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 reductions in drug intake.
OBJECTIVES: To evaluate the effects of increasing the volume/delivery (v/d) of saccharin on oral phencyclidine (PCP) self-administration in rhesus monkeys given continuous access to PCP and saccharin during daily sessions using a behavioral economic analysis. The effects of availability of a saccharin solution during the inter-session 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 available 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 remained constant (0.6 ml). In a second experiment, subjects had access to water or 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 concurrent FR schedules. The FR for both liquids was increased (FR16-128).
RESULTS: PCP intake was reduced at all FRs and magnitude conditions when saccharin (versus water) was concurrently available. Varying the v/d of saccharin 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 greater at the higher FR values.
CONCLUSIONS: The magnitude of the saccharin delivery had an effect on PCP consumption at higher FRs, suggesting that economic factors such as high drug cost (FR) and low cost (responses/ml) of the alternative reinforcer (saccharin) interact to produce a maximum suppression of drug intake. Between-session availability of saccharin also effectively reduced drug intake, and it had a greater effect on the maintenance levels of drug self-administration when the unit price of drug was high.