Abstract
Purpose:
Dopamine (DA), released from amacrine and interplexiform neurons in the inner nuclear layer of the retina, contributes to light adaptive reconfiguration of retinal circuits for high resolution vision. Interestingly, the retinal circadian clock appears to play a role in regulating the synthesis and release of DA, with a high DA level during the day-phase and low level during the night-phase. DA neurons express all six core clock genes and circadian rhythms of the clock gene Per1 are exhibited by DA neurons within retinal whole-mounts. Hereby, we hypothesize that DA neurons sustain circadian rhythms autonomously and circadian components in those DA neurons are involved in retinal physiology.
Methods:
Single-cell recording: Isolated primary cell cultures of retinal DA neurons were obtained from postnatal day 3-6 Th::RFP and Period 2 (Per2)::LUC bioluminescence on a C57bl/6J (C57) background. Bioluminescence is considered as the readout for Per2 gene expression by single cell recordings. Retinal function: Animals, aged 2-4 months, with DA neuron-specific knockout of Bmal1 gene (DA-Bmal1KO) and controls underwent light-adapted electroretinogram (ERG) tests following 42 (CT6) or 54 hr (CT18) of dark-adaption, to examine the circadian regulation of the light-adapted b-wave. DA content: The same cohort of knockout and control animals were sacrificed either at ZT22 or ZT14 after one-week reentrainment. The retinal DA and its metabolites were measure by HPLC.
Results:
A total of 6 DA neurons were recorded: 2 out of 6 neurons sustained PER2::LUC bioluminescence oscillations independently over ~3 days of recording, with an average period of 32.30 hrs. Comparatively, the other 4 neurons appeared arrhythmic with low amplitudes. DA-Bmal1KO and control animals both exhibit circadian rhythms in light-adapted ERGs, with an elevation of b-wave response amplitude in day-phase, and do not display significant differences between genotypes. Also, both groups show higher levels of DA and DOPAC at ZT 14 compared to ZT22, with again no difference found between knockouts and controls.
Conclusions:
The single cell recordings suggest that DA neurons may exhibit autonomous circadian rhythms. However, this potential circadian oscillator may not directly control overt rhythms of DA release and overall retinal electrical responses. More validations are needed to be conducted before any solid conclusions are made.
Keywords: 688 retina •
694 retinal culture