The effect of dopamine depletion on light-evoked and circadian retinomotor movements in the teleost retina

Abstract

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 casual 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.

Rearing in different photic and spectral environments changes the optomotor response to chromatic stimuli in the cichlid fish Aequidens pulcher

“…The myoids of rods are elongated and their outer segments are screened from incoming light by the melanosomes of the retinal pigment epithelium. Once entrained to a daily light:dark cycle, rods and cones remain in their light-adapted positions throughout the entire light phase, even during prolonged darkness (Douglas et al, 1992). During morphological studies (Kröger and Wagner, 1996;Braun et al, 1997;Kröger et al, 1999), we observed that cones contracted fully in animals reared in long-wavelength light.…”

Synchronizing multiphasic circadian rhythms of rhodopsin promoter expression in rod photoreceptor cells

“…Dopamine plays important roles in the regulation of photoreceptor cell functions. For example, activation of dopamine D2 receptors regulates daily photomechanical movement of both rod and cone myoids (Douglas et al, 1992;McCormack and Burnside, 1992;Hillman et al, 1995). Rod and cone photoreceptor cells may synapse with each other via gap junctions.…”

Retinoic acid has light-adaptive effects on horizontal cells in the retina

“…1). However, not all lightinduced effects are attributable to dopamine, because some occur in dopamine-depleted retinae or are at least in part insensitive to dopamine antagonists (2)(3)(4). This has spurred the search for additional light-adaptive signals and nitric oxide (NO) has been suggested as a possible candidate.…”