A color-coding amacrine cell may provide a blue-off signal in a mammalian retina

Abstract

Retinal amacrine cells are thought to lack chromatic or color-selective light responses and have only a minor role in color processing. We found that a type of mammalian (Ictidomys tridecemlineatus) amacrine cell selectively carries a blue-On signal, which is received from a blue or short wavelength-sensitive (S) cone On bipolar cell. This glycinergic inhibitory S-cone amacrine cell is ideally placed for driving blue-Off responses in downstream ganglion cells.

Figure 1. SCA light responses

a) An amacrine cell is unresponsive to SCIS (upper) but responsive to green LED (lower); one–tier ramification (right). b) Sample SCA unresponsive to green LED stimulus (upper) but responsive to SCIS (lower). c) Sample SCA current (upper) and voltage (lower) responses to a 1s blue light pulse. Inset:spontaneous EPSCs.

Figure 2. SCA morphology

a) Tracer–filled SCA (green) against DIC background. b) Dual dendritic arbors shown as flatmounts in c) and d). Arrows indicate sample varicosities likely to be synaptic sites. Inset d) SCAs immunolabeled by anti– glycine transporter (magenta). f,g)Additional examples of SCAs. e) An SCB (magenta) contacts a single S–cone (blue). Aligning IPLs of e and f, SCA varicosities co–stratify with SCB axon terminals (box).

Figure 3. SCA pharmacology

a) Proposed SCA pathway; below: SCB axon terminals (magenta) opposite puncta immunolabeled for GluR2/3 (green). Right: abolition of SCA responses to the SCIS ) by L–AP4 and NBQX. b) AII pathway. Graphics summarize abolition of SCA responses to SCIS by L–AP4 and NBQX, but not MFA . Mean ± S.D.; *p<0.01.