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Circuit analysis of thalamic visual processing and its modulation by long-range projections

Periodic Reporting for period 1 - Thalamic Circuits (Circuit analysis of thalamic visual processing and its modulation by long-range projections)

Reporting period: 2017-09-01 to 2019-08-31

The thalamus, a region located deep inside the brain, is a major communication hub for sensory information propagating to cortex and consciousness. One of the thalamic nuclei, the dorsal lateral geniculate nucleus (dLGN), is often referred to as the visual “relay” nucleus, even though it not only relays visual information, but is the first brain region in which visual information coming from the retina and traveling towards visual cortex is integrated and modified. Yet, how exactly information is modified at this stage is not fully understood.

In addition to retinal information, the dLGN integrates top-down information and is modulated by behavioral state, arousal, and attention. Recent studies have shown that attentional modulation of visual processing is mediated by an interplay between inhibition and excitation in the dLGN. Inhibition in the dLGN is provided by a local population of inhibitory interneurons, as well as inputs from inhibitory neurons located in the thalamic reticular nucleus (TRN).

Understanding visual information processing in the dLGN is of great clinical relevance. In patients with retinal or optic nerve damage, the dLGN is largely preserved, and therefore thalamic visual prostheses could, by stimulating dLGN neurons, potentially mimic the no longer available visual information. Moreover, schizophrenic patients show abnormalities in early visual processing and deficits in visual attention. The TRN has been implicated with these clinical symptoms of schizophrenia, and a possible role of the dLGN is debated. A better understanding of the neuronal circuits underlying function and malfunction of early visual processing would be needed to approach an understanding of the neurobiological disease mechanisms.

The aim of “Thalamic Circuits” was to describe the neuronal circuits underlying early-stage thalamic visual processing in the mouse dLGN and its modulation by long-range projections. For gaining a deeper biophysical understanding of information processing in the LGN and the basic rules of signal integration in LGN neurons, two methods needed to be established: deep brain monosynaptic rabies tracing (to resolve functional connectivity with monosynaptic resolution in vivo) and deep 2-photon calcium imaging (to achieve in vivo recordings of both cellular activity and subcellular signals). Applying these methods, we find that dLGN interneurons play an unexpectedly complex role for information processing.
Deep brain monosynaptic rabies tracing, which maps the functional connectivity of individual neurons with monosynaptic resolution, we had established shortly before start of the funding period (Rompani et. al., Neuron 2017). For “Thalamic Circuits”, we re-designed the strategy for dLGN interneurons and successfully performed targeted single-cell initiated transsynaptic tracing from individual interneurons in the dLGN.

Inhibition onto thalamocortical neurons is thought to be one major mechanism of how long-range inputs modify visual information. In the case of attentional modulation, attention has been shown to relieve inhibition mediated by the TRN. As a potential mediator of both top-down modulatory influences and bottom-up signal processing, the intrinsic population of inhibitory interneurons was thought to perform local processing of incoming information along its wide dendrites which carry dendro-dendritic output synapses. Based on anatomical evidence, each cell was thought to participate in diverse local functions. In contrast, our monosynaptic rabies tracing shows that, while the overall population of interneuron participates in a variety of computations, individual dLGN interneurons are much more specialized than expected.

Our mono-synaptic tracing data provides us with an unprecedented and large dataset of mono-synaptic long-range inputs to specified cell-types in the dLGN. Assigning individual neurons to specific brain regions based on anatomical coordinates requires both a high-resolution atlas as well as tools to align each dataset to this atlas. To make use of the high-resolution brain atlas that has been very recently developed by the Allen institute (Common Coordinate Framework v3), we developed an interactive software, which allows the semi-automated mapping of the reference atlas to the data, enables the manual online correction of distortions caused by biological variation and preparation artifacts, and provides a simplified procedure to extract important parameters of each annotated region. This valuable tool accelerates the analysis of the data and allows us to obtain detailed, cell-type specific long-range input connectivity maps of individual dLGN neurons. Together with mono-synaptically connected inputs to thalamocortical neurons, these brain regions are candidate sources for behavioral modulation.

Finally, we established deep 2-photon calcium imaging in the dLGN of awake-behaving mice. Our preparation allows us to image the visual activity of dLGN interneurons not only at their soma, but also has the resolution to image subcellular structures and their ongoing activity while the mouse is performing visual tasks. These experiments will advance our biophysical understanding of early visual processing.
In the progress of “Thalamic Circuits”, new methods were established and existing methods refined, thereby expanding the methodological toolbox for the scientific community. Firstly, deep brain monosynaptic rabies tracing maps the functional connectivity of individual neurons with monosynaptic resolution. For “Thalamic Circuits”, we expanded the application to dLGN interneurons. This is the first application of monosynaptic rabies tracing to cerebral inhibitory interneurons. Secondly, we developed a visualization and analysis software for mapping brain slices to the CCFv3 reference brain atlas. This tool will be useful also for other researchers who intend to obtain high-resolution input maps for any given brain region based on conventional or novel tracing technologies. Finally, we established for the first time deep 2-photon calcium imaging in the dLGN of awake-behaving mice. This allows in vivo recordings of both cellular and subcellular signals and opens an important avenue towards understanding thalamic functional architecture.

“Thalamic Circuits” contributes to one of the key questions of our time – how brain circuits implement sensory information processing. One of the biggest challenges for today’s society is finding treatments for neurological diseases and mental disorders. Understanding the physiological function of neural circuits is a prerequisite for developing causal treatments for neural pathologies. The results of the project contribute to the knowledge basis, which could potentially aid medical applications in at least two areas: blindness and schizophrenia. The knowledge acquired by “Thalamic Circuits” might also promote advanced information technologies mimicking the neural implementation of our brain.

A better understanding of neural circuits in the thalamus is the first step towards the development of visual prosthetics to ease the lives of blind people, for which retinal treatments or prostheses are not applicable, and will lead to a great societal benefit. Our research group has ongoing projects and collaborations with clinicians to translate the basic research findings of “Thalamic Circuits” and other projects into therapeutic applications.
Single-cell initiated rabies tracing labels inputs to an individual dLGN neuron in a mouse brain