The pharmacological basis of contemporary pain management

Mitigating the Shadow of the Worldwide Opioid Crisis: A Review for the Foot and Ankle Specialist

The foot and ankle physician is no stranger to the difficulties in achieving optimal pain therapy. There remains much confusion and conflicting information available to nonspecialist prescribers regarding opioid therapy as well as great deal of fear or opiophobia during the prescribing and monitoring of opioids worldwide. The role of the lower extremity specialist provider is to responsibly provide pain management to their patients in an error-free environment. The purpose of this article is to explore the central theme of responsible opioid pain management worldwide. This review focuses on the prescribing strategies of opioid analgesics to treat lower-extremity pain. Pharmacology of opioid agents and opioid prescribing strategies will be presented. Then, the concept of multimodal pain relief criteria for selecting appropriate opioid analgesics and use of adjunctive therapies to prevent opioid misuse as presented in the current medical literature is reported. Finally, a commentary and discussion centered on the actions of pharmaceutical companies of promoting their opioid products and the negative outcomes of their actions in the United States that may go worldwide if behaviors of these companies are not recognized by the foot and ankle specialist.

Targeting peripheral ϰ-opioid receptors for the non-addictive treatment of pain

Drug addiction to prescription mu-opioid agonists used in the setting of pain is a major public health threat, affecting millions of Americans. Kappa opioid agonists (KOAs) may serve as a possible solution. KOAs have demonstrated indistinguishable analgesic activity relative to mu-opioid agonists in models of acute and chronic pain; however, conventional KOAs suffer from central nervous system-mediated psychoactive side-effects. In this review, we discuss our efforts, as well as other's efforts, in developing peripherally-restricted kappa opioid agonists with retained or improved efficacy in rodent models of pain. Results indicate that our lead compound JT09 acts as efficacious as morphine in alleviating peripheral pain, while failing to produce undesired central nervous system-mediated side-effects. In this review, we discuss our former results and future directions.

Cannabidiol Is a Potential Therapeutic for the Affective-Motivational Dimension of Incision Pain in Rats

Background: Pain involves different brain regions and is critically determined by emotional processing. Among other areas, the rostral anterior cingulate cortex (rACC) is implicated in the processing of affective pain. Drugs that interfere with the endocannabinoid system are alternatives for the management of clinical pain. Cannabidiol (CBD), a phytocannabinoid found in Cannabis sativa, has been utilized in preclinical and clinical studies for the treatment of pain. Herein, we evaluate the effects of CBD, injected either systemically or locally into the rACC, on mechanical allodynia in a postoperative pain model and on the negative reinforcement produced by relief of spontaneous incision pain. Additionally, we explored whether CBD underlies the reward of pain relief after systemic or rACC injection.

Methods and Results: Male Wistar rats were submitted to a model of incision pain. All rats had mechanical allodynia, which was less intense after intraperitoneal CBD (3 and 10 mg/kg). Conditioned place preference (CPP) paradigm was used to assess negative reinforcement. Intraperitoneal CBD (1 and 3 mg/kg) inverted the CPP produced by peripheral nerve block even at doses that do not change mechanical allodynia. CBD (10 to 40 nmol/0.25 μL) injected into the rACC reduced mechanical allodynia in a dose-dependent manner. CBD (5 nmol/0.25 μL) did not change mechanical allodynia, but reduced peripheral nerve block-induced CPP, and the higher doses inverted the CPP. Additionally, CBD injected systemically or into the rACC at doses that did not change the incision pain evoked by mechanical stimulation significantly produced CPP by itself. Therefore, a non-rewarding dose of CBD in sham-incised rats becomes rewarding in incised rats, presumably because of pain relief or reduction of pain aversiveness.

Conclusion: The study provides evidence that CBD influences different dimensions of the response of rats to a surgical incision, and the results establish the rACC as a brain area from which CBD evokes antinociceptive effects in a manner similar to the systemic administration of CBD. In addition, the study gives further support to the notion that the sensorial and affective dimensions of pain may be differentially modulated by CBD.

Drug Formulation Advances in Extended-Release Medications for Pain Control

Prescription opioid abusers frequently tamper with opioid tablets in order to either accelerate the delivery of the euphoria-inducing agent or to alter the route of delivery, such that it may be delivered intranasally or intravenously. As one strategy to combat the opioid epidemic in the USA, drug manufacturers have begun to explore formulations which resist such tampering by abusers. Techniques to prevent tampering consist of physical barriers to crushing, chewing, and drug extraction, or aversive or antagonistic agents, incorporated within the formulation itself. Recent years have seen the development of numerous extended-release opioid agents, which are described in this review. This article provides a comprehensive summary of the pharmacology, benefits, risks, and processes behind the development of currently available extended-release opioid drugs, as well as a glimpse into promising future formulations.

The Clinical Applications of Extended-Release Abuse-Deterrent Opioids

Opioids are the mainstay for treatment of acute pain and cancer pain, and also have a role in the treatment of chronic non-malignant pain. There has been, however, a growing public health problem stemming from the misuse of opioid analgesics leading to serious consequences. To deter abuse, new formulations of extended-release opioid analgesics and tamper-resistant opioids have recently been developed. The concept of abuse-deterrent extended-release opioids is relatively new and, although abuse may not be completely prevented, the utilization of such abuse-deterrent extended-release opioids could reduce this risk. Extended-release abuse-deterrent opioids have been found to have important clinical applications in cancer, acute pain, and chronic non-malignant pain for analgesia control with decreased incidence of tampering and abuse. In this review, different extended-release formulations of opioids available for clinical applications are presented with descriptions of the formulations, their physical properties, and the clinical studies performed to provide physicians with a better understanding of their uses.

Development of a Peptide-derived orally-active kappa-opioid receptor agonist targeting peripheral pain

Kappa-opioid agonists are particularly efficacious in the treatment of peripheral pain but suffer from central nervous system (CNS)-mediated effects that limit their development. One promising kappa-agonist is the peptidic compound CR665. Although not orally available, CR665 given i.v. exhibits high peripheral to CNS selectivity and benefits patients with visceral and neuropathic pain. In this study we have generated a series of derivatives of CR665 and screened them for oral activity in the acetic acid-induced rat writhing assay for peripheral pain. Five compounds were further screened for specificity of activation of kappa receptors as well as agonism and antagonism at mu and delta receptors, which can lead to off-target effects. All active derivatives engaged the kappa receptor with EC50s in the low nM range while agonist selectivity for kappa over mu or delta was >11,000-200,000-fold. No antagonist activity was detected. One compound was chosen for further analysis (Compound 9). An oral dose response of 9 in rats yielded an EC50 of 4.7 mg/kg, approaching a druggable level for an oral analgesic. To assess the peripheral selectivity of this compound an i.v. dose response in rats was assessed in the writhing assay and hotplate assay (an assay of CNS-mediated pain). The EC50 in the writhing assay was 0.032 mg/kg while no activity was detectable in the hotplate assay at doses as high as 30 mg/kg, indicating a peripheral selectivity of >900-fold. We propose that compound 9 is a candidate for development as an orally-available peripherally-restricted kappa agonist.

Morphine induces albuminuria by compromising podocyte integrity

Morphine has been reported to accelerate the progression of chronic kidney disease. However, whether morphine affects slit diaphragm (SD), the major constituent of glomerular filtration barrier, is still unclear. In the present study, we examined the effect of morphine on glomerular filtration barrier in general and podocyte integrity in particular. Mice were administered either normal saline or morphine for 72 h, then urine samples were collected and kidneys were subsequently isolated for immunohistochemical studies and Western blot. For in vitro studies, human podocytes were treated with morphine and then probed for the molecular markers of slit diaphragm. Morphine-receiving mice displayed a significant increase in albuminuria and showed effacement of podocyte foot processes. In both in vivo and in vitro studies, the expression of synaptopodin, a molecular marker for podocyte integrity, and the slit diaphragm constituting molecules (SDCM), such as nephrin, podocin, and CD2-associated protein (CD2AP), were decreased in morphine-treated podocytes. In vitro studies indicated that morphine modulated podocyte expression of SDCM through opiate mu (MOR) and kappa (KOR) receptors. Since morphine also enhanced podocyte oxidative stress, the latter seems to contribute to decreased SDCM expression. In addition, AKT, p38, and JNK pathways were involved in morphine-induced down regulation of SDCM in human podocytes. These findings demonstrate that morphine has the potential to alter the glomerular filtration barrier by compromising the integrity of podocytes.

Double-blind, placebo-controlled, randomized pilot study of cerebral blood flow patterns employing SPECT imaging in dental postsurgical pain patients with and without pain relief

BACKGROUND

Single-photon emission computed tomography (SPECT) has been employed in the study of altered regional cerebral blood flow (CBF) in experimental and chronic pain. CBF patterns have not been evaluated in patients with acute postoperative pain.

OBJECTIVE

The purpose of this pilot study was to employ SPECT to measure CBF distribution associated with postoperative dental pain and to compare these CBF patterns to subsequent images in the same patients who were experiencing pain relief versus continued or worsening pain who had received active or placebo analgesic interventions. The primary outcome measure was the percentage change in blood flow in various regions of interest.

METHODS

Twenty-two healthy individuals (10 males and 12 females, age range 20-29 years) who underwent the removal of ≥1 partial or full bony impacted mandibular third molars were evaluated for pain intensity as the local anesthesia dissipated, employing a 0 to10 numeric rating scale (0 = no pain; 10 = worst imaginable). When the subjects' pain level reached ≥4/10, they were injected intravenously with 260 MBq of technetium Tc 99m bicisate (ethyl cysteinate dimer). Under double-blind conditions and 10 minutes before being placed in the SPECT scanner, the first 10 subjects were randomized to receive intravenous ketorolac 15 mg or saline while the remaining 12 subjects were randomized to receive by mouth either ibuprofen 400 mg, ibuprofen 200 mg, acetaminophen 1000 mg, or placebo. One hour after drug administration, subjects were reevaluated for pain, injected with 925 MBq of technetium Tc 99m bicisate, given rescue medication if required, and then rescanned. CBF ratios were obtained for regions of interest and by normalizing to average whole brain activity.

RESULTS

Subjects generally had a moderate degree (mean [SD], 7.3% [4.0%]) of thalamic asymmetry on initial scans with pain; after treatment, subjects reporting worsening pain regardless of the intervention had higher thalamic asymmetry (8.1% vs 2.8%) than those reporting relief of pain. Subjects who reported reduced pain after the intervention had significantly different (P < 0.05) mean CBF changes compared with those reporting worsening pain in the left prefrontal cortex, left sensorimotor area, right anterior cingulate, and right caudate.

CONCLUSIONS

Acute postoperative dental pain was associated with moderate thalamic asymmetry that improved following successful pain management. Sustained or worsening pain was associated with increased CBF in brain regions associated with pain pathways, whereas pain relief was associated with decreased activity in the same areas.

A treatment algorithm for neuropathic pain: an update

OBJECTIVE

The purpose of this review is to provide an update of the neuropathic pain treatment algorithm previously published by Namaka et al. in 2004. This algorithm focuses on the strategic incorporation of the latest pain therapies while providing an update of any recent developments involving medications previously listed in the algorithm.

DATA SOURCES

PubMed, MEDLINE, Cochrane, and Toxnet databases were used to conduct all literature searches on neuropathic pain and targeted treatment strategies. Comprehensive search efforts in the identified databases included studies published between 1980 and 2009. The search term "neuropathic pain" was used along with each of the agents outlined in this review: pregabalin, paroxetine CR, duloxetine, tramadol XL, Tramacet, Sativex, and nabilone.

STUDY SELECTION

A total of 90 studies were reviewed and selected based on level 1, 2, and 3 search strategies.

DATA EXTRACTION

Level 1 search strategies were initially aimed at evidence-based trials of large sample size (N > 100), with a randomized, double-blind, placebo-controlled design conducted by investigators well versed in the specialty area of interest. A level 2 search was conducted for additional trials that had many, but not all, of the desirable traits of evidence-based trials. In addition, a level 3 search strategy was conducted to compare key findings stated in anecdotal reports of very small (N < 15), poorly designed trials with the results of well-designed, evidence-based trials identified in level 1 and/or level 2 searches.

DATA SYNTHESIS

Based on a thorough evaluation of the literature, pregabalin, paroxetine CR, and duloxetine have been placed in the updated algorithm as first-line agents, while tramadol XL, Tramacet, Sativex, and nabilone function primarily as adjunctive agents.

CONCLUSION

The updated algorithm provides a baseline framework from which clinicians can justify the medication they prescribe.

Effect of Bacopa monniera on liver and kidney toxicity in chronic use of opioids

In the present study, we investigated the protective effect of Bacopa monniera, an indigenous Ayurvedic medicinal plant in India, against morphine-induced liver and kidney toxicity in rats. Morphine intoxicated rats received 10-160 mg/kg body weight of morphine hydrochloride intraperitoneally for 21 days. Bacopa monniera Extract (BME) pretreated rats were administered with BME (40 mg/kg) orally once a day 2 h before the injection of morphine for 21 days. Pretreatment with BME has shown to possess a significant protective effect against morphine-induced liver and kidney functions in terms of serum glutamate oxaloacetate transaminase, serum glutamate pyruvate transaminase, alkaline phosphatase, lactate dehydrogenases and gamma-glutamyl transferase activities and urea, creatinine and uric acid level respectively. Histopathological changes of liver and kidney were also in accordance with the biochemical findings. The results of this study indicate that Bacopa monniera extract exerted a protection against morphine-induced liver and kidney toxicity.

The combined analgesic effect of gabapentin and transdermal fentanyl patch on acute and chronic pain after maxillary cancer surgeries

Purpose of this study was to evaluate the combined analgesic effect of gabapentin and transdermal fentanyl patch, on acute and chronic pain after surgery for maxillary cancer.

STUDY DESIGN

The Study was double blind and prospective. 100 subjects belonging to ASA grade I and II, 30-50 years age group, scheduled for maxillary cancer surgery were randomized into two groups; treatment group (GT): to receive gabapentin, transdermal fentanyl patch or control group ©: two placebos. For acute postoperative pain (Visual Analogue Score) and analgesic requirements were assessed 2, 4, 8 hours and 7 days after surgery. Subjects were also assessed for chronic pain 2, 4, 6 months later.

RESULTS

Subjects in treatment group required lesser dose of analgesic, as compared to control group, in the post operative period. Visual Analogue Scores were also significantly lower in the treatment group throughout the post operative period. Occurrence of side effects was non significant between both groups. 2, 4 and 6 months after surgery, 40, 35 and 28 subjects respectively, out of total 45 subjects of the control group, reported chronic pain. In comparison, 25, 10 and 4 subjects out of 42 subjects in the treatment group reported chronic pain 2, 4, 6 months respectively after surgery. 15, 10 and 6 out of 45 of the control group required analgesics, whereas 2, 0 and 0 out of 42 in the treatment group, required analgesics respectively 2, 4 and 6 months after surgery

CONCLUSION

Acute and chronic pain after maxillary cancer surgery is significantly reduced by multimodal analgesia.

Effects of intravitreal morphine administered at different time points after reperfusion in a rabbit model of ischemic retinopathy

PURPOSE

To investigate the effects of morphine administered after reperfusion in a rabbit model of ischemic retinopathy.

METHODS

The right eyes of 54 albino New Zealand rabbits were randomly allocated into nine treatment groups (n = 6 in each group). The eyes in saline-control group received 0.1 mL of phosphate-buffered saline solution intravitreally. In the ischemia-saline group, ischemia was induced by raising the intraocular pressure to 150 mmHg for 60 minutes. Then 0.1 mL of phosphate-buffered saline solution was administered intravitreally 5 minutes after reperfusion. The eyes in three ischemia-morphine groups (ischemia-morphine 0 hour, 1 hour, and 18 hours) received 0.1 mL of morphine (10 micromol/L) intravitreally 5 minutes, 1 hour, or 18 hours after termination of 60 minutes of ischemia, respectively. The eyes in ischemia-naloxone-morphine group received 0.05 mL of naloxone (10 micromol/L) intravitreally followed by injection of 0.05 mL morphine (10 micromol/L) 5 minutes after termination of ischemia. Toxicity controls were performed with morphine (10 micromol/L) and naloxone (10 micromol/L) without ischemia. Histologic evaluation was performed for all groups on the seventh postoperative day.

RESULTS

Sixty minutes of ischemia led to severe cell loss in ganglion cell layer and thinning of the inner nuclear layer in ischemia-saline group compared with that of the saline-control group (P < 0.001). Thickness of the inner plexiform layer to the inner limiting membrane (a measure of inner retinal thickness) was significantly increased due to edema (P < 0.001). Administration of morphine 5 minutes after reperfusion significantly improved all of the above mentioned indices compared with ischemia-saline group (P < 0.001). Administration of morphine 1 hour after reperfusion had also a significant effect on the improvement of above mentioned indices compared with saline-control group (P < 0.05). However, the number of ganglion cells was significantly higher in ischemia-morphine 0 hour group compared with ischemia-morphine 1 hour group (P < 0.001). Morphine treatment 18 hours after reperfusion did not change the amount of injury. Administration of naloxone 5 minutes before morphine abolished most of the morphine protective effects.

CONCLUSION

Intravitreal administration of morphine immediately after reperfusion maximally protects retina against ischemia-reperfusion injury. Pharmacologic evidence suggests that this protective phenomenon may be mediated in part by opioid receptors.

Acupoint stimulation with diluted bee venom (apipuncture) potentiates the analgesic effect of intrathecal clonidine in the rodent formalin test and in a neuropathic pain model

Although intrathecal (i.t.) administration of the alpha(2)-adrenoceptor agonist clonidine has a pronounced analgesic effect, the clinical use of clonidine is limited by its side effects. Previously, our laboratory has demonstrated that the subcutaneous injection of diluted bee venom (DBV) into an acupoint (termed apipuncture) produces significant analgesic effect in various pain animal models. The present study was designed to examine whether DBV injection into the Zusanli acupoint (ST-36) could enhance lower-dose clonidine-induced analgesic effects without the development of hypotension, bradycardia, or sedation. In the mouse formalin test, DBV injection produced a dramatic leftward shift in the dose-response curve for clonidine-induced analgesia. In a rat neuropathic pain model i.t. clonidine dose dependently suppressed chronic constriction injury (CCI)-induced mechanical allodynia and thermal hyperalgesia, and this clonidine-induced analgesic effect was significantly potentiated by apipuncture pretreatment. DBV apipuncture alone or in combination with a low dose of i.t. clonidine produced an analgesic effect similar to that of the high dose of clonidine, but without significant side effects. The analgesic effect produced by the combination of i.t. clonidine and apipuncture was completely blocked by pretreatment with an alpha(2)-adrenoceptor antagonist. These data show that DBV-apipuncture significantly enhances clonidine-induced analgesia and suggest that a combination of low dose clonidine with acupuncture therapy represents a novel strategy for pain management that could eliminates clonidine's side effects.

PERSPECTIVE

This study demonstrated that intrathecal clonidine-induced analgesia is significantly enhanced when it is combined with chemical acupuncture treatment. The administration of low-dose clonidine in combination with acupuncture produced a potent analgesic effect without significant side effects and thus represents a potential novel strategy for the management of chronic pain.

An update on the pharmacological management of post-herpetic neuralgia and painful diabetic neuropathy

Neuropathic pain is a persistent pain condition that develops secondary to nerve injury. The two most common types of peripheral neuropathic pain are post-herpetic neuralgia (PHN) and painful diabetic neuropathy (PDN). Amitriptyline, nortriptyline, desipramine and imipramine are TCAs that have been shown to be effective for the symptomatic relief of PHN and PDN. Serotonin noradrenaline reuptake inhibitors (SNRIs) such as venlafaxine and duloxetine have been shown to be very promising for the treatment of PDN with fewer adverse effects than TCAs. Selective serotonin reuptake inhibitors (SSRIs) were shown in a number of studies to have some efficacy in relieving PDN-related pain, yet other studies of the SSRIs have demonstrated conflicting outcomes. Most of the older antiepileptic studies were performed in patients with PDN; consequently, little is known about the efficacy of these drugs in patients with PHN. Carbamazepine, phenytoin and valproic acid were shown to be effective in ameliorating PDN-related pain. Other antiepileptic agents, including lamotrigine, oxcarbazepine and topiramate, have demonstrated some beneficial effects for the treatment of PDN, although they were also found to be ineffective in some PDN studies. alpha2delta Ligands such as gabapentin and pregabalin have been proven to be effective for the treatment of PHN and PDN in a number of large placebo-controlled trials. These drugs are useful not only in relieving pain but also in improving quality of life. Although the use of opioids for the treatment of neuropathic pain is controversial, a number of studies support the efficacy and safety of opioids in the treatment of neuropathic pain. Of these, oxycodone and tramadol have been shown to be superior to placebo for the treatment of PHN and PDN. A number of small studies have shown that dextromethorphan was effective in patients with PDN but not in patients with PHN. Topical agents such as lidocaine 5% patches and topical capsaicin are useful in ameliorating pain in patients with PHN but these agents are unsatisfactory for use as a sole agent. Although a number of drug treatments are available for the symptomatic relief of neuropathic pain symptoms, these agents do not provide satisfactory relief in all patients. For these patients, other treatment alternatives such as combination drug therapy that produces pain relief via distinctly different mechanisms may be successful. The purpose of this review is to compare the efficacy and limitations of currently available pharmacological treatments for the symptomatic relief of PHN and PDN, and to discuss the potential of combination therapy in PHN and PDN.

Overview on pathophysiology and newer approaches to treatment of peripheral neuropathies

Peripheral neuropathies are extremely heterogeneous nosological entities. One of the most common symptoms is pain, the underlying mechanisms of which are numerous and complex. Inflammation, reparative processes, and anatomical and gene expression alterations lead to chronic pain, the persistence of which is sustained by peripheral and central sensitisation mechanisms. Treatment of peripheral neuropathies is targeted to its symptomatic and aetiological features. For pain relief, several types of drugs may be used, notably antidepressants (e.g. tricyclic antidepressants, selective serotonin reuptake inhibitors, and both serotonin and noradrenaline [norepinephrine] reuptake inhibitors), antiepileptic drugs (e.g. carbamazepine, phenytoin, lamotrigine, valproic acid, gabapentin, topiramate and pregabalin), NSAIDs and opioid analgesics. Aetiological therapy is aimed at modifying the pathophysiological mechanisms underlying the neuropathy, some of which are common in different neuropathic conditions. Certain drugs are known to exert more than one action on different pathophysiological mechanisms. This is the case with acetyl-L-carnitine (ALC), which can be considered both a symptomatic therapy that can be used in any kind of painful neuropathy, and an aetiological therapy, at least in diabetic neuropathy and neuropathies induced by nucleoside reverse transcriptase inhibitors and cancer chemotherapeutic agents. ALC acts via several mechanisms, inducing regeneration of injured nerve fibres, reducing oxidative stress, supporting DNA synthesis in mitochondria, and enhancing nerve growth factor concentrations in neurons.

Opioid receptor-independent protection of ischemic rat hepatocytes by morphine

We studied the role of morphine in anoxia/reoxygenation injury to hepatocytes. Overnight cultured rat hepatocytes were incubated in anoxic buffer at pH 6.2 for 4h and reoxygenated at pH 7.4 for 2h to simulate anoxia/reoxygenation. Some hepatocytes were preincubated with 50 microM morphine for 10 min prior to onset of anoxia/reoxygenation. To study the effect of morphine on nitric oxide (NO), hepatocytes were loaded with 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM). Changes in NO concentration were assessed with a multi-well fluorescence reader and confocal microscopy. Morphine substantially improved cell viability after reoxygenation and increased NO generation, which was blocked by ATP-sensitive potassium channel blockers. Confocal images revealed that the increase in NO occurred mainly at the cytosol. However, treatment with opioid receptor antagonists did not reverse cytoprotection by morphine. These results indicate that morphine prevents anoxia/reoxygenation injury to hepatocytes. Protective mechanisms are associated with the potassium channels and NO, but are independent of opioid receptor-mediated signaling.

Pharmacological correlation between the formalin test and the neuropathic pain behavior in different species with chronic constriction injury

Research on mechanisms of drug action, and preclinical screening of molecules with a potential activity on neuropathic pain requires extensive animal work. The chronic constriction injury model is one of the best-characterized models of neuropathic pain behavior in rats, but requires extensive time consuming operations and animal handling. The formalin test is easier to perform, and a well validated model. The latter may serve as an effective prescreening test of molecules and may facilitate drug targeting. In the present study the activity of different pharmacological reference compounds was tested in rats and gerbils on the cold plate for animals that had undergone chronic constriction injury and in the second phase of the formalin test. In rats, a comparable outcome in both test conditions was observed for morphine, fentanyl, MK-801 and flunarizine. Clonidine had more activity in the second phase of the formalin test, whereas baclofen, tramadol, amitryptiline, ketamine and topiramate showed more activity in the cold plate. In gerbils, both test conditions yielded comparable results for fentanyl and ketoprofen. Tramadol and CP-96345 tended to have more activity in the second phase of the formalin test, whereas morphine, SR-48968, SR-142801 and R116301 demonstrated more activity in the cold plate test. This study demonstrates a good correlation between the second phase of the formalin test and the cold allodynia in the CCI model for, both for rats and gerbils. Drugs with a proven activity in humans, used as reference compounds, also showed good pharmacological activity in this animal study.

Clonidine increases membrane-associated phospholipase A 2

BACKGROUND AND OBJECTIVE

An anti-inflammatory effect of alpha2-adrenoreceptor agonists has been suggested. Phospholipase A2 is a key enzyme in the production of precursors of inflammatory lipid mediators. The aim of the present study was to investigate the effect of clonidine on phospholipase A2 activity in an established in vitro model.

METHODS

Human being platelet membranes containing active phospholipase A2 were exposed to buffer control or to three increasing concentrations of clonidine. Phospholipase A2 was measured by a radioisotope technique.

RESULTS

A massive increase in phospholipase A2 activity was measured after clonidine exposure leading to final values of 92.5 +/- 3.1 pmol mg protein(-1) min(-1) (4.5-fold higher than control values; P < or = 0.01 vs. control). After clonidine exposure the maximal reaction velocity increased, while the Michaelis-Menten constant did not change. The Lineweaver-Burk representation suggested an interaction of clonidine with the phospholipase A2-substrate complex as well as the phospholipase A2 molecule.

CONCLUSION

We conclude that the putative anti-inflammatory effect of clonidine was not caused by inhibition of phospholipase A2.

Understanding pain, part 2: pain management

This article is the second in a two-part series which explores pain and its management from a physiological perspective. Nurses play an important role in assessing and managing pain. Effective pain management by nurses requires them to have an understanding of the biological basis of the pain interventions which may be used to control pain. This article emphasizes the importance of pain assessment as a precursor for effective pain management and explores the biological basis of pain interventions which contribute to pain control. The role of non-pharmacological approaches in alleviating pain and their actions which contribute to pain relief are explored. The three main types of pharmaceutical agents used, non-opioids, opioids and adjuvant drugs, are introduced and their mechanisms of actions discussed.

Controversies in pharmacotherapy of pain management

Since the establishment of the WHO three-step ladder for management of cancer pain, several controversies have arisen, which are partly due to new drug development, reformulations of older analgesics, and technological advancements. As a result, clinicians need clarification of several questions. Is morphine the opioid of choice for moderate to severe pain in cancer? Should combinations of opioids be used? When should spinal opioids be used to treat pain in cancer? What are the appropriate opioid doses for breakthrough pain? Should selective cyclo-oxygenase (COX) 2 inhibitors be used? What is the best tactic to treat neuropathic pain, and what first-line adjuvant analgesic should be used? And do bisphosphonates relieve bone pain in cancers other than breast cancer and myeloma? This review addresses these questions.

HS014, a selective melanocortin-4 (MC4) receptor antagonist, modulates the behavioral effects of morphine in mice

RATIONALE

Melanocortin and opioid systems regulate feeding as well as other behaviors; however, the relationship between the two systems is not yet defined. Since agonist-induced stimulation of melanocortin receptors blocks the behavioral effects of mu opioid receptor agonists, and melanocortin-4 (MC4) receptors and mu opioid receptors share a similar anatomical distribution in the central nervous system, MC4 receptor blockade may increase opioid responsiveness.

OBJECTIVES

The goal of this study was to test the hypothesis that blockade of MC4 receptors increases the behavioral effects of morphine.

METHODS

The effects of HS014 (0.0032, 0.032, and 1 nmol, i.c.v.), a selective MC4 antagonist, on morphine-induced (3.2, 10, and 32 mg/kg, i.p.) locomotor activity (measured in the open field for 15 min) and antinociception (measured in the hot plate at 55 degrees C) were assessed in C57Bl/6 mice. In addition, the effects of morphine were evaluated in A(y) mice, a genetic model for MC4 receptor blockade.

RESULTS

The dose-effect curve of morphine for locomotor activity was shifted downwards in C57Bl/6 mice pretreated with HS014 and in A(y) mice. The dose-effect curve of morphine for antinociception was shifted two- and threefold to the left in C57Bl/6 mice pretreated with HS014 and in A(y) mice, respectively.

CONCLUSIONS

These results indicate that blockade of MC4 receptors increases the antinociceptive effects of morphine without changing the potency of morphine for locomotor activity, suggesting that MC4 receptor antagonists may be candidate drugs that can be clinically used for the treatment of pain.

A treatment algorithm for neuropathic pain

BACKGROUND

Neuropathic pain is a chronic pain syndrome caused by drug-, disease-, or injury-induced damage or destruction of sensory neurons within the dorsal root ganglia of the peripheral nervous system. Characteristic clinical symptoms include the feeling of pins and needles; burning, shooting, and/or stabbing pain with or without throbbing; and numbness. Neuronal hyperexcitability represents the hallmark cellular mechanism involved in the underlying pathophysiology of neuropathic pain. Although the primary goal is to alleviate pain, clinicians recognize that even the most appropriate treatment strategy may be, at best, only able to reduce pain to a more tolerable level.

OBJECTIVE

The purpose of this review is to propose a treatment algorithm for neuropathic pain that health care professionals can logically follow and adapt to the specific needs of each patient. The algorithm is intended to serve as a general guide to assist clinicians in optimizing available therapeutic options.

METHODS

A comprehensive review of the literature using the PubMed, MEDLINE, Cochrane, and Toxnet databases was conducted to design and develop a novel treatment algorithm for neuropathic pain that encompasses agents from several drug classes, including antidepressants, antiepileptic drugs, topical antineuralgic agents, narcotics, and analgesics, as well as various treatment options for refractory cases.

RESULTS

Any of the agents in the first-line drug classes (tricyclic antidepressants, antiepileptic drugs, topical antineuralgics, analgesics) may be used as a starting point in the treatment of neuropathic pain. If a patient does not respond to treatment with at least 3 different agents within a drug class, agents from a second drug class may be tried. When all first-line options have been exhausted, narcotic analgesics or refractory treatment options may provide some benefit. Patients who do not respond to monotherapy with any of the first- or second-line agents may respond to combination therapy or may be candidates for referral to a pain clinic. Because the techniques used at pain clinics tend to be invasive, referrals to these clinics should be reserved for patients who are truly refractory to all forms of pharmacotherapy.

CONCLUSIONS

Neuropathic pain continues to be one of the most difficult pain conditions to treat. With the proposed algorithm, clinicians will have a framework from which to design a pain treatment protocol appropriate for each patient. The algorithm will also help streamline referrals to specialized pain clinics, thereby reducing waiting list times for patients who are truly refractory to traditional pharmacotherapy.

Dextromethorphan potentiates morphine-induced antinociception at both spinal and supraspinal sites but is not related to the descending serotoninergic or adrenergic pathways

Morphine is a strong and widely used opioid analgesic in pain management, but some adverse effects limit its clinical use at high doses. The clinically available non-opioid antitussive, dextromethorphan (DM) can potentiate the analgesic effect of morphine and decrease the dose of morphine in acute postoperative pain. However, the mechanism underlying this synergistic phenomenon is still not clear. To examine if the potentiation by DM occurs through the descending pain-inhibitory pathways, ketanserin (a 5-HT2 receptor antagonist) and yohimbine (an alpha2-adrenergic receptor antagonist) were employed and found to have no significant effect on the potentiation by DM. Using local delivery of drugs in rats in the present study, potentiation of morphine-induced antinociception by DM was observed via both intrathecal and intracerebroventricular routes, suggesting that both spinal and supraspinal sites are involved. This suggests that the potentiation of morphine-induced antinociception by DM is not mediated by the serotoninergic or adrenergic descending pain-inhibitory pathways. The present results are consistent with findings in clinical studies, which showed that DM can effectively decrease the consumption of morphine in patients suffering from pain. Since DM has excellent clinical potential as a synergistic agent with morphine, further investigating and clarifying the possible pharmacological mechanism of DM are of great importance for future studies.

Is the Second Phase of the Formalin Test Useful to Predict Activity in Chronic Constriction Injury Models? A Pharmacological Comparison in Different Species

This study presents data of several reference drugs in rats and gerbils for both the second phase of the formalin test and the cold allodynia in animals with a constriction injury of the sciatic nerve. A pharmacological validation of the formalin test and the CCI model in gerbils was performed. It was evaluated whether the second phase of the formalin test could be used as a pharmacological screening to predict outcome in the cold plate test in CCI animals. Male Sprague Dawley and Wistar rats and male gerbils were used for both tests. For the formalin test, animals were injected in the right hind paw (5% formalin rat: 0.05 microl; gerbil: 0.01 microl) and flinching and licking or biting were recorded. For CCI testing, a Bennett operation was performed on the left hind paw 7 days before testing. Cold plate allodynia was evaluated before and after drug treatment. In rats, a good correlation between both test conditions for morphine, fentanyl, MK-801 and flunarizine was found. Clonidine tends to have more activity in the second phase of the formalin test, whereas baclofen, tramadol, amitryptiline, ketamine and topiramate demonstrate to be more active in the cold plate. In gerbils, a good correlation between both test conditions for fentanyl and ketoprofen was found. Tramadol and CP-96345 tend to have more activity in the second phase of the formalin test, whereas morphine, SR-48968, SR-142801 and R116301 demonstrates to be more active in the cold plate test. In the present acute test conditions, there is a correlation in the pharmacological activity in rats and gerbils for the tested compounds a correlation between the second phase of the formalin test and the cold allodynia in CCI animals is found. Comparing to human data the screening drugs tested in this study show a correlation between animal and human studies in these specific circumstances. Further validation studies are needed to make these correlations clinical applicable.

Synthesis and biological evaluation of 14-alkoxymorphinans. 20. 14-phenylpropoxymetopon: an extremely powerful analgesic

The synthesis and the biological and pharmacological evaluation of several 14-phenylpropoxy analogues of 14-methoxymetopon are described. Most of the new compounds were nonselective and exhibited binding affinities in the subnanomolar or low nanomolar range at opioid receptors mu, kappa, delta), with 14-phenylpropoxymetopon (PPOM; 7) displaying the highest affinity for all three opioid receptor types. The most striking finding of this study is that the derivatives from the novel series of N-methyl-14-phenylpropoxymorphinans acted as extremely powerful antinociceptives with potencies higher than that of 14-methoxymetopon (1) and even etorphine. 14-Phenylpropoxymetopon (PPOM; 7) showed considerably increased potency in the in vivo assays in mice (25-fold in the tail-flick assay, 10-fold in the hot-plate assay, and 2.5-fold in the paraphenylquinone writhing test) when compared to etorphine, while it was equipotent to dihydroetorphine in the hot-plate assay and the paraphenylquinone writhing test and ca. twice as potent in the tail-flick assay than this reference compound. The 3-O-alkyl ethers of PPOM, compounds 6 and 8, showed less potency in in vivo assays, but partly surpassed the potency of the 3-OH analogue 14-methoxymetopon (1).

The role of spinal muscarinic acetylcholine receptors in clonidine-induced anti-nociceptive effects in rats

We have examined the effects of intrathecal (i.t.) injection of the muscarinic acetylcholine receptor antagonist atropine on the clonidine-induced nociceptive effect in formalin-induced nociception in rats. The injection of 5% formalin into the hind paw caused biphasic nociceptive responses, and i.t. injection of clonidine inhibited both phases of the nociceptive response in a dose-dependent manner. Pretreatment with atropine (i.t.) only partially inhibited the nociceptive effect of clonidine. These results suggest that the nociceptive effect of clonidine in the rat formalin model may be at least partly mediated by muscarinic acetylcholine receptors in the spinal cord.

A comparison of the pharmacological modulation of hyperalgesia and bronchial hyperresponsiveness

It has been proposed that there may be similarities in the mechanisms contributing toward hyperalgesia/allodynia observed in various clinical pain syndromes and bronchial hyperresponsiveness (BHR) in asthma. Both features are characterized by an increase in responsivity to various stimuli. In the case of pain, these include increase in responsiveness to mechanical and thermal stimuli whilst asthmatics bronchoconstrict in response to substances that are otherwise innocuous. A variety of mediators can induce hyperalgesia and pain when injected into the skin and these same mediators are also released during an inflammatory insult in the airways and in many cases can induce hyperresponsiveness experimentally. In this review, a comparison of the pharmacological modulation of afferent activity in hyperalgesia and in BHR will be made to ascertain whether any commonality exists between the treatment of these two disease states.

Binding characteristics of [3H]14-methoxymetopon, a high affinity mu-opioid receptor agonist

The highly potent micro -opioid receptor agonist 14-methoxymetopon (4,5alpha-epoxy-3-hydroxy-14beta-methoxy-5beta,17-dimethylmorphinan-6-one) was prepared in tritium labelled form by a catalytic dehalogenation method resulting in a specific radioactivity of 15.9 Ci/mmol. Opioid binding characteristics of [3H]14-methoxymetopon were determined using radioligand binding assay in rat brain membranes. [3H]14-Methoxymetopon specifically labelled a single class of opioid sites with affinity in low subnanomolar range (Ki = 0.43 nm) and maximal number of binding sites of 314 fmol/mg protein. Binding of [3H]14-methoxymetopon was inhibited by ligands selective for the micro -opioid receptor with high potency, while selective kappa-opioids and delta-opioids were weaker inhibitors. 14-Methoxymetopon increased guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]GTPgammaS) binding with an EC50 of 70.9 nm, thus, providing evidence for the agonist character of this ligand. The increase of [35S]GTPgammaS binding was inhibited by naloxone and selective micro -opioid antagonists, indicating a micro -opioid receptor-mediated action. [3H]14-Methoxymetopon is one of the few nonpeptide mu-opioid receptor agonists available in radiolabelled form up to now. Due to its high affinity and selectivity, high stability and extremely low nonspecific binding (<10%), this radioligand would be an important and useful tool in probing mu-opioid receptor mechanisms, as well as to promote a further understanding of the opioid system at the cellular and molecular level.

Epibatidine: impact on nicotinic receptor research

Epibatidine is a natural product that was isolated and identified by Daly and coworkers in 1992. Since that time, it has had a profound influence on the investigation of alpha4beta2 nicotinic cholinergic (nACh) receptor pharmacophore models, and has inspired the development of novel agents with therapeutic potential in CNS disorders. Apart from acetylcholine and nicotine, probably no other agent has had as much recent impact on nACh research as has epibatidine.

Topical and peripherally acting analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, alpha-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and alpha-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

In vivo activation of a mutant mu-opioid receptor by antagonist: future direction for opiate pain treatment paradigm that lacks undesirable side effects

Based on the in vitro ability of opioid antagonists to activate a mu-opioid receptor mutant, S196A, we reasoned that when expressed in the appropriate sites in vivo, this mutant receptor could be used to elicit the analgesic effects of the opioids without the accompanying side effects, such as tolerance and dependence. To test this hypothesis, we introduced the S196A mutation into the mouse mu-opioid receptor by a knock-in strategy to test the ability of the opioid antagonist to produce analgesic effects. In these homozygous mice, we observed increased intrinsic efficacy of opioid analgesics with two antinociceptive tests: hot water tail-withdrawal and acetic acid-induced writhing tests. Opioid antagonists, such as naloxone and naltrexone, elicited antinociceptive effects similar to that of partial agonists. Most importantly, chronic treatment of the homozygous mice with naltrexone did not produce the expected tolerance response, whereas less physical dependence was observed than with chronic morphine treatment. Such in vivo properties suggest the possibility of using the S196A mutant of the mu-opioid receptor and opioid antagonists to minimize the spectrum of unwarranted side effects in pain management when opiate analgesics are used.

The heritability of antinociception: common pharmacogenetic mediation of five neurochemically distinct analgesics

The heritability of nociception and antinociception has been well established in the mouse. The pharmacogenetics of morphine analgesia are fairly well characterized, but far less is known about other analgesics. The purpose of this work was to begin the systematic genetic study of non-mu-opioid analgesics. We tested mice of 12 inbred mouse strains for baseline nociceptive sensitivity (49 degrees C tail-withdrawal assay) and subsequent antinociceptive sensitivity to systemic administration of (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methane-sulfonate hydrate (U50,488; 10-150 mg/kg), a kappa-opioid receptor agonist; (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55,212-2; 0.5-480 mg/kg), a synthetic cannabinoid receptor agonist; epibatidine (7.5-150 microg/kg), a nicotinic receptor agonist; clonidine (0.1-5 mg/kg), an alpha(2)-adrenergic receptor agonist; and, for purposes of comparison, the prototypic mu-opioid receptor agonist, morphine (5-200 mg/kg). Robust interstrain variability was observed in nociceptive sensitivity and in the antinociceptive effects of each of the drugs, with extreme-responding strains exhibiting antinociceptive potencies differing up to 37-fold. Unexpectedly, we observed moderate-to-high genetic correlations of strain sensitivities to the five drugs (r = 0.39-0.77). We also found moderate-to-high correlations between baseline nociceptive sensitivity and subsequent analgesic response to each drug (r = 0.33-0.68). The generalizability of these findings was established in follow-up experiments investigating morphine and clonidine inhibition of formalin test nociception. Despite the fact that each drug activates a unique receptor, our results suggest that the potency of each drug is affected by a common set of genes. However, the genes in question may affect antinociception indirectly, via a primary action on baseline nociceptive sensitivity.

New horizons in the development of antiepileptic drugs

Significant advances have been made in the treatment of epilepsy over the past decades. However, despite the development of various novel antiepileptic drugs, about one third of patients with epilepsy is resistant to current pharmacotherapies. Even in patients in whom pharmacotherapy is efficacious, current antiepileptic drugs do not seem to affect the progression or underlying natural history of epilepsy. Furthermore, there is currently no drug available which prevents the development of epilepsy, e.g. after head trauma. Thus, there are at least three important goals for the future. (1) Better understanding of processes leading to epilepsy, thus allowing to create therapies aimed at the prevention of epilepsy in patients at risk; (2) improved understanding of biological mechanisms of pharmacoresistance, allowing to develop drugs for reversal or prevention of resistance; and (3) development of disease-modifying therapies, inhibiting the progression of epilepsy. The ultimate goal would be a drug combining these three properties, thus resulting in a complete cure for epilepsy. In this review, the current status of antiepileptic therapies is critically assessed, and innovative approaches for future therapies are highlighted.

Intrathecal alpha2 adrenoceptor agonist clonidine inhibits mechanical transmission in mouse spinal cord via activation of muscarinic M1 receptors

We examined the role of the spinal muscarinic receptor subtype in the anti-nociceptive effect of intrathecal (i.t.) alpha2 adrenoceptor agonist clonidine in mice. I.t. injection of the muscarinic receptor antagonist atropine completely inhibited i.t. clonidine-induced increase in the mechanical threshold, but did not affect the increase in tail-flick latency induced by i.t. clonidine. The clonidine-induced increase in mechanical threshold was inhibited by i.t. injection of the M1 receptor antagonist pirenzepine in a dose-dependent manner, and by the M3 receptor antagonist 4-DAMP, but not by the M2 receptor antagonist methoctramine. The potency of pirenzepine was greater than that of 4-DAMP. These results suggest that the clonidine-induced increase in mechanical threshold is mediated via the activation of M1 receptors in the spinal cord.

Effects of 14-methoxymetopon, a potent opioid agonist, on the responses to the tail electric stimulation test and plus-maze activity in male rats: neuroendocrine correlates

We have studied the effects of 14-methoxymetopon (HS 198), a potent opioid agonist, on the responses to the tail electric stimulation test and plus-maze activity of adult male rats. The prototype mu agonist morphine was used as the drug of reference. Besides we addressed the effects of HS 198 on the serum corticosterone levels and on serotonergic systems of discrete brain regions. Both drugs were administered subcutaneously. Morphine (5 mg/kg) and HS 198 (30 microg/kg) induced a similar effect on the nociceptive test, with both drugs significantly increasing the threshold for the vocalization afterdischarge, which is related to the emotional component of pain. In the plus-maze, morphine (5 mg/kg) and HS 198 (20 and 30 microg/kg) induced similar increases in the percentages of entries and time in the open arms, two parameters related to the anxiety state of the animals. The results indicate that HS 198 is far more potent than morphine in reducing the emotive/affective component of pain and in inducing an anxiolytic effect. HS 198 (30 microg/kg) also induced parallel increases in the serum corticosterone levels and the hypothalamic serotonin content. A possible correlation between the anxiolytic action of the drug and its effect on the hypothalamic serotonergic system is suggested.

Treatment of pain in acutely burned children

The child with burns suffers severe pain at the time of the burn and during subsequent treatment and rehabilitation. Pain has adverse physiological and emotional effects, and research suggests that pain management is an important factor in better outcomes. There is increasing understanding of the private experience of pain, and how children benefit from honest preparation for procedures. Developmentally appropriate and culturally sensitive pain assessment, pain relief, and reevaluation have improved, becoming essential in treatment. Pharmacological treatment is primary, strengthened by new concepts from neurobiology, clinical science, and the introduction of more effective drugs with fewer adverse side effects and less toxicity. Empirical evaluation of various hypnotic, cognitive, behavioral, and sensory treatment methods is advancing. Multidisciplinary assessment helps to integrate psychological and pharmacological pain-relieving interventions to reduce emotional and mental stress, and family stress as well. Optimal care encourages burn teams to integrate pain guidelines into protocols and critical pathways for improved care.

Current status and future directions in the pharmacotherapy of epilepsy

Considerable progress in the pharmacotherapy of epilepsy has been witnessed during the 20th century. However, despite the development of various antiepileptic drugs, about a third of patients are resistant to current pharmacotherapies. Even in patients in whom pharmacotherapy is efficacious, current antiepileptic drugs do not affect the progression or underlying natural history of the condition. Furthermore, currently there are no drugs available that prevent the development of epilepsy following, for example, head trauma. The rapid expansion of information about the cellular, molecular and genetic mechanisms of epilepsy is expected to lead to more effective therapies, prevention or even a cure for different types of epilepsy. In this article, I assess the current status of antiepileptic therapies and highlight innovative approaches for future treatments.

Neuropathic pain: a possible role for the melanocortin system?

In humans, damage to the nervous system can lead to a pain state referred to as neuropathic pain. Here, we give a short overview of the clinical picture and classification of neuropathic pain and highlight some of the currently known pathophysiological mechanisms involved, with special emphasis on neuropeptide plasticity. In this context, we discuss a specific group of neuropeptides, the melanocortins. These peptides have been demonstrated to play a role in nociception and to functionally interact with the opiate system. Recently, we demonstrated that spinal melanocortin receptors are upregulated in a rat model of neuropathic pain and that blockade of the melanocortin MC(4) receptor has anti-allodynic effects in this condition, suggesting that the melanocortin system plays a role in neuropathic pain. A natural agonist of melanocortin receptors is alpha-melanocyte-stimulating hormone (alpha-MSH), derived from the precursor molecule pro-opiomelanocortin (POMC). Cleavage of this precursor also yields beta-endorphin, which is co-released with alpha-MSH in nociception-associated areas of the spinal cord. We hypothesise that melanocortin receptor blockade attenuates a tonic influence of alpha-MSH on nociception, thus allowing the analgesic effects of beta-endorphin to develop, resulting in the alleviation of allodynia. In this way, treatment with melanocortin receptor antagonists might enhance opioid efficacy in neuropathic pain, which would be of great benefit in clinical practice.