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Abstract:
Purpose : There is growing evidence that intrinsically photosensitive retinal ganglion cells (ipRGCs) modulate retinal activity. It has been demonstrated that selective stimulation of the optic nerve head with blue light (BL) can activate the melanopsin-containing ipRGCs axons located there. Stimulation of ipRGC axons also improves contrast sensitivity and alters retinal electrical activity, which may indirectly reflect an increase in retinal dopamine release. Retrograde signalling from ipRGCs to dopaminergic amacrine cells can influence the retinal dopaminergic system. This study aimed to directly investigate any change in dopamine release as a result of ipRGC activation. It was hypothesized that dopamine levels would be elevated after stimulation of the optic nerve head with BL.
Methods : Adult male New Zealand rabbits were stimulated for one and ten minutes with and without BL, being included five rabbits in each group. With a self-constructed stereotactic system, it was able to specifically stimulate the optic nerve head with BL (peak 470 nm) in rabbits’ eyes. The BL stimulus was delivered by an optical fiber of 200 µm diameter inserted into the globe and positioned over the optic nerve head in the experimental eye only. In the control condition with no light, the optical fiber was inserted without light stimulation. The concentration of dopamine in the tears, aqueous humor, vitreous humor, and retina was measured using high-performance liquid chromatography. After normal distribution analysis, statistical comparisons were conducted between the contralateral eye and the experimental eye after stimulation using paired Student’s t-tests, being considered p < 0.05 as statistical significance.
Results : No significant difference in absolute dopamine levels between eyes was found in the tears, aqueous humor, or retina (p > 0.05). In the vitreous humor, dopamine concentration was significantly higher in the experimental eye compared to the contralateral eye after BL stimulation (p < 0.05).
Conclusions : These results are consistent with research that suggests vitreal content is a better measure of the rate of retinal dopamine release than retinal dopamine levels. This is the first direct evidence that activation of melanopsin in the axons of ipRGCs at the head of the optic nerve results in measurable changes in dopamine concentration in the eye.
