Imprinted genes and neuroendocrine function

Imprinted genes are monoallelically expressed in a parent-of-origin dependent manner. Whilst the full functional repertoire of these genes remains obscure, they are generally highly expressed in the brain and are often involved in fundamental neural processes. Besides influencing brain neurochemistry, imprinted genes are important in the development and function of the hypothalamus and pituitary gland, key sites of neuroendocrine regulation. Moreover, imprinted genes may directly modulate hormone-dependent signalling cascades, both in the brain and elsewhere. Much of our knowledge about imprinted gene function has come from studying knockout mice and human disorders of imprinting. One such disorder is Prader-Willi syndrome, a neuroendocrine disorder characterised by hypothalamic abnormalities and aberrant feeding behaviour. Through examining the role of imprinted genes in neuroendocrine function, it may be possible to shed light on the neurobiological basis of feeding and aspects of social behaviour and underlying cognition, and to provide insights into disorders where these functions go awry.

Epigenetics: what is it and why is it important to mental disease?

INTRODUCTION

The chemical marking of the DNA and surrounding histone proteins represent some of the means by which gene expression is controlled. Many of these epigenetic modifications are pre-programmed and are an important part of the control of development.

SOURCES OF DATA

There is an accumulating body of evidence from clinical genetics and animal work that suggests some epigenetic processes may also be labile.

AREAS OF AGREEMENT

A number of these studies have demonstrated that the epigenetic status of genes can be altered through environmental events such as in vitro culture of embryos and exposure to toxins, sometimes resulting in disease.

AREAS OF CONTROVERSY

More routine variations in life events may also be encoded by changes in the epigenetic status of genes, and as such these processes may provide a mechanism mediating interplay between genes and the environment, including the now recognized idea of gene-environment interactions.

GROWING POINTS AND AREAS TIMELY FOR DEVELOPING RESEARCH

The significance of epigenetics for mental disease is becoming increasingly clear. It is important that the techniques developed to analyse the epigenome are now applied to the study of the molecular basis of mental disease to assess the contribution of gene-environment interactions to brain function.

What are imprinted genes doing in the brain?

As evidence for the existence of brain-expressed imprinted genes accumulates, we need to address exactly what they are doing in this tissue, especially in terms of organisational themes and the major challenges posed by reconciling imprinted gene action in brain with current evolutionary theories attempting to explain the origin and maintenance of genomic imprinting. We are at the beginning of this endeavor and much work remains to be done but already it is clear that imprinted genes have the potential to influence diverse behavioral processes via multiple brain mechanisms. There are also grounds to believe that imprinting may contribute to risk of mental and neurological disease. As well as being a source of basic information about imprinted genes in the brain (e.g., via the newly established website, www.bgg.cardiff.ac.uk/imprinted_tables/index. html), we have used this chapter to identify and focus on a number of key questions. How are brain-expressed imprinted genes organised at the molecular and cellular levels? To what extent does imprinted action depend on neurodevelopmental mechanisms? Do imprinted gene effects interact with other epigenetic influences, especially early on in life? Are imprinted effects on adult behaviors adaptive or just epiphenomena? If they are adaptive, what areas of brain function and behavior might be sensitive to imprinted effects? These are big questions and, as shall become apparent, we need much more data, arising from interactions between behavioral neuroscientists, molecular biologists and evolutionary theorists, if we are to begin to answer them.

What are imprinted genes doing in the brain?

As evidence for the existence of brain-expressed imprinted genes accumulates, we need to address exactly what they are doing in this tissue, especially in terms of organizational themes and the major challenges posed by reconciling imprinted gene action in brain with current evolutionary theories attempting to explain the origin and maintenance of genomic imprinting. We are at the beginning of this endeavor and much work remains to be done but already it is clear that imprinted genes have the potential to influence diverse behavioral processes via multiple brain mechanisms. There are also grounds to believe that imprinting may contribute to risk of mental and neurological disease. As well as being a source of basic information about imprinted genes in the brain (e.g., via the newly established website, www.bgg.cardiff.ac.uk/imprinted_tables/index.html), we have used this chapter to identify and focus on a number of key questions. How are brain-expressed imprinted genes organised at the molecular and cellular levels? To what extent does imprinted action depend on neurodevelopmental mechanisms? Do imprinted gene effects interact with other epigenetic influences, especially early on in life? Are imprinted effects on adult behaviors adaptive or just epiphenomena? If they are adaptive, what areas of brain function and behavior might be sensitive to imprinted effects? These are big questions and, as shall become apparent, we need much more data, arising from interactions between behavioral neuroscientists, molecular biologists and evolutionary theorists, if we are to begin to answer them.

X-monosomy effects on visuospatial attention in mice: a candidate gene and implications for Turner syndrome and attention deficit hyperactivity disorder

BACKGROUND

The loss of all, or part of an X chromosome, in Turner syndrome (TS, 45,XO) results in deficits in attentional functioning.

METHODS

Using a 39,XO mouse model, we tested the hypothesis that X-monosomy and/or parental origin of the single X chromosome may influence visuospatial attentional functioning in a 5-choice serial reaction time task (5-CSRTT).

RESULTS

Under attentionally demanding conditions 39,XO mice displayed impaired discriminative response accuracy and slowed correct reaction times relative to 40,XX mice; these deficits were alleviated in a version of the task with reduced attentional demands. Parental origin of the X did not affect performance of the 5-CSRTT. In contrast, the attentional phenotype was rescued in 40,XY*X mice possessing a single maternally inherited X chromosome and a small Y*X chromosome that comprises a complete pseudoautosomal region (PAR), and a small X-specific segment.

CONCLUSIONS

Our findings are consistent with an X-monosomy effect on attention and suggest the existence of X-linked gene(s) that escape X-inactivation, are present on the small Y*X chromosome and impact on attentional functioning; the strongest candidate gene is Sts, encoding steroid sulfatase. The data inform the TS literature and indicate novel genetic mechanisms that may be of general significance to the neurobiology of attention.

Impairments in impulse control in mice transgenic for the human FTDP-17 tau V337M mutation are exacerbated by age

Abnormalities in microtubule-associated tau protein are a key neuropathological feature of both Alzheimer's disease and many frontotemporal dementias (FTDs), including hereditary FTD with Parkinsonism linked to chromosome 17 (FTDP-17). In these disorders, tau becomes aberrantly phosphorylated, leading to the development of filamentous neurofibrillary tangles in the brain. Here we report, in a longitudinal ageing study, the sensorimotor and cognitive assessment of transgenic mice expressing the human tau(V337M) ('Seattle Family A') FTDP-17 mutation, which we have previously shown to demonstrate abnormalities in brain tau phosphorylation. The data indicated highly specific effects of transgene expression on the ability to withhold responding in a murine version of the 5-choice serial reaction time task, behaviour consistent with deficits in impulse control. Ageing exacerbated these effects. In young tau(V337M) mice, increased impulsivity was present under task conditions making inhibition of premature responding more difficult (longer inter-trial intervals) but not under baseline conditions. However, when older, the tau(V337M) mice showed further increases in premature responding, including under baseline conditions. These impulse control deficits were fully dissociable from sensorimotor or motivation effects on performance. The findings recapitulate core abnormalities in impulsive responding observed in both frontal variant FTD and FTDP-17 linked to the tau(V337M) mutation in humans.

Genomic imprinting and the social brain

Genomic imprinting refers to the parent-of-origin-specific epigenetic marking of a number of genes. This epigenetic mark leads to a bias in expression between maternally and paternally inherited imprinted genes, that in some cases results in monoallelic expression from one parental allele. Genomic imprinting is often thought to have evolved as a consequence of the intragenomic conflict between the parental alleles that occurs whenever there is an asymmetry of relatedness. The two main examples of asymmetry of relatedness are when there is partiality of parental investment in offspring (as is the case for placental mammals, where there is also the possibility of extended postnatal care by one parent), and in social groups where there is a sex-biased dispersal. From this evolutionary starting point, it is predicted that, at the behavioural level, imprinted genes will influence what can broadly be termed bonding and social behaviour. We examine the animal and human literature for examples of imprinted genes mediating these behaviours, and divide them into two general classes. Firstly, mother-offspring interactions (suckling, attachment and maternal behaviours) that are predicted to occur when partiality in parental investment in early postnatal offspring occurs; and secondly, adult social interactions, when there is an asymmetry of relatedness in social groups. Finally, we return to the evolutionary theory and examine whether there is a pattern of behavioural functions mediated by imprinted genes emerging from the limited data, and also whether any tangible predictions can be made with regards to the direction of action of genes of maternal or paternal origin.

It is not all hormones: Alternative explanations for sexual differentiation of the brain

Males and females of many species differ with regard to neurodevelopment, ongoing brain function and behavior. For many years, it was assumed that these differences primarily arose due to hormonal masculinization of the male brain (and to a lesser extent hormonal feminization of the female brain). Recent elegant experiments in model systems have revealed that, while gonadal hormones undoubtedly play an important role in sexual differentiation of the brain, they are not the only possible mechanism for this phenomenon. In the present review, we discuss the concept that genes residing upon the sex chromosomes (which are asymmetrically inherited between males and females) may influence sexually dimorphic neurobiology directly, and suggest possible mechanisms. Future work will be directed towards understanding the extent and specificity with which sex-linked genes and hormones define brain structure and function, and towards elucidating potential interactions between the two mechanisms. Ultimately, it is hoped that such studies will provide insights into why men and women are differentially vulnerable to certain mental disorders, and will enable the development of effective sex-tailored therapeutics.

Pathological changes in dopaminergic nerve cells of the substantia nigra and olfactory bulb in mice transgenic for truncated human alpha-synuclein(1-120): implications for Lewy body disorders

Dysfunction of the 140 aa protein alpha-synuclein plays a central role in Lewy body disorders, including Parkinson's disease, as well as in multiple system atrophy. Here, we show that the expression of truncated human alpha-synuclein(1-120), driven by the rat tyrosine hydroxylase promoter on a mouse alpha-synuclein null background, leads to the formation of pathological inclusions in the substantia nigra and olfactory bulb and to a reduction in striatal dopamine levels. At the behavioral level, the transgenic mice showed a progressive reduction in spontaneous locomotion and an increased response to amphetamine. These findings suggest that the C-terminal of alpha-synuclein is an important regulator of aggregation in vivo and will help to understand the mechanisms underlying the pathogenesis of Lewy body disorders and multiple system atrophy.

A comparison of multiple 5-HT receptors in two tasks measuring impulsivity

Impulsivity has often been assumed to be a unitary construct. However dissociable forms of impulsive behaviour may exist, each with distinct neurochemical underpinnings. To test this hypothesis, behavioural effects of three partially selective serotonergic (5-HT) ligands, ketanserin (5-HT2(A, C) receptor antagonist), SER-082 (5-HT2(C, B) receptor antagonist) and SB-270146-A (5-HT6 receptor antagonist) were compared in two tests of impulsivity. The five-choice serial reaction time task (5-csrtt) and a delayed reward task were chosen as they measure theoretically different types of impulsivity, behavioural inhibition versus choice preference for a delayed reward. Dissociation was seen between the effects of ketanserin, which decreased impulsivity in the 5-csrtt, but had no effect on the delayed reward task, and SER-082, which had no effect on the 5-csrtt, but decreased impulsive responding in the delayed reward task. SB-270146-A had no effect in either paradigm. The results suggest that the 5-csrtt and the delayed reward task do in fact measure different types of impulsive behaviour, which are at least partially neurochemically distinct.

An mTph2 SNP gives rise to alterations in extracellular 5-HT levels, but not in performance on a delayed-reinforcement task

5-Hydroxytryptamine (5-HT) is an important neurotransmitter mediating many aspects of cognition and behaviour. One psychology in which 5-HT plays an important role is impulsive responding. Recently, we have demonstrated that variation in an aspect of impulsive behaviour, namely delayed gratification, has a clear genetic contribution. Here, we examined the neurobiological relevance of a recently discovered single nucleotide polymorphism (SNP) in the murine gene tryptophan hydroxylase (mTph2) by analysing extracellular levels of 5-HT in medial prefrontal cortex (mPFC) and ventral striatum (VS), key brain regions for impulsive behaviours. The allelic variants were associated with systematic effects on baseline 5-HT efflux in the mPFC and VS. We then went on to examine whether the mTph2 allelic variants gave rise to differences in impulsive behaviour. However, the mTph2 genotype, and therefore presumably baseline brain levels of 5-HT, did not predict impulsive choice, as indexed by sensitivity to delayed reinforcement. Consequently, the data do not support a role for the mTph2 C1473G polymorphism on this aspect of impulsive behaviour. Instead, they indicate that perturbations of the 5-HT system via heritable traits may have differential consequences for qualitatively distinct aspects of impulsive behaviour.

Increasing the diagnosis of multifocal primary breast cancer by the use of bilateral whole-breast ultrasound

AIM

The aim of this study was to evaluate the contribution of bilateral whole-breast ultrasound (BBUS) to the diagnosis and management of women with newly diagnosed breast cancer.

METHODS

Over a period of 6 months, 102 women presenting with breast cancer underwent BBUS. Data were collected on clinical findings, radiology, histology and surgical outcome. These women were compared with a control group of 124 women presenting over a similar 6-month period 1 year previously, who had undergone targeted breast ultrasound.

RESULTS

Multicentric/multifocal tumours were demonstrated in 35 (34%) of the 102 participants and in 18 (15%) of the 124 controls, a statistically significant difference (Fisher's exact test, p = 0.001). Multiple tumours were diagnosed preoperatively in 18% of the study population compared with 8% of the controls, and BBUS identified invasive multifocal/multicentric tumours in significantly more women in the study population (11 versus 1 control) (Fisher's exact test, p = 0.019). Contralateral cancer was diagnosed in 4 women in the study population and none in the control population (Fisher's exact test, p = 0.040). Surgical review showed that the surgical management changed significantly in 8% (95% confidence interval 4 to 14%) of cases in the study population following BBUS. The increase in the number of women undergoing benign biopsies in the study population (10 versus 5 controls) was not statistically significant (Fisher's exact test, p = 0.11).

CONCLUSION

BBUS increased the preoperative diagnosis of multiple tumours in women presenting with primary breast cancer, resulting in a management change in 8% of cases.

Imprinted Nesp55 influences behavioral reactivity to novel environments

Genomic imprinting results in parent-of-origin-dependent monoallelic expression of selected genes. Although their importance in development and physiology is recognized, few imprinted genes have been investigated for their effects on brain function. Gnas is a complex imprinted locus whose gene products are involved in early postnatal adaptations and neuroendocrine functions. Gnas encodes the stimulatory G-protein subunit Gsalpha and two other imprinted protein-coding transcripts. Of these, the Nesp transcript, expressed exclusively from the maternal allele, codes for neuroendocrine secretory protein 55 (Nesp55), a chromogranin-like polypeptide associated with the constitutive secretory pathway but with an unknown function. Nesp is expressed in restricted brain nuclei, suggesting an involvement in specific behaviors. We have generated a knockout of Nesp55 in mice. Nesp55-deficient mice develop normally, excluding a role of this protein in the severe postnatal effects associated with imprinting of the Gnas cluster. Behavioral analysis of adult Nesp55 mutants revealed, in three separate tasks, abnormal reactivity to novel environments independent of general locomotor activity and anxiety. This phenotype may be related to prominent Nesp55 expression in the noradrenergic locus coeruleus. These results indicate a role of maternally expressed Nesp55 in controlling exploratory behavior and are the first demonstration that imprinted genes affect such a fundamental behavior.

Common genetic effects on variation in impulsivity and activity in mice

Impulsivity is a complex psychological construct that impacts on behavioral predispositions in the normal range and has been shown to have a genetic element through the examination of hereditary patterns of abnormal conditions such as attention deficit/hyperactivity disorder and obsessive compulsive disorder. In this study, we took advantage of the isogenic nature of inbred strains of mice to determine the contribution of genes to impulsive behaviors by examining the performance of four separate mouse strains in a novel murine delayed-reinforcement paradigm, during which the animals had to choose between rewards that were relatively small but available immediately and larger but progressively delayed rewards. To control for maternal effects, all the mice were cross-fostered to a common strain immediately after birth. Under these conditions, we found significant differences between the strains on behaviors indexing impulsive choice and on independent measures of locomotor activity, which subsequent heritability analysis showed could be related, in part, to genetic effects. Moreover, the two aspects of behavior were found to co-vary, with the more active animals also displaying more impulsive behavior. This was not attributable to mundane confounds related to individual task requirements but instead indicated the existence of common genetic factors influencing variation in both impulsivity and locomotor activity. The data are discussed in terms of the coexistence of impulsivity and hyperactivity, interactions between environmental and genetic effects, and possible candidate genes.

Imprinted gene expression in the brain

In normal mammals, autosomal genes are present in duplicate (i.e. two alleles), one inherited from the father, and one from the mother. For the majority of genes both alleles are transcribed (or expressed) equally. However, for a small subset of genes, known as imprinted genes, only one allele is expressed in a parent-of-origin dependent manner (note that the 'imprint' here refers to the epigenetic mechanism through which one allele is silenced, and is completely unrelated to classical 'filial imprinting' manifest at the behavioural level). Thus, for some imprinted genes expression is only (or predominantly) seen from the paternally inherited allele, whilst for the remainder, expression is only observed from the maternally inherited allele. Early work on this class of genes highlighted their importance in gross developmental and growth phenotypes. Recent studies in mouse models and humans have emphasised their contribution to brain function and behaviour. In this article, we review the literature concerning the expression of imprinted genes in the brain. In particular, we attempt to define emerging organisation themes, especially in terms of the direction of imprinting (i.e. maternal or paternal expression). We also emphasise the likely role of imprinted genes in neurodevelopment. We end by pointing out that, so far as discerning the precise functions of imprinted genes in the brain is concerned, there are currently more questions than answers; ranging from the extent to which imprinted genes might contribute to common mental disorders, to wider issues related to how easily the new data on brain may be accommodated within the dominant theory regarding the origins and maintenance of imprinting, which pits the maternal and paternal genomes against each other in an evolutionary battle of the sexes.

Increased tau phosphorylation on mitogen-activated protein kinase consensus sites and cognitive decline in transgenic models for Alzheimer's disease and FTDP-17: evidence for distinct molecular processes underlying tau abnormalities

Abnormal tau phosphorylation occurs in several neurodegenerative disorders, including Alzheimer's disease (AD) and frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). Here, we compare mechanisms of tau phosphorylation in mouse models of FTDP-17 and AD. Mice expressing a mutated form of human tau associated with FTDP-17 (tau(V337M)) showed age-related increases in exogenous tau phosphorylation in the absence of increased activation status of a number of kinases known to phosphorylate tau in vitro. In a "combined" model, expressing both tau(V337M) and the familial amyloid precursor protein AD mutation APP(V717I) in a CT100 fragment, age-dependent tau phosphorylation occurred at the same sites and was significantly augmented compared to "single" tau(V337M) mice. These effects were concomitant with increased activation status of mitogen-activated protein kinase (MAPK) family members (extracellular regulated kinases 1 and 2, p38, and c-Jun NH(2)-terminal kinase) but not glycogen synthase kinase-3alphabeta or cyclin-dependent kinase 5. The increase in MAPK activation was a discrete effect of APP(V717I)-CT100 transgene expression as near identical changes were observed in single APP(V717I)-CT100 mice. Age-dependent deficits in memory were also associated with tau(V337M) and APP(V717I)-CT100 expression. The data reveal distinct routes to abnormal tau phosphorylation in models of AD and FTDP-17 and suggest that in AD, tau irregularities may be linked to processing of APP C-terminal fragments via specific effects on MAPK activation status.

X-linked imprinting: effects on brain and behaviour

Imprinted genes are monoallelically expressed in a parent-of-origin-dependent manner and can affect brain and behavioural phenotypes. The X chromosome is enriched for genes affecting neurodevelopment and is donated asymmetrically to male and female progeny. Hence, X-linked imprinted genes could potentially influence sexually dimorphic neurobiology. Consequently, investigations into such loci may provide new insights into the biological basis of behavioural differences between the sexes and into why men and women show different vulnerabilities to certain mental disorders. In this review, we summarise recent advances in our knowledge of X-linked imprinted genes and the brain substrates that they may act upon. In addition, we suggest strategies for identifying novel X-linked imprinted genes and their downstream effects and discuss evolutionary theories regarding the origin and maintenance of X-linked imprinting.

Expression patterns of the novel imprinted genes Nap1l5 and Peg13 and their non-imprinted host genes in the adult mouse brain

Recent work has implicated imprinted gene functioning in neurodevelopment and behaviour and defining the expression patterns of these genes in brain tissue has become a key prerequisite to establishing function. In this work we report on the expression patterns of two novel imprinted loci, Nap1l5 and Peg13, in adult mouse brain using in situ hybridisation methods. Nap1l5 and Peg13 are located, respectively, within the introns of the non-imprinted genes Herc3 and the Tularik1 (T1)/KIAA1882 homologue in two separate microimprinted domains on mouse chromosomes 6 and 15. These 'host' genes are highly expressed in brain and consequently we were interested in assessing their expression patterns in parallel to the imprinted genes. The brain expression of all four genes appeared to be mainly neuronal. The detailed expression profiles of Nap1l5 and Peg13 were generally similar with widespread expression that was relatively high in the septal and hypothalamic regions, the hippocampus and the cerebral cortex. In contrast, there was some degree of dissociation between the imprinted genes and their non-imprinted hosts, in that, whilst there was again widespread expression of Herc3 and the T1/KIAA1882 homologue, these genes were also particularly highly expressed in Purkinje neurons and piriform cortex. We also examined expression of the novel imprinted genes in the adrenal glands. Nap1l5 expression was localised mainly to the adrenal medulla, whilst Peg13 expression was observed more generally throughout the adrenal medulla and the outer cortical layers.

Effects on fear reactivity in XO mice are due to haploinsufficiency of a non-PAR X gene: implications for emotional function in Turner's syndrome

Recent work has indicated altered emotional functioning in Turner's syndrome (TS) subjects (45,XO). We examined the role of X-chromosome deficiency on fear reactivity in X-monosomic mice (39,XO), and found that they exhibited anxiogenic behaviour relative to normal females (40,XX). A molecular candidate for this effect is Steroid sulfatase (Sts) as this is located in the pseudoautosomal region (PAR) of the X-chromosome and consequently is normally biallelically expressed. In addition, the steroid sulfatase enzyme (STS) is putatively linked to fear reactivity by an effect on GABAA receptors via the action of neurosteroids. Real-time PCR demonstrated that levels of Sts mRNA were reduced by half in the brains of 39,XO mice compared with 40,XX, and that expression levels of a number of GABAA subunits previously shown to be important components of fear processing (Gabra3, Gabra1 and Gabrg2) were also altered. However, 40,XY*X mice, in which the Y*X is a small chromosome comprising of a complete PAR and a small non-PAR segment of the X-chromosome, exhibited the same pattern of fear reactivity behaviour as 39,XO animals, but equivalent expression levels of Sts, Gabra1, Gabra3 and Gabrg2 to 40,XX females. This showed that although Sts may cause alterations in GABAA subunit expression, these changes do not result in increased fear reactivity. This suggests an alternative X-chromosome gene, that escapes inactivation, is responsible for the differences in fear reactivity between 39,XO and 40,XX mice. These findings inform the TS data, and point to novel genetic mechanisms that may be of general significance to the neurobiology of fear.

Effects of acute and chronic buspirone on impulsive choice and efflux of 5-HT and dopamine in hippocampus, nucleus accumbens and prefrontal cortex

RATIONALE

Reduced central serotonin (5-HT) activity has been associated with impulsive choice behaviour, but there is no consensus about the precise nature of these effects. Behavioural and neurochemical effects of 5-HT(1A) agonists such as buspirone depend critically on the dose and the duration of treatment. We thus undertook a parametric study of the effects of acute and chronic buspirone on the performance on a test of delayed gratification, as well as on the efflux of serotonin and dopamine (DA) in cortical and subcortical regions in rats.

OBJECTIVES

Three experiments examined (i) the effects of acute buspirone on impulsive choice and how such effects were modified by prior chronic exposure to buspirone; (ii) the effects of chronic buspirone on impulsive choice; (iii) the effects on impulsive choice of a selective 5-HT(1A) antagonist, WAY-100635 tested alone and in combination with buspirone; (iv) the effects of chronic and acute buspirone on 5-HT and DA efflux in anaesthetised rats.

METHODS

In experiment 1, rats previously trained on the delayed gratification task were tested with acute buspirone (0.5, 1 and 2 mg/kg). The same rats were then treated with chronic buspirone (1 mg/kg/day) over the next 65 days, and the effects of acute buspirone (1 mg/kg) re-determined at 20, 45 and 65 days of chronic treatment. In experiment 2, two groups of rats trained on the delayed gratification task were treated either with saline or buspirone (1 mg/kg/day) continually for 65 days before being tested with acute buspirone (1 mg/kg), WAY-100635 (0.08 mg/kg), or a combination of the two drugs. In experiment 3, rats received the same regimen of buspirone dosing as in experiment 2, before receiving in-vivo microdialysis for 5-HT and DA in the ventral hippocampus, nucleus accumbens and medial prefrontal cortex.

RESULTS

Acute buspirone dose dependently increased the choice for the small, immediate reinforcer (impulsive choice) but the effects of 1 mg/kg were reversed on chronic administration of buspirone. This increased choice of the large, delayed reinforcer, which was not accompanied by any changes in baseline (non-drugged) performance, was blocked by the 5-HT(1A) receptor antagonist WAY-100635. The chronic buspirone regimen did not alter buspirone-evoked reductions in 5-HT efflux in hippocampus but did lead to a differential effect of acute buspirone in medial prefrontal cortex, with the chronic buspirone and saline groups exhibiting decreases and increases in efflux, respectively. There were no systematic changes in DA efflux under any condition.

CONCLUSIONS

These findings show that the effects of acute buspirone on impulsive choice are reversed following chronic treatment and are mediated by 5-HT(1A) receptors, and suggest, in addition, that the behavioural effects may involve changes in 5-HT functioning in medial prefrontal cortex.

Measuring impulsivity in mice using a novel operant delayed reinforcement task: effects of behavioural manipulations and d-amphetamine

RATIONALE

The increasing use of genetically modified mice to probe genetic contributions to normal and abnormal behaviours requires the development of sensitive and selective behavioural tasks.

OBJECTIVES

To develop a discrete trial assay of impulsivity (delayed reinforcement) that is tractable in mice utilising a mouse operant nine-hole box apparatus and to specify the task with respect to behavioural and pharmacological manipulations.

METHODS

Mice were trained to respond with a nose-poke to one of two visual stimuli; one response resulted in a small quantity of reinforcer, the other in a larger quantity of reinforcer. As the session proceeded increasing delay was introduced onto the response leading to the large reward. Hence, the nature of the choice was a small quantity of reinforcer immediately versus a larger but progressively delayed amount of reinforcer. At stable baseline performance the mice were challenged with a variety of task manipulations and systemic d-amphetamine in order to discern aspects of the underlying psychological and neurochemical substrates of the choice behaviour.

RESULTS

The mice showed a systematic shift in responding away from the large reinforcer with increasing delay (0, 2, 4, 8, 12 s), such that at the longest delay >80% of nose-pokes were for the smaller, immediate reinforcer. Task manipulations indicated that behaviour was controlled in a trial discrete manner by the contingency between delay and reward and was not due to non-specific factors such as satiation. d-Amphetamine had complex, dose dependent effects on choice behaviour which revealed dissociations between impulsive choice and hyperactivity.

CONCLUSIONS

We have successfully developed an assay of impulsivity in mice that will be of utility to examine impulsive behaviours and their genetic substrates. In addition, our data provided evidence of distinct dopaminergic mechanisms mediating aspects of impulsivity and hyperactivity.

Somatostatin receptor 2 knockout/lacZ knockin mice show impaired motor coordination and reveal sites of somatostatin action within the striatum

The peptide somatostatin can modulate the functional output of the basal ganglia. The exact sites and mechanisms of this action, however, are poorly understood, and the physiological context in which somatostatin acts is unknown. Somatostatin acts as a neuromodulator via a family of five 7-transmembrane G protein-coupled receptors, SSTR1-5, one of which, SSTR2, is known to be functional in the striatum. We have investigated the role of SSTR2 in basal ganglia function using mice in which Sstr2 has been inactivated and replaced by the lacZ reporter gene. Analysis of Sstr2lacZ expression in the brain by beta-galactosidase histochemistry demonstrated a widespread pattern of expression. By comparison to previously published in situ hybridization and immunohistochemical data, Sstr2lacZ expression was shown to accurately recapitulate that of Sstr2 and thus provided a highly sensitive model to investigate cell-type-specific expression of Sstr2. In the striatum, Sstr2 expression was identified in medium spiny projection neurons restricted to the matrix compartment and in cholinergic interneurons. Sstr2 expression was not detected in any other nuclei of the basal ganglia except for a sparse number of nondopaminergic neurons in the substantia nigra. Microdialysis in the striatum showed Sstr2-null mice were selectively refractory to somatostatin-induced dopamine and glutamate release. In behavioural tests, Sstr2-null mice showed normal levels of locomotor activity and normal coordination in undemanding tasks. However, in beam-walking, a test of fine motor control, Sstr2-null mice were severely impaired. Together these data implicate an important neuromodulatory role for SSTR2 in the striatum.