Disease Models of ADHD

Attention Deficit Hyperactivity Disorder (ADHD) is a disorder that is characterized by inattention, hyperactivity, and impulsivity.^[1] Symptoms include the inability to complete tasks, follow instructions, or sit still. It is most prominent in children and is accompanied by poor school performance, which is one of the reasons why it is typically noticed and diagnosed at this stage. However, symptoms persist through adulthood, although they decrease in intensity with appropriate treatment. It is relatively common among children and adolescents, with a prevalence that ranges between 5 and 10% in different countries.^[2]

For the past few decades, Methylphenidate, commonly known as Ritalin, has been the gold standard in the treatment of ADHD due to its efficacy and safety.^[3] It falls under the category of stimulant since it increases the activity of dopamine and norepinephrine in the prefrontal cortex. Although it has high tolerability and mild side effects, there has been concern regarding the potential abuse of Methylphenidate due to its stimulatory properties, in addition to possible negative long-term effects that have yet to be investigated. ^[4][5] The effects of Ritalin on rodents have been reviewed in detail here. Another drug that falls under the category of stimulant is commonly known as Adderall. The main difference between Adderall and Ritalin is that the former consists of a mixture of two stimulants (Amphetamine and Dextroamphetamine), while the latter consists of only one stimulant (Methylphenidate). A summary of the ADHD effects of Adderall on rodents can be found in this article.

The causes underlying ADHD are largely unknown, but researchers have identified various genetic and environmental factors that may contribute to its pathology. Twin studies revealed that ADHD has a mean heritability of around 76%.^[6] In addition, large-scale genome analysis studies have been performed on individuals with ADHD, leading to the discovery that certain genetic variations, such as single nucleotide polymorphisms (SNPs), are associated with increased susceptibility or risk for developing ADHD. Many of these affected genes encode molecules that are important for dopamine and norepinephrine activity in the brain.^[7] However, these genetic variations are present in a very small subset of cases and are therefore insufficient to explain the cause behind ADHD.

Various environmental and psychosocial factors have been found to be associated with an increased risk of developing ADHD. Some of these are prenatal factors, such as exposure to toxins, alcohol, or drugs during pregnancy. Postnatal risk factors include low birth weight and household poverty that is typically associated with low intellectual stimulation.^[8] The mechanisms through which these factors could cause ADHD is unclear. It should be noted that because these studies are mainly observational, they cannot yield concrete conclusions regarding the role of certain factors in contributing to ADHD. Interestingly, recent studies showed that these risk factors may have only short-term effects, and therefore cannot explain the persistence of ADHD throughout life.^[9]

Many studies have identified significant changes throughout the brain, although it is unclear whether these changes are causes for or effects of ADHD. Some of the consistent findings include changes in neurotransmitter activity, disruptions in circuits, structural changes, and altered connectivity patterns.^[10][11] Accordingly, the overarching question – that is far from being answered – is how so many widespread neurological changes could occur and what triggers them. Researchers are currently using many models and techniques in an attempt to answer this question.

Human Studies

Human studies are performed on individuals with ADHD using non-invasive methods, such as Magnetic Resonance Imaging (MRI) or functional MRI (fMRI). These studies investigate structural or volumetric changes in particular areas of the brain, such as the frontal and temporal lobes, in an effort to characterize possible neurobiological etiologies of ADHD.^[12] Since fMRI allows the visualization of activation in different areas of the brain while the research subject performs a task, various fMRI studies have shown the positive effects of Methylphenidate in re-establishing activity in the prefrontal cortex.^[13]

Animal Models

The nature of non-invasive human studies precludes the possibility of investigating the underlying neurobiological causes of ADHD and the effects that it can have on the brain. Accordingly, because of this need for more accessible research models, various ADHD rodent models have been established and characterized in recent years. A careful analysis of the different models indicates that they each have strengths and flaws, based on the method used to create the model and the accuracy of the resulting phenotypes.

However, it is important to note that it has been challenging for researchers to establish ADHD models that exhibit all three categories of symptoms (inattention, impulsivity, and hyperactivity). Most rodent models are successful in mimicking one symptom and very few replicate two or all three. Nevertheless, the models are still highly valuable for studying neurobiological changes that can lead to each symptom in particular. In addition, new and better models are being established each year.

Spontaneously Hypertensive Rat (SHR) Model

One of the most well-studied rodent models for ADHD is the Spontaneously Hypertensive Rat. Although it was initially developed as a hypertension model, it is also used to study ADHD due to the observation that these rats exhibit symptoms of inattention, hyperactivity, and impulsivity. The presence of all three symptoms is rare in ADHD animal models, which explains the popularity of the SHR model due to its face validity. The model was created by breeding brother and sister Wistar-Kyoto rats.^[14]

SHRs have shown hyperactivity in open field tests, although the face validity of this type of test in the context of behavioral symptoms of ADHD has been questioned. Impulsivity of the SHR has been tested through a variety of more adequate methods. In a modified T-maze that forces rats to choose between a small, immediate reward or a large, delayed reward, SHRs show a preference for immediate rewards. The effect of methylphenidate and other similar ADHD drugs have also been tested in treated versus untreated SHRs. The results have been conflicting, as some studies have reported an improvement in measures of impulsivity in treated SHRs, while others did not see a significant change. ^[14] For this reason, the predictive validity of the SHR is problematic, since it does not seem to be suitable for drug testing. This model may therefore be more appropriate for shedding light on neural substrates that underlie symptoms of ADHD, since they are well-portrayed in SHRs. For example, future studies could evaluate the status of neurotransmitter activity, quantify the expression of certain candidate genes that may play a role in the behavioral symptoms, and analyze any possible changes in structures related to inhibition and attention, such as the prefrontal and anterior cingulate cortices.

Lister Hooded Rat (LHR) Model

The LHR model was developed very recently and appears to show promise. LHRs are outbred strains that have been used in epilepsy models. In a first study that was performed to characterize their potential as an ADHD model, researchers found that LHRs show hyperactivity in open field and elevated plus-maze tests, even when compared to SHRs.^[15] They also showed poor performance in light/dark box and drop tests, which may be related to impulsivity or inattention. The LHRs did not show learning or memory deficits, as tested by the Morris water maze test, which led the researchers to conclude that the model is specific and only shows deficits similar to those seen in ADHD.

Interestingly, LHRs that were treated with Methylphenidate did not show any changes in these tests compared to non-treated controls, but treatment with Atomoxetine, a common non-stimulant ADHD treatment, led to decreased impulsivity as seen through performance in light/dark box and drop tests that was comparable to that in non-treated controls. At the level of the brain, LHRs showed altered expression levels of multiple genes that have been found to be affected in ADHD. This strengthens the construct validity of this model. Although this model still requires further validation, its initial characterization indicates it has solid face validity and predictive validity with respect to treatment screening.

DAT-Knockout (DAT-KO) Mouse

Dopamine transporters (DATs) are crucial for the termination of dopamine neurotransmission at the synapse, since they reuptake dopamine after an action potential has occurred. Accordingly, defects in DAT activity can result in a dysregulation of dopamine levels at the synapse. In the DAT-KO mouse model, the gene encoding the dopamine transporter is knocked out. These mice show hyperactivity in the open field test that was decreased upon treatment with Methylphenidate.^[14] No other symptoms of ADHD have been described, although these mice exhibit impaired learning and memory, which is not representative of ADHD symptoms. This may be a sign that there are other processes that are disrupted in the brain, which may act as confounding factors.

6-OHDA Lesion Model

The neurotoxin 6-hydroxydopamine (6-OHDA) is administered to neonatal rats or mice to induce damage to dopaminergic neurons. These animals then show locomotor hyperactivity that remains in adulthood. Moreover, treatment with Methylphenidate showed a decrease in hyperactivity.^[14] However, this model remains to be characterized fully, since other symptoms of ADHD have not been shown yet and the use of locomotor hyperactivity as the sole observable outcome can be problematic.

Coloboma Mutant Mouse

The coloboma mutant mouse is established by inducing a deletion mutation in the Snap25 gene, which is a pre-synaptic protein that is important for neurotransmitter release and has been shown to be affected in ADHD genetic screens. These mice show hyperactivity that is decreased upon treatment with Methylphenidate. Interestingly, these mice also show signs of impulsivity as demonstrated by poor performance in a delayed reinforcement test and signs of inattention that were observed through deficits in latent inhibition in a conditioning paradigm.^[16] Therefore, the coloboma mutant mouse may exhibit all three symptoms of ADHD, which is not common in the animal models, and strengthens its face validity.

One important issue to note is that genetic models force the assumption that ADHD is caused by a mutation in one single gene, which is far from accurate and may weaken the construct validity of the model. Rather, a network of genes may cooperatively contribute to the disorder. Therefore, animal models that are based on single gene mutations are not representative and are overly simplistic. Another flaw in this model is that genes are often dysregulated rather than mutated and accordingly, the animal model may represent a more severe type of disorder that does not mirror the etiology of ADHD. They may be useful if the specific purpose is to study the role of a certain gene in the pathophysiology of ADHD. This type of data can be valuable for the overarching goal of identifying how each risk gene is connected to the underlying mechanisms that may drive the disorder.

Isolation Rearing Model

The isolation rearing model is established by raising rats in an isolated condition (IC) by keeping them in individual cages. These rats then show impulsive behavior in various tasks with rewards, such as an adjusting delay task.^[17] Upon treatment with Methylphenidate, the IC rats show improved performance in the adjusting delay task, indicating decreased impulsivity. IC rats also exhibit altered levels of dopamine in the brain, and this is also rescued upon treatment with Methylphenidate.^[18] Accordingly, this model may be useful for testing treatments. However, this model has been criticized by many researchers due to the flawed method that is used to establish it. Namely, it lacks construct validity, as social isolation has not been shown to be an etiology or a factor in the development of ADHD.

The existence of a wide array of ADHD models will undoubtedly facilitate the development of specific novel drugs, in addition to contributing to our understanding of the neurobiological substrates that lead to the development of ADHD. It is apparent that combining all three domains of deficits seen in ADHD in the same model, while maintaining adequate face, construct, and predictive validity, is a challenge. However, the appropriate model can be chosen based on the specific research question and a subsequent analysis of the strengths and flaws of each.

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