In the retinas of many species of lower vertebrates, retinal photoreceptors and pigment epithelium pigment granules undergo daily movements in response to both diurnal, and in the case of teleost cone photoreceptors, endogenous circadian signals. Typically, these cone movements take place at dawn and at dusk when teleosts are maintained on a cyclic light (LD) regime, and at expected dawn and expected dusk when animals are maintained in continuous darkness (DD). Because these movements are so strictly controlled, they provide an overt indicator of the stage of the underlying clock mechanism. In this study we report that both light-induced and circadian-driven cone myoid movements in the Midas cichlid (Cichlasoma citrinellum), occur normally in vitro. Many of the features of retinomotor movements found in vivo also occur in our culture conditions, including responses to light and circadian stimuli and dopamine. Circadian induced predawn contraction and maintenance of expected day position in response to circadian modulation, are also normal. Our studies suggest that circadian regulation of cone myoid movement in vitro is mediated locally by dopamine, acting via a D2 receptor. Cone myoid contraction can be induced at midnight and expected mid-day by dark culture with dopamine or the D2 receptor agonist LY171555. Further, circadian induced predawn contraction can be increased with either dopamine or LY171555, or may be reversed with the dopamine D2 antagonist, sulpiride. Sulpiride will also induce cone myoid elongation in retinal cultures at expected mid-day, but will not induce cone myoid elongation at dusk. In contrast, circadian cone myoid movements in vitro were unaffected by the D1 receptor agonist SCH23390, or the D1 receptor antagonist SKF38393. Our short-term culture experiments indicate that circadian regulation of immediate cone myoid movement does not require humoral control but is regulated locally within the retina. The inclusion of dopamine, or dopamine receptor agonists and antagonists in our cultures, has indicated that retinal circadian regulation may be mediated by endogenously produced dopamine, which acts via a D2 mechanism.

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