The emergence and plasticity of a balance between excitation and inhibition along dendrites

A single neuron in the brain can receive signals from thousands of other neurons. Each signal arrives at specialised sites, known as synapses that are distributed across a tree-like structure known as the dendrite. Every neuron must then decide, based on these incoming signals, whether or not to generate an output signal of its own. The integration of input signals by a neuron is therefore crucial for determining how information flows through the brain. Recent evidence has shown than not all synapses are alike however. The strength of a synapse, i.e. its ability to influence the generation of an output signal, can vary according to position on the dendrite. What's more remarkable though is that neurons receive two types of input: excitatory signals that promote output, and inhibitory signals that prevent it. The behaviour of a neuron is therefore precisely regulated by how these two opposing forces interact with one another in space and time. The importance of developing and maintaining a correct balance between excitation and inhibition is underscored by the fact that an incorrect balance may underlie many neurological disorders such as epilepsy, autism and schizophrenia. However, we know very little about precisely how excitatory and inhibitory synapses are distributed across neurons in the mature brain or how the interaction between them influences neuronal output. We know even less about how the spatial relationship of excitatory and inhibitory synapses is established as the neurons in the brain wire up during development nor how they are adjusted the sensory experience the animal is subjected to as they grow. This is particularly important given that many neurological disorders arise as a consequence of abnormal brain development. In this proposal we will engineer neurons in the mouse brain to express fluorescent markers of different colours to label excitatory and inhibitory synapses. This will allow us to visualize, in a single neuron, how both types of synapse are distributed and how this distribution develops during the wiring of the brain. We will also examine how the distribution of excitatory and inhibitory inputs changes in response to chronic changes in experience (in this case visual input) that results in changes in the activity of neurons. These fluorescent markers will also serve as a guide that will allow us to selectively activate identified synapses and ask specific questions - how does stimulation of single excitatory synapse influence the activity of a neuron and how does this change when neighbouring or distant inhibitory synapses are also active? These studies will provide important insight into how neurons and circuits develop and how excitatory and inhibitory signals are integrated in the healthy brain. This, in-turn, will lay the foundations for studying these processes in models of neurological disorders.

Neurons in the brain process information by activating both excitatory and inhibitory inputs. It is the fine interplay between the two that controls whether a neuron depolarises sufficiently to reach the threshold for firing an action potential. Here, we aim to provide a complete description of the distribution of excitatory and inhibitory synapses along entire dendritic tress of principal cells in the mouse visual cortex. The strength of this proposal lies in the use of different tools that will provide a structural and functional map of the balance between excitation and inhibition. Our aim is to understand whether the balance between excitation and inhibition reported at the single cell level, can occur at the subcellular level, endowing dendrites the possibility of integrating information locally and in a stable manner. Here, we will use recently developed antibody-like proteins, termed Fibronectin intrabodies (FingRs), able to bind to the endogenous proteins PSD-95 and Gephyrin, each a key scaffolding protein in excitatory and inhibitory synapses, respectively. Using these new genetic tools, we will provide a description of excitatory and inhibitory inputs onto the entire dendritic tree of principal neurons in the mouse cortex, in mature and also developing brains. In addition, we will also characterise how chronic alterations in neuronal activity through, for example, sensory deprivation experiments, change the distribution of these connections during development. Following the structural description of synapse distribution, we will also provide a functional description of the evolution of both excitatory and inhibitory synapses by recording synaptic inputs electrophyisologically during development and following sensory deprivation. Finally, we will explore the spatio-temporal role that a single GABAergic input has on neighbouring excitatory inputs.

This proposal will carry out basic research to understand the basic properties of how the brain wires up. Aside from the academic interest this research will generate, there are a number of potential non-academic beneficiaries that can be divided into two main groups: (1) the general public and (2) public sector and commercial enterprises linked to the provision of health care.

1. Communications and engagement with the general public: Our research has the potential to benefit the UK public, by generating open source data and publications of novel finding about how the brain wires up during development. My lab is committed to public engagement to promote scientific research through a number of different routes. Each year we host summer students from King's University and other UK academic institutions to carry out research projects and discuss science with lab members. I have also given talks at the Judd Grammar School (Kent) and discussed career opportunities with budding scientists about University attendance. In addition, our Centre hosts an annual research summer school where sixth form students, from non-selective state schools, identified as having low progression to Higher Education, are able to experience first-hand what it is like to work in a laboratory. Collaborating schools for the 2017/2018 academic year are: City of London Academy, St Michael's Catholic College and Harris Academy Bermondsey. Our lab is involved with this project and will continue to participate in the future. We will also participate in science festivals to engage with the wider general public, such as Brain Awareness and British Science weeks, and Pint of Science Fest. Finally, the Centre for Developmental Neurobiology regularly publishes news about our activities and publications in the Centre's website and social media account (Twitter: @dev_neuro). The Centre coordinates with King's College Communications Centre, as well as with the press offices of particular journals and funding agencies, in generating and promoting media coverage of the most relevant findings and achievements. By bringing science to life through lab visits, talks and exhibitions we hope to generate enthusiasm and an appreciation of scientific research within the general public.

2. Contribution to the nation's health: Studying how E/I balance develops and is maintained in the healthy brain has particular relevance for understanding the aetiology of neurodevelopmental disorders such as autism, epilepsy and schizophrenia since dysregulation of E/I balance has been implicated in all these conditions. This proposal has a direct impact on these disorders and meets the BBSRCs Key Strategy Priority of 'Bioscience for Health'. By establishing the tools and techniques to study the spatial and functional relationship between excitatory and inhibitory synapses in the mouse brain, we will provide important insight into how neurons and circuits develop and how excitatory and inhibitory signals are integrated in the healthy brain. This, in-turn, will lay the foundations for studying these processes in models of neurological disorders which, collectively, place a major burden on society. Our work can therefore inform public sector bodies responsible for health care or pharmaceutical companies to develop new strategies or targets for their drug discovery units.