DSP4 alters the effect of d-amphetamine on variable-interval performance: analysis in terms of Herrnstein's equation

The effect of d-amphetamine (0.1-3.2 mg/kg) on performance in variable-interval 1-min and variable-interval 12-min schedules of positive reinforcement was examined in ten rats treated with the selective noradrenaline neurotoxin DSP4 and 12 control rats. In the control group d-amphetamine had a dose-dependent suppressant effect on response rates maintained under variable-interval 1-min; under variable-interval 12-min, response rates were increased by low doses and suppressed by higher doses of the drug. In the case of both schedules, lower doses of d-amphetamine were more suppressant and higher doses less suppressant in the DSP4-treated group than in the control group. The levels of noradrenaline in the parietal cortex, hippocampus and cerebellum (determined by high-performance liquid chromatography) were reduced to approximately 15% of control levels in the DSP4-treated rats. The results indicate that treatment with DSP4 attenuated both the facilitatory and the suppressant effects of d-amphetamine on variable-interval performance. A formal model couched in terms of Herrnstein's (1970) equation is put forward to account for these results. It is suggested that the noradrenergic pathways emanating from the locus caeruleus are involved in both the facilitatory and suppressant effects of d-amphetamine on operant behaviour.