We study the cellular, molecular and network processes through which the brain processes and stores information. Our approach is interdisciplinary, examining mechanisms that operate at a variety of levels. Most of our effort, however, involves experimental studies of synaptic signalling and plasticity. We use advanced optical methods including fluorescent ion and voltage indicators in conjunction with multiphoton-, confocal- and fast CCD-imaging to investigate neurotransmission at individual synapses in brain tissue. In this way, we discovered that single synaptic activation evokes calcium-induced calcium release (CICR) from internal stores within spines (Emptage et al., 1999).
Activation of individual excitatory synapses can be studied by observing rapid changes in fluorescence in postsynaptic dendritic spines (compare bottom right with top right panels) of neurons filled with fluorescent calcium indicators. The generation of these large but localised Ca2+ transients is important because of the diverse and crucial signalling functions of Ca2+ within nerve cells, and has important implications for our understanding of the mechanisms inducing synaptic plasticity in development and learning. CICR also occurs in synaptic terminals, where it is a major source of the residual calcium that mediates short-term synaptic facilitation; moreover, the ubiquitous but mysterious phenomenon of spontaneous transmitter release is in large part due to the spontaneous release of Ca2+ from these presynaptic Ca2+ stores (Emptage et al., 2001). Ca2+ fluxes in the extracellular space also influence transmission: we recently showed that the synaptically-evoked influx of Ca2+ into dendritic spines can briefly reduce extracellular Ca2+ levels sufficiently to decrease the probability of subsequent transmission at the activated synapse (Rusakov and Fine, 2003). This previously unrecognised mechanism imposes an important limit on the frequency response of synaptic transmission, and may provide a signal to neighbouring synapses.
The large synaptically evoked Ca2+ transients in dendritic spines have given us a direct way to monitor transmission at individual synapses, and we are now using this to resolve fundamental and long-standing controversies concerning forms of long-term synaptic plasticity that may underlie memory. Thus we have established that long-term potentiation (LTP) is expressed, at least in part, through a persistent increase in the probability of transmission at potentiated hippocampal associational synapses (Emptage et al., 2003) as well as, in the particular case of hippocampal mossy fibre synapses, by recruitment of additional release sites and activation of silent synapses (Reid et al., 2004). We continue to resolve the properties of these “quanta of memory,” most recently showing that individual synapses can undergo both potentiation and depression, via graded (rather than binary) changes in strength.
We are now working to extend our understanding of these phenomena. For example, we are investigating whether LTP is associated with morphological changes in spines and/or boutons, and are determining the degree to which long-term plasticity is synapse-specific. Our abilities to observe at the electron microscopic level the same synapses we previously characterised by optical physiological means (Reid, 2001), to activate chemical processes at discrete subcellular sites by focal photolysis, and to introduce exogenous genes into our preparations, are helping us to clarify the roles of specific structures and molecules in synaptic function and plasticity.
Releasable calcium stores represent an excitable medium within the cell, capable of entirely new modes of local signalling and integration within the neuron; we are examining their role in coincidence-detection and neural computation. We are also investigating the ability of other inputs (particularly acetylcholine) to influence synaptic plasticity, and, increasingly, the expression of plasticity in functioning networks in intact brain during learning.