Drinking of Ethanol by Rhesus Monkeys: Experimental Strategies for Establishing Ethanol as a Reinforcer

Abstract

Ethanol drinking by three male rhesus monkeys was established using a procedure termed food-induced drinking. The monkeys were maintained at approximately 80% of their free-feeding weights. During daily 3-hour sessions the monkeys had access to water and were provided with their daily food rations. Within approximately 20 minutes after obtaining their food ration, the monkeys drank up to 500 ml of water. In subsequent sessions, ethanol in increasing concentrations (0.5, 1, 2, 4, 5.6, 8% W/V) was substituted for water. When the monkeys were regularly drinking 100 to 200 ml of 8% ethanol, access to food was shifted from within the 3-hour sessions to a period beginning 1-hour after the sessions. Thereafter, daily sessions consisted of simply three hours of access to 8% ethanol; between sessions the monkeys had ad lib access to water plus their daily food ration. Under these conditions drinking of ethanol persisted at substantial rates, e.g., blood ethanol levels were usually 100 to 200 mg%. In the next series of manipulations, the monkeys were required to emit a fixed-number of responses per each delivery of approximately 0.5 ml of liquid, i.e., fixed-ratio schedules (FR) were instated. At FR 16 8% ethanol but not water (the control vehicle) maintained substantial performance. Fixed-ratio performance was similar to that observed using more commonly studied reinforcers in that when responding occurred, response rates were high and relatively constant. Ethanol-maintained behavior was similar to that reported in earlier studies from this laboratory: Long drinking bouts occurred at the beginning of sessions, and the initial drinking was later followed by several shorter drinking bouts. In contrast, water maintained markedly less responding, and the responding that did occur was spaced irregularly over the session. These findings confirm findings with other rhesus monkeys. That is, ethanol can function as an effective positive reinforcer via the oral route and can maintain intermittently reinforced behavior.