The effects of acutely administered cocaine on responding maintained by a progressive-ratio schedule of food...

Lever pressing in rats (N=5) was reinforced under a progressive-ratio (PR) schedule of food presentation, in which the number of responses required increased exponentially. The session was terminated when 1 h passed without completion of the scheduled ratio. Doses of cocaine (5.6-42.0 mg/kg; one subject received a dose of 56.0 mg/kg) as well as saline were administered i.p. prior to the session. Under non-drug conditions, breakpoints were typically less than 100, and substantial responding usually occurred only during about the first 10 min of the session. The rate of responding usually increased over the first 2-8 reinforcers and then decreased for the last few reinforcers obtained. For four of five rats, breakpoint, overall rate of response, and session duration were first increased above control and vehicle levels by increasing doses of cocaine. Larger doses produced smaller increases, no effect, or decreases. Cocaine, in the range of doses near the apex of the breakpoint dose-effect functions, suppressed rates of responding at the small ratios present at the beginning of the session. It is suggested that cocaine increases low rates of response if: (1). rates are low due to extinction; and (2). the stimuli present are those present when the response is reinforced.