MK801 increases retinotectal arbor size in developing zebrafish without affecting kinetics of branch elimination and addition

Abstract

Visual activity refines the retinotopic map formed on tectum during regeneration and development in goldfish through an N-methyl-D-aspartate (NMDA) receptor-mediated mechanism. Retinal arbors are enlarged in fish with unrefined maps. Here, we examined the effect of NMDA receptor blockers on the development of retinotectal arbors in zebrafish. Since visual behaviors begin 68-79 h postfertilization, we blocked NMDA receptors by immersion of larvae in MK801, AP5, or CPP starting at either 48 or 72 h. We then labeled axons with DiI at 72 or 96 h and examined them 5-9 h later. Arbors at 101-105 h (31 cases) were larger than at 77-79 h (11 cases): The average number of branches increased from 4.0 to 7.6 and the area (convex polygon method) increased by 42%. Blocking NMDA receptors with MK801 from 72 to 101-105 h significantly enlarged arbor size, but the number of branches remained roughly the same. The length and area of the arbors were both significantly increased (21% and 36%), whereas the width increased by a smaller amount (6%). This increase was reflected in longer distances between branches within the arbor (interbranch segments, +13%) as well as in the summed length of all branches (+28%). This selective effect on the extent but not number of branches is in agreement with our previous report of strobe effects in both developing and regenerating projections in goldfish, and supports the role of NMDA receptors in the first 24 h of synaptic transmission. We also used DiO to label arbors in time-lapse images taken at hourly intervals from 77 to 112 h. These sequences confirmed that individual arbors grew during this time, but showed that rates of branch addition and deletion and branch lifetimes were unaltered by the MK801 treatment. This is consistent with a simple model of random insertion of new branches and selective activity-driven elimination of those at the periphery to keep the normal arbor focused. Blocking NMDA receptors is postulated to randomize the elimination allowing the periphery to expand, thus accounting for the enlarged areas, without change in branch numbers or branch dynamics.