Absence of circadian clock regulation of horizontal cell gap junctional coupling reveals two dopamine systems in the goldfish retina

An average of approximately 180 cells were coupled in each dark-adapted condition in the goldfish retina.

• Dopamine release in the retina is influenced by both light and an internal circadian clock, with light having a greater effect.
• The retinal clock activates D2-like receptors during the subjective day.
• H1 cell coupling remains unchanged in dark-adapted conditions and across subjective day and night.
• Bright light stimulation or the D1 receptor agonist SKF38393 significantly reduces H1 cell coupling.
• D1 receptor activity does not appear to be affected by endogenous dopamine under dark-adapted conditions.
• Two distinct dopamine systems are suggested to exist in the goldfish retina, one influenced by the circadian clock and the other by light.

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