The Wistar-Kyoto rat as a model relevant to inattentive ADHD: substrain variability and affective comorbidity

Abstract

Wistar-Kyoto (WKY) rats, originally used as a control group for spontaneously hypertensive rats, are increasingly recognized as a valuable model for studying selected neuropsychiatric disorders. This strain exhibits behavioral and neurobiological traits relevant to both depression and inattentive attention-deficit/hyperactivity disorder (ADHD), including behavioral inhibition, increased stress sensitivity, anhedonic behaviors, and deficits in sustained attention. At the same time, a significant challenge remains in the form of considerable variability among WKY substrains, resulting from genetic differences and the breeding practices employed by individual suppliers. This variability may limit the reproducibility of results, but it also holds research value, enabling the analysis of distinct dimensions of mental disorders within related animal models. The aim of this review is to integrate available data on the genealogy, genetic variability, and behavioral phenotypes of WKY rats, with particular emphasis on their relevance to ADHD research and on depression-related traits that may influence the interpretation of WKY rats as a reference or disease-relevant strain. The added value of the present review is therefore to provide an integrated ADHD-oriented perspective on the WKY strain, while also considering depression-related and treatment-response traits as biologically relevant components of its complex behavioral phenotype.

Introduction

Animal models are a critical approach in the study of the neurobiological basis of attention-deficit/hyperactivity disorder (ADHD); however, their translation into clinical practice remains limited, especially considering the complexity and heterogeneity of ADHD [1, 2]. ADHD comprises a wide range of behavioral and neurobiological dimensions, including attention regulation, impulsivity, activity level, emotional regulation, cognitive functioning, and neuroendocrine responses [2–5]. In addition, ADHD is frequently associated with affective and anxiety-related symptoms, which may complicate the interpretation of behavioral phenotypes in animal models.

In this context, the Wistar-Kyoto (WKY) strain of rats is an interesting model for studying ADHD-relevant behavioral dimensions; these rats also display a set of features that are important for understanding depression-related traits that may overlap with or modify ADHD-like phenotypes [6–10].

Even though these rats were used as a neutral control strain for hypertensive rats (SHR) [11–13], a specific profile has been revealed in these animals in recent years: behavioral inhibition, stress sensitivity, anhedonic behaviors, and a deficit in sustained attention; these features are closely associated with the dimensions of depressive disorders and inattentive type ADHD [6, 7, 9, 10, 14]. Notably, however, these behavioral characteristics are not always expressed in all WKY substrains [11, 15–18]. The genetic background and breeding practices employed by the breeders have led to significant variability in the substrains. This variability in the substrains may be viewed as a limitation of the model. On the other hand, it may also be viewed as a potential advantage. The various substrains of the WKY may be seen as manifestations of various aspects of psychiatric illness. This may provide an opportunity to investigate these aspects.

Although WKY rats have been discussed separately in the context of hypertension, ADHD, and depression, an integrated perspective linking their genealogy, substrain-dependent variability, and ADHD-relevant behavioral phenotype remains needed [4, 6, 11, 17]. Therefore, the aim of this review is to integrate available information on the genealogy, genetic variability, and behavioral phenotypes of WKY rats, with particular emphasis on their relevance to ADHD research, while considering depression-related traits as an integral component of the WKY phenotype that may influence the interpretation of this strain as a reference or disease-relevant model.

Genealogy of the Wistar-Kyoto rat strain and substrain variability

The WKY strain is closely associated with the spontaneously hypertensive rat (SHR) due to their shared genetic origin. Both strains were derived from the same outbred Wistar parental stock maintained at Kyoto University School of Medicine [11]. In 1963, Okamato and Aoki first reported the development of a hypertensive Wistar rat strain obtained through selective inbreeding [12]. The strain was transferred to the U.S. National Institutes of Health (NIH) in 1966, where continued brother–sister mating established the inbred SHR-NIH line. This U.S.-based colony subsequently became the source of most commercial SHR stocks outside Japan [11].

To provide a normotensive comparator for hypertension studies, the NIH obtained WKY breeding stock in 1971 from the original Wistar colony maintained in Kyoto. Two factors associated with the derivation of the WKY strain contributed to later heterogeneity. First, the nearly ten-year delay between the initiation of inbreeding for SHR and WKY increased the likelihood of genetic divergence between the parental breeding populations. Second, WKY breeding stock was distributed to multiple vendors before a fully stabilized inbred line had been established [11, 15].

Major commercial suppliers of WKY rats included Charles River, Harlan Laboratories, and Taconic Biosciences. Because these facilities received breeding stock at different generations, distinct WKY substrains gradually emerged. WKY colonies maintained by Charles River (WKY/NCrl) and Harlan (WKY/NHsd) are generally preserved as inbred lines, whereas Taconic historically maintained a more outbred colony [11, 16, 19].

Although behavioral variability has also been reported across SHR colonies in relation to traits relevant to substance use disorder (SUD) and attention-deficit/hyperactivity disorder (ADHD) [20], the hypertensive phenotype of SHRs remains relatively stable across colonies [11].

In contrast, WKY rats display pronounced behavioral and genetic variability not typically observed in other inbred strains. Importantly, this heterogeneity is not limited to differences between vendors but has also been reported within colonies maintained by the same breeding facility [16, 20].

For example, analyses of single-nucleotide polymorphisms (SNPs) revealed that the German WKY/NCrl substrain differs substantially from other WKY and SHR lines, including WKY/NCrl colonies maintained in the United States and the United Kingdom [17]. This German WKY/NCrl substrain shows pronounced genetic divergence, with approximately one-third of assessed genomic regions differing from the WKY/NHsd strain. Due to its distinct behavioral phenotype and genetic profile, it has been proposed as a potential model for studying the predominantly inattentive presentation of ADHD [6].

By contrast, the U.S. WKY/NCrl strain exhibits a much smaller degree of genetic divergence, with approximately 2.5% discordant SNPs relative to the WKY/NHsd strain [17]. Substrain differences among WKY rats have also been documented in behavioral responses to stress. In general, greater similarity has been observed between WKY/NCrl and WKY/NHsd rats than between these inbred substrains and the more outbred WKY colonies obtained from Taconic Laboratories [5, 19, 21].

In summary, the complex breeding history of the WKY strain and the resulting phenotypic variability between suppliers underscores the importance of reporting information on the origin of experimental animals. Therefore, careful consideration of the source and characteristics of the substrain is crucial for proper experimental design and interpretation of results obtained using the WKY rat model.

WKY strain as a normotensive control

As described previously, the WKY rat strain was originally developed to serve as a normotensive control for the SHR model in hypertension research. Although multiple rodent models of hypertension are currently available, the SHR remains the most widely used genetic model of essential hypertension. Indeed, the number of studies employing SHR substantially exceeds those using other models; for example, the Dahl salt-sensitive rat (DSS), the second most frequently cited model, appears nearly nine times less often in the literature [13]. According to the original description by Okamato and Aoki, SHRs develop stable systolic blood pressure values exceeding 150 mmHg, with hypertension typically emerging between 7 and 15 weeks of age [12]. In this context, the use of WKY rats as a normotensive reference strain has been central to the interpretation of experimental findings.

However, several studies have highlighted important limitations associated with the use of WKY rats as a control strain. Notably, WKY strain exhibits certain physiological characteristics that partially overlap with features observed in hypertensive models. As a result, some researchers have proposed outbred normotensive Wistar rats as an alternative reference strain. A recent meta-analysis comparing WKY and Wistar rats suggested that both strains may serve as acceptable controls in hypertension studies, provided that their specific limitations are carefully considered [22].

In that meta-analysis, two major observations were reported: first, Wistar rats displayed blood pressure values more consistent with typical normotensive ranges, whereas WKY rats exhibited intermediate levels between Wistar and SHR, approaching borderline hypertensive values; second, WKY rats demonstrated earlier reductions in cardiac function compared with Wistar controls [23].

Additional evidence suggests that WKY rats may exhibit structural cardiac alterations similar to those observed in hypertensive models. For example, Aiello et al. reported greater myocardial hypertrophy in both WKY rats and SHRs compared with Wistar rats. In particular, left ventricular hypertrophy was observed, a condition that may develop independently of elevated blood pressure and is recognized as an independent risk factor for cardiovascular morbidity and mortality. Parameters such as cardiomyocyte size, fibrosis, and diastolic dysfunction were found to be more similar to those of age-matched SHRs than to those of Wistar rats [24].

The suitability of WKY rats as a control strain may become increasingly problematic with age. Blood pressure in WKY rats gradually increases during aging and can reach values classified as hypertensive in older animals (approximately 52–60 weeks of age). Age-related cardiac remodeling has also been observed, including enlargement of the ventricles and thickening of the interventricular septum and left ventricular wall when compared with baseline echocardiographic values reported for Sprague-Dawley (SD) male rats [25, 26].

Moreover, echocardiographic features suggestive of early heart failure—such as mitral regurgitation, right ventricular dilation, left ventricular enlargement, and atrial dilation—have occasionally been detected even at relatively young ages [25]. Given that several physiological parameters in aged WKY rats resemble those observed in SHRs, their use as a control strain in studies of age-related hypertension may be inappropriate.

Nevertheless, because hypertension in SHR typically peaks around 16 weeks of age, most experimental studies employ relatively young animals (approximately four months old) together with age-matched controls [22, 25]. Comparative studies examining juvenile (8–9 weeks) and adult (12–18 weeks) SHRs and WKY rats have also revealed age-related structural changes in cardiac innervation, including reductions in intracardiac neuron size observed in both strains. However, many of these alterations appear to be specifically associated with the development of hypertension in adult SHRs [26].

Further analyses of aged animals (48–52 weeks) have demonstrated neuropathic changes in myelinated cardiac fibers in both strains. Although these alterations were more pronounced in SHRs, the findings suggest that WKY rats may also be susceptible to cardiac dysfunction that is partially independent of blood pressure [23, 24]. The above studies indicate that although WKY rats are still widely used as a normotensive reference strain in hypertension studies, their physiological characteristics and age-related cardiovascular changes should be taken into account when interpreting experimental results.

WKY as a control strain in ADHD research

In addition to its role as a normotensive control in hypertension studies, the WKY rat strain is commonly used as the closest genetic reference strain in studies employing the SHRs as an animal model of ADHD. However, because WKY rats exhibit a distinct behavioral phenotype—particularly in the context of depression research—the suitability of this strain as a control in neuropsychiatric studies has been questioned [7, 27].

According to the DSM-5 classification, ADHD comprises three main clinical presentations: predominantly inattentive (ADHD-I), predominantly hyperactive/impulsive (ADHD-H), and combined (ADHD-C) [3, 28]. SHRs display several behavioral characteristics consistent with ADHD symptom domains, including impaired attention, hyperactivity, and impulsivity. Consequently, SHRs are widely used as an animal model of ADHD-C and ADHD-H, with the SHR/NCrl substrain gaining broad acceptance for this purpose [4, 29].

In such studies, the control strain is typically obtained from the same breeding facility as the experimental animals—for example, WKY/NCrl rats from Charles River or WKY/NHsd rats from Harlan Laboratories. Currently, Charles River maintains WKY/NCrl breeding facilities in the United States, Germany, and China, whereas the SHR/NCrl strain is bred in the United States and China [30]. Harlan Laboratories was renamed Envigo in 2015 and later acquired by Inotiv in 2021; therefore, WKY/NHsd and SHR/NHsd strains referenced in the current literature generally correspond to colonies maintained by Envigo or Inotiv in the United States [31].

Previous work by Sagvolden and colleagues suggested that a combination of the German SHR/NCrl strain and the UK WKY/NHsd strain may provide a reliable experimental framework for studying ADHD-C, particularly because the German WKY/NCrl substrain was reported to display traits resembling inattentive ADHD [7]. Genetically, the Charles River-derived SHR strain shows relatively limited divergence across colonies, whereas the SHR/NHsd strain has been reported to exhibit a higher proportion of heterozygous single-nucleotide polymorphisms (approximately 2.2%). Consequently, the SHR/NCrl substrain has been suggested to provide more consistent experimental results [17, 32].

This recommendation is further supported by recent findings demonstrating phenotypic and genomic differences between SHR substrains in relation to compulsive-like behaviors associated with substance use disorder, a condition frequently comorbid with ADHD. In contrast to the SHR strain, WKY substrains display a more complex breeding history and greater phenotypic variability, which warrants caution when selecting appropriate control animals in neuropsychiatric research [11, 19].

Except for the German Charles River WKY/NCrl substrain, most WKY/NCrl and WKY/NHsd colonies exhibit minimal heterozygosity, consistent with a high degree of inbreeding and relatively small genetic divergence between substrains. Nevertheless, even modest genomic differences between WKY substrains may have functional relevance. For example, although only approximately 2.5% of single-nucleotide polymorphisms differ between WKY/NHsd and WKY/NCrl strains, these regions are enriched in genes associated with autism spectrum disorder (ASD) [17, 33].

Behavioral studies further illustrate these differences. When female rats were assessed across several behavioral paradigms, WKY strains generally displayed lower locomotor activity and reduced interest in novel social stimuli compared with SHR and SD rats. However, the WKY/NCrl substrain showed more pronounced deficits in social behavior that may be relevant to ASD-like traits. In the three-chamber social interaction test, WKY/NCrl rats exhibited greater avoidance of familiar conspecifics compared with WKY/NHsd rats. They also produced fewer ultrasonic vocalizations, suggesting reduced social communication and lower interest in mating-related interactions. Moreover, WKY/NCrl rats displayed elevated anxiety-like behavior in the elevated plus maze (EPM) and open-field test (OFT) [33].

Comparative genomic analyses have further shown that WKY/NCrl rats harbor polymorphisms in genes associated with depression and anxiety when compared with WKY/NHsd rats, with enrichment analyses revealing overrepresentation of depression-related genes within divergent genomic regions [17]. Nevertheless, both substrains originate from the parental WKY/N strain, which carries disruptive variants in several proteins implicated in human major depressive disorder. Phylogenetic analyses indicate that WKY/NHsd is genetically closer to the ancestral WKY/N lineage than WKY/NCrl, suggesting that both substrains may retain genetic features predisposing them to depression-like phenotypes [27].

The observation that the WKY/NCrl substrain may exhibit features relevant to ADHD-I, ASD, or depressive traits may reflect the substantial overlap and comorbidity among these psychiatric conditions. In clinical populations, internalizing disorders such as depression and anxiety frequently co-occur with the inattentive presentation of ADHD, and similar patterns of comorbidity are observed in ASD [3, 34]. Recent genetic studies further indicate that psychiatric disorders share a substantial proportion of risk variants, suggesting that the WKY phenotype may reflect a cluster of partially overlapping behavioral and neurobiological traits rather than a single disorder-specific profile [5, 35].

In this context, WKY rats may also provide insights into the overlap between ADHD and treatment-resistant depression. Comorbid ADHD has been associated with reduced responsiveness to selective serotonin reuptake inhibitors (SSRIs), and WKY rats show limited responsiveness to SSRI treatment following exposure to the chronic mild stress (CMS) paradigm [36, 37].

It should be noted, however, that most detailed genomic comparisons of WKY substrains were conducted more than a decade ago, and the extent to which these findings reflect current breeding stocks remains uncertain. Commercial breeders may employ strategies such as clone-back breeding to maintain genetic stability within inbred lines. Consequently, the distinct genetic profile reported for the German WKY/NCrl substrain may have been limited to a specific breeding period, although these findings remain important for interpreting earlier experimental results [17, 38].

Nevertheless, divergence between WKY/NCrl and WKY/NHsd substrains is expected to persist to some degree, as these lines were originally derived from separate breeding nuclei at different generations [11, 18].

Behavioral phenotype of the WKY strain in ADHD research

There is a general claim that WKY rats exhibit lower locomotor activity when compared with SHRs. Nonetheless, concerns have been raised that the pronounced hyperactivity observed in SHRs may appear exaggerated if WKY rats are used as the sole comparator, as WKY rats may not only be less active than SHRs but also generally hypoactive when compared with outbred strains [8, 9, 39].

When diurnal activity patterns were examined, all three strains—Wistar, WKY, and SHR—displayed a circadian rhythm of spontaneous activity. In addition, telemetry-based measurements in freely moving rats showed clear circadian rhythms of blood pressure, heart rate, and locomotor activity in SHR and WKY strains, with higher values during the dark phase and lower values during the light phase. Apart from elevated blood pressure in SHR, the overall circadian pattern of locomotor activity and heart rate was broadly similar between the strains, suggesting that some behavioral differences may depend on measurement conditions and stress associated with experimental procedures [40].

However, WKY rats showed lower mean activity during the active phase compared with both Wistar rats and SHRs, although these differences were evident only at certain time points during the active phase in the actimeter test [41]. Other studies have reported increased locomotor activity in SHRs compared with WKY rats during both light and dark phases, based on measures such as total distance travelled in the open-field test (OFT) and the number of arm entries in the Y-maze [42, 43]. Similar differences have also been observed at the beginning of the light phase in the OFT [44].

Overall, locomotor activity in the OFT tends to be lower in WKY rats than in both SHRs and Wistar rats [45–48]. It has also been suggested that WKY and Wistar rats habituate to a novel environment more rapidly than SHRs. Consequently, the pronounced hyperactivity of SHRs relative to Wistar rats may become more apparent after prolonged or repeated observation of behavior in the open-field arena [39, 49].

Moreover, locomotor activity patterns in SHR and WKY rats appear to vary across developmental stages and are therefore age-sensitive. For example, increased locomotor activity in SHRs relative to WKY rats in the OFT has been reported to be most pronounced before six months of age, during longer observation periods, and particularly during the dark phase [50]. When assessed for disruptions in time perception and impulsive choice behavior, WKY rats have been reported to resemble Wistar rats more closely than SHRs. Unlike SHRs, WKY rats did not show accelerated temporal processing in the fixed-interval temporal bisection task (also referred to as the “switch task”) or increased impulsivity in the delay-discounting paradigm [51].

SHRs typically display a steeper discounting curve compared with WKY rats under ascending delay conditions, meaning that as reward delays progressively increase, SHRs tend to shift more rapidly toward choosing smaller immediate rewards. Under descending delay conditions, in which delays are presented in reversed order, this strain effect has been reported either to disappear or again indicate greater impulsivity in SHRs [52, 53].

In general, WKY rats appear to exhibit more self-controlled behavior in delay-discounting paradigms relative to both SHRs and Wistar rats [51, 54–56]. WKY rats also perform better than SHRs in the differential reinforcement of low-rate responding (DRL) task and more frequently choose larger but delayed rewards in the T-maze [39, 57, 58, 59]. Additionally, WKY rats spend more time in the central area of the Y-maze, which has been interpreted as an indication of lower impulsivity [42].

Although some studies have reported no differences between SHR and WKY rats in large-later reward preference, SHRs have been shown to consist of more and less impulsive subpopulations. WKY rats also display inter-individual variability; however, this variability is generally lower than that observed in SHRs, and WKY rats tend to show more consistent non-impulsive behavior [60, 61].

WKY rats have also been reported to perform worse than SD rats in spatial learning paradigms. In the win-shift version of the water radial arm maze, WKY rats made more reference errors consistently and working memory errors than SD rats across testing weeks and during the initial learning phase, compared with SHRs. However, WKY and SD rats displayed similar learning curves overall. It has therefore been suggested that the greater number of errors observed in SHRs during later stages of testing may reflect impulsive responding rather than true memory deficits, while the errors observed in WKY rats may indicate impairments in spatial memory [62].

This interpretation is supported by findings from Sontag et al., who suggested that the elevated locomotor activity of SHRs may act as a confounding factor when assessing spatial working memory performance. In the holeboard paradigm designed to control for the influence of activity levels, WKY rats made significantly more working memory and spatial reference memory errors [63].

Similarly, WKY rats performed worse than SHRs, but comparably to SD rats, in the NCTR complex maze and the Morris water maze [64]. However, other studies have reported superior performance of SHRs relative to WKY rats in the Morris water maze and in the delayed alternation task in the figure-eight maze [65, 66]. WKY rats have also been shown to exhibit lower spatial memory performance during both light and dark phases in the Y-maze spontaneous alternation test compared with SHRs [42]. When compared with both SHRs and Wistar rats in the Y-maze, WKY rats performed significantly worse than Wistar rats and similarly or worse than SHRs, which has been interpreted as reflecting inattentive behavior [41, 45, 56]. Taken together, the behavioral differences between SHR and WKY rats appear to depend strongly on methodological aspects of the experimental paradigm, including observation duration, circadian phase (light versus dark), age, and the origin of the animals. Inter-strain variability and the inclusion of outbred comparator strains are additional factors that may influence experimental outcomes. Nevertheless, WKY rats generally display behavioral tendencies toward a hypoactive, non-impulsive, and inattentive phenotype. The poorer performance of WKY rats in several behavioral tasks may therefore reflect motivational and psychomotor alterations associated with their depression-related phenotype. Taking this into consideration, it would be wise to include outbred control strains when assessing behavioral traits in SHRs as a rodent model of ADHD. The main behavioral findings comparing WKY rats with SHRs, Wistar, and SD strains in ADHD-related paradigms are summarized in Table 1.

Table 1.

Behavioral phenotypes of the WKY rat strain compared with other rat strains in ADHD research

Behavioral phenotype	Vendor/Substrain	Compared to SHR	Compared to Wistar	Compared to SD	References
Decreased locomotor activity in the open field test	WKY/NCrl	+	+	+	[17, 42, 46–48]
Reduced impulsiveness in the delay-discounting paradigm	WKY/NCrl	+	+	N/A	[46, 51, 54, 55]
Short-term and spatial memory deficits	WKY/NCrl, WKY/NHsd	+/-	N/A	+/=	[63–66]
Reduced spontaneous alternation in the Y-maze (inattentiveness)	WKY/NCrl, WKY from the Institute of Neurobiology, BAS	+/=	+	N/A	[41, 42, 45, 56]

Abbreviations: ADHD, attention-deficit/hyperactivity disorder; BAS, Bulgarian Academy of Sciences; N/A, not applicable; SD, Sprague-Dawley; SHR, spontaneously hypertensive rat; WKY, Wistar-Kyoto rat; WKY/NCrl, Wistar-Kyoto rat, Charles River substrain; WKY/NHsd, Wistar-Kyoto rat, Harlan substrain

(+) - phenotype more pronounced in WKY relative to comparator strain, (-) – phenotype less pronounced in WKY relative to comparator strain, (=) – similar phenotype; (N/A) – not applicable

Molecular phenotype of the WKY strain in ADHD research

There is substantial evidence indicating that alterations in dopamine signaling are involved in the etiopathology of ADHD and its comorbid neuropsychiatric disorders [35].

The WKY/NCrl substrain possesses specific sequence variants within the Slc9a9 (NHE9) and Slc6a3 (DAT1) gene regions that differ from those observed in the WKY/NHsd substrain [17, 67, 68]. Mutations in the Slc9a9 gene have been strongly associated with the occurrence of ASD and ADHD, and it has been suggested that this gene may contribute to the co-occurrence of these conditions. Furthermore, Slc9a9 variants have also been linked to comorbid epilepsy. In line with this observation, WKY rats have been reported to show higher susceptibility to rhythmic metrazol activity induced by pentylenetetrazol compared with SHR and Wistar rats [41, 69].

WKY/NCrl rats exhibit differences in the regulation of dopamine release and uptake in the striatum and nucleus accumbens (NAc) when compared with WKY/NHsd, SHR, and SD rats. The highest KCl-evoked dopamine release in the dorsal striatum has been observed in the WKY/NCrl substrain, accompanied by faster dopamine uptake in the NAc core compared with SD controls. In contrast, the WKY/NHsd substrain displays significantly elevated evoked dopamine release in the intermediate region of the striatum relative to SD rats. Despite these regional differences, the two WKY substrains did not differ significantly from each other overall. Nevertheless, both SHR/NCrl and WKY/NCrl strains lack the dorsal–ventral gradient in striatal dopamine uptake observed in control strains, suggesting altered regional DAT expression or activity [70].

Consistent with these findings, both the WKY/NCrl and SHR/NCrl strains have been reported to display elevated baseline levels of DAT and tyrosine hydroxylase in the substantia nigra and ventral tegmental area (VTA). In contrast, WKY/NHsd rats do not differ significantly from Wistar or SD controls. Additionally, radioligand binding studies have shown that striatal DAT binding is significantly higher in SHRs than in WKY/NCrl rats at postnatal days 50 and 90, while no significant differences were detected at postnatal day 25 [71].

WKY/NCrl rats also show altered distribution of dopamine receptors. Higher dopamine D2 receptor binding has been reported in the prefrontal cortex (PFC), NAc shell, and VTA compared with Wistar rats, whereas lower D2 receptor binding has been observed in the NAc core, hypothalamus, and caudate–putamen [72, 73].

Among dopamine receptor subtypes, D2 receptors are considered to play a key role in regulating neuronal firing rate, dopamine synthesis, and presynaptic dopamine release. Given their function as autoreceptors in the VTA, elevated D2 receptor expression may lead to reduced dopamine availability in projection regions receiving input from this structure [72, 74].

Gene expression studies have also revealed that WKY/NCrl rats share several differentially expressed genes in the prefrontal cortex with the SHR/NCrl strain when compared with Wistar rats. These include genes involved in transcription (Creg1, Thrsp, Zeb2), synaptic transmission (Atp2b2, Syt12, Chrna5), neuronal processes (Atg7, Cacnb4, Grin3a), and immune-related pathways (Atg7, Ip6k2, Mx2). Notably, downregulation of the Grin3a gene, which encodes the GluN3A subunit of the NMDA receptor, has been observed in WKY rats [56].

Collectively, these findings suggest an altered molecular profile in the WKY strain, with distinct dopamine signaling possibly shaped by substrain-specific genetic variation. Key molecular alterations reported in WKY rats in the context of ADHD research are summarized in Table 2.

Table 2.

Molecular characteristics of the WKY rat strain in comparison to other rat strains in ADHD research

Molecular domain	Vendor/substrain	Compared to SHR	Compared to Wistar	Compared to SD	References
Slc9a9 (NHE9) gene variants	WKY/NCrl vs. WKY/NHsd	N/A	N/A	N/A	[17, 67, 68]
Slc6a3 (DAT1) gene variants	WKY/NCrl vs. WKY/NHsd	N/A	N/A	N/A	[17, 67, 68]
KCl-evoked dopamine release (dorsal striatum)	WKY/NCrl	N/A	N/A	+	[70]
KCl-evoked dopamine release (intermediate striatum)	WKY/NHsd	N/A	N/A	+	[70]
Dopamine uptake in NAc core	WKY/NCrl	N/A	N/A	+ (faster uptake)	[70]
Baseline DAT and tyrosine hydroxylase levels (SN and VTA)	WKY/NCrl WKY/NHsd	= (elevated) -	+ (elevated) =	+ (elevated) =	[71]
Striatal DAT binding (PND50 and 90)	WKY/NCrl	=	N/A	N/A	[71]
D2 receptor binding	WKY/NCrl	N/A	+ in PFC, NAc shell, VTA - in NAc core, hypothalamus, caudate-putamen	N/A	[72, 73]
Grin3a (GluN3A) expression	WKY/NCrl	= (elevated)	-	N/A	[56]

Abbreviations: ADHD, attention-deficit/hyperactivity disorder; DAT, dopamine transporter; D2, dopamine D2 receptor; GluN3A, glutamate ionotropic receptor NMDA type subunit 3 A; KCl, potassium chloride; N/A, not applicable; NAc, nucleus accumbens; PFC, prefrontal cortex; PND, postnatal day; SD, Sprague-Dawley; SHR, spontaneously hypertensive rat; Slc6a3 (DAT1), solute carrier family 6 member 3; Slc9a9 (NHE9), solute carrier family 9 member 9; SN, substantia nigra; VTA, ventral tegmental area; WKY, Wistar-Kyoto rat; WKY/NCrl, Wistar-Kyoto rat, Charles River substrain; WKY/NHsd, Wistar-Kyoto rat, Harlan substrain

(+) - phenotype more pronounced in WKY relative to comparator strain, (-) – phenotype less pronounced in WKY relative to comparator strain, (=) – similar phenotype; (N/A) – not applicable

Affective and stress-related traits of the WKY strain relevant to ADHD interpretation

As we described above, the WKY rat strain was originally introduced as a reference line in hypertension research [75] and later widely adopted as a genetic control in ADHD models [6], detailed behavioral assessments conducted in the late 1980s and early 1990s revealed a distinct affective profile that has since been replicated across multiple behavioral paradigms.

Systematic phenotyping demonstrated a characteristic behavioral profile including depression- and anxiety-like behaviors that distinguish WKY rats from commonly used outbred strains such as Wistar and SD. These findings led to a substantial reinterpretation of the strain’s role, shifting it from a neutral physiological control toward a genetically predisposed model characterized by persistent affective vulnerability [76–78].

One of the earliest and most replicable findings was the markedly increased immobility of WKY rats in the forced swim test (FST) compared with Wistar and SD rats [77, 79, 80]. In the original characterization by Paré, WKY rats exhibited prolonged immobility and reduced active escape attempts despite the absence of prior stress exposure, suggesting a basal behavioral disposition rather than an induced state [77]. Importantly, this immobility was not accompanied by significant motor deficits, as baseline locomotor activity did not account for the magnitude of the differences observed in the FST [78]. Later studies confirmed that WKY rats display a consistent passive coping strategy across repeated exposures to the FST, indicating the stability of this phenotype [19, 79].

At the time, immobility in the FST was interpreted within the framework of the “behavioral despair” hypothesis. Later conceptualizations of the test, however, shifted the emphasis toward coping strategies rather than despair per se [81, 82]. From this perspective, the WKY phenotype may reflect a predisposition to passive coping under conditions of unavoidable stress. This distinction is important, as it positions the strain as a vulnerability-based model rather than a simple analogue of acute depressive episodes.

A particularly distinctive feature of the WKY strain is that depressive-like behaviors can be observed without the need for prior experimental manipulation. This clearly contrasts with stress-based paradigms such as chronic mild stress (CMS), learned helplessness, or chronic social defeat, in which behavioral changes are induced by adverse environmental conditions [1, 83]. In CMS models, anhedonia and behavioral inhibition develop gradually and may subside following antidepressant treatment. In WKY rats, by contrast, behavioral inhibition and increased stress sensitivity appear to represent intrinsic traits [9, 81].

This interpretation is further supported by studies using stress exposure paradigms. Following exposure to uncontrollable stress, WKY rats display stronger behavioral withdrawal than SD rats [14]. In learned helplessness paradigms, this strain develops escape deficits more rapidly and shows greater persistence of passive behaviors [9]. Together, these findings suggest that the basal passive coping strategy of WKY rats may interact synergistically with stress exposure, modeling stress vulnerability rather than stress resistance.

The distinction between innate susceptibility and induced state is of major translational significance. In clinical populations, not all individuals exposed to stress develop depression, as susceptibility factors play a major role in modulating risk [2]. The WKY rat may therefore reflect a genetically determined vulnerability that is shaped by environmental stressors, rather than a purely stress-induced pathology.

Anxiety-like behaviors are also consistently observed in the WKY strain and appear to represent an integral component of its affective phenotype. Compared with Wistar and SD rats, WKY rats display reduced exploration of the centre in the open field test and reduced exploration of open arms in the EPM [79, 82]. Increased thigmotaxis and avoidance-like behaviors are evident even under baseline conditions. This anxiety-prone profile complicates the interpretation of immobility in the FST. Elevated anxiety may promote energy-conserving and risk-avoidant strategies, thereby influencing FST performance. Nevertheless, anxiety and depression are highly comorbid in humans, and a shared neurobiological background has been proposed [2]. The co-occurrence of anxiety- and depression-like behaviors in WKY rats may therefore enhance, rather than diminish, the translational value of this model.

Data concerning baseline anhedonia in WKY rats are more nuanced. Some studies have reported reduced sucrose preference relative to Wistar controls [20]. In contrast, others have found comparable baseline intake but increased susceptibility to stress-induced reduction [84]. Importantly, following the CMS procedure, WKY rats display a pronounced and stable reduction in sucrose intake, often accompanied by a blunted response to classical antidepressant treatment [10]. These findings suggest that reward-related processes in the WKY strain may function relatively normally under neutral conditions, while remaining highly sensitive to stress. Such stress-dependent dysregulation may more closely resemble clinical anhedonia. The pharmacological profile of WKY rats further supports their relevance to affective and motivational dimensions that may interact with ADHD-like phenotypes. WKY rats often show attenuated behavioral responses to SSRIs and variable responses to tricyclic antidepressants, whereas kappa-opioid receptor antagonists and rapid-acting compounds such as ketamine may produce more pronounced antidepressant-like effects [79, 85–89]. This selective responsiveness suggests that monoaminergic pathways may be dysregulated in a way that weakens the effects of classical antidepressants, while glutamatergic and stress-related systems remain susceptible to modulation.

Despite its advantages as a model of trait-like affective vulnerability, the WKY strain also has important limitations that require careful consideration. First, the depressive phenotype is not expressed uniformly across behavioral domains. Although increased immobility in the FST is highly replicable [79, 82], evidence for baseline anhedonia remains inconsistent. Some studies report reduced sucrose preference compared with Wistar rats [20], whereas others indicate comparable baseline intake but increased stress sensitivity [84]. This variability suggests that the WKY phenotype may reflect altered stress reactivity or motivational processes rather than a global deficit in reward function.

Second, the increased anxiety-like behavior observed in WKY rats complicates the interpretation of depression-related tests. Reduced exploration in OFT and EPM [79, 82] may affect performance in paradigms based on novelty or risk assessment. Although the co-occurrence of anxiety and depression may increase translational validity, it also requires careful experimental design in order to distinguish anxiety-related withdrawal from the depressive phenotype.

Third, differences between substrains and suppliers introduce an additional source of variability. Subtle phenotypic differences may depend on breeding origin, and therefore explicit reporting of the supplier, substrain, and age of the animals is essential.

Finally, it is important to emphasize that the WKY strain models vulnerability-related traits rather than the full complexity of MDD. Key clinical features such as cognitive impairment, circadian rhythm dysregulation, and social withdrawal have not been comprehensively characterized across laboratories. The WKY strain should therefore not be regarded as a universal model of depression, but rather as a genetically predisposed model in which the effects of environmental and pharmacological manipulations can be studied systematically.

The WKY strain does not represent a simple model of acute depressive episodes. Rather, it reflects a stable, genetically anchored constellation of traits that includes behavioral withdrawal, increased stress sensitivity, overlap between depression- and anxiety-like features, and a distinctive pharmacological response profile. Unlike outbred strains such as Wistar or SD, which typically require environmental manipulation for a depressive-like phenotype to emerge, WKY rats express an intrinsic susceptibility that can be further exacerbated by stress. This distinction places the WKY strain in the context of models of vulnerability and chronic affective sensitivity. Its divergence from the SHR strain, despite their shared origin, further highlights the specificity of its behavioral profile. Whereas SHRs exhibit hyperactivity and impulsivity consistent with ADHD-related phenotypes [6], WKY rats display behavioral withdrawal and passive coping strategies, indicating distinct neurobiological trajectories within closely related strains. In the context of contemporary depression research, which increasingly emphasizes heterogeneity, treatment resistance, and stress susceptibility, the WKY strain provides a valuable experimental framework for investigating mechanisms underlying trait-specific affective vulnerability.

Behavioral inhibition, stress coping, and reward-related traits in WKY rats

The behavioral phenotype of WKY rats is best understood through systematic comparisons with other commonly used laboratory strains, particularly Wistar, SD, and the genetically related SHR strain. Across paradigms assessing stress coping, anxiety, reward processing, and pharmacological response, WKY rats consistently exhibit a pattern of behavioral withdrawal and stress hypersensitivity that distinguishes them from these comparator strains.

The most pronounced behavioral differences are observed in stress coping paradigms. In the FST, WKY rats consistently display prolonged immobility and reduced active behavior compared with Wistar and SD controls [78, 79]. Importantly, this phenotype persists across repeated testing sessions, indicating trait stability rather than situational adaptation [19]. When compared directly with the SHR strain, these discrepancies become even more pronounced. While SHRs often display hyperactivity and increased active escape attempts in the FST [78], WKY rats adopt a passive stress coping strategy characterized by energy-conserving behaviors. Considering the shared origin of both strains, this contrast highlights the specificity of their affective divergence despite common genetic ancestry.

Stress exposure further amplifies these differences. Rittenhouse et al. (2002) demonstrated that WKY rats show stronger behavioral withdrawal following stress compared with SD rats [14]. Similarly, in learned helplessness paradigms, WKY rats acquire escape deficits more rapidly and exhibit a greater persistence of passive responses [9]. Together, these findings suggest that the WKY strain combines a baseline passive coping strategy with heightened reactivity to uncontrollable stress.

When interpreting depression-related behavioral tests, it is essential to assess baseline locomotor activity and exploratory drive, as motor impairments may confound measures such as immobility in the FST. In OFT, WKY rats often display shorter distances travelled and lower mean speed compared with Wistar and SD strains [82]. However, these differences are not consistently observed across laboratories and, when present, are generally modest relative to the pronounced immobility observed in stress paradigms [78]. In contrast, SHRs exhibit the opposite behavioral profile, characterized by increased locomotion and impulsive exploration [6]. Importantly, numerous studies have demonstrated that increased immobility in the FST in WKY rats cannot be fully explained by reduced baseline activity [78, 77]. Following pharmacological treatment, WKY rats show normal climbing and swimming behaviors [82], indicating preserved motor abilities despite altered coping strategies. Thus, increased immobility in WKY rats appears to reflect emotional inhibition rather than a simple limitation of movement.

Anxiety-like behaviors represent another consistent and integral component of the WKY phenotype. In the OFT, WKY rats exhibit reduced exploration of the central area and increased preference for the periphery compared with Wistar and SD strains [82]. Similarly, in the EPM, decreased exploration of open arms and enhanced risk avoidance are observed [79]. Such behaviors are not consistently present in SHRs, which are more often characterized by increased locomotion and impulsivity [6]. Given that the SHR strain is widely used as a model of ADHD, the behavioral profile of WKY rats appears to represent an important affective and anxiety-related background that may modify the interpretation of ADHD-relevant behaviors.

Compared with outbred strains, WKY rats show reduced exploration of novel environments, prolonged latency in the novelty-suppressed feeding test (NSF), and decreased social interaction [82]. These findings suggest a more generalized pattern of behavioral withdrawal, potentially associated with dysregulation of stress-related neuroendocrine systems.

Reward processing in WKY rats appears more complex. Baseline sucrose preference is generally comparable to that observed in Wistar rats [84], yet susceptibility to stress-induced reductions in sucrose intake is markedly greater [20, 83]. In chronic mild stress (CMS) paradigms, WKY rats develop more persistent anhedonia and show weaker remission following antidepressant treatment compared with outbred strains [83]. These results seem to indicate that reward-related systems in WKY rats may be particularly sensitive to stress-induced dysregulation. Rather than modeling severe baseline anhedonia, the strain appears to reflect a lowered threshold for stress-induced motivational deficits.

Behavioral responses to antidepressant drugs further differentiate WKY rats from other strains. Compared with SD rats, WKY rats show weaker behavioral responses to classical monoaminergic antidepressants but retain responsiveness to pharmacological approaches targeting kappa-opioid, glutamatergic, and plasticity-related mechanisms [79, 87–89]. This profile is relevant to ADHD interpretation because altered motivation, stress coping, and reward responsiveness may influence performance in attention- and activity-based behavioral paradigms. Age-related changes may also influence the behavioral phenotype of WKY rats. Although most depression-related studies employ young adult animals (8–16 weeks of age), behavioral inhibition and stress sensitivity appear to persist into adulthood [19]. However, systematic analyses across the lifespan remain limited, and age-dependent modulation of affective traits requires further investigation. Given that physiological parameters such as blood pressure increase with age in WKY rats [90], potential interactions between cardiovascular alterations and behavioral outcomes cannot be excluded. For this reason, experimental comparisons between groups should be carefully age-matched.

A summary of the behavioral characteristics of WKY rats in depression-related paradigms is presented in Table 3.

Table 3.

Depression-related behavioral characteristics of the WKY rat strain in comparison to other rat strains, with relevance to ADHD interpretation

Behavioral phenotype	Vendor/ Substrain	Compared to SHR	Compared to Wistar	Compared to SD	References
Increased immobility in FST (baseline)	Various substrains (e.g., WKY/NCrl)	-	+	+	[76, 78]
Increased anxiety-like behavior in EPM / OFT	WKY/NCrl; BAS strains	-	+	+	[78]
Reduced locomotor/ exploratory activity	WKY/NCrl	-	+	+	[9, 78]
Enhanced vulnerability to CMS	Various laboratory strains	+/-	+	+	[10, 83]
Blunted SSRI response	Experimental CMS controls	N/A	+/-	+/-	[89, 94]

Abbreviations: BAS, Bulgarian Academy of Sciences; CMS, chronic mild stress; EPM, elevated plus maze; FST, forced swim test; N/A, not applicable; OFT, open-field test; SD, Sprague-Dawley; SHR, spontaneously hypertensive rat; SSRI, selective serotonin reuptake inhibitor; WKY, Wistar-Kyoto rat; WKY/NCrl, Wistar-Kyoto rat, Charles River substrain

(+) - phenotype more pronounced in WKY relative to comparator strain, (-) – phenotype less pronounced in WKY relative to comparator strain, (N/A) – not applicable

Molecular mechanisms linking affective vulnerability and ADHD-relevant phenotypes in WKY rats

The behavioral profile of the WKY line is supported by a complex molecular architecture that extends beyond classical monoaminergic hypotheses and involves coordinated alterations in stress axis regulation, neurotransmitter systems, synaptic plasticity mechanisms, and inflammatory signaling. Comparisons with the Wistar, SD, and SHR strains indicate that WKY rats do not simply exhibit enhanced versions of typical stress responses but rather display strain-specific regulatory differences across multiple neurobiological domains.

Stress axis dysfunction represents one of the most consistently described molecular features of WKY rats. Compared with SD rats, they exhibit enhanced corticosterone and adrenocorticotropic hormone (ACTH) responses following acute stress exposure [14, 91, 92]. Differences in the basal circadian corticosterone rhythm have also been reported, including altered amplitude and feedback sensitivity [84]. Redei et al. demonstrated altered glucocorticoid receptor (GR) expression and sensitivity in WKY rats, suggesting impaired negative feedback regulation of the hypothalamic–pituitary–adrenal axis (HPA) [84]. In contrast, Wistar and SD rats typically show more efficient glucocorticoid-dependent inhibition of the stress axis [91]. Importantly, SHRs, despite their increased sympathetic tone, do not consistently display a pattern of dysregulated corticosterone reactivity similar to that observed in WKY rats, highlighting the divergence of neuroendocrine trajectories between these strains [91].

At the level of corticotropin-releasing factor (CRF) signaling, increased sensitivity of the CRF system has been observed in WKY rats, particularly in limbic structures involved in stress responses [93]. Enhanced CRF-induced serotonergic dysregulation in the dorsal raphe nucleus has also been reported compared with SD rats, suggesting altered integration between stress peptides and monoaminergic nuclei [93].

Monoaminergic changes in WKY rats are relatively subtle, yet functionally significant. Electrophysiological recordings from the dorsal raphe nucleus have revealed altered firing patterns of serotonergic neurons and increased sensitivity to CRF compared with SD rats [93]. Chronic escitalopram administration increases serotonergic transmission in WKY rats; however, the extent and nature of these neurochemical adaptations differ from those observed in the Wistar strain [94].

Differences are also evident in the noradrenergic system. Bruzos-Cidón et al. demonstrated altered reactivity of locus coeruleus neurons and impaired inhibitory regulation in WKY rats compared with SD rats [95, 96]. In particular, feedback mechanisms dependent on α2-adrenergic receptors appear dysregulated, which may contribute to altered stress reactivity and behavioral inhibition. In contrast, SHRs display a distinct monoaminergic profile characterized by elevated dopaminergic activity and dysregulated catecholamine regulation associated with hyperactivity [8]. Thus, within closely related genetic strains, the monoaminergic configuration of WKY rats appears more consistent with affective phenotypes.

Dopaminergic pathways are also relevant to the affective and motivational dimensions of the WKY phenotype. As discussed above, WKY rats show alterations in dopamine transporter function and dopamine receptor distribution, although their profile differs from the elevated dopaminergic tone typically associated with SHRs [72, 73, 97–99, 100]. In the context of ADHD-related studies, these differences are important because altered dopaminergic regulation may influence exploratory drive, reward sensitivity, and task engagement.

Under conditions of chronic stress, dopaminergic alterations in WKY rats become more pronounced, and their susceptibility to stress-induced motivational deficits is greater than that observed in Wistar rats [101]. This suggests that the dopaminergic system in this strain may be particularly sensitive to stress-dependent modulation rather than constitutively hypoactive.

Recent evidence also highlights the habenula as a region exhibiting strain-specific molecular alterations. In WKY rats, differential expression of miRNAs and their target transcripts has been reported in both medial and lateral habenular nuclei compared with Wistar rats, including genes such as Htr7, associated with serotonergic signaling, and Slc12a5, which regulates neuronal chloride homeostasis and inhibitory neurotransmission. These findings further support the role of habenular dysfunction in the treatment-resistant depression-like phenotype observed in this strain [102].

Growing evidence supports a role for glutamatergic dysregulation in the pathophysiology of depression [103, 104]. Lei et al. (2009) reported strain-dependent differences in the distribution of NMDA and GABA-A receptor subunits between WKY and Wistar rats, particularly in cortical and limbic regions [105]. Such differences may influence the balance between excitation and inhibition and stress reactivity.

Electroconvulsive stimulation induces distinct neuroplastic adaptations in WKY rats compared with Wistar rats, including differential regulation of plasticity-related genes [106]. Furthermore, rapid-acting antidepressants such as ketamine improve behavioral outcomes in WKY rats and are accompanied by changes in synaptic plasticity within prefrontal and hippocampal circuits [89]. Recent studies involving psychedelic compounds, including psilocybin, suggest that plasticity-related signaling pathways may also respond differently in WKY rats despite their baseline vulnerability [107]. This pattern further supports the notion that glutamatergic and plasticity-related mechanisms remain modifiable in this strain, even when monoaminergic responses are attenuated.

Altered expression of brain-derived neurotrophic factor (BDNF) has long been associated with depression [108]. Region-specific differences in BDNF expression have been reported in WKY rats, particularly in the hippocampus and cortex [83]. Chronic antidepressant treatment modulates BDNF signaling in WKY rats, although baseline expression patterns may differ from those observed in outbred strains [109, 110]. In CMS paradigms, decreases in hippocampal BDNF levels are often more pronounced in WKY rats than in Wistar rats [101], suggesting increased sensitivity of neuroplasticity-related pathways to stress. Given the well-documented interaction between glucocorticoids and BDNF transcription, dysregulation of the HPA axis may contribute to these neurotrophic disturbances.

Accumulating evidence suggests that WKY rats exhibit an atypical inflammatory profile rather than a clearly elevated proinflammatory state. Farinha-Ferreira et al. (2024) described subtle changes in cytokine expression and neuroimmune signaling compared with the Wistar strain [111]. Transcriptomic analyses across multiple tissues have also revealed coordinated differences in stress-related gene expression in both the brain and peripheral tissues [112].

The principal molecular features associated with depression-related traits of WKY rats, with potential relevance to the interpretation of ADHD-related phenotypes, are summarized in Table 4.

Table 4.

Depression-related molecular characteristics of the WKY rat strain in comparison to other rat strains, with relevance to ADHD interpretation

Molecular domain	Vendor/ substrain	Compared to SHR	Compared to Wistar	Compared to SD	References
Hyperreactive HPA axis /impaired feedback	Multiple substrains	+	+	+	[14, 91]
Altered glucocorticoid receptor	Multiple substrains	+	+	+	[7, 84, 92]
Altered dorsal raphe CRF sensitivity	WKY/NCrl	-	+/-	+/-	[93]
Restrained dopaminergic profile	WKY/NCrl	-	+/-	+/-	[28, 97]
NMDA distribution	WKY/NCrl, Perry Point	N/A	- in the anterior cingulate cortex, caudate putamen, NAc, CA1 region of the hippocampus, the SN pars reticulata	N/A	[105]
GABA-A distribution	WKY/NCrl, Perry Point	N/A	+ in amygdala, caudate putamen, dentate gyrus, CA2 and CA3 fields of the hippocampus, periaqueductal gray, SN pars reticulata	N/A	[105]
Region-specific BDNF alterations (stress-sensitive)	Multiple substrains	N/A	+	+	[7, 10, 101]
Altered habenular gene expression (Htr7, Slc12a5)	WKY/NCrl	N/A	-	N/A	[102]

Abbreviations: BDNF, brain-derived neurotrophic factor; CA1, cornu ammonis 1; CA2, cornu ammonis 2; CA3, cornu ammonis 3; CRF, corticotropin-releasing factor; GABA-A, gamma-aminobutyric acid type A receptor; HPA, hypothalamic-pituitary-adrenal axis; Htr7, 5-hydroxytryptamine receptor 7 gene; N/A, not applicable; NAc, nucleus accumbens; NMDA, N-methyl-D-aspartate receptor; SD, Sprague-Dawley; SHR, spontaneously hypertensive rat; Slc12a5, solute carrier family 12 member 5 gene; SN, substantia nigra; WKY, Wistar-Kyoto rat; WKY/NCrl, Wistar-Kyoto rat, Charles River substrain

(+) - phenotype more pronounced in WKY relative to comparator strain, (-) – phenotype less pronounced in WKY relative to comparator strain, (N/A) – not applicable. WKY/NCrl, Perry Point; WKY rats were raised in the Perry Point laboratory from the breeding stock initially obtained from Charles River Laboratories (Kingston, NY)

Conclusion

The WKY rat strain has undergone a conceptual transformation from a neutral reference strain in hypertension research to a strain with a complex and biologically relevant behavioral phenotype in neuropsychiatric research. Although WKY rats continue to be used as comparator animals in cardiovascular and ADHD research, extensive evidence suggests that this strain cannot be considered a behaviorally neutral control strain. Instead, WKY rats display ADHD-relevant characteristics, particularly reduced sustained attention, hypoactivity, and low impulsivity, together with depression- and anxiety-related traits such as behavioral inhibition, increased stress reactivity, altered reward response, and attenuated responsiveness to classical antidepressant treatment. This profile supports the relevance of WKY rats for ADHD research, particularly for the inattentive presentation and for studies addressing comorbid affective traits that may influence the interpretation of ADHD-like phenotypes.

Abbreviations

ACTH

Adrenocorticotropic hormone

ADHD

Attention-deficit/hyperactivity disorder

ADHD-H

Predominantly hyperactive/impulsive attention-deficit/hyperactivity disorder

ADHD-C

Attention-deficit/hyperactivity disorder: combined subtype

ADHD-I

Predominantly inattentive attention-deficit/hyperactivity disorder

ASD

Autism spectrum disorder

Atg7

Autophagy-related 7 gene

Atp2b2

ATPase plasma membrane Ca2 + transporting 2 gene

BDNF

Brain-derived neurotrophic factor

Cacnb4

Calcium voltage-gated channel auxiliary subunit beta 4 gene

Chrna5

Cholinergic receptor nicotinic alpha 5 subunit

CMS

Chronic mild stress

Creg1

Cellular repressor of E1A stimulated genes 1 gene

CRF

Corticotropin-releasing factor

DRL

Differential reinforcement of low-rate responding

DAT

Dopamine transporter

DSS

Dahl salt-sensitive rat

EPM

Elevated plus maze

GABA-A

Gamma-aminobutyric acid type A receptor

GluN3A

Glutamate ionotropic receptor NMDA type subunit 3 A

Grin3a

Glutamate ionotropic receptor NMDA type subunit 3 A gene

Htr7

5-Hydroxytryptamine receptor 7 gene

Ip6k2

Inositol hexakisphosphate kinase 2 gene

MDD

Major depressive disorder

Mx2

MX dynamin-like GTPase 2 gene

NAc

Nucleus accumbens

NIH

U.S. National Institutes of Health

NMDA

N-methyl-D-aspartate receptor

NSF

Novelty-suppressed feeding test

OFT

Open-field test

PFC

Prefrontal cortex

SD

Sprague-Dawley

SHR

Spontaneously hypertensive rat

SHR/NCrl

Spontaneously hypertensive rat, inbred SHR from Charles River, Germany

Slc12a5 (KCC2)

Solute carrier family 12 member 5 gene

Slc6a3 (DAT1)

Solute carrier family 6 member 3 gene

Slc9a9 (NHE9)

Solute carrier family 9 member 9 gene

SN

Substantia nigra

SNPs

Single-nucleotide polymorphisms

SSRI

Selective serotonin reuptake inhibitor

SUD

Substance use disorder

Syt12

Synaptotagmin 12 gene

Thrsp

Thyroid hormone-responsive gene

VTA

Ventral tegmental area

WKY

Wistar-Kyoto rat

WKY/NCrl

Wistar-Kyoto rat, inbred from Charles River, Germany

WKY/NHsd

Wistar-Kyoto rat, inbred from Harlan Europe, UK

Zeb2

Zinc finger E-box binding homeobox 2 gene

Author contributions

N.G. reviewed and synthesized the literature on the use of WKY rats in ADHD research, whereas E.D. reviewed and synthesized the literature on the use of WKY rats in depression research. A.F.-G. supervised the conceptual development of the manuscript and contributed to its interpretation and final structure. All authors contributed to writing the first version of the manuscript, revising the text, addressing the reviewers’ comments, and preparing the revised version. All authors critically reviewed and approved the final manuscript.

Funding

This work was conducted as part of a project 2024/53/B/NZ7/00671 (PI Agata Faron-Górecka) funded by the National Science Centre (NCN), Poland. Natalia Gaik and Emil Dzierkacz are scholarship recipients in this project.

Data availability

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

Declarations

Ethical approval

The work did not involve animals and human subjects directly and did not require ethical approval.

Consent to participate

Not applicable.

Competing interests

The authors declare no competing interests.