Behavioral effects of intra-cranial self-stimulation in an animal model of attention-deficit/hyperactivity disorder (ADHD)

Previous experience with delays affects delay discounting in animal model of ADHD

BACKGROUND

ADHD is a disorder where a common symptom is impulsive behaviour, a broad term associated with making sub-optimal choices. One frequently used method to investigate impulsive behaviour is delay discounting, which involves choosing between a small, immediate reinforcer and a delayed, larger one. Choosing the small immediate reinforcer is by itself, however, not sufficient for terming the choice impulsive, as all organisms eventually switch to choosing the small, immediate reinforcer when the delay to the larger reinforcer becomes long. This switch can be termed impulsive only when it occurs more frequently, or at shorter LL delays, than typically observed in normal controls. A poorly understood aspect is how choice is influenced by previous experience with delays. Using an animal model of Attention-Deficit/Hyperactivity Disorder, the Spontaneously Hypertensive Rat, we manipulated the order of exposure to delays in a delay discounting task. Following a preference test, the Ascending group experienced gradually increasing delays between choice and reinforcer while the Descending group were exposed to these delays in reverse order.

RESULTS

The results showed that the Descending group chose the small, immediate reinforcer over the larger delayed to a much larger extent than the Ascending group, and continued to do so even when the delay component was ultimately removed. Strain effects were found in the Ascending group, with SHRs switching to the small, immediate reinforcer earlier than controls as the delay to the larger reinforcer increased.

CONCLUSION

The data suggests that delay discounting is affected by history of exposure to delayed consequences. When reinforcement contingencies are incrementally changed from having no response-reinforcer delay to a long delay, discounting of delayed consequences is gradual. However, a sudden change from no delay to a long delay, without intermediate training, results in a rapid switch to the small, immediate reinforcer option, and this behaviour is somewhat resilient to the shortening and eventual removal of the large reinforcer delay. The implication is that attempting to reduce already existing impulsive behaviour in children with ADHD will require gradual habituation and not sudden changes in reinforcement contingencies.

Integrated wireless fast-scan cyclic voltammetry recording and electrical stimulation for reward-predictive learning in awake, freely moving rats

OBJECTIVE

Fast-scan cyclic voltammetry (FSCV) is commonly used to monitor phasic dopamine release, which is usually performed using tethered recording and for limited types of animal behavior. It is necessary to design a wireless dopamine sensing system for animal behavior experiments.

APPROACH

This study integrates a wireless FSCV system for monitoring the dopamine signal in the ventral striatum with an electrical stimulator that induces biphasic current to excite dopaminergic neurons in awake freely moving rats. The measured dopamine signals are unidirectionally transmitted from the wireless FSCV module to the host unit. To reduce electrical artifacts, an optocoupler and a separate power are applied to isolate the FSCV system and electrical stimulator, which can be activated by an infrared controller.

MAIN RESULTS

In the validation test, the wireless backpack system has similar performance in comparison with a conventional wired system and it does not significantly affect the locomotor activity of the rat. In the cocaine administration test, the maximum electrically elicited dopamine signals increased to around 230% of the initial value 20 min after the injection of 10 mg kg(-1) cocaine. In a classical conditioning test, the dopamine signal in response to a cue increased to around 60 nM over 50 successive trials while the electrically evoked dopamine concentration decreased from about 90 to 50 nM in the maintenance phase. In contrast, the cue-evoked dopamine concentration progressively decreased and the electrically evoked dopamine was eliminated during the extinction phase. In the histological evaluation, there was little damage to brain tissue after five months chronic implantation of the stimulating electrode.

SIGNIFICANCE

We have developed an integrated wireless voltammetry system for measuring dopamine concentration and providing electrical stimulation. The developed wireless FSCV system is proven to be a useful experimental tool for the continuous monitoring of dopamine levels during animal learning behavior studies of freely moving rats.

Extinction learning deficit in a rodent model of attention-deficit hyperactivity disorder

Background

Deficient operant extinction has been hypothesized to be constitutive of ADHD dysfunction. In order to elucidate the behavioral mechanisms underlying this deficit, the performance of an animal model of ADHD, the spontaneously hypertensive rat (SHR), was compared against the performance of a control strain, the Wistar-Kyoto rat (WKY) during extinction.

Method

Following extensive training of lever pressing under variable interval schedules of food reinforcement (reported previously), SHR and WKY rats were exposed to two sessions of extinction training. Extinction data was analyzed using the Dynamic Bi-Exponential Refractory Model (DBERM) of operant performance. DBERM assumes that operant responses are organized in bouts separated by pauses; during extinction, bouts may decline across multiple dimensions, including frequency and length. DBERM parameters were estimated using hierarchical Bayesian modeling.

Results

SHR responded more than WKY during the first extinction session. DBERM parameter estimates revealed that, at the onset of extinction, SHR produced more response bouts than WKY. Over the course of extinction, response bouts progressively shortened for WKY but not for SHR.

Conclusions

Based on prior findings on the sensitivity of DBERM parameters to motivational and schedule manipulations, present data suggests that (1) more frequent response bouts in SHR are likely related to greater incentive motivation, and (2) the persistent length of bouts in SHR are likely related to a slower updating of the response-outcome association. Overall, these findings suggest specific motivational and learning deficits that may explain ADHD-related impairments in operant performance.

Rat models of ADHD

Showing that an animal is hyperactive is not sufficient for it to be accepted as a model of ADHD. Based on behavioral, genetic, and neurobiological data, the spontaneously hypertensive rat (SHR) obtained from Charles River, Germany, (SHR/NCrl) is at present the best-validated animal model of ADHD. One Wistar Kyoto substrain (WKY/NHsd), obtained from Harlan, UK, is its most appropriate control. Another WKY substrain (WKY/NCrl) obtained from Charles River, Germany, is inattentive, has distinctly different genetics and neurobiology, and provides a promising model for the predominantly inattentive subtype of ADHD (ADHD-I) if one wants to investigate categorical ADHD subtypes. In this case, also, the WKY/NHsd substrain should be used as control. Although other rat strains may behave like WKY/NHsd rats, neurobiological results indicate significant differences when compared to the WKY/NHsd substrain, making them less suitable as controls for the SHR/NCrl. Thus, there are no obvious behavioral differences among the various SHRs, but there are behavioral and neurobiological differences among the WKY strains. The use of WKY/NCrl, outbred Wistar, Sprague Dawley, or other rat strains as controls for SHR/NCrl may produce spurious neurobiological effects and erroneous conclusions. Finally, model data yield support to independent hyperactivity and inattention dimensions in ADHD behavior.

Electrophysiological evidence of atypical motivation and reward processing in children with attention-deficit hyperactivity disorder

Behavioral and neurophysiological evidence suggest that attention-deficit hyperactivity disorder (ADHD) is characterized by the impact of abnormal reward prediction error signals carried by the midbrain dopamine system on frontal brain areas that implement cognitive control. To investigate this issue, we recorded the event-related brain potential (ERP) from typical children and children with ADHD as they navigated a "virtual maze" to find monetary rewards, and physically gave them their accumulated rewards halfway through the task and at the end of the experiment. We found that the amplitude of a reward-related ERP component decreased somewhat for typical children after they received their first payment, but increased for children with ADHD following the payment. This result indicates that children with ADHD are unusually sensitive to the salience of reward and suggests that such sensitivity may be mediated in part by the midbrain dopamine system.

Reprint of "Problems with spontaneously hypertensive rats (SHR) as a model of attention-deficit/hyperactivity disorder (AD/HD)"

Spontaneously hypertensive rats (SHR) have been promoted as animal models of attention-deficit hyperactivity disorder (AD/HD). Free-operant experiments have compared the effects of simple schedules, extinction, delayed reinforcers, and non-contingent reinforcement on the behaviour of SHR and Wistar Kyoto (WKY) controls. Differences between SHR and WKY performance have been paralleled with the overactivity, impulsiveness, and inattention that characterize AD/HD. Re-examination of selected studies suggests that differences in these strains' overall response rates can exaggerate other differences between SHR and WKY performance. Removing this confounding factor can reduce differences between these two strains' behaviours.

Behavioral variability, elimination of responses, and delay-of-reinforcement gradients in SHR and WKY rats

Background

Attention-deficit/hyperactivity disorder (ADHD) is characterized by a pattern of inattention, hyperactivity, and impulsivity that is cross-situational, persistent, and produces social and academic impairment. Research has shown that reinforcement processes are altered in ADHD. The dynamic developmental theory has suggested that a steepened delay-of-reinforcement gradient and deficient extinction of behavior produce behavioral symptoms of ADHD and increased behavioral variability.

Method

The present study investigated behavioral variability and elimination of non-target responses during acquisition in an animal model of ADHD, the spontaneously hypertensive rat (SHR), using Wistar Kyoto (WKY) rats as controls. The study also aimed at providing a novel approach to measuring delay-of-reinforcement gradients in the SHR and the WKY strains. The animals were tested in a modified operant chamber presenting 20 response alternatives. Nose pokes in a target hole produced water according to fixed interval (FI) schedules of reinforcement, while nose pokes in the remaining 19 holes either had no consequences or produced a sound or a short flickering of the houselight. The stimulus-producing holes were included to test whether light and sound act as sensory reinforcers in SHR.

Data from the first six sessions testing FI 1 s were used for calculation of the initial distribution of responses. Additionally, Euclidean distance (measured from the center of each hole to the center of the target hole) and entropy (a measure of variability) were also calculated.

Delay-of-reinforcement gradients were calculated across sessions by dividing the fixed interval into epochs and determining how much reinforcement of responses in one epoch contributed to responding in the next interval.

Results

Over the initial six sessions, behavior became clustered around the target hole. There was greater initial variability in SHR behavior, and slower elimination of inefficient responses compared to the WKY. There was little or no differential use of the stimulus-producing holes by either strain. For SHR, the reach of reinforcement (the delay-of-reinforcement gradient) was restricted to the preceding one second, whereas for WKY it extended about four times as far.

Conclusion

The present findings support previous studies showing increased behavioral variability in SHR relative to WKY controls. A possibly related phenomenon may be the slowed elimination of non-operant nose pokes in SHR observed in the present study. The findings provide support for a steepened delay-of-reinforcement gradient in SHR as suggested in the dynamic developmental theory of ADHD. Altered reinforcement processes characterized by a steeper and shorter delay-of-reinforcement gradient may define an ADHD endophenotype.

Problems with spontaneously hypertensive rats (SHR) as a model of attention-deficit/hyperactivity disorder (AD/HD)

Spontaneously hypertensive rats (SHR) have been promoted as animal models of attention-deficit hyperactivity disorder (AD/HD). Free-operant experiments have compared the effects of simple schedules, extinction, delayed reinforcers, and non-contingent reinforcement on the behaviour of SHR and Wistar Kyoto (WKY) controls. Differences between SHR and WKY performance have been paralleled with the overactivity, impulsiveness, and inattention that characterize AD/HD. Re-examination of selected studies suggests that differences in these strains' overall response rates can exaggerate other differences between SHR and WKY performance. Removing this confounding factor can reduce differences between these two strains' behaviours.

Involvement of Tissue Plasminogen Activator-Plasmin System in Depolarization-Evoked Dopamine Release in the Nucleus Accumbens of Mice

Tissue plasminogen activator (tPA), a serine protease, catalyzes the conversion of plasminogen to plasmin. In the present study, we investigated the role of the tPA-plasmin system in depolarization-evoked dopamine (DA) and acetylcholine (ACh) release in the nucleus accumbens (NAc) and hippocampus, respectively, of mice, by using in vivo microdialysis. Microinjection of either tPA or plasmin significantly potentiated 40 mM KCl-induced DA release without affecting basal DA levels. In contrast, plasminogen activator inhibitor-1 dose-dependently reduced 60 mM KCl-induced DA release. The 60 mM KCl-evoked DA release in the NAc was markedly diminished in tPA-deficient (tPA-/-) mice compared with wild-type mice, although basal DA levels did not differ between the two groups. Microinjections of either exogenous tPA (100 ng) or plasmin (100 ng) into the NAc of tPA-/-mice restored 60 mM KCl-induced DA release, as observed in wild-type mice. In contrast, there was no difference in either basal or 60 mM KCl-induced ACh release in the hippocampus between wild-type and tPA-/-mice. Our findings suggest that the tPA-plasmin system is involved in the regulation of depolarization-evoked DA release in the NAc.

Slower extinction of responses maintained by intra-cranial self-stimulation (ICSS) in an animal model of attention-deficit/hyperactivity disorder (ADHD)

Children with attention-deficit/hyperactivity disorder (ADHD) show performance deficits and excessive motor activity during extinction and in situations where no reinforcer can be identified, suggesting an extinction deficit in ADHD possibly linked to dopamine dysfunction. The present study examined extinction of responding previously maintained by intra-cranial self-stimulation (ICSS) in spontaneously hypertensive rats (SHR), an animal model of ADHD using three different extinction procedures. Delivery of electrical pulses were terminated altogether or presented independently of responding using two different current intensities. The results showed that more responses were retained in the SHR, especially during the initial transition from ICSS-maintained responding to response-independent delivery of electrical pulses with current reduced relative to that given during reinforcement. Slower extinction of previously reinforced behavior is suggested as an alternative explanation for the frequently observed increased behavioral output that has previously been interpreted as "disinhibition" of behavior in ADHD.

Effects of delayed reinforcers on the behavior of an animal model of attention-deficit/hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD), affecting 3-5% of grade-school children, is a behavioral disorder characterized by developmentally inappropriate levels of inattention, hyperactivity, and impulsivity. It has been suggested that the symptoms are caused by altered reinforcement and extinction processes, behaviorally described as an abnormally short and steep delay-of-reinforcement gradient in ADHD. The present study tested predictions from the suggested shortened and steepened delay gradient in ADHD in an animal model, the spontaneously hypertensive rats (SHRs). It was predicted that SHR responding during baseline would mainly consist of responses with short inter-response times, and that responding would be more rapidly reduced in the SHR than in the controls by the introduction of a time interval between the response and reinforcer delivery. Effects of a resetting delay of reinforcement procedure with water as the reinforcer were tested on two baseline reinforcement schedules: variable interval 30 s (VI 30 s) and conjoint variable interval 60 s differential reinforcement of high rate 1s (VI 60 s DRH 1 s). The results showed a higher rate of responses in the SHR than in the controls during baseline, mainly consisting of responses with short inter-response times. The statistical analyses showed that response rates decreased more rapidly as a function of reinforcer delay in the SHR than in the controls. The analyses of the estimates of the reinforcer decay parameter showed no strain differences during the VI 30 s schedule but showed a significant strain difference at the end, but not at the start, of the sessions during the VI 60 s DRH 1 s schedule. In general, the results support predictions from the suggested steepened delay gradient in SHR. However, the predictions were only partly confirmed by the analyses of the decay parameter.