We are investigating the cellular and molecular mechanisms regulating synaptic transmission and the way these determine the processing of information by a neural circuit. The circuit we are concentrating on is the retina and the question that guides our research is “how do synapses in the retina extract and transfer the information in a visual stimulus?”.
We approach this question from two angles. The first views the synaptic terminal as a machine and seeks to understand how this machine works down to the molecular level. To achieve this we are using cultured mammalian neurons and, more recently, isolated neurons from the retina of transgenic zebrafish expressing fluorescent fusion proteins. Our assays are based on electrophysiology and total internal reflection fluorescence microscopy (TIRFM) - an imaging technique with a potential resolution of single molecules. We monitor key events in the synaptic vesicle cycle in real-time, including the translation of individual synaptic vesicles to and from the membrane, docking at the active zone, fusion in response to a calcium signal, and the endocytic mechanisms that recycle vesicles. Our aim is to understand how these processes in the presynaptic terminal determine the output from the neuron.
Our second approach is to ask how these synaptic machines function within the retinal circuit. We have developed fluorescent reporter proteins by which the electrical activation of synapses and the resulting output - vesicle fusion – can be monitored across hundreds of neurons simultaneously. By applying multiphoton microcopy to transgenic zebrafish expressing these reporters we can observe synaptic activity in vivo as the retinal network responds to visual stimuli. Our general aim is to understand how the synaptic properties of different retinal neurons contribute to basic computations, such as detection of variations in light intensity. An important part of this goal is to understand the plasticity of retinal synapses and the way this contributes to “network adaptation” - changes in the way that visual stimuli are processed by the retinal circuit according to the recent history of activity. To investigate microcircuits in the retina we also combine imaging with electrophysiology in retinal slices. These investigations of synaptic function in the retinal circuit are informed by our experiments in isolated neurons, and often use the same lines of transgenic zebrafish.
