COMPARISON OF THE REINFORCING EFFICACY OF COCAINE AND PROCAINE IN RHESUS-MONKEYS RESPONDING UNDER A PROGRESSIVE RATIO SCHEDULE

Rhesus monkeys (n = 5) were prepared with chronic IV catheters and tra ined to lever press under a PR schedule of drug injection. The schedul e consisted of five components, each made up of four trials (i.e., 20 trials total). Each trial within a component had the same response req uirement. The response requirement in the first component was 120/tria l and doubled in successive components to a maximum of 1920 in the fif th. A trial ended with an injection or the expiration of a 12-min limi ted hold (LH). The inter-trial interval (ITI) was 15 or 30 min. Follow ing an injection or expiration of the LH, all stimulus lights were ext inguished and responding had no consequence for the remainder of the t rial. A session ended when either all 20 injections were self-administ ered or the response requirement was not met within the LH for two con secutive trials. The number of injections/session and responses/sessio n increased with dose for cocaine (0.012-0.1 mg/kg per injection) and procaine (0.12-2.0 mg/kg per injection) at both ITI values. At the 15- min ITI, responding decreased again at higher doses in some monkeys wi th cocaine and in all monkeys with procaine. At maximum, cocaine maint ained significantly more injections and responses/session when the ITI was 30 min than when it was 15 min. In contrast, the increase in ITI did not increase the maximum maintained by procaine. Cocaine was appro ximately 10-fold more potent than procaine and maintained at maximum s ignificantly more injections and responses than procaine when the ITI was 30 min but not when the ITI was 15 min. These results are consiste nt with previous studies demonstrating that cocaine is a more efficaci ous positive reinforcer than procaine. Moreover, they extend recent fi ndings suggesting that number of injections/session provides a measure of PR performance that is amenable to statistical analysis and may, t herefore, be useful in establishing reliable differences among drugs i n terms of relative reinforcing efficacy. Reliable quantification of b etween-drug differences in reinforcing efficacy can enhance not only e stimates of relative abuse liability but also pharmacological analysis of central mechanisms mediating reinforcing effects.