Dopamine inhibits forskolin- and 3-isobutyl-1-methylxanthine-induced dark-adaptive retinomotor movements in isolated teleost retinas.

We have been investigating the mechanisms of diurnal and circadian regulation of teleost retinomotor movements. In the retinas of lower vertebrates, photoreceptors and melanin pigment granules of the retinal pigment epithelium (RPE) undergo movements at dawn and dusk. These movements continue to occur at subjective dawn and dusk in animals maintained in constant darkness. Cone myoids contract at dawn and elongate at dusk; RPE pigment disperses into the epithelial cells' long apical processes at dawn and aggregates into the cell bodies at dusk. We report here that forskolin, an adenylate cyclase activator, and 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor, each induces dark-adaptive cone and RPE retinomotor movements in isolated light-adapted green sunfish retinas cultured in constant light. Forskolin induces a 22-fold elevation in retinal cyclic AMP content. Forskolin- and IBMX-induced movements are inhibited approximately 65% and 95%, respectively, by 3,4-dihydroxyphenylethylamine (dopamine). However, dopamine does not inhibit dark-adaptive movements induced by dibutyryl cyclic AMP. Epinephrine is much less effective than dopamine in inhibiting forskolin-induced movements, while phenylephrine and clonidine are totally ineffective. These results are consistent with our previous findings that treatments that increase intracellular cyclic AMP content promote dark-adaptive retinomotor movement. They further suggest that dopamine inhibits adenylate cyclase activity in photoreceptors and RPE cells and thereby favors light-adaptive retinomotor movements.