NK1R knockout mouse model and human genetics of Attention Deficit Hyperactivity Disorder

Attention Deficit Hyperactivity Disorder (ADHD) affects about 2-5% of children and adults in the UK. Its main features are hyperactivity, impulsivity and inattentiveness. ADHD is heritable and often co-exists with other psychiatric problems, especially alcoholism and bipolar disorder (manic depression). Front-line medication includes drugs that are described as ?psychostimulants? (e.g amphetamine). However, little is known about their long-term effects, especially in children, and there is a pressing need for alternative drug treatments. Recently, we discovered that a mutant mouse, which lacks so-called NK1 receptors (?NK1R knockout?), is hyperactive. Moreover, this striking behaviour is prevented by drugs used to treat humans with ADHD. Arising from these findings, we are developing the NK1R knockout mouse as an animal model of ADHD. In this project, we shall refine this model by investigating whether these mice are impulsive and/or inattentive, as well as hyperactive. We shall also test our proposal that their abnormal behaviour is explained by disrupted regulation of the neurotransmitters, dopamine and acetylcholine, in two brain regions that have been linked with ADHD.
Prompted by these studies in mice, we have also discovered mutations (single nucleotide polymorphisms) in the human equivalent of the NK1R gene (known as TACR1) in DNA from patients with ADHD. Our evidence suggests that these polymorphisms can be used as a diagnostic marker for ADHD. It is also possible that certain mutations in the TACR1 gene increase susceptibility of ADHD patients to alcoholism and/or bipolar disorder. In this project, we shall map the TACR1 gene in DNA from ADHD patients and confirm which of the mutations we have identified are associated with ADHD and/or alcoholism or bipolar disorder.
The combined results of these experiments will help to refine the use of both mutant (NKR1 /-) mice, as a much needed animal model of ADHD, and mutations in the TACR1 gene as a diagnostic test for ADHD. They will also cast light on the functional abnormalities in the brain arising from a deficit in NK1R/TACR1 function. All this information will inform future investigations aimed at developing novel treatment strategies (including the potential for a new class of drug treatment) for ADHD.

Mice with functional ablation of the NK1 receptor (NK1R-/-) express hyperactivity, which is prevented by psychostimulants. They also have deficits in dopaminergic transmission in the prefrontal cortex and dorsal striatum. All these abnormalities are consistent with NK1R /- mice being a model of Attention Deficit Hyperactivity Disorder (ADHD). This proposal was confirmed in studies in which we found an association between four single nucleotide polymorphisms in the TACR1 (the human NK1R) gene and a diagnosis of ADHD. We have further evidence for polymorphisms in the TACR1 gene and increased susceptibility to alcoholism and/or bipolar disorder (BPAD). One aim of this project is to refine this mouse model of ADHD. We shall use the 5-Choice Serial Reaction Time task to establish whether NK1R-/- mice express other core features of ADHD, namely, impulsivity / inattentiveness, as well as hyperactivity. We shall also use microdialysis to test our hypothesis that, as well as disruption of dopaminergic transmission in NK1R-/- mice, there is a deficit in acetylcholine release from striatal interneurones: these neurons govern striatal output and so have a key role in regulating behaviour. We shall go on to investigate whether behavioural and neurochemical abnormalities are abolished by psychostimulants, which are used to treat ADHD, and mimicked by giving an NK1R antagonist to wild type mice. All these experiments will have a multifactorial design and the mice will be assigned randomly to the different groups. In parallel studies, we shall carry out genotyping of human DNA to test whether a clinical subgroup of ADHD is strongly associated with NKR1/TACR1 polymorphisms, as our evidence suggests. This will involve association studies using DNA from patients with ADHD, BPAD, alcoholism and healthy controls. We shall look for allelic and haplotype associations and identify region(s) of NK1R/TACR1 to be screened for mutations in the exons and promoter regions of NK1R/TACR1. We shall then genotype the entire sample to assess the likely role of novel mutations in susceptibility to ADHD, BPAD and alcoholism. This work will guide bioinformatics and in vitro studies of the functional consequences of promising mutations. The combined results of this translational project will help to develop the NK1R-/- mice as a much-needed mouse model of ADHD, provide essential information on TACR1 polymorphisms as a causal factor in this disorder and its comorbidity, and could open up new avenues of research in the hunt for novel treatments.