LTP promotes formation of multiple spine synapses between a single axon terminal and a dendrite

Structural remodelling of synapses(1-4) and formation of new synaptic conta cts(5-8) has been postulated as a possible mechanism underlying the late ph ase of long-term potentiation (LTP), a form of plasticity which is involved in learning and memory(9). Here we use electron microscopy to analyse the morphology of synapses activated by high-frequency stimulation and identifi ed by accumulated calcium in dendritic spines. LTP induction resulted in a sequence of morphological changes consisting of a transient remodelling of the postsynaptic membrane followed by a marked increase in the proportion o f axon terminals contacting two or more dendritic spines. Three-dimensional reconstruction revealed that these spines arose from the same dendrite. As pharmacological blockade of LTP prevented these morphological changes, we conclude that LTP is associated with the formation of new, mature and proba bly functional synapses contacting the same presynaptic terminal and thereb y duplicating activated synapses.