Pre‐ and postsynaptic correlates of interocular competition and segregation in the frog

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Abstract
Segregated zones of termination between converging inputs that arise from different presynaptic populations are a common property of topographically organized zones within the vertebrate central nervous system. Increasing evidence suggests that such segregation is at least in part established on the basis of competitive interactions that depend upon the activity patterns within each afferent population. However, the cellular mechanisms of these interactions are poorly understood. We have used a preparation in which a stereotyped interdigitating pattern of retina-specific termination stripes are produced in frog tecta innervated by two retinas as a result of embryonic implantation of a third eye primordia. In these animals it has been possible to examine the relationship between the number of retinal ganglion cells in each of the retinas innervating a striped tectum, the volumetric changes in the tectum as a result of this double innervation, and the pattern of eye-specific segregation that is produced. Counts of retinal ganglion cells in the retinas of the three-eyed frogs with one completely striped tectal lobe revealed no significant differences between cell numbers in the doubly innervating retinas and the normal retinas of the same animals. The average increase in retinal ganglion cell innervation to the striped tecta of these animals was 100%. However the tecta only increased in total volume by 26%. This later increase consisted of a 25% increase in the volume of the deep lying and predominately cellular tectal laminae and a 37% increase in the superficial retinotectal synaptic zone. In many of these same animals HRP and 3H-proline were used to differentially label the set of stripes from each retina and measurements of the extent of each projection were performed. We found that the volume of tectal neuropil occupied by a striped projection is relatively unrelated to the number of ganglion cells making up that projection. Observations of the striping pattern after HRP processing to visualize stripes in whole unsectioned tecta indicate that the periodicities and rostrocaudal orientation of stripes are robust over a wide range of relative innervation densities. When one projection is much smaller than the other, stripes appear to break down into a series of “puffs” or islands of retina-specific termination zones. Nevertheless, these puffs still have a rostrocaudal alignment and the spacing of fully formed stripes. These observations suggest that the formation of exclusive termination zones may be a threshold phenomenon: so after a certain innervation density is reached one input can take over a unit of target neuropil in an all-or-none manner. The data are discussed in the context of how activity and functional circuits in the tectum may control the segregation pattern.