The effect of intravitreal cholinergic drugs on motor control
The retina bears embryological, neurochemical and functional similarities to the circadian and dopamine systems of the brain. Recent studies have shown that the intravitreal injection of minute quantities of L-dopa and of the melatonin receptor antagonist ML-23 have anti-Parkinsonian potential. Furthermore, it has been suggested that light therapy may be potentially useful in treating some aspects of Parkinson's disease (PD) and it is hypothesized that this treatment works via the circadian system. Given that little is known about the mechanism by which such treatments work the present study was designed to examine the role of the acetyl cholinergic system of the retina in gross bodily movement. While IVIT atropine was shown to improve movement in intact rats Cogentin treated rats showed impairment of motor function compared to control rats or to rats treated with any other cholinergic drug. Furthermore, a link between the phase of the light/dark cycle and the efficacy of these drugs in altering movement was demonstrated. These results show that anticholinergic systems in the retina can exert control over movement which has been solely attributed to the function of deep brain structures.