Stability of dendritic spines and axon terminals in diverse sensory regions of adult mouse cortex.

A.K. Majewska*; J. R. Newton; M. Sur

Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge , MA , USA

Cortical synapses consist of postsynaptic dendritic spines and presynaptic axon terminals. Although plastic changes are thought to occur in adult cortex, it remains unclear whether these changes require remodeling of cortical circuitry whereby synapses are formed and eliminated, or whether they rely on changes in synaptic strength of existing synapses. In order to determine the structural stability of synapses in vivo, several groups have used chronic in vivo imaging to monitor synaptic structure over time. These experiments have yielded contradictory results, and it remains unclear to what extent dendritic spines turn over in the adult cortex. Additionally, the stability of the presynaptic partner has not been explored. Here, we report that dendritic spines on the apical tuft of layer 5 cortical neurons are remarkably stable (>80%; n=2644; 26 animals) even in young adult animals (P40) over a period of 1 to 3 weeks. At these ages spine loss (13%) is more prominent than spine gain (5%). Over a one week period, spines in different sensory cortical areas (primary visual, somatosensory, and auditory cortex) had similar rates of turnover. After 3 weeks of observation, most spines (70-80%) could still be identified in all areas studied. Spines in auditory cortex, however, appeared more stable than those in visual or somatosensory cortices (p<0.05). Interestingly, rewiring visual input into auditory cortex did not alter this increased stability, suggesting that intrinsic cues may determine the level of spine turnover in different cortical areas. Axon terminals were equally stable (>90%; n=2137; 26 animals) in all cortical areas studied and showed less turnover than dendritic spines (p<0.01), with matched levels of terminal loss (~5%) and gain (~5%). These data suggest that limited remodeling of connectivity occurs in the adult cortex, and that this remodeling is importantly mediated by turnover of the postsynaptic partner.

Supported by: Burroughs Wellcome Fund (AM), NIH Grant EY13900 (JN) and NIH Grants EY15068 and EY07023.