Background: The break point (BP) of the progressive ratio (PR) schedule of drug delivery is a well-recognized parameter in self-administration studies. Nonetheless, two problems remain unresolved: there is no rationally justified criterion for the last response at BP; the both commonly used definitions of BP as the number of deliveries or the last complete progressive ratio requirement are not the best assuming that BP is a measure of motivation.
New method: A criterion for the last lever press is proposed in this study using intravenous cocaine self-administration in rats. The rationale is based on the finding that long inter-press intervals have initially very low probability to occur during the self-administration phase of the session under the PR schedule. But this probability dramatically increases when inter-injection intervals increase due to high ratio requirements.
Results: For cocaine these critical inter-press intervals were 7.5 min and longer. This novel criterion was applied to measure BP according to all four theoretically plausible definitions of BP including the new one: the higher of the two numbers of presses before or after the last delivery of the reinforcer.
Comparison with existing method: The conventionally defined BP is significantly lower (by 12 %) than BP defined according to the new proposed definition. The new definition of BP provides not only a more accurate value of BP but now the variance of BP at different cocaine doses is homogeneous as required by many statistical tests.
Conclusion: These new definitions of the last press and BP provide more accurate and statistically homogenous measure of BP.
Keywords: Breakpoint; Cocaine; Definitions; Drug addiction; Inter-press intervals; Progressive ratio schedule; Self-administration.
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