Functional effects of systemically administered agonists and antagonists of mu, delta, and kappa opioid receptor subtypes on body temperature in mice

Morphine-induced fever: a case report and review of the literature

BACKGROUND

Morphine is widely used to treat moderate-to-severe cancer pain. However, it causes various adverse effects, with morphine-induced fever being an extremely rare and poorly understood symptom.

CASE PRESENTATION

We report the case of a 58-year-old Chinese woman with advanced lung cancer. Due to the ineffectiveness of tramadol for pain relief, her treatment regimen was switched to morphine. Following the change, she developed nausea, vomiting, dizziness, and elevated body temperature. A similar episode occurred subsequently. After a drug review, the pharmacist speculated that morphine was the most likely causative agent. Upon discontinuation of morphine, her body temperature returned to baseline levels.

CONCLUSIONS

This case highlights the need for healthcare providers to consider morphine as a potential cause of unexplained fever in patients. The fever may be caused by a hypersensitive response, as there was a significant increase in eosinophils during the fever episodes.

Effect of different concentrations of sufentanil combined with ropivacaine on epidural fever during labor: A single-center retrospective study

Labor epidural analgesia (LEA) is associated with increased maternal body temperature; however, the responsible mechanism is unknown. Recent studies suggest that changes in EA affect the incidence of fever and that epidural sufentanil supplementation enhances analgesia and reduces the amount of local anesthetic. The aim of this study was to evaluate the effect of different concentrations of sufentanil combined with ropivacaine on intrapartum fever during delivery. We performed a retrospective study comparing maternal fever rates in patients receiving labor analgesia between December 2018 and January 2019. Each patient receiving different concentrations of sufentanil in their EA received either proposal H (0.08% ropivacaine + 0.4 µg/mL sufentanil) or proposal L (0.08% ropivacaine + 0.2 µg/mL sufentanil), with the same nulliparous status. The primary outcome of this study was the incidence of intrapartum maternal fever, which was defined as any temperature ≥ 38°C during labor using Fisher exact test. Secondary outcome measures included visual analog scale (VAS) pain scores, birth events, and neonatal outcomes. We observed a perinatal fever incidence rate of 11.7% in the group receiving proposal L, while the incidence rate was 19.8% in the group receiving proposal H (P = .001). Five hours after administration, the average body temperature of the puerpera decreased significantly in the proposal L group compared with proposal H group. In addition, treatment with 0.2 µg/mL sufentanil provided satisfactory pain relief during labor, shortened the first stage of labor and total labor time, reduced oxytocin use, and had no significant adverse effects on neonatal outcomes. EA may increase the risk of intrapartum epidural-associated fever. Compared with the 0.4 µg/mL sufentanil group, the 0.2 µg/mL sufentanil group can provide better analgesia and improve maternal fever. These retrospective results highlighted the importance of prospective and mechanistic studies of maternal fever associated with intraspinal analgesia.

Natural product P57 induces hypothermia through targeting pyridoxal kinase

Induction of hypothermia during hibernation/torpor enables certain mammals to survive under extreme environmental conditions. However, pharmacological induction of hypothermia in most mammals remains a huge challenge. Here we show that a natural product P57 promptly induces hypothermia and decreases energy expenditure in mice. Mechanistically, P57 inhibits the kinase activity of pyridoxal kinase (PDXK), a key metabolic enzyme of vitamin B6 catalyzing phosphorylation of pyridoxal (PL), resulting in the accumulation of PL in hypothalamus to cause hypothermia. The hypothermia induced by P57 is significantly blunted in the mice with knockout of PDXK in the preoptic area (POA) of hypothalamus. We further found that P57 and PL have consistent effects on gene expression regulation in hypothalamus, and they may activate medial preoptic area (MPA) neurons in POA to induce hypothermia. Taken together, our findings demonstrate that P57 has a potential application in therapeutic hypothermia through regulation of vitamin B6 metabolism and PDXK serves as a previously unknown target of P57 in thermoregulation. In addition, P57 may serve as a chemical probe for exploring the neuron circuitry related to hypothermia state in mice.

Theophylline reverses oxycodone's but not fentanyl's respiratory depression in mice while caffeine is ineffective against both opioids

RATIONALE

The opioid epidemic remains a pressing public health crisis in the United States. Most of these overdose deaths are a result of lethal respiratory depression. In recent years the increasing incidence of opioid-involved overdose deaths has been driven by fentanyl, which is more resistant to adequate reversal by naloxone (NARCAN ®) than semi-synthetic or classical morphinan predecessors like oxycodone and heroin. For this and other reasons (e.g., precipitating withdrawal) non-opioidergic pharmacotherapies to reverse opioid-depressed respiration are needed. Methylxanthines are a class of stimulant drugs including caffeine and theophylline which exert their effects primarily via adenosine receptor antagonism. Evidence suggests methylxanthines can stimulate respiration by enhancing neural activity in respiratory nuclei in the pons and medulla independent of opioid receptors. This study aimed to determine whether caffeine and theophylline can stimulate respiration in mice when depressed by fentanyl and oxycodone.

METHODS

Whole-body plethysmography was used to characterize fentanyl and oxycodone's effects on respiration and their reversal by naloxone in male Swiss Webster mice. Next, caffeine and theophylline were tested for their effects on basal respiration. Finally, each methylxanthine was evaluated for its ability to reverse similar levels of respiratory depression induced by fentanyl or oxycodone.

RESULTS AND CONCLUSIONS

Oxycodone and fentanyl dose-dependently reduced respiratory minute volume (ml/min; MVb) that was reversible by naloxone. Caffeine and theophylline each significantly increased basal MVb. Theophylline, but not caffeine, completely reversed oxycodone-depressed respiration. In contrast, neither methylxanthine elevated fentanyl-depressed respiration at the doses tested. Despite their limited efficacy for reversing opioid-depressed respiration when administered alone, the methylxanthines safety, duration, and mechanism of action supports further evaluation in combination with naloxone to augment its reversal of opioid-depressed respiration.

Kappa opioid receptor activation increases thermogenic energy expenditure which drives increased feeding

Opioid receptors, including the kappa opioid receptor (KOR), exert control over thermoregulation and feeding behavior. Notably, activation of KOR stimulates food intake, leading to postulation that KOR signaling plays a central role in managing energy intake. KOR has also been proposed as a target for treating obesity. Herein, we report studies examining how roles for KOR signaling in regulating thermogenesis, feeding, and energy balance may be interrelated using pharmacological interventions, genetic tools, quantitative thermal imaging, and metabolic profiling. Our findings demonstrate that activation of KOR in the central nervous system causes increased energy expenditure via brown adipose tissue activation. Importantly, pharmacologic, or genetic inhibition of brown adipose tissue thermogenesis prevented the elevated food intake triggered by KOR activation. Furthermore, our data reveal that KOR-mediated thermogenesis elevation is reversibly disrupted by chronic high-fat diet, implicating KOR signaling as a potential mediator in high-fat diet-induced weight gain.

COMPARISON OF KETAMINE-MIDAZOLAM AND KETAMINE-MIDAZOLAM-BUTORPHANOL PREMEDICATION PRIOR TO SEVOFLURANE ANESTHESIA IN WOODCHUCKS (MARMOTA MONAX)

Cardiovascular disease is a frequent cause of death in the critically endangered Vancouver Island marmots (Marmota vancouverensis). This warrants the use of anesthetic protocols with minimal cardiovascular adverse effects. In this study, 12 adult male woodchucks (Marmota monax) were used as models for Vancouver Island marmots. The objective was to compare the physiological effects of two premedication protocols during induction and maintenance of anesthesia with sevoflurane. The two premedications were ketamine 10 mg/kg and midazolam 0.5 mg/kg (KM) or ketamine 10 mg/kg, midazolam 0.5 mg/kg, and butorphanol 1.0 mg/kg (KMB), administered intramuscularly prior to mask induction. Each marmot underwent three anesthetic events and protocols were assigned using a blinded randomized crossover design. Heart rate, respiratory rate, oxygen saturation, and body temperature were recorded throughout, and blood gases were assessed following induction. Resistance to induction was scored and time to induction was recorded. Although mask induction with sevoflurane was successful in all events (mean induction time of 2.1 min), KMB premedication resulted in a faster induction (mean induction time reduced by 1.2 ± 0.3 min) and lower resistance scores. Both protocols resulted in significant cardiovascular and respiratory depression; however, animals that received KMB were more hypercapnic than KM by 8.8 ± 2.8 mm Hg (P = 0.03) (mean venous partial pressure of carbon dioxide [PvCO2] for all: 79.9 mm Hg). In conclusion, if shorter induction times are desired, KMB premedication is preferred. However, cardiorespiratory variables including blood pressure should be monitored, and endotracheal intubation is recommended to allow for ETCO2 monitoring and provision of intermittent positive pressure ventilation.

Microglial knockdown does not affect acute withdrawal but delays analgesic tolerance from oxycodone in male and female C57BL/6J mice

Opioid Use Disorder (OUD) affects approximately 8%-12% of the population. In dependent individuals, abrupt cessation of opioid taking results in adverse withdrawal symptoms that reinforce drug taking behavior. Considerable unmet clinical need exists for new pharmacotherapies to treat opioid withdrawal as well as improve long-term abstinence. The neuroimmune system has received much scientific attention in recent years as a potential therapeutic target to combat various neurodegenerative and psychiatric disorders including addiction. However, the specific contribution of microglia has not been investigated in oxycodone dependence. Chronic daily treatment with the CSF1R inhibitor Pexidartinib (PLX3397) was administered to knockdown microglia expression and evaluate consequences on analgesia and on naloxone induced withdrawal from oxycodone. In vivo results indicated that an approximately 40% reduction in brain IBA1 staining was achieved in the PLX treatment group, which was associated with a delay in the development of analgesic tolerance to oxycodone and maintained antinociceptive efficacy. Acute withdrawal behavioral symptoms, brain astrocyte expression, and levels of many neuroinflammatory markers were not affected by PLX treatment. KC/GRO (also known as CXCL1) was significantly enhanced in the somatosensory cortex in oxycodone-treated mice receiving PLX. Microglial knock-down did not affect the expression of naloxoneinduced opioid withdrawal but affected antinociceptive responsivity. The consequences of increased KC/GRO expression within the somatosensory cortex due to microglial reduction during opioid dependence are unclear but may be important for neural pathways mediating opioid-induced analgesia.

µ-Opioid Receptors Expressed by Intrinsically Photosensitive Retinal Ganglion Cells Contribute to Morphine-Induced Behavioral Sensitization

Opioid drugs are the most effective tools for treating moderate to severe pain. Despite their analgesic efficacy, long-term opioid use can lead to drug tolerance, addiction, and sleep/wake disturbances. While the link between opioids and sleep/wake problems is well-documented, the mechanism underlying opioid-related sleep/wake problems remains largely unresolved. Importantly, intrinsically photosensitive retinal ganglion cells (ipRGCs), the cells that transmit environmental light/dark information to the brain's sleep/circadian centers to regulate sleep/wake behavior, express μ-opioid receptors (MORs). In this study, we explored the potential contribution of ipRGCs to opioid-related sleep/circadian disruptions. Using implanted telemetry transmitters, we measured changes in horizontal locomotor activity and body temperature in mice over the course of a chronic morphine paradigm. Mice lacking MORs expressed by ipRGCs (McKO) exhibited reduced morphine-induced behavioral activation/sensitization compared with control littermates with normal patterns of MOR expression. Contrastingly, mice lacking MORs globally (MKO) did not acquire morphine-induced locomotor activation/sensitization. Control mice also showed morphine-induced hypothermia in both the light and dark phases, while McKO littermates only exhibited morphine-induced hypothermia in the dark. Interestingly, only control animals appeared to acquire tolerance to morphine's hypothermic effect. Morphine, however, did not acutely decrease the body temperature of MKO mice. These findings support the idea that MORs expressed by ipRGCs could contribute to opioid-related sleep/wake problems and thermoregulatory changes.

Decursinol-mediated antinociception and anti-allodynia in acute and neuropathic pain models in male mice: Tolerance and receptor profiling

Korean scientists have shown that oral administration of Angelica gigas Nakai (AGN) root alcoholic extract and the metabolite of its pyranocoumarins, decursinol, have antinociceptive properties across various thermal and acute inflammatory pain models. The objectives of this study were 1) to assess whether tolerance develops to the antinociceptive effects of once-daily intraperitoneally administered decursinol (50 mg/kg) in acute thermal pain models, 2) to establish its anti-allodynic efficacy and potential tolerance development in a model of chemotherapy-evoked neuropathic pain (CENP) and 3) to probe the involvement of select receptors in mediating the pain-relieving effects with antagonists. The results show that decursinol induced antinociception in both the hot plate and tail-flick assays and reversed mechanical allodynia in mice with cisplatin-evoked neuropathic pain. Tolerance was detected to the antinociceptive effects of decursinol in the hot plate and tail-flick assays and to the anti-allodynic effects of decursinol in neuropathic mice. Pretreatment with either the 5-HT2 antagonist methysergide, the 5-HT2A antagonist volinanserin, or the 5-HT2C antagonist SB-242084 failed to attenuate decursinol-induced antinociception in the tail-flick assay. While pretreatment with the cannabinoid inverse agonists rimonabant and SR144528 failed to modify decursinol-induced anti-allodynia, pretreatment with the opioid antagonist naloxone partially attenuated the anti-allodynic effects of decursinol. In conclusion, our data support decursinol as an active phytochemical of AGN having both antinociceptive and anti-allodynic properties. Future work warrants a more critical investigation of potential receptor mechanisms as they are likely more complicated than initially reported.

In Vitro and In Vivo Pharmaco-Toxicological Characterization of 1-Cyclohexyl-x-methoxybenzene Derivatives in Mice: Comparison with Tramadol and PCP

1-cyclohexyl-x-methoxybenzene is a novel psychoactive substance (NPS), first discovered in Europe in 2012 as unknown racemic mixture of its three stereoisomers: ortho, meta and para. Each of these has structural similarities with the analgesic tramadol and the dissociative anesthetic phencyclidine. In light of these structural analogies, and based on the fact that both tramadol and phencyclidine are substances that cause toxic effects in humans, the aim of this study was to investigate the in vitro and in vivo pharmacodynamic profile of these molecules, and to compare them with those caused by tramadol and phencyclidine. In vitro studies demonstrated that tramadol, ortho, meta and para were inactive at mu, kappa and delta opioid receptors. Systemic administration of the three stereoisomers impairs sensorimotor responses, modulates spontaneous motor activity, induces modest analgesia, and alters thermoregulation and cardiorespiratory responses in the mouse in some cases, with a similar profile to that of tramadol and phencyclidine. Naloxone partially prevents only the visual sensorimotor impairments caused by three stereoisomers, without preventing other effects. The present data show that 1-cyclohexyl-x-methoxybenzene derivatives cause pharmaco-toxicological effects by activating both opioid and non-opioid mechanisms and suggest that their use could potentially lead to abuse and bodily harm.

Parabrachial opioidergic projections to preoptic hypothalamus mediate behavioral and physiological thermal defenses

Maintaining stable body temperature through environmental thermal stressors requires detection of temperature changes, relay of information, and coordination of physiological and behavioral responses. Studies have implicated areas in the preoptic area of the hypothalamus (POA) and the parabrachial nucleus (PBN) as nodes in the thermosensory neural circuitry and indicate that the opioid system within the POA is vital in regulating body temperature. In the present study we identify neurons projecting to the POA from PBN expressing the opioid peptides dynorphin and enkephalin. Using mouse models, we determine that warm-activated PBN neuronal populations overlap with both prodynorphin (Pdyn) and proenkephalin (Penk) expressing PBN populations. Here we report that in the PBN Prodynorphin (Pdyn) and Proenkephalin (Penk) mRNA expressing neurons are partially overlapping subsets of a glutamatergic population expressing Solute carrier family 17 (Slc17a6) (VGLUT2). Using optogenetic approaches we selectively activate projections in the POA from PBN Pdyn, Penk, and VGLUT2 expressing neurons. Our findings demonstrate that Pdyn, Penk, and VGLUT2 expressing PBN neurons are critical for physiological and behavioral heat defense.

Respiratory effects of low and high doses of fentanyl in control and β-arrestin 2-deficient mice

We have investigated the potential acute desensitizing role of the β arrestin 2 (β-arr2) pathway on the ventilatory depression produced by levels of fentanyl ranging from analgesic to life-threatening (0.1 to 60 mg/kg ip) in control and β-arr2-deficient nonsedated mice. Fentanyl at doses of 0.1, 0.5, and 1 mg/kg ip-corresponding to the doses previously used to study the role of β-arr2 pathway-decreased ventilation, but along the V̇e/V̇co₂ relationship established in baseline conditions. This reduction in ventilation was therefore indistinguishable from the decrease in breathing during the periods of spontaneous immobility. Above 1.5 mg/kg, however, ventilation was depressed out of proportion of the changes in metabolic rate, suggesting a specific depression of the drive to breathe. The ventilatory responses were similar between the two groups. At high doses of fentanyl (60 mg/kg ip) 1 out of 20 control mice died by apnea versus 8 out of 20 β-arr2-deficient mice (P = 0.008). In the surviving mice, ventilation was however identical in both groups. The ventilatory effects of fentanyl in β-arr2-deficient mice, reported in the literature, are primarily mediated by the "indirect" effects of sedation/hypometabolism on breathing control. There was an excess mortality at very high doses of fentanyl in the β-arr2-deficient mice, mechanisms of which are still open to question, as the capacity of maintaining a rhythmic, although profoundly depressed, breathing activity remains similar in all of the surviving control and β-arr2-deficient mice.NEW & NOTEWORTHY When life-threatening doses of fentanyl are used in mice, the β-arrestin 2 pathway appears to play a critical role in the recovery from opioid overdose. This observation calls into question the use of G protein-biased μ-opioid receptor agonists, as a strategy for safer opioid analgesic drugs.

Comparison of the effects of morphine-lidocaine-ketamine and fentanyl-lidocaine-ketamine combinations administered as constant rate infusions on postprocedure rectal temperature in dogs

OBJECTIVE

To compare the effects of morphine-lidocaine-ketamine (MLK) and fentanyl-lidocaine-ketamine (FLK) combinations administered as constant rate infusions (CRIs) during and after veterinary procedures on postprocedure rectal temperature in dogs.

ANIMALS

32 clinically normal client-owned dogs undergoing nonemergent procedures.

PROCEDURES

Dogs were randomly assigned to receive an MLK or FLK combination (16 dogs/group). During the procedure, each dog received 2% lidocaine hydrochloride (1 mg/kg/h; both groups), ketamine hydrochloride (0.6 mg/kg/h; both groups), and morphine (0.36 mg/kg/h; MLK group) or fentanyl (4 μg/kg/h; FLK group) via CRI for analgesia; esophageal temperature was maintained at 37° to 39°C. At extubation, each drug dose in each assigned combination was halved and administered (via CRI) for 12 additional hours for postprocedure analgesia. Rectal temperature and other data were recorded at baseline (prior to administration of premedicants), extubation (0 hours), and 0.5, 1.5, 3, 6, and 12 hours thereafter.

RESULTS

Mean postprocedure rectal temperature was significantly lower at each postextubation time point for the MLK group, compared with corresponding values for the FLK group. Compared with the baseline value, mean postprocedure rectal temperature was significantly lower at 0, 0.5, 1.5, and 3 hours for the FLK group and at all postprocedure time points for the MLK group. Hypothermia (rectal temperature < 37°C) was detected at ≥ 1 postprocedure time point more often in dogs in the MLK group (9/16) than in the FLK group (1/16).

CONCLUSIONS AND CLINICAL RELEVANCE

Dogs that received an MLK combination for analgesia during and after a veterinary procedure developed hypothermia more commonly than did dogs that received an FLK combination under similar conditions.

Neprilysin inhibition, endorphin dynamics, and early symptomatic improvement in heart failure: a pilot study

Aim

Sacubitril/valsartan is a first‐in‐class angiotensin receptor‐neprilysin inhibitor developed for the treatment of heart failure with reduced ejection fraction. Its benefits are achieved through the inhibition of neprilysin (NEP) and the specific blockade of the angiotensin receptor AT1. The many peptides metabolized by NEP suggest multifaceted potential consequences of its inhibition. We sought to evaluate the short‐term changes in serum endorphin (EP) values and their relation with patients' physical functioning after initiation of sacubitril/valsartan treatment.

Methods and results

A total of 105 patients with heart failure with reduced ejection fraction, who were candidates for sacubitril/valsartan treatment, were included in this prospective, observational, multicentre, and international study. In a first visit, and in agreement with current guidelines, treatment with angiotensin‐converting enzyme inhibitors or angiotensin receptor blocker was replaced by sacubitril/valsartan because of clinical indication by the responsible physician. By protocol, patients were reevaluated at 30 days after the start of sacubitril/valsartan. Serum levels of α‐ (α‐EP), γ‐Endorphin (γ‐EP), and soluble NEP (sNEP) were measured using enzyme‐linked immunoassays. New York Heart Association (NYHA) functional class was used as an indicator of patient's functional status. Baseline median levels of circulating α‐EP, γ‐EP, and sNEP were 582 (160–772), 101 (37–287), and 222 pg/mL (124–820), respectively. There was not a significant increase in α‐EP nor γ‐EP serum values after sacubitril/valsartan treatment (P value = 0.194 and 0.102, respectively). There were no significant differences in sNEP values between 30 days and baseline (P value = 0.103). Medians (IQR) of Δα‐EP, Δγ‐EP, and ΔsNEP between 30 days and baseline were 9.3 (−34 − 44), −3.0 (−46.0 − 18.9), and 0 units (−16.4 − 157.0), respectively. In a pre–post sacubitril/valsartan treatment comparison, there was a significant improvement in NYHA class, with 36 (34.3%) patients experiencing improvement by at least one NYHA class category. Δα‐EP and ΔsNEP showed to be significantly associated with NYHA class after 30 days of treatment (P = 0.014 and P < 0.001, respectively). Δα‐EP was linear and significantly associated with NYHA class improvement after 30 days of sacubitril/valsartan treatment.

Conclusions

These preliminary data suggest that beyond the haemodynamic benefits achieved with sacubitril/valsartan, the altered cleavage of endorphin peptides by NEP inhibition may participate in patients' symptoms improvement.

Activation of Kappa Opioid Receptor Regulates the Hypothermic Response to Calorie Restriction and Limits Body Weight Loss

Mammals maintain a nearly constant core body temperature (T_b) by balancing heat production and heat dissipation. This comes at a high metabolic cost that is sustainable if adequate calorie intake is maintained. When nutrients are scarce or experimentally reduced such as during calorie restriction (CR), endotherms can reduce energy expenditure by lowering T_b [1-6]. This adaptive response conserves energy, limiting the loss of body weight due to low calorie intake [7-10]. Here we show that this response is regulated by the kappa opioid receptor (KOR). CR is associated with increased hypothalamic levels of the endogenous opioid Leu-enkephalin, which is derived from the KOR agonist precursor dynorphin [11]. Pharmacological inhibition of KOR, but not of the delta or the mu opioid receptor subtypes, fully blocked CR-induced hypothermia and increased weight loss during CR independent of calorie intake. Similar results were seen with DIO mice subjected to CR. In contrast, inhibiting KOR did not change T_b in animals fed ad libitum (AL). Chemogenetic inhibition of KOR neurons in the hypothalamic preoptic area reduced the CR-induced hypothermia, whereas chemogenetic activation of prodynorphin-expressing neurons in the arcuate or the parabrachial nucleus lowered T_b. These data indicate that KOR signaling is a pivotal regulator of energy homeostasis and can affect body weight during dieting by modulating T_b and energy expenditure.

Therapeutic Hypothermia and Neuroprotection in Acute Neurological Disease

Therapeutic hypothermia has consistently been shown to be a robust neuroprotectant in many labs studying different models of neurological disease. Although this therapy has shown great promise, there are still challenges at the clinical level that limit the ability to apply this routinely to each pathological condition. In order to overcome issues involved in hypothermia therapy, understanding of this attractive therapy is needed. We review methodological concerns surrounding therapeutic hypothermia, introduce the current status of therapeutic cooling in various acute brain insults, and review the literature surrounding the many underlying molecular mechanisms of hypothermic neuroprotection. Because recent work has shown that body temperature can be safely lowered using pharmacological approaches, this method may be an especially attractive option for many clinical applications. Since hypothermia can affect multiple aspects of brain pathophysiology, therapeutic hypothermia could also be considered a neuroprotection model in basic research, which would be used to identify potential therapeutic targets. We discuss how research in this area carries the potential to improve outcome from various acute neurological disorders.

Proenkephalin in Heart Failure

The opioid system is activated in heart failure, which may be cardioprotective but may also be counter-regulatory. Recently, systemic proenkephalin activation has been investigated in various conditions predicting mortality and kidney injury. In acute heart failure, proenkephalin independently predicts mortality and heart failure rehospitalization in addition to traditional risk markers. It also predicts worsening renal function, increasingly recognized as an important risk predictor for poor outcome in heart failure. This article explores the role of enkephalins and delta-opioid receptors in the heart, then reviews studies measuring proenkephalin levels in the circulation and their associations with prognosis.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Pharmacological hypothermia: a potential for future stroke therapy?

Mild physical hypothermia after stroke has been associated with positive outcomes. Despite the well-studied beneficial effects of hypothermia in the treatment of stroke, lack of precise temperature control, intolerance for the patient, and immunosuppression are some of the reasons which limit its clinical translation. Pharmacologically induced hypothermia has been explored as a possible treatment option following stroke in animal models. Currently, there are eight classes of pharmacological agents/agonists with hypothermic effects affecting a multitude of systems including cannabinoid, opioid, transient receptor potential vanilloid 1 (TRPV1), neurotensin, thyroxine derivatives, dopamine, gas, and adenosine derivatives. Interestingly, drugs in the TRPV1, neurotensin, and thyroxine families have been shown to have effects in thermoregulatory control in decreasing the compensatory hypothermic response during cooling. This review will briefly present drugs in the eight classes by summarizing their proposed mechanisms of action as well as side effects. Reported thermoregulatory effects of the drugs will also be presented. This review offers the opinion that these agents may be useful in combination therapies with physical hypothermia to achieve faster and more stable temperature control in hypothermia.

Behavioral Characterization of κ Opioid Receptor Agonist Spiradoline and Cannabinoid Receptor Agonist CP55940 Mixtures in Rats

Pain is a significant clinical problem, and there is a need for more effective treatments with reduced adverse effects that currently limit the use of μ opioid receptor agonists. Synthetic κ opioid receptor agonists have no abuse liability and well-documented antinociceptive effects; however, adverse effects (diuresis, dysphoria) preclude their use in the clinic. Combining κ opioids with nonopioid drugs (cannabinoid receptor agonists) allows for smaller doses of each drug to produce antinociception. This study tested whether a potentially useful effect of the κ opioid receptor agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(5R,7S,8S)-7-pyrrolidin-1-yl-1-oxaspiro[4.5]decan-8-yl] (spiradoline; antinociception) is selectively enhanced by the cannabinoid receptor agonist 2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol (CP55940). Cumulative dose-response functions were determined in eight male Sprague-Dawley rats for spiradoline (0.032-32.0 mg/kg, i.p.) and CP55940 (0.0032-1.0 mg/kg, i.p.) for antinociception, hypothermia, food-maintained responding, and diuresis. Alone, each drug dose dependently increased tail withdrawal latencies from 50°C water, decreased body temperature by ∼4°C, and eliminated food-maintained responding. Spiradoline, but not CP55940, significantly increased urine output at doses that eliminated responding. Smaller doses of spiradoline and CP55940 in mixtures (3:1, 1:1, and 1:3 spiradoline:CP55940) had effects comparable to those observed with larger doses of either drug administered alone: the interaction was additive for antinociception and additive or greater than additive for hypothermia and food-maintained responding. Collectively, these data fail to provide support for the use of these mixtures for treating acute pain; however, κ opioid/cannabinoid mixtures might be useful for treating pain under other conditions (e.g., chronic pain), but only if the adverse effects of both drugs are not enhanced in mixtures.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

The subjective experience of acute, experimentally-induced Salvia divinorum inebriation

This study examined the overall psychological effects of inebriation facilitated by the naturally-occurring plant hallucinogen Salvia divinorum using a double-blind, randomized, placebo-controlled trial. Thirty healthy individuals self-administered Salvia divinorum via combustion and inhalation in a quiet, comfortable research setting. Experimental sessions, post-session interviews, and 8-week follow-up meetings were audio recorded and transcribed to provide the primary qualitative material analyzed here. Additionally, post-session responses to the Hallucinogen Rating Scale provided a quantitative groundwork for mixed-methods discussion. Qualitative data underwent thematic content analysis, being coded independently by three researchers before being collaboratively integrated to provide the final results. Three main themes and 10 subthemes of acute intoxication emerged, encompassing the qualities of the experience, perceptual alterations, and cognitive-affective shifts. The experience was described as having rapid onset and being intense and unique. Participants reported marked changes in auditory, visual, and interoceptive sensory input; losing normal awareness of themselves and their surroundings; and an assortment of delusional phenomena. Additionally, the abuse potential of Salvia divinorum was examined post hoc. These findings are discussed in light of previous research, and provide an initial framework for greater understanding of the subjective effects of Salvia divinorum, an emerging drug of abuse.

Telemetry monitoring for non-invasive assessment of changes in core temperature after spinal drug administration in freely moving rats

INTRODUCTION

There are no data available about the effects of spinally administered drugs on thermoregulation in freely moving animals. The first goal of the present study was to throw light on the consequences of intrathecally administered saline as a vehicle on core temperature and motor activity in unrestrained conditions. The second goal was to characterize the effects of morphine on these parameters as a widely used antinociceptive drug in spinal anesthesia, and reveal the potential role of the N-methyl-d-aspartate (NMDA) receptors in these processes.

METHODS

For these purposes, male Wistar rats were catheterized intrathecally, and E-Mitter battery-free transponders were implanted intraabdominally to continuously monitor core temperature and locomotor activity.

RESULTS

Saline induced a short-lasting hyperactivity accompanied by significant and prolonged hyperthermia that was blunted by systemic paracetamol administration. Morphine had no impact on motor activity; however, it caused high but equivalent degree hyperthermia in a wide dose range (1-15 μg), suggesting that it reached its peak effect. In the highest applied dose (25 μg), the NMDA receptor antagonist kynurenic acid blunted the saline-induced hyperthermia, and all doses caused higher hyperactivity compared to vehicle or morphine injections. In combination, kynurenic acid significantly inhibited the morphine-induced hyperthermia.

DISCUSSION

These data suggest that this method might be a valuable tool for investigating the thermoregulatory and locomotor effects of different drugs at spinal level; however, the prolonged effects of intrathecal vehicle injections should also be considered. The results point out that morphine is a very potent hyperthermic drug that may act primarily on the efferent limb of thermoregulation, at least partially, via an indirect NMDA-receptor mediated action mechanism.

Biphasic effect of melanocortin agonists on metabolic rate and body temperature

The melanocortin system regulates metabolic homeostasis and inflammation. Melanocortin agonists have contradictorily been reported to both increase and decrease metabolic rate and body temperature. We find two distinct physiologic responses occurring at similar doses. Intraperitoneal administration of the nonselective melanocortin agonist MTII causes a melanocortin-4 receptor (Mc4r)-mediated hypermetabolism/hyperthermia. This is preceded by a profound, transient hypometabolism/hypothermia that is preserved in mice lacking any one of Mc1r, Mc3r, Mc4r, or Mc5r. Three other melanocortin agonists also caused hypothermia, which is actively achieved via seeking a cool environment, vasodilation, and inhibition of brown adipose tissue thermogenesis. These results suggest that the hypometabolic/hypothermic effect of MTII is not due to a failure of thermoregulation. The hypometabolism/hypothermia was prevented by dopamine antagonists, and MTII selectively activated arcuate nucleus dopaminergic neurons, suggesting that these neurons may contribute to the hypometabolism/hypothermia. We propose that the hypometabolism/hypothermia is a regulated response, potentially beneficial during extreme physiologic stress.

Drug-induced hypothermia in stroke models: does it always protect?

Ischemic stroke is a common neurological disorder lacking a cure. Recent studies show that therapeutic hypothermia is a promising neuroprotective strategy against ischemic brain injury. Several methods to induce therapeutic hypothermia have been established; however, most of them are not clinically feasible for stroke patients. Therefore, pharmacological cooling is drawing increasing attention as a neuroprotective alternative worthy of further clinical development. We begin this review with a brief introduction to the commonly used methods for inducing hypothermia; we then focus on the hypothermic effects of eight classes of hypothermia-inducing drugs: the cannabinoids, opioid receptor activators, transient receptor potential vanilloid, neurotensins, thyroxine derivatives, dopamine receptor activators, hypothermia-inducing gases, adenosine, and adenine nucleotides. Their neuroprotective effects as well as the complications associated with their use are both considered. This article provides guidance for future clinical trials and animal studies on pharmacological cooling in the setting of acute stroke.

Mechanisms that underlie μ-opioid receptor agonist-induced constipation: differential involvement of μ-opioid receptor sites and responsible regions

Reducing the side effects of pain treatment is one of the most important strategies for improving the quality of life of cancer patients. However, little is known about the mechanisms that underlie these side effects, especially constipation induced by opioid receptor agonists; i.e., do they involve naloxonazine-sensitive versus -insensitive sites or central-versus-peripheral μ-opioid receptors? The present study was designed to investigate the mechanisms of μ-opioid receptor agonist-induced constipation (i.e., the inhibition of gastrointestinal transit and colonic expulsion) that are antagonized by the peripherally restricted opioid receptor antagonist naloxone methiodide and naloxonazine in mice. Naloxonazine attenuated the fentanyl-induced inhibition of gastrointestinal transit more potently than the inhibition induced by morphine or oxycodone. Naloxone methiodide suppressed the oxycodone-induced inhibition of gastrointestinal transit more potently than the inhibition induced by morphine, indicating that μ-opioid receptor agonists induce the inhibition of gastrointestinal transit through different mechanisms. Furthermore, we found that the route of administration (intracerebroventricular, intrathecally, and/or intraperitoneally) of naloxone methiodide differentially influenced the suppressive effect on the inhibition of colorectal transit induced by morphine, oxycodone, and fentanyl. These results suggest that morphine, oxycodone, and fentanyl induce constipation through different mechanisms (naloxonazine-sensitive versus naloxonazine-insensitive sites and central versus peripheral opioid receptors), and these findings may help us to understand the characteristics of the constipation induced by each μ-opioid receptor agonist and improve the quality of life by reducing constipation in patients being treated for pain.

Pharmacological traits of delta opioid receptors: pitfalls or opportunities?

RATIONALE

Delta opioid receptors (DORs) have been considered as a potential target to relieve pain as well as treat depression and anxiety disorders and are known to modulate other physiological responses, including ethanol and food consumption. A small number of DOR-selective drugs are in clinical trials, but no DOR-selective drugs have been approved by the Federal Drug Administration and some candidates have failed in phase II clinical trials, highlighting current difficulties producing effective delta opioid-based therapies. Recent studies have provided new insights into the pharmacology of the DOR, which is often complex and at times paradoxical.

OBJECTIVE

This review will discuss the existing literature focusing on four aspects: (1) Two DOR subtypes have been postulated based on differences in pharmacological effects of existing DOR-selective ligands. (2) DORs are expressed ubiquitously throughout the body and central nervous system and are, thus, positioned to play a role in a multitude of diseases. (3) DOR expression is often dynamic, with many reports of increased expression during exposure to chronic stimuli, such as stress, inflammation, neuropathy, morphine, or changes in endogenous opioid tone. (4) A large structural variety in DOR ligands implies potential different mechanisms of activating the receptor.

CONCLUSION

The reviewed features of DOR pharmacology illustrate the potential benefit of designing tailored or biased DOR ligands.

The in vivo pharmacodynamics of the novel opioid receptor antagonist, TD-1211, in models of opioid-induced gastrointestinal and CNS activity

The in vivo preclinical pharmacodynamic profile of TD-1211, a selective opioid receptor antagonist currently under development for the treatment of opioid-induced constipation, was compared to that of the clinically studied opioid antagonists, naltrexone, alvimopan, and ADL 08-0011 (the primary active metabolite of alvimopan). The oral activity of TD-1211 was evaluated in models of gastrointestinal (GI) and central nervous system (CNS) function in the rat and dog. Oral administration of TD-1211, naltrexone, and ADL 08-0011 reversed loperamide-induced inhibition of gastric emptying and castor oil-induced diarrhea in rats and nonproductive GI circular smooth muscle contractility in dogs. Alvimopan was only efficacious in the castor oil model. Oral administration of naltrexone and ADL 08-0011, but not TD-1211 or alvimopan, was associated with a CNS withdrawal response in morphine-dependent mice, inhibition of morphine-induced anti-nociception in rat and dog hot plate tests, and hypothermia and sedation in dogs. It is concluded that TD-1211 has potent in vivo GI activity, consistent with opioid receptor antagonism, but has no significant CNS activity. The data from these studies support the clinical development of TD-1211 as a novel treatment for opioid-induced GI dysfunction.

A comparison of the antinociceptive and temperature responses to morphine and fentanyl derivatives in rats

In addition to producing antinociception, opioids exert profound effects on body temperature. This study aimed at comparing antinociceptive and hyperthermic responses between two groups of μ-opioid receptor agonists: fentanyl (4-anilinopiperidine-type) and morphine (phenanthrene-type) derivatives in rats. Analgesic activity was assessed by tail immersion test and the body temperature by insertion of a thermometer probe into the colon. Fentanyl (F), (±)-cis-3-methyl fentanyl (CM), (±)-cis-3-carbomethoxy fentanyl (C), (±)trans-3-carbomethoxy fentanyl (T) and (±)-cis-3 butyl fentanyl (B) produced dose-dependent increase in antinociception and hyperthermia. The relative order of analgesic potency was: CM(11.27)>F(1)>C(0.35)≥T(0.11)≥B(0.056). Similar to this, the relative order of hyperthermic potency was: CM(8.43)>F(1)>C(0.46)≥T(0.11)≥B(0.076). Morphine (M), oxycodone (O), thebacon (T) and 6,14-ethenomorphinan-7-methanol, 4,5-epoxy-6-fluoro-3-hydroxy-α,α,17-trimethyl-, (5α,7α) (E) also produced dose-dependent increase in antinociception and hyperthermia. Among morphine derivatives the relative order of analgesic potency was: E(56)>O(5)≥T(2.6)>M(1), and similar to this, the relative order of hyperthermic potency was: E(37)>O(3)≥T(2.3)>M(1). Morphine (phenanthrene-type) and fentanyl (4-anilinopiperidine-type) derivatives produced hyperthermia in rats at doses about 2 times lower, and 6-11 times higher, than their median antinociceptive doses, respectively. This study is first to identify difference between these two classes of opioid drugs in their potencies in producing hyperthermia. Further studies are needed to clarify the significance of these findings.

δ-opioid receptor function in the dorsal striatum plays a role in high levels of ethanol consumption in rats

Binge-like patterns of excessive drinking during young adulthood increase the propensity for alcohol use disorders (AUDs) later in adult life; however, the mechanisms that drive this are not completely understood. Previous studies showed that the δ-opioid peptide receptor (DOP-R) is dynamically regulated by exposure to ethanol and that the DOP-R plays a role in ethanol-mediated behaviors. The aim of this study was to determine the role of the DOP-R in high ethanol consumption from young adulthood through to late adulthood by measuring DOP-R-mediated [(35)S]GTPγS binding in brain membranes and DOP-R-mediated analgesia using a rat model of high ethanol consumption in Long Evans rats. We show that DOP-R activity in the dorsal striatum and DOP-R-mediated analgesia changes during development, being highest during early adulthood and reduced in late adulthood. Intermittent access to ethanol but not continuous ethanol or water from young adulthood leads to an increase in DOP-R activity in the dorsal striatum and DOP-R-mediated analgesia into late adulthood. Multiple microinfusions of naltrindole into the dorsal striatum or multiple systemic administration of naltrindole reduces ethanol consumption, and following termination of treatment, DOP-R activity in the dorsal striatum is attenuated. These findings suggest that DOP-R activity in the dorsal striatum plays a role in high levels of ethanol consumption and suggest that targeting the DOP-R is an alternative strategy for the treatment of AUDs.

Life span extension in Drosophila melanogaster induced by morphine

The influence of morphine on the life span of Drosophila melanogaster fruit flies has been investigated. Morphine hydrochloride (MH) at concentrations of 0.01, 0.05 and 0.25 mg/ml was added to a medium starting from day 5 or 54 of imaginal life. Supplementation with MH starting from day 5 of imaginal life has resulted in significant increases in the mean life span of males at all concentrations studied. In females, a significant increase in life span compared with control was obtained only for those treated with 0.25 mg/ml MH. In flies with MH feeding from day 54, residual life span was significantly increased in both males and females after treatment with 0.05 mg/ml MH. The present data, together with those of our earlier study in mice (Dubiley et al. Probl Aging Longvity 9:331–332, 2000) suggest that morphine supplementation can result in life extension in both vertebrate and invertebrate animal species.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Effects of opioids, cannabinoids, and vanilloids on body temperature

Cannabinoid and opioid drugs produce marked changes in body temperature. Recent findings have extended our knowledge about the thermoregulatory effects of cannabinoids and opioids, particularly as related to delta opioid receptors, endogenous systems, and transient receptor potential (TRP) channels. Although delta opioid receptors were originally thought to play only a minor role in thermoregulation compared to mu and kappa opioid receptors, their activation has been shown to produce hypothermia in multiple species. Endogenous opioids and cannabinoids also regulate body temperature. Mu and kappa opioid receptors are thought to be in tonic balance, with mu and kappa receptor activation producing hyperthermia and hypothermia, respectively. A particularly intense research focus is TRP channels, where TRPV1 channel activation produces hypothermia whereas TRPA1 and TRPM8 channel activation causes hyperthermia. The marked hyperthermia produced by TRPV1 channel antagonists suggests these warm channels tonically control body temperature. A better understanding of the roles of cannabinoid, opioid, and TRP systems in thermoregulation may have broad clinical implications and provide insights into interactions among neurotransmitter systems involved in thermoregulation.

Strategies to discover unexpected targets for drugs active at G protein-coupled receptors

G protein-coupled receptors (GPCRs) are an evolutionarily conserved family of signaling molecules comprising approximately 2% of the human genome; this receptor family remains a central focus in basic pharmacology studies and drug discovery efforts. Detailed studies of drug action at GPCRs over the past decade have revealed existing and novel ligands that exhibit polypharmacology-that is, drugs with activity at more than one receptor target for which they were designed. These "off-target" drug actions can be a liability that causes adverse side effects; however, in several cases, drugs with less selectivity demonstrate better clinical efficacy. Here we review physical screening and cheminformatic approaches that define drug activity at the GPCR receptorome. In many cases, such profiling has revealed unexpected targets that explain therapeutic actions as well as off-targets underlying drug side effects. Such drug-receptor profiling has also provided new insights into mechanisms of action of existing drugs and has suggested directions for future drug development.

Study of adrenergic, imidazoline, and endothelin receptors in clonidine-, morphine-, and oxycodone-induced changes in rat body temperature

OBJECTIVES

The potentiation of morphine or oxycodone analgesia by endothelin-A (ET(A)) receptor antagonists and imidazoline/α(2)-adrenergic agonists is well documented. However, the effect of morphine or oxycodone in combination with an ET(A) receptor antagonist or an imidazoline/α(2) adrenergic agonist on body temperature is not known. The present study was carried out to study the role of ET(A) and imidazoline/α(2) adrenergic receptors in body temperature effects of morphine, oxycodone, and clonidine in rats.

METHODS

Body temperature was determined in male Sprague-Dawley rats treated with morphine, oxycodone, or clonidine. Yohimbine, idazoxan, and BMS182874 were used to determine the involvement of α(2)-adrenergic, imidazoline, and ET(A) receptors, respectively.

KEY FINDINGS

Morphine and oxycodone produced hyperthermia which was not affected by α(2)-adrenergic antagonist yohimbine, imidazoline/α(2)-adrenergic antagonist idazoxan, or ET(A) receptor antagonist BMS182874. Clonidine alone produced hypothermia that was comparable to the hypothermia observed with clonidine plus morphine or oxycodone. The hypothermic effect of clonidine was blocked by idazoxan and yohimbine. The blockade by idazoxan was more pronounced compared to yohimbine. Clonidine hypothermia was not affected by BMS182874.

CONCLUSIONS

This is the first report demonstrating that ET(A) receptors do not influence morphine- and oxycodone- induced hyperthermia or clonidine-induced hypothermia. Imidazoline receptors and α(2)-adrenergic receptors are involved in clonidine-induced hypothermia, but not in morphine- and oxycodone-induced hyperthermia.

Effects of intracerebroventricular application of the delta opioid receptor agonist [D-Ala2, D-Leu5] enkephalin on neurological recovery following asphyxial cardiac arrest in rats

The delta opioid receptor (DOR) agonist [D-Ala2, D-Leu5] enkephalin (DADLE) has been implicated as a novel neuroprotective agent in the CNS. The current study was designed to evaluate the effects of intracerebroventricular (ICV) application of DADLE on neurological outcomes following asphyxial cardiac arrest (CA) in rats. Male Sprague-Dawley rats were randomly assigned to four groups: Sham group, CA group, DADLE group (DADLE+CA), and Naltrindole group (Naltrindole and DADLE+CA). All drugs were administered into the left cerebroventricle 30 min before CA. CA was induced by 8-min asphyxiation and the animals were resuscitated with a standardized method. DOR protein expression in the hippocampus was significantly increased in the CA group at 1 h after restoration of spontaneous circulation (ROSC) compared with the Sham group. As time progressed, expression of DOR proteins decreased gradually in the CA group. Treatment with DADLE alone or co-administration with Naltrindole reversed the down-regulation of DOR proteins in the hippocampus induced by CA at 24 h after ROSC. Compared with the CA group, the DADLE group had persistently better neurological functional recovery, as assessed by neurological deficit score (NDS) and Morris water maze trials. The number of surviving hippocampal CA1 neurons in the DADLE group was significantly higher than those in the CA group. However, administration of Naltrindole abolished most of the neuroprotective effects of DADLE. We conclude that ICV administration of DADLE 30 min before asphyxial CA has significant protective effects in attenuating hippocampal CA1 neuronal damage and neurological impairments, and that DADLE executes its effects mainly through DOR.

Opioid, cannabinoid CB1 and NOP receptors do not mediate APAP-induced hypothermia in rats

Acetaminophen (APAP) produces antinociception and hypothermia. Because the antinociceptive effect in rats is partially dependent on opioid and cannabinoid CB1 receptor activation, we determined if activation of these receptors also contributes to the hypothermic effect of APAP. Rats injected with APAP (100, 250, 375 or 500 mg/kg, i.p.) displayed dose-related hypothermia. For combined administration, the hypothermic effect of APAP (400 mg/kg, i.p.) was not altered by pretreatment with: naltrexone (10 mg/kg, s.c.), a non-selective opioid antagonist; naltrindole (1 mg/kg, s.c.), a delta opioid antagonist; nor-binaltorphimine (10 mg/kg, i.p.), a kappa opioid antagonist; SR 141716A (3 mg/kg, i.m.), a cannabinoid CB1 receptor antagonist; or JTC-801(1 mg/kg, i.p.), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist. The demonstration that APAP produces hypothermia independent of opioid, cannabinoid CB1 or NOP receptor activation is contrary to its antinociceptive effect, which requires opioid and cannabinoid CB1 receptor activation.

Endogenous cardiac opioids: enkephalins in adaptation and protection of the heart

Opiates have been used for thousands of years in the form of opium for relief of pain or fever and to induce sleep. However, it was only in the 1970s that the endogenous ligands for the opiate receptors were identified and termed opioid peptides. Opioid peptides activate G protein-coupled receptors in the central and autonomic nervous system, with marked effects on the regulation of pain perception, body temperature, respiration, heart rate and blood pressure. Cardiovascular regulatory effects of endogenous opioids were initially considered to originate from neural centres in the central nervous system, facilitating a regulatory role in neuro-transmission, as demonstrated by the presynaptic co-release from sympathetic neurones of norepinephrine with enkephalin or acetylcholine with enkephalin. However, opioid peptides of myocardial origin have also recently been shown to play a key role in local regulation of the heart. This brief review highlights the key features of the enkephalin opioids in the heart and the current understanding of their role in development, ageing, cardioprotection, hypertension, hypertrophy, and heart failure.

Neuropeptide FF receptors antagonist, RF9, attenuates opioid-evoked hypothermia in mice

The present study used the endpoint of hypothermia to investigate opioid and neuropeptide FF (NPFF) interactions in conscious animals. Both opioid and NPFF systems played important roles in thermoregulation, which suggested a link between opioid receptors and NPFF receptors in the production of hypothermia. Therefore, we designed a study to investigate the relationship between opioid and NPFF in control of thermoregulation in mice. The selective NPFF receptors antagonist RF9 (30nmol) injected into the third ventricle failed to induce significant effect, but it completely antagonized the hypothermia of NPFF (45 nmol) after cerebral administration in mice. In addition, RF9 (30 nmol) co-injected i.c.v. in the third ventricle reduced the hypothermia induced by morphine (5nmol,) or nociceptin/orphanin FQ (N/OFQ) (2 nmol). Neither the classical opioid receptors antagonist naloxone (10 nmol) nor NOP receptor antagonist [Nphe(1)]NC(1-13)NH(2) (7.5 nmol) reduced the hypothermia induced by the central injection of NPFF at dose of 45 nmol. Co-injected with a low dose of NPFF (5 nmol), the hypothermia of morphine (5 nmol) or N/OFQ (2 nmol) was not modified. These results suggest that NPFF receptors activation is required for opioid to produce hypothermia. In contrast, NPFF-induced hypothermia is mainly mediated by its own receptors, independent of opioid receptors in the mouse brain. This interaction, quantitated in the present study, is the first evidence that NPFF receptors mediate opioid-induced hypothermia in conscious animals.

Peripherally acting mu-opioid receptor agonist attenuates neuropathic pain in rats after L5 spinal nerve injury

Studies in experimental models and controlled patient trials indicate that opioids are effective in managing neuropathic pain. However, side effects secondary to their central nervous system actions present barriers to their clinical use. Therefore, we examined whether activation of the peripheral mu-opioid receptors (MORs) could effectively alleviate neuropathic pain in rats after L5 spinal nerve ligation (SNL). Systemic loperamide hydrochloride (0.3-10 mg/kg, s.c.), a peripherally acting MOR-preferring agonist, dose-dependently reversed the mechanical allodynia at day 7 post-SNL. This anti-allodynic effect produced by systemic loperamide (1.5mg/kg, s.c.) was blocked by systemic pretreatment with either naloxone hydrochloride (10 mg/kg, i.p.) or methyl-naltrexone (5 mg/kg, i.p.), a peripherally acting MOR-preferring antagonist. It was also blocked by ipsilateral intraplantar pretreatment with methyl-naltrexone (43.5 microg/50 microl) and the highly selective MOR antagonist CTAP (5.5 microg/50 microl). However, this anti-allodynic effect of systemic loperamide was not blocked by intraplantar pretreatment with the delta-opioid receptor antagonist naltrindole hydrochloride (45.1 microg/50 microl). The anti-allodynic potency of systemic loperamide varied with time after nerve injury, with similar potency at days 7, 28, and 42 post-SNL, but reduced potency at day 14 post-SNL. Ipsilateral intraplantar injection of loperamide also dose-dependently (10-100 microg/50 microl) reversed mechanical allodynia on day 7 post-SNL. We suggest that loperamide can effectively attenuate neuropathic pain, primarily through activation of peripheral MORs in local tissue. Therefore, peripherally acting MOR agonists may represent a promising therapeutic approach for alleviating neuropathic pain.