Regional research priorities in brain and nervous system disorders.

The characteristics of neurological, psychiatric, developmental and substance-use disorders in low- and middle-income countries are unique and the burden that they have will be different from country to country. Many of the differences are explained by the wide variation in population demographics and size, poverty, conflict, culture, land area and quality, and genetics. Neurological, psychiatric, developmental and substance-use disorders that result from, or are worsened by, a lack of adequate nutrition and infectious disease still afflict much of sub-Saharan Africa, although disorders related to increasing longevity, such as stroke, are on the rise. In the Middle East and North Africa, major depressive disorders and post-traumatic stress disorder are a primary concern because of the conflict-ridden environment. Consanguinity is a serious concern that leads to the high prevalence of recessive disorders in the Middle East and North Africa and possibly other regions. The burden of these disorders in Latin American and Asian countries largely surrounds stroke and vascular disease, dementia and lifestyle factors that are influenced by genetics. Although much knowledge has been gained over the past 10 years, the epidemiology of the conditions in low- and middle-income countries still needs more research. Prevention and treatments could be better informed with more longitudinal studies of risk factors. Challenges and opportunities for ameliorating nervous-system disorders can benefit from both local and regional research collaborations. The lack of resources and infrastructure for health-care and related research, both in terms of personnel and equipment, along with the stigma associated with the physical or behavioural manifestations of some disorders have hampered progress in understanding the disease burden and improving brain health. Individual countries, and regions within countries, have specific needs in terms of research priorities.

Glutamate-stimulated release of norepinephrine in hippocampal slices of animal models of attention-deficit/hyperactivity disorder (spontaneously hypertensive rat) and depression/anxiety-like behaviours (Wistar-Kyoto rat).

Norepinephrine is known to play an integral role in different aspects of behaviour, such as attention and arousal. It has also been implicated in the neurobiology of attention-deficit/hyperactivity disorder (ADHD). The present study was undertaken to determine the differential effects of glutamate on norepinephrine release in hippocampal slices of several rat strains. Two of the strains used in this study model behavioural disorders i.e. spontaneously hypertensive rats (SHR) mimic the behavioural characteristics of ADHD and Wistar-Kyoto (WKY) rats have been used to model depression/anxiety-like behaviours. To achieve the aims of this study, an in vitro superfusion technique was used to determine glutamate-stimulated release of radioactively labelled norepinephrine in hippocampal slices. The results show (1) SHR and Wistar rats released significantly more [(3)H]norepinephrine in response to a 1-min pulse of glutamate (1 mM) than WKY, Sprague-Dawley and Long-Evans rats. (2) Glutamate-stimulated release of [(3)H]norepinephrine was reduced by the AMPA receptor antagonist, CNQX (1 muM), suggesting that AMPA receptors are involved. (3) Exposure of hippocampal slices to a second and third 1-min pulse of glutamate revealed significant decreases in the peaks of [(3)H]norepinephrine release suggesting internalization of AMPA receptors. The rate of AMPA receptor internalization was slower in SHR than in WKY. (4) The NMDA receptor antagonist, MK-801 (10 microM) increased glutamate-stimulated release of [(3)H]norepinephrine in SHR hippocampus. This effect was blocked by CNQX, suggesting that AMPA receptors were required for the NMDA effect and that there was an NMDA component of AMPA receptor internalization in SHR hippocampus which was not evident in WKY. The present findings reveal a novel NMDA component that influences AMPA receptor-mediated regulation of norepinephrine release in SHR hippocampus.

Reprint of "Neurobiology of animal models of attention-deficit hyperactivity disorder".

Attention-deficit hyperactivity disorder (ADHD) is a heterogeneous, highly heritable, disorder resulting from complex gene-gene and gene-environment interactions. The defining symptoms of hyperactivity, impulsivity and impaired sustained attention are not unique to ADHD. It is therefore not surprising that animals with distinctly different neural defects model the behavioural characteristics of the disorder. Consistent with ADHD being a developmental disorder, animal models are either genetic (spontaneously hypertensive rats (SHR), dopamine transporter (DAT) knock-out mice, SNAP-25 mutant mice, mice expressing a mutant thyroid receptor) or have suffered an insult to the central nervous system during the early stages of development (anoxia, 6-hydroxydopamine). It appears that neural transmission is impaired by either direct disruption of dopaminergic transmission or a more general impairment of neurotransmission that gives rise to compensatory changes in monoaminergic systems that are not sufficient to completely normalize neural function. In general, results obtained with animal studies suggest that dopamine neurons are functionally impaired. However, evidence obtained from some animal models suggests that the noradrenergic and serotonergic neurotransmitter systems may be the target of drugs that ameliorate ADHD symptoms.

Neurobiology of animal models of attention-deficit hyperactivity disorder.

Attention-deficit hyperactivity disorder (ADHD) is a heterogeneous, highly heritable, disorder resulting from complex gene-gene and gene-environment interactions. The defining symptoms of hyperactivity, impulsivity and impaired sustained attention are not unique to ADHD. It is therefore not surprising that animals with distinctly different neural defects model the behavioural characteristics of the disorder. Consistent with ADHD being a developmental disorder, animal models are either genetic (spontaneously hypertensive rats (SHR), dopamine transporter (DAT) knock-out mice, SNAP-25 mutant mice, mice expressing a mutant thyroid receptor) or have suffered an insult to the central nervous system during the early stages of development (anoxia, 6-hydroxydopamine). It appears that neural transmission is impaired by either direct disruption of dopaminergic transmission or a more general impairment of neurotransmission that gives rise to compensatory changes in monoaminergic systems that are not sufficient to completely normalize neural function. In general, results obtained with animal studies suggest that dopamine neurons are functionally impaired. However, evidence obtained from some animal models suggests that the noradrenergic and serotonergic neurotransmitter systems may be the target of drugs that ameliorate ADHD symptoms.

Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder--the spontaneously hypertensive rat.

RUSSELL, V.A. Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder-the spontaneously hypertensive rat. NEUROSCI. BIOBEHAV. REV.27(2003). Disturbances in glutamate, dopamine and norepinephrine function in the brain of a genetic animal model for attention-deficit hyperactivity disorder (ADHD), the spontaneously hypertensive rat (SHR), and information obtained from patients with ADHD, suggest a defect in neuronal circuits that are required for reward-guided associative learning and memory formation. Evidence derived from (i). the neuropharmacology of drugs that are effective in treating ADHD symptoms, (ii). molecular genetic and neuroimaging studies of ADHD patients, as well as (iii). the behaviour and biochemistry of animal models, suggests dysfunction of dopamine neurons. SHR have decreased stimulation-evoked release of dopamine as well as disturbances in the regulation of norepinephrine release and impaired second messenger systems, cAMP and calcium. In addition, evidence supports a selective deficit in the nucleus accumbens shell of SHR which could contribute to impaired reinforcement of appropriate behaviour.

Hypodopaminergic and hypernoradrenergic activity in prefrontal cortex slices of an animal model for attention-deficit hyperactivity disorder--the spontaneously hypertensive rat.

Evidence supports dysfunction of dopaminergic and noradrenergic systems in patients with attention-deficit hyperactivity disorder (ADHD). Noradrenergic and dopaminergic systems exert distinct modulatory actions on the transfer of information through neural circuits that connect functionally distinct cortical areas with separate striatal regions and remain segregated in parallel striato-pallidal-thalamic and striato-substantia nigra pars reticulata-thalamic pathways. Prefrontal cortex performance is maximal at moderate stimulation of postsynaptic dopaminergic and noradrenergic receptors, and is reduced by either higher or lower levels of receptor stimulation. Spontaneously hypertensive rats (SHR) are generally considered to be a suitable genetic model for ADHD, since they display hyperactivity, impulsivity, poor stability of performance, impaired ability to withhold responses and poorly sustained attention, when compared with their normotensive Wistar-Kyoto (WKY) control rats. Evidence suggests that terminals of mesocortical, mesolimbic and nigrostriatal dopaminergic neurons of SHR release less dopamine in response to electrical stimulation and/or depolarization as a result of exposure to high extracellular K+ concentrations, than WKY. Vesicular storage of dopamine was suggested to be impaired in SHR, causing leakage of dopamine into the cytoplasm and increased d-amphetamine-induced transporter-mediated release. While electrically stimulated release of dopamine appears to be decreased in prefrontal cortex of SHR suggesting hypodopaminergic function, autoreceptor-mediated inhibition of norepinephrine release appears to be impaired in SHR, suggesting that noradrenergic function may be poorly regulated in the prefrontal cortex of the SHR. These findings are consistent with the hypothesis that the behavioral disturbances of ADHD are the result of an imbalance between noradrenergic and dopaminergic systems in the prefrontal cortex, with inhibitory dopaminergic activity being decreased and noradrenergic activity increased relative to controls.

Overview of animal models of attention deficit hyperactivity disorder (ADHD).

Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous, highly heritable, behavioral disorder that affects ∼5% to 10% of children worldwide. Although animal models cannot truly reflect human psychiatric disorders, they can provide insight into the disorder that cannot be obtained from human studies because of the limitations of available techniques. Genetic models include the spontaneously hypertensive rat (SHR), the Naples High Excitability (NHE) rat, poor performers in the 5-choice serial reaction time (5-CSRT) task, the dopamine transporter (DAT) knock-out mouse, the SNAP-25 deficient mutant coloboma mouse, mice expressing a human mutant thyroid hormone receptor, a nicotinic receptor knock-out mouse, and a tachykinin-1 (NK1) receptor knock-out mouse. Chemically induced models of ADHD include prenatal or early postnatal exposure to ethanol, nicotine, polychlorinated biphenyls, or 6-hydroxydopamine (6-OHDA). Environmentally induced models have also been suggested; these include neonatal anoxia and rat pups reared in social isolation. The major insight provided by animal models was the consistency of findings regarding the involvement of dopaminergic, noradrenergic, and sometimes also serotonergic systems, as well as more fundamental defects in neurotransmission.

Voluntary exercise reduces the neurotoxic effects of 6-hydroxydopamine in maternally separated rats.

Maternal separation has been associated with development of anxiety-like behaviour and learning impairments in adult rats. This has been linked to changes in brain morphology observed after exposure to high levels of circulating glucocorticoids during the stress-hyporesponsive period (P4-P14). In the present study, adult rats that had been subjected to maternal separation (180 min/day for 14 days) during the stress-hyporesponsive period, received unilateral infusions of a small dose of 6-hydroxydopamine (6-OHDA, 5 microg/4 microl saline) into the medial forebrain bundle. The results showed that voluntary exercise had a neuroprotective effect in both non-stressed and maternally separated rats in that there was a decrease in forelimb akinesia (step test) and limb use asymmetry (cylinder test). Maternal separation increased forelimb akinesia and forelimb use asymmetry and reduced the beneficial effect of exercise on forelimb akinesia. It also reduced exploratory behaviour, consistent with anxiety-like behaviour normally associated with maternal separation. Exercise appeared to reduce dopamine neuron destruction in the lesioned substantia nigra when expressed as a percentage of the non-lesioned hemisphere. However, this appeared to be due to a compensatory decrease in completely stained tyrosine hydroxylase-positive neurons in the contralateral, non-lesioned substantia nigra. In agreement with reports that maternal separation increases the 6-OHDA-induced loss of dopamine terminals in the striatum, there was a small increase in dopamine neuron destruction when expressed as a percentage of the non-lesioned hemisphere but there was no difference in dopamine cell number, suggesting that exposure to maternal separation did not exacerbate dopamine cell loss.

Methylphenidate does not increase ethanol consumption in a rat model for attention-deficit hyperactivity disorder-the spontaneously hypertensive rat.

Attention-deficit hyperactivity disorder (ADHD) is a behavioural disorder that has been suggested to result from disturbances in the dopaminergic system of the brain. The most effective drugs used to treat ADHD are the psychostimulants, methylphenidate and amphetamine. They block dopamine transporters and increase dopamine release, thereby increasing the extracellular concentration of dopamine and altering dopamine signaling. Drugs of abuse, such as cocaine, also block dopamine transporters, which raises the concern that treatment of children with ADHD with psychostimulants might increase their susceptibility to drug addiction. The present study was aimed at investigating whether treatment with methylphenidate at an early stage of development increased preference for ethanol in a widely used rat model for ADHD, the spontaneously hypertensive rat (SHR). SHR display the three major characteristics of ADHD (hyperactivity, impulsivity, poor sustained attention) compared to their progenitor Wistar-Kyoto (WKY) rat strain. Ethanol increased locomotor activity of SHR slightly more than WKY when injected intraperitoneally (0.6 g/kg). SHR also spent more time in the inner zone of the open field than WKY, consistent with SHR being less anxious than WKY. When given free access to ethanol-containing solutions of increasing concentration, SHR consumed less ethanol than WKY. Treatment with methylphenidate at an early age (P21 to P35) did not alter ethanol consumption in adult SHR or WKY, suggesting that it does not increase susceptibility to ethanol addiction in these rats. In vitro superfusion studies further demonstrated that preadolescent methylphenidate treatment did not have long-term effects on dopamine release in adult SHR and WKY striatum. A major finding of this study is the fact that methylphenidate treatment did not increase alcohol use in SHR.

A dynamic developmental theory of attention-deficit/hyperactivity disorder (ADHD) predominantly hyperactive/impulsive and combined subtypes.

Attention-deficit/hyperactivity disorder (ADHD) is currently defined as a cognitive/behavioral developmental disorder where all clinical criteria are behavioral. Inattentiveness, overactivity, and impulsiveness are presently regarded as the main clinical symptoms. The dynamic developmental behavioral theory is based on the hypothesis that altered dopaminergic function plays a pivotal role by failing to modulate nondopaminergic (primarily glutamate and GABA) signal transmission appropriately. A hypofunctioning mesolimbic dopamine branch produces altered reinforcement of behavior and deficient extinction of previously reinforced behavior. This gives rise to delay aversion, development of hyperactivity in novel situations, impulsiveness, deficient sustained attention, increased behavioral variability, and failure to "inhibit" responses ("disinhibition"). A hypofunctioning mesocortical dopamine branch will cause attention response deficiencies (deficient orienting responses, impaired saccadic eye movements, and poorer attention responses toward a target) and poor behavioral planning (poor executive functions). A hypofunctioning nigrostriatal dopamine branch will cause impaired modulation of motor functions and deficient nondeclarative habit learning and memory. These impairments will give rise to apparent developmental delay, clumsiness, neurological "soft signs," and a "failure to inhibit" responses when quick reactions are required. Hypofunctioning dopamine branches represent the main individual predispositions in the present theory. The theory predicts that behavior and symptoms in ADHD result from the interplay between individual predispositions and the surroundings. The exact ADHD symptoms at a particular time in life will vary and be influenced by factors having positive or negative effects on symptom development. Altered or deficient learning and motor functions will produce special needs for optimal parenting and societal styles. Medication will to some degree normalize the underlying dopamine dysfunction and reduce the special needs of these children. The theory describes how individual predispositions interact with these conditions to produce behavioral, emotional, and cognitive effects that can turn into relatively stable behavioral patterns.

In vitro glutamate-stimulated release of dopamine from nucleus accumbens core and shell of spontaneously hypertensive rats.

Spontaneously hypertensive rats (SHR) are used as a model for attention-deficit hyperactivity disorder (ADHD) since SHR display the major symptoms of ADHD (hyperactivity, impulsivity, inablity to sustain attention during behavioral tasks). We previously showed that electrical and/or K+-stimulated release of dopamine (DA) from nerve terminals in the prefrontal cortex, nucleus accumbens, and caudate-putamen of SHR was significantly lower than that of Wistar-Kyoto (WKY) control rats. The aim of the present investigation was to determine whether glutamate-stimulated release of DA from nucleus accumbens core and shell of SHR was significantly different from that of WKY. Using an in vitro superfusion technique, we showed that glutamate-stimulated release of [3H]DA from striatal slices is mediated by glutamate activation of AMPA receptors and that glutamate-stimulated release of [3H]DA from nucleus accumbens core and shell of 4-6-week-old SHR and WKY is not significantly different. Glutamate-stimulated release of [3H]DA from SHR shell is significantly lower than SHR core and there is also a tendency for glutamate-stimulated [3H]DA release from SHR shell to be lower than release from WKY shell.

Effect of enriched environment on Ca2+ uptake via NMDA receptors into barrel cortex slices of spontaneously hypertensive rats.

Exposure to an enriched environment provides animals with informal learning opportunities and is associated with increases in brain size, cortical thickness, neuron size, dendritic branching, spine density, and number of synapses per neuron. The NMDA receptor is involved in synaptic plasticity. This study sought to determine the effect of exposure to an enriched environment on NMDA receptor function in barrel cortex slices of spontaneously hypertensive rats (SHR) and their control Wistar-Kyoto (WKY) rats. An assortment of items such as PVC pipes, metal pipes, metal boxes, metal ladders, and a polystyrene maze, were placed successively in the cages of test animals to create an enriched environment. After 2 weeks, the rats were killed. Their brains were rapidly removed, cooled in continuously oxygenated HEPES buffer (pH 7.4), and sliced in a vibratome to produce 0.35-mm thick slices. The barrel cortex was dissected from slices corresponding to 8.6-4.8 mm anterior to the interaural line and incubated with 45Ca2+ and 100 microM NMDA for 2 min. There was no difference between rats exposed to an enriched environment and rats kept in standard cages. Enrichment of environment did not alter NMDA-stimulated Ca2+ uptake into barrel cortex of SHR and WKY.

NMDA receptor function in the prefrontal cortex of a rat model for attention-deficit hyperactivity disorder.

The spontaneously hypertensive rat (SHR) is an accepted model for attention-deficit hyperactivity disorder (ADHD) since it displays the major symptoms of ADHD (hyperactivity, impulsivity, and poor performance in tasks that require sustained attention). We have previously shown that glutamate activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors released significantly more norepinephrine from SHR prefrontal cortex slices than control Wistar-Kyoto (WKY) rats. The aim of this study was to determine whether N-methyl-D-aspartate (NMDA) receptor function is disturbed in the prefrontal cortex of SHR. Prefrontal cortex slices were incubated with 45Ca2+ in the presence or absence of 100 microM NMDA for 2 min. Activation of NMDA receptors stimulated significantly less Ca2+ uptake into prefrontal cortex slices of SHR than control WKY (2.8 +/- 0.17 vs. 3.7 +/- 0.38 nmol/mg protein, respectively, P < 0.05). Basal Ca2+ uptake into SHR slices was not significantly different from WKY. These findings are consistent with suggestions that the intracellular concentration of calcium is elevated and therefore the concentration gradient that drives calcium into the cell is decreased in SHR compared to WKY. Impaired NMDA receptor function in the prefrontal cortex of SHR could give rise to impaired cognition and an inability to sustain attention.

Voluntary running provides neuroprotection in rats after 6-hydroxydopamine injection into the medial forebrain bundle.

Neurotoxic drugs such as 6-hydroxydopamine (6-OHDA) have been used to mimic a Parkinsonian state in a rat model. The toxic effect of 6-OHDA has been shown to be reduced in rats that were forced to use the impaired limb immediately after unilateral 6-OHDA injection. The aim of this study was to determine whether dopamine neurons in the substantia nigra are spared in rats that exercise voluntary. Two groups of rats were placed in cages with attached running wheels 7 days prior to injection of 6-OHDA (10 microg/4 microL saline) into the medial forebrain bundle. The running wheels of the control group were immobilized for the duration of the study. After 6-OHDA injection, the rats were returned to their respective cages where they remained for a further period of 14 days. Wheel revolutions during free running were recorded daily in the experimental group. At the end of this period the rats were injected with apomorphine (0.5 mg/kg, s.c.) and the number and direction of rotations was recorded. Rats that exercised in the running wheels did not rotate contralaterally in response to apomorphine injection, suggesting that dopamine neurons had been spared sufficiently from the toxic effects of 6-OHDA injection to prevent upregulation of postsynaptic dopamine receptors in the striatum.