Abstract
Purpose: :
The objective is to characterize the spectral responses, rod/cone inputs, receptive fields and patterns of dye coupling of various types of color opponent second-order cells in the salamander retina.
Methods: :
Bipolar cells (BCs) and horizontal cells (HCs) were impaled with microelectrodes filled with Neurobiotin in superfused flatmount retinas of the tiger salamander, and voltage responses to light spots and annuli of various diameters and wavelengths were recorded. Cell morphology and patterns of dye coupling were examined with a confocal microscope, and the cells’ receptive fields were determined by responses to moving light bars across the retina.
Results: :
We found four types of color opponent second-order cells in the salamander retina. The first are medium field HCs (receptive field diameter about 1,300 µm) that are dye coupled with adjacent HCs. These cells depolarized to dim-intermediate 500nm light and hyperpolarized to 700nm light and bright 500 nm lights. The second type are wide field HCs with a receptive field diameter over 2,000µm, and these cells hyperpolarized to 500 nm light with large voltage tails and depolarized to 700 nm light steps. The third type are wide field (receptive field diameter about 1,800 µm) second-order cells displaced in the proximal margin of the outer nuclear layer. These cells have several dendritic processes about 200-300 µm long, and they are dye coupled to one another. They exhibited biphasic responses (transient hyperpolarization followed by a more sustained depolarization) to 500 nm light and depolarizing responses to 700nm light. The fourth type are bipolar cells that exhibited a center-surround antagonistic receptive field. These cells gave hyperpolarizing responses to 400-450 nm light steps, biphasic responses (depolarization followed by hyperpolarization) to 500-550 nm light steps and depolarizing responses to 600-700 nm light steps.
Conclusions: :
Our results suggest that color opponency of salamander second-order neurons is likely to be mediated by differential sign-preserving/inverting synapses from rods, red-sensitive cones and blue-sensitive (S) cones.
Keywords: horizontal cells • electrophysiology: non-clinical • color vision