Light‐Induced Dopamine Release from Teleost Retinas Acts as a Light‐Adaptive Signal to the Retinal Pigment Epithelium
- 1 September 1989
- journal article
- research article
- Published by Wiley in Journal of Neurochemistry
- Vol. 53 (3) , 870-878
- https://doi.org/10.1111/j.1471-4159.1989.tb11785.x
Abstract
In the retinal pigment epithelium (RPE) of lower vertebrates, melanin pigment granules migrate in and out of the cells'' long apical projections in response to changes in light condition. When the RPE is in its normal association with the retina, light onset induces pigment granules to disperse into the apical projections; dark onset induces pigment granules to aggregate into the cell bodies. However, when the RPE is separated from the retina, pigment granule movement in the isolated RPE is insensitive to light onset. It thus seems likely that a signal from the retina communicates light onset to the RPE to initiate pigment dispersion. We have examined the nature of this retina-to-RPE signal in green sunfish, Lepomis cyanellus. In isolated retinas with adherent RPE, light-induced pigment dispersion in the RPE is blocked by treatments known to block Ca2+-dependent transmitter release in the retina. In addition, the medium obtained from incubating previously dark-adapted retinas in the light induces light-adaptive pigment dispersion when added to isolated RPE. In contrast, the medium obtained from incubating dark-adapted retinas in constant darkness does not affect pigment distribution when added to isolated RPE. These results are consistent with the idea that RPE pigment dispersion is triggered by a substance that diffuses from the retina at light onset. The capacity of the conditioned medium from light-incubated retinas to induce pigment dispersion in isolated RPE is inhibited by a D2 dopamine antagonist, but not by D1 or .alpha.-adrenergic antagonists. Light-induced pigment dispersion in whole RPE-retinas is also blocked by a D2 dopamine antagonist. These results suggest that the diffusible substance is dopamine and that retinal dopamine stimulates pigment dispersion by interacting with D2 dopaminergic receptors on the RPE cell surface. Direct measurement of the endogenous catecholamine content of isolated dark-adapted retinas and of the medium surrounding isolated retinas incubated in the dark or in the light demonstrates that dopamine is the predominant catecholamine of green sunfish retinas, and that light onset stimulates the overflow of endogenous dopamine-but not that of norepinephrine or epinephrine-from isolated retinas. These results suggest that illumination increases dopamine release in teleost retinas. Together, our findings are consistent with the idea that dopamine acts as a paracrinic messenger of light-adaptive stimuli by diffusing from its site of release in the retina to distant nonsynaptic receptors on RPE cells.Keywords
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