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

Abstract

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 ( G D) 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, G L, ranges between 28 and 56. Linear regression analysis indicates that G D and G L are strongly, positively, and linearly correlated. The average G L/ G D ratio is 0.73, suggesting that, on average, G L in any given AC is about 73% of G D. 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.