astigmatism. Orientation, spatial frequency and temporal frequency interacted differently in determining contrast sensitivity in the two groups of observers. For the controls, an oblique effect was present for both the steady and counterphase modulated gratings of high spatial frequencies, and there was no orientation-dependent loss of sensitivity for low spatial frequencies. For the observers with MS, there was no oblique effect, but sensitivity was dependent on orientation for the low spatial frequencies. Most patients with MS had reduced contrast sensitivity, compared to the controls, at one or more orientations. Counterphase modulation increased sensitivity to the low spatial frequencies and decreased sensitivity to the high spatial frequencies for both normal controls and patients with MS. In patients with MS this effect of temporal modulation on contrast sensitivity was markedly enhanced.

Dynamics of adaptation for vision with a stabilized image.

The addition of a uniform increment of light to a high-contrast image that has been stabilized on the retina reveals marked perceptual nonlinearities. When the increment is small, the pattern appears in its original phase (OP), large increments produce an apparent phase reversal (APR), and intermediate increments may yield an apparently blank field or an oscillation of the apparent phase. In the present series of studies the threshold values used to produce a stable OP and APR were determined as a function of adaptation time before the application of the increment. The stabilized target had a luminance profile consisting of the difference of two Gaussians. A model of detection incorporating a multiplicative gain controlled by a filtered version of the stimulus was used to account for the occurrence of the OP and the APR and the transitory phenomena following the uniform increment. It is argued that the midpoint of the transition zone between the OP and APR, corresponding to blanking, enables us to estimate the shape of the step response function of the gain filter independently of the subsequent detection processes.

Two-dimensional approach to psychophysical orientation tuning.

Orientation bandwidths of psychophysical channels in the human visual system were inferred from contrast thresholds for a special class of polar-separable, two-dimensional patterns. Detection thresholds for these patterns conformed to a model with linear filtering by orientation-selective channels followed by probability summation across these channels. The number of channels (6-8 pairs) and their half-bandwidths at half-sensitivity (approximately 17 degrees) did not differ at 4 and 10 cycles/degree. The results extend the many one-dimensional, linear system models of vision to two dimensions.

Threshold and suprathreshold spatiotemporal response throughout adulthood.

The effect of age on spatiotemporal contrast sensitivity at both threshold and suprathreshold levels was studied in 63 adults ranging in age from 13 to 67 years. Sensitivity for low spatial frequencies modulated at 0 to 15 Hz was not affected by age, but a progressive age-related elevation of threshold was found for combinations of high spatial and temporal frequencies. The magnitude of the effect ascribed to age on spatial frequency was larger than the effect of age on temporal frequency. For most combinations of spatial and temporal frequencies, elevation of thresholds started at approximately 45 years of age. No age-related effects were observed in a contrast-matching task performed at suprathreshold levels.

Apparent phase reversal during stabilized image fading.

A pattern with a horizontal luminance profile described by the difference of Gaussians (DOG) was viewed under conditions of retinal image stabilization. When a uniform increment was applied after the image had disappeared, the pattern reappeared, but with the reversed phase. This phenomenon of apparent phase reversal (APR) is indicative of changes in local sensitivity. It was studied as a function of the time at which the increment was applied and the space constant of the DOG pattern. It was found that the threshold increment necessary to evoke an APR was an exponential function of time. A slight dependence on the spatial dimensions of the stimulus was also demonstrated. The data were examined according to a model of spatial vision that includes a gain that is inversely proportional to a spatially and temporally filtered version of the stimulus. The data provided estimates of the time constant and the spatial extent of the gain mechanism.

Infant speech recognition in multisyllabic contexts.

In 2 infant speech recognition experiments using trisyllabic sequences, the amount of redundancy within nontarget, context syllables was manipulated. Infants 6 1/2 months old were trained to discriminate the syllables [ba] versus [du] in contexts that were either redundant (e.g., [ko ba ko] or [ti ba ti]) or mixed (e.g., [ko ba ti] or [ti ba ko]) A visually reinforced head-turning procedure was employed. In Experiment 1, context was manipulated between subjects, but in Experiment 2 each subject received all 4 contexts (2 redundant, 2 mixed). Infants consistently recognized the familiar target in all contexts, but did so more successfully in redundant than in mixed trisyllablic contexts. These results suggest that amount of speech redundancy may be an important factor in infants' perceptual capabilities.