Chemical stimulation of rat retinal neurons: feasibility of an epiretinal neurotransmitter-based prosthesis.

OBJECTIVE: No cure currently exists for photoreceptor degenerative diseases, which cause partial or total blindness in millions of people worldwide. Electrical retinal prostheses have been developed by several groups with the goal of restoring vision lost to these diseases, but electrical stimulation has limitations. It excites both somas and axons, activating retinal pathways nonphysiologically, and limits spatial resolution because of current spread. Chemical stimulation of retinal ganglion cells (RGCs) using the neurotransmitter glutamate has been suggested as an alternative to electrical stimulation with some significant advantages. However, sufficient scientific data to support developing a chemical-based retinal prosthesis is lacking. The goal of this study was to investigate the feasibility of a neurotransmitter-based retinal prosthesis and determine therapeutic stimulation parameters. APPROACH: We injected controlled amounts of glutamate into rat retinas from the epiretinal side ex vivo via micropipettes using a pressure injection system and recorded RGC responses with a multielectrode array. Responsive units were identified using a spike rate threshold of 3 Hz. MAIN RESULTS: We recorded both somal and axonal units and demonstrated successful glutamatergic stimulation across different RGC subtypes. Analyses show that exogenous glutamate acts on RGC synapses similar to endogenous glutamate and, unlike electrical prostheses, stimulates only RGC somata. The spatial spread of glutamate stimulation was ≈ 290 µm from the injection site, comparable to current electrical prostheses. Further, the glutamate injections produced spatially differential responses in OFF, ON, and ON-OFF RGC subtypes, suggesting that differential stimulation of the OFF and ON systems may be possible. A temporal resolution of 3.2 Hz was obtained, which is a rate suitable for spatial vision. SIGNIFICANCE: We provide strong support for the feasibility of an epiretinal neurotransmitter-based retinal prosthesis. Our findings suggest that chemical stimulation of RGCs is a viable alternative to electrical stimulation and could offer distinct advantages such as the selective stimulation of RGC somata.

Beta-blocker migraine prophylaxis affects the excitability of the visual cortex as revealed by transcranial magnetic stimulation.

The objective of this study is to assess effects of beta-blocker migraine prophylaxis on cortical excitability determined by transcranial magnetic stimulation (TMS). Phosphene and motor thresholds (PT, MT) were investigated in 29 patients with migraine, in 15 of them prior to and following preventive medication with metoprolol and in 14 patients without prophylaxis. Following prophylaxis headache frequency significantly decreased (p = 0.005) and mean PT were significantly increased (51.5 ± 7.5 vs. 63.6 ± 8.4%) compared to patients without preventive treatment (53.7 ± 5.3 vs. 52.3 ± 6.3%; p = 0.040). Mean MT did not significantly differ either between groups or due to treatment. In the group of all patients, a significant inverse correlation between headache frequency and the level of PT was found (R = -0.629; p < 0.01). There was, however, no significant correlation in the subgroups of patients. We conclude that (a) clinical efficacy of beta-blocker treatment in migraine could be (at least partly) linked to its ability to modulate the excitability of the visual cortex and (b) the PT determined by TMS appears suitable to assess the effects of prophylaxis on cortical excitability in the individual patient. This may be useful in clinical trials investigating migraine preventive drugs.

Visual hallucinations in dementia with Lewy bodies: transcranial magnetic stimulation study.

BACKGROUND: The aetiology of visual hallucinations is poorly understood in dementia with Lewy bodies. Pathological alterations in visual cortical excitability may be one contributory mechanism. AIMS: To determine visual cortical excitability in people with dementia with Lewy bodies compared with aged-matched controls and also the relationship between visual cortical excitability and visual hallucinations in dementia with Lewy bodies. METHOD: Visual cortical excitability was determined by using transcranial magnetic stimulation (TMS) applied to the occiput to elicit phosphenes (transient subjective visual responses) in 21 patients with dementia with Lewy bodies and 19 age-matched controls. RESULTS: Phosphene parameters were similar between both groups. However, in the patients with dementia with Lewy bodies, TMS measures of visual cortical excitability correlated strongly with the severity of visual hallucinations (P = 0.005). Six patients with dementia with Lewy bodies experienced visual hallucination-like phosphenes (for example, seeing people or figures on stimulation) compared with none of the controls (P = 0.02). CONCLUSIONS: Increased visual cortical excitability in dementia with Lewy bodies does not appear to explain visual hallucinations but it may be a marker for their severity.

Modulation of visual cortex excitability in migraine with aura: effects of valproate therapy.

We explored the effects of valproate treatment on visual cortex excitability changes in migraine with aura patients. Abnormal cortical excitability has been suggested to play an important role in the etiopathogenesis of migraine; in particular, it has been suggested a failure of inhibitory circuits in migraine with aura. Valproate acts as a central GABA agonist and it is reasonable suppose that VPA could modify cortical excitability state. Phosphene threshold (PT) was assessed at baseline and after 1Hz rTMS before and after one month therapy. We found that low-frequency rTMS in drug-free migraineurs decreased PT, while the treatment with the GABA agonist valproate is able to revert the effect of 1Hz rTMS over the occipital cortex. If the paradoxical increasing of PT to 1Hz rTMS is consequent upon the deficiency of intracortical inhibitory circuits in migraine, it seems reasonable to suppose that the effect of valproate is due to a recovery of activity of these circuits.

Correlation of increase in phosphene threshold with reduction of migraine frequency: observation of levetiracetam-treated subjects.

OBJECTIVES: To correlate the reduction in migraine frequency with change in phosphene threshold of transcranial magnetic stimulation during levetiracetam treatment. BACKGROUND: Several case series have suggested levetiracetam efficacy may be effective in the management of migraine. Phosphene threshold is reduced in patients with migraine with aura, migraine without aura, and menstrual migraine. Preventive treatment may raise phosphene threshold while reducing headache frequency. METHODS: Subjects experiencing 4-10 migraine attacks per month and not currently receiving preventive treatment for the indication of migraine were recruited into an open-label trial using levetiracetam, and asked to record headache symptoms, severity, duration, and acute medication use in a daily diary. Following a 28-day qualifying baseline period, subjects were titrated over 6 weeks to either a total daily dose of 3000 mg or their maximum tolerated dose (minimum tolerated daily dose of 1000 mg required). Transcranial magnetic stimulation was performed at day 28 and days 26, 28, 84, and 154. The visual cortex of each subject was stimulated 2 times at 20% power. Power was increased by 10% increments until at least one of the 2 stimulations produced a positive phosphene response. Once a positive response was achieved, a random order of 5 stimulation intensities surrounding the initial positive threshold was generated and given 3 times per session. Stimulation intensities were -10%, -5%, 0%, +5%, and +10% in relation to the positive threshold achieved. To eliminate a learning curve distortion, only observations at days 28, 84, and 154 were used for analysis. The mean phosphene threshold was defined as the average of the lowest positive threshold of the 3 stimulation sequences per visit. Ordinary least squares regression was used to evaluate the association between the change in mean daily headache rate from visit 3 to visit 7 and the change in mean transcranial magnetic stimulation threshold during the same period. RESULTS: Sixty-one subjects were enrolled. Twenty-one subjects were discontinued (because of poor study compliance or attack frequency) during the baseline phase prior to study drug initiation, and an additional subject whose data were not analyzed because of suspect quality. During the first 6 weeks on study drug (titration phase), 8 subjects dropped out (20.5%). Full analysis of the remaining 31 subjects, who reached a maintenance dose after 6 weeks on study medication, was performed. Subjects were largely white, female, and had a mean age of 41 +/- 13 years. Increasing age (beta = 1.27, P = .09), nonwhite race (beta = 6.90, P = .03), and diagnosis of tension-type headache (beta = 6.12, P = .095) were found to be associated with a higher mean transcranial magnetic stimulation threshold. Conversely, increasing body mass index was found to be associated with a lower mean transcranial magnetic stimulation threshold (beta = -1.19, P = .005). The number of migraine attacks decreased from 4.24 during the baseline interval to 2.53 during the interval preceding visit 7 (P = .001). There was a small but significant increase in transcranial magnetic stimulation threshold from visit 3 to visit 5 (P = .03) and visit 3 to visit 7 (P = .03 omnibus test). However,the difference between visit 5 and visit 7 was not statistically significant (P = .88). The mean transcranial magnetic stimulation threshold did not change from visit 5 to visit 7. CONCLUSION: Phosphene threshold increased during treatment with levetiracetam. At the 10% significance level, headache frequency and phosphene threshold were negatively correlated.

Cellular mechanisms underlying the pharmacological induction of phosphenes.

Visual sensations evoked by stimuli other than luminance changes are called phosphenes. Phosphenes may be an early symptom in a variety of diseases of the retina or of the visual pathways, but healthy individuals may perceive them as well. Phosphene-like phenomena are perhaps the most common side effect reported in clinical pharmacology. Ivabradine, a novel anti-anginal drug that reduces heart-rate by inhibiting the hyperpolarization activated current expressed in cardiac sinoatrial node cells (I(f)) induces phosphenes in some patients. One hypothesis is that ivabradine interacts with the visual system by inhibiting hyperpolarization-activated current in retinal cells (Ih). An Ih current with properties similar to cardiac I(f) has been reported in retinal neurones. Under normal circumstances most of the random fluctuations generated within the retinal circuits do not reach the level of conscious perception because they are filtered out. Presumably, filtering occurs mostly within the retina and one serious candidate for this action is the ability of Ih to act as a negative-feedback mechanism. Ih activation in the membrane of visual cells causes dampening of responses to slow noisy inputs thus tuning the visual system to perceptually more relevant signals of higher frequency. Ih inhibition, by altering at the retinal synapses the filtering of signals generated by thermal breakdown of rhodopsin or other fluctuations, is expected to increase the probability of phosphene occurrence. It is the purpose of the present paper to outline and discuss the features of the visual system and the pharmacological conditions relevant to phosphene perception.

Increased visual cortical excitability in ecstasy users: a transcranial magnetic stimulation study.

OBJECTIVES: To test the presence of abnormalities of visual cortical excitability in people using ecstasy as a recreational drug. METHODS: Ecstasy users and control subjects underwent single pulse transcranial magnetic stimulation (TMS) of the occipital cortex. The phosphene threshold was analysed and compared in the two groups. RESULTS: Phosphene thresholds were significantly lower in ecstasy users compared with control subjects, and were correlated negatively with frequency of ecstasy use. Frequency of use was positively correlated with the presence of visual hallucinations. The phosphene threshold of subjects with hallucinations was significantly lower than that of subjects without hallucinations. CONCLUSIONS: The use of ecstasy as a recreational drug is associated with an increased excitability of the visual cortex, possibly linked with massive serotonin release, followed by serotonin depletion, in this cortical area.

Optic neuritis with residual tunnel vision in perchloroethylene toxicity.

CASE REPORT: In a 57-year-old female owner of a dry-cleaning shop, we describe the association of severe bilateral optic neuritis with unexpectedly high concentrations of perchloroethylene/metabolites in the blood and of chloroform in urine. Visual disturbances consisted of complete blindness for 9 days in the left eye, for 11 days in the right eye, with bright phosphenes and pain on eye rotation. Only central (2-3 degrees radius) vision recovered in the following months. CONCLUSION: Although environmental concentrations of perchloroethylene were within normal limits, we measured five-fold increases in vapors emitted when ironing freshly dry-cleaned fabrics, and suggest that inhalation of perchloroethylene vapors was the cause of this case of ocular nerve toxicity, recapitulating a previous report of major perchloroethylene toxicity.