Effects of morphine on fixed interval-induced escape from food reinforcement

Abstract

Pigeons were trained to respond on a fixed-interval 3-min (FI 3 min) schedule of food reinforcement. During each 1-hr session responses on a second response key produce a timeout from the FI schedule. mThe food reinforcement schedule maintained characteristic FI responding. The escape-timeout responses occurred during the period of low response rates following reinforcement. Morphine (0.25, 0.5, 1.0 mg/kg) produced dose-related rate-dependent decreases in mean fixed-interval response rates. As morphine dose increased, there was an increased tendency for the rates within the interval to converge from a variety of predrug rates to a common, low rate of responding. The effect of morphine on the schedule-induced escape responses was to increase the mean number of escapes at doses of 0.25–0.5 mg/kg and to decrease the escapes at the highest dose of 1.0 mg/kg. The mean duration of the escape-timeouts was increased by the 3 doses of the drug, with the longest durations occurring at the 0.5 mg/kg dose. There was a great deal of variability between subjects on these measures and the dose effects did not reach statistical significance. The present study extends the analysis of drug effects on schedule-induced escape to fixed-interval-induced escape responding and includes the drug morphine.