Evidence for learned skill during cocaine self-administration in rats.

RATIONALE: It has been proposed that cocaine abuse results in skilled or "automatic" drug-taking behaviors. Brain regions important for skill learning are implicated in cocaine self-administration. However, the development of skill during self-administration has not been investigated.
OBJECTIVES: The present experiment investigated the development of skilled self-administration over extended drug use by employing a novel operant vertical head movement under discriminative stimulus (S(D)) control. In addition, the capacity of the head movement to serve as an operant was tested by manipulating drug levels above or below satiety drug levels via frequent noncontingent microinfusions (0.2 s) of cocaine.
RESULTS: Animals acquired the vertical head movement operant, which increased in number over days. Task learning was demonstrated by reduced reaction time in response to the S(D), increased propensity to self-administer upon S(D) presentation, and escalated drug consumption over days. Skill learning was demonstrated by (1) an increase over days in the velocity of operant movements, as a function of shorter duration but not altered distance, and (2) an increase over days in the probability of initiating the operant at the optimal starting position. Evidence that responding was specific to self-administration was revealed during periods of experimenter-manipulated drug level: maintaining drug levels above satiety decreased responding while maintaining drug levels below satiety increased responding.
CONCLUSIONS: Under the specific set of circumstances tested herein, cocaine self-administration became skilled over extended drug use. The vertical head movement can be used as an operant comparable to lever pressing with the additional benefit of quantifying skill learning.