Effects of delta-opioid receptor agonist pretreatment on the cardiotoxicity of bupivacaine in rats

Background Delta-opioid receptor is widely expressed in human and rodent hearts, and has been proved to protect cardiomyocytes against ischemia/reperfusion and heart failure. The antagonist of delta-opioid receptor could block the rescue effect of lipid emulsion against local anesthetic cardiotoxicity. However, no evidence is available for the direct effect of delta-opioid-receptor agonists on the cardiotoxicity of local anesthetics. Methods Anesthetized Sprague Dawley rats were divided into five groups. Group NS received 2 ml·kg−1·min−1 normal saline, group LE received 2 ml·kg−1·min−1 30% lipid emulsion and group BW received 0.1, 1.0, or 5.0 mg/kg BW373U86, a delta-opioid-receptor agonist, for 5 min. Then 0.5% bupivacaine was infused intravenously at a rate of 3.0 mg·kg−1·min−1 until asystole. The time of arrhythmia, 50% mean arterial pressure-, 50% heart rate-reduction and asystole were recorded, and the dose of bupivacaine at each time point was calculated. Results All three different doses of BW373U86 did not affect the arrhythmia, 50% mean arterial pressure-reduction, 50% heart rate-reduction and asystole dose of bupivacaine compared with group NS. 30% LE significantly increased the bupivacaine threshold of 50% mean arterial pressure-reduction (17.9 [15.4–20.7] versus 7.2 [5.9–8.7], p = 0.018), 50% heart rate-reduction (18.7 ± 4.2 versus 8.8 ± 1.7, p < 0.001) and asystole (26.5 [21.0–29.1] versus 11.3 [10.7–13.4], p = 0.008) compared with group NS. There was no difference between group LE and group NS in the arrhythmia dose of bupivacaine (9.9 [8.9–11.7] versus 5.6 [4.5–7.0], p = 0.060). Conclusions Our data show that BW373U86 does not affect the cardiotoxicity of bupivacaine compared with NS control in rats. 30% LE pretreatment protects the myocardium against bupivacaine-induced cardiotoxicity.

Investigating the anti-cocaine, anti-nociceptive properties and side effects of MP1104, a novel mixed opioid receptor agonist

<p>Rationale: Drug addiction is a chronic, relapsing disease with great socioeconomic and morbidity costs. There are limited treatments, with no Food and Drug Administration approved pharmacotherapies available for psychostimulant addiction. In addition, the use of prescription opioid medications has reached epidemic proportions in the world. More than 40,000 deaths from prescription opioid overdose was reported in USA alone in the year 2017. There is an urgent need for the development of effective, non-addictive pain medications and addiction treatments. The opioid receptors play an important role in the modulation of pain and addiction. Mu opioid receptor (MOPr) agonists are widely used to treat pain, however, can also induce respiratory depression, tolerance and addiction. In contrast, drugs activating the kappa opioid receptor (KOPr) attenuate the rewarding properties of drugs, hence are promising non-addictive analgesics. However, side effects like aversion, sedation, anxiety and depression limit their clinical utility. Delta opioid receptor (DOPr) agonists have rewarding, anti-nociceptive and anti-depressive properties, but can also cause seizures. We hypothesise that development of mixed opioid receptor ligands may have therapeutic properties with reduced side effects. Therefore, this thesis evaluated MP1104, a potent mixed opioid receptor agonist, with full efficacy at all three receptors and 3- and 13-fold higher binding affinity for KOPr compared to MOPr and DOPr, respectively. MP1104 was evaluated for the ability to modulate cocaine-induced behaviours, the anti-nociceptive effects and side effects. Methods: Male Sprague-Dawley rats were used to investigate the effects of acute MP1104 treatment on cocaine self-administration and drug seeking behaviour. To determine the mechanism, the modulatory effect of MP1104 on dopamine transporter (DAT) function was assessed using rotating disk electrode voltammetry to measure dopamine uptake in rat dorsal striatum (dStr) and nucleus accumbens (NAc) tissue. Evaluation of side effects included sedation (spontaneous locomotor activity), anxiety (elevated plus maze (EPM)), aversion (conditioned place aversion (CPA)) and depression (forced swim tests (FST)) in rats. The anti-nociceptive effects were measured in the warm-water tail withdrawal assay in rats and male C57BL/6 mice. Acute and chronic administration of MP1104 were evaluated in the paclitaxel-induced neuropathic pain model in mice. Results: In rats trained to self-administer cocaine, acute MP1104 (0.3 and 1 mg/kg, i.p.) administration reduced cocaine-primed reinstatement of drug seeking behaviour and caused a significant downward shift in the cocaine dose-response curve. The anti-cocaine effects exerted by MP1104 are in part due to increased dopamine uptake by DAT in the NAc, which was KOPr-mediated. In the warm-water tail withdrawal assay in rats, acute administration of MP1104 (0.3 and 0.6 mg/kg, i.p.) was 4 times longer acting (8 h) than morphine (2 h). These effects were both KOPr and DOPr dependent. In the dose-response tail withdrawal assay, MP1104 was found to be potent in both rats (ED₅₀ = 0.58 mg/kg, s.c.) and mice (ED₅₀ = 0.35 mg/kg, s.c.). In the paclitaxel-induced neuropathic pain model, mice treated with MP1104 showed potent reductions in both mechanical (ED₅₀ = 0.449 mg/kg, s.c.) and cold (ED₅₀ = 0.479 mg/kg, s.c.) allodynia compared to morphine. Following chronic daily administration of the ED₈₀ dose, MP1104 (1.2 mg/kg, i.p.) was more potent than morphine in reducing mechanical and cold allodynia. Surprisingly, MP1104 reversed responding back to baseline (non-disease) levels. The most remarkable finding was that MP1104, unlike morphine did not produce tolerance when administered chronically. When the side effects of MP1104 were evaluated in rats, no significant anxiogenic effects were seen in the EPM, nor pro-depressive effects in the FST, nor aversion in CPA tests in rats. Furthermore, pre-treatment with a DOPr antagonist, led to MP1104 producing aversive effects. This data suggests that the DOPr agonist actions of MP1104 attenuate the KOPr-mediated aversive effects of MP1104. However, at higher doses, MP1104 (1 mg/kg, i.p.) was found to be sedative. Conclusions: MP1104 exerts potent anti-cocaine properties in self-administration tests. The reduced cocaine reward is at least in part due to the ability of MP1104 to modulate DAT function by increasing dopamine uptake in the NAc. MP1104 is also a potent and long-lasting anti-nociceptive agent in rats. Significantly, when evaluated in a chronic neuropathic pain model, MP1104 was potent with no tolerance to the anti-nociceptive effects observed. Moreover, MP1104 showed fewer side effects with reduced sedative effects and no observed anxiety, aversive, nor pro-depressive effects, unlike pure KOPr agonists. This data supports the therapeutic development of mixed opioid receptor agonists, particularly mixed KOPr/DOPr agonists as non-addictive pain medications and anti-cocaine pharmacotherapies with fewer side effects.</p>

Investigating the anti-cocaine, anti-nociceptive properties and side effects of MP1104, a novel mixed opioid receptor agonist

<p>Rationale: Drug addiction is a chronic, relapsing disease with great socioeconomic and morbidity costs. There are limited treatments, with no Food and Drug Administration approved pharmacotherapies available for psychostimulant addiction. In addition, the use of prescription opioid medications has reached epidemic proportions in the world. More than 40,000 deaths from prescription opioid overdose was reported in USA alone in the year 2017. There is an urgent need for the development of effective, non-addictive pain medications and addiction treatments. The opioid receptors play an important role in the modulation of pain and addiction. Mu opioid receptor (MOPr) agonists are widely used to treat pain, however, can also induce respiratory depression, tolerance and addiction. In contrast, drugs activating the kappa opioid receptor (KOPr) attenuate the rewarding properties of drugs, hence are promising non-addictive analgesics. However, side effects like aversion, sedation, anxiety and depression limit their clinical utility. Delta opioid receptor (DOPr) agonists have rewarding, anti-nociceptive and anti-depressive properties, but can also cause seizures. We hypothesise that development of mixed opioid receptor ligands may have therapeutic properties with reduced side effects. Therefore, this thesis evaluated MP1104, a potent mixed opioid receptor agonist, with full efficacy at all three receptors and 3- and 13-fold higher binding affinity for KOPr compared to MOPr and DOPr, respectively. MP1104 was evaluated for the ability to modulate cocaine-induced behaviours, the anti-nociceptive effects and side effects. Methods: Male Sprague-Dawley rats were used to investigate the effects of acute MP1104 treatment on cocaine self-administration and drug seeking behaviour. To determine the mechanism, the modulatory effect of MP1104 on dopamine transporter (DAT) function was assessed using rotating disk electrode voltammetry to measure dopamine uptake in rat dorsal striatum (dStr) and nucleus accumbens (NAc) tissue. Evaluation of side effects included sedation (spontaneous locomotor activity), anxiety (elevated plus maze (EPM)), aversion (conditioned place aversion (CPA)) and depression (forced swim tests (FST)) in rats. The anti-nociceptive effects were measured in the warm-water tail withdrawal assay in rats and male C57BL/6 mice. Acute and chronic administration of MP1104 were evaluated in the paclitaxel-induced neuropathic pain model in mice. Results: In rats trained to self-administer cocaine, acute MP1104 (0.3 and 1 mg/kg, i.p.) administration reduced cocaine-primed reinstatement of drug seeking behaviour and caused a significant downward shift in the cocaine dose-response curve. The anti-cocaine effects exerted by MP1104 are in part due to increased dopamine uptake by DAT in the NAc, which was KOPr-mediated. In the warm-water tail withdrawal assay in rats, acute administration of MP1104 (0.3 and 0.6 mg/kg, i.p.) was 4 times longer acting (8 h) than morphine (2 h). These effects were both KOPr and DOPr dependent. In the dose-response tail withdrawal assay, MP1104 was found to be potent in both rats (ED₅₀ = 0.58 mg/kg, s.c.) and mice (ED₅₀ = 0.35 mg/kg, s.c.). In the paclitaxel-induced neuropathic pain model, mice treated with MP1104 showed potent reductions in both mechanical (ED₅₀ = 0.449 mg/kg, s.c.) and cold (ED₅₀ = 0.479 mg/kg, s.c.) allodynia compared to morphine. Following chronic daily administration of the ED₈₀ dose, MP1104 (1.2 mg/kg, i.p.) was more potent than morphine in reducing mechanical and cold allodynia. Surprisingly, MP1104 reversed responding back to baseline (non-disease) levels. The most remarkable finding was that MP1104, unlike morphine did not produce tolerance when administered chronically. When the side effects of MP1104 were evaluated in rats, no significant anxiogenic effects were seen in the EPM, nor pro-depressive effects in the FST, nor aversion in CPA tests in rats. Furthermore, pre-treatment with a DOPr antagonist, led to MP1104 producing aversive effects. This data suggests that the DOPr agonist actions of MP1104 attenuate the KOPr-mediated aversive effects of MP1104. However, at higher doses, MP1104 (1 mg/kg, i.p.) was found to be sedative. Conclusions: MP1104 exerts potent anti-cocaine properties in self-administration tests. The reduced cocaine reward is at least in part due to the ability of MP1104 to modulate DAT function by increasing dopamine uptake in the NAc. MP1104 is also a potent and long-lasting anti-nociceptive agent in rats. Significantly, when evaluated in a chronic neuropathic pain model, MP1104 was potent with no tolerance to the anti-nociceptive effects observed. Moreover, MP1104 showed fewer side effects with reduced sedative effects and no observed anxiety, aversive, nor pro-depressive effects, unlike pure KOPr agonists. This data supports the therapeutic development of mixed opioid receptor agonists, particularly mixed KOPr/DOPr agonists as non-addictive pain medications and anti-cocaine pharmacotherapies with fewer side effects.</p>

Kappa opioids inhibit the GABA/glycine terminals of rostral ventromedial medulla projections in the superficial dorsal horn of the spinal cord

Background and Purpose: Descending projections from neurons in the rostral ventromedial medulla (RVM) make synapses within the superficial dorsal horn of the spinal cord that are involved in acute nociception and the development of chronic pain and itch. In addition, this projection plays an important role in mediating the analgesic effects of opioids. However, our knowledge about the spinal synaptic targets of RVM projections and their modulation by opioids is unknown. Experimental Approach: We used ex vivo optogenetic stimulation of RVM descending fibres and whole-cell patch-clamp recordings from superficial dorsal horn (SDH) neurons to identify the target neurons and to investigate their descending synaptic inputs. Key Results: We demonstrate that SDH neurons are targeted by descending GABA/glycine inhibitory inputs from the RVM, although glycinergic inputs predominate. These SDH neurons had diverse morphological and electrical properties. This inhibitory synapse was presynaptically suppressed by the kappa opioid receptor agonist U69593. By contrast, the mu-opioid receptor agonist DAMGO inhibited only a subset of RVM-SDH synapses, acting both pre- and postsynaptically, while the delta-opioid receptor agonist deltorphin II had little effect. Conclusion and Implications: Developing reliable and effective alternatives to opioid analgesics requires a detailed, mechanistic understanding of how opioids interact with nociceptive circuits. This study selectively and systematically characterises the synaptic connections between RVM projection neurons and their SDH targets to advance our knowledge of how this descending projection is organised and modulated. In addition, it improves our understanding of how opioids alter spinal pathways involved in the sensations of pain and itch.

Nalfurafine, a G-Protein–Biased KOR (Kappa Opioid Receptor) Agonist, Enhances the Diuretic Response and Limits Electrolyte Losses to Standard-of-Care Diuretics

Nalfurafine is a G-protein–biased KOR (kappa opioid receptor) agonist that produces analgesia and lacks CNS adverse effects. Here, we examined the cardiovascular and renal responses to intravenous and oral nalfurafine alone and in combination with furosemide, hydrochlorothiazide, or amiloride. We hypothesized that nalfurafine, given its distinct mechanism of vasopressin inhibition, would increase urine output to these diuretics and limit electrolyte loss. Following catheterization, conscious Sprague-Dawley rats received an isotonic saline infusion and were then administered an intravenous bolus of nalfurafine, a diuretic, or a combination. Mean arterial pressure, heart rate, and urine output were recorded for 90 minutes. In another study, rats were placed in metabolic cages and administered drug in an oral volume load. Hourly urine samples were then collected for 5 hours. Intravenous and oral nalfurafine produced a marked diuresis, antinatriuresis, antikaliuresis, and a decrease in mean arterial pressure. Compared with diuretic treatment alone, intravenous coadministration with nalfurafine significantly increased urine output to furosemide and hydrochlorothiazide and decreased sodium and potassium excretion. Notably, mean arterial pressure was reduced with nalfurafine/diuretic combination therapy compared to diuretics alone. Similarly, oral coadministration of nalfurafine significantly increased urine output to hydrochlorothiazide and decreased sodium and potassium excretion, whereas combination with furosemide only limited the amount of sodium excreted. Further, both intravenous and oral coadministration of nalfurafine enhanced the diuresis to amiloride and decreased sodium excretion. Together, these findings demonstrate that nalfurafine enhances the diuresis to standard-of-care diuretics without causing an excessive loss of electrolytes, offering a new approach to treat several cardiovascular conditions.

Identification of a Novel Delta Opioid Receptor Agonist Chemotype with Potential Negative Allosteric Modulator Capabilities

The δ-opioid receptor (δOR) holds great potential as a therapeutic target. Yet, clinical drug development, which has focused on δOR agonists that mimic the potent and selective tool compound SNC80 have largely failed. It has increasingly become apparent that the SNC80 scaffold carries with it potent and efficacious β-arrestin recruitment. Here, we screened a relatively small (5120 molecules) physical drug library to identify δOR agonists that underrecruit β-arrestin, as it has been suggested that compounds that efficaciously recruit β-arrestin are proconvulsant. The screen identified a hit compound and further characterization using cellular binding and signaling assays revealed that this molecule (R995045, compound 1) exhibited ten-fold selectivity over µ- and κ-opioid receptors. Compound 1 represents a novel chemotype at the δOR. A subsequent characterization of fourteen analogs of compound 1, however did not identify a more potent δOR agonist. Computational modeling and in vitro characterization of compound 1 in the presence of the endogenous agonist leu-enkephalin suggest compound 1 may also bind allosterically and negatively modulate the potency of Leu-enkephalin to inhibit cAMP, acting as a ‘NAM-agonist’ in this assay. The potential physiological utility of such a class of compounds will need to be assessed in future in vivo assays.

Antinociceptive and Antipruritic Effects of HSK21542, a Peripherally-Restricted Kappa Opioid Receptor Agonist, in Animal Models of Pain and Itch

Kappa opioid receptor (KOR) agonists have been promising therapeutic candidates, owing to their potential for relieving pain and treating intractable pruritus. Although lacking morphine-like central nervous system (CNS) effects, KOR agonists do elicit sedation, dysphoria and diuresis which seriously impede their development. Peripherally-restricted KOR agonists have a poor ability to penetrate into the CNS system, so that CNS-related adverse effects can be ameliorated or even abolished. However, the only approved peripherally-restricted KOR agonist CR845 remains some frequent CNS adverse events. In the present study, we aim to address pharmacological profiles of HSK21542, with an expectation to provide a safe and effective alternative for patients who are suffering from pain and pruritus. The in vitro experimental results showed that HSK21542 was a selective and potent KOR agonist with higher potency than CR845, and had a brain/plasma concentration ratio of 0.001, indicating its peripheral selectivity. In animal models of pain, HSK21542 significantly inhibited acetic acid-, hindpaw incision- or chronic constriction injury-induced pain-related behaviors, and the efficacy was comparable to CR845 at 15 min post-dosing. HSK21542 had a long-lasting analgesic potency with a median effective dose of 1.48 mg/kg at 24 h post-drug in writhing test. Meanwhile, the antinociceptive activity of HSK21542 was effectively reversed by a KOR antagonist nor-binaltorphimine. In addition, HSK21542 had powerful antipruritic activities in compound 48/80-induced itch model. On the other hand, HSK21542 had a weak ability to produce central antinociceptive effects in a hot-plate test and fewer effects on the locomotor activity of mice. HSK21542 didn’t affect the respiratory rate of mice. Therefore, HSK21542 might be a safe and effective KOR agonist and promising candidate for treating pain and pruritus.

Microdose induction of buprenorphine-naloxone in a patient using high dose methadone: A case report

Background Buprenorphine is a partial mu-opioid receptor agonist approved for the treatment of opioid dependence. The risk of withdrawal symptoms and wait time required to safely initiate buprenorphine provides challenges to both patients and providers. Microdose induction is proposed as a possible solution to ease the transition to buprenorphine; however, little data has been published to date on patients stabilized on methadone doses greater than 100 mg. Case Report A 29-year-old patient stabilized on methadone 105 mg was successfully transitioned to sublingual buprenorphine-naloxone using a 7-day microdose protocol on an inpatient psychiatric service. During the transition, the patient reported only minimal symptoms. Conclusion This report adds to the growing literature supporting the use of a microdose induction to initiate buprenorphine-naloxone. Additionally, this approach may be significant for patients stabilized on high doses of methadone who may not be able to tolerate a traditional buprenorphine induction.