title: Temporal determinants of intravenous cocaine self-administration in rhesus monkeys'
abstract: The operant behavior of four rhesus monkeys was studies during 1-h sessions in which intravenous cocaine or saline was available on a fixed-ratio 10 schedule of reinforcement. Examination of the temporal characteristics of the behavior revealed differences between drug and saline, and between the two doses of cocaine, 33 and 100 µg/kg/infusion. Rates of saline self-administration decreased rapidly after the first 15 min of the session, whereas rates of cocaine-maintained responding changed less in successive segments of the session. The frequency of inter-infusion times (IITs) falling into successive 30-sec bins confirmed that short IITs were likely to be produced, but a substantial proportion of very long IITs also occurred when saline or a high (100 µg/kg/infusion) dose of cocaine was the available substance. The occurrence of intermediate inter-infusion times was low under all test conditions. The results of an IIT-per-opportunity analysis also suggested that the selection of IITs by the monkeys was orderly and dose-related. Examination of IIT distributions during successive 15-min portions of the session revealed a progressive shift in the relative probabilities of different IITs; however, the occurrence of short IITs late in the session suggested that the negatively accelerating rate of drug self-administration was not a consequence of response disruption. The latencies to the first five infusions of the session were markedly different between saline and 33 µ/kg/infusion of cocaine, suggesting that the drug's effects were manifested at a very early stage of the session. Individual differences were also apparent. These results suggest a temporal complexity in drug self-administration behavior which may help to explain some of its determinants. Examination of such temporal indices may be helpful in the laboratory study of drug-taking behavior.
authors.affiliation: Department of Pharmacology and Toxicology, Commonwealth of Virginia Center for Drug Abuse Research, Medical College of Virginia, Virginia Commonwealth University, Box 613, MCV Station, Richmond, VA 23298-0613, USA Correspondence to R.S. Mansbach at above address.
authors.forename: R.S.
authors.initials: RS
authors.lastname: Mansbach
authors.forename: R.L.
authors.initials: RL
authors.lastname: Balster