(+)-CPCA - Pharmacology

Pharmacology

Like cocaine, (–)-cis-CPCA and (+)-CPCA bind to the dopamine transporter and inhibit dopamine uptake, stimulate motor activity in rodents and completely substitute for cocaine in discrimination tests. Pretreatment with (–)-cis-CPCA or (+)-CPCA enhances the cocaine discriminative stimulus in rats. However there are a number of differences; the locomotor stimulant effects of the piperidine derivatives are much less than those induced by cocaine, and pretreating mice with (–)-cis-CPCA or (+)-CPCA does not increase cocaine induced convulsions, and actually reduced cocaine induced locomotor stimulation. The (–)-cis-CPCA isomer has similar reinforcing effects to cocaine as shown by fixed-ratio self-administration tests in rats, but (+)-CPCA has a flat dose-response curve, and similarly while (–)-cis-CPCA and cocaine had nearly identical break points in a "punished responding" (?) self administration test, (+)-CPCA had a lower break point than either of the other drugs.

The generally lower efficacy of (+)-CPCA in locomotor and methamphetamine discrimination tests could result from the differential selectivity of the two isomers for the DAT relative to the SERT. That is, if serotonin receptor activation is requisite for maximal efficacy, the difference SERT affinity between (–)-cis-CPCA and (+)-CPCA might play a contributory role in accounting for the differences in the observed pharmacology. Catecholamine selective drugs, like TMP (methylphenidate), are reported to possess decent abuse potential though, so it is not easy to gauge why (+)-CPCA does not entice a strong self-administration propensity.

A possible explanation might be nocaine preferentially binds to the ↓ DAT, in which case it would be expected to behave somewhat differently to cocaine. Some sort of cholinergic effect might also be aversive. For example, muscarinic activity of benztropine analogs is known to limit their reinforcing potential. Ion-channel activity is another factor that can be used to explain certain differences in pharmacology.

It is possible that sigma receptor activity might also account for some of the differences between cocaine and these piperidine mimics (R. Matsumoto, et al. 2001, (Ping and Teruo, 2003 rev). Sigma receptors are not specific to cocaine, other psychostimulants like methamphetamine (E. Nguyen, et al. 2005), and phencyclidine are also linked to this neural target. An increased understanding of this receptor recently led to a novel AD being reported that is based around its pharmacology.

In summary, (+)-CPCA has lower potency and efficacy than cocaine in increasing locomotor activity in rodents. (+)-CPCA only manages to produce partial methamphetamine-like discriminative stimulus effects, although it is fully cocaine-like in cocaine-trained animals. (+)-CPCA has lower reinforcing potential than cocaine as assessed by fixed and progressive ratio IV self-administration tests in rats, with its reinforcing effects confirmed by rhesus monkeys. Furthermore, (+)-CPCA dose dependently antagonizes cocaine-induced locomotion and potentiates the discriminative stimulus effects of a low dose of cocaine. (+)-CPCA, unlike cocaine, does not enhance cocaine-induced convulsions. These results suggest that (+)-CPCA completely mimics certain behavioral actions of cocaine, whereas acting like a weak partial agonist in others, including its ability to attenuate cocaine-induced increase in locomotion and to serve as a positive reinforcing agent in rodents. Thus, (+)-CPCA may have potential utility in the treatment of cocaine addiction, and also offer valuable pharmacological information, furthering our understanding of cocaines mechanism of action, because it exhibits fundamental differences from other related DARI molecules.