Plasticity of neurone to glial signalling in the cerebellum

The brain is composed of billions of neurones, which conduct electrical signals and are connected by specialized junctions known as synapses. At the synapse, an incoming electrical signal causes the release of a chemical called a neurotransmitter, which crosses the synapse and binds to specific receptor proteins, thereby initiating an electrical impulse in the cell on the other side of the synapse. An important property of synapses is that the strength of synaptic transmission can be modified, depending on the frequency with which electrical excitation occurs. This principle is known as 'synaptic plasticity' and is thought to be the cellular basis of learning and memory. Neurones are outnumbered in the brain ten-fold by a second class of cells known as astrocytes, which provide support to the neurones, and are electrically non-excitable. However, in recent years it has been discovered that astrocytes also have receptor molecules similar to those found at neuronal synapses. Astrocytes form a sheath around synapses, and can respond to release of neurotransmitter by initiating biochemical signalling pathways - in particular, by accumulating calcium ions - and can release other transmitters to modulate synaptic strength. In this way, astrocytes and neurones can communicate information with one another. We have been investigating neurone to astrocyte signalling in a region of the brain responsible for muscle coordination called the cerebellum. We discovered that the strength of neurone to astrocyte signalling could also be changed by the frequency of activity at the synapse: astrocytes also express plasticity. Interestingly, the pattern of plasticity of astrocyte receptors was very different from the plasticity at the synapse that the astrocyte ensheathed. The aim of this project is to investigate the plasticity of neurone to astrocyte signalling in more detail. The biochemical mechanism by which astrocyte signalling is altered will be explored, and the impact of the plasticity on the strength and timing of synaptic transmission will be determined. We will then investigate another type of synapse, to see if the plasticity we have observed is a general mechanism for modifying cell to cell communication in the brain.

Bergmann glial cells enclose synapses throughout the molecular layer of the cerebellum and express extrasynaptic AMPA receptors and glutamate transporters which are activated during synaptic transmission. Elimination of AMPAR calcium permeability leads to the withdrawal of glial processes and synaptic dysfunction, suggesting that AMPAR-mediated calcium signalling is essential for glial support of the neuronal network. Glial glutamate transporters clear transmitter from the extrasynaptic space, limiting crosstalk between synapses through spillover, moderating activation of perisynaptic receptors, and protecting against excitotoxicity. We recently monitored BGC extrasynaptic currents (ESC) during parallel fibre (PF) stimulation over a range of frequencies. We found a selection of short- and long-term plasticity in PF-BGC signalling, which differed from the plasticity of adjacent synapses. We also discovered a slow NBQX and GDP-beta-S sensitive current in BGC, apparently linked to activation of neuronal receptors, the identity of which is unknown. We propose to investigate the plasticity of neurone to glial signalling in more detail. Specifically, we aim to identify the mechanism of plasticity at a molecular level. Thereafter, we will examine the impact of ESC depression on transmission at the PF synapse. Depression of glial glutamate transporters should change the dynamics of the extrasynaptic glutamate transient, affecting spillover between synapses and activation of perisynaptic mGluR. This is a putative mechanism by which ESC depression could be linked to synaptic plasticity. Finally, the second input shared by Purkinje neurones and BGC, the climbing fibre, will be tested for plasticity. The site specificity of BGC plasticity will be explored for the possibility of PF depression spreading to climbing fibre inputs, and the effect of synchronous climbing and PF stimulation (the paradigm for classical cerebellar LTD) on BGC will be determined.