Regulation of plateau potentials by dendritically targeted inhibitory synaptic transmission.

Experience-dependent memory is the foundation on which we make all our decisions. Reliable memory encoding is therefore essential for good decision making and our mental health. But what determines the durability of memories and how are they protected from interference by subsequent events? Our brains are not like computers which reliably transcribe all information faithfully and equally - we have a much greater capacity for flexibility. But how do we balance the needs for flexibility and adaptation with reliability and stability?

Memory representations in the brain are thought to be encoded in the strength of connections (synapses) between neurons creating assemblies where each neuron represents a distinct aspect of the memory. An excellent example of this are place cells of the hippocampus which each represent one specific location but can group together by strengthening their synaptic connections into assemblies that provide a representation or memory of the whole environment. When we experience a new environment the place cell assemblies must reorganise to form a new representation where each place cell may now "re-map" to a different location. The hippocampus is therefore an excellent system to study the flexibility and stability of memory representations.

The biological substrate for memory formation is therefore modifications in the strength of synaptic connections. This plasticity enables the reorganisation of cell assemblies. Synaptic plasticity is triggered by the influx of calcium ions across the synaptic membrane through proteins called NMDA receptors. If multiple excitatory synaptic inputs are activated simultaneous, a plateau potential is generated which is a long-lasting activation of NMDA receptors and calcium influx. These plateau potentials are known to be important in triggering synaptic plasticity to encode new aspects of our environment into place cells.

We propose that plateau potentials are controlled by inhibition provided by a specialised subtype of inhibitory neuron termed an OLM interneuron. These inhibitory cells can counteract excitatory synaptic input and are therefore perfectly positioned to regulate plateau potentials and the resulting synaptic plasticity and memory formation. Furthermore, we propose that OLM adaptation is important for creating stable memory representations.

In this BBSRC project, we will test the hypothesis that OLM interneurons can control when new place cells can incorporate new information by regulating plateau potentials and synaptic plasticity. To do this we will fill neurons with dyes that fluoresce when calcium ions are present and measure whether a synapse has strengthened or weakened by recording electrical activity from the neurons. We will do this while activating OLM interneurons to test how these cells regulate neuronal calcium and synapse strength. We will then record place cells in the hippocampus and investigate if OLM inputs can keep a place cell stable and prevent new information from destabilising previously encoded representations of the world.

This work is important because it will lead to a wealth of new information about place cells and synaptic plasticity. Dysfunctional synaptic plasticity is thought to underlie the altered neuronal activity in several brain diseases, such as Alzheimer's disease and schizophrenia. Therefore, the mechanisms that we will study in this research will add to our knowledge about these debilitating diseases and may contribute to developing novel therapies.

A central function of the brain is its ability to incorporate new information into memories to enable experience-dependent adaptations in behaviour. It is critical that the brain accurately decides which pieces of information should be incorporated and which can be discarded. Underpinning this process is the regulation of memory formation and stability through modulation of synaptic plasticity. As synaptic plasticity is determined by the excitability of post-synaptic neurons, activity of inhibitory synaptic inputs can have a profound impact on synaptic plasticity generation. The aim of this project is to investigate how dendritically targeted OLM inhibitory synaptic inputs regulate excitatory synaptic plasticity and memory formation.

We will use the hippocampus of mice where individual excitatory neurons encode aspects of the mouse's environment in the form of place cells. New information is incorporated into place cells via the formation of plateau potential driven plasticity. Plateau potentials are the result of synchronous excitatory input leading to NMDA receptor activation and large Ca2+ elevations. Plateau potentials are primarily driven by the temporoammonic input to the hippocampus from entorhinal cortex and we have shown that these inputs, and the plateau potentials they generate, can be directly reduced by increases in OLM interneuron activity and inhibitory synapse strength. Therefore, we hypothesise that OLM interneuron activity through modulation of plateau potentials regulates synaptic plasticity within the hippocampus and in doing so controls the formation and stability of place cells.

We will address this hypothesis using ex vivo brain slice electrophysiology recordings and Ca2+ imaging of plateau potentials paired with optogenetic stimulation of OLM interneurons. These findings will integrate with in vivo Ca2+ imaging of neuron activity in freely moving mice using head fixed miniscopes to study OLM regulation of place cell stability.

Who will benefit from the research?

As well as specific academic beneficiaries, the public (particularly school pupils and teachers) and wider academic community will benefit from the increase in knowledge about the role of synaptic plasticity in new memory formation and the mechanisms by which it is stabilised. In addition, sectors of the hi-tech industry working to develop more efficient machine learning algorithms and pharmaceutical industries working to develop effective drug therapies for neurological diseases will also benefit from the proposed work. Indirectly, and in the long term, people suffering from such diseases may also benefit. Therefore, there is the potential for beneficial impact on both the health and wealth of the UK.

How will they benefit from this research?

Public: Our work will impact several public audiences, including school pupils, teachers and the general public. As mentioned above, we know that understanding more about the functioning of the brain, including fundamental processes like learning and memory, is of significant interest to many groups. The biennial Bristol Neuroscience Festival open to the general public with emphasis on school children from diverse backgrounds is attended by over 4000 people and the range of hands-on science stalls focused on neuroscience are always very popular. School pupils are engrossed by testing their memories, and these activities could easily be expanded to include testing memory stability in the face of interference. Other neuroscience activities led by Bristol researchers - for example during Brain Awareness Week and Discover - are equally popular with public audiences, as are public talks on neuroscience topics held regularly by Bristol Neuroscience.

Teachers: Impacts on the teachers with whom we engage are likely to be significant. The Science Learning Centres are developing continuing professional development programmes that introduce teachers to neuroscience, and organisations such as the University of Bristol-based Neuroeducation Network provide resources for teachers interested in integrating the latest neuroscience research into educational practice. We anticipate that our research could impact how teachers manage emotional states in the classroom to facilitate learning.

Hi-tech industry: The mechanisms by which error signals are back-propagated in machine learning are critical components of artificial neural networks. Our data on inhibitory plasticity provide a novel and potentially more efficient mechanism to achieve this leading to better algorithms requiring less computer power. This would be of huge potential benefit to these companies and to society.

Pharmaceutical industry: Research into numerous neurological diseases such as schizophrenia, autism and Alzheimer's disease has found deficits in synaptic plasticity that could contribute to disease symptoms. Our close working relationship with specific pharmaceutical companies means our work is likely to enhance their understanding of the fundamental science of learning and memory, pharmacological approaches to manipulating it and putative novel drugs and targets.

The social impact and economic costs of the diseases mentioned above are enormous. Therefore our work will benefit society from the advances we make in investigating mechanisms that may underlie such diseases, and will benefit the economy both in terms of costs saved in care for patients suffering from these conditions, and also in profits from pharmaceuticals developed and sold by UK-based companies. We acknowledge that these indirect benefits may take several years before they are realised.

Educational impact: We will train a new generation of scientists by training the staff in our laboratory and by teaching at summer schools. By teaching at Bristol, JRM will train a new generation of non-academic workers in the UK, teaching a solid skillset for working in pharmaceutical or biotechnological companies.