Neurochemical Systems of the Retina Involved in the Control of Movement

Abstract

Recent studies have revealed that the retina may exert control over deep brain function and may be importantly involved in the etiology, progression, and treatment of disorders such as Parkinson’s disease (PD). While such a concept is uncharted territory and even less is known about the mechanism by which this might be achieved, this study was undertaken to determine how retinal dopamine (DA), serotonin (5-HT), and melatonin (MEL) neurotransmitter systems might be involved in the control of movement in their own right. To explore these further, intravitreal (IVIT) injections of DA, 5-HT, and MEL were made 0.5 or 3 h prior to testing horizontal and vertical movement in the open field as well as assessment on three motor tests used routinely to evaluate movement as a preclinical model of PD. The doses of DA (2 µl of 25 and 75 µg/µl), 5-HT (2 µl of 5 and 15 µg/µl), and MEL (2 µl of 5 µg/µl) were chosen because of previous work demonstrating an anatomically precise effect of these transmitters after they were injected directly into the brain. The postinjection times of testing were also chosen on the basis of previous intracerebral and IVIT work intimating the importance of the circadian cycle in determining the efficacy of such effects. 0.5 h after IVIT injection of DA at the 25 and 75 µg/µl doses, significant inhibition of motor function was observed. While IVIT injection of 10 or 30 µg of 5-HT also inhibited motor performance, this was significantly less than that seen with DA. In fact, IVIT injection increases motor performance compared to vehicle injection on some parameters. The IVIT injection of 10 µg of MEL facilitated motor function on many parameters compared to DA, 5-HT, and vehicle injection. When rats were tested 3 h after IVIT injection, the inhibition of vertical movement was also observed compared to controls. The present results illustrate that specific retinal neurotransmitter systems participate in the normal control of bodily motor function. The possible involvement of these systems in movement disorders such as PD is the subject of ongoing research.

Polychromatic Light Exposure as a Therapeutic in the Treatment and Management of Parkinson's Disease: A Controlled Exploratory Trial

“…Their interrelationship has been hypothesized to mediate the effects of toxins in causing PD (29) and in the therapeutic effect of DA replacement (28, 39). In fact, recent studies have defined neurochemical systems in the retina that exert control over deep brain systems controlling movement (40) and embrace circadian involvement (41). However, caution should be exercised in ascribing a therapeutic response solely to retinal DA function given that we have never observed enhanced dyskinaesia and other adverse effects with extended use of light in PD patients experiencing DA overdosing phenomena (6, 8, 22).…”

“…The present results also provide further support for the recent suggestion that the circadian system is involved in PD (6–8, 11, 35). While the mechanism by which this is achieved remains somewhat elusive, we hypothesize that the retina may play a major role in defining how the circadian system is involved (28, 40, 41, 47) and from this will emerge less invasive therapeutic approaches. Whatever the outcome of such exploration, it is becoming increasingly evident that very subtle changes in neurochemistry at targets distant from the NSD can evoke a significant therapeutic response in PD.…”

Preparation and Neuroprotective Activity of Glucuronomannan Oligosaccharides in an MPTP-Induced Parkinson’s Model

“…Recently, several studies indicated that bioactive polysaccharides or oligosaccharides from animals, plants, or fungi presented potential effective treatment in the prevention of development of neurodegenerative disorders [ 11 , 12 , 13 ]. Li and colleagues found that Astragalus polysaccharides suppressed 6-hydroxydopamine-induced neurotoxicity in Caenor-habditis elegans [ 14 ], and Kim and colleagues found that fucoidan attenuated 6-hydroxydopamine-induced neurotoxicity by exerting anti-oxidative and anti-apoptotic effects in SH-SY5Y cells [ 15 ].…”

Olfaction and Colour Vision: What Can They Tell Us about Parkinson’s Disease?

“…Colour discrimination has been shown to be impaired in PD patients (Muller et al, 1998). Despite this, it remains unclear how retinal function is related to substantia nigra dopamine deficiency and whether and to what extent are retinal dopaminergic pathways affected in PD (Willis and Freelance, 2017). In a study employing optical coherence tomography, it has been shown that retinal thickness is not altered in PD patients with or without visual hallucinations compared to healthy controls (Kopal et al, 2015).…”