[Effects of sodium iodate, iodoacetic acid and ethambutol on electroretinogram and visual evoked potential in rats]

Oxidative Model of Retinal Neurodegeneration Induced by Sodium Iodate: Morphofunctional Assessment of the Visual Pathway

Sodium iodate (NaIO3) has been shown to cause severe oxidative stress damage to retinal pigment epithelium cells. This results in the indirect death of photoreceptors, leading to a loss of visual capabilities. The aim of this work is the morphological and functional characterization of the retina and the visual pathway of an animal model of retinal neurodegeneration induced by oxidative stress. Following a single intraperitoneal dose of NaIO3 (65 mg/kg) to C57BL/6J mice with a mutation in the Opn4 gene (Opn4-/-), behavioral and electroretinographic tests were performed up to 42 days after administration, as well as retinal immunohistochemistry at day 57. A near total loss of the pupillary reflex was observed at 3 days, as well as an early deterioration of visual acuity. Behavioral tests showed a late loss of light sensitivity. Full-field electroretinogram recordings displayed a progressive and marked decrease in wave amplitude, disappearing completely at 14 days. A reduction in the amplitude of the visual evoked potentials was observed, but not their total disappearance. Immunohistochemistry showed structural alterations in the outer retinal layers. Our results show that NaIO3 causes severe structural and functional damage to the retina. Therefore, the current model can be presented as a powerful tool for the study of new therapies for the prevention or treatment of retinal pathologies mediated by oxidative stress.

P-VECP can reveal visual toxicity in pigmented rats of repeated doses of ethambutol

We determined if pattern visually evoked cortical potentials (P-VECPs) in pigmented rats would reveal visual toxicity induced by a drug even when in cases of repeated doses. We obtained appropriate conditions of P-VECPs measurement; the spatial frequency, 0.16 cycle/degree; the mean stimulation luminance, 25 cd/m(2); and the stimulation frequency, 2 Hz. Twelve adult male pigmented rats (Iar: Long-Evans), weighing 210-301 g, were grouped into two (six per group): the control and the ethambutol (EB) 500 mg/kg administered groups. In the EB 500 mg/kg group, the rats were administered EB subcutaneously once daily for 6 weeks. Rats in the control group were given the vehicle subcutaneously once daily for 6 weeks. P-VECPs were carried out prior to initiation of drug administration and at the first, second, third, fourth, and sixth week of the administration. Prolongation of P1 latency in the P-VECPs was evident in the EB 500 mg/kg at fourth and sixth weeks, and there were no marked changes in the control group and no marked changes in P1N1 amplitude in either group. These findings suggest that P-VECPs in pigmented rats can detect chemically induced visual toxicity even in cases of repeated dosing of a drug. This approach is useful for evaluating the visual toxicity of drugs given repeatedly.

The effects of certain H(1)-antagonists on visual evoked potential in rats

The present study was undertaken to clarify the effects of certain H(1)-antagonists on visual evoked potential (VEP) in rats. Pyrilamine (5 and 10 mg/kg), diphenhydramine (5 and 10 mg/kg) and chlorpheniramine (10 and 20 mg/kg) caused a significant reduction in the amplitude of late VEP components (P(3)-N(3), N(3)-P(4)), although these drugs showed no significant changes in early VEP components (P(1)-N(1), N(1)-P(2)). Cyproheptadine caused a slight enhancement of late components of VEP at a dose of 20 mg/kg. On the other hand, epinastine caused no significant effect on late VEP components even at a dose of 20 mg/kg. The reduction in the late VEP components induced by pyrilamine and diphenhydramine was significantly antagonised by pre-treatment of histidine (200 and 500 mg/kg), but not by physostigmine even at a dose of 0.01 mg/kg. The effect induced by cyproheptadine was significantly potentiated by histidine (500 mg/kg), and significantly reduced by DOI (2 mg/kg). These results indicate that an inhibition of the late VEP components induced by H(1)-antagonist pyrilamine, diphenhydramine and chlorpheniramine may be due to an inhibition of specific sensory system relating the histaminergic mechanisms. In addition, slight enhancement of these components induced by cyproheptadine may be attributable to its anti-serotonergic effects.

Visually evoked cortical potentials obtained using checker patterns can detect ethambutol-induced visual toxicity in albino rats

We determined whether visually evoked cortical potentials obtained using checker patterns (P-VECPs) and albino rats would reveal visual damage induced by ethambutol (EB). Findings were compared in cases of detection of visual damage between by P-VECPs and by flash visually evoked cortical potentials (F-VECPs). Twelve adult albino male Crj:CD(SD)IGS rats were grouped into four, three per group: control, 250PS, 500PS, and 500SC groups. In the 250PS and 500PS groups, rats were administered EB orally for the first 2 weeks and then subcutaneously for the second 2 weeks to 250 and 500 mg/kg, respectively. In the 500SC group, rats were given 500 mg/kg of EB subcutaneously for 4 weeks. Rats in the control group were given the vehicle orally for the first 2 weeks and then subcutaneouly for the second 2 weeks. P-VECPs and F-VECPs were carried out prior to initiation of drug administration and at the 1st, 2nd, 3rd, and 4th weeks of the administration. Prolongation of P1 latency in the P-VECPs was evident in both the 500PS and the 500SC groups at the 4th week, while no marked changes were observed in the F-VECPs. Thus, P-VECPs in albino rats can detect visual damage induced by EB even when F-VECPs cannot do so. These studies suggest that P-VECPs are useful for evaluating the visual toxicity of drugs.

Recovery after severe ethambutol intoxication--psychophysical and electrophysiological correlations

Six patients with severe ocular side effects caused by therapeutical doses of the tuberculostatic drug ethambutol were investigated during the course of recovery with psychophysical and electrophysiological methods. Three patients developed an optic atrophy with permanently reduced vision as a likely consequence of additional risk factors such as diabetes, alcohol abuse, and reduced kidney function. The severity of the neuritis of the optic nerve was not related to the total intake of ethambutol. The likelihood of a permanent ocular damage increased sharply if the visual acuity had dropped below a value of 1/10. permanently prolonged latency of the P-100 component was found in visual evoked potentials even in cases with good recovery from ethambutol-induced damage. The recovery of color vision could be monitored very well with the Farnsworth-Munsell 100-Hue Test which revealed a diffuse impairment of color discrimination with a slight prevalence of the red-green axis. In addition to the known disturbances of the red-green antagonistic neurons, it could be demonstrated by measuring transient tritanopia and spectral sensitivity functions that ethambutol also affects the blue-yellow antagonism at the retinal level.