Abstract

Rats were trained to bar-press on either a fixed-interval 15-second (FI 15) or fixed-ratio (FR 30) schedule for water reinforcement. Rats were allowed to bar-press on either of two levers (left and right). Most rats showed consistent side preferences. Various doses (0.125-4.0 mg/kg i.p.) of d-amphetamine differentially affected performance on the two schedules. Rats on the FR 30 schedule were much more sensitive to a drug-induced rate decrement than rats on the FI 15 schedule; as rates decreased with increasing drug dosage, side preferences reliably increased or decreased depending upon the particular pattern of paw usage and the relationship between paw and side preferences. In both FI 15 and FR 30 rats, base-line rates were directly related to the strength of side preferences; higher rates were significantly correlated with greater preferences. It was observed that FR 30 rats with high base-line rates usually used both paws in rapid, alternating, well-coordinated movements to bar-press; this kind of activity was disrupted by d-amphetamine such that paws were used individually while bar-pressing at the lower rates. FI 15 rats were observed to use only one paw to make most responses; this mode of responding was unaffected by d-amphetamine. It is suggested that subtle motor actions of d-amphetamine, possibly a function of altered dopamine metabolism in the nigro-striatal system, may be somewhat responsible for the effects of d-amphetamine on different schedule-dependent rates of responding as well as on spatial and paw preferences.