Response-inhibition capacity in spontaneously hypertensive and Wistar rats: acquisition of fixed minimum interval performance and responsiveness to D-amphetamine

Reduced response-inhibition capacity is a defining feature of attention-deficit hyperactivity disorder. The fixed minimum interval (FMI) schedule has been systematically validated to assess such capacity in rats. On each FMI trial, the first lever press initiates an inter-response time (IRT); a potentially consummatory response terminates the IRT; only IRTs longer than a target interval result in access to food. Despite task validity, steady-state FMI performance in the most common animal model of attention-deficit hyperactivity disorder, the spontaneously hypertensive rat (SHR), is similar to normotensive control performance, even though SHR performs at lower levels, especially during acquisition, in similar response-withholding tasks. To determine whether such limitations of the model are specific to stable-state performance, this experiment compared FMI 6-s performance in SHR and Wistar rats during acquisition and in steady state, and assessed the effect of acute D-amphetamine (AMP) administration (0.1, 0.5, and 1.0 mg/kg) on steady-state performance. Median latencies to first lever press were consistently shorter in SHR than in Wistar rats; IRTs were shorter for SHR than for Wistar rats during acquisition, but substantially less so during asymptotic performance. AMP dose-dependently reduced latencies, shortened IRTs, and, at the highest dose, increased the proportion of IRTs under schedule control. These results suggest that, relative to Wistar rats, SHR have a reduced capacity to learn to withhold a reinforced response; once the FMI is acquired, high doses of D-AMP disrupt withholding performance in both strains, but they also enhance the responsiveness of both strains to reinforcement contingencies.

Differential Behavioral and Biochemical Responses to Caffeine in Male and Female Rats from a Validated Model of Attention Deficit and Hyperactivity Disorder

Epidemiological studies suggest sex differences in attention deficit and hyperactivity disorder (ADHD) symptomatology. The potential benefits of caffeine have been reported in the management of ADHD, but its effects were not properly addressed with respect to sex differences. The present study examined the effects of caffeine (0.3 g/L) administered since childhood in the behavior and brain-derived neurotrophic factor (BDNF) and its related proteins in both sexes of a rat model of ADHD (spontaneously hypertensive rats-SHR). Hyperlocomotion, recognition, and spatial memory disturbances were observed in adolescent SHR rats from both sexes. However, females showed lack of habituation and worsened spatial memory. Although caffeine was effective against recognition memory impairment in both sexes, spatial memory was recovered only in female SHR rats. Besides, female SHR rats showed exacerbated hyperlocomotion after caffeine treatment. SHR rats from both sexes presented increases in the BDNF, truncated and phospho-TrkB receptors and also phospho-CREB levels in the hippocampus. Caffeine normalized BDNF in males and truncated TrkB receptor at both sexes. These findings provide insight into the potential of caffeine against fully cognitive impairment displayed by females in the ADHD model. Besides, our data revealed that caffeine intake since childhood attenuated behavioral alterations in the ADHD model associated with changes in BDNF and TrkB receptors in the hippocampus.

Toward a Unifying Account of Impulsivity and the Development of Self-Control

Impulsivity has traditionally been thought to involve various behavioral traits that can be measured using different laboratory protocols. Whereas some authors regard different measures of impulsivity as reflecting fundamentally distinct and unrelated behavioral tendencies (fragmentation approach), others regard those different indexes as analogue forms of the same behavioral tendency, only superficially different (unification approach). Unifying accounts range from mere intuitions to more sophisticated theoretical systems. Some of the more complete attempts at unifying are intriguing but have validity weaknesses. We propose a new unifying attempt based on theoretical points posed by other authors and supplemented by theory and research on associative learning. We then apply these assumptions to characterize the paradigms used to study impulsivity in laboratory settings and evaluate their scope as an attempt at unification. We argue that our approach possesses a good balance of parsimony and empirical and theoretical grounding, as well as a more encompassing scope, and is more suitable for experimental testing than previous theoretical frameworks. In addition, the proposed approach is capable of generating a new definition of impulsivity and outlines a hypothesis of how self-control can be developed. Finally, we examine the fragmentation approach from a different perspective, emphasizing the importance of finding similarities among seemingly different phenomena.

Validation of a method to assess ADHD-related impulsivity in animal models

BACKGROUND

Response inhibition capacity (RIC), the ability to withhold instrumentally reinforced responses, is compromised in ADHD. Most standard methods for assessing RIC in rodents potentially confound motivational, motor, learning, and inhibitory processes, lack sensitivity to pharmacological treatment, and have unknown reliability.

NEW METHOD

The fixed minimum interval (FMI) schedule of reinforcement and its associated analytical techniques are designed to dissociate inhibitory processes from incentive-motivational and timing processes. This study is aimed at validating the FMI as a method for assessing RIC in animal models. FMI performance was compared across different withholding requirements (0.5, 3, 6 and 21s), deprivation levels, reinforcement rates, and reinforcer magnitudes.

RESULTS AND COMPARISON WITH EXISTING METHODS

Motivational manipulations differentially affected estimates of incentive motivation but not the FMI-derived index of RIC, θ. Changes in the withholding requirement influenced timed IRTs in a manner consistent with extant timing theories. Individual estimates of RIC were resilient to prolonged changes in motivation but not to changes in FMI schedule. Results indicate that the FMI schedule is not vulnerable to the same limitations associated with existing methods for assessing RIC.

CONCLUSIONS

These results support the use of the FMI schedule and associated analytic techniques as tools for assessing RIC in animal models.

Detrimental effects of acute nicotine on the response-withholding performance of spontaneously hypertensive and Wistar Kyoto rats

RATIONALE

Attention-deficit hyperactivity disorder (ADHD) is associated with a higher prevalence of smoking, which may be related to potential therapeutic effects of nicotine on ADHD symptoms. Whereas nicotine offers robust improvements in sustained attention, the effects of nicotine on impulsivity are unclear.

OBJECTIVES

The present study examined the effects of nicotine on the response inhibition capacity of spontaneously hypertensive rats (SHR), an animal model of ADHD, compared to that of a normotensive control Wistar Kyoto (WKY), using the fixed minimum interval (FMI) schedule of reinforcement.

METHODS

Tests were conducted following acute injections of subcutaneous nicotine (0.1-0.6 mg/kg). On each FMI trial, the first lever press initiated an inter-response time (IRT); a head entry into a food receptacle terminated the IRT. IRTs longer than 6 s were intermittently reinforced with sucrose.

RESULTS

A model that assumes that only a proportion of IRTs are sensitive to the timing contingencies of the FMI provided a close fit to the data, regardless of strain or treatment. No baseline difference in FMI performance was observed between SHR and WKY. Nicotine reduced the duration of timed IRTs and the duration of latencies to the IRT-initiating lever press similarly for both strains. Nicotine dose-dependently increased the proportion of timed IRTs; the dose-response curve was shifted leftwards in SHR relative to WKY.

CONCLUSIONS

These results suggest that nicotine (a) reduces response-inhibition capacity, (b) enhances the reinforcing efficacy of sucrose, and (c) dose-dependently enhances attention-like sensitivity to contingencies of reinforcement, through mechanisms that are yet unknown.

Reinforcement, dopamine and rodent models in drug development for ADHD

Attention deficit hyperactivity disorder (ADHD) presents special challenges for drug development. Current treatment with psychostimulants and nonstimulants is effective, but their mechanism of action beyond the cellular level is incompletely understood. We review evidence suggesting that altered reinforcement mechanisms are a fundamental characteristic of ADHD. We show that a deficit in the transfer of dopamine signals from established positive reinforcers to cues that predict such reinforcers may underlie these altered reinforcement mechanisms, and in turn explain key symptoms of ADHD. We argue that the neural substrates controlling the excitation and inhibition of dopamine neurons during the transfer process are a promising target for future drug development. There is a need to develop animal models and behavioral paradigms that can be used to experimentally investigate these mechanisms and their effects on sensitivity to reinforcement. More specific and selective targeting of drug development may be possible through this approach.

Animal models to guide clinical drug development in ADHD: lost in translation?

We review strategies for developing animal models for examining and selecting compounds with potential therapeutic benefit in attention-deficit hyperactivity disorder (ADHD). ADHD is a behavioural disorder of unknown aetiology and pathophysiology. Current understanding suggests that genetic factors play an important role in the aetiology of ADHD. The involvement of dopaminergic and noradrenergic systems in the pathophysiology of ADHD is probable. We review the clinical features of ADHD including inattention, hyperactivity and impulsivity and how these are operationalized for laboratory study. Measures of temporal discounting (but not premature responding) appear to predict known drug effects well (treatment validity). Open-field measures of overactivity commonly used do not have treatment validity in human populations. A number of animal models have been proposed that simulate the symptoms of ADHD. The most commonly used are the spontaneously hypertensive rat (SHR) and the 6-hydroxydopamine-lesioned (6-OHDA) animals. To date, however, the SHR lacks treatment validity, and the effects of drugs on symptoms of impulsivity and inattention have not been studied extensively in 6-OHDA-lesioned animals. At the present stage of development, there are no in vivo models of proven effectiveness for examining and selecting compounds with potential therapeutic benefit in ADHD. However, temporal discounting is an emerging theme in theories of ADHD, and there is good evidence of increased value of delayed reward following treatment with stimulant drugs. Therefore, operant behaviour paradigms that measure the effects of drugs in situations of delayed reinforcement, whether in normal rats or selected models, show promise for the future.

Sex, ADHD symptoms, and smoking outcomes: an integrative model

Both females and individuals with Attention-Deficit/Hyperactivity Disorder (ADHD) have been found to be at increased risk for a range of smoking outcomes, and recent empirical findings have suggested that women with ADHD may be particularly vulnerable to nicotine dependence. On a neurobiological level, the dopamine reward processing system may be implicated in the potentially unique interaction of nicotine with sex and with ADHD status. Specifically, nicotine appears to mitigate core ADHD symptoms through interaction with the dopamine reward processing system, and ovarian hormones have been found to interact with nicotine within the dopamine reward processing system to affect neurotransmitter release and functioning. This article synthesizes data from research examining smoking in women and in individuals with ADHD to build an integrative model through which unique risk for cigarette smoking in women with ADHD can be systematically explored. Based upon this model, the following hypotheses are proposed at the intersection of each of the three variables of sex, ADHD, and smoking: (1) individuals with ADHD have altered functioning of the dopamine reward system, which diminishes their ability to efficiently form conditioned associations based on environmental contingencies; these deficits are partially ameliorated by nicotine; (2) nicotine interacts with estrogen and the dopamine reward system to increase the positive and negative reinforcement value of smoking in female smokers; (3) in adult females with ADHD, ovarian hormones interact with the dopamine reward system to exacerbate ADHD-related deficits in the capacity to form conditioned associations; and (4) during different phases of the menstrual cycle, nicotine and ovarian hormones may interact differentially with the dopamine reward processing system to affect the type and value of reinforcement smoking provides for women with ADHD. Understanding the bio-behavioral mechanisms underlying cigarette addiction in specific populations will be critical to developing effectively tailored smoking prevention and cessation programs for these groups. Overall, the goal of this paper is to examine the interaction of sex, smoking, and ADHD status within the context of the dopamine reward processing system not only to elucidate potential mechanisms specific to female smokers with ADHD, but also to stimulate consideration of how the examination of such individual differences can inform our understanding of smoking more broadly.

The effect of methylphenidate and rearing environment on behavioral inhibition in adult male rats

RATIONALE

The ability to withhold reinforced responses-behavioral inhibition-is impaired in various psychiatric conditions including Attention Deficit Hyperactivity Disorder (ADHD). Methodological and analytical limitations have constrained our understanding of the effects of pharmacological and environmental factors on behavioral inhibition.

OBJECTIVES

To determine the effects of acute methylphenidate (MPH) administration and rearing conditions (isolated vs. pair-housed) on behavioral inhibition in adult rats.

METHODS

Inhibitory capacity was evaluated using two response-withholding tasks, differential reinforcement of low rates (DRL) and fixed minimum interval (FMI) schedules of reinforcement. Both tasks made sugar pellets contingent on intervals longer than 6 s between consecutive responses. Inferences on inhibitory and timing capacities were drawn from the distribution of withholding times (interresponse times, or IRTs).

RESULTS

MPH increased the number of intervals produced in both tasks. Estimates of behavioral inhibition increased with MPH dose in FMI and with social isolation in DRL. Nonetheless, burst responding in DRL and the divergence of DRL data relative to past studies, among other limitations, undermined the reliability of DRL data as the basis for inferences on behavioral inhibition.

CONCLUSIONS

Inhibitory capacity was more precisely estimated from FMI than from DRL performance. Based on FMI data, MPH, but not a socially enriched environment, appears to improve inhibitory capacity. The highest dose of MPH tested, 8 mg/kg, did not reduce inhibitory capacity but reduced the responsiveness to waiting contingencies. These results support the use of the FMI schedule, complemented with appropriate analytic techniques, for the assessment of behavioral inhibition in animal models.

Intra-individual variability in genetic and environmental models of attention-deficit/hyperactivity disorder

The frequent observation of intra-individual variability (IIV) in the expression of ADHD symptoms suggest that IIV is an integral component of the disorder. We tested IIV in ADHD-like phenotype from five different studies of rodent models of ADHD, including studies with Spontaneous Hypertensive Rats (SHR/NCrl and SHR/N), Wistar-Kyoto Hyperactive Rats (WKHA/N), Wistar-Kyoto Hypertensive rat (WKHT), PCB-126 and -153-treated Lewis rats and behaviorally normal Wistar/Mol, Wistar-Kyoto (WKY/N and WKY/NMol), and untreated Lewis rats. Averages of the absolute residual deviation of ADHD-like behavior from individual means ("individual phenotypic dispersion," PD(i)) were used to represent IIV in the fixed-interval (FI) and extinction (EXT) phases of operant behavioral activity. Across all studies, SHR rats had higher PD(i) than WKY rats (P < 0.0001) for all ADHD-like traits, and higher PD(i) for hyperactivity than WKHT and WKHA/N rats. Male SHR rats in particular had higher PD(i) for hyperactivity than male or female WKYs, SHR females for EXT hyperactivity, and higher dispersion for inattention than WKY females. These findings strongly suggest the genetic control of IIV, and suggest that the SHR may be a useful model for the identification of genes for IIV in human ADHD. These findings also obliquely support the SHR as a useful model for ADHD overall.

Potential hormonal mechanisms of attention-deficit/hyperactivity disorder and major depressive disorder: a new perspective

Hormonal influences on the organization of behavior are apparent to neuroendocrinologists but under-examined in relation to childhood and adolescent mental disorders. A central mystery in the field of developmental psychopathology is the preferential male vulnerability to behavior disorders in childhood and female vulnerability to emotional disorders in adolescence. Relative neglect of a hormonal explanation may be due to lack of simple or unifying conceptual paradigms to guide studies. This paper seeks to stimulate research in this area by drawing upon clinical psychology and neuroscience literatures to offer a heuristic paradigm for clinical research. Two syndromes are selected here for illustration: Attention-Deficit/Hyperactivity Disorder (ADHD) and Major Depressive Disorder (MDD), because they have opposite gender risk profiles. Two guiding theories are evaluated. First, prenatal organizational effects of testosterone may modulate striatally-based dopaminergic circuits in such a way as to place boys at greater risk for early developing inattention and disruptive behavioral disorders. Second, activational effects of estradiol at puberty may modulate amygdalar and other circuitry, with particular effects on serotonergic pathways, in such a way as to place girls at greater risk for internalizing and mood disorders. Hypotheses from these theories are evaluated based on the current available literature, and limitations of, and future directions for, this literature are discussed.

Effects of sex hormones on associative learning in spontaneously hypertensive rats

Pavlovian conditioning of a visual stimulus paired with food was examined in spontaneously hypertensive rats (SHR), which are a commonly used model for Attention-Deficit/Hyperactivity Disorder (ADHD), and in Wistar rats (normoactive control). In gonadally intact rats of both strains, males spent more time in the food cup following onset of the light than did females, indicating a stronger association of the conditioned stimulus (CS) with reward. Gonadectomy carried out in adulthood affected conditioning differently in the two strains. In Wistar rats, gonadectomy had no effect on conditioned responding in females, but reduced conditioned responding in males, effectively eliminating the sex difference in behavior. This result suggests that circulating androgens in male Wistar rats normally aid conditioning in this task. In contrast, gonadectomy enhanced conditioning in both sexes in the SHR rats, indicating that androgens and/or estrogens impair conditioned associations in this strain. These data indicate that gonadal steroids can influence conditioning in rats and that the valence of steroid action on this behavior is strain-dependent. To the extent that SHR serves as a model of ADHD in humans, the influence of steroids on associative learning may play a role in the expression of ADHD-like behaviors.

Sex differences in learning and inhibition in spontaneously hypertensive rats

Few studies have addressed potential differences in the nature of cognitive impairment observed in males and females with ADHD. In Experiment 1, we examined sex differences in conditioned inhibitory behaviour in spontaneously hypertensive rats (SHR strain), a purported animal model of ADHD. Rats were presented with two types of trials during each of the 15 conditioning sessions. On some trials an auditory stimulus (a tone) was presented and followed immediately by delivery of food reward. On the remaining trials the tone was preceded by presentation of a visual stimulus and on those trials food was not delivered after the tone was presented. As training progressed, conditioned responding during presentation of the tone increased on reinforced trials and decreased during the non-reinforced trials, indicative of successful discrimination and inhibition. Overall, female SHR rats exhibited less conditioned overall food cup behaviour compared to male rats. Female SHR rats also required more training sessions until they responded significantly more during presentation of the tone on reinforced trials versus non-reinforced trials. In addition, the magnitude of the discrimination was smaller in female SHR rats compared to males. In contrast, no sex differences were observed in WKY rats (commonly-used control strain) in Experiment 2. Importantly, there were no significant sex differences in baseline activity or motivation during either experiment, indicating that performance differences could not account for the observed results. These results suggest that male and female SHR rats differ in their ability to form conditioned associations and inhibit behavioural responses and may provide a useful model for sex differences in cognitive dysfunction specific to ADHD.

Baseline behavior, but not sensitivity to stimulant drugs, differs among spontaneously hypertensive, Wistar-Kyoto, and Sprague-Dawley rat strains

Deficits in temporal processing are implicated in Attention Deficit Hyperactivity Disorder (ADHD) for which the most common rodent model is the Spontaneously Hypertensive Rat (SHR). To assess strain differences in temporal processing, males and females of the SHR, Wistar-Kyoto (WKY), and Sprague-Dawley (SD) strains were compared on two timing tasks: one requiring maintenance of a lever press for 10-14 s (TRD, temporal response differentiation) and the other requiring withholding of a lever press for 10-14 s (DRL, differential reinforcement of low rates). Performance of the progressive ratio (PR) task more directly assessed food-motivated behavior. Strains did not differ in task acquisition; however, steady state TRD and DRL performance of the SHR and WKY strains was less accurate which was related to increased burst (non-timing related) responses in those strains relative to the SD. PR performance demonstrated that the SHR and WKY strains exhibited higher response rates and breakpoints than the SD. Subsequently, methylphenidate (1, 3.25, 4.50, 7.50, and 12.0 mg/kg) and d-amphetamine (0.1, 0.25, 0.65, 1.0, and 2.0 mg/kg) were administered intraperitoneally pre-testing. Both drugs disrupted TRD and DRL performances by increasing burst response frequency; however, the strains were not differentially sensitive to either drug. Strain differences were generally maintained throughout the drug and extinction portions of the study. These results indicate increased similarity between the SHR and WKY strains relative to the SD in performance of timing and motivation tasks. Further, the current results do not support continued use of the SHR as a model for ADHD.

Early androgen treatment influences the pattern and amount of locomotion activity differently and sexually differentially in an animal model of ADHD

Higher testosterone level in males is one of the most obvious possibilities for the development of a clear gender difference in ADHD. The present study focused on the influence of excessive androgen exposure in the developmental stage on the hyperactivity feature of ADHD. The study used the spontaneously hypertensive rat (SHR) as an animal model. The amount of locomotion activity previously used as an over-activity measure in the SHR has resulted in a complicated picture. While the general activity level of SHR was significantly higher than its progenitor-the Wistar Kyoto rat (WKY), comparative differences with the Wistar rat could be observed only under certain experimental conditions. The present study applied the scaling approach to assess open field behaviors from a qualitative aspect. Although SHR and Wistar rats showed similar locomotion amounts, movement patterns differed significantly, as indicated by the spatial scaling exponent. Androgen treatment during the early postnatal developmental stage significantly increased total path lengths only in the male SHR. Effects of the hormone manipulation were not expressed in the scaling measurement. The scaling approach conclusively provides a different aspect of open field behaviors and also reacts differently as the total path length to excessive early testosterone exposure.

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.

Rodent models of attention-deficit/hyperactivity disorder

An ideal animal model should be similar to the disorder it models in terms of etiology, biochemistry, symptomatology, and treatment. Animal models provide several advantages over clinical research: simpler nervous systems, easily interpreted behaviors, genetic homogeneity, easily controlled environment, and a greater variety of interventions. Attention-deficit/hyperactivity disorder (ADHD) is a neurobehavioral disorder of childhood onset that is characterized by inattentiveness, hyperactivity, and impulsiveness. Its diagnosis is behaviorally based; therefore, the validation of an ADHD model must be based in behavior. An ADHD model must mimic the fundamental behavioral characteristics of ADHD (face validity), conform to a theoretical rationale for ADHD (construct validity), and predict aspects of ADHD behavior, genetics, and neurobiology previously uncharted in clinical settings (predictive validity). Spontaneously hypertensive rats (SHR) fulfill many of the validation criteria and compare well with clinical cases of ADHD. Poor performers in the five-choice serial reaction time task and Naples high-excitability rats (NHE) are useful models for attention-deficit disorder. Other animal models either focus on the less important symptom of hyperactivity and might be of limited value in ADHD research or are produced in ways that would not lead to a clinical diagnosis of ADHD in humans, even if ADHD-like behavior is displayed.

Dissociation of hypertension and fixed interval responding in two separate strains of genetically hypertensive rat

The behaviour of spontaneously hypertensive rats (SHR) may model attention-deficit hyperactivity disorder. For example, SHR are hyperactive in an open field and show high terminal rates of responding on certain fixed-interval schedules. Open field behaviour has been dissociated from the accompanying spontaneous hypertension but fixed interval responding has not. We compared the fixed interval responding of two unrelated strains of genetically hypertensive rat, the SHR (n = 6) and the New Zealand genetically hypertensive rat (GH, n = 5), with their normotensive control strains, the Wistar Kyoto (WKY, n = 6) and Wistar (n = 5), respectively. Both hypertensive rat strains showed increased terminal lever-pressing rates on a multiple fixed-interval schedule (FI-EXT) compared to controls. In order to investigate the association of hypertension and the behavioural characteristics in question, an F-2 hybrid strain was obtained by cross-breeding GH and Wistar rats. When these F-2 hybrids (n = 33) were tested on the FI-EXT schedule, terminal lever-pressing rate was not correlated with blood pressure. The independent segregation of these phenotypical characteristics in the hybrids suggests independent genetic control. By contrast, other behavioural characteristics, including high lever-pressing rates during the extinction component and a tendency to emit responses in bursts, did cosegregate with terminal lever-pressing rates. Taken together, these findings suggest that the genetic loci for high blood pressure and responding on the FI-EXT schedule in these two unrelated rat strains are close but distinct.

Behavioural and pharmacological relevance of stroke-prone spontaneously hypertensive rats as an animal model of a developmental disorder

The present study evaluates juvenile stroke-prone spontaneously hypertensive rats (SHRSP) as an animal model of a developmental disorder, which is diagnosed according to hyperactivity-impulsivity and/or inattention. To characterize behavioural alterations, we studied motor activity, as well as emotional and cognitive behaviours in juvenile SHRSP, with and without methylphenidate, a psychostimulant. Ambulatory and rearing activities in the open-field environment were significantly higher in SHRSP than in Wistar-Kyoto rats (WKY). In the elevated plus maze task, the entries into open arms, as an index of impulsivity, were significantly increased in SHRSP. In the Y-maze task, spontaneous alternation behaviour, as an index of attention, was significantly lowered in the male SHRSP, but not in the female SHRSP, indicating that spontaneous alternation deficit is gender specific. Methylphenidate (0.01-1 mg/kg, i.p.) significantly attenuated locomotor hyperactivity at low doses and dose-dependently improved the spontaneous alternation deficit in SHRSP. Our findings reveal that juvenile SHRSP manifest problematic behaviours resembling a developmental disorder, attention-deficit/hyperactivity disorder (ADHD), namely hyperactivity-impulsivity and/or inattention. Methylphenidate alleviated the behavioural symptoms of hyperactivity and inattention. We propose that juvenile SHRSP are an appropriate animal model of a developmental disorder resembling ADHD, from behavioural and pharmacological perspectives.

Evaluation of the spontaneously hypertensive rat as a model of attention deficit hyperactivity disorder: acquisition and performance of the DRL-60s test

CD. However, the pattern of responding at DRL-60s suggested poor schedule control for the WKY rats. Therefore, the performance of SHR in the DRL test does not appear to represent a valid model of ADHD. Further, our findings with the WKY rat suggest that this strain is a poor behavioural control for the SHR.

Behavioural effects in female rats of postnatal exposure to sub-toxic doses of polychlorinated biphenyl congener 153

Polychlorinated biphenyls (PCBs) are widespread environmental contaminants that are also present in human tissues and breast milk. Behavioural disturbances have been reported in both children and animals exposed perinatally to PCBs. The present study assessed the behavioural consequences in female rats of postnatal exposure to the di-ortho-substituted 2,2',4,4',5,5'-hexachlorobiphenyl (IUPAC no. 153), which is one of the PCB congeners most frequently detected in human milk. The different groups of mothers were dosed via gavage with 5 mg/kg bodyweight of PCB 153 in corn oil or 5 ml/kg bodyweight corn oil vehicle every second day from day 3 to day 13 after delivery. The exposure did not affect the bodyweight of the dams nor the physical development of the pups. Operant behavioural testing of the female offspring by two different schedules of reinforcement was performed. First, the animals were tested by a multiple schedule with two components: fixed interval (FI) and extinction (EXT), which has proved sensitive in revealing changes in activity level. There were no statistically significant differences in frequency or interresponse times of lever pressing between the PCB-exposed female rats and the controls. These results were in contrast to a previous, analogous study where PCB 153 produced an increased frequency of lever presses during the FI in male rats, indicating a sex-specific behavioural effect of PCB 153. The female offspring was also tested by a conjunctive schedule with two components: variable interval (VI) and differential reinforcement of low rate (DRL). This schedule revealed slower acquisition of time discrimination in the PCB 153-exposed females as compared with the controls. The VI-DRL results showed that PCB 153 may also produce long-lasting behavioural effects in female rats following postnatal exposure through the mother's milk.

Chronic L-deprenyl treatment alters brain monoamine levels and reduces impulsiveness in an animal model of Attention-Deficit/Hyperactivity Disorder

Effects of chronic L-deprenyl administration on hyperactive behaviour and brain monoamine levels were studied in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. SHR were hyperactive, impulsive and had impaired sustained attention when tested with a multiple 2-min fixed interval (FI) 5-min extinction (EXT) schedule of reinforcement. Even low, 0.25 mg/kg, doses of chronically-administered L-deprenyl reduced the impulsiveness (bursts of responses with short interresponse times) of SHR, without altering the general hyperactivity or the impaired sustained attention. The drug had no effect on WKY behaviour. The levels of noradrenaline (NA), dopamine (DA), serotonin (5-hydroxytryptamine, 5-HT) and their metabolites, measured in neostriatum, nucleus accumbens and frontal cortex, showed that L-deprenyl effectively inhibited monoamine oxidase (MAO) activity. These results suggest that impulsiveness is a behavioural component that may be operating independent of the other components, like hyperactivity and deficient sustained attention, and that can be reduced by chronic MAO-B inhibition with L-deprenyl in this strain of rats. The positive effect of L-deprenyl on impulsiveness is discussed as due either to normalization of an asymmetric dopaminergic activity in the nucleus accumbens, or to a restoration of normal DA function in the prefrontal cortex.

Sex differences in operant discrimination behaviour in an animal model of attention-deficit hyperactivity disorder

The present study was aimed at determining whether the behaviour of the spontaneously hypertensive rat (SHR), an animal model of attention-deficit hyperactivity disorder (ADHD), showed sex differences parallel to those seen in ADHD children. The experimental protocol contained an operant discrimination task, a two-component multiple (mult) 2-min fixed interval (FI) 5-min extinction (EXT) schedule of water reinforcement, a reliable behavioural paradigm for testing activity levels, discrimination behaviour and impulsiveness. The results indicated that both male and female SHRs show some of the most important behavioural traits of ADHD. Both were hyperactive and showed discrimination problems in terms of a behavioural extinction deficit towards the end of the EXT component. Still their behaviour differed markedly, which was probably due to quite different underlying mechanisms. The behavioural characteristics of the female SHRs may be compatible with an attention-deficit interpretation, whereas the behavioural characteristics of the male SHRs may be due to a shorter than normal delay-of-reinforcement gradient. The present study strengthens the position of SHR as an animal model of ADHD for future studies that may elucidate details in the underlying neurobiological deficits and for testing various treatment strategies.