Effects of chlorpromazine on rats' acquisition of lever-press responding with immediate and delayed reinforcement

Response acquisition with signaled delayed reinforcement in a rodent model of ADHD

Previous research by Hand et al. [10] showed that acquisition of lever pressing was retarded in spontaneously hypertensive rats (SHRs) relative to Wistar-Kyoto rats (WKYs) when unsignaled delays of 15s separated lever presses from food delivery. The SHRs took longer to begin responding, exhibited a slower increase in response rates, responded at a lower asymptotic response rate and earned fewer reinforcers than the WKYs. The present experiment examined whether similar strain differences in acquisition would be observed if the same delay to reinforcement was signaled. Signaled delays of reinforcement typically result in lesser disruption of steady-state operant behavior than unsignaled delays, presumably because the signals function as conditioned reinforcers. Under a response-acquisition procedure, signals might be expected to facilitate acquisition which could minimize SHR-WKY strain differences. The present study exposed SHR and WKY rats to a procedure where a single lever press illuminated the houselight and delivered a food pellet 15s later. Response acquisition was similar between SHR and WKY rats under 15-s signaled delays of reinforcement; the responses emitted, delay resets and pellets earned by both strains were similar. Removal of the delay signal immediately decreased responding for both strains with the SHRs showing a significantly slower recovery over time. Overall the results suggest that signals occurring during response-reinforcer delays can mitigate the response-weakening effects of delayed reinforcement in a rodent model of ADHD.

Contingency tracking during unsignaled delayed reinforcement

Three experiments were conducted with rats in which responses on one lever (labeled the functional lever) produced reinforcers after an unsignaled delay period that reset with each response during the delay. Responses on a second, nonfunctional, lever did not initiate delays, but, in the first and third experiments, such responses during the last 10 s of a delay did postpone food delivery another 10 s. In the first experiment, the location of the two levers was reversed several times. Responding generally was higher on the functional lever, though the magnitude of the difference diminished with successive reversals. In the second experiment, once a delay was initiated by a response on the functional lever, in different conditions responses on the nonfunctional lever either had no effect or postponed food delivery by 30 s. The latter contingency typically lowered response rates on the nonfunctional lever. In the first two experiments, both the functional and nonfunctional levers were identical except for their location; in the third experiment, initially, a vertically mounted, pole-push lever defined the functional response and a horizontally mounted lever defined the nonfunctional response. Higher response rates occurred on the functional lever. These results taken together suggest that responding generally tracked the response-reinforcer contingency. The results further show how nonfunctional-operanda responses are controlled by a prior history of direct reinforcement of such responses, by the temporal delay between such responses and food delivery, and as simple generalization between the two operanda.

Response acquisition with delayed reinforcement in Lewis and Fischer 344 rats

Previous work has shown neurochemical and behavioral differences between Lewis rats and Fischer 344 rats. Some of this work suggests that there might be differential sensitivity to delayed reinforcement between the two strains. To further explore this possibility, Lewis (n=8) and Fischer 344 (n=8) rats were exposed to a response-acquisition task with a non-resetting 20s delay to reinforcement. A tandem fixed-ratio 1, fixed-time 20s schedule of reinforcement was programmed for one of two levers; presses on the alternate lever had no programmed consequences. A greater number of Lewis rats (5/8) acquired lever pressing compared to the Fischer 344 rats (2/8). Future work with these strains may lead to a better understanding of the genetic and/or neurochemical factors involved in temporal control of behavior.

Differential acquisition of lever pressing in inbred and outbred mice: comparison of one-lever and two-lever procedures and correlation with differences in locomotor activity

Recent progress in mouse genetics has led to an increased interest in developing procedures for assessing mouse behavior, but relatively few of the behavioral procedures developed involve positively reinforced operant behavior. When operant methods are used, nose poking, not lever pressing, is the target response. In the current study differential acquisition of milk-reinforced lever pressing was observed in five inbred strains (C57BL/6J, DBA/2J, 129X1/SvJ, C3H/HeJ, and BALB/cJ) and one outbred stock (CD-1) of mice. Regardless of whether one or two levers (an "operative" and "inoperative" lever) were in the operant chamber, a concomitant variable-time fixed-ratio schedule of milk reinforcement established lever pressing in the majority of mice within two 120-min sessions. Substantial differences in lever pressing were observed across mice and between procedures. Adding an inoperative lever retarded acquisition in C57BL/6J, DBA/2J, 129X1/SvJ, and C3H/HeJ mice, but not in CD-1 and BALB/cJ mice. Locomotor activity was positively correlated with number of lever presses in both procedures. Analyses of durations of the subcomponents (e.g., time to move from hopper to lever) of operant behavior revealed further differences among the six types of mice. Together, the data suggest that appetitively reinforced lever pressing can be acquired rapidly in mice and that a combination of procedural, behavioral, and genetic variables contributes to this acquisition.

Acquisition of cocaine self-administration with unsignaled delayed reinforcement in rhesus monkeys

Six experimentally naive rhesus monkeys produced 0.01 mg/kg/infusion cocaine by lever pressing under a tandem fixed-ratio 1 differential-reinforcement-of-other-behavior schedule. One lever press initiated an unsignaled 15- or 30-s delay culminating in cocaine delivery. Each press made during the delay reset the delay interval. With two exceptions, responding was acquired and maintained at higher rates than responding on a second (inoperative) lever. For the exceptions, a cancellation contingency was arranged in which each formerly inoperative-lever response reset the tandem schedule. This manipulation reduced presses on the inoperative lever. Subsequently, the consequences of responding on the two levers were reversed, and the monkeys again responded at higher rates on the operative lever. As a comparison, 3 additional experimentally naive monkeys received response-independent cocaine deliveries. Although lever pressing was observed, it extinguished and was subsequently reestablished under the tandem schedule. The results suggest that although response-reinforcer contiguity is not required for cocaine to acquire reinforcing functions, a response-reinforcer relation appears necessary.

Response acquisition with immediate and delayed conditioned reinforcement

Groups comprising eight rats initially were exposed to response-independent water deliveries, then to conditions under which a lever-press response raised an empty dipper immediately or after a resetting delay of 15, 30, or 45 s. When their performance was compared to that of control animals using a 90% confidence level, six rats in the immediate-reinforcement group met the primary criterion for response acquisition during a single 6-h session; 4, 4, and 3 did so in the 15, 30, and 45 s delay groups, respectively. Similar evidence of acquisition was obtained when a 95% confidence level was used. With a 99% confidence level, however, evidence of acquisition was not compelling. Although these data appear to provide the first demonstration of response acquisition in the absence of handshaping or autoshaping under conditions where the putative reinforcer is both conditioned and delayed, they also demonstrate that whether response acquisition occurs depends, in part, on how it is defined.

The effects of behavioral history on response acquisition with immediate and delayed reinforcement

Effects of prior exposure to the experimental chamber with levers present or absent and variable-time (VT) 60-s water deliveries arranged during one, five, or no 1-hr sessions were examined in rats during a 6-hr response-acquisition session in which presses on one lever produced water delivery immediately or after a 15-s resetting delay, and presses on the other lever canceled scheduled water deliveries. Response acquisition was (a) slower to occur when water deliveries were delayed, (b) most consistent in groups that had received five VT sessions, and (c) impaired by the presence of levers only when there had been five VT sessions and water deliveries were delayed during the acquisition session.

Mouse strains differ under a simple schedule of operant learning

Recent advances in understanding the composition of the human and mouse genomes have paved the way to a more detailed understanding of the influence of genes on behavior, particularly learning and memory. One problem with many learning paradigms is the great length of training time required to generate a stable baseline. Our goal for the current studies was to develop a method of rapidly assessing learning and to use it to compare various strains of mice. The acquisition of a simple nose-poke was determined in operant chambers with two nose-poke holes illuminated: a single nose poke in one hole resulted in the presentation of 0.01 ml evaporated milk; responses in the other hole did not result in dipper presentation. All mice of the B6JxImJ F1, C57BL/6J, 129X1/SvJ and C3H/HeJ mice emitted 50 or more correct operant responses, whereas fewer than 50% of 129X1/SvJ and 75-90% of mice of Balb/cByJ, DBA/2J and the outbred CD-1 mice emitted 50 or more correct operant responses. On average, C57BL/6J emitted 50 operative responses in less than 30 min, whereas DBA/2J mice required nearly 1 h to complete 50 operative responses. Other strains performed at intermediate levels. There was no apparent relationship between operant activity and locomotor activity that may have influenced response acquisition. These findings are consistent with those reported using other learning paradigms and provide a rapid method of learning assessment.

MDMA and learning: effects of acute and neurotoxic exposure in the rat

In two experiments, the effects of MDMA on the acquisition of lever-press responding of rats were examined under procedures in which water delivery was delayed by 0, 10, or 20 s relative to the response that produced it. In the first study, experimentally naive, water-deprived rats received an intraperitoneal injection of MDMA (0, 1.0, 3.2, or 5.6 mg/kg) prior to one 8-h experimental session. Response acquisition was observed under all conditions at all drug doses. MDMA increased the total number of responses emitted and the total number of water deliveries earned in dose-dependent fashion, but only when reinforcement was immediate. Under conditions of delay, MDMA had no effect on either measure. Under all reinforcement conditions, higher doses of MDMA typically produced an initial reduction in lever pressing, and in that sense interfered with learning. In the second study, rats received an MDMA injection regimen previously shown to be neurotoxic. Control rats received saline solution according to the same injection schedule. Two weeks after completing the regimen, rats were water deprived and exposed to behavioral procedures as described for the first experiment. Although MDMA significantly reduced 5-HT and 5-HIAA levels in the striatum and prefrontal cortex, mean performance of rats exposed to MDMA did not differ from that of rats exposed to vehicle. Twenty-five percent of the rats exposed to MDMA and delayed reinforcement did fail to acquire responding, which suggests that further study of the effects of neurotoxic doses of MDMA on initial response acquisition is warranted.

Response acquisition with delayed reinforcement: a comparison of two-lever procedures

Groups of 8 experimentally naive rats were exposed during 8-hr sessions to resetting delay procedures in which responses on one lever (the reinforcement lever) produced water after a delay of 8, 16, 32, or 64 s. For rats in one condition, responses on a second (no-consequences) lever had no programmed consequences. For rats in another condition, responses on a second (cancellation) lever during a delay initiated by a response on the reinforcement lever prevented delivery of the scheduled reinforcer; responses on the cancellation lever at other times had no programmed consequences. Under both conditions and at all delays, most subjects emitted more responses on the reinforcement lever than did control rats that never received water emitted on either lever. At 8-s delays, both conditions engendered substantially more responding on the reinforcement lever than on the other lever, and performance closely resembled that of immediate-reinforcement controls. At delays of 16 and 32 s, however, there was clear differential responding on the two levers under the cancellation condition but not under the other condition. When the delay was 64 s, differential responding on the two levers did not occur consistently under either condition. These findings provide strong evidence that the behavior of rats is sensitive to consequences delayed by 8, 16, and 32 s, but only equivocal evidence of such sensitivity to consequences delayed 64 s. They also indicate that acquisition depends, in part, on the measure of performance used to index it.