Discriminative Stimulus Effects Research Articles

In vitro and in vivo pharmacology of nine novel synthetic cannabinoid receptor agonists

Synthetic cannabinoid receptor agonists (SCRAs) are novel psychoactive substances that bind to and activate CB1 receptors in the brain. The structural manipulations observed in newer SCRAs suggest that manufacturers have incorporated modern drug development techniques into their repertoire, often producing higher CB1 receptor affinity than Δ9-tetrahydrocannabinol (Δ9-THC). This study examined nine SCRAs recently detected by forensic surveillance, some of which caused fatalities: 5F-MDMB-PICA, FUB-144, 5F-MMB-PICA, MMB-4en-PICA, MMB-FUBICA, 5F-EDMB-PINACA, APP-BINACA, MDMB-4en-PINACA, and FUB-AKB48. Compounds were evaluated for CB1 and CB2 receptor binding affinity and functional activation and for their effects on body temperature, time course, and pharmacological equivalence with Δ9-THC in Δ9-THC drug discrimination in mice. All SCRAs bound to and activated CB1 and CB2 receptors with high affinity, with similar or greater affinity for CB2 than CB1 receptors and stimulated [35S]GTPγS binding in CB1 and CB2 expressing cell membranes. All compounds produced hypothermia, with shorter latency to peak effects for SCRAs than Δ9-THC. All SCRAs fully substituted for Δ9-THC in drug discrimination at one or more doses. Rank order potency in producing in vivo effects mostly aligned with rank order CB1 receptor affinities. Potencies for Δ9-THC-like discriminative stimulus effects were similar across sex except Δ9-THC was more potent in females and 5F-MMB-PICA was more potent in males. In summary, 5F-EMDB-PINACA, 5F-MDMB-PICA, MDMB-4en-PINACA, FUB-144, FUB-AKB48, 5F-MMB-PICA, MMB-4en-PICA, and MMB-FUBICA are potent and efficacious SCRAs with pharmacology like that of past SCRAs that have been abused in humans. In contrast, APP-BINACA was efficacious, but had lower potency than most past SCRAs.

Assessment of dependence potential and abuse liability of Δ8-tetrahydrocannabinol in mice

Delta-8-tetrahydrocannabinol (Δ8-THC) is a psychotropic cannabinoid produced in low quantities in the cannabis plant. Refinements in production techniques, paired with the availability of inexpensive cannabidiol substrate, have resulted in Δ8-THC being widely marketed as a quasi-legal, purportedly milder alternative to Δ9-THC. Yet, little research has probed the behavioral and physiological effects of repeated Δ8-THC use. The present study aimed to evaluate the effects of acute and repeated exposure to Δ8-THC. We hypothesized that Δ8-THC produces effects similar to Δ9-THC, including signs of drug tolerance and dependence. Adult male and female C57BL/6J mice were treated acutely with Δ8-THC (6.25–100 mg/kg, i.p.) or vehicle and tested in the tetrad battery to quantify cannabimimetic effects (i.e., catalepsy, antinociception, hypothermia, immobility) as compared with a non-selective synthetic cannabinoid (WIN 55,212–2) and Δ9-THC. As previously reported, Δ8-THC (≥12.5 mg/kg) induced cannabimimetic effects. Pretreatment with the CB1 receptor-selective antagonist rimonabant (3 mg/kg, i.p.) blocked each of these effects. In addition, repeated administration of Δ8-THC (50 mg/kg, s.c.) produced tolerance, as well as cross-tolerance to WIN 55,212–2 (10 mg/kg, s.c.) in tetrad, consistent with downregulated CB1 receptor function. Behavioral signs of physical dependence in the somatic signs, tail suspension, and marble burying assays were also observed following rimonabant-precipitated withdrawal from Δ8-THC (≥10 mg/kg BID for 6 days). Lastly, Δ8-THC produced Δ9-THC-like discriminative stimulus effects in both male and female mice. Together, these findings demonstrate that Δ8-THC produces qualitatively similar effects to Δ9-THC, including risk of drug dependence and abuse liability.

The discriminative stimulus effects of baclofen and gamma hydroxybutyrate in C57BL/6J mice.

Baclofen and γ-hydroxybutyrate (GHB) exert γ-aminobutyric acid (GABA)B receptor agonism and have therapeutic utility but possess different pharmacological activities. We examined whether separate groups of mice could be trained to discriminate either baclofen or GHB, and the contribution of GABAB receptors to discriminative stimulus effects. Male C57BL/6J mice were trained to discriminate either baclofen (3.2 mg/kg, intraperitoneal) or GHB (178 mg/kg, intraperitoneal) from saline under a fixed-ratio 10 schedule. The GABAB antagonist 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348) was used to pharmacologically assess GABAB receptor involvement. The selectivity of the resulting discriminations was assessed with the opioid agonist morphine and the benzodiazepine midazolam. In baclofen-trained mice, both baclofen and GHB were readily discriminated. Baclofen produced a maximum of 86% baclofen-appropriate responding. CGP 35348 (320 mg/kg, i.p.) produced a 4.7-fold rightward shift in the dose-effect function. GHB produced a maximum of 85.8% baclofen-appropriate responding. In GHB-trained mice, both GHB and baclofen were readily discriminated. In GHB-trained mice, GHB produced a maximum of 85.3% drug-appropriate responding; CGP 35348 (320 mg/kg, i.p.) produced a 1.8-fold rightward shift in the GHB discrimination dose-effect function. Baclofen produced up to 70.0% GHB-appropriate responding. CGP 35348 (320 mg/kg, i.p.) significantly antagonized baclofen discrimination and baclofen produced up to 37% GHB-appropriate responding up to doses that disrupted operant responding. Morphine did not produce substitution for either baclofen or GHB. Midazolam produced partial substitution for both. GHB and baclofen discrimination assays in mice provide a useful approach for examining different receptor types mediating the effects of these two drugs.

Modulation of morphine physical dependence and discriminative stimulus effects by ovarian hormones: Role of estradiol

Ovarian hormones influence the activity of endogenous opioids, and exogenous administration of estradiol reduces opioid intake and opioid seeking in animal models of opioid reward and reinforcement. The purpose of this study was to examine the effects of ovarian hormones on the discriminative stimulus effects of morphine and naloxone-precipitated opioid withdrawal. To this end, separate groups of ovariectomized female rats were trained to discriminate the stimulus effects of either 3.0 or 10 mg/kg morphine, and substitution tests were conducted with estradiol or progesterone alone and in combination with morphine. At the conclusion of discrimination testing, rats were treated chronically with estradiol, progesterone, or their combination, and challenged with naloxone to measure opioid-like withdrawal symptoms. Finally, the effects of estradiol, progesterone, and their combination were examined on naloxone-precipitated withdrawal in morphine-dependent rats. Neither estradiol nor progesterone substituted for the morphine discriminative stimulus, but estradiol significantly increased the potency of morphine in rats trained to discriminate 10 mg/kg but not 3 mg/kg morphine. When administered chronically, neither hormone nor their combination produced an opioid-like withdrawal syndrome following a naloxone challenge. Acute administration of estradiol, but not progesterone or a combination of estradiol and progesterone, significantly reduced naloxone-precipitated weight loss in morphine-dependent rats. These data indicate that estradiol influences the behavioral effects of morphine, possibly by increasing endogenous tone at mu opioid receptors.

Sodium Oxybate: a Substitute for Alcohol?

Gamma-hydroxybutyrate (GHB) is a neurotransmitter found naturally in the human brain. Sodium oxybate (SO) is the sodium salt of GHB. In 2000 GHB was classified a Schedule I controlled substance, while SO became a Schedule III controlled substance for medicinal use under the Controlled Substances Act. SO and alcohol share a similar pharmacological profile. GHB acts on GABAB receptors and extrasynaptic GABAA receptors resulting in alcohol-mimetic effects in several CNS actions. It substitutes the discriminative stimulus effects of alcohol in rats, and has cross tolerance with alcohol. All together, this leads to think of SO as a substitution therapy for alcohol use disorders. SO was initially studied in the prevention of alcohol withdrawal, and it showed similar efficacy to benzodiacepines. The studies on relapse prevention were developed later and the results are mixed and more complex to understand. While open label studies show a positive effect, RCTs have not been able to show a significant effect for the whole sample. Nevertheless, post-hoc analysis show a robust effect in the subsample of patients with high risk drinking levels, that would be the preferred target for a substitution treatment. The potential for abuse of GHB is well documented, which should be no surprise for a substitution treatment. Nevertheless, when correctly prescribed the risk of abuse of SO remains very low, as shown both in clinical trials and in the pharmacovigilance database, with more than 260000 cases documented. SO can be considered a substitution treatment, effective in patients with high risk drinking levels.DisclosureI was investigator in the SMO study on sodium oxybate, funded by D&A Pharma.

Acute nicotine reinforcement requires ability to discriminate the stimulus effects of nicotine.

This review of research on behavioral discrimination of nicotine and how it informs public health policy for reducing risk of tobacco dependence is adapted from Kenneth A. Perkins's American Psychological Association Division 28 (Psychopharmacology and Substance Abuse) 2020 Med Associates Brady/Schuster Award Lecture. The author's initial programmatic clinical research on nicotine is introduced, especially efforts to develop and validate a novel method of acute nicotine dosing. After the public health rationale for characterizing the discriminative stimulus effects of nicotine in humans are described, details from two separate programs of research on nicotine discrimination in humans are presented. The first, conducted with nicotine dosing by nasal spray, documented that humans could discriminate nicotine administered rapidly, examined nicotine's neuropharmacological specificity, identified discrimination threshold dose in smokers and nonsmokers, and explored other conditions that might alter ability to discriminate its effects. The second, more recent program focused on threshold doses for discrimination of nicotine by cigarette smoking, a program that was very difficult to do until the past decade, and how nicotine's self-reported "reward" and preference via choice behavior relate to its discriminability. Differences due to menthol and degree of tobacco dependence were also examined. For each of these two programs, the main findings of selected studies are noted, followed by very recent work on nicotine discrimination and choice that informs Food and Drug Administration's efforts to formulate public policy to improve health and reduce the nearly half million American deaths per year due to persistent tobacco use. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Effects of Cannabinoid Agonists and Antagonists in Rats Discriminating Fentanyl

Extensive evidence suggests the existence of a functional interaction between endogenous cannabinoids and opioid systems. Targeting cannabinoid CB1 receptors might be a promising approach for developing new medications for opioid addiction. The present studies were undertaken to evaluate the effects of CB1 agonists and antagonists in rats (n=8) trained to discriminate, i.p., injections of 0.032 mg/kg fentanyl from saline on a 10‐response fixed‐ratio (FR‐10) schedule of food reinforcement. Results show that the µ‐opioid agonists (fentanyl, morphine, and oxycodone) substituted fully for fentanyl, whereas the muscarinic antagonist atropine did not substitute for fentanyl. The CB1 agonists Δ9‐THC, AM2201, and AM8936 all partially substituted for fentanyl’s discriminative‐stimulus effects suggesting some overlap in the discriminative‐stimulus effects of cannabinoid agonists and fentanyl. Pretreatment studies with a µ‐opioid receptor antagonist show that naltrexone antagonized fentanyl’s effects. Interestingly, pretreatment studies with CB1 antagonists show that rimonabant (inverse agonist) produced some attenuation of fentanyl’s discriminative‐stimulus effects at doses that significantly decreased response rates, whereas the neutral CB1 antagonist AM4113 blocked fentanyl discrimination at doses that did not modify rates of responding. Our findings are consistent with our recent work showing that AM4113 attenuates heroin self‐administration in rats, without producing depressive‐like effects. Collectively, these data suggests that CB1 neutral antagonists that block CB1 receptors with rimonabant‐like potency, devoid of unwanted side‐effects, may be therapeutically advantageous for countering the abuse‐related behavioral effects of opioids.

Effects of cannabidiol (CBD), ∆9‐tetrahydrocannabinol (THC), and CBD/THC mixtures on fentanyl self‐administration in a food drug choice procedure in rhesus monkeys

The use of cannabinoids as adjuvants to opioid therapies to mitigate the problematic side effects of opioids or to decrease the doses of opioids necessary to relieve pain is an area of intense interest. In rhesus monkeys, ∆9‐tetrahydrocannabinol (THC) enhances the acute antinociceptive effects of opioids. However, THC does not enhance other effects of opioids including physical dependence, ventilatory depression, reinforcing effects, or discriminative stimulus effects. Evidence from rodent studies suggest other phytocannabinoids like cannabidiol (CBD) might decrease the reinforcing effects of several classes of drugs of abuse, including opioids. The present studies sought to assess the impact of THC, CBD, and THC/CBD mixtures on i.v. fentanyl self‐administration in rhesus monkeys (n=4) in a food drug choice procedure. Experimental sessions consisted of 2 forced trials followed by up to 30 choice trials where animals had the opportunity to respond on one lever for a sucrose pellet and another for an i.v. infusion (fentanyl or vehicle). Fentanyl dose was adjusted for each animal to find the largest dose that occasioned <20% drug choice (0.0001 mg/kg/infusion), and the smallest dose that elicited >80% drug choice (0.001 or 0.0032 mg/kg/infusion). CBD (10‐17.8 mg/kg), THC (0.032‐1 mg/kg), and THC/CBD mixtures (dose ratios of 1:10 and 1:32) were delivered i.v. 15 minutes prior to testing sessions. CBD or THC alone did not affect choice for either dose of fentanyl. THC/CBD mixtures similarly did not produce an increase in choice for the small dose of fentanyl, or a decrease for the large dose of fentanyl. The present studies suggest that phytocannabinoids such as THC or CBD, alone or in combination, do not enhance or diminish the rewarding effects of fentanyl. These results add to the body of evidence that phytocannabinoids might be a safe option to enhance the antinociceptive effects of opioids without enhancing their rewarding properties.

Does clozapine affect the discriminative stimulus effects of food deprivation in rats trained to discriminate between 2 and 22 hours food deprivation?

In humans, weight gain is a side effect of several atypical antipsychotics. Previous research with rodents indicates clozapine may increase food consumption for the first 2‐4 hours after injection (Antelman et al. 1977; Benvenga & Leander 1997; Lee & Clifton 2002) as well as increase motivation to acquire food under a progressive ratio schedule (Abela et al. 2020; Loh et al. 1992; Mobini et al. 2000). In other studies, however, clozapine has been shown to reduce response rates in lever pressing operant procedures (Ford et al., 1997; Kaempf & Porter, 1997; Varvel et al., 2002; Wiley et al., 1994; Abela 2020) and decrease food intake in rodents (Baptista et al., 1993). The current study investigated if clozapine would affect the discriminative stimulus effects of food deprivation. Male, Sprague‐Dawley rats (n = 11) were trained to discriminate 22 hrs of food deprivation from 2 hrs of food deprivation in a two‐lever, operant choice task. Rats were trained to respond on the right lever after 2 hrs of food deprivation to earn a 45 mg food pellet under an FR 15 schedule. Left lever presses were punished with 8 seconds of darkness. Under 22 hr deprivation conditions, the contingencies were reversed (left lever presses were reinforced with food delivery and right lever presses resulted in 8 seconds of darkness). Training sessions lasted until subjects earned 10 reinforcers or 15 minutes elapsed. Acquisition of discrimination was defined as 80% or greater condition appropriate responding both before the first reinforcer was earned and for the entire session for 8 out of 10 consecutive days. After acquiring the discrimination, subjects were food deprived for 2 or 22 hrs and administered clozapine (1.0 ‐ 5.6 mg/kg, s.c.). or vehicle (30% DMSO; 1.0 ml/kg, s.c.). During test sessions, responses toward either lever were reinforced under the FR 15. The session lasted until subjects earned 5 reinforcers or 5 minutes elapsed, whichever occurred first. The discriminative stimulus effects of food deprivation and response rates were assessed every 15 min for the succeeding 2 hrs. After test sessions, each subjects’ food intake was recorded for 1 hour. Clozapine had no effect on the discriminative stimulus effects of 2 hr deprivation, however, clozapine (1.0 ‐ 5.6 mg/kg) significantly decreased response rates. After 22 hrs food deprivation, clozapine (1.0 ‐ 5.6 mg/kg) significantly decreased % 22 hr deprivation‐appropriate responding. Response rates were significantly increased by 1.0 mg/kg clozapine, whereas response rates were decreased by 5.6 mg/kg clozapine compared to rates of responding following 30% DMSO. Clozapine (3.2 ‐ 5.6 mg/kg) decreased food intake in rats under both food deprivation conditions. These data provide further support for the notion that clozapine decreases feeding‐related behaviors in rats.

How to account for hallucinations in the interpretation of the antidepressant effects of psychedelics: a translational framework

RationaleRecent trials with psychedelics in major depressive disorder and treatment-resistant depression showed remarkable improvements in depressive symptoms that can last for up to several months after even a single administration. The lack of an appropriate placebo control group—as patients are often able to discriminate the subjective effects of the drug—and an incomplete understanding of the role of the hallucinogenic and mystical experience, hampers the interpretation of these therapeutic effects.ObjectivesTo control for these factors, we developed a translational framework based on establishing pharmacokinetic/pharmacodynamic (PK/PD) relationships in rodents and humans for hallucinogenic (i.e., discriminative stimulus effects in rodents and humans; head twitch responses in rodents; questionnaires in humans) and therapeutic effects. For the latter, we selected the pattern separation and attentional set-shifting tasks as measures for cognitive flexibility because of their high translational value. We predict that these PK/PD analyses will lead to a more objective evaluation of improvements in patients compared to relying only on the currently used self-reported questionnaires. We hypothesize that—if the role of the hallucinogenic experience is not central in the antidepressant effects of psychedelics—the ED50’s for the therapeutic effects will be significantly lower than for the hallucinogenic and mystical effects.ConclusionOur framework will help to inform future studies that aim at the elucidation of the mechanism(s) of action of psychedelics in depression, and the role of the acute subjective and/or hallucinogenic experience in their effects.

Correction to: Discriminative stimulus effects of an imidazolidine-derived appetite suppressant

Correction to: Discriminative stimulus effects of an imidazolidine-derived appetite suppressant

Discriminative stimulus effects of an imidazolidine-derived appetite suppressant

Discriminative stimulus effects of an imidazolidine-derived appetite suppressant

Correction to "Antinociceptive and Discriminative Stimulus Effects of Six Novel Psychoactive Opioid Substances in Male Rats".

The above article [Walker EA, Chambers C, Korber MG, Tella SR, Prioleau C, and Fang L (2021) J Pharmacol Exp Ther 379: [1-11][1]; DOI: <https://doi.org/10.1124/jpet.121.000689>] was published with several errors introduced by the compositor during the proofing process. [Fig. 1][2] and [Diagram 1][3

Human behavioral pharmacology of stimulant drugs: An update and narrative review.

Stimulant use disorders present an enduring public health concern. Chronic stimulant use is associated with a range of health problems, with notable increases in stimulant overdose that disproportionately affect marginalized populations. With these persistent problems, it is important to understand the behavioral and pharmacological factors that contribute to stimulant use in humans. The purpose of this chapter is to provide an update and narrative review on recent human laboratory research that has evaluated the behavioral pharmacology of stimulant drugs. We focus on two prototypic stimulants: cocaine as a prototype monoamine reuptake inhibitor and d-amphetamine as a prototype monoamine releaser. As such, placebo controlled human laboratory studies that involved administration of doses of cocaine or d-amphetamine and were published in peer reviewed journals within the last 10 years (i.e., since 2011) are reviewed. Primary outcomes from these studies are subjective effects, reinforcing effects, cognitive/behavioral effects, and discriminative stimulus effects. Both cocaine and d-amphetamine produce classical stimulant-like behavioral effects (e.g., increase positive subjective effects, function as reinforcers), but there are notable gaps in the literature including understanding sex differences in response to stimulant drugs, cognitive-behavioral effects of stimulants, and influence of use history (e.g., relatively drug naïve vs drug experienced) on stimulant effects.

Characterization of the discriminative stimulus effect of quinpirole: Further evidence for functional interaction between central dopamine D1/D2-receptors

Dysfunction of the central dopamine D2-receptor-related network has been proposed to play a critical role in dopamine-related diseases, such as schizophrenia and drug dependence. Generally, the stimulation of dopamine D2-receptors on medium spiny neurons (MSN) induces several behavioral effects, such as sedation, hallucination, aversion and motivation. Furthermore, such physiological responses through dopamine D2-receptor-containing MSN (D2-MSN) may be synchronized with the activity of dopamine D1-receptor-containing MSN (D1-MSN), or both may exhibit dual agonistic/antagonistic innervation. In the present study, we characterized the discriminative stimulus effect of the selective dopamine D2-receptor agonist quinpirole to further investigate the “D1/D2-MSN” interaction using dopamine-related agents, hallucinogens and sedatives in rats. Among dopamine receptor agonists, only selective dopamine D2-receptor agonists substituted for the discriminative stimulus effects of quinpirole. Neither the δ-opioid receptor agonist SNC80 nor the adenosine A2A-receptor antagonist istradefylline, both of which may act on D2-MSNs, substituted for the discriminative stimulus effects of quinpirole. Interestingly, the dopamine D1-receptor antagonist SCH23390 and the GABAB-receptor agonist baclofen, but not hallucinogens or sedatives, substituted for the discriminative stimulus effects of quinpirole. These results suggest that stimulation of central dopamine D2-receptors exerts a distinct discriminative stimulus effect, and blockade of dopamine D1-receptors and agonistic modulation of GABAB-receptors may share the discriminative stimulus effect via the activation of central dopamine D2-receptors.

Characterization of the Discriminative Stimulus Effects of Binary Mixtures of Mu Opioid Receptor Agonists in Rats Discriminating Fentanyl.

Drug overdose deaths involving synthetic opioids, primarily fentanyl, have risen dramatically over the past decade and are currently the driving force of the opioid epidemic in the United States. Fentanyl analogs with greater potency than fentanyl (e.g., carfentanil) pose serious risk to public health. Although fentanyl analogs are primarily encountered by humans as constituents of a mixture of drugs, research has primarily evaluated the effects of these drugs alone. The present study characterized interactions between mu opioid receptor agonists in seven male Sprague-Dawley rats trained to discriminate 10 μg/kg fentanyl from saline while responding under a fixed-ratio 10 schedule of food presentation. Dose-effect curves were determined for each drug alone and in binary mixtures (fentanyl:heroin, fentanyl:carfentanil, and heroin:carfentanil) at fixed-dose ratios of 3:1, 1:1, and 1:3 relative to the ED50 for each drug when given alone. Dose addition analyses were used to determine the nature of the drug-drug interaction for each mixture. Additive interactions were observed for all binary mixtures at each fixed dose ratio, except the 1:3 fentanyl:carfentanil mixture, which exhibited supra-additive effects at the 80% effect level. These results suggest a lack of a significant interaction between the discriminative stimulus effects of these mu opioid receptor agonists at the doses tested in this study. Future studies expanding these findings to the respiratory depressant effects of these drugs are of significant importance to rule out possible interactions directly relevant to opioid overdose that occur at doses much larger than those tested in this study. SIGNIFICANCE STATEMENT: In the United States, drug overdose deaths involving synthetic opioids, primarily fentanyls including superpotent fentanyl analogs (e.g., carfentanil), have increased 12-fold over the past decade. Although previous studies have evaluated the effects of carfentanil alone, fentanyl analogs are encountered by humans as a mixture with other drugs; this study determined the effects of mixtures of carfentanil and other opioids (fentanyl and heroin) to characterize interactions between these drugs that might contribute to their apparent increased lethality in humans.

In Vitro and In Vivo Pharmacological Comparison of Mu‐Opioid Receptor Activity of the Kratom (Mitragyna speciosa) Alkaloid Mitragynine and its Metabolite 7‐Hydroxymitragynine

AbstractBackgroundThe µ‐opioid receptor (MOR) pharmacology of mitragynine and its 7‐hydroxy metabolite have not been fully established.MethodFor both we assessed binding affinity and efficacy in vitro, as well as discriminative stimulus and antinociceptive effects in rats.ResultBoth mitragynine (MG) and 7‐hydroxymitragynine (7‐MG) had higher binding affinities at human MOR (Ki values 77.9 and 709 nM, respectively) than at the human k‐ (KOR) or d‐opioid receptors (DOR). MG was identified as a MOR antagonist while 7‐MG was identified as a MOR partial agonist (Emax=41%) using the [35S]GTPγS functional assay. In Sprague Dawley rats trained to discriminate 3.2 mg/kg morphine from vehicle (i.p.), MG produced a maximum of 72% morphine‐lever responding. Whereas morphine produced a maximum of 65% MG‐lever responding in a separate group of rats trained to discriminate 32 mg/kg MG from vehicle (i.p.), other MOR agonists (i.p.) produced high percentages of morphine‐lever and MG‐lever responding (respectively): 7‐MG (100% and 98%), fentanyl (100% and 80%), buprenorphine (100% and 79%), and nalbuphine (99% and 98%). MG (up to 56 mg/kg, i.p.) did not produce significant antinociception, whereas 7‐MG (17.8 mg/kg, i.p.) produced robust antinociception in the hotplate assay with the temperature set at 52 °C. Naltrexone (0.032 mg/kg, i.p.) antagonized the discriminative stimulus, rate‐decreasing, and antinociceptive effects of morphine and 7‐MG. Conversly, naltrexone (0.032 mg/kg, i.p.) antagonized the discriminative stimulus effects of MG, but not its rate‐decreasing effects. Pretreatment with MG (5.6 mg/kg, i.p.) potentiated the discriminative‐stimulus effects of morphine but antagonized the antinociceptive effects of morphine.ConclusionStriking differences in MOR efficacy in vitro are consistent with 7‐MG having greater MOR efficacy than MG in vivo. MG exerts both MOR antagonist and agonist activity depending on the MOR efficacy required.

Characterization of DREADD receptor expression and function in rhesus macaques trained to discriminate ethanol.

Circuit manipulation has been a staple technique in neuroscience to identify how the brain functions to control complex behaviors. Chemogenetics, including designer receptors exclusively activated by designer drugs (DREADDs), have proven to be a powerful tool for the reversible modulation of discrete brain circuitry without the need for implantable devices, thereby making them especially useful in awake and unrestrained animals. This study used a DREADD approach to query the role of the nucleus accumbens (NAc) in mediating the interoceptive effects of 1.0 g/kg ethanol (i.g.) in rhesus monkeys (n = 7) using a drug discrimination procedure. After training, stereotaxic surgery was performed to introduce an AAV carrying the human muscarinic 4 receptor DREADD (hM4Di) bilaterally into the NAc. The hypothesis was that decreasing the output of the NAc by activation of hM4Di with the DREADD actuator, clozapine-n-oxide (CNO), would potentiate the discriminative stimulus effect of ethanol (i.e., a leftward shift the ethanol dose discrimination curve). The results showed individual variability shifts of the ethanol dose-response determination under DREADD activation. Characterization of the expression and function of hM4Di with MRI, immunohistochemical, and electrophysiological techniques found the selectivity of NAc transduction was proportional to behavioral effect. Specifically, the proportion of hM4Di expression restricted to the NAc was associated with the potency of the discriminative stimulus effects of ethanol. Together, these experiments highlight the NAc in mediating the interoceptive effects of ethanol, provide a framework for validation of chemogenetic tools in primates, and underscore the importance of robust within-subjects examination of DREADD expression for interpretation of behavioral findings.

Modulation of relapse-like drinking in male Sprague-Dawley rats by ligands targeting the α5GABAA receptor

Preclinical evidence suggests a key role for GABAA receptors containing the α5 subunit (i.e., α5GABAA receptors) in the abuse-related effects of alcohol, including the reinforcing and discriminative stimulus effects, as well as cue-induced alcohol-seeking behavior. However, the contribution of this GABAA receptor subtype to relapse-like drinking behavior remains unknown. The present study evaluated the capacity of ligands targeting α5GABAA receptors to modulate the alcohol deprivation effect (ADE), a model of relapse-like drinking. Groups of Sprague-Dawley rats underwent repeated cycles of long-term access to alcohol solutions (5%, 10%, 20% v/v) and water in the home cage followed by water only deprivation periods. Upon evidence that the ADE could be reliably expressed across cycles, drug treatment was initiated. One group received the α5GABAA receptor-preferring agonist QH-ii-066 and the other group received the α5GABAA receptor-selective inverse agonist L-655,708. At the end of ADE testing, rats underwent testing in the elevated zero maze under vehicle or L-655,708 treatment for assessment of anxiety-like behavior. The ADE was reliably expressed across repeated cycles of alcohol access/deprivation in a subset of rats. Low doses of QH-ii-066 enhanced expression of the ADE; whereas, L-655,708 dose-dependently inhibited expression of the ADE. L-655,708 did not engender anxiogenic effects in the elevated zero maze under the conditions evaluated. These findings suggest a key role for α5GABAA receptor mechanisms in relapse-like drinking. Moreover, they suggest that α5GABAA receptors may represent a novel pharmacological target for the development of medications to prevent or reduce alcohol relapse.

Characterization of 3,4-methylenedioxypyrovalerone discrimination in female Sprague-Dawley rats.

3,4-Methylenedioxypyrovalerone (MDPV), one of several synthetic cathinones, is a popular constituent of illicit 'bath salts'. In preclinical studies utilizing drug discrimination methods with male rodents, MDPV has been characterized as similar to both cocaine and 3,4-methylenedioxymethamphetamine-hydrochloride (MDMA). Whereas few drug discrimination studies have utilized female rats, the current study evaluated the discriminative stimulus effects of MDPV in 12 adult female Sprague-Dawley rats trained to discriminate 0.5 mg/kg MDPV from saline under a fixed ratio 20 schedule of food reinforcement. Stimulus substitution was assessed with MDPV and its enantiomers, other synthetic cathinones [alpha pyrrolidinopentiophenone-hydrochloride(α-PVP), 4-methylmethcathinone (4-MMC)], other dopamine agonists (cocaine, [+)-methamphetamine] and serotonin agonists [MDMA, lysergic acid diethylamide (LSD)] Stimulus antagonism was assessed with the dopamine D1 receptor antagonist, Sch 23390 and the D2 receptor antagonist, haloperidol. Cocaine and (+)-methamphetamine engendered full stimulus generalization to MDPV with minimal effects on response rate. LSD produced partial substitution, whereas MDMA and 4-MMC produced complete substitution, and all these serotonergic compounds produced dose-dependent response suppression. (S)-MDPV and α-PVP engendered full substitution with similar potency to the racemate, while (R)-MDPV failed to substitute up to 5 mg/kg. Both Sch 23390 and haloperidol attenuated the discrimination of low MDPV doses and essentially shifted the dose-response curve to the right but failed to block discrimination of the training dose. These findings are generally consistent with previous reports based exclusively on male rodents. Moreover, they confirm the contribution of dopaminergic mechanisms but do not rule out the possible contribution of other neurotransmitter actions to the interoceptive stimulus effects of MDPV.