Abstract
We previously showed both in vivo and in vitro that the corticospinal (CS) synapses are at first distributed in the whole spinal cord but eliminated from the ventrolateral side later. In this study we investigated the daily progress of the synapse elimination from P7 to P11 in vivo rat by electrophysiological and histological methods. Field EPSPs were evoked by stimulation of the medullary pyramid and recorded from the lower cervical cord (C7). They were widely distributed at P7, and eliminated from the ventrolateral side of spinal cord at P8. Their distribution was limited to the dorsomedial area at P10 and P11. To label the CS terminals anterogradely, biotin dextran amine (BDA) was injected to the sensorimotor cortex. Sections prepared from the C7 spinal cord studied morphologically. First, peroxidase reaction with diaminobenzidine was used for counting CS fibers in the ventrolateral area. The CS fibers spread to the entire spinal cord in fan-shape at P7. Terminals in the ventrolateral area began to regress as early as at P8 and further decreased at P10 and P11. This regression of the CS terminals paralleled closely with field EPSPs change. Second, localization of presynaptic structure on the labeled terminals in the ventrolateral area was confirmed by double immunostaining of BDA and synaptophysin, which was observed with laser confocal microscope. These findings suggest that the CS synapses are formed in the whole spinal cord at P7, but immediately after this the elimination begins in the ventrolateral area. The start of elimination is much earlier than previously reported. [Jpn J Physiol 55 Suppl:S153 (2005)]
