MORPHOLOGICAL TYPES OF HORIZONTAL CELL IN RODENT RETINAE - A COMPARISON OF RAT, MOUSE, GERBIL, AND GUINEA-PIG

Retinal horizontal cells of four rodent species, rat, mouse, gerbil, a nd guinea pig were examined to determine whether they conform to the b asic pattern of two horizontal cell types found in other mammalian ord ers. Intracellular injections of Lucifer-Yellow were made to reveal th e morphologies of individual cells. Immunocytochemistry with antisera against the calcium-binding proteins calbindin D-28k and parvalbumin w as used to assess population densities and mosaics. Lucifer-Yellow inj ections showed axonless A-type and axon-bearing B-type horizontal cell s in guinea pig, but revealed only B-type cells in rat and gerbil reti nae. Calbindin immunocytochemistry labeled the A- and B-type populatio ns in guinea pig, but only a homogeneous regular mosaic of cells with B-type features in rat, mouse, and gerbil. All calbindin-immunoreactiv e horizontal cells in the latter species were also parvalbumin-immunor eactive; comparison with Nissl-stained retinae showed that both antise ra label ah of the horizontal cells. Taken together, the data from cel l injections and the population studies provide strong evidence that r at, mouse, and gerbil retinae have only one type of horizontal cell, t he axon-bearing B-type, whereas the guinea pig has both A- and B-type cells. Thus, at least three members of the family Muridae differ from other rodents and deviate from the proposed mammalian scheme of horizo ntal cell types. The absence of A-type cells is apparently not linked to any peculiarities in the photoreceptor populations, and there is no consistent match between the topographic distributions of the horizon tal cells and those of the cone photoreceptors or ganglion cells acros s the four rodent species. However, the cone to horizontal cell ratio is rather similar in the species with and without A-type cells.