In teleosts, retinomotor movements of photoreceptors and retinal pigment epithelium are regulated both by light and by an endogenous circadian rhythm. Light induces cones to contract, rods to elongate and RPE cells to disperse their pigment granules into their long apical projections; darkness induces opposite movements. When fish are maintained in prolonged constant darkness, appropriate movements nonetheless occur at subjective dusk and dawn. To explore the mechanisms of this light and circadian regulation, we have been investigating effects of several extracellular messengers known to be present in retina on retinomotor movements in the green sunfish (Lepomis cyanellus). Here we report that prostaglandin E1 (PGE1) can induce movements characteristic of dark onset (or night) in both cones and RPE in isolated light-adapted retinas in the light; ie, PGE1 induces cone elongation and RPE pigment granule aggregation. The extent of PGE1-induced cone and RPE movements were dose-dependent with maximal movement occurring at 250-500 nM; higher concentrations were not as effective. Incubations with PGE2 and PGD2 also induced dark-adaptive cone and RPE retinomotor movements, but PGF2 alpha did not. Further observations suggest that prostaglandins may play a role in mediating the induction of cone and RPE movements by dark onset: dark-induced movements were inhibited by pretreating light-adapted isolated retinas before dark culture with agents which inhibit endogenous prostaglandin synthesis. Both indomethicin (50 microM) and acetylsalicylic acid (50 microM), two inhibitors of the cyclooxygenase component of specific prostaglandin synthase, inhibited dark-induced cone elongation and pigment aggregation in cultured sunfish retinas. Another cyclooxygenase inhibitor, ibuprofen (50 microM) had no effect. Together the effectiveness of PGE1 inducing dark-adaptive movement and the inhibition of dark adaptive movement by cyclooxygenase inhibitors suggest that prostaglandins may play a role in vivo in mediating the induction of dark-adapted RPE and cone retinomotor movements by dark onset.