Abstract

Rats were housed and tested under a controlled cycle of 12 hours light-12 hours dark. Effects of drugs on free-operant avoidance and fixed-interval performance varied from enhancement to depression, depending on the stage of the light-dark cycle at which the drug was administered. Methamphetamine produced the greatest increase in lever pressing when administered during the dark period, regardless of whether responding was maintained by food or by electric shock. This effect appeared to be independent of diurnal differences in base-line response rates. Methamphetamine increased locomotor activity at least as much during the light as during the dark, which suggests a dissociation between effects on operant behavior and effects on locomotor activity. α-Methyltyrosine was a more effective antagonist of methamphetamine effects in the light than in the dark. Unlike methamphetamine, p-chloromethamphetamine was more effective in increasing responding during the light period. The dose of scopolamine which was most effective in increasing avoidance responding varied from 0.2 mg/kg in the dark to 0.8 mg/kg in the light. Results are discussed with reference to reported diurnal rhythms in behavioral toxicity and in brain amines.