Chronic postsurgical pain (CPSP): an underestimated problem after incisional hernia treatment.

BACKGROUND: Chronic postsurgical pain (CPSP) is a potential long-term problem following open incisional hernia repair which may affect the quality of life of patients despite successful anatomical repair of the hernia. The aim of this manuscript was to identify the incidence and outcome of patients following open incisional hernia repair in respect of risk factors to develop CPSP. METHODS: A single-center retrospective analysis of patients who underwent open incisional hernia repair between 2015 and 2021 was performed. Pre-existing conditions (e.g., diabetes mellitus and malignancy), hernia complexity, postoperative complications, and postoperative pain medication were analyzed using the local database. Quality of life and CPSP were assessed using the EuraHS Quality of Life (QoL) questionnaire. RESULTS: A total of 182 cases were retrospectively included in a detailed analysis based on the complete EuraHS (QoL) questionnaire. During the average follow-up period of 46 months, this long-term follow-up revealed a 54.4% incidence of CPSP and including a rate of 14.8% for severe CPSP (sCPSP) after open incisional hernia surgery. The complexity of the hernia and the demographic variables were not different between the group with and without CPSP. Patients with CPSP reported significantly reduced QoL. The analgesics score which includes the need of pain medication in the initial days after surgery was significantly higher in patients with CPSP than in those without (no CPSP: 2.86 vs. CPSP: 3.35; p = 0.047). CONCLUSION: The presence of CPSP after open incisional hernia repair represents a frequent and underestimated long-term problem which has been not been recognized to this extent before. CPSP impairs QoL in these patients. Patients at risk to develop CPSP can be identified in the perioperative setting by the need of high doses of pain medication using the analgesics score. Possibly timely adjustment of pain medication, even in the domestic setting, could alleviate the chronicity or severity of CPSP.

Postoperative Analgesics Score as a Predictor of Chronic Postoperative Inguinal Pain After Inguinal Hernia Repair: Lessons Learned From a Retrospective Analysis.

BACKGROUND: Chronic postoperative inguinal pain (CPIP) is a common complication after inguinal hernia surgery and occurs in up to 10-14% of cases. CPIP has a significant impact on daily life, work ability and thus compromises quality of life. The aim of this retrospective study was an in-depth analysis of patients undergoing inguinal hernia repair to further refine the prediction of the onset of CPIP reliably. METHODS: A single center retrospective analysis of patients with who underwent open or minimally invasive inguinal hernia repair from 2016 to 2021 was carried out. Complication rates, detailed analysis of postoperative pain medication and quality of life using the EuraHS Quality of Life questionnaire were assessed. RESULTS: Out of 596 consecutive procedures, 344 patients were included in detailed analyses. While patient cohorts were different in terms of age and co-morbidities, and the prevalence of CPIP was 12.2% without differences between the surgical procedures (Lichtenstein: 12.8%; TEP 10.9%; TAPP 13.5%). Postoperative pain was evaluated using a newly developed analgesic score. Patients who developed CPIP later had a significant higher consumption of analgesics at discharge (p = 0.016). As additional risk factors for CPIP younger patient age and postoperative complications were identified. CONCLUSION: The prospective use of the analgesic score established here could be helpful to identify patients that are at risk to develop CPIP. These patients could benefit from a structured follow-up to allow early therapeutic intervention to prevent chronification and restore the quality of life.

[Chronic pain in elderly people during the COVID-19 pandemic]. / Chronische Schmerzen im Seniorenalter vor dem Hintergrund der COVID-19-Pandemie.

BACKGROUND: International studies have shown negative effects of the COVID-19 pandemic on mood and levels of distress. Correlations between the pandemic and higher levels of pain as well as greater pain-related disability have also been found; however, studies report ambiguous results about whether elderly people cope differently with the pandemic and its effects. METHODS: The University Hospital of Würzburg offers multimodal pain therapy for older adults. The current study performed a retrospective analysis of routine data measured during an interdisciplinary multimodal assessment. We compared n = 75 patients taking part in the therapy during 2018 and 2019 to n = 42 patients assessed in 2020-2021. We measured pain, mental distress and physical functioning using the German Pain Questionnaire, clinical diagnosis, and geriatric tests of physical fitness. RESULTS: Both subgroups did not differ in demographic characteristics, neither did we find significant differences regarding pain intensity, pain-related disability, and mental health; however, patients before the pandemic reported a higher number of days on which they felt limited due to pain. In the physical performance test, we even found significantly better results during the COVID-19 pandemic. DISCUSSION: The current data do not support an aggravation of pain or mental and physical well-being. Possible explanations could be better resilience in elderly people due to their experience of life, financial security or less change in their daily life.

Characteristics of the nerve barrier and the blood dorsal root ganglion barrier in health and disease.

A variety of barriers ensures the protection of the peripheral nervous system from noxious blood-borne or surrounding stimuli. In this review, anatomy and functioning of the blood nerve barrier (BNB) and the blood DRG barrier (BDB) will be presented and key tight junction proteins described: ZO-1, claudin-1, -3, -5, -11, -12, -19, occludin, and tricellulin. Different diseases can lead to or be accompanied by nerve barrier disruption; impairment of nerve barriers in turn worsens pathology. Peripheral nerve injury, diabetic neuropathy and inflammatory polyneuropathy cause an increased permeability of BNB and BDB. Knowledge and understanding of these mechanisms might ultimately lead to the invention of drugs to control barrier function and help ameliorating neurological diseases.

[Recommendations for the perioperative use of dipyrone : Expert recommendation of the working group on acute pain of the German Pain Society, the scientific working group on pain medicine of the German Society for Anesthesiology and Intensive Care Medicine and the surgical working group on acute pain of the German Society for Surgery with participation of representatives of the Drug Commission of the German Medical Association]. / Empfehlungen zur perioperativen Anwendung von Metamizol : Expertenempfehlung des Arbeitskreises Akutschmerz der Deutschen Schmerzgesellschaft, des Wissenschaftlichen Arbeitskreises Schmerzmedizin der Deutschen Gesellschaft für Anästhesiologie und Intensivmedizin und der Chirurgischen Arbeitsgemeinschaft Akutschmerz der Deutschen Gesellschaft für Chirurgie unter Beteiligung von Vertretern der Arzneimittelkommission der deutschen Ärzteschaft.

BACKGROUND: Dipyrone (metamizole) is widely used for perioperative pain management in countries where it is marketed; however, uncertainty exists concerning the safe use of this drug, specifically considering the rare adverse event of an agranulocytosis. METHODS: As evidence from published studies was lacking, an expert panel developed recommendations for the perioperative use of dipyrone. After a formal, structured consensus process, the recommendations were approved by the involved medical societies. RESULTS: The panel agreed that blood cell counts shall not be standard for short-term perioperative use in patients unless they are at risk for neutropenia. The medical staff shall be aware of the symptoms and course of action when agranulocytosis is suspected. Patients shall be informed about the risks and benefits of dipyrone and about potential alternatives. The expert group concluded that dipyrone has a relatively positive risk-benefit ratio compared to other nonopioid analgesics. The group strongly recommended educating patients about the symptoms of agranulocytosis if they have received dipyrone over several days and/or treatment is to be continued after discharge, because agranulocytosis can occur several days after discontinuation of metamizole. Further recommendations refer to the information of the physician taking over the patient's care after discharge and the avoidance of re-exposure in patients having previously suffered from dipyrone-induced agranulocytosis. CONCLUSION: The group's recommendations shall be communicated in order to raise medical staff's and patients' awareness of the appropriate use of dipyrone in the perioperative period.

[Recommendations for the perioperative use of dipyrone : Expert recommendation of the working group on acute pain of the German Pain Society, the scientific working group on pain medicine of the German Society for Anesthesiology and Intensive Care Medicine and the surgical working group on acute pain of the German Society for Surgery with participation of representatives of the Drug Commission of the German Medical Association]. / Empfehlungen zur perioperativen Anwendung von Metamizol : Expertenempfehlung des Arbeitskreises Akutschmerz der Deutschen Schmerzgesellschaft, des Wissenschaftlichen Arbeitskreises Schmerzmedizin der Deutschen Gesellschaft für Anästhesiologie und Intensivmedizin und der Chirurgischen Arbeitsgemeinschaft Akutschmerz der Deutschen Gesellschaft für Chirurgie unter Beteiligung von Vertretern der Arzneimittelkommission der deutschen Ärzteschaft.

BACKGROUND: Dipyrone (metamizole) is widely used for perioperative pain management in countries where it is marketed; however, uncertainty exists concerning the safe use of this drug, specifically considering the rare adverse event of an agranulocytosis. METHODS: As evidence from published studies was lacking, an expert panel developed recommendations for the perioperative use of dipyrone. After a formal, structured consensus process, the recommendations were approved by the involved medical societies. RESULTS: The panel agreed that blood cell counts shall not be standard for short-term perioperative use in patients unless they are at risk for neutropenia. The medical staff shall be aware of the symptoms and course of action when agranulocytosis is suspected. Patients shall be informed about the risks and benefits of dipyrone and about potential alternatives. The expert group concluded that dipyrone has a relatively positive risk-benefit ratio compared to other nonopioid analgesics. The group strongly recommended educating patients about the symptoms of agranulocytosis if they have received dipyrone over several days and/or treatment is to be continued after discharge, because agranulocytosis can occur several days after discontinuation of metamizole. Further recommendations refer to the information of the physician taking over the patient's care after discharge and the avoidance of re-exposure in patients having previously suffered from dipyrone-induced agranulocytosis. CONCLUSION: The group's recommendations shall be communicated in order to raise medical staff's and patients' awareness of the appropriate use of dipyrone in the perioperative period.

[Recommendations for the perioperative use of dipyrone : Expert recommendation of the working group on acute pain of the German Pain Society, the scientific working group on pain medicine of the German Society for Anesthesiology and Intensive Care Medicine and the surgical working group on acute pain of the German Society for Surgery with participation of representatives of the Drug Commission of the German Medical Association]. / Empfehlungen zur perioperativen Anwendung von Metamizol : Expertenempfehlung des Arbeitskreises Akutschmerz der Deutschen Schmerzgesellschaft, des Wissenschaftlichen Arbeitskreises Schmerzmedizin der Deutschen Gesellschaft für Anästhesiologie und Intensivmedizin und der Chirurgischen Arbeitsgemeinschaft Akutschmerz der Deutschen Gesellschaft für Chirurgie unter Beteiligung von Vertretern der Arzneimittelkommission der deutschen Ärzteschaft.

BACKGROUND: Dipyrone (metamizole) is widely used for perioperative pain management in countries where it is marketed; however, uncertainty exists concerning the safe use of this drug, specifically considering the rare adverse event of an agranulocytosis. METHODS: As evidence from published studies was lacking, an expert panel developed recommendations for the perioperative use of dipyrone. After a formal, structured consensus process, the recommendations were approved by the involved medical societies. RESULTS: The panel agreed that blood cell counts shall not be standard for short-term perioperative use in patients unless they are at risk for neutropenia. The medical staff shall be aware of the symptoms and course of action when agranulocytosis is suspected. Patients shall be informed about the risks and benefits of dipyrone and about potential alternatives. The expert group concluded that dipyrone has a relatively positive risk-benefit ratio compared to other nonopioid analgesics. The group strongly recommended educating patients about the symptoms of agranulocytosis if they have received dipyrone over several days and/or treatment is to be continued after discharge, because agranulocytosis can occur several days after discontinuation of metamizole. Further recommendations refer to the information of the physician taking over the patient's care after discharge and the avoidance of re-exposure in patients having previously suffered from dipyrone-induced agranulocytosis. CONCLUSION: The group's recommendations shall be communicated in order to raise medical staff's and patients' awareness of the appropriate use of dipyrone in the perioperative period.

[Epigenetics : Important aspects for anesthesiologists, pain and intensive care physicians]. / Epigenetik : Wichtiges für Anästhesisten, Schmerz- und Intensivmediziner.

Epigenetics, i.e. an altered reading of the genome without altering the genes themselves is a growing scientific field. A distinction is made between changes in the DNA by modification of the histones and non-coding RNA that alter the messenger (m)RNAs. Epigenetic modifications can be triggered by personal circumstances or other external factors and therefore influence the occurrence of diseases. Epigenetics are therefore of particular interest to anesthesiologists, pain specialists and intensive care physicians, as anesthetic drugs may have a long-term influence on protein transcription leading for example to alterations in neurocognition after anesthesia, chronification of postoperative pain and immune response in sepsis. Non-coding microRNAs known to be altered in a variety of perioperatively relevant diseases e. g. heart infarct, might serve as prognostic factors of perioperative outcome. Moreover, there are ways to influence epigenetic changes through life style and certain medications. In this review article, examples of anesthesia, intensive care and pain medicine-relevant diseases and the influence of epigenetics on them are presented.

Barrier function in the peripheral and central nervous system-a review.

The peripheral (PNS) and central nervous system (CNS) are delicate structures, highly sensitive to homeostatic changes-and crucial for basic vital functions. Thus, a selection of barriers ensures the protection of the nervous system from noxious blood-borne or surrounding stimuli. In this chapter, anatomy and functioning of the blood-nerve (BNB), the blood-brain (BBB), and the blood-spinal cord barriers (BSCB) are presented and the key tight junction (TJ) proteins described: claudin-1, claudin-3, claudin-5, claudin-11, claudin-12, claudin-19, occludin, Zona occludens-1 (ZO-1), and tricellulin are by now identified as relevant for nerval barriers. Different diseases can lead to or be accompanied by neural barrier disruption, and impairment of these barriers worsens pathology. Peripheral nerve injury and inflammatory polyneuropathy cause an increased permeability of BNB as well as BSCB, while, e.g., diseases of the CNS such as amyotrophic lateral sclerosis, multiple sclerosis, spinal cord injury, or Alzheimer's disease can progress and worsen through barrier dysfunction. Moreover, the complex role and regulation of the BBB after ischemic stroke is described. On the other side, PNS and CNS barriers hamper the delivery of drugs in diseases when the barrier is intact, e.g., in certain neurodegenerative diseases or inflammatory pain. Understanding of the barrier - regulating processes has already lead to the discovery of new molecules as drug enhancers. In summary, the knowledge of all of these mechanisms might ultimately lead to the invention of drugs to control barrier function to help ameliorating or curing neurological diseases.

Differential transcriptional profiling of damaged and intact adjacent dorsal root ganglia neurons in neuropathic pain.

Neuropathic pain, caused by a lesion in the somatosensory system, is a severely impairing mostly chronic disease. While its underlying molecular mechanisms are not thoroughly understood, neuroimmune interactions as well as changes in the pain pathway such as sensitization of nociceptors have been implicated. It has been shown that not only are different cell types involved in generation and maintenance of neuropathic pain, like neurons, immune and glial cells, but, also, intact adjacent neurons are relevant to the process. Here, we describe an experimental approach to discriminate damaged from intact adjacent neurons in the same dorsal root ganglion (DRG) using differential fluorescent neuronal labelling and fluorescence-activated cell sorting (FACS). Two fluorescent tracers, Fluoroemerald (FE) and 1-dioctadecyl-3,3,3,3-tetramethylindocarbocyanine perchlorate (DiI), were used, whose properties allow us to distinguish between damaged and intact neurons. Subsequent sorting permitted transcriptional analysis of both groups. Results and qPCR validation show a strong regulation in damaged neurons versus contralateral controls as well as a moderate regulation in adjacent neurons. Data for damaged neurons reveal an mRNA expression pattern consistent with established upregulated genes like galanin, which supports our approach. Moreover, novel genes were found strongly regulated such as corticotropin-releasing hormone (CRH), providing novel targets for further research. Differential fluorescent neuronal labelling and sorting allows for a clear distinction between primarily damaged neuropathic neurons and "bystanders," thereby facilitating a more detailed understanding of their respective roles in neuropathic processes in the DRG.

Long-term antinociception by electroacupuncture is mediated via peripheral opioid receptors in free-moving rats with inflammatory hyperalgesia.

BACKGROUND: Electroacupuncture (EA) has been widely accepted and applied as an important acupuncture-related technique for acupuncture analgesia (AA) research. The involvement of opioid peptides and receptors in acute AA has been shown via pre-EA application of opioid receptor/peptide antagonists. In this study, we intended to reproducibly institute acupoint position and needling excluding influences from anaesthesia or restrainers on rats with complete Freund's adjuvant (CFA) hind paw inflammatory pain, as well as to explore opioid-dependency and anti-inflammatory effects in sustained acupuncture analgesia. METHODS: Accurate position and needling approach on acupoint GB30 was modelled by computer-based three-dimensional (3D) images and followed by an optimal EA treatment protocol (100 Hz, 2-3 mA, 20 min) at 0 and 24 h post-CFA in conscious free-moving rats. Opioid receptor antagonists, naloxone (NLX) and naltrindole (NTI) were applied intraplantarly post-EA at late phase (96 h) of CFA. Nociceptive thresholds were assessed by paw pressure threshold (Randall-Sellito) or paw withdrawal latency (Hargreaves), and anti-inflammatory effects were evaluated by measurement of plantar temperature and paw volume. RESULTS: EA elicited significant sustained mechanical and thermal antinociception up to 144 h. Mechanical antinociception of EA was suppressed by peripheral intraplantar application of NLX and NTI. EA also reduced paw temperature and volume during the same time frame indicating anti-inflammatory effects. CONCLUSIONS: By employing a reproducible EA treatment model on GB30 in free-moving rats, we demonstrated the involvement of peripheral opioid receptors mediated EA-induced long-term antinociception. Future studies should examine the specific neuroimmunological connection of EA-induced sustained antinociception in inflammation.

Modulation of tight junction proteins in the perineurium for regional pain control.

Peripheral neurons are surrounded by the perineurium that forms the blood-nerve barrier and protects the nerve. Although the barrier serves as protection, it also hampers drug delivery of analgesic drugs to the peripheral nerve. We previously showed that opening of the barrier using hypertonic solutions facilitates drug delivery, for example, of hydrophilic opioids, which selectively target nociceptors. The perineurial barrier is formed by tight junction proteins, including claudin-1, claudin-5, and occludin. Under pathophysiological conditions such as nerve crush injury, the perineurial barrier is opened and tight junction proteins are no longer present. After several days, tight junction proteins reappear and the barrier reseals. Similarly, perineurial injection of hypertonic saline transiently opens the barrier, claudin-1 disappears, and hydrophilic analgesic drugs are effective. In the future, these findings could be used to reseal the barrier breakdown and could be applied to other barriers like the blood-brain or the intestinal mucosal barrier.

Recruitment of opioid peptide-containing neutrophils is independent of formyl peptide receptors.

In complete Freund's adjuvants (CFA) inflammation opioid containing neutrophils release opioid peptides upon stimulation and mediate peripheral analgesia. Neutrophil migration is regulated partially by chemokines, but other mediators e.g. formyl peptides could also contribute. In vitro, formyl peptides but not Mycobacterium butyricum (CFA component) induced migration of neutrophils. In contrast, local formyl peptide injection did not induce leukocyte recruitment in vivo due to insufficient up-regulation of adhesion molecule expression. Furthermore, leukocyte recruitment and peripheral opioid-mediated analgesia were unaffected by systemic formyl peptide receptor blockade in CFA inflammation. Thus, while formyl peptides do not regulate migration they directly stimulate opioid peptide release.

[What can we learn from the Scott Reuben case? Scientific misconduct in anaesthesiology]. / Was lernen wir aus dem Fall Scott Reuben? Wissenschaftliches Fehlverhalten in der Anästhesiologie.

In February 2009 a major case of scientific misconduct was discovered. The American pain researcher Dr. S. Reuben had published 21 papers over a period of 15 years that were found to be fraudulent. Suddenly many advances in postoperative pain therapy which had been assumed to be correct seemed questionable. In this review article the lessons which can be learnt from this case are described. This review also reveals that it is almost impossible for reviewers or readers of scientific journals to detect scientific fraud. However, several warning signs can be identified that might be useful when reading clinical papers. In retrospect many of these signs were detectable in Reuben's studies. Based on the fraudulent papers of Reuben it will be shown how and to what extent falsified results can affect other types of literature, such as practice guidelines, meta-analyses, review articles and oral presentations.

Antinociception by neutrophil-derived opioid peptides in noninflamed tissue--role of hypertonicity and the perineurium.

Inflammatory pain can be controlled by intraplantar opioid injection or by secretion of endogenous opioid peptides from leukocytes in inflamed rat paws. Antinociception requires binding of opioid peptides to opioid receptors on peripheral sensory nerve terminals. In the absence of inflammation, hydrophilic opioid peptides do not penetrate the perineurial barrier and, thus, do not elicit antinociception. This study was designed to examine the conditions under which endogenous, neutrophil-derived hydrophilic opioid peptides (i.e. Met-Enkephalin and beta-endorphin) can raise nociceptive thresholds in noninflamed tissue in rats. Intraplantar injection of the chemokine CXCL2/3 (macrophage inflammatory protein-2) induced selective neutrophil recruitment without overt signs of inflammation or changes in mechanical nociceptive thresholds (paw pressure threshold). Following intraplantar injection of hypertonic saline, the perineurial barrier was permeable for hours and intraplantar injection of opioid peptides increased mechanical nociceptive thresholds. While formyl-Met-Leu-Phe (fMLP) triggered opioid peptide release from neutrophils in vitro, nociceptive thresholds were unchanged in vivo. In vitro, hypertonicity interfered with fMLP-induced p38 mitogen activated kinase (MAPK) phosphorylation and opioid peptide release from neutrophils. These inhibitory effects were fully reversible by washout. In vivo, return to normotonicity occurred within 30min while the perineurium remained permeable for hours. Under these conditions, fMLP triggered MAPK phosphorylation and induced opioid peptide-mediated increases in nociceptive thresholds in the noninflamed paw. Taken together, antinociception mediated by endogenous opioids in noninflamed tissue has two important requirements: (i) opening of the perineurial barrier for opioid peptide access and (ii) opioid peptide release from neutrophils involving p38 MAPK.

[Opioid-induced immunosuppression. A clinically relevant problem?]. / Opioidinduzierte Immunsuppression. Ein klinisch relevantes Problem?

Several in vitro and animal studies have demonstrated the immunosuppressive effects of opioids and an increased risk of infection. The clinical relevance of these findings is unclear. In this review the relevant animal and human studies on the relationship of opioid use and risk of infection are summarized. The areas of retroviral infections (i.e. human immunodeficiency virus, HIV), sepsis and pneumonia, postoperative and chronic pain therapy are covered. In the majority of animal studies an increased risk of infection was demonstrated but in human studies these findings were contradictory. However, these studies were frequently underpowered because they involved small patient collectives and do not reflect the standards of evidence-based medicine. In summary, a causal relationship between opioid therapy and an increased risk of infection could neither be conclusively demonstrated nor fully excluded.

The other side of the medal: how chemokines promote analgesia.

Chemokines are chemotactic mediators controlling cell trafficking under physiological and pathological conditions. Chemokines are not only important under various inflammatory conditions but also play a role in pain and analgesia. While many studies examined the hyperalgesic action of chemokines, recent evidence also points towards antinociceptive effects of chemokines. Such effects are indirect by recruitment of opioid containing leukocytes and stimulation of release of opioid peptides. Opioid peptides then bind to opioid receptors on peripheral sensory neurons eliciting potent analgesia. This review focuses on the analgesic role of chemokines in the periphery under inflammatory and non-inflammatory conditions.

Pain and the immune system.

In inflammation, leucocytes containing opioid peptides migrate into the tissue. Opioid peptides can be released and bind to opioid receptors on peripheral nerve terminals, which counteracts inflammatory pain. Migration of opioid peptide-containing leucocytes is controlled by chemokines and adhesion molecules. Neurokinins, such as, substance P also contribute to the recruitment of these cells. Opioid peptide release from granulocytes can be stimulated by chemokines, such as, CXCR2 ligands. The release is dependent on intracellular calcium and activation of phosphoinositol-3 kinase and p38 mitogen activated kinase. Endogenous opioid peptides produced by leucocytes not only confer analgesia but recent evidence supports the concept that they also prevent the development of tolerance at peripheral opioid receptors. This review presents the discoveries that led to the concept of analgesia produced by immune-derived opioids.

Interleukin-1 beta contributes to the upregulation of kappa opioid receptor mrna in dorsal root ganglia in response to peripheral inflammation.

During local painful inflammation, axonal transport of opioid receptors from dorsal root ganglia toward the periphery is increased, associated with a higher receptor density and enhanced efficacy of opioid analgesics at the injured site. To examine whether this increase is related to transcription, mRNA of the kappa opioid receptor in lumbar dorsal root ganglia was quantified by real time light cycler polymerase chain reaction. In dorsal root ganglia of naive rats, kappa opioid receptor mRNA expression was three-fold higher than previously shown for delta opioid receptor and two times lower than mu opioid receptor mRNA, respectively. After induction of unilateral paw inflammation by Freund's complete adjuvant, kappa opioid receptor mRNA was significantly upregulated with a peak at 12 h in ipsilateral dorsal root ganglia. This effect could be mimicked by intraplantar injection of the proinflammatory cytokine interleukin-1 beta. Kappa opioid receptor mRNA upregulation lasted longer in interleukin-1 beta-treated rats compared with Freund's complete adjuvant-treated rats. Furthermore, a significant increase in kappa opioid receptor positive neurons was detected by immunohistochemistry 24 h after local injection of Freund's complete adjuvant or interleukin-1 beta. In Freund's complete adjuvant-induced inflammation, kappa opioid receptor upregulation was blocked by treatment with interleukin-1 receptor antagonist without changing the leukocyte infiltration in the paw. In conclusion, kappa opioid receptor mRNA and protein in dorsal root ganglia are upregulated in response to peripheral inflammation. This effect can be mimicked by a single local injection of interleukin-1 beta, and Freund's complete adjuvant-induced upregulation in kappa opioid receptor mRNA and protein can be prevented by treatment with interleukin-1 receptor antagonist. These data suggest that the peripheral production of the proinflammatory cytokine interleukin-1 beta is a specific inducer of kappa opioid receptor expression in the dorsal root ganglia.

Leukocytes in the regulation of pain and analgesia.

When tissue is destroyed or invaded by leukocytes in inflammation, numerous mediators are delivered by the circulation and/or liberated from resident and immigrated cells at the site. Proalgesic mediators include proinflammatory cytokines, chemokines, protons, nerve growth factor, and prostaglandins, which are produced by invading leukocytes or by resident cells. Less well known is that analgesic mediators, which counteract pain, are also produced in inflamed tissues. These include anti-inflammatory cytokines and opioid peptides. Interactions between leukocyte-derived opioid peptides and opioid receptors can lead to potent, clinically relevant inhibition of pain (analgesia). Opioid receptors are present on peripheral endings of sensory neurons. Opioid peptides are synthesized in circulating leukocytes, which migrate to inflamed tissues directed by chemokines and adhesion molecules. Under stressful conditions or in response to releasing agents (e.g., corticotropin-releasing factor, cytokines, noradrenaline), leukocytes can secrete opioids. They activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of excitatory neuropeptides. This review presents discoveries that led to the concepts of pain generation by mediators secreted from leukocytes and of analgesia by immune-derived opioids.