The electroretinogram as a biomarker of central dopamine and serotonin: potential relevance to psychiatric disorders

Is dopamine transporter-mediated dopaminergic signaling in the retina a noninvasive biomarker for attention-deficit/ hyperactivity disorder? A study in a novel dopamine transporter variant Val559 transgenic mouse model

Background

Dopamine (DA) is a critical neuromodulator in the retina. Disruption of retinal DA synthesis and signaling significantly attenuates light-adapted, electroretinogram (ERG) responses, as well as contrast sensitivity and acuity. As these measures can be detected noninvasively, they may provide opportunities to detect disease processes linked to perturbed DA signaling. Recently, we identified a rare, functional DA transporter (DAT, SLC6A3) coding substitution, Ala559Val, in subjects with attention-deficit/hyperactivity disorder (ADHD), demonstrating that DAT Val559 imparts anomalous DA efflux (ADE) with attendant physiological, pharmacological, and behavioral phenotypes. To understand the broader impact of ADE on ADHD, noninvasive measures sensitive to DAT reversal are needed.

Methods

Here, we explored this question through ERG-based analysis of retinal light responses, as well as HPLC measurements of retinal DA in DAT Val559 mice.

Results

Male mice homozygous (HOM) for the DAT Val559 variant demonstrated increased, light-adapted ERG b-wave amplitudes compared to wild type (WT) and heterozygous (HET) mice, whereas dark-adapted responses were indistinguishable across genotypes. The elevated amplitude of the photopic light responses in HOM mice could be mimicked in WT mice by applying D₁ and D₄ DA receptor agonists and suppressed in HOM mice by introducing D₄ antagonist, supporting elevated retinal DA signaling arising from ADE. Following the challenge with amphetamine, WT exhibited an increase in light-adapted response amplitudes, while HOM did not. Total retinal DA content was similar across genotypes. Interestingly, female DAT Val559 HOM animals revealed no significant difference in photopic ERG responses when compared with WT and HET littermates.

Conclusions

These data reveal that noninvasive, in vivo evaluation of retinal responses to light can reveal physiological signatures of ADE, suggesting a possible approach to the segregation of neurobehavioral disorders based on the DAT-dependent control of DA signaling.

Looking into the brain through the retinal ganglion cells in psychiatric disorders: A review of evidences

Psychiatry and neuroscience research need novel approaches to indirectly investigate brain function. As the retina is an anatomical and developmental extension of the central nervous system (CNS), changes in retinal function may reflect neurological dysfunctions in psychiatric disorders. The last and most integrated retinal relay before visual information transfer to the brain is the ganglion cell layer. Here, based on collected arguments, we argue that these cells offer a crucial site for indirectly investigating brain function. We describe the anatomical and physiological properties of these cells together with measurements of their functional properties named pattern electroretinogram (PERG). Based on ganglion cell dysfunctions measured with PERG in neurological disorders, we argue for the relevance of studying ganglion cell function in psychiatric research. We review studies that have evaluated ganglion cell function in psychiatric and addictive disorders and discuss how changes in PERG measurements could be functional markers of pathophysiological mechanisms of psychiatric disorders.

Neurotoxic effects of silver nanoparticles and the protective role of rutin

The toxicological studies on silver nanoparticles (Ag-NPs) have become a hot topic over the past few decades due to their unique properties on the nanoscale and widespread in many commercial products that launched into the market recently. This study was undertaken to shed light on Ag-NPs toxicity on neurotransmitters with special emphasis on the impact of concurrent administration of rutin with Ag-NPs in the experimental rats. The oral administration of Ag-NPs in rats induced brain oxidative stress, significant alterations in neurotransmitters and amino acids. Furthermore, transcriptional levels of glutamatergic N-methyl-d-aspartate (NMDA) receptors, monoamino oxidases (MAO-A, MAO-B) and metallothionein-III (MT-III) showed a significant elevation in Ag-NPs intoxicated rats. Moreover, histological examinations revealed astrogliosis and demyelination of neurons concomitant with neuronal degeneration and vacuolation. Strikingly, oral administration of rutin counterbalanced the toxic effects triggered by Ag-NPs. Taken together, our findings suggested that oral administration of Ag-NPs induced neurotoxicity in rats and rutin mitigates these effects.

Electroretinographic anomalies in medicated and drug free patients with major depression: Tagging the developmental roots of major psychiatric disorders

The retina is tagged as an approachable part of the brain due to its common embryonic origin and appears as a promising site of investigation for psychiatric disorders. Retinal function is assessed best with the electroretinogram (ERG), which was obtained in a large sample of patients with major depressive disorder and matched controls. ERG cone and rod luminance response functions were recorded in non-dilated eyes in 100 major depressive disorder patients (MDD) and 100 controls, (mean age of 42.8 and 40.9y. o. respectively). Amongst MDD patients, 17 were drug free (mean age 41.2y. o). In medicated patients, at the cone level, a prolonged b-wave was observed (p≤0.01). In drug free patients a prolonged b-wave was discovered only when averaging the implicit time of the 3 highest b-wave amplitudes of the photopic hill. For the medicated patients, the mixed rods/cones a-wave was reduced (p=0.01) whereas a trend (p=0.06) was observed for the pure rod b-wave (reduced) and the mixed rods/cones (reduced and prolonged; p=0.05). In drug free patients, a similar pattern could be observed in terms of effect sizes. Overall, medicated and drug free MDD patients shared some deficits suggesting that some anomalies are present above and beyond the effect of medication. Of interest, the prolonged cone and reduced rod amplitude were reported by our group in schizophrenia patients, suggesting a common neurodevelopmental root of major psychiatric disorders.

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.

Effects of dopamine D1 receptor blockade on the ERG b- and d-waves during blockade of ionotropic GABA receptors

Background

Some data indicate that the dopaminergic and GABAergic systems interact in the vertebrate retina, but the type of interactions is not well understood.

Methods

In this study we investigated the effect of dopamine D₁ receptor blockade by 75 μM SCH 23390 on the electroretinographic ON (b-wave) and OFF (d-wave) responses in intact frog eyecup preparations and in eyecups where the ionotropic GABA receptors were blocked by 50 μM picrotoxin. Student’s t-test, One-way repeated measures ANOVA with Bonferroni post-hoc test and Two-way ANOVA were used for statistical evaluation of the data.

Results

We found that SCH 23390 alone significantly enhanced the amplitude of the b- and d-waves without altering their latency. The effect developed rapidly and was fully expressed within 8-11 min after the blocker application. Picrotoxin alone also markedly enhanced the amplitude of the ERG ON and OFF responses and increased their latency significantly. The effect was fully expressed within 25-27 min after picrotoxin application and remained very stable in the next 20 min. The effects of SCH 23390 and picrotoxin are similar to that reported in our previous studies. When SCH 23390 was applied on the background of the fully developed picrotoxin effect, it diminished the amplitude of the b- and d-waves in comparison to the corresponding values obtained during application of picrotoxin alone.

Conclusion

Our results demonstrate that the enhancing effect of D₁ receptor blockade on the amplitude of the ERG b- and d-waves is not evident during the ionotropic GABA receptor blockade, indicating an interaction between these neurotransmitter systems in the frog retina. We propose that the inhibitory effect of endogenous dopamine mediated by D₁ receptors on the ERG ON and OFF responses in the frog retina may be due to the dopamine-evoked GABA release.

Electronic supplementary material

The online version of this article (doi:10.1186/s40662-016-0064-4) contains supplementary material, which is available to authorized users.

Optical coherence tomography and neurodegeneration: are eyes the windows to the brain?

INTRODUCTION

Central nervous system (CNS) pathologies have ocular manifestations due to direct and/or retrograde degeneration of the visual pathways, most often related to a direct injury to the optic nerve, retinal ganglion cells and/or its surrounding cells. These ocular manifestations can be recognized and monitored by a non-invasive technique called Optical Coherence Tomography (OCT).

AREAS COVERED

This review article describes the OCT technique and its application in various neurological pathologies. OCT helps in the measurement of retinal nerve fiber layer thickness (RNFLT) and macular thickness which consists of retinal ganglion cells. OCT provides a near-histological level of image resolution up to 5µm by using principles of interferometry that can detect CNS inflammatory, as well as degenerative, pathologies (i.e Multiple Sclerosis, Parkinson's disease and Alzheimer's Disease etc.) at initial clinical and possibly subclinical stages. Expert Commentary: RNFLT and macular volumes measured by OCT can serve as biomarkers for early recognition and progression of a variety of neurological disease, although the specificity of these findings in clinical diagnosis requires further refinement. An early biomarker can help in an earlier therapeutic intervention and thus preventing further progression and provide the opportunity for possible regenerative interventions of the underlying disease process. With the advent of "next-generation" OCT technology an increase in use is foreseen in research as well as in clinical practice.

Retinal Electrophysiology Is a Viable Preclinical Biomarker for Drug Penetrance into the Central Nervous System

Objective. To examine whether retinal electrophysiology is a useful surrogate marker of drug penetrance into the central nervous system (CNS). Materials and Methods. Brain and retinal electrophysiology were assessed with full-field visually evoked potentials and electroretinograms in conscious and anaesthetised rats following systemic or local administrations of centrally penetrant (muscimol) or nonpenetrant (isoguvacine) compounds. Results. Local injections into the eye/brain bypassed the blood neural barriers and produced changes in retinal/brain responses for both drugs. In conscious animals, systemic administration of muscimol resulted in retinal and brain biopotential changes, whereas systemic delivery of isoguvacine did not. General anaesthesia confounded these outcomes. Conclusions. Retinal electrophysiology, when recorded in conscious animals, shows promise as a viable biomarker of drug penetration into the CNS. In contrast, when conducted under anaesthetised conditions confounds can be induced in both cortical and retinal electrophysiological recordings.

Loss of Bmi1 causes anomalies in retinal development and degeneration of cone photoreceptors

Retinal development occurs through the sequential but overlapping generation of six types of neuronal cells and one glial cell type. Of these, rod and cone photoreceptors represent the functional unit of light detection and phototransduction and are frequently affected in retinal degenerative diseases. During mouse development, the Polycomb group protein Bmi1 is expressed in immature retinal progenitors and differentiated retinal neurons, including cones. We show here that Bmi1 is required to prevent post natal degeneration of cone photoreceptors and bipolar neurons and that inactivation of Chk2 or p53 could improve but not overcome cone degeneration in Bmi1(-/-) mice. The retinal phenotype of Bmi1(-/-) mice was also characterized by loss of heterochromatin, activation of tandem repeats, oxidative stress and Rip3-associated necroptosis. In the human retina, BMI1 was preferentially expressed in cones at heterochromatic foci. BMI1 inactivation in human embryonic stem cells was compatible with retinal induction but impaired cone terminal differentiation. Despite this developmental arrest, BMI1-deficient cones recapitulated several anomalies observed in Bmi1(-/-) photoreceptors, such as loss of heterochromatin, activation of tandem repeats and induction of p53, revealing partly conserved biological functions between mouse and man.

From Imaging the Brain to Imaging the Retina: Optical Coherence Tomography (OCT) in Schizophrenia

Optical Coherence Tomography (OCT) is a noninvasive imaging method, which provides an in vivo image of the retina. It allows for quantitative measurements of retinal and macular thickness, including single-layer analysis. Because the retinal nerve fibre layer comprises the first axons of the visual pathway and is unmyelinated, it can be considered a unique anatomical model, which may provide insight into the pathophysiological processes of diseases with a neurodegenerative character. In fact, past OCT studies have emphasized the role of the visual pathway as an ideal structure for exploring neurodegeneration and have demonstrated the potential of the method as an instrument for longitudinally monitoring structural changes in neurological disorders such as multiple sclerosis. Progress in signal processing and advancements to the OCT technique enables the illustration of structural changes in the retinal layers in a quick, reproducible, and objective manner with a spatial resolution comparable to those of histological slices.Findings from computer-based magnetic resonance imaging analyses and neuropathological studies support the hypothesis of a degenerative component of certain psychiatric disorders such as schizophrenia. Studies in schizophrenia incorporating OCT are currently rare and have yielded further heterogeneous results. This article elucidates the method of OCT and the retina's role as a "window to the brain". Furthermore, in delineating the degenerative components of schizophrenia, we discuss the possible applications of OCT in the schizophrenia population.

The emerging field of retinal electrophysiological measurements in psychiatric research: A review of the findings and the perspectives in major depressive disorder

Major depressive disorder (MDD) is a severe mental illness leading to long-term disabilities. One of the current challenges in psychiatric research is to develop new approaches to investigate the pathophysiology of MDD and monitor drug response in order to provide better therapeutic strategies to the patients. Since the retina is considered as part of the central nervous system, it was suggested that it constitutes an appropriate site to investigate mental illnesses. In the past years, several teams assessed the retinal function of patients with mood disorders and many relevant abnormalities have been reported. Investigation of the retinal electrophysiological abnormalities in MDD remains a young emerging field, but we believe that the current findings are very promising and we argue that objective retinal electrophysiological measurements may eventually become relevant tools to investigate the pathophysiology of MDD. Here, we review the retinal abnormalities detected with objective electrophysiological measurements such as the flash electroretinogram (fERG), the pattern electroretinogram (PERG) and the electrooculogram (EOG) in patients with MDD. We discuss how these changes might reflect the pathophysiology of MDD in both clinical and scientific points of view, according especially to the monoamine neurotransmission deficiency hypothesis. We also discuss the technical details that must be taken into consideration for a potential use of the objective retinal electrophysiological measurements as tools to investigate the pathophysiology of MDD.

Dopamine receptors - IUPHAR Review 13

The variety of physiological functions controlled by dopamine in the brain and periphery is mediated by the D1, D2, D3, D4 and D5 dopamine GPCRs. Drugs acting on dopamine receptors are significant tools for the management of several neuropsychiatric disorders including schizophrenia, bipolar disorder, depression and Parkinson's disease. Recent investigations of dopamine receptor signalling have shown that dopamine receptors, apart from their canonical action on cAMP-mediated signalling, can regulate a myriad of cellular responses to fine-tune the expression of dopamine-associated behaviours and functions. Such signalling mechanisms may involve alternate G protein coupling or non-G protein mechanisms involving ion channels, receptor tyrosine kinases or proteins such as β-arrestins that are classically involved in GPCR desensitization. Another level of complexity is the growing appreciation of the physiological roles played by dopamine receptor heteromers. Applications of new in vivo techniques have significantly furthered the understanding of the physiological functions played by dopamine receptors. Here we provide an update of the current knowledge regarding the complex biology, signalling, physiology and pharmacology of dopamine receptors.

Schizophrenia and the eye

Although visual processing impairments are common in schizophrenia, it is not clear to what extent these originate in the eye vs. the brain. This review highlights potential contributions, from the retina and other structures of the eye, to visual processing impairments in schizophrenia and high-risk states. A second goal is to evaluate the status of retinal abnormalities as biomarkers for schizophrenia. The review was motivated by known retinal changes in other disorders (e.g., Parkinson’s disease, multiple sclerosis), and their relationships to perceptual and cognitive impairments, and disease progression therein. The evidence reviewed suggests two major conclusions. One is that there are multiple structural and functional disturbances of the eye in schizophrenia, all of which could be factors in the visual disturbances of patients. These include retinal venule widening, retinal nerve fiber layer thinning, dopaminergic abnormalities, abnormal ouput of retinal cells as measured by electroretinography (ERG), maculopathies and retinopathies, cataracts, poor acuity, and strabismus. Some of these are likely to be illness-related, whereas others may be due to medication or comorbid conditions. The second conclusion is that certain retinal findings can serve as biomarkers of neural pathology, and disease progression, in schizophrenia. The strongest evidence for this to date involves findings of widened retinal venules, thinning of the retinal nerve fiber layer, and abnormal ERG amplitudes. These data suggest that a greater understanding of the contribution of retinal and other ocular pathology to the visual and cognitive disturbances of schizophrenia is warranted, and that retinal changes have untapped clinical utility.

The cannabinoid system and visual processing: a review on experimental findings and clinical presumptions

Cannabis is one of the most prevalent drugs used worldwide. Regular cannabis use is associated with impairments in highly integrative cognitive functions such as memory, attention and executive functions. To date, the cerebral mechanisms of these deficits are still poorly understood. Studying the processing of visual information may offer an innovative and relevant approach to evaluate the cerebral impact of exogenous cannabinoids on the human brain. Furthermore, this knowledge is required to understand the impact of cannabis intake in everyday life, and especially in car drivers. Here we review the role of the endocannabinoids in the functioning of the visual system and the potential involvement of cannabis use in visual dysfunctions. This review describes the presence of the endocannabinoids in the critical stages of visual information processing, and their role in the modulation of visual neurotransmission and visual synaptic plasticity, thereby enabling them to alter the transmission of the visual signal. We also review several induced visual changes, together with experimental dysfunctions reported in cannabis users. In the discussion, we consider these results in relation to the existing literature. We argue for more involvement of public health research in the study of visual function in cannabis users, especially because cannabis use is implicated in driving impairments.

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.

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Stimulation of 5-HT2C receptors improves cognitive deficits induced by human tryptophan hydroxylase 2 loss of function mutation

Polymorphisms in the gene encoding the serotonin synthesis enzyme Tph2 have been identified in mental illnesses, including bipolar disorder, major depression, autism, schizophrenia, and ADHD. Deficits in cognitive flexibility and perseverative behaviors are shared common symptoms in these disorders. However, little is known about the impact of Tph2 gene variants on cognition. Mice expressing a human TPH2 variant (Tph2-KI) were used to investigate cognitive consequences of TPH2 loss of function and pharmacological treatments. We applied a recently developed behavioral assay, the automated H-maze, to study cognitive functions in Tph2-KI mice. This assay involves the consecutive discovery of three different rules: a delayed alternation task, a non-alternation task, and a delayed reversal task. Possible contribution of locomotion, reward, and sensory perception were also investigated. The expression of loss-of-function mutant Tph2 in mice was associated with impairments in reversal learning and cognitive flexibility, accompanied by perseverative behaviors similar to those observed in human clinical studies. Pharmacological restoration of 5-HT synthesis with 5-hydroxytryptophan or treatment with the 5-HT(2C) receptor agonist CP809.101 reduced cognitive deficits in Tph2-KI mice and abolished perseveration. In contrast, treatment with the psychostimulant methylphenidate exacerbated cognitive deficits in mutant mice. Results from this study suggest a contribution of TPH2 in the regulation of cognition. Furthermore, identification of a role for a 5-HT(2) receptor agonist as a cognition-enhancing agent in mutant mice suggests a potential avenue to explore for the personalized treatment of cognitive symptoms in humans with reduced 5-HT synthesis and TPH2 polymorphisms.

Electroretinographic modifications induced by agomelatine: a novel avenue to the understanding of the claimed antidepressant effect of the drug?

BACKGROUND

Agomelatine, the first melatonergic antidepressant, has been postulated to enhance the dopaminergic activity at the central nervous system by 5-hydroxytryptamine receptor type 2C (5-HT2C) antagonism, yet the impact of melatonergic agonism on this pathway is unclear. Previous studies employing simplified, yet reliable, proxy (retinal) measures of the central nervous system dopaminergic activity, namely the standard electroretinogram (ERG) technique, suggested a reduction of the dopaminergic activity of the main ERG parameter, the b-wave, by pure melatonin, notably a hormone devoid of any antidepressant activity. Therefore, the antidepressant effects of the melatonergic antidepressant drug agomelatine should be reflected by a differential b-wave trend at ERG versus the effect exerted by pure melatonin, which was eventually found to be due to a contrasting effect on central dopaminergic transmission between the two drugs.

OBJECTIVE AND METHODS

The aim of the present preliminary ERG study carried out on healthy volunteers (n=23) receiving agomelatine was to explore the impact of this antidepressant drug on b-wave amplitude and latency of cones in daylight conditions using standard ERG.

RESULTS

As postulated, agomelatine induced an enhancement of retinal dopaminergic activity, in contrast to what has been previously documented for melatonin.

CONCLUSION

Given the limits of this explorative study, especially the lack of a control group and that of a luminance response function to measure retinal sensitivity, further studies in clinical samples are recommended to allow more tenable conclusions about the potential role of ERG in discriminating between 5-HT antagonism and melatonergic (MT) agonism in relationship to the claimed antidepressant effect of agomelatine.

Effects of dopamine receptor blockade on the intensity-response function of electroretinographic b- and d-waves in light-adapted eyes

The effects of dopamine receptor blockade by sulpiride (D2-class antagonist) and sulpiride plus SCH 23390 (D1-class antagonist) on the V - log I function of the electroretinographic (ERG) b- and d-waves were investigated in light-adapted frog eyes. Sulpiride significantly decreased the absolute sensitivity of the b- and d-waves. The amplitude of the both waves was diminished over the whole intensity range studied. A similar effect on the b-, but not d-wave amplitude was seen during the perfusion with sulpiride plus SCH 23390. The effect on the d-wave amplitude depended on stimulus intensity. The threshold of the d-wave was not significantly altered. The suprathreshold d-wave amplitude was enhanced at the lower stimulus intensities and remained unchanged at the higher ones. The results obtained indicate that the action of endogenous dopamine on the photopic ERG shows clear ON-OFF asymmetry. Participation of different classes of dopamine receptors is probably responsible for this difference.