Hold-down as an alternative to unit dose in cocaine self-administration experiments: Characterization using a progressive ratio schedule.

Rationale: Virtually all cocaine self-administration studies have used a "unit dose" as a reinforcing stimulus; the subject is a passive recipient of an experimenter-selected dose.
Objectives: The present experiments examined the consequence of requiring the subject to actively determine the dose and speed of each injection.
Methods: A two-lever procedure was used in which responding on a progressive ratio (PR) schedule provided access to cocaine on a hold down (HD) schedule. With HD, the pump is turned on for the duration that the lever is held down, thus the dose and speed of injection is determined by the behavior of the subject. The procedure allows for the evaluation of both drug taking and drug seeking responses.
Results: The results were qualitatively different from PR self-administration studies using unit dose. The self-administered HD dose varied across the session; the self-administered dose was found to inversely correlate with drug levels at the time of access. Importantly, the 2 L-PR-HD procedure identified a subpopulation of subjects that showed extremes in both drug seeking and drug taking. Subjects at the top end of the distribution displayed unprecedented final ratios (> 900) and rapidly self-administered very large doses (> 1.4 mg; ~ 4.2 mg/kg). Manipulation of drug-taking variables (HD access duration and concentration of drug in the pump) showed that the immediacy of a cocaine bolus, not the duration of access, is the major determinant of drug seeking.
Conclusions: Incorporating a consummatory response into a PR procedure provides a unique perspective on the interactions of drug-seeking and drug-taking.