DISTRIBUTION OF TAG-1 AND SYNAPTOPHYSIN IN THE DEVELOPING CEREBELLAR CORTEX - RELATIONSHIP TO PURKINJE-CELL DENDRITIC DEVELOPMENT

During postnatal cerebellar development, differentiating Purkinje cell (PC) dendrites extend towards the pial surface and progressively cont act immature granule cell parallel fiber (PF) axons in the deep extern al granule layer (EGL), thus forming a zone of synaptic contact called the molecular layer (ML). The neuronal cell adhesion molecule, TAG-1, is transiently expressed on PF axons in the deep EGL (Yamamoto et al. [1986] J. Neurosci. 12:3576-3594). To determine the spatiotemporal re lationship between Purkinje cell dendritic differentiation and the ces sation of TAG-1 expression, sagittal sections from developing rat cere bellum were double-labeled for TAG-1 and the Purkinje cell-specific ma rker, calbindin, by using indirect immunofluorescence. At postnatal da y 2 (P2) and P5, confocal microscopy revealed that TAG-1 immunoreactiv ity began above the furthest superficial extent of the Purkinje cell a pical dendritic cap. By P10, PC dendrites penetrated partially into th e TAG-1-positive deep EGL, creating a narrow region of overlap in TAG- 1/calbindin staining at the deep EGL/ML border. In contrast, at P15 an d P20, TAG-1 staining began directly above the furthest superficial ex tent of the Purkinje cell dendrites, with little or no overlap in TAG- 1/calbindin staining. Staining for the synaptic vesicle glycoprotein, synaptophysin, was dim throughout most of the TAG-1-positive deep EGL, although bright synaptophysin immunoreactivity was observed throughou t the ML. Overlap in TAG-1/synaptophysin staining was observed primari ly at the deep EGL/ML border, suggesting that robust expression of syn aptophysin in granule cells does not begin until after contact with Pu rkinje cell dendrites has been initiated. Our results suggest that fac tors present in the developing ML may influence the cessation of TAG-1 expression and the initiation of synaptophysin expression at the bord er region between the ML and the deep EGL. (C) 1998 Wiley-Liss, Inc.