Graph theory analysis identified two hubs that connect sensorimotor and cognitive and cortical and subcortical nociceptive networks in the non-human primate

Multimodal Correspondence between Optogenetic fMRI, Electrophysiology, and Anatomical Maps of the Secondary Somatosensory Cortex in Nonhuman Primates

Optogenetic neuromodulation combined with functional MRI (opto-fMRI) enables noninvasive monitoring of brain-wide activity and probes causal connections. In this study, we focused on the secondary somatosensory (S2) cortex, a hub for integrating tactile and nociceptive information. By selectively stimulating excitatory neurons in the S2 cortex of monkeys using optogenetics, we observed widespread opto-fMRI activity in regions beyond the somatosensory system, as well as a strong spatial correspondence between opto-fMRI activity map and anatomical connections of the S2 cortex. Locally, optogenetically evoked fMRI BOLD signals from putative excitatory neurons exhibited standard hemodynamic response function. At low laser power, graded opto-fMRI signal changes are closely correlated with increases in local field potential (LFP) signals, but not with spiking activity. This indicates that LFP changes in excitatory neurons more accurately reflect the opto-fMRI signals than spikes. In summary, our optogenetic fMRI and anatomical findings provide causal functional and anatomical evidence supporting the role of the S2 cortex as a critical hub connecting sensory regions to higher-order cortical and subcortical regions involved in cognition and emotion. The electrophysiological basis of the opto-fMRI signals uncovered in this study offers novel insights into interpreting opto-fMRI results. Nonhuman primates are an invaluable intermediate model for translating optogenetic preclinical findings to humans.

Modulating nociception networks: the impact of low-intensity focused ultrasound on thalamocortical connectivity

Pain engages multiple brain networks, with the thalamus serving as a critical subcortical hub. This study aims to explore the effects of low-intensity transcranial focused ultrasound-induced suppression on the organization of thalamocortical nociceptive networks. We employed MR-guided focused ultrasound, a potential non-invasive therapy, with real-time ultrasound beam localization feedback and fMRI monitoring. We first functionally identified the focused ultrasound target at the thalamic ventroposterior lateral nucleus by mapping the whole-brain blood oxygenation level-dependent responses to nociceptive heat stimulation of the hand using fMRI in each individual macaque monkey under light anaesthesia. The blood oxygenation level-dependent fMRI signals from the heat-responsive thalamic ventroposterior lateral nucleus were analysed to derive thalamocortical effective functional connectivity network using the psychophysical interaction method. Nineteen cortical regions across sensorimotor, cognitive, associative and limbic networks exhibited strong effective functional connectivity to the thalamic ventroposterior lateral during heat nociceptive processing. Focused ultrasound-induced suppression of heat activity in the thalamic ventroposterior lateral nucleus altered nociceptive responses in most of the 19 regions. Data-driven hierarchical clustering analyses of blood oxygenation level-dependent time courses across all thalamocortical region-of-interest pairs identified two effective functional connectivity subnetworks. The concurrent suppression of thalamic heat response with focused ultrasound reorganized these subnetworks and modified thalamocortical connection strength. Our findings suggest that the thalamic ventroposterior lateral nucleus has extensive and causal connections to a wide array of cortical areas during nociceptive processing. The combination of MR-guided focused ultrasound with fMRI enables precise dissection and modulation of nociceptive networks in the brain, a capability that no other device-based neuromodulation methods have achieved. This presents a promising non-invasive tool for modulating pain networks with profound clinical relevance. The robust modulation of nociceptive effective functional connectivity networks by focused ultrasound strongly supports the thalamic ventroposterior lateral as a viable target for pain management strategies.

TMC6 functions as a GPCR-like receptor to sense noxious heat via Gαq signaling

Thermosensation is vital for the survival, propagation, and adaption of all organisms, but its mechanism is not fully understood yet. Here, we find that TMC6, a membrane protein of unknown function, is highly expressed in dorsal root ganglion (DRG) neurons and functions as a Gαq-coupled G protein-coupled receptor (GPCR)-like receptor to sense noxious heat. TMC6-deficient mice display a substantial impairment in noxious heat sensation while maintaining normal perception of cold, warmth, touch, and mechanical pain. Further studies show that TMC6 interacts with Gαq via its intracellular C-terminal region spanning Ser780 to Pro810. Specifically disrupting such interaction using polypeptide in DRG neurons, genetically ablating Gαq, or pharmacologically blocking Gαq-coupled GPCR signaling can replicate the phenotype of TMC6 deficient mice regarding noxious heat sensation. Noxious heat stimulation triggers intracellular calcium release from the endoplasmic reticulum (ER) of TMC6- but not control vector-transfected HEK293T cell, which can be significantly inhibited by blocking PLC or IP3R. Consistently, noxious heat-induced intracellular Ca2+ release from ER and action potentials of DRG neurons largely reduced when ablating TMC6 or blocking Gαq/PLC/IP3R signaling pathway as well. In summary, our findings indicate that TMC6 can directly function as a Gαq-coupled GPCR-like receptor sensing noxious heat.

Secondary somatosensory cortex glutamatergic innervation of the thalamus facilitates pain

ABSTRACT

Although the secondary somatosensory cortex (SII) is known to be involved in pain perception, its role in pain modulation and neuropathic pain is yet unknown. In this study, we found that glutamatergic neurons in deep layers of the SII (SII Glu ) responded to bilateral sensory inputs by changing their firing with most being inhibited by contralateral noxious stimulation. Optical inhibition and activation of unilateral SII Glu reduced and enhanced bilateral nociceptive sensitivity, respectively, without affecting mood status. Tracing experiments revealed that SII Glu sent dense monosynaptic projections to the posterolateral nucleus (VPL) and the posterior nucleus (Po) of the thalamus. Optical inhibition and activation of projection terminals of SII Glu in the unilateral VPL and Po inhibited and facilitated pain on the contralateral side, respectively. After partial sciatic nerve ligation, SII Glu became hyperactive as evidenced by higher frequency of spontaneous firing, but the response patterns to peripheral stimulation remained. Optical inhibition of SII Glu alleviated not only bilateral mechanical allodynia and thermal hyperalgesia but also the negative affect associated with spontaneous pain. Inhibition of SII Glu terminals in the VPL and Po also relieved neuropathic pain. This study revealed that SII Glu and the circuits to the VPL and Po constitute a part of the endogenous pain modulatory network. These corticothalamic circuits became hyperactive after peripheral nerve injury, hence contributes to neuropathic pain. These results justify proper inhibition of SII Glu and associated neural circuits as a potential clinical strategy for neuropathic pain treatment.

fMRI, LFP, and anatomical evidence for hierarchical nociceptive routing pathway between somatosensory and insular cortices

The directional organization of multiple nociceptive regions, particularly within obscure operculoinsular areas, underlying multidimensional pain processing remains elusive. This study aims to establish the fundamental organization between somatosensory and insular cortices in routing nociceptive information. By employing an integrated multimodal approach of high-field fMRI, intracranial electrophysiology, and transsynaptic viral tracing in rats, we observed a hierarchically organized connection of S1/S2 → posterior insula → anterior insula in routing nociceptive information. The directional nociceptive pathway determined by early fMRI responses was consistent with that examined by early evoked LFP, intrinsic effective connectivity, and anatomical projection, suggesting fMRI could provide a valuable facility to discern directional neural circuits in animals and humans non-invasively. Moreover, our knowledge of the nociceptive hierarchical organization of somatosensory and insular cortices and the interface role of the posterior insula may have implications for the development of targeted pain therapies.

Disrupting nociceptive information processing flow through transcranial focused ultrasound neuromodulation of thalamic nuclei

BACKGROUND

MRI-guided transcranial focused ultrasound (MRgFUS) as a next-generation neuromodulation tool can precisely target and stimulate deep brain regions with high spatial selectivity. Combined with MR-ARFI (acoustic radiation force imaging) and using fMRI BOLD signal as functional readouts, our previous studies have shown that low-intensity FUS can excite or suppress neural activity in the somatosensory cortex.

OBJECTIVE

To investigate whether low-intensity FUS can suppress nociceptive heat stimulation-induced responses in thalamic nuclei during hand stimulation, and to determine how this suppression influences the information processing flow within nociception networks.

FINDINGS

BOLD fMRI activations evoked by 47.5 °C heat stimulation of hand were detected in 24 cortical regions, which belong to sensory, affective, and cognitive nociceptive networks. Concurrent delivery of low-intensity FUS pulses (650 kHz, 550 kPa) to the predefined heat nociceptive stimulus-responsive thalamic centromedial_parafascicular (CM_para), mediodorsal (MD), ventral_lateral (VL_ and ventral_lateral_posteroventral (VLpv) nuclei suppressed their heat responses. Off-target cortical areas exhibited reduced, enhanced, or no significant fMRI signal changes, depending on the specific areas. Differentiable thalamocortical information flow during the processing of nociceptive heat input was observed, as indicated by the time to reach 10% or 30% of the heat-evoked BOLD signal peak. Suppression of thalamic heat responses significantly altered nociceptive processing flow and direction between the thalamus and cortical areas. Modulation of contralateral versus ipsilateral areas by unilateral thalamic activity differed. Signals detected in high-order cortical areas, such as dorsal frontal (DFC) and ventrolateral prefrontal (vlPFC) cortices, exhibited faster response latencies than sensory areas.

CONCLUSIONS

The concurrent delivery of FUS suppressed nociceptive heat response in thalamic nuclei and disrupted the nociceptive network. This study offers new insights into the causal functional connections within the thalamocortical networks and demonstrates the modulatory effects of low-intensity FUS on nociceptive information processing.

Interconnected sub-networks of the macaque monkey gustatory connectome

Macroscopic taste processing connectivity was investigated using functional magnetic resonance imaging during the presentation of sour, salty, and sweet tastants in anesthetized macaque monkeys. This examination of taste processing affords the opportunity to study the interactions between sensory regions, central integrators, and effector areas. Here, 58 brain regions associated with gustatory processing in primates were aggregated, collectively forming the gustatory connectome. Regional regression coefficients (or β-series) obtained during taste stimulation were correlated to infer functional connectivity. This connectivity was then evaluated by assessing its laterality, modularity and centrality. Our results indicate significant correlations between same region pairs across hemispheres in a bilaterally interconnected scheme for taste processing throughout the gustatory connectome. Using unbiased community detection, three bilateral sub-networks were detected within the graph of the connectome. This analysis revealed clustering of 16 medial cortical structures, 24 lateral structures, and 18 subcortical structures. Across the three sub-networks, a similar pattern was observed in the differential processing of taste qualities. In all cases, the amplitude of the response was greatest for sweet, but the network connectivity was strongest for sour and salty tastants. The importance of each region in taste processing was computed using node centrality measures within the connectome graph, showing centrality to be correlated across hemispheres and, to a smaller extent, region volume. Connectome hubs exhibited varying degrees of centrality with a prominent leftward increase in insular cortex centrality. Taken together, these criteria illustrate quantifiable characteristics of the macaque monkey gustatory connectome and its organization as a tri-modular network, which may reflect the general medial-lateral-subcortical organization of salience and interoception processing networks.