Induction of dark-adaptive retinomotor movement (cell elongation) in teleost retinal cones by cyclic adenosine 3','5-monophosphate.

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Abstract
In the teleost retina, the photoreceptors and retinal pigment epithelium (RPE) undergo extensive movements (called retinomotor movements) in response to changes in light conditions and to an endogenous circadian rhythm. Photoreceptor movements serve to reposition the light-receptive outer segments and are effected by changes in inner segment length. Melanin granule movements within the RPE cells provide a movable melanin screen for rod outer segments. In the dark (night), cones elongate, rods contract, and pigment granules aggregate to the base of the RPE cell; in the light (day), these movements are reversed. Treatments that elevate cytoplasmic cAMP provoke retinomotor movements characteristic of nighttime dark adapation, even in bright light at midday. To illustrate this response, a quantitative description of the effects of cyclic nucleotides on cone length in the green sunfish, Lepomis cyanellus, are reported. Cone elongation is induced when light-adapted retinas are exposed to exogenous cAMP analogues accompanied by phosphodiesterase (PDE) inhibitors (either by intraocular injection or in retinal organ culture). Cone movement is not affected by cGMP analogues. Dose-response studies indicate that the extent, but not the rate, of cone elongation is proportional to the concentration of exogenous cAMP and analogue presented. As reported for other species, levels of cAMP are significantly higher in dark- than in light-adapted green sunfish retinas. Apparently, cAMP plays a role in the light and circadian regulation of teleost cone length.