Figure 3.
The effects of varying rate of infusion (5, 25, 50 and 100 sec infusion duration) on responding for cocaine of amphetamine maintained on different FR schedules of reinforcement. After acquisition of stable responding for 0.4 mg/kg/inf cocaine or 0.2 mg/kg/inf amphetamine, each rat was tested on 4 consecutive sessions with each of 4 infusion rates (in random order) for responding on, first an FR1 schedule (n=15 and 19 for the cocaine and amphetamine conditions, respectively), and subsequently on an FR2 (cocaine; n=13) or FR5 (amphetamine; n = 14) schedule. Asterisks’ indicate significant differences between the 5 and 100 sec infusion conditions (p < 0.05). Contrary to expectations, both cocaine and amphetamine intake decreased, by 20% and 21%, respectively, when infused over 100 sec versus 5 sec and no effects were evident on responding maintained on higher order schedules of drug reinforcement. At the same time responding into the cocaine or amphetamine reinforced nose-poke manipulandum significantly increases during the 100 sec infusion/time-out period (this effect only approached significance in rats responding for cocaine on an FR2 schedule).