Effects of drugs on stimulus control of behavior. I. Independent assessment of effects on response rates and stimulus control.

617 0022-3565/82/2233-0617$02.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THxRAPEUTICS Copyright © 1982 by The American Society for Pharmacology and Experimental Therapeutics Effects of Drugs on Stimulus Control of Behavior. I. Independent Assessment of Effects on Response Rates and Stimulus Control1 Pigeons were trained to peck one of two concurrently available, differently colored response keys (red or amber) depending on the presence or absence of a houselight that provided general illumination of the experimental chamber. When the houselight was illuminated, only responses on a red key were reinforced (red-key responses designated SD responses; amber-key responses designated Sa responses); when the houselight was not illuminated, only responses on an amber key were reinforced (amber-key responses designated 5D responses; redkey responses designated Sa responses). Under one procedure, every 5D response was reinforced (fixed-ratio 1 schedule), whereas under a second procedure, the first SD response after the elapse of a 5-mm interval was reinforced (fixed-interval 5-mm schedule). Both the rates of responding and the stimulus control over responding exerted by the houselight were assessed. Under the fixed-ratio 1 schedule, all of the drugs decreased rates of SD responding and, with the exception of pentobarbital, did not affect low rates of S’ responding. Pentobarbital increased S’ responding and decreased stimulus control over responding at doses below those that produced substantial decreases in responding. Under the fixed-interval schedule, low to intermediate doses of d-amphetamine and cocaine increased 5D responding but did not affect 5a responding. Pentobarbital produced small increases in both SD and Sa responding. Promazine increased red-key 5D responding and substantially increased red-key 5a responding. Under the fixedinterval schedule, stimulus control was decreased only at the highest doses that also produced substantial decreases in response rates. Thus, changes in response rate occurred at doses below those that decreased stimulus control. Effects of pentobarbital on stimulus control of responding, however, were modified by the schedule under which responding was maintamed. Schedule-controlled behavior is usually studied as repetitive responses of a subject that vary in rate of occurrence depending on experimental conditions such as the prevailing discriminative stimuli, the schedule of reinforcement and the history of the subject (see reviews by Morse, 1966; Zeiler, 1977). Whereas drugs can have pronounced effects on schedule-controlled responding, the mechanisms by which they exert those effects are not always clear (Laties and Weiss, 1969). One possible type of drug effect is an alteration of the control exerted by discriminative stimuli. Discriminative control over behavior is typically evidenced by a high rate of responding in the presence of one stimulus and a low rate in the presence of another stimulus. Many drugs decrease the difference between the rates of responcling in the presence of the two stimuli suggesting that the control over behavior exerted by the stimuli is diminished. Changes in rates of responding after drug administration, Received for publication June 14, 1982. I Thia work was supported by U.S. Public Health Service Grants DA-02873 and AA-02104 awarded to Dr. J. E. Barrett. however, often are related to the rate of responding before drug administration; normally low rates of responding often are increased whereas higher rates are decreased (Dews, 1958a; Kelleher and Morse, 1968). Thus, when discriminative stimuli differentially control high and low response rates, changes interpreted as changes in stimulus control can aLso be interpreted as rate-dependent drug effects (Thompson, 1978). For example, Laties and Weiss (1966) found that promazine and chlorpromazine increased low rates of responding during portions of a fixed-interval schedule that were correlated with distinctive stimuli and no likelihood ofreinforcement. As Laties (1972) notes, the results from that study could be interpreted as either a decrease in control exerted by the stimuli or an increase in the normally low response rates. The present study was designed to determine the degree to which drug-induced changes in rates of schedule-controlled behavior might be due to changes in discriminative control over responding. Changes in rates of responding and changes in stimulus control were assessed independently in a procedure that closely approximated those typically used in studying at A PE T Jornals on Jne 3, 2017 jpet.asjournals.org D ow nladed from