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D.-Q. Zhang, D. G. McMahon; Cellular Mechanisms of Light-Induced Dopamine Release in the Mammalian Retina. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4911.
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A transgenic mouse model expressing red fluorescence protein (RFP) under the control of the tyrosine hydroxylase (TH) promoter (Zhang et al., 2004) was used for the experiments. TH::RFP expressing cells were identified by fluorescence microscopy for loose patch voltage-clamp recordings. Full-field illumination was uniformly delivered from the LED to the retina, via the microscope condenser.
525-nm light (2.67 cd/m2, 1 second) only evoked increases in firing frequency (ON responses) in 28/47 DA cells tested. The majority of light-responsive DA cells (18/28) exhibited ON-transient responses, whereas the rest of them (10/28) exhibited ON-sustained responses. To determine whether light signals from photoreceptors to DA cells pass through ON-type bipolar cells, we applied L-AP4 (10 - 30 µM) to 22 light-responsive DA cells (14 ON-transient DA cells and 8 ON-sustained DA cells) and found that light responses were completely blocked by L-AP4 in all 14 ON-transient DA cells. L-AP4 failed to block light responses in 7 out of 8 ON-sustained DA cells. Furthermore, light responses of DA cells persisted under blockade of GABAA (GABAzine, 20 µM) or glycinergic (strychnine, 1µM) neurotransmission suggesting that ON bipolar cells do not require a serial inhibitory network to activate DA cells. In addition, ON-transient DA cells responded to flickering light (0.33 Hz) whereas their firing rates were not significantly changed during prolonged stimuli (20 s).
Fluorescent labeling of dopaminergic neurons with a genetically encoded RFP reporter construct has enabled targeted electrophysiological recordings of these neurons in intact retinal circuits and characterization of their spontaneous activity, synaptic inputs and light responses. Our findings indicate that, in situ, DA neurons exhibit diversity in their light responses, producing transient, sustained and null responses mediated by distinct synaptic mechanisms. These findings provide the basis for unifying previously disparate results regarding dopaminergic regulation and function in the retina.
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