Temporal determinants of intravenous cocaine self-administration in rhesus monkeys'
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.
Mansbach
R.S.
RS
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.
Balster
R.L.
RL