The eel retina: Receptor classes and spectral mechanisms

Light and electron microscopy revealed that there are both rods and cones in the retina of the eel Anguilla rostrata. The rods predominate with a rod to cone ratio of 150: 1. The spectral sensitivity of the dark-adapted eyecup ERG had a peak at about 520 nm and was well fit by a vitamin A₂ nomogram pigment with a λ_max = 520 nm. This agrees with the eel photopigment measurements of other investigators. This result implies that a single spectral mechanism the rods provides the input for the dark-adapted ERG. The spectral sensitivity of the ERG to flicker in the light-adapted eyecup preparation was shifted to longer wavelengths; it peaked at around 550 nm. However, there was evidence that this technique might not have completely eliminated rod intrusion. Rod responses were abolished in a bleached isolated retina preparation, in which it was shown that there were two classes of cone-like mechanisms, one with A.max of 550 nm and the other with Amax of <450 nm. Ganglion cell recording provided preliminary evidence for opponent-color processing. Horizontal cells were only of the L type with both rod and cone inputs.