Complex dependence of dopamine release on light in chicken retina

Purpose. In vertebrate retina, dopaminergic neurons are the pivot in a dark-light switch consisting of two mutually inhibitory loops, one involving the photoreceptors (melatonin) and the other a population of enkephalinergic amacrine cells (Morgan & Boelen, 19%, Vis Neurosci 13-399-409) The properties of the dark-light switch tend to confer two-state non-adapting responses on the neurons involved The available evidence, however, suggests that dopaminergic functioning is considerably more complex The aim of this study is to investigate the diurnal pattern of dopamine release and its dependence on light intensity. Methods. Ten-day old chickens were reared under controlled light-dark conditions. Retinal and vitreal samples were assayed for dopamine and DOP AC content by HPLC with electrochemical detection. Results. Vitreal DOPAC levels appeared to be a reliable index of retinal dopamine release. These levels rose approximately two-fold within 30min of light coming on. The rise continued and the levels then stabilised after 3h for the remainder of the 12h light phase. Upon return to the dark, vitreal levels of DOPAC fell and reached a plateau within 6h. During continous dark, vitreal DOPAC remained low, although a small, transient rise was detected early in the subjective light phase The activation by light occurred at 0.4 lux with vitreal DOPAC tripling in 3h as compared to dark levels. With higher light intensities, to a maximum of 200 lux tested, there was a further progressive two-fold increase of vitreal DOPAC. Conclusions. Light appears tobe the main activator of dopamine release. The minor circadian activation may be due to the circadian fall of melatonin, which would reduce inhibition of the dopaminergic neurons. Dopamine release shows a simple 'on-ofT component, as indicated by the step-like activation at a low light intensity, with further increases in dopamine release at higher light intensities.