ACTIVITY-DEPENDENT RETINOTOPIC REFINEMENT IN A LOW-DENSITY RETINOTECTAL PROJECTION IN THE GOLDFISH - EVIDENCE FAVORING SYNAPTIC COOPERATIONOVER COMPETITION

During optic nerve regeneration in goldfish, the label from a small re tinal spot injection of WGA-HRP has been previously reported to be ini tially widely dispersed in the tectum and subsequently to condense int o a small patch in the retinotopically appropriate location of tectum. This refinement involves two separate processes: one is activity inde pendent and generates gross retinotopy; the other is activity dependen t and mediates the formation of fine retinotopy. Since the number of s ynapses remains constant during this refinement, one or both of these processes may involve some form of competition for a limited number of synaptic sites. To clarify the role of synaptic competition, we creat ed a low-density retinotectal projection in goldfish by deflecting abo ut 20% of the optic fibers from one tectum into the opposite tectum, w hich was denervated of all other optic fibers. Under this condition, i t was previously shown that less than half the normal density of synap ses is formed. If competition for synaptic sites is a requirement of r efinement, refinement should be prevented or significantly hindered. T o monitor refinement during regeneration, 2 nl spot injections of WGA- HRP were made into the retina at various times after deflection. To di stinguish between activity-dependent and activity-independent refineme nt, retinal impulse activity was blocked in some fish with repeated in jections of TTX into the eye for the duration of the experiment. It wa s found that considerable activity-independent refinement occurred und er continuous TTX blockade although the fibers remained more dispersed than in previous TTX studies when normal numbers of fibers were prese nt. Surprisingly, in fish with normal impulse activity, the degree of activity-dependent refinement was almost normal. Labeled fibers conden sed into a small area roughly comparable in size to that observed when the full complement of fibers was regenerating into tectum. These res ults suggest that competition for limited synaptic sites is not essent ial for activity-dependent refinement, which may, instead, be mediated by a cooperative process that actively promotes convergence. The find ings further suggest that if synaptic competition plays a role in this system, it is in regulating activity-independent mechanisms that dete rmine the large-scale distribution of fibers within tectum.