Altered Brainstem Pain-Modulation Circuitry Connectivity During Spontaneous Pain Intensity Fluctuations

The roles of 5-HT in orofacial pain

OBJECTIVES

Acute and chronic orofacial pain are very common and remain a vexing health problem that has a negative effect on the quality of life. Serotonin (5-HydroxyTryptamine, 5-HT) is a kind of monoamine neurotransmitter that is involved in many physiological and pathological processes. However, its role in orofacial pain remains inconclusive. Therefore, this review aims to summarize the recent advances in understanding the effect exerted by 5-HT on the modulation of orofacial pain.

SUBJECTS AND METHODS

An extensive search was conducted on PubMed and Web of Science for pertinent studies focusing on the effects of 5-HT on the modulation of orofacial pain.

RESULTS

In this review, we concisely review how 5-HT mediates orofacial pain, how 5-HT is regulated and how we can translate these findings into clinical applications for the prevention and/or treatment of orofacial pain.

CONCLUSIONS

5-HT plays a key role in the modulation of orofacial pain, implying that 5-HT modulators may serve as effective treatment for orofacial pain. However, further research on the precise mechanisms underlying the modulation of orofacial pain is still warranted.

Altered Corticobrainstem Connectivity during Spontaneous Fluctuations in Pain Intensity in Painful Trigeminal Neuropathy

Chronic neuropathic pain can result from nervous system injury and can persist in the absence of external stimuli. Although ongoing pain characterizes the disorder, in many individuals, the intensity of this ongoing pain fluctuates dramatically. Previously, it was identified that functional magnetic resonance imaging signal covariations between the midbrain periaqueductal gray (PAG) matter, rostral ventromedial medulla (RVM), and spinal trigeminal nucleus are associated with moment-to-moment fluctuations in pain intensity in individuals with painful trigeminal neuropathy (PTN). Since this brainstem circuit is modulated by higher brain input, we sought to determine which cortical sites might be influencing this brainstem network during spontaneous fluctuations in pain intensity. Over 12 min, we recorded the ongoing pain intensity in 24 PTN participants and classified them as fluctuating (n = 13) or stable (n = 11). Using a PAG seed, we identified connections between the PAG and emotional-affective sites such as the hippocampal and posterior cingulate cortices, the sensory-discriminative posterior insula, and cognitive-affective sites such as the dorsolateral prefrontal (dlPFC) and subgenual anterior cingulate cortices that were altered dependent on spontaneous high and low pain intensity. Additionally, sliding-window functional connectivity analysis revealed that the dlPFC-PAG connection anticorrelated with perceived pain intensity over the entire 12 min period. These findings reveal cortical systems underlying moment-to-moment changes in perceived pain in PTN, which likely cause dysregulation in the brainstem circuits previously identified, and consequently alter the appraisal of pain across time.

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.

Function and biochemistry of the dorsolateral prefrontal cortex during placebo analgesia: how the certainty of prior experiences shapes endogenous pain relief

Prior experiences, conditioning cues, and expectations of improvement are essential for placebo analgesia expression. The dorsolateral prefrontal cortex is considered a key region for converting these factors into placebo responses. Since dorsolateral prefrontal cortex neuromodulation can attenuate or amplify placebo, we sought to investigate dorsolateral prefrontal cortex biochemistry and function in 38 healthy individuals during placebo analgesia. After conditioning participants to expect pain relief from a placebo "lidocaine" cream, we collected baseline magnetic resonance spectroscopy (1H-MRS) at 7 Tesla over the right dorsolateral prefrontal cortex. Following this, functional magnetic resonance imaging scans were collected during which identical noxious heat stimuli were delivered to the control and placebo-treated forearm sites. There was no significant difference in the concentration of gamma-aminobutyric acid, glutamate, Myo-inositol, or N-acetylaspartate at the level of the right dorsolateral prefrontal cortex between placebo responders and nonresponders. However, we identified a significant inverse relationship between the excitatory neurotransmitter glutamate and pain rating variability during conditioning. Moreover, we found placebo-related activation within the right dorsolateral prefrontal cortex and altered functional magnetic resonance imaging coupling between the dorsolateral prefrontal cortex and the midbrain periaqueductal gray, which also correlated with dorsolateral prefrontal cortex glutamate. These data suggest that the dorsolateral prefrontal cortex formulates stimulus-response relationships during conditioning, which are then translated to altered cortico-brainstem functional relationships and placebo analgesia expression.

Effect of High-Definition Transcranial Direct Current Stimulation on Headache Severity and Central µ-Opioid Receptor Availability in Episodic Migraine

Objective

The current understanding of utilizing HD-tDCS as a targeted approach to improve headache attacks and modulate endogenous opioid systems in episodic migraine is relatively limited. This study aimed to determine whether high-definition transcranial direct current stimulation (HD-tDCS) over the primary motor cortex (M1) can improve clinical outcomes and endogenous µ-opioid receptor (µOR) availability for episodic migraineurs.

Methods

In a randomized, double-blind, and sham-controlled trial, 25 patients completed 10-daily 20-min M1 HD-tDCS, repeated Positron Emission Tomography (PET) scans with a selective agonist for µOR. Twelve age- and sex-matched healthy controls participated in the baseline PET/MRI scan without neuromodulation. The primary endpoints were moderate-to-severe (M/S) headache days and responder rate (≥50% reduction on M/S headache days from baseline), and secondary endpoints included the presence of M/S headache intensity and the use of rescue medication over 1-month after treatment.

Results

In a one-month follow-up, at initial analysis, both the active and sham groups exhibited no significant differences in their primary outcomes (M/S headache days and responder rates). Similarly, secondary outcomes (M/S headache intensity and the usage of rescue medication) also revealed no significant differences between the two groups. However, subsequent analyses showed that active M1 HD-tDCS, compared to sham, resulted in a more beneficial response predominantly in higher-frequency individuals (>3 attacks/month), as demonstrated by the interaction between treatment indicator and baseline frequency of migraine attacks on the primary outcomes. These favorable outcomes were also confirmed for the secondary endpoints in higher-frequency patients. Active treatment also resulted in increased µOR concentration compared to sham in the limbic and descending pain modulatory pathway. Our exploratory mediation analysis suggests that the observed clinical efficacy of HD-tDCS in patients with higher-frequency conditions might be potentially mediated through an increase in µOR availability.

Conclusion

The 10-daily M1 HD-tDCS can improve clinical outcomes in episodic migraineurs with a higher baseline frequency of migraine attacks (>3 attacks/month). This improvement may be, in part, facilitated by the increase in the endogenous µOR availability.

Clinical Trial Registration

www.ClinicalTrials.gov, identifier - NCT02964741.

Bulbospinal nociceptive ON and OFF cells related neural circuits and transmitters

The rostral ventromedial medulla (RVM) is a bulbospinal nuclei in the descending pain modulation system, and directly affects spinal nociceptive transmission through pronociceptive ON cells and antinociceptive OFF cells in this area. The functional status of ON and OFF neurons play a pivotal role in pain chronification. As distinct pain modulative information converges in the RVM and affects ON and OFF cell excitability, neural circuits and transmitters correlated to RVM need to be defined for an in-depth understanding of central-mediated pain sensitivity. In this review, neural circuits including the role of the periaqueductal gray, locus coeruleus, parabrachial complex, hypothalamus, amygdala input to the RVM, and RVM output to the spinal dorsal horn are discussed. Meanwhile, the role of neurotransmitters is concluded, including serotonin, opioids, amino acids, cannabinoids, TRPV1, substance P and cholecystokinin, and their dynamic impact on both ON and OFF cell activities in modulating pain transmission. Via clarifying potential specific receptors of ON and OFF cells, more targeted therapies can be raised to generate pain relief for patients who suffer from chronic pain.

Altered Functional Activity and Functional Connectivity of Seed Regions Based on ALFF Following Acupuncture Treatment in Patients with Stroke Sequelae with Unilateral Limb Numbness

OBJECTIVE

Limb numbness is a frequent symptom of post-stroke somatosensory dysfunction, which may be alleviated by non-invasive therapy such as acupuncture. However, the precise mechanism via acupuncture remains unknown. The goal of this study was to investigate how the amplitude of low-frequency fluctuations (ALFF) and functional connectivity (FC) changed between stroke patients with limb numbness and healthy people, as well as how acupuncture might work.

METHODS

24 stroke sequelae patients with unilateral limb numbness and 14 matched healthy controls were enrolled in the study. The patients with limb numbness received acupuncture therapy three days a week for four weeks. We mainly assessed the clinical outcomes via the visual analogue scale (VAS). In addition, fMRI data from patients with unilateral limb numbness at baseline and after treatment (4th week) were collected, as well as data from healthy controls at baseline.

RESULTS

Compared with the healthy subjects, the patient group demonstrated significantly decreased ALFF in several brain regions, mainly associated with the sensorimotor network (SMN) and default mode network (DMN), including left superior frontal gyrus (SFG), right temporal fusiform cortex (TFC), right middle frontal gyrus (MFG), bilateral middle temporal gyrus (MTG), right putamen (PUT), right precentral gyrus (preCG), right planum polare (PP), and left supplementary motor area (SMA). These regions were chosen as the seeds for investigating the FC alteration induced by acupuncture. Several sensorimotor-related brain regions were activated by acupuncture, and the FC of the left supramarginal gyrus (SMG) with right MTG, as well as brain-stem, cerebellum vermis 9 with right MFG showed enhancement following acupuncture in the patient group, which had a significant correlation with clinical outcomes.

CONCLUSION

Acupuncture treatment may be used to stimulate brain areas associated with somatosensory processing and to strengthen the FC of sensorimotor and cognitive brain networks in order to achieve therapeutic effect.

NMDARs mediate peripheral and central sensitization contributing to chronic orofacial pain

Peripheral and central sensitizations of the trigeminal nervous system are the main mechanisms to promote the development and maintenance of chronic orofacial pain characterized by allodynia, hyperalgesia, and ectopic pain after trigeminal nerve injury or inflammation. Although the pathomechanisms of chronic orofacial pain are complex and not well known, sufficient clinical and preclinical evidence supports the contribution of the N-methyl-D-aspartate receptors (NMDARs, a subclass of ionotropic glutamate receptors) to the trigeminal nociceptive signal processing pathway under various pathological conditions. NMDARs not only have been implicated as a potential mediator of pain-related neuroplasticity in the peripheral nervous system (PNS) but also mediate excitatory synaptic transmission and synaptic plasticity in the central nervous system (CNS). In this review, we focus on the pivotal roles and mechanisms of NMDARs in the trigeminal nervous system under orofacial neuropathic and inflammatory pain. In particular, we summarize the types, components, and distribution of NMDARs in the trigeminal nervous system. Besides, we discuss the regulatory roles of neuron-nonneuronal cell/neuron-neuron communication mediated by NMDARs in the peripheral mechanisms of chronic orofacial pain following neuropathic injury and inflammation. Furthermore, we review the functional roles and mechanisms of NMDARs in the ascending and descending circuits under orofacial neuropathic and inflammatory pain conditions, which contribute to the central sensitization. These findings are not only relevant to understanding the underlying mechanisms, but also shed new light on the targeted therapy of chronic orofacial pain.

Mechanisms of Peripheral and Central Pain Sensitization: Focus on Ocular Pain

Persistent ocular pain caused by corneal inflammation and/or nerve injury is accompanied by significant alterations along the pain axis. Both primary sensory neurons in the trigeminal nerves and secondary neurons in the spinal trigeminal nucleus are subjected to profound morphological and functional changes, leading to peripheral and central pain sensitization. Several studies using animal models of inflammatory and neuropathic ocular pain have provided insight about the mechanisms involved in these maladaptive changes. Recently, the advent of new techniques such as optogenetics or genetic neuronal labelling has allowed the investigation of identified circuits involved in nociception, both at the spinal and trigeminal level. In this review, we will describe some of the mechanisms that contribute to the perception of ocular pain at the periphery and at the spinal trigeminal nucleus. Recent advances in the discovery of molecular and cellular mechanisms contributing to peripheral and central pain sensitization of the trigeminal pathways will be also presented.

Brainstem Pain-Modulation Circuitry and Its Plasticity in Neuropathic Pain: Insights From Human Brain Imaging Investigations

Acute pain serves as a protective mechanism that alerts us to potential tissue damage and drives a behavioural response that removes us from danger. The neural circuitry critical for mounting this behavioural response is situated within the brainstem and is also crucial for producing analgesic and hyperalgesic responses. In particular, the periaqueductal grey, rostral ventromedial medulla, locus coeruleus and subnucleus reticularis dorsalis are important structures that directly or indirectly modulate nociceptive transmission at the primary nociceptive synapse. Substantial evidence from experimental animal studies suggests that plasticity within this system contributes to the initiation and/or maintenance of chronic neuropathic pain, and may even predispose individuals to developing chronic pain. Indeed, overwhelming evidence indicates that plasticity within this circuitry favours pro-nociception at the primary synapse in neuropathic pain conditions, a process that ultimately contributes to a hyperalgesic state. Although experimental animal investigations have been crucial in our understanding of the anatomy and function of the brainstem pain-modulation circuitry, it is vital to understand this system in acute and chronic pain states in humans so that more effective treatments can be developed. Recent functional MRI studies have identified a key role of this system during various analgesic and hyperalgesic responses including placebo analgesia, offset analgesia, attentional analgesia, conditioned pain modulation, central sensitisation and temporal summation. Moreover, recent MRI investigations have begun to explore brainstem pain-modulation circuitry plasticity in chronic neuropathic pain conditions and have identified altered grey matter volumes and functioning throughout the circuitry. Considering the findings from animal investigations, it is likely that these changes reflect a shift towards pro-nociception that ultimately contributes to the maintenance of neuropathic pain. The purpose of this review is to provide an overview of the human brain imaging investigations that have improved our understanding of the pain-modulation system in acute pain states and in neuropathic conditions. Our interpretation of the findings from these studies is often guided by the existing body of experimental animal literature, in addition to evidence from psychophysical investigations. Overall, understanding the plasticity of this system in human neuropathic pain conditions alongside the existing experimental animal literature will ultimately improve treatment options.

Monoaminergic and Opioidergic Modulation of Brainstem Circuits: New Insights Into the Clinical Challenges of Pain Treatment?

The treatment of neuropathic pain remains a clinical challenge. Analgesic drugs and antidepressants are frequently ineffective, and opioids may induce side effects, including hyperalgesia. Recent results on brainstem pain modulatory circuits may explain those clinical challenges. The dual action of noradrenergic (NA) modulation was demonstrated in animal models of neuropathic pain. Besides the well-established antinociception due to spinal effects, the NA system may induce pronociception by directly acting on brainstem pain modulatory circuits, namely, at the locus coeruleus (LC) and medullary dorsal reticular nucleus (DRt). The serotoninergic system also has a dual action depending on the targeted spinal receptor, with an exacerbated activity of the excitatory 5-hydroxytryptamine 3 (5-HT3) receptors in neuropathic pain models. Opioids are involved in the modulation of descending modulatory circuits. During neuropathic pain, the opioidergic modulation of brainstem pain control areas is altered, with the release of enhanced local opioids along with reduced expression and desensitization of μ-opioid receptors (MOR). In the DRt, the installation of neuropathic pain increases the levels of enkephalins (ENKs) and induces desensitization of MOR, which may enhance descending facilitation (DF) from the DRt and impact the efficacy of exogenous opioids. On the whole, the data discussed in this review indicate the high plasticity of brainstem pain control circuits involving monoaminergic and opioidergic control. The data from studies of these neurochemical systems in neuropathic models indicate the importance of designing drugs that target multiple neurochemical systems, namely, maximizing the antinociceptive effects of antidepressants that inhibit the reuptake of serotonin and noradrenaline and preventing desensitization and tolerance of MOR at the brainstem.

Favorite Music Mediates Pain-related Responses in the Anterior Cingulate Cortex and Skin Pain Thresholds

Purpose

Music therapy is widely used to enhance well-being, reduce pain, and distract patients from unpleasant symptoms in the clinical setting. However, the degree to which music modulates pain perception is unknown. The medial pain pathway including the limbic system is associated with emotion, but how music alters pathway activity is unclear. The aim of the study was to investigate pain thresholds and pain-related responses in the anterior cingulate cortex (ACC) and whether they were modulated when subjects listened to their favorite music genre.

Subjects and Methods

First, 30 subjects were examined for left forearm pain threshold using electrical stimulation with Pain Vision PS-2011N. The pain thresholds with and without music were compared. Second, when an 80-μA current from Pain Vision was applied to the left ankle of eight women, the pain-related responses of the ACC with and without music were observed with functional magnetic resonance device (fMRI). The changes in the pain-related activity in both parameters were discussed.

Results

The median pain threshold with favorite music was 38.9 μA, compared to 29.0 μA without, which was significantly different (p<0.0001). The men’s thresholds were significantly higher than women’s both with music (p<0.05) and without music (p<0.01). The pain threshold in women was more strongly affected by music than in men. The fMRI results showed that the pain-related response in the ACC in five of eight subjects was attenuated while they listened to their favorite music. No change was observed in the other three subjects.

Conclusion

The present findings suggest that pain perception might be strongly affected by listening to favorite music, possibly through modulation of pain-related responses in the ACC.