How is diverse sensory information encoded within the simple circuitry of the thalamus?

Lead Research Organisation: King's College London
Department Name: Neuroscience

Abstract

The scientific communities' basic understanding of the brain is still too limited to offer any hope of comprehensively curing age-related cognitive decline and other common neurological disorders. To reach an adequate understanding of the basic mechanisms that control our brains, science is reliant on access to animal models that enable neuroscientists to study the cellular and molecular aspects of this most complex of organs. In this proposal, we aim to reach a greater understanding of how visual information is processed by our brains. Everything we see, hear and feel must pass through a relatively small part of our brain called the thalamus. We have known for several decades that a key role of the thalamus is to distribute salient features of our sensory world across the outer part of our brains called the neocortex to enable higher-order brain functions such as perception, cognition, generation of motor commands, spatial reasoning and language. Importantly, the thalamus stops relaying this sensory information to the neocortex when we are asleep. One of the most important steps in this process is the binding of a small inhibitory molecule called GABA (gamma amino butyric acid) to a type of membrane bound protein called the GABA-A receptor. The inhibitory neurotransmitter GABA is often released from local interneurons and high-resolution imaging studies from our laboratories have shown that thalamic interneurons are mainly found in a region of the thalamus called the dorsal lateral geniculate nucleus that receives sensory information from our eyes. We believe, therefore, that these interneurons have a specialised role to play in vision, but surprisingly we do not know what that role is. In most brain areas, the release of GABA is under the control of many distinct types of interneurons each doing slightly different jobs. For example, in the retina there are over 60 types of interneurons each responsible for converting the visual scene into electrical messages that are passed to the thalamus. Surprisingly, thalamic interneurons appear to be far less diverse, which rises the interesting question of how they may process the complex information arriving from the retina. To tackle this question, we first need to find out what type of visual information they are receiving. We have developed an innovative technique for doing this by using novel tracers that just label the inputs onto these thalamic interneurons. We now want to measure the electrical activity of these interneurons with specialized equipment that mimics how they would behave if they were receiving a visual stimulus. We are now in the unique position of being able to find out how these interneurons are controlled by visual input and discover for the first time what these cells are doing as they relay sensory information to the neocortex. Our work will go some way to explain why disturbances to the thalamus, for example during the ageing process, can be associated with cognitive decline especially during certain forms of vascular dementia. However, the central focus of this research is to reach a deeper understanding of how sensory information is encoded within the seemingly simple neuronal circuitry of the thalamus.

Technical Summary

The Sox14cre/+ line has enabled us to identify the monosynaptic inputs arriving onto interneurons of the visual thalamus (dLGN-INsox14) using cre-dependent rabies virus tracing (RVCre). Our pilot data has highlighted the potential for dLGN-INsox14 neurons to receive input from multiple retinal ganglion cell (RGC) types; morphologically identified based upon criteria such as dendrite stratification in the internal plexiform layer. This data revealed also a previously unappreciated synaptic input to dLGN-INsox14 neurons from the reticular thalamic nucleus (RTN) that could be the dominant input to dLGN-INsox14 neurons in terms of connectivity. This proposal will study how morphologically identified RGC-types control dLGN-INsox14-mediated inhibition within the thalamus and examine the RTNs influence on dLGN-INsox14 neurons. Following morphological identification of RGC-types we will record from fluorescently labelled dLGN-INsox14 neurons in the Sox14gfp/+ line and use a dynamic-clamp approach to mimic the presence of specific RGC inputs. Paired simultaneous recording between dLGN-INsox14 neurons and relay neurons will help reveal how RGC input from ON, OFF and ON/OFF receptive fields can influence tonic and phasic inhibition. Preliminary data suggests a frequency-dependent switch occurs in the GABA release mechanism that could explain how separation of visual information from distinct RGC-types is maintained within the relatively simple thalamic circuitry. We will deliver channel rhodopsin to the RTN using a modified RVcre technique to specifically activate the RTN input onto dLGN-INsox14 neurons. Finally, we will use pharmacogenetics to silence dLGN-INsox14 neurons during visual stimulation to test a specific hypothesis that a push-pull mechanism operates through the dLGN-INsox14 circuitry to enable de novo reconstruction of the OFF response associated with visual receptive fields.

Planned Impact

Who might benefit from this research? The immediate beneficiaries of the work undertaken during this proposal will be the research scientists involved, including the two PIs, two PDRAs and PhD students. Additionally, other research scientists who are involved in similar research in the life sciences as well as the wider academic community and the pharmaceutical/biotechnology industry will benefit from our findings. It also hoped that members of the public will feel sufficiently motivated to engage with the more accessible aspects of this research.
2. How might they benefit from this research? The career progression of the two PIs involved in this research will be enhanced due to the publications resulting from this work. The two PDRAs employed on the grant will become trained in several highly desirable skills that will enable them to continue a successful research career. The PDRAs will similarly benefit from high-profile publications that result from this work. Additionally, the transferable skills obtained in both written and oral communication along with the analytical skills obtained during this research will enable the PDRAs to pursue careers inside or outside of academia. For example, in the biotechnology or pharmaceutical industry. The PhD students involved in this work would also greatly benefit from the training obtained as well as their involvement in a successful research project of this type. Other academics in the life sciences and beyond will benefit from this research in ways highlighted in the preceding "Academic beneficiaries" section.
3. What will be done to ensure that beneficiaries have the opportunity to engage with this research? Firstly, we will present our work at national conferences (British Neuroscience Association and Physiological Society) and international conferences (Society for Neuroscience, Federation European Neuroscience Associations) before publishing our work in peer-reviewed open-access journals. We will issue press releases to explain our peer-reviewed publications to the wider public - Imperial College and Kings College London are strong in this activity and their websites contain daily advertising for the new research emerging from the many researchers. Imperial College holds an annual Festival weekend (in May) at South Kensington (next to the Natural History and Science Museums) where members of the public come into the College for interactive and fun scientific displays in the campus grounds. By Googling "Imperial College" and "Festival" you can see a movie with highlights of this year's festival events. The Natural History Museum also runs late ("party") nights for the public, and the museum staff often ask Imperial to provide installations. King's has recently inaugurated the Museum of Life Sciences, at KCL (Guy's Campus) which aims, among other things, to deliver impact towards education in the local society. The Museum uses its range of expertise and specimens to promote the Life Sciences to communities outside King's College and especially to local schools with which it has special links. The Museum also runs special lectures and workshops for teachers as part of their continuing professional development. We would plan to be involved in these organisations' activities to highlight the work we are doing on this research.

Publications

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Description We have discovered a novel synaptic organisation in the part of the brain responsible for sensory perception, alertness and consciousness. We are conducting further experiments to understand the precise mechanism of action of this novel synapse.
Exploitation Route Fundamental discovery will advance our understanding of brain function.
Sectors Education,Healthcare