Slow light and dark adaptation of horizontal cells in the Xenopus retina: A role for endogenous dopamine

Oct 1, 1990Visual neuroscience

Slow adjustment of light and dark responses in eye cells of Xenopus linked to natural dopamine

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Abstract

Dopamine may play a critical role in regulating the contributions of rods and cones to horizontal cell activity in the Xenopus retina.

Cone input to horizontal cells decreased over hours when the retina transitioned from light to darkness.
Rod input to horizontal cells increased slowly during the same period, balancing the decline in cone input.
D-amphetamine, a dopamine releasing agent, restored normal cone input waveforms to horizontal cells.
When tested in darkness, cone input increased for 2-4 hours before declining, but continued to rise at approximately 10% per hour in the presence of a weak background field.
The increase in cone input was blocked by a dopamine receptor antagonist, leading to enhanced rod input to horizontal cells.
Depleting dopamine with reserpine or a dopamine neurotoxin resulted in decreased cone input to horizontal cells, even in light-adapted retinas.

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A role for endogenous dopamine in the control of rod and cone contributions to a second-order retinal neuron, the horizontal cell (HC) was studied in the Xenopus retina. Relative rod and cone contributions were estimated from HC responses to scotopically balanced 491- and 650-nm flashes. In eyecups prepared in light then placed in darkness, cone input to the HC slowed and diminished on a time scale of hours. The decline in cone input was balanced by a slow growth of rod input to the HC. Administration of D-amphetamine, a dopamine releasing agent, restored the light-adapted waveform. The kinetics of slow light adaptation were examined by recording HC responses from eyecups that had been dark-adapted previously for 11-14 h. When test flashes fell on a dark field, cone input to the HC grew for 2-4 h, reached a plateau, and later declined. If, however, flashes were superimposed on a weak background field, cone input to the HC continued to increase monotonically at about 10%/h. This increase was abolished by superfusion with a nonspecific dopamine receptor blocker, cis-flupenthixol (50 microM), resulting in the complete suppression of cone-to-horizontal cell synaptic transfer and the enhancement of rod-to-horizontal cell communication. Subcutaneous injection of reserpine, a drug that depletes dopamine stores (2 mg/kg on 1-4 successive days), or intraocular injection of the dopamine neurotoxin, 6-hydroxydopamine (10-30 micrograms) slowed and reduced the amplitude of cone input to the HC, even in completely light-adapted eyes.(ABSTRACT TRUNCATED AT 250 WORDS)

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Slow light and dark adaptation of horizontal cells in the Xenopus retina: A role for endogenous dopamine

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