Methionine-enkephalin in bone and joint tissues

Pre-proenkephalin 1 is Downregulated Under Unloading and is Involved in Osteoblast Biology

Pre-proenkephalin 1 (Penk1) is a pro-neuropeptide that belongs to the typical opioid peptide's family, having analgesic properties. We previously found Penk1 to be the most downregulated gene in a whole gene profiling analysis performed in osteoblasts subjected to microgravity as a model of mechanical unloading. In this work, Penk1 downregulation was confirmed in the bones of two in vivo models of mechanical unloading: tail-suspended and botulinum toxin A (botox)-injected mice. Consistently, in the sera from healthy volunteers subjected to bed rest, we observed an inverse correlation between PENK1 and bed rest duration. These results prompted us to investigate a role for this factor in bone. Penk1 was highly expressed in mouse bone, but its global deletion failed to impact bone metabolism in vivo. Indeed, Penk1 knock out (Penk1-/-) mice did not show an overt bone phenotype compared to the WT littermates. Conversely, in vitro Penk1 gene expression progressively increased during osteoblast differentiation and its transient silencing in mature osteoblasts by siRNAs upregulated the transcription of the Sost1 gene encoding sclerostin, and decreased Wnt3a and Col1a1 mRNAs, suggesting an altered osteoblast activity due to an impairment of the Wnt pathway. In line with this, osteoblasts treated with the Penk1 encoded peptide, Met-enkephalin, showed an increase of Osx and Col1a1 mRNAs and enhanced nodule mineralization. Interestingly, primary osteoblasts isolated from Penk1-/- mice showed lower metabolic activity, ALP activity, and nodule mineralization, as well as a lower number of CFU-F compared to osteoblasts isolated from WT mice, suggesting that, unlike the transient inhibition, the chronic Penk1 deletion affects both osteoblast differentiation and activity. Taken together, these results highlight a role for Penk1 in the regulation of the response of the bone to mechanical unloading, potentially acting on osteoblast differentiation and activity in a cell-autonomous manner.

Methionine Enkephalin Suppresses Osteocyte Apoptosis Induced by Compressive Force through Regulation of Nuclear Translocation of NFATc1

Mechanical stress stimulates bone remodeling, which occurs through bone formation and resorption, resulting in bone adaptation in response to the mechanical stress. Osteocytes perceive mechanical stress loaded to bones and promote bone remodeling through various cellular processes. Osteocyte apoptosis is considered a cellular process to induce bone resorption during mechanical stress-induced bone remodeling, but the underlying molecular mechanisms are not fully understood. Recent studies have demonstrated that neuropeptides play crucial roles in bone metabolism. The neuropeptide, methionine enkephalin (MENK) regulates apoptosis positively and negatively depending on cell type, but the role of MENK in osteocyte apoptosis, followed by bone resorption, in response to mechanical stress is still unknown. Here, we examined the roles and mechanisms of MENK in osteocyte apoptosis induced by compressive force. We loaded compressive force to mouse parietal bones, resulting in a reduction of MENK expression in osteocytes. A neutralizing connective tissue growth factor (CTGF) antibody inhibited the compressive force-induced reduction of MENK. An increase in osteocyte apoptosis in the compressive force-loaded parietal bones was inhibited by MENK administration. Nuclear translocation of NFATc1 in osteocytes in the parietal bones was enhanced by compressive force. INCA-6, which inhibits NFAT translocation into nuclei, suppressed the increase in osteocyte apoptosis in the compressive force-loaded parietal bones. NFATc1-overexpressing MLO-Y4 cells showed increased expression of apoptosis-related genes. MENK administration reduced the nuclear translocation of NFATc1 in osteocytes in the compressive force-loaded parietal bones. Moreover, MENK suppressed Ca²⁺ influx and calcineurin and calmodulin expression, which are known to induce the nuclear translocation of NFAT in MLO-Y4 cells. In summary, this study shows that osteocytes expressed MENK, whereas the MENK expression was suppressed by compressive force via CTGF signaling. MENK downregulated nuclear translocation of NFATc1 probably by suppressing Ca²⁺ signaling in osteocytes and consequently inhibiting compressive force-induced osteocyte apoptosis, followed by bone resorption. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Hip Fracture Risk among Hemodialysis-Dependent Patients Prescribed Opioids and Gabapentinoids

BACKGROUND

Despite opioids' known association with hip fracture risk in the general population, they are commonly prescribed to patients with ESKD. Whether use of opioids or gabapentinoids (also used to treat pain in patients with ESKD) contributes to hip fracture risk in patients with ESKD on hemodialysis remains unknown.

METHODS

In a case-control study nested within the US Renal Data System, we identified all hip fracture events recorded among patients dependent on hemodialysis from January 2009 through September 2015. Eligible cases were risk-set matched on index date with ten eligible controls. We required >1 year of Medicare Parts A and B coverage and >3 years of part D coverage to study cumulative longer-term exposure. To examine new, short-term exposure, we selected individuals with >18 months of Part D coverage and no prior opioid or gabapentinoid use between 18 and 7 months before index. We used conditional logistic regression to estimate unadjusted and multivariable-adjusted odds ratios (ORs) and 95% confidence intervals (95% CI).

RESULTS

For the longer-term analyses, we identified 4912 first-time hip fracture cases and 49,120 controls. Opioid use was associated with increased hip fracture risk (adjusted OR, 1.39; 95% CI, 1.26 to 1.53). Subgroups of low, moderate, and high use yielded adjusted ORs of 1.33 (95% CI, 1.20 to 1.47), 1.53 (95% CI, 1.36 to 1.72), and 1.66 (95% CI, 1.45 to 1.90), respectively. The association with hip fractures was also elevated with new, short-term use (adjusted OR, 1.38; 95% CI, 1.25 to 1.52). There were no associations between gabapentinoid use and hip fracture.

CONCLUSIONS

Among patients dependent on hemodialysis in the United States, both short-term and longer-term use of opioid analgesics were associated with hip fracture events.

A study on OPG/RANK/RANKL axis in osteoporotic bile duct-ligated rats and the involvement of nitrergic and opioidergic systems

Chronic liver disease (CLD) affects millions of people and its impact on bone loss has become a subject of interest. Nitric oxide and endogenous opioids are suggested to increase during cholestasis/cirrhosis and may impact bone resorption by different mechanisms. The receptor activator of nuclear factor-κB (RANK)/RANK-ligand (RANKL)/osteoprotegerin (OPG) signaling pathway regulates bone resorption, but its role in metabolic bone disease subsequent to CLD is unknown. We aimed to investigate the involvement of nitrergic and opioidergic systems in bone loss relative to the RANK/RANKL/OPG pathway, in bile duct-ligated (BDL) rats. Eighty BDL/sham-operated (SO) rats received injections of 3 mg/kg/day Nω-Nitro-L-arginine methyl ester ± naltrexone (10 mg/kg/day) or saline for 28 days. Plasma bone turnover markers, OPG, RANK, and RANKL along with mRNA expression levels of the latter three were assessed. Plasma bone turnover markers and OPG level increased, but RANKL decreased in the BDL group compared with their SO controls (both: P ≤ 0.05). Administration of naltrexone reduced bone turnover markers and OPG level while increased RANKL content in comparison to BDL rats (P ≤ 0.05). As compared to untreated BDL rats, nitric oxide inhibition showed no effect on bone turnover marker i.e. OPG, RANK, and RANKL levels. BDL significantly increased RANK mRNA, but had no significant effect on RANKL and OPG mRNA expression. The lack of association between plasma levels and quantitative gene expression of RANKL and OPG suggests an indirect function of these markers in BDL rats. Considering that opioid receptor blockage by naltrexone in BDL animals caused a significant decrease in OPG and an increase in RANKL plasma contents, it could be postulated that the opioidergic system may have a regulatory effect on these bone markers.

Antagonism of the Met5-enkephalin-opioid growth factor receptor-signaling axis promotes MSC to differentiate into osteoblasts

Chronic opioid therapy is associated with bone loss. This led us to hypothesize that the opioid antagonists, that include naloxone, would stimulate bone formation by regulating MSC differentiation. The opioid growth factor receptor (OGFR) is a non-canonical opioid receptor that binds naloxone with high affinity whereas the native opioid growth factor, met5-enkephalin (met5), binds both the OGFR and the canonical delta opioid receptor (OPRD). Naloxone and an shRNA OGFR lentivirus were employed to disrupt the OGFR-signaling axis in cultured MSC. In parallel, naloxone was administered to bone marrow using a mouse unicortical defect model. OPRD, OGFR, and the met5-ligand were highly expressed in MSC and osteoblasts. A pulse-dose of naloxone increased mineral formation in MSC cultures in contrast to MSC treated with continuous naloxone or OGFR deficient MSC. Importantly, SMAD1 and SMAD8/9 expression increased after a pulse dose of naloxone whereas SMAD1, SMAD7, and ID1 were increased in the OGFR deficient MSC. Inhibited OGFR signaling decreased proliferation and increased p21 expression. The addition of naloxone to the unicortical defect resulted in increased bone formation within the defect. Our data suggest that novel mechanism through which signaling through the OGFR regulates osteogenesis via negative regulation of SMAD1 and p21. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1195-1205, 2016.

Effects of Remifentanil Preconditioning on Osteoblasts under Hypoxia-Reoxygenation Condition

BACKGROUND

Ischemia-reperfusion of bone occurs in a variety of clinical conditions, such as orthopedic arthroplasty, plastic gnathoplasty, spinal surgery, and amputation. Usually, cellular models of hypoxia-reoxygenation reflect in vivo models of ischemia-reperfusion. With respect to hypoxia-reoxygenation conditions, the effects of remifentanil on osteogenesis have received little attention. Therefore, we investigated the effects of remifentanil on the proliferation and differentiation of osteoblasts during hypoxic-reoxygenation.

METHODS

After remifentanil (0.1, 1 ng/mL) preconditioning for 2 hours, human osteoblasts were cultured under 1% oxygen tension for 24 hours. Thereafter, the cells were reoxygenated for 12 hours at 37 °C. The naloxone groups were treated with naloxone for 30 minutes before remifentanil treatment. We measured cell viability via MTT assay. Osteoblast maturation was determined by assay of bone nodular mineralization. Quantitative PCR and western blot methods were used to determine BMP-2, osteocalcin, Akt, type I collagen, osterix, TGF-β1, HIF-1α, and RUNX2 expression levels.

RESULTS

Osteoblast viability and bone nodular mineralization by osteoblasts is recovered by remifentanil preconditioning from hypoxia-reoxygenation insult. During hypoxic-reoxygenation condition, remifentanil preconditioning induced the expression of BMP-2, osteocalcin, Akt, type I collagen, osterix, TGF-β1, HIF-1α, and RUNX2 in osteoblasts.

CONCLUSIONS

Under hypoxia-reoxygenation conditions, remifentanil preconditioning enhanced the cell viability and maturation of osteoblasts, and stimulated the expression of proteins associated with osteoblast proliferation and differentiation of the osteoblast. Our results suggest that remifentanil may help in the treatment of bone stress injuries.

Osteoporosis and multiple sclerosis: risk factors, pathophysiology, and therapeutic interventions

Multiple sclerosis (MS) is a chronic inflammatory-demyelinating disease of the nervous system. There has been mounting evidence showing that MS is associated with increased risk of osteoporosis and fractures. The development of osteoporosis in MS patients can be related to the cumulative effects of various factors. This review summarizes the common risk factors and physiologic pathways that play a role in development of osteoporosis in MS patients. Physical inactivity and reduced mechanical load on the bones (offsetting gravity) is likely the major contributing factor for osteoporosis in MS. Additional possible factors leading to reduced bone mass are low vitamin D levels, and use of medications such as glucocorticoids and anticonvulsants. The role of the inflammatory processes related to the underlying disease is considered in the context of the complex bone metabolism. The known effect of different MS disease-modifying therapies on bone health is limited. An algorithm for diagnosis and management of osteoporosis in MS is proposed.

Bisphosphonate treatment may reduce osteoporosis risk in female cancer patients with morphine use: a population-based nested case-control study

Chronic use of morphine is a risk factor for endocrinopathy and osteoporosis. Bisphosphonates accentuated the protective effect to develop osteoporosis in female patients with malignancy with morphine treatment.

INTRODUCTION

This study investigates the risk of osteoporosis associated with morphine use by comparing the incidence of osteoporosis in female cancer patients treated with and without morphine.

METHODS

A population-based nested case-control retrospective analysis was performed using the Longitudinal Health Insurance Database 2000 and Registry for Catastrophic Illness Patients of Taiwan. A malignancy cohort of 12,467 female patients without a history of osteoporosis during 1998-2010, and then 639 patients who subsequently developed osteoporosis as the osteoporosis group, were evaluated. Control-group patients were selected from the malignancy cohort without osteoporosis and frequency matched to each osteoporosis case 2:1 for age, year of cancer diagnosis, and index year. Logistic regression was used to estimate the odds ratios and 95% confidence intervals, and the multivariable model was applied to control for age.

RESULTS

Female cancer patients who received morphine had a 10% lower risk of developing osteoporosis than non-morphine users, but this risk reduction was not significant. For patients treated with bisphosphonates, the morphine group had significantly lower odds in developing osteoporosis than the non-morphine group.

CONCLUSION

Morphine treatment is not associated with the incidence of osteoporosis, and bisphosphonates accentuated the protective effect of morphine in the development of osteoporosis in female patients with malignancy in Taiwan.

The association of opioid use with incident lower extremity fractures in spinal cord injury

OBJECTIVE

To determine the association between opioid use and lower extremity fracture risk in men with spinal cord injury (SCI).

DESIGN

Retrospective cohort study.

SETTING

Veterans Affairs Healthcare System.

PARTICIPANTS

In total, 7447 male Veterans with a history of a traumatic SCI identified from the Veterans Affairs (VA) Spinal Cord Dysfunction Registry (SCD) from September 2002 through October 2007 and followed through October 2010.

OUTCOME MEASURES

Incident lower extremity fractures by use of opioids.

RESULTS

In individuals identified from the VA SCD Registry 2002-2007, opioid use was quite common, with approximately 70% of the cohort having received a prescription for an opioid. Overall, there were 892 incident lower extremity fractures over the time period of this study (597 fractures in the opioid users and 295 fractures in the non-opioid users). After adjusting for covariates, there was a statistically significant relationship between opioid use and increased risk for lower extremity fractures (hazard ratio 1.82 (95% confidence interval 1.59-2.09)). Shorter duration of use (<6 months) and higher doses were positively related to fracture risk (P < 0.0001).

CONCLUSIONS

Opioid use is quite common in SCI and is associated with an increased risk for lower extremity fractures. Careful attention to fracture prevention is warranted in patients with SCI, particularly upon initiation of an opioid prescription and when higher doses are used.

Bone mineral density in young women on methadone substitution

Little is known about bone mineral density (BMD) in patients with heroin addiction and subsequent methadone substitution. The goal of this study was to compare bone mass density of young HIV-negative women on long-term methadone treatment to a local group of young healthy women. Eleven women (aged 20-29) with previous heroin dependence and current methadone substitution (20-140 mg, median 60, daily) for 1.5-9 (median 3) years were compared to 30 healthy women (aged 20-28). Participants were examined with dual-energy X-ray absorptiometry of the lumbar spine (L2-L4), of the total proximal hip area, and of the femoral neck. Patients and controls had neither current nor lifetime underweight condition, had comparable ages at menarche, and did not differ significantly in current body mass index (21.9 ± 4.0, respectively, 20.5 ± 1.5 kg/m(2)) in spite of a largely unhealthy lifestyle (cigarette, alcohol, and cocaine consumption in patients). Patients' total-hip parameters were marginally lower than those of controls (BMD P = 0.054, T score P = 0.049), whereas the femoral neck and lumbar spine parameters did not differ significantly between the two groups. Long-term methadone substitution in HIV-negative women seems to slightly affect bone mass density.

Prospective study of bone mineral density changes in aging men with or at risk for HIV infection

OBJECTIVE

To investigate rates and predictors of change in bone mineral density (BMD) in a cohort of aging men with or at risk for HIV infection.

DESIGN

A prospective cohort study among 230 HIV-infected and 159 HIV-uninfected men aged at least 49 years.

METHODS

Longitudinal analyses of annual change in BMD at the femoral neck, total hip, and lumbar spine.

RESULTS

At baseline, 46% of men had normal BMD, 42% had osteopenia, and 12% had osteoporosis. Of those men with normal BMD, 14% progressed to osteopenia and 86% continued to have normal BMD. Of the men initially with osteopenia, 12% progressed to osteoporosis and 83% continued to have osteopenia. Osteopenia incidence per 100 person-years at risk was 2.6 for HIV-uninfected men and 7.2 for HIV-infected men; osteoporosis incidence was 2.2 per 100 person-years at risk among men with osteopenia, regardless of HIV status. In multivariable analysis of annual change in BMD at the femoral neck, we found a significant interaction between heroin use and AIDS diagnosis, such that the greatest bone loss occurred with both AIDS and heroin use (adjusted predicted mean annual bone loss 0.0196 g/cm). Hepatitis C virus seropositivity was also associated with femoral neck bone loss (P = 0.04). The interaction between AIDS and heroin use also was associated with bone loss at the total hip, as was current methadone use (P < 0.01).

CONCLUSION

We found an association of heroin use and AIDS with BMD change, suggesting that heroin users with AIDS may be at particular risk for bone loss.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Orally administered H-Dmt-Tic-Lys-NH-CH2-Ph (MZ-2), a potent mu/delta-opioid receptor antagonist, regulates obese-related factors in mice

Orally active dual mu-/delta-opioid receptor antagonist, H-Dmt-Tic-Lys-NH-CH(2)-Ph (MZ-2) was applied to study body weight gain, fat content, bone mineral density, serum insulin, cholesterol and glucose levels in female ob/ob (B6.V-Lep<ob>/J homozygous) and lean wild mice with or without voluntary exercise on wheels for three weeks, and during a two week post-treatment period under the same conditions. MZ-2 (10mg/kg/day, p.o.) exhibited the following actions: (1) reduced body weight gain in sedentary obese mice that persisted beyond the treatment period without effect on lean mice; (2) stimulated voluntary running on exercise wheels of both groups of mice; (3) decreased fat content, enhanced bone mineral density (BMD), and decreased serum insulin and glucose levels in obese mice; and (4) MZ-2 (30 microM) increased BMD in human osteoblast cells (MG-63) comparable to naltrexone, while morphine inhibited mineral nodule formation. Thus, MZ-2 has potential application in the clinical management of obesity, insulin and glucose levels, and the amelioration of osteoporosis.

Joint capsule treatment with enkephalin-encoding HSV-1 recombinant vector reduces inflammatory damage and behavioural sequelae in rat CFA monoarthritis

This study assessed enkephalin expression induced by intra-articular application of recombinant, enkephalin-encoding herpes virus (HSV-1) and the impact of expression on nociceptive behaviours and synovial lining inflammation in arthritic rats. Replication-conditional HSV-1 recombinant vectors with cDNA encoding preproenkephalin (HSV-ENK), or control transgene beta-galactosidase cDNA (HSV-beta-gal; control) were injected into knee joints with complete Freund's adjuvant (CFA). Joint temperatures, circumferences and nociceptive behaviours were monitored on days 0, 7, 14 and 21 post CFA and vector treatments. Lumbar (L4-6) dorsal root ganglia (DRG) and spinal cords were immunostained for met-enkephalin (met-ENK), beta-gal, HSV-1 proteins and Fos. Joint tissues were immunostained for met-ENK, HSV-1 proteins, and inflammatory mediators Regulated on Activation, Normal T-cell Expressed and Secreted (RANTES) and cyclo-oxygenase-2, or stained with haematoxylin and eosin for histopathology. Compared to exuberant synovial hypertrophy and inflammatory cell infiltration seen in arthritic rats treated with CFA only or CFA and HSV-beta-gal, the CFA- and HSV-ENK-treated arthritic rats had: (i) striking preservation of synovial membrane cytoarchitecture with minimal inflammatory cell infiltrates; (ii) significantly improved nociceptive behavioural responses to mechanical and thermal stimuli; (iii) normalized Fos staining in lumbar dorsal horn; and (iv) significantly increased met-ENK staining in ipsilateral synovial tissue, lumbar DRG and spinal cord. The HSV-1 and transgene product expression were confined to ipsilateral lumbar DRG (HSV-1, met-ENK, beta-gal). Only transgene product (met-ENK and beta-gal) was seen in lumbar spinal cord sections. Targeted delivery of enkephalin-encoding HSV-1 vector generated safe, sustained opioid-induced analgesia with protective anti-inflammatory blunting in rat inflammatory arthritis.

[Bone mineral loss in patients with human immunodeficiency virus infection]

Life expectancy and quality of life among patients infected with human immunodeficiency virus (HIV) has dramatically improved with the advent of highly active antiretroviral therapy. Nonetheless, with the transformation of HIV infection into a chronic disease, a series of long-term consequences have been observed, among them osteoporosis. The prevalence of this condition is increasing among individuals with HIV infection. Although an epidemic of fragility fractures has not been described in these patients, an increase in the incidence of such fractures is anticipated as the HIV population ages, and this will undoubtedly have an impact on their quality of life. The numerous factors that favor the development of osteoporosis in these patients are related to the infection, itself, and the treatment received.

Opioid peptides and receptors in joint tissues: study in the rat

Using immunohistochemical and biochemical techniques, the occurrence of endogenous opioid peptides and their receptors in normal rat bone and joint tissues was investigated. Opioid receptors were detected, quantified, and characterized in homogenates from capsule/synovium and periosteum using radioligand binding assays. Receptor binding of the nonselective opioid [3H]naloxone to tissue homogenates was stereospecific and saturable, showing similar characteristics to that of brain tissue, although with lower binding capacities. By immunohistochemistry, the neuronal occurrence of four different enkephalins was demonstrated in synovium, bone marrow, periosteum, and juxta-articular bone, whereas no neuronal dynorphin immunoreactivity was detected. Double-staining studies disclosed that enkephalins coexisted with substance P in primary afferent fibers. The applied techniques can be used to assess changes in the distribution of endogenous opioids and their receptors in joint tissues in conditions associated with pain and inflammation. The endogenous opioid system now demonstrated might be targeted and exploited therapeutically to obtain peripheral control of symptoms in joint disorders.

Tissue levels of leu-enkephalin in rats with adjuvant arthritis

To study the level of leu-enkephalin in bone and joint tissues and in the spinal cord of rats with adjuvant arthritis, arthritis was induced in Lewis rats by the injection of Mycobacterium butyricum in Freund's incomplete adjuvant (FIA). Immunoelectron microscopy (IEM) was used to monitor the cellular distribution of leu-enkephalin in control and arthritis groups, and radioimmunoassay (RIA) was used to measure the concentration in the tissues. The results of IEM showed increased levels of leu-enkephalin in the matrix of the sciatic nerve, in nerve fibres in the synovial membrane and periosteum, as well as in fibroblasts and endothelial cells of the periosteum in arthritic groups. In macrophage-like cells of the synovial membrane as well as monocyte and polymorphonuclear lineage cells in the bone marrow, the level of leu-enkephalin was decreased in the arthritic group. The results of RIA showed that the concentration of leu-enkephalin was lower in the ankle and increased in the spinal cord of arthritic animals compared with controls. In conclusion, leu-enkephalin levels were decreased in joints and in bone marrow, but increased in nerve tissues in the group with arthritis. Further studies are needed to show whether leu-enkephalin is involved in a process that serves to limit the effect of immunisation.

Purification and quantification of opioid peptides in bone and joint tissues--a methodological study in the rat

The occurrence of methionine-enkephalin-Arg(6)-Phe(7) (MEAP) and dynorphin B (DYNB) representing two main precursors of opioids was analyzed in specimens from rat cortical bone, periosteum, bone marrow and joint tissue by radioimmunoassay (RIA). MEAP and DYNB were extracted in a solution of 4% EDTA in 2 M acetic acid previously proven suitable for extraction of sensory and autonomic neuropeptides in bone and joints. In crude extracts of cortical bone, the immunoreactive (ir) levels of both opioids were under the detection limit of RIA. As for DYNB this also applied to crude extracts of joints and periosteum. Therefore, two purification methods were tested and compared, i.e. reverse phase C 18 and ion exchange chromatography. RIA of the elution fraction disclosed a significant difference between the two methods in terms of recovery, i.e. <5% and 50%, respectively. Thus, purification by ion exchange chromatography prior to RIA appeared to be the most suitable by providing measurable levels of both MEAP and DYNB in all tissues analyzed (highest in bone marrow, lowest in cortical bone). The described method offers a means of quantifying opioid peptides in bone and joints, which may be utilized in the analysis of regulatory mechanisms of nociception, growth and immune responses in different conditions.

Distribution of leucine-enkephalin in bone and joint tissues

The distribution and concentration of leu-enkephalin in periosteum, cortical bone, bone marrow and synovial membrane of normal rats were analysed. Periosteum, cortical bone and bone marrow of the rat femurs were collected as well as the ankles. The distribution of leu-enkephalin was analysed by immunoelectron microscopy and the concentration of leu-enkephalin was measured with radioimmunoassay. Immunoelectron microscopy showed that leu-enkephalin is abundant in monocytes of bone marrow, nerve fibers and endothelial cells in the periosteum and also in macrophage-like-cells of the synovial membrane. The concentration of leu-enkephalin measured by RIA showed highest concentration in bone marrow followed by periosteum and cortical bone. The study supports that leu-enkephalin is present and can be quantified in different compartments of bone and joint tissues suggesting that leu-enkephalin may be involved in the physiological regulation of nociception and immunoregulation.

An opioid system in connective tissue: a study of achilles tendon in the rat

The occurrence of endogenous opioids and their receptors in rat achilles tendon was analyzed by immunohistochemistry (IHC), radioimmunoassay (RIA), and in vitro binding assays. The investigation focused on four enkephalins, dynorphin B, and nociceptin/orphanin FQ. Nerve fibers immunoreactive to all enkephalins (Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Gly-Lys, Met-enkephalin-Arg-Phe) were consistently found in the loose connective tissue and the paratenon, whereas dynorphin B and nociceptin/orphanin FQ could not be detected. The majority of enkephalin-positive nerve fibers exhibited varicosities predominantly seen in blood vessel walls. Measurable levels of Met-enkephalin-Arg-Phe and nociceptin/orphanin FQ were found in tendon tissue using RIA, whereas dynorphin B could not be detected. In addition to the endogenous opioids identified, delta-opioid receptors on nerve fibers were also detected by IHC. Binding assays to characterize the opioid binding sites showed that they were specific and saturable for [3H]-naloxone (Kd 7.01 +/- 0.98 nM; Bmax 23.52 +/- 2.23 fmol/mg protein). Our study demonstrates the occurrence of an opioid system in rat achilles tendon, which may be assumed to be present also in other connective tissues of the locomotor apparatus. This system may prove to be a useful target for pharmacological therapy in painful and inflammatory conditions by new drugs acting selectively in the periphery.

Therapeutic efficacy in experimental polyarthritis of viral-driven enkephalin overproduction in sensory neurons

Rheumatoid arthritis is characterized by erosive inflammation of the joints, new bone proliferation, and ankylosis, leading to severely reduced locomotion and intense chronic pain. In a model of this disease, adjuvant-induced polyarthritis in the rat, neurons involved in pain transmission and control undergo plastic changes, especially at the spinal level. These changes affect notably neurons that contain opioids, such as enkephalins deriving from preproenkephalin A (PA) precursor protein. Using recombinant herpes simplex virus containing rat PA cDNA, we enhanced enkephalin synthesis in sensory neurons of polyarthritic rats. This treatment markedly improved locomotion and reduced hyperalgesia. Furthermore, the progression of bone destruction slowed down, which is the most difficult target to reach in the treatment of patients suffering from arthritis. These data demonstrate the therapeutic efficacy of enkephalin overproduction in a model of systemic inflammatory and painful chronic disorder.

Gene therapy of pain: emerging strategies and future directions

Gene therapy to alleviate pain could appear surprising and perhaps not appropriate when opioids and other active molecules are available. However, the possibility of introducing a therapeutic protein into some targeted structures, where it would be continuously synthesised and exert its biological effect in the near vicinity of, or inside the cells, might avoid some drawbacks of "classical" drugs. Moreover, the gene-transfer techniques might improve present therapies or lead to novel ones. The recent significant and constant advances in vector systems design suggest that these techniques will be available in the near future for safe application in humans. The first experimental protocols attempting the transfer of opioid precursors genes, leading to their overexpression at the spinal level, demonstrated the feasibility and the potential interest of these approaches. Indeed, overproduction of opioid peptides in primary sensory neurones or spinal cord induced antihyperalgesic effects in various animal models of persistent pain. However, numerous other molecules involved in pain processing or associated with chronic pain have been identified and the gene-based techniques might be particularly adapted for the evaluation of the possible therapeutic interest of these new potential targets.

Met-enkephalin is preferentially transported into the peripheral processes of primary afferent fibres in both control and HSV1-driven proenkephalin A overexpressing rats

The demonstration of preproenkephalin A gene expression in rat dorsal root ganglia has raised the question of the physiological role of met-enkephalin-containing primary afferent fibres. Recently, we showed that systemic infection with a recombinant Herpes simplex virus encoding preproenkephalin A (HSVLatEnk1) yielded a marked increase in the density of met-enkephalin-like material synthesising neurons in rat dorsal root ganglia. This study further investigated the synthesis, transport and release of met-enkephalin-like material in the central and/or peripheral processes of primary afferent fibres in HSVLatEnk1-infected and control rats. In controls, dorsal root ganglia neurons containing met-enkephalin-like material were scarce and only a few positively labelled processes were seen at the peripheral output of the dorsal root ganglia. Met-enkephalin-like material accumulated at the proximal side of ligatured sciatic nerve, but not in ligatured L4-L5 dorsal roots. In HSVLatEnk1-infected rats with numerous somas and fibres stained for met-enkephalin-like material in dorsal root ganglia, met-enkephalin immunoreactive material largely accumulated at the proximal side of the ligatured sciatic nerve and few positively stained fibres were also observed in ligatured dorsal roots. Electrical stimulation of L4-L5 dorsal roots attached to a dorsal slice of the lumbar enlargement produced an overflow of met-enkephalin-like material which was approximately 70% higher in HSVLatEnk1-infected rats compared to controls. At the periphery, subcutaneous microdialysis showed higher basal levels of met-enkephalin-like material in the interstitial fluid of hindpaw plantar area in HSVLatEnk1-infected rats, and electrical stimulation of the ipsilateral sciatic nerve resulted in an approximately three-fold-higher overflow of this material than in control rats. These data demonstrated that met-enkephalin synthesised in dorsal root ganglion of both control and preproenkephalin A overexpressing rats is preferentially transported into the peripheral processes of primary afferent fibres where the peptide reaches a releasable compartment, thus providing a neuronal source of peripheral met-enkephalin.

Identification of dipeptidyl peptidase III in human neutrophils

We have found activity of dipeptidyl peptidase (DPP) III, one of the most important enkephalin-degrading enzymes in the central nervous system, in human neutrophils. HPLC analysis of the peptide fragments produced by treatment of leucine-enkephalin with isolated neutrophils in the presence of inhibitors of other enkephalin-degrading enzymes revealed that the enzyme in human neutrophils cleaved dipeptides from the NH(2) terminus of leucine-enkephalin, suggesting the presence of DPPIII activity in human neutrophils. Using a specific synthesized substrate and proteinase inhibitors, it was found that the neutrophils have 19.2 +/- 3.6 microM/h/5 x 10(6) cells of beta-naphthylamine for the enzyme. It was also confirmed that spinorphin and tynorphin, both reported to inhibit the activities of enkephalin-degrading enzymes, had potent inhibitory activities (IC(50): 4.0 and 0.029 microg/ml, respectively) against the enzyme. The presence of DPPIII activity in human neutrophils suggests that the biologically active peptides which are associated with enkephalin play a physiological role in regulating enkephalin or inflammatory mechanisms in peripheral tissues.