NMDA and non-NMDA sensitive [L-3H]glutamate receptor binding in the brain of the Naples high- and low-excitability rats: an autoradiographic study

Attention-Deficit/Hyperactivity Disorder: Focus upon Aberrant N-Methyl-D-Aspartate Receptors Systems

Attention-deficit/hyperactivity disorder (ADHD) pathophysiology persists in an obscure manner with complex interactions between symptoms, staging, interventions, genes, and environments. Only on the basis of increasing incidence of the disorder, the need for understanding is greater than ever. The notion of an imbalance between central inhibitory/excitatory neurotransmitters is considered to exert an essential role. In this chapter, we first review how the default mode network functions and dysfunction in individuals diagnosed with ADHD. We also present and briefly review some of the animal models used to examine the neurobiological aspects of ADHD. There is much evidence indicating that compounds/interventions that antagonize/block glutamic acid receptors and/or block the glutamate signal during the "brain growth spurt" or in the adult animal may induce functional and biomarker deficits. Additionally, we present evidence suggesting that animals treated with glutamate blockers at the period of the "brain growth spurt" fail to perform the exploratory activity, observed invariably with control mice, that is associated with introduction to a novel environment (the test cages). Later, when the control animals show less locomotor and rearing activity, i.e., interest in the test cages, the MK-801, ketamine and ethanol treated mice showed successively greater levels of locomotion and rearing (interest), i.e., they fail to "habituate" effectively, implying a cognitive dysfunction. These disturbances of glutamate signaling during a critical period of brain development may contribute to the ADHD pathophysiology. As a final addition, we have briefly identified new research venues in the interaction between ADHD, molecular studies, and personality research.

Panglial gap junctional communication is essential for maintenance of myelin in the CNS

In this study, we have investigated the contribution of oligodendrocytic connexin47 (Cx47) and astrocytic Cx30 to panglial gap junctional networks as well as myelin maintenance and function by deletion of both connexin coding DNAs in mice. Biocytin injections revealed complete disruption of oligodendrocyte-to-astrocyte coupling in the white matter of 10- to 15-d-old Cx30/Cx47 double-deficient mice, while oligodendrocyte-to-oligodendrocyte coupling was maintained. There were no quantitative differences regarding cellular networks in acute brain slices obtained from Cx30/Cx47 double-null mice and control littermates, probably caused by the upregulation of oligodendrocytic Cx32 in Cx30/Cx47 double-deficient mice. We observed early onset myelin pathology, and ∼40% of Cx30/Cx47 double-deficient animals died within 42 to 90 d after birth, accompanied by severe motor impairments. Histological and ultrastructural analyses revealed severe vacuolization and myelination defects in all white matter tracts of the CNS. Furthermore, Cx30/Cx47 double-deficient mice exhibited a decreased number of oligodendrocytes, severe astrogliosis, and microglial activation in white matter tracts. Although less affected concerning motor impairment, surviving double-knock-out (KO) mice showed behavioral alterations in the open field and in the rotarod task. Vacuole formation and thinner myelin sheaths were evident also with adult surviving double-KO mice. Since interastrocytic coupling due to Cx43 expression and interoligodendrocytic coupling because of Cx32 expression are still maintained, Cx30/Cx47 double-deficient mice demonstrate the functional role of both connexins for interastrocytic, interoligodendrocytic, and panglial coupling, and show that both connexins are required for maintenance of myelin.

Brain processes in discounting: consequences of adolescent methylphenidate exposure

Traits of inattention, impulsivity, and motor hyperactivity characterize children diagnosed with attention-deficit/hyperactivity disorder (ADHD), whose inhibitory control is reduced. In animal models, crucial developmental phases or experimental transgenic conditions account for peculiarities, such as sensation-seeking and risk-taking behaviors, and reproduce the beneficial effects of psychostimulants. An "impulsive" behavioral profile appears to emerge more extremely in rats when forebrain dopamine (DA) systems undergo remodeling, as in adolescence, or with experimental manipulation tapping onto the dopamine transporter (DAT). Ritalin(®) (methylphenidate, MPH), a DAT-blocking drug, is prescribed for ADHD therapy but is also widely abused by human adolescents. Administration of MPH during rats' adolescence causes a long-term modulation of their self-control, in terms of reduced intolerance to delay and diminished proneness for risk when reward is uncertain. Exactly the opposite profile emerges when exogenous alteration of DAT levels is achieved via lentiviral transfection. Both adolescent MPH exposure and DAT-targeting transfection lead to enduring hyperfunction of dorsal striatum and hypofunction of ventral striatum. Together with upregulation of prefronto-cortical phospho-creatine, striatal upregulation of selected genes (like serotonin 7 receptor gene) suggests that enhanced inhibitory control is generated by adolescent MPH exposure. Operant tasks, which assess the balance between motivational drives and inhibitory self-control, are thus useful for investigating reward-discounting processes and their modulation by DAT-targeting tools. In summary, due to the complexity of human studies, preclinical investigations of rodent models are necessary to understand better both the neurobiology of ADHD-like symptoms' etiology and the long-term therapeutic safety of adolescent MPH exposure.

DRD4 and DAT1 in ADHD: Functional neurobiology to pharmacogenetics

Attention deficit/hyperactivity disorder (ADHD) is a common and potentially very impairing neuropsychiatric disorder of childhood. Statistical genetic studies of twins have shown ADHD to be highly heritable, with the combination of genes and gene by environment interactions accounting for around 80% of phenotypic variance. The initial molecular genetic studies where candidates were selected because of the efficacy of dopaminergic compounds in the treatment of ADHD were remarkably successful and provided strong evidence for the role of DRD4 and DAT1 variants in the pathogenesis of ADHD. However, the recent application of non-candidate gene strategies (eg, genome-wide association scans) has failed to identify additional genes with substantial genetic main effects, and the effects for DRD4 and DAT1 have not been replicated. This is the usual pattern observed for most other physical and mental disorders evaluated with current state-of-the-art methods. In this paper we discuss future strategies for genetic studies in ADHD, highlighting both the pitfalls and possible solutions relating to candidate gene studies, genome-wide studies, defining the phenotype, and statistical approaches.

METHYLPHENIDATE (RITALIN): BEHAVIORAL STUDIES IN THE RAT

Attention Deficit Hyperactivity Disorder (ADHD) is a neuropsychiatric syndrome with an onset in childhood characterized by an inability to remain focused or to concentrate for prolonged periods of time. Children suffering from this disease are many times described as either inattentive or as hyperactive-impulsive depending on what form of the disease they manifest. Methylphenidate is the preferred treatment for this behavioral disorder and is used for long term disease management. Much still remains unknown concerning this stimulant and its effects on behavior and future abuse potential are pertinent questions. Since animal models are used to study the mechanism of drug action and rats are used often in drug studies, the objective of this review is to summarize the research reports that mainly have used rats as the model to investigate the action of methylphenidate. Topics discussed in this review include: (1) What effect does a single dose of methylphenidate have on locomotion activity; (2) Does repeated administration of methylphenidate result in tolerance or sensitization; and (3) Does methylphenidate have rewarding properties as measured by the self-administration and condition placed preference paradigms.

Excitatory amino acids in the forebrain of the Naples high-excitability rats: neurochemical and behavioural effects of subchronic D-aspartate and its diethyl ester prodrug

The excitatory amino acids (EAA) L-glutamate (L-Glu), L-aspartate (L-Asp) and D-aspartate (D-Asp) are thought to play a neurotransmitter/neuromodulator role in neuronal communications. Recently, a high level of EAA L-Glu, D- and L-Asp isomers has been found in the forebrain of Naples high-excitability (NHE) rat line that models the mesocortical variant of Attention-Deficit Hyperactivity Disorder (ADHD). The aim of this study was to assess the functions of D-Asp using two forms, i.e. free D-Asp or D-Asp diethyl ester (DEE) as prodrug, on brain and behaviour. Thus, prepuberal rats were given, for two weeks daily, an i.p. injection of D-Asp or DEE or vehicle. Then rats were exposed to two spatial novelties i.e. Làt and radial Olton maze. Behaviour was monitored for indices of activity, non-selective attention (NSA), selective spatial attention (SSA) and emotional reactivity. L-Glu and D- and L-Asp were detected by HPLC in cognitive and non-cognitive brain areas such as prefrontal cortex, striatum, hippocampus and hypothalamus. Results indicate that subchronic D-Asp or DEE (i) reduced EAA levels in the NHE and increased it in the random-bred controls (NRB) rats, (ii) in the Làt-maze D-Asp increased horizontal activity in NHE but DEE decreased it in NRB rats, (iii) in the Olton maze D-Asp and DEE decreased vertical activity in NHE and NRB rats respectively, (iv) D-Asp impaired attention only in NRB decreasing number of arms visited before first repetition. Therefore, data demonstrate differential effects of prepuberal subchronic D-Asp and DEE that may be related to different basal EAA levels in NHE and NRB rats.

Acute and chronic methylphenidate dose-response assessment on three adolescent male rat strains

Methylphenidate (MPD), commonly known as Ritalin, is the most frequently prescribed drug to treat children and adults with attention deficit hyperactivity disorder (ADHD). Adolescence is a period of development involving numerous neuroplasticities throughout the central nervous system (CNS). Exposure to a psychostimulant such as MPD during this crucial period of neurodevelopment may cause transient or permanent changes in the CNS. Genetic variability may also influence these differences. Thus, the objective of the present study was to determine whether acute and chronic administration of MPD (0.6, 2.5, or 10.0mg/kg, i.p.) elicit effects among adolescent WKY, SHR, and SD rats and to compare whether there were strain differences. An automated, computerized, open-field activity monitoring system was used to study the dose-response characteristics of acute and repeated MPD administration throughout the 11-day experimental protocol. Results showed that all three adolescent rat groups exhibited dose-response characteristics following acute and chronic MPD administration, as well as strain differences. These strain differences depended on the MPD dose and locomotor index. Chronic treatment of MPD in these animals did not elicit behavioral sensitization, a phenomenon described in adult rats that is characterized by the progressive augmentation of the locomotor response to repeated administration of the drug. These results suggest that the animal's age at time of drug treatment and strain/genetic variability play a crucial role in the acute and chronic effect of MPD and in the development of behavioral sensitization.

The role of age, genotype, sex, and route of acute and chronic administration of methylphenidate: A review of its locomotor effects

Children with attention deficit hyperactivity disorder (ADHD) are treated for extended periods of time with the psychostimulant methylphenidate (MPD). The psychostimulants cocaine, amphetamine, and MPD exhibit similar structural configuration and pharmacological profile. The consequence of the long-term use of psychostimulants such as MPD as treatment for ADHD in the developing brain of children is unknown. Repeated treatment with psychostimulants has been shown to elicit adverse effects in behavior, such as dependence, paranoia, schizophrenia, and behavioral sensitization. Behavioral sensitization and cross-sensitization between two drugs are used as experimental markers to determine the potential of a drug to develop dependence/addiction. Although there are many reviews written about behavioral sensitization involving psychostimulants, scarcely any have focused specifically on MPD-elicited behavioral sensitization and cross-sensitization with other psychostimulants. Moreover, the response to MPD and the expression of ADHD vary among females and males and among different populations due to genetic variability. Since the interpretation and synthesis of the data reported are controversial, this review focuses on the adverse effects of MPD and the role of age, sex, and genetic composition on the acute and chronic effects of MPD, such as MPD-elicited behavioral sensitization and cross-sensitization with amphetamine in animal models. Animal models of drug-induced locomotor stimulation, particularly locomotor sensitization, can be used to understand the mechanisms underlying human drug-induced dependence.

A family based study implicates solute carrier family 1-member 3 (SLC1A3) gene in attention-deficit/hyperactivity disorder

BACKGROUND

The glutamatergic system, the major excitatory neurotransmitter system in the central nervous system (CNS) has been proposed as contributing a possible role in the etiology of attention deficit hyperactivity disorder (ADHD). This is based upon observations from animal, neuroimaging, neuroanatomical and neuropsychological studies. Genes related to glutamate function are therefore good functional candidates for this disorder. The SLC1A3 (Solute Carrier Family 1, member 3) gene encodes a glial glutamate transporter which maps to chromosome 5p12, a region of linkage that coincides in two published ADHD genome scans so far. SLC1A3 is thus both a functional and positional candidate gene for ADHD.

METHODS

We have undertaken detailed association analysis of SLC1A3 using a multi-stage approach for candidate gene analysis.

RESULTS

In a family-based sample (n = 299) we found a significant association between marker rs2269272 (p = .007) and ADHD. Two, two-marker haplotypes, rs2269272/rs3776581 (p = .016) and rs2269272/rs2032893 (p = .013) also yielded evidence of association.

CONCLUSIONS

The results of our study suggest that genetic variation in SLC1A3 may contribute to susceptibility to ADHD.

Behavioral correlates of differences in neural metabolic capacity

Cytochrome oxidase is a rate-limiting enzyme in oxidative phosphorylation, the major energy-synthesizing pathway used by the central nervous system, and cytochrome oxidase histochemistry has been extensively utilized to map changes in neural metabolism following experimental manipulations. However, the value of cytochrome oxidase activity in predicting behavior has not been analyzed. We argue that this endeavor is important because genetic composition and embryonic environment can engender differences in baseline neural metabolism in pertinent neural circuits, and these differences could represent differences in the degree to which specific behaviors are 'primed.' Here we review our studies in which differences in cytochrome oxidase activity and in behavior were studied in parallel. Using mammalian and reptilian models, we find that embryonic experiences that shape the propensity to display social behaviors also affect cytochrome oxidase activity in limbic brain areas, and elevated cytochrome oxidase activity in preoptic, hypothalamic, and amygdaloid nuclei correlates with heightened aggressive and sexual tendencies. Selective breeding regimes were used to create rodent genetic lines that differ in their susceptibility to display learned helplessness and in behavioral excitability. Differences in cytochrome oxidase activity in areas like the paraventricular hypothalamus, frontal cortex, habenula, septum, and hippocampus correlate with differences in susceptibility to display learned helplessness, and differences in activity in the dentate gyrus and perirhinal and posterior parietal cortex correlate with differences in hyperactivity. Thus, genetic and embryonic manipulations that engender specific behavioral differences produce specific neurometabolic profiles. We propose that knowledge of neurometabolic differences can yield valuable predictions about behavioral phenotype in other systems.

Neurobehavioural deficits associated with apoptotic neurodegeneration and vulnerability for ADHD

Several studies involving postnatal administration of the N-methyl-D-aspartate (NMDA) antagonists, dizocilpine (MK-801; 3 x 0.5 mg/kg, at 08.00, 16.00 and 24.00 h) on Postnatal day 11, or Ketamine (1 x 50 mg/kg) or Ethanol (1 x 2.5 g/kg, Ethanol-Low, or 2 x 2.5 g/kg, 2-h interval, Ethanol-High) on Postnatal day 10, are described. Some mice from each treatment/vehicle group were sacrificed 24 h after NMDA antagonist treatment and brain regions were taken for fluoro-jade staining analysis. Functional analysis was initiated at 60 days of age. All three treatments inducing an antagonistic action at NMDA receptors, MK-801, Ketamine and Ethanol-High induced a similar pattern of initial hypoactivity followed by marked and lasting hyperactivity in the motor activity test chambers. In each case, the basal hyperactivity level was abolished by acute treatment with a low dose of D-amphetamine (0.25 mg/kg). All three treatments, MK-801, Ketamine and Ethanol-High, induced a deficit in acquisitive performance in the radial arm maze test of instrumental learning. The deficit induced by postnatal MK-801 was abolished by acute treatment with the low dose of D-amphetamine. All three treatments, MK-801, Ketamine and Ethanol-High, resulted in normal acquisitive performance during the first three test days in the circular swim with the submerged platform maintained in a constant position, but on the fourth test day, with the platform position shifted to a different "quadrant", induced marked deficits. Fluoro-jade staining analyses indicated a devastating cell degeneration in several brain regions of mice administered NMDA antagonists postnatally, including the hippocampus, frontal cortex, parietal cortex, and cerebellum. Severe cell degeneration in the laterodorsal thalamus due to Ethanol or diazepam (5 mg/kg) appeared not to affect the different aspects of function. The pattern of dysfunctional outcome and apoptotic cell loss following postnatal NMDA antagonist treatment offers a plausible similarity to the major aspects of 'syndromatic continuity' in ADHD, hyperactivity, inattention and impulsivity, thereby providing an interesting animal model of the disorder.

The dopaminergic system in attention deficit/hyperactivity disorder

Numerous studies have shown the importance of the mesocorticolimbic dopamine system in the pathophysiology of attention deficit/hyperactivity disorder. However, there has been inconsistency in the findings of those studies. Varied and sometimes contradictory interpretation has been made on the basis of similar results. It is, therefore, still unclear whether the dopaminergic system is hypo- or hyperfunctioning in attention deficit/hyperactivity disorder. The majority of the functional brain imaging studies in both clinical and experimental settings support hypofunction of the basal ganglia which receive abundant dopaminergic afferent. The experimental studies, using dopamine-depleted animals, also support the hypodopaminergic hypothesis, whereas recent studies with the dopamine transporter knockout/knockdown mouse suggest hyperdopaminergic function as the underlying abnormality. In this review we attempt to clarify the issues raised by previous neuroimaging and functional neuroimaging studies. Research involving animal models with genetic traits, genetic manipulation or with brain lesions is analysed, concentrating on the significance of the dopaminergic system in the abnormal behavior of attention deficit/hyperactivity disorder. In addition, the functional state of the dopaminergic system is considered through the speculated mechanism of psychostimulant therapy of the disorder. No final conclusions have been reached regarding the pathological, biochemical and physiological mechanisms responsible for various symptoms. Inconsistency in the findings and their interpretations may indicate the heterogeneity of the pathogenesis of this syndrome.

Animal models of attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) involves clinically heterogeneous dysfunctions of sustained attention, with behavioral overactivity and impulsivity, of juvenile onset. Experimental models, in addition to mimicking syndromal features, should resemble the clinical condition in pathophysiology, and predict potential new treatments. One of the most extensively evaluated animal models of ADHD is the spontaneously hypertensive rat. Other models include additional genetic variants (dopamine transporter gene knock-out mouse, coloboma mouse, Naples hyperexcitable rat, acallosal mouse, hyposexual rat, and population-extreme rodents), neonatal lesioning of dopamine neurons with 6-hydroxydopamine, and exposure to other neurotoxins or hippocampal irradiation. None is fully comparable to clinical ADHD. The pathophysiology involved varies, including both deficient and excessive dopaminergic functioning, and probable involvement of other monoamine neurotransmitters. Improved models as well as further testing of their ability to predict treatment responses are required.

Behavioral validation of the spontaneously hypertensive rat (SHR) as an animal model of attention-deficit/hyperactivity disorder (AD/HD)

A good model of a disorder is one that: (a) mimics, although in a simpler form than the full-blown clinical case, the fundamentals of the behavioral characteristics, in this case of people with Attention-Deficit/Hyperactivity Disorder (AD/HD;face validity); (b) conforms with a theoretical rationale for the disorder (construct validity); and (c) is able to predict aspects of behavior, genetics and neurobiology previously uncharted in the clinics (predictive validity). This article discusses the Spontaneously Hypertensive Rat (SHR) and some other putative animal models of AD/HD. It is argued that although other strains and species may be hyperactive and/or show attention deficits following genetic, environmental or pharmacological interventions, the SHR is presently the only strain shown to have the major behavioral symptoms of AD/HD. This does not mean that investigating other models cannot give valuable information.