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Zhijing Zhang, Grant Hopping, Xiaoqin Liu, Christophe Ribelayga; Photoperiod Dependence of the Mouse Retinal Dopaminergic System. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3423.
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Dopamine (DA) is the main catecholamine of the retina and fulfills critical roles in neural adaptation to time of day and photic illumination. Basal DA release in the dark is higher during the day compared to that of the night due to the action of a circadian clock intrinsic to the retina. In addition, DA is released from the dopaminergic amacrine cells (DACs) upon photic illumination of the retina. Although DACs and DA release and action in the retina have been extensively studied, DAC morphology and physiology have not been investigated under different day lengths (i.e. photoperiods). The present study was undertaken to test the influence of the photoperiod on the dopaminergic system in the mouse retina.
Melatonin-proficient CBA/CaJ and melatonin-deficient C57Bl/6J mice born under a 12 hr light/12 hr dark cycle (normal photoperiod, NP) were transferred at the adult age (2 months-old) to either an 18 hr light/06 hr dark cycle (long photoperiod, LP) or a 06 hr light/18 hr dark cycle (short photoperiod, SP) or kept under NP, for 8 weeks. At the end of the 8-week period, retinas were isolated and processed for morphological analysis of DACs by immunohistochemical staining with an antibody against the rate limiting enzyme in DA synthesis-tyrosine hydroxylase (TH) or HPLC analysis of the retinal content of DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). The ratio DA/DOPAC was taken as an index of DA release.
In light-adapted retinas collected at midday, DA content was ~30% higher and DOPAC content and DA release were ~50% higher under LP compared to that of SP in both CBA/CaJ and C57Bl/6J mice. Furthermore, in CBA/CaJ mice, circadian variations of DA release were observed under both SP and LP, even though DA and DOPAC content and DA release were generally lower under SP compared to that of LP. We also found a ~50% difference in the density of TH+ cells between SP and LP in both strains. DA and DOPAC content, DA release, and DAC density in NP were at intermediate levels between SP and LP.
Our data demonstrate that in the adult mammalian retina, a change in the photoperiodic regime has dramatic consequences on the dopaminergic system. These changes were observed in both CBA/CaJ and C57Bl/6J mouse strains, indicating that the modulations can occur independently of the circadian rhythm of melatonin.
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