Antinociceptive interactions between Mu-opioid receptor agonists and the serotonin uptake inhibitor clomipramine in rhesus monkeys: role of Mu agonist efficacy

Functional modular networks identify the pivotal genes associated with morphine addiction and potential drug therapies

BACKGROUND

Chronic morphine usage induces lasting molecular and microcellular adaptations in distinct brain areas, resulting in addiction-related behavioural abnormalities, drug-seeking, and relapse. Nonetheless, the mechanisms of action of the genes responsible for morphine addiction have not been exhaustively studied.

METHODS

We obtained morphine addiction-related datasets from the Gene Expression Omnibus (GEO) database and screened for Differentially Expressed Genes (DEGs). Weighted Gene Co-expression Network Analysis (WGCNA) functional modularity constructs were analyzed for genes associated with clinical traits. Venn diagrams were filtered for intersecting common DEGs (CDEGs). Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis for functional annotation. Protein-protein interaction network (PPI) and CytoHubba were used to screen for hub genes. Potential treatments for morphine addiction were figured out with the help of an online database.

RESULTS

Sixty-five common differential genes linked to morphine addiction were identified, and functional enrichment analysis showed that they were primarily involved in ion channel activity, protein transport, the oxytocin signalling pathway, neuroactive ligand-receptor interactions, and other signalling pathways. Based on the PPI network, ten hub genes (CHN2, OLIG2, UGT8A, CACNB2, TIMP3, FKBP5, ZBTB16, TSC22D3, ISL1, and SLC2A1) were checked. In the data set GSE7762, all of the Area Under Curve (AUC) values for the hub gene Receiver Operating Characteristic (ROC) curves were greater than 0.8. We also used the DGIdb database to look for eight small-molecule drugs that might be useful for treating morphine addiction.

CONCLUSIONS

The hub genes are crucial genes associated with morphine addiction in the mouse striatum. The oxytocin signalling pathway may play a vital role in developing morphine addiction.

Effects of methadone, buprenorphine, and naltrexone on actigraphy-based sleep-like parameters in male rhesus monkeys

Opioid use disorder (OUD) has been associated with the emergence of sleep disturbances. Although effective treatments for OUD exist, evidence suggests that these treatments also may be associated with sleep impairment. The extent to which these effects are an effect of OUD treatment or a result of chronic opioid use remains unknown. We investigated the acute effects of methadone, buprenorphine, and naltrexone on actigraphy-based sleep-like parameters in non-opioid-dependent male rhesus monkeys (Macaca mulatta, n = 5). Subjects were fitted with actigraphy monitors attached to primate collars to measure sleep-like parameters. Actigraphy recordings were conducted under baseline conditions, or following acute injections of vehicle, methadone (0.03-1.0 mg/kg, i.m.), buprenorphine (0.01-1.0 mg/kg, i.m.), or naltrexone (0.03-1.0 mg/kg, i.m.) in the morning (4 h after "lights on") or in the evening (1.5 h before "lights off"). Morning and evening treatments with methadone or buprenorphine significantly increased sleep latency and decreased sleep efficiency. The effects of buprenorphine on sleep-like measures resulted in a biphasic dose-response function, with the highest doses not disrupting actigraphy-based sleep. Buprenorphine induced a much more robust increase in sleep latency and decrease in sleep efficiency compared to methadone, particularly with evening administration, and detrimental effects of buprenorphine on sleep-like measures were observed up to 25.5 h after drug injection. Treatment with naltrexone, on the other hand, significantly improved sleep-like measures, with evening treatments improving both sleep latency and sleep efficiency. The currently available pharmacotherapies for OUD significantly alter sleep-like parameters in non-opioid-dependent monkeys, and opioid-dependent mechanisms may play a significant role in sleep-wake regulation.

Magnesium Salt, a Simple Strategy to Improve Methadone Analgesia in Chronic Pain: An Isobolographic Preclinical Study in Neuropathic Mice

Analgesic efficacy of methadone in cancer and chronic non-cancer pains is greater than that of other opioids, probably because of its unique pharmacokinetics properties and also because it targets glutamatergic receptors in addition to µ-opioid receptors. However, methadone has drawbacks which are clearly related to dosing and treatment duration. The authors hypothesized that the antinociceptive efficacy of methadone could be synergistically potentiated by magnesium and copper salts in a preclinical mouse model of chronic pain, using the intraplantar formalin test as algesimetric tool. The spared nerve injury mice model was used to generate mononeuropathy. A low dose (0.25%) formalin was injected in the neuropathic limb in order to give rise only to Phase I response, resulting from direct activation by formalin of nociceptive primary afferents. Licking/biting of the formalin-injected limb was evaluated as nociceptive behavior during a 35-min observation period. Dose-response curves for intraperitoneal magnesium sulfate (10, 30, 100, and 300 mg/kg i.p.), copper sulfate (0.1, 0.3, 1, and 3 mg/kg i.p.) and methadone (0.1, 0.3, 1, and 3 mg/kg i.p.) allowed to combine them in equieffective doses and to determine their interaction by isobolographic analysis. Magnesium sulfate, copper sulfate and methadone dose-dependently decreased the nociceptive response evoked by formalin injection, the respective ED₅₀ being 76.38, 1.18, and 0.50 mg/kg i.p. Isobolographic analysis showed a superadditive interaction for magnesium and methadone. Indeed, despite that both ED₅₀ are obviously equieffective, the ED₅₀ for the MgSO4/methadone combination contained less than one third of the methadone having the ED₅₀ for methadone alone. For the CuSO₄/methadone combination, the interaction was only additive. Extrapolated to clinical settings, the results suggest that magnesium salts might be used to improve synergistically the efficacy of methadone in neuropathy, which would allow to reduce the dose of methadone and its associated side effects.

Evaluation of a Dual Fentanyl/Heroin Vaccine on the Antinociceptive and Reinforcing Effects of a Fentanyl/Heroin Mixture in Male and Female Rats

Opioid-targeted vaccines represent an emerging treatment strategy for opioid use disorder. To determine whether concurrent vaccination against two commonly abused opioids (fentanyl and heroin) would confer broader spectrum opioid coverage, the current study evaluated dual fentanyl/heroin conjugate vaccine effectiveness using a warm water tail-withdrawal and a fentanyl/heroin-vs-food choice procedure in male and female rats across a 105-day observation period. Vaccine administration generated titers of high-affinity antibodies to both fentanyl and heroin sufficient to decrease the antinociceptive potency of fentanyl (25-fold), heroin (4.6-fold), and a 1:27 fentanyl/heroin mixture (7.5-fold). Vaccination did not alter the antinociceptive potency of the structurally dissimilar opioid agonist methadone. For comparison, continuous treatment with a naltrexone dose (0.032 mg/kg/h) shown previously to produce clinically relevant plasma-naltrexone levels decreased the antinociceptive potency of fentanyl, heroin, and the 1:27 fentanyl/heroin mixture by approximately 20-fold. Naltrexone treatment also shifted the potency of 1:27 fentanyl/heroin mixture in a drug-vs-food choice self-administration procedure 4.3-fold. In contrast, vaccination did not attenuate 1:27 fentanyl/heroin mixture self-administration in the drug-vs-food choice procedure. These data demonstrate that a vaccine can simultaneously attenuate the thermal antinociceptive effects of two structurally dissimilar opioids. However, the vaccine did not attenuate fentanyl/heroin mixture self-administration, suggesting a greater magnitude of vaccine responsiveness is required to decrease opioid reinforcement relative to antinociception.

Vaccine blunts fentanyl potency in male rhesus monkeys

One proposed factor contributing to the increased frequency of opioid overdose deaths is the emergence of novel synthetic opioids, including illicit fentanyl and fentanyl analogues. A treatment strategy currently under development to address the ongoing opioid crisis is immunopharmacotherapies or opioid-targeted vaccines. The present study determined the effectiveness and selectivity of a fentanyl-tetanus toxoid conjugate vaccine to alter the behavioral effects of fentanyl and a structurally dissimilar mu-opioid agonist oxycodone in male rhesus monkeys (n = 3-4). Fentanyl and oxycodone produced dose-dependent suppression of behavior in an assay of schedule-controlled responding and antinociception in an assay of thermal nociception (50 °C). Acute naltrexone (0.032 mg/kg) produced an approximate 10-fold potency shift for fentanyl to decrease operant responding. The fentanyl vaccine was administered at weeks 0, 2, 4, 9, 19, and 44 and fentanyl or oxycodone potencies in both behavioral assays were redetermined over the course of 49 weeks. The vaccine significantly and selectively shifted fentanyl potency at least 10-fold in both assays at several time points over the entire experimental period. Mid-point titer levels correlated with fentanyl antinociceptive potency shifts. Antibody affinity for fentanyl as measured by a competitive binding assay improved over time to approximately 3-4 nM. The fentanyl vaccine also increased fentanyl plasma levels approximately 6-fold consistent with the hypothesis that the vaccine sequesters fentanyl in the blood. Overall, these results support the continued development and evaluation of this fentanyl vaccine in humans to address the ongoing opioid crisis.

The Effects of Morphine, Baclofen, and Buspirone Alone and in Combination on Schedule-Controlled Responding and Hot Plate Antinociception in Rats

Better therapeutic options are needed for pain. Baclofen, buspirone, and morphine are characterized as having analgesic properties. However, little is known about potential interactions between analgesic effects of these drugs when combined. Furthermore, it is not known if the magnitude of these potential interactions will be similar for all drug effects. Thus, we tested the effects of these drugs alone and in combination for their capacity to produce thermal antinociception and to decrease food-maintained responding. Four male and four female Sprague-Dawley rats responded for food under a fixed-ratio 10 schedule; afterward they were immediately placed on a 52°C hot plate. Morphine, baclofen, and buspirone were examined alone and in 1:1 combinations, based upon ED₅₀ values. Morphine and baclofen effects were evaluated with the opioid antagonist naltrexone and the GABA_B antagonist (3-Aminopropyl)(diethoxymethyl)phosphinic acid (CGP35348), respectively. Morphine, baclofen, and buspirone dose dependently decreased operant responding, with the calculated ED₅₀ values being 7.09, 3.42, and 0.57 mg/kg, respectively. The respective antinociception ED₅₀ values were 16.15, 8.75, and 2.20 mg/kg. Analysis of 1:1 combinations showed the effects of morphine plus baclofen to decrease schedule-controlled responding and to produce thermal antinociception were synergistic. Effects of morphine plus buspirone and baclofen plus buspirone to decrease schedule-controlled responding were additive. Effects of the two combinations to produce thermal antinociception were synergistic. Naltrexone and CGP35348 antagonized the effects of morphine and baclofen, respectively. Synergistic antinociceptive effects, in conjunction with additive effects on food-maintained responding, highlight the therapeutic utility of opioid and non-opioid drug combinations.

Additive and subadditive antiallodynic interactions between μ-opioid agonists and N-methyl D-aspartate antagonists in male rhesus monkeys

μ-Opioid agonists are clinically effective analgesics, but also produce undesirable effects such as sedation and abuse potential that limit their clinical utility. Glutamatergic systems also modulate nociception and N-methyl D-aspartate (NMDA) receptor antagonists have been proposed as one useful adjunct to enhance the therapeutic effects and/or attenuate the undesirable effects of μ-opioid agonists. Whether NMDA antagonists enhance the antiallodynic effects of μ-agonists in preclinical models of thermal hypersensitivity (i.e. capsaicin-induced thermal allodynia) are unknown. The present study determined the behavioral effects of racemic ketamine, (+)-MK-801, (-)-nalbuphine, and (-)-oxycodone alone and in fixed proportion mixtures in assays of capsaicin-induced thermal allodynia and schedule-controlled responding in rhesus monkeys. Ketamine, nalbuphine, and oxycodone produced dose-dependent antiallodynia. MK-801 was inactive up to doses that produced undesirable effects. Ketamine, but not MK-801, enhanced the potency of μ-agonists to decrease rates of operant responding. Ketamine and nalbuphine interactions were additive in both procedures. Ketamine and oxycodone interactions were additive or subadditive depending on the mixture. Furthermore, oxycodone and MK-801 interactions were subadditive on antiallodynia and additive on rate suppression. These results do not support the broad clinical utility of NMDA receptor antagonists as adjuncts to μ-opioid agonists for thermal allodynic pain states.

Combining opioids and non-opioids for pain management: Current status

Pain remains a global health challenge. For decades, clinicians have been primarily relying on μ-opioid receptor (MOR) agonists and nonsteroidal anti-inflammatory drugs (NSAIDs) for pain management. MOR agonists remain the most efficacious analgesics available; however, adverse effects related to MOR agonists use are severe which often lead to forced drug discontinuation and inadequate pain relief. The recent opioid overdose epidemic urges the development of safer analgesics. Combination therapy is a well-established clinical pharmacotherapeutic strategy for the treatment of various clinical disorders. The combination of MOR agonists with non-MOR agonists may increase the analgesic potency of MOR agonists, reduce the development of tolerance and dependence, reduce the diversion and abuse, overdose, and reduce other clinically significant side effects associated with prolonged opioid use such as constipation. Overall, the combination therapy approach could substantially improve the therapeutic profile of MOR agonists. This review summarizes some recent developments in this field. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Pharmacological interaction between NSAIDS with clomipramine and risperidone in mice visceral pain

Nonsteroidal anti-inflammatory drugs (NSAIDs) possess as primary action mechanism the inhibition of cyclooxygenases (COX-1, COX-2, and COX-3), thus producing a decreasing prostaglandin synthesis. This study was designed to evaluate whether the antinociception induced by NSAIDs could be modulated by clomipramine or risperidone using a chemical model of inflammatory acute visceral pain, the abdominal acetic acid induced a writhing test in mice. Dose-response curves, intraperitoneal, or intrathecal for the antinociceptive activity displayed by ketoprofen, piroxicam, nimesulide, parecoxib, and paracetamol were analyzed in order to obtain the ED₅₀ of each drug. Pretreatment of mice with either clomipramine or risperidone, increased antinociceptive potency of ketoprofen, piroxicam, nimesulide, parecoxib, and paracetamol, expressed by a decrease in the values of antinociceptive ED_50. The results that were obtained are in line with those where the inhibition of COXs provides a justification for most of the pharmacological actions. Nevertheless, several findings suggest other molecular mechanisms, among which may be mentioned, L-selecting shedding; inhibition of i-NOS; inhibition of NF-Kappa B; suppression metaloproteinasas; inhibition of ß2 integrin activation; activation of α₂ -adrenoceptor; increase of IL-1ß; upregulation IL-6. In conclusion, the data generated in this study demonstrated that risperidone and clomipramine, separately, increase antinociceptive potency of NSAIDs in a chemical model of inflammatory acute visceral tonic pain.

Effectiveness comparisons of G-protein biased and unbiased mu opioid receptor ligands in warm water tail-withdrawal and drug discrimination in male and female rats

One emerging strategy to address the opioid crisis is the development of mu opioid receptor (MOR) ligands that preferentially signal the G-protein vs. β-arrestin pathway. The present study compared the relative potency and effectiveness of two G-protein biased (GPB)-MOR ligands TRV130 and SR-14968 to five unbiased MOR ligands (NAQ, nalbuphine, buprenorphine, morphine, and methadone) on therapeutic-related (e.g. antinociception) and abuse-related (e.g. discriminative stimulus effects) endpoints. Male and female rats were tested in a warm water tail-withdrawal procedure (50 °C) or trained to discriminate fentanyl (0.04 mg/kg, SC) from saline in a two-lever food-reinforced discrimination procedure. TRV130 and SR-14968 were approximately two-fold more potent to produce fentanyl stimulus effects vs. antinociception. Morphine, fentanyl, and methadone were significantly more potent in the fentanyl discrimination vs. tail withdrawal procedure. In addition, maximum antinociceptive and discriminative stimulus effects of fixed-proportion fentanyl/naltrexone mixtures (1:0.018, 1:0.054, 1:0.18, 1:0.3, and 1:0.54) were used to quantify 1) the relative in vivo efficacy of the two GPB-MOR agonists and five unbiased MOR ligands, and 2) potential species differences in MOR ligand effects between rats and monkeys. The efficacy-effect function generated from the fentanyl/naltrexone mixtures stratified the five unbiased ligands consistent with agonist-stimulated GTPγS binding (NAQ = nalbuphine < buprenorphine < morphine < methadone). However, TRV130 and SR-14968 produced greater antinociception and less fentanyl-like stimulus effects than was predicted. Furthermore, there was a significant positive correlation between rat and monkey antinociceptive effects. Overall, these results demonstrate GPB-MOR agonists produce undesirable abuse-related effects, albeit with slightly better potency and efficacy ratios compared to unbiased agonists. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

Role of mu-opioid agonist efficacy on antinociceptive interactions between mu agonists and the nociceptin opioid peptide agonist Ro 64-6198 in rhesus monkeys

Mu-opioid receptor agonists are clinically effective analgesics, but also produce undesirable effects that limit their clinical utility. The nociceptin opioid peptide (NOP) receptor system also modulates nociception, and NOP agonists might be useful adjuncts to enhance the analgesic effects or attenuate the undesirable effects of mu-opioid agonists. The present study determined behavioral interactions between the NOP agonist (-)-Ro 64-6198 and mu-opioid ligands that vary in mu-opioid receptor efficacy (17-cyclopropylmethyl-3,14β-dihyroxy-4,5α-epoxy-6α-[(3 ́-isoquinolyl)acetamindo]morphinan (NAQ) < buprenorphine < nalbuphine < morphine = oxycodone < methadone) in male rhesus monkeys. For comparison, Ro 64-6198 interactions were also examined with the kappa-opioid receptor agonist nalfurafine. Each opioid ligand was examined alone and following fixed-dose Ro 64-6198 pretreatments in assays of thermal nociception (n = 3-4) and schedule-controlled responding (n = 3). Ro 64-6198 alone failed to produce significant antinociception up to doses (0.32 mg/kg, IM) that significantly decreased rates of responding. All opioid ligands, except NAQ and nalfurafine, produced dose- and thermal intensity-dependent antinociception. Ro 64-6198 enhanced the antinociceptive potency of buprenorphine, nalbuphine, methadone, and nalfurafine. Ro 64-6198 enhancement of nalbuphine antinociception was NOP antagonist SB-612111 reversible and occurred under a narrow range of dose and time conditions. All opioid ligands, except NAQ and buprenorphine, produced dose-dependent decreases in rates of responding. Ro 64-6198 did not significantly alter mu-opioid ligand rate-decreasing effects. Although these results suggest that NOP agonists may selectively enhance the antinociceptive vs. rate-suppressant effects of some mu-opioid agonists, this small enhancement occurred under a narrow range of conditions dampening enthusiasm for NOP agonists as candidate "opioid-sparing" adjuncts.

Application of Receptor Theory to the Design and Use of Fixed-Proportion Mu-Opioid Agonist and Antagonist Mixtures in Rhesus Monkeys

Receptor theory predicts that fixed-proportion mixtures of a competitive, reversible agonist (e.g., fentanyl) and antagonist (e.g., naltrexone) at a common receptor [e.g., mu-opioid receptors (MORs)] will result in antagonist proportion-dependent decreases in apparent efficacy of the agonist/antagonist mixtures and downward shifts in mixture dose-effect functions. The present study tested this hypothesis by evaluating behavioral effects of fixed-proportion fentanyl/naltrexone mixtures in a warm-water tail-withdrawal procedure in rhesus monkeys (n = 4). Fentanyl (0.001-0.056 mg/kg) alone, naltrexone (0.032-1.0 mg/kg, i.m.) alone, and fixed-proportion mixtures of fentanyl/naltrexone (1:0.025, 1:0.074, and 1:0.22) were administered in a cumulative-dosing procedure, and the proportions were based on published fentanyl and naltrexone Kd values at MOR in monkey brain. Fentanyl alone produced dose-dependent antinociception at both 50 and 54°C thermal intensities. Up to the largest dose tested, naltrexone alone did not alter nociception. Consistent with receptor theory predictions, naltrexone produced a proportion-dependent decrease in the effectiveness of fentanyl/naltrexone mixtures to produce antinociception. The maximum effects of fentanyl, naltrexone, and each mixture were also used to generate an efficacy-effect scale for antinociception at each temperature, and this scale was evaluated for its utility in quantifying 1) efficacy requirements for antinociception at 50 and 54°C and 2) relative efficacy of six MOR agonists that vary in their efficacies to produce agonist-stimuated GTPγS binding in vitro (from lowest to highest efficacy: 17-cyclopropylmethyl-3,14β-dihyroxy-4,5α-epoxy-6α-[(3'-isoquinolyl)acetamindo]morphine, nalbuphine, buprenorphine, oxycodone, morphine, and methadone). These results suggest that fixed-proportion agonist/antagonist mixtures may offer a useful strategy to manipulate apparent drug efficacy for basic research or therapeutic purposes.

Concurrent Assessment of the Antinociceptive and Behaviorally Disruptive Effects of Opioids in Squirrel Monkeys

Although the clinical application of opioids for pain management is often hindered by undesired behavioral impairment, preclinical assays of antinociception typically do not provide information regarding the behaviorally disruptive effects of opioids that may accompany their antinociceptive effects. To address this, we modified a warm water tail withdrawal procedure to determine concurrently the effects of opioids on tail withdrawal latency (antinociception) and indices of food-maintained operant behavior (rates of responding and reinforcement density) in squirrel monkeys. Six opioid agonists were tested, and all produced dose-dependent antinociception and impairment of operant behavior. The ratio of median effective dose (ED₅₀) values for both measures (behavioral impairment:antinociception) was used as a quantitative measure of therapeutic index. Nalbuphine had the highest ED₅₀ ratio (4.88), reflecting antinociception with minimal behavioral disruption. Oxycodone, heroin, buprenorphine, and methadone all produced similar ED₅₀ ratios (.82-1.14), whereas butorphanol yielded a significantly lower ED₅₀ ratio (.17) reflecting behavioral disruption at doses producing only minimal antinociception. The antinociceptive and behaviorally disruptive effects of oxycodone and buprenorphine were further characterized using Schild analysis to calculate apparent pA₂ values for antagonism of the 2 drugs by naltrexone. These analyses suggest that µ-receptor mechanisms likely mediate the antinociceptive as well as behaviorally disruptive effects of oxycodone (pA₂ values: 8.13 and 8.57) and buprenorphine (pA₂ values: 8.6 and 7.9).

PERSPECTIVE

This article presents an assay that allows for the concurrent assessment of the antinociceptive and behaviorally disruptive effects of opioids. Our results show that the tail withdrawal assay in squirrel monkeys can provide a useful index of the behavioral selectivity with which opioids produce antinociception.

Maintenance on naltrexone+amphetamine decreases cocaine-vs.-food choice in male rhesus monkeys

BACKGROUND

Cocaine use disorder remains a significant public health issue for which there are no FDA-approved pharmacotherapies. Amphetamine maintenance reduces cocaine use in preclinical and clinical studies, but the mechanism of this effect is unknown. Previous studies indicate a role for endogenous opioid release and subsequent opioid receptor activation in some amphetamine effects; therefore, the current study examined the role of mu-opioid receptor activation in d-amphetamine treatment effects in an assay of cocaine-vs-food choice.

METHODS

Adult male rhesus monkeys with double-lumen intravenous catheters responded for concurrently available food pellets and cocaine injections (0-0.1mg/kg/injection) during daily sessions. Cocaine choice and overall reinforcement rates were evaluated during 7-day treatments with saline or test drugs.

RESULTS

During saline treatment, cocaine maintained a dose-dependent increase in cocaine-vs.-food choice. The mu-opioid receptor agonist morphine (0.032-0.32mg/kg/h) dose-dependently increased cocaine choice and decreased rates of reinforcement. A dose of the mu-selective opioid receptor antagonist naltrexone (0.0032mg/kg/h) that completely blocked morphine effects had no effect on cocaine choice when it was administered alone, but it enhanced the effectiveness of a threshold dose of 0.032mg/kg/h amphetamine to decrease cocaine choice without also enhancing nonselective behavioral disruption by this dose of amphetamine. Conversely, the kappa-selective opioid antagonist norbinalorphimine did not enhance amphetamine effects on cocaine choice.

CONCLUSIONS

These results suggest that amphetamine maintenance produces mu opioid-receptor mediated effects that oppose its anti-cocaine effects. Co-administration of naltrexone may selectively enhance amphetamine potency to decrease cocaine choice without increasing amphetamine potency to produce general behavioral disruption.

Development of a Clinically Viable Heroin Vaccine

Heroin is a highly abused opioid and incurs a significant detriment to society worldwide. In an effort to expand the limited pharmacotherapy options for opioid use disorders, a heroin conjugate vaccine was developed through comprehensive evaluation of hapten structure, carrier protein, adjuvant and dosing. Immunization of mice with an optimized heroin-tetanus toxoid (TT) conjugate formulated with adjuvants alum and CpG oligodeoxynucleotide (ODN) generated heroin "immunoantagonism", reducing heroin potency by >15-fold. Moreover, the vaccine effects proved to be durable, persisting for over eight months. The lead vaccine was effective in rhesus monkeys, generating significant and sustained antidrug IgG titers in each subject. Characterization of both mouse and monkey antiheroin antibodies by surface plasmon resonance (SPR) revealed low nanomolar antiserum affinity for the key heroin metabolite, 6-acetylmorphine (6AM), with minimal cross reactivity to clinically used opioids. Following a series of heroin challenges over six months in vaccinated monkeys, drug-sequestering antibodies caused marked attenuation of heroin potency (>4-fold) in a schedule-controlled responding (SCR) behavioral assay. Overall, these preclinical results provide an empirical foundation supporting the further evaluation and potential clinical utility of an effective heroin vaccine in treating opioid use disorders.

Repeated 7-Day Treatment with the 5-HT2C Agonist Lorcaserin or the 5-HT2A Antagonist Pimavanserin Alone or in Combination Fails to Reduce Cocaine vs Food Choice in Male Rhesus Monkeys

Cocaine use disorder is a global public health problem for which there are no Food and Drug Administration-approved pharmacotherapies. Emerging preclinical evidence has implicated both serotonin (5-HT) 2C and 2A receptors as potential mechanisms for mediating serotonergic attenuation of cocaine abuse-related neurochemical and behavioral effects. Therefore, the present study aim was to determine whether repeated 7-day treatment with the 5-HT_2C agonist lorcaserin (0.1-1.0 mg/kg per day, intramuscular; 0.032-0.1 mg/kg/h, intravenous) or the 5-HT_2A inverse agonist/antagonist pimavanserin (0.32-10 mg/kg per day, intramuscular) attenuated cocaine reinforcement under a concurrent 'choice' schedule of cocaine and food availability in rhesus monkeys. During saline treatment, cocaine maintained a dose-dependent increase in cocaine vs food choice. Repeated pimavanserin (3.2 mg/kg per day) treatments significantly increased small unit cocaine dose choice. Larger lorcaserin (1.0 mg/kg per day and 0.1 mg/kg/h) and pimavanserin (10 mg/kg per day) doses primarily decreased rates of operant behavior. Coadministration of ineffective lorcaserin (0.1 mg/kg per day) and pimavanserin (0.32 mg/kg per day) doses also failed to significantly alter cocaine choice. These results suggest that neither 5-HT_2C receptor activation nor 5-HT_2A receptor blockade are sufficient to produce a therapeutic-like decrease in cocaine choice and a complementary increase in food choice. Overall, these results do not support the clinical utility of 5-HT_2C agonists and 5-HT_2A inverse agonists/antagonists alone or in combination as candidate anti-cocaine use disorder pharmacotherapies.

Distinct BOLD fMRI Responses of Capsaicin-Induced Thermal Sensation Reveal Pain-Related Brain Activation in Nonhuman Primates

Background

Approximately 20% of the adult population suffer from chronic pain that is not adequately treated by current therapies, highlighting a great need for improved treatment options. To develop effective analgesics, experimental human and animal models of pain are critical. Topically/intra-dermally applied capsaicin induces hyperalgesia and allodynia to thermal and tactile stimuli that mimics chronic pain and is a useful translation from preclinical research to clinical investigation. Many behavioral and self-report studies of pain have exploited the use of the capsaicin pain model, but objective biomarker correlates of the capsaicin augmented nociceptive response in nonhuman primates remains to be explored.

Methodology

Here we establish an aversive capsaicin-induced fMRI model using non-noxious heat stimuli in Cynomolgus monkeys (n = 8). BOLD fMRI data were collected during thermal challenge (ON:20 s/42°C; OFF:40 s/35°C, 4-cycle) at baseline and 30 min post-capsaicin (0.1 mg, topical, forearm) application. Tail withdrawal behavioral studies were also conducted in the same animals using 42°C or 48°C water bath pre- and post- capsaicin application (0.1 mg, subcutaneous, tail).

Principal Findings

Group comparisons between pre- and post-capsaicin application revealed significant BOLD signal increases in brain regions associated with the ‘pain matrix’, including somatosensory, frontal, and cingulate cortices, as well as the cerebellum (paired t-test, p<0.02, n = 8), while no significant change was found after the vehicle application. The tail withdrawal behavioral study demonstrated a significant main effect of temperature and a trend towards capsaicin induced reduction of latency at both temperatures.

Conclusions

These findings provide insights into the specific brain regions involved with aversive, ‘pain-like’, responses in a nonhuman primate model. Future studies may employ both behavioral and fMRI measures as translational biomarkers to gain deeper understanding of pain processing and evaluate the preclinical efficacy of novel analgesics.

Antinociceptive Interactions between the Imidazoline I2 Receptor Agonist 2-BFI and Opioids in Rats: Role of Efficacy at the μ-Opioid Receptor

Although μ-opioids have been reported to interact favorably with imidazoline I2 receptor (I2R) ligands in animal models of chronic pain, the dependence on the μ-opioid receptor ligand efficacy on these interactions had not been previously investigated. This study systematically examined the interactions between the selective I2 receptor ligand 2-(2-benzofuranyl)-2-imidazoline hydrochloride (2-BFI) and three μ-opioid receptor ligands of varying efficacies: fentanyl (high efficacy), buprenorphine (medium-low efficacy), and 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-[(3'-isoquinolyl) acetamido] morphine (NAQ; very low efficacy). The von Frey test of mechanical nociception and Hargreaves test of thermal nociception were used to examine the antihyperalgesic effects of drug combinations in complete Freund's adjuvant-induced inflammatory pain in rats. Food-reinforced schedule-controlled responding was used to examine the rate-suppressing effects of each drug combination. Dose-addition and isobolographical analyses were used to characterize the nature of drug-drug interactions in each assay. 2-BFI and fentanyl fully reversed both mechanical and thermal nociception, whereas buprenorphine significantly reversed thermal but only slightly reversed mechanical nociception. NAQ was ineffective in both nociception assays. When studied in combination with fentanyl, NAQ acted as a competitive antagonist (apparent pA2 value: 6.19). 2-BFI/fentanyl mixtures produced additive to infra-additive analgesic interactions, 2-BFI/buprenorphine mixtures produced supra-additive to infra-additive interactions, and 2-BFI/NAQ mixtures produced supra-additive to additive interactions in the nociception assays. The effects of all combinations on schedule-controlled responding were generally additive. Results consistent with these were found in experiments using female rats. These findings indicate that lower-efficacy μ-opioid receptor agonists may interact more favorably with I2R ligands than high-efficacy μ-opioid receptor agonists.

Nalbuphine could decrease the rewarding effect induced by tramadol in mice while enhancing its antinociceptive activity

Nalbuphine, a kappa-opioid agonist and mu-opioid partial agonist, has been used as an analgesic or an adjuvant with morphine to attenuate the development of morphine dependence and rewarding effect. In this study, we investigated the effect of nalbuphine on tramadol rewarding effect and antinociception. Using the conditioned place preference (CPP) paradigm in mice, we demonstrated that co-administration of nalbuphine (7mg/kg, s.c.) with tramadol (70mg/kg, s.c.) during conditioning completely blocked the CPP induced by tramadol. Co-administration of nalbuphine blocked the increase in dopamine level in the nucleus accumbens induced by tramadol. These actions were accompanied by an increase rather than attenuation of the antinociceptive effect of tramadol. These results suggest that nalbuphine could have a great potential as a pharmacotherapy for tramadol abuse.

The effect of prolonged exposure to morphine on canine cerebral 5-HT2A receptors measured with (123)I-R91150 SPECT

Down-stream neuronal alterations, including changes in the 5-HT-2A receptor system, play an important role in the etiology and treatment of depression. The present study examined the effect of prolonged opioid treatment on cerebral 5-HT2A receptors. Cerebral 5-HT2A receptor availability was estimated in seven healthy five-year-old female neutered Beagle dogs pre and post 10-day morphine treatment (oral sustained release morphine 20mg twice daily for 10 days) with (123)I-R-91150, a 5-HT2A selective radioligand, and SPECT. 5-HT2A receptor binding indices (BI) for the frontal, parietal, temporal and occipital cortex and the subcortical region were calculated. Statistical analysis was performed using a linear mixed-effect model with treatment as fixed effect and dog as random effect. Morphine treatment significantly (P≤0.05) lowered 5-HT2A BIs in the right and left frontal cortex, the right and left temporal cortex, the right and left parietal cortex, and the subcortical region. The decreased cerebral 5-HT2A receptor availability following prolonged morphine exposure provides further evidence for an interaction between the opioid and serotonergic system.

Interaction of different antidepressants with acute and chronic methadone in mice, and possible clinical implications

We studied the interaction of a single dose of different antidepressant medications with a single (acute) dose or implanted mini-pump (chronic) methadone administration in mice, using the hotplate assay. For the acute experiment, subthreshold doses of six antidepressant drugs were administered separately with a single dose of methadone. The addition of a subthreshold dose of desipramine or clomipramine to methadone produced significant augmentation of the methadone effect with each drug (p < 0.05). Fluvoxamine given at a fixed subthreshold dose induced a synergistic effect only with a low methadone dose. Escitalopram, reboxetine and venlafaxine given separately, each at a fixed subthreshold dose, induced no interaction. Possible clinical implications of these findings are that while escitalopram, reboxetine and venlafaxine do not affect methadone's antinociception in mice and are safe to be given together with methadone when indicated, fluvoxamine, clomipramine and desipramine considerably augment methadone-induced effects and should be avoided in this population due to the risk of inducing opiate overdose. For the chromic experiment, when a subthreshold dose of either escitalopram, desipramine or clomipramine was injected to mice following 2 weeks of methadone administration with the mini-pump, none of the antidepressant drugs strengthened methadone's analgesic effect. Further studies are needed before possible clinical implications can be drawn.

Personalized medicine and opioid analgesic prescribing for chronic pain: opportunities and challenges

Use of opioid analgesics for pain management has increased dramatically over the past decade, with corresponding increases in negative sequelae including overdose and death. There is currently no well-validated objective means of accurately identifying patients likely to experience good analgesia with low side effects and abuse risk prior to initiating opioid therapy. This paper discusses the concept of data-based personalized prescribing of opioid analgesics as a means to achieve this goal. Strengths, weaknesses, and potential synergism of traditional randomized placebo-controlled trial (RCT) and practice-based evidence (PBE) methodologies as means to acquire the clinical data necessary to develop validated personalized analgesic-prescribing algorithms are overviewed. Several predictive factors that might be incorporated into such algorithms are briefly discussed, including genetic factors, differences in brain structure and function, differences in neurotransmitter pathways, and patient phenotypic variables such as negative affect, sex, and pain sensitivity. Currently available research is insufficient to inform development of quantitative analgesic-prescribing algorithms. However, responder subtype analyses made practical by the large numbers of chronic pain patients in proposed collaborative PBE pain registries, in conjunction with follow-up validation RCTs, may eventually permit development of clinically useful analgesic-prescribing algorithms.

PERSPECTIVE

Current research is insufficient to base opioid analgesic prescribing on patient characteristics. Collaborative PBE studies in large, diverse pain patient samples in conjunction with follow-up RCTs may permit development of quantitative analgesic-prescribing algorithms that could optimize opioid analgesic effectiveness and mitigate risks of opioid-related abuse and mortality.

Effects of 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) and quipazine on heroin self-administration in rhesus monkeys

RATIONALE

The serotonin (5-HT) system is involved in pain modulation, and 5-HT receptor agonists can enhance antinociceptive effects of mu opioid receptor agonists. Less is known about the actions of 5-HT receptor agonists on other effects of opioids.

OBJECTIVE

This study examined the effects of non-contingent and contingent administration of the 5-HT(2A) receptor agonists DOM and quipazine on i.v. heroin self-administration in rhesus monkeys.

RESULTS

Heroin (0.0001-0.1 mg/kg/infusion) generated an inverted U-shaped dose-response function. Non-contingent administration of DOM (0.1-0.32 mg/kg) flattened the dose-response function in three monkeys and eliminated heroin self-administration in a fourth monkey. Contingent DOM (0.0032-0.032 mg/kg/infusion) alone did not maintain responding above that maintained by saline, and, when added to self-administered heroin, monkeys responded less than for the same unit doses of heroin alone. Non-contingent (0.32-3.2 mg/kg) and contingent (0.0032-0.56 mg/kg/infusion) administration of quipazine flattened the dose-response function in two monkeys, increasing responding maintained by small unit doses of heroin and saline, but failed to enhance responding for heroin in two other monkeys.

CONCLUSION

This study shows that DOM does not enhance, and might attenuate, the positive reinforcing effects of the mu opioid receptor agonist heroin. Quipazine increased responding for saline and small doses of heroin; those effects were modest and observed in only two subjects. Taken together, these data suggest that 5-HT(2A) receptor agonists do not significantly enhance the reinforcing effectiveness of mu opioid receptor agonists and support the view that administering 5-HT drugs in combination with opioids to treat pain might not enhance abuse liability.

Interaction between Mu and Delta Opioid Receptor Agonists in an Assay of Capsaicin-Induced Thermal Allodynia in Rhesus Monkeys

Delta opioid agonists enhance antinociceptive effects of mu-opioid agonists in many preclinical assays of acute nociception, but delta/mu interactions in preclinical models of inflammation-associated pain have not been examined. This study examined interactions between the delta agonist SNC80 [(+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide] and the mu agonist analgesics methadone, morphine, and nalbuphine in an assay of capsaicin-induced thermal allodynia in rhesus monkeys. Thermal allodynia was produced by topical application of capsaicin to the tail. Antiallodynic effects of methadone, morphine, and nalbuphine were evaluated alone or in combination with fixed proportions of SNC80 identical to proportions previously shown to enhance acute thermal antinociceptive effects of these mu agonists in rhesus monkeys (0.9 : 1 SNC80/methadone; 0.29 : 1 SNC80/morphine; 3.6 : 1 SNC80/nalbuphine). Methadone, morphine, and nalbuphine each produced dose-dependent antiallodynia. SNC80 produced partial antiallodynia up to the highest dose tested (5.6 mg/kg). SNC80 produced a modest, enantioselective, and naltrindole-reversible enhancement of methadone-induced antiallodynia. However, SNC80 did not enhance morphine antiallodynia and only weakly enhanced nalbuphine antiallodynia. Overall, SNC80 produced modest or no enhancement of the antiallodynic effects of the three mu agonists evaluated. These results suggest that delta agonist-induced enhancement of mu agonist antiallodynia may be weaker and less reliable than previously demonstrated enhancement of mu agonist acute thermal nociception.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Sertraline inhibits formalin-induced nociception and cardiovascular responses

The objective of the present study was to determine the antihyperalgesic effect of sertraline, measured indirectly by the changes of sciatic afferent nerve activity, and its effects on cardiorespiratory parameters, using the model of formalin-induced inflammatory nociception in anesthetized rats. Serum serotonin (5-HT) levels were measured in order to test their correlation with the analgesic effect. Male Wistar rats (250-300 g) were divided into 4 groups (N = 8/per group): sertraline-treated group (Sert + Saline (Sal) and Sert + Formalin (Form); 3 mg·kg-1·day-1, ip, for 7 days) and saline-treated group (Sal + Sal and Sal + Form). The rats were injected with 5% (50 µL) formalin or saline into the right hind paw. Sciatic nerve activity was recorded using a silver electrode connected to a NeuroLog apparatus, and cardiopulmonary parameters (mean arterial pressure, heart rate and respiratory frequency), assessed after arterial cannulation and tracheotomy, were monitored using a Data Acquisition System. Blood samples were collected from the animals and serum 5-HT levels were determined by ELISA. Formalin injection induced the following changes: sciatic afferent nerve activity (+50.8 ± 14.7%), mean arterial pressure (+1.4 ± 3 mmHg), heart rate (+13 ± 6.8 bpm), respiratory frequency (+4.6 ± 5 cpm) and serum 5-HT increased to 1162 ± 124.6 ng/mL. Treatment with sertraline significantly reduced all these parameters (respectively: +19.8 ± 6.9%, -3.3 ± 2 mmHg, -13.1 ± 10.8 bpm, -9.8 ± 5.7 cpm) and serum 5-HT level dropped to 634 ± 69 ng/mL (P < 0.05). These results suggest that sertraline plays an analgesic role in formalin-induced nociception probably through a serotonergic mechanism.

Effects of direct- and indirect-acting serotonin receptor agonists on the antinociceptive and discriminative stimulus effects of morphine in rhesus monkeys

Serotonergic (5-HT) systems modulate pain, and drugs acting on 5-HT systems are used with opioids to treat pain. This study examined the effects of 5-HT receptor agonists on the antinociceptive and discriminative stimulus effects of morphine in monkeys. Morphine increased tail-withdrawal latency in a dose-related manner; 5-HT receptor agonists alone increased tail-withdrawal latency at 50 °C but not 55 °C water. The antinociceptive effects of morphine occurred with smaller doses when monkeys received an indirect-acting (fenfluramine) or direct acting (8-OH-DPAT, F13714, buspirone, quipazine, DOM, and 2C-T-7) agonist. The role of 5-HT receptor subtypes in these interactions was confirmed with selective 5-HT(1A) (WAY100635) and 5-HT(2A) (MDL100907) receptor antagonists. None of the 5-HT drugs had morphine-like discriminative stimulus effects; however, fenfluramine and 5-HT(2A) receptor agonists attenuated the discriminative stimulus effects of morphine and this attenuation was prevented by MDL100907. The 5-HT(1A) receptor agonists did not alter the discriminative stimulus effects of morphine. Thus, 5-HT receptor agonists increase the potency of morphine in an assay of antinociception, even under conditions where 5-HT agonists are themselves without effect (ie, 55 °C water), without increasing (and in some cases decreasing) the potency of morphine in a drug discrimination assay. Whereas 5-HT(2A) receptor agonists increase the potency of morphine for antinociception at doses that have no effect on the rate of operant responding, 5-HT(1A) receptor agonists increase the potency of morphine only at doses that eliminate operant responding. These data suggest that drugs acting selectively on 5-HT receptor subtypes could help to improve the use of opioids for treating pain.