Ectopic transmission in the cerebellar cortex

The brain is composed of billions of electrically excitable cells, called neurons, which are connected together in an astonishingly complex network by specialized junctions known as synapses. At synapses, one neuron releases chemical neurotransmitters, which activate receptor proteins on the other neuron. In this way, the electrical signal can "jump" the synapse in the form of a chemical intermediate. This idea - of synapses being a dedicated site for neuron to neuron communication - has been a bedrock concept in neuroscience for more than a century. In recent years, however, the simplicity of this idea has been challenged. One of the challenges has come from the discovery that neurons can release neurotransmitter from many parts of the cell, not just the synaptic terminal. This form of release has been termed "ectopic" transmission, as the neurotransmitter will not be targeted to the synapse, but will instead enter the bulk medium surrounding the cells.
The role of ectopic release in brain physiology is largely unknown. One concept that has good supporting evidence is that ectopic release of transmitter activates receptor proteins on non-neuronal cells termed glia. Glial cells outnumber neurons in the brain by ten to one, and are crucial for the maintenance of a healthy microenvironment. Key roles played by glia are the control of ion concentration gradients, the clearance of neurotransmitter from the extracellular space after release, and the supply of nutrients to neurons. Breakdown in any of these functions leads to disease, and glial dysfunction has been implicated in many neurological disorders, including epilepsy, Alzheimer's, Parkinson's and multiple sclerosis.
Ectopic release is thought to promote the interaction of glial cells with neurons, both physically and functionally. In particular, evidence suggests that ectopic release promotes the projection of the glial cell membrane around synapses, encouraging the clearance of neurotransmitter that escapes the synapse. Other evidence suggests that ectopic transmission may alter the electrical properties of the glial cells, altering their ability to redistribute potassium ions from sites of high neuronal activity to sites of low activity.
Recent work from our laboratory has shown that the strength of ectopic release can vary according to the frequency at which neurons are stimulated. As such, it appears that the extent of ectopic transmission will continually change over time, and that the strength of coupling between neurons and glia will similarly change. We also have evidence that ectopic release can activate receptors on neurons, altering the strength of transmission at the synapse.
Understanding the interplay between stimulation frequency, the strength of ectopic release, the strength of synaptic transmission, and the strength of neuron-glial coupling is a formidable puzzle. It is also difficult to make sense of this puzzle, as there are currently too many unanswered questions about ectopic transmission to make informed judgements about the importance of the phenomenon in healthy brain function. We wish to answer some of the questions - specifically: where precisely does ectopic release occur, does ectopic release activate receptors on neurons as well as glia, and what short-term and long-term effects does ectopic transmission have on glial cells? Answering these questions will give us a much better chance of understanding what the purpose of this previously uninvestigated form of neurotransmitter release is in normal brain function, and whether dysfunction in ectopic transmission may have a role in the onset of brain diseases.

Ectopic release of transmitter at sites distant from the synaptic active zone is a newly discovered, and poorly understood, mode of transmission in many brain regions. The best characterized example to date is at the parallel and climbing fibres in the cerebellar cortex, where ectopic release of glutamate activates AMPA receptors on the ensheathing Bergmann glia. This mechanism for neuron-glial communication is thought to promote the interaction of glial processes with sites of transmitter release.
We have recently characterized the plasticity of ectopic release, and found a range of activity-dependent changes in strength of transmission to Bergmann glia. Most notably, ectopic transmission shows long-term depression during repetitive stimulation at frequencies >0.1 Hz, due to a failure to recycle vesicles to the ectopic release sites. The consequence of this pattern of plasticity is that the strength of ectopic transmission will be inversely proportional to the firing rate of the ensheathed terminal, providing an index of the average firing rate over several minutes. What advantage this form of signal may convey to glia is unknown, as there is little evidence for the consequences of AMPA receptor activation on glial cell function.
In addition to neuron-glial communication, we have preliminary evidence that ectopic glutamate also activates AMPA and metabotropic glutamate receptors on Purkinje neurons. This suggests that ectopic release may play a role in the induction of synaptic plasticity, and we also have evidence for a form of metaplasticity - where ectopic release can change the sensitivity of the parallel fibre to exhibiting long-term changes in strength.
To better understand the role of this atypical mode of neuronal signalling, we aim to answer three key questions: where does ectopic release occur, does ectopic release contribute to synaptic plasticity, and what is the consequence of AMPA receptor activation for Bergmann glial cell function?

The proposed programme of work is basic research into an unconventional mode of neuronal communication, termed ectopic release. The principal outcome anticipated is a deeper understanding of the role of this phenomenon in normal, healthy brain physiology. Neuroscience is almost universally recognised as a key area for the advancement of medical technologies, drug development and improved diagnostic and prognostic capabilities. More recently, the importance of mathematical modelling approaches have been realized in, for example, decreasing reliance on animal testing and more targeted drug development. The goals of this research proposal would contribute to further development of these areas.
Specific aspects of neurophysiology touched on by this proposal are neuron-glial communication and synaptic plasticity, abnormalities in which contribute to many diseases. Examples include ataxia, epilepsy, dementia and other forms of neurodegenerative disease. The economic impact of these diseases is considerable, both through loss of productivity and the market for drug development.
As ectopic release is a previously unappreciated aspect of neurophysiology, it may offer novel drug targets for discovery and pharmaceutical development. This target identification stage of drug discovery is receiving dwindling support from big pharma, offering the possibility for greater impact in the future from basic research of this type. Potential secondary economic and quality of life impacts may therefore arise, affecting beneficiaries in clinical research, veterinary sciences, pharmaceutical industries, and patient groups.
Finally, because the programme of work includes close collaboration between physiology and mathematics, it offers a valuable training opportunity for staff in this important area of interdisciplinary research.