External discriminative stimuli can modify the behavioral effects of d-amphetamine. Previous work with the pigeon has demonstrated that some aspects of performance on the fixed consecutive number schedule are changed less if a discriminative stimulus indicates when reinforcement is available. This effect has now been replicated with the rat using both simple and multiple schedules. Moderate doses of d-amphetamine (0.56--1.0 mg/kg) usually produced large decreases in reinforced runs when no external cue indicated the possibility of reinforcement. Adding discriminative stimuli when the number requirement was met decreased the drug effect. As was true in the pigeon, response rate measures did not differ between the two stimulus control conditions. Thus, external stimulus control diminishes the drug effect in both species, despite the fact that key pecking was studied in the pigeon and lever pressing in the rat. Evidence was also seen of a possible increase in discriminative stimulus control by d-amphetamine.

Sensitization in the neuroscience and pharmacology literatures is defined as progressive increase in the size of a response over repeated presentations of a stimulus. Types of sensitization include stimulant drug-induced time-dependent sensitization (TDS), an animal model related to substance abuse, and limbic kindling, an animal model for temporal lobe epilepsy. Neural sensitization (primarily nonconvulsive or subconvulsive) to the adverse properties of substances has been hypothesized to underlie the initiation and subsequent elicitation of heightened sensitivity to low levels of environmental chemicals. A corollary of the sensitization model is that individuals with illness from low-level chemicals are among the more sensitizable members of the population. The Working Group on Sensitization and Kindling identified two primary goals for a research approach to this problem: to perform controlled experiments to determine whether or not sensitization to low-level chemical exposures occurs in multiple chemical sensitivity (MCS) patients; and to use animal preparations for kindling and TDS as nonhomologous models for the initiation and elicitation of MCS.

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