F-actin is the polymer form of actin, and its presence in dendritic spines is associated with their change in shape and structure.
Stimulation of the neuron that promotes LTP causes larger spine volume, increased cell communication, and a greater ratio of F-actin to G-actin.
G-actin are the monomer building blocks that assemble via weak noncovalent interactions to form F-actin.
F-actin can be found in the presynaptic bouton surrounding synaptic vesicle clusters and acting as scaffolding.
The active zone is the portion of the presynaptic membrane opposite the postsynaptic density across the synaptic cleft.
Non-LTP inducing stimuli cause alterations in spine morphology due to changes in actin polymerization.
Postsynaptically, actin filaments traffic AMPA receptors to the PSDZ, while also providing scaffolding for plasticity products such as CAMKII.
[5] F-actin could serve as a synaptic tag because the scaffolding space for plasticity products is increased during LTP actin polymerization.
LTP inducing high frequency stimulation leads to NMDA receptor activation and calcium influx.
[7] The increase in polymerized F-actin is due to the recruitment of G-actin monomers and the translation of actin mRNA in the dendrite.
Being associated with long term structural changes at the synapse and LTP, it is no surprise that actin dynamics influence learning and memory.
Experiments have shown that drugs like cytochalasin C and Latrunculin A that inhibit the assembly of G-actin into F-actin disrupt both the acquisition and extinction of fear responses in mice.
[9] LIMK1 knockout neurons are unable to form a cytoskeletal matrix within the dendritic spine,[6] which has interesting implications for learning.
[6] In humans, many heritable disorders characterized by mental retardation are linked to mutations in genes important to the actin polymerization pathway.