Regulation of spine Ca2+ dynamics and spike timing-dependent synaptic plasticity by muscarinic acetylcholine receptors

Our memories define who we are and are therefore fundamental to our existence and mental health. Furthermore, having a "good" memory is perceived to be a major advantage throughout life. Conversely, the loss of memory in pathological diseases such as Alzheimer's disease is tremendously debilitating and stressful. One of the key mechanisms that underlie memory is the ability of nerve cells (neurons) to change the strength of their connections with other neurons. These connections are called synapses and so any change in their strength is called "synaptic plasticity". This process is thought to underlie learning and memory, because memories are likely to be stored in a circuit of interconnected neurons.
Synaptic plasticity is triggered by the influx of calcium ions into small compartments of neurons called spines. Calcium ions pass across the synaptic membrane through proteins called NMDA receptors which are activated by coincident activity in the two neurons that form the synapse. The sensitivity to coincident activity regulates how many calcium ions are allowed into the neuron by the NMDA receptor and therefore controls the induction of synaptic plasticity.
We have recently found that the neurotransmitter acetylcholine that is released in the brain during specific behavioural states can regulate the intrinsic properties of spines and thus control the opening of NMDA receptors and induction of synaptic plasticity. This provides an explanation for the common observation that behavioural states play a major role in determining whether we remember things, or forget them.
We are going to investigate the mechanisms by which acetylcholine controls memory by performing experiments to find out how acetylcholine regulates calcium ion influx through NMDA receptors and therefore the induction of synaptic plasticity. To do this we will fill neurons with dyes that fluoresce when calcium ions are present. We will also measure whether a synapse has strengthened or weakened by recording electrical activity from the neurons. These techniques will enable us to visualize the influx of calcium ions during the process of synaptic plasticity.
This work is important because it will lead to a wealth of new information about synaptic plasticity, and hence learning and memory mechanisms. Dysfunctional synaptic plasticity is thought to underlie the altered neuronal activity in several brain diseases, such as Alzheimer's disease, schizophrenia and autism. The most common and effective treatment currently available for Alzheimer's patients are drugs that mimic or enhance the actions of acetylcholine. 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.

The influx of Ca2+ through NMDA receptors (NMDARs) into postsynaptic dendritic spines is known to be critical for the induction of both long-term potentiation (LTP) and depression (LTD). It has been hypothesized that the amount of Ca2+ entering a postsynaptic spine through NMDARs determines the direction of synaptic plasticity with high [Ca2+]i leading to LTP and lower [Ca2+]i leading to LTD. This hypothesis is fundamental to the study of the mechanisms underlying synaptic plasticity therefore it is remarkable that it has not been directly tested at the synapse most often used for studies on synaptic plasticity - the Schaffer collateral synapse in the hippocampus.
We have recently shown that acetylcholine, acting at M1 muscarinic receptors, facilitates the induction of NMDAR-dependent (LTP) at Schaffer collateral synapses in the hippocampus via an inhibition of SK channels (Buchanan et al., 2010). An intriguing outcome of this research is the concept that acetylcholine controls the induction of synaptic plasticity by regulating postsynaptic excitability. However, it is unclear how this regulates spine Ca2+ dynamics during the induction of synaptic plasticity.
We will directly test these questions by imaging spine Ca2+ dynamics during the induction of synaptic plasticity using 2-photon laser scanning microscopy. We will investigate the role of muscarinic M1 receptors and SK channels in regulating Ca2+ entry during synaptic plasticity using a combination of Ca2+ imaging and electrophysiology.
The purpose of the proposed work is to test the regulation of synaptic plasticity by acetylcholine and determine the mechanisms by which this occurs. The outcome of the experiments will provide important data on the role of acetylcholine in synaptic plasticity and by inference the role of acetylcholine in cognitive function in health and disease.

Who will benefit from the research?
The public (particularly school pupils and teachers) and wider academic community will benefit from the increase in knowledge about the role of acetylcholine in synaptic plasticity. In addition, sectors of the pharmaceutical industry 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.

How will they benefit from this research?
Since memory is so integral to all our lives, gaining knowledge about the mechanisms of synaptic plasticity is of interest not only to the academic community but also to the wider public.
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. At a recent public engagement event for schools and families (Changing Perspectives) neuroscience activities were one of the most popular of the range of hands-on science stalls on offer. 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.
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, along with other colleagues working in this field, our research could impact how teachers manage emotional states in the classroom to facilitate learning.
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. JRM has ongoing collaborations with GSK and Eli Lilly & co (through the Centre for Cognitive Neuroscience) to study the effects of putative muscarinic receptor agonists on synaptic transmission in the hippocampus. Dr John Isaac and JRM co-supervise a postdoctoral researcher at Eli Lilly & co since October 2010 and a CASE award studentship at Bristol University since October 2011. The M1 selective antagonist used in this proposal is part of the ongoing collaboration with GSK. We also collaborate with Neurosearch through an EU funded Marie Curie studentship to study the role of SK channels in synaptic plasticity, an interaction that will have direct impact on the work proposed here. Through the research described in this proposal we can offer these companies academic expertise to further this goal. This is particularly important since GSK and Eli Lilly & co are both developing M1 receptor selective agonists for use in the treatment of cognitive disorders.
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.