THE EFFECT OF DOPAMINE DEPLETION ON LIGHT-EVOKED AND CIRCADIAN RETINOMOTOR MOVEMENTS IN THE TELEOST RETINA

The retinae of lower vertebrates undergo a number of structural changes during light adaptation, including the photomechanical contraction of cone myoids and the dispersion of melanin granules within the epithelial pigment. Since the application of dopamine to dark-adapted retinae is known to produce morphological changes that are characteristic of light adaptation, dopamine is accepted as a causal mechanism for such retinomotor movements. However, we report here that in the teleost fish, Aequidens pulcher, the intraocular injection of 6-hydroxydopamine (6-OHDA), a substance known to destroy dopaminergic retinal cells, has no effect on the triggering of light-adaptive retinomotor movements of the cones and epithelial pigment and only slightly depresses the final level of light adaptation reached. Furthermore, the retina continues to show circadian retinomotor changes even after 48 h in continual darkness that are similar in both control and 6-OHDA injected fish. Biochemical assay and microscopic examination showed that 6-OHDA had destroyed dopaminergic retinal cells. We conclude, therefore, that although a dopaminergic mechanism is probably involved in the control of light-induced retinomotor movements, it cannot be the only control mechanism, nor can it be the cause of circadian retinomotor migrations. Interestingly, 6-OHDA injected eyes never reached full retinomotor dark adaptation, suggesting that dopamine has a role to play in the retina's response to darkness.