Reduction in the percentage of serotoninergic axons making synapses during the development of the superficial layers of the hamster's superior colliculus.

Immunocytochemistry with an antibody directed against a serotonin (5-HT)-bovine serum albumin conjugate was combined with electron microscopy 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 these ages. In the newborn (P0) animals, 36.6% of the 5-HT-positive profiles made conventional synapses. This percentage remained fairly constant until P15 when only 17.3% of the profiles made conventional synaptic contacts. On P20, this value decreased to 10.5% and in the adult animals, it fell to 4.4%. These results thus indicate a dramatic age-related change in the synaptic organization of the 5-HT input to the hamster's superior colliculus.