Interspecies and gender differences in multifocal electroretinograms of cynomolgus and rhesus macaques

Comparison of retinal parameters between rhesus and cynomolgus macaques

Nonhuman primates are important research models for basic vision research, preclinical pathogenesis, and treatment studies due to strong similarities in retinal structure and function with humans. We compared retinal parameters between 10 healthy normal rhesus macaques (Macaca mulatta) and 10 cynomolgus macaques (Macaca fascicularis) by optical coherence tomography and electroretinography. The Heidelberg Spectralis® HRA+OCT and Roland multifocal electrophysiometer were used to analyze retinal morphology, multifocal electroretinograms (mfERGs), and full-field electroretinograms (ff-ERGs). Mean retinal thickness was lowest in the central fovea of macaques and did not differ significantly between species, but the retinal thicknesses of the nerve fiber ganglion cell layer and the inner plexiform layer were significantly different. The amplitude density of the N1 wave was lower in rhesus macaques than in cynomolgus macaques in ring and quadrant areas. Dark-adapted 3.0 oscillatory potentials (reflection of amacrine cell activity) and light-adapted 30-hz flicker ERG (a sensitive cone-pathway-driven response) waveforms of the ff-ERG were similar in both species, while the times to peaks in dark-adapted 0.01 ERG (the rod-driven response of bipolar cells) and dark-adapted 3.0 ERG (combined rod and cone system responses) as well as the implicit times of the a- and b-waves in light-adapted 3.0 ERG (the single-flash cone response) were substantially different. This study provides normative retinal parameters for nonhuman primate research on basic and clinical ophthalmology, as well as a reference for researchers in the appropriate selection of rhesus or cynomolgus macaques as models for ophthalmology studies.

Age and Sex-Related Changes in Retinal Function in the Vervet Monkey

Among the deficits in visual processing that accompany healthy aging, the earliest originate in the retina. Moreover, sex-related differences in retinal function have been increasingly recognized. To better understand the dynamics of the retinal aging trajectory, we used the light-adapted flicker electroretinogram (ERG) to functionally assess the state of the neuroretina in a large cohort of age- and sex-matched vervet monkeys (N = 35), aged 9 to 28 years old, with no signs of obvious ocular pathology. We primarily isolated the cone–bipolar axis by stimulating the retina with a standard intensity light flash (2.57 cd/s/m²) at eight different frequencies, ranging from 5 to 40 Hz. Sex-specific changes in the voltage and temporal characteristics of the flicker waveform were found in older individuals (21–28 years-old, N = 16), when compared to younger monkeys (9–20 years-old, N = 19), across all stimulus frequencies tested. Specifically, significantly prolonged implicit times were observed in older monkeys (p < 0.05), but a significant reduction of the amplitude of the response was only found in old male monkeys (p < 0.05). These changes might reflect ongoing degenerative processes targeting the retinal circuitry and the cone subsystem in particular. Altogether, our findings corroborate the existing literature in humans and other species, where aging detrimentally affects photopic retinal responses, and draw attention to the potential contribution of different hormonal environments.

Advanced Retinal Imaging and Ocular Parameters of the Rhesus Macaque Eye

Purpose

To determine the range of normal ocular biometry and perform advanced retinal imaging and functional assessment of the rhesus macaque eye.

Methods

We performed ocular phenotyping on rhesus macaques at the California National Primate Research Center. This process consisted of anterior and posterior segment eye examination by ophthalmologists, advanced retinal imaging, and functional retinal electrophysiology.

Results

Full eye examinations were performed on 142 animals, consisting of pupillary light reflex, tonometry, external examination and photography, anterior slit lamp examination, and posterior segment examination by indirect ophthalmoscopy. Ages of the rhesus macaques ranged from 0.7 to 29 years (mean, 16.4 ± 7.5 years). Anterior segment measurements such as intraocular pressure (n = 142), corneal thickness (n = 84), lens thickness (n = 114), and axial length (n = 114) were acquired. Advanced retinal imaging in the form of fundus photography (n = 78), optical coherence tomography (n = 60), and quantitative autofluorescence (n = 44) was obtained. Electroretinography (n = 75) was used to assay retinal function. Quantitative analyses of the macular structure, retinal layer segmentation, and rod and cone photoreceptor electrical responses are reported. Quantitative assessments were made and variations between sexes were analyzed to compare with established sex changes in human eyes.

Conclusions

The rhesus macaque has an ocular structure and function very similar to that of the human eye. In particular macular structure and retinal function is very similar to humans, making this species particularly useful for the study of macular biology and development of therapies for cone photoreceptor disorders.

Translational Relevance

Rhesus macaques are an ideal model for future vision science studies of human eye diseases.

Factors Affecting Photopic Negative Response Recorded with RETeval System: Study of Young Healthy Subjects

Purpose

To determine whether there is a significant correlation between the amplitude of the photopic negative response (PhNR) and the peripapillary retinal nerve fiber layer thickness (pRNFLT) in eyes of young, healthy subjects.

Methods

We analyzed 136 eyes of 136 young, healthy subjects (89 males and 47 females; age, 20–29 years). The PhNRs were recorded with the RETeval system without mydriasis using red flashes on a blue background. PhNR amplitude was measured at two points: at 72 ms (P₇₂) and at the negative trough following the b-wave (P_min). Univariate and multivariable linear regression analyses were performed to identify the independent variables that were significantly correlated with P₇₂ and P_min. The variables included age, sex, axial length, pRNFLT, intraocular pressure (IOP), a-wave amplitude, b-wave amplitude, and pupillary area during the electroretinogram recordings.

Results

The amplitudes of P₇₂ and P_min were significantly larger in female subjects (P = 0.021 and P = 0.001, respectively). Univariate analyses showed that PhNR amplitudes were significantly correlated with pRNFLT (P₇₂: r = 0.246, P = 0.004; P_min: r = 0.219, P = 0.011). Female sex was significantly and negatively correlated with P₇₂ (r = –0.206; P = 0.016) and P_min (r = –0.271; P = 0.001). Multivariable regression analyses showed that greater pRNFLT was an independent factor significantly associated with a larger P₇₂ (r = 0.283; P = 0.004) and P_min (r = 0.299; P = 0.002). Female sex was an independent factor that was significantly associated with a larger P_min (r = –0.208; P = 0.022).

Conclusions

These findings indicate that PhNR amplitude is significantly associated with pRNFLT and female sex in young, healthy subjects.

Translational Relevance

The amplitude of the PhNR recorded with RETeval is smaller in subjects with thinner pRNFLT not only in glaucoma patients but also in young healthy subjects.

Structural and functional effects of hemiretinal endodiathermy axotomy in cynomolgus macaques

PURPOSE

Outer retinal injury has been well described in glaucoma. To better understand the source of this injury, we wanted to develop a reliable model of partial retinal ganglion cell (RGC) axotomy.

METHODS

Endodiathermy spots were placed along the inferior 180° adjacent to the optic nerve margin in the right eyes of four cynomolgus monkeys. Fluorescein angiography, spectral domain optical coherence tomography (SD-OCT), and multifocal electroretinography (mfERG) were performed at various intervals. Two animals were sacrificed at 3 months. Two animals were sacrificed at 4 months, at which time they underwent an injection of fluorescent microspheres to measure regional choroidal blood flow. Retinal immunohistochemistry for glial fibrillary acidic protein (GFAP), rhodopsin, S-cone opsin, and M/L-cone opsin were performed, as were axon counts of the optic nerves.

RESULTS

At 3 months, there was marked thinning of the inferior nerve fiber layer on SD-OCT. The mfERG waveforms were consistent with inner but not outer retinal injury. Greater than 95% reduction in axons was seen in the inferior optic nerves but no secondary degeneration superiorly. There was marked thinning of the nerve fiber and ganglion cell layers in the inferior retinas. However, the photoreceptor histology was similar in the axotomized and nonaxotomized areas. Regional choroidal blood flow was not affected by the axotomy.

CONCLUSIONS

Unlike experimental glaucoma, hemiretinal endodiathermy axotomy (HEA) of the RGCs produces no apparent anatomic, functional, or blood flow effects on the outer retina and choroid.

The effect of pentobarbital sodium and propofol anesthesia on multifocal electroretinograms in rhesus macaques

We compared the suitability of pentobarbital sodium (PB) and propofol (PF) anesthetics for multifocal electroretinograms (mfERGs) in rhesus macaques. mfERGs were collected from 4 ocularly normal rhesus macaques. All animals were pre-anesthetized with intramuscular ketamine (10-15 mg/kg). Intravenous PB induction/maintenance levels were 15 mg/kg/2-10 mg/kg and for PF, 2-5 mg/kg/6-24 mg/kg/h. There were 3 testing sessions with PB anesthesia and 5-7 testing sessions with PF anesthesia. All PB sessions were carried out before PF. First-order (K1) and second-order (first slice) kernels (K2.1) response density amplitude (RDA), implicit time (IT), and root mean square signal-to-noise ratios (RMS SNR) of the low-frequency (LFC) and high-frequency (HFC) components were evaluated. The use of PF or PB anesthesia resulted in robust, replicable mfERGs in rhesus macaques; however, RMS SNR of K1 LFC in ring and quadrant analyses was significantly larger for PF than for PB. Additionally, K1 RDA under PF was significantly larger than under PB for N1, P1, and P2 components (ring and quadrant) and for N2 (quadrant). PF IT was significantly prolonged (<1 ms) relative to PB IT for N1, P1 (ring), and N1 (quadrant), while PB IT was significantly prolonged (0.8-4.2 ms) relative to PF IT for N2 and P2 (ring and quadrant). K1 HFC and K2.1 LFC did not differ significantly between PB and PF in the ring or quadrant analyses. The response differences found with PB and PF anesthesia likely arise from variable relative effects of the anesthetics on retinal γ-aminobutyric acid (GABA(A)) receptors, and in part, on glycine and on glutamate receptors. Given the advantages of a stable anesthetic plane with continuous intravenous infusion and a smoother, more rapid recovery, PF is an appealing alternative for mfERG testing in rhesus macaques.

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.

Serial multifocal electroretinograms during long-term elevation and reduction of intraocular pressure in non-human primates

The purpose of this study was to evaluate the relationship between elevations of intraocular pressure (IOP) and the multifocal electroretinogram (mfERG) in non-human primates. Experimental glaucoma was induced in 4 rhesus and 4 cynomolgus monkeys by laser trabecular meshwork destruction (LTD) in one eye. To evaluate the contribution of ganglion cells to mfERG changes, one monkey of each species had previously underwent unilateral optic nerve transection (ONT). After >or=44 weeks of elevation, the IOP was reduced by trabeculectomy in 2 non-transected animals. In the intact (non-transected) animals, there was an increase in the amplitude of the early mfERG waveforms (N1 and P1) of the first-order kernel (K1) throughout the period of IOP elevation in all of the rhesus, but not all of the cynomolgus monkeys. A species difference was also present as a decrease of the second-order kernel, first slice (K2.1) in all of the cynomolgus monkeys but only in 1 of the rhesus monkeys (the 1 with the ONT). Similar IOP effects on the mfERG were seen in the ONT animals. Surgical lowering of IOP resulted in a return of the elevated K1 amplitudes to baseline levels. However, the depressed K2.1 RMS in the cynomolgus monkeys did not recover. These results demonstrate species-specific changes in cone-driven retinal function during periods of elevated IOP. These IOP-related effects can occur in the absence of retinal ganglion cells and may be reversible.

The multifocal electroretinogram (mfERG) in the pig

PURPOSE

To establish a method allowing multifocal electroretinography (mfERG) recording with simultaneous fundus monitoring on anaesthetized pigs. In addition we characterize the peaks of the porcine mfERG trace, and compare the visual streak area with the optic nerve head, a known non-response area. Finally we illustrate the feasibility of the method by performing mfERG after an induced laser burn in the visual streak.

METHODS

Fifteen pigs underwent mfERG recordings at baseline, and after 1 and 6 weeks of observation. One pig was evaluated before and after retinal diode laser treatment in the visual streak.

RESULTS

The porcine mfERG trace appears similar to the human mfERG trace, and can be described by three peaks named N1, P1 and N2. Significantly faster implicit time was found in the visual streak regarding N1 (P < 0.001) than in areas outside the visual streak. Amplitudes of all three peaks were increased in the visual streak (P < 0.005). The laser-treated area was characterized by a response similar to what is found at the location of the optic nerve head.

CONCLUSION

Porcine mfERG is similar in appearance to the human response and can be described by the same three peaks. Significantly higher amplitudes of all three peaks are found in the visual streak when compared to the optic nerve head and inferior retina. We have detected the functional deficit caused by a laser burn at the size of 3 x 3 mm.

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.

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.

Effects of reference electrode location on monopolar-derived multifocal electroretinograms in cynomolgus monkeys

The purpose of this study is to determine the effect of reference electrode location on the multifocal electroretinographic waveform. Multifocal electroretinograms (mfERGs) were recorded from 20 ocularly normal cynomolgus monkeys. The corneal electrode was an ERG-jet referenced to an ipsilaterally (outer canthus) situated subdermal needle electrode and to the contralateral corneal electrode. Testing was monocular and recordings from both montages were obtained simultaneously. The stimulus array consisted of 103 equal-sized hexagonal elements, which subtended +/-44 degrees about the central visual axis. Mean luminance of the display was 100 cd/m2. First-order (K1) and second-order (first slice) kernels (K2.1) of the mfERG were grouped in (a) 4 rings, representing the central 56 degrees of visual field and (b) in 15-element quadrants. The mfERG waveform measures included amplitude, implicit time, and root mean square (RMS) of the oscillatory potentials (OP) and response waveform. K1 and K2.1 ring and quadrant amplitudes were larger with the contralateral than with the ipsilateral reference, but more notably signal-to-noise ratios (S:N) of the response waveform were always larger with the ipsilateral reference. Implicit times were longer for the contralateral than ipsilateral reference montage. K1 and K2.1 implicit times in males were longer than in females. Quadrant groupings revealed generally larger K1 and K2.1 amplitudes in nasal than in temporal retina.