Original articleValproate attenuates hyperactive and perseverative behaviors in mutant mice with a dysregulated dopamine system
Introduction
Dysregulation of the dopamine (DA) system is thought to underlie several psychiatric disorders including schizophrenia, bipolar disorder, and attention-deficit/hyperactivity disorder (ADHD). Psychotic behavior, mania, and hyperactivity can be triggered in humans by dopaminergic stimulants such as amphetamine, suggesting that a DA-related mechanism may be involved in these behaviors. Based on the idea that a hyperdopaminergic state underlies some psychiatric illnesses, several studies have focused on the DA transporter (DAT) as a possible candidate gene in linkage studies in patients with schizophrenia, bipolar disorder, and ADHD Cook et al 1995, Crowe and Vieland 1998, Fujiwara et al 1997, Gill et al 1997, Kelsoe 1999, Kelsoe et al 1996, Persico and Catalano 1998, Persico and Macciardi 1997. Animal model paradigms have been used to examine certain aspects of the neurobiological substrates of these disorders. In recent years, DA transporter (DAT) mutant mouse lines have been generated that may prove useful in behavioral studies related to these disease states.
A DAT “knockout” (KO) mouse has been created that completely lacks the gene coding for the DAT (Giros et al 1996). These mice exhibit a chronic hyperdopaminergic tone because, upon stimulation, DA remains in the extracellular space 300 times longer than it does in the wildtype (WT) control animals. In addition, these animals also have profound physiologic alterations Gainetdinov et al 1998, Giros et al 1996. The DAT KO mutants are dramatically hyperactive in a novel environment, have impairments in spatial cognitive function, and exhibit locomotor hypoactivity in response to psychostimulants such as amphetamine and methylphenidate (Gainetdinov et al 1999). Recently, a second kind of DAT mutant mouse has been created. Unlike the full DAT KO, these “knockdown” (KD) mice have an approximately 90% loss of DAT (Zhuang et al 2001). The generation of the DAT knockdown has been described previously (Zhuang et al 2001). Briefly, the reduction in DAT expression is due to the insertion of an extra 4-kb DNA sequence into the second exon in the 5′-untranslated region of DAT, resulting in a reduction in gene expression efficiency while keeping the coding sequence intact. These mice also have a chronic hyperdopaminergic tone (e.g., a 70% increase in extracellular striatal DA), but they do not show gross physical changes. Behaviorally, the DAT KD mice are hyperactive in a novel open field and show impaired response inhibition in a number of paradigms (Zhuang et al 2001). The DAT KO and KD mutant mice have been proposed to model aspects of ADHD Gainetdinov et al 1999, Zhuang et al 2001, but they may more generally model disease states characterized by a hyperdopaminergic tone. Hence, we set out to characterize further the DAT KD mice, mice that had shown no overt compensatory changes based on the DAT mutation, in both startle and locomotor behavioral paradigms.
Although no animal model can fully account for the complexity of psychiatric disorders, selected behaviors that are observed experimentally in humans have been reproduced in animal models. One such behavior is prepulse inhibition (PPI) of the startle response, a form of sensorimotor gating in which the startle response is reduced when the startling stimulus is preceded by a low intensity prepulse Graham 1975, Hoffman and Ison 1980. Several psychiatric populations characterized by dysregulated DA systems have deficits in PPI, including schizophrenia, psychotic bipolar affective disorder, and ADHD comorbid with other disorders Braff et al 1992, Castellanos et al 1996, Ornitz et al 1999, Perry et al 2001. Comparable deficits in PPI are produced in rodents by stimulating the DA system with agonists such as amphetamine and apomorphine Dulawa and Geyer 1996, Mansbach et al 1988, Ralph et al 1999, suggesting that DA is involved in the modulation of PPI. We have previously described similar deficits in PPI in DAT KO mice, mice that were also hyperactive and highly perseverative in their motor behavior (Ralph et al 2001a). Based on the reported deficits in PPI in disorders such as bipolar disorder and ADHD, the evidence of DA-stimulated disruptions in PPI in mice, and the observed hyperactivity phenotype in the DAT KD mice, we hypothesized that the DAT KD mice would show deficits in PPI.
Locomotor hyperactivity has been used for many years as one aspect of modeling the mania phenotype in animals (for review, see Lyon 1991). Because the DAT KD mutant mice exhibit locomotor hyperactivity and because PPI deficits have been reported in patients with bipolar disorder (Perry et al 2001), we hypothesized that the hyperdopaminergic state of the DAT KD mice is consistent with a manic state in humans with bipolar disorder. One approach to address this hypothesis is to examine whether standard pharmacologic interventions to treat manic episodes in bipolar disorder patients are also effective in reducing locomotor hyperactivity in these mice. Valproate is a standard treatment for manic and hypomanic episodes in subjects with bipolar disorder. Similar to the effects of lithium, valproate reduces hyperactivity, racing thoughts, and other key symptoms of mania within days of treatment Motohashi 1999, Tohen and Grundy 1999. It has been suggested that the manic episodes during the course of bipolar disorder are consistent with a temporarily dysregulated DA system Diehl and Gershon 1992, Emilien et al 1999. If indeed a dysregulated DA system underlies some of the key symptoms of mania, and if the DAT KD mice model a dysregulated DA system, we hypothesized that pharmacological agents that successfully treat manic symptoms would attenuate the hyperactivity displayed by the DAT KD mutant mice. Hence, we conducted a valproate dose-response study and observed that 100 mg/kg valproate had no effect on the overall levels of locomotor activity in control mice. We then treated the DAT KD mice with this dose of valproate in an attempt to attenuate their hyperactivity motor phenotype.
Section snippets
Animals
The DAT KD cohort (Zhuang et al 2001) used in the initial phenotypic characterization was sent to our laboratory from Columbia University (New York, NY; n = 12 male WT and DAT KD). All subsequent mice were derived from breedings at the vivarium at the University of California, San Diego. The DAT KD mice were generated using embryonic stem cells from the 129SvJ strain and were inserted in C57BL/6J blastocyst cells; one of the chimeras was mated with 129SvJ females to generate heterozygous
DAT knockdown baseline characterization
The first group of DAT KD mice tested had comparable levels of PPI and acoustic startle reactivity compared with WT control mice (see Figure 1A and 1B). Similarly, testing of a second cohort also revealed no significant difference in PPI between DAT WT and KD mice, indicating that the lack of a PPI phenotype was reliable (see Figure 1C). There was, however, a significant main effect of genotype on acoustic startle responding only in the second cohort [F(1,64) = 11.4, p < .01]. Specifically,
Discussion
In this study, we examined the behavioral phenotypes of DAT KD mice and their role as possible models for a dysregulated DA system, a state that is thought to contribute to schizophrenia, bipolar disorder, and ADHD. We obtained four main results. Unlike the full DAT KO, DAT KD mice do not show deficits in PPI compared with WT controls. Similar to the DAT KO mice, however, the DAT KD mice are hyperactive and display perseverative motor patterns. Following a dose-response study, the DAT KD mice
Acknowledgements
These studies were supported by the Veterans Affairs VISN 22 Mental Illness Research, Education, and Clinical Center, and by the National Institute on Drug Abuse (Grant No. DA02925) and the National Institute of Mental Health (Grant No. F31-MH12806, MH61326, MH42228). M.A. Geyer holds an equity interest in San Diego Instruments. The authors thank Virginia Lehmann-Masten for her excellent technical assistance.
References (42)
- et al.
Anterior pituitary hypoplasia and dwarfism in mice lacking the dopamine transporter
Neuron
(1997) - et al.
Human studies of prepulse inhibition of startleNormal subjects, patient groups, and pharmacological studies
Psychopharmacology (Berl)
(2001) - et al.
Gating and habituation of the startle reflex in schizophrenic patients
Arch Gen Psychiatry
(1992) - et al.
Sensorimotor gating in boys with Tourette’s syndrome and ADHDPreliminary results
Biol Psychiatry
(1996) - et al.
Association of attention-deficit disorder and the dopamine transporter gene
Am J Hum Genet
(1995) - et al.
Chromosome 5 workshop
Psychiatr Genet
(1998) - et al.
Effects of phencyclidine (PCP) and (+)MK-801 on sensorimotor gating in CD-1 mice
Prog Neuropsychopharmacol Biol Psychiatry
(1998) - et al.
The role of dopamine in mood disorders
Compr Psychiatry
(1992) - et al.
Psychopharmacology of prepulse inhibition in mice
Chin J Physiol
(1996) - et al.
Serotonin1B receptor modulation of startle reactivity, habituation, and prepulse inhibition in wild-type and serotonin1B knockout mice
Psychopharmacology (Berl)
(1997)