Psilocybin as a discriminative stimulus: lack of specificity in an animal behavior model for 'hallucinogens'

Fifteen rats were trained to discriminate between the tryptamine hallucinogen psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine; 1.0 mg/kg) and saline in a two-lever choice task. Dose-response and time-response curves were obtained. The psilocybin cue generalized to psilocin (the dephosphorylated congener of psilocybin) and to the prototypical indoleamine hallucinogen LSD, but not to the phenylethylamine hallucinogen mescaline. These results indicate that the hallucinogenic effects of these drugs in humans may not be identical with their discriminative stimulus functions in animals, and that these four compounds may not be members of a single drug class. The term 'hallucinogen' may thus be a misnomer in the context of drug discrimination studies in nonhumans.