The present study provides behavioral and genotyping data to support the suggestion that the WKY/NCrl rat is a suitable animal model of the inattentive subtype of ADHD (ADHD-PI). This provides researchers with a new tool for studying the neurobiology and genetics of ADHD-PI. Such a model has, heretofore, been unavailable. Our findings are consistent with the fact that the WKY/NCrl and WKY/NHsd substrains had been separated before they became fully inbred [22
The present, as well as other recently published results [9
], show that overactivity, impulsiveness and deficient sustained attention of the SHR/NCrl strain are independent behaviors that may be affected differently by drugs. Further, the present study shows that the WKY/NHsd rat is inattentive without being overactive. Thus, overactivity does not account for impulsiveness and deficient sustained attention in either SHR/NCrl, or in WKY/NHsd rats.
Compared to WKY/NHsd and SD/NTac rat, SHR/NCrl rats showed striking overactivity and poor sustained attention. This result is in accordance with previous studies [8
]. In addition, WKY/NCrl rats showed poor sustained attention, but no overactivity or impulsiveness. This result has not been described before.
There were substantial genomic differences between the WKY/NCrl and WKY/NHsd rats for eight SSLP loci and 2625 SNPs. We estimate that 33.5% of the genome differs between the two rat strains, with large stretches of divergence on each chromosome. These data provide solid evidence that the WKY/NCrl and WKY/NHsd rats should be considered as separate strains.
Our genomic results for the SHR/NCrl rats also provide some insights into the possible source of the genetic divergence between the WKY/NHsd and WKY/NCrl rats. 10.5% of the SNPs did not match in comparisons between the SHR/NCrl and WKY/NHsd rat data. Most of these SNPs (99%) also did not match in comparisons between the WKY/NHsd and WKY/NCrl strains. However, of the 276 SNPs that did not match when comparing WKY/NHsd and SHR/NCrl rats, almost all (99%) did match when comparing the WKY/NCrl and SHR/NCrl rats. These data suggest that at least part of the genetic divergence between the WKY/NHsd and WKY/NCrl rats could have been caused by intercrossing with an SHR or SHR-derived rat line. However, the background of the SHR rat alone clearly cannot account for all of the divergence that we have detected.
A more likely explanation of the genetic divergence between the WKY/NHsd and WKY/NCrl rats is that the WKY was not fully inbred when shipped to various breeders [22
]. The NIH Animal Genetic Resource stock was obtained in 1971 as non-inbred Wistar stock from the Kyoto School of Medicine, Japan. The breeding stock of this strain was distributed before F20, possibly resulting in the emergence of a number of strains or substrains. The WKY/NCrl used in the present study arrived at Charles River in 1974 from NIH at F11. The WKY/NMol, used in several of our previous studies (for a review see [8
]), arrived at Møllegaard Breeding Centre, Denmark, from the NIH in 1975 at F13. It is unclear exactly when the Wistar Kyoto rat, later known as WKY/NHsd arrived at Harlan Sprague Dawley, US. The WKY/NCrl appears to now offer the opportunity to evaluate the phenotype and genotype of ADHD-PI in an animal model. It is therefore essential that subline codes are always used in designating this strain [25
The SHR [32
] arrived NIH in 1966 at F13 from the Kyoto School of Medicine. It was bred from an outbred Wistar Kyoto male with marked elevation of blood pressure mated to female with slightly elevated blood pressure; brother-sister mating with continued selection for spontaneous hypertension. The SHR/NCrl came to Charles River from NIH in 1973 at F32. There is no evidence for substrain differentiation among SHR stocks from the major commercial suppliers in the USA both respect to phenotype and DNA fingerprints [25
The SD/NTac (NTac:SD), Taconic Sprague Dawley rats, were first obtained in 1970 from the NIH Animal Genetic Resource. The NIH stock originated from Sprague Dawley, Inc. in 1945 and has since been maintained as an outbred closed colony. The WH/HanTac stock was hysterectomy derived at RCC Ltd, Switzerland, in 1989. Genetic drift in RCC's colony of Wistar Hannovers is minimized through the use of the Poiley rotational breeding system and revitalization of the stock with cryopreserved embryos (most recent revitalization completed in 1998). Taconic replaced its former WH stock with the GALAS Wistar Hannover rat in June 2000 [25
The appropriateness of WKY as a control for SHR has systematically been investigated in our laboratory [8
]. When using operant schedules of reinforcement, there was no significant behavioral difference between SHR/Mol and SHR/N, but these rats were more active than various comparison groups that did not differ: WKY/Mol, WKY/N, WKHA, WKHT, Wistar/Mol, SD/Mol, hooded PVG/Mol, and offsprings of female DA/OlaHsd crossed with male LEW/NHsd. In some less well-controlled experimental conditions like open fields however, Wistar/Mol and SD/Mol could be as active as SHR/Mol [33
]. In conclusion, under well-controlled operant schedules of reinforcement, both the present as well as our previous results [8
] indicate that carefully chosen WKY substrains are adequate controls for SHRs.
SD/NTac rats behaved like WKY/NHsd rats in the present study, but the genetic results indicated significant differences between this strain and the WKY/NHsd strain. Thus, Sprague Dawley rats may be a poor control for the SHR in neurobiological studies. Given that the present results indicate that the WH/HanTac rats and WKY/NCrl deviated both genetically as well as behaviorally from the WKY/NHsd, we conclude that the use of these strains as controls for SHRs may also produce spurious neurobiological differences. Thus, WKY/NHsd is the most appropriate control for SHR/NCrl.
There are significant neurobiological differences between the SHR/NCrl and WKY/NHsd rats [35
]. Current theories of ADHD relate symptom development to factors that alter learning [21
]. N-methyl-D-aspartate receptor (NMDAR) dependent long term changes in synaptic efficacy in the mammalian CNS are thought to represent underlying cellular mechanisms for some forms of learning [38
]. Physiological and anatomical aspects of hippocampal CA3-to-CA1 synapses in age-matched SHR/NCrl and WKY/NHsd rats showed functional impairments in glutamatergic synaptic transmission that may be one of the underlying mechanisms leading to the abnormal behavior in SHR/NCrl and possibly in ADHD [35