Optogenetic-fUSI reveals brain-wide neural pathways downstream of specific cell-types

How visual information is processed by the brain to orient our attention and guide behavior is central to our understanding of sensory - motor transformations. Transgenic mouse models together with molecular and genetic tools and high-throughput recording techniques have allowed us to start to understand how local brain circuits and their direct projections are organized to integrate sensory information and guide behavior. However, these approaches, as currently applied, have fostered a reductionist conceptualization of these processes, identifying specific pathways as necessary and sufficient for a particular behavior. We believe that continuing in this direction will limit our understanding of the neural basis of these behaviors; creating observational biases where the field increasingly focuses attention where it is easiest to look and ignoring the role of potentially critical unobserved cell-populations, pathways, and the interactions between them. Here we present an experimental solution to this problem that combines optogenetics, to manipulate specific cell-types, and functional ultrasound imaging (fUSI) to observe the brain-wide consequence of manipulating these cell-types with high spatiotemporal resolution (~100μm3 , 100ms). We use this framework to investigate the contribution of distinct cell-types in the mouse superior colliculus to behavior and map the brain-wide neural activity they produce in awake mice. This approach allowed us to identify new brain areas involved in colliculus-driven behaviors and demonstrate that each celltypes mediates behavior via a broad yet distinct downstream network of brain regions. The superior colliculus is part of the network of brain areas involved in implementing the motor consequences of attention; playing a central role in directing attention and triggering the motor outputs that orient us toward objects of interest, or away from threats. In addition, dysfunction of the superior colliculus and its downstream partners has been associated with psychiatric and developmental disorders including anxiety, post-traumatic stress disorders, autism, and schizophrenia. In mice, there is evidence of a strong relationship between individual cell-types and orienting, hunting and defensive behaviors. We use this relationship to delineate the cell-type specific brain-wide functional networks that lie downstream of the superior colliculus. We first establish that optogenetic activation of 4 different cell populations each trigger a distinct behavior. Subsequently, we map out the brain-wide neuronal activity in 264 brain regions that occurs as a consequence of optogenetically activating these populations. These recordings allowed us to unravel the different spatial and temporal functional networks that lie downstream of each cell-type and link these to differences in the evoked behaviors. The results of this work highlighted that each cell-type of the colliculus is functionally connected to at least 82 brain areas, forming a large brain-wide network that acts to guide a distinct behavior. Critically, fUSI allowed us to inspect the brain-wide activity of awake animals in an unbiased manner. This revealed brain areas that had not previously been considered as part of these networks. A selection of these were targeted for electrophysiological recordings and chemogenetic manipulations. Our electrophysiological recordings supported the fUSI findings and demonstrated that neurons in the downstream nuclei preferentially respond to ecologically relevant visual stimuli. Chemogenetic manipulation of an are identified with opto-fUSI, the posterior paralaminar nuclei of the thalamus (PPnT), illustrated the role it plays in mediating habituation to repeated input from the superior colliculus. Finally, using the recently developed volumetric fUSI, that allows simultaneous imaging of the entire brain, combined with optogenetics (opto-vfUSI), we obtained results consistent with opto-fUSI, with a fraction of the time and stimulations. Importantly, this allowed us to identify the neural pathwaysdownstream of an additional cell-type that is very sensitive to adaptation, and thus to repeated stimulations. We believe this is an important step towards understanding the role of the superior colliculus. Although the superior colliculus has been implicated in the coordination of attention and orienting behaviors, the results of this study demonstrate that this is accomplished through a set of downstream networks that are far broader than the individual pathways recent manipulations have highlighted. This has implications for understanding all functions of the superior colliculus including how it directs the brains attention to objects of interest, triggers defensive behaviors and the unconscious processing of fear. Being able to systematically investigate the causal link between specific cell-types and brain-wide neuronal activity is critical for comprehending the role the superior colliculus plays in these processes and how its dysfunction could contribute to psychiatric disorders. This experimental framework is versatile enough to be applied to many small brain mammals including mice, Peromyscus, rats and marmosets. We believe that the combination of optogenetics and functional ultrasound imaging (fUSI and vfUSI) bridges the gap between precise circuits manipulations and brain-wide activity measurements and provides an opportunity to gain a mechanistic understanding of how neural activity in specific cell-types causally relates to brain-wide activity and behavior.

Jaar van publicatie:2022

Toegankelijkheid:Open