Abstract
Abstract: :
Purpose: In fish and other vertebrate retinas, dopamine release is under the control of both light and an endogenous circadian (24-hour) clock. However, light increases dopamine release to a greater extent than the clock. Under constant dark conditions, the extracellular level of dopamine is higher during the subjective day compared to the night so that D2-like receptors are activated (Ribelayga et al., 2002). It is not known, however, whether the retinal clock also activates D1 receptors, which display a much lower sensitivity to dopamine in intact tissue. Because the gap junctions between fish cone horizontal cells can be uncoupled by activation of the D1 receptors on the cells, we studied whether a circadian clock regulates the extent of biocytin tracer coupling between cone (H1) horizontal cells. Methods: Goldfish retinas were isolated and superfused under different lighting conditions throughout the circadian and the regular light/dark cycles. Standard intracellular recording, biocytin injection and histochemical techniques were used. Results: Tracer coupling between goldfish H1 cone horizontal cells was extensive under dark-adapted conditions. No significant difference was observed between the subjective day, subjective night, day and night, that is, an average of about 180 cells were coupled in each dark-adapted condition. However, bright light stimulation or application of the D1 agonist SKF38393 (10 µM) dramatically reduced H1 cell coupling down to 1-5 cells. The D2 agonist quinpirole (1 µM) or the concomitant application of the D1 antagonist SCH23390 (10 µM) and the D2 antagonist spiperone (10 µM) had no effect on the coupling of the cells in dark-adapted retinas. Conclusions: These observations demonstrate that H1 cell gap junctional coupling and thus D1 receptor activity are not affected by endogenous dopamine under dark-adapted conditions. We conclude that two different dopamine systems are present in the fish retina. One system is controlled by an endogenous clock that activates low threshold D2-like receptors in the day, while the second system is controlled by light and involves activation of higher threshold D1 receptors.
Keywords: dopamine • gap junctions/coupling • horizontal cells