0331

The Role of Inhibitory Thalamic Reticular Nucleus (TRN) in Brain-wide Resting-state Functional MRI (rsfMRI) Connectivity
Alex T L Leong1,2, Xunda Wang1,2, Yankai Zhang1,2, Linshan Xie1,2, and Ed X Wu1,2,3
1Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong SAR, China, 2Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong SAR, China, 3School of Biomedical Sciences, The University of Hong Kong, Hong Kong SAR, China

Synopsis

Keywords: Task/Intervention Based fMRI, fMRI (resting state), fMRI(task based), functional connectivity, neuroscience

Motivation: Despite the enormous potential inherent in rsfMRI, the neural basis underlying rsfMRI connectivity remains unclear.

Goal(s): We aim to dissect the role of the TRN inhibitory neural population in rsfMRI connectivity given its prominent role in maintaining/regulating thalamo-cortical oscillations.

Approach: We examined brain-wide activity and rsfMRI connectivity changes after optogenetically manipulating neural activity in rodent TRN.

Results: We demonstrate that somatosensory-specific TRN inhibitory networks play a role in modulating rsfMRI connectivity of sensorimotor and default mode networks.

Impact: Present studies examining neural basis of rsfMRI have primarily focused on excitatory networks. Here, we investigated the role of a major inhibitory thalamic nucleus to advance our understanding of the contributions of inhibitory inputs in regulating brain-wide rsfMRI networks.

Purpose

It is increasingly clear that resting-state functional MRI (rsfMRI) is a powerful tool for analyzing large-scale functional brain networks. We can now, non-invasively and at the resting state, detect robust and functionally coupled networks in both humans and animals through simple analysis of low frequency temporal oscillations in MRI signals1,2. Thereby, we can interrogate, at a large-scale, whole-brain networks that remain outside of the realm of traditional behavioral/cognitive tasks and neuroimaging approaches. Despite its enormous potential, rsfMRI connectivity remains a correlative/observational measure, because the poorly understood neural basis of rsfMRI poses a barrier for its precise interpretation and effective applications.

Our previous rsfMRI studies showed that low frequency (1 Hz) optogenetic stimulation in thalamocortical excitatory neurons in the somatosensory-specific ventral posteromedial (VPM) increased interhemispheric rsfMRI connectivity in all sensory cortices3,4. However, we believe that our findings are likely an oversimplification - when we described the contributions of thalamo-cortical networks in mediating brain-wide rsfMRI connectivity we did so without manipulating and/or monitoring complementary inhibitory activities in the thalamo-cortical circuit.

Here, we propose to dissect the neural basis of rsfMRI connectivity through the thalamic reticular nucleus (TRN), a major source of feedforward and feedback inhibition that executes processing of sensory and associative input to the frontal cortex.

Methods

Animal preparation and MRI experimental setup (Figure 1): 3μl rAAV5-hsyn-hChR2(H134R)-mcherry-WPRE-PA was injected to somatosensory-specific subdivision of the TRN in adult SD rats. After 4 weeks, opaque optical fiber cannulas were implanted at the injection sites. All fMRI experiments were performed under 1.0% isoflurane. FMRI data was acquired at 7T using GE-EPI (FOV=32×32mm2, matrix=64×64, α=56°, TE/TR=20/1000ms, and 16 contiguous slices with 1mm thickness).

Optogenetic fMRI experiments (Figure 2): Three types of sinusoidal stimuli were implemented to mimic spindle (8Hz, duration=3s, ramp-modulated amplitude), slow (1Hz, duration=12s, constant amplitude) and gamma oscillations (i.e., 40Hz, duration=10s, constant amplitude). An additional long-duration pulse was also presented (duration=5s, constant amplitude) following a paradigm established in a previous work stimulating TRN5. All stimuli were blue (473nm) light with peak amplitudes at 40mW/mm2 and interleaved. Typically, 3-4 baseline rsfMRI scans were acquired before commencement of TRN optogenetic stimulations. GLM analysis was applied to identify significant BOLD responses. Seed-based analysis with task regressors was utilized to quantify rsfMRI connectivity measures during TRN optogenetic stimulation.

Results and Discussion

TRN initiated neural oscillations drive distinct brain-wide responses (Figure 3): Optogenetic stimulation of TRN at 1Hz and 8Hz evoked robust positive BOLD responses mainly in ipsilateral sensorimotor cortical and thalamic regions, including somatosensory (primary: barrel field S1BF; limb region S1Limb; upper lip region S1ULp; secondary: S2) and visual (VC) cortices, and somatosensory (ventral posteromedial, VPM) and visual (LGN) thalamus. Meanwhile, at 40Hz, positive BOLD responses were significantly weaker at ipsilateral sensorimotor cortices. Notably, strong negative BOLD responses were evoked at ipsilateral caudate putamen (CPu). In the presence of a long-5s continuous pulse stimulus, strong negative BOLD responses spread to CPu and ipsilateral VC. TRN can drive different oscillations5-7 (slow vs. spindle vs. gamma oscillations) at different levels of synchronization, contributing to the differential fMRI responses ranging from local VPM/S1 responses and brain-wide responses to feedforward inhibition in remote regions, demonstrating the role of TRN in regulating inhibition-excitation balance in thalamo-cortical networks.

TRN modulates brain-wide rsfMRI networks (Figure 4): RsfMRI connectivity in bilateral sensorimotor cortices and thalamic nuclei, and within default mode network (DMN; comprising of higher-order and limbic cortices ROIs) were consistently increased in all TRN stimulation paradigms. Interestingly, we observed specific deviations in the modulatory effects on rsfMRI connectivity. For example, connectivity from thalamus & brainstem to DMN was decreased during 8Hz but increased during 5s square-pulse stimulations. Given the dense reciprocal projections between the excitatory and inhibitory neural populations in VPM and TRN8,9 and that dysfunction in the thalamic reticular nucleus (TRN) significantly disrupts thalamo-cortical oscillations6,10, we hypothesize that an overall changes in rsfMRI connectivity is likely mediated by reciprocal projections between both excitatory and inhibitory neural populations in the VPM and the TRN, respectively.

Conclusion

In this study, we investigated the neural basis of rsfMRI connectivity through the thalamic reticular nucleus (TRN). We found that TRN and its inhibitory networks play a central role in modulating brain-wide functional networks as measured with rsfMRI. Although our present findings revealed varied rsfMRI connectivity changes during different stimulation paradigms, subsequent electrophysiological recordings are needed at the identified key ROIs studies to elucidate the underlying neural events that are associated with the altered rsfMRI networks.

Acknowledgements

This work was supported in part by Hong Kong Research Grant Council (HKU17112120, HKU17127121, HKU17127022 and HKU17127523 to E.X.W.; and HKU17104020, HKU17127021, HKU17127723 to A.T.L.L.), Lam Woo Foundation, and Guangdong Key Technologies for AD Diagnostic and Treatment of Brain (2018B030336001) to E.X.W.

References

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Figures

Figure 1. Histological confirmation of ChR2 expression in inhibitory neurons of the thalamic reticular nucleus (TRN). Confocal images of ChR2-mCherry expression in TRN with lower (left) and higher (right) magnification. Note that the higher magnification images are located at the subregion that has been shown to be involved in somatosensory processing. Overlay of images reveals colocalization of the nuclear marker DAPI, inhibitory marker GABA, and ChR2-mCherry in the cell body of inhibitory TRN neurons (white arrows).

Figure 2. Illustration of TRN optogenetic fMRI experiment setup and stimulation paradigm. Optical fiber cannula was implanted at somatosensory-specific TRN for subsequent optogenetic fMRI experiments (left). Four different stimulation paradigms were employed (right). Note that for 8Hz sinusoidal stimulation, the intensity of blue light pulse was modulated by a linear ramp envelope (i.e., 0.5s to ramp-up to maximum intensity of 40mW/mm2 and 2.5s to ramp-down to 0mW/mm2).

Figure 3. Distinct brain-wide BOLD responses upon different stimulation paradigms. (A) Illustration of atlas-based ROI definitions. (B) Averaged activation maps across four different stimulation frequencies (n=10; t>3.1 corresponding to P<0.001). TRN can drive different oscillations (slow vs. spindle vs. gamma) contributing to varied fMRI responses ranging from i) local VPM/S1 responses to ii) brain-wide responses at other sensorimotor regions, higher-order/limbic regions, and iii) feed-forward inhibition in sensory modalities beyond somatosensory regions.

Figure 4. Brain-wide rsfMRI connectivity changes driven by TRN-initiated neural oscillations. Matrices summarizing significant changes in rsfMRI connectivity during (A) 1 Hz, (B) 8 Hz and (C) 40 Hz sinusoidal stimulations, and (D) 5s square-pulse stimulation. One-sample t-tests (P < 0.05) within each condition (i.e., “Baseline” and “During”) were performed to identify significant source-target rsfMRI connectivity. Subsequently, two-tailed paired t-tests with FDR correction (P < 0.05) were applied to identify altered rsfMRI connectivity during optogenetic stimulations.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
0331
DOI: https://doi.org/10.58530/2024/0331