Reduction of drug self-administration by an alternative non-drug reinforcer in rhesus monkeys: magnitude and temporal effects

Rationale: Recent studies have shown that non-drug alternative reinforcers reduce drug self-administration. A goal of the present study was to explore factors such as magnitude of the alternative reinforcer and inter-session access to the alternative to identify conditions that lead to optimal reduc tions in drug intake. Objectives: To evaluate the effects of increasing the volume/delivery (v/d) of saccharin on oral phencyclidine (PCP) self-admini stration in rhesus monkeys given continuous access to PCP and saccharin dur ing daily sessions using a behavioral economic analysis. The effects of ava ilability of a saccharin solution during the intersession period on session PCP consumption in drug-experienced monkeys was also investigated. Methods : Subjects had access to PCP (0.25 mg/ml) and either water or saccharin (0. 03%) from two drinking spouts under concurrent and independent fixed-ratio (FR) schedules during daily 3-h sessions. The FR requirements for both avai lable liquids were simultaneously increased (FR4-64). The v/d of saccharin or water was increased (from 0.3 ml to 1.2 ml), while the v/d of PCP remain ed constant (0.6 ml. In a second experiment, subjects had access to water o r saccharin and water during the inter-session period (17.5 h) under an FR1 schedule. PCP and water were available during daily 3-h sessions under con current FR schedules. The FR for both liquids was increased (FR16-128). Res ults: PCP intake was reduced at all FRs and magnitude conditions when sacch arin (versus water) was concurrently available. Varying the v/d of sacchari n only had a modest effect on the extent to which PCP intake was decreased at the higher FR values. Inter-session saccharin availability (versus water ) reduced session PCP intake and the magnitude of this effect was also grea ter at the higher FR values. Conclusions: The magnitude of the saccharin de livery had an effect on PCP consumption at higher FRs, suggesting that econ omic factors such as high drug cost (FR) and low cost (responses/ml) of the alternative reinforcer (saccharin) interact to produce a maximum suppressi on of drug intake. Between-session availability of saccharin also effective ly reduced drug intake, and it had a greater effect on the maintenance leve ls of drug self-administration when the unit price of drug was high.