Effects of clonidine on progressive ratio schedule performance in Fmr1 knockout mice.

RATIONALE: Fragile X syndrome (FXS), the most prevalent genetic form of intellectual disability, is characterized by intellectual impairment, impaired sociability, aggression, self-injury, hyperactivity, and attention deficits. A consequence of the hyperactivity and attention deficits is that individuals with FXS are frequently diagnosed with attention deficit hyperactivity disorder (ADHD) and treated with medications approved for ADHD (e.g., the α2-agonist clonidine). The pharmacotherapy of FXS is often accompanied with behavioral therapies that rely on positive reinforcement and other operant principles. Despite the commonplace mixture of drug and behavioral therapy, little attention has been paid to the observation that clonidine or other psychotropic drugs may alter operant processes. OBJECTIVES: In the present progressive ratio study, we used a knockout mouse model to test the effects of the fragile X genotype, the α2-agonist clonidine, and the fragile X genotype and clonidine together on operant processes in a positive reinforcement task. RESULTS: We found that clonidine decreased the progressive ratio breakpoint, increased the length of post-reinforcement pauses, and slowed the run rate. None of these effects varied by genotype. The effect on breakpoint suggests that clonidine alters motivation, but analysis using mathematical principles of reinforcement (MPR) did not rule out motor parameters as a contributor. CONCLUSIONS: Our findings show that clonidine alters operant behavior and serve as a caution for combining clonidine with behavioral therapies that rely on positive reinforcement. Further research using different murine behaviors (e.g., touchscreen tasks) or different animal models (e.g., knockout rats) is needed to explore the interaction between pharmaco- and behavioral therapy.

The role of temporal intervals on reinforcer accumulation.

Animals accumulate reinforcers when they forgo the opportunity to consume available food in favor of acquiring additional food for later consumption. Laboratory research has shown that reinforcer accumulation is facilitated when an interval (either spatial or temporal) separates earning from consuming reinforcers. However, there has been no systematic investigation on the interval separating consuming reinforcers from earning additional reinforcers. This oversight is problematic because this second interval is an integral part of much of the previous research on reinforcer accumulation. The purpose of the current study was to determine the independent contributions of these two temporal intervals on reinforcer accumulation in rats. Each left lever press earned a single food pellet; delivery of the accumulated pellet(s) occurred upon a right lever press. Conditions varied based on the presence of either an intertrial interval (ITI) that separated pellet delivery from the further opportunity to accumulate more pellets, or a delay-to-reinforcement that separated the right lever press from the delivery of the accumulated pellet(s). Delay and ITI values of 0, 5, 10 and 20 s were investigated. The delay-to-reinforcement conditions produced greater accumulation relative to the ITI conditions, despite accumulation increasing the density of reinforcement more substantially in the ITI conditions. This finding suggests that the temporal separation between reinforcer accumulation and subsequent delivery and consumption was a more critical variable in controlling reinforcer accumulation.

On the nature of directed behavior to drug-associated light cues in rhesus monkeys (Macaca mulatta).

The present study investigated the role of drug-paired stimuli in controlling the behavior of rhesus monkeys. Systematic observations were made with nine monkeys who had a history of drug self-administration; they had been lever pressing to produce intravenous infusions of various drugs. These observations revealed that the stimulus light co-occurring with drug infusion produced robust and cue-directed behavior such as orienting, touching and biting. Experiment 1 showed that this light-directed behavior would occur in naïve monkeys exposed to a Pavlovian pairing procedure. Four monkeys were given response-independent injections of cocaine. In two monkeys, a red light preceded cocaine injections by 5 s, and a green light co-occurred with the 5-s cocaine injections. In the other two monkeys, the light presentations and cocaine injections occurred independently. Light-directed behavior occurred in all four monkeys within the first couple of trials and at high levels but decreased across sessions. The cocaine-paired stimulus maintained behavior longer and at higher levels than the uncorrelated stimuli. Furthermore, light-directed behavior was not maintained when cocaine was replaced with saline. Light-directed behavior did not occur in the absence of the lights. When these monkeys were subsequently trained to lever press for cocaine, light-directed behavior increased to levels higher than previously observed. Behavior directed towards drug-paired stimuli is robust, reliable and multiply determined; the mechanisms underlying this activity likely include Pavlovian conditioning, stimulus novelty, habituation and operant conditioning.

A within-subject between-apparatus comparison of impulsive choice: T-maze and two-lever chamber.

Whereas intertemporal choice procedures are a common method for examining impulsive choice in nonhuman subjects, the apparatus used to implement this procedure varies across studies. The purpose of the present study was to compare impulsive choice between a two-lever chamber and a T-maze. In Experiment 1, rats chose between a smaller, immediate reinforcer and a larger, delayed reinforcer, first in a two-lever chamber and then in a T-maze. Delay to the larger reinforcer changed in an ascending and descending order (0-32 s) across sessions. Experiment 2 examined the same between-apparatus comparison but under steady-state conditions with the delay fixed at 32 s. In Experiment 1, choice for the larger, delayed reinforcer was generally higher in the T-maze than in the two-lever chamber. Similarly in Experiment 2, steady-state choice for the larger, delayed reinforcer was higher in the T-maze. Choice for the 32-s delayed reinforcer was also greater in Experiment 2 than in Experiment 1, suggesting that extended exposure to the delay is required for the T-maze to yield reliable impulsive choice data. While the reasons for the between-apparatus discrepancies are at present unknown, results from both experiments clearly demonstrate that the apparatus matters when assessing overall level and reliability of impulsive choice data.

A quantitative analysis of the behavior maintained by delayed reinforcers.

Random-interval reinforcement was arranged for a sequence of pigeon first-key pecks followed by second-key pecks. First-key pecks, separated from reinforcers by delays that included number of second-key pecks and time, decreased in rate as delays increased. Delay functions, or gradients, were obtained in one experiment with reinforced sequences consisting of M first-key pecks followed by N second-key pecks (M + N = 16), in a second where required first-key pecks were held constant (M = 8), and in a third where minimum delay between most recent first-key pecks and reinforcers varied. In each, gradients were equally well fitted by exponential, hyperbolic and logarithmic functions. Performances were insensitive to reinforcer duration and functions were consistent across varied random-interval values. In one more experiment, time and number delays were independently varied using differential reinforcement of rate of second-key pecks. Delay gradients depended primarily on time rather than on number of second-key pecks. Thus, reinforcers have effects based on earlier responses, not just the ones that produced them, with the contribution of each response weighted by the time separating it from the reinforcer rather than by intervening behavior. Situations where unwanted responses (e.g., errors) often precede reinforced corrects can maintain them unless designed to avoid such effects of delay.

Intertrial interval duration and impulsive choice.

Discrete-trial intertemporal choice procedures assess impulsive choice or preference for a smaller, immediate reinforcer over a larger, delayed one. The effect of the delay associated with the larger reinforcer has been the focus of much research. It, however, is not the only delay in the context of discrete-trial procedures. Often separating each choice trial is an intertrial interval (ITI) that maintains equal trial spacing of the two alternatives. The removal of this ITI has been shown to increase impulsive choice, perhaps because choosing the small alternative results in another choice trial immediately following reinforcer delivery. Impulsive choice has not been affected when the ITI duration is manipulated in conditions that equate the trial presentation rate across the two alternatives. These null results could have been due to floor effects and/or an inadequate range of ITI durations. To address these possibilities, three experiments were conducted to determine how changes in ITI duration affected impulsive choice in rats and pigeons. All three experiments found that preference for the large delayed alternative decreased (i.e., impulsive choice increased) when the ITI was shortened. Satiation was not a likely explanation since preference for the large alternative at the 0-s delay was not affected by ITI duration. Trial spacing, like other temporal properties of choice situations, is an important variable underlying the occurrence of impulsive choice.

Flash rate discrimination in rats: rate bisection and generalization peak shift.

Two experiments were conducted to determine whether responding by albino rats can be brought under the stimulus control of different flash rates. In the first experiment, a conditional discrimination procedure was employed whereby two different flash rates (fast or slow) signaled the availability of reinforcement on one of two levers (left or right). Stimulus control emerged rapidly and improved with continued training. When intermediate flash rates were presented during probe sessions, the bisection point of the fast and slow flash rates was near their geometric mean, consistent with research employing other stimulus types. In the second experiment, a successive discrimination procedure was employed whereby responding in the presence of one flash rate (S(+) ) was reinforced while responding in the presence of another flash rate (S(-) ) was not reinforced. Again, stimulus control emerged quickly and improved with continued training. Test sessions in which many different flash rates were presented for brief periods in extinction revealed the peak shift phenomenon, in which peak response rates are shifted from the S(+) in a direction away from the S(-) . Flash rate is endorsed as a continuous stimulus dimension that is useful for differentially signaling schedule components.

Rats (Rattus norvegicus) and pigeons (Columbia livia) are sensitive to the distance to food, but only rats request more food when distance increases.

Three experiments investigated foraging by rats and pigeons. In Experiment 1, each response on a manipulandum delivered food to a cup, with the distance between the manipulandum and the cup varying across conditions. The number of responses made before traveling to collect and eat the food increased with distance for rats, but not for pigeons. In Experiment 2, two manipulanda were placed at different distances from a fixed food source; both pigeons and rats preferentially used the manipulandum closest to the food source. Experiment 3 was a systematic replication of Experiment 1 with pigeons. In different conditions, each peck on the left key increased the upcoming hopper duration by 0.5, 1.5 or 2.5s. Completing a ratio requirement on the right key of 1, 4, 8, 16 or 32 pecks, depending on the condition, then produced the food hopper for a duration that depended on the number of prior left pecks. As the ratio requirement increased on the right key, pigeons responded more on the left key and earned more food. Overall, the results replicate previous research, underlining similarities and differences between these species. The results are discussed in terms of optimal foraging, reinforcer sensitivity and delay discounting.

Early cognitive dysfunction in the HD 51 CAG transgenic rat model of Huntington's disease.

Huntington's disease (HD) is a neurodegenerative disorder in humans caused by an expansion of a CAG trinucleotide repeat that produces choreic movements, which are preceded by cognitive deficits. The HD transgenic rat (tgHD), which contains the human HD mutation with a 51 CAG repeat allele, exhibits motor deficits that begin when these rats are 12 months of age. However, there are no reports of cognitive dysfunction occurring prior to this. To assess whether cognitive dysfunction might precede motor deficits in tgHD rats, one group of 9-month-old male rats with homozygotic mutated genes and one group of wild-type (WT) rats underwent three testing phases in a unique Spatial Operant Reversal Test (SORT) paradigm, as well as assessment of spontaneous motor activity. After testing, morphological and histological examination of the brains were made. Results indicated that tgHD rats acquired the cued-response (Phase 1) portion of the SORT, but made significantly more errors during the reversal (Phase 2) and during the pseudorandomized reversals (Phase 3) portion of the study, when compared to WT rats. Analysis of the data using mathematical principles of reinforcement revealed no memory, motor, or motivational deficits. These results indicate that early cognitive dysfunction, as measured by the SORT, occur prior to motor deficits, gross anatomical changes, or cell loss in the tgHD rat with 51 CAG repeats, and suggest that this protocol could provide a useful screen for therapeutic studies.

Immediate postsession feeding reduces operant responding in rats.

Three experiments investigated the effects of immediate and delayed postsession feeding on progressive-ratio and variable-interval schedule performance in rats. During Experiments 1 and 2, immediate postsession feeding decreased the breakpoint, or largest completed ratio, under progressive-ratio schedules. Experiment 3 was conducted to extend the results of the first two experiments to responding maintained by variable-interval schedules with different session lengths (15 and 60 min). Response rates decreased in all 4 subjects when postsession feeding immediately followed a 15-min session and in 3 of 4 subjects when postsession feeding immediately followed a 60-min session. The implications of this research are twofold: (1) The functional context in which within-session reinforcers are embedded extends outside the experimental chamber, and (2) supplemental postsession feedings should be sufficiently delayed from the end of a session to avoid weakening operant behavior in the experimental sessions.

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.

Differential effects of d-amphetamine on impulsive choice in spontaneously hypertensive and Wistar-Kyoto rats.

The spontaneously hypertensive rat (SHR) has been shown to exhibit three of the behavioral characteristics of attention-deficit/hyperactivity disorder: hyperactivity, attention deficit and impulsivity. This study used SHRs and a control strain to assess the effects of the commonly prescribed psychomotor stimulant, d-amphetamine, on impulsivity, defined as choice for a small, immediate over a large, delayed reinforcer. d-Amphetamine (1.0, 3.2 and 5.6 mg/kg) was administered to SHR and Wistar-Kyoto rats (WKY; their progenitor strain) before sessions of a choice task involving small, immediate and larger, delayed food reinforcers. As reported earlier, SHRs were more impulsive than WKYs (they preferred the smaller, immediate reinforcer). d-Amphetamine had no effect on preference for the SHRs, but increased choices for the small, immediate reinforcer for the WKYs at the 1.0 and 3.2 doses. Thus, d-amphetamine did not reduce impulsivity in the already impulsive SHRs, but did increase impulsivity in rats that were not already impulsive.

Evaluating timing in spontaneously hypertensive and Wistar-Kyoto rats using the peak procedure.

A core deficit in timing may underlie the symptoms of attention-deficit/hyperactivity disorder (ADHD). Timing deficits have been observed in ADHD-diagnosed children but have yet to be fully explored in the spontaneously hypertensive rat (SHR), a purported model of ADHD. We asked whether SHRs demonstrate ADHD-like timing deficits using the peak procedure. Response rates across peak intervals were modeled using the sum of two Gaussian curves. Results showed that SHRs peaked earlier than Wistar-Kyotos based on 4s intervals that contained the individuals' maximum response rates but not based on model parameters. The strains showed approximately equal precision of timing based on Weber fractions derived from model parameters, a result that replicates previous findings and does not support the use of SHRs to model this aspect of ADHD.

Modeling operant behavior in the Parkinsonian rat.

Mathematical principles of reinforcement (MPR; Killeen, 1994) is a quantitative model of operant behavior that contains three parameters representing motor capacity (delta), motivation (a), and short term memory (lambda). The present study applied MPR to characterize the effects of bilateral infusions of 6-OHDA into the substantia nigra pars compacta in the rat, a model of Parkinson's disease. Rats were trained to lever press under a 5-component fixed-ratio (5, 15, 30, 60, and 100) schedule of food reinforcement. Rats were tested for 15 days prior to dopamine lesions and again for 15 days post-lesion. To characterize functional loss relative to lesion size, rats were grouped according to the extent and the degree of lateralization of their dopamine loss. Response rates decreased as a function of dopamine depletion, primarily at intermediate ratios. MPR accounted for 98% of variance in pre- and post-lesion response rates. Consistent with reported disruptions in motor behavior induced by dopaminergic lesions, estimates of delta increased when dopamine was severely depleted. There was no support for different estimates of a based on pre- and post-lesion performance of any lesion group, suggesting that dopamine loss has negligible effects on incentive motivation. The present study demonstrates the usefulness of combining operant techniques with a theoretical model to better understand the effects of a neurochemical manipulation.

Impulsive choice in a rodent model of attention-deficit/hyperactivity disorder.

The spontaneously hypertensive rat (SHR) has been studied extensively as a purported rodent model of attention-deficit/hyperactivity disorder (ADHD). Because ADHD in humans is partially defined by marked impulsivity, SHRs, if a valid model of ADHD, ought to behave more impulsively than their normotensive parent strain, Wistar Kyoto (WKY). This prediction was evaluated in two experiments that employed an intertemporal choice procedure in which SHRs and WKYs made repeated choices between a single food pellet delivered immediately and three food pellets delivered after a delay. Four or five delays were investigated (1, 3, 6, 12 and 24s); the experiments differed in the manner in which the delays were experienced. In Experiment 1, the delay values changed after each session and were presented in ascending then descending order. SHRs chose more small/immediate reinforcers than WKYs at the longest delays during the ascending series and at nearly all delays during the descending series. In Experiment 2, the delay values remained in effect for several sessions and were presented in random order. Again, the SHRs chose more small/immediate reinforcers than the WKYs at the longest delays. Thus, in the present study, the SHRs were shown to be more impulsive than the WKYs as defined by preference for smaller, immediate reinforcers over larger, delayed ones in an intertemporal choice procedure.

Effects of intensity and type of prepulse stimulus on prepulse inhibition in scopolamine treated rats.

Prepulse inhibition (PPI) of the auditory startle response (ASR) is a behavioral test that has been used to measure auditory thresholds, to assess sensory-motor integration functions, and its use has been recommended in the United States Environmental Protection Agency Developmental Neurotoxicity Guideline (OPPTS 870.6300). The purpose of the present study was to determine to what extent the intensity and/or type of prepulse stimuli modulate PPI in scopolamine-treated rats. The PPI of the ASR peak amplitude was measured when the intensity of a 10-kHz prepulse tone was varied (69-, 80-, and 90 dB[A]; Experiment 1) and when both the intensity and type of auditory prepulse (a 10-kHz tone vs. a white noise burst) were varied (Experiment 2). Scopolamine treatment attenuated PPI in both experiments and interacted significantly with the prepulse stimulus intensity in Experiment 1. In Experiment 2, the percent of PPI was linearly related to prepulse stimulus intensity for trials using a tone, but was biphasic on trials using a white-noise prepulse stimulus. Prepulse stimuli of certain intensities elicited a response, and this response was greater when the prepulse stimulus was a white noise burst versus a tone of the same intensity. Further, the response to the prepulse altered the amount of inhibition and, therefore, confounded the overall measure of PPI at the higher prepulse stimulus intensity levels. Overall, these results indicate that careful consideration of the intensity and type of prepulse stimuli be taken in the context of their potential to induce a prepulse-elicited response, as well as providing the appropriate measures of such a response, when designing and interpreting PPI experiments.

Response acquisition with delayed reinforcement in a rodent model of attention-deficit/hyperactivity disorder (ADHD).

The spontaneously hypertensive rat (SHR) has been shown to exhibit behavioral characteristics analogous to those exhibited by humans diagnosed with attention-deficit/hyperactivity disorder (ADHD). The present study was conducted to further evaluate the validity of the SHR model of ADHD by characterizing learning of a novel response under conditions of delayed reinforcement. Seven experimentally naïve SHRs and a control group of seven normotensive Wistar-Kyoto (WKY) rats were exposed to a contingency where one lever press initiated pellet delivery after a 15-s, resetting delay. Rats in both groups acquired lever pressing, and the pattern of acquisition was well described with a three-parameter, sigmoidal equation. Response acquisition was retarded in the SHRs; they took longer to acquire the behavior, exhibited lower response rates and earned fewer reinforcers over the course of the experiment. When reinforcer delivery was made immediate in a subsequent condition, the SHRs exhibited higher response rates than the WKY, suggesting that the lower rates of responding seen in the SHRs were due to the reinforcer delay. The results replicate previous research on response acquisition with delayed reinforcement and provide further validation of the SHR strain as a model of ADHD. Like humans diagnosed with ADHD, the SHRs appear to be hypersensitive to delayed consequences, which in the present context, interfered with learning a novel behavior.

Disruption of responding maintained by conditioned reinforcement: alterations in response-conditioned-reinforcer relations.

An observing procedure was used to investigate the effects of alterations in response-conditioned-reinforcer relations on observing. Pigeons responded to produce schedule-correlated stimuli paired with the availability of food or extinction. The contingency between observing responses and conditioned reinforcement was altered in three experiments. In Experiment 1, after a contingency was established in baseline between the observing response and conditioned reinforcement, it was removed and the schedule-correlated stimuli were presented independently of responding according to a variable-time schedule. The variable-time schedule was constructed such that the rate of stimulus presentations was yoked from baseline. The removal of the observing contingency reliably reduced rates of observing. In Experiment 2, resetting delays to conditioned reinforcement were imposed between observing responses and the schedule-correlated stimuli they produced. Delay values of 0, 0.5, 1, 5, and 10 s were examined. Rates of observing varied inversely as a function of delay value. In Experiment 3, signaled and unsignaled resetting delays between observing responses and schedule-correlated stimuli were compared. Baseline rates of observing were decreased less by signaled delays than by unsignaled delays. Disruptions in response-conditioned-reinforcer relations produce similar behavioral effects to those found with primary reinforcement.

Within-session delay-of-reinforcement gradients.

Within-session delay-of-reinforcement gradients were generated with pigeons by progressively increasing delays to reinforcement within each session. In Experiment 1, the effects of imposing progressive delays on variable-interval and fixed-interval schedules were investigated while controlling for simultaneous decreases in reinforcer rate across the session via a within-subject yoked-control procedure. Rate of key pecking decreased as a negatively decelerated function of delay of reinforcement within a session. These rate decreases were greater than those during a yoked-interval session in which the rate of immediate reinforcement decreased at the same rate as it did under the progressive-delay procedure. In Experiment 2, delay-of-reinforcement gradients were shallower when the progressive delay intervals were signaled by a blackout than when they were unsignaled. The delay gradients obtained in each experiment were similar to those generated under conditions in which different delays of reinforcement are imposed across blocks of sessions. The present procedure offers a technique for rapidly generating delay-of-reinforcement gradients that might serve as baselines for assessing the effects of other behavioral and pharmacological variables.