Causal effects of acute cell-specific neuromodulation on resting-state functional connectivity and brain dynamics

Abstract

The brain is a complex network of anatomically connected and perpetually interacting neural populations. To comprehensively understand complex brain function, it is necessary to investigate the brain across different scales – from cellular and (micro)circuit levels to distinct brain networks – and to define relationships among these levels. Noninvasive resting-state functional magnetic resonance imaging (rsfMRI) is a common approach used to understand intrinsic brain network organization in healthy humans. RsfMRI is also widely utilized to investigate altered brain network organization in numerous psychiatric and neurodevelopmental disorders. However, the neural basis of brain network organization in healthy and diseased brains remains elusive, making it difficult to associate functional macroscopic observations with underlying cellular level alterations. The aim of this PhD thesis was to bridge this gap. To do so required invasive interventional approaches and controlled experimental conditions achievable with animal models. We provide causal insights into how altered neuronal signaling of specific cell populations map onto alterations of rsfMRI brain network organization, by combining rsfMRI with cell-specific neuromodulation of Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) in mice. DREADDs are modified G protein coupled receptors insensitive to endogenous ligands but activated by an otherwise inert low dose pharmacological agent, allowing targeted control of neuronal signaling. This PhD thesis starts with a review of emerging methods to study neuronal activity of the rodent brain at the system and network levels and continues with two projects designed to bridge the gap between the cellular and network levels. In the first project we investigated the effects of perturbing the excitation-inhibition (E:I) ratio, the putative mechanism in a number of neurodevelopmental disorders, on whole-brain functional connectivity (FC) and dynamics. Using DREADDs we increased the E:I ratio either by i) overexciting excitatory neurons, or ii) inhibiting inhibitory Parvalbumin (PV) interneurons within cortical microcircuits. Conventional FC analyses revealed significant reductions in long-range functional connectivity within anatomically connected areas, regardless of the E:I perturbation approach. Moreover, using an advanced machine learning approach, a trained classifier was able to correctly identify regions with perturbed E:I ratio elicited by our DREADD manipulation. We further validated the same classifier on an independent cohort of Fmr1y/- knockout mice, a mouse model for autism with well-documented loss of PV neurons and chronic alterations in E:I. Our findings demonstrate a novel approach towards inferring microcircuit abnormalities from macroscopic fMRI measurements. In the second project we explored the effects of excitation or inhibition of D1 medium spiny neurons (MSNs) of the right dorsomedial striatum on brain FC and dynamics. Dynamics of each region constituting a striato-thalamic-cortical circuit were assessed using an advanced machine learning approach and FC analyses. We showed that modulation of D1 MSNs in the striatum propagates through anatomically connected networks, thereby shaping the dynamics of multiple thalamic and cortical regions within an anatomically defined circuit, while only perturbing cortico-striatal functional connectivity. Our findings demonstrate a complementary approach to FC analyses, which provides rich region-specific information in the context of altered brain dynamics. Finally, some general implications of the major findings are discussed followed by an outlook of possible future directions to address open questions. Taken together, our results indicate that local cell-specific neuromodulation, whether of cortical or subcortical origin, shapes the brain dynamics and functional connectivity of anatomically connected regions, revealing distinct patterns of cell-specific neuromodulation across scales.