Distinct nociception processing in the dysgranular and barrel regions of the mouse somatosensory cortex

Regulation of nociception by long-term potentiation of inhibitory postsynaptic currents from insular cortical parvalbumin-immunopositive neurons to pyramidal neurons

ABSTRACT

The insular cortex (IC) processes various sensory information, including nociception, from the trigeminal region. Repetitive nociceptive inputs from the orofacial area induce plastic changes in the IC. Parvalbumin-immunopositive neurons (PVNs) project to excitatory neurons (pyramidal neurons [PNs]), whose inputs strongly suppress the activities of PNs. This study investigated how PVNs in the IC modulate pain-related behaviors using optogenetics. To evaluate the effect of PVN activation on pain-related behavior, we applied nociceptive heat stimulation to the whisker pads of PV-Cre rats that received an injection of adeno-associated virus-Flex-channelrhodopsin-2-mCherry into the IC. Exposure to nociceptive heat stimulation significantly increased the amount of pain-related escape behavior, and PVN activation by optogenetics did not significantly decrease pain-related behavior. We next examined the possibility that long-term potentiation (LTP) of PVN→PN synapses suppresses pain-related behaviors. We recorded light-evoked inhibitory postsynaptic currents (IPSCs) from PNs in the IC slice preparation to examine whether optogenetic activation of PVNs can induce LTP. Repetitive optogenetic stimulation (ROS) of PVNs in a manner analogous to theta burst stimulation increased the amplitude of IPSCs for at least 50 minutes. Long-term potentiation was induced by either the -45 or -60 mV membrane potential of PNs. Then, the IC received ROS to induce LTP of IPSCs from PVNs to PNs, and we evaluated pain-related behaviors. Compared to those before ROS, the pain-related behaviors were further reduced after ROS. These results suggest that LTP induction of PVN→PN synapses in the IC could be a possible treatment for abnormal pain in the orofacial area.

Role of Posterior Medial Thalamus in the Modulation of Striatal Circuitry and Choice Behavior

The posterior medial (POm) thalamus is heavily interconnected with sensory and motor circuitry and is likely involved in behavioral modulation and sensorimotor integration. POm provides axonal projections to the dorsal striatum, a hotspot of sensorimotor processing, yet the role of POm-striatal projections has remained undetermined. Using optogenetics with slice electrophysiology, we found that POm provides robust synaptic input to direct and indirect pathway striatal spiny projection neurons (D1- and D2-SPNs, respectively) and parvalbumin-expressing fast spiking interneurons (PVs). During the performance of a whisker-based tactile discrimination task, POm-striatal projections displayed learning-related activation correlating with anticipatory, but not reward-related, pupil dilation. Inhibition of POm-striatal axons across learning caused slower reaction times and an increase in the number of training sessions for expert performance. Our data indicate that POm-striatal inputs provide a behaviorally relevant arousal-related signal, which may prime striatal circuitry for efficient integration of subsequent choice-related inputs.

Beyond Barrels: Diverse Thalamocortical Projection Motifs in the Mouse Ventral Posterior Complex

Thalamocortical pathways from the rodent ventral posterior (VP) thalamic complex to the somatosensory cerebral cortex areas are a key model in modern neuroscience. However, beyond the intensively studied projection from medial VP (VPM) to the primary somatosensory area (S1), the wiring of these pathways remains poorly characterized. We combined micropopulation tract-tracing and single-cell transfection experiments to map the pathways arising from different portions of the VP complex in male mice. We found that pathways originating from different VP regions show differences in area/lamina arborization pattern and axonal varicosity size. Neurons from the rostral VPM subnucleus innervate trigeminal S1 in point-to-point fashion. In contrast, a caudal VPM subnucleus innervates heavily and topographically second somatosensory area (S2), but not S1. Neurons in a third, intermediate VPM subnucleus innervate through branched axons both S1 and S2, with markedly different laminar patterns in each area. A small anterodorsal subnucleus selectively innervates dysgranular S1. The parvicellular VPM subnucleus selectively targets the insular cortex and adjacent portions of S1 and S2. Neurons in the rostral part of the lateral VP nucleus (VPL) innervate spinal S1, while caudal VPL neurons simultaneously target S1 and S2. Rostral and caudal VP nuclei show complementary patterns of calcium-binding protein expression. In addition to the cortex, neurons in caudal VP subnuclei target the sensorimotor striatum. Our finding of a massive projection from VP to S2 separate from the VP projections to S1 adds critical anatomical evidence to the notion that different somatosensory submodalities are processed in parallel in S1 and S2.

Optogenetic Approach in Trigeminal Neuralgia and Potential Concerns: Preclinical Insights

The integration of optogenetics in the trigeminal pain circuitry broadens and reinforces existing pain investigations. Similar to research on spinal neuropathic pain, the exploration of the underlying determinants of orofacial pain is expanding. Optogenetics facilitates more direct, specific, and subtle investigations of the neuronal circuits involved in orofacial pain. One of the most significant concerns of both dentistry and medicine is trigeminal neuralgia (TN) management due to its substantial impact on a patient's quality of life. Our objective is to gather insights from preclinical studies conducted in TN employing an optogenetic paradigm, thereby extending the prospects for in-depth neurobiological research. This review highlights optogenetic research in trigeminal pain circuitry involving TN. We outline the central and peripheral regions associated with pain-that have been investigated using optogenetics in the trigeminal pain circuitry. The study further reports its scope and limitations as well as its potential for future applications from bench to bedside.

Comprehensive Analysis of the Spatial Distribution of Gadolinium, Iron, Manganese, and Phosphorus in the Brain of Healthy Rats After High-Dose Administrations of Gadodiamide and Gadobutrol

OBJECTIVES

After the administration of gadolinium-based contrast agents (GBCAs), residual gadolinium (Gd) has been detected in a few distinct morphological structures of the central nervous system (CNS). However, a systematic, comprehensive, and quantitative analysis of the spatial Gd distribution in the entire brain is not yet available. The first aim of this study is to provide this analysis in healthy rats after administration of high GBCA doses. The second aim is to assess the spatial distributions and possible Gd colocalizations of endogenous iron (Fe), manganese (Mn), and phosphorus (P). In addition, the presence of Gd in proximity to blood vessels was assessed by immunohistochemistry.

MATERIALS AND METHODS

Male rats were randomly assigned to 3 groups (n = 3/group): saline (control), gadodiamide (linear GBCA), and gadobutrol (macrocyclic GBCA) with cumulative Gd doses of 14.4 mmol/kg of body mass. Five weeks after the last administration, the brains were collected and cryosectioned. The spatial distributions of Gd, Fe, Mn, and P were analyzed in a total of 130 sections, each covering the brain in 1 of the 3 perpendicular anatomical orientations, using laser ablation coupled with inductively coupled plasma mass spectrometry. Quantitative spatial element maps were generated, and the concentrations of Gd, Fe, and Mn were measured in 31 regions of interest covering various distinct CNS structures. Correlation analyses were performed to test for possible colocalization of Gd, Fe, and Mn. The spatial proximity of Gd and blood vessels was studied using metal-tagged antibodies against von Willebrand factor with laser ablation coupled with inductively coupled plasma mass spectrometry.

RESULTS

After administration of linear gadodiamide, high Gd concentrations were measured in many distinct structures of the gray matter. This involved structures previously reported to retain Gd after linear GBCA, such as the deep cerebellar nuclei or the globus pallidus, but also structures that had not been reported so far including the dorsal subiculum, the retrosplenial cortex, the superior olivary complex, and the inferior colliculus. The analysis in all 3 orientations allowed the localization of Gd in specific subregions and layers of certain structures, such as the hippocampus and the primary somatosensory cortex. After macrocyclic gadobutrol, the Gd tissue concentration was significantly lower than after gadodiamide. Correlation analyses of region of interest concentrations of Gd, Fe, and Mn revealed no significant colocalization of Gd with endogenous Fe or Mn in rats exposed to either GBCA. Immunohistochemistry revealed a colocalization of Gd traces with vascular endothelium in the deep cerebellar nuclei after gadobutrol, whereas the majority of Gd was found outside the vasculature after gadodiamide.

CONCLUSIONS

In rats exposed to gadodiamide but not in rats exposed to gadobutrol, high Gd concentrations were measured in various distinct CNS structures, and structures not previously reported were identified to contain Gd, including specific subregions and layers with different cytoarchitecture and function. Knowledge of these distinct spatial patterns may pave the way for tailored functional neurological testing. Signs for the localization of the remaining Gd in the vascular endothelium were prominent for gadobutrol but not gadodiamide. The results also indicate that local transmetalation with endogenous Fe or Mn is unlikely to explain the spatial patterns of Gd deposition in the brain, which argues against a general role of these metals in local transmetalation and release of Gd ions in the CNS.

Cortical Reorganization after Limb Loss: Bridging the Gap between Basic Science and Clinical Recovery

Despite the increasing incidence and prevalence of amputation across the globe, individuals with acquired limb loss continue to struggle with functional recovery and chronic pain. A more complete understanding of the motor and sensory remodeling of the peripheral and central nervous system that occurs postamputation may help advance clinical interventions to improve the quality of life for individuals with acquired limb loss. The purpose of this article is to first provide background clinical context on individuals with acquired limb loss and then to provide a comprehensive review of the known motor and sensory neural adaptations from both animal models and human clinical trials. Finally, the article bridges the gap between basic science researchers and clinicians that treat individuals with limb loss by explaining how current clinical treatments may restore function and modulate phantom limb pain using the underlying neural adaptations described above. This review should encourage the further development of novel treatments with known neurological targets to improve the recovery of individuals postamputation.Significance Statement In the United States, 1.6 million people live with limb loss; this number is expected to more than double by 2050. Improved surgical procedures enhance recovery, and new prosthetics and neural interfaces can replace missing limbs with those that communicate bidirectionally with the brain. These advances have been fairly successful, but still most patients experience persistent problems like phantom limb pain, and others discontinue prostheses instead of learning to use them daily. These problematic patient outcomes may be due in part to the lack of consensus among basic and clinical researchers regarding the plasticity mechanisms that occur in the brain after amputation injuries. Here we review results from clinical and animal model studies to bridge this clinical-basic science gap.

Effects of pain associated with orthodontic tooth movement on tactile sensation of periodontal ligaments

OBJECTIVES

Pain associated with orthodontic tooth movement reportedly reduces periodontal ligament tactile sensation. However, the mechanism associated with the central nervous system remains unclear. This study was conducted by measuring somatosensory evoked magnetic fields (SEFs) during mechanical stimulation of teeth as they were being moved by separator elastics. Findings clarified the effects of pain on periodontal ligament tactile sensation during orthodontic tooth movement.

MATERIALS AND METHODS

Using magnetoencephalography, SEFs were measured during the application of mechanical stimuli to the mandibular right first molars of 23 right-handed healthy participants (0 h). Separator elastics were subsequently inserted into the mesial and distal interdental portions of the mandibular right first molars. The same mechanical stimuli were applied again 24 h later while the SEFs were measured (24 h). After each SEF measurements, pain was also evaluated using the Visual Analog Scale (VAS).

RESULTS

The VAS values were significantly higher at 24 h than at 0 h (p < 0.05). No significant difference in the peak latencies was found between those obtained at 0 h and 24 h, but the intensities around 40.0 ms in the contralateral hemisphere were significantly lower at 24 h than at 0 h (p < 0.01).

CONCLUSIONS

Pain associated with orthodontic tooth movement might suppress periodontal ligament tactile sensation in the primary somatosensory cortex.

CLINICAL RELEVANCE

Pain associated with orthodontic tooth movement might affect periodontal ligament sensation, consequently causing discomfort during occlusion.

Functional and structural synaptic remodeling mechanisms underlying somatotopic organization and reorganization in the thalamus

The somatosensory system organizes the topographic representation of body maps, termed somatotopy, at all levels of an ascending hierarchy. Postnatal maturation of somatotopy establishes optimal somatosensation, whereas deafferentation in adults reorganizes somatotopy, which underlies pathological somatosensation, such as phantom pain and complex regional pain syndrome. Here, we focus on the mouse whisker somatosensory thalamus to study how sensory experience shapes the fine topography of afferent connectivity during the critical period and what mechanisms remodel it and drive a large-scale somatotopic reorganization after peripheral nerve injury. We will review our findings that, following peripheral nerve injury in adults, lemniscal afferent synapses onto thalamic neurons are remodeled back to immature configuration, as if the critical period reopens. The remodeling process is initiated with local activation of microglia in the brainstem somatosensory nucleus downstream to injured nerves and heterosynaptically controlled by input from GABAergic and cortical neurons to thalamic neurons. These fruits of thalamic studies complement well-studied cortical mechanisms of somatotopic organization and reorganization and unveil potential intervention points in treating pathological somatosensation.

Pain sensitivity related to gamma oscillation of parvalbumin interneuron in primary somatosensory cortex in Dync1i1-/- mice

Cytoplasmic dynein is an important intracellular motor protein that plays an important role in neuronal growth, axonal polarity formation, dendritic differentiation, and dendritic spine development among others. The intermediate chain of dynein, encoded by Dync1i1, plays a vital role in the dynein complex. Therefore, we assessed the behavioral and related neuronal activities in mice with dync1i1 gene knockout. Neuronal activities in primary somatosensory cortex were recorded by in vivo electrophysiology and manipulated by optogenetic and chemogenetics. Nociception of mechanical, thermal, and cold pain in Dync1i1^-/- mice were impaired. The activities of parvalbumin (PV) interneurons and gamma oscillation in primary somatosensory were also impaired when exposed to mechanical nociceptive stimulation. This neuronal dysfunction was rescued by optogenetic activation of PV neurons in Dync1i1^-/- mice, and mimicked by suppressing PV neurons using chemogenetics in WT mice. Impaired pain sensations in Dync1i1^-/- mice were correlated with impaired gamma oscillations due to a loss of interneurons, especially the PV type. This genotype-driven approach revealed an association between impaired pain sensation and cytoplasmic dynein complex.