Thalamic low frequency activity facilitates resting-state cortical interhemispheric MRI functional connectivity
Introduction
The brain consists of numerous interconnected parallel and hierarchical networks subserving sensory, behavioral and cognitive functions (Park and Friston, 2013). Resting-state functional MRI (rsfMRI) enables non-invasive visualization of long-range brain-wide functional connectivity networks between anatomically separated, but functionally related, brain regions during rest based on coherent spontaneous blood-oxygen-level-dependent (BOLD) activity (Biswal et al., 1995; Chan et al., 2017; Fox and Raichle, 2007; Smith et al., 2013). A fast-growing body of literature utilizes rsfMRI connectivity as a robust tool to study complex brain-wide functional networks. In recent years, these intrinsic rsfMRI connectivity networks and their changes in normal and diseased brains have emerged in the scientific and clinical community as important biomarkers to predict cognitive performance and disease state (Chuang and Nasrallah, 2017; Cole et al., 2016; Drysdale et al., 2017; Miller et al., 2016; Shen et al., 2018). Since cortical interhemispheric rsfMRI connectivity can be detected robustly across species, it has gained prominence as a key model to explore the neural bases of rsfMRI connectivity. For example, several human and animal studies have reported preserved cortical interhemispheric rsfMRI connectivity in subjects with congenital agenesis (Tovar-Moll et al., 2014; Tyszka et al., 2011) or partial damage (O’Reilly et al., 2013; Roland et al., 2017; Zhou et al., 2014) of the interhemispheric corpus callosum connections. In-depth interpretations of these rsfMRI findings are challenging, as little is explicitly known about the neural bases underlying rsfMRI connectivity. These observations do strongly suggest the involvement of subcortical regions and polysynaptic connections in maintaining and supporting brain-wide rsfMRI connectivity. However, the exact contributions of subcortical regions (e.g., thalamus) and polysynaptic connections to brain-wide rsfMRI connectivity are still poorly understood, primarily owing to the lack of studies directly interrogating their roles in brain-wide cortical rsfMRI connectivity.
Thalamus relays and modulates diverse long-range functional neural integrations involved in cortical processing (Hwang et al., 2017; Rikhye et al., 2018; Sherman, 2016). Meta-analyses of large task-based neuroimaging dataset indicated thalamic engagement in numerous cognitive functions (Crossley et al., 2013; Hwang et al., 2017). Electrophysiology studies revealed that the thalamus can control cortical states and modulate the flow of information processing during cognitive processes (Alonso and Swadlow, 2015; Nakajima and Halassa, 2017; Poulet et al., 2012; Sherman, 2016). Unfortunately, previous attempts to infer thalamic contributions to brain-wide rsfMRI connectivity using graph theory-based analyses have generated inconclusive results. Some studies found that the thalamus is a critical region to promote global communication and information integration in brain-wide rsfMRI connectivity (Cole et al., 2010; Crossley et al., 2013; Hwang et al., 2017), while several other found the opposite (X. Liang et al., 2018; Power et al., 2013; van den Heuvel and Sporns, 2013; Zalesky et al., 2014). To determine whether and how the thalamus contributes to brain-wide rsfMRI connectivity, it is imperative to directly perturb thalamus and monitor the rsfMRI connectivity with a multi-modal approach of rsfMRI, neuromodulation (e.g., optogenetics) and electrophysiology recordings.
Polysynaptic connections may underlie and maintain brain-wide rsfMRI connectivity, especially cortical interhemispheric rsfMRI connectivity (O’Reilly et al., 2013; Roland et al., 2017; Tovar-Moll et al., 2014; Tyszka et al., 2011; Zhou et al., 2014). The cerebral cortex contains an intricate laminar and columnar architecture, which has extensive polysynaptic connections within itself and with the thalamus to form brain-wide thalamo-cortical networks (Sherman, 2017). Such properties enable diverse long-range functional neural interactions. For example, somatosensory cortical neurons first receive somatosensory thalamic inputs at layer IV and V before transmission to other cortical neurons across multiple layers and columns in somatosensory cortices (Douglas and Martin, 2004; Feldmeyer, 2012; Thomson and Bannister, 2003) and other sensory cortices (M. Liang et al., 2013), before returning to the thalamus (Sherman, 2016, 2017). Hence, neural activity initiated in the thalamus could recruit multiple cortical regions via cortico-cortical pathway, which consists of interhemispheric corpus callosum, cross-modal, and cortico-thalamo-cortical connections that involve multiple cortical layers (Douglas and Martin, 2004; Leong et al., 2016; Sherman, 2016; Thomson and Bannister, 2003; Zingg et al., 2014). Meanwhile, all thalamic nuclei are also connected to the thalamic reticular nucleus within the same hemisphere (Halassa et al., 2014; Lam and Sherman, 2007, 2011; Lewis et al., 2015). Hence, thalamic nucleus could also activate multiple thalamic nuclei through this thalamo-thalamic pathway and subsequently influence multiple cortical regions without first directly recruiting multiple cortical layers across cortico-cortical connections. Thus, examining whether multiple cortical regions and layers are involved could help to elucidate the predominant polysynaptic pathway(s) recruited by the thalamus to mediate cortical interhemispheric rsfMRI connectivity. We will exploit this thalamo-cortical architecture and network as a model to study the contributions of subcortical inputs and polysynaptic connections to brain-wide rsfMRI connectivity.
Recent studies suggested low frequency (<10 Hz) spontaneous oscillatory neuronal activities as potential candidates to constrain and elicit rsfMRI BOLD activity, since these oscillations resemble spatiotemporal patterns of rsfMRI signals during anesthesia, sleep and awake states (Ma et al., 2016; Matsui et al., 2016; Pan et al., 2013; Schölvinck et al., 2010; Wang et al., 2012). Slow (0.5–1.5 Hz), delta (1.5–4 Hz) and spindle (7–14 Hz) oscillations have been identified as the unique characteristics in thalamo-cortical networks (Crunelli et al., 2015; Fernandez et al., 2017; Mak-McCully et al., 2017; Neske, 2015). Specifically, the thalamus participates in long-range brain-wide propagation of slow cortical activity (David et al., 2013; Stroh et al., 2013; Xiao et al., 2017) and brain-wide coupling of spontaneous oscillatory neural events (Dudai et al., 2015; Latchoumane et al., 2017; Staresina et al., 2015) through thalamo-cortical or thalamo-hippocampal-cortical networks. Our recent fMRI investigation directly demonstrated that low frequency (1 Hz) neural activity optogenetically initiated at somatosensory thalamus robustly propagates throughout the brain to bilateral sensory cortices (Leong et al., 2016). These studies highlight the pivotal roles the thalamus plays in propagating and synchronizing neural oscillations across the brain. They also indicate the thalamus is an ideal target to perturb for delineating the relationship between rsfMRI connectivity and neural oscillations.
Investigation of whether and how ascending sensory inputs influence cortical rsfMRI connectivity can provide in-depth understanding into how subcortical regions and polysynaptic connections contribute to brain-wide rsfMRI connectivity. There has been numerous research on revealing the relevance of rsfMRI connectivity networks to task activations (Cole et al., 2016; Smith et al., 2009; Tavor et al., 2016), and showing that either sensory stimulation or task performance can modulate intrinsic rsfMRI connectivity networks (Chuang and Nasrallah, 2017; Harmelech et al., 2013; J. Li et al., 2015; Nasrallah et al., 2016). However, such external task-based paradigms cannot identify the key sources or players (e.g., thalamic activities) in the ascending sensory pathways that contribute to brain-wide rsfMRI connectivity. An intuitive way to identify some of these sources is to examine the changes in rsfMRI functional connectivity by selectively modulating the thalamocortical neurons of sensory thalamic nuclei. Structural connections and characteristics of activity propagation between ventral posteromedial thalamus (VPM) and the cortical layers of primary somatosensory barrel field (S1BF) are well-studied in the rodent brain (Constantinople and Bruno, 2013; Cruikshank et al., 2010; Viaene et al., 2011a, 2011b). Furthermore, recent studies have demonstrated that the VPM can recruit brain-wide polysynaptic activity and modulate functions beyond the somatosensory system (Leong et al., 2016; Xiao et al., 2017), indicating the potentially important role of VPM in complex polysynaptic networks that support brain-wide cortical rsfMRI connectivity. Taken together, we chose VPM as a neuromodulation target to provide pivotal insights into the contributions of sensory thalamus to brain-wide rsfMRI connectivity.
Here, we examined whether and how the somatosensory thalamus influences cortical interhemispheric rsfMRI connectivity within and beyond the somatosensory modality. We directly perturbed the VPM and monitored brain-wide resting-state functional connectivity changes using rsfMRI and electrophysiology recordings at bilateral somatosensory cortices before and after low frequency (1 Hz) optogenetic stimulation of VPM thalamocortical excitatory neurons. We also examined whether pharmacological inhibition of VPM thalamocortical neurons would decrease cortical interhemispheric rsfMRI connectivity.
Section snippets
Animal subjects, virus packaging and stereotactic surgery for viral injection
All animal experiments were approved by the University of Hong Kong’s Committee on the Use of Live Animals in Teaching and Research (CULATR). Three groups of age-matched adult male Sprague Dawley rats were used in this study (i.e., two groups comprising of optogenetically transfected animals and the other with naïve animals). Two of these animal groups underwent rsfMRI experiments (optogenetic: n = 8; tetrodotoxin, TTX: n = 10), whereas the other underwent local field potential (LFP) recording
Optogenetic stimulation of VPM thalamocortical excitatory neurons increases brain-wide cortical interhemispheric rsfMRI connectivity
After confirming the specific expression of ChR2-mCherry in VPM thalamocortical excitatory neurons through colocalization of mCherry with CaMKIIα staining (Fig. 1a), we examined the modulatory effects of thalamic low frequency optogenetic stimulation on interhemispheric rsfMRI connectivity in lightly anesthetized rats (Fig. 1b). We found that after (post) 1 Hz VPM stimulation, the strength of interhemispheric rsfMRI connectivity increased significantly in primary somatosensory barrel field
Discussion
In summary, we directly reveal the pivotal contribution of the thalamus to brain-wide interhemispheric rsfMRI connectivity. Our findings demonstrated that thalamically-evoked low frequency activity by optogenetic stimulation of VPM thalamocortical excitatory neurons increased interhemispheric rsfMRI connectivity and local cortical intrahemispheric BOLD activity in all sensory cortices at infraslow frequency 0.01–0.1 Hz. In parallel, our electrophysiological recordings after low frequency (1 Hz)
Acknowledgements
This work was supported by the Hong Kong Research Grant Council (C7048-16G to E.X.W.), Guangdong Provincial Brain Research Key Projects (No. 2018B030332001 and No. 2018B030336001), and Lam Woo Foundation. We would also like to thank Dr. K. Deisseroth who provided us with the ChR2 viral construct.
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Authors contributed equally to this work.