Interactions between kappa and mu opioid receptor agonists: effects of the ratio of drugs in mixtures

Behavioral Effects of Vaporized Delta-8 Tetrahydrocannabinol, Cannabidiol, and Mixtures in Male Rats

Background: The popularity of delta-8 tetrahydrocannabinol (THC) and cannabidiol (CBD) products has seen a sharp increase in use during recent years. Despite the rise in use of these minor cannabinoids, there are little to no pre-clinical behavioral data on their effects, with most pre-clinical cannabis research focusing on the behavioral effects of delta-9 THC. The current experiments aimed to characterize the behavioral effects of delta-8 THC, CBD, and mixtures of these two drugs using a whole-body vapor exposure route of administration in male rats. Methods: Rats were exposed to vapor that contained different concentrations of delta-8 THC, CBD, or CBD/delta-8 THC mixtures during 10 min of exposure. Following 10 min of vapor exposure, locomotor behavior was monitored, or the warm-water tail withdrawal assay was conducted to measure the acute analgesic effects of the vapor exposure. Results: CBD and CBD/delta-8 THC mixtures resulted in a significant increase in locomotion across the entire session. Although delta-8 THC alone had no significant effect on locomotion across the session, the 10 mg concentration of delta-8 THC had a hyperlocomotion effect in the first 30 min of the session followed by a hypolocomotor effect later in the session. In the tail withdrawal assay, a 3/1 mixture of CBD/delta-8 THC resulted in an immediate analgesic effect compared to vehicle vapor. Finally, immediately following vapor exposure, all drugs had a hypothermic effect on body temperature compared to vehicle. Conclusion: This experiment is the first to characterize the behavioral effects of vaporized delta-8 THC, CBD, and CBD/delta-8 THC in male rats. While data were generally congruent with previous research investigating delta-9 THC, future studies should explore abuse liability and validate plasma blood concentrations of these drugs following administration through whole-body vapor exposure.

Antinociceptive and adverse effects of morphine:ketamine mixtures in rats

Prescription opioids are the gold standard for treating moderate to severe pain despite their well-documented adverse effects. Of all prescription medications, opioids are abused most widely, and fatal overdoses have reached epidemic levels. One strategy for improving the margin of safety of opioids is combining them with non-opioid drugs to decrease the opioid dose needed for pain relief, thereby reducing adverse effects that occur with larger doses. The N-methyl-D-aspartate receptor antagonist ketamine has been used safely as an analgesic but only under a very limited range of conditions. The current studies characterized the antinociceptive, behavioral suppressant, and gastrointestinal effects of morphine and ketamine alone and in mixtures to determine their interaction in 24 adult male Sprague-Dawley rats (n = 8 per assay). Given alone, both morphine and ketamine produced antinociception, decreased responding for food, and reduced gastrointestinal transit (i.e. produced constipation). The effects of morphine:ketamine mixtures generally were additive, except for the antinociceptive effects of 1:1 mixtures for which the difference in slope (i.e. non-parallel shift) between the observed and predicted effects suggested synergy at smaller doses and additivity at larger doses. The potency of morphine to produce constipation was not enhanced by administration of morphine:ketamine mixtures with antinociceptive effects. The nature of the interaction between morphine and ketamine for adverse effects such as dependence, withdrawal, abuse, or respiratory depression remains unknown but also might be related to the ratio of each drug in mixtures. It will be important to identify conditions that produce the largest potential therapeutic window in humans.

Self-Administration of Fentanyl-Alprazolam Combinations by Rhesus Monkeys Responding under a Progressive-Ratio Schedule

This study evaluated the reinforcing effects of fentanyl, alone or in combination with the benzodiazepine alprazolam, in rhesus monkeys (3 females, 3 males). Subjects were trained to self-administer the opioid remifentanil (0.3 µg/kg/injection) under a progressive-ratio schedule of reinforcement. The reinforcing effects of fentanyl (0.1-10 µg/kg/injection) or alprazolam (1.0-100 µg/kg/injection) alone, or in combinations of fixed proportions (1:1, 1:3, and 3:1 fentanyl:alprazolam, with 1:1 based on the potencies of drugs alone) were evaluated in single-day test sessions (with double determinations). Dose-equivalence analysis was used to determine the extent to which fentanyl and alprazolam combinations differed from additivity. Fentanyl functioned as a positive reinforcer in all monkeys, while alprazolam was a reinforcer in 3 of 6 monkeys only. Therefore, drug combination data were grouped as "alprazolam-taking" and "non-alprazolam-taking" monkeys. For alprazolam-taking monkeys, we observed additive effects for the 3:1 and 1:3 combinations, and a significant supra-additive interaction for the 1:1 combination of fentanyl and alprazolam. For 2 of the 3 non-alprazolam-taking monkeys, the combination of fentanyl and alprazolam resulted in enhanced reinforcing effects relative to either drug alone. However, the one monkey showed primarily inhibitory, or suppressive effects, with the 3:1 dose combination resulting in a relatively modest rightward shift in the fentanyl dose-response function. In summary, our findings show that combining fentanyl and alprazolam generally result in proportion-dependent additive or supra-additive enhancements. These data raise the possibility that the prevalence of opioid-benzodiazepine polydrug abuse may reflect a unique enhancement of these drugs' reinforcing effects, although individual differences may exist. SIGNIFICANCE STATEMENT: Addressing the critical question of the degree to which benzodiazepines can modulate the abuse-related effects of opioids may provide improved pathways to treatment of this common form of polydrug addiction. In the present study, we show that combinations of the opioid fentanyl and the benzodiazepine alprazolam can be more reinforcing than either drug alone in a rhesus monkey model, suggesting that enhancement of reinforcement processes may underlie this prevalent form of polydrug use disorder.

The Potential of Methocinnamox as a Future Treatment for Opioid Use Disorder: A Narrative Review

The opioid epidemic is an ongoing public health crisis, and the United States health system is overwhelmed with increasing numbers of opioid-related overdoses. Methocinnamox (MCAM) is a novel mu opioid receptor antagonist with an extended duration of action. MCAM has potential to reduce the burden of the opioid epidemic by being used as an overdose rescue treatment and a long-term treatment for opioid use disorder (OUD). The currently available treatments for OUD include naloxone, naltrexone, and methadone. These treatments have certain limitations, which include short duration of action, patient non-compliance, and diversion. A narrative review was conducted using PubMed and Google Scholar databases covering the history of the opioid epidemic, pain receptors, current OUD treatments and the novel drug MCAM. MCAM could potentially be used as both a rescue and long-term treatment for opioid misuse. This is due to its pseudo-irreversible antagonism of the mu opioid receptor, abnormally long duration of action of nearly two weeks, and the possibility of using kappa or delta opioid receptor agonists for pain management during OUD treatment. MCAM’s novel pharmacokinetic and pharmacodynamic properties open a new avenue for treating opioid misuse.

The kappa-opioid receptor agonist, triazole 1.1, reduces oxycodone self-administration and enhances oxycodone-induced thermal antinociception in male rats

RATIONALE

Triazole 1.1 is a novel kappa-opioid receptor (KOR) agonist reported to produce antinociception without KOR-typical adverse effects. When combined with the mu-opioid receptor (MOR) agonist, oxycodone, triazole 1.1 blocks oxycodone-induced pruritis without producing sedation-like effects in nonhuman primates. However, it is unknown if triazole 1.1 can reduce the abuse-related effects or enhance the antinociceptive effects of oxycodone similarly to other KOR agonists.

OBJECTIVES

The aim of the present study was to quantitatively compare the behavioral effects of triazole 1.1 to the KOR agonists, U50,488h and nalfurafine, on oxycodone self-administration and oxycodone-induced thermal antinociception when administered as mixtures with oxycodone.

METHODS

In the self-administration study, male Sprague-Dawley (SD) rats (n = 6) self-administered intravenous (i.v.) oxycodone alone (0.056 mg/kg/inj) or combined with U50,488 h (0.032-0.32 mg/kg/inj), nalfurafine (0.00032-0.0032 mg/kg/inj), or triazole 1.1 (0.32-1.8 mg/kg/inj) under a progressive-ratio schedule of reinforcement. In a hot plate assay, male SD rats (n = 6) received i.v. injections of oxycodone (1.0-5.6 mg/kg), U50,488h (1.0-18.0 mg/kg), nalfurafine (0.01-1.0 mg/kg), or triazole 1.1 (3.2-32.0 mg/kg) alone or in combinations of fixed proportion with oxycodone based on the relative potencies of the single drugs. Each study concluded with administration of the KOR antagonist nor-BNI and some degree of retesting of the previous conditions to verify that the behavioral effects were mediated by KOR activation.

RESULTS

All KOR agonists reduced oxycodone self-administration in a dose-dependent manner. Moreover, all single drugs and drug combinations produced dose-dependent, fully efficacious thermal antinociception. All KOR agonist:oxycodone combinations produced either additive or super-additive thermal antinociception. Finally, each KOR agonist was blocked in effect by nor-BNI in both behavioral measures.

CONCLUSION

This study demonstrates that triazole 1.1 reduces oxycodone's reinforcing effects and enhances oxycodone-induced antinociception to degrees that are comparable to typical KOR agonists. Given triazole 1.1's mild adverse-effect profile, developing MOR-KOR agonist combinations from the triazole 1.1 series may render new pain therapeutics with reduced abuse liability.

Distribution, excretion and metabolic pathways of a single parenteral administration of kappa-opioid receptor agonist RU-1205

Introduction: The purpose was to study the pharmacokinetic properties of RU-1205 with the previously identified kappa-agonistic and analgesic effects after parenteral administration.

Materials and methods: Pharmacokinetic parameters of RU-1205 after intravenous and subcutaneous administration at doses of 10 mg/kg and 50 mg/kg, respectively, were investigated, using the method of high-performance liquid chromatography with measurement of the compound according to a pre-established calibration curve. The indices of the area under the pharmacokinetic curve, clearance, half-life, residence time of the drug molecule in the body, total (apparent) volume of distribution, as well as the indicator of absolute bioavailability for subcutaneous administration were calculated. Tissue distribution and excretion of RU-1205 were also studied. Evaluation of metabolism of RU-1205 was conducted in silico, using the PALLAS 3.00 software, with the use of specific tests with CYP 450 substrates and by studying the ability of RU-1205 to form conjugates with endogenous acids.

Results and discussion: It was found that after a single intravenous administration, the investigated substance was determined in the blood for 12 h; the half-life was 8.49 hours. The absolute bioavailability after subcutaneous administration is 57.35%. RU-1205 is eliminated within 3–4 days. The main route of excretion is extrarenal. The biotransformation of the substance probably proceeds mainly with the formation of oxidized forms of the initial molecule according to the reactions of the first phase of metabolic transformation, so the chance to observe phase 2 of the metabolism could be very low.

Conclusion: The test substance undergoes a long process of elimination, has the highest tropism to the elimination organs and undergoes active biotransformation processes in the body of animals.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Quantification of observable behaviors induced by typical and atypical kappa-opioid receptor agonists in male rhesus monkeys

RATIONALE

Kappa-opioid receptor (KOR) agonists are antinociceptive but have side effects that limit their therapeutic utility. New KOR agonists have been developed that are fully efficacious at the KOR but may produce fewer or reduced side effects that are typical of KOR agonists.

OBJECTIVES

We determined behavioral profiles for typical and atypical KOR agonists purported to differ in intracellular-signaling profiles as well as a mu-opioid receptor (MOR) agonist, oxycodone, using a behavioral scoring system based on Novak et al. (Am J Primatol 28:124-138, 1992, Am J Primatol 46:213-227, 1998) and modified to quantify drug-induced effects (e.g., Duke et al. J Pharmacol Exp Ther 366:145-157, 2018).

METHODS

Six adult male rhesus monkeys were administered a range of doses of the typical KOR agonists, U50-488H (0.0032-0.1 mg/kg) and salvinorin A (0.00032-0.01 mg/kg); the atypical KOR agonists, nalfurafine (0.0001-0.001 mg/kg) and triazole 1.1 (0.01-0.32 mg/kg); the MOR agonist, oxycodone (0.0032-0.32 mg/kg); and as controls, cocaine (0.032-0.32 mg/kg) and ketamine (0.32-10 mg/kg). For time-course determinations, the largest dose of each KOR agonist or MOR agonist was administered across timepoints (10-320 min). In mixture conditions, oxycodone (0.1 mg/kg) was followed by KOR-agonist administration.

RESULTS

Typical KOR agonists produced sedative-like and motor-impairing effects. Nalfurafine was similar to typical KOR agonists on most outcomes, and triazole 1.1 produced no effects on its own except for reducing scratch during time-course determinations. In the mixture, all KOR agonists reduced oxycodone-induced scratching, U50-488H and nalfurafine reduced species-typical activity, and U50-488H increased rest/sleep posture.

CONCLUSIONS

Atypical "biased" KOR agonists produce side-effect profiles that are relatively benign (triazole 1.1) or reduced (nalfurafine) compared to typical KOR agonists.

Effects of opioid/cannabinoid mixtures on impulsivity and memory in rhesus monkeys

The opioid epidemic underscores the need for safer and more effective treatments for pain. Combining opioid receptor agonists with drugs that relieve pain through nonopioid mechanisms could be a useful strategy for reducing the dose of opioid needed to treat pain, thereby reducing risks associated with opioids alone. Opioid/cannabinoid mixtures might be useful in this context; individually, opioids and cannabinoids have modest effects on cognition, and it is important to determine whether those effects occur with mixtures. Delay discounting and delayed matching-to-sample tasks were used to examine effects of the mu-opioid receptor agonist morphine (0.32-5.6 mg/kg), the cannabinoid CB1/CB2 receptor agonist CP55940 (0.0032-0.1 mg/kg), and morphine/CP55940 mixtures on impulsivity (n = 3) and memory (n = 4) in rhesus monkeys. Alone, each drug decreased rate of responding without modifying choice in the delay-discounting task, and morphine/CP55940 mixtures reduced choice of one pellet in a delay dependent manner, with monkeys instead choosing delayed delivery of the larger number of pellets. With the exception of one dose in one monkey, accuracy in the delayed matching-to-sample task was not altered by either drug alone. Morphine/CP55940 mixtures decreased accuracy in two monkeys, but the doses in the mixture were equal to or greater than doses that decreased accuracy or response rate with either drug alone. Rate-decreasing effects of morphine/CP55940 mixtures were additive. These data support the notion that opioid/cannabinoid mixtures that might be effective for treating pain do not have greater, and might have less, adverse effects compared with larger doses of each drug alone.

Psychotropic Drugs for the Management of Chronic Pain and Itch

Clinical observations have shown that patients with chronic neuropathic pain or itch exhibit symptoms of increased anxiety, depression and cognitive impairment. Such patients need corrective therapy with antidepressants, antipsychotics or anticonvulsants. It is known that some psychotropic drugs are also effective for the treatment of neuropathic pain and pruritus syndromes due to interaction with the secondary molecular targets. Our own clinical studies have identified antipruritic and/or analgesic efficacy of the following compounds: tianeptine (atypical tricyclic antidepressant), citalopram (selective serotonin reuptake inhibitor), mianserin (tetracyclic antidepressant), carbamazepine (anticonvulsant), trazodone (serotonin antagonist and reuptake inhibitor), and chlorprothixene (antipsychotic). Venlafaxine (serotonin-norepinephrine reuptake inhibitor) is known to have an analgesic effect too. The mechanism of such effect of these drugs is not fully understood. Herein we review and correlate the literature data on analgesic/antipruritic activity with pharmacological profile of these compounds.