Evaluation of the reinforcing properties and phencyclidine-like discriminative stimulus effects of dextromethorphan and dextrorphan in rats and rhesus monkeys.
BACKGROUND
Dextromethorphan (DXM) and its metabolite, dextrorphan (DXO) have neuroprotective and anticonvulsant properties through their activity as N-methyl-D-aspartate (NMDA) receptor channel blockers. Based on this receptor activity, coupled with reports of DXM abuse, both were evaluated for abuse potential and phencyclidine (PCP)-like behavioral effects in two animal models.


OBJECTIVE
The discriminative stimulus properties of DXO and DXM were tested in rats (3-56 mg/kg DXM, i.p. and 2.2-40.9 mg/kg DXO, i.p.) and rhesus monkeys (0.3-10 mg/kg DXM, i.m. and 0.25-8.0 mg/kg DXO, i. m.) trained to discriminate PCP from saline using a standard two-lever drug-discrimination paradigm under a fixed-ratio (FR) schedule of food reinforcement. In a second set of experiments, i.v. self-administration of DXO (10-100 microg/kg/infusion) and DXM (10-1000 microg/kg/infusion) were tested under a FR schedule of reinforcement in monkeys trained to lever press for infusions of PCP during daily 1-h sessions.


RESULTS
In rats, both DXM and DXO produced a dose-dependent substitution for PCP. When tested in monkeys, DXM yielded partial (1 monkey) and full (2 monkeys) substitution for PCP, while DXO substituted fully for PCP in all four subjects tested. In the self-administration study, in five of the six subjects, at least one dose of DXM served as a positive reinforcer, maintaining infusion rates above those for saline. For DXO, at least one dose maintained infusion numbers well above mean saline infusion numbers in all subjects.


CONCLUSIONS
Taken together, these data show that DXM has some PCP-like effects in rats and monkeys, but that they are more reliably produced by its metabolite, DXO. Thus, high doses of DXM may have some PCP-like abuse potential in humans but this potential may be associated with, or enhanced by, metabolism of DXM to DXO.
Nicholson
K L
KL
Department of Pharmacology and Toxicology, Medical College of Virginia, Box 980310, Virginia Commonwealth University, Richmond, VA 23298-0310, USA,
Hayes
B A
BA
Balster
R L
RL
D000818:Animals; D000996:Antitussive Agents / Q000494:pharmacology*; D003915:Dextromethorphan / Q000378:metabolism / Q000494:pharmacology*; D004193:Discrimination Learning / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D008253:Macaca mulatta; D008297:Male; D010622:Phencyclidine / Q000494:pharmacology*; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors; D012054:Reinforcement, Psychology; D012646:Self Administration
D000996:Antitussive Agents; D016194:Receptors, N-Methyl-D-Aspartate; D003915:Dextromethorphan; D010622:Phencyclidine