Nicotinic modulation of descending pain control circuitry

α4 nicotinic receptors on GABAergic neurons mediate a cholinergic analgesic circuit in the substantia nigra pars reticulata

Nicotinic acetylcholine receptors (nAChRs) regulate pain pathways with various outcomes depending on receptor subtypes, neuron types, and locations. But it remains unknown whether α4β2 nAChRs abundantly expressed in the substantia nigra pars reticulata (SNr) have potential to mitigate hyperalgesia in pain states. We observed that injection of nAChR antagonists into the SNr reduced pain thresholds in naïve mice, whereas injection of nAChR agonists into the SNr relieved hyperalgesia in mice, subjected to capsaicin injection into the lower hind leg, spinal nerve injury, chronic constriction injury, or chronic nicotine exposure. The analgesic effects of nAChR agonists were mimicked by optogenetic stimulation of cholinergic inputs from the pedunculopontine nucleus (PPN) to the SNr, but attenuated upon downregulation of α4 nAChRs on SNr GABAergic neurons and injection of dihydro-β-erythroidine into the SNr. Chronic nicotine-induced hyperalgesia depended on α4 nAChRs in SNr GABAergic neurons and was associated with the reduction of ACh release in the SNr. Either activation of α4 nAChRs in the SNr or optogenetic stimulation of the PPN-SNr cholinergic projection mitigated chronic nicotine-induced hyperalgesia. Interestingly, mechanical stimulation-induced ACh release was significantly attenuated in mice subjected to either capsaicin injection into the lower hind leg or SNI. These results suggest that α4 nAChRs on GABAergic neurons mediate a cholinergic analgesic circuit in the SNr, and these receptors may be effective therapeutic targets to relieve hyperalgesia in acute and chronic pain, and chronic nicotine exposure.

Use of Prescribed and Non-Prescribed Treatments for Cluster Headache in a Swedish Cohort

BACKGROUND

Cluster headache (CH) is a debilitating condition, but current therapies leave CH patients in pain. The extent of this problem in Sweden is unknown.

METHODS

An anonymized questionnaire was sent to 479 Swedish CH patients to investigate patterns and perceived effects of treatments.

RESULTS

Three hundred fourteen answers were analyzed. The population was representative regarding age of onset and sex. Less than half (46%) were satisfied with their abortive treatments, 19% terminated functioning abortive treatments due to side effects. Additionally, 17% of chronic CH patients had not tried the first-line preventive drug verapamil. A small subset had tried illicit substances to treat their CH (0-8% depending on substance). Notably, psilocybin was reported effective as an abortive treatment by 100% (n = 8), and with some level of effect as a preventive treatment by 92% (n = 12). For verapamil, some level of preventive effect was reported among 68% (n = 85).

CONCLUSIONS

Our descriptive data illustrate that many Swedish CH patients are undertreated, lack functional therapies, and experience side effects. Further studies are warranted to search for new treatment strategies as well as a revision of current treatment guidelines with the aim of reducing patient disease burden to the greatest extent possible.

Restoration of the Activity of the Prefrontal Cortex to the Nucleus Accumbens Core Pathway Relieves Fentanyl-Induced Hyperalgesia in Male Rats

Purpose

Functional connectivity between the prelimbic medial prefrontal cortex (PL-mPFC) and the core of the nucleus accumbens (NAc core) predicts pain chronification. Inhibiting the apoptosis of oligodendrocytes in the PL-mPFC prevents fentanyl-induced hyperalgesia in rats. However, the role of prefrontal cortex (PFC)-NAc projections in opioid-induced hyperalgesia (OIH) remains unclear. Herein, we explored the role of the PL-NAc core circuit in fentanyl-induced hyperalgesia.

Methods

An OIH rat model was established, and patch-clamp recording, immunofluorescence, optogenetics, and chemogenetic methods were employed for neuron excitability detection and nociceptive behavioral assessment.

Results

Our results showed decreased activity of the right PL-mPFC layer V output neurons in rats with OIH. Similarly, the excitability of the NAc core neurons receiving glutamatergic projections from the PL-mPFC decreased in OIH rats, observed by the light-evoked excitatory postsynaptic currents/light-excited inhibitory postsynaptic currents ratio (eEPSC/eIPSC ratio). Fentanyl-induced hyperalgesia was reversed by optogenetic activation of the PL-NAc core pathway, and chemogenetic suppression of this pathway induced hyperalgesia in control (saline-treated) rats. However, behavioral hyperalgesia was not aggravated by this chemogenetic suppression in OIH (fentanyl-treated) rats.

Conclusion

Our findings indicate that inactivation of the PL-NAc core pathway may be a cause of OIH and restoring the activity of this pathway may provide a strategy for OIH treatment.

Anti-hyperalgesic and anti-inflammatory effects of 4R-tobacco cembranoid in a mouse model of inflammatory pain

4R is a tobacco cembranoid that binds to and modulates cholinergic receptors and exhibits neuroprotective and anti-inflammatory activity. Given the established function of the cholinergic system in pain and inflammation, we propose that 4R is also analgesic. Here, we tested the hypothesis that systemic 4R treatment decreases pain-related behaviors and peripheral inflammation via modulation of the alpha 7 nicotinic acetylcholine receptors (α7 nAChRs) in a mouse model of inflammatory pain. We elicited inflammation by injecting Complete Freund's Adjuvant (CFA) into the hind paw of male and female mice. We then assessed inflammation-induced hypersensitivity to cold, heat, and tactile stimulation using the Acetone, Hargreaves, and von Frey tests, respectively, before and at different time points (2.5 h - 8d) after a single systemic 4R (or vehicle) administration. We evaluated the contribution of α7 nAChRs 4R-mediated analgesia by pre-treating mice with a selective antagonist of α7 nAChRs followed by 4R (or vehicle) administration prior to behavioral tests. We assessed CFA-induced paw edema and inflammation by measuring paw thickness and quantifying immune cell infiltration in the injected hind paw using hematoxylin and eosin staining. Lastly, we performed immunohistochemical and flow cytometric analyses of paw skin in α7 nAChR-cre::Ai9 mice to measure the expression of α7 nAChRs on immune subsets. Our experiments show that systemic administration of 4R decreases inflammation-induced peripheral hypersensitivity in male and female mice and inflammation-induced paw edema in male but not female mice. Notably, 4R-mediated analgesia and anti-inflammatory effects lasted up to 8d after a single systemic administration on day 1. Pretreatment with an α7 nAChR-selective antagonist prevented 4R-mediated analgesia and anti-inflammatory effects, demonstrating that 4R effects are via modulation of α7 nAChRs. We further show that a subset of immune cells in the hind paw expresses α7 nAChRs. However, the number of α7 nAChR-expressing immune cells is unaltered by CFA or 4R treatment, suggesting that 4R effects are independent of α7 nAChR-expressing immune cells. Together, our findings identify a novel function of the 4R tobacco cembranoid as an analgesic agent in both male and female mice that reduces peripheral inflammation in a sex-dependent manner, further supporting the pharmacological targeting of the cholinergic system for pain treatment.

The Anti-Nociceptive Effects of Nicotine in Humans: A Systematic Review and Meta-Analysis

BACKGROUND

Pain can have a serious impact on a patient's physical, mental, and social health, often causing their quality of life to decline. Various nicotine dosage forms, such as nicotine patches and nasal spray, have been developed and used as analgesics in clinical settings. However, there is controversy over the anti-nociceptive effects of nicotine among different clinical trials. The purpose of this meta-analysis is to quantify the analgesic effect of nicotine patches, nicotine nasal spray, and tobacco smoking on pain in humans.

METHODS

Relevant articles published in English prior to July 2023 were identified using the PubMed, Cochrane Library, and Embase online databases in accordance with PRISMA (2020) guidelines. Two reviewers independently screened and selected studies, extracted data, and assessed the quality of the included studies using version 2 of the Cochrane risk-of-bias tool for randomized trials (RoB 2). RStudio was used for data synthesis, heterogeneity assessment, sensitivity analysis, publication bias assessment, trim-and-fill analyses, and generating forest plots.

RESULTS

Sixteen eligible articles, including k = 5 studies of pain tolerance (n = 210), k = 5 studies of pain threshold (n = 210), and k = 12 studies of pain scores (N = 1249), were included for meta-analysis. Meta-analytic integration for pain threshold (Hedges' g = 0.28, 95% CI = 0-0.55, Z = 1.99, p = 0.05) and pain tolerance (Hedges' g = 0.32, 95% CI = 0.05-0.59, Z = 2.30, p = 0.02) revealed that nicotine administered via tobacco smoke generated acute analgesic effects to thermal stimuli. Meta-analytic integration for pain scores revealed that nicotine had a weak anti-nociceptive effect on postoperative pain of -0.37 (95% CI = -0.77 to 0.03, Z = -1.80) but with no statistical significance (p = 0.07). In addition, a limited number of included studies revealed that long-term smoking produced hyperalgesia that may be characterized as small to medium in magnitude (Hedges' g = 0.37, 95% CI = 0.29-0.64, Z = 5.33, p < 0.01).

CONCLUSION

These results help to clarify the mixed outcomes of trials and may ultimately inform the treatment of pain. We observed that acute nicotine administration prolonged the laboratory-induced pain threshold and tolerance time and may mildly relieve postoperative pain. In addition, long-term tobacco smoking may have a nociceptive effect on different types of chronic pain. More research is needed to determine the anti-nociceptive effects of nicotine in humans, and to understand the optimal timing, dose, and method of delivery of nicotine.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

The Antinociceptive Activity of (E)-3-(thiophen-2-yl)- N -(p-tolyl)acrylamide in Mice Is Reduced by (E)-3-(furan-2-yl)- N -methyl- N -(p-tolyl)acrylamide Through Opposing Modulatory Mechanisms at the α7 Nicotinic Acetylcholine Receptor

BACKGROUND

The primary objective of this study was to characterize the pharmacological and behavioral activity of 2 novel compounds, DM497 [(E)-3-(thiophen-2-yl)- N -(p-tolyl)acrylamide] and DM490 [(E)-3-(furan-2-yl)- N -methyl- N -(p-tolyl)acrylamide], structural derivatives of PAM-2, a positive allosteric modulator of the α7 nicotinic acetylcholine receptor (nAChR).

METHODS

A mouse model of oxaliplatin-induced neuropathic pain (2.4 mg/kg, 10 injections) was used to test the pain-relieving properties of DM497 and DM490. To assess possible mechanisms of action, the activity of these compounds was determined at heterologously expressed α7 and α9α10 nAChRs, and voltage-gated N-type calcium channel (Ca V 2.2) using electrophysiological techniques.

RESULTS

Cold plate tests indicated that 10 mg/kg DM497 was able to decrease neuropathic pain in mice induced by the chemotherapeutic agent oxaliplatin. In contrast, DM490 induced neither pro- nor antinociceptive activity but inhibited DM497's effect at equivalent dose (30 mg/kg). These effects are not a product of changes in motor coordination or locomotor activity. At α7 nAChRs, DM497 potentiated whereas DM490 inhibited its activity. In addition, DM490 antagonized the α9α10 nAChR with >8-fold higher potency than that for DM497. In contrast, DM497 and DM490 had minimal inhibitory activity at the Ca V 2.2 channel. Considering that DM497 did not increase the mouse exploratory activity, an indirect anxiolytic mechanism was not responsible for the observed antineuropathic effect.

CONCLUSIONS

The antinociceptive activity of DM497 and the concomitant inhibitory effect of DM490 are mediated by opposing modulatory mechanisms on the α7 nAChR, whereas the involvement of other possible nociception targets such as the α9α10 nAChR and Ca V 2.2 channel can be ruled out.

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

Nicotinic acetylcholine receptors: Therapeutic targets for novel ligands to treat pain and inflammation

Nicotinic acetylcholine receptors (nAChRs) have been historically defined as ligand-gated ion channels and function as such in the central and peripheral nervous systems. Recently, however, non-ionic signaling mechanisms via nAChRs have been demonstrated in immune cells. Furthermore, the signaling pathways where nAChRs are expressed can be activated by endogenous ligands other than the canonical agonists acetylcholine and choline. In this review, we discuss the involvement of a subset of nAChRs containing α7, α9, and/or α10 subunits in the modulation of pain and inflammation via the cholinergic anti-inflammatory pathway. Additionally, we review the most recent advances in the development of novel ligands and their potential as therapeutics.

Neural Subtype-dependent Cholinergic Modulation of Neural Activities by Activation of Muscarinic 2 Receptors and G Protein-activated Inwardly Rectifying Potassium Channel in Rat Periaqueductal Gray Neurons

Acetylcholine plays a pivotal role in the regulation of functions such as pain and the sleep and wake cycle by modulating neural activities of the ventrolateral periaqueductal gray (vlPAG). Electrophysiological studies have shown that cholinergic effects are inconsistent among recorded neurons, particularly in the depolarization and hyperpolarization of the resting membrane potential (RMP). This discrepancy may be due to the neural subtype-dependent cholinergic modulation of the RMP. To examine this possibility, we performed whole-cell patch-clamp recordings from subtype-identified neurons using vesicular GABA transporter (VGAT)-Venus × ChAT-TdTomato rats and elucidated cellular mechanisms of cholinergic effects on the RMP. The application of carbachol hyperpolarized the RMP of cholinergic neurons in a dose-dependent manner but had much less of an effect on other neural subtypes, including GABAergic/glycinergic and glutamatergic neurons. Cholinergic hyperpolarization was accompanied by a decrease in input resistance. These cholinergic effects were blocked by AF-DX384 or gallamine and were mimicked by arecaidine but-2-ynyl ester tosylate, suggesting that the carbachol-induced hyperpolarization of the RMP in cholinergic neurons is mediated via M₂ receptors. Tertiapin suppressed the carbachol-induced G protein-activated inwardly rectifying potassium channel (GIRK) currents and hyperpolarization of the RMP in cholinergic neurons. Intracellular application of GDP-β-S blocked the carbachol-induced hyperpolarization of the RMP. Neostigmine slowly hyperpolarized the RMP in cholinergic neurons. These results suggest that neural firing of vlPAG cholinergic neurons is suppressed by GIRK currents induced via M₂ receptor activation, and this negative feedback regulation of cholinergic neuronal activities can be induced by acetylcholine, which is intrinsically released in the vlPAG.

Targeting α7 nicotinic acetylcholine receptors for chronic pain

Despite rapid advances in the field of chronic pain, it remains extremely challenging in the clinic. Pain treatment strategies have not improved for decades as opioids remain the main prescribed drugs for chronic pain management. However, long-term use of opioids often leads to detrimental side effects. Therefore, uncovering the mechanisms underlying the development and maintenance of chronic pain may aid the discovery of novel therapeutics to benefit patients with chronic pain. Substantial evidence indicates downregulation of α7 nicotinic acetylcholine receptors (α7 nAChR) in the sciatic nerve, dorsal root ganglia, and spinal cord dorsal horn in rodent models of chronic pain. Moreover, our recent study and results from other laboratories demonstrate that potentiation of α7 nAChR attenuates pain behaviors in various murine models of chronic pain. This review summarized and discussed the preclinical evidence demonstrating the therapeutic potential of α7 nAChR agonists and allosteric modulators in chronic pain. This evidence indicates that potentiation of α7 nAChR is beneficial in chronic pain, mostly by alleviating neuroinflammation. Overall, α7 nAChR-based therapy for chronic pain is an area with great promise, but more research regarding its detailed mechanisms is warranted.

Consensus statement on smoking cessation in patients with pain

Smoking is closely associated with the development of various cancers and tobacco-related illnesses such as cardiovascular and respiratory disorders. However, data are scarce on the relationship between smoking and both acute and chronic pain. In addition to nicotine, tobacco smoke contains more than 4000 different compounds. Although nicotine is not the sole cause of smoking-induced diseases, it plays a critical role in pain-related pathophysiology. Despite the acute analgesic effects of nicotine, long-term exposure leads to tolerance and increased pain sensitivity due to nicotinic acetylcholine receptor desensitization and neuronal plastic changes. The purpose of smoking cessation interventions in smoking patients with pain is primarily not only to reduce their pain and associated limitations in activities of daily living, but also to improve the outcomes of underlying pain-causing conditions and reduce the risks of tobacco-related disorders. This statement aims to summarize the available evidence on the impact of smoking on pain and to inform medical professionals of the significance of smoking cessation in patients with pain.

Smoking cessation after cancer diagnosis reduces the risk of severe cancer pain: A longitudinal cohort study

Background

Whether abstinence from smoking among cancer patients reduces cancer pain is still unclear. Opioids can act as a surrogate index for evaluating the incidence of severe cancer pain in countries where opioid abuse is infrequent. This study aimed to investigate whether changed smoking behavior after cancer diagnosis influences the incidence of severe cancer pain as determined by strong opioid use.

Methods

Using a large Japanese insurance claims database (n = 4,797,329), we selected 794,702 insured employees whose annual health checkup data could be confirmed ≥6 times between January 2009 and December 2018. We selected 591 study subjects from 3,256 employees who were diagnosed with cancer pain and had health checkup data at the year of cancer pain diagnosis.

Results

A significantly greater proportion of patients who continued smoking after cancer diagnosis (“current smoker”, n = 133) received strong opioids (36.8%) compared with patients who had never smoked or had stopped before cancer diagnosis (“non-smoker”, n = 383, 20.6%; p<0.05) but also compared with patients who had quit smoking after cancer diagnosis (“abstainer:”, n = 75, 24.0%; p<0.05). In multivariable Cox proportional hazards regression analysis, abstainers had a significantly lower risk of receiving strong opioids than current smokers (hazard ratio: 0.57, 95% CI: 0.328 to 0.997). These findings were consistent across multiple sensitivity analyses.

Conclusion

Our study demonstrated that patients who quit smoking after cancer diagnosis have a lower risk of severe cancer pain. This information adds clinical incentives for improving quality of life among those who smoked at the time of cancer diagnosis.

Adolescent nicotine exposure increases nociceptive behaviors in rat model of formalin test: Involvement of ventrolateral periaqueductal gray neurons

Among the major life-threatening factors, smoking tobacco is the leading cause of death worldwide. Adolescence is a sensitive stage of brain development, and smoking at this age is thought to be associated with neural and behavioral alterations. Currently the association between adolescent tobacco use and pain perception remained to be addressed. It is also important to consider that the periaqueductal gray (PAG) is a major component of the descending pain inhibitory system. The present study was performed to reveal the possible effects of adolescent nicotine consumption on pain-related behaviors and also the antinociceptive effect of a single dose of morphine administration besides the ventrolateral PAG (vlPAG) firing assessment in adulthood during formalin test. Adolescent male Wistar rats were administered with either a nicotine or saline injection (s.c.), and after 30 days of washout period, formalin test was performed. The vlPAG neuronal responses to formalin injection were recorded via in vivo extracellular single-unit recording. The results demonstrated that adolescent nicotine exposure enhances behavioral responses to pain. It also reduced morphine-induced antinociceptive behavior in the formalin test during adulthood. Moreover, adolescent nicotine exposure attenuates the extent of vlPAG inhibitory response to formalin. Our data provided a further conclusion that adolescent nicotine exposure may alter the pain modulatory systems and their subsequent response to painful stimuli.

Nicotine and opioid co-dependence: Findings from bench research to clinical trials

Concomitant use of tobacco and opioids represents a growing public health concern. In fact, the mortality rate due to smoking-related illness approaches 50% among SUD patients. Cumulative evidence demonstrates that the vulnerability to drugs of abuse is influenced by behavioral, environmental, and genetic factors. This review explores the contribution of genetics and neural mechanisms influencing nicotine and opioid reward, respiration, and antinociception, emphasizing the interaction of cholinergic and opioid receptor systems. Despite the substantial evidence demonstrating nicotine-opioid interactions within the brain and on behavior, the currently available pharmacotherapies targeting these systems have shown limited efficacy for smoking cessation on opioid-maintained smokers. Thus, further studies designed to identify novel targets modulating both nicotinic and opioid receptor systems may lead to more efficacious approaches for co-morbid nicotine dependence and opioid use disorder.

Evidence for Alpha7 Nicotinic Receptor Activation During the Cough Suppressing Effects Induced by Nicotine and Identification of ATA-101 as a Potential Novel Therapy for the Treatment of Chronic Cough

Studies performed in healthy smokers have documented a diminished responsiveness to tussive challenges, and several lines of experimental evidence implicate nicotine as an antitussive component in both cigarette smoke and the vapors generated by electronic cigarettes (eCigs). We set out to identify the nicotinic receptor subtype involved in the antitussive actions of nicotine and to further evaluate the potential of nicotinic receptor-selective agonists as cough-suppressing therapeutics. We confirmed an antitussive effect of nicotine in guinea pigs. We additionally observed that the alpha-4 beta-2 (α ₄ β ₂)-selective agonist Tc-6683 was without effect on evoked cough responses in guinea pigs, while the α ₇-selective agonist PHA 543613 dose-dependently inhibited evoked coughing. We subsequently describe the preclinical evidence in support of ATA-101, a potent and highly selective (α ₇) selective nicotinic receptor agonist, as a potential candidate for antitussive therapy in humans. ATA-101, formerly known as Tc-5619, was orally bioavailable and moderately central nervous system (CNS) penetrant and dose-dependently inhibited coughing in guinea pigs evoked by citric acid and bradykinin. Comparing the effects of airway targeted administration versus systemic dosing and the effects of repeated dosing at various times prior to tussive challenge, our data suggest that the antitussive actions of ATA-101 require continued engagement of α ₇ nicotinic receptors, likely in the CNS. Collectively, the data provide the preclinical rationale for α ₇ nicotinic receptor engagement as a novel therapeutic strategy for cough suppression. The data also suggest that α ₇ nicotinic acetylcholine receptor (nAChR) activation by nicotine may be permissive to nicotine delivery in a way that may promote addiction. SIGNIFICANCE STATEMENT: This study documents the antitussive actions of nicotine and identifies the α₇ nicotinic receptor subtype as the target for nicotine during cough suppression described in humans. We additionally present evidence suggesting that ATA-101 and other α₇ nicotinic receptor-selective agonists may be promising candidates for the treatment of chronic refractory cough.

The role of endogenous opioid neuropeptides in neurostimulation-driven analgesia

Due to the prevalence of chronic pain worldwide, there is an urgent need to improve pain management strategies. While opioid drugs have long been used to treat chronic pain, their use is severely limited by adverse effects and abuse liability. Neurostimulation techniques have emerged as a promising option for chronic pain that is refractory to other treatments. While different neurostimulation strategies have been applied to many neural structures implicated in pain processing, there is variability in efficacy between patients, underscoring the need to optimize neurostimulation techniques for use in pain management. This optimization requires a deeper understanding of the mechanisms underlying neurostimulation-induced pain relief. Here, we discuss the most commonly used neurostimulation techniques for treating chronic pain. We present evidence that neurostimulation-induced analgesia is in part driven by the release of endogenous opioids and that this endogenous opioid release is a common endpoint between different methods of neurostimulation. Finally, we introduce technological and clinical innovations that are being explored to optimize neurostimulation techniques for the treatment of pain, including multidisciplinary efforts between neuroscience research and clinical treatment that may refine the efficacy of neurostimulation based on its underlying mechanisms.

Brain circuits for pain and its treatment

[Figure: see text].

Strong Twirling-Rotating Manual Acupuncture with 4 r/s Is Superior to 2 r/s in Relieving Pain by Activating C-Fibers in Rat Models of CFA-Induced Pain

Background

Manual acupuncture (MA) with different stimulus frequencies may give rise to varying acupuncture effects. However, the intensity-effect relationship and the underlying mechanisms of MA remain unclear.

Objective

To compare the analgesic effects of different frequencies of twirling-rotating MA on rats with complete Freund's adjuvant- (CFA-) induced pain and explore the underlying mechanism via peripheral sensory nerves.

Methods

First, 36 healthy male Wistar rats were randomly divided into 6 groups: control group, 2 r/s MA group (twirling-rotating MA with the frequency of 2 revolutions per second), 4 r/s MA group (twirling-rotating MA with the frequency of 4 revolutions per second), CFA group, CFA + 2 r/s MA group, and CFA + 4 r/s MA group. Rats in three CFA groups received an intraplantar injection of CFA to establish a pain model, while the rats in other three groups received an intraplantar injection of saline. Rats in the 2 r/s MA group and 4 r/s MA group were treated with the corresponding frequencies of twirling-rotating MA on bilateral Zusanli (ST36) and Kunlun (BL60) for 7 days. The ipsilateral nociceptive thresholds (paw withdrawal latency; PWL) were tested to evaluate the analgesic effects. Second, 9 healthy male Wistar rats were randomly divided into 3 groups: control group, 2 r/s MA group, and 4 r/s MA group. The proportion of C-fiber neurons (calcitonin gene-related peptide- (CGRP-) positive neurons) and A-fiber neurons (neurofilament 200- (NF200-) positive neurons) in the dorsal root ganglia (DRG) activated by MA were quantitatively analyzed with the morphological immunofluorescence staining method. Third, 30 healthy male Wistar rats were randomly divided into 6 groups: control group, CFA group, CFA + 2 r/s MA group, CFA + 2 r/s MA + RTX group, CFA + 4 r/s MA group, and CFA  + 4 r/s MA + RTX group. Resiniferatoxin (RTX) was injected into the acupoints before acupuncture. PWL was evaluated to investigate the analgesic effect.

Results

Both types of MA treatment increased the PWL of saline-injecting rats and pain model rats. Moreover, 4 r/s MA was superior to 2 r/s MA in increasing PWL. A higher quantity of excited C-fiber neurons was observed following 4 r/s MA than 2 r/s MA, while the reverse was observed in the activation of A-fiber neurons. Following the injection of RTX to inhibit the activation of C-fibers, the analgesic effect of 4 r/s MA reduced significantly but not of 2 r/s MA.

Conclusion

Strong MA (4 r/s MA) has superior analgesic effects to gentle MA (2 r/s MA) on CFA model rats, which is associated with C-fiber activation.

The Periaqueductal Gray and Its Extended Participation in Drug Addiction Phenomena

The periaqueductal gray (PAG) is a complex mesencephalic structure involved in the integration and execution of active and passive self-protective behaviors against imminent threats, such as immobility or flight from a predator. PAG activity is also associated with the integration of responses against physical discomfort (e.g., anxiety, fear, pain, and disgust) which occurs prior an imminent attack, but also during withdrawal from drugs such as morphine and cocaine. The PAG sends and receives projections to and from other well-documented nuclei linked to the phenomenon of drug addiction including: (i) the ventral tegmental area; (ii) extended amygdala; (iii) medial prefrontal cortex; (iv) pontine nucleus; (v) bed nucleus of the stria terminalis; and (vi) hypothalamus. Preclinical models have suggested that the PAG contributes to the modulation of anxiety, fear, and nociception (all of which may produce physical discomfort) linked with chronic exposure to drugs of abuse. Withdrawal produced by the major pharmacological classes of drugs of abuse is mediated through actions that include participation of the PAG. In support of this, there is evidence of functional, pharmacological, molecular. And/or genetic alterations in the PAG during the impulsive/compulsive intake or withdrawal from a drug. Due to its small size, it is difficult to assess the anatomical participation of the PAG when using classical neuroimaging techniques, so its physiopathology in drug addiction has been underestimated and poorly documented. In this theoretical review, we discuss the involvement of the PAG in drug addiction mainly via its role as an integrator of responses to the physical discomfort associated with drug withdrawal.

Projections from the lateral parabrachial nucleus to the lateral and ventral lateral periaqueductal gray subregions mediate the itching sensation

ABSTRACT

Lateral and ventral lateral subregions of the periaqueductal gray (l/vlPAG) have been proved to be pivotal components in descending circuitry of itch processing, and their effects are related to the subclassification of neurons that were meditated. In this study, lateral parabrachial nucleus (LPB), one of the most crucial relay stations in the ascending pathway, was taken as the input nucleus to examine the modulatory effect of l/vlPAG neurons that received LPB projections. Anatomical tracing, chemogenetic, optogenetic, and local pharmacological approaches were used to investigate the participation of the LPB-l/vlPAG pathway in itch and pain sensation in mice. First, morphological evidence for projections from vesicular glutamate transporter-2-containing neurons in the LPB to l/vlPAG involved in itch transmission has been provided. Furthermore, chemogenetic and optogenetic activation of the LPB-l/vlPAG pathway resulted in both antipruritic effect and analgesic effect, whereas pharmacogenetic inhibition strengthened nociceptive perception without affecting spontaneous scratching behavior. Finally, in vivo pharmacology was combined with optogenetics which revealed that AMPA receptor-expressing neurons in l/vlPAG might play a more essential role in pathway modulation. These findings provide a novel insight about the connections between 2 prominent transmit nuclei, LPB and l/vlPAG, in both pruriceptive and nociceptive sensations and deepen the understanding of l/vlPAG modulatory roles in itch sensation by chosen LPB as source of ascending efferent projections.

Stimulation of brain α7-nicotinic acetylcholine receptors suppresses the rat micturition through brain GABAergic receptors

Brain nicotinic acetylcholine receptors (nAChRs) reportedly suppress the micturition, but the mechanisms responsible for this suppression remain unclear. We previously reported that intracerebroventricularly administered (±)-epibatidine (non-selective nAChR agonist) activated the sympatho-adrenomedullary system, which can affect the micturition. Therefore, we investigated (1) whether intracerebroventricularly administered (±)-epibatidine-induced effects on the micturition were dependent on the sympatho-adrenomedullary system, and (2) brain nAChR subtypes involved in the (±)-epibatidine-induced effects in urethane-anesthetized male Wistar rats. Plasma noradrenaline and adrenaline (catecholamines) were measured just before and 5 min after (±)-epibatidine administration. Evaluation of urodynamic parameters, intercontraction intervals (ICI) and maximal voiding pressure (MVP) by cystometry was started 1 h before (±)-epibatidine administration or intracerebroventricular pretreatment with other drugs and continued 1 h after (±)-epibatidine administration. Intracerebroventricularly administered (±)-epibatidine elevated plasma catecholamines and prolonged ICI without affecting MVP, and these changes were suppressed by intracerebroventricularly pretreated mecamylamine (non-selective nAChR antagonist). Acute bilateral adrenalectomy abolished the (±)-epibatidine-induced elevation of plasma catecholamines, but had no effect on the (±)-epibatidine-induced ICI prolongation. The latter was suppressed by intracerebroventricularly pretreated methyllycaconitine (selective α7-nAChR antagonist), SR95531 (GABAA antagonist), and SCH50911 (GABAB antagonist), but not by dihydro-β-erythroidine (selective α4β2-nAChR antagonist). Intracerebroventricularly administered PHA568487 (selective α7-nAChR agonist) prolonged ICI without affecting MVP, similar to (±)-epibatidine. These results suggest that stimulation of brain α7-nAChRs suppresses the rat micturition through brain GABAA/GABAB receptors, independently of the sympatho-adrenomedullary outflow modulation.

[Relation between tobacco smoking and pain: A narrative review of the scientific literature]

INTRODUCTION

Tobacco smoking and pain have an intimate, complex, two-way interaction. The purpose of this narrative review of the literature is to present what is currently understood about the relationship.

STATE OF KNOWLEDGE

Tobacco smoking (and the associated chronic exposure to nicotine) has been defined as a risk factor for chronic pain, involving nociceptive sensitisation. For people who smoke, pain will be both a motivational factor for tobacco consumption and a barrier to tobacco use cessation. Conversely, nicotine (acute exposure) has clearly demonstrated analgesic properties, mediated in particular by activation of nicotinic acetylcholine receptors.

PERSPECTIVES

The management of pain in people who smoke is still largely unaddressed, and further studies will be needed to develop effective strategies for tobacco use cessation in this context. Nicotine and modulators of nicotinic acetylcholine receptors represent innovative strategies for the discovery of new analgesics.

CONCLUSIONS

The effects of smoking on pain, chronic nociceptive sensitisation and acute analgesia, serve to maintain tobacco consumption via negative reinforcement. A holistic therapeutic strategy is necessary to maximise the likelihood of successful smoking cessation.

Acetylcholine and spinal locomotor networks: The insider

This article aims to review studies that have investigated the role of neurons that use the transmitter acetylcholine (ACh) in controlling the operation of locomotor neural networks within the spinal cord. This cholinergic system has the particularity of being completely intraspinal. We describe the different effects exerted by spinal cholinergic neurons on locomotor circuitry by the pharmacological activation or blockade of this propriospinal system, as well as describing its different cellular and subcellular targets. Through the activation of one ionotropic receptor, the nicotinic receptor, and five metabotropic receptors, the M1 to M5 muscarinic receptors, the cholinergic system exerts a powerful control both on synaptic transmission and locomotor network neuron excitability. Although tremendous advances have been made in our understanding of the spinal cholinergic system's involvement in the physiology and pathophysiology of locomotor networks, gaps still remain, including the precise role of the different subtypes of cholinergic neurons as well as their pre- and postsynaptic partners. Improving our knowledge of the propriospinal cholinergic system is of major relevance to finding new cellular targets and therapeutics in countering the debilitating effects of neurodegenerative diseases and restoring motor functions after spinal cord injury.

The nAChR Chaperone TMEM35a (NACHO) Contributes to the Development of Hyperalgesia in Mice

Pain is a major health problem, affecting over fifty million adults in the US alone, with significant economic cost in medical care and lost productivity. Despite evidence implicating nicotinic acetylcholine receptors (nAChRs) in pathological pain, their specific contribution to pain processing in the spinal cord remains unclear given their presence in both neuronal and non-neuronal cell types. Here we investigated if loss of neuronal-specific TMEM35a (NACHO), a novel chaperone for functional expression of the homomeric α7 and assembly of the heteromeric α3, α4, and α6-containing nAChRs, modulates pain in mice. Mice with tmem35a deletion exhibited thermal hyperalgesia and mechanical allodynia. Intrathecal administration of nicotine and the α7-specific agonist, PHA543613, produced analgesic responses to noxious heat and mechanical stimuli in tmem35a KO mice, respectively, suggesting residual expression of these receptors or off-target effects. Since NACHO is expressed only in neurons, these findings indicate that neuronal α7 nAChR in the spinal cord contributes to heat nociception. To further determine the molecular basis underlying the pain phenotype, we analyzed the spinal cord transcriptome. Compared to WT control, the spinal cord of tmem35a KO mice exhibited 72 differentially-expressed genes (DEGs). These DEGs were mapped onto functional gene networks using the knowledge-based database, Ingenuity Pathway Analysis, and suggests increased neuroinflammation as a potential contributing factor for the hyperalgesia in tmem35a KO mice. Collectively, these findings implicate a heightened inflammatory response in the absence of neuronal NACHO activity. Additional studies are needed to determine the precise mechanism by which NACHO in the spinal cord modulates pain.

Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain

Chronic, pathological pain remains a global health problem and a challenge to basic and clinical sciences. A major obstacle to preventing, treating, or reverting chronic pain has been that the nature of neural circuits underlying the diverse components of the complex, multidimensional experience of pain is not well understood. Moreover, chronic pain involves diverse maladaptive plasticity processes, which have not been decoded mechanistically in terms of involvement of specific circuits and cause-effect relationships. This review aims to discuss recent advances in our understanding of circuit connectivity in the mammalian brain at the level of regional contributions and specific cell types in acute and chronic pain. A major focus is placed on functional dissection of sub-neocortical brain circuits using optogenetics, chemogenetics, and imaging technological tools in rodent models with a view towards decoding sensory, affective, and motivational-cognitive dimensions of pain. The review summarizes recent breakthroughs and insights on structure-function properties in nociceptive circuits and higher order sub-neocortical modulatory circuits involved in aversion, learning, reward, and mood and their modulation by endogenous GABAergic inhibition, noradrenergic, cholinergic, dopaminergic, serotonergic, and peptidergic pathways. The knowledge of neural circuits and their dynamic regulation via functional and structural plasticity will be beneficial towards designing and improving targeted therapies.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

The α9α10 Nicotinic Acetylcholine Receptor Antagonist αO-Conotoxin GeXIVA[1,2] Alleviates and Reverses Chemotherapy-Induced Neuropathic Pain

Oxaliplatin is a third-generation platinum drug and is widely used as a first-line therapy for the treatment of colorectal cancer (CRC). However, a large number of patients receiving oxaliplatin develop dose-limiting painful neuropathy. Here, we report that αO-conotoxin GeXIVA[1,2], a highly potent and selective antagonist of the α9α10 nicotinic acetylcholine receptor (nAChR) subtype, can relieve and reverse oxaliplatin-induced mechanical and cold allodynia after single and repeated intramuscular (IM) injections in rats. Treatments were started at 4 days post oxaliplatin injection when neuropathic pain emerged and continued for 8 and 16 days. Cold score and mechanical paw withdrawal threshold (PWT) were detected by the acetone test and von Frey test respectively. GeXIVA[1,2] significantly relieved mechanical and cold allodynia in oxaliplatin-treated rats after a single injection. After repeated treatments, GeXIVA[1,2] produced a cumulative analgesic effect without tolerance and promoted recovery from neuropathic pain. Moreover, the long lasting analgesic effect of GeXIVA[1,2] on mechanical allodynia continued until day 10 after the termination of the 16-day repeated treatment procedure. On the contrary, GeXIVA[1,2] did not affect acute mechanical and thermal pain behaviors in normal rats after repeated injections detected by the von Frey test and tail flick test. GeXIVA[1,2] had no influence on rat hind limb grip strength and body weight after repeated treatments. These results indicate that αO-conotoxin GeXIVA[1,2] could provide a novel strategy to treat chemotherapy-induced neuropathic pain.

Acupuncture-Analgesia-Mediated Alleviation of Central Sensitization

Pain can trigger central amplification called central sensitization, which ultimately results in hyperalgesia and/or allodynia. Many reports have showed acupuncture has an analgesic effect. We searched the related article on PubMed database and Cochrane database to discover central sensitization pathway in acupuncture analgesia. We summarized that acupuncture enhances the descending inhibitory effect and modulates the feeling of pain, thus modifying central sensitization. The possible mechanisms underlying the analgesic effects of acupuncture include segmental inhibition and the activation of the endogenous opioid, adrenergic, 5-hydroxytryptamine, and N-methyl-D-aspartic acid, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate pathways. Moreover, acupuncture can locally reduce the levels of inflammatory mediators. In clinical settings, acupuncture can be used to treat headache, neuropathic pain, low back pain, osteoarthritis, and irritable bowel syndrome. These mechanisms of acupuncture analgesia may be involved in the alleviation of central sensitization.

Nicotine and sleep deprivation: impact on pain sensitivity and immune modulation in rats

Repeated nicotine administration has been associated with increased paradoxical sleep in rats and antinociceptive properties, whereas paradoxical sleep deprivation (PSD) elicits pronociceptive and inflammatory responses. Thus, we aimed to evaluate the effect of repeated nicotine administration and its withdrawal combined with PSD on pain sensitivity and inflammatory markers. Sixty adult male Wistar rats were subjected to repeated injections of saline (SAL) or nicotine (NIC) for 12 days or 7 days of nicotine followed by acute mecamylamine administration on day 8 to precipitate nicotine abstinence (ABST). On day 9, the animals were submitted to PSD for 72 h or remained in control condition (CTRL); on day 12, thermal pain threshold was assessed by the hot plate test. PSD significantly decreased the latency to paw withdrawal in all groups compared to their respective controls. ABST-PSD animals presented higher levels of interleukin (IL)-6 compared to all groups, except ABST-CTRL. After adjustment for weight loss, IL-6, IL-4 and tumor necrosis factor alpha, ABST-PSD was associated with the lowest pain threshold. Nicotine and IL-4 levels were predictors of higher pain threshold. Hyperalgesia induced by PSD prevailed over the antinociceptive action of nicotine, while the association between PSD and ABST synergistically increased IL-6 concentrations and decreased pain threshold.

Hypersensitivity of Prelimbic Cortex Neurons Contributes to Aggravated Nociceptive Responses in Rats With Experience of Chronic Inflammatory Pain

Previous experience of chronic pain causes enhanced responses to upcoming noxious events in both humans and animals, but the underlying mechanisms remain unclear. In the present study, we found that rats with complete Freund's adjuvant (CFA)-induced chronic inflammatory pain experience exhibited aggravated pain responses to later formalin test. Enhanced neuronal activation upon formalin assaults and increased phosphorylated cAMP-response element binding protein (CREB) were observed in the prelimbic cortex (PL) of rats with chronic inflammatory pain experience, and inhibiting PL neuronal activities reversed the aggravated pain. Inflammatory pain experience induced persistent p38 mitogen-activated protein kinase (MAPK; p38) but not extracellular regulated protein kinase (ERK) or c-Jun N-terminal kinase (JNK) hyperphosphorylation in the PL. Inhibiting the p38 phosphorylation in PL reversed the aggravated nociceptive responses to formalin test and down-regulated enhanced phosphorylated CREB in the PL. Chemogenetics identified PL-periaqueductal gray (PAG) but not PL-nucleus accumbens (NAc) as a key pathway in inducing the aggravated formalin pain. Our results demonstrate that persistent hyperphosphorylation of p38 in the PL underlies aggravated nociceptive responses in rats with chronic inflammatory pain experience.