Canonical PKCα-immunoreactive rod bipolar cells are present in nocturnal snakes but not in diurnal snakes.

A series of studies have described aspects of outer-retinal morphology and visual opsins in different snake species. Among squamate reptiles, snakes stand out for their diversity in photoreceptor morphotypes and pronounced differences in retinal architecture between diurnal and nocturnal species. Diurnal snakes have a cone-dominated retina, lacking typical rods, but containing a transmuted, cone-like rod and exhibiting a low photoreceptor density. Nocturnal species have rod-dominated retinas with high rod densities and two or three types of cones. Despite these striking differences, no previous study has examined inner-retinal neuron types or aspects of neural circuitry in snakes. In this study, we used immunohistochemistry to investigate a specific type of bipolar cell immunoreactive to the anti-protein kinase C alpha (PKCα) antibody, a well-established marker for rod bipolar cells, and to assess its connectivity with photoreceptors in eight nocturnal and 13 diurnal species of snakes from eight families. Nocturnal snakes exhibited a canonical rod-selective PKCα-immunoreactive rod bipolar cell, whereas in diurnal species, the anti-PKCα antibody labeled a distinct population of long-wavelength-sensitive, cone-selective bipolar cells. This study reveals clear differences between diurnal and nocturnal species and highlights the remarkable plasticity and evolutionary innovation of the visual system in this group.