The push-pull action of dopamine on spatial tuning of the monkey retina: the effects of dopaminergic deficiency and selective D1 and D2 receptor ligands on the pattern electroretinogram

Multimodal brain and retinal imaging of dopaminergic degeneration in Parkinson disease

Parkinson disease (PD) is a progressive disorder characterized by dopaminergic neurodegeneration in the brain. The development of parkinsonism is preceded by a long prodromal phase, and >50% of dopaminergic neurons can be lost from the substantia nigra by the time of the initial diagnosis. Therefore, validation of in vivo imaging biomarkers for early diagnosis and monitoring of disease progression is essential for future therapeutic developments. PET and single-photon emission CT targeting the presynaptic terminals of dopaminergic neurons can be used for early diagnosis by detecting axonal degeneration in the striatum. However, these techniques poorly differentiate atypical parkinsonian syndromes from PD, and their availability is limited in clinical settings. Advanced MRI in which pathological changes in the substantia nigra are visualized with diffusion, iron-sensitive susceptibility and neuromelanin-sensitive sequences potentially represents a more accessible imaging tool. Although these techniques can visualize the classic degenerative changes in PD, they might be insufficient for phenotyping or prognostication of heterogeneous aspects of PD resulting from extranigral pathologies. The retina is an emerging imaging target owing to its pathological involvement early in PD, which correlates with brain pathology. Retinal optical coherence tomography (OCT) is a non-invasive technique to visualize structural changes in the retina. Progressive parafoveal thinning and fovea avascular zone remodelling, as revealed by OCT, provide potential biomarkers for early diagnosis and prognostication in PD. As we discuss in this Review, multimodal imaging of the substantia nigra and retina is a promising tool to aid diagnosis and management of PD.

Retinal Degeneration: A Window to Understand the Origin and Progression of Parkinson’s Disease?

Parkinson’s disease (PD), the second most prevalent neurodegenerative disorder, manifests with motor and non-motor symptoms associated with two main pathological hallmarks, including the deterioration of dopaminergic cells and aggregation of alpha-synuclein. Yet, PD is a neurodegenerative process whose origin is uncertain and progression difficult to monitor and predict. Currently, a possibility is that PD may be secondary to long lasting peripheral affectations. In this regard, it has been shown that retinal degeneration is present in PD patients. Although it is unknown if retinal degeneration precedes PD motor symptoms, the possibility exists since degeneration of peripheral organs (e.g., olfaction, gut) have already been proven to antedate PD motor symptoms. In this paper, we explore this possibility by introducing the anatomical and functional relationship of retina and brain and providing an overview of the physiopathological changes of retinal structure and visual function in PD. On the basis of the current status of visual deficits in individuals with PD, we discuss the modalities and pathological mechanism of visual function or morphological changes in the retina and focus on the correlation between visual impairment and some representative structural features with clinical significance. To consider retinal degeneration as a contributor to PD origin and progress is important because PD evolution may be monitored and predicted by retinal studies through state-of-the-art techniques of the retina. It is significant to integrally understand the role of retinal morphological and functional changes in the neurodegenerative process for the diagnosis and therapeutic strategies of PD.

Dopamine, Alpha-Synuclein, and Mitochondrial Dysfunctions in Parkinsonian Eyes

Parkinson’s disease (PD) is characterized by motor dysfunctions including bradykinesia, tremor at rest and motor instability. These symptoms are associated with the progressive degeneration of dopaminergic neurons originating in the substantia nigra pars compacta and projecting to the corpus striatum, and by accumulation of cytoplasmic inclusions mainly consisting of aggregated alpha-synuclein, called Lewy bodies. PD is a complex, multifactorial disorder and its pathogenesis involves multiple pathways and mechanisms such as α-synuclein proteostasis, mitochondrial function, oxidative stress, calcium homeostasis, axonal transport, and neuroinflammation. Motor symptoms manifest when there is already an extensive dopamine denervation. There is therefore an urgent need for early biomarkers to apply disease-modifying therapeutic strategies. Visual defects and retinal abnormalities, including decreased visual acuity, abnormal spatial contrast sensitivity, color vision defects, or deficits in more complex visual tasks are present in the majority of PD patients. They are being considered for early diagnosis together with retinal imaging techniques are being considered as non-invasive biomarkers for PD. Dopaminergic cells can be found in the retina in a subpopulation of amacrine cells; however, the molecular mechanisms leading to visual deficits observed in PD patients are still largely unknown. This review provides a comprehensive analysis of the retinal abnormalities observed in PD patients and animal models and of the molecular mechanisms underlying neurodegeneration in parkinsonian eyes. We will review the role of α-synuclein aggregates in the retina pathology and/or in the onset of visual symptoms in PD suggesting that α-synuclein aggregates are harmful for the retina as well as for the brain. Moreover, we will summarize experimental evidence suggesting that the optic nerve pathology observed in PD resembles that seen in mitochondrial optic neuropathies highlighting the possible involvement of mitochondrial abnormalities in the development of PD visual defects. We finally propose that the eye may be considered as a complementary experimental model to identify possible novel disease’ pathways or to test novel therapeutic approaches for PD.

New players in basal ganglia dysfunction in Parkinson's disease

The classical model of the basal ganglia (BG) circuit has been recently revised with the identification of other structures that play an increasing relevant role especially in the pathophysiology of Parkinson's disease (PD). Numerous studies have supported the spreading of the alpha-synuclein pathology to several areas beyond the BG and likely even before their involvement. With the aim of better understanding PD pathophysiology and finding new targets for treatment, the spinal cord, the pedunculopontine nucleus, the substantia nigra pars reticulata, the retina, the superior colliculus, the cerebellum, the nucleus parabrachialis and the Meynert's nucleus have been investigated both in animal and human studies. In this chapter, we describe the main anatomical and functional connections between the above structures and the BG, the relationship between their pathology and PD features, and the rational of applying neuromodulation treatment to improve motor and non-motor symptoms in PD. Some of these new players in the BG circuits might also have a potential intriguing role as early biomarkers of PD.

Retinal α-synuclein deposits in Parkinson's disease patients and animal models

Despite decades of research, accurate diagnosis of Parkinson's disease remains a challenge, and disease-modifying treatments are still lacking. Research into the early (presymptomatic) stages of Parkinson's disease and the discovery of novel biomarkers is of utmost importance to reduce this burden and to come to a more accurate diagnosis at the very onset of the disease. Many have speculated that non-motor symptoms could provide a breakthrough in the quest for early biomarkers of Parkinson's disease, including the visual disturbances and retinal abnormalities that are seen in the majority of Parkinson's disease patients. An expanding number of clinical studies have investigated the use of in vivo assessments of retinal structure, electrophysiological function, and vision-driven tasks as novel means for identifying patients at risk that need further neurological examination and for longitudinal follow-up of disease progression in Parkinson's disease patients. Often, the results of these studies have been interpreted in relation to α-synuclein deposits and dopamine deficiency in the retina, mirroring the defining pathological features of Parkinson's disease in the brain. To better understand the visual defects seen in Parkinson's disease patients and to propel the use of retinal changes as biomarkers for Parkinson's disease, however, more conclusive neuropathological evidence for the presence of retinal α-synuclein aggregates, and its relation to the cerebral α-synuclein burden, is urgently needed. This review provides a comprehensive and critical overview of the research conducted to unveil α-synuclein aggregates in the retina of Parkinson's disease patients and animal models, and thereby aims to aid the ongoing discussion about the potential use of the retinal changes and/or visual symptoms as biomarkers for Parkinson's disease.

Reading deficits in schizophrenia and their relationship to developmental dyslexia: A review

Although schizophrenia and developmental dyslexia are considered distinct disorders in terms of clinical presentation and functional outcome, they both involve disruption in the processes that support skilled reading, including language, auditory perception, visual perception, oculomotor control, and executive function. Further, recent work has proposed a common neurodevelopmental basis for the two disorders, as suggested by genetic and pathophysiological overlap. Thus, these lines of research suggest that reading may be similarly impacted in schizophrenia and dyslexia. In this review, we survey research on reading abilities in individuals with schizophrenia, and review the potential mechanisms underlying reading deficits in schizophrenia that may be shared with those implicated in dyslexia. Elucidating the relationship between reading impairment in schizophrenia and dyslexia could allow for a better understanding of the pathophysiological underpinnings of schizophrenia, and could facilitate remediation of cognitive deficits that impact day-to-day functioning.

Spatial Frequency Selectivity Is Impaired in Dopamine D2 Receptor Knockout Mice

Dopamine is a neurotransmitter implicated in several brain functions, including vision. In the present study, we investigated the impacts of the lack of D2 dopamine receptors on the structure and function of the primary visual cortex (V1) of D2-KO mice using optical imaging of intrinsic signals. Retinotopic maps were generated in order to measure anatomo-functional parameters such as V1 shape, cortical magnification factor, scatter, and ocular dominance. Contrast sensitivity and spatial frequency selectivity (SF) functions were computed from responses to drifting gratings. When compared to control mice, none of the parameters of the retinotopic maps were affected by D2 receptor loss of function. While the contrast sensitivity function of D2-KO mice did not differ from their wild-type counterparts, SF selectivity function was significantly affected as the optimal SF and the high cut-off frequency (p < 0.01) were higher in D2-KO than in WT mice. These findings show that the lack of function of D2 dopamine receptors had no influence on cortical structure whereas it had a significant impact on the spatial frequency selectivity and high cut-off. Taken together, our results suggest that D2 receptors play a specific role on the processing of spatial features in early visual cortex while they do not seem to participate in its development.

Novel Therapeutics in Glaucoma Management

BACKGROUND

Glaucoma is a progressive optic neuropathy characterized by retinal ganglion cell death and alterations of visual field. Elevated intraocular pressure (IOP) is considered the main risk factor of glaucoma, even though other factors cannot be ruled out, such as epigenetic mechanisms.

OBJECTIVE

An overview of the ultimate promising experimental drugs to manage glaucoma has been provided.

RESULTS

In particular, we have focused on purinergic ligands, KATP channel activators, gases (nitric oxide, carbon monoxide and hydrogen sulfide), non-glucocorticoid steroidal compounds, neurotrophic factors, PI3K/Akt activators, citicoline, histone deacetylase inhibitors, cannabinoids, dopamine and serotonin receptors ligands, small interference RNA, and Rho kinase inhibitors.

CONCLUSIONS

The review has been also endowed of a brief chapter on last reports about potential neuroprotective benefits of anti-glaucoma drugs already present in the market.

Visual Dysfunction in Parkinson's Disease

This chapter describes the visual problems likely to be encountered in Parkinson's disease (PD) and whether such signs are useful in differentiating the parkinsonian syndromes. Visual dysfunction in PD may involve visual acuity, contrast sensitivity, color discrimination, pupil reactivity, saccadic and pursuit eye movements, motion perception, visual fields, and visual processing speeds. In addition, disturbance of visuospatial orientation, facial recognition problems, rapid eye movement (REM) sleep behavior disorder, and chronic visual hallucinations may be present. Problems affecting pupil reactivity, stereopsis, pursuit eye movement, and visuomotor adaptation, when accompanied by REM sleep behavior disorder, could be early features of PD. Dementia associated with PD is associated with enhanced eye movement problems, visuospatial deficits, and visual hallucinations. Visual dysfunction may be a useful diagnostic feature in differentiating PD from other parkinsonian symptoms, visual hallucinations, visuospatial dysfunction, and variation in saccadic eye movement problems being particularly useful discriminating features.

Retinal biomarkers provide "insight" into cortical pharmacology and disease

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

A combination of retinal morphology and visual electrophysiology testing increases diagnostic yield in Parkinson's disease

BACKGROUND

Impaired vision and remodeled foveal pit have been demonstrated in Parkinson's disease (PD) patients using different techniques.

METHODS

Ten PD (20 eyes) and eight healthy controls (HC) subjects (16 eyes) were enrolled. Subjects were evaluated for N70 and P100 latencies using two-channel VEP with pattern reversal and on/off pattern; Contrast sensitivity (CS) using Pelli-Robson chart; macular thickness measured using Zeiss-HD optical coherence tomography (OCT).

RESULTS

PD patients had a significantly delayed N70 (reversal pattern) and P100 (on/off pattern), lower CS score, and decreased retinal thickness at temporal 1.5-2.5 mm from the foveola. N70 latency was negatively correlated with CS (R = -0.419, P = 0.01) and average GCL-IPL thickness (R = -0.529, P = 0.001). CS was positively correlated with parafoveal thickness (R = 0.490, P = 0.002). A combination of parafoveal thickness and CS score yielded an AUC of 0.784 for PD discrimination which increased to 0.844 when combined with N70 and P100 measures.

CONCLUSION

A combination of pattern reversal VEP latency, CS score, and inner retinal foveal thickness measures has a high diagnostic yield for PD.

Oculo-Visual Dysfunction in Parkinson's Disease

This review describes the oculo-visual problems likely to be encountered in Parkinson's disease (PD) with special reference to three questions: (1) are there visual symptoms characteristic of the prodromal phase of PD, (2) is PD dementia associated with specific visual changes, and (3) can visual symptoms help in the differential diagnosis of the parkinsonian syndromes, viz. PD, progressive supranuclear palsy (PSP), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and corticobasal degeneration (CBD)? Oculo-visual dysfunction in PD can involve visual acuity, dynamic contrast sensitivity, colour discrimination, pupil reactivity, eye movement, motion perception, and visual processing speeds. In addition, disturbance of visuo-spatial orientation, facial recognition problems, and chronic visual hallucinations may be present. Prodromal features of PD may include autonomic system dysfunction potentially affecting pupil reactivity, abnormal colour vision, abnormal stereopsis associated with postural instability, defects in smooth pursuit eye movements, and deficits in visuo-motor adaptation, especially when accompanied by idiopathic rapid eye movement (REM) sleep behaviour disorder. PD dementia is associated with the exacerbation of many oculo-visual problems but those involving eye movements, visuo-spatial function, and visual hallucinations are most characteristic. Useful diagnostic features in differentiating the parkinsonian symptoms are the presence of visual hallucinations, visuo-spatial problems, and variation in saccadic eye movement dysfunction.

Dopamine D2 receptors preferentially regulate the development of light responses of the inner retina

Retinal light responsiveness measured via electroretinography undergoes developmental modulation, and is thought to be critically regulated by both visual experience and dopamine. The primary goal of this study was to determine whether dopamine D2 receptors regulate the visual experience-dependent functional development of the retina. Accordingly, we recorded electroretinograms from wild-type mice and mice with a genetic deletion of the gene that encodes the D2 receptor raised under normal cyclic light conditions and constant darkness. Our results demonstrate that D2 receptor mutation preferentially increases the amplitude of the inner retinal light responses evoked by high-intensity light measured as oscillatory potentials in adult mice. During postnatal development, all three major components of electroretinograms, i.e. a-waves, b-waves, and oscillatory potentials, increase with age. Comparatively, D2 receptor mutation preferentially reduces the age-dependent increase in b-waves evoked by low-intensity light. Light deprivation from birth reduces b-wave amplitudes and completely abolishes the increased amplitude of oscillatory potentials of D2 receptor mutants. Taken together, these results demonstrate that D2 receptors play an important role in the activity-dependent functional development of the mouse retina.

Cognitive control predicted by color vision, and vice versa

One of the most important functions of cognitive control is to continuously adapt cognitive processes to changing and often conflicting demands of the environment. Dopamine (DA) has been suggested to play a key role in the signaling and resolution of such response conflict. Given that DA is found in high concentration in the retina, color vision discrimination has been suggested as an index of DA functioning and in particular blue-yellow color vision impairment (CVI) has been used to indicate a central hypodopaminergic state. We used color discrimination (indexed by the total color distance score; TCDS) to predict individual differences in the cognitive control of response conflict, as reflected by conflict-resolution efficiency in an auditory Simon task. As expected, participants showing better color discrimination were more efficient in resolving response conflict. Interestingly, participants showing a blue-yellow CVI were associated with less efficiency in handling response conflict. Our findings indicate that color vision discrimination might represent a promising predictor of cognitive controlability in healthy individuals.

Are patients with Parkinson's disease blind to blindsight?

In Parkinson's disease, visual dysfunction is prominent. Visual hallucinations can be a major hallmark of late stage disease, but numerous visual deficits also occur in early stage Parkinson's disease. Specific retinopathy, deficits in the primary visual pathway and the secondary ventral and dorsal pathways, as well as dysfunction of the attention pathways have all been posited as causes of hallucinations in Parkinson's disease. We present data from patients with Parkinson's disease that contrast with a known neuro-ophthalmological syndrome, termed 'blindsight'. In this syndrome, there is an absence of conscious object identification, but preserved 'guess' of the location of a stimulus, preserved reflexive saccades and motion perception and preserved autonomical and expressive reactions to negative emotional facial expressions. We propose that patients with Parkinson's disease have the converse of blindsight, being 'blind to blindsight'. As such they preserve conscious vision, but show erroneous 'guess' localization of visual stimuli, poor saccades and motion perception, and poor emotional face perception with blunted autonomic reaction. Although a large data set on these deficits in Parkinson's disease has been accumulated, consolidation into one specific syndrome has not been proposed. Focusing on neuropathological and physiological data from two phylogenetically old and subconscious pathways, the retino-colliculo-thalamo-amygdala and the retino-geniculo-extrastriate pathways, we propose that aberrant function of these systems, including pathologically inhibited superior colliculus activity, deficient corollary discharges to the frontal eye fields, dysfunctional pulvinar, claustrum and amygdaloid subnuclei of the amygdala, the latter progressively burdened with Lewy bodies, underlie this syndrome. These network impairments are further corroborated by the concept of the 'silent amygdala'. Functionally being 'blind to blindsight' may facilitate the highly distinctive 'presence' or 'passage' hallucinations of Parkinson's disease and can help to explain handicaps in driving capacities and dysfunctional 'theory of mind'. We propose this synthesis to prompt refined neuropathological and neuroimaging studies on the pivotal nuclei in these pathways in order to better understand the networks underpinning this newly conceptualized syndrome in Parkinson's disease.

Parkinson's disease, lights and melanocytes: looking beyond the retina

Critical analysis of recent research suggesting that light pollution causes Parkinson's disease (PD) reveals that such a hypothesis is unsustainable in the context of therapeutic use of light in treating various neuropsychiatric conditions. Reinterpretation of their findings suggests that retinal damage caused by prolonged light exposure may have contributed to the observed enhancement of experimental PD. To test this hypothesis further, forty-two Sprague Dawley rats received microinjections of 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-2, 4, 6-tetrahydropyridine (MPTP), paraquat or rotenone into the vitreal mass in doses so minute that the effects could not be attributed to diffusion into brain. Significant changes in five motor parameters consistent with symptoms of experimental PD were observed. These findings support the interpretation that the retina is involved in the control of motor function and in the aetiology of PD.

Development of a prediction formula of Parkinson disease severity by optical coherence tomography

The aims of this study were to assess the peripapillary retinal nerve fiber layer (RNFL) thickness in patients with Parkinson's disease (PD), to determine its correlation with disease severity, and to define a simple biomarker for predicting clinical severity. One hundred two eyes from 52 patients affected by PD were compared with 97 eyes from 50 age-comparable controls. In all patients, peripapillary RNFL thickness was measured by optical coherence tomography (OCT). We used the Unified Parkinson's Disease Rating Scale (UPDRS) total score and measured responses in the on medication state. Eyes from patients with PD had a statistically significant decrease in average peripapillary RNFL thickness compared with control eyes (P < 0.001). This reduction was observed in every quadrant (inferior, superior, nasal [P < 0.001], and temporal [P = 0.017]) in patients with PD. Furthermore, a strong inverse correlation was found between the PD severity measured according to the UPDRS score and the average peripapillary RNFL thickness (r = -0.615; P < 0.001) and PD duration (r = -0.303; P = 0.002). From these results, we defined a regression equation that predicts the UPDRS score from the above-mentioned variables: UPDRS = 81.6 + 29.6 * log PD duration (years) - 0.6 * RFNL thickness (μm). We observed that, as the evolution and severity of PD progress, the peripapillary RNFL layer thickness, as evaluated by OCT, gradually diminishes. These results suggest that the average peripapillary RNFL thickness measured by OCT might be useful as a biomarker to detect the early onset and progression of PD.

Retinal contrast transfer functions in adults with and without ADHD

In previous studies, we found a strong reduction in contrast perception and retinal contrast gain in patients with major depression, which normalized after remission of depression. We also identified a possible role of the dopaminergic system in this effect, because visual contrast perception depends on dopaminergic neurotransmission. Dopamine is also known to play an important role in the pathogenesis of attention deficit hyperactivity disorder (ADHD). Therefore, in order to explore the specificity of retinal contrast gain as a marker of depression in comparison with other psychiatric diseases, we recorded the pattern electroretinogram (PERG) in patients with ADHD. Twenty patients diagnosed with ADHD and 20 matched healthy subjects were studied. Visual pattern electroretinograms were recorded from both eyes. The contrast gain of the patients with attention deficit disorder (ADD) did not differ from the control group, nor did the contrast gain of any ADHD subgroup (predominantly inattentive or combined patients). In the healthy subjects, a significant correlation between depression score and contrast gain was found. As the contrast gain in an earlier study clearly separated the patients with depression from the controls, we assume that retinal contrast gain might be a specific marker in depression.

Blue-yellow colour vision impairment and cognitive deficits in occasional and dependent stimulant users

Specific blue-yellow colour vision impairment has been reported in dependent cocaine users and it was postulated that drug-induced changes in retinal dopamine neurotransmission are responsible. However, it is unclear whether these changes are confined to chronic cocaine users, whether they are specific for dopaminergic stimulants such as cocaine and amphetamine and whether they are related to cognitive functions such as working memory, encoding and consolidation. In 47 occasional and 29 dependent cocaine users, 23 MDMA (commonly known as 'ecstasy') users and 47 stimulant-naive controls, colour vision discrimination was measured with the Lanthony Desaturated Panel D-15 Test and memory performance with the Auditory Verbal Learning Test. Both occasional and dependent cocaine users showed higher colour confusion indices than controls. Users of the serotonergic stimulant MDMA (26%), occasional (30%) and dependent cocaine users (34%) exhibited more frequent blue-yellow colour vision disorders compared to controls (9%). Inferior performance of MDMA users was caused by a subgroup with high amphetamine co-use (55%), while MDMA use alone was not associated with decreased blue-yellow discrimination (0%). Cognitive performance was worse in cocaine users with colour vision disorder compared to users and controls with intact colour vision and both colour vision impairment and cognitive deficits were related to cocaine use. Occasional cocaine and amphetamine use might induce blue-yellow colour vision impairment, whereas the serotonergic stimulant MDMA does not impair colour vision. The association between colour vision impairment and cognitive deficits in cocaine users may reflect that retinal and cerebral dopamine alterations are linked to a certain degree.

Identifying prodromal Parkinson's disease: pre-motor disorders in Parkinson's disease

Increasing recognition that Parkinson's disease (PD) may start outside of the substantia nigra has led to a rapidly expanding effort to define prodromal stages of PD, before motor signs permit classical diagnosis. Many of these efforts center around the identification of clinical non-motor symptoms and signs of disease. There is now direct evidence that olfaction, rapid eye movement (REM) sleep behavior disorder (RBD), constipation, and depression can be present in prodromal PD. In addition, there is suggestive evidence that visual changes, other autonomic symptoms, and subtle cognitive changes may also be present at prodromal stages. A critical issue in utility of these prodromal markers will be assessment of sensitivity, specificity, and positive and negative predictive values. Although these have yet to be fully defined, olfactory deficits, some visual changes, and autonomic symptoms occur in the majority of PD patients at diagnosis, suggesting good potential sensitivity. However, with the exception of RBD and perhaps some specific autonomic measures, specificity, and positive predictive value of these markers may be insufficient to be used alone as identifiers of prodromal disease. The evidence for the utility of olfaction, RBD, autonomic markers, visual changes, mood disorders, and cognitive loss as markers of prodromal PD and the potential sensitivity and specificity of these markers are summarized.

Parkinson's disease: acquired frailty of archaic neural networks?

In Parkinson's disease (PD) many motor and non-motor symptoms are difficult to explain in terms of a purely ascending degeneration process as described by Braak. This essay proposes phylogenetic considerations for consolidating the multidimensional elements of PD. Subtle clinical analysis paired with ethological comparisons as well as patho-anatomical data suggests that disrupted automatic gait control, olfactory deficits, selected visual deficits, impaired emotional face recognition, and REM sleep behavior disorder could be due to dysfunction of phylogenetically ancient networks. Neuroanatomical and behavioral findings lead to a reconsideration of the basal ganglia, to be viewed as the nuclear core of a widely distributed neural network that arborizes throughout the primordial core of the neuraxis, including the brainstem. Fragility of the resulting multiple, closed, ancillary loops that link brainstem and forebrain components of the basal ganglia may be a nodal point, pivotal to the pathogenesis of PD. Other primitive neural networks, such as those located at cardiac or gastro-intestinal levels, may share the same vulnerability. Such a network-based hypothesis overrides the need of a fixed temporal ordering of symptoms based on putative caudal-cephalic propagation patterns of pathological lesions. It also creates testable, secondary hypotheses such as differential gene expression in different neural networks, potential early epigenetic influences, concepts of "overuse" or maladaptation of primitive networks to the constraints of adult life, and system frailty due to irreparable mitochondrial "exhaustion" in highly energy consuming postmitotic cells.

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.

Atypical pattern of rod electroretinogram modulation by recent light history: a possible biomarker of seasonal affective disorder

Our goal was to challenge both normal controls and patients with seasonal affective disorders (SAD) to various light histories and then measure their retinal response modulation using the electroretinogram (ERG) in both winter and summer. In winter and summer, 11 normal controls and 12 SAD patients were exposed to three different light conditions for 1 h (10,000, 100 and 5 lux) followed by an ERG. Groups showed similar ERG amplitudes in the 100 lux condition. Compared with the 100-lux condition, in controls, the ERG response was significantly increased in the 5-lux condition; in SAD, it was significantly decreased in the 10,000-lux condition. This pattern was present in both seasons. This is the first time a retinal response modulation anomaly has been observed in SAD patients in both the depressed and euthymic states. Retinal response modulation may represent an interesting biomarker of the disease for future research.

Visual symptoms in Parkinson's disease

Parkinson's disease (PD) is a common disorder of middle-aged and elderly people in which degeneration of the extrapyramidal motor system causes significant movement problems. In some patients, however, there are additional disturbances in sensory systems including loss of the sense of smell and auditory and/or visual problems. This paper is a general overview of the visual problems likely to be encountered in PD. Changes in vision in PD may result from alterations in visual acuity, contrast sensitivity, colour discrimination, pupil reactivity, eye movements, motion perception, visual field sensitivity, and visual processing speeds. Slower visual processing speeds can also lead to a decline in visual perception especially for rapidly changing visual stimuli. In addition, there may be disturbances of visuospatial orientation, facial recognition problems, and chronic visual hallucinations. Some of the treatments used in PD may also have adverse ocular reactions. The pattern electroretinogram (PERG) is useful in evaluating retinal dopamine mechanisms and in monitoring dopamine therapies in PD. If visual problems are present, they can have an important effect on the quality of life of the patient, which can be improved by accurate diagnosis and where possible, correction of such defects.

Specific impairment of visual spatial covert attention mechanisms in Parkinson's disease

Visual deficits in early and high level processing nodes have been documented in Parkinson's disease (PD). Non-motor high level visual integration deficits in PD seem to have a cortical basis independently of a low level retinal contribution. It is however an open question whether sensory and visual attention deficits can be separated in PD. Here, we have explicitly separated visual and attentional disease related patterns of performance, by using bias free staircase procedures measuring psychophysical contrast sensitivity across visual space under covert attention conditions with distinct types of cues (valid, neutral and invalid). This further enabled the analysis of patterns of dorsal-ventral (up-down) and physiological inter-hemispheric asymmetries. We have found that under these carefully controlled covert attention conditions PD subjects show impaired psychophysical performance enhancement by valid attentional cues. Interestingly, PD patients also show paradoxically increased visual homogeneity of spatial performance profiles, suggesting flattening of high level modulation of spatial attention. Finally we have found impaired higher level attentional modulation of contrast sensitivity in the visual periphery, where mechanisms of covert attention are at higher demands. These findings demonstrate a specific loss of attentional mechanisms in PD and a pathological redistribution of spatial mechanisms of covert attention.

Expression in the mammalian retina of parkin and UCH-L1, two components of the ubiquitin-proteasome system

The ubiquitin-proteasome system (UPS) functions as a major degradation pathway for misfolded and damaged proteins with an important neuroprotective role in the CNS against a variety of cellular stresses. Parkin and ubiquitin C-terminal hydrolase L1 (UCH-L1) are two relevant components of the UPS associated with a number of neurodegenerative disorders. We here address the expression profile of parkin and UCH-L1 in the mammalian retina, with special emphasis on primates. We describe for the first time the presence of parkin in the retina of mammals, including humans. Parkin and UCH-L1 genes were expressed at the mRNA and protein levels in the retina of all species examined. The immunolocalization pattern of parkin was quite widespread, being expressed by most retinal neuronal types, including photoreceptors. UCH-L1 was localized to horizontal cells and specific subtypes of bipolar and amacrine cells, as well as to ganglion cells and their axons forming the nerve fiber layer. In rodents no UCH-L1 immunoreactivity was found in cone or rod photoreceptors, whereas this protein was present along the whole length of cones in all other mammals. Remarkably, UCH-L1 was expressed by dopaminergic amacrine cells of primates. The ample distribution of parkin and UCH-L1 in the mammalian retina, together with the crucial role played by the UPS in normal neuronal physiology in the brain, points to a participation of these two proteins in the ubiquitin-proteasomal pathway of protein degradation in most retinal cell types, where they could exert a protective function against neuronal stress.

Parkinson's disease as a neuroendocrine disorder of circadian function: dopamine-melatonin imbalance and the visual system in the genesis and progression of the degenerative process

For more than 50 years, Parkinson's disease (PD) has been conceptualized as a product of nigro-striatal dopamine (NSD) system degeneration. In spite of a growing body of evidence depicting the mammalian brain as an interrelated complexity of circuitous systems, dopamine (DA) deficiency of the NSD is still regarded as the main problem, with DA replacement being the purpose of therapeutic intervention. For at least 191 years circadian involvement in various aspects of PD, including depression and insomnia, has been recognized as an integral part of the symptom matrix of PD and yet attempts to elucidate the involvement of this system is uncharted territory. The present review attempts a major reorganization of mammalian brain into a coordinated complex involving the NSD and the retinal hypothalamic tract (RHT) as the primary systems involved in the retino-diencephalic/mesencephalic-pineal (RDMP) axis. Secondary systems including the lateral hypothalamus (LH), the area postraema (AP) and the subthalamic nucleus (STN) also form an integral part of this system as they have been shown to be either intimately related to the primary systems of the RDMP axis or have been shown to be significantly involved in the expression and treatment of PD. A large volume of evidence suggests that the RDMP axis is activated during the course of PD and during therapeutic intervention. Four types of neurotoxicity associated with melatonin are identified and the susceptibility of various parts of the RDMP axis to undergo neuropathological change, the tendency for melatonin to induce PD-like behavioural toxicity, and the relationship of this to PD symptomotology are described. This includes adverse effects of melatonin on motor function, hypotension, the adjuvant use of benzodiazepines, depression, insomnia, body weight regulation and various biochemical effects of melatonin administration: all problems currently facing the proposal to introduce melatonin as an adjuvant. It is suggested further that traditional DA replacement may well work by exerting its effect upon the circadian system, rather than simply replacing deficient DA. Activation of the circadian function by antagonizing melatonin with bright light not only has therapeutic value in treating the primary symptoms of PD but it shares a common mechanism with L-dopa in reducing the occurrence of seborrheic dermatitis. Concepts at the centre of understanding pineal function in PD, including pineal calcification, melatonin deficiency, symptomatic versus protective features of melatonin and antioxidative effects, are explained in a counterintuitive context. Intriguing propositions including the role of the retina in the aetiology of PD and that the nigra functions as a retina in this disorder are presented with the intention to provide a new understanding of the underlying compromised function in PD and to provide new treatment strategies. For the first time, abundant evidence is presented describing PD as an endocrine disorder of melatonin hyperplasia. The role of circadian interventive therapies and internal desynchrony in the aetiology and progression of PD provides a new direction for understanding the underlying physiology of a disease which is currently in a state of impasse and provides new hope for those who suffer from its debilitating effects.

The retina in Parkinson's disease

As a more complete picture of the clinical phenotype of Parkinson's disease emerges, non-motor symptoms have become increasingly studied. Prominent among these non-motor phenomena are mood disturbance, cognitive decline and dementia, sleep disorders, hyposmia and autonomic failure. In addition, visual symptoms are common, ranging from complaints of dry eyes and reading difficulties, through to perceptual disturbances (feelings of presence and passage) and complex visual hallucinations. Such visual symptoms are a considerable cause of morbidity in Parkinson's disease and, with respect to visual hallucinations, are an important predictor of cognitive decline as well as institutional care and mortality. Evidence exists of visual dysfunction at several levels of the visual pathway in Parkinson's disease. This includes psychophysical, electrophysiological and morphological evidence of disruption of retinal structure and function, in addition to disorders of 'higher' (cortical) visual processing. In this review, we will draw together work from animal and human studies in an attempt to provide an insight into how Parkinson's disease affects the retina and how these changes might contribute to the visual symptoms experienced by patients.

'Gamma' band oscillatory response to chromatic stimuli in volunteers and patients with idiopathic Parkinson's disease

The signal structure of the responses to equiluminant chromatic and achromatic (contrast) stimuli was studied in normal volunteers and patients with mild to moderate idiopathic Parkinson's disease. Visual stimuli were full-field (14 x 16 deg) achromatic or equiluminant (red-green or blue-yellow) sinusoidal gratings at 2c/deg and 90% contrast presented in onset-offset mode. The signal was processed offline by DFT and factor analysis was performed in the frequency domain. The conventional VEPs to chromatic onset stimuli showed a monophasic negative wave, while the response to offset stimuli was comparable in shape to the on-/offset achromatic responses; latencies were longer and amplitudes higher than those of responses to contrast stimulation. In patients, latencies were longer than in controls after achromatic and (to a lesser extent) red-green stimulations, but not after blue-yellow stimulation; amplitudes were comparable in all stimulus conditions. In healthy subjects, two non-overlapping factors accounted for the approximately 2-30.0 Hz and approximately 25.0-50.0 Hz signal components (representative of the low-frequency VEP and gamma oscillatory responses, respectively); the frequency of the approximately 25.0-50.0 Hz factor was lower after color than after contrast stimulation. The same factor structure was identified in patients, but the peak frequency of the factor on gamma activity was higher than in controls and did not vary with color-opponent stimulation. These observations indicate that stimulus-related gamma activity originates in cortex irrespective of the activated (magno-, parvo-, or konio-cellular) visual pathway, consistent with the suggested role in the phase coding of neuronal activities. Some dopaminergic modulation of gamma activity is conceivable.

The phenomenon of drug craving

The phenomenology of drug craving has become the focus of much research within addictive disorders because of the belief that desire plays a role in maintaining the addiction. Many of the studies have focused on the activation of neural pathways, particularly within the dopamine system in response to specific events or stimuli. While many of these studies have focused on a particular drug of choice, little has been done across addictive disorders. This article will present and review phenomena that induce drug craving, as well as delineate precise neural pathways which are activated during craving and specific neurobiological markers which are associated with an increased risk for drug craving and other forms of addictive behavior.

Perception measurement in clinical trials of schizophrenia: promising paradigms from CNTRICS

The third meeting of the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia (CNTRICS) focused on selecting promising measures for each of the cognitive constructs selected in the first CNTRICS meeting. In the domain of perception, the 2 constructs of interest were gain control and visual integration. CNTRICS received 5 task nominations for gain control and three task nominations for visual integration. The breakout group for perception evaluated the degree to which each of these tasks met prespecified criteria. For gain control, the breakout group for perception believed that 2 of the tasks (prepulse inhibition of startle and mismatch negativity) were already mature and in the process of being incorporated into multisite clinical trials. However, the breakout group recommended that steady-state visual-evoked potentials be combined with contrast sensitivity to magnocellular vs parvocellular biased stimuli and that this combined task and the contrast-contrast effect task be recommended for translation for use in clinical trial contexts in schizophrenia research. For visual integration, the breakout group recommended the Contour Integration and Coherent Motion tasks for translation for use in clinical trials. This manuscript describes the ways in which each of these tasks met the criteria used by the breakout group to evaluate and recommend tasks for further development.

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.

Visual and cognitive predictors of driving safety in Parkinson's disease patients

This study assessed the clinical utility of contrast sensitivity (CS) relative to attention, executive function, and visuospatial abilities for predicting driving safety in participants with Parkinson's disease (PD). Twenty-five, non-demented PD patients completed measures of contrast sensitivity, visuospatial skills, executive functions, and attention. All PD participants also underwent a formal on-road driving evaluation. Of the 25 participants, 11 received a marginal or unsafe rating on the road test. Poorer driving performance was associated with worse performance on measures of CS, visuospatial constructions, set shifting, and attention. While impaired driving was associated with a range of cognitive and visual abilities, only a composite measure of executive functioning and visuospatial abilities, and not CS or attentional skills, predicted driving performance. These findings suggest that neuropsychological tests, which are multifactorial in nature and require visual perception and visual spatial judgments are the most useful screening measures for hazardous driving in PD patients.

Contrast sensitivity and magnocellular functioning in schizophrenia

It has been suggested that schizophrenia is associated with a magnocellular deficit. This would predict a loss of contrast sensitivity at low spatial and/or at high temporal frequencies. We here review research that tested contrast sensitivity in individuals with schizophrenia. We find that the results of this research tend to show uniform reductions in contrast sensitivity that are generally not consistent with a magnocellular deficit. While much of this data may be consistent with an attentional deficiency on the part of the schizophrenic individuals, it is difficult to link such an attentional deficiency specifically to the magnocellular system. The conclusion of the present review is that contrast sensitivity data do not indicate the existence of an association between magnocellular deficits and schizophrenia.

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.

Subcortical visual dysfunction in schizophrenia drives secondary cortical impairments

Visual processing deficits are an integral component of schizophrenia and are sensitive predictors of schizophrenic decompensation in healthy adults. The primate visual system consists of discrete subcortical magnocellular and parvocellular pathways, which project preferentially to dorsal and ventral cortical streams. Subcortical systems show differential stimulus sensitivity, while cortical systems, in turn, can be differentiated using surface potential analysis. The present study examined contributions of subcortical dysfunction to cortical processing deficits using high-density event-related potentials. Event-related potentials were recorded to stimuli biased towards the magnocellular system using low-contrast isolated checks in Experiment 1 and towards the magnocellular or parvocellular system using low versus high spatial frequency (HSF) sinusoidal gratings, respectively, in Experiment 2. The sample consisted of 23 patients with schizophrenia or schizoaffective disorder and 19 non-psychiatric volunteers of similar age. In Experiment 1, a large decrease in the P1 component of the visual event-related potential in response to magnocellular-biased isolated check stimuli was seen in patients compared with controls (F = 13.2, P = 0.001). Patients also showed decreased slope of the contrast response function over the magnocellular-selective contrast range compared with controls (t = 9.2, P = 0.04) indicating decreased signal amplification. In Experiment 2, C1 (F = 8.5, P = 0.007), P1 (F = 33.1, P < 0.001) and N1 (F = 60.8, P < 0.001) were reduced in amplitude to magnocellular-biased low spatial frequency (LSF) stimuli in patients with schizophrenia, but were intact to parvocellular-biased HSF stimuli, regardless of generator location. Source waveforms derived from inverse dipole modelling showed reduced P1 in Experiment 1 and reduced C1, P1 and N1 to LSF stimuli in Experiment 2, consistent with surface waveforms. These results indicate pervasive magnocellular dysfunction at the subcortical level that leads to secondary impairment in activation of cortical visual structures within dorsal and ventral stream visual pathways. Our finding of early visual dysfunction is consistent with and explanatory of classic literature showing subjective complaints of visual distortions and is consistent with early visual processing deficits reported in schizophrenia. Although deficits in visual processing have frequently been construed as resulting from failures of top-down processing, the present findings argue strongly for bottom-up rather than top-down dysfunction at least within the early visual pathway. Deficits in magnocellular processing in this task may reflect more general impairments in neuronal systems functioning, such as deficits in non-linear amplification and may thus represent an organizing principle for predicting neurocognitive dysfunction in schizophrenia.

Influence of Dopamine Deficiency in Early Parkinson’s Disease on the Slow Stimulation Multifocal-ERG

PURPOSE

In animal studies intravitreal injection of tetrodotoxin (TTX) results in mfERG waveform changes similar to those observed in glaucoma. As TTX blocks amacrine as well as ganglion cells, there is still a question regarding the underlying cell population responsible for these changes in waveform. In an attempt to assess the contribution of the amacrine cells to these changes, a mfERG was obtained from patients with Parkinson's disease as some amacrine cells are mediated by dopamine, a substance lacking in Parkinson's.

METHODS

Eight patients with early Parkinson's disease underwent ophthalmologic examination, testing of contrast sensitivity and electrophysiological examination according to ISCEV standard at least 12 h following their last medication with Dopamine. A slow stimulation mfERG was obtained with a stimulus base interval of 53.3 ms and with a stimulus base interval of 106.6 ms. During MF-ERG recordings 103 hexagons stimulated the central 50 deg of the retina simultaneously and independently (m-sequence 2(13), L(max): 200 cd/m(2), approximately 100% contrast).

RESULTS

Contrast sensitivity and ISCEV standard electrophysiological testing was unremarkable. When the mfERG was analyzed, only four patients had an adequate signal-to-noise ratio to allow further data analysis - one of whom was diagnosed with a multi system atrophy in retrospect. The first order response component was analyzed at a filter setting of 10-300 Hz and at 100-300 Hz (OPs) and compared to mfERGs of a control group. On average, in patients, the amplitude of N1P1 was slightly lower in the central and nasal response averages. When the three OPs at a latency of 72-89 ms were analyzed in the 53.3 ms base interval recording, the most marked difference in amplitude was observed in the superior nasal response average of the first OP. Here a mean amplitude of 1.3 nV/deg(2) in patients compared to a mean amplitude of 1.9 nV/deg(2) in the control group (P: 0.08).

DISCUSSION

In contrast to our previous findings in NTG, there was a consistent presence of three OPs. Under the stimulus conditions applied, we did not find an influence of dopaminergic amacrine cells on the mfERG in our patients with moderate stages of Parkinsion's. The difficulties in obtaining an adequate signal-to noise ratio due to e.g. muscle artifacts even in Parkinson patients of moderate disease stages render a success of mfERG recording in patients with more advanced stages unlikely. The question of the influence of dopaminergic amacrine cells on the mfERG could possibly be addressed using MPDT in animal research.

[Multimodal evoked potential abnormalities in patients with Wilson's disease]

The aim of this study was to investigate the involvement of the following functional systems: somatosensory evoked potentials (SSEP), visual evoked potentials (VEP), and event related potentials (ERP), in twenty patients with Wilson's disease (WD). VEP and SSEP abnormalities were discovered in 8 patients respectively (40%), whereas ERP were either absent or, in the case of 10 patients (50%), had significantly prolonged P-300 latencies. Taken together, at least one evoked potential abnormality was discovered in 17 patients (85%). Only in 3 patients (15%), involving either the isolated hepatic type of disease or short illness duration of the neurological type, were normal evoked potential findings observed. Our findings suggest the usefulness of multimodal evoked potential abnormalities in the evaluation of subclinical manifestations in patients with WD.