Introduction

Attention-deficit hyperactivity disorder (ADHD) is the most common neurobehavioral disorder in childhood and is characterized by inattention, impulsivity, and hyperactivity. Although ADHD is thought to be a crippling and frequent illness that only arises in infancy, recent studies indicate that it persists in adulthood in 30–70% of patients [1, 2]. Furthermore, poor cognitive impulsiveness, forgetfulness, planning deficits, poor time management, and impulsive conduct are prevalent in children with ADHD. Adults are diagnosed with ADHD by examining clinical abnormalities, such as: hyperactive-impulsive (ADHD-HI), predominantly inattentive (ADHD-PI), or combination (ADHD-C) subtypes.

ADHD is one of the most frequent juvenile disorders, with a prevalence of 3–5%. About half of the children affected by ADHD continue to experience symptoms as adults [3]. Numerous ADHD symptoms, such as-hyperactivity/impulsivity and attention impairment- must appear before the age of 12 years.

ADHD has been demonstrated to be comorbid with a number of different mental diseases in addition to this primary symptomatology. Mood, anxiety, oppositional defiance, and conduct disorders are the most frequent among children [4]; whereas in adults, different comorbidities occur, such as: major depressive disorder, social phobia, and substance abuse [5]. Emotional lability or dysregulation plays an underlying role in the development of ADHD symptoms [6]. In literature, there are various risk factors that play in elevating the proportion of ADHD patients including genetic [7, 8], environmental factors [9,10,11] and other related factors [12,13,14] that are briefly discussed and highlighted by various researcher obtained from different studies (Fig. 1).

Fig. 1: The common risk factor of ADHD.
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SNP single-nucleotide polymorphism, SLC6A3 solute carrier family 6 member 3, FOXP2 Forkhead box P2, LPHN3 Latrophilin-3, SORCS3 Sortilin-related VPS10 domain containing receptor 3, SCZ schizophrenia, ASD autism spectrum disorder; Odd oppositional defiant disorder, BP bipolar disorder, MDD major depressive disorder, BDNF brain-derived neurotrophic factor, GDNF glial cell line-derived neurotrophic factor, NGF nerve growth factor.

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Seo et al. [15] used national representative data collected between 2008 and 2018 to investigate the prevalence and comorbidities of ADHD among adults, children, and adolescents in Korea. They reported that ADHD prevalence rates for children/adolescents had increased steeply over that decade, from 127.1/100,000 in 2008 to 192.9/100,000 in 2018, increasing 1.47 and 10.1 times in children/adolescents (≤18 years) and adults (>18 years), respectively. According to the study, a significant proportion of ADHD patients in Korea are either misdiagnosed or undertreated.

The cause of ADHD remains unknown, but mounting evidence points to a hereditary component to its occurrence. Now with the help of recently developed genetic models, we may be able to comprehend the behavior of animals manifested by the presence of an attention-deficit disorder, hyperactivity, impulsivity, or all three traits in a single animal. Several animal models of ADHD have been proposed; however, genetically modified animals are the most promising models for displaying ADHD symptoms. ADHD models differ in terms of pathophysiological abnormalities and the capacity to imitate behavioral symptoms and predict pharmaceutical responses. Their varied nature could be attributed to the lack of sufficient knowledge on ADHD biology from clinical data based on human studies, which is why researchers are unable to determine which model best mimics ADHD or other subtypes. As per the recent research on ADHD, the models used should be classified as animal models of symptoms similar to ADHD rather than exact models of ADHD [16].

In this review, we discuss the most notable animal models that could be valuable for studying ADHD with a particular focus on genetic models. Various models include: dopamine transporter (DAT) knockout mice, spontaneously hypertensive rats (SHR), steroid sulfatase, coloboma mice, and alpha-synuclein-lacking mice.

Animal model and criteria for good animal models

McKinney (1988) stated the requirement of animal models for “experimental preparations developed in one species for the purpose of studying phenomena occurring in another species [17, 18]”. This definition is still valid for clinical researchers. Certain criteria from animal models- such as: etiology, genetic resemblance, physiological processes, and treatment-can be used to study human psychological disorders. Three forms of validity were chosen: predictive, face, and construct. The predictive validity of a model is determined by whether it properly selects a pharmacological treatment with equivalent clinical potency without omission or commission errors. Face validity is determined by how closely a model mimics illnesses in different ways. Construct validity is evaluated by whether both the model’s behavior and the features of the disorder traits can be unambiguously interpreted and are homologous, and whether the features being modeled have a well-established empirical and theoretical relationship with the disorder [19].

Dopamine transporters knock out mice

DAT is expressed in all dopamine (DA) neurons but is known to reuptake extracellular DA in the synaptic cleft of the dopamine system [20,21,22]. Therefore, dopamine transporter knockout (DAT-KO) using various methods causes an increase in DA by reducing extracellular DA clearance [22]; hence, the level of extracellular DA can be increased by nearly five times [23].

Figure 2 shows the dopamine homeostasis in normal and DAT-KO mice. The right picture depicts DAT-KO mice, where the synthesis of DA is double as compared to that in normal mice, and the neuronal DA concentration is drastically lower, whereas extracellular DA is increased five-fold. DAT-KO mice lacked auto-receptor function.

Fig. 2: Differences in dopamine transmission in normal and dopamine transporter knock out (DAT-KO).
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Figure was modified and redrawn from Efimova et al. [37].

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DA is thought to play an important role in ADHD; however, other neurotransmitters are also involved. Transgenic mice have become indispensable tools for analyzing the role of genetic factors in the pathogenesis of human diseases. Although rodent models cannot fully recapitulate complex human psychiatric disorders such as ADHD, transgenic mice offer an opportunity to directly investigate the specific roles of novel candidate genes identified in patients with ADHD in vivo. Targeting genes implicated in DA transmission, such as the gene encoding the dopamine transporter (DAT1), has led to the development of several knockout and transgenic mouse models proposed as ADHD models. These mutant animals provide researchers with the opportunity to assess the role of dopamine-related processes in brain diseases, analyze the molecular and neuronal mechanisms at play, and test new ADHD treatments.

Due to the defects in DAT, DAT-KO mice exhibit spontaneous hyperlocomotion [22, 24]. DAT-KO mice showed an elevation in hyperactivity and velocity, along with less time of immobility, with a breakdown or failing in habituating over time in the open field. DAT-KO mice also buried fewer marbles than respective controls of DAT wild-type (DAT-WT) and -heterozygous (HET) mice in appraisal of obsessive or compulsive-like behaviors, likely because of severe hyperactivity and related attention deficit [25] representing attention deficit hyperactivity (ADHA)-related phenotypes.

Multiple studies have reported a relationship between DAT variants and ADHD [26,27,28]. It was previously recognized that in patients with ADHD, psychostimulants might interact with the DAT: for example, amphetamine and methylphenidate drugs improve behavioral deficits. However, no clear indication of decreased DAT was identified in patients with ADHD when different researchers compared models to patients, but an increment in the DAT level was seen in the striatum of adults and children [29, 30].

DAT is one of the dopamine-related genes that is known to be a candidate for ADHD risk [31] and is also included in the Na+/Cl- dependent transporter family that uptakes dopamine into neurons [20]. DA reuptake by the DAT mainly occurs from the synaptic cleft to the presynaptic terminal and plays an important role in the functioning of the dopamine system [32].

DAT is predominantly expressed in nigrostriatal and mesolimbic dopaminergic neurons of the central nervous system, with the highest levels in the striatum and nucleus accumbens (NAc) [33]. In addition, subcellular ultrastructural studies have confirmed that most DAT in striatal dopamine axons are disseminated at the synaptic periphery and nonsynaptic membrane regions [34].

DAT deficiency in DAT-KO mice results in changes in the DA system. Compared to WT mice, DAT-KO mice had a five-fold higher extracellular DA concentration [23], which is consistent with the 300-fold slower DA clearance in DAT-KO mice [35]. It was also confirmed that electrically stimulated dopamine release in DAT-KO mice was reduced by approximately 75% compared with that in WT controls [36]. Giros et al. [22] reported a quantitative in situ hybridization study that confirmed a reduction in the messenger ribonucleic acid (mRNA) levels of postsynaptic DA receptors D1 and D2, which were further downregulated by almost 50% in the striatum [22]. These studies have shown that the release of dopamine and its receptors is controlled in the brain. Regarding physiological functions, DAT-KO mice had a significantly slower breathing rate with extended inspiration time. DAT-KO mice show a decreased response to hypoxia compared to WT mice; however, CO2 production is unaffected in the mutants [37]. Body temperature of DAT-KO mice doesn’t follow a circadian variation. Circadian analysis revealed a decrease in body temperature during the daytime in DAT-KO mice. The exclusion of DAT in DAT-KO mice resulted in delayed weight gain compared to HET and WT mice. Females without DAT exhibited poor lactation and diminished ability to care for their young. Deletion of DAT causes anterior pituitary hypoplasia and a number of changes in the hypothalamo-pituitary axis characteristics, emphasizing the function of hypothalamic DA reuptake in developmental events [35].

DAT-KO mice exhibit ADHD-related behavioral changes in various psychological experiments [38, 39]. In the open field test, the movement speed and hyperactivity of DAT-KO mice increased, whereas the immobility time decreased [40]. Fewer marbles were buried during the marble-burying test, which was attributed to hyperactivity and inattention [41, 42]. In the cliff avoidance reaction test, it was confirmed that they showed slightly more impulsive behavior, unlike WT mice, which tried to avoid falling [43]. In addition, behavioral experiments such as -the Y-maze and pre-pulse inhibition have confirmed: poor attention, learning, and memory [44,45,46]. Moreover, very poor learning and memory abilities were confirmed through an eight-arm maze, novel object recognition task, and social food preference transmission tests [35, 47]. Contradictory results were also observed in these animals, such as- the alleviation of hyperactivity by amphetamine, methylphenidate, and cocaine- which act on DAT [23, 48, 49]; further suggesting that the effects of these compounds in ADHD do not target the DA system alone. In addition, methylphenidate-induced increases in DA concentration in the synaptic cleft were not observed in DAT-KO mice [35], suggesting that the reduction in hyperactivity in DAT-KO mice could be due to the targeting of noradrenergic systems other than the dopaminergic system.

As per the results of various studies, DAT impairment could lead to ADHD-related behaviors- such as in multiple genetic studies that revealed the association between DAT gene mutations in ADHD patients [18]; and further brain imaging studies also showed a reduction in DAT levels in ADHD patients [50]. However, other studies have confirmed the opposite results, such as an increase in DAT levels in the striatum of patients [51,52,53]. Therefore, the specific role of DAT in ADHD pathogenesis remains unclear. However, the DAT-KO mouse model is by far the most well-known and reliable ADHD model, and has provided several clues about the function of this gene, which may be related to psychological disturbances.

Spontaneously hypertensive rat (SHR) model

Spontaneously hypertensive (SH) rats are one of the most studied animal models of ADHD. This model characterization is based on several findings which recommend that the SHR model could be possibly one of the promising hyperactive model for studying ADHD [54, 55]. The SHR strain was developed by Okamoto and Aoki (1963) [56]. They obtained F1 by crossing male Wistar rats with spontaneous hypertension and females with moderately high blood pressure, and selected and mated the hypertensive rats again, eventually reaching F3, where almost 100% of the rats had spontaneous hypertension [56]. SHRs and Wistar-Kyoto (WKY) controls differed in their home-cage circadian activities, with SHRs being more active than WKYs at numerous time points. Interindividual variance in impulsivity was virtually absent in the WKY strain during the test; however, SHRs showed significant inter individual variability [57].

Generally, in the preliminary stage, the SHR model was developed to study patients with hypertension and related comorbidities in animal settings [56]. Nevertheless, Sagolden et al. reported resultant hyperactivity and spontaneous motor activity during experiments, suggesting that this animal could be used as a model for ADHD [58].

According to several studies, the SHR validated the key symptoms of ADHD, such as attention deficit, hyperactivity, and impulsiveness [59,60,61,62,63,64,65]. SHR have been demonstrated to be similar to children with ADHD [66] in that they are more sensitive to delays in reinforcement, which is consistent with a steeper gradient of delays in reinforcement observed in SHR compared to controls [64, 67]. Aase et al. (2006) and Aase and Sagvolden (2005) found higher intraindividual variability in SHR behavior than in controls [68, 69]. This is similar to that observed in children with ADHD [61, 63, 68,69,70].

Experiential alterations or deficits have been found to be directly associated with frontostriatal system dysfunction. References [54, 71] previously showed that the impaired release of DA was witnessed in SHR in specific areas- clearly observed as affected regions in ADHD, namely: the prefrontal caudate-putamen cortex and NAc [71]. In young male SHRs, D5 and D1 receptor density is typically increased in the neostriatum and NAc -according to a previous study by ref. [72]; which demonstrated that the prefrontal cortex (PFC) of SHR has decreased expression of the D4 receptor gene. Furthermore, alterations in noradrenergic system release have been observed in the PFC and LC (locus coeruleus) [73]. In other words, the noradrenergic system is overactive in the prefrontal cortex of SHR. The production of noradrenaline (NA) in the prefrontal cortex induced by glutamatergic stimulation is elevated in SHRs compared to their respective control WKY rats [74]. Collectively, from the above studies, we recommend SHR as a favorable model for studying ADHD. However, the modifications to this model that affect hypertension may also function as variables. Although this model is valuable, considering the impact of hypertension on it is also important.

The SHR strain was generated as described by Okamoto and Aoki (1963) [56]. They obtained F1 by crossing male Wistar rats with spontaneous hypertension and females with moderately high blood pressure, and selected and bred hypertensive individuals among them again, eventually reaching F3 to such an extent that almost 100% of the individuals showed spontaneous hypertension [56]. As such, SHR was created for the study of hypertension; however, it shows ADHD symptoms such as : impulsivity, learning and memory deficits, hyperactivity, deficient sustained attention, and increased impulsiveness [63, 75]. For example, SHR were confirmed to be hyperactive compared to control WKY rats in an open field test, and this increased activity was observed in both male and female rats [74]. Moreover, similar to children with ADHD, SHRs are less sensitive to delayed reinforcement and more sensitive to immediate behavioral reinforcement than nonhypertensive WKY control rats [63]. The behavioral responsiveness of SHR mice was altered by psychomotor stimulants such as methylphenidate hydrochloride (ritalin) or d-amphetamine, which treat childhood ADHD with major symptoms of attention problems and hyperkinesis. This is consistent with the clinical findings in children with ADHD [76]. In addition, behavioral deficits, such as hyperactivity and impulsiveness, can be alleviated by monoaminergic agents [77, 78].

An in vitro superfusion technique revealed that depolarization (25 mM K1)induced the release of DA from the NAc slices of SHR, which was significantly lower than that in WKY controls [79]. Compared to WKY rats, electrical stimulation required less [3 H]DA in the PFC and caudate-putamen slices of SHR [80]. Miller et al. reported that SHR/NCrl exhibited decreased KCl-evoked DA release versus the WKY/NCrl model of inattentive subtype (ADHD-PI) in the dorsal striatum (Str). The SHR/NCrl model of ADHD-PI showed quicker DA uptake in the ventral Str and NAc compared to both control strains, but the WKY/NCrl model of ADHD-PI had faster DA uptake in the NAc compared to the SD control. These findings show that higher surface expression of DA transporters could elucidate the faster DA absorption in the Str and NAc of these ADHD animal models. [81].

Next, SHR exhibited changes in several brain systems. One of them is the dopamine system. Depolarization (25 mM K1)-induced electrical stimulation and KCl-evoked release of dopamine are significantly lower in the NAc, cortex, caudate-putamen, and striatum than in WKY controls [80,81,82]. In addition, in SHR, D5 and D1 receptor subtypes showed high levels in the NAc and caudate-putamen [83]; and the expression of the D4 receptor gene in the PFC and the protein synthesis of it, was significantly low [72]; and Moreover, in a study of dopamine-related genes (Drd2, Drd4, and Dat1) in WKY and SHR by Mill- who found several mutations in the DAT1 gene, which could explain some of the behavioral differences between the two animals by DAT1 sequence changes [84].

In addition to changes in the dopamine system, changes in the norepinephrine (NE) system have been observed in SHR. Basal NE concentrations were significantly higher in the frontal cortex, LC, A2 nucleus, and substantia nigra of SHR than in WKY [85]. During early development of SHR, we found higher NE uptake in the frontal cortex, cerebellum, and hypothalamus, and reduced [3H]DHA binding, indicating downregulation of beta-adrenergic receptors in these regions [86]. In the induction of NE release through glutamate, SHR showed more release compared to the WKY controls [87], whereas UK 14,304 (alpha 2 agonists- an adrenergic agonist) and neuropeptides showed less NE release inhibition [88]. The inhibition of alpha2-adrenoceptor-mediated NE release was also reduced, suggesting that auto-receptor function in the PFC was disrupted [82]. PNMT, NAT, and 1A-R mRNA expression levels were higher in SHR, and PNMT mRNA in SHR was three-fold higher than in WKY rats. In contrast, for 2A-R the mRNA expression was three-fold lower in the spinal cord [89]. Enhanced DAT was observed in SHR before the onset of hypertension, whereas enhanced DAT and Dl receptors were observed in posthypertensive SH rats. These findings imply that the dopamine system is involved in the pathophysiology and development of hypertension [90].

In conclusion, SHR can be used as a good ADHD research model based on ADHA-related behavioral characteristics and changes in the brain. However, hypertension-related changes in this model can act as a cause or variable of change. Therefore, the effects of high blood pressure on behavioral changes and brain damage should be considered. We have included Table 1 to briefly describe the different models that can be used to study the different symptoms of ADHD in mutant mice, genes involved, and behavioral changes.

Table 1 Examining the reliability of various proposed Attention-Deficit Hyperactivity Disorder (ADHD) transgenic animal models the table was modified and adopted from Pena et al. [154].
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Steroid sulfatase

Steroid sulfatase (STS) is an enzyme encoded by the X-linked gene STS in humans and the pseudoautosomal gene STS in mice [91]. STS functions in the desulfation of neurosteroids by hydrolyzing dehydroepiandrosterone sulfate (DHEA-S) to DHEA [92]. DHEA-S and DHEA act as negative regulators of GABA A receptors and positive regulators of NMDA-receptors [93, 94]. STS expression has been confirmed in brain regions important for attention and impulsivity, which are thought to be problematic in ADHD, such as the PFC, thalamus, and basal nucleus [95].

The X and Y chromosomes are joined end-to-end by pseudoautosomal regions in a single large sex chromosome of 39XY*O mice. All other X and Y genes are present in their normal complement despite the deletion of the STS gene [96]. STS is expressed in key areas of the developing brain that are vital for attention and impulsivity as well as in the frontal cortex, thalamus, and basal ganglia. However, the aforementioned regions are likely to be dysregulated in ADHD [95].

Several studies have shown that 39XY*O mice exhibit ADHD-related behaviors, such as hyperactivity, inattention, and occasional aggression [97,98,99] which have been linked to an increase in serotonin (5-hydroxytryptamine,5-HT) levels in the striatum and hippocampus as a consequence of decreased DHEA [98]. Trent et al. showed that 39XY*O mice had higher ratios for progressive ratio (PR) task thought to index motivation compared to WT mice. However, no variation were observed between the two groups in the behavioral tasks that were thought to index compulsivity [99]. A neurobiological explanation for the behavioral differences between 40,XY and 39,X(Y)*O mice is the regionally specific perturbations of the 5-HT system, which are associated with significant correlations between hippocampal 5-HT levels and PR performance, as well as between striatal 5-HT levels and locomotor activity. These findings imply that functional variations and inactivating mutations within STS may affect ADHD vulnerability and disease endophenotypes by altering the serotonergic system.

Therefore, although 39XY*O mice have some validity as ADHD models based on ADHD-related behavioral phenotypes and altered serotonergic systems, more evidence is needed to establish them as ADHD models.

There is a male bias in ADHD [100]. According to previous reports by Szatmari et al. and Gomez et al., the male-to-female ratios were 3:1 and 5:1, respectively [101, 102]. In addition, if there is a male bias, the association between the X-linked gene and ADHD may also be reflected, as indicated by several studies reporting that patients with Xp deletions exhibit ADHD-like cognitive-behavioral characteristics [103, 104].

Moreover, female with Turners syndrome (45XO) who are haplo-insufficient for genes which results in the escape of X-inactivation- further establish that with different cognitive deficits consisting of: social, visuospatial, memory, cognitive and attentional deficits [105, 106].

Kent et al. (2008) reported the neurobehavioral characteristics of 25 boys with X-linked Ichthyosis, a genetic skin disorder caused by deletion or point mutation of the STS gene, confirming the diagnostic and statistical manual of mental disorders IV ADHD with no comorbidity: 32% (8 cases) of patients were diagnosed with the inattentive subtype [107]. ADHD has been found in boys with both STS deletions and putative point mutations, indicating that STS insufficiency may be the cause of the high risk of inattentive symptoms in these populations [100].

Coloboma mice

Coloboma mutant mice were first described by Searle et al., developed through irradiation caused by a mutation on chromosome 2 [108] and reported to be mutated by approximately 20 genes [109,110,111] such as: phospholipase C beta-1 (Plcb1), jagged 1 (Jag1), and synaptosomal-associated protein 25 kDa (Snap25). Among all the genetic disarrangements, SNAP25 gene is attracting attention owing to its association with ADHD in terms of pathophysiology [112,113,114,115,116].

SNAP25 is a component of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, which facilitates the fusion and docking of postsynaptic vesicles to enable the release of neurotransmitters [117, 118]. It was previously shown that variations in the SNAP-25 gene could lead to symptoms of ADHD by altering the levels of dopamine and other neurotransmitters at the synapse [119].

SNAP25 dysfunction causes changes in the dopaminergic system. Coloboma mutant mice exhibit a marked decrease in dopamine release from the dorsal striatum compared to their respective controls [120]. In addition, mRNA expression of the dopamine D2 receptor increased in the ventral tegmental area and substantia nigra, which is consistent with the inhibition of dopamine neurons [121, 122]. In addition to the dopamine system, SNAP25 anomalies result in altered NE elevations in the noradrenergic system [121]. The reduction of NE in mice with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride reduced hyperactivity but did not improve impulsivity, demonstrating a link between the noradrenergic system and hyperactivity in this model [123, 124]. In a coloboma mouse study, Bruno et al (2006) discovered that the alpha (2 C)-adrenergic receptor (ADRA2C) was involved in hyperactivity [125].

Hence, these synaptic differences in coloboma mutant mice can serve as the foundation for the basic approval of this model for triggering behavioral anomalies such as hyperactivity, inattention, and impulsivity. This mice model displayed spontaneous locomotor hyperactivity in an open-field experiment [126] and less patience than the control group in a delayed reinforcement task, demonstrating the characteristics of inattention and impulsivity [123].

Studies have suggested that coloboma mutant mice show a reduction in hyperactivity with d-amphetamine and not with methylphenidate; therefore, it works as a moderate to conventional ADHD treatment [109, 110, 127]. Taking all these considerations into account, we propose that coloboma mutant mice or Snap25-mutant mice could be used as promising models for ADHD.

Alpha-synuclein lacking mice

Alpha-, beta-, and gamma-synucleins belong to the synuclein family and are small, soluble proteins that have been found only in vertebrates and are expressed in nerve tissues and some tumors [128]. Among these, mutations in alpha-synuclein are associated with rare familial cases of Parkinson’s disease as well as the accumulation of this protein in AD and several neurodegenerative diseases [129, 130]. Alpha-synuclein is predominantly expressed in the brain tissues of the neocortex, hippocampus, striatum, thalamus, and cerebellum, and is found at presynaptic terminals [131]. The human and rodent sequences were 95.3% identical except of six amino acids [132]. The amino acid residue at position 53 is typically alanine in humans and threonine in rodents. Surprisingly, the same change, Ala-53-Thr, has been found in some family cases of Parkinson’s disease (PD) [129].

Phospholipase D2 (PLD2) has been reported to function in cytoskeletal regulation and/or endocytosis [133]. It has been reported that a- and b synuclein can selectively inhibit PLD2 through direct interaction on the membrane surface, suggesting that synuclein may play an important role in regulating the vesicle transport process [134]. PLD2 overexpression in the rat substantia nigra pars compacta (SNc) results in: severe neurodegeneration of DA neurons, loss of striatal DA, and ipsilateral amphetamine-induced rotational asymmetry [135]. Other studies have found that alpha-synuclein expression in the rat brain-especially in the cerebral cortex, hippocampus, and dentate gyrus- is related to the localization of molecules associated with the phosphoinositol (PI) secondary messenger pathway, such as phospholipase C1 (PLC1) and muscarinic cholinergic receptor types m1 and m3 [136]. This discovery gave them the idea that alpha-synuclein could be involved in synaptic vesicle release and/or recycling in response to PI stimulation. This notion is supported by the discovery that a-synuclein can bind to tiny, unilamellar phospholipid vesicles. [137]. Therefore, alpha-synuclein may also play a role in the dopamine system because it is associated with Parkinson’s disease, along with functions related to the vesicles of alpha-synuclein.

In fact, the depletion of alpha-synuclein in primary hippocampal neurons treated with antisense oligonucleotides reduces the pool of presynaptic vesicles [138]. Several other studies have suggested that alpha-synuclein is involved in the regulation of DA homeostasis [139, 140]. Tissue cultures have shown that alpha-synuclein inhibits DA synthesis by regulating the activity of tyrosine hydroxylase, protein phosphatase 2 A, and aromatic amino acid decarboxylase [141,142,143]. Alpha-synuclein KO mice, in contrast, showed increased dopamine release in the nigrostriatal terminals as a result of paired electrical stimuli- indicating that alpha-synuclein acts as a negative regulator of DA neurotransmission [144]. Alpha-synuclein KO mice showed a reduced effect of D-amphetamine compared to WT, further supporting the fact that alpha-synuclein is a negative regulator of DA neurotransmission [144].

Despite these changes in the dopamine system, behavioral phenotypes related to ADHD are not easily observed in alpha-synuclein-related mutant mice. When comparing alpha-synuclein-lacking mice and WT mice, no significant difference in amphetamine-induced activity was observed, and the rearing was the same [145]. In a study on the association between alpha-synuclein and anxiety, no significant difference was observed between alpha-synuclein knockout and WT mice in emotionality tests, such as the open field, elevated plus maze, and light–dark box. Therefore, alpha-synuclein is not involved in anxiety in mice [146]. In Senior’s study, significant results were found, and it was confirmed that alpha-synuclein and gamma-synuclein double-null mice were hyperactive in the novel environment and alternated at a lower rate in the T-maze spontaneous alternation task. In addition, the concentration of extracellular DA in the striatum doubled in double-null mice after discrete electrical stimulation [147]. However, this does not only target alpha-synuclein. Although the behavioral characteristics of hyperactivity have been identified, studies on other ADHD-related behavioral phenotypes-such as inattention and anxiety- are lacking. Therefore, further studies are needed using alpha-synuclein-deficient mice as ADHA-related models. As related behavioral phenotypes and changes in the dopamine system were confirmed in double-null mice; if the study was expanded to the synuclein family rather than limited to alpha-synuclein, it may provide another clue to research its relationship with ADHD.

Conclusion and future prospective

Animal models are vital research tools that may help us better understand the possible complex mechanisms involved in the development of a disease and enable us to screen and report new effective medications for therapy that can be translated to humans. Animal models of ADHD are categorized as perfect mimics of all disease-inducing features at both the behavioral and physiological levels. Abundant evidence regarding the genetics of ADHD exists; however, these findings appear to be inconsistent. Studies have shown that ADHD may develop from the interaction of many polygenic genes. Mooney et al. combined the results of numerous analysis methods to determine the pathways most likely to be connected with ADHD as well as to evaluate different types of route methodologies and the benefits involved in this approach [148].

This study acknowledges seven pathways, including :RhoA signaling, glycosaminoglycan biosynthesis, fibroblast growth factor receptor activity, and pathways containing potassium channel genes, reported as nominally significant by multiple analysis methods using two GWAS databases. This study confirmed earlier beliefs regarding how controlling neurotransmitters, neurite outgrowth, and axon regulation contribute to the ADHD phenotype; and stressed the importance of cross-method convergence when evaluating route analysis results. The polygenic model of illness risk was consistent with the excess minor SNP effects found in each of these pathways. These pathway correlations offer additional support for earlier hypotheses concerning the etiology of ADHD, particularly those associated with the regulation of neurotransmitter release and neurodevelopmental processes; however, further studies are required to confirm this hypothesis.

To study the mechanism and to understand the etiopathology of ADHD studies have highlighted the importance of using induced pluripotent stem cells (iPSCs) for disease modeling. This method allows us to analyze individual-specific neuronal cell lines in vitro in order to research cellular malfunction and identify the underlying genetic variables [149,150,151]. Reference [152] developed a methodology for generating iPSCs from hair-derived keratinocytes as beginning somatic cells from patients in order to circumvent the invasive aspect of sample collection in the research of early neurodevelopment diseases such as ADHD [152].

Another pathway discussed by Ohki et al. regarding the cause and pathophysiology of ADHD supported the hypothesis of the Wnt and mTOR signaling pathways [153]. Cellular proliferation, polarity, and differentiation are controlled by the Wnt signaling system, whereas synaptic plasticity and several other important neurodevelopmental processes are controlled by the mTOR pathway. Therefore, dysregulation of these time-dependent pathways may result in neurodevelopmental delays and ADHD phenotypes.

Additional genetic variations are present in other models (dopamine transporter gene knockout mice, coloboma mice, Naples hyperexcitable rats, steroid sulfatase, alpha-synuclein-lacking mice, and neonatal lesioning of dopaminergic neurons with 6-hydroxydopamine). However, none of them are fully comparable to clinical ADHD. The pathophysiology involved varies, including both deficient and excessive dopaminergic functioning, and there is probable involvement of other monoamine neurotransmitters such as dopamine, serotonin, and noradrenaline. Therefore, improved models as well as further testing of their ability to predict treatment responses are required. Some aspects of ADHD behavior may result from an imbalance between increased noradrenergic activity and decreased dopaminergic regulation of neural circuits that involve the prefrontal cortex. In addition to providing unique insights into the neurobiology of ADHD, animal models are also used to test new drugs that can alleviate ADHD symptoms.

The evidence addressed in this study suggests that currently available animal models may be useful for studying human behavioral disorders. Furthermore, our current knowledge of ADHD neurobiology is insufficient, making it challenging to identify an optimal model for investigating ADHD.