Altered Neurotransmitter Ratio in the Prefrontal Cortex is Associated with Pain in Fibromyalgia Syndrome

The central mechanisms underlying fibromyalgia syndrome (FMS) remain undetermined. The dorsolateral prefrontal cortex (DLPFC) is particularly relevant to FMS because it is implicated in cognitive, affective, and top-down pain regulation. Imbalances in excitatory (Glutamate) and inhibitory (Gamma aminobutyric acid; GABA) neurochemicals may play a critical role in the pathophysiology of the condition and more generally in homeostatic function within cortical circuits. Although the balance of excitation and inhibition are intrinsically linked no investigations to date have investigated the E/I ratio in FMS. Thus, the primary objective of this study was to determine whether the E/I ratio in the DLPFC is altered in participants with FMS compared to healthy controls using magnetic resonance spectroscopy. Additionally, we examined the relationship between E/I ratio and pain metrics. We hypothesized that the E/I ratio within the DLPFC would be altered in participants with FMS compared to controls and, secondly, that E/I ratio would be associated with both clinical pain and thermal pain sensitivity. The Brief Pain Inventory (BPI) self-assessment was used to evaluate pain severity and impact on physical functioning and acute pain sensitivity was determined via quantitative sensory testing to define thermal (heat) pain threshold and tolerance. Our results revealed an elevation in the E/I ratio in FMS compared to controls. A positive relationship between E/I ratio and thermal pain sensitivity measures was identified in the FMS cohort. Collapsing across groups, there was a positive relationship between E/I ratio and BPI score. These findings suggest that dysfunction in the balance between excitation and inhibition within cognitive brain circuitry may play a role in pain processing in FMS.

Preserved Thermal Pain in 3xTg-AD Mice With Increased Sensory-Discriminative Pain Sensitivity in Females but Affective-Emotional Dimension in Males as Early Sex-Specific AD-Phenotype Biomarkers

The increase of the aging population, where quite chronic comorbid conditions are associated with pain, draws growing interest across its investigation and the underlying nociceptive mechanisms. Burn injuries associated problems might be of relevance in the older adult’s daily life, but in people with dementia, exposure to high temperatures and heat sources poses a significantly increased risk of burns. In this brief report, the hind paws and tail pain withdrawal reflexes and the emotional responses to thermal nociception in 3xTg-AD mice were characterized for the first time in the plantar test and compared to their non-transgenic (NTg) counterparts. We studied a cohort of male and female 3xTg-AD mice at asymptomatic (2 months), early (6 months), middle (9 months), and advanced (12 and 15 months) stages of the disease and as compared to sex- and age-matched NTg control mice with normal aging. At 20 and 40W intensities, the sensorial-discriminative thresholds eliciting the withdrawal responses were preserved from asymptomatic to advanced stages of the disease compared to NTg counterparts. Moreover, 3xTg-AD females consistently showed a greater sensory-discriminative sensitivity already at premorbid ages, whereas increased emotionality was shown in males. False-negative results were found in “blind to sex and age” analysis, warning about the need to study sexes independently. The current results and previous report in cold thermal stimulation provide two paradigms unveiling sex-specific early AD-phenotype nociceptive biomarkers to study the mechanistic underpinnings of sex-, age- and AD-disease-dependent thermal pain sensitivity.

The Effect of Intrathecal Injection of Dextromethorphan on the Experimental Neuropathic Pain Model

Background: Peripheral glucocorticoid receptors (GRs) are altered by peripheral nerve injury and may modulate the development of neuropathic pain. Two central pathogenic mechanisms underlying neuropathic pain are neuroinflammation and N-methyl-D-aspartate receptor (NMDAR)-dependent neural plasticity in the spinal cord. Objectives: This study examined the effect of the non-competitive NMDAR antagonist dextromethorphan on partial sciatic nerve ligation (PSL)-induced neuropathic pain and the spinal expression of the glucocorticoid receptor (GR). Methods: Male mice were randomly assigned into a sham group and two groups receiving PSL followed by intrathecal saline vehicle or dextromethorphan (iDMP). Vehicle or iDMP was administered 8 - 14 days after PSL. The hotplate paw-withdrawal latency was considered to measure thermal pain sensitivity. The spinal cord was then sectioned and immunostained for GR. Results: Thermal hyperalgesia developed similarly in the vehicle and iDMP groups prior to the injections (P = 0.828 and 0.643); however, it was completely mitigated during the iDMP treatment (P < 0.001). GR expression was significantly higher in the vehicle group (55.64 ± 4.50) than in the other groups (P < 0.001). The iDMP group (9.99 ± 0.66) showed significantly higher GR expression than the sham group (6.30 ± 1.96) (P = 0.043). Conclusions: The suppression of PLS-induced thermal hyperalgesia by iDMP is associated with the downregulation of GR in the spinal cord, suggesting that this analgesic effect is mediated by inhibiting GR-regulated neuroinflammation.

Differential alteration of fMRI signal variability in the ascending trigeminal somatosensory and pain modulatory pathways in migraine

Background The moment-to-moment variability of resting-state brain activity has been suggested to play an active role in chronic pain. Here, we investigated the regional blood-oxygen-level-dependent signal variability (BOLDSV) and inter-regional dynamic functional connectivity (dFC) in the interictal phase of migraine and its relationship with the attack severity. Methods We acquired resting-state functional magnetic resonance imaging from 20 migraine patients and 26 healthy controls (HC). We calculated the standard deviation (SD) of the BOLD time-series at each voxel as a measure of the BOLD signal variability (BOLDSV) and performed a whole-brain voxel-wise group comparison. The brain regions showing significant group differences in BOLDSV were used to define the regions of interest (ROIs). The SD and mean of the dynamic conditional correlation between those ROIs were calculated to measure the variability and strength of the dFC. Furthermore, patients’ experimental pain thresholds and headache pain area/intensity levels during the migraine ictal-phase were assessed for clinical correlations. Results We found that migraineurs, compared to HCs, displayed greater BOLDSV in the ascending trigeminal spinal-thalamo-cortical pathways, including the spinal trigeminal nucleus, pulvinar/ventral posteromedial (VPM) nuclei of the thalamus, primary somatosensory cortex (S1), and posterior insula. Conversely, migraine patients exhibited lower BOLDSV in the top-down modulatory pathways, including the dorsolateral prefrontal (dlPFC) and inferior parietal (IPC) cortices compared to HCs. Importantly, abnormal interictal BOLDSV in the ascending trigeminal spinal-thalamo-cortical and frontoparietal pathways were associated with the patient’s headache severity and thermal pain sensitivity during the migraine attack. Migraineurs also had significantly lower variability and greater strength of dFC within the thalamo-cortical pathway (VPM-S1) than HCs. In contrast, migraine patients showed greater variability and lower strength of dFC within the frontoparietal pathway (dlPFC-IPC). Conclusions Migraine is associated with alterations in temporal signal variability in the ascending trigeminal somatosensory and top-down modulatory pathways, which may explain migraine-related pain and allodynia. Contrasting patterns of time-varying connectivity within the thalamo-cortical and frontoparietal pathways could be linked to abnormal network integrity and instability for pain transmission and modulation.

TailTimer: an open-source device for automating the rodent tail immersion assay

The tail immersion assay is a widely used method for measuring acute thermal pain in a way which is quantifiable and reproducible. It is non-invasive and measures response to a stimulus that may be encountered by an animal in its natural environment. However, tail withdrawal latency data are usually collected manually, and precise temperatures of the water at the time of measurement are most often not recorded. These two factors can reduce the reproducibility of tail immersion assay data. We designed a device, TailTimer, which uses the Raspberry Pi single-board computer and a temperature sensor, to automatically record both tail withdrawal latency and water temperature. The device has a radio frequency identification (RFID) system that can record the ID of animals. Our software recognizes several specific RFID keys as user interface commands, which allows TailTimer to be operated via RFID fobs. We also programmed the device to only allow tests to be conducted when the water is within ± 0.25 °C of the target temperature. Data recorded using the TailTimer device showed a linear relationship between tail withdrawal latency and water temperature when tested between 47 - 50 °C. We also observed a profound effect of water mixing speed on tail withdrawal latency. Our data further revealed significant strain and sex differences, valorizing TailTimer in its ability to detect genetically-determined variations in thermal pain sensitivity.Significance StatementQuantification of tail withdrawal latency in response to thermal pain has essentially remained the same since the method was first introduced decades ago and relies on manual recording of water temperature and tail withdrawal latency. Such manual methods engender relatively substantial variability and are potential contributors to some of the discrepancies present among relevant research. The open source TailTimer device we report here is simple and inexpensive to manufacture. The RFID-based user interface is ergonomic, especially in animal facilities where space is limited and gloves are mandatory. We anticipate that the increased reproducibility of tail withdrawal latency provided by TailTimer will augment its utility in nociception and addiction research.

Dexmedetomidine Improves Locomotor Function and Alleviates Thermal Hyperalgesia Following Sciatic Nerve Crush Injury in Rats

Purpose: The effects of dexmedetomidine on locomotor function and thermal hyperalgesia in sciatic nerve crush injury (SNCI) were investigated using rats.Methods: After exposing the right sciatic nerve, the sciatic nerve was crushed for 1 minute by a surgical clip. One day after nerve injury, dexmedetomidine (5, 25, and 50 µg/kg) was directly applied to the injured sciatic nerve once a day for 14 days. Walking track analysis was used to assess locomotor function and plantar test was conducted to assess thermal pain sensitivity. Immunohistochemistry was performed to determine the expression of c-Fos in the ventrolateral periaqueductal gray (vlPAG) and paraventricular nucleus (PVN). Western blot was used to evaluate the expression level of nerve growth factor (NGF) and myelin basic protein (MBP) in the sciatic nerve.Results: SNCI resulted in deterioration of locomotor function and increased thermal pain sensitivity. The level of c-Fos expression in the PVN and vlPAG was increased and the level of NGF and MBP expression in the sciatic nerve was enhanced by SNCI. Dexmedetomidine treatment improved locomotor function and upregulated expression of NGF and MBP in the sciatic nerve of SNCI. Dexmedetomidine treatment alleviated thermal hyperalgesia and downregulated expression of c-Fos in the vlPAG and PVN after SNCI.Conclusions: Dexmedetomidine may be used as a potential new treatment drug for recovery of locomotion and control of pain in peripheral nerve injury.

Lysozyme elicits pain during nerve injury by neuronal Toll-like receptor 4 activation and has therapeutic potential in neuropathic pain

The role of neuronal Toll-like receptor 4 (TLR4) in nerve injury is being pursued actively. However, the endogenous activation of neuronal TLR4 during neuroinflammation, in absence of the participation of glial TLR4, remains elusive. Here, we identified lysozyme as an endogenous activator of neuronal TLR4 signaling during nerve injury. Upon nerve injury, enhanced expression of lysozyme promoted neuronal hyperexcitability and neuropathic pain. Injections of lysozyme in healthy rats increased their mechanical and thermal pain sensitivity. Likewise, infusion of spinal cord slices with lysozyme increased neuronal excitability typical of neuropathic pain. Our results also showed that lysozyme activated excitability of both Aδ- and C-fibers. Thus, in addition to the discovery of lysozyme as an endogenous ligand for regulating neuronal TLR4 signaling, this study also lays the foundation of our understanding of its role in nervous system pathologies, providing multiple avenues for treating neuroinflammation.