Dendritic spine expansion induced by hippocampal long-term potentiation

Dendritic spines are small protrusions from dendritic shafts that contain the postsynaptic sites of glutamatergic synapses in the brain Spines undergo dramatic activity-dependent structural changes that are particularly prominent during neuronal development. Although changes in spine shape or number have been proposed to contribute to forms of synaptic plasticity that underlie learning and memory, the extent to which spines remain plastic in the adult brain is unclear. I find that induction of long-term potentiation (LTP) using one train of 1 sec, 100 Hz stimulation in acute hippocampal slices of young adult mice evokes a reliable, transient expansion in spines that are synaptically activated, as determined with calcium imaging. Similar to LTP, transient spine expansion requires N-methyl-D-aspartate receptor (NMDAR)-mediated Ca2+ influx and actin polymerization. Moreover, like the early phase of LTP induced by the stimulation protocol, spine expansion does not require Ca2+ influx through L-type voltage-gated Ca 2+ channels, nor does it require protein synthesis. Thus, transient spine expansion is a characteristic feature of the initial phases of plasticity at mature synapses and so may contribute to synapse remodeling important for LTP.; Transient spine expansion in response to LTP induction was also observed by Matsuzaki et al. (2004) in organotypic slice cultures prepared from young rat pups. In slice cultures, glutamate uncaging results in both transient and persistent enlargement of dendritic spines, whereas in acute slices from adults, a 1-sec tetanus at 100 Hz elicits only a transient enlargement. Does persistent spine expansion reflect an effect of developmental age, slice preparation, or stimulation protocol? Here, I examine these factors by looking at spine size one hour following induction of LTP in acute hippocampal slices prepared from juvenile or young adult mice, using either a standard one-train or stronger two-train 1 sec, 100 Hz LTP induction protocol. I find that persistence of spine expansion is both age-dependent and stimulation-dependent. Furthermore, the extent of persistent spine expansion correlates with the percentage of LTP in adult, but not juvenile, slices. Thus, persistent spine expansion may be a mechanism for postsynaptic expression of LTP at mature synapses, but reflect a more general process at juvenile synapses.