Dopamine depletion slows retinal transmission

Durable recovery from amblyopia with donepezil

An elevated threshold for neuroplasticity limits visual gains with treatment of residual amblyopia in older children and adults. Acetylcholinesterase inhibitors (AChEI) can enable visual neuroplasticity and promote recovery from amblyopia in adult mice. Motivated by these promising findings, we sought to determine whether donepezil, a commercially available AChEI, can enable recovery in older children and adults with residual amblyopia. In this open-label pilot efficacy study, 16 participants (mean age 16 years; range 9-37 years) with residual anisometropic and/or strabismic amblyopia were treated with daily oral donepezil for 12 weeks. Donepezil dosage was started at 2.5 or 5.0 mg based on age and increased by 2.5 mg if the amblyopic eye visual acuity did not improve by 1 line from the visit 4 weeks prior for a maximum dosage of 7.5 or 10 mg. Participants < 18 years of age further patched the dominant eye. The primary outcome was visual acuity in the amblyopic eye at 22 weeks, 10 weeks after treatment was discontinued. Mean amblyopic eye visual acuity improved 1.2 lines (range 0.0-3.0), and 4/16 (25%) improved by ≥ 2 lines after 12 weeks of treatment. Gains were maintained 10 weeks after cessation of donepezil and were similar for children and adults. Adverse events were mild and self-limited. Residual amblyopia improves in older children and adults treated with donepezil, supporting the concept that the critical window of visual cortical plasticity can be pharmacologically manipulated to treat amblyopia. Placebo-controlled studies are needed.

The Relationship between Visual-Evoked Potential and Optic Coherence Tomography and Clinical Findings in Parkinson Patients

Background

In Parkinson's disease (PD), dopamine deficiency is present not only in the nigrostriatal pathway but also in the retinal and visual pathways. Optic coherence tomography (OCT) can be used as morphological evidence of visual influence from early nonmotor symptoms. The aim of this study was to investigate the relationship of OCT and visual evoked potentials (VEPs) of eyes with the severity of clinical findings and ocular findings in PD.

Methods

A group of 42 patients diagnosed with idiopathic PD and a control group of 29 people between the ages of 45-85 were included in our study. VEP was recorded in the patient and control groups. OCT measurement was made with the Optovue spectral-domain device. Foveal thickness and macular volume were measured in the foveal region and in the parafoveal and perifoveal regions in the temporal, superior, nasal, and inferior quadrants. RNFL (retinal nerve fiber layer) was measured in temporal, superior, nasal, and inferior quadrants. Ganglion cell complex (GCC) was evaluated in the superior and inferior quadrants. Using the UPDRS clinical scale, the relationship between measurements and the differences between the control group and the patient group were evaluated.

Results

Among the OCT values in our study, foveal, parafoveal, perifoveal thickness, macular volume, RNFL, and GCC measurements were performed for the right and left eyes, and no difference was found between the patient group and the control group. There was no difference in VEP amplitude and latency values between the patient and control groups. The relationships between UPDRS and modified Hoehn Yahr staging and OCT and VEP measurements in the patient revealed no correlation.

Conclusions

Studies on whether OCT measurements can functionally be a marker or which segments are more valuable for disease progression in patients with PD are needed. Visual dysfunction in PD cannot be attributed only to retinal pathology; however, the retina may provide monitoring of the status of dopaminergic neurodegeneration and axonal loss in PD.

Does retina play a role in Parkinson's Disease?

Visual disorder is one of the non-motor symptoms found in Parkinson's disease (PD). It can be easily identified in the early stages even before the spread of pathological conditions to the brain parts. Studies have revealed that loss of dopamine (DA) cells in retinal layers is a prime cause for both retinal disturbance and pathological conditions of PD. This reduction of DA in retina is due to the aggregation of phosphorylated α-synuclein (aSyn) in the intra-retinal region, which eventually results in visual impairment in PD. Until now, very limited studies have been focused on the mechanism of aSyn influence and DA depletion as a cause for both retinal layer dysfunction and PD. Thus, more research is warranted to provide the missing connection between the exact role of DA and aSyn as a risk factor for visual problems in PD. Hence, the current review's focus is on the function and effects of DA degeneration in retinal cells of PD. Further, we suggest that iron plays a major role in regulating the aggregation of aSyn in the DA cells of retina and brain in PD. The study finds that the unidentified pathophysiological role of retinal degeneration in PD is an essential biomarker that needs further investigation to use it as a novel therapy in treating retinal dysfunctions in PD.

Evaluation of neurodegeneration through visual evoked potentials in restless legs syndrome

Restless legs syndrome (RLS) is a disease characterized by some type of dysesthesia, an indescribable abnormal sensation in the extremities. Our objective was to determine whether the visual evoked potentials (VEP) can be used as a quantitative monitoring method to evaluate demyelination-remyelination and neurodegeneration in the patients with RLS. The present study was carried out prospectively. It was planned to determine normal or pathological conditions in the form of increased latency or decreased amplitude of VEP and to evaluate possible pathologies in the visual and retinal pathways at early stages and at months 3 and 6 of follow-up in the patients with RLS (with or without iron deficiency anemia), in those without RLS (at the time of diagnosis prior to any medical therapy) without any visual symptoms. It was observed that latency of VEP improved but didn't return to normal limits following treatment with dopamin agonists, iron, or combination of both and that there was no significant difference between the post-treatment data and those of the control group. These results in combination with the fact that the latencies and amplitudes didn't return to normal levels despite the 6-month-treatment but showed a progressive course with partial regeneration suggests that there was incomplete remyelination. It should be kept in mind that this syndrome is likely to be a part of neurodegenerative process.

Colour vision in ADHD: part 1--testing the retinal dopaminergic hypothesis

OBJECTIVES

To test the retinal dopaminergic hypothesis, which posits deficient blue color perception in ADHD, resulting from hypofunctioning CNS and retinal dopamine, to which blue cones are exquisitely sensitive. Also, purported sex differences in red color perception were explored.

METHODS

30 young adults diagnosed with ADHD and 30 healthy young adults, matched on age and gender, performed a psychophysical task to measure blue and red color saturation and contrast discrimination ability. Visual function measures, such as the Visual Activities Questionnaire (VAQ) and Farnsworth-Munsell 100 hue test (FMT), were also administered.

RESULTS

Females with ADHD were less accurate in discriminating blue and red color saturation relative to controls but did not differ in contrast sensitivity. Female control participants were better at discriminating red saturation than males, but no sex difference was present within the ADHD group.

CONCLUSION

Poorer discrimination of red as well as blue color saturation in the female ADHD group may be partly attributable to a hypo-dopaminergic state in the retina, given that color perception (blue-yellow and red-green) is based on input from S-cones (short wavelength cone system) early in the visual pathway. The origin of female superiority in red perception may be rooted in sex-specific functional specialization in hunter-gather societies. The absence of this sexual dimorphism for red colour perception in ADHD females warrants further investigation.

Does levodopa improve vision in albinism? Results of a randomized, controlled clinical trial

BACKGROUND

Dopamine is an intermediate product in the biosynthesis of melanin pigment, which is absent or reduced in albinism. Animal research has shown that supplying a precursor to dopamine, levodopa, may improve visual acuity in albinism by enhancing neural networks. This study examines the safety and effectiveness of levodopa on best-corrected visual acuity in human subjects with albinism.

DESIGN

Prospective, randomized, placebo-controlled, double-masked clinical trial conducted at the University of Minnesota.

PARTICIPANTS

Forty-five subjects with albinism.

METHODS

Subjects with albinism were randomly assigned to one of three treatment arms: levodopa 0.76 mg/kg with 25% carbidopa, levodopa 0.51 mg/kg with 25% carbidopa, or placebo and followed for 20 weeks, with best-corrected visual acuity measured at enrollment, and at weeks 5, 10, 15, and 20 after enrollment. Side-effects were recorded with a symptom survey. Blood was drawn for genotyping.

MAIN OUTCOME MEASURES

Side-effects and best-corrected visual acuity 20 weeks after enrolment.

RESULTS

All subjects had at least one mutation found in a gene known to cause albinism. Mean age was 14.5 years (range: 3.5 to 57.8 years). Follow up was 100% and compliance was good. Minor side-effects were reported; there were no serious adverse events. There was no statistically significant improvement in best-corrected visual acuity after 20 weeks with either dose of levodopa.

CONCLUSIONS

Levodopa, in the doses used in this trial and for the time course of administration, did not improve visual acuity in subjects with albinism.

Visual functions in phenylketonuria-evaluating the dopamine and long-chain polyunsaturated fatty acids depletion hypotheses

BACKGROUND

In phenylketonuria presymptomatic treatment following newborn screening prevents severe mental and physical impairment. The reasons for subtle impairments of cerebral functions despite early treatment remain unclear. We assessed a broad spectrum of visual functions in early-treated patients with phenylketonuria and evaluated two hypotheses-the dopamine and the long-chain polyunsaturated fatty acids (LCPUFAs) depletion hypotheses.

METHODS

Contrast sensitivity, colour vision, electroretinography, frequency doubling technology campimetry (FDT), and their relation with blood phenylalanine and docosahexaenoic acid levels were assessed in 36 patients with phenylketonuria and 18 age-matched healthy controls.

RESULTS

Contrast sensitivity was significantly lower and total error scores in colour vision significantly higher in patients than controls. Electroretinography results differed significantly between patients and controls. We found a trend for the effect of phenylalanine-levels on contrast sensitivity and a significant effect on colour vision/FDT results. Docosahexaenoic acid levels in erythrocytes were not associated with visual functions.

CONCLUSION

This is the first evaluation of visual functions in phenylketonuria using a comprehensive ophthalmological test battery. We found no evidence supporting the long-chain polyunsaturated fatty acids depletion hypothesis. However, the effect of phenylalanine-levels on visual functions suggests that imbalance between phenylalanine and tyrosine may affect retinal dopamine levels in phenylketonuria. This is supported by the similar patterns of visual functions in patients with phenylketonuria observed in our study and patients with Parkinson's disease.

Acute administration of dopaminergic drugs has differential effects on locomotion in larval zebrafish

Altered dopaminergic signaling causes behavioral changes in mammals. In general, dopaminergic receptor agonists increase locomotor activity, while antagonists decrease locomotor activity. In order to determine if zebrafish (a model organism becoming popular in pharmacology and toxicology) respond similarly, the acute effects of drugs known to target dopaminergic receptors in mammals were assessed in zebrafish larvae. Larvae were maintained in 96-well microtiter plates (1 larva/well). Non-lethal concentrations (0.2-50 μM) of dopaminergic agonists (apomorphine, SKF-38393, and quinpirole) and antagonists (butaclamol, SCH-23390, and haloperidol) were administered at 6 days post-fertilization (dpf). An initial experiment identified the time of peak effect of each drug (20-260 min post-dosing, depending on the drug). Locomotor activity was then assessed for 70 min in alternating light and dark at the time of peak effect for each drug to delineate dose-dependent effects. All drugs altered larval locomotion in a dose-dependent manner. Both the D1- and D2-like selective agonists (SKF-38393 and quinpirole, respectively) increased activity, while the selective antagonists (SCH-23390 and haloperidol, respectively) decreased activity. Both selective antagonists also blunted the response of the larvae to changes in lighting conditions at higher doses. The nonselective drugs had biphasic effects on locomotor activity: apomorphine increased activity at the low dose and at high doses, while butaclamol increased activity at low to intermediate doses, and decreased activity at high doses. This study demonstrates that (1) larval zebrafish locomotion can be altered by dopamine receptor agonists and antagonists, (2) receptor agonists and antagonists generally have opposite effects, and (3) drugs that target dopaminergic receptors in mammals appear, in general, to elicit similar locomotor responses in zebrafish larvae.

The reorganized brain: how treatment strategies for stroke and amblyopia can inform our knowledge of plasticity throughout the lifespan

Neural plasticity plays a crucial role in human development. During development, neural networks are shaped by experience-dependent processes that selectively strengthen and prune connections so that those that remain match the environment and process it optimally. Over time, neural connections become more stable, forming widely distributed, interconnected networks involving balanced excitation and inhibition and structural stabilizers like myelin. It was long believed that the potential for organization or reorganization existed only during early development. However, the successful treatments for adults with stroke or amblyopia discussed in this issue suggest that the potential for significant reorganization persists well into adulthood. Thus, development can be thought of as the stabilization of connections to match the current environment but with considerable residual plasticity that can be revealed if there is a shift in the excitatory: inhibitory balance or the removal of the structural stabilizers.

Potential role of retina as a biomarker for progression of Parkinson's disease

Optical coherence tomography (OCT) noninvasively quantifies the thickness of the retinal nerve fiber layer (RNFL). OCT has been studied in several neuro-ophthalmic conditions, including Parkinson's disease (PD). Recent studies suggest that the quantitative analysis of RNFL can be precisely and noninvasively done by OCT scans and the results suggest that the thickness of RNFL is significantly decreased in patients with PD compared with age-matched controls and the foveal retinal thickness correlates with disease severity in PD. In this article, the application of OCT imaging of the retina in PD was reviewed. Literature survey of PubMed was carried out using the search terms of "Optical Coherence Tomography" combined with "Parkinson's Disease" and "retinal nerve fiber layer" (without restriction to the year of publication). Some related articles were also included. The search was completed in Jul. 2011 and revised and updated as necessary. The aim of this article is to review the current literatures on the use of optical coherence tomography in patients affected by PD and to enhance its use in clinical practice in neuro-ophthalmology.

Morphologic changes and functional retinal impairment in patients with Parkinson disease without visual loss

PURPOSE

To investigate the anatomic and electrophysiologic changes of the macula and the optic nerve in patients with Parkinson disease (PD) without visual impairment.

METHODS

Thirty-two eyes of 16 patients with PD (group A) without visual impairment were tested. Visual acuity was 20/20 or better and visual fields as well as color vision testing results were normal. Also, no retinal lesions were assessed. Patients in group B (40 eyes of 20 patients) were age- and sex-matched control subjects. All study participants underwent a comprehensive ophthalmic examination, multifocal electroretinogram (mfERG) recording, and optical coherence tomography (OCT) scan. Thickness of retinal nerve fiber layer (RNFL) along a 3.4-mm-diameter circle centered on the optic nerve head was evaluated using third-generation OCT.

RESULTS

The mean P1-response density amplitude of ring 1 of mfERG was 136.69 nV/deg2 in patients with PD and 294 nV/deg2 in control subjects and the difference was highly significant. On the contrary, these values in ring 2 and 3 did not differ statistically between controls and patients with PD. The mean inferior and temporal RNFL thickness was significantly lower in patients with PD than in control subjects (p<0.0001 and p=0.0045, respectively).

CONCLUSIONS

In patients with PD with normal vision, we found a decrease in the electrical activity of the fovea as well as in the thickness of the RNFL. Multifocal electroretinogram and OCT scan objectively detect early subclinical PD-associated visual functional impairment.

Levodopa/carbidopa in the treatment of amblyopia

PURPOSE

To evaluate the role of levodopa/carbidopa in the treatment of amblyopia.

METHODS

Thirty patients with strabismic amblyopia between the ages of 3 and 12 years were part of this double-blind, randomized study. Patients were divided into two groups. Group A received 0.50 mg + 1.25 mg of levodopa/carbidopa per kilogram body weight three times daily after meals, with a protein rich drink, whereas Group B received placebo. Both groups received full-time conventional occlusion until a visual acuity of 6/6 was achieved or for a maximum of 3 months.

RESULTS

The authors observed more than two lines improvement in visual acuity that was greater in the levodopa group (15 of 15) than in the placebo group (9 of 15) (P < .005). Furthermore, improvement in visual acuity of more than two lines was greater in patients younger than 8 years (100%) than in patients older than 8 years of age (60%) (P = .0026). There was also no significant reversal of the improved visual acuity in up to 6 months of follow-up.

CONCLUSION

Levodopa/carbidopa improves visual acuity in patients with amblyopia and maintains improved visual acuity, especially in patients younger than 8 years.

The possible role of retinal dopaminergic system in visual performance

It is a well-known fact that the retina is one of the tissues in the body, which is richest in dopamine (DA), yet the role of this system in various visual functions remains unclear. We have identified 13 types of DA retinal pathologies, and 15 visual functions. The pathologies were arranged in this review on a net grid, where one axis was "age" (i.e., from infancy to old age) and the other axis the level of retinal DA (i.e., from DA deficiency to DA excess, from Parkinson disorder to Schizophrenia). The available data on visual dysfunction(s) is critically presented for each of the DA pathologies. Special effort was made to evaluate whether the site of DA malfunction in the different DA pathologies and visual function is at retinal level or in higher brain centers. The mapping of DA and visual pathologies demonstrate the pivot role of retinal DA in mediating visual functions and also indicate the "missing links" in our understanding of the mechanisms underlying these relationships.

A rat model of Parkinsonism shows depletion of dopamine in the retina

The retinal dopamine (DA) deficiency is an important feature of the pathogenesis in Parkinson's disease (PD) visual dysfunction. Systemic inhibition of complex I (rotenone) in rats has been proposed as a model of PD. In this study, we investigated whether systemic inhibition of complex I can induce impairment of DA-ergic cells in the retina, similar to the destruction of retinal cells found in PD patients. Rotenone (2.5mg/kg i.p., daily) was administered over 60 days. Neurochemically, rotenone treated rats showed a depletion of DA in the striatum and substantia nigra (SN). In addition, the number of retinal DA-ergic amacrine cells was significantly reduced in the rotenone treated animals. This study is the first one giving highlight towards a deeper understanding of systemic complex I inhibition (rotenone as an environmental toxin) and the connection between both, DA-ergic degeneration in the nigrostriatal pathway, and in the DA-ergic amacrine cells of the retina.

Functional MRI of amblyopia before and after levodopa

Functional magnetic resonance imaging (fMRI) was applied to five older amblyopes with monocular amblyopia before and after levodopa treatment. During the experiment, images were acquired in two runs with visual stimulation delivered through the sound and the amblyopic eyes, respectively. The experiment was performed on each of the subjects, before and after their oral administration of levodopa/carbidopa (0.5/0.12 mg/kg) three times per day for 7 weeks. Our study demonstrated that there was no effect on the spatial extent of the visual cortical activation during the sound eye stimulation (P=0.17), but some improvement during the amblyopic eye stimulation (P=0.06). The volume ratio between the amblyopic and sound eye stimulation significantly increased after the treatment (P<0.05). This finding supports the previous studies of levodopa effect on amblyopia at the visual cortical level, and suggests that fMRI can be a useful tool in assessing changes of visual cortical activity after the treatment

Effect of levodopa and carbidopa in human amblyopia

BACKGROUND

To assess the role of continuous therapy for 3 weeks with levodopa and carbidopa in the management of human amblyopia in children and adults.

METHOD

There were 88 amblyopic eyes of 82 subjects included in this double masked randomized prospective clinical trial. Levodopa and carbidopa combination in 2 different dosage schedules were given to both adults and children. The response was monitored of the improvement in visual acuity, contrast sensitivity, and visually evoked potentials.

RESULT

Patients receiving higher dosages of levodopa and carbidopa in both adults and children showed a better response to treatment. However, the effect did not last beyond 9 weeks of stopping treatment.

CONCLUSION

Though levodopa and carbidopa therapy may not be able to ameliorate amblyopia on its own on a long-term basis, it may be considered nonetheless to be an important adjunct to conventional therapy because it may improve patient compliance for occlusion by improving visual acuity in the amblyopic eye. Thus, it offers promise of improving the functional outcome in these cases. However, longer follow-up trials are needed to substantiate these conclusions.

Effects of ethanol on flash-evoked potentials of rats: lack of antagonism by naltrexone

The present study examined the effects of ethanol and naltrexone hydrochloride (a nonselective opiate receptor antagonist) on flash-evoked potentials recorded from both the visual cortex (VC) and the superior colliculus (SC) of chronically implanted hooded rats. There were four treatment conditions administered on separate days: Either saline or naltrexone (10 mg/kg; volume of 1.0 ml/kg) was given 10 min before either saline or ethanol (2.0 g/kg; 20% ethanol solution in a volume of 1.26 ml/100 g). Evoked potentials were recorded 15 min after the intraperitoneal injections were completed. Animals were tested at 23.1 degrees C room temperature. In the VC, ethanol significantly decreased the amplitude of components N1, P3, and N3, whereas it increased the amplitude of P2. Components P1 and N2 were unaffected by ethanol treatment. The SC components P3 and N4 were reduced in amplitude by ethanol, but component P1 was not altered. Latencies of all components in both structures were increased by ethanol. Naltrexone alone did not significantly affect the potentials, nor did naltrexone pretreatment significantly alter the effects of ethanol on the potentials. Naltrexone produced a modest hypothermia of about 0.25 degrees C, whereas ethanol resulted in hypothermia of about 1.0 degrees C. Ethanol, either alone or in combination with naltrexone, significantly reduced body movement during the evoked-potential recording sessions. The results indicate that endogenous opioid systems do not play a major role in the acute effects of ethanol on flash-evoked potentials recorded from primary areas of the visual system.

Treatment with AC pulsed electromagnetic fields normalizes the latency of the visual evoked response in a multiple sclerosis patient with optic atrophy

Visual evoked response (VER) studies have been utilized as supportive information for the diagnosis of multiple sclerosis (MS) and may be useful in objectively monitoring the effects of various therapeutic modalities. Delayed latency of the VER, which reflects slowed impulse transmission in the optic pathways, is the most characteristic abnormality associated with the disease. Brief transcranial applications of AC pulsed electromagnetic fields (EMFs) in the picotesla flux density are efficacious in the symptomatic treatment of MS and may also reestablish impulse transmission in the optic pathways. A 36 year old man developed an attack of right sided optic neuritis at the age of 30. On presentation he had blurring of vision with reduced acuity on the right and fundoscopic examination revealed pallor of the optic disc. A checkerboard pattern reversal VER showed a delayed latency to right eye stimulation (P100 = 132 ms; normal range: 95-115 ms). After he received two successive applications of AC pulsed EMFs of 7.5 picotesla flux density each of 20 minutes duration administered transcranially, there was a dramatic improvement in vision and the VER latency reverted to normal (P100= 107 ms). The rapid improvement in vision coupled with the normalization of the VER latency despite the presence of optic atrophy, which reflects chronic demyelination of the optic nerve, cannot be explained on the basis of partial or full reformation of myelin. It is proposed that in MS synaptic neurotransmitter deficiency is associated with the visual impairment and delayed VER latency following optic neuritis and that the recovery of the VER latency by treatment with pulsed EMFs is related to enhancement of synaptic neurotransmitter functions in the retina and central optic pathways. Recovery of the VER latency in MS patients may have important implications with respect to the treatment of visual impairment and prevention of visual loss. Specifically, repeated pulsed applications of EMFs may maintain impulse transmission in the optic nerve and thus potentially sustain its viability.

The relationship between the putative optic pathway potential and the electroretinogram

The putative optic pathway flash visual evoked potential (FVEP) and the electroretinogram (ERG) were recorded consecutively in the lightly anesthetised rat. The purpose was to test the hypothesis that the optic pathway FVEP is only an artifact created by distorted volume-conducted retinal activity. A comparison of the timing of the ERG with that of the optic pathway FVEP confirmed this suspicion. It is shown that there is a close temporal correspondence between individual subcomponents of the optic pathway potential and those of the ERG (i.e., the a-wave, b-wave, and the oscillatory potentials). In addition, it was found that when ERG currents are recorded far field from the vicinity of the optic nerve or tract, they acquire a triphasic positive-negative-positive waveform, thereby heightening the illusion that the optic pathway FVEP is a genuine compound action potential. It is concluded that experimental findings derived from the recording of the optic pathway FVEP must be treated with caution.

Therapeutic effects of alternating current pulsed electromagnetic fields in multiple sclerosis

Multiple sclerosis is the third most common cause of severe disability in patients between the ages of 15 and 50 years. The cause of the disease and its pathogenesis remain unknown. The last 20 years have seen only meager advances in the development of effective treatments for the disease. No specific treatment modality can cure the disease or alter its long-term course and eventual outcome. Moreover, there are no agents or treatments that will restore premorbid neuronal function. A host of biological phenomena associated with the disease involving interactions among genetic, environmental, immunologic, and hormonal factors, cannot be explained on the basis of demyelination alone and therefore require refocusing attention on alternative explanations, one of which implicates the pineal gland as pivotal. The pineal gland functions as a magnetoreceptor organ. This biological property of the gland provided the impetus for the development of a novel and highly effective therapeutic modality, which involves transcranial applications of alternating current (AC) pulsed electromagnetic fields in the picotesla flux density. This review summarizes recent clinical work on the effects of transcranially applied pulsed electromagnetic fields for the symptomatic treatment of the disease.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Retinal dopamine depletion in young quail mimics some of the effects of ageing on visual function

The hypothesis that retinal dopaminergic (DA) neurones are involved in the visual functions of interest was tested. The retinal DA in young quail was partially depleted by intravitreal injection of 6-hydroxydopamine (6-OHDA). It was found that the refractive state of 6-OHDA-treated birds became more myopic than normal (untreated) young, whereas the pupil diameter was not affected. The contrast sensitivity of 6-OHDA treated quail was significantly lowered (two to three times) at all spatial frequencies studied (0.25-5 c/d), and the peak latency of pattern electro-retinogram (PERG) response was prolonged by 3-4 msec (9%). Furthermore, the visual acuity and maximal amplitude of PERG response of the 6-OHDA-treated young quail were lower than those of normals. From histochemical studies, it was revealed that the morphology of the DA cells of 6-OHDA-treated young appeared similar to those of the old quail; the DA cells of 6-OHDA-treated retinae were less fluorescent and 2.5-5 times less numerous than respective controls. Combining the PERG and the morphological results, it would seem that the retinal DA plays an important role in the visual functions studied, and that loss of retinal DA could underlie some of the visual changes which occur during ageing.

Effects of ageing on spatial aspects of the pattern electroretinogram in male and female quail

This study deals with age-related functional changes in the eye of quail. Measurements of lens refractive state and transmission, as well as pupil diameter, showed that the quail's eye became more myopic with increasing age, but no change could be detected in lens transmission and pupil diameter. Three aspects of the pattern electroretinogram (PERG) were studied after correcting for ocular refraction: (i) spatial characteristics (visual acuity and contrast sensitivity); (ii) maximal amplitude; and (iii) peak latency. The PERG results suggested that the visual acuity was age-independent for both sexes. However, the contrast sensitivity of the old quail (16-20 months) was much lower (1.5-3 times) than that of the young (3-6 months) at low to intermediate (< 2 c/d), but not at high spatial frequencies. The peak latency of the PERG response was 3-4 msec longer in the old birds compared with the young, while the maximal amplitude of the PERG response was age-independent. These results clearly suggested that at least some of the age-induced changes are located in the neural retina.

Effect of L-dopa on visual evoked potentials and neuropsychological tests in adult phenylketonuria patients

Eight adult, untreated patients with classical phenylketonuria received L-dopa and a decarboxylase inhibitor for 2 weeks. No effect of L-dopa therapy on choice reaction time tasks, sustained attention, frontal lobal function as well as latencies of visual evoked potentials was found. The results raise the question if adult patients with phenylketonuria really suffer from functional dopamine deficiency.

Weak electromagnetic fields increase the amplitude of the pattern reversal VEP response in patients with multiple sclerosis

Visual evoked potential (VEP) studies are widely used for the diagnosis of multiple sclerosis (MS) and are also useful in monitoring the effects of various therapeutic modalities in the disease. Brief, extracerebral applications of picotesla (pT) range flux intensity electromagnetic fields (EMFs) of low frequency have been shown efficacious in the treatment of motor and cognitive symptoms in MS implying that this treatment modality improves action potential transmission in demyelinating pathways. This report documents three MS patients with a remitting-progressive course in whom two successive brief extracerebral applications of pT range EMFs caused an immediate increase (and normalization) of the amplitudes of the visual evoked response in the eye previously affected by optic neuritis. However, the pretreatment prolonged latencies of the evoked responses remained essentially unchanged after the administration of EMFs. Since the latency of the VEP reflects the degree of conduction velocity and the amplitude the degree of conduction block in demyelinating optic pathways, the report demonstrates that extracerebral applications of these EMFs may rapidly reverse conduction block in demyelinating fibers. Reversal of the conduction block, which is though to be related to changes in axonal Na+ and K+ channels and synaptic neurotransmitter release, accounts for the immediate improvement of vision and other neurological deficits observed in MS patients following exposure to these EMFs.

Effect of weak electromagnetic fields on the amplitude of the pattern reversal VEP response in Parkinson's disease

Visual evoked potential (VEP) studies are widely used for the diagnosis of multiple sclerosis (MS) and are also useful in monitoring the effects of various therapeutic modalities in the disease. Prolongation of the VEP latencies has been demonstrated in patients with MS and in other neurodegenerative disorders including Parkinson's disease (PD), a disorder characterized by deficient cerebral dopamine (DA) functions. Pharmacological and biochemical studies have demonstrated a positive correlation between the amplitude of the VEP response and cerebral DA levels. Since brief, extracerebral applications of picotesla (pT) range flux intensity electromagnetic fields (EMFs) of low frequency have been shown to produce rapid improvement in motor and cognitive symptoms in PD, it is expected that application these EMFs would lead also to an increase in the amplitude of VEP response. This report documents three randomly selected PD patients who, following two successive brief extracerebral applications of pT range EMFs, showed an almost 3-fold increase of the mean pretreatment amplitude of the pattern reversal VEP in response to monocular stimulation. One patient underwent also a placebo EMF treatment which did not result in a significant change in the posttreatment amplitude. The study demonstrates that in Parkinsonian patients extracerebral application of these EMFs rapidly increases in amplitude of the VEP response and, by inference, cerebral DA levels presumably by increasing DA release.

Perceptual abnormalities in Parkinson's disease: top-down or bottom-up processes?

A wide variety of perceptual impairments have been reported in patients with Parkinson's disease (PD) in recent years; the underlying causes of these impairments have been variously attributed to different levels of the visual-cognitive system, from the retina to frontal cortex. Parkinsonian perceptual abnormalities could thus be interpreted as indirectly caused either by 'bottom-up' effects, stemming from dopaminergic dysfunction in the retina, or by 'top-down' effects, stemming from deficits in attention due to disturbances in the striatal-frontal system. Alternatively, a direct visuospatial impairment, perhaps related to the motor symptoms, has been considered. Data on three basic aspects of visual perception (3-D stereo vision, figure-ground discrimination, and pattern perception) which might be expected to give difficulty to PD subjects suffering changes in early processing mechanisms are reported. Visual complexity and the degree of mental manipulation of the material required both varied in different parts of the tests. PD patients were on stable drug regimens. It was found that disease severity interacted with performance: patients with mild PD showed little perceptual abnormality, patients with moderate PD showed only top-down effects, and patients with severe PD showed evidence both of top-down and of bottom-up deficits. Thus it appears that any retinal effects on perception in PD occur only in the advanced stages of the disease; in earlier stages any visual dysfunction probably reflects top-down disturbances from higher levels of the cognitive-behavioural system.

Rapid normalization of visual evoked potentials by picoTesla range magnetic fields in chronic progressive multiple sclerosis

This communication concerns a 55 year old woman with a 5 year history of chronic progressive multiple sclerosis (MS) in whom a single external application of extremely low magnetic fields (MF) (7.5 picoTesla; 5 Hz frequency) of 20 minutes duration resulted in a rapid improvement in symptoms including vision, cerebellar symptomatology (ataxia and dysarthria), bladder functions, mood, sleep, cognitive functions and fatigue. Improvement in the patient's symptoms was associated with normalization of the pretreatment abnormal visual evoked potential (VEP) latencies within 24 hours after magnetic treatment. The rapid normalization of the VEP latencies suggests that recovery did not occur as a result of remyelination but probably was related to enhancement of neurotransmitter functions. MF have been shown to alter cellular calcium metabolism which may facilitate axonal conduction in demyelinating plaques. Furthermore, as MF affects the release of the pineal gland's principal hormone, melatonin, which influences the release of monoamines, it is also hypothesized that the effects of picoTesla MF in MS are partly mediated by the pineal gland which has recently been implicated in the pathogenesis of MS (Sandyk, 1992 a).

Visual and auditory evoked potentials in early onset Parkinson's disease and their relationship to cerebrospinal fluid monoamine metabolites

We studied visual (VEP) and brainstem auditory (BAEP) evoked potential changes in 23 patients with early onset Parkinson's disease (EOPD) to establish the nature of the changes as well as their relationship to dopaminergic (DA) and serotonergic (5-HT) disturbances, as determined by cerebrospinal fluid levels of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA). We also compared these parameters between the young onset (YOPD) and juvenile Parkinsonism (JP), the two subgroups of EOPD, to look for any possible differences between the two. In EOPD, the mean P100 latency of the VEP was significantly prolonged compared to controls (p < 0.001). However, within EOPD the evoked potential parameters were not significantly different between YOPD and the JP subgroups. P100 latency was abnormal in six patients (YOPD: 5, JP: 1) (26%). Six patients (YOPD: 3, JP:3) (26%) had abnormal BAEP. A significant negative correlation (r: -0.89, p < 1%) was observed between the P100 latency and CSF HVA levels. No correlation was observed between the BAEP interpeak latencies and either CSF HVA or 5-HIAA levels. This study suggests that VEP and BAEP abnormalities do occur in EOPD (in both YOPD and JP), and that the prolongation of P100 latency is secondary to DA deficiency as in PD. The cause of BAEP abnormalities is probably independent of DA and 5-HT disturbances. The only difference between EOPD and classical PD was the higher incidence of BAEP abnormalities in EOPD. There was no correlation between the VEP or BAEP changes to either the age at onset or duration of EOPD.

Magnetic fields normalize visual evoked potentials and brainstem auditory evoked potentials in multiple sclerosis

The present communication concerns a 46 year old woman with a 10 year history of chronic progressive multiple sclerosis (MS) in whom external application of magnetic fields (MF) (7.5 picoTesla; 5 Hz) during a period of remission resulted in a rapid and dramatic improvement in symptoms including vision, cerebellar symptomatology (ataxia and dysarthria), mood, sleep, bowel and bladder functions as well as fatigue. Improvement in the patient's symptoms was associated with normalization of the pretreatment latencies of the visual evoked potentials and brainstem auditory evoked potential responses within a week after initiation of magnetic treatment. This report demonstrates that treatment with picoTesla MF is an effective, nonpharmacological modality in the management of MS and for the first time documents reversal of abnormal evoked potential responses by this treatment. The pineal gland is a magnetosensor. As MF affect the release of the pineal gland's principal hormone, melatonin, it is hypothesized that the effects of picoTesla MF in MS are partly mediated by the pineal gland which has recently been implicated in the pathogenesis of MS (Sandyk, 1992 a; b).

The effects of low-pass filtering on the flash visual evoked potential of the albino rat

Flash visual evoked potentials (FVEPs) were recorded from the rat in order to determine the effects of low-pass filtering on the wave form. The low-frequency (high pass) filter remained fixed at 3.2 Hz while the setting of the high-frequency (low-pass) filter was progressively raised from 32 Hz to 3.2 kHz. The amplitude of the primary cortical potential (P30) steadily increased while its latency decreased until asymptotic values were recorded with a low-pass cut-off of 320 Hz. Thereafter, there was little additional change in wave form. It is concluded that a bandpass of 3.2-320 Hz is optimal to record the primary cortical response of the FVEP, and this is consistent with the theory that the P30 potential is generated by comparatively slow post-synaptic activity. In a second experiment the effects of low-pass filtering were examined on the later and more labile secondary components of the FVEP wave form. These were found to be less responsive to low-pass filtering than the early components and assumed a near optimal configuration when the high-frequency cut-off was raised to 80 Hz. The high-frequency filter setting which is most appropriate to record the primary component of the FVEP therefore appears to be more than adequate also to record the secondary responses. It is also shown that the same principles of low-pass filtering on the FVEP will apply irrespective of whether the subject is awake or anaesthetised.

Low contrast stimuli enhance PERG sensitivity to the visual dysfunction in Parkinson's disease

The pattern electroretinogram (PERG) was recorded at different contrast levels (96%, 71%, 47%) in 10 Parkinson's disease patients before and during dopaminergic monotherapy. The data were compared to a control group of 8 normal subjects recorded with the same procedure. PERG P50 latency progressively increased as contrast was decreased both in normal subjects and patients; however, this trend was much more pronounced in PD patients without therapy; consequently in this group the difference between P50 latency obtained with 96% and 47% contrast was statistically significant (P = 0.01, analysis of variance corrected by post-hoc Tukey test). By contrast this was not seen in the control group. Statistical analysis (Bonferroni's t test) showed at the 47% contrast level a significant P50 latency increase (P less than 0.01) in PD patients without therapy if compared with the control group. Dopaminergic monotherapy induced a P50 latency recovery in PD patients. We conclude that low contrast stimuli enhance PERG sensitivity to the visual dysfunction of PD patients. Moreover, the effects observed after therapy confirm that abnormal contrast response functions in PD patients are linked to dopaminergic deficiency.

Localization and function of dopamine in the adult vertebrate retina

Dopamine (DA) has satisfied many of the criteria for being a major neurochemical in vertebrate retinae. It is synthesized in amacrine and/or interplexiform cells (depending on species) and released upon membrane depolarization in a calcium-dependent way. Strong evidence suggests that it is normally released within the retina during light adaptation, although flickering and not so much steady light stimuli have been found to be most effective in inducing endogenous dopamine release. DA action is not restricted to those neurones which appear to be in "direct" contact with pre-synaptic dopaminergic terminals. Neurones that are several microns away from such terminals can also be affected, presumably by short diffusion of the chemical. DA thus affects the activity of many cell types in the retina. In photoreceptors, it induces retinomotor movements, but inhibits disc shedding acting via D2 receptors, without significantly altering their electrophysiological responses. DA has two main effects upon horizontal cells: it uncouples their gap junctions and, independently, enhances the efficacy of their photoreceptor inputs, both effects involving D1 receptors. In the amphibian retina, where horizontal cells receive mixed rod and cone inputs, DA alters their balance in favour of the cone input, thus mimicking light adaptation. Light-evoked DA release also appears to be responsible for potentiating the horizontal cell-->cone negative feed-back pathway responsible for generation of multi-phasic, chromatic S-potentials. However, there is little information concerning action of DA upon bipolar and amacrine cells. DA effects upon ganglion cells have been investigated in mammalian (cat and rabbit) retinae. The results suggest that there are both synaptic and non-synaptic D1 and D2 receptors on all physiological types of ganglion cell tested. Although the available data cannot readily be integrated, the balance of evidence suggests that dopaminergic neurones are involved in the light/dark adaptation process in the mammalian retina. Studies of the DA system in vertebrate retinae have contributed greatly to our understanding of its role in vision as well as DA neurobiology generally in the central nervous system. For example, the effect of DA in uncoupling horizontal cells is one of the earliest demonstrations of the uncoupling of electrotonic junctions by a neurally released chemical. The many other, diverse actions of DA in the retina reviewed here are also likely to become model modes of neurochemical action in the nervous system.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of a dopaminergic agonist, piribedil (Trivastal 50 mg LP), on visual and spatial integration in elderly subjects

Dopamine acts as a neuromodulator in the retina. Dopaminergic deficiency of any origin, as observed in elderly subjects, is associated with altered visual performances, and more specifically with altered perception of contrasts. The goal of this study was to compare contrast vision in elderly subjects and young subjects (first phase, n = 20), then to compare this function in elderly subjects before and after 3 months of treatment with a dopaminergic agonist, piribedil (Trivastal 50 mg LP), administered once daily (second phase, n = 20). The perception of contrast was analysed using a test measuring sensitivity to colour contrast yielding threshold values for sensitivity to contrast in eight spatial frequencies and three colours (red, green, blue) and in two directions (horizontal and vertical). The results of the first phase of the study demonstrated that elderly subjects showed a decrease in contrast perception in comparison with young subjects, primarily in the high frequency range, and over the full range of stimulation for direction and colour. In the second phase, elderly subjects, in comparison with young subjects, showed altered visual contrast, again in the high frequency range, but also in the low frequency range for horizontal simulation with red and blue. After 3 months of treatment with piribedil the mean contrast sensitivity threshold, over the entire frequency range, had significantly increased (P less than 0.05) for all stimulations, apart for red in the vertical direction. These results underline the value of treatment with a dopaminergic agonist, piribedil in visual disturbances in patients with dopaminergic deficiency (Parkinson patients or elderly subjects).

Impairment of high-contrast visual acuity in Parkinson's disease

Several studies have shown that the visual system is affected in Parkinson's disease (PD) with reduced contrast sensitivity, low-contrast acuity, and flicker sensitivity, as well as altered electroretinograms (ERGs) and pattern visual evoked potentials (VEPs). Apparently, however, no study has yet specifically determined whether visual acuity to high-contrast stimuli is impaired in PD. Visual acuity was measured in a group of 16 patients with PD, both on and off drugs (for 24 h), and 16 age- and sex-matched normal control subjects. Acuity was impaired in the PD group both on standard Snellen chart and on a screen in a computerized test of visual resolution. The degree of impairment was 24 and 25%, respectively, in the two tests. The PD patients had marginally better acuity on both tests while receiving drugs, but the differences were not significant. The difference between the two groups was consistent with impaired resolution and could not be accounted for by any perceptual dysfunction that may also have been present in the PD group. Conversely, however, impaired acuity may be implicated in studies that have reported mild deficits of visuospatial/visuoperceptual function in PD. Reduced acuity appears to be a subtle sequela of dopaminergic deficiency in the visual system.

Are the differential effects of chloral hydrate on hooded rats vs. albino rats due to pigmentation or strain differences?

Effects of chloral hydrate anesthesia on EEG power spectra and VEP components were examined as a function of both pigmentation and strain differences in rats. Ten albino Westenberg Long Evans rats (WLE A) were compared to ten pigmented Westenberg Long Evans rats (WLE P), and to ten Wistar albino (Wis A) rats. Albino rats required less chloral hydrate to reach a deep level of anesthesia than pigmented rats. Wistar rats remained anesthetized longer than WLE rats. During deep levels of anesthesia, the lowest EEG frequency band contained more power in Wistar rats than in WLE rats. During moderate levels of anesthesia, frequencies less than 4 Hz lost power while frequencies greater than 13 Hz gained across all rats. Wistar rats had more power in the frequencies less than 8 Hz than did WLE rats; pigmented rats had more power in the frequencies greater than 13 Hz than did albinos. VEP component latencies of pigmented rats were shorter than albinos. Component amplitudes were not significantly different between groups.

Effects of chlorpromazine on pattern and flash ERGs and VEPs compared to oxazepam and to placebo in normal subjects

Antidopaminergic drugs delay the pattern-reversal VEP (P-VEP) and the flash VEP (F-VEP) and, in separate studies, reductions in the amplitude and increases in the latencies of scotopic ERGs have been reported. This study investigated the effects of chlorpromazine (CPZ) on the pattern ERG (P-ERG), P-VEP, flash ERGs and VEPs and oscillatory potentials (OPs). Normal volunteers (N = 15) were administered a placebo, or a single dose of CPZ 100 mg or oxazepam (OZP) 15 mg at weekly intervals, in a double-blind crossover design. A gold foil-ipsilateral ear derivation and an Oz'-Fz derivation were used for the ERG and VEP recordings, respectively. The latencies of 'mixed' and cone ERGs were significantly prolonged after CPZ compared to both placebo and to OZP. Amplitudes of rod- and cone-dominated ERGs were reduced following CPZ administration. All components of the OPs were significantly delayed after CPZ administration. No significant intertreatment differences were found in the F-VEP results. The P-ERG P50 peak and the P-VEP N70 and P100 peaks were significantly delayed after CPZ in the case of 28' checks but not 55' checks. Retinocortical times and P-ERG and P-VEP amplitudes were not significantly affected. In contrast to CPZ, the administration of OZP had virtually no significant effects compared to placebo. These findings suggest that the antidopaminergic CPZ has a primary effect on retinal electrophysiology. Similar findings have been reported in Parkinson's disease and in animal models.

Visual deficits related to dopamine deficiency in experimental animals and Parkinson's disease patients

In patients affected by Parkinson's disease, and in the monkey model of this disease, visual defects have been shown using psychophysical and electrophysiological measures of spatial and temporal contrast sensitivity. These studies imply an essential role for dopamine in primate vision. There is electrophysiological and neurochemical evidence to suggest that at least part of the problem is impaired retinal processing caused by systemic dopaminergic deficiency. Some of the deficits that have been demonstrated, consistent with physiological studies, suggest that center-surround interaction of neurons may suffer as a consequence of dopaminergic deficiency. The role of the regulation of retinal dopamine (D1 and D2) receptors in primate vision and of the balance of these receptors in presynaptic dopaminergic deficiency is not yet determined. Using sinusoidal grating stimuli in cognitively loaded tasks may increase understanding of the behavioral consequences of visual deficits seen in dopamine deficiency syndromes.

Age and fitness effects on EEG, ERPs, visual sensitivity, and cognition

Measures of EEG, event-related potentials (ERPs), visual sensitivity, and cognition were obtained from 30 young (20-31 years) and 30 older (50-62 years) healthy men. Age groups were evenly divided between subjects with low and high fitness levels documented by VO2max during a maximal exercise test. Age comparisons revealed that, compared to young adults, the older men had reduced visual sensitivity, delayed ERP latencies, greater homogeneity of EEG activity across recording sites, more positive visual-evoked potential (VEP) amplitude-intensity (A/I) slope, and poorer performance on a battery of neurocognitive tests. The EEG and VEP A/I slope findings are believed to reflect weakened central inhibition for the older men. In general, the measures that differentiated groups on the basis of age were also sensitive to differences in aerobic fitness. Compared to low fit men, the physically active men had shorter ERP latencies, stronger central inhibition, better neurocognitive performance, and better visual sensitivity. We speculate the performance superiority of the physically active men was, at least in part, the result of more oxygen being available for cerebral metabolism.

Haloperidol delays visually evoked cortical potentials but not electroretinograms in mice

Visually evoked cortical potentials (VECPs) and electroretinograms (ERG) to flash stimulation were studied in one ICR strain mouse and 23 ICR mice, respectively. The effect of a dopamine receptor blocker, haloperidol, on both responses was investigated. Significant peak latency prolongation was found in VECPs following injection of haloperidol, while the amplitude reduction of the b-wave was found in ERG. No effect of anesthetics only on ERG was found. It has been assumed that haloperidol affects VECPs related function mainly the higher visual pathway. However, the slight decrease of the b-wave amplitude of ERG could not exclude the possibility of the toxic effect on the retina.

Comparative studies on the retinal dopamine response to altered magnetic fields in rodents

Previously it was demonstrated that experimental alteration of the ambient magnetic field at night significantly reduced catecholamine levels in the retinae of Sprague-Dawley rats. As this effect appeared to depend on intact photoreceptors, it seemed of interest to examine whether the dopamine response to altered magnetic fields (MFs) differed between rod-dominant retinae and cone-dominant retinae. Furthermore, the effect of MFs on daytime dopamine content was explored. As in previous nocturnal investigations, dopamine levels in light-adapted (i.e. daytime) retinae from albino rats were significantly reduced by MFs. In the cone-dominant retina of the pigment ground squirrel, a similar MF effect was observed. However, in the rod-dominant retina of the golden hamster, dopamine levels increased significantly following daytime MF exposure. These results indicate that the retinal dopaminergic system is differentially responsive to MFs in various rodent species. Hence, the retina may play an important role in the perception of MFs by mammals.

Effect of levodopa on the human pattern electroretinogram and pattern visual evoked potentials

Pattern electroretinograms (P-ERGs) and pattern visual evoked potentials (P-VEPs) were recorded at three luminance levels and five different check sizes in a group of 16 control subjects before and after the oral administration of levodopa. At the lower luminance levels, significant decrease in P-ERG and P-VEP latencies were found. For P-VEPs the latency changes occurred only at small check sizes. No changes were observed in control experiments without levodopa administration. Our results show that levodopa-induced changes even occur at the retinal level and support a dopaminergic involvement in light and dark adaptation. Our observations are in agreement with a VEP delay found in Parkinson's disease and with a VEP latency increase in rats after dopamine depletion.