RETINOHYPOTHALAMIC TRACT DEVELOPMENT IN THE HAMSTER AND RAT

The development of the retinohypothalamic tract (PI-IT) in the albino rat and golden hamster was studied using anterograde transport of chol era toxin conjugated to horseradish peroxidase (CT-HRP). The RHT has t hree components in the adult: (1) a dense projection to the ventrolate ral subdivision of the suprachiasmatic nucleus (SCN) with some fibers extending into the dorsomedial SCN; (2) a projection to adjacent areas , the anterior hypothalamic area (AHA) and retrochiasmatic area (RCA) and in the hamster, into the preoptic area (POA); (3) a projection to the lateral hypothalamic area (LHA). In the rat, the projection to the SCN and adjacent areas first appears as scattered Varicosities at the ventral border of the SCN at postnatal day 1 (Pi) and gradually incre ases until the adult pattern is achieved at approximately P10. The pro jections to the AHA and RCA are seen first at P2-P3 and gradually incr ease to the adult appearance by P15. Both the projection to the SCN an d adjacent areas and to the LHA, initially are more extensive than in the adult. Many of the axons extend well beyond the zone of the adult pattern but these anomalous fibers are eliminated by P6-P10. The LHA p rojection first appears at embryonic day 21-22 (E 21-22) and gradually increases in density from P1-P6. In the hamster the projections to th e SCN, AHA and LHA appear first on P4 and gradually increase in densit y to reach the adult pattern by P15. The projections to the RCA and PO A are present by P6 and reach the adult pattern by P15. None of the RH T projections in the hamster has the initial extended growth followed by pruning back that characterizes RHT development in the rat. Thus, t he development of the RHT in both the rat and the hamster is complex w ith components of the projection appearing at different times with dif fering patterns of development that indicate specialized interactions of the developing axons with their target neurons. Synaptogenesis in t he hamster hypothalamus was analyzed using an antiserum to synapsin I. Few synapses are present at E16, the last day of gestation, in the LH A, SCN and AHA. From P1-P3, synaptogenesis proceeds rapidly and the ad ult pattern is achieved in all three areas by P4.