Opioid Receptors and Their Ligands in the Musculoskeletal System and Relevance for Pain Control

Peripheralization Strategies Applied to Morphinans and Implications for Improved Treatment of Pain

Opioids are considered the most effective analgesics for the treatment of moderate to severe acute and chronic pain. However, the inadequate benefit/risk ratio of currently available opioids, together with the current 'opioid crisis', warrant consideration on new opioid analgesic discovery strategies. Targeting peripheral opioid receptors as effective means of treating pain and avoiding the centrally mediated side effects represents a research area of substantial and continuous attention. Among clinically used analgesics, opioids from the class of morphinans (i.e., morphine and structurally related analogues) are of utmost clinical importance as analgesic drugs activating the mu-opioid receptor. In this review, we focus on peripheralization strategies applied to N-methylmorphinans to limit their ability to cross the blood-brain barrier, thus minimizing central exposure and the associated undesired side effects. Chemical modifications to the morphinan scaffold to increase hydrophilicity of known and new opioids, and nanocarrier-based approaches to selectively deliver opioids, such as morphine, to the peripheral tissue are discussed. The preclinical and clinical research activities have allowed for the characterization of a variety of compounds that show low central nervous system penetration, and therefore an improved side effect profile, yet maintaining the desired opioid-related antinociceptive activity. Such peripheral opioid analgesics may represent alternatives to presently available drugs for an efficient and safer pain therapy.

Periarticular Injection in Total Knee Arthroplasty

Optimal pain management after total knee arthroplasty (TKA) can expedite postoperative recovery, improve perioperative outcomes, and increase patient satisfaction. Periarticular injections (PAIs) have become increasingly used to improve pain management after TKA. Similar to peripheral nerve blocks, the use of intraoperative PAIs can lower pain scores and expedite discharge from the hospital. However, there is notable variability in the ingredients and administration techniques of PAIs. Currently, no standard of care exists for PAIs, especially in the setting of adjuvant peripheral nerve blocks. This study seeks to evaluate the ingredients, administration techniques, and outcomes of PAIs used during TKA.

Surgeon-Performed Intraoperative Peripheral Nerve Blocks and Periarticular Infiltration During Total Hip and Knee Arthroplasty: A Critical Analysis Review

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Surgeon-performed intraoperative peripheral nerve blocks may improve operating room efficiency and reduce hospital resource utilization and, ultimately, costs. Additionally, these blocks can be safely performed intraoperatively by most orthopaedic surgeons, while only specifically trained physicians are able to perform ultrasound-guided peripheral nerve blocks.

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IPACK (infiltration between the popliteal artery and capsule of the knee) blocks are at least noninferior to periarticular infiltration when combined with an adductor canal block for analgesia following total knee arthroplasty.

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Surgeon-performed intraoperative adductor canal blocks are technically feasible and offer reliable anesthesia comparable with ultrasound-guided blocks performed by anesthesiologists. While clinical studies have shown promising results, additional Level-I studies are required.

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A surgeon-performed intraoperative psoas compartment block has been described as a readily available and safe technique, although there is some concern for femoral nerve analgesia, and temporary sensory changes have been reported.

GIRK3 deletion facilitates kappa opioid signaling in chondrocytes, delays vascularization and promotes bone lengthening in mice

Long bones are formed and repaired through the process of endochondral ossification. Activation of G protein-coupled receptor (GPCR) signaling pathways is crucial for skeletal development and long bone growth. G protein-gated inwardly-rectifying K+ (GIRK) channel genes are key functional components and effectors of GPCR signaling pathways in excitable cells of the heart and brain, but their roles in non-excitable cells that directly contribute to endochondral bone formation have not been studied. In this study, we analyzed skeletal phenotypes of Girk2-/-, Girk3-/- and Girk2/3-/- mice. Bones from 12-week-old Girk2-/- mice were normal in length, but femurs and tibiae from Girk3-/- and Girk2/3-/- mice were longer than age-matched controls at 12-weeks-old. Epiphyseal chondrocytes from 5-day-old Girk3-/- mice expressed higher levels of genes involved in collagen chain trimerization and collagen fibril assembly, lower levels of genes encoding VEGF receptors, and produced larger micromasses than wildtype chondrocytes in vitro. Girk3-/- chondrocytes were also more responsive to the kappa opioid receptor (KOR) ligand dynorphin, as evidenced by greater pCREB expression, greater cAMP and GAG production, and upregulation of Col2a1 and Sox9 transcripts. Imaging studies showed that Kdr (Vegfr2) and endomucin expression was dramatically reduced in bones from young Girk3-/- mice, supporting a role for delayed vasculogenesis and extended postnatal endochondral bone growth. Together these data indicate that GIRK3 controls several processes involved in bone lengthening.

The Efficacy of Moxibustion on the Serum Levels of CXCL1 and β-EP in Patients with Rheumatoid Arthritis

Objective

This study aims to evaluate the efficacy of moxibustion on joint swelling and pain and the levels of C-X-C motif chemokine ligand 1 (CXCL1), β-endorphin (β-EP) in serum of rheumatoid arthritis (RA) patients and to investigate the anti-inflammatory and analgesic mechanism of moxibustion on improving RA.

Methods

Sixty-eight patients with RA were randomly and equally classified into the control and treatment groups. The control group was treated with routine drug therapy, while the treatment group received routine drug therapy and moxibustion. Both groups were treated for eight weeks. The symptoms and laboratory indicators of RA patients were compared in the two groups before and after intervention.

Results

Sixty-one patients completed the study: four patients dropped out from the treatment group and three from the control group. Trial endpoints were change (∆) in symptoms, measured by Ritchie's articular index (RAI), swollen joint count (SJC), and laboratory indicators, measured by the level of CXCL1, β-EP, tumor necrosis factor-a (TNF-α), and interleukin-1β (IL-1β). ∆RAI, ∆SJC, ∆CXCL1, ∆β-EP, ∆TNF-α, and ∆IL-1β in the treatment group were superior to the control group (13.50 [14.50] versus 6.00 [13.00] in ∆RAI, 4.00 [3.00] versus 2.00 [4.00] in ∆SJC, 0.04 ± 0.79 ng/mL versus -0.01 ± 0.86 ng/mL in ∆CXCL1, -2.43 [5.52] pg/mg versus -0.04 [4.09] pg/mg in ∆β-EP, 3.45 [5.90] pg/mL versus 1.55 [8.29] pg/mL in ∆TNF-α, and 6.15 ± 8.65 pg/mL versus 1.28 ± 8.51 pg/mL in ∆IL-1β; all P < 0.05).

Conclusion

Moxibustion can improve the joint swelling and pain symptoms in patients with RA, which may be related to the fact that moxibustion can reduce the release of inflammatory factors in patients with RA and downregulate the level of CXCL1 and increase the level of β-EP at the same time. This trial is registered with ChiCTR-IOR-17012282.

Tai Chi as a Therapy of Traditional Chinese Medicine on Reducing Blood Pressure: A Systematic Review of Randomized Controlled Trials

Objective

This study systematically evaluated the effects of Tai Chi exercise on blood pressure, body mass index (BMI), and quality of life (QOL) in patients with hypertension. A meta-analysis was performed to provide a reliable reference for clinical practice.

Methods

We searched for randomized controlled trials (RCTs) in five English databases and two Chinese databases, with the earliest data dated December 5, 2020. A quality assessment of the methods and a meta-analysis were also conducted.

Results

The meta-analysis of 24 studies showed that the intervention group showed better outcomes in terms of systolic blood pressure (SBP) (SMD −1.05, 95% CI −1.44 to −0.67, P ≤ 0.001; I² = 93.7%), diastolic blood pressure (DBP) (SMD −0.91, 95% CI −1.24 to −0.58, P ≤ 0.001; I² = 91.9%), and QOL (physical functioning (SMD 0.86, 95% CI 0.36 to 1.37, P=0.001; I² = 91.3%), role-physical (SMD 0.86, 95% CI 0.61 to 1.11, P ≤ 0.001; I² = 65%), general health (SMD 0.75, 95% CI 0.32 to 1.17, P=0.001; I² = 88.1%), bodily pain (SMD 0.65, 95% CI 0.29 to 1.00, P ≤ 0.001; I² = 83.1%), vitality (SMD 0.71, 95% CI 0.34 to 1.07, P ≤ 0.001; I² = 84.3%), social functioning (SMD 0.63, 95% CI 0.07 to 1.19, P=0.027; I² = 93.1%), role-emotional (SMD 0.64, 95% CI 0.22 to 1.06, P=0.003; I² = 88.1%), and mental health (SMD 0.73, 95% CI 0.31 to 1.16, P=0.001; I² = 88.2%)) compared to those of the control group. However, no significant improvements were seen in BMI of the intervention group (SMD −0.08, 95% CI −0.35 to −0.19, P=0.554; I² = 69.4%) compared to that of the control group.

Conclusion

Tai Chi is an effective intervention to improve SBP and DBP in patients with essential hypertension.

Emerging Natural-Product-Based Treatments for the Management of Osteoarthritis

Osteoarthritis (OA) is a complex degenerative disease in which joint homeostasis is disrupted, leading to synovial inflammation, cartilage degradation, subchondral bone remodeling, and resulting in pain and joint disability. Yet, the development of new treatment strategies to restore the equilibrium of the osteoarthritic joint remains a challenge. Numerous studies have revealed that dietary components and/or natural products have anti-inflammatory, antioxidant, anti-bone-resorption, and anabolic potential and have received much attention toward the development of new therapeutic strategies for OA treatment. In the present review, we provide an overview of current and emerging natural-product-based research treatments for OA management by drawing attention to experimental, pre-clinical, and clinical models. Herein, we review current and emerging natural-product-based research treatments for OA management.

Effects of Tai Chi on beta endorphin and inflammatory markers in older adults with chronic pain: an exploratory study

The purpose of this exploratory study was to examine the effects of Tai Chi on blood levels of beta endorphin (β-endorphin) and inflammatory markers in older adults with chronic pain. Forty community-dwelling older adults with chronic pain were randomized to Tai Chi or light physical exercise, and each offered twice weekly for 12 weeks. Following the 12-week intervention, neither Tai Chi nor light physical exercise changed levels of β-endorphin and inflammatory markers. However, in older adults who completed 70% or more classes, Tai Chi significantly lowered levels of β-endorphin (p < 0.05), whereas light physical exercise did not change levels of β-endorphin. The results suggest that Tai Chi may reduce levels of β-endorphin in older adults with chronic pain. Future studies are needed to better understand the role of the opioid analgesic system and immune system in regulating pain with aging and the long-term effects of Tai Chi on pain-related biomarkers.

Modulation of Hedgehog Signaling by Kappa Opioids to Attenuate Osteoarthritis

OBJECTIVE

Inhibition of hedgehog (HH) signaling prevents cartilage degeneration and promotes repair in animal models of osteoarthritis (OA). This study, undertaken in OA models and in human OA articular cartilage, was designed to explore whether kappa opioid receptor (KOR) modulation via the inhibition of HH signaling may have therapeutic potential for achieving disease-modifying activity in OA.

METHODS

Primary human articular cartilage and synovial tissue samples from patients with knee OA undergoing total joint replacement and from healthy human subjects were obtained from the National Disease Research Interchange. For in vivo animal studies, a partial medial meniscectomy (PMM) model of knee OA in rats was used. A novel automated 3-dimensional indentation tester (Mach-1) was used to quantify the thickness and stiffness properties of the articular cartilage.

RESULTS

Inhibition of HH signaling through KOR activation was achieved with a selective peptide agonist, JT09, which reduced HH signaling via the cAMP/CREB pathway in OA human articular chondrocytes (P = 0.002 for treated versus untreated OA chondrocytes). Moreover, JT09 markedly decreased matrix degeneration induced by an HH agonist, SAG, in pig articular chondrocytes and cartilage explants (P = 0.026 versus untreated controls). In vivo application of JT09 via intraarticular injection into the rat knee joint after PMM surgery significantly attenuated articular cartilage degeneration (60% improvement in the tibial plateau; P = 0.021 versus vehicle-treated controls). In JT09-treated rats, cartilage content, structure, and functional properties were largely maintained, and osteophyte formation was reduced by 70% (P = 0.005 versus vehicle-treated controls).

CONCLUSION

The results of this study define a novel mechanism for the role of KOR in articular cartilage homeostasis and disease, providing a potential unifying mechanistic basis for the overlap in disease processes and features involving opioid and HH signaling. Moreover, this study identifies a potential novel therapeutic strategy in which KOR modulation can improve outcomes in patients with OA.

Histopathological analysis of the synovium in trapeziometacarpal osteoarthritis

Dorsoradial and anterior oblique ligaments were harvested during surgery in 13 patients with symptomatic trapeziometacarpal osteoarthritis, which had been graded preoperatively by a modified Eaton-Littler radiographic grading. Ligaments, including the periligamentous synovium, were stained with S100 protein, neurotrophic receptor p75, protein gene product 9.5, calcitonin gene related peptide, acetylcholine, substance P, neuropeptide Y, noradrenaline, N-methyl-D-aspartate-receptor and Met/Leu-enkephalin. The synovium was classified as showing no, low-grade or high-grade synovitis. Free nerve endings had higher immunoreactivity for substance P than for N-methyl-D-aspartate-receptor, enkephalin and noradrenaline. The synovial stroma had less immunoreactivity for N-methyl-D-aspartate-receptor than for noradrenaline, substance P and calcitonin gene related peptide. There was no relation between the grade of osteoarthritis and the visual pain analogue scale, synovitis score, immunoreactivity of all antibodies and quantity of free nerve endings or blood vessels. Synovium in trapeziometacarpal joint osteoarthritis produces several neuromediators causing a polymodal neurogenic inflammation and which may serve as biomarkers for osteoarthritis or therapeutic targets.

The Efficacy of Liposomal Bupivacaine Over Traditional Local Anesthetics in Periarticular Infiltration and Regional Anesthesia During Total Knee Arthroplasty: A Systematic Review and Meta-Analysis

BACKGROUND

Since its Food and Drug Administration approval in 2011 as a local anesthetic for postsurgical analgesia, liposomal bupivacaine (LB) has been incorporated into the periarticular injection (PAI) of many knee surgeons. The slow release of this medication from vesicles should significantly extend the duration of its analgesic effect, but current evidence has not clearly demonstrated this benefit.

METHODS

We systematically searched electronic databases including PubMed, MEDLINE, Cochrane Library, EMBASE, ScienceDirect, and Scopus, as well as the Journal of Arthroplasty web page for relevant articles. All calculations were made using Review Manager 5.3.

RESULTS

We identified 42 studies that compared LB to an alternate analgesic modality. Seventeen of these studies were controlled trials that were included in meta-analysis. Significant differences were seen in pain scores with LB over a peripheral nerve block (mean difference = 0.45, P = .02) and LB over a traditional PAI (standard mean difference = -0.08, P = .004).

CONCLUSION

While LB may offer a statistically significant benefit over a traditional PAI, the increase in pain control may not be clinically significant and it does not appear to offer a benefit in reducing opioid consumption. However, there is no standardization among current studies, as they vary greatly in design, infiltration technique, and outcome measurement, which precludes any reliable summarization of their results. Future independent studies using a standardized protocol are needed to provide clear unbiased evidence.

Peripherally Acting μ-Opioid Receptor Antagonists for the Treatment of Opioid-Related Side Effects: Mechanism of Action and Clinical Implications

Opioid receptors are distributed throughout the central and peripheral nervous systems and on many nonneuronal cells. Therefore, opioid administration induces effects beyond analgesia. In the enteric nervous system (ENS), stimulation of µ-opioid receptors triggers several inhibitory responses that can culminate in opioid-induced bowel dysfunction (OBD) and its most common side effect, opioid-induced constipation (OIC). OIC negatively affects patients' quality of life (QOL), ability to work, and pain management. Although laxatives are a common first-line OIC therapy, most have limited efficacy and do not directly antagonize opioid effects on the ENS. Peripherally acting µ-opioid receptor antagonists (PAMORAs) with limited ability to cross the blood-brain barrier have been developed. The PAMORAs approved by the U S Food and Drug Administration for OIC are subcutaneous and oral methylnaltrexone, oral naloxegol, and oral naldemedine. Although questions of cost-effectiveness and relative efficacy versus laxatives remain, PAMORAs can mitigate OIC and improve patient QOL. PAMORAS may also have applications beyond OIC, including reducing the increased cardiac risk or potential tumorigenic effects of opioids. This review discusses the burden of OIC and OBD, reviews the mechanism of action of new OIC therapies, and highlights other potential opioid-related side effects mediated by peripheral opioid receptors in the context of new OIC therapies.

Effects of intramuscular morphine in men and women with temporomandibular disorder with myofascial pain

OBJECTIVES

This placebo-controlled randomized double-blinded clinical study assessed the analgesic efficacy of intramuscular morphine in TMD patients with myofascial pain and sex-dependent responses of the morphine treatment.

SUBJECTS AND METHODS

Men and women with TMD were treated with morphine (1.5 or 5 mg), lidocaine, or saline in the masseter muscle. VAS of pain intensity, PPT, and PPtol were compared between treatment groups and gender. An additional group was treated with morphine in the trapezius muscle to evaluate the systemic effect of morphine that may reduce pain in the masseter muscle.

RESULTS

There was a significant difference in VAS scores between the morphine 5 mg group and the saline group favoring morphine, but not between the morphine 5 mg and lidocaine. Morphine 1.5 and 5 mg treatments led to consistently and significantly elevated PPT and PPtol measures in men, but not in women. Morphine administered in the trapezius muscle did not affect the outcome measures.

CONCLUSION

A single dose intramuscular morphine produced analgesic effects up to 48 hr in patients with myofascial pain. Intramuscular morphine elevated mechanical pain threshold and tolerance in the masseter only in male patients, suggesting sex differences in local morphine effects. No systemic effect of intramuscular morphine was detected.

Local Efficacy of Periarticular Morphine Injection in Simultaneous Bilateral Total Knee Arthroplasty: A Prospective, Randomized, Double-Blind Trial

BACKGROUND

The periarticular multimodal cocktail injection including morphine is currently commonly used to reduce postoperative pain following total knee arthroplasty (TKA). Despite its analgesic effect, it frequently causes nausea, which is an adverse effect of opioids. It is inconclusive whether the intraoperative injection of periarticular morphine is effective peripherally. The aim of this study was to assess whether the addition of morphine to unilateral periarticular knee injections improves postoperative pain, range of motion, and swelling in patients undergoing simultaneous bilateral TKA.

METHODS

A prospective, single-center, double-blinded, randomized, controlled trial was conducted to assess the local efficacy of adding morphine to intraoperative periarticular anesthesia in patients undergoing simultaneous bilateral TKA. Fifty-three patients undergoing 106 TKAs received an intraoperative periarticular injection in randomly selected one knee with added morphine (0.1 mg/kg) and the other knee without added morphine. The periarticular injection was composed of ropivacaine (a local anesthetic), epinephrine, ketoprofen, and methylprednisolone sodium. Visual analog scale pain scores at rest and on motion, range of motion (ROM), thigh swelling, the Western Ontario and McMaster Universities Osteoarthritis Index score, and adverse outcomes were compared between the 2 knees.

RESULTS

There were no statistically significant differences in the visual analog scale score, ROM, thigh girth, Western Ontario and McMaster Universities Osteoarthritis Index score, and adverse events between the 2 sides.

CONCLUSION

Adding morphine to periarticular injections is ineffective locally for relieving pain, reducing swelling, and improving the postoperative ROM.

Ethnic Differences in the Effects of Naloxone on Sustained Evoked Pain: A Preliminary Study

Ethnic differences in pain response have been well documented, with non-Hispanic Black (NHB) participants reporting enhanced clinical pain and greater laboratory-evoked pain sensitivity to a variety of quantitative sensory testing (QST) methods compared to non-Hispanic Whites (NHW). One potential mechanism that may contribute to these disparities is differential functioning of endogenous pain-regulatory systems. To evaluate endogenous opioid (EO) mechanisms in pain responses, we examined group differences in response to tonic capsaicin pain following double-blinded crossover administration of saline and the opioid antagonist, naloxone. Ten percent topical capsaicin cream and a thermode were applied to the dorsum of the non-dominant hand, maintaining a constant temperature of 40°C for 90 min. Naloxone (0.1 mg/kg) or saline placebo was administered at the 25 min mark and post-drug pain intensity ratings were obtained every 5 min thereafter. As an index of EO function, blockade effects were derived for each participant, reflecting the difference between mean post-drug pain intensity ratings under the saline versus naloxone conditions, with higher positive scores reflecting greater EO inhibition of pain. Thirty-nine healthy, young individuals (19 non-Hispanic Black [NHB], 20 non-Hispanic White [NHW]) participated. Group difference in EO function were identified, with NHB participants displaying lower EO function scores (mean=-10.8, SD=10.1) as compared to NHW participants (mean=-0.89, SD=11.5; p=0.038). NHB participants experienced significant paradoxical analgesia with naloxone. Thirty five percent of the NHW participants showed a positive blockade effect indicating EO analgesia (i.e., an increase in pain with naloxone), while only 10% of the NHB participants exhibited evidence of EO analgesia. These findings suggest differential functioning of the endogenous opioid pain regulatory system between NHB and NHW participants. Future research is warranted to examine whether these differences contribute to the disparities observed in clinical pain between groups.

Periarticular Injections in Knee and Hip Arthroplasty: Where and What to Inject

BACKGROUND

Periarticular injections have become a valuable adjunct to multimodal pain control regimens after knee and hip arthroplasties. Injection techniques vary greatly among surgeons with little standardization of practice.

METHODS

We performed an extensive literature search to determine where nociceptive pain fibers are located in the hip and the knee and also to explore the pharmacology of periarticular cocktail ingredients.

RESULTS

Large concentrations of nociceptors are present throughout the various tissues of the knee joint with elevated concentrations in the infrapatellar fat pad, fibrous capsule, ligament insertions, periosteum, subchondral bone, and lateral retinaculum. Less empiric evidence is available on nociceptor locations in the hip joint, but they are known to be located diffusely throughout the hip capsule with elevated concentrations at the labral base and central ligamentum teres. Local anesthetics are the base ingredient in most injection cocktails and function by blocking voltage-gated sodium channels. Liposomal anesthetics may offer longer duration of action over traditional anesthetics. Nonsteroidal anti-inflammatory agents and corticosteroids block peripheral production of inflammatory mediators and may desensitize nociceptors. Opioid receptors are present in lower densities peripherally as compared with the central nervous system, but their inclusion in injections can lead to pain relief. Sympathetic drugs can provide adjunct effects to periarticular cocktails to increase duration of action and effectiveness of medications.

CONCLUSION

Targeting specific sites of nociceptors may help to further decrease pain after knee and hip arthroplasties. Altering periarticular cocktail ingredients may aid in multimodal pain control with injections.

Kappa opioid receptor signaling protects cartilage tissue against posttraumatic degeneration

Osteoarthritis is the most common form of arthritis, and pain relief with opioid-like drugs is a commonly used therapeutic for osteoarthritic patients. Recent studies published by our group showed that the kappa opioid receptor (KOR) is highly expressed during human development in joint-forming cells. However, the precise role of this receptor in the skeletal system remains elusive. The main aim of the current study was to investigate the role of KOR signaling in synovial and cartilaginous tissues in pathological conditions. Our data demonstrate that KOR null mice exhibit accelerated cartilage degeneration after injury when compared with WT mice. Activation of KOR signaling increased the expression of anabolic enzymes and inhibited cartilage catabolism and degeneration in response to proinflammatory cytokines such as TNF-α. In addition, selective KOR agonists increased joint lubrication via the activation of cAMP/CREB signaling in chondrocytes and synovial cells. Taken together, these results demonstrate direct effects of KOR agonists on cartilage and synovial cells and reveals a protective effect of KOR signaling against cartilage degeneration after injury. In addition to pain control, local administration of dynorphin or other KOR agonist represents an attractive therapeutic approach in patients with early stages of osteoarthritis.

Opioid receptor agonists may favorably affect bone mechanical properties in rats with estrogen deficiency-induced osteoporosis

The results of epidemiological, clinical, and in vivo and in vitro experimental studies on the effect of opioid analgesics on bone are inconsistent. The aim of the present study was to investigate the effect of morphine (an agonist of opioid receptors), buprenorphine (a partial μ opioid receptor agonist and κ opioid receptor antagonist), and naloxone (an antagonist of opioid receptors) on the skeletal system of female rats in vivo. The experiments were carried out on 3-month-old Wistar rats, divided into two groups: nonovariectomized (intact; NOVX) rats and ovariectomized (OVX) rats. The bilateral ovariectomy was performed 7 days before the start of drug administration. Morphine hydrochloride (20 mg/kg/day s.c.), buprenorphine (0.05 mg/kg/day s.c.), or naloxone hydrochloride dihydrate (2 mg/kg/day s.c.) were administered for 4 weeks to NOVX and OVX rats. In OVX rats, the use of morphine and buprenorphine counteracted the development of osteoporotic changes in the skeletal system induced by estrogen deficiency. Morphine and buprenorphine beneficially affected also the skeletal system of NOVX rats, but the effects were much weaker than those in OVX rats. Naloxone generally did not affect the rat skeletal system. The results confirmed the role of opioid receptors in the regulation of bone remodeling processes and demonstrated, in experimental conditions, that the use of opioid analgesics at moderate doses may exert beneficial effects on the skeletal system, especially in estrogen deficiency.

Involvement of the dynorphin/KOR system on the nociceptive, emotional and cognitive manifestations of joint pain in mice

Joint pain is a major clinical problem mainly associated to osteoarthritis, and characterized by articular cartilage degradation resulting in a complex chronic pain state that includes nociceptive, emotional and cognitive manifestations. Memory impairment, depressive- and anxiety-like symptoms have been reported to be associated with chronic pain, leading to a decrease of life quality. In this study, we evaluated the involvement of the endogenous dynorphin/kappa opioid receptor (KOR) system on the nociceptive, emotional, cognitive, neurochemical and epigenetic manifestations of joint pain. The murine model of monosodium iodoacetate (MIA) was used to induce joint pain in knockout mice for KOR (KOR-KO), prodynorphin (PDYN-KO) and their wild-type (WT) littermates. KOR-KO and PDYN-KO mice developed mechanical allodynia after intra-articular injection of MIA. This allodynia was significantly increased in both KOR-KO and PDYN-KO when compared to WT mice. Accordingly, both mutants showed increased microglial activation on the lumbar section of the spinal cord after MIA. The emotional responses were evaluated by measuring anxiety-like behaviour in the elevated plus maze and anhedonia as depressive-like behaviour, and cognitive alterations in the object recognition paradigm. Emotional and cognitive impairments after joint pain were differently modified in KOR-KO and PDYN-KO mice. Alterations of corticotropin-releasing factor (CRF) on the amygdala and hippocampus and down regulation of histone 3 acetylation on the amygdala suggest a possible mechanism to explain these emotional and cognitive manifestations. Our results reveal a specific involvement of the dynorphin/KOR system on joint pain manifestations that are usually associated to osteoarthritis.

A background for the management of osteoarthritic knee pain

Knee osteoarthritis (KOA) is the most common degenerative arthritis and is treated by a wide range of practitioners. Treatment planning requires knowledge of the knee joint components and the influence of systemic and environmental factors. The treatment of KOA has changed little in 50 years. We are entering a new stage where KOA is now being viewed as an organ in failure. Neurotransmission of pain is both peripheral and central. Medical treatment can influence both pathways. Current guidelines for treatment have more rigid criteria based on the literature. In the future, the use of genetic-based biomarkers, clinical patterns of response and imaging characteristics will likely create subgroups of individuals who could benefit from improved designer therapies.