EVIDENCE FOR D-4 RECEPTOR REGULATION OF RETINOMOTOR MOVEMENT IN ISOLATED TELEOST CONE INNER-OUTER SEGMENTS

In the retinas of teleost fish, cone photoreceptors change shape in re sponse to light and circadian signals. They elongate in the dark, cont ract in the light, and under conditions of constant darkness undergo a ppropriate movements at expected dusk and dawn. Dopamine induces cones to contract, thus mimicking the effect of light or expected dawn. To identify the receptor subtype responsible for mediating dopamine regul ation of cone retinomotor movements, we have carried out pharmacologic al studies using isolated fragments of teleost cones consisting of con e inner segments-cone outer segments (CIS-COS). Isolated CIS-COS retai n the ability to elongate in dark culture and contract when subsequent ly exposed to light or dopamine. We report that dark-induced elongatio n of CIS-COS was inhibited by dopamine and its agonists with an effect iveness ranking of dopamine = quinpirole > bromocriptine much greater than SKF-38393. After 60 min of elongation in dark culture, CIS-COS my oids contracted when subsequently cultured in the dark with dopamine o r quinpirole. Quinpirole-induced inhibition of elongation and quinpiro le-induced contraction were completely blocked by clozapine at 1 mu M or by sulpiride at 100 mu M. These effectiveness profiles for dopamine agonists and antagonists suggest that dopamine regulation of cone ret inomotor movement is mediated by a D-4-like receptor.