tetrodotoxin and Glaucoma

To determine retinal pathway origins of pattern electroretinogram (PERG) in macaque monkeys using pharmacologic dissections, uniform-field flashes, and PERG simulations.. Transient (2 Hz, 4 reversals/s) and steady state (8.3 Hz, 16.6 reversals/s) PERGs and uniform-field ERGs were recorded before and after intravitreal injections of L-AP4 (not APB) (2-amino-4-phosphonobutyric acid, 1.6-2.0 mM), to prevent ON pathway responses; PDA (cis-2,3-piperidinedicarboxylic acid, 3.3-3.8 mM), to block activity of hyperpolarizing second- and all third-order retinal neurons; and TTX (tetrodotoxin, 6 μM), to block Na+-dependent spiking. PERGs were also recorded from macaques with advanced unilateral experimental glaucoma, and were simulated by averaging ON and OFF responses to uniform-field flashes.. For 2-Hz stimulation, L-AP4 reduced both negative- and positive-going (N95 and P50) amplitudes in transient PERGs, and their counterparts, N2 and P1 in simulations, to half-amplitude. PDA eliminated N95 and N2, but increased P50 and P1 amplitudes, in that it enhanced b-waves. As previously reported, severe experimental glaucoma or TTX eliminated photopic negative responses, N95, and N2; glaucoma eliminated P50 and reduced P1 amplitude; TTX reduced P50 and hardly altered P1. For 8.3-Hz stimulation, L-AP4 eliminated the steady state PERG and reduced simulated PERG amplitude, whereas PDA enhanced both responses. TTX reduced PERG amplitude to less than half; simulations were less reduced. Blockade of all postreceptoral activity eliminated transient and steady state PERGs, but left small residual P1 in simulations.. Transient PERG receives nearly equal amplitude contributions from ON and OFF pathways. N95 reflects spiking activity of ganglion cells; P50 reflects nonspiking activity as well. Steady state PERG, in contrast, reflects mainly spike-related ON pathway activity.

Topics: Action Potentials; Animals; Computer Simulation; Disease Models, Animal; Electroretinography; Glaucoma; Intravitreal Injections; Macaca mulatta; Retina; Retinal Ganglion Cells; Sodium Channel Blockers; Tetrodotoxin; Visual Pathways

This study used the Matching Pursuit (MP) method, a time-frequency analysis, to identify and characterize oscillatory potentials (OPs) in the primate electroretinogram (ERG). When the slow-sequence mfERG from the macular region of the retina was matched with Gabor functions, OPs were identified in two distinct bands: a high-frequency band peaking around 150 Hz that contributes to early OPs, and a low-frequency band peaking around 80 Hz that contributes to both early and late OPs. Pharmacological blockade and experimental glaucoma studies showed that the high-frequency OPs depend upon sodium-dependent spiking activity of retinal ganglion cells, whereas the low-frequency OPs depend primarily upon non-spiking activity of amacrine cells, and more distal retinal activity.

Topics: Animals; Disease Models, Animal; Electroretinography; gamma-Aminobutyric Acid; Glaucoma; Macaca; N-Methylaspartate; Photic Stimulation; Pipecolic Acids; Retina; Retinal Ganglion Cells; Signal Processing, Computer-Assisted; Tetrodotoxin

To determine flash and background colors that best isolate the photopic negative response (PhNR) and maximize its amplitude in the primate ERG.. Photopic full-field flash ERGs were recorded from anesthetized macaque monkeys before and after pharmacologic blockade of Na(+)-dependent spiking activity with tetrodotoxin (TTX, 1 to 2 muM, n = 3), blockade of ionotropic glutamatergic transmission with cis-2,3 piperidine dicarboxylic acid (PDA, 3.3-3.8 mM, n = 3) or laser-induced monocular experimental glaucoma (n = 6), and from six normal human subjects. Photopically matched colored flashes of increasing stimulus strengths were presented on scotopically matched blue, white, or yellow backgrounds of 100 scot cd/m(2) using an LED-based stimulator.. PhNRs that could be eliminated by TTX or severe experimental glaucoma were present in responses to brief (<5 ms) and long-duration (200 ms) stimuli of all color combinations. In normal monkey and human eyes for brief low-energy flashes, PhNR amplitudes were highest for red flashes on blue backgrounds and blue flashes on yellow backgrounds. For high-energy flashes, amplitudes were more similar for all color combinations. For long-duration stimuli, the PhNR(on) at light onset in monkeys was larger for red and blue stimuli, regardless of background color, than for spectrally broader flashes, except for stimuli >17.7 cd/m(2) when PhNR(on)s were all of similar amplitude. For red flashes, eliminating the PhNR(on) pharmacologically or by glaucoma removed the slowly recovering negative wave that normally followed the transient b-wave and elevated the whole ON response close to the level of the b-wave peak. However, for white, blue, and green flashes, a lower-amplitude plateau that could be removed by PDA remained.. For weak to moderate flash strengths, the best stimulus for maximizing PhNR amplitude is one that primarily stimulates one cone type, on a background with minimal adaptive effect on cones. For stronger stimuli, differences in amplitude are smaller. For long-duration stimuli, red best isolates the PhNR(on) because it minimizes the overlapping lower-level plateau that originates from the activity of second-order hyperpolarizing retinal neurons.

Topics: Adult; Animals; Electroretinography; Glaucoma; Humans; Macaca mulatta; Photic Stimulation; Pipecolic Acids; Receptors, Glutamate; Retina; Retinal Cone Photoreceptor Cells; Tetrodotoxin

The purposes of this article are to gain insight into the cellular origins of the multifocal pattern electroretinogram (mPERG) and evaluate its potential for clinical use.. mPERGs were recorded from four anesthetized monkeys before and after pharmacologic blockade of light-driven activity of inner-retinal neurons and from 55 normal human subjects (19-91 years) and six patients with glaucoma (43-77 years of age). Stimuli consisted of counterphase-modulated black and white triangles organized in 61-scaled hexagons with mean luminance 100 cd/m2 and 100% contrast. The stimulus array subtended 31 degrees vertically and 37 degrees horizontally at 48 cm. The amplifier cutoff frequencies were 3 and 100 Hz. Responses were grouped as quadrants and the first slice of the second-order kernel was analyzed.. The mPERG responses of monkeys and humans were similar. In the monkey responses, there was an early positive potential (P1) around 25 ms and a later positive potential (P2) found selectively in the nasal field quadrants around 31 ms. These responses were seen around 22 and 36 ms in the human responses. After blockade of inner-retinal activity in monkeys, P1 amplitude was greatly reduced at all retinal locations and P2 was eliminated. P1 and P2 amplitudes were significantly reduced in the glaucomatous eyes relative to amplitudes of age-matched controls. Reductions in the amplitudes of P1 and P2 could easily discriminate between glaucomatous visual field quadrants with and without behavioral sensitivity losses. However, these alterations are likely to reflect diffuse losses.. mPERG responses contain prominent contributions from inner-retinal neurons that can be reduced in glaucomatous eyes. These findings raise the possibility that the mPERG could be potentially useful in the objective estimation of neural damage in glaucoma. However, further refinement of recording techniques will be required if the mPERG is to be used to detect focal damage.

Topics: Adult; Aged; Aged, 80 and over; Animals; Electroretinography; Evoked Potentials, Visual; Excitatory Amino Acid Agonists; Female; Glaucoma; Humans; Male; Middle Aged; N-Methylaspartate; Photic Stimulation; Retinal Ganglion Cells; Tetrodotoxin; Visual Fields

With the increasing use of the rat as an animal model for glaucoma and for the evaluation of neuroprotective treatments, there is a need for a sensitive test of retinal ganglion cell (RGC) function in this species. The aims of this study were to detect functional abnormalities of the inner retina in a rat model of high intraocular pressure (IOP) using the pattern electroretinogram (PERG), and to correlate them with morphometric analysis of RGC survival and the functional integrity of the inner retina. Unilateral ocular hypertension was induced in 17 Lewis rats through laser photocoagulation. Pattern ERGs were recorded prior to lasering and 3 weeks later, using a series of shifting patterns of decreasing spatial frequency projected directly onto the animals' fundus. IOP was measured at the same intervals, and the number of surviving RGCs estimated. Low amplitude PERG signals could be recorded in response to a narrow grating of 0.368 cycles per degree (cpd), and increased with stimulus size. Lasering caused mean (+/-s.d.) IOP to increase significantly from 18.3+/-4.5 (baseline) to 29.8+/-8.8 mmHg within 3 weeks (p<0.0001). At this time, PERG amplitudes were significantly reduced (p<0.05), declining an average of 45% compared to the normotensive, control eyes. No outer retinal damage was observed, but the mean number of RGCs decreased significantly (p<0.001), from 2 525.0+/-372.4 to 1 542.8+/-333.8 cells per mm2. This decrease in RGC number was significantly (p=0.03) correlated the decrease in PERG amplitude. The correlation between functional integrity of the inner retina and the rat PERG was further demonstrated by intravitreal tetrodotoxin injections, which temporarily abolished the PERG but did not affect outer retinal activity, reflected in the flash ERG. The evidence for early functional deficits, combined with tonometry and documentation of correlated ganglion cells loss, confirms the sensitivity of this diagnostic tool and the validity and importance of this animal model in glaucoma research.

Topics: Animals; Cell Survival; Disease Models, Animal; Electroretinography; Glaucoma; Male; Rats; Rats, Inbred Lew; Retinal Diseases; Retinal Ganglion Cells; Tetrodotoxin; Tonometry, Ocular

This study assessed the inter-ocular and inter-session variability of the transient pattern electroretinogram (PERG) in a group of non-human primates. The transient PERG was measured both eyes of 29 non-human primates, and again after three months in 23 eyes of 23 of these animals. Signals were elicited using a contrast (90%, 75 cdm(-2)) reversing (5 reversals sec(-1)) checkerboard pattern (0.56 cpd). PERGs were also measured for stimuli of varied spatial frequency (n=8, 0.07-2.22 cpd), contrast (n=4, 20-100%), mean luminance (n=4, 4.7-75 cdm(-2)) and defocus (n=5, +1, +2, +3 diopters). The inter-eye and inter-session limits-of-agreement (LOA; 95%) were determined for each PERG parameter. Variability was also compared with previous studies using the coefficient-of-variability (COV). Pharmacological blockade of the inner retinal contributions to the PERG measured under these conditions was conducted in one animal using intravitreal injection of tetrodotoxin (approximately 6 microM) and N-methyl-D-aspartic acid (approximately 6 microM). The N95 component of the primate transient PERG showed spatial tuning, with a peak between 0.14 and 0.28cpd. This spatial tuning was not as apparent for the P50 component. A linear relationship between P50 and N95 amplitude was found with contrast and mean luminance. Both components were attenuated with the introduction of +2 diopters or more of defocus. The inter-session COV for the P50 and N95 components were 23.8 and 19.2%, respectively, while the LOA were 58 and 46%, respectively. The N95:P50 ratio had smaller inter-session variability, was robust to changes in contrast, mean luminance and defocus, and was effective for characterization of inner-retinal dysfunction after pharmacologic block.

Topics: Animals; Electroretinography; Female; Glaucoma; Macaca mulatta; N-Methylaspartate; Pattern Recognition, Visual; Photic Stimulation; Reproducibility of Results; Tetrodotoxin

To determine whether the uniform field and pattern ERGs that are reduced in macaque eyes with experimental glaucoma have the same inner-retinal origins.. ERGs were recorded from 14 anesthetized adult macaques using DTL electrodes. Six monkeys had laser-induced experimental glaucoma, and two others received intravitreal injections of tetrodotoxin (TTX, 6 microM) to block spiking activity of inner-retinal neurons. The remaining 6 animals were normal. Uniform fields and grating patterns (0.1-3 cpd) were square-wave modulated at 1.7 Hz (transient) and 8 Hz (steady state). The test field (42 degrees x 32 degrees) had a mean luminance of 44 cd/m2 and a contrast of 10% to 82%.. In normal eyes transient ERGs to uniform fields contained photopic negative responses (PhNR) after the b-wave and after the d-wave. Transient pattern electroretinograms (PERGs) at each contrast reversal showed positive (P50) potentials followed by negative (N95) potentials of time course similar to that of the PhNR. The PhNR and N95 were greatly reduced or eliminated by experimental glaucoma and by TTX. Summing responses to luminance increments and decrements of the uniform field could simulate the PERG to low spatial frequency stimuli. Further, the PERG responses to high spatial frequencies were similar to the simulation in shape but slightly delayed in time. Experimental glaucoma and TTX had similar effects on the N95 of the simulated PERG as to those on the actual PERG. However, P50 was more reduced by experimental glaucoma than by TTX, indicating a nonspiking contribution to P50. For the steady state condition, the uniform field ERG, the simulated PERG, and the actual PERG all were affected by experimental glaucoma and TTX, indicating that they contained contributions from the spiking activity of ganglion cells.. The changes in the uniform field and PERG responses produced by experimental glaucoma are related and are largely a consequence of reduced spiking activity of ganglion cells and their axons. These findings raise the possibility that the uniform field ERG could serve as a useful alternative to the PERG in the assessment of clinical glaucomatous neuropathy.

Topics: Animals; Disease Models, Animal; Electroretinography; Glaucoma; Intraocular Pressure; Laser Therapy; Macaca mulatta; Microelectrodes; Pattern Recognition, Visual; Photic Stimulation; Retinal Ganglion Cells; Tetrodotoxin; Trabecular Meshwork; Visual Field Tests; Visual Fields

Multifocal ERGs (MERGs) of 5 adult monkeys (Macaca mulatta) with inner retinal defects caused by laser-induced glaucoma were compared to MERGs from 3 monkeys with inner retinal activity suppressed pharmacologically. MERGs were recorded with DTL fiber electrodes from anesthetized monkeys. Stimuli consisted of 103 equal size hexagons within 17 degrees of the fovea. Stimuli at each location passed through a typical VERIS m-sequence of white (200 cd/m2) and black (12 cd/m2) presentations. In animals with laser-induced glaucoma, visual field sensitivity was assessed by static perimetry using the Humphrey C24-2 full-threshold program modified for animal behavior. Inner retinal (amacrine and ganglion cell) activity was suppressed by intravitreal injection of TTX (4.7-7.6 microM) and NMDA (1.6-5 mM). In normal eyes the first order response (1st order kernel) was larger and more complex, with more distinct oscillations (>60 Hz) in central than in peripheral locations. The 2nd order kernel also was dominated by oscillatory activity. There were naso-temporal variations in both kernels. Pharmacological suppression of inner retinal activity reduced or eliminated the oscillatory behavior, and naso-temporal variations. The 1st order kernel amplitude was increased most and was largest at the fovea. Removed inner retinal responses also were largest at the fovea. The 2nd order kernel was greatly reduced at all locations. In eyes with advanced glaucoma, the effects were similar to those produced by suppressing inner retinal activity, but the later portion of the 1st order kernel waveform was different, lacking a dip after the large positive wave. Visual sensitivity losses and MERG changes both increased over the timecourse of glaucoma, with changes in the MERG being more diffusely distributed across the visual field. We conclude that 1st and 2nd order responses of the primate MERG can be identified that originate from inner retina and are sensitive indicators of glaucomatous neuropathy.

Topics: Animals; Disease Models, Animal; Electroretinography; Glaucoma; Injections; Intraocular Pressure; Macaca mulatta; N-Methylaspartate; Retina; Tetrodotoxin; Visual Field Tests; Vitreous Body

To investigate the photopic flash electroretinograms (ERGs) of macaque monkeys in which visual field defects developed as a consequence of experimental glaucoma.. Unilateral experimental glaucoma was induced in 10 monkeys by argon laser treatment of the trabecular meshwork. Visual field sensitivity was assessed behaviorally by static perimetry. Photopic ERGs were recorded to brief- (< or = 5 msec) and long-duration (200 msec) red ganzfeld flashes on a rod-suppressing blue-adapting background. Electroretinograms were recorded in four other monkeys, after intravitreal injection of tetrodotoxin (TTX; 3.8-8 p.M) to suppress action potentials of retinal ganglion and amacrine cells, and in six normal adult human subjects.. Experimental glaucoma removed a cornea-negative response, the photopic-negative response (PhNR), from the ERG. The PhNR in control eyes was maximal approximately 60 msec after a brief flash, 100 msec after onset, and 115 msec after offset of the long-duration stimulus. The PhNR in experimental eyes was greatly reduced when the mean deviation of the visual field sensitivity was as little as -6 dB. As visual sensitivity declined further, the PhNR was reduced only slightly more. The a- and b-waves were unchanged, even when sensitivity decreased by more than 16 dB. Tetrodotoxin also selectively reduced the PhNR. The PhNR was observed in normal human ERGs.. The cornea-negative PhNR of the photopic ERG depends on spiking activity and is reduced in experimental glaucoma when visual sensitivity losses are still mild. The PhNR most likely arises from retinal ganglion cells and their axons, but its slow timing raises the possibility that it could be mediated by glia. Regardless of the mechanism of its generation, the PhNR holds promise as an indicator of retinal function in early glaucomatous optic neuropathy.

Topics: Adult; Animals; Electroretinography; Female; Glaucoma; Humans; Intraocular Pressure; Macaca mulatta; Male; Middle Aged; Photic Stimulation; Reference Values; Retina; Sensitivity and Specificity; Tetrodotoxin; Visual Fields