Test for a neurochemically specific mechanism mediating drug discriminations and for stimulus masking.

Five groups of rats were trained to discriminate methysergide, morphine, nicotine, pentobarbital, or scopolamine, respectively, from no injection in a T-maze shock-escape drug discrimination task. After these discriminations were learned, rats received substitution tests, before which they were injected with the training drug and with an additional drug. The additional drugs were mainly transmitter depletors, antagonists, or agonists. The experiment was designed to test whether drug discriminations might be mediated by brain mechanisms operated by one of the manipulated neurotransmitters, and to test whether one discriminable drug could mask or occlude the effects of a second discriminable drug. Neither hypothesis was supported by the results.