ADHD in animal models - Introduction

Rodents are very popular as animal models because they are very similar to humans, are easy to keep and their development cycles and metabolism are much faster. One day in the life of a rat is equivalent to about 27 days in a human.¹ However, due to the accelerated metabolism, the required dose of stimulants is also increased, so that rats require almost twice as high doses of MPH as humans, which must be taken into account when assessing drug studies.

There are various rat breeds that represent ADHD-HI, ADHD-I and non-affected individuals as animal models. They are used to investigate which neurophysiological changes are associated with which symptoms. There are two groups of model animals: those that have been bred or selected for specific symptoms and those in which a singular change has been induced (usually a single gene is deactivated (e.g. DAT-KO) or a single neurotransmitter (e.g. dopamine) is eliminated).

In the first group, the Spontaneous(ly) Hypertensive Rat (SHR) represents a form of ADHD-HI (with hyperactivity), while Wistar-Kyoto rats (WKY) usually represent non-affected individuals. In addition, the SHR/NCrl is an SHR strain that shows symptoms of ADHD-C, and the WKY/NCrl is a strain that shows symptoms of ADHD-I (attention deficit without hyperactivity).²³⁴⁵ If studies do not differentiate this, it can generally be assumed that the Wistar-Kyoto rats (WKY) refer to the non-affected model.
The fact that these animal models express their symptoms solely due to their genetic make-up and without the influence of early childhood stress,⁶ shows that certain gene variants alone can also represent a separate pathway for the development of mental disorders such as ADHD and that the two pathways of genes alone and genes + environment coexist.
Interestingly, SHRs already have a disturbed HPA axis (stress axis) due to their genetic predisposition alone.

In rodents, the predominant glucocorticoid is corticosterone, rather than cortisol, which is predominant in humans. Otherwise, the stress systems of rodents and hominids are surprisingly similar, despite the approximately 90 million years that separated their evolutionary lineage.
ADHD is also discussed in dogs.⁷⁸ An ADHD rating scale for dogs showed reliable results.⁹ Another scale for measuring ADHD in dogs is currently being developed.¹⁰

Drug dosage in the animal model

Drug doses cannot be linearly adjusted to animals according to weight. The conversion is usually based on body surface area or other pharmacokinetic scaling.¹¹¹²
Pharmacokinetics in rodents is significantly faster than in humans.

1. Human Equivalent Dose (HED): Conversion according to body surface area¹³

The most common method is the Human Equivalent Dose (HED), based on body surface area:

HED (mg/kg) = animal dose (mg/kg) × (𝐾𝑚 (animal) / 𝐾𝑚 (human))

Km values according to FDA guidelines (based on body surface area):

• Mouse: 3
• Rat: 6
• Human (70 kg): 37

Sample calculation

• Mouse:
  • 1 mg/kg MPH corresponds for a person weighing 70 kg: HED = 1 × (3 / 37) = 0.081 mg/kg = 5.7 mg
  • 10 mg/kg MPH corresponds for a person weighing 70 kg: HED = 10 × (3 / 37) = 0.81 mg/kg = 57 mg
  • 15 mg/kg MPH corresponds for a person weighing 70 kg: HED = 15 × (3 / 37) = 1.21 mg/kg = 85.5 mg
• Rat
  • 1 mg/kg MPH, for a person weighing 70 kg this corresponds to: HED = 1 × (6 / 37) = 0.16 mg/kg = 11.4 mg
  • 5 mg/kg MPH, for a person weighing 70 kg this corresponds to: HED = 5 × (6 / 37) = 0.81 mg/kg = 57 mg
  • 10 mg/kg MPH, for a person weighing 70 kg this corresponds to: HED = 10 × (6 / 37) = 1.62 mg/kg = 114 mg

Only doses administered by the same route are equivalent. An injection is metabolized faster than oral intake. Therefore, 15 mg/kg of orally administered MPH in a mouse corresponds to a very high drug dose, whereas 15 mg/kg of subcutaneously injected MPH corresponds to a drug dose.

2. Typical oral doses of methylphenidate in animal studies

These doses are commonly used in animal studies to investigate therapeutic or subtoxic effects.

• Mice (oral, common in the literature):
  • Low: 1-2 mg/kg
  • Medium: 5-10 mg/kg
  • High: 20-30 mg/kg
• Rats (oral):
  • Low: 0.5-2 mg/kg
  • Medium: 2-10 mg/kg
  • High: 10-20 mg/kg

Typical therapeutic doses in humans are 0.3-1 mg/kg orally (e.g. 18-72 mg per day for ADHD treatment). As a result, animal doses of 5-10 mg/kg mouse or 2-10 mg/kg rat are within the range of human equivalence using the HED conversion.

Animal models allow analysis of the causes of various symptoms

The various animal models clearly show that symptoms such as hyperactivity, impulsivity or attention problems can be triggered by very different causes. The mediation of symptoms must be distinguished from the causes (e.g. a specific genetic defect). For example, very different causes (e.g. genetic defects) can lead to a dopamine deficit or others to a dopamine excess, both of which in turn cause almost identical symptoms due to the deviation from the optimal dopamine level (inverted-U).¹⁴ To illustrate this, we have divided the animal models, as far as we know, into those with a dopamine deficiency and those with a dopamine excess. Although dopamine is the most important factor in ADHD, the other influences also contribute.
Whether a hypo- or hyperdopaminergic state prevails extracellularly is of little relevance in our understanding - both are associated with ADHD symptoms. Extracellular dopamine levels are too low in overactive DAT, and the phasically released dopamine is already reabsorbed by overactive DAT before it can transmit its signal to the receptors. In underactive DAT, extracellular dopamine levels are too high, so that it interferes with signal transmission as noise. Most importantly, there is a lack of reuptaken dopamine to replenish the vesicles for phasic dopamine release.
Both extracellular hyperdopaminergic and hypodopaminergic states impair signal transmission through phasic dopamine.

The most commonly used criteria for model validity include face validity, construct validity and predictive validity.
Apparent validity: Reliability with which the model reproduces certain state characteristics (e.g. the behavioral symptoms of a psychiatric disorder).
Construct validity: Reliability with which the model measures what it is supposed to measure (etiology and pathogenesis of a Disorder)
Predictive validity (predictive validity): Reliability with which the model predicts the relationship between a triggering factor and an observable effect of the disease on the one hand and the observable effects of a therapeutic agent on the disease on the other.

Interestingly, mouse models in which the dopamine receptors were deactivated barely show ADHD symptoms, further emphasizing the importance of DAT and its influence on extracellular and phasic dopamine.

Animal models according to extracellular dopamine levels

We subdivided the animal models according to extracellular dopamine levels:

1. ADHD animal models with reduced extracellular dopamine
2. ADHD animal models with increased extracellular dopamine
3. ADHD animal models with (to us) unknown dopamine alterations
4. Animal models that inadequately represent ADHD

Cabana-Domínguez et al. have compiled almost 200 mouse models in which the manipulation of individual genes triggered hyperactivity, hyper- and hypoactivity, impulsivity or inattention.¹⁵ More on this at Monogenetic animal models for ADHD symptoms in the article Monogenetic causes of ADHD