Memories are stored, at least partly, as patterns of strong synapses. Given
molecular turnover, how can synapses maintain strong for the years that
memories can persist? Some models postulate that biochemical bistability
maintains strong synapses. However, bistability should give a bimodal
distribution of synaptic strength or weight, whereas current data show unimodal
distributions for weights and for a correlated variable, dendritic spine
volume. Bistability of single synapses has also never been empirically
demonstrated. Thus it is important for models to simulate both unimodal
distributions and long-term memory persistence. Here a model is developed that
connects ongoing, competing processes of synaptic growth and weakening to
stochastic processes of receptor insertion and removal in dendritic spines. The
model simulates long-term (in excess of 1 yr) persistence of groups of strong
synapses. A unimodal weight distribution results. For stability of this
distribution it proved essential to incorporate resource competition between
synapses organized into small clusters. With competition, these clusters are
stable for years. These simulations concur with recent data to support the
clustered plasticity hypothesis, which suggests clusters, rather than single
synaptic contacts, may be a fundamental unit for storage of long-term memory.
The model makes empirical predictions, and may provide a framework to
investigate mechanisms maintaining the balance between synaptic plasticity and
stability of memory.Comment: 17 pages, 5 figure