Ultrastructural Analysis of Spine Plasticity

Introduction of Spine Plasticity

Most excitatory synapses in the central nervous system (CNS) are formed on the dendritic spine, that is, tiny protrusions on the dendrites of excitatory neurons. Adult CNS has strong spinal plasticity, and this plasticity is mainly concentrated in the dendritic spines, which are the postsynaptic parts of excitatory connections. The increase in synaptic strength tends to produce new spine and expand and stabilize the existing spine, while the decrease in synaptic strength leads to the reduction and contraction of the spine. Changes in spinal density, morphology, and motility have been shown to be related to the paradigm that induces synaptic plasticity, as well as sensory experience and changes in neuronal activity. These changes may cause changes in the synaptic connectivity and strength between neuronal partners and affect the effectiveness of synaptic communication.

Fig.1 Dendritic spines in the mouse motor cortex. (Harms, 2007)

Ultrastructural Analysis of the Spine

Many studies have shown that long-term potentiation (LTP) and long-term depression (LTD) can cause changes in the number and morphology of the spine. The development of two-photon microscopy, the use of important dyes, and the application of green fluorescence technology make it possible to image dendritic spines in living tissues and track basic dynamics in living vertebrate organisms. Compared with the optical microscope method, the visualization of the ultrastructural spine will vary greatly depending on the type of sample used, the electron microscope, and the image acquisition protocol. Transmission electron microscope (TEM) and scanning electron microscope (SEM) enable the spine to realize large-scale system reconstruction with nanometer precision in 3D. Computer vision algorithms are used to analyze the superstructure reconstruction of mouse neocortical neurons, which proves that most spine structures can be strictly separated into the head and neck, to realize the morphological measurement of the spine and neck.

Fig.2 3D reconstruction of the CA1 dendrites of a representative mouse hippocampus. (Colombo, 2021)

Spinal Plasticity is Related to Synaptic Strength

The strength of excitatory synapses can be modified in an experience-dependent way, and some forms of learning have been shown to enhance synaptic efficacy in the hippocampus, amygdala, and neocortex. Structural changes related to synaptic strength can be tested by inducing LTP stimulation patterns. LTP induces changes in the number and morphology of the spine in important areas of learning and memory (such as the hippocampus and cortex). EM analysis after induction of LTP showed that the size and number of spines in the hippocampus and neocortex increased. The presence of calcium deposits identifies stimulated synapses from the ultrastructure. LTP leads to an increase in the number of multiple synaptic buttons (MSBs), and a single presynaptic button contacts multiple spines from the same dendrites. It indicates that the changes in the morphology of dendritic spines are related to the stimulation of synapse enhancement, and similar mechanisms play a role in the sensory enhancement of learning and synaptic connections.

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References

1. Harms, K.J.; Dunaevsky, A. Dendritic spine plasticity: looking beyond development. Brain research. 2007, 1184: 65-71.
2. Colombo, M.N.; et al. Comparative 2D and 3D Ultrastructural Analyses of Dendritic Spines from CA1 Pyramidal Neurons in the Mouse Hippocampus. International Journal of Molecular Sciences. 2021, 22(3): 1188.