Concurrent progressive-ratio schedules to compare reinforcing effectiveness of different phencyclidine (PCP) concentrations in rhesus monkeys

The Abuse Potential of Novel Synthetic Phencyclidine Derivative 1-(1-(4-Fluorophenyl)Cyclohexyl)Piperidine (4'-F-PCP) in Rodents

The dissociative anesthetic phencyclidine (PCP) and PCP derivatives, including 4′-F-PCP, are illegally sold and abused worldwide for recreational and non-medical uses. The psychopharmacological properties and abuse potential of 4′-F-PCP have not been fully characterized. In this study, we evaluated the psychomotor, rewarding, and reinforcing properties of 4′-F-PCP using the open-field test, conditioned place preference (CPP), and self-administration paradigms in rodents. Using Western immunoblotting, we also investigated the expression of dopamine (DA)-related proteins and DA-receptor-mediated downstream signaling cascades in the nucleus accumbens (NAc) of 4′-F-PCP-self-administering rats. Intraperitoneal administration of 10 mg/kg 4′-F-PCP significantly increased locomotor and rearing activities and increased CPP in mice. Intravenous administration of 1.0 mg/kg/infusion of 4′-F-PCP significantly enhanced self-administration during a 2 h session under fixed ratio schedules, showed a higher breakpoint during a 6 h session under progressive ratio schedules of reinforcement, and significantly altered the expression of DA transporter and DA D1 receptor in the NAc of rats self-administering 1.0 mg/kg 4′-F-PCP. Additionally, the expression of phosphorylated (p) ERK, pCREB, c-Fos, and FosB/ΔFosB in the NAc was significantly enhanced by 1.0 mg/kg 4′-F-PCP self-administration. Taken together, these findings suggest that 4′-F-PCP has a high potential for abuse, given its robust psychomotor, rewarding, and reinforcing properties via activation of DAergic neurotransmission and the downstream signaling pathways in the NAc.

Determinants of conditioned reinforcing effectiveness: Dopamine D2-like receptor agonist-stimulated responding for cocaine-associated stimuli

Environmental stimuli associated with drug use can take on conditioned properties capable of promoting drug-seeking behaviors during abstinence. This study investigated the relative importance of the amount of reinforced responding, number of cocaine-stimulus pairings, total cocaine intake, and reinforcing effectiveness of the self-administered dose of cocaine to the conditioned reinforcing effectiveness of cocaine-associated stimuli (CS). Male rats were trained to self-administer cocaine (0.1 [small] or 1.0mg/kg/inf [large]) under a fixed ratio schedule of reinforcement. A progressive ratio (PR) schedule was used to quantify the reinforcing effectiveness of each dose of cocaine, as well as the conditioned reinforcing effectiveness of the CS following treatment with saline or the dopamine D2-like receptor agonist pramipexole (0.1-3.2mg/kg). The large dose of cocaine maintained larger final ratios and greater levels of cocaine intake, whereas the small dose resulted in more cocaine-CS pairings. The total amount of responding was comparable between groups. During PR tests of conditioned reinforcement, pramipexole increased responding for CS presentations in both groups; however, the final ratio completed was significantly greater in large- as compared to small-dose group. In addition to highlighting a central role for dopamine D2-like receptors in modulating the effectiveness of cocaine-paired stimuli to reinforce behavior, these results suggest that conditioned reinforcing effectiveness is primarily determined by the reinforcing effectiveness of the self-administered dose of cocaine and/or total cocaine intake, and not the total amount of responding or number cocaine-stimulus pairings. These findings have implications for understanding how different patterns of drug-taking might impact vulnerability to relapse.

Measurement of food reinforcement in preschool children. Associations with food intake, BMI, and reward sensitivity

Progressive ratio (PR) schedules of reinforcement have been used to measure the relative reinforcing value (RRV) of food in humans as young as 8 years old; however, developmentally appropriate measures are needed to measure RRV of food earlier in life. Study objectives were to demonstrate the validity of the RRV of food task adapted for use among for preschool children (3-5 y), and examine individual differences in performance. Thirty-three children completed the RRV of food task in which they worked to access graham crackers. They also completed a snack task where they had free access these foods, liking and hunger assessments, and their heights and weights were measured. Parents reported on their child's reward sensitivity. Overall, children were willing work for palatable snack foods. Boys and older children made more responses in the task, while children with higher BMI z-scores and reward sensitivity responded at a faster rate. Children who worked harder in terms of total responses and response rates consumed more calories in the snack session. This study demonstrates that with slight modifications, the RRV of food task is a valid and developmentally appropriate measure for assessing individual differences in food reinforcement among very young children.

Effects of altering reinforcer magnitude and reinforcement schedule on phencyclidine (PCP) self-administration in monkeys using an adjusting delay task

Compared to nondrug reinforcers, few studies have examined delay discounting for drug reinforcers. The purpose of the present study was to examine delay discounting in rhesus monkeys using orally delivered phencyclidine (PCP) as the reinforcer and to examine the effects of manipulating reinforcer magnitude and cost on delay discounting for PCP using an adjusting delay task. Monkeys could choose between a single delivery of PCP available immediately or a bundle of PCP deliveries available following a titrated delay. The average of the delays, or the mean adjusted delay (MAD), served as the quantitative measure of delay discounting. In Experiment 1, reinforcer magnitude was manipulated by varying the PCP concentration and the size of the delayed reinforcer (6 or 12 deliveries). The concentration-effect curve for PCP deliveries assumed an inverted U-shaped function, but varying PCP concentration had little effect on MAD values or the choice between immediate and delayed reinforcers. Increasing the size of the delayed reinforcer produced an upward and leftward shift in the concentration-effect curve. In Experiment 2, the cost of reinforcers was manipulated by increasing the fixed ratio (FR) requirement for each choice. Increasing the FR led to increased MAD values and decreased PCP self-administration.

Social stimuli enhance phencyclidine (PCP) self-administration in rhesus monkeys

Environmental factors, including social interaction, can alter the effects of drugs of abuse on behavior. The present study was conducted to examine the effects of social stimuli on oral phencyclidine (PCP) self-administration by rhesus monkeys. Ten adult rhesus monkeys (M. mulatta) were housed side by side in modular cages that could be configured to provide visual, auditory, and olfactory stimuli provided by another monkey located in the other side of a paired unit. During the first experiment, monkeys self-administered PCP (0.25 mg/ml) and water under concurrent fixed ratio (FR) 16 schedules of reinforcement with either a solid or a grid (social) partition separating each pair of monkeys. In the second experiment, a PCP concentration-response relationship was determined under concurrent progressive ratio (PR) schedules of reinforcement during both the solid and grid partition conditions. Under the concurrent FR 16 schedules, PCP and water self-administration were significantly higher during exposure to a cage mate through a grid partition than when a solid partition separated the monkeys. The relative reinforcing strength of PCP, as measured by PR break points, was greater during the grid partition condition compared to the solid partition condition indicated by an upward shift in the concentration-response curve. To determine whether the social stimuli provided by another monkey led to activation of the hypothalamic-pituitary-adrenal (HPA) axis, which may have evoked the increase of PCP self-administration during the grid partition condition, a third experiment was conducted to examine cortisol levels under the two housing conditions. A modest, but nonsignificant increase in cortisol levels was found upon switching from the solid to the grid partition condition. The results suggest that social stimulation among monkeys in adjoining cages leads to enhanced reinforcing strength of PCP.

Sex differences in the escalation of oral phencyclidine (PCP) self-administration under FR and PR schedules in rhesus monkeys

RATIONALE

Studies with male rats indicate that long access (LgA) vs short access (ShA) to i.v. cocaine and heroin self-administration leads to an escalation of drug intake and a subsequent upward shift of the dose-response function.

OBJECTIVE

The purpose of this experiment was to extend these results to male and female rhesus monkeys and oral phencyclidine (PCP) self-administration under fixed-ratio (FR) and progressive-ratio (PR) schedules.

METHODS

Adult rhesus monkeys (seven females and nine males) orally self-administered PCP (0.25 mg/ml) and water under concurrent FR 16 FR 16 schedules during daily ShA 3-h sessions. Since females weighed less than males, each liquid delivery (0.6 ml) represented a higher unit dose mg/kg for females than males, but drug concentration mg/ml remained constant. Concurrent PR PR schedules were then used to obtain a concentration-response function (0.125, 0.25, 0.5, and 1.0 mg/ml). Next, PCP and water were available during LgA 6-h sessions under concurrent FR 16 FR 16 schedules for 21 days. The monkeys were then retested under the concurrent FR 16 FR 16 and PR PR conditions during ShA sessions.

RESULTS

Under the initial ShA concurrent FR 16 FR 16 schedules, females and males did not differ on PCP deliveries or intake (mg/kg); however, during LgA, males and females had more PCP deliveries compared with ShA. During LgA, males exceeded females in PCP deliveries, but females were higher than males in mg/kg PCP intake. Also, PCP (but not water) deliveries and mg/kg PCP intake significantly increased from the first 3 days to the last 3 days of the 21-day LgA period in both males and females. The subsequent ShA FR 16 FR 16 performance did not differ by sex, but it was significantly elevated above the first ShA period in both sexes. The concentration-response function for PCP break point under the PR PR schedules and PCP intake (mg/kg) were significantly shifted upward during the second (vs first) ShA period, and females' mg/kg intake significantly exceeded males'.

CONCLUSIONS

Male and female rhesus monkeys both showed escalation of PCP self-administration during LgA to PCP and during ShA that occurred after (vs before) LgA. Both showed vertical upward shifts in the concentration x intake (mg/kg) function under the PR schedule, and females exceeded males on this measure. These findings with PCP and monkeys are consistent with vertical upward shifts of cocaine dose-response functions in previous escalation studies in male rats and reports of sex differences (F>M) during several other phases of drug abuse.

Positron emission tomography studies of potential mechanisms underlying phencyclidine-induced alterations in striatal dopamine

Positron emission tomography (PET), in combination with (11)C-raclopride, was used to examine the effects of phencyclidine (PCP) on dopamine (DA) in the primate striatum. In addition, we explored the hypotheses that GABAergic pathways as well as molecular targets beyond the N-methyl-D-aspartate (NMDA) receptor complex (ie dopamine transporter proteins, DAT) contribute to PCP's effects. In the first series of experiments, (11)C-raclopride was administered at baseline and 30 min following intravenous PCP administration. In the second series of studies, gamma-vinyl GABA (GVG) was used to assess whether enhanced GABAergic tone altered NMDA antagonist-induced changes in DA. Animals received an initial PET scan followed by pretreatment with GVG (300 mg/kg), then PCP 30 min prior to a second scan. Finally, we explored the possible contributions of DAT blockade to PCP-induced increases in DA. By examining (11)C-cocaine binding a paradigm in which PCP was coadministered with the radiotracer, we assessed the direct competition between these two compounds for the DAT. At 0.1, 0.5, and 1.0 mg/kg, PCP decreased (11)C-raclopride binding by 2.1, 14.9+/-2.2 and 8.18+/-1.1%, respectively. These effects were completely attenuated by GVG (3.38+/-3.1% decrease in (11)C-raclopride binding). Finally, PCP (0.5 mg/kg) decreased (11)C-cocaine binding by 25.5+/-4.3%, while at 1.0 mg/kg this decrease was 13.5%, consistent with a competitive interaction at the DAT. These results suggest that PCP may be exerting some direct effects through the DAT and that GABA partially modulates NMDA-antagonist-induced increases in striatal DA.

NMDA antagonist effects on striatal dopamine release: microdialysis studies in awake monkeys

Brain imaging studies have suggested that the NMDA antagonist ketamine is as potent a releaser of striatal dopamine as amphetamine. This conclusion contradicts microdialysis findings in the rodent that NMDA antagonists, in contrast to amphetamine, have little or no effect on striatal dopamine release. The present study addressed two mechanisms that could account for this discrepancy: 1) whether there is a species difference, i.e., rodents vs. primates, in the responsivity of striatal dopamine to NMDA antagonists, and 2) whether rapid uptake of dopamine prevents reliable measures of synaptic dopamine release by microdialysis in response to NMDA antagonists. MRI-directed in vivo microdialysis was used to compare the effects of psychotomimetic NMDA antagonists phencyclidine (PCP), ketamine, and amphetamine on extracellular striatal dopamine levels in awake rhesus monkeys. The effect of PCP was also investigated in the presence of intrastriatally applied nomifensine, a dopamine uptake blocker. Amphetamine (0.1 or 0.4 mg/kg) produced robust and dose-dependent increases in dopamine release ranging 2-10-fold above baseline. PCP at 0.1 mg/kg had no effect and at 0.3 mg/kg produced a small 50% increase over baseline. Ketamine, at the relatively high dose of 5 mg/kg, produced only a 30% increase in dopamine release. Intrastriatal application of nomifensine did not influence the effect of PCP, suggesting that rapid uptake of dopamine is not preventing the detection of a PCP-induced increase in dopamine release. These findings suggest that in the primate, ketamine and PCP are not effective dopamine releasers, as has been suggested by previous imaging studies.

Oral drug self-administration: an overview of laboratory animal studies

Many abused drugs can be established as orally delivered reinforcers for rhesus monkeys and other animals. Benzodiazepines, barbiturates, opioids, psychomotor stimulants, dissociative anesthetics, and ethanol can come to serve as reinforcers when taken by mouth. The principal problems in establishing drugs as reinforcers by the oral route of administration are (1) aversive taste, (2) delay in onset of central nervous system effects, and (3) consumption of low volumes of drug solution. Strategies have been devised to successfully overcome these problems, and orally delivered drugs can be established as effective reinforcers. Reinforcing actions are demonstrated by consumption of greater volumes of drug solution than volumes of the water vehicle, and supporting evidence for reinforcing effects consists of the maintenance of behavior under intermittent schedules of reinforcement and the generation of orderly dose-response functions. This article presents an overview of studies of behavior reinforced by oral drug reinforcement. Factors that control oral drug intake include dose, schedule of reinforcement, food restriction, and alternative reinforcers. Many drugs, administered by the experimenter, can alter oral drug reinforcement. Relative reinforcing effects can be assessed by choice procedures and by persistence of behavior across increases in schedule size. In general, reinforcing effects increase directly with dose. Rhesus monkeys prefer combinations of reinforcing drugs to the component drugs. The taste of drug solutions may act as a conditioned reinforcer and a discriminative stimulus. Consequences of drug intake include tolerance and physiological dependence. Findings with orally self-administered drugs are similar to many findings with other positive reinforcers, including intravenously self-administered drugs.