Growth behavior of retinotectal axons in live zebrafish embryos under TTX-induced neural impulse blockade.

The growth dynamics of individual DiO-labeled retinal axons deprived of normal neural impulse activity by TTX was monitored in the tectum of living zebrafish embryos with time-lapse video microscopy and compared 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 ruffling lamellipodia and filopodia, but become streamlined when advancing. The activity-deprived axons grow directly towards their retinotopic target sites in the tectum as do their normal counterparts and very rarely extend branches en route. Much like normal axons, TTX-blocked axons begin to branch and develop their terminal arbors only at their retinotopic target area. They emit and retract numerous short side branches over a period of several hours. The area they contact (the "exploration 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 small as arbors of normal active axons and retinotopically correct. Thus, the typical exploratory growth behavior of developing retinal axons in the tectum, the dynamics of terminal arbor formation at retinotopically correct sites, the dimension of the exploration field, and the shaping of the arbors in zebrafish embryos are unaffected by TTX-induced neural impulse blockade.