GROWTH-BEHAVIOR OF RETINOTECTAL AXONS IN LIVE ZEBRAFISH EMBRYOS UNDERTTX-INDUCED NEURAL IMPULSE BLOCKADE

The growth dynamics of individual DiO-labeled retinal axons deprived o f normal neural impulse activity by TTX was monitored in the tectum of living zebrafish embryos with time-lapse video microscopy and compare d with normal active axons. Growth cones of TTX-blocked axons advance intermittently with an average velocity similar to normal axons. While exploring their local environment, they are broadened and bear ruffll ing lamellipodia and filopodia, but become streamlined when advancing. The activity-deprived axons grow directly towards their retinotopic t arget sites in the tectum as do their normal counterparts and very rar ely extend branches en route. Much like normal axons, TTX-blocked axon s begin to branch and develop their terminal arbors only at their reti notopic target area. They emit and retract numerous short side branche s over a period of several hours. Thearea they contact (the ''explorat ion field'') is of similar dimension as that of active axons, covering from 1% to 7.4% of the tectal neuropil surface, but the final arbors cover an area only one-half to one-sixth as large. TTX arbors are as s mall as arbors of normal active axons and retinotopically correct. Thu s, the typical exploratory growth behavior of developing retinal axons in the tectum, the dynamics of terminal arbor formation at retinotopi cally correct sites, the dimension of the exploration field, and the s haping of the arbors in zebrafish embryos are unaffected by TTX-induce d neural impulse blockade. (C) 1994 John Wiley & Sons, Inc.