The effects of melatonin and L-DOPA on the diurnal rhythms of free amino acids content in the rat retina.

The effects of melatonin and dopamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA) intraperitoneal administration on the rhythms of free amino acids content in the retina of rats were studied. The authors found that the levels of those amino acids, which are protein constituents but not neurotransmitters in the rat retina, change diurnally with maximum at 3-6 h after light onset. Diurnal changes of Ala, Arg, Asn, Ile, Met, Ser, Trp, and Val content persisted in the retina of rats maintained at constant darkness. This fact confirms the true circadian nature of these rhythms. Constant lighting abolished diurnal changes of the content of all amino acids with the exception of Trp. Daytime but not nighttime administration of melatonin decreased the levels of Ala, Asn, Gln, Ile, Met, and Ser down to nocturnal values. Diurnal changes of amino acids content vanished in melatonin-injected rats. The effect of melatonin administration disappeared when the protein synthesis was inhibited by cycloheximide. The effect of intraperitoneal administration of L-DOPA on the levels of free amino acids was opposite the effect of melatonin administration. L-DOPA increased nocturnal levels of Gly, Thr, Trp, and Val but had no effect on the daytime amino acids content. As in the case of melatonin administration, significant diurnal changes of amino acid levels disappeared in L-DOPA-injected rats. The authors hypothesize that melatonin and dopamine can serve as zeitgebers-antagonists of amino acids content rhythms in the rat retina.

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.