SCH23390 and a humanized anti-cocaine mAb decrease the latency to cocaine-induced reinstatement of lever pressing behavior in rats that self-administer cocaine.

In rats that self-administer cocaine, the latency to the reinstatement of lever pressing behavior induced by a single dose of cocaine is due to the time taken for cocaine levels to fall to the satiety threshold. The D1 dopamine receptor antagonist SCH23390, and the recombinant humanized anti-cocaine mAb h2E2 increase the cocaine satiety threshold and would be expected to alter the latency to reinstatement. Male rats acquired cocaine self-administration behavior on an FR1 schedule. These rats received a single injection of cocaine (12 µmol/kg i.v.) after an i.v. injection of SCH23390 or an infusion of h2E2 or vehicle. The latency to, and the duration of, lever pressing was measured but the presses had no consequence. SCH23390 decreased the latency to lever pressing consistent with dose-dependent increases in satiety threshold. The duration of lever pressing behavior was inversely proportional to the SCH23390 dose suggesting that SCH23390 also increased the cocaine compulsion zone. The mAb h2E2 also produced a similar decrease in latency to responding that gradually reversed over 2 weeks. SCH23390 and h2E2 had an additive effect on the decreased latency to cocaine-induced lever pressing. The single cocaine dose reinstatement paradigm within the context of the compulsion zone theory is a useful pharmacological bioassay system to explore potential pharmacotherapies for relapse prevention in cocaine use disorder.

The cocaine compulsion zone theory explains the reinstatement of lever pressing behavior in rats in response to a single cocaine dose.

A single non-contingent dose of cocaine reinstates extinguished lever pressing behavior in rats trained to self-administer cocaine. This represents a model of relapse in cocaine use disorder and the number of lever presses has been the standard measure. Lever pressing behavior during self-administration occurs only when cocaine levels are below the satiety threshold and above the remission/priming threshold, a range termed the compulsion zone. Calculated cocaine levels at the time of each lever press during an FR1 self-administration session and following a single non-contingent dose of cocaine were compared. The mean latency to lever pressing behavior was dose dependent and ranged from 1 to 11 min after cocaine doses of 2 or 12 µmol/kg, respectively. This is consistent with higher cocaine doses producing levels above satiety threshold that take more time to fall back to that threshold. The level of cocaine when lever pressing occurred was similar whether cocaine was self-administered or after a single dose of cocaine. The number of lever presses after a single cocaine dose was variable and poorly dose dependent. The latency to the start of lever pressing behavior is a more reliable dependent measure than the number of lever presses. In addition, lever pressing behavior occurs only when cocaine levels are within the compulsion zone. The compulsion zone theory not only explains maintained cocaine self-administration behavior, but also explains the reinstatement of lever pressing behavior in response to a single non-contingent cocaine dose.

Effects of a recombinant humanized anti-cocaine monoclonal antibody on the metabolism and distribution of cocaine in vitro and in mice.

The anti-cocaine monoclonal antibody, h2E2, is a candidate for treating cocaine-use disorder. h2E2 binds to and sequesters cocaine in the plasma compartment, effectively decreasing cocaine concentrations in the brains of rats and mice. Despite the binding of cocaine to h2E2, plasma cocaine concentrations decline rapidly in rodents over time, but there was a drastic decrease in the urinary elimination of cocaine in the presence of h2E2. Since cocaine is not being renally excreted, the apparent disappearance of cocaine from the plasma must be explained by either metabolism or distribution. However, binding of cocaine to h2E2 may restrict the availability of cocaine for hydrolysis by endogenous esterases. Therefore, the antibody would be expected to extend the elimination half-life of cocaine. In contrast, previous studies reported h2E2 as having no effect on the rate of cocaine clearance. It is important to examine the ultimate clearance of the cocaine to ascertain its half-life and potential for re-intoxication. Therefore, we investigated the effects of h2E2 on cocaine hydrolysis in vitro and on cocaine metabolism and disposition in vivo over a 6-h time course. The spontaneous and enzyme-mediated in vitro hydrolysis of cocaine was drastically decreased in the presence of h2E2 in vitro. Additionally, in mice, h2E2 significantly increased the distribution and elimination half-lives of cocaine relative to vehicle controls over an extended time course. Therefore, we concluded that h2E2 slowing the distribution and elimination of cocaine is the most appropriate explanation for the initial disappearance of cocaine from the plasma in vivo.

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 t1/2 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 t1/2. Additionally, the relative durations of lever-pressing following termination of drug access correlated with the t1/2 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.