Remodeling of synaptic membranes after induction of long-term potentiation

Several morphological changes of synapses have been reported to be associat ed with the induction of long-term potentiation (LTP) in the CA1 hippocampu s, including an transient increase in the proportion of synapses with perfo rated postsynaptic densities (PSDs) and a later occurrence of multiple spin e boutons (MSBs) in which the two spines arise from the same dendrite. To i nvestigate the functional significance of these modifications, we analyzed single sections and reconstructed 134 synapses labeled via activity using a calcium precipitation approach. Analyses of labeled spine profiles showed changes of the spine head area, PSD length, and proportion of spine profile s containing a coated vesicle that reflected variations in the relative pro portion of different types of synapses. Three-dimensional reconstruction in dicated that the increase of perforated spine profiles observed 30 min afte r LTP induction essentially resulted from synapses exhibiting segmented, co mpletely partitioned PSDs. These synapses had spine head and PSD areas appr oximately three times larger than those of simple synapses. They contained coated vesicles in a much higher proportion than that of any other type of synapse and exhibited large spinules associated with the PSD. Also the MSBs with two spines arising from the same dendrite that were observed 1-2 hr a fter LTP induction included a spine that was smaller and a PSD that was sma ller than those of simple synapses. These results support the idea that LTP induction is associated with an enhanced recycling of synaptic membrane an d that this process could underlie the formation of synapses with segmented PSDs and eventually result in the formation of a new, immature spine.