Quantal and structural analysis of transmission and plasticity at individual synapses in the hippocampus

Abstract

Synapses are the primary sites of information processing in the brain and effective communication between neurons relies on the efficacy of their transmission. The excitatory glutamatergic connections between CA3-CA1 neurons in the hippocampus have become a standard model system to investigate electrophysiological properties and morphological aspects of synaptic behaviour implicated in learning and memory. Detailed structure-function relationships of synapses, however, remain difficult due to the paucity of techniques capable of examining both functional and morphological aspects of synaptic behaviour simultaneously in intact tissue preparations. Overall, my thesis aimed i. to examine, using optical quantal analyses, the quantal components of transmission in relation to key morphological aspects of mature synapses during basal states, ii. to assess these relationships during forms of activity-dependent plasticity, such as long-term potentiation (LTP), and iii. to compare functional and morphological modifications of single synapses using remote extracellular stimulation and two-photon glutamate uncaging approaches to plasticity. In this thesis, and within the confines of these experiments, I provide evidence that: i. transmitter release probability, pr, but not potency (the postsynaptic response amplitude when release occurs), scales with spine size; ii. that pr and potency are uncorrelated and independent quantal components of transmission; and iii. that potency is inversely correlated with spine neck length. I further demonstrate that iv. long-term potentiation (LTP), when induced via remote synaptic stimulation, is associated with persistent enhancements in pr, but not potency, and v. that persistent morphological changes are not necessary for the expression of LTP at CA3-CA1 synapses. By contrast, two-photon glutamate uncaging-induced LTP (uLTP), at synapses that are functionally and structurally similar, is associated with increases in responsiveness to photo-released glutamate, persistent enlargement of spine head volume and reductions in spine neck length. Lastly, I demonstrate vi. that uLTP induction produces an elevation of spine Ca2+ that is not present when a more physiological mode of stimulation (i.e., remote stimulation) is used. Taken together, my thesis clarifies several divergent results among laboratories with respect to the loci of expression of LTP and the necessity of morphological modifications to synapses as they undergo activity-dependent changes in synaptic strength.