Interactive effects of (±)-trans-U50488 and its stereoisomers with cannabinoids.

The adverse effects of mu opioid agonists have spurred a renewed interest in using kappa opioid receptor (KOR) agonists as analgesics. KOR agonists also have potential for development as diuretics for the treatment of edema and hypertension. Here, we evaluated the discriminative stimulus, antinociceptive, and diuretic effects of the kappa agonist (±)-trans-U-50488 and its stereoisomers (-)-(1S,2S)-U-50488 or (+)-(1R,2R)-U-50488) alone and in combination with the cannabinoid agonist (-)-CP 55,940. To establish (±)-U-50488 as a discriminative stimulus, rats (n = 12) were trained to discriminate intraperitoneal (i.p.) administration of 5.6 mg/kg of (±)-trans-U-50488 from saline under a fixed-ratio 20 (FR-20) schedule of food reinforcement. Then, antinociception was assessed using two procedures: warm water tail withdrawal and von Frey paw withdrawal. Diuretic effects were assessed in separate rats (n = 6/group). Doses of (±)-U-50488 and (-)-U-50488 that served as discriminative stimuli produced significant increases in urine output, but at lower doses than those that produced antinociception. In contrast, (+)-U-50488 alone had no discriminative stimulus or diuretic effects at the doses tested, but did produce antinociception in the von Frey assay. When three cannabinoids and morphine were tested in the (±)-U-50488 discrimination procedure to determine the similarity of these drugs' discriminative stimulus effects to those for (±)-U-50488, the rank order similarity was (-)-CP 55,940 > (-)-trans-THC > (+)-WIN 55,212-2 ≥ morphine. (-)-CP 55,940 alone (0.056 mg/kg) partially substituted for the discriminative stimulus effects of (±)-U-50488 and produced significant diuretic and antinociceptive effects. (-)-CP 55,940 in combination with (±)-U-50488 also produced a two-fold leftward shift in the discriminative stimulus curve for (±)-U-50488, and near-additive antinociception with (±)-U-50488 and (+)-U-50488. Further, the diuretic effect of (-)-CP 55,940 was enhanced by a dose of (+)-U50488, which itself did not alter urine output. These data together indicate that a combination of cannabinoid and kappa opioid agonists can enhance diuresis, but may have limited potential for serving as opioid-sparing pharmacotherapeutics for treatment of pain.

Effects of noncontingent ethanol, DHEA, and pregnanolone administration on ethanol self-administration in outbred female rats.

Previous research from this laboratory demonstrated that male outbred rats (Long-Evans) can be trained to prefer ethanol (10% v/v) over water during 30-min home-cage sessions and that higher ethanol concentrations (18-32% v/v) can serve as a reinforcer under various operant schedules. Further, we have shown that two neurosteroids, dehydroepiandrosterone (DHEA) and pregnanolone, can readily decrease ethanol self-administration in males. The present study used the same procedures in an attempt to systematically replicate the previous findings in female outbred rats. Rats were first trained to self-administer ethanol in the home cage using a saccharin-fading procedure. Subsequently, a two-bottle preference test was initiated by substituting different ethanol concentrations after subjects reliably consumed 10% ethanol alone. Water was always available during this phase. Next, subjects were transitioned to a fixed-ratio 10 (FR-10) schedule of reinforcement with 0.1 mL of ethanol (18% v/v) serving as the reinforcer so that a concentration-effect curve could be established. Upon completion, subjects were transitioned to an FR-10 FR-20 multiple schedule of ethanol (32% v/v) and food reinforcement to determine whether noncontingent ethanol, DHEA, and pregnanolone could selectively decrease ethanol intake. Not surprisingly, female subjects preferentially consumed ethanol over water at concentrations of 3.2-18% (v/v) during the home-cage procedure, and significantly increased the mean dose of ethanol consumed and blood ethanol concentration (BEC). Similarly, increasing concentrations under an FR-10 schedule significantly increased the dose of ethanol presented and BEC compared to control (water). Finally, under the multiple schedule, noncontingent injections of ethanol (0.32-1.8 g/kg), DHEA (10-100 mg/kg), and pregnanolone (1.8-32 mg/kg) dose-dependently decreased food- and ethanol-maintained responding and the dose of ethanol presented. BEC was significantly decreased by the neurosteroids, but increased by ethanol due to its noncontingent administration. Together, these data replicate only a subset of the data previously obtained in males, suggesting there are sex differences particularly with respect to the effects of DHEA and pregnanolone.

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

RATIONALE: 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. OBJECTIVE: 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. METHODS: Rats were trained to discriminate mephedrone (3.2 mg/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 (5.6-32 mg/kg), cocaine (1.8-18 mg/kg), morphine (0.56-10 mg/kg), and amitriptyline (3.2-32 mg/kg). In addition, cocaine (3.2-18 mg/kg) and the serotonin type-2 (5-HT2) receptor antagonist ritanserin (1, 3.2, and 10 mg/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. RESULTS: 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. CONCLUSION: 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.

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.