Abstract
We previously showed the rat corticospinal (CS) synapses were formed all the gray matter at P7; but the synapses in the ventrolateral (VL) area were eliminated from P8 to P10 and CS synapses were distributed predominantly in the dorsomedial (DM) area. This was accompanied by CS axonal regression from the VL area, whereas axons were almost unchanged in the DM area. However, our subsequent study with anterograde labeling of CS axons showed that after this synapse elimination, the " second wave " of the CS innervation arrived at P12: CS terminals again increase in the ventral and also dorsal side. In this study, we investigated the development of CS synaptic responses directly by optical imaging with the voltage sensitive dye. The ventralmost part of posterior funiculus of the spinal cord slice was stimulated to activate CS tract axons and the response signal of the membrane potential change was recorded, which is referred to as optically detected EPSP (optEPSP). The optEPSP intensity in the VL area once peaked at P6–7, but later reduced at P10–P11. This suggests the early synapse formation and succeeding elimination in the VL area. However, the optEPSP intensity in the VL area increased again from P13 and continued to increase until P18. The optEPSP intensity in the DM area was fairly constant for P6–P11, and also increased from P13. Those findings indicate that the second wave of synapse formation surges both in the DM and VL area after elimination of synapses in the VL area formed by the first wave. [J Physiol Sci. 2007;57 Suppl:S83]
