The dendritic patterning of retinal horizontal cells has been shown to be specified by the cone photoreceptor afferents. The present investigation has addressed whether this specification is due to visually dependent synaptic transmission in the outer plexiform layer or to some other early, pre-visual, neural activity. Individually labeled horizontal cells from dark-reared mice, as well as from mice carrying a mutation in the Cacna1f gene, which encodes the pore-forming calcium channel subunit Ca(v)1.4, were assessed for various morphological features. The dark-reared mice showed no alteration in any of these features, despite showing a compromised maximal voltage response in the electroretinograms. The retinas of Cacna1f mutant mice, by contrast, showed conspicuous morphological changes that mimicked the effects observed previously in coneless transgenic mice. These changes were present as early as postnatal day 10, when the shape and density of the cone pedicles appeared normal. Ultrastructurally, however, the pedicles at this early stage, as well as in maturity, lacked synaptic ribbons and the invaginations associated with postsynaptic processes. These results suggest a role for this calcium channel subunit in ribbon assembly in addition to its role in modulating calcium influx and glutamate release. Together, they suggest a complex cascade of interactions between developing cone pedicles and horizontal cell dendrites involving early spontaneous activity, dendritic attraction, ribbon assembly, and pedicle invagination.

The present investigation has sought to determine whether the population of retinal horizontal cells undergoes an increase in the precision of its mosaic patterning during postnatal development, and if so, whether this increase is compatible with three different mechanistic accounts of retinal mosaic formation. Horizontal cells were labeled with antibodies to neurofilaments or calbindin at different developmental stages, and then visualized in retinal wholemounts. Multiple fields were sampled from each retina to determine horizontal cell density, while the X-Y coordinates of each cell in a field were determined. An estimate of total horizontal cell number was calculated for each retina, while the Voronoi domain regularity index and the packing factor were computed for each field. Two strains of mice showing a two-fold difference in the size of their horizontal cell population in maturity were sampled, C57BL/6J and A/J. Horizontal cell number in C57BL/6J was approximately twice that observed in A/J at all postnatal stages, with neither strain showing an effect of age on horizontal cell number. In both strains, however, the Voronoi domain regularity index and the packing factor were significantly lower at P-1 relative to later developmental stages. These results show that accounts of mosaic formation proposing the selective death of irregularly positioned cells, or the periodic occurrence of fate-determining events, are insufficient to establish the final patterning achieved by horizontal cells. Rather, they support the hypothesis that tangential dispersion enhances mosaic patterning during postnatal development.

The pedicles of cone photoreceptors, labeled with an antibody to mouse cone arrestin (blue), stratify in the outer plexiform layer, where they form synapses with the dendrites of horizontal and bipolar cells. Those synaptic sites are evidenced by the co‐localization of the synaptic ribbon protein, piccolo (red), with the cone arrestin labeling. The remaining red profiles in the outer plexiform layer indicate the sites of the rod spherules. An antibody to cytochrome oxidase (green) labels the mitochondrion‐rich inner segments of all photoreceptors and yields punctate peri‐nuclear labeling within the outer nuclear layer. Many of these features of the outer retina are altered in the Cacna1f‐mutant retina, expressing a defective calcium channel subunit that prevents normal neurotransmission in the outer plexiform layer. J. Comp. Neurol. 506:745–758, 2007. © 2007 Wiley‐Liss, Inc.

The pedicles of cone photoreceptors, labeled with an antibody to mouse cone arrestin (blue), stratify in the outer plexiform layer, where they form synapses with the dendrites of horizontal and bipolar cells. Those synaptic sites are evidenced by the co‐localization of the synaptic ribbon protein, piccolo (red), with the cone arrestin labeling. The remaining red profiles in the outer plexiform layer indicate the sites of the rod spherules. An antibody to cytochrome oxidase (green) labels the mitochondrion‐rich inner segments of all photoreceptors and yields punctate peri‐nuclear labeling within the outer nuclear layer. Many of these features of the outer retina are altered in the Cacna1f‐mutant retina, expressing a defective calcium channel subunit that prevents normal neurotransmission in the outer plexiform layer. J. Comp. Neurol. 506:745–758, 2007. © 2007 Wiley‐Liss, Inc.