Signal transmission from cones to amacrine cells in dark- and light-adapted tiger salamander retina.

Amacrine cells (ACs) are third-order interneurons in the retina that mediate antagonistic surround inputs to retinal ganglion cells and motion-related signals in the inner retina. Previous studies have revealed that rod-to-AC signals in dark-adapted retina are mediated by a nonlinear high-gain synaptic pathway. In this study, we investigated how cone signals are transmitted to ACs under dark- and light-adapted conditions. By using the spectral subtraction method, we found that the voltage gain of the cone-AC synaptic pathway in dark-adapted salamander retina (GD) is between 28 and 72, which is about one order of magnitude lower than the voltage gain of the rod-AC pathway. This suggests that, in darkness, rod signals are more efficiently transmitted to the ACs than cone signals. The voltage gain of the cone-AC synaptic pathway in the presence of 500 nm/-2.4 background light, GL, ranges between 28 and 56. Linear regression analysis indicates that GD and GL are strongly, positively, and linearly correlated. The average GL/GD ratio is 0.73, suggesting that, on average, GL in any given AC is about 73% of GD. This adaptation-induced change in cone-AC voltage gain exemplifies use-dependent modulations of synaptic transmission in the retina, and possible mechanisms underlying light-mediated alterations of retinal synaptic function are discussed.