CALBINDIN IMMUNOREACTIVITY OF HORIZONTAL CELLS IN THE DEVELOPING RABBIT RETINA

Horizontal cells are retinal interneurons that establish inhibitory fe edback loops within the outer plexiform layer of the primary visual pa thway. Most mammalian retinas contain two types of horizontal cells. A -type horizontal cells have neuritic branches that contact cone photor eceptors exclusively, while the B-type horizontal cells have dendritic branches that contact cones, in addition to axons that form synapses with rod photoreceptors. Immunoreactivity for calbindin, a calcium bin ding protein involved in calcium transport, was used as a marker for h orizontal cells during post-natal development of the rabbit retina. On post-natal days 1, 3 and 5, calbindin immunoreactivity is limited to a single population of A-type horizontal cells. They appear as a monol ayer of cells with broad tapering processes, establishing the proximal border of the nascent outer plexiform layer and forming a target for ingrowing cone photoreceptor terminals. The size and density of the ce ll bodies and the length of neuritic processes are essentially unchang ed during this period, which corresponds to the time of peak expressio n of GABAergic markers in horizontal cells. Coincident with a decrease in GABAergic markers and the completion of cone-to-horizontal cell sy naptogenesis by day 7, changes within the horizontal cell mosaic are d etected morphometrically. A delayed phase of overall cell growth resul ts in a 70% increase in average somal diameter (representing a 3.7-fol d increase in spherical volume), a six-fold increase in mean neurite l ength and a decrease in cell density to one-third of that found in the newborn. We conclude that the process of terminal differentiation of horizontal cells is not complete until some time after the second post -natal week. Furthermore, the expression of GABAergic markers is assoc iated primarily with early maturational events, whereas expression of calbindin is sustained throughout post-natal development, suggesting a prominent role for calcium dependent mechanisms at all developmental stages. (C) 1995 Academic Press Limited