Light and electron microscopic study of corticorubral synapses in adult cat: Evidence for extensive synaptic remodeling during postnatal development

Spine-like dendritic protrusions (SLDPs) emanating from developing dendrite s have been proposed to play an important role in early synaptogenesis. We previously analyzed synaptic termination sites on soma-dendritic membrane o f newborn cats and found that corticorubral (CR) axons form synapses prefer entially on SLDPs (Saito et al., 1997). In the present study, we examined C R synapses in adult cats to elucidate the maturation process of CR synapses in relation to SLDPs. Electron microscopic observation of serial thin sect ions of Phaseolus vulgaris-leucoagglutinin-labeled axons revealed that appr oximately 60% of CR terminals in adult cats formed synapses on dendritic sp ines. We also found that CR axons terminate on dendritic spines originating from the intermediate or distal dendrites of rubrospinal cells (more than 200 mum apart from the soma), in contrast to kittens in which CR fibers ter minate on SLDPs originating from the proximal dendrites (less than 100 mum apart from the soma) of rubrospinal cells (Saito et al. [1997] J. Neurosci. 17:8792-8803). These results suggest that CR synapses undergo remarkable r emodeling after initial termination on SLDP during postnatal development. J . Comp. Neurol. 440:236-244, 2001. (C) 2001 Wiley-Liss, Inc.