The self-administration of WIN 35,428 and cocaine: comparisons of satiety threshold and elimination half-life in rats

The Effects of Drugs on Behavior Maintained by Social Contact: Role of Monoamines in Social Reinforcement

Drug use is highly concordant among members of adolescent and young adult peer groups. One potential explanation for this observation is that drugs may increase the reinforcing effects of social contact, leading to greater motivation to establish and maintain contact with other members of the peer group. Several classes of drugs, particularly drugs that increase synaptic dopamine, increase the reinforcing effects of contextual stimuli, but the extent to which these drugs enhance the reinforcing effects of social contact is not known. The purpose of this study was to determine the extent to which drugs that increase synaptic dopamine, norepinephrine, and serotonin enhance the positive reinforcing effects of social contact. To this end, male and female Long-Evans rats were pretreated with acute doses of the selective dopamine reuptake inhibitor, WIN-35,428, the selective norepinephrine reuptake inhibitor, atomoxetine, the selective serotonin reuptake inhibitor, fluoxetine, the non-selective monoamine reuptake inhibitor, cocaine, and the non-selective monoamine releasers d-amphetamine and (±)-MDMA. Ten minutes later, the positive reinforcing effects of 30-s access to a same-sex social partner was examined on a progressive ratio schedule of reinforcement. To determine whether the reinforcement-altering effects of these drugs were specific to the social stimulus, the reinforcing effects of a non-social stimulus (30-s access to an athletic sock of similar size and coloring as another rat) was determined in control subjects. WIN-35,428, d-amphetamine, and cocaine, but not atomoxetine, fluoxetine, or MDMA, dose-dependently increased breakpoints maintained by a social partner under conditions in which responding maintained by a non-social stimulus was not affected. These data indicate that increases in extracellular dopamine, but not extracellular norepinephrine or serotonin, increases the positive reinforcing effects of social contact in both male and female rats. These data also provide support for the hypothesis that some drugs with high abuse liability increase the motivation to establish and maintain contact with social peers.

The compulsion zone explains the self-administration of cocaine, RTI-55 and bupropion in rats

Rats that reliably self-administered cocaine also reliably self-administered the cocaine analog RTI-55 and bupropion. The inter-injection intervals of these dopamine transporter (DAT) inhibitors were regular at a given unit dose and increased as a function of unit dose. However, the mean rate of intake differed widely, ranging from 731 to 459 to 2.1 nmol/kg∙min^-1 for bupropion, cocaine and RTI-55 respectively, a dramatic 348-fold range. An analysis of inter-injection intervals as a function of unit dose generated values for the mean satiety threshold of 50.6, 5.1 and 0.7 nmol/kg and t_1/2 of 56.7, 9.3 and 255.6 min for bupropion, cocaine and RTI-55, respectively. The difference in rate of intake of bupropion and RTI-55 relative to cocaine is a product of their 0.1 and 7.3 fold difference in PD potency and their 6.1 and 27.5 fold difference in t_1/2. Additionally, the relative durations of lever-pressing following termination of drug access correlated with the t_1/2 estimates. It is hypothesized this duration represents the time required for the drug concentration to fall from the satiety threshold below the priming threshold (the minimum DAT inhibitor level that will induce lever-pressing). This indicates that the time needed for an animal to cease lever pressing following termination of access to the DAT inhibitor is predominately a function of the PK properties of the agonist. The self-administration behavior paradigm in the context of the compulsion zone theory can be used as a bioassay to determine the PK/PD properties of indirect dopamine receptor agonists.

Effects of 5-HT2A receptor stimulation on economic demand for fentanyl after intermittent and continuous access self-administration in male rats

The relative value of and motivation for abused drugs often increases with drug experience and differs based on drug availability. Here, we determined how different intake patterns of fentanyl, a μ-opioid agonist, alter economic demand for fentanyl and how 5-HT_2A receptor stimulation affects economic demand for fentanyl. We used a within-session demand threshold procedure to characterize changes in economic demand for fentanyl before and after intermittent or continuous access schedules. We subsequently tested the acute effects of 5-HT_2A receptor stimulation with psychedelic 2,5-dimethoxy-4-iodoamphetamine (DOI) on economic demand for fentanyl. Extended fentanyl experience with both intermittent and continuous schedules increased fentanyl consumption at low cost (Q₀ ), increased total fentanyl consumption, and decreased demand elasticity (α), indicating both schedules elevated motivation to self-administer fentanyl. Overall, the two schedules produced similar alterations in economic demand for fentanyl, although low-cost consumption (Q₀ ) increased more in the continuous access group. Systemic injections of DOI (0.0-0.4 mg/kg, i.p.) dose-dependently produced economic demand changes in the opposite direction produced by fentanyl experience. DOI decreased motivation (increased "α"), decreased Q₀ , and decreased total fentanyl consumption. The selective 5-HT_2A antagonist, M100907 (0.3 mg/kg, i.p.), blocked the effects of DOI, indicating that DOI is acting through 5-HT_2A receptors to alter economic demand for fentanyl. In an economic food demand experiment, DOI (0.4 mg/kg) also increased demand elasticity and reduced food consumption. These results demonstrate that both intermittent and continuous fentanyl experience raise the economic demand for fentanyl, and acute 5-HT_2A receptor activation reduces economic demand for fentanyl and food.

Withdrawal from extended-access cocaine self-administration results in dysregulated functional activity and altered locomotor activity in rats

Much work has focused on determining the consequences of cocaine self-administration on specific neurotransmitter systems, thus neglecting the global changes that occur. Previous imaging studies have focused on the effects of cocaine self-administration in the presence of high blood levels of cocaine, but have not determined the functional effects of cocaine self-administration after cocaine has cleared. Extended-access cocaine self-administration, where animals administer cocaine for 6 h each day, results in escalation in the rate of cocaine intake and is believed to model the transition from recreational use to addiction in humans. We aimed to determine the functional changes following acute (48 h) withdrawal from an extended-access, defined-intake self-administration paradigm (5 days, 40 injections/day, 6 h/day), a time point when behavioral changes are present. Using the 2-[(14) C]deoxyglucose method to measure rates of local cerebral glucose metabolism, an indicator of functional activity, we found reductions in circuits related to learning and memory, attention, sleep, and reward processing, which have important clinical implications for cocaine addiction. Additionally, lower levels of functional activity were found in the dorsal raphe and locus coeruleus, suggesting that cocaine self-administration may have broader effects on brain function than previously noted. These widespread neurochemical reductions were concomitant with substantial behavioral differences in these animals, highlighted by increased vertical activity and decreased stereotypy. These data demonstrate that behavioral and neurochemical impairments following cocaine self-administration are present in the absence of drug and persist after cocaine has been cleared.

Evidence for learned skill during cocaine self-administration in rats

RATIONALE

It has been proposed that cocaine abuse results in skilled or "automatic" drug-taking behaviors. Brain regions important for skill learning are implicated in cocaine self-administration. However, the development of skill during self-administration has not been investigated.

OBJECTIVES

The present experiment investigated the development of skilled self-administration over extended drug use by employing a novel operant vertical head movement under discriminative stimulus (S(D)) control. In addition, the capacity of the head movement to serve as an operant was tested by manipulating drug levels above or below satiety drug levels via frequent noncontingent microinfusions (0.2 s) of cocaine.

RESULTS

Animals acquired the vertical head movement operant, which increased in number over days. Task learning was demonstrated by reduced reaction time in response to the S(D), increased propensity to self-administer upon S(D) presentation, and escalated drug consumption over days. Skill learning was demonstrated by (1) an increase over days in the velocity of operant movements, as a function of shorter duration but not altered distance, and (2) an increase over days in the probability of initiating the operant at the optimal starting position. Evidence that responding was specific to self-administration was revealed during periods of experimenter-manipulated drug level: maintaining drug levels above satiety decreased responding while maintaining drug levels below satiety increased responding.

CONCLUSIONS

Under the specific set of circumstances tested herein, cocaine self-administration became skilled over extended drug use. The vertical head movement can be used as an operant comparable to lever pressing with the additional benefit of quantifying skill learning.

Regulation of cocaine self-administration in humans: lack of evidence for loading and maintenance phases

BACKGROUND

In rodents, cocaine self-administration under a fixed-ratio schedule and with timeout intervals limited to the duration of the infusions is characterized by an initial burst of drug intake (loading) followed by more stable infusion rates (maintenance). We sought to examine whether similar phases might characterize self-regulated cocaine use in humans.

METHODS

31 Non-treatment seeking, cocaine dependent subjects participated in three (8, 16, and 32 mg/70 kg/infusion), self-regulated, 2-h cocaine self-administration sessions under a fixed-ratio 1, 5-min timeout schedule. Data were assessed for visual (e.g., by graphs of cumulative numbers of infusions) and statistical evidence of change in phase (by step-function analyses of individual infusion rates).

RESULTS

Graphs of cumulative infusions over time suggested a single, linear rate of self-administration over 2h at each cocaine dose. Statistical analyses of infusion data by generalized estimating equation (GEE) models also failed to support a loading/maintenance pattern (suggesting, if anything, the possibility of increasing infusion rates over time).

CONCLUSIONS

Our findings fail to support the existence of distinct loading and maintenance phases of self-regulated cocaine administration in humans at behaviorally relevant doses. Several factors may account for these observations including differences between humans and rodents in self-regulated drug intake.

The compulsion zone: a pharmacological theory of acquired cocaine self-administration

In rats trained to reliably self-administer cocaine, the cumulative drug level was calculated during sessions in which cocaine was administered either contingently or non-contingently. During both types of sessions a high rate of responding was observed only when cocaine levels were above the priming threshold but below the satiety threshold. When the levels of non-contingently administered cocaine were maintained between the priming and satiety thresholds for at least 5 h rats continuously maintained high rates of responding. Although it is generally assumed that rats are responding for cocaine during self-administration sessions, the persistence of responding during non-contingent administration is consistent with responding being induced by cocaine. Therefore, in contrast to the basic assumptions underlying the operant theory of self-administration behavior, choice, contingency and reinforcement are not necessary to explain acquired cocaine self-administration. The presented data demonstrate that there is no ascending limb of the dose-response curve and that the cocaine priming and satiety thresholds delineate the lower and upper limits, respectively, of a cocaine "compulsion zone". It is concluded that the self-administration paradigm is the sum of cocaine induced responding and cocaine induced satiety and which of these cocaine-induced effects occur at any time is dependent on the cocaine level. This novel pharmacokinetic/pharmacodynamic theory provides a basis for a comprehensive understanding of the cocaine self-administration paradigm.

A reduced rate of in vivo dopamine transporter binding is associated with lower relative reinforcing efficacy of stimulants

A slow onset of action has been hypothesized to weaken the reinforcing effects of drugs. The present study evaluated this hypothesis with slow-onset cocaine analogs, WIN 35428, RTI 31, and RTI 51. When cocaine or a cocaine analog was made available to rhesus monkeys (n = 4 or 5) for self-administration under a progressive-ratio (PR) schedule with a 1-h time-out between injections, all the drugs functioned as positive reinforcers. The maximum number of injections was in the order of cocaine > WIN 35428 > RTI 31 > RTI 51. In in vivo binding in rat striatum, equipotent doses of cocaine, WIN 35428, RTI 31, and RTI 51 were estimated to displace 25% of [(3)H]WIN 35428 binding at the dopamine transporters (DAT), respectively, 5.8, 22.4, 30.8, and 44.1 min after the intravenous injection. Further, relative reinforcing efficacy was correlated with rate of DAT binding such that slower displacement of [(3)H]WIN 35428 was associated with a weaker reinforcing effect. In in vitro binding in monkey brain tissue, the cocaine analogs had higher affinity for monoamine transporter sites, but similar affinity ratios of 5-HTT/DAT, compared to cocaine. Lastly, RTI 31 was shown to function as a positive reinforcer in drug-naïve rhesus monkeys under a fixed-ratio 1 schedule. Collectively, the data support the hypothesis that a slow onset at the DAT is associated with reduced reinforcing efficacy of DAT ligands. The data under both the PR and FR schedules, however, suggest that a slow onset at the DAT influence reinforcing effect only to a limited extent.