Optogenetic and pharmacological resting-state functional MRI reveals thalamic modulation of brain-wide functional connectivity

Abstract

The brain is a highly complex, interconnected structure with parallel and hierarchical networks distributed within and between neural systems. Functional connectivity mapping using resting-state functional MRI (rsfMRI) enables non-invasive visualization of brain-wide networks in humans and animals through coherent infra-slow (<0.1Hz) hemodynamic/BOLD activity. However, the exact underlying neural bases and functional significance remain unclear. Previous studies integrating large-scale electrical or Ca2+ recordings imply that slow, oscillating neural activity (<1Hz) constraints and elicits these rsfMRI BOLD activities. This indicates that interrogating the brain’s underlying electrical activity can provide mechanistic insights into brain-wide rsfMRI connectivity. Numerous long-range networks exist, subserving the functions of particular systems and their interactions. One classical example is the thalamo-cortical-thalamic network, which interconnects the thalamus and neocortex via thalamo-cortical and cortico-thalamic projections. In this study, we optogenetically stimulate ventral posteromedial thalamus (VPM) thalamocortical excitatory neurons and pharmacologically inactivate VPM thalamocortical neurons, to investigate whether modulating and/or disrupting the neural activity of such an integral brain network could uncover the neural bases of rsfMRI. Optogenetic control of VPM thalamocortical excitatory neurons in adult male Sprague-Dawley rats was enabled through AAV5-CaMKIIα-ChR2(H134R). RsfMRI scans were performed before, during (i.e., 1Hz, 473nm, 10% duty cycle, 40mW/mm2) and after optogenetic stimulation. Inactivation of VPM was enabled through the use of Tetrodotoxin (TTX; 5ul, 5ng/uL). Our results demonstrated that optogenetic excitation of the VPM thalamocortical excitatory neurons at 1Hz enhanced brain-wide rsfMRI connectivity, specifically somatosensory, visual and auditory networks, whereas, TTX inactivation of VPM decreased rsfMRI connectivity. Spectral analysis indicated that such changes were underpinned by neuromodulatory effects on the infra-slow rsfMRI BOLD activity. Our study suggests that 1Hz optogenetic stimulation of VPM initiates slow oscillations that may couple with infra-slow oscillations, leading to enhanced rsfMRI connectivity. Whereas, TTX inactivation of VPM disrupts the initiation or propagation of these oscillations, decreasing rsfMRI connectivity. By manipulating the specific activities within the thalamo-cortical-thalamic network, optogenetic rsfMRI presents a powerful approach to dissect the neural bases of rsfMRI connectivity.