Factors Affecting Behavior Maintained by Response-Contingent Intravenous Infusions of Amphetamine in Squirrel Monkeys

Behavior developed and maintained in previously untrained monkeys by a modified progressive-ratio schedule of response-contingent intravenous infusions of d-amphetamine is described. Availability of amphetamine infusions can be restricted to relatively brief (2 h) daily sessions, once responding has been established, without disruption of self-administration behavior. When amphetamine infusions are replaced by saline, responding is reduced immediately to a low rate; when amphetamine infusions are replaced by saline and the session is preceded by an intramuscular injection of the drug (1.5 mg/kg), the pattern of responding is indistinguishable from that observed when drug infusions are available. If an external discriminative stimulus, normally associated with those periods in which infusions are available, is withdrawn, responding is reduced to a low rate. The discriminative stimulus entered into control of the behavior only when amphetamine had been administered by intravenous infusion or by intramuscular injection beforehand, reflecting, therefore, some form of 'state dependence'. Possible sources of interoceptive stimulation, arising from an infusion per se, did not control behavior to any significant extent. Results emphasize the need for specific control procedures when drug self-administration is to be conceptualized as an operant reinforcement effect.