The effect of chronic methylphenidate administration on presynaptic dopaminergic parameters in a rat model for ADHD

doi:10.1016/j.euroneuro.2010.04.007

European Neuropsychopharmacology

Volume 20, Issue 10, October 2010, Pages 714-720

https://doi.org/10.1016/j.euroneuro.2010.04.007 Get rights and content

Abstract

Dysregulations in monoaminergic systems have been implicated in attention deficit/hyperactivity disorder. The spontaneous hypertensive rats (SHR) are used as an animal model for ADHD. Juvenile SHR rats exhibited low dopamine transporter (DAT) density, low vesicular monoamine transporter 2 (VMAT2) density and lower unstimulated dopamine (DA) release in comparison to their corresponding WKY controls. Chronic methylphenidate treatment of the young SHR rats was associated with lower DAT density and lower unstimulated basal dopamine release but with enhanced potassium- and amphetamine-induced dopamine release. These neurochemical alterations might be relevant to the pathophysiology and to the beneficial effect of methylphenidate in ADHD.

Introduction

Attention deficit/hyperactivity disorder (ADHD) is a neurobehavioral disorder most common in youth. A recent report has indicated that the prevalence of ADHD in school aged children is 6–8% worldwide (Biederman, 2005).

Even though the disorder was introduced over 100 years ago, its neurobiology is still not completely understood. Fronto-subcortical pathways (lateral prefrontal cortex, dorsal anterior cingulate cortex, caudate and putamen), which are rich in catecholamines, were suggested to play a major role in this disorder. These amines, especially dopamine (DA) and norepinephrine (NE), are involved in the mechanism of action of the stimulant medications that are used to treat the disorder (Biederman, 2005, Curatolo et al., 2009). This led to the hypothesis of catecholamine dysfunction in ADHD (Pliszka, 2005); inefficient function of the posterior cortical attentional system is suggested to be due to dysregulation of the norepinephrine system, while the impaired function of the anterior executive system is related to dopamine dysregulation (Curatolo et al., 2009).

Goto et al. (2007) hypothesized that the tonic DA release is reduced in ADHD. As a consequence, the phasic dopamine release is increased. This excessive phasic release overstimulates reward centers, which leads to excessive sensation seeking behavior (Pliszka, 2005). A strong validity to this hypothesis, or a weak validity to the dopamine deficiency hypothesis, is the fact that drugs with dopaminergic effect are ineffective for treating ADHD. Drugs for the treatment of ADHD selectively potentiate dopaminergic and noradrenergic neurotransmission in the brain, especially in the striatum. Stimulant drugs, such as methylphenidate, raise extracellular dopamine levels supposedly by blockade of the dopamine transporter (DAT). This mechanism prevents reuptake of dopamine into the neuron, which results in higher extracellular dopamine levels (Madras et al., 2005, Volz et al., 2005). Additionally, according to Volz and Fleckenstein (Fleckenstein et al., 2009, Volz et al., 2008), acute administration of methylphenidate redistributes vesicular monoamine transporter 2 (VMAT2) protein from membrane associated vesicles fraction to the cytoplasmic vesicles fraction, which results in an increase in dopamine content in both fractions. Imaging studies using fMRI showed reduced striatal activation in ADHD children in comparison to controls. These differences disappeared after methylphenidate treatment (Wilens, 2008). Methylphenidate treatment was associated with improved spatial working memory and response inhibition in the prefrontal cortex in ADHD children and adults (Arnsten, 2006). It was shown that age at the time of drug treatment and pharmacokinetic differences in absorption, distribution and metabolism could influence both the acute and chronic effect of psychostimulants (Yang et al., 2006).

As ADHD is highly heterogeneous disorder, both genetically and environmentally, using animal models can simplify and promote the understanding of this disorder. The spontaneously hypertensive rats (SHR) strain was developed in Japan by selective inbreeding of rats from the Wistar Kyoto strain (WKY) that exhibited high systolic blood pressure. During the selective inbreeding, the new strain was more active in comparison to its progenitor, the normotensive WKY strain (Kantak et al., 2008). The SHR strain fulfills the validation criteria which make it an adequate animal model for ADHD; face validity — SHRs mimic the behavioral characteristics of ADHD described in children. Impulsiveness, which is absent initially and develop gradually over time, sustained attention a deficit that is demonstrated only when stimuli are widely spaced in time, hyperactivity, which is not observed in a novel non-treating environment and develop over time. Construct, validity — the two main behavioral processes that are proposed to be major contributory factors in the etiology of ADHD, altered reinforcement of novel behavior and deficient extinction of previously reinforced behavior, are demonstrated. These processes are associated with hypofunction of the dopamine system. Predictive validity — predictions are currently focused on the behavioral level, although the usefulness of the model might become apparent in areas like genetics, neurobiology and pharmacology (Sagvolden et al., 2005).

A dilemma concerning the usage of SHR rats revolves around their development of hypertension. It is unclear whether the neurocognitive deficits are result of hypertension. Since the hypertension develops around weeks 12–14 and is not present in young hyperactive rats, it is accepted that the neurocognitive alterations appear prior to the hypertension (Kantak et al., 2008, Russell, 2002, Sagvolden et al., 2005).

Studies on methylphenidate and its neurochemical effects have been done before, but the majority of them were conducted in juvenile or adult rats without ADHD-like symptoms. In the first part of this study, we assessed the differences in dopaminergic parameters between juvenile SHR and their corresponding controls, juvenile WKY rats. Secondly, we examined the effect of chronic methylphenidate treatment on the same dopaminergic parameters in juvenile SHR rats.

Section snippets

Animals

Male Wistar Kyoto rats (WKY strain) and spontaneous hypertensive rats (SHR strain) at the age of 4 weeks (80–100 g) were purchased from Harlan laboratories. The rats were housed 4 per cage at 22 ± 2 °C and a 12 light:12 dark cycle (lights on at 05:00 h) with unlimited access to commercial pellet food and tap water.Animal procedures were approved by the Animal Care Committees of Tel-Aviv University (approval numbers: M-08-087 and M-09-034).

Materials

[³H]GBR12935 (specific activity: 38.5 Ci/mmol) and [³H]dopamine

[³H]GBR12935 binding

The density of [³H]GBR12935 binding sites was significantly lower in the striatum of SHR rats in comparison to WKY rats (915 ± 87 vs. 1120 ± 91 fmol/mg protein; t = 3.8, df = 9, p = 0.004) (Fig. 1). There was no difference between the two strains in the affinity of [³H]GBR12935 to its binding site in the striatal membranes (2.8 ± 0.7 vs. 2.8 ± 0.6 nM; t = − 0.13, df = 9, p = 0.89).

[³H]Dopamine uptake

No difference was observed between the two strains in the maximal [³H]dopamine uptake rate (V_max) to brain synaptosomes or in the K_m

Discussion

In this study, we used spontaneous hypertensive rats (SHR) as an animal model for ADHD. We characterized several key synaptic dopaminergic parameters; dopamine transporter level, dopamine uptake, vesicular monoamine transporter 2 level and dopamine release in brains of SHR rats in comparison to their controls, normotensive Wistar Kyoto rats (WKY). The dopaminergic parameters in SHR rats were also studied before and after chronic methylphenidate administration.

We found that the density of the

Role of the funding source

There is no involvement of the funding source.

Contributors

Yaarit Simchon performed the entire research as part of her MSc thesis. She participated in designing the experiments, treating the rats, performing all of the biochemical assays. Abraham Weizman participated in writing the final draft and the statistics. Moshe Rehavi designed the project and was Yaarit Simchon mentor in the project, he wrote the first draft. All authors contributed to and have approved the final manuscript.

Conflict of interests

There are no conflicts of interest.

Acknowledgment

The study was supported by a grant from the Dr. Miriam and Sheldon G. Adelson Center for the Biology of Addictive Diseases, Tel-Aviv University, Israel.

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The effect of chronic methylphenidate administration on presynaptic dopaminergic parameters in a rat model for ADHD

Abstract

Introduction

Section snippets

Animals

Materials

[3H]GBR12935 binding

[3H]Dopamine uptake

Discussion

Role of the funding source

Contributors

Conflict of interests

Acknowledgment

Biol Psychiatry

Anal Biochem

Eur J Paediatr Neurol

Neuropharmacology

Brain Res Dev Brain Res

Eur J Pharmacol

Neuropharmacology

Neurosci Biobehav Rev

J Biol Chem

Biol Psychiatry

Biol Psychiatry

Brain Res

Behav Brain Res

Behav Brain Res

Brain Res Bull

[³H]GBR12935 binding

[³H]Dopamine uptake