REDUCTION IN THE PERCENTAGE OF SEROTONINERGIC AXONS MAKING SYNAPSES DURING THE DEVELOPMENT OF THE SUPERFICIAL LAYERS OF THE HAMSTER SUPERIOR COLLICULUS

Immunocytochemistry with an antibody directed against a serotonin (5-H T)-bovine serum albumin conjugate was combined with electron microscop y to assess the synaptic organization of the serotoninergic projection to the stratum griseum superficiale (SGS) of the superior colliculus of hamsters killed on postnatal days (P) 0, 3, 7, 12, 15, 20 and > 60. At least 200 5-HT-immunoreactive profiles were examined at each of th ese ages. In the newborn (PO) animals, 36.6% of the 5-HT-positive prof iles made conventional synapses. This percentage remained fairly const ant until P15 when only 17.3% of the profiles made conventional synapt ic contacts. On P20, this value decreased to 10.5% and in the adult an imals, it fell to 4.4% These results thus indicate a dramatic age-rela ted change in the synaptic organization of the 5-HT input to the hamst er's superior colliculus.