Postnatal development of the retinal projection to the nucleus of the optic tract and accessory optic nuclei in the hooded rat

Retinal projections to the nucleus of the optic tract (NOT) and accessory o ptic nuclei (AON) were studied in the postnatal hooded rat after monocular injection of cholera toxin B subunit (CTB) into the vitreous chamber of the eye. At all postnatal ages, retinal axons were labeled sensitively; they r evealed dense projections to the contralateral, and sparse but distinct pro jections to the ipsilateral, NOT and AON, The CTB labeling enabled the firs t delineation of the complete morphology of developing retinal axons in the ipsilateral NOT and AON. From postnatal day (P) 1 to P3, axons with comple x growth cones were seen, and unbranched collaterals with simple growth con es increased and extended gradually, At P6, complex growth cones disappeare d while branched collaterals with simple growth cones as well as small-size d varicosities increased. By P12 (two days before eye-opening) the adult-li ke pattern of terminal arbors appeared. The branched collaterals with tiny, small-sized varicosities present probably represented developing synaptic boutons. At P16 (after eye opening), the pattern of terminal arbors was wel l developed, almost to the same extent as in the adult. By contrast, a broa dly distributed, transient retinal projection around NOT and BON was gradua lly eliminated; it started to disappear during the first few postnatal days , and was fully retracted by the time of eye-opening time to a pattern norm al for the adult.