Synaptogenesis and ultrastructural localization of the polysialylated neural cell adhesion molecule in the developing striatum

The polysialylated neural cell adhesion molecule (PSA-NCAM) plays a role in axonal development and synaptic plasticity. Its pattern of expression is r egulated temporally and topographically in the brain during development. Ho wever, it is unclear whether or not its subcellular location also changes. We have examined PSA-NCAM expression in relation to synapse formation in th e developing rat striatum with immunohistochemistry and electron microscopy . Early in development, PSA-NCAM was present along the cytoplasmic membrane s of neurons and in growth cones. PSA-NCAM expression became progressively confined to pre- and postsynaptic elements as neurons matured morphological ly. Confirming previous results, a marked increase in the density of asymme tric synapses determined by using the physical dissector method was observe d in the dorsolateral striatum between postnatal day 14 (P14) and P18. It w as followed by a reduction between P18 and P25, when asymmetric synapse den sity reached adult levels. In contrast, the density of symmetric synapses h ad surpassed adult levels by P14. In the dorsomedial striatum, the density of asymmetric and symmetric synapses was similar at P18, at P25, and in adu lts. PSA-NCAM was associated with most asymmetric and symmetric synapses at P14 and P18 and was expressed in both pre- and postsynaptic elements of a majority (P14) or approximately half(P18) of the synapses. Most synapses lo st PSA-NCAM expression between P18 and P25 in the dorsolateral striatum and between P25 and adult in the dorsomedial striatum. The data indicate that PSA-NCAM expression becomes restricted topographically during neuronal matu ration but remains strategically associated with developing synapses during late postnatal development in the striatum. J. Comp. Neurol. 405:216-232, 1999. (C) 1999 Wiley-Liss, Inc.