Pair housing, but not using a controlled reinforcer frequency procedure, attenuates the modulatory effect of probability presentation order on amphetamine-induced changes in risky choice

Probability discounting is often measured with independent schedules. Independent schedules have several limitations, such as confounding preference for one alternative with frequency of reward presentation and generating ceiling/floor effects at certain probabilities. To address this potential caveat, a controlled reinforcer frequency schedule can be used, in which the manipulandum that leads to reinforcement is pseudo-randomly determined before each trial. This schedule ensures subjects receive equal presentations of the small and large magnitude reinforcers across each block of trials. A total of 24 pair-housed and 11 individually housed female Sprague Dawley rats were tested in a controlled reinforcer frequency procedure. For half of the rats, the odds against (OA) receiving the large magnitude reinforcer increased across the session (ascending schedule); the OA decreased across the session for half of the rats (descending schedule). Following training, rats received treatments of amphetamine (AMPH; 0, 0.25, 0.5, 1.0 mg/kg; s.c.). For pair-housed rats, AMPH (0.5 mg/kg) increased risky choice, regardless of probability presentation order, whereas a higher dose of AMPH (1.0 mg/kg) decreased discriminability of reinforcer magnitude for rats trained on the descending schedule only. For individually housed rats, probability presentation order modulated the effects of AMPH on probability discounting, as AMPH (0.25 and 0.5 mg/kg) increased risky choice in rats trained on the ascending schedule but not on the descending schedule. These results show that pair-housing animals, but not using a controlled reinforcer frequency procedure, attenuates the modulatory effects of probability presentation order on drug effects on risky choice.