Functional and Morphological Plasticity of Dendritic Spines in the Hippocampus

On CA1 pyramidal neurons, the majority of excitatory synapses are
located on dendritic spines. Previous experiments demonstrated that
the induction of LTP can modify spine numbers and morphology.
However, there was no direct proof if and when the newly grown
spines are contacted by a presynaptic terminal and potentially form
a functional synapse. To address this, the extent of colocalization
of newly grown spines with antibody staining for either synapsin, a
marker of mature presynaptic terminals or GluR2, a subunit of
postsynaptic AMPA receptors common at functional synapses was
determined. Growth of dendritic spines was induced by extracellular
local high frequency stimulation or by using a “chemical LTP”
induction protocol. Overall, the number of spines that colocalized
with synapsin puncta seemed to increase with the age of the spine.
This indicates that new spines grown upon LTP induction initially
lack presynaptic innervation suggesting that new spines do not
emerge from pre-existing shaft synapses but protrude towards an
existing presynaptic contact. Most of the newly grown spines are
GluR2 negative, suggesting that these spines do not contain a fully
functional synapse within the first six hours of existence. Because
LTP leads to spine growth, the question was if LTD induces the
retraction of previously existing spines. Using two-photon
time-lapse microscopy, it was observed that low-frequency
stimulation induced NMDA receptor-dependent spine retractions,
while theta-burst stimulation led to the formation of new spines,
as reported previously. Thus, spines on CA1 pyramidal neurons from
organotypic slice cultures can undergo bidirectional morphological
plasticity; spines can be formed and eliminated in an
activity-dependent way.