Cold Spring Harbor
Optical studies of synaptic function at single spines
The importance of the spatial and temporal structure of synaptic activity to information processing by single neurons are controversial, as are many significant aspects of the mechanisms of synaptic transmission itself. Through anatomical localization of individual active synapses, optical studies can provide windows into synaptic function which are unavailable through electrophysiological measurements.
We are using 2-photon laser scanning microscopy to study transmission at the single synapse level. Using imaging and whole-cell recording from CA1 pyramidal neurons in acute hippocampal slices, glutamate receptor-mediated  transients can be monitored at sub-micron and millisecond scales. The resolution, stability and signal-to-noise of these recordings is sufficient to monitor quantal transmission (failures and successes of release) at multiple individual synapses for tens of minutes to hours. We have used this approach to measure presynaptic release properties and to quantify postsynaptic receptor opening. We are also working to apply these techniques to study synaptic activity in vivo.
We have used single-synapse imaging to study a particularly controversial issue in synaptic physiology, the possible saturation of postsynaptic NMDA receptors. Although numerical considerations (particularly, a large ratio of transmitter molecules to receptors) suggest that saturation is likely, the physiological evidence is weak and indirect. Using a relatively direct paired-pulse test, we find that NMDA receptors are in fact not saturated by single quantal release: a pair of releases evokes nearly twice the influx of a single release. The implication of this finding is that individual spines can differentiate electrically and biochemically between different numbers of presynaptic action potentials, with possible consequences for syanptic signal processing and plasticity.