Functional Dissection of the CA3-CA1 Learning Rule
Department of Mathematics, University of British Columbia
(June 8, 2004 3:30 PM - 4:30 PM)
In populations of synapses, long-term potentiation and long-term depression reflect the sum of many individual plasticity events. We have found that at individual hippocampal CA3-CA1 synapses, upward or downward transitions in strength are all-or-none and sudden, thus allowing each synapse two levels of strength. Under native conditions, three-fourths of synapses begin in a low-strength state. Downward transitions are reversible, but after upward transitions synapses can be locked quickly into a high-strength state. Upward and downward transitions could be isolated by blocking or saturating potentiation or depression. This resolves plasticity into component processes that, when recombined, yield the native learning rule. Under realistic spiking conditions, these processes have activity- and timing-dependence predicting that when a rat runs through a place field the only possible form of plasticity is LTP. A three-state model (low, high and locked-in) accounts for our observations and for a variety of previous physiological, pharmacological and genetic manipulations.