Dose and elasticity of demand for self-administered cocaine in rats

Abstract

The present experiment tested whether the elasticity of demand for self-administered cocaine in rats is dose-dependent. Subjects lever pressed for three different doses of intravenous cocaine - 0.11, 0.33, and 1.0 mg/kg/infusion - on a demand procedure where the number of lever presses required per infusion increased within a session. The main finding was that demand for the 0.11 mg/kg dose was more elastic than it was for the two larger doses. There was no difference in demand elasticity between the 0.33 and 1.0 mg/kg doses. These results parallel findings previously reported in monkeys. The present study also demonstrated that a within-session procedure can be used to generate reliable demand curves.

Figure 1

The top panel presents the mean (+ SEM) numbers of infusions self-administered at each fixed-ratio (FR) value for each dose tested. The results presented are the average of the two within-session demand tests. The middle panel presents these results expressed in terms of mg/kg consumed. The bottom panel presents the same results as log-log normalized demand curves, where consumption is expressed as a percentage of Q₀, the expected amount consumed as price approaches 0, and price is expressed as the number of responses required at a particular FR value to produce 1% of Q₀ [that is, normalized price = (FR × Q₀)/100]. Hursh and Silberberg's (2008) exponential demand model was used to fit curves to the data from each dose. Values of α and R² are presented for each of the curves.