Schedule Of Food Presentation Research Articles

Discriminative stimulus properties of Cannabis sativa terpenes in rats.

Cannabis is a pharmacologically complex plant consisting of hundreds of potentially active compounds. One class of compounds present in cannabis that has received little attention are terpenes. Traditionally thought to impart aroma and flavor to cannabis, it has become increasingly recognized that terpenes might exert therapeutic effects themselves. Several recent reports have also indicated terpenes might behave as cannabinoid type 1 (CB1) receptor agonists. This study aimed to investigate whether several terpenes present in cannabis produce discriminative stimulus effects similar to or enhance the effects of Δ 9 -tetrahydrocannabinol (THC). Subsequent experiments explored other potential cannabimimetic effects of these terpenes. Rats were trained to discriminate THC from vehicle while responding under a fixed-ratio 10 schedule of food presentation. Substitution testing was performed with the CB receptor agonist JWH-018 and the terpenes linalool, limonene, γ-terpinene and α-humulene alone. Terpenes were also studied in combination with THC. Finally, THC and terpenes were tested in the tetrad assay to screen for CB1-receptor agonist-like effects. THC and JWH-018 dose-dependently produced responding on the THC-paired lever. When administered alone, none of the terpenes produced responding predominantly on the THC-paired lever. When administered in combination with THC, none of the terpenes enhanced the potency of THC, and in the case of α-humulene, decreased the potency of THC to produce responding on the THC-paired lever. While THC produced effects in all four tetrad components, none of the terpenes produced effects in all four components. Therefore, the terpenes examined in this report do not have effects consistent with CB1 receptor agonist properties in the brain.

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.

Interactions Between Mu Opioid Receptor Agonists in Rats Discriminating Fentanyl

The opioid epidemic persists as a major public health challenge with grave consequences. In 2018 two-thirds of the 47,000 opioid overdose deaths in the US involved a synthetic opioid such as fentanyl or one of its many derivatives, e.g. carfentanil, which is reportedly 100-fold more potent than fentanyl. Clinical reports suggest that larger doses of the opioid antagonist naloxone are required to reverse opioid overdoses involving carfentanil, compared with other opioid receptor agonists. Highly potent synthetic opioids such as fentanyl and carfentanil are most frequently encountered as adulterants of another drug such as heroin, suggesting that drug interactions might underlie the reported difficulty in reversing overdoses. This study evaluated the discriminative stimulus effects of opioid mixtures to assess interactions between mu opioid receptor agonists. Eight male Sprague Dawley rats were trained to discriminate 0.01 mg/kg fentanyl under a fixed-ratio 10 schedule of food presentation. Dose-effect curves were determined for fentanyl (0.001- 0.032 mg/kg), heroin (0.032-1.0 mg/kg), and carfentanil (0.000032-0.001 mg/kg) alone, and as binary mixtures (fentanyl and heroin, fentanyl and carfentanil, heroin and carfentanil) at fixed-dose ratios of 3:1. 1:1, and 1:3 relative to the ED50 for occasioning fentanyl-appropriate responding for each drug when given alone. Fentanyl, heroin, and carfentanil dose-dependently increased responding on the fentanyl-associated lever and reduced rate of responding. The kappa opioid receptor agonist spiradoline (0.1-0.32 mg/kg) did not occasion fentanyl-appropriate responding, but dose-dependently reduced rate of responding. Dose-addition analyses were used to determine the nature of drug interactions (additive, supra-additive, subadditive) for each mixture. Some mixtures of mu opioid receptor agonists, particularly those containing carfentanil, had supra-additive discriminative stimulus effects relative to predictions based on the effects of each drug alone. These results suggest that not only the increased potency of synthetic opioids, but also interactions with other opioid drugs likely play a role in the apparent increased lethality of these mixtures in humans.

Discriminative stimulus effects of carfentanil in rats discriminating fentanyl: Differential antagonism by naltrexone

BackgroundA significant number of deaths caused by opioids involve fentanyl and/or one of its very potent analogs (e.g., carfentanil). Some clinical reports suggest larger doses of opioid receptor antagonists may be required to reverse the effects of carfentanil compared with other opioid receptor agonists, although this has not been examined extensively in vivo. The current study compared the discriminative stimulus effects of fentanyl, carfentanil, and heroin, and their antagonism by naltrexone. MethodsEight male Sprague-Dawley rats were trained to discriminate 10.0 μg/kg fentanyl from saline while responding under a fixed-ratio 10 schedule of food presentation. Dose-effect curves were determined for the opioid receptor agonists fentanyl (1.0–32.0 μg/kg), carfentanil (0.1–3.2 μg/kg), and heroin (10.0–320.0 μg/kg), then redetermined following a 15-minute pretreatment with 0.1 mg/kg naltrexone. ResultsFentanyl, carfentanil, and heroin dose-dependently increased responding on the fentanyl-associated lever and decreased the rate of lever pressing. Carfentanil and heroin were approximately 10-fold more and less potent, respectively, than fentanyl at eliciting >80 % responding on the fentanyl-associated lever. Pretreatment with 0.1 mg/kg naltrexone resulted in a significant rightward shift in the fentanyl and heroin but not carfentanil dose-effect curves. ConclusionsDifferences in the effectiveness of naltrexone to attenuate the discriminative stimulus effects of carfentanil, compared with fentanyl and heroin, suggest that there may be differences in how carfentanil exerts its discriminative stimulus effects compared with other opioids. Further evaluation is needed of potential pharmacological and behavioral differences between carfentanil and other opioids, particularly in the context of toxicity.

Behavioral effects of benzylideneoxymorphone (BOM), a low efficacy µ opioid receptor agonist and a δ opioid receptor antagonist.

Opioids remain the drugs of choice for treating moderate to severe pain, although adverse effects often limit use. Drugs acting concomitantly as agonists at μ opioid receptors and antagonists at δ opioid receptors produce antinociceptive effects with a reduced profile of adverse effects; one such drug, benzylideneoxymorphone (BOM), might further limit adverse effects because it appears to have lower pharmacological efficacy than other μ opioid receptor agonists. The current study compared the acute behavioral effects of BOM with the effects of other μ opioid receptor agonists. Discriminative stimulus and rate-decreasing effects were studied in 1 group of 7 rats discriminating 3.2 mg/kg morphine while responding under a fixed-ratio 10 schedule of food presentation. Antinociceptive effects were determined in a second group of 8 rats using a warm water tail withdrawal procedure. Reinforcing effects were evaluated in a third group of 12 rats with a history of remifentanil self-administration. BOM produced morphine-lever responding and both discriminative stimulus and rate-decreasing effects were antagonized by naltrexone. BOM did not markedly increase tail-withdrawal latencies from water maintained at 50 °C and did not substantially attenuate the antinociceptive effects of morphine. BOM was not self-administered and did not change remifentanil self-administration. Some effects of BOM (e.g., discriminative stimulus effects) appear to be mediated by μ opioid receptors; however, BOM is not self-administered by rats, suggesting that it might have limited abuse liability and a reduced profile of adverse effects compared with currently prescribed opioids.

Effects of SCH‐23390 and Alpha‐Methyl‐Tyrosine on the Discriminative Stimulus Effects of Mephedrone in Male Rats

The illicit synthetic cathinone mephedrone (4‐methylmethcathinone) often termed a “bath salt” produces discriminative stimulus effects that are similar, but not identical, to those for central nervous system stimulants such as cocaine and methamphetamine. This may be due to its capacity to bind atypically at both dopamine and serotonin transporters compared to those drugs. The purpose of the present study was to further elucidate the mechanism of action underlying mephedrone’s discriminative stimulus effects by administering it with the dopamine type‐1 receptor antagonist SCH‐23390 (n=11) or the catecholamine synthesis inhibitor alpha‐methyl‐tyrosine (alpha‐MT; n=8). Male Long‐Evans rats were trained to discriminate intraperitoneal injections of mephedrone (3.2 mg/kg) from saline under a fixed‐ratio 20 (FR‐20) schedule of food presentation. Mephedrone dose‐effect curves (0.32 – 10 mg/kg) were then established by administering increasing cumulative doses of mephedrone on test days, and comparing those effects with multiple (4–5) injections of saline. The interaction of SCH‐23390 (0.032 – 0.1 mg/kg) and alpha‐MT (200 mg/kg) with mephedrone was examined by administering an acute injection of these drugs prior to increasing cumulative doses of mephedrone. As shown previously by our laboratory, saline administration produced almost exclusively saline‐lever responding while increasing cumulative doses of mephedrone dose‐dependently increased mephedrone‐lever responding. When SCH‐23390 preceded cumulative doses of mephedrone, neither dose alone produced mephedrone‐lever responding and only the larger of the two doses tested (0.1 mg/kg) shifted the mephedrone dose‐effect curve rightward; however, this shift was still less than 2‐fold and produced rate‐decreasing effects when administered alone. Acute administration of alpha‐MT alone also did not produce mephedronelever responding nor decrease rate, and in combination did not produce a marked rightward shift in the mephedrone dose‐effect curve. Taken together, these data indicate that neither SCH‐23390 nor alpha‐MT effectively antagonize the discriminative stimulus effects of mephedrone, and the alpha‐MT data in particular suggest that mephedrone’s discriminative stimulus effects do not depend on newly synthesized catecholamines.

Aversive Effects of the Kappa Opioid Agonist U‐50488 Alone and in Combination with the Cannabinoid Agonist CP 55,940 in Male Rats

Kappa (κ) opioid receptor agonists can produce prominent analgesic and diuretic effects when administered alone and these effects can be enhanced by the co‐administration of cannabinoids such as CP 55,940. The purpose of the present experiment was to determine if cannabinoids can also enhance the aversive effects of kappa agonists, because these effects substantially reduce their clinical utility as analgesics and aquaretics. To study the aversive effects of kappa agonists and their interaction with cannabinoids, nine male Long‐Evans rats were trained to respond for food under a fixed‐ratio (FR) schedule of food presentation and then implanted with intravenous (i.v.) catheters. Following recovery, the subjects were transitioned to a multiple FR, FR schedule. More specifically, in the presence of a single white stimulus light, food was presented under a fixed‐ratio 30 (FR‐30) schedule (non‐infusion component), whereas presence of both this white stimulus light and a green stimulus light, an i.v. infusion was presented on the first response of each FR and food was presented on the thirtieth FR response (infusion component). When responding was consistent in both components, varying infusion doses of U‐50488 (0.032 – 0.18 mg/kg) replaced the baseline saline infusion until one of three criterion was met: 3 days in which the variability of responding did not vary by ± 20% of the mean, 3 days of responding at less than 0.3 responses per sec, or a maximum of 8 days. After the criterion was met for each dose substitution, the subjects returned to the baseline in which saline served as the infusion and responding was similar between components. To study the interaction of CP 55,940 with the effects of U‐50488, single acute injections of a dose of CP 55,940 (0.056 mg/kg) were administered prior to sessions in which a dose of U‐50488 was available in the infusion component. Each dose‐infusion combination was administered until subjects met the criterion. These effects were then compared to sessions in which one of two doses of naltrexone (1, 3.2 mg/kg) was administered prior the same U‐50488 infusion doses. When infusions of U‐50488 replaced saline, low infusion doses (e.g., 0.056 mg/kg/infusion) selectively suppressed responding in the infusion component, while higher infusion doses (e.g., 0.18 mg/kg/infusion) decreased responding in both components as the total dose of U‐50488 for the session increased. Administration of CP 55,940 did not produce a marked enhancement of the suppressive effects of U‐50488 at the dose tested, and surprisingly, neither dose of naltrexone antagonized its suppressive effects. Together, these data demonstrate the aversive effects of U‐50488 alone, the absence of a marked enhancement by at least one cannabinoid agonist, and the surprising absence of antagonism by the opioid receptor antagonist naltrexone.

Discriminative Stimulus Effects of Mu Opioid Receptor Agonists in Rats Discriminating Fentanyl: Differential Antagonism by Naltrexone

The opioid crisis continues despite the availability of medications that are effective in some patients. A significant number of deaths caused by opioids involve fentanyl and/or one of its very potent analogues (e.g., carfentanil). Many fentanyl analogs have not been studied extensively in vivo. The current study compared the discriminative stimulus effects of fentanyl and carfentanil and their antagonism by naltrexone. Seven male Sprague‐Dawley rats were trained to discriminate a 0.01 mg/kg fentanyl from saline while responding under a fixed‐ratio 10 schedule of food presentation. Dose effect curves were determined for the opioid receptor agonists alone: fentanyl (0.001–0.1 mg/kg) and carfentanil (0.0001–0.0032 mg/kg). Dose effect curves for fentanyl and carfentanil were then redetermined following a 15‐minute pretreatment with 0.1 mg/kg naltrexone. All drugs were delivered intraperitoneally. Fentanyl and carfentanil dosedependently increased responding on the fentanyl‐associated lever and at larger doses decreased the rate of lever pressing. Carfentanil was approximately 10‐fold more potent than fentanyl at eliciting >90% responding on the fentanyl‐associated lever. Pretreatment with 0.1 mg/kg naltrexone shifted the fentanyl dose‐effect curve 10‐fold rightward whereas the same dose of naltrexone shifted the carfentanil curve only 3‐fold rightward. Differences in the effectiveness of naltrexone to attenuate the discriminative stimulus effects of fentanyl and carfentanil suggests that there may be differences in how fentanyl and carfentanil exert their discriminative stimulus effects. Further evaluation of potential pharmacological and behavioral differences between fentanyl and carfentanil is needed and might provide important insights regarding the apparently increased toxicity of carfentanil, compared with other opioids, in humans.Support or Funding InformationAQ‐0039 from the Welch Foundation

Sex differences in the effectiveness of buprenorphine to decrease rates of responding in rhesus monkeys

Sex differences in μ-opioid receptor (MOR) agonist-induced antinociception have been reported in nonhuman primates. The degree to which μ-opioid receptor agonist sex differences in nonhuman primates extend to other behavioral endpoints remains unknown. The present study compared the behavioral effects of three MOR ligands (fentanyl, buprenorphine, and naltrexone) that varied in efficacy to stimulate [S]-GTPγS binding (from highest to lowest: fentanyl, buprenorphine, and naltrexone) in male and female rhesus monkeys. Male (n=3) and female (n=3) monkeys were trained to respond under a fixed-ratio 10 schedule of food presentation during daily sessions consisting of multiple components. Once rates of responding were stable, cumulative dose-effect functions were determined for intramuscular fentanyl (0.00032-0.032 mg/kg), buprenorphine (0.001-1 mg/kg), and naltrexone (0.01-0.1 mg/kg). Fentanyl dose-dependently decreased rates of responding in both sexes and the corresponding ED50 values were not significantly different. Buprenorphine dose-dependently decreased rates of responding in females, but not males. Naltrexone did not significantly alter behavior in either females or males. Overall, these results suggest that the expression of sex differences in MOR pharmacology depends upon both the efficacy of the MOR ligand and the behavioral endpoint.

Abuse Potential of Phendimetrazine and its Effects on Cocaine Self‐Administration in Rhesus Monkeys

Cocaine use disorder (CUD) continues to be a major public health and social problem with >1.5 million users resulting in 14,556 deaths in 2017 in the United States and with no FDA‐approved medications available (Volkow and Koob, 2016). Preclinical studies in nonhuman primates have shown that monoamine releasers such as amphetamine can reduce the reinforcing strength of cocaine under several schedules of cocaine self‐administration (SA; Czoty et al. 2010; Negus et al. 2003a, b). However, the high abuse potential of this category of drugs has hindered advancement of their clinical use for treatment of CUD. More recently, preclinical researchers have investigated phendimetrazine (PDM), an FDA‐approved Schedule III anorectic agent, with evidence of efficacy in animal models (Banks and Negus, 2013; Czoty et al. 2016). As a pro‐drug for the monoamine releaser phenmetrazine, PDM has a slow onset and long duration of action. Only two studies (Griffiths et al. 1976; Corwin et al. 1987) have evaluated the reinforcing effects of PDM, with opposite conclusions. The goal of the present study was to compare, in the same subjects, the efficacy of PDM to decrease cocaine SA and the reinforcing strength of PDM relative to cocaine. For 5 consecutive days, three adult male rhesus monkeys responded under a daily fixed‐ratio 20 schedule of food presentation, which, on the fifth day, was followed by a progressive‐ratio (PR) schedule of cocaine (0.003–0.3 mg/kg/injection) presentation (limited hold 30‐min, maximum 4‐hr). PDM was administered daily (1.0–6.0 mg/kg, PO, BID). The primary dependent variable under the PR schedule was the number of injections delivered. Under baseline conditions, monkeys typically earned 10 food reinforcers and the number of cocaine injections was characterized as an inverted U‐shaped function of dose. Under treatment conditions, in two subjects, food‐maintained responding was disrupted at higher PDM doses that also decreased cocaine SA. In the third subject, food reinforcement was not affected and PDM did not show efficacy in decreasing cocaine SA. When PDM (0.1–1.0 mg/kg/injection) was substituted for cocaine, it functioned as a reinforcer in all three subjects; the number of injections observed at peak doses of PDM (0.3 and 1.0 mg/kg/injection) were similar to those observed at the peak dose of cocaine (0.03 mg/kg/injection). These findings suggest that PDM has abuse liability and was ineffective in decreasing cocaine SA. These data do not support the use of PDM as a potential treatment for CUD.Support or Funding InformationP50 DA06634‐27This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effects of cocaine on the discriminative stimulus and reinforcing effects of mephedrone in male rats.

Abuse of cathinones has been a worldwide health concern for some time. Their chemical structures and wide variation in pharmacodynamic effects have led to clinical and preclinical effects that can be both similar to and different from other psychoactive substances such as methylenedioxymethamphetamine (MDMA), methamphetamine, and cocaine. The present study examined the discriminative stimulus and reinforcing effects of mephedrone to further characterize the behavioral and pharmacological profile of this first-generation substituted methcathinone. Rats were trained to discriminate mephedrone (3.2mg/kg) from saline under a fixed-ratio 20 (FR-20) schedule of food presentation. After establishing dose-effect curves for increasing cumulative doses of mephedrone, substitution tests were conducted with bupropion (5.6-32mg/kg), cocaine (1.8-18mg/kg), morphine (0.56-10mg/kg), and amitriptyline (3.2-32mg/kg). In addition, cocaine (3.2-18mg/kg) and the serotonin type-2 (5-HT2) receptor antagonist ritanserin (1, 3.2, and 10mg/kg) were administered prior to the cumulative doses of mephedrone. Lastly, varying infusion doses of cocaine were substituted for mephedrone in subjects trained to self-administer mephedrone, and varying infusion doses of mephedrone were substituted for cocaine in subjects trained to self-administer cocaine to assess the importance of drug history on the reinforcing effects of mephedrone. Of the drugs tested, cocaine had the highest level of mephedrone-lever responding when administered alone (73.5%). In combination with mephedrone, cocaine shifted the mephedrone dose-effect curve upwards in an infra-additive manner. Ritanserin had a small, but non-significant, effect on mephedrone's discriminative stimulus effects. An extensive history (baseline) of cocaine self-administration increased mephedrone self-administration compared to that obtained in mephedrone-trained subjects, whereas a baseline of mephedrone self-administration decreased cocaine self-administration compared to that obtained in cocaine-trained subjects. The similarity between the discriminative stimulus effects of cocaine and mephedrone in male rats suggests an important overlap and the relative importance of the dopamine (DAT) and serotonin (SERT) transporters. The self-administration data suggest that mephedrone is less reinforcing than cocaine, but that a history of responding for cocaine can increase the reinforcing effects of mephedrone.

Estrogen‐ and Delay‐Dependent Effects of Δ9‐THC on Memory in Female Rats

The cannabinoid receptor agonist, Δ9‐tetrahydrocannabinol (Δ9‐THC), is a psychoactive constituent of marijuana, and has been shown to produce sex‐dependent changes in learning and memory. This study examined the effects of estrogen on memory, and how these effects alter the acute disruptive effects of Δ9‐THC (0.32–3.2 mg/kg) on memory in female rats. To do this, female rats were trained to respond under a repeated acquisition and delayed‐performance procedure and then ovariectomized. During the acquisition phase of this procedure, rats acquired a 4‐response sequence on three response keys (center, left and right), and sequences changed daily (CRLC, LRCL, RLCR, etc). Sequence acquisition was followed by a delay phase (1 minute to 24 hours), and a delayed‐performance phase in which rats were required to emit the previously acquired sequence. Responding in the acquisition and delayed‐performance phases was maintained under a second‐order fixed‐ratio 3 schedule of food presentation. Δ9‐THC or vehicle (control) was administered 30 minutes prior to the delayed‐performance phase, regardless of delay duration, in order to specifically assess memory, which was quantified by a percent savings measure. Response rate and the percentage of errors were also recorded. To further examine the effects of estrogen, OVX female rats were chronically administered estradiol using a subcutaneous capsule containing a 25/75% ratio of estradiol to cholesterol. The disruptive effects of Δ9‐THC on memory were assessed prior to and during estradiol administration. Prior to estradiol administration, increasing delays ranging from 1 minute to 24 hours produced delay‐dependent decreases in percent savings. When the delay was 1 hour, acute administration of Δ9‐THC produced dose‐dependent decreases in response rate and percent savings, and increases in percent errors, that were significantly attenuated by estradiol administration. Increasing the delay from 1 to 3 hours enhanced the effects of low doses of Δ9‐THC (e.g., 0.56 mg/kg) and produced a significant decrease in percent savings. These results demonstrate the delay‐dependent effects of Δ9‐THC on memory and the involvement of estrogen in modulating Δ9‐THC's acute memory‐disrupting effects in female rats.Support or Funding Information–NIH‐NIDA: R01‐DA037255‐02S1This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Chronic Δ9-THC in Rhesus Monkeys: Effects on Cognitive Performance and Dopamine D2/D3 Receptor Availability.

Cannabis-related impairments to cognitive function may represent novel therapeutic targets for cannabis-use disorder, although the nature, persistence, and reversibility of such deficits remain unclear. Adult male rhesus monkeys (N = 6) responded in the morning on tasks designed to assess different cognitive domains using the Cambridge Neuropsychological Test Automated Battery (CANTAB) touchscreens followed by responding maintained under a fixed-ratio (FR) 10 schedule of food presentation in different operant chambers. First, the acute effects of Δ9-tetrahydrocannabinol (THC; 0.01-0.56 mg/kg, i.v.) on cognitive performance, FR responding, and body temperature were determined. Next, THC (1.0-2.0 mg/kg, s.c.) was administered daily after FR 10 sessions for 12 weeks, during which the residual effects of THC (i.e., 22 hours after administration) on cognition were examined and the acute effects of THC were redetermined. In a subgroup of monkeys, dopamine D2/D3 receptor availability was assessed after 4 weeks of chronic THC exposure and compared with drug-naive controls using positron emission tomography and [11C]-raclopride (N = 4/group). Acute THC pretreatments dose-dependently decreased FR responding and body temperature, and impairment to cognitive performance was task specific. During chronic treatment, THC produced persistent residual impairment only to working memory; tolerance differentially developed to acute cognitive impairments. There was recovery from residual cognitive impairments to working memory within 2 weeks of abstinence. Compared with controls, D2/D3 receptor availability was not altered during chronic THC treatment. In conclusion, THC-induced disruptions in cognition were task-specific, as was tolerance development, and not related to changes in D2/D3 receptor availability. Intervention strategies for cannabis-use disorder that enhance working memory performance may facilitate positive treatment outcomes.

Atypical binding at dopamine and serotonin transporters contribute to the discriminative stimulus effects of mephedrone

Mephedrone (4-methylmethcathinone), a constituent of the recreational substances known as “bath salts”, is a synthetic cathinone that can produce auditory and visual hallucinations, as well as problematic cardiovascular effects. This study compared the discriminative stimulus effects of mephedrone (0.32–10 mg/kg) with other prototypical drugs of abuse: cocaine (0.56–32 mg/kg), d-amphetamine (0.18–3.2 mg/kg), ketamine (1.8–18 mg/kg), phencyclidine (PCP, 1–5.6 mg/kg), heroin (1–10 mg/kg), 2,5-dimethoxy-4-iodoamphetamine (R-DOI, 0.1–1 mg/kg), Δ9-tetrahydrocannabinol (Δ9−THC 0.56–5.6 mg/kg), 3,4-methylenedioxyamphetamine (MDA, 0.32–5.6 mg/kg), methylphenidate (1–10 mg/kg), and 3,4-methylenedioxypyrovalerone (MDPV, 0.56–5.6 mg/kg). The discriminative stimulus effects of mephedrone were also assessed after administration of the sigma receptor antagonist rimcazole (0.32–10 mg/kg), the relatively selective norepinephrine transporter (NET) inhibitor desipramine (1.8–18 mg/kg), and the selective serotonin transporter (SERT) inhibitor fluoxetine (1–18 mg/kg). Initially, rats were trained to discriminate an intraperitoneal injection of mephedrone (3.2 mg/kg) from saline under a fixed-ratio 20 schedule of food presentation. Following training, cumulative doses of mephedrone and the other drugs were administered to test for substitution (80% drug-lever responding). Of the drugs tested, including those that were tested in combination with mephedrone (i.e., rimcazole, desipramine, and fluoxetine), only cocaine fully substituted for mephedrone without substantially decreasing response rate. In addition, the three drugs administered in combination with mephedrone shifted the cumulative dose-effect curves leftward (percent drug-lever responding) and down (response rate), although fluoxetine did so in a dose-dependent manner ranging from antagonism to potentiation. In summary, the discriminative stimulus effects of mephedrone were most similar to those for the central nervous system (CNS) stimulant, cocaine, and SERT and DAT activity were necessary for these effects.

Interactive Effects of Hormone Status and Δ9‐THC Administration on Memory in Female Rats

The cannabinoid receptor agonist, Δ9‐tetrahydrocannabinol (Δ9‐THC), is a psychoactive constituent of marijuana, and a popular recreational drug that has been shown to produce disruptions in learning and memory. Conversely, estrogen has been found to produce a positive effect on learning and memory in female rats. The purpose of this study was to examine the effects of hormone status on memory, and how these effects alter the acute disruptive effects of Δ9‐THC (0.32–3.2 mg/kg) on memory in female rats. To do this, intact and ovariectomized (OVX) female rats were trained on a repeated acquisition and delayed‐performance procedure. During the acquisition phase of this procedure, rats acquired a 4‐response sequence (CRLC, LRCL, RLCR, etc.) that changed daily. Responding was maintained under a second‐order fixed‐ratio 3 schedule of food presentation. Sequence acquisition was followed by a delay phase and a delayed‐performance phase in which rats emitted the previously acquired sequence. Delay phase under baseline conditions was 1 hour. In order to examine the effect of delay, delays ranging from 1 minute to 24 hours were used. Δ9‐THC or vehicle was administered 30 minutes prior to the delayed‐performance phase in order to specifically target retention, which was assessed by percent savings. Response rate and the percentage of errors were also recorded. When the delay was varied under non‐drug conditions, delay‐dependent decreases in percent savings were observed in intact and OVX rats. However, the disruption of retention was greater in OVX than intact rats. Acute administration of Δ9‐THC produced dose‐dependent decreases in response rate and percent savings, and increases in percent errors in both intact and OVX rats under baseline conditions. After administration of 0.56 mg/kg Δ9‐THC, intact rats showed less sequence retention than OVX rats. Increasing the delay to 3 hours produced a leftward shift in the percent‐savings curve for both intact and OVX rats. These results suggest that hormone status directly impacts retention under an acquisition and delayed‐performance procedure, and acute administration of Δ9‐THC produces both delay‐dependent and dose‐dependent effects on memory in intact and OVX female rats.Support or Funding InformationNIH grant R01‐DA037255‐02S1

Behavioral Determinants of Cannabinoid Self-Administration in Old World Monkeys.

Reinforcing effects of Δ9-tetrahydrocannabinol (THC), the primary active ingredient in marijuana, as assessed with self-administration (SA), has only been established in New World primates (squirrel monkeys). The objective of this study was to investigate some experimental factors that may enhance intravenous SA of THC and the cannabinoid receptor (CBR) agonist CP 55 940 in Old World monkeys (rhesus and cynomolgus), a species that has been used extensively in biomedical research. In one experiment, male rhesus monkeys (N=9) were trained to respond under a fixed-ratio 10 schedule of food presentation. The effects of CP 55 940 (1.0-10 μg/kg, i.v.) and THC (3.0-300 μg/kg, i.v.) on food-maintained responding and body temperature were determined in these subjects prior to giving them access to self-administer each drug. Both drugs dose-dependently decreased food-maintained responding. CP 55 940 (0.001-3.0 μg/kg) functioned as a reinforcer in three monkeys, whereas THC (0.01-10 μg/kg) did not have reinforcing effects in any subject. CP 55 940 was least potent to decrease food-maintained responding in the monkeys in which CP 55 940 functioned as a reinforcer. Next, THC was administered daily to monkeys until tolerance developed to rate-decreasing effects. When THC SA was reexamined, it functioned as a reinforcer in three monkeys. In a group of cocaine-experienced male cynomolgus monkeys (N=4), THC SA was examined under a second-order schedule of reinforcement; THC functioned as reinforcer in two monkeys. These data suggest that SA of CBR agonists may be relatively independent of their rate-decreasing effects in Old World monkeys. Understanding individual differences in vulnerability to THC SA may lead to novel treatment strategies for marijuana abuse.

Discriminative Stimulus Effects of Mephedrone, A Constituent of “Bath salts”

Mephedrone (4‐methylmethcathinone) is one of the major constituents of the recreational substances known as “bath salts”. It is a synthetic cathinone and psychostimulant that has been reported to cause auditory and visual hallucinations, as well as problematic cardiovascular effects. The purpose of this study was to compare the discriminative stimulus effects of mephedrone (0.32–10 mg/kg) alone with other prototypical drugs of abuse, such as cocaine (0.56–32 mg/kg), delta‐9‐tetrahydrocannabinol (THC, 0.56–10 mg/kg), ketamine (1.8–18 mg/kg), phencyclidine (PCP, 1–5.6 mg/kg), 2,5‐dimethoxy‐4‐iodoamphetamine (R‐DOI, 0.1‐1 mg/kg), heroin (1–10 mg/kg) and d‐amphetamine (0.18–3.2 mg/kg). The discriminative stimulus effects of mephedrone were also examined after the administration of rimcazole (0.32–10 mg/kg), a sigma receptor antagonist, and desipramine (1.8–18 mg/kg), a tricyclic antidepressant with relatively selective affinity for norepinephrine transporters (NET). Rats were trained to discriminate an intraperitoneal injection of mephedrone from saline under a fixed‐ratio 20 schedule of food presentation. The training dose of mephedrone (3.2 mg/kg) was reliably discriminated from saline by all subjects. Following training, substitution of varying doses of mephedrone produced dose‐dependent increases in mephedrone‐lever responding, with full substitution considered to be greater than 80% responding on this lever. Of the drugs tested, only cocaine and THC fully substituted for mephedrone. However, full substitution of THC occurred in only 2 of 9 animals and at a dose that substantially decreased response rate. Both THC and R‐DOI produced dose‐dependent rate‐decreasing effects (≥ 20% decrease in response rate compared to vehicle). In addition, although rimcazole and desipramine did not substitute for mephedrone, both drugs potentiated mephedrone's discriminative stimulus effects by producing leftward shifts of the dose‐effect curve for mephedrone‐lever responding. In summary, the discriminative stimulus effects of mephedrone most closely approximate those for the stimulant, cocaine, and these effects may be mediated, in part, by the sigma receptor and NET.

The Discriminative‐Stimulus Effects of Mephedrone in Comparison with Other Drugs of Abuse

Mephedrone (4‐methylmethcathinone) is one of the major constituents of the recreational drug, “bath salts”. It is a synthetic cathinone and potent psychostimulant that has been reported to cause auditory and visual hallucinations. The purpose of this study was to compare the discriminative stimulus effects of mephedrone alone with those of other prototypical drugs of abuse, such as cocaine, delta‐9‐tetrahydrocannabinol (THC), phencyclidine (PCP), and 2,5‐dimethoxy‐4‐iodoamphetamine (DOI). Rats were trained to discriminate an intraperitoneal injection of mephedrone from saline under a fixed‐ratio 20 schedule of food presentation. The final training dose of mephedrone was 3.2 mg/kg and it was reliably discriminated from saline by all subjects. Following training, substitution tests indicated that cocaine (0.56‐32 mg/kg) fully substituted for mephedrone (0.32‐10 mg/kg) by producing over 80% mephedrone‐lever responding, whereas THC (0.56‐10 mg/kg), PCP (1‐5.6 mg/kg) and DOI(0.1‐1 mg/kg) produced a maximum of 73%, 30% and 50% mephedrone‐lever responding, respectively. Only THC produced dose‐dependent rate‐decreasing effects (蠅 20% decrease in response rate compared to THC vehicle). These results indicate that the discriminative stimulus effects of mephedrone most closely approximate those for the stimulant drug, cocaine, and are only modestly similar to THC.

The Discriminative Stimulus Effects of Nicotine, Epibatidine, and Varenicline in Mice: Involvement of β2 Containing Nicotinic Acetylcholine Receptor Subtypes

Nicotine is a high efficacy β2 containing nicotinic acetylcholine receptor (β2 nAChR) agonist, whereas varenicline has low efficacy at β2 nAChRs. To better understand the receptor pharmacology of varenicline, male C57BL/6J mice (n=8) were trained to discriminate varenicline (3.2 mg/kg s.c.) under an FR10 schedule of food presentation. Separate groups of mice (n=8 per group) were trained to discriminate nicotine (1 mg/kg base weight s.c.) or the β2 nAChR agonist epibatidine (0.0032 mg/kg s.c.). Mice trained with nicotine and epibatidine satisfied the test criteria after a mean of 87 and 89 sessions, respectively, whereas varenicline‐trained mice required significantly more (i.e., 113) sessions. Varenicline was less effective than nicotine and epibatidine in maintaining stimulus control as evidenced by the number of training sessions required between tests. The nicotine and epibatidine dose‐effect functions did not vary as a function of training drug; the functions were steep and characterized by low inter‐subject variability. In contrast, the varenicline dose‐effect functions were shallow and highly variable among mice. Varenicline produced a maximum 63% and 61% drug‐appropriate responding in the nicotine and epibatidine discriminations, respectively. The nAChR antagonist mecamylamine (3.2 mg/kg) antagonized the effects of nicotine, epibatidine, and varenicline. In contrast, the β2 antagonist DHβE (3.2 mg/kg) antagonized the effects of nicotine and epibatidine, but not varenicline. These results suggest that the discriminative stimulus effects of varenicline are not as robust as those of nicotine and epibatidine and are not mediated by β2 nAChRs.*Supported by USPHS grant DA25267

Role of gamma-aminobutyric acid type A (GABAA) receptor subtypes in acute benzodiazepine physical dependence-like effects: evidence from squirrel monkeys responding under a schedule of food presentation

Assays of schedule-controlled responding can be used to characterize the pharmacology of benzodiazepines and other GABAA receptor modulators, and are sensitive to changes in drug effects that are related to physical dependence. The present study used this approach to investigate the role of GABAA receptor subtypes in mediating dependence-like effects following benzodiazepine administration. Squirrel monkeys (n = 6) were trained on a fixed-ratio schedule of food reinforcement. Initially, the response rate-decreasing effects of chlordiazepoxide (0.1-10mg/kg; nonselective GABAA receptor agonist), zolpidem (0.032-1.0mg/kg; α1 subunit-containing GABAA subtype-preferring agonist), and HZ-166 (0.1-10mg/kg; functionally selective α2 and α3 subunit-containing GABAA receptor agonist) were assessed. Next, acute dependence-like effects following single injections of chlordiazepoxide, zolpidem, and HZ-166 were assessed with flumazenil (0.1-3.2mg/kg; nonselective GABAA receptor antagonist). Finally, acute dependence-like effects following zolpidem administration were assessed with βCCt and 3-PBC (0.1-3.2mg/kg and 0.32-10mg/kg, respectively; α1 subunit-containing GABAA receptor antagonists). Chlordiazepoxide, zolpidem, and HZ-166 produced dose- and time-dependent decreases in response rates, whereas flumazenil, βCCT, and 3-PBC were ineffective. After the drug effects waned, flumazenil produced dose-dependent decreases in response rates following administration of 10mg/kg chlordiazepoxide and 1.0mg/kg zolpidem, but not following any dose of HZ-166. Further, both βCCT and 3-PBC produced dose-dependent decreases in response rates when administered after 1.0mg/kg zolpidem. These data raise the possibility that α1 subunit-containing GABAA receptors play a major role in physical dependence-related behaviors following a single injection of a benzodiazepine.