Impact of chronic nicotine on the development and maintenance of neuropathic hypersensitivity in the rat

Chronic nicotine exposure elicits pain hypersensitivity through activation of dopaminergic projections to anterior cingulate cortex

BACKGROUND

Cigarette smoking is commonly reported among chronic pain patients in the clinic. Although chronic nicotine exposure is directly linked to nociceptive hypersensitivity in rodents, underlying neurobiological mechanisms remain unknown.

METHODS

Multi-tetrode recordings in freely moving mice were used to test the activity of dopaminergic projections from the ventral tegmental area (VTA) to pyramidal neurones in the anterior cingulate cortex (ACC) in chronic nicotine-treated mice. The VTA→ACC dopaminergic pathway was inhibited by optogenetic manipulation to detect chronic nicotine-induced allodynia (pain attributable to a stimulus that does not normally provoke pain) assessed by von Frey monofilaments (force units in g).

RESULTS

Allodynia developed concurrently with chronic (28-day) nicotine exposure in mice (0.36 g [0.0141] vs 0.05 g [0.0018], P<0.0001). Chronic nicotine activated dopaminergic projections from the VTA to pyramidal neurones in the ACC, and optogenetic inhibition of VTA dopaminergic terminals in the ACC alleviated chronic nicotine-induced allodynia in mice (0.06 g [0.0064] vs 0.28 g [0.0428], P<0.0001). Moreover, optogenetic inhibition of Drd2 dopamine receptor signalling in the ACC attenuated nicotine-induced allodynia (0.07 g [0.0082] vs 0.27 g [0.0211], P<0.0001).

CONCLUSIONS

These findings implicate a role of Drd2-mediated dopaminergic VTA→ACC pathway signalling in chronic nicotine-elicited allodynia.

Exploring Cholinergic Compounds for Peripheral Neuropathic Pain Management: A Comprehensive Scoping Review of Rodent Model Studies

Neuropathic pain affects about 7-8% of the population, and its management still poses challenges with unmet needs. Over the past decades, researchers have explored the cholinergic system (muscarinic and nicotinic acetylcholine receptors: mAChR and nAChR) and compounds targeting these receptors as potential analgesics for neuropathic pain management. This scoping review aims to provide an overview of studies on peripheral neuropathic pain (PNP) in rodent models, exploring compounds targeting cholinergic neurotransmission. The inclusion criteria were original articles on PNP in rodent models that explored the use of compounds directly targeting cholinergic neurotransmission and reported results of nociceptive behavioral assays. The literature search was performed in the PubMed and Web of Science databases (1 January 2000-22 April 2023). The selection process yielded 82 publications, encompassing 62 compounds. The most studied compounds were agonists of α4β2 nAChR and α7 nAChR, and antagonists of α9/α10 nAChR, along with those increasing acetylcholine and targeting mAChRs. Studies mainly reported antinociceptive effects in traumatic PNP models, and to a lesser extent, chemotherapy-induced neuropathy or diabetic models. These preclinical studies underscore the considerable potential of cholinergic compounds in the management of PNP, warranting the initiation of clinical trials.

Outcomes of Surgical Treatment for Sural Neuritis: A Retrospective Case Series

BACKGROUND

Somatic nerve pain is one of the most common complications following surgery of the foot and ankle but may also arise following traumatic injury or chronic nerve compression. The sural nerve is a commonly affected nerve in the foot and ankle; it is at risk given the proximity to frequently used surgical approaches, exposure to crush injuries, and traction from severe ankle inversion injuries. The purpose of this study is to investigate the outcomes of sural nerve neurectomy with proximal implantation for sural neuromas (SN) and chronic sural neuritis (CSN).

METHODS

Patients that underwent neurectomy with proximal implantation (20 muscle, 1 adipose tissue) by 2 foot and ankle specialists for isolated SN- and CSN-related pain at a single tertiary institution were included. Demographic data, baseline outcomes including 36-Item Short Form Health Survey (SF-36), Foot and Ankle Ability Measure (FAAM), and visual analog scale (VAS) were recorded. Final follow-up questionnaires using Patient Reported Outcomes Measurement Information System (PROMIS) lower extremity function, pain interference (PI), and neuropathic pain quality, FAAM, and VAS were administered using REDCap. Perioperative factors including neuropathic medications, diagnostic injections, the use of collagen wraps, and perioperative ketamine were collected from the medical record. Descriptive statistics were performed and potential changes in patient-reported outcome measure scores were evaluated using Wilcoxon signed-rank tests.

RESULTS

The 21 patients meeting inclusion criteria for this study had a median age of 47 years (interquartile range [IQR], 43-49) and had median follow-up duration of 33.7 months (IQR, 4.5-47.6). Median FAAM activities of daily living score improved from 40.6 (38.7-50.7) preoperatively to 66.1 (53.6-83.3) postoperatively, P = .032. FAAM sports scores improved from 14.1 (7.8-21.9) to 41.1 (25.0-60.9) postoperatively, P = .002. VAS scores improved from a median of 9.0 (8.0-9.0) to 3.0 (3.0-6.0), P < .001. At final follow-up, patients reported PROMIS lower extremity function score median of 43.8 (35.6-54.9), PROMIS neuropathic pain quality score of 54.1 (43.6-61.6), and PROMIS PI of 57.7 (41.1-63.8). Patients with both anxiety and depression reported less improvement in pain and physical. Other perioperative factors lacked sufficient numbers for statistical analysis.

CONCLUSION

Sural nerve neurectomy and proximal implantation (20 muscle, 1 adipose) provided significant improvement in pain and function for patients with sural neuromas and chronic sural neuritis at median follow-up of 33.7 months. Anxiety and depression were associated with significantly poorer outcomes following surgery. Patients with CRPS as well as recent nicotine use tended to report less improvement in pain and worse function after surgery, although this sample size was too limited for statistical analysis of these variables. Further research is needed to identify the ideal surgical candidates and perioperative factors to optimize patient outcomes.

LEVEL OF EVIDENCE

Level IV, retrospective case series.

Microglia sustain anterior cingulate cortex neuronal hyperactivity in nicotine-induced pain

BACKGROUND

Long-term smoking is a risk factor for chronic pain, and chronic nicotine exposure induces pain-like effects in rodents. The anterior cingulate cortex (ACC) has been demonstrated to be associated with pain and substance abuse. This study aims to investigate whether ACC microglia are altered in response to chronic nicotine exposure and their interaction with ACC neurons and subsequent nicotine-induced allodynia in mice.

METHODS

We utilized a mouse model that was fed nicotine water for 28 days. Brain slices of the ACC were collected for morphological analysis to evaluate the impacts of chronic nicotine on microglia. In vivo calcium imaging and whole-cell patch clamp were used to record the excitability of ACC glutamatergic neurons.

RESULTS

Compared to the vehicle control, the branch endpoints and the length of ACC microglial processes decreased in nicotine-treated mice, coinciding with the hyperactivity of glutamatergic neurons in the ACC. Inhibition of ACC glutamatergic neurons alleviated nicotine-induced allodynia and reduced microglial activation. On the other hand, reactive microglia sustain ACC neuronal excitability in response to chronic nicotine, and pharmacological inhibition of microglia by minocycline or liposome-clodronate reduces nicotine-induced allodynia. The neuron-microglia interaction in chronic nicotine-induced allodynia is mediated by increased expression of neuronal CX3CL1, which activates microglia by acting on CX3CR1 receptors on microglial cells.

CONCLUSION

Together, these findings underlie a critical role of ACC microglia in the maintenance of ACC neuronal hyperactivity and resulting nociceptive hypersensitivity in chronic nicotine-treated mice.

Patient-Reported Outcomes following Surgical Treatment of Symptomatic Digital Neuromas

BACKGROUND

Many surgical techniques are used to treat symptomatic neuroma, but options are limited for digital neuromas because of a paucity of soft-tissue coverage and/or the absence of the terminal nerve end. The authors assessed factors that influence patient-reported outcomes after surgery for symptomatic digital neuroma.

METHODS

The authors retrospectively identified 29 patients with 33 symptomatic digital neuromas that were treated surgically. Patients completed the Patient-Reported Outcomes Measurement Information System (PROMIS) Upper Extremity and Pain Interference scales, a numeric rating scale for pain, and the PROMIS Depression scale at a median follow-up of 7.6 years postoperatively (range, 3.2 to 16.8 years). Surgical treatment for neuroma included excision with nerve repair/reconstruction (n =13; 39 percent), neuroma excision alone (n =10; 30 percent), and excision and implantation (n =10; 30 percent). Multivariable linear regression was performed to identify the factors that independently influenced patient-reported outcomes.

RESULTS

The mean postoperative PROMIS Upper Extremity score was 45.2 ± 11.2, the mean Pain Interference score was 54.3 ± 10.7, and the mean numeric rating scale pain score was 3 (interquartile range, 1 to 5). Compared with other treatment techniques, neuroma excision with nerve repair/reconstruction was associated with lower numeric rating scale pain scores; lower Pain Interference scores, corresponding to less daily impact of pain; and higher Upper Extremity scores, reflecting better upper extremity function. Older age and higher Depression scores were associated with lower Upper Extremity scores and higher Pain Interference scores. Smoking was associated with higher Pain Interference and numeric rating scale pain scores.

CONCLUSIONS

Neuroma excision followed by nerve repair/reconstruction resulted in better outcomes compared with neuroma excision alone with or without implantation. Patient age and psychosocial factors influenced patient-reported outcomes.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Risk, III.

Surgery for lower extremity symptomatic neuroma: Long-term outcomes

INTRODUCTION

Traumatic neuroma caused by injuries or surgery can result in neuropathic pain, functional impairment, and psychological distress, which has an impact on quality of life. The aim of this study was to identify the factors related to successful treatment of symptomatic lower extremity symptomatic neuromas using patient-reported outcome measures (PROMs).

METHODS

Thirty-two patients with 48 symptomatic neuromas completed the PROMIS mobility, PROMIS pain interference (PI), Numeric Rating Scale (NRS) for pain (0-10) for both pre- and post-operative pain, and the PROMIS depression at a mean of 8.9±4.5 years following neuroma surgery. Neuromas were located around the foot and ankle (n=18, 38%), leg (n=14, 29%), around the knee (n=13, 27%), and in the thigh (n=3, 6.3%). Surgical treatment included neuroma excision and implantation (n=29, 60%) followed by neuroma excision alone or excision with placement in the subcutaneous tissue (n=12, 25%). We performed multivariable analysis to identify the factors influencing the PROMs.

RESULTS

Patients reported significant reduction in mean NRS pain after surgery (7.3 vs 4.9, p=0.0013). Higher PROMIS depression scores were independently associated with inferior PROMIS mobility scores (β=-0.38, p=0.001), higher PROMIS PI scores (β=0.68, p<0.001), and higher NRS pain scores (β=0.1, p=0.001). Additionally, smoking was independently associated with higher NRS pain scores (β=1.59, p=0.049) CONCLUSION: Surgical treatment of symptomatic neuromas of the lower extremity provides a long-term improvement in 59% of patients, but 19% of patients still reported severe persistent pain despite surgical treatment. Smoking and negative mood have negative effects on patient-reported outcomes after neuroma surgery.

Relationship between pain outcomes and smoking history following video-assisted thoracic surgery for lobectomy: a retrospective study

Purpose

The relationship between chronic smoking history and postoperative pain remains controversial. This study aimed to elucidate this relationship in non-small cell lung cancer (NSCLC) patients who underwent video-assisted thoracic surgery (VATS) lobectomy.

Patients and methods

This retrospective observational study included NSCLC patients treated with VATS lobectomy between January 2011 and July 2017. Demographic and clinical information, including preoperative smoking history, was collected. The primary goal was to investigate the relationship between smoking history and postoperative pain outcomes (oral morphine equivalent [OME] consumption and pain score). Multivariate linear regression analysis was performed, and P<0.05 was considered as statistically significant.

Results

A total of 1,785 patients were included in the final analysis. Multivariate linear regression analysis revealed that total smoking amount (in packs), status as current smoker, and cessation time did not have an association with OME consumption (mg) or pain scores on postoperative days 0–2 (P>0.05). However, patients who had never smoked consumed less morphine equivalent analgesics (mg) on postoperative days 0–2 (coefficient: −17.48, 95% CI [−33.83, −1.13], P=0.036) compared to patients who had a history of smoking.

Conclusion

Patients who had never smoked had lower opioid analgesics consumption on the days immediately following surgery, while being a current smoker or the total amount of smoking in packs did not affect postoperative pain outcomes after VATS lung lobectomy.

Interleukin-17A Acts to Maintain Neuropathic Pain Through Activation of CaMKII/CREB Signaling in Spinal Neurons

Immunity and neuroinflammation play major roles in neuropathic pain. Spinal interleukin (IL)-17A, as a mediator connecting innate and adaptive immunity, has been shown to be an important cytokine in neuroinflammation and acute neuropathic pain. However, the effects and underlying mechanisms of spinal IL-17A in the maintenance of neuropathic pain remain unknown. This study was designed to investigate whether spinal IL-17A acted to maintain neuropathic pain and to elucidate the underlying mechanisms in IL-17A knockout or wild-type (WT) mice following L4 spinal nerve ligation (L4 SNL). WT mice were treated with anti-IL-17A neutralized monoclonal antibody (mAb) or recombinant IL-17A (rIL-17A). We showed that IL-17A levels were significantly increased 1, 3, 7, and 14 days after SNL in spinal cord. Double immunofluorescence staining showed that astrocytes were the major cellular source of spinal IL-17A. IL-17A knockout or anti-IL-17A mAb treatment significantly ameliorated hyperalgesia 7 days after SNL, which was associated with a significant reduction of p-CaMKII and p-CREB levels in spinal cord, whereas rIL-17A treatment conferred the opposite effects. Furthermore, we showed that blocking CaMKII with KN93 significantly reduced SNL- or rIL-17A-induced hyperalgesia and p-CREB expression. Our in vitro data showed that KN93 also significantly inhibited rIL-17A-induced CREB activation in primary cultured spinal neurons. Taken together, our study indicates that astrocytic IL-17A plays important roles in the maintenance of neuropathic pain through CaMKII/CREB signaling pathway in spinal cord, and thus targeting IL-17A may offer an attractive strategy for the treatment of chronic persistent neuropathic pain.

Nicotine stimulates expression of proteins implicated in peripheral and central sensitization

Pain patients who are nicotine dependent report a significantly increased incidence and severity of pain intensity. The goal of this study was to determine the effects of prolonged nicotine administration on inflammatory proteins implicated in the development of peripheral and central sensitization of the trigeminal system. Behavioral, immunohistochemical, and microarray studies were utilized to investigate the effects of nicotine administered daily for 14 days via an Alzet® osmotic pump in Sprague Dawley rats. Systemic nicotine administration caused a significant increase in nocifensive withdrawals to mechanical stimulation of trigeminal neurons. Nicotine stimulated expression of the pro-inflammatory signal transduction proteins phosphorylated-extracellular signal-regulated kinase (p-ERK), phosphorylated-c-Jun N-terminal kinase (p-JNK), and protein kinase A (PKA) in the spinal trigeminal nucleus. Nicotine also promoted elevations in the expression of glial fibrillary acidic protein (GFAP), a biomarker of activated astrocytes, and the microglia biomarker ionized calcium-binding adapter molecule 1 (Iba1). Similarly, levels of eleven cytokines were significantly elevated with the largest increase in expression of TNF-α. Levels of PKA, p-ERK, and p-JNK in trigeminal ganglion neurons were increased by nicotine. Our findings demonstrate that prolonged systemic administration of nicotine promotes sustained behavioral and cellular changes in the expression of key proteins in the spinal trigeminal nucleus and trigeminal ganglion implicated in the development and maintenance of peripheral and central sensitization.

Increased pain in response to mechanical or thermal stimulation in a rat model of incision-induced pain with nicotine dependence and withdrawal

The aim of this study was to observe the changes in mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) in a rat model of incisional pain with nicotine dependence and withdrawal. Twelve Wistar rats were randomly divided into a control and a withdrawal group, with 6 rats per group. In the control group, the rats were raised in normal conditions for 7 days without any treatment. A model of plantar incisional pain was established in the right lower extremity and changes in the plantar MWT and TWL of the healthy and operative sides were observed for 7 successive days. In the withdrawal group, the rats were raised in normal conditions and treated with a subcutaneous injection of pure nicotine (3 mg/kg), 3 times each day for 7 days. The model of plantar incisional pain in the right lower extremity was established, and changes in bilateral plantar MWT and TWL were observed for 7 days. The operative side plantar MWT and TWL in the withdrawal group were significantly lower than those in the control group on postoperative days 1–7, respectively (P<0.05). Compared with the healthy side in the control group, the healthy plantar MWT was significantly reduced on postoperative days 1–7 (P<0.05) and TWL was significantly decreased in postoperative days 1–6 (P<0.05) in the withdrawal group. The pain sensitivity to mechanical and thermal stimulation significantly increased in the rat model of incisional pain with nicotine dependence and withdrawal. This is consistent with the clinical increase of postoperative pain observed in patients after quitting smoking.

Surgical management of neuroma pain: a prospective follow-up study

Painful neuromas can cause severe loss of function and have great impact on the daily life of patients. Surgical management remains challenging; despite improving techniques, success rates are low. To accurately study the success of surgical neuroma treatment and factors predictive of outcome, a prospective follow-up study was performed. Between 2006 and 2009, pre- and post-operative questionnaires regarding pain (VAS, McGill), function (DASH), quality of life (SF-36), symptoms of psychopathology (SCL-90), epidemiologic determinants and other outcome factors were sent to patients surgically treated for upper extremity neuroma pain. Pain scores after diagnostic nerve blocks were documented at the outpatient clinic before surgery. Thirty-four patients were included, with an average follow up time of 22 months. The mean VAS score decreased from 6.8 to 4.9 after surgery (p<0.01), 19 (56%) of patients were satisfied with surgical results. Upper extremity function improved significantly (p=0.001). Neuroma patients had significantly lower quality of life compared to a normal population. Employment status, duration of pain and CRPS symptoms were found to be prognostic factors. VAS scores after diagnostic nerve block were predictive of post-operative VAS scores (p=0.001). Furthermore, smoking was significantly related to worse outcome (relative risk: 2.10). The results could lead to improved patient selection and treatment strategies. If a diagnostic nerve block is ineffective in relieving pain, patients will most likely not benefit from surgical treatment. Patients should be encouraged to focus on activity and employment instead of their symptoms. Smoking should be discouraged in patients who will undergo surgical neuroma treatment.

Alteration of primary afferent activity following inferior alveolar nerve transection in rats

Background

In order to evaluate the neural mechanisms underlying the abnormal facial pain that may develop following regeneration of the injured inferior alveolar nerve (IAN), the properties of the IAN innervated in the mental region were analyzed.

Results

Fluorogold (FG) injection into the mental region 14 days after IAN transection showed massive labeling of trigeminal ganglion (TG). The escape threshold to mechanical stimulation of the mental skin was significantly lower (i.e. mechanical allodynia) at 11-14 days after IAN transection than before surgery. The background activity, mechanically evoked responses and afterdischarges of IAN Aδ-fibers were significantly higher in IAN-transected rats than naive. The small/medium diameter TG neurons showed an increase in both tetrodotoxin (TTX)-resistant (TTX-R) and -sensitive (TTX-S) sodium currents (I_Na) and decrease in total potassium current, transient current (I_A) and sustained current (I_K) in IAN-transected rats. The amplitude, overshoot amplitude and number of action potentials evoked by the depolarizing pulses after 1 μM TTX administration in TG neurons were significantly higher, whereas the threshold current to elicit spikes was smaller in IAN-transected rats than naive. Resting membrane potential was significantly smaller in IAN-transected rats than that of naive.

Conclusions

These data suggest that the increase in both TTX-S I_Na and TTX-R I_Na and the decrease in I_A and I_k in small/medium TG neurons in IAN-transected rats are involved in the activation of spike generation, resulting in hyperexcitability of Aδ-IAN fibers innervating the mental region after IAN transection.

SIP30 is regulated by ERK in peripheral nerve injury-induced neuropathic pain

ERK plays an important role in chronic neuropathic pain. However, the underlying mechanism is largely unknown. Here we show that in chronic constriction injury-treated rat spinal cords, up-regulation of SIP30 (SNAP25-interacting protein 30), which is involved in the development and maintenance of chronic constriction injury-induced neuropathic pain, correlates with ERK activation and that the up-regulation of SIP30 is suppressed by intrathecal delivery of the MEK inhibitor U0126. In PC12 cells, up-regulation of SIP30 by nerve growth factor is also dependent on ERK activation. We found that there is an ERK-responsive region in the rat sip30 promoter. Activation of ERK promotes the recruitment of the transcription factor cyclic AMP-response element-binding protein to the sip30 gene promoter. Taken together, our results provide a potential downstream target of ERK activation-mediated neuropathic pain.

Increased spinal dynorphin contributes to chronic nicotine-induced mechanical hypersensitivity in the rat

Chronic nicotine administration has been shown previously to produce mechanical hypersensitivity in the rat although the mechanism of this effect is unknown. Rats treated with chronic systemic nicotine 3.6 or 8.6 mg/(kg day) for 14-21 days displayed mechanical hypersensitivity coincident with an increase of prodynorphin immunoreactivity and dynorphin content within the spinal cord. The administration of dynorphin antiserum intrathecally significantly attenuated chronic nicotine-induced mechanical hypersensitivity. Our results suggest that chronic nicotine administration produces an increase in spinal dynorphin content and release that contributes to mechanical hypersensitivity.

Impact of chronic nicotine on sciatic nerve injury in the rat

Chronic nicotine exposure and the immune response to peripheral nerve injury has not been investigated thoroughly. Rats were exposed to chronic nicotine or saline followed by chronic constriction injury (CCI) of the sciatic nerve. Mechanical sensitivity was measured at various time points and the immune response was investigated at 21 days post-CCI. Chronic nicotine increased mechanical hypersensitivity, microglia activation, and the production of IL-1beta, but not the number of immune cells at the site of injury. These results suggest that chronic nicotine increases mechanical hypersensitivity following peripheral nerve injury through a mechanism that may involve an increased production and release of central and peripheral cytokines.