Progressive ratio schedules are used to determine the "breakpoint" or limit to the amount of "work" that a subject is willing to perform to obtain a reinforcer. Reinforcing efficacy is inferred from the breakpoint values, which are typically measured in a single session by increasing the number of responses required for successive reinforcer presentations. This procedure is not feasible, however, when assessing the reinforcing efficacy of a substance that can change as a function of its physiological actions during self-administration, as in the case of ethanol.
The present study made use of a procedure that increased the response requirement across single daily sessions rather than within a session. Completion of the response requirement in each daily session resulted in the presentation of a drinking tube that allowed for self-administration of ethanol for a 20-min period. This procedure made possible the assessment of ethanol-directed appetitive (number of lever presses) and consummatory (number of licks and intake volume) behaviors. Reliable responding for 10% ethanol was initiated using sucrose-substitution on a fixed ratio (FR) 4 schedule in male Long Evans rats. Then four successive breakpoint determinations were made which were separated by a return to the FR4 schedule to re-establish baseline responding.
The results indicated that there was an increase in breakpoint values from the first to the second determination, which was then stable over the following three determinations. Individual rats reached breakpoints as high as 240 lever presses to receive access to 10% ethanol and maintained ethanol intake over sessions in the 1.0 g/kg range. Ethanol intake (g/kg), however, was stable across all four determinations (mean 0.86 +/- 0.06 to 1.01 +/- 0.10). Moreover, ethanol intake was not related to the preceding appetitive responding, as no differences between intake on the session before a breakpoint (high FR) and the following baseline period (FR4) were observed.
This model provides an assessment of the distinct mechanisms that mediate ethanol-seeking versus ethanol consumption in subjects that drink measurable amounts of ethanol, with the appetitive behaviors not altered by the pharmacological effects of ethanol.