Progressive ratio and behavioural economic evaluation of the reinforcing efficacy of orally delivered phencyclidine and ethanol in monkeys: Effects of feeding conditions

The effect of feeding conditions on the reinforcing efficacy of orally-delivered drugs was evaluated using a progressive-ratio (PR) paradigm and a behavioral economic analysis of demand. Seven monkeys self-administered phencyclidine (PCP) (0.06, 0.12, 0.25, 0.5, and 1.0 mg/ml) or ethanol (2, 4, 8, 16, and 32% wt/vol) and concurrent water from two drinking spouts under concurrent PR schedules. The ratios increased from 8 to 4096, and 40 liquid deliveries were available after completion of each ratio schedule. The entire range of drug concentrations was presented in nonsystematic order under two feeding conditions, food restriction and food satiation. Drug maintained responses, deliveries and break points were significantly greater than those maintained by water. Food restriction significantly increased the rate of PCP-maintained responses, deliveries and PR break points over the food satiation baseline. There was also a significant interaction between feeding condition and drug concentration. Although ethanol-maintained responses, liquid deliveries and break points consistently increased in five of seven monkeys during food restriction, only drug concentration produced significant differences in these measures. Using break point as a measure of reinforcing efficacy, food restriction increased the reinforcing efficacy of PCP and had a more pronounced effect at higher drug unit prices.