Adults with excess weight or obesity, but not with overweight, report greater pain intensities than individuals with normal weight: a systematic review and meta-analysis

Context

Over 1.9 billion adult people have overweight or obesity. Considered as a chronic disease itself, obesity is associated with several comorbidities. Chronic pain affects approximately 60 million people and its connection with obesity has been displayed in several studies. However, controversial results showing both lower and higher pain thresholds in subjects with obesity compared to individuals with normal weight and the different parameters used to define such association (e.g., pain severity, frequency or duration) make it hard to draw straight forward conclusions in the matter. The objective of this article is to examine the relationship between overweight and obesity (classified with BMI as recommended by WHO) and self-perceived pain intensity in adults.

Methods

A literature search was conducted following PRISMA guidelines using the databases CINAHL, Cochrane Library, EMBASE, PEDro, PubMed, Scopus and Web of Science to identify original studies that provide BMI values and their associated pain intensity assessed by self-report scales. Self-report pain scores were normalized and pooled within meta-analyses. The Cochrane's Q test and I2 index were used to clarify the amount of heterogeneity; meta-regression was performed to explore the relationship between each outcome and the risk of bias.

Results

Of 2194 studies, 31 eligible studies were identified and appraised, 22 of which provided data for a quantitative analysis. The results herein suggested that adults with excess weight (BMI ≥ 25.0) or obesity (BMI ≥ 30.0) but not with overweight (pre-obesity) alone (BMI 25.0-29.9), are more likely to report greater intensities of pain than individuals of normal weight (BMI 18.5-24.9). Subgroup analyses regarding the pathology of the patients showed no statistically significant differences between groups. Also, influence of age in the effect size, evaluated by meta-regression, was only observed in one of the four analyses. Furthermore, the robustness of the findings was supported by two different sensitivity analyses.

Conclusion

Subjects with obesity and excess weight, but not overweight, reported greater pain intensities than individuals with normal weight. This finding encourages treatment of obesity as a component of pain management. More research is required to better understand the mechanisms of these differences and the clinical utility of the findings.

Systematic Review Registration

https://doi.org/10.17605/OSF.IO/RF2G3, identifier OSF.IO/RF2G3.

SHORT-TERM STRESS SIGNIFICANTLY DECREASES MORPHINE ANALGESIA IN TRIGEMINAL BUT NOT IN SPINAL INNERVATED AREAS IN RATS

Plenty information exists regarding the effects of chronic stress, although few data exist on the effects of short-lasting stressors, which would mimic daily challenges. Differences in craniofacial and spinal nociception have been observed, thus those observations obtained in spinally innervated areas cannot be directly applied to the orofacial region. Although, opioids are considered amongst the most effective analgesics, their use is sometimes hampered by the constipation they induce. Thus, our aims were to study if a short-lasting stressor, forced swim stress (FSS), modifies nociception, morphine antinociception and constipation in rats. Animals were submitted to 10-20min of FSS for three days, nociception and gastrointestinal transit were studied 24h after the last swimming session. Nociception and morphine (0.6-5mg/kg) antinociception were evaluated in the formalin and hypertonic saline tests in the orofacial area and limbs. Morphine-induced modifications in the GI transit were studied through radiographic techniques. Naloxone was administered, before each swimming session, to analyse the involvement of the endogenous opioid system on the effect of stress. Overall, stress did not alter nociception, although interestingly it reduced the effect of morphine in the orofacial tests and in the inflammatory phase of the formalin tests. Naloxone antagonized the effect of stress and normalized the effect of morphine. Stress did not modify the constipation induced by morphine. Opioid treatment may be less effective under a stressful situation, whilst adverse effects, such as constipation, are maintained. The prevention of stress may improve the level of opioid analgesia. Keywords.

Antinociceptive and modulatory effect of pathoplastic changes in spinal glia of a TLR4/CD14 blocking molecule in two models of pain in rat

The role of spinal glia in the development and maintenance of chronic pain has become over the last years a subject of increasing interest. In this regard, toll-like receptor 4 (TLR4) signaling has been proposed as a major trigger mechanism. Hence, in this study we explored the implications of TLR4 inhibition in the periphery and primarily in the CNS, focusing on the impact this inhibition renders in pain development and glia activation in the dorsal horn in two models of pain. Making use of a synthetic cluster of differentiation 14 (CD14)/TLR4 antagonist, the effect of TLR4 blockade on tactile allodynia and heat hyperalgesia was evaluated in osteoarthritic and postoperative rat models. An in vitro parallel artificial membrane permeation assay was performed to determine the proneness of the drug to permeate the blood-brain barrier prior to systemic and central administration. Findings suggest a dominant role of peripheral TLR4 in the model of incisional pain, whilst both peripheral and central TLR4 seem to be responsible for osteoarthritic pain. That is, central and peripheral TLR4 may be differently involved in the etiopathology of diverse types of pain what potentially seems a promising approach in the management of pain.

Monoclonal Antibodies for Chronic Pain Treatment: Present and Future

Chronic pain remains a major problem worldwide, despite the availability of various non-pharmacological and pharmacological treatment options. Therefore, new analgesics with novel mechanisms of action are needed. Monoclonal antibodies (mAbs) are directed against specific, targeted molecules involved in pain signaling and processing pathways that look to be very effective and promising as a novel therapy in pain management. Thus, there are mAbs against tumor necrosis factor (TNF), nerve growth factor (NGF), calcitonin gene-related peptide (CGRP), or interleukin-6 (IL-6), among others, which are already recommended in the treatment of chronic pain conditions such as osteoarthritis, chronic lower back pain, migraine, or rheumatoid arthritis that are under preclinical research. This narrative review summarizes the preclinical and clinical evidence supporting the use of these agents in the treatment of chronic pain.

TLR4 Antagonism Reduces Movement-Induced Nociception and ATF-3 Expression in Experimental Osteoarthritis

Introduction

Toll-like receptor 4 (TLR4) is a pattern recognition receptor involved in the detection of pathogen-associated molecular patterns (PAMPs), but also a "danger-sensing" receptor that recognizes host-derived endogenous molecules called damage-associated molecular patterns (DAMPs). The involvement of TLR4 in rheumatic diseases is becoming evident, as well as its potential role as a target for therapeutic intervention. Moreover, increasing evidence also suggests that TLR4 is implicated in chronic pain states. Thus, in this study, we evaluated whether a systemic administration of a synthetic antagonist of TLR4 (TLR4-A1) could decrease nociception and cartilage degradation in experimental osteoarthritis (OA). Furthermore, as the activation transcription factor (ATF)-3 serves as a negative regulator for TLR4-stimulated inflammatory response, we also evaluated the effect of TLR4 inhibition on ATF-3 expression in primary afferent neurons at the dorsal root ganglia (DRG).

Methods

OA was induced in adult male Wistar rats through an intra-articular injection of 2 mg of sodium mono-iodoacetate (MIA) into the left knee. From days 14 to 28 after OA induction, animals received an intraperitoneal injection of either TLR4-A1 (10 mg/kg) or vehicle. Movement- and loading-induced nociception was evaluated in all animals, by the Knee-Bend and CatWalk tests, before and at several time-points after TLR4-A1/vehicle administration. Immunofluorescence for TLR4 and ATF-3 was performed in L3-L5 DRG. Knee joints were processed for histopathological evaluation.

Results

Administration of TLR4-A1 markedly reduced movement-induced nociception in OA animals, particularly in the Knee-Bend test. Moreover, the increase of ATF-3 expression observed in DRG of OA animals was significantly reduced by TLR4-A1. However, no effect was observed in cartilage loss nor in the neuronal cytoplasmic expression of TLR4 upon antagonist administration.

Conclusion

The TLR4 antagonist administration possibly interrupts the TLR4 signalling cascade, thus decreasing the neurotoxic environment at the joint, which leads to a reduction in ATF-3 expression and in nociception associated with experimental OA.

Behavior and electrophysiology studies of the peripheral neuropathy induced by individual and co-administration of paclitaxel and oxaliplatin in rat

AIMS

Antitumor agents, as taxanes and platinum compounds, induce peripheral neuropathies which can hamper their use for cancer treatment. The study of chemotherapy-induced neuropathies in humans is difficult because of ethical reasons, differences among administration protocols and intrinsic characteristics of patients. The aim of the present study is to compare the neuropathic signs induced by individual or combined administration of paclitaxel and oxaliplatin.

MAIN METHODS

Oxaliplatin and paclitaxel were administered individually and combined to induce peripheral neuropathy in rats, sensory neuropathic signs were assessed in the hind limbs and orofacial area. The in vitro skin-saphenous nerve preparation was used to record the axonal activity of Aδ sensory neurons.

KEY FINDINGS

Animals treated with the combination developed mechanical allodynia in the paws and muscular hyperalgesia in the orofacial area, which was similar to that in animals treated with monotherapy, the latter also developed cold allodynia in the paws. Aδ-fibers of the rats treated with the combination were hyperexcited and presented hypersensitivity to pressure stimulation of the innervated skin, also similar to that recorded in the fibers of the animals treated with monotherapy.

SIGNIFICANCE

Our work objectively demonstrates that the combination of a platinum compound with a taxane does not worsen the development of sensorial neuropathies in rats, which is an interesting data to take into account when the combination of antitumor drugs is necessary. Co-administration of antitumor drugs is more effective in cancer treatment without increasing the risk of the disabling neuropathic side effects.

Maternal separation affects the electrophysiological properties of Aδ-fibres and nociceptive behaviours in male and female mice

AIM

Early life adverse effects have been associated with an increased risk of suffering pain syndromes in adulthood. Although animal models are of great importance to study modifications of pain sensitivity, up to date the results obtained are contradicting due to the varied methodologies used. Therefore, the aim of the present study was to characterise, as a whole, possible modifications in visceral and somatic nociceptive responses in male and female ICR mice, submitted to two different protocols of maternal separation (MS), and possible modifications in the electrophysiological properties of peripheral nociceptive Aδ-primary afferents.

MAIN METHODS

Male and female mice were submitted to 3 or 4-8 hr of daily MS from postnatal day (PND) 2-17 and early weaned. On PND 67 von Frey, hot plate and writhing tests were performed. Afterwards electrophysiological recordings were carried out, using the in vitro skin-saphenous nerve preparation in males.

KEY FINDINGS

The short separation protocol of MS did not modify nociceptive sensitivity; but when mice were separated from their dams for the long separation, mechanical pain thresholds were modified in male and female mice and visceral nociception was increased in female mice. Electrophysiological recordings showed that cutaneous Aδ-fibres were sensitised and their mechanotransduction properties were altered in both MS protocols.

SIGNIFICANCE

Although MS increases the activity and the mechanosensitivity of cutaneous Aδ-afferent fibres at both short and long periods of separation, only the longer interval of time induces nociceptive sensitivity alterations during adulthood. These results highlight the possible influence of a stress free environment during childhood to reduce nociceptive alterations in adulthood.

Toll-like receptor 4: A promising crossroads in the diagnosis and treatment of several pathologies

Toll-like receptor 4 (TLR4) is expressed in a wide variety of cells and is the central component of the mammalian innate immune system. Since its discovery in 1997, TLR4 has been assigned an ever-increasing number of functions that extend from pathogen recognition to tissue damage identification and promotion of the intrinsic "damage repair response" in pain, intestinal, respiratory and vascular disorders. Precisely, the finding of conserved sequence homology among species along with the molecular and functional characterisation of the TLR4 gene enabled researchers to envisage a common operating system in the activation of innate immunity and the initiation of plastic changes at the onset of chronic pain. Malfunctioning in other conditions was conceived in parallel. In this respect, "pivot" proteins and pathway redundancy are not just evolutionary leftovers but essential for normal functioning or cell survival. Indeed, at present, TLR4 single nucleotide polymorphisms (SNP) and their association with certain dysfunctions and diseases are being confirmed in different pools of patients. However, despite its ability to trigger pathogen infection or alternatively tissue injury communications to immune system, TLR4 targeting might not be considered a panacea. This review article represents a compilation of what we know about TLR4 from clinics and basic research on the 20th anniversary of its discovery. Understanding how to fine-tune the interaction between TLR4 and its specific ligands may lead in the next decades to the development of promising new treatments, reducing polypharmacy and probably having an impact on drug use in numerous pathologies.

Animal models in the study and treatment of orofacial pain

Background

Pain is one of the first causes of medical consultation in the world and by extension of dental consultation too. Orofacial pain comprehends the oral and facial regions including teeth, oral mucosa, gingiva, tongue and lips, but also the muscles of the jaw and neck, the temporomandibular joint, face, head and neck. Despite its highly estimated prevalence, it appears controversial and hard to quantify given the lack of common criteria to select the population under study and the difficulties to classify the different types of pain. Although for many patients the problem eventually fades after tissue healing, certain sub-chronic and chronic pain conditions remain notoriously undertreated. In this respect, animal models can be of great help.

Material and Methods

A systematic search was conducted in PubMed-Medline with appropriate keywords: orofacial pain, prevalence and dentist. Seven groups were generated and a second search based on each of these groups and on animal models was made. Search was restricted to English and Spanish, but no time restriction was applied.

Results

There are as yet few experimental models of orofacial pain: there hardly exists no other than trigeminal nerve injury for neuropathic pain, a bunch of oral squamous cell carcinoma models (mainly referred to the tongue) for cancer pain and none for the painful swelling of salivary glands. Similarly occurs for the burning mouth syndrome. A few more exist for inflammatory odontalgiae, aphthae, joint, myofascial and muscle inflammatory pains, although scarcely diverse as regards the nature of the noxious stimulus.

Conclusions

Given the relevance of envisaging the mechanistic of the various types of orofacial pain, new experimental models are needed on the basis of the dentist's perspective for their correct management. Key words:Orofacial pain, neuralgia, odontalgia, oral cancer, animal models.

Comparison of the antinociceptive profiles of morphine and oxycodone in two models of inflammatory and osteoarthritic pain in rat

Oxycodone and morphine are two opioid drugs commonly used for the treatment of moderate to severe pain. However, their use in the management of noncancer pain remains a controversial issue and, in this respect, the evidence on their effectiveness and safety, particularly in osteoarthritis, is being questioned. In order to analyse their analgesic profile, two different pain models in rats were used: the formalin-induced inflammatory pain and the monosodium iodoacetate (MIA)-induced knee osteoarthritic pain. Drugs were administered systemically (i.p.) and their antinociceptive effect and potency were assessed. In the formalin test, both morphine and oxycodone produced a dose-dependent antinociceptive effect, but oxycodone outdid morphine in terms of effectiveness and potency (nearly two times) in the early (acute nociceptive) as in the late phase (inflammatory). In the osteoarthritis model, both drugs reduced movement-evoked pain (knee-bend test), mechanical allodynia (von Frey test) and heat hyperalgesia (Plantar test). Pretreatment with naloxone and naloxone methiodide reduced morphine and oxycodone effects. Peripheral mu-opioid receptors play a crucial role in the antinociceptive effect of both drugs on movement-evoked pain and heat hyperalgesia, but not on tactile allodynia. The main finding of our study is that oxycodone has a better antinociceptive profile in the inflammatory and osteoarthritic pain, being more effective than morphine at 14 days post-MIA injection (phase with neuropathic pain); it overcame the morphine effect by improving the movement-induced pain, tactile allodynia and heat hyperalgesia. Therefore, oxycodone could be an interesting option to treat patients suffering from knee osteoarthritis when opioids are required.

May a sigma-1 antagonist improve neuropathic signs induced by cisplatin and vincristine in rats?

BACKGROUND

The antineoplastic drugs cisplatin and vincristine induce peripheral neuropathies. The sigma-1 receptor (σ1R) is expressed in areas of pain control, and its blockade with the novel selective antagonist MR-309 has shown efficacy in nociceptive and neuropathic pain models. Our goal was to test whether this compound reduces neuropathic signs provoked by these antitumoural drugs.

METHODS

Rats were treated with cisplatin or vincristine to induce neuropathies. The effects of acute or repeated administration of MR-309 were tested on mechanical and thermal sensitivity, electrophysiological activity of Aδ-primary afferents in the rat skin-saphenous nerve preparation, and gastrointestinal or cardiovascular functions.

RESULTS

Rats treated with antitumourals developed tactile allodynia, while those treated with vincristine also developed mechanical hyperalgesia. These in vivo modifications correlated with electrophysiological hyperactivity (increased spontaneous activity and hyperresponsiveness to innocuous and noxious mechanical stimulation). Animals treated with cisplatin showed gastrointestinal impairment and those receiving vincristine showed cardiovascular toxicity. A single dose of MR-309 strongly reduced both nociceptive behaviour and electrophysiological changes. Moreover, its concomitant administration with the antitumourals blocked the development of neuropathic symptoms, thus restoring mechanical sensitivity, improving the impairment of feeding behaviour and gastrointestinal transit in the cisplatin-treated group along with ameliorating the altered vascular reactivity recorded in rats treated with vincristine.

CONCLUSION

σ1R antagonist, MR-309, reduces sensorial and electrophysiological neuropathic signs in rats treated with cisplatin or vincristine and, in addition, reduces gastrointestinal and cardiovascular side effects.

SIGNIFICANCE

σ1R antagonism could be an interesting and new option to palliate antitumoural neuropathies.

The role of Omega-3 and Omega-9 fatty acids for the treatment of neuropathic pain after neurotrauma

Omega-3 polyunsaturated fatty acids (PUFAs), such as docosaexaenoic acid (DHA) and eicosapentaenoic acid (EPA), mediate neuroactive effects in experimental models of traumatic peripheral nerve and spinal cord injury. Cellular mechanisms of PUFAs include reduced neuroinflammation and oxidative stress, enhanced neurotrophic support, and activation of cell survival pathways. Bioactive Omega-9 monounsaturated fatty acids, such as oleic acid (OA) and 2-hydroxy oleic acid (2-OHOA), also show therapeutic effects in neurotrauma models. These FAs reduces noxious hyperreflexia and pain-related anxiety behavior following peripheral nerve injury and improves sensorimotor function following spinal cord injury (SCI), including facilitation of descending inhibitory antinociception. The relative safe profile of neuroactive fatty acids (FAs) holds promise for the future clinical development of these molecules as analgesic agents. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Blockade of sigma 1 receptors alleviates sensory signs of diabetic neuropathy in rats

BACKGROUND

E-52862 (S1RA, 4-[2-[[5-methyl-1-(2-naphthalenyl)-1H-pyrazol-3-yl]oxy]ethyl]-morpholine), a novel selective sigma 1 receptor (σ1R) antagonist, has demonstrated efficacy in nociceptive and neuropathic pain models. Our aim was to test if σ1R blockade with E-52862 may modify the signs of neuropathy in Zucker diabetic fatty (ZDF) rats, a type 2 diabetes model.

METHODS

Mechanical and thermal response thresholds were tested on 7-, 13-, 14- and 15-week-old ZDF rats treated with saline or with E-52862 acutely administered on week 13, followed by sub-chronic administration (14 days). Axonal peripheral activity (skin-saphenous nerve preparation) and isolated aorta or mesenteric bed reactivity were analysed in 15-week-old ZDF rats treated with saline or E-52862 and in LEAN rats.

RESULTS

Zucker diabetic fatty rats showed significantly decreased thermal withdrawal latency and threshold to mechanical stimulation on week 13 compared to week 7 (prediabetes) and with LEAN animals; single-dose and sub-chronic E-52862 administration restored both parameters to those recorded on week 7. Regarding axonal peripheral activity, E-52862 treatment increased the mean mechanical threshold (77.3 ± 21 mN vs. 19.6 ± 1.5 mN, saline group) and reduced the response evoked by mechanical increasing stimulation (86.4 ± 36.5 vs. 352.8 ± 41.4 spikes) or by repeated mechanical supra-threshold steps (39.4 ± 1.4 vs. 83.5 ± 0.9). E-52862 treatment also restored contractile response to phenylephrine in aorta and mesenteric bed.

CONCLUSIONS

E-52862 administration reverses neuropathic (behavioural and electrophysiological) and vascular signs in the ZDF rat.

SIGNIFICANCE

Blockade of σ1R avoids the development of diabetic neuropathy in rats, and may represent a potentially useful therapeutic approach to peripheral neuropathies in diabetic patients. WHAT DOES THIS STUDY ADD?: This study presents evidences for the potential usefulness of sigma receptor blockade on diabetic neuropathy in rats. The methodology includes behavioural evidences, electrophysiological data and vascular-isolated models.

Adamantyl analogues of paracetamol as potent analgesic drugs via inhibition of TRPA1

Paracetamol also known as acetaminophen, is a widely used analgesic and antipyretic agent. We report the synthesis and biological evaluation of adamantyl analogues of paracetamol with important analgesic properties. The mechanism of nociception of compound 6a/b, an analog of paracetamol, is not exerted through direct interaction with cannabinoid receptors, nor by inhibiting COX. It behaves as an interesting selective TRPA1 channel antagonist, which may be responsible for its analgesic properties, whereas it has no effect on the TRPM8 nor TRPV1 channels. The possibility of replacing a phenyl ring by an adamantyl ring opens new avenues in other fields of medicinal chemistry.

Pain relief by applying transcutaneous electrical nerve stimulation (TENS) during unsedated colonoscopy: a randomized double-blind placebo-controlled trial

Transcutaneous electrical nerve stimulation (TENS) is a noninvasive alternative to traditional pain treatments. TENS has been studied in the past as a pain reduction modality in colonoscopy with limited success. Reviews and meta-analysis have shown that the inconclusive results of TENS may be due to the lack of randomized controlled trials and the difficulty in defining precise output parameters. The objective of this double-blind randomized placebo-controlled trial was to investigate the pain-relieving effect of a new application of TENS in unsedated screening colonoscopy. Ninety patients undergoing unsedated screening colonoscopy were randomly allocated to one of three groups: a control group (n=30), a group to receive active TENS (n=30), or a group to receive placebo TENS (n=30). A visual analogue scale (VAS) and a five-point Likert scale were used to assess pain 5 min into the procedure and at the end of the procedure. The patient's bloating sensation during colonoscopy and the effect on the duration of the procedure were also evaluated. Throughout the procedure, the active TENS group experienced a VAS pain score reduction ≥50% compared to the placebo TENS group (P<0.001) and the control group (P<0.001). On the five-point Likert scale, there was also a significant reduction in pain score in the active TENS group compared to the placebo TENS and control groups (P=0.009). No significant differences were found between the study groups as to the bloating sensation and the duration of the procedure. We conclude that TENS can be used as a pain relief therapy in unsedated screening colonoscopy.

Antinociceptive effect of the cannabinoid agonist, WIN 55,212-2, in the orofacial and temporomandibular formalin tests

Orofacial pain disorders are frequent in the general population and their pharmacological treatment is not always adequately resolved. Cannabinoids have demonstrated their analgesic effect in several pain conditions, both in animal models and in clinical situations. The aim of the present study was to evaluate the cannabinoid-mediated antinociception in two inflammatory models of orofacial pain (orofacial and temporomandibular joint (TMJ) formalin test) and to compare it with a spinal inflammatory model (paw formalin test). WIN 55,212-2 (0.5, 1mg/kg), a synthetic cannabinoid agonist, was intraperitoneally (i.p.) administered prior to formalin and significantly reduced the nociceptive behavioural responses in these inflammatory tests. To elucidate which subtype of receptor could be involved in such effect, two selective cannabinoid antagonists were administered prior to WIN. SR141716A (1mg/kg i.p.), the CB1 receptor-selective antagonist, was able to prevent the cannabinoid-induced analgesia in all three models, whereas SR144528 (1mg/kg i.p.), the CB2 receptor-selective antagonist, only prevented it in the paw formalin test. A comparison with the antinociceptive effects of morphine (2.5, 5, 10mg/kg, i.p.), indomethacin (2.5, 5mg/kg, i.p.) and ketamine (25, 50mg/kg, i.p.) was also performed. Morphine displayed a dose-dependent reduction of acute and inflammatory pain in all three models, whereas indomethacin and ketamine only attenuated inflammatory pain at the highest tested doses. These results indicate that the cannabinoid-induced antinociception in the orofacial region is mediated by activation of CB1 cannabinoid receptor. Moreover WIN was as effective as morphine and more effective than indomethacin and ketamine, in oral inflammatory pain.

Analgesic properties of oleoylethanolamide (OEA) in visceral and inflammatory pain

Oleoylethanolamide (OEA) is a natural fatty acid amide that mainly modulates feeding and energy homeostasis by binding to peroxisome proliferator-activated receptor-alpha (PPAR-alpha) [Rodríguez de Fonseca F, Navarro M, Gómez R, Escuredo L, Navas F, Fu J, et al. An anorexic lipid mediator regulated by feeding. Nature 2001;414:209-12; Fu J, Gaetani S, Oveisi F, Lo Verme J, Serrano A, Rodríguez de Fonseca F, et al. Oleoylethanolamide regulates feeding and body weight through activation of the nuclear receptor PPAR-alpha. Nature 2003;425:90-3]. Additionally, it has been proposed that OEA could act via other receptors, including the vanilloid receptor (TRPV1) [Wang X, Miyares RL, Ahern GP. Oleoylethanolamide excites vagal sensory neurones, induces visceral pain and reduces short-term food intake in mice via capsaicin receptor TRPV1. J Physiol 2005;564:541-7.] or the GPR119 receptor [Overton HA, Babbs AJ, Doel SM, Fyfe MC, Gardner LS, Griffin G, et al. Deorphanization of a G protein-coupled receptor for oleoylethanolamide and its use in the discovery of small-molecule hypophagic agents. Cell Metab 2006;3:167-175], suggesting that OEA might subserve other physiological roles, including pain perception. We have evaluated the effect of OEA in two types of nociceptive responses evoked by visceral and inflammatory pain in rodents. Our results suggest that OEA has analgesic properties reducing the nociceptive responses produced by administration of acetic acid and formalin in two experimental animal models. Additional research was performed to investigate the mechanisms underlying this analgesic effect. To this end, we evaluated the actions of OEA in mice null for the PPAR-alpha receptor gene and compared its actions with those of PPAR-alpha receptor wild-type animal. We also compared the effect of MK-801 in order to evaluate the role of NMDA receptor in this analgesia. Our data showed that OEA reduced visceral and inflammatory responses through a PPAR-alpha-activation independent mechanism. Co-administration of subanalgesic doses of MK-801 and OEA produced an analgesic effect, suggesting the participation of glutamatergic transmission in the antinociceptive effect of OEA. This study represents a novel approach to the examination of the effectiveness of OEA in nociceptive responses and provides a framework for understanding its biological functions and endogenous targets in visceral and inflammatory pain.

A cannabinoid agonist, WIN 55,212-2, reduces neuropathic nociception induced by paclitaxel in rats

Paclitaxel is an effective antineoplastic drug treatment used as an anti-tumoral therapy. Unfortunately its use is associated with unwanted side effects, which include the development of peripheral neuropathies and neuropathic pain, greatly affecting the quality of life of patients. It is well known that agonists of the cannabinoid receptor are able to reduce hyperalgesia and allodynia that develop after nerve injury. Our aim was to evaluate the efficacy of the cannabinoid agonist WIN 55,212-2 to reduce the thermal hyperalgesia and the tactile allodynia induced by administration of paclitaxel in rats. Present results demonstrate that WIN 55,212-2 (1 mg/kg i.p.) significantly reduced the heat (P<0.0001) and the mechanical (P=0.0003) withdrawal thresholds, the dose being smaller than that required to reach similar effects in the sciatic nerve constriction model (1.5 mg/kg). When the cannabinoid tetrad test was evaluated to measure behavioral modifications, it was found that WIN 55,212-2 (1mg/kg) did not induce changes either in body temperature or in immobility time, and only a reduction in spontaneous motility was recorded. This effect was antagonized by SR 141716A, suggesting the involvement of the CB1 receptor, although the participation of CB2 receptors cannot be excluded from this study. When WIN 55,212-2 was administered intraplantar, no differences were observed between the injected paw and the contralateral paw, suggesting that systemic mechanisms are needed to reach effectiveness. From these results we suggest that cannabinoids may be an interesting alternative to reduce neuropathic symptoms induced by paclitaxel, however more work is required to assess this possibility.

Blockade of Gi/o proteins modifies electrical activity of S-myenteric neurons from guinea-pig ileum

We have investigated the effect of blockade of G(i/o) proteins on the electrical activity of S-myenteric neurons from guinea-pig ileum longitudinal muscle-myenteric plexus preparations. Intracellular recordings were made from either control or pertussis toxin (PTX) treated tissues. PTX is known to disrupt the function of G(i/o) proteins. Incubation with PTX (300 ng/ml) induced three main changes in the electrophysiological characteristics of S-neurons: (1) resting membrane potential was more depolarised; (2) fast excitatory postsynaptic potentials (EPSPs) were faster and narrower; and (3) spontaneous fast EPSPs and action potentials were more frequent. Amplitude of slow EPSPs was not modified. This general increase in excitability after blockade of G(i/o) proteins shares characteristics with the slow depolarisation induced by long-lasting trains of low-frequency electrical stimulation and could be mediated by similar mechanisms. Electrophysiological activity of S-myenteric neurons could be modulated by an inhibitory tone, which PTX could modify by either pre- or post-synaptic mechanisms, such as inhibition of the tonic release of inhibitory neurotransmitters and/or intracellular signal transduction involving PTX-sensitive G(i/o) proteins.

Synthesis and analgesic activity of a series of new azaalkane bis-guanidinium and bis(2-aminoimidazolinium) compounds

In the present paper, we wish to report the synthesis and antinociceptive activity of a series of new azaalkane bis(2-aminoimidazolinium) compounds from which, N,N'-di(4,5-dihydro-1H-imidazol-2-yl)-3-aza-1,6-hexanediamine 2a has shown the best analgesic properties in vivo in two different assays (i.e., acetic acid-induced writhing test and hot-plate test in mice), as well as oral bioavailability.

Age-related changes in the antinociception induced by taurine in mice

Taurine is a nonessential amino acid that is of medical interest for the nutrition of infants. Taurine has been found in the central nervous system of rodents and humans, and among its potential therapeutic uses, it is interesting to remark its analgesic actions. It is also well known that concentration levels during the fetal and prenatal periods are higher than in adulthood. The data obtained so far indicate that taurine is involved in the development process of the brain and possibly other organs. The taurine levels in old age are still unknown, but it is presumed that they will be different from those of younger animals. Data about age-related alterations and functional modifications of this and other amino acids are still scarce. The aim of the present work was to study the antinociceptive effect of taurine and its relationship with aging in mice. No differences were found between prepubertal and young adult animals; on the contrary, old animals showed significantly reduced sensitivity to the antinociception induced by taurine; in fact, at the tested doses, taurine did not induce antinociception in this group of mice. The mechanism underlying this effect has not been clarified because there are several mechanisms and neurotransmitter systems involved in the antinociception induced by taurine.

Synthesis and opioid activity of new fentanyl analogs

Three new fentanyl analogs (compounds 3-4-5) have been synthesized and evaluated for antinociceptive properties using the writhing test. The analgesic property of the active compound, N-[1-phenylpyrazol-3-yl]-N-[1-(2-phenethyl)-4-piperidyl)] propenamide (compound 4), was tested using the hot plate test in mice. Its opioid agonistic activity was characterized using three isolated tissues: guinea pig ileum, mouse vas deferens, and rabbit vas deferens. Compound 4 was as effective as fentanyl or morphine and it showed less antinociceptive potency than fentanyl but it was more potent than morphine. The duration of the antinociception was similar to that of fentanyl. This compound inhibited the electrically evoked contractions of myenteric plexus-longitudinal muscle strips of guinea pig ileum and of mouse vas deferens but not those of rabbit vas deferens. These effects could be reversed by micro selective antagonists (naloxone and/or CTOP) but not by the delta selective antagonist naltrindole, thus indicating that the compound acted as a micro opioid agonist. Finally, the binding data confirmed that compound 4 had high affinity and selectivity for the micro-receptor.

The involvement of 5-HT3 and 5-HT4 receptors in two models of gastrointestinal transit in mice

Our aim was to study the involvement of 5-hydroxytryptamine (5-HT)(3) and 5-HT(4) receptors in two models of gastrointestinal transit (GIT) in mice: the 5-hydroxytryptophan (5-HTP)-induced diarrhea and intestinal inflammation produced by an irritant agent, croton oil (CO). 5-HTP (10 mg/kg) produced diarrhea that was significantly inhibited after pretreatment with ondansetron (5-HT(3) antagonist) or RS 39604 (5-HT(4) antagonist) (1-5 mg/kg). The GIT speed was increased after CO and 5-HTP administration. 5-HT(3-4) antagonists decreased GIT after 5-HTP-treatment but not after CO-treatment. Our results show that 5-HT(3) and 5-HT(4) receptors are involved in 5-HTP-induced diarrhea. This may be the reason why 5-HT(3-4) antagonists could be useful in the treatment of carcinoid syndrome diarrhea. 5-HT(3-4) antagonists were not effective in the modifications of GIT; nevertheless, they could be useful in the treatment of inflammatory bowel diseases because some symptoms as abdominal pain, discomfort or abnormal bowel function are modulated via 5-HT(3).

Guanidinium and aminoimidazolinium derivatives of N-(4-piperidyl)propanamides as potential ligands for mu opioid and I2-imidazoline receptors: synthesis and pharmacological screening

Derivatives of N-(1-phenethyl-4-piperidyl)propanamides incorporating guanidinium and 2-aminoimidazolinium groups have been prepared by a synthetic approach involving first introduction of a spacer between the piperidine and the functional group by reductive amination of piperidinone. The formation of each of these functional groups was carried out using N-N'-di(tert-butoxycarbonyl)thiourea and 2-methylthioimidazolinium iodide, respectively. These structures have been designed to incorporate two pharmacologic goals into one entity. Radioligand binding assays have been used to study their affinity for opioid (mu, delta and kappa) and I2-imidazoline receptors. Two of them, 10 and 16, showed high affinity for mu opioid receptors and functionally they had moderate analgesic properties in the hot plate and writhing tests. The in vitro studies on guinea pig ileum (GPI) indicated that both compounds are mu opioid agonists. In what concerns I2-imidazoline receptor activity, these derivatives showed low affinity around 6 to 7 times less than idazoxan.

Long-acting fentanyl analogues: synthesis and pharmacology of N-(1-phenylpyrazolyl)-N-(1-phenylalkyl-4-piperidyl)propanamides

The synthesis of new fentanyl analogues in which the benzene ring of the propioanilido group has been replaced by phenylpyrazole is described. Antinociceptive activity was evaluated using the writhing and hot plate tests in mice. Two compounds, and, showed interesting analgesic properties, being more potent than morphine and less than fentanyl but with longer duration of action. These compounds inhibited the electrically evoked muscle contraction of guinea pig ileum and mouse vas deferens but not that of rabbit vas deferens and the effects could be reversed by antagonists (naloxone and/or CTOP), thus indicating that the compounds acted as mu agonists. Finally, the binding data confirmed that the compounds had high affinity and selectivity for the mu receptor.

Salmon calcitonin potentiates the analgesia induced by antidepressants

Antidepressants are used in the treatment of a variety of pain syndromes. Most of them act by blocking noradrenaline (NA) and serotonin (5-HT) reuptake. It is also well known that the serotonergic system is also involved in calcitonin (CT) analgesia. Taking these two evidences into account, the modification of the analgesic effect of nortriptyline, amitriptyline, and paroxetine in the presence of salmon CT (s-CT) was examined in mice. The forced-swimming test was carried out in order to choose doses of each drug that did not induce an antidepressant effect under our experimental conditions (nortriptyline: 0.2-5 mg/kg ip, amitriptyline: 2.5-20 mg/kg ip, and paroxetine: 5-30 mg/kg ip). The analgesic effect of each antidepressant was then evaluated using the acetic acid test. At the doses tested, the antidepressants induced a dose-dependent analgesic effect. When mice were pre-treated with a subanalgesic dose of s-CT (2.5 IU/kg), the analgesic effect of amitriptyline and paroxetine was significantly increased though no modification was found for nortriptyline. In summary, s-CT was able to increase the analgesic effect of the antidepressant drugs that reduce the uptake of 5-HT, suggesting that the joint administration of antidepressants and CT may be an interesting alternative in pain management.

Blockade by pertussis toxin of the opioid effect on guinea pig ileum. Contractility and electrophysiological neuronal recording

The aim of the present study was to analyse the correlation between functional and electrophysiological effects of the opioids in guinea pig ileum. Preparations of guinea pig ileum myenteric plexus-longitudinal muscle strips were used to compare the effect of two opioids, morphine (a mu-agonist) and U-50,488H (a kappa-agonist) on the electrically-induced contractile response and the excitatory postsynaptic potentials exhibited by the myenteric neurones when the internodal strands are electrically stimulated. Both opioids dose-dependently inhibited the contractile responses and the fast excitatory postsynaptic potentials (fEPSPs) in S neurones but did not modify the amplitude of the slow postsynaptic potentials. Pertussis toxin pretreatment was able to antagonise the effects of both morphine and U-50,448H. From our results it can be suggested that the effect of the opioids on guinea pig ileum involves the inhibition of neuronal fEPSPs.

Study of mechanisms of calcitonin analgesia in mice. Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT(1A) agonist (+/-)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT(1A) antagonist (WAY 100, 135). As the stimulation of 5-HT(1A) autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT(2A-2C) agonist (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT(2A) antagonist ketanserin failed to prevent this effect. The 5-HT(3) agonist (+/-)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT(3) antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT(3) receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT(3) receptors.

Calcitonin reverts pertussis toxin blockade of the opioid analgesia in mice

The aim of this paper is to study the influence of salmon calcitonin (SCT) on opioid analgesia when opioid transduction pathways are functionally uncoupled from Gi/o proteins by treatment with pertussis toxin (PTX). The antinociceptive effect of morphine and three selective opioid agonists, [D-Ala2,N-Me-Phe2,Gly5-ol]enkephalin (DAMGO) (OP(3-mu receptor agonist), [D-Pen2.5]-enkephalin (OP-1-delta receptor agonist) and trans-( +/- )-3,4-dichloro-N-methyl-N-[2-1(-pyrrolidinyl)-cyclohexyl]-benzene-acetam ide methane sulfonate (U-50, 488H) (OP1-kappareceptor agonist) was evaluated, using the tail flick test, in mice treated with PTX or with PTX and SCT. PTX blocked the antinociceptive effect of the opioids, being the antinociception similar in control animals and in mice treated with PTX and SCT. Thus, SCT prevents the effect of the blockade of Gi/o-proteins. From this it could be suggested that calcitonin activates alternative antinociceptive mechanisms that are not dependent on Gi/o-proteins.

Alendronate induces antinociception in mice, not related with its effects in bone

The antinociceptive effect of alendronate was studied. The bisphosphonate was i.p. administered and two tests were carried out: acetic acid in mice and formalin test in rats. In the acetic acid test, alendronate induced a dose-dependent antinociceptive effect that was statistically significant for the doses of 10, 20 and 40 mg/kg, and could be detected 48 hr after its administration. In the formalin test, however, alendronate, at the doses of 10 and 20 mg/kg, did not modify the pain score nor the number of flinches, when it was administered either 30 or 60 min before the test. However it must be noted that doses inducing analgesic effect are close to those inducing toxicity.

Effect of salmon-calcitonin on the analgesic effect of selective mu, delta and kappa opioid agonists in mice

The analgesic effect of three different opioid agonists, DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin), U-50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidynyl)cyclohexyl] benzene-aceramide methane sulphonate), and [D,Pen2-D,Pen5]-enkephalin, which act upon mu, delta and kappa opioid receptors, respectively, was compared in the presence and absence of salmon-calcitonin (s-CT). The analgesic test used was the writhing test in mice. The analgesic effect of the opioids was significantly enhanced by pretreatment of the animals with s-CT intraperitoneally (i.p.) administered. This effect was more evident for the delta and kappa-agonists. The present result suggests that the joint administration of s-CT and opioids may be a useful and interesting alternative in the treatment of painful diseases resistant to other treatments.

Effect of butanedione monoxime on the contractility of guinea pig ileum and on the electrophysiological activity of myenteric S-type neurones

2,3-Butanedione monoxime (BDM) has demonstrated protective effects on isolated cardiac tissues, and on smooth muscle but its mechanism of action is not fully understood. To simultaneously study the effect of BDM on muscle contractility and on neuronal activity, the effect of BDM was tested in the contractile force of myenteric plexus-longitudinal muscle strips and in electrophysiological activity of myenteric S-type neurones of guinea pig ileum. BDM reduces, in a dose-dependent manner, the force of the spontaneous motility and the contractions induced by acetylcholine, bethanechol and electrical stimulation. The same BDM concentrations depolarize the neuronal membrane and reduce the rate of evoked firing. The effect of BDM can be attributed to a direct effect on the smooth muscle and to modifications of the neuronal activity.

Effect of salmon-calcitonin on in vitro opioid withdrawal

The effect on the in vitro withdrawal sign induced by naloxone in morphine-dependent guinea pig ileum has been analyzed. Salmon calcitonin (s-CT) dose-dependently reduced the force of the contraction induced by naloxone in morphine incubated tissues, but did not modify the contraction induced by administration of acetylcholine or substance P in non-treated tissues. Therefore, the effect of s-CT in morphine incubated tissues may not be attributable to postsynaptic mechanisms, and an inhibitory modulation of the excitatory pathways triggered by naloxone would be suggested. In conclusion, s-CT is able to prevent the withdrawal sign in vitro.

Age-related changes in nociception, behavior, and monoamine levels in rats

1. Pain threshold, behavioral parameters, and monoamine levels were compared in two groups of rats: adult (12 months old) and old rats (25 months old). 2. No differences in nociception were found between the two groups using the tail-shock test. 3. Behavioral experiments with the holeboard test showed that locomotor activity and exploration activity were lower in aged animals, whereas no significant differences were found in emotivity. 4. Using high-performance liquid chromatography (HPLC) techniques, we found that serotonin and dopamine showed lower levels in the old group.

Influence of pertussis toxin on the calcitonin-opioid interaction in isolated tissues

1. In order to clarify one of the mechanisms involved in the analgesic effect of calcitonin, we have tested the in vitro modifications induced by calcitonin on the effect of opioids. 2. The inhibition of the contractions induced by opioids or clonidine, in guinea-pig ileum or in mouse vas deferens, were significantly reduced in tissues incubated with pertussis toxin (PTX). When tissues were incubated with PTX and calcitonin, the inhibitory effect was restored. 3. These results suggest that calcitonin is able to potentiate a non-PTX-sensitive mechanism of transduction and support the possibility of involvement of similar G-proteins in the effects of opioid and alpha 2-adrenoceptor agonists.

Behavioral and analgesic effects induced by administration of nifedipine and nimodipine

Evidence exists that calcium antagonists can have effects on neural function. The aim of this work is to analyze the effect of two dihydropyridines, nifedipine and nimodipine, administered for 11 days on the behavior and pain sensitivity of rats. Nociception was tested using the tail electric stimulation test, and behavior parameters using a holeboard. Our results show that chronic administration of nifedipine or nimodipine induces analgesia that can be evaluated by tail withdrawal. However, neither the vocalization nor the vocalization after discharge were modified, so the analgesia may be mediated by spinal mechanisms. Rats treated with nifedipine or nimodipine exhibited a dose-dependent tendency to avoid the center of the field without modification of other parameters, suggesting an increased emotivity in the rats. This conclusion is supported by the fact that anxiogenic or anxiolytic drugs modify the pattern of locomotion without significant changes in other parameters related with the motility. The results from this study suggest the view of a complex mechanism of action underlying nifedipine- and nimodipine-mediated behavioral effects.

Effect of the intraperitoneal administration of salmon-calcitonin on the "in vitro" actions of opioid agonists

1. The interaction of intraperitoneal administration of salmon-calcitonin with opioids was studied. The study was carried out using guinea pig ileum (mu and kappa-opioid receptors), rabbit vas deferens (kappa-opioid receptors) and mouse vas deferens (delta-opioid receptors), and selective mu, delta and kappa agonists were used in the pertinent tissues. 2. The treatment with salmon-calcitonin increased, in a dose-dependent manner, the effect of U-50,488H in guinea pig ileum and rabbit vas deferens and the effects of [D-Pen2, D-Pen5] enkephalin in mouse vas deferens. 3. The treatment with analgesic doses of salmon-calcitonin enhances the in vitro effects of kappa- and delta-opioid agonists. The increase of the effectiveness of the opioid agonists may be one of the mechanisms involved on the analgesia induced by salmon-calcitonin.

Analgesic effect of two calcitonins and in vitro interaction with opioids

1. When the analgesic effect of salmon-calcitonin (S-CT) and of eel-calcitonin (E-CT), as well as their influence on the morphine-analgesia were compared, no significant differences were found. 2. While on isolated tissues, E-CT induced a significant increase on the effect of bremazocine, [D-Pen2,D-Pen5]enkephalin and [Met5]enkephalin and no changes were observed on the effect of DAMGO, suggesting that E-CT increases the effects of opioids acting on delta or kappa receptors but not on mu receptors. 3. These findings corroborate the possibility of interactions between calcitonin and the opioid system.