Diurnal changes of tyrosine, dopamine, and dopamine metabolites content in the retina of rats maintained at different lighting conditions.

The aim of the study was to investigate the daily rhythms of tyrosine (Tyr), dopamine (DA), 3,4-dioxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) content in the retina of rats maintained under different lighting conditions. Studied substances were measured by highly sensitive high-performance liquid chromatography (HPLC) with an electrochemical detection method. It was found that the levels of Tyr, DA, DOPAC, and HVA change diurnally in the retina of adult Wistar rats maintained under standard cyclic lighting (12-h light/12-h dark). The maximum was found during the light phase of 12:12 light:dark photoperiod. Diurnal changes of DA, DOPAC, and HVA persisted in constant darkness. When the rats were maintained in constant lighting conditions for 2 d (light intensity, 200 lx), the rhythms of tyrosine, DA, and DOPAC completely disappeared. However the level of HVA was higher during the subjective day. The presence of diurnal rhythm of melatonin content in the retina of rats was confirmed by HPLC with an amperometric detection method. Since the presence of time cues is not necessary for manifestation of diurnal changes of DA and its metabolite content, it was concluded that the circadian oscillator drives these rhythms. Probably DA can serve as a zeitgeber for retinal circadian rhythms.