Non-selective attention in a rat model of hyperactivity and attention deficit: subchronic methylphenydate and nitric oxide synthesis inhibitor treatment

Modafinil Administration to Preadolescent Rat Impairs Non-Selective Attention, Frontal Cortex D2 Expression and Mesolimbic GABA Levels

The misuse of psychostimulants is an increasing behavior among young people, highlighting in some countries the abuse of modafinil (MOD) as a neuropotentiator. However, several clinical trials are investigating MOD as an alternative pharmacological treatment for attentional deficit and hyperactivity disorder (ADHD) in children and adolescents. On the other hand, the early use of psychostimulants and the misdiagnosis rates in ADHD make it crucial to investigate the brain effects of this type of drug in young healthy individuals. The aim of this work was to evaluate the effects of chronic MOD treatment on neurochemicals (γ-aminobutyric acid and glutamate), dopamine receptor 2 (D₂) expression and behavior (non-selective attention "NSA") in the mesocorticolimbic system of young healthy Sprague-Dawley rats. Preadolescent male rats were injected with MOD (75 mg/kg, i.p.) or a vehicle for 14 days (from postnatal day 22 to 35). At postnatal day 36, we measured the GLU and GABA contents and their extracellular levels in the nucleus accumbens (NAc). In addition, the GLU and GABA contents were measured in the ventral tegmental area (VTA) and D₂ protein levels in the prefrontal cortex (PFC). Chronic use of MOD during adolescence induces behavioral and neurochemical changes associated with the mesocorticolimbic system, such as a reduction in PFC D₂ expression, VTA GABA levels and NSA. These results contribute to the understanding of the neurological effects of chronic MOD use on a young healthy brain.

Review of rodent models of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a polygenic neurodevelopmental disorder that affects 8-12 % of children and >4 % of adults. Environmental factors are believed to interact with genetic predispositions to increase susceptibility to ADHD. No existing rodent model captures all aspects of ADHD, but several show promise. The main genetic models are the spontaneous hypertensive rat, dopamine transporter knock-out (KO) mice, dopamine receptor subtype KO mice, Snap-25 KO mice, guanylyl cyclase-c KO mice, and latrophilin-3 KO mice and rats. Environmental factors thought to contribute to ADHD include ethanol, nicotine, PCBs, lead (Pb), ionizing irradiation, 6-hydroxydopamine, neonatal hypoxia, some pesticides, and organic pollutants. Model validation criteria are outlined, and current genetic models evaluated against these criteria. Future research should explore induced multiple gene KOs given that ADHD is polygenic and epigenetic contributions. Furthermore, genetic models should be combined with environmental agents to test for interactions.

Motor Function Deficits in the Estrogen Receptor Beta Knockout Mouse: Role on Excitatory Neurotransmission and Myelination in the Motor Cortex

BACKGROUND

Male estrogen receptor beta (ERβ) knockout (BERKO) mice display anxiety and aggression linked to, among others, altered serotonergic signaling in the basolateral amygdala and dorsal raphe, impaired cortical radial glia migration, and reduced GABAergic signaling. The effects on primary motor cortex (M1 cortex) and locomotor activity as a consequence of ERβ loss have not been investigated.

OBJECTIVE

The aim of this study was to determine whether locomotor activity is altered as a consequence of the changes in the M1 cortex.

METHODS

The locomotor activity of male wild-type (WT) and BERKO mice was evaluated using the open-field and rotarod tests. Molecular changes in the M1 cortex were analyzed by RNA sequencing, electron microscopy, electrophysiology, and immunohistological techniques. In addition, we established oligodendrocyte (OL) cultures from WT and BERKO mouse embryonic stem cells to evaluate OL function.

RESULTS

Locomotor profiling revealed that BERKO mice were more active than WT mice but had impaired motor coordination. Analysis of the M1 cortex pointed out differences in synapse function and myelination. There was a reduction in GABAergic signaling resulting in imbalanced excitatory and inhibitory neurotransmission as well as a defective OL differentiation accompanied by myelin defects. The effects of ERβ loss on OL differentiation were confirmed in vitro.

CONCLUSION

ERβ is an important regulator of GABAergic interneurons and OL differentiation, which impacts on adult M1 cortex function and may be linked to increased locomotor activity and decreased motor coordination in BERKO mice.

Effects of agmatine, glutamate, arginine, and nitric oxide on executive functions in children with attention deficit hyperactivity disorder

In this study, we aimed to investigate the effects of agmatine, nitric oxide (NO), arginine, and glutamate, which are the metabolites in the polyamine pathway,  on the performance of executive functions (EF) in attention deficit hyperactivity disorder (ADHD). The ADHD group included 35 treatment-naive children (6-14 years old) who were ewly diagnosed with ADHD. The control group consisted of 35 healthy children with the same age and sex, having no previous psychiatric disorders. In the study groups, Stroop test (ST) and trail making test (TMT) were used to monitor EF, and blood samples were collected to measure agmatine with ultra-high-performance liquid chromatography and NO, glutamate, and arginine with enzyme-linked immunosorbent assay (ELISA). The EFs were significantly impaired in the ADHD group. The agmatine and arginine levels of the ADHD group were significantly higher than their peers. The NO and glutamate levels were also higher in the ADHD group compared to the control group, but these differences did not reach statistical significance. Children with ADHD had more difficulties during EF tasks compared to healthy children. The elevated NO and glutamate levels may be related with the impairment during EF tasks. Therefore, agmatine and arginine may increase to improve EF tasks through its inhibitory effect on the synthesis of NO and glutamate. Further studies are needed about polyamine pathway molecules to shed light on the pathophysiology of ADHD.

Behavioral Experimental Paradigms for the Evaluation of Drug’s Influence on Cognitive Functions: Interpretation of Associative, Spatial/Nonspatial and Working Memory

Background:

Currently, a large number of people throughout the world are affected by neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease which appear with a lapse in recall, attention and altered cognitive functions. Learning and memory, the fundamental indices defining cognitive functions, are the complex psychological processes governing acquisition, consolidation, and retrieval of stored information. These processes are synchronized by the coordination of various parts of the brain including hippocampus, striatum and amygdala.

Objective:

The present review is centered on different behavioral paradigms in rodents interpreting learning and memory both explicitly and implicitly. Furthermore, it is also emphasizing on the interaction of various brain structures during different stages of associative, spatial and non-spatial memory.

Methods:

We embarked on an objective review of literature relevant to screening methods for evaluation of drug’s influence on a wide range of cognitive functions (learning and memory) as well as the underlying mechanism responsible for modulation of these functions.

Results:

Our review highlighted the behavioral paradigms based on associative, spatial/nonspatial and working memory. The cited research acknowledged the hippocampal and striatal control on learning and memory.

Conclusion:

Since the neurodegenerative disorders and dementia have continuously been increasing, a wide range of therapeutic targets have been developed at the cellular and molecular level. This arises the necessity of screening of these targets in different cognitive behavioral paradigms which reflect their memory enhancing potential. The understanding of behavioral models and the involvement of brain structures in cognitive functions highlighted in the present review might be helpful to advance therapeutic interventions.

Systematic review and meta-analysis of the behavioral effects of methylphenidate in the spontaneously hypertensive rat model of attention-deficit/hyperactivity disorder

The spontaneously hypertensive rats (SHR) are the most widely used model for ADHD. While face and construct validity are consolidated, questions remain about the predictive validity of the SHR model. We aim at summarizing the evidence for the predictive validity of SHR by evaluating its ability to respond to methylphenidate (MPH), the most well documented treatment for ADHD. A systematic review was carried out to identify studies evaluating MPH effects on SHR behavior. Studies (n=36) were grouped into locomotion, attention, impulsivity or memory, and a meta-analysis was performed. Meta-regression, sensitivity, heterogeneity, and publication bias analyses were also conducted. MPH increased attentional and mnemonic performances in the SHR model and decreased impulsivity in a dose-dependent manner. However, MPH did not reduce hyperactivity in low and medium doses, while increased locomotor activity in high doses. Thus, since the paradoxical effect of stimulant in reducing hyperactivity was not observed in the SHR model, our study does not fully support the predictive validity of SHR, questioning their validity as an animal model for ADHD.

Stress response genes associated with attention deficit hyperactivity disorder: A case-control study in Chinese children

To explore the associations between stress response genes and attention deficit hyperactivity disorder (ADHD) in children, we conducted a case-control study consisting of 406 newly diagnosed ADHD cases and 432 controls in Wuhan, China. We genotyped the candidate genes, nuclear receptor subfamily 3 group C member 1(NR3C1) and solute carrier family 6 member 4(SLC6A4), using the Sequenom MassARRAY technology. After correction by Bonferroni (α' = 0.05/6 = 0.008), the rs6191 SNP was found to be associated with a reduced risk of ADHD in the dominant model (OR = 0.564, 95% CI = 0.389-0.819, P = 0.003) while the rs25531 SNP was associated with an increased risk of ADHD in the multiplicative model (OR = 1.380, 95% CI = 1.111-1.714, P = 0.004). Additionally, both the rs6191 and rs25531 SNPs were significantly associated with the attention deficit factor (P = 0.006, P = 0.003, respectively) but not with the hyperactivity/impulsivity factor in the Swanson, Nolan and Pelham-IV Questionnaire (SNAP-IV) scale. Furthermore, we found that these two SNPs were significantly associated with pure ADHD, and not affected by the comorbidities (P =  0.001, P =  0.007, respectively). Besides, there was an interaction between these two SNPs. This study demonstrated the role of NR3C1 and SLC6A4 polymorphisms in ADHD, yet independent replication of the findings of this study in multi-center and multi-stage studies with large samples is warranted in the future.

Behavioural effects of methylphenidate in the spontaneously hypertensive rat model of attention-deficit/hyperactivity disorder: a systematic review and meta-analysis protocol

INTRODUCTION

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent condition related to several negative outcomes, and its pathophysiology is still poorly understood. The spontaneously hypertensive rats (SHRs) are the most commonly used animal model of ADHD. How ever, its validity, and especially its predictive validity, has been questioned. Therefore, the current protocol discloses the background, aims and methods of a systematic review and meta-analysis of studies reporting the behavioural effects of methylphenidate (MPH), the most commonly prescribed treatment for ADHD, in the SHR.

SEARCH STRATEGY

Studies will be identified through a literature search using three different electronic databases: Medline, Embase and Web of Science. There will be no language restrictions. All s tudies that administered MPH to SHR and evaluated locomotion, attention, impulsivity or memory will be included.

SCREENING AND ANNOTATION

Studies will be prescreened based on title and abstract, and a full-text review will be performed if necessary. Screening will be performed by two authors, and any disagreement will be discussed with a third author.

DATA MANAGEMENT AND REPORTING

Data extraction will be performed by two independent authors according to a standardised form. Studies will be grouped according to the behavioural outcomes reported, and a meta-analysis will be performed for each group. The influence of predefined covariates on the effects of MPH will be evaluated using meta-regression and sensitivity analyses. Data will be reported following PRISMA guidelines.

The probable role of adrenomedullin and nitric oxide in childhood attention deficit hyperactivity disorder

BACKGROUND

The role of adrenomedullin hormone, which has been shown to be associated with many psychiatric disorders, in the etiology of ADHD and its relation to disease is not yet known.

AIM

In this study, it was aimed to compare plasma adrenomedullin and nitric oxide (NO) levels of newly diagnosed, treatment-naive patients with ADHD with healthy children.

METHODS

A total of 45 children with ADHD and 45 healthy children were included. The Schedule for Affective Disorders and Schizophrenia Present and Lifetime Version (K-SADS), a semi-structured interview, was applied to all cases by child and adolescent psychiatrist. Age and gender matched participants who admitted to the hospital for any other reasons without any psychiatric diagnosis according to K-SADS were selected as a control group. Sociodemographic data form and The Turgay DSM-IV-Based Child and Adolescent Disruptive Behavioral Disorders Screening and Rating Scale-parental form were applied to the all groups. NO and adrenomedullin levels were analysed by ELISA method with specific commercial kits.

RESULTS

There was no statistically significant difference in NO and adrenomedullin levels, neither between the groups nor ADHD subtypes. A positive correlation between adrenomedullin and NO levels was found in both the case (r = 0.659) and the control groups (r = 0.494).

CONCLUSIONS

Besides being the first study to evaluate adrenomedullin levels to elucidate the etiology of childhood ADHD as well as NO, significant differences was not found between the case and the control groups in terms of NO and adrenomedullin levels.

Attentional deficits and altered neuronal activation in medial prefrontal and posterior parietal cortices in mice with reduced dopamine transporter levels

The executive control function of attention is regulated by the dopaminergic (DA) system. Dopamine transporter (DAT) likely plays a role in controlling the influence of DA on cognitive processes. We examined the effects of DAT depletion on cognitive processes related to attention. Mice with the DAT gene genetically deleted (DAT+/- heterozygotes) were compared to wild type (WT) mice on the Attentional Set-Shifting Task (ASST). Changes in neuronal activity during the ASST were shown with early growth response genes 1 and 2 (egr-1 and egr-2) immunohistochemistry in the medial prefrontal cortex (mPFC) and in the posterior parietal cortex (PPC). Heterozygotes were impaired in tasks that tax reversal learning, attentional-set formation and set-shifting. Densities of egr-2 labeled cells in the mPFC were lower in mutant mice when compared with wild-types in intradimensional shift of attention (IDS), extradimensional shift of attention and extradimensional shift of attention-reversal phases of the ASST task, and in PPC in the IDS phase of the task. The results demonstrate impairments of the areas associated with attentional functions in DAT+/- mice and show that an imbalance of the dopaminergic system has an impact on the complex attention-related executive functions.

Chronic corticosterone treatment enhances extinction-induced depression in aged rats

Withdrawal and avoidance behavior are common symptoms of depression and can appear as a consequence of absence of reward, i.e. extinction-induced depression (EID). This is particularly relevant for the aged organism subjected to pronounced loss of former rewards. Avoidance of the former site of reward and increased withdrawal into a distant compartment accompany extinction of food-rewarded behavior in rodent models. During extinction, behavioral markers for re-learning dissociate from indicators of extinction-induced depression. Here we examined the effect of a chronic treatment with corticosterone (CORT), a well-known inducer of depression-related behavior, on EID in adult and aged rats. Adult (3-4months) and aged (18months) male rats were treated with CORT via drinking water for 3weeks prior to extinction of a cued food-reward task. CORT treatment increased the distance from the site of reward and decreased goal tracking behavior during extinction, especially in the aged rats. Plasma hormone levels measured before and after restraint stress showed a decline in basal ACTH- and CORT-levels after chronic CORT treatment in aged animals. The treatment significantly impaired the HPA-axis activation after acute stress in both, adult and aged animals, alike. Altogether, these findings show an enhancement of EID after chronic CORT treatment in the aged organism, which may be mediated by an impaired HPA-axis sensitivity. These findings may have special relevance for the investigation of human geriatric depression.

Nr3C1-Bhlhb2 Axis Dysregulation Is Involved in the Development of Attention Deficit Hyperactivity

Attention deficit hyperactivity disorder (ADHD) is a child developmental and behavioral disorder which seriously hinders their education and development. To investigate the key regulators in the prefrontal cortex (PFC), the major affected areas of ADHD, microRNA (miR)-138,138*, 34c*, 296, and 494, were noted for their significant downregulation in ADHD model rats spontaneously hypertensive rats (SHRs) compared to Wistar Kyoto (WKY) rat control. Based on promoter sequence analysis and activity assay, glucocorticoid receptor (Nr3c1) was identified for the inhibition of the promoter activity of miR-138-1, 34c*, 296, and 494 genes and their transcription. In the PFC of ADHD model rats SHR, Nr3c1 expression was abnormally elevated and reversely correlated with the levels of miR-138-1, 34c, 296, and 494 expression. Luciferase report assays indicated that all miR-138, 138*, 34c*, 296, and 494 targeted the 3′ untranslated region of transcription factor Bhlhb2 (Bhlhe40) messenger RNA (mRNA) in common and ectopic expression of miR-138,138*, 34c*, 296, and 494 further suppressed the expression of Bhlhb2 gene. Consistently, Bhlhb2 expression was significantly higher in PFC of ADHD model SHR than control. Overexpressed Bhlhb2 in vitro significantly suppressed PC12 cell differentiation, and silence of Bhlhb2 enhanced the growth of neurite axon and dendrite. To observe the roles of Bhlhb2 further in vivo, Bhlhb2 was silenced in the PFC of nine SHR rats. Interestingly, knockdown of Bhlhb2 significantly improved the hyperactivity behaviors in SHRs compared to control. These findings show that Nr3c1-Bhlhb2 axis dysregulation was involved in the development of attention deficit and hyperactivity.

Diversification of behavior and postsynaptic properties by netrin-G presynaptic adhesion family proteins

BACKGROUND

Vertebrate-specific neuronal genes are expected to play a critical role in the diversification and evolution of higher brain functions. Among them, the glycosylphosphatidylinositol (GPI)-anchored netrin-G subfamily members in the UNC6/netrin family are unique in their differential expression patterns in many neuronal circuits, and differential binding ability to their cognate homologous post-synaptic receptors.

RESULTS

To gain insight into the roles of these genes in higher brain functions, we performed comprehensive behavioral batteries using netrin-G knockout mice. We found that two netrin-G paralogs that recently diverged in evolution, netrin-G1 and netrin-G2 (gene symbols: Ntng1 and Ntng2, respectively), were responsible for complementary behavioral functions. Netrin-G2, but not netrin-G1, encoded demanding sensorimotor functions. Both paralogs were responsible for complex vertebrate-specific cognitive functions and fine-scale regulation of basic adaptive behaviors conserved between invertebrates and vertebrates, such as spatial reference and working memory, attention, impulsivity and anxiety etc. Remarkably, netrin-G1 and netrin-G2 encoded a genetic "division of labor" in behavioral regulation, selectively mediating different tasks or even different details of the same task. At the cellular level, netrin-G1 and netrin-G2 differentially regulated the sub-synaptic localization of their cognate receptors and differentiated the properties of postsynaptic scaffold proteins in complementary neural pathways.

CONCLUSIONS

Pre-synaptic netrin-G1 and netrin-G2 diversify the complexity of vertebrate behaviors and differentially regulate post-synaptic properties. Our findings constitute the first genetic analysis of the behavioral and synaptic diversification roles of a vertebrate GPI protein and presynaptic adhesion molecule family.

Distinct lncRNA expression profiles in the prefrontal cortex of SD rats after exposure to methylphenidate

Methylphenidate (MPH) is a central nervous system stimulant that is widely used to treat attention deficit hyperactivity disorder (ADHD) and has been shown to improve attention, cognitive function and behaviors in both patients and animal models of ADHD. Even among normal healthy people, MPH can facilitate the consolidation of memories and improve declarative memory. Using microarray techniques, we aimed to find new pharmacology profile of MPH. A Làt maze experiment showed that locomotor activity and non-selective attention were affected by 2 weeks of exposure to MPH. Then, we identified long non-coding RNA (lncRNA) signatures in the prefrontal cortex of rats; 461 up-regulated lncRNAs and 97 down-regulated lncRNAs were found in the MPH-exposed group compared with the control group using fold-change >1.5. GO and KEGG pathway analyses indicated biological functions related to the metabolism of neural chemical compounds and nerve cell development. Furthermore, we reported changes in uc.173+ related to the UBE2B gene, which may affect neurite outgrowth and axonal regeneration. At the same time, MRAK081997 associated with the DHFR gene may be involved in axon regeneration in the rodent central nervous system through DNA methylation. Our study showed distinct expression profiles of lncRNAs in the normal rat prefrontal cortex after exposure to MPH, offering information for further research of MPH and may suggesting a new therapeutic target for ADHD.

Motivational disturbances and effects of L-dopa administration in neurofibromatosis-1 model mice

Children with neurofibromatosis type 1 (NF1) frequently have cognitive and behavioral deficits. Some of these deficits have been successfully modeled in Nf1 genetically-engineered mice that develop optic gliomas (Nf1 OPG mice). In the current study, we show that abnormal motivational influences affect the behavior of Nf1 OPG mice, particularly with regard to their response to novel environmental stimuli. For example, Nf1 OPG mice made fewer spontaneous alternations in a Y-maze and fewer arm entries relative to WT controls. However, analysis of normalized alternation data demonstrated that these differences were not due to a spatial working memory deficit. Other reported behavioral results (e.g., open-field test, below) suggest that differential responses to novelty and/or other motivational influences may be more important determinants of these kinds of behavior than simple differences in locomotor activity/spontaneous movements. Importantly, normal long-term depression was observed in hippocampal slices from Nf1 OPG mice. Results from elevated plus maze testing showed that differences in exploratory activity between Nf1 OPG and WT control mice may be dependent on the environmental context (e.g., threatening or non-threatening) under which exploration is being measured. Nf1 OPG mice also exhibited decreased exploratory hole poking in a novel holeboard and showed abnormal olfactory preferences, although L-dopa (50 mg/kg) administration resolved the abnormal olfactory preference behaviors. Nf1 OPG mice displayed an attenuated response to a novel open field in terms of decreased ambulatory activity and rearing but only during the first 10 min of the session. Importantly, Nf1 OPG mice demonstrated investigative rearing deficits with regard to a novel hanging object suspended on one side of the field which were not rescued by L-dopa administration. Collectively, our results provide new data important for evaluating therapeutic treatments aimed at ameliorating NF1-associated cognitive/behavioral deficits.

Increased locomotor activity and non-selective attention and impaired learning ability in SD rats after lentiviral vector-mediated RNA interference of Homer 1a in the brain

Our previous studies found that Homer 1a, a scaffolding protein localized at the post-synaptic density (PSD) of glutamatergic excitatory synapses, is significantly down-regulated in the brain of spontaneous hypertensive rats (SHR), an animal model of attention deficit hyperactivity disorder (ADHD). Furthermore, a first-line treatment drug for ADHD, methylphenidate, can up-regulate the expression of Homer 1a. To investigate the possible role of Homer 1a in the etiology and pathogenesis of ADHD, a lentiviral vector containing miRNA specific for Homer 1a was constructed in this study. Intracerebroventricular injection of this vector into the brain of Sprague Dawley (SD) rats significantly decreased Homer 1a mRNA and protein expression levels. Compared to their negative controls, these rats displayed a range of abnormal behaviors, including increased locomotor activity and non-selective attention and impaired learning ability. Our results indicated that Homer 1a down-regulation results in deficits in control over behavioral output and learning similar to ADHD.

Changes in oxidative stress and cellular immunity serum markers in attention-deficit/hyperactivity disorder

AIMS

Attention-deficit/hyperactivity disorder (ADHD) is a developmental disorder with an etiopathogeny not fully understood. According to the prevailing view, the main factors contributing to the disorder are prefrontal dopamine deficiency and central dopaminergic dysfunction, but the factors/mechanisms involved in the brain dysfunction and its consequences are not well known. We suggest that changes in oxidative metabolism and cellular immunity may be involved. In this study, we aimed to investigate whether there are associations between ADHD and changes in serum levels of nitric oxide synthase (NOS), xanthine oxidase (XO), glutathione S-transferase (GST) and paraoxonase-1 (PON-1) activities, which are important markers of oxidative stress, and adenosine deaminase (ADA) activity, marker of cellular immunity.

METHODS

The study sample consisted of 35 child or adolescent patients diagnosed with ADHD according to DSM-IV-TR criteria. Thirty-five healthy subjects were also included in the study as controls. Venous blood samples were collected, and NOS, XO, GST, PON-1 and ADA activities were measured.

RESULTS

NOS, XO and ADA activities of the patients were significantly higher than those of the controls. GST and PON-1 activities of the patients were significantly lower than those of the controls.

CONCLUSIONS

Changes in oxidative metabolism and cellular immunity may have a role in the etiopathogenesis of ADHD.

Attention-modulating effects of cognitive enhancers

Attention can be readily measured in experimental animal models. Animal models of attention have been used to better understand the neural systems involved in attention, how attention is impaired, and how therapeutic treatments can ameliorate attentional deficits. This review focuses on the ways in which animal models are used to better understand the neuronal mechanism of attention and how to develop new therapeutic treatments for attentional impairment. Several behavioral test methods have been developed for experimental animal studies of attention, including a 5-choice serial reaction time task (5-CSRTT), a signal detection task (SDT), and a novel object recognition (NOR) test. These tasks can be used together with genetic, lesion, pharmacological and behavioral models of attentional impairment to test the efficacy of novel therapeutic treatments. The most prominent genetic model is the spontaneously hypertensive rat (SHR). Well-characterized lesion models include frontal cortical or hippocampal lesions. Pharmacological models include challenge with the NMDA glutamate antagonist dizocilpine (MK-801), the nicotinic cholinergic antagonist mecamylamine and the muscarinic cholinergic antagonist scopolamine. Behavioral models include distracting stimuli and attenuated target stimuli. Important validation of these behavioral tests and models of attentional impairments for developing effective treatments for attentional dysfunction is the fact that stimulant treatments effective for attention deficit hyperactivity disorder (ADHD), such as methylphenidate (Ritalin®), are effective in the experimental animal models. Newer lines of treatment including nicotinic agonists, α4β2 nicotinic receptor desensitizers, and histamine H₃ antagonists, have also been found to be effective in improving attention in these animal models. Good carryover has also been seen for the attentional improvement caused by nicotine in experimental animal models and in human populations. Animal models of attention can be effectively used for the development of new treatments of attentional impairment in ADHD and other syndromes in which have attentional impairments occur, such as Alzheimer's disease and schizophrenia.

Abnormal synaptic plasticity in basolateral amygdala may account for hyperactivity and attention-deficit in male rat exposed perinatally to low-dose bisphenol-A

If the pregnant and lactating female rats are exposed to environmental levels of bisphenol-A (BPA), their male offspring will display hyperactivity and attention-deficit. In patients with attention-deficit/hyperactivity disorder (ADHD), the size of the amygdala is reported to be reduced. This study examined functional alterations in the basolateral amygdala (BLA) of the postnatal 28-day-old male offspring exposed perinatally to BPA (BPA-rats). We specifically focused on the synaptic properties of GABAergic/dopaminergic systems in the BLA. A single electrical stimulation of the capsule fibers evoked multispike responses with an enhanced primary population spikes (1st-PS) in the BPA-rats. A single train of high-frequency stimulation of the fibers induced NMDA receptor (NMDAR) dependent long-term potentiation (LTP) in BPA-rats, but not in control rats. Also, paired-pulse inhibition (PPI, GABA-dependent) in control rats was reversed to paired-pulse facilitation (PPF) in BPA-rats. Perfusion of slices obtained from BPA-rats with the GABA(A) receptor (GABA(A)R) agonist muscimol blocked the multispike responses and LTP, and recovered PPI. By contrast, the dopamine D1 receptor antagonist SCH23390 abolished LTP and attenuated the increased amplitude of 1st-PS in BPA-rats. Conversely, blockade of GABA(A)R by bicuculline could produce the multispike responses and PPF in BLA in control rats. Furthermore, in BLA the infusion of SCH23390, muscimol or the NMDAR blocker MK801 ameliorated the hyperactivity and improved the deficits in attention. These findings suggest that the perinatal exposure to BPA causes GABAergic disinhibition and dopaminergic enhancement, leading to an abnormal cortical-BLA synaptic transmission and plasticity, which may be responsible for the hyperactivity and attention-deficit in BPA-rats. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Oxidative imbalance in child and adolescent patients with attention-deficit/hyperactivity disorder

Various psychological, social, genetic, and biochemical factors are thought to be involved in the aetiology of attention-deficit/hyperactivity disorder (ADHD). However, few studies have evaluated the biochemical basis of ADHD. In the present study, we evaluate whether levels of nitric oxide pool (NO+NO(2)(-)) and malondialdehyde (MDA) oxidants as well as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) antioxidant enzyme activities are associated with ADHD. The sample population consisted of thirty-five child or adolescent patients diagnosed with ADHD according to DSM-IV-TR criteria. Thirty-five healthy subjects also were included in the study as controls. Venous blood samples were collected, and NO pool and MDA levels as well as SOD, GSH-Px, and CAT activities were measured. NO and MDA levels of the patients were significantly higher than the controls. GSH-Px activities of the patients were significantly lower than the controls. CAT activities of the patients were higher than the controls; however, the difference was not statistically significant. There were no significant differences in SOD activity between the patient and control groups. Remarkably high levels of NO pool and MDA oxidants as well as low GSH-Px activities suggest an oxidative imbalance in paediatric patients with ADHD. CAT activities may be increased in response to increased oxidant levels.

Reduced striatal dopamine underlies the attention system dysfunction in neurofibromatosis-1 mutant mice

Learning and behavioral abnormalities are among the most common clinical problems in children with the neurofibromatosis-1 (NF1) inherited cancer syndrome. Recent studies using Nf1 genetically engineered mice (GEM) have been instructive for partly elucidating the cellular and molecular defects underlying these cognitive deficits; however, no current model has shed light on the more frequently encountered attention system abnormalities seen in children with NF1. Using an Nf1 optic glioma (OPG) GEM model, we report novel defects in non-selective and selective attention without an accompanying hyperactivity phenotype. Specifically, Nf1 OPG mice exhibit reduced rearing in response to novel objects and environmental stimuli. Similar to children with NF1, the attention system dysfunction in these mice is reversed by treatment with methylphenidate (MPH), suggesting a defect in brain catecholamine homeostasis. We further demonstrate that this attention system abnormality is the consequence of reduced dopamine (DA) levels in the striatum, which is normalized following either MPH or l-dopa administration. The reduction in striatal DA levels in Nf1 OPG mice is associated with reduced striatal expression of tyrosine hydroxylase, the rate-limited enzyme in DA synthesis, without any associated dopaminergic cell loss in the substantia nigra. Moreover, we demonstrate a cell-autonomous defect in Nf1+/- dopaminergic neuron growth cone areas and neurite extension in vitro, which results in decreased dopaminergic cell projections to the striatum in Nf1 OPG mice in vivo. Collectively, these data establish abnormal DA homeostasis as the primary biochemical defect underlying the attention system dysfunction in Nf1 GEM relevant to children with NF1.

A novel function of microRNA let-7d in regulation of galectin-3 expression in attention deficit hyperactivity disorder rat brain

In this study we investigated the locomotor activity and non-selective attention in spontaneously hypertensive rats (SHR) with control Wistar-Kyoto (WKY) rats, which were employed as an attention deficit hyperactivity disorder (ADHD) model. In open-field test and làt maze, SHR rats were found to be much more spontaneously active than WKY rats. As compared with WKY rats, a lower level of galectin-3 was observed in SHR brain prefrontal cortex (PFC), which was the major affected brain area of ADHD. Through miRNA microarray screening, rno-let-7d was noted to be solely upregulated in SHR PFC. Interestingly, rno-let-7d had a binding site at galectin-3 mRNA and was shown to regulate galectin-3 3' untranslated region (UTR) directly. Mutation of galectin-3 3'UTR by one nucleotide of the seed sequence prevented rno-let-7d regulation of the 3' UTR completely. Although rno-let-7d did not directly regulate tyrosine hydroxylase (TH) 3'UTR, the level of galectin-3 was important for cAMP response element binding protein, the major transcript factor for TH gene. Either overexpression or downexpression of galectin-3 could result in modulation of TH expression in both PC12H and PC12L cells. In conclusion, our data suggested a novel function of rno-let-7d in regulation of galectin-3 and in ADHD development. Rno-let-7d, which is increased in the PFC of SHR brain, negatively regulated galectin-3, which is coupled with TH expression regulation.

Prefrontal cortex Homer expression in an animal model of attention-deficit/hyperactivity disorder

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is a pervasive neurobehavioral disorder affecting approximately 5% of children and adolescents and 3% of adults, and the prefrontal cortex (PFC) may play the most critical role in the expression of ADHD. Converging previous studies indicate a potential role of Homer--a scaffolding protein family localized at the postsynaptic density (PSD) of glutamatergic excitatory synapses--in behavioral pathologies associated with neuropsychiatric disorders. Accordingly, we speculate that these Homer isoforms might contribute to the etiology and development of ADHD.

METHOD

We investigated the differential mRNA and protein expressions of several Homer isoforms in the PFC of the spontaneous hypertensive rat/Wistar-Kyoto rats (SHR/WKY), the most frequently used animal model of ADHD, using RT-PCR and western blotting. Furthermore, we examined the effects of methylphenidate (MPH) exposure on the behaviors and the expression of different Homer isoforms in the PFC of SHR, using Làt maze, RT-PCR and western blotting, respectively.

RESULTS

Homer 1a and Homer 2a/b, but not Homer 1b/c, were expressed at a significantly lower levels in the PFC of SHR compared with WKY. MPH exposure decreased the locomotor activity and non-selective attention of SHR, and it up regulated the expression of Homer 1a and Homer 2a/b, but not Homer 1b/c, in the PFC of SHR.

CONCLUSION

It is plausible that Homer 1a and Homer 2a/b may be involved in the etiology and pathogenesis of ADHD. Future work will focus on elucidating the specific mechanisms of Homer 1a and Homer 2a/b in ADHD.

Attenuation of pharmacologically-induced attentional impairment by methylphenidate in rats

Methylphenidate is widely used as a treatment option for attention deficit hyperactivity disorder. In animal models of attentional impairment, it is an important validation to determine whether this clinically effective treatment attenuates deficits. The purpose of the current study was to determine whether methylphenidate can diminish attentional impairment induced by three pharmacological agents with different mechanisms of action: scopolamine, mecamylamine, and dizocilpine. Female rats were trained on an operant visual signal detection task. Ten min before the test, the rats were injected subcutaneously with methylphenidate (0, 0.1, 0.3 mg/kg), scopolamine (0, 0.005, 0.01 mg/kg), mecamylamine (0, 2, 4 mg/kg), dizocilpine (0, 0.025, 0.05 mg/kg) or combinations of methylphenidate with these drugs. In each of the experiments, all rats received every treatment in a repeated measures counterbalanced order. Correction rejection accuracy was impaired by all three of the antagonists and these effects were attenuated by methylphenidate. Both scopolamine at 0.01 and dizocilpine at 0.05 mg/kg significantly impaired percent correct rejection choice accuracy, an effect that was ameliorated by methylphenidate. Mecamylamine (4 mg/kg) impaired attentional performance by reducing percent hit and percent correct rejection. Co-administration of methylphenidate failed to significantly affect the mecamylamine-induced attentional impairment. Methylphenidate alone at 0.3 mg/kg significantly improved percent hit choice accuracy only in low-performing rats in one experiment, an effect which was reversed by scopolamine. These data show that methylphenidate effectively reverses the attentional impairment caused by scopolamine and dizocilpine. These findings further validate the operant visual signal detection task for assessing attentional impairments and their reversal.

Galactosylated dopamine enters into the brain, blocks the mesocorticolimbic system and modulates activity and scanning time in Naples high excitability rats

Pathological conditions, such as Parkinson's disease and attention deficit hyperactivity disorder, have been linked to alterations of specific dopamine (DA) pathways. However, since exogenous DA does not cross the blood-brain barrier, DA levels can be modulated e.g. by DA precursors or DA reuptake blockers. Hereby histochemical, analytical and behavioral evidence shows that a galactosylated form of DA (GAL-DA) carries DA into the brain, thus modulating activity and nonselective attention in rats. To this aim adult male rats of the Naples high-excitability (NHE) and random bred controls (NRB) lines were given a single i.p. injection of GAL-DA (10 or 100 mg/kg). Three hours later the behavior was videotaped and analyzed for horizontal activity, orienting frequency and scanning duration. The dose of 100 mglkg of GAL-DA reduced by 25% the horizontal activity in NHE rats, mainly in the first part of the testing period. No effect was observed on orienting frequency or on scanning duration. However, GAL-DA 100 mg/kg was associated with longer rearing episodes in the second part of the testing period in NHE rats. In parallel experiments histochemistry with a galactose-specific lectin showed 10% increase in galactose residues into the striatum between 0.5 and 3.0 h. To quantify the level of GAL-DA, its metabolite DA-succinate and DA in the prefrontal cortex, neostriatum, and cerebellum, rats were killed 2.0 h after the injection of prodrug. Mass high performance liquid chromatography (HPLC) was used for analysis of GAL-DA and DA succinate whereas electrochemical HPLC for DA. Both HPLC techniques demonstrate that GAL-DA carries and releases DA into the brain. Specifically 100 mg/kg of GAL-DA increased DA level in the striatum in the NHE rats only. Moreover, DA in the mesencephalon (MES) was correlated positively with striatal and prefrontal cortex DA in NHE rats. In contrast DA in the MES was negatively correlated with striatal DA in NRB. GAL-DA disrupted these correlations in both rat lines. Thus, this new DA prodrug may modify DA neurotransmission and might have a potential clinical application.

Oxidative imbalance in adult attention deficit/hyperactivity disorder

OBJECTIVE

There are few studies evaluating the biochemical basis of adult attention deficit/hyperactivity disorder (A-ADHD). In the present study, we evaluated whether nitric oxide (NO), an oxidant, level and superoxide dismutase (SOD), an antioxidant, activity are associated with A-ADHD or not.

METHODS

Twenty A-ADHD patients from Gaziantep University Sahinbey Research Hospital, Psychiatry Clinic, diagnosed according to The Turkish version of Adult ADD/ADHD DSM IV-Based Diagnostic Screening and Rating Scale by two psychiatrists (H.A.S. and S.S.), and twenty-one healthy volunteer controls were included. Blood samples were collected; NO levels and SOD activities were measured.

RESULTS

The mean NO levels in patients were significantly higher than those of controls and SOD activity of patients was significantly lower than controls.

CONCLUSIONS

Remarkable high levels of oxidant NO, and low SOD activities suggest an oxidative imbalance in A-ADHD. This is the first study evaluating the oxidative metabolism in A-ADHD.

Gabrb3 gene deficient mice exhibit impaired social and exploratory behaviors, deficits in non-selective attention and hypoplasia of cerebellar vermal lobules: a potential model of autism spectrum disorder

OBJECTIVE

GABA(A) receptors play an important regulatory role in the developmental events leading to the formation of complex neuronal networks and to the behaviors they govern. The primary aim of this study was to assess whether gabrb3 gene deficient (gabrb3(-/-)) mice exhibit abnormal social behavior, a core deficit associated with autism spectrum disorder.

METHODS

Social and exploratory behaviors along with non-selective attention were assessed in gabrb3(-/-), littermates (gabrb3(+/+)) and progenitor strains, C57BL/6J and 129/SvJ. In addition, semi-quantitative assessments of the size of cerebellar vermal lobules were performed on gabrb3(+/+) and gabrb3(-/-) mice.

RESULTS

Relative to controls, gabrb3(-/-) mice exhibited significant deficits in activities related to social behavior including sociability, social novelty and nesting. In addition, gabrb3(-/-) mice also exhibited differences in exploratory behavior compared to controls, as well as reductions in the frequency and duration of rearing episodes, suggested as being an index of non-selective attention. Gabrb3(-/-) mice also displayed significant hypoplasia of the cerebellar vermis compared to gabrb3(+/+) mice.

CONCLUSIONS

The observed behavioral deficits, especially regarding social behaviors, strengthens the face validity of the gabrb3 gene deficient mouse as being a model of autism spectrum disorder.

Nitric oxide levels in disruptive behavioral disorder

There are various evidences of the role of nitric oxide (NO) in several neuropsychiatric disorders. However, there is no clinical study which investigated the role of NO in disruptive behavioral disorders (DBD). The aim of this study is to investigate the relation between NO levels and DBD. NO levels were measured in serum from 45 patients diagnosed as having DBD (30 patients with a diagnosis of attention deficit and hyperactivity disorder [ADHD] and 15 with ADHD + oppositional defiant disorder [ODD]) and 51 healthy control subjects. It is statistically significant that the pure ADHD group's blood NO levels are lower than those of both the ADHD + ODD and control groups. There was no significant difference between the ADHD + ODD group and the controls. The difference of the NO levels in DBD may indicate the effect of NO in the etiology of this disorder spectrum.

Algorithmically designed peptides ameliorate behavioral defects in animal model of ADHD by an allosteric mechanism

This study exemplifies the use of three ADHD-relevant methodological innovations. (1) The use of novel, patented, computational peptide design techniques to generate peptides targeting the extra-cellular and para-transmembrane amino acid loops of the putatively ADHD-involved, D(2) dopamine receptor, D(2)DAR; (2) experimental evidence that these peptides in L-amino acid/ortho ordered or D-amino acid/reverse ordered (retro-inverso), D(2)DAR, hydrophobic eigenmode matched forms, evoked positive allosteric and indirect agonist influences on in vitro stably receptor transfected CHO and LtK cells and on in vivo, brain mediated activity; (3) a representative 15 residue all-D-amino acid, D(2) mode matched peptide, given parenterally, was found to "repair" a key aberrant ADHD behavioral characteristic in a standard animal model of ADHD, the Spontaneously Hypertensive Rat, SHR, relative to its progenitor species control, the Wistar-Kyoto rat, WKY. The representative, retro-inverso peptide, all-D-LLYKNKPRYPKRNRE, reversed SHR's relative deficiency in sensory motor gating (pre-pulse inhibition, PPI) while leaving SHR's nonselective attention (rearings), impulsive behavior (time in center), and activity level (timed total motor behavior) unchanged. Amphetamine also reversed SHRs sensory gating defect, but with significant increases in nonselective attention, impulsivity and hyperactivity. These preliminary results suggest the possibility of a new, "softer" pharmacological approach to ADHD: hydrophobic mode matched peptide allosteric augmentation of the activity of indigenous dopamine with respect to D(2)DAR mediated function, in place of stimulant drug-induced presynaptic dopamine release or impairment of dopamine uptake.

Deletion of dopamine D1 and D3 receptors differentially affects spontaneous behaviour and cocaine-induced locomotor activity, reward and CREB phosphorylation

Co-localization of dopamine D1 and D3 receptors in striatal neurons suggests that these two receptors interact at a cellular level in mediating dopaminergic function including psychostimulant-induced behaviour. To study D1 and D3 receptor interactions in cocaine-mediated effects, cocaine-induced locomotion and reward in mice lacking either D1, D3 or both receptors were analysed. Spontaneous locomotor activity was increased in D1-/- and D1-/-D3-/- mice and D1-/-D3-/- mice did not exhibit habituation of spontaneous rearing activity. Cocaine (20 mg/kg) increased locomotor activity in wild-type and D3-/- mice, failed to stimulate activity in D1-/- mice and reduced activity in D1-/-D3-/- mice. In the conditioned place preference, all groups exhibited reward at 5, 10 and 20 mg/kg of cocaine. D1-/-D3-/- mice did not demonstrate preference at 2.5 mg/kg of cocaine although preference was observed in wild-type, D1-/- and D3-/- mice. The transcription factor cAMP-responsive element binding protein (CREB) is activated by phosphorylation in striatal regions following dopamine receptor activation. Striatal pCREB levels following acute cocaine were increased in wild-type and D3-/- mice and decreased in D1-/- and D1-/-D3-/- mice. After repeated administration of 2.5 mg/kg of cocaine, D1-/- mice had lower pCREB levels in caudate-putamen and nucleus accumbens. Our findings suggest that, although spontaneous and cocaine-induced horizontal activity depended mainly on the presence of the D1 receptor, there may be crosstalk between D1 and D3 receptors in rearing habituation and the perception of cocaine reward at low doses of the drug. Furthermore, alterations in pCREB levels were associated with changes in cocaine-induced locomotor activity but not reward.

Acetyl-L-carnitine reduces impulsive behaviour in adolescent rats

The attention deficit/hyperactivity disorder (ADHD) can affect human infants and adolescents. One important feature of this disorder is behavioural impulsivity. This study assessed the ability of chronic acetyl-L-carnitine (ALC, saline or 100 mg/kg SC, plus 50 mg/kg orally) to reduce impulsivity in a validated animal model for ADHD. Food-restricted rats were tested during adolescence (postnatal days, pnd, 30-45) in operant chambers with two nose-poking holes, one delivering one food pellet immediately, and the other five pellets after a delay. Delay length was increased over days (from 0 to 80 s). Individual differences in the preference-delay curve emerged, with the identification of two distinct subpopulations, i.e. one with a nearly horizontal curve and another with a very steep ("impulsive") slope. The impulsivity profile was slightly but consistently reduced by chronic ALC administration. Consistent results were also obtained with methylphenidate (MPH, saline or 3 mg/kg IP twice daily). Impulsive rats exhibited a lower metabolite/serotonin (5HIAA/5HT) ratio in the medial frontal cortex (MFC) and lower noradrenaline (NA) levels in the MFC and cingulate cortex (CC) when compared with the other subgroup. The ALC treatment increased NA levels in the CC and the 5HIAA/5HT ratio in both CC and MFC. Present data suggest that ALC, a drug devoid of psychostimulant properties, may have some beneficial effects in the treatment of ADHD children.

Locomotor hyperactivity following prenatal exposure to 5-bromo-2'-deoxyuridine: neurochemical and behavioral evidence of dopaminergic and serotonergic alterations

Prenatal exposure to 5-bromo-2'-deoxyuridine (BrdU) has been reported to induce abnormal behaviors in offspring, including marked hyperactivity. In this study, the contribution of the serotonin (5-HT) and dopamine (DA) systems to BrdU-induced developmental neurotoxicity was investigated. Sprague-Dawley rats were treated with BrdU on gestational days 9 through 15 (50mg/kg, i.p.) and male offspring (BrdU-rats) were examined. The BrdU-rats exhibited a 3.5-fold increase in locomotor activity. The dopamine D2 receptor antagonist sulpiride increased locomotor activity in the BrdU-rats, but decreased it in control rats. The BrdU-rats responded to the 5-HT1A receptor antagonist NAN190 much more than the controls. The measurement of monoamines revealed significant decreases in DA, dihydroxyphenylacetic acid, and homovanilic acid, and significant increases in 5-HT and 5-hydroxy-3-indolacetic acid, with a decrease in the 5-HT turnover ratio in the striatum of BrdU-rats. Thus, prenatal exposure to BrdU induced alterations in both the DA and 5-HT systems.

The spontaneously hypertensive-rat as an animal model of ADHD: evidence for impulsive and non-impulsive subpopulations

Attention-deficit hyperactivity disorder (ADHD) is a neuropsychiatric syndrome, affecting human infants and adolescents. Two main behavioural features are reported: (1). impaired attention and (2). an impulsive-hyperactive behavioural trait. The latter has been studied in a series of experiments, using the spontaneously hypertensive-rat (SHR) strain (which is regarded as a validated animal model for ADHD) in operant tasks. Food-restricted SHRs and their Wistar-Kyoto (WKY) controls were tested during adolescence (i.e. post-natal days 30-45), in operant chambers provided with two nose-poking holes. Nose-poking in one hole (H1) resulted in the immediate delivery of a small amount of food, whereas nose-poking in the other hole (H5) delivered a larger amount of food after a delay, which was increased progressively each day (0-100 s). As expected, all animals showed a shift in preference from the large (H5) to the immediate (H1) reinforcer as the delay length increased. Impulsivity can be measured by the steepness of this preference-delay curve. The two strains differed in home-cage circadian activity, SHRs being more active than WKYs at several time-points. During the test for impulsivity, inter-individual differences were completely absent in the WKY strain, whereas a huge inter-individual variability was evident for SHRs. On the basis of the median value of average hole-preference, we found an 'impulsive' SHR subgroup, with a very quick shift towards the H1 hole, and a flat-slope ('non-impulsive') SHR subgroup, with little or no shift. The impulsive subpopulation also presented reduced noradrenaline levels in both cingulated and medial-frontal cortex, as well as reduced serotonin turnover in the latter. Also, cannabinoid CB1 receptor density resulted significantly lower in the prefrontal cortex of impulsive SHRs, when compared to both the non-impulsive subgroup and control WKYs. Interestingly, acute administration of a cannabinoid agonist (WIN 55,212, 2 mg/kg s.c.) normalized the impulsive behavioural profile, without any effect on WKY rats. Thus, two distinct subpopulations, differing for impulsive behaviour and specific neurochemical parameters, were evidenced within adolescent SHRs. These results support the notion that a reduced cortical density of cannabinoid CB1 receptors is associated with enhanced impulsivity. This behavioural trait can be positively modulated by administration of a cannabinoid agonist. Present results confirm and extend previous literature, indicating that adolescent SHRs represent a suitable animal model for the preclinical investigation of the early-onset ADHD syndrome.

Behavioural, pharmacological, morpho-functional molecular studies reveal a hyperfunctioning mesocortical dopamine system in an animal model of attention deficit and hyperactivity disorder

Clinical and experimental evidence suggest an involvement of dopamine systems, mainly the mesocorticolimbic one (MCL), in Attention-Deficit Hyperactivity Disorder (ADHD). However, it remains to be ascertained whether the systems are hyper- or hypo-functioning, for the implications of the functional state. Indeed, differential functional states of the MCL branches are suggested to be the neural substrate of different ADHD variants. This review covers published and unpublished data from the Naples-High Excitability (NHE) rat, an animal model of ADHD, featuring its main aspects, with no hypertension. Therefore, a multiple approach based on morphological studies of dopamine, norepinephrine, glutamate, acetylcholine and GABA systems, synaptic (Calcium/Calmodulin kinase II) and extrasynaptic (chondroitin sulphates) environments, and molecular biology and pharmacological studies on the dopamine system has been carried out. Morphological findings suggest dopamine neurons in the Ventral Tegmental Area (VTA) to be hypertrophic in NHE rats. The mesostriatal and mesolimbic dopamine branches appear to be normal in basal conditions. However, the striatal interface is probably defective following activation. Conversely, the prefrontal cortex, which represents the second main target of VTA dopamine neurons, has many alterations at the basal level. Therefore, the emerging picture is the association of a hyperinnervating and hyperfunctioning mesocortical branch of the dopamine system. Thus, the evidence gathered so far might improve our understanding of the neural substrates of neuropsychiatric disorders such as ADHD, schizophrenia and drug addiction.

Dopamine phenotype and behaviour in animal models: in relation to attention deficit hyperactivity disorder

The phenotypic expression of behaviour is the outcome of interacting neuronal networks and is modulated by different subcortical systems. In the present paper the role of a major subcortical neurochemical system, dopamine (DA), is reviewed. In particular, knockout (KO) technology has given an overwhelming insight into the effects of specific component of the dopaminergic system. Therefore, the behavioural profile of dopamine transporter (DAT), tyrosine hydroxylase (TH), DA and cAMP-regulated phosphoprotein (DARPP 32), and D1, D2, D3, D4 and D5 dopamine receptors knockouts (and their combination) is reviewed.TH, D1, D2, D4 KO mice exhibit decreased locomotor activity, perhaps due to decreased motivational level. D3 KO and DAT KO mice show an increase in basal and novelty-induced activity respectively. It is possible that the increased dopamine levels in DAT KO mice enhance motivation. These observations support the hyperDA hypothesis in hyperactive phenotypes. Moreover, they suggest that the inhibitory effect of psychostimulant drugs, such as methylphenidate and amphetamines, in Attention Deficit Hyperactivity Disorder may be the outcome of an altered balance between auto- and hetero-receptors. However, since KO technology is hampered by blockade of the target at early stages of development, some alternatives have been proposed, such as inducible mutagenesis and inhibitory small RNAs conveyed to target by viral vectors in adulthood.

Prenatal elevation of endocannabinoids corrects the unbalance between dopamine systems and reduces activity in the Naples High Excitability rats

Several evidences suggest that endocannabinoids exert a neurotrophic effect on developing mesencephalic dopamine neurons. Since an altered mesocorticolimbic system seems to underlie hyperactivity and attention deficit in clinical and animal studies of attention deficit hyperactivity disorder (ADHD), prenatal elevation of anandamide has been induced in Naples high excitability (NHE) rats by inhibition of its reuptake. To this aim, pregnant NHE and random-bred females received a subcutaneous injection of AM-404 (1 mg/kg) or vehicle daily from E11 until E20. Young adult male offsprings were exposed to a spatial novelty (Làt-maze) for 30 min and the behavior was videotaped and analysed for indices of activity (travelled distance, rearing frequency) and attention (rearing duration). Moreover, morphological analysis of the brains was carried out that pertained to cytochrome oxydase as marker of metabolic activity and thyrosine hydroxylase as marker of the dopamine systems. The results indicate that prenatal AM-404 treatment significantly reduces activity by about 20% during the entire testing period and modifies the distribution of scanning times towards short duration episodes in the first part of the test only in NHE-treated rats. In addition, image analysis revealed a significant increase in relative optical density of TH+terminals in the dorsal striatum and substantia nigra of AM-404 treated NHE rats and minor changes in the dorsal cortex of AM-404 treated NRB rats. The data suggest a corrected unbalance between the two dopamine systems that apparently leads to reduced hyperactivity and modified scanning times in this animal model of ADHD. This, in turn, might open new strategies in the treatment of a subset of ADHD cases.

Effects of a neurosteroid, pregnenolone, during the neonatal period on adenosine A1 receptor, dopamine metabolites in the fronto-parietal cortex and behavioral response in the open field

Neuronal dysfunction in the frontal cortex has been reported in the etiology of mental disorders, including schizophrenia. The adenosine A(1) receptor system, as well as the dopaminergic system, are important in the control of cortical neuronal activity. We hypothesize that neuroexcitability in early life is critical to the normal development of the brain, and neurosteroids are factors that modulate neuroexcitability during the development period. In this study, we treated neonatal rats with a neurosteroid, pregnenolone (10 microg/g) from postnatal day (PD) 3 until PD 7. In pregnenolone-treated male and female rats, adenosine A(1) receptor density, the amount of dopamine (DA), serotonin (5-HT) and their metabolites in the fronto-parietal cortex and behavioral responses in the open field were examined pre- and post-puberty. A decrease in K(d) values for the adenosine A(1) receptor binding assay using [(3)H]1,3-dipropyl-8-cyclopentylxanthine (DPCPX), increased formation of DA metabolites and hyper-locomotor activity in the open field were found in pregnenolone-treated rats compared with controls in pre- and post-puberty. An increase in 5-HT metabolites was found in the pregnenolone-treated rats in pre-puberty, but not post-puberty. These effects of pregnenolone were not different between males and females. However, correlations between horizontal and vertical activities in the open field were disrupted only in pregnenolone-treated females. The present results indicate that pregnenolone treatment during the neonatal period influences the cortical dopaminergic and adenosinergic systems as well as behavioral responses in the open field.

Activity, non-selective attention and emotionality in dopamine D2/D3 receptor knock-out mice

In order to assess the role of dopamine (DA) D2 and D3 receptors in the modulation of behaviour, we analysed exploration in a spatial novelty in mouse model systems. Genetically engineered mice mutants have been used that carry normal, partial or no expression of D2R, D3R, or both D2R/D3R (double mutants) DA receptor subtypes. Adult male mice were exposed for 30 min to a Làte-maze. The behaviour was analysed for indices of activity, orienting (rearing frequency), scanning times (rearing duration) and defecation score (emotionality). D2R - / - and + / - as well as the D2R/D3R double homozygous mutants were less active than wild-type (WT) controls in travelled distance. In contrast D3R + / - were more active than WT mice in the first part of the test. As to orienting frequency, the D2R - / - were less active than WT during the entire test-period, whereas the D2 + / - mutants were less active than WT only in the second part of the test. Moreover, the D3R - / - and + / - mutants showed less and more rearing frequency than WT, respectively, during the entire test. Finally, the D2/D3R - / - double mutants were also less active than WT during the entire test period. As to scanning times, D2R + / - and - / - mutants were higher than WT during the entire test or only in the second part, respectively. The D3R + / - and - / - were not different from WT, whereas the D2/D3R - / - double mutants showed shorter scanning times only in the first part of the test. As to emotionality index, the defecation score, was lower only in D3R + / - mutants. Thus, the dopamine D2 and D3 receptor subtypes appear to be differentially involved in the modulation of activity, orienting and scanning phases of attention. Lastly double mutation experiments reveal an interaction between D2R and D3R with the former prevailing on the latter.

A rostro-caudal dissociation in the dorsal and ventral striatum of the juvenile SHR suggests an anterior hypo- and a posterior hyperfunctioning mesocorticolimbic system

Functional molecular neuroimaging techniques have been applied to the study of the neural substrates of Attention-Deficit Hyperactivity Disorder (ADHD) in an animal model, the juvenile SHR rat. They include quantitative receptor autoradiography and immunocytochemistry for neuronal markers such as Ca2+/Calmodulin Dependent Kinase II (CaMKII) and transcription factors. Multiple evidence emerges for a rostro caudal dissociation within the dorsal (DS) and ventral striatum (VS) (n. accumbens) and olfactory tubercle (OT). It consists in (i) a higher density of dopamine (DA) D-1/D-5 receptor binding sites in a discrete segment of the anterior forebrain that comprises the DS, VS and OT, (ii) a lower density of DA D-2/D-3 autoreceptors in the caudal portion of the n. accumbens shell subterritory, (iii) a reduced number of CaMKII and c-FOS positive elements only in the anterior portion of DS and VS (iv) reversal by repeated injections of methylphenidate (MP) (3 mg/kg, 14 days) with 'downregulation' in SHR and 'up-regulation' in the WKY control rats of DS and VS of DA D-1/D-5 receptors. Thus, under basal conditions the mesocorticolimbic (MCL) DA system appears to be hyperfunctioning rather than hypofunctioning, as demonstrated (i) by subsensitivity of presynaptic D-3 autoreceptors and (ii) by phasic inhibition of MCL activity induced by acute blockade of endocannabinoid reuptake using AM404. Following MP treatment, the hyperfunctioning MCL DA system turns into a hypofunctioning one, as earlier suggested by Solanto. Since the target neurons of MCL fibers seem to be uncoupled to D-1 receptors, the medium spiny GABA neurons projecting to the ventral pallidum and ventral tegmental area (VTA) exert a weak feedback inhibition on the neurons of origin of MCL system. Therefore, MCL neurons maintain a high basal activity with consequences on the cortico-striato-pallido-thalamo-cortical system and amygdala complex through the 'extended amygdala system'. While the former explains the attention, motivation and activity alterations of this rat model of ADHD, the latter explains the emotional symptoms of the syndrome. It remains to be ascertained the starting point in the network leading eventually to the segmental defect as well as its significance in humans.

Selective inhibition of neuronal nitric oxide synthesis reduces hyperactivity and increases non-selective attention in the Naples High-Excitability rat

The involvement of neuron-derived NO in the process of orienting and scanning times (non-selective attention: NSA) towards environmental stimuli has been investigated in the Naples High-Excitability rat (NHE), a putative animal model of Hyperactivity and Attention Deficit (ADHD). To this aim, orienting and scanning times have been monitored by the frequency and duration of rearing episodes, respectively. Adult male NHE rats were tested in a novelty situation (Làt-maze) for 30 min following single or repeated injections of the non competitive inhibitor 7-Nitroindazole (7-NINA) of the neuronal isoform of the enzyme nitric oxide synthase (n-NOS). In the acute experiments, rats received a single injection of 7-NINA (1 mg/kg) intraperitonealy in a saline vehicle (exp. 1, fast release) or subcutaneously in a lipid carrier, dimethyl sulfoxide (DMSO; exp. 2, slow release) or the vehicles alone as controls 30 min before testing. In the repeated injection experiments, rats received a subcutaneus injection of 1 mg/kg in DMSO or DMSO alone daily for 14 days, and tested 24 h after the last injection (exp. 3, slow release). The results showed a significant differential effect of the drug that was dependent on the release rate, i.p. saline-diluted 7-NINA increased the duration of individual rearing episodes whereas, both single and repeated subcutaneous DMSO-carried 7-NINA exerted an opposite effect. Thus, selective inhibition of n-NOS by an allosteric inhibitor that increases arginine availability without displacing the inhibitor from n-NOS, strengthens the hypothesized role of NO in NSA. These findings may shed light on the mechanism of action of drug treatment of and be useful in the treatment of ADHD in children.

Multiple evidence of a segmental defect in the anterior forebrain of an animal model of hyperactivity and attention deficit

Molecular biology and microscope imaging techniques were used to map putative neural substrates of hyperactivity and attention deficit in an animal model, the juvenile prehypertensive male spontaneously hypertensive rat (SHR). We have studied in anterior forebrain sections of SHR and Wistar-Kyoto Normotensive (WKY) controls the spatial distribution of neural markers such as: (i) dopamine (DA) D-1 and D-2 receptor families by radioligand binding studies; (ii) the Ca2+/calmodulin-dependent protein kinase II (CaMKII); and (iii) the transcription regulators of gene expression (TFs) c-FOS and JUN-B by Immunocytochemistry (ICC). Microcomputer-assisted high-resolution image analysis showed in the SHR a higher density of DA D-1 receptors and a lower density of D-3 autoreceptors paralleled by a reduced number of elements positive for CaMKII and TFs in a restricted segment of the anterior forebrain that included the most rostral portions of the caudate-putamen, pole and shell of the nucleus accumbens and olfactory tubercle. The differential rostro-caudal distribution of D-1 receptors and D-3 autoreceptors is discussed in the light of current hypotheses of DA mesocorticolimbic system functioning. In addition, the segmental defect was partially reversed by subchronic treatment with a DA re-uptake blocker, Methylphenidate (MPH; 3 mg/kg) and by environmental stimulation during the fifth and sixth postnatal week. The findings are consistent with the role of genetic determinants and environmental factors in the phenotypic expression of hyperactivity and attention deficit.