The endogenous opioid dynorphin is required for normal bone homeostasis in mice

Two Weeks of Continuous Opioid Treatment in an Adenine-Induced Mouse Model of Chronic Kidney Disease Exacerbates the Bone Inflammatory State and Increases Osteoclasts

Patients with chronic kidney disease (CKD) report high pain levels, but reduced renal clearance eliminates many analgesic options; therefore, 30-50% of CKD patients have chronic opioid prescriptions. Opioid use in CKD is associated with higher fracture rates. Opioids may directly alter bone turnover directly through effects on bone cells and indirectly via increasing inflammation. We hypothesized that continuous opioid exposure would exacerbate the high bone turnover state of CKD and be associated with elevated measures of inflammation. Male C57Bl/6J mice after 8 weeks of adenine-induced CKD (AD) and non-AD controls (CON) had 14-day osmotic pumps (0.25-µL/hr release) containing either saline or 50-mg/mL oxycodone (OXY) surgically implanted in the subscapular region. After 2 weeks, all AD mice had elevated blood urea nitrogen, parathyroid hormone, and serum markers of bone turnover compared to controls with no effect of OXY. Immunohistochemical staining of the distal femur showed increased numbers of osteocytes positive for the mu opioid and for toll-like receptor 4 (TLR4) due to OXY. Osteocyte protein expression of tumor necrosis factor-α (TNF-α) and RANKL were higher due to both AD and OXY so that AD + OXY mice had the highest values. Trabecular osteoclast-covered surfaces were also significantly higher due to both AD and OXY, resulting in AD + OXY mice having 4.5-fold higher osteoclast-covered surfaces than untreated CON. These data demonstrate that opioids are associated with a pro-inflammatory state in osteocytes which increases the pro-resorptive state of CKD.

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.

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.

From Pharmacology to Physiology: Endocrine Functions of μ-Opioid Receptor Networks

The steady rise in opioid users and abusers has uncovered multiple detrimental health consequences of perturbed opioid receptor signaling, thereby creating the need to better understand the biology of these systems. Among endogenous opioid networks, μ-receptors have received special attention due to their unprecedented biological complexity and broad implications in homeostatic functions. Here, we review the origin, molecular biology, and physiology of endogenous opioids with a special focus on μ-opioid receptor networks within the endocrine system. Moreover, we summarize the current evidence supporting an involvement of the latter in regulating distinct endocrine functions. Finally, we combine these insights to present an integrated perspective on μ-opioid receptor biology and provide an outlook on future studies and unresolved questions in this field.

The blockade of kappa opioid receptors exacerbates alveolar bone resorption in rats

OBJECTIVES

Bone resorption associated to chronic diseases, such as arthritis and periodontitis, results from exacerbated immuno-inflammatory host response that leads to tissue breakdown. The significance of opioid pathways as endogenous modulators of inflammatory events has already been described. Thus, the aim of this work is to determine whether some of the main three opioid receptors are endogenously activated to prevent bone loss during experimentally-induced alveolar bone resorption.

DESIGN

This study used an experimental model of alveolar bone resorption induced by ligature in rats. A silk thread was placed around the 2nd maxillary molar of male Wistar rats. In the 3rd, 4th and 5th day after ligation the rats received a local injection of different concentrations of opioid antagonists Cyprodime, Naltrindole, or Nor-binaltorphimine, which specifically block mü, delta and kappa opioid receptors, respectively. In the 7th experimental day, rats were euthanized and their maxillae collected for evaluation of alveolar bone and fiber attachment loss, morphometric counting of osteoclasts and osteoblasts, as well as the levels of cytokines IL-1β, IFN-γ, and IL-6 by ELISA.

RESULTS

Selective antagonism of kappa opioid receptors, but not mü and delta, exacerbated alveolar bone resorption induced by ligature in rats. The increased bone loss associated with higher number of osteoclasts surrounding alveolar bone, although osteoblasts' counting remained unchanged. The concentrations of IL-1β and IL-6 in periodontal tissues were also significantly higher in the rats treated with the kappa antagonist.

CONCLUSION

Inhibiting kappa opioid receptors exacerbates alveolar bone resorption.

Intracrine Endorphinergic Systems in Modulation of Myocardial Differentiation

A wide variety of peptides not only interact with the cell surface, but govern complex signaling from inside the cell. This has been referred to as an "intracrine" action, and the orchestrating molecules as "intracrines". Here, we review the intracrine action of dynorphin B, a bioactive end-product of the prodynorphin gene, on nuclear opioid receptors and nuclear protein kinase C signaling to stimulate the transcription of a gene program of cardiogenesis. The ability of intracrine dynorphin B to prime the transcription of its own coding gene in isolated nuclei is discussed as a feed-forward loop of gene expression amplification and synchronization. We describe the role of hyaluronan mixed esters of butyric and retinoic acids as synthetic intracrines, controlling prodynorphin gene expression, cardiogenesis, and cardiac repair. We also discuss the increase in prodynorphin gene transcription and intracellular dynorphin B afforded by electromagnetic fields in stem cells, as a mechanism of cardiogenic signaling and enhancement in the yield of stem cell-derived cardiomyocytes. We underline the possibility of using the diffusive features of physical energies to modulate intracrinergic systems without the needs of viral vector-mediated gene transfer technologies, and prompt the exploration of this hypothesis in the near future.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

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.

The brain in bone and fuel metabolism

Obesity and osteoporosis have become major public health challenges worldwide. The brain is well established as a pivotal regulator of energy homeostasis, appetite and fuel metabolism. However, there is now clear evidence for regulation between the brain and bone. Similarly, evidence also indicates that the involvement of the brain in bone and adipose regulation is both related and interdependent. The hypothalamus, with its semi-permeable blood brain barrier, is one of the most powerful regulatory regions within the body, integrating and relaying signals not only from peripheral tissues but also from within the brain itself. Two main neuronal populations within the arcuate nucleus of the hypothalamus regulate energy homeostasis: The orexigenic, appetite-stimulating neurons that co-express neuropeptide Y and agouti-related peptide and the anorexigenic, appetite-suppressing neurons that co-express proopiomelanocortin and cocaine- and amphetamine related transcript. From within the arcuate, these four neuropeptides encompass some of the most powerful control of energy homeostasis in the entire body. Moreover, they also regulate skeletal homeostasis, identifying a co-ordination network linking the processes of bone and energy homeostasis. Excitingly, the number of central neuropeptides and neural factors known to regulate bone and energy homeostasis continues to grow, with cannabinoid receptors and semaphorins also involved in bone homeostasis. These neuronal pathways represent a growing area of research that is identifying novel regulatory axes between the brain and the bone, and links with other homeostatic networks; thereby revealing a far more complex, and interdependent bone biology than previously envisioned. This review examines the current understanding of the central regulation of bone and energy metabolism.

Evaluating the affect and reversibility of opioid-induced androgen deficiency in an orthopaedic animal fracture model

BACKGROUND

Opioid pain medications are the basis for analgesia after orthopaedic injuries and procedures. However, opioids have many adverse effects, including opioid-induced androgen deficiency.

QUESTIONS/PURPOSES

We evaluated the occurrence and affect of opioid-induced androgen deficiency on osseous union and its ability to be reversed. Additional focus was placed on its perioperative onset and its duration in an orthopaedic animal model.

METHODS

A femoral osteotomy was created in 75 Sprague-Dawley rats. Postoperatively, animals were randomized into three treatment groups: control, morphine, and morphine plus testosterone. Testosterone levels were recorded preoperatively and at 48 hours, 4 weeks, and 8 weeks postoperatively. Some animals were euthanized at 4 weeks and others at 8 weeks postoperatively. Histology and micro-CT scans were used to evaluate callus. Three-point bend testing was performed to evaluate callus strength.

RESULTS

Serum testosterone levels in the morphine group showed decreased baseline levels of 2.2 ng/mL, 1.4 ng/mL, and 1.4 ng/mL (p < 0.001), whereas the morphine plus testosterone supplementation group showed increased serum levels at 41.7 ng/mL, 11.8 ng/mL, and 19.8 ng/mL (p < 0.001) compared with control animals (3.3 ng/mL, 5.8 ng/mL, 5.2 ng/mL) at 48 hours, 4 weeks, and 8 weeks, respectively. Compared with control animals, histology and micro-CT showed an impedance of callus maturation in the two experimental groups. Morphine-treated animals showed reduction in callus strength at 8 weeks (30% of the contralateral unfractured femur strength compared with 49% seen in the control animals at 8 weeks; p = 0.048); this finding was not fully reversed by testosterone supplementation (33% of the contralateral femur strength; p = 0.171).

CONCLUSIONS

Opioid-induced androgen deficiency occurred in this orthopaedic animal model. Although we previously showed that morphine inhibits callus maturation, the current study did not show a rescue of the morphine-treated animals with testosterone supplementation in either morphologic or mechanical testing. Testosterone suppression associated with opioid administration occurred almost immediately (within 48 hours) and was suppressed continually throughout the 8-week duration of study.

CLINICAL RELEVANCE

Opioid-induced androgen deficiency occurs during the perioperative orthopaedic period. Although its clinical relevance remains unknown, further evaluation is needed to determine if supplementation is warranted during the perioperative period.

Double deletion of orexigenic neuropeptide Y and dynorphin results in paradoxical obesity in mice

OBJECTIVE

Orexigenic neuropeptide Y (NPY) and dynorphin (DYN) regulate energy homeostasis. Single NPY or dynorphin deletion reduces food intake or increases fat loss. Future developments of obesity therapeutics involve targeting multiple pathways. We hypothesised that NPY and dynorphin regulate energy homeostasis independently, thus double NPY and dynorphin ablation would result in greater weight and/or fat loss than the absence of NPY or dynorphin alone.

DESIGN AND METHODS

We generated single and double NPY and dynorphin knockout mice (NPYΔ, DYNΔ, NPYDYNΔ) and compared body weight, adiposity, feeding behaviour, glucose homeostasis and brown adipose tissue uncoupling protein-1 (UCP-1) expression to wildtype counterparts.

RESULTS

Body weight and adiposity were significantly increased in NPYDYNΔ, but not in NPYΔ or DYNΔ. This was not due to increased food intake or altered UCP-1 expression, which were not significantly altered in double knockouts. NPYDYNΔ mice demonstrated increased body weight loss after a 24-h fast, with no effect on serum glucose levels after glucose injection.

CONCLUSIONS

Contrary to the predicted phenotype delineated from single knockouts, double NPY and dynorphin deletion resulted in heavier mice, with increased adiposity, despite no significant changes in food intake or UCP-1 activity. This indicates that combining long-term opioid antagonism with blockade of NPY-ergic systems may not produce anti-obesity effects.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).