The nature of the scheduled reinforcer and adjunctive drinking in nondeprived rhesus monkeys

Adjunctive drinking was generated in three free-feeding rhesus monkeys by the contingent and intermittent delivery of flavored pellets. The amount of drinking generated was greater when pellet availability was restricted under fixed-interval schedules compared to a massed-reinforcer control condition. The volume of water consumed depended upon the fixed-interval of pellet delivery (FI 180 sec to FI 1800 sec). Peak amounts of water consumed ranged from 532 ml to 650 ml during the 2 hr sessions and the schedule which generated the most drinking was either FI 420 or FI 600 sec, across monkeys. Variables which did not appear to influence the amount of drinking generated within the session were the amount of water-consumed outside the session, the rates of responding maintained by pellet delivery and the pattern of responding for pellet delivery. However, when either cocaine or diazepam was the scheduled reinforcer, these same free-feeding monkeys did not engage in adjunctive drinking. The ability of cocaine and diazepam to generate adjunctive drinking was determined first by gradually decreasing the frequency of pellet delivery while keeping drug delivery constant using a second-order schedule of pellet delivery [FR n (FI 300 sec: drug delivery) with n ranging from 1 to 6]. Second, a range of drug doses was tested under a FI 300 sec schedule (cocaine: 0.01-0.3 mg/kg/injection; diazepam: 0.01-0.56 mg/kg/injection). These results suggest that there may be some restriction on the generation of adjunctive drinking depending upon the nature of the scheduled reinforcer.