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Jie Feng, Cameron L. Atkinson, Dao-Qi Zhang; Dopaminergic Amacrine Cells Are Inhibited by Melatonin through Activation of MT1 and MT2 Receptors in The Mammalian Retina. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6326.
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© ARVO (1962-2015); The Authors (2016-present)
Dopamine and melatonin are two key neuromodulators in the vertebrate retina and play critical roles in retinal function. It is known that dopamine release from dopaminergic amacrine cells (DA cells) is suppressed by melatonin. However, the underlying mechanisms involved are poorly understood. We sought to determine whether and how spontaneous and light-evoked activity of DA cells is regulated by melatonin in the intact mouse retina.
A transgenic mouse model expressing red fluorescence protein (RFP) under the control of the tyrosine hydroxylase (TH) promoter was used for the experiments. TH::RFP expressing DA cells were identified by fluorescence microscopy for loose patch voltage-clamp recordings in whole-mount retina preparations.
The spontaneous spike activity of DA cells in situ was substantially suppressed by application of 100 nM melatonin. The spike frequency was reduced from 3.6 ± 0.4 Hz to 1.0 ± 0.2 Hz (n=31, p<0.001). To determine whether this suppression is mediated by activation of melatonin receptors, luzindole, a competitive MT1/MT2 antagonist, and 4-P-PDOT, a specific MT2 antagonist, were used. 100 nM luzindole decreased melatonin inhibition from 78.0 ± 7.2% to 46.3 ± 7.5% (n=8, p<0.05), whereas 100 nM 4-P-PDOT decreased it from 87.8 ± 7.1% to 58.5 ± 8.4% (n=8, p<0.05). To determine whether melatonin indirectly inhibits DA cells, a cocktail of synaptic blockers (CNQX, GABAzine and strychnine) was used. The cocktail partially blocked melatonin inhibition (n=3). In addition, melatonin profoundly reduced the light-evoked responses of transient DA cells that are driven by rods and cones (n=9). In contrast, melatonin had a slight inhibitory effect on the light-evoked responses of sustained DA cells that are mediated by the intrinsically photosensitive retinal ganglion cells (n=5).
Our results suggest that DA cells are inhibited by melatonin via activation of MT1 and MT2 receptors in the resting state, providing insight into the mechanisms responsible for melatonin regulation of dopamine release in darkness and at night. Our results also suggest that melatonin preferentially regulates transient DA cells, providing information on how melatonin modulates excitatory synaptic transmission from photosensitive neurons to DA cells in the retina.
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