Effects of haloperidol, methylergometrine and phentolamine on the frog ERG

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.

The brain through the retina: the flash electroretinogram as a tool to investigate psychiatric disorders

Investigating the living brain remains one of the major obstacles in psychiatry research in order to better understand the biological underpinning of brain disorders. Novel approaches are needed to study brain functions indirectly. Since it is part of the central nervous system, retinal functions as measured with the flash electroretinogram (ERG) may reflect the central dysfunctions reported in psychiatric disorders. This review describes the flash ERG anomalies reported in patients with psychiatric disorders such as seasonal affective disorder, schizophrenia, autism spectrum disorder and drug addiction and discusses how changes in retinal functions might be used as biomarkers for psychiatric disorder as well as a potential aid to diagnosis in psychiatry.

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.

Effects of dopamine receptor blockade on the intensity-response function of ERG b- and d-waves in dark 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 ERG b- and d-waves were investigated in dark adapted frog eyes. We observed that sulpiride enhanced the amplitude of the suprathreshold b- and d-waves in the lower intensity range, where the responses were mediated by rods, but diminished it in the higher intensity range, where the responses were mediated by cones. A similar effect on the b-, but not d-wave amplitude was seen during the perfusion with sulpiride plus SCH 23390. The d-wave amplitude was enhanced over the whole intensity range with the exception of the highest intensities during the combined D1 and D2 receptor blockade. The results obtained indicate that the endogenous dopamine has an overall inhibitory action on the suprathreshold rod-mediated ON and OFF responses, while its action on the cone-mediated responses shows clear ON-OFF asymmetry. It is excitatory upon the ON responses, but inhibitory upon the OFF responses except for those in the highest intensity range. Participation of different types of dopamine receptors (predominantly D2 for the ON versus D1 for the OFF response) is probably responsible for this difference.

Electroretinographic assessment in major depressed patients receiving duloxetine: might differences between responders and non-responders indicate a differential biological background?

INTRODUCTION

Despite intense research efforts, still too little is known about the biological determinants of depression, thus soliciting diverse study approaches. Among others, the electroretinography (ERG) has been proposed even as a putative proxy (retinal) measurement of central dopaminergic activity for Major Depressive Disorder (MDD) both in drug-naïve patients and subjects receiving antidepressant treatments. Nonetheless, current evidences are merely preliminary, essentially considering just older classes of antidepressants, thus requiring confirmation studies even with newer agents as duloxetine.

METHOD

Twenty MDD subjects and 20 matched controls received duloxetine 60 mg/day for 12 weeks, being monitored both by standard ERG recording and by administration of the Hamilton scales for Depression and Anxiety and the Young Mania Rating Scale at baseline and week 12 (end of the study).

RESULTS

ERG mean rod b-wave amplitude significantly reduced from baseline to week 12 in those depressed subjects achieving final response (p=.024), decreasing from the highest rank values to the ones, substantially unmodified, seen among non-responders and controls.

LIMITATIONS

Small sample size and lack of multiple assessments.

CONCLUSIONS

At least some MDD patients responding to duloxetine might exhibit a peculiar ERG pattern, hypothetically indicating a specific biological background. If confirmed by larger-sampled studies, these results might shed further light in the understanding of the biological determinants of different subtypes of depression, ideally showing alternative patterns of response upon different treatment interventions.

The effects of dopamine blockade on the human flash electroretinogram

Single-cell electrophysiologic studies have shown that dopamine modulates retinal activity, but its role in human retinal processing is unclear. We investigated the effects of short-term oral administration of dopaminergic receptor blocking agents on the flash electroretinogram in humans. Both chlorpromazine (25 and 50 mg) and fluphenazine (1 and 2 mg) significantly reduced electroretinogram b-wave amplitudes and also selectively reduced the amplitude of the first oscillatory potential. Implicit times were not altered. Metoclopramide (10 and 20 mg) had no effect on any electroretinographic variable. Our study indicates that dopamine receptor blocking agents with both D-1 and D-2 receptor affinities reduce the amplitude of the electroretinogram in humans.

Electroretinographic effects of haloperidol on the mouse retina

The effects of haloperidol on the electroretinogram were studied in mice injected intraperitoneally with haloperidol. A dose-related study was performed. A significant decrease in the amplitude of the b-wave was found in the mice injected with more than 20 mg/kg of haloperidol. However, the peak latency of the b-wave and the stimulus intensity required to elicit a b-wave of one half of the maximum b-wave amplitude remained unchanged. No significant changes were found in the a-wave. This change in the electroretinogram recovered 24 hours after the injection. These results suggest that functional change occurred at the retinal level after intraperitoneal injection of haloperidol.

Electroretinographic effects induced in humans by psychopharmacologic agents

We studied the effects of single doses of different dopamine agonists and antagonists on the electroretinogram of a group of healthy volunteers. The results demonstrated significant b-wave amplitude changes after drug administration, suggesting that electroretinograms can be employed to evaluate the effects on retinal dopaminergic activity induced by psychotropic drugs and that the study of the electroretinographic effects of psychopharmacologic agents can provide new insights into the relationship between retinal dopaminergic mechanisms and the electroretinogram b-wave origin.

The effect of fluphenazine on rod-mediated retinal responses

The effect of various concentrations of the dopamine antagonist fluphenazine on ocular field potentials, recorded under scotopic conditions from isolated, arterially perfused cat eyes, was studied. Responses from outer (isolated PIII-component of the electroretinogram, ERG), middle (b-wave), and inner (optic nerve response, ONR) retinal layers were separated. Neither the fast or slow PIII-amplitude nor the temporal characteristics of the response were influenced by any of the drug concentrations tested. In contrast, fluphenazine reversibly increased the rod b-wave amplitude over a large range of concentrations. Only very high drug concentrations led to an irreversible loss of the b-wave. In the ONR the initial transient on-response increased during drug injection, whereas the sustained on-response and off-response decreased. In summary, the dopamine antagonist fluphenazine affects mainly the signal processing of the rod pathway in the inner retinal layers, while responses from outer retinal layers are not influenced. On- and off-responses of the ONR are affected differently.

Dopamine hyperpolarizes and reduces the light responses of rod ON-centre bipolar cells in the retina of the dogfish, Scyliorhinus canicula

Intracellular recordings were obtained from horizontal cells and bipolar cells of the dark-adapted, virtually all-rod retina of the dogfish. Eyecups were superfused with Ringer's solution containing micromolar concentrations of dopamine (DA). It was found that 10 microM DA hyperpolarized rod ON-centre bipolar cells and reduced their responses to light flashes. Detectable effects could be obtained with DA concentrations as low as 1 microM. A decrease in the light responses of bipolar cells due to the effects of DA was concomitant with a decrease in the amplitude of the b-wave of the electroretinogram. DA had no effect on the membrane potential or the light response of rod horizontal cells. These results demonstrate a selective action on rod ON-centre bipolar cells and suggest the existence of a dopaminergic pathway which affects the sensitivity of the rod visual system at the retinal level.