Methohexital-reinforced responding in rats: effects of fixed ratio size and injection dose

The rate and pattern of lever pressing were studied in nine rats during daily 6-hr sessions in which responding led to the i.v. injection of sodium methohexital. Level pressing was shown to be established and maintained by the response-drug contingency in the absence of physiological dependence or aversive stimulation. Effects of 1.0, 2.0, 3.0 and 4.0 mg/kg/injection of methohexital and fixed ratio schedules of 1, 5, 10, 15 and 20 responses per injection were determined on response rate and drug intake. At 1.0, 2.0 and 3.0 mg/kg/injection, increases in fixed ratio value produced systematic decreases in drug intake; the magnitude of the effect was dose-dependent. More total drug was taken at each fixed ratio value as injection dose was increased. At 4.0 mg/kg/injection, drug intake remained relatively stable over 1, 5, 10 and 15 responses per injection, and decreased at 20 responses per injection. Response rate was inversely related in injection dose at 1 and 5 responses per injection and directly related to injection dose at 15 and 20 responses per injection.