Approaches and Limitations in the Investigation of Synaptic Transmission and Plasticity

Abstract

The numbers and strengths of synapses in the brain change throughout development, and even into adulthood, as synaptic inputs are added, eliminated, and refined in response to ongoing neural activity. A number of experimental techniques can assess these changes, including single-cell electrophysiological recording which offers measurements of synaptic inputs with high temporal resolution. Coupled with electrical stimulation, photoactivatable opsins, and caged compounds, to facilitate fine spatiotemporal control over release of neurotransmitters, electrophysiological recordings allow for precise dissection of presynaptic and postsynaptic mechanisms of action. Here, we discuss the strengths and pitfalls of various techniques commonly used to analyze synapses, including miniature excitatory/inhibitory (E/I) postsynaptic currents, evoked release, and optogenetic stimulation. Together, these techniques can provide multiple lines of convergent evidence to generate meaningful insight into the emergence of circuit connectivity and maturation. A full understanding of potential caveats and alternative explanations for findings is essential to avoid data misinterpretation.

Keywords: LTP (long term potentiation); analysis; electrophysiology; evoked potential; mEPSCs; spontaneous release; synapse.

Figure 1

Changes associated with long-term potentiation and synaptic maturation. Under basal conditions synapses are adhesive junctions between a presynaptic terminal containing neurotransmitter vesicles and Ca²⁺-dependent release machinery, in close proximity to a postsynaptic site where various neurotransmitter receptors are present, often on electrotonically and biochemically isolated structures called dendritic spines. Long-term potentiation can be induced by specific stimulation paradigms resulting in the activation of specific biochemical signaling cascades. This has consequences on the spine structure, as well as on the trafficking and localization of neurotransmitter receptors, especially AMPA receptors. In this figure, we illustrate a number of events associated with synaptic plasticity. In the phenomenon of developmental synapse maturation many similar events are observed. Presynaptic changes: (a) Changes in high voltage-activated (HVA) voltage-dependent calcium channels (VDCCs) single-channel conductance affect Pr, changing the PPR and miniature postsynaptic current recordings (mPSCs) frequencies. (b) Changes in residual Ca²⁺ clearance affect Pr and therefore PPR, mPSCs frequencies. (c) Modulation of the vesicle fusion machinery would affect Pr and therefore PPR, mPSCs frequencies. (d) Depletion of the pool of readily releasable vesicles might reduce mPSC frequency and impact PPR. Due to synapses variability, could influence recorded mPSCs amplitude. (e) Changes in T-type low voltage-activated (LVA) VDCC properties would impact mEPSC frequency. Postsynaptic changes: (f) Nanodomains align active zones for vesicular release with postsynaptic receptors, constituting potential sites for the rapid exchange of receptors. (g) α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) phosphorylation modulates single-channel conductance and (g′) membrane translocation. (h) Polyamine blockade: mediates rectification of Ca²⁺-permeable AMPARs, can produce changes in PPR. (i) Alteration of synapse geometry such as spine neck shortening can decrease resistance and facilitate cooperativity of synaptic responses.