Elsevier

Neuropharmacology

Volume 57, Issues 7–8, December 2009, Pages 678-686
Neuropharmacology

Behavioural characterisation of the effects of acute and repeated administration of GBR 12909 in rats: Further evaluation of a potential model of ADHD

https://doi.org/10.1016/j.neuropharm.2009.08.006Get rights and content

Abstract

Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric disorder characterised by excessive levels of hyperactivity, inattentiveness and impulsivity. Stimulant drugs which increase dopamine neurotransmission are treatments for ADHD. Hypodopaminergic fronto-striatal function with associated overactivity of the dopamine transporter (DAT) represents one possible neurobiological mechanism underlying ADHD. Few, if any, of the existing animal models of ADHD mimic the underlying neurobiology of the disorder. In this study we have further characterised the behavioural profile of a model of a hyperactive inattentive animal through manipulation of the DAT.

The behavioural effects of acute treatment and following withdrawal from sub-chronic treatment with GBR 12909 (30 mg/kg i.p.), a potent and highly selective DAT inhibitor, were examined in juvenile rats. GBR 12909 treatment was used to produce a compensatory up regulation following withdrawal. Acute treatment with GBR 12909 (30 mg/kg i.p.) resulted in a marked increase in locomotor and rearing behaviours on the first and fourth days during a 4 consecutive bi-daily drug treatment regime in postnatal weaned rats. Adolescent rats after 10, 20 and 30 days withdrawal from GBR 12909 pre-treatment maintained mild increases in locomotor activity and failed to discriminate a familiar over a novel object in the novel object discrimination task (using both 1 min and 3 h inter-trial intervals) indicating impaired learning and memory. Prepulse inhibition of acoustic startle was unaltered following withdrawal from GBR 12909 treatment.

These data reinforce the potential role of the DAT in the underlying neurobiology of ADHD. They also add further evidence to suggest that postnatal changes in the DAT following withdrawal from treatment with the DAT inhibitor, GBR 12909, may prove to be a useful animal model of ADHD with potential for examining the effectiveness of novel ADHD treatments.

Introduction

Attention deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder of childhood, characterised by developmentally inappropriate and pervasive symptoms of impulsiveness, inattention and overactivity (Barkley, 1998). ADHD has been proposed to involve, in part, a malfunction of the dopaminergic system (Sagvolden and Sergeant, 1998, Swanson et al., 2007, Viggiano et al., 2004), in particular, decreased frontostriatal dopaminergic function (Sagvolden et al., 2005). Increased striatal dopamine transporter (DAT) levels have been reported using radioligands and single positron emission computed tomography (SPECT) or positron emission tomography (PET) imaging (Madras et al., 2002, Krause et al., 2003, Krause, 2008) although not all studies have replicated this finding it is clear that DAT levels are plastic and closely related to synaptic DA homeostasis (Swanson et al., 2007). Furthermore, polymorphisms of several dopamine genes and of the dopamine transporter have been suggested to play a major role in the aetiology of ADHD (Faraone et al., 2005, Winsberg and Comings, 1999, Rommelse et al., 2008, Xu et al., 2009). It is due to this involvement of the DAT and increased dopamine functioning that many of the existing animal models of ADHD do not conform fully to all 3 validity criteria for modelling a psychiatric disorder in a rodent (face validity – ability to mimic the clinical behavioural characteristics of ADHD; construct validity – to model the underlying neurobiological causes of ADHD; and predictive validity – to predict how yet unidentified aspects of ADHD, such as behaviour, neurobiology and genetics may fair). Many different animal models are currently used to investigate behavioural components of ADHD and attempt to evaluate the efficacy of novel ADHD compounds. The Spontaneous Hypertensive rat has long been the most popular preclinical model of ADHD (Wultz et al., 1990, Sagvolden et al., 1992; Pardey et al., 2009), however these animals do not accurately replicate the underlying neurobiology of the disorder (and develop hypertension with age which is not a feature of ADHD) and thus lack robust construct validity. Likewise, DAT knock out mice (Giros et al., 1996) and 6-OHDA lesioned animals (Archer et al., 1988) have often been used to model ADHD but show reduced rather than elevated striatal DAT thought to be associated with this disorder. The current research has attempted to produce a model with long-term up-regulation of DAT to mimic the putative frontostriatal hypodopaminergic function accompanying ADHD.

This study further characterises, the behaviour repertoire of animals withdrawn from adolescent treatment with the selective DAT inhibitor, GBR 12909, as a potential rodent model of ADHD (Hewitt et al., 2001, Hewitt et al., 2005). Treatment of young rats with the potent and selective DAT inhibitor GBR 12909 (van der Zee et al., 1980, Heikkila and Manzino, 1984, Andersen, 1989) would be expected to initially reduce DAT activity but subsequently upon drug withdrawal might induce a rebound DAT over-expression similar to that reported by imaging studies in ADHD patients (Krause et al., 2003, Krause, 2008). GBR 12909 produces behavioural effects similar to other psychomotor stimulants, such as cocaine which also inhibit DAT (Kelley and Lang, 1989, van Gaalen et al., 2006). Indeed, DAT mRNA is upregulated in the ventral tegmental area and the substantia nigra of rats following repeated amphetamine treatment (Shilling et al., 1997) consistent with the rebound hypothesis. Furthermore, withdrawal from GBR 12909 treatment should result in some of the behavioural characteristics of ADHD, such as hyperactivity, decreased attention and impulsivity. We previously reported that 30 days after GBR 12909 treatment there were marked alterations in behaviour (activity) and regional blood flow activity (BOLD) including a blunted response to the stimulant effects of an acute methylphenidate challenge (Hewitt et al., 2005). Here we further characterise the behavioural profile of withdrawal from sub-chronic treatment with GBR 12909 in juvenile rats. To this extent we employed the use of locomotor activity paradigm to examine hyperactivity. The novel object discrimination test examined changes and differences in learning and memory, which may also reflect alterations in attention, in the GBR 12909 treated animals compared to controls. This test, which utilises the amount of time a rodent spends exploring different objects (familiar and novel), is a becoming increasingly popular for its powerful assessment of complex behaviours relying on attentional processes and the consolidation of memory without placing undue stress on the animal. Barnett et al. (2005) have reported that ADHD children have impaired memory and attention for visuo-spatial information Finally prepulse inhibition (PPI) of the startle response was used to assess any possible alterations in response following treatment regime as this test is sensitive to dopaminergic manipulation. PPI was also used as a negative control, it has been reported that ADHD patients exhibit no fundamental alteration in PPI (Feifel et al., 2009) so we would hypothesis there should be no alteration in the startle response in this rodent model of ADHD.

Section snippets

Animals

Male Lister hooded rats age post natal day (PND) 24 at the start of each experiment were used. For all behavioural experiments animals were obtained from Charles River UK and Biomedical Services Unit, University of Nottingham (a colony derived from Charles River UK stock). Animals were housed in groups of four, in mixed treatment groups, and maintained on a 12/12 h light/dark cycle (lights on at 07.00 h) with a red light providing illumination during the dark period. Temperature was maintained

Body weight during treatment regime

GBR 12909 significantly reduced body weight gain in rats during the period of drug treatment (PND 24–27) compared to vehicle controls (Fig. 1) such that there was a main effect of drug [F(1,88) = 69.30, p < 0.0001], and a drug × time course interaction [F(3,88) = 3.964, p = 0.0106]. Thus GBR 12909 treated rats had significantly lower body weights, on days 2 (p < 0.001), 3 and 4 (both p < 0.0005) of treatment. There was a similar trend for all groups of rats in the different experiments. Even

Discussion

The aim of the present study was to further characterise a novel animal model of ADHD, produced by developmental manipulation of the dopamine transporter protein. This was achieved by sub-chronic treatment with followed by withdraw from the irreversible dopamine transporter inhibitor, GBR 12909. The aim was to produce a rebound increase in DAT synthesis following acute irreversible inhibition and thus a subsequent state of hypodopaminergic function, as previously reported (Hewitt et al., 2005).

Acknowledgements

KNH was funded by a PPP Healthcare Trust grant. Kind donation of GBR 12909 by Novo Nordisk and financial donation of GBR 12909 was made by GSK. The authors would like to thank Mrs Magrit Mitchell for care of the animals. Mr Ian Topham and Susan Ramsey for assistance with behavioural studies and Dr Marie Pardon for comments. Sources of support: Grant support from PPP healthcare trust. Financial support for purchase of GBR 12909 from Dr Declan Jones at Glaxo Smith Kline and Novo Nordisk for kind

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