Pause for thought: The role of corticostriatal circuitry and its dopamine innervation in the inhibitory modulation of associative learning

The nuts and bolts of everyday thinking, provided by the formation of connections between related events, enable us to establish clear trains of thought. Associative learning mechanisms, common to all animals, support not only the cueing of thoughts, words and deeds, but also the ability to hesitate through the use of a 'mental brake' to suppress inappropriate thoughts and actions. When the ability to restrain the impulse generated by an association is lost, diverse symptoms may result. Clinically, impaired inhibitory modulation has been identified as contributing to a number of disorders, including addiction, anxiety, obesity and schizophrenia. Disinhibited behaviours are also a well-documented feature of the cognitive decline characteristic of normal ageing.

Learning procedures developed in the laboratory rat provide an excellent model system to study the gating of unwanted associations. If a particular event predicts an outcome, learning is normally demonstrated by the animal's behavioural reaction to the first event. However, if the first event is presented in conjunction with another cue which means that the expected outcome will not now occur, the normal behavioural and cognitive reactions are inhibited. Impairments in such inhibition could explain a variety of symptoms, from over-eating when the consequences of eating more food will no longer be pleasant, to some of the disordered thought patterns identified with schizophrenia. We propose to use translational procedures in the rat, adapting the experimental design which we have used to test inhibitory learning in humans, in order to delineate the brain areas involved in inhibitory learning. The experimental brain treatments will be precisely targeted to areas known to be important for behavioural regulation and which receive projections containing the brain chemical dopamine implicated in the modulation of inhibitory learning. Highly controlled experiments of the kind proposed are necessary to identify the mechanisms underlying inhibitory learning deficits.

We will (1) examine the effects of temporarily switching off small areas of cortex and interconnected areas (injecting a shortlived inactivating drug); (2) test for cross-talk in the patterns of electrical activity in interconnected brain areas; (3) use drug treatments to selectively manipulate the brain chemical dopamine, to determine the precise pathways through which inhibitory learning is modulated; and (4) examine how the observed behavioural effects depend on cross-talk between interconnected brain regions.

The present project will advance previous findings in that we will selectively interfere with chemical signalling in specific brain pathways, by targeted drug delivery to areas first identified by temporarily inactivating small brain regions. One unique contribution of the present project will be the delineation of the role of the chemical modulator dopamine in the neural circuitry necessary for inhibitory learning. Other studies in our laboratory have shown that localised drug treatments with some selectivity to particular sites of action can have distinct effects on behaviour, often quite different to the effects of general brain damage in the same brain regions. Importantly, the behavioural procedures to be used in this project work in humans too; although the experimental details differ, there is sufficient similarity to translate findings from the animal laboratory to the clinic (and vice versa). Experiments in rats delineate the brain substrates of inhibitory learning and give vital clues as to where and how new treatments should work, with minimum side-effects.

In parallel with the experimental programme, we will visit schools and use sixth form outreach programmes, as well as liaise with mental health professionals and contacts in the pharmaceutical industry, to explain the importance of animal work of this kind, and how it translates to our understanding of human health.

Conditioned inhibition is a form of learning seen when an otherwise expected event does not occur in the presence of the inhibitor. Such inhibitory modulation is fundamental to many aspects of normal psychological function, such as the control of food intake, while impairments in this process underpin a wide variety of mental health conditions. We have found that this kind of inhibitory learning is impaired in humans with schizophrenia (as well as those with particular personality profiles) and have adapted the experimental design for use in rats, with a refined appetitive procedure, to investigate the role of the dopamine (DA) system and interconnected cortical structures, specifically medial prefrontal cortex (mPFC). We will use this translational task to analyse the role of corticostriatal DA in inhibitory modulation, with a view to developing novel therapeutic strategies. The mPFC and DA systems have been independently identified as being involved in aspects of inhibition. The proposed plan of work will advance on these findings by testing for dissociable effects in mPFC sub-regions. Moreover, the mPFC projects topographically to nucleus accumbens (NAc) in the striatum, which is also a functionally heterogenous structure. We will compare the effects of (1) regional inactivation and (2) electrophysiological profiles in NAc and the corresponding sub-regions in mPFC, to identify functional interactions between the two structures. We will go on to investigate the role of particular DA receptor sub-types in a series of micro-infusion studies at the coordinates identified in the regional inactivation studies. This will allow us to examine the modulatory role of DA in the key brain regions of interest. The proposed use of a translational task in a rat model allows for localised interventions (including disconnection studies using crossed unilateral infusions) to establish cause and effect, combined with correlational studies of interconnectivity of small sub-regions.

In addition to the academic beneficiaries described above, the research proposed here will have wider benefits. The workplan attachment (Gantt chart) shows how the proposed impact activities will be interleaved with the experimental milestones and scientific objectives over the course of the project.

PHARMACEUTICAL INDUSTRY: A wide variety of conditions are characterised by inhibition deficits of one kind or another. In some cases, inhibitory impairment is fundamental to the condition, in others it is symptomatic. Disinhibition is nonetheless troubling and potentially very disruptive to normal cognitive function. Prospectively, selective neural interventions will be developed as cognitive enhancers to counteract deficits in inhibitory modulation. Identifying the role of neuromodulation in inhibitory learning and its expression will be key to future development of selective treatment interventions either to increase activity or enhance neuroplasticity in identified brain circuitries, depending on the nature of the target impairment. Many of the scientific meetings we attend offer opportunities to discuss our findings with representatives of the pharmaceutical industry. In addition, Elaine Anderson, University of Nottingham Business Engagement and Innovation Services, is approaching a range of companies, to negotiate material transfer agreements that will enable us to test whether inhibitory learning deficits can be reversed by novel compounds of interest. Using the same experimental design in analogous behavioural tasks, we have conducted a series of studies of inhibitory learning in diverse human populations, some with diagnosed clinical conditions, also to examine the role of individual differences in the normal range. Data, of the kind to be generated by the present project, on the underlying neural substrates (in medial prefrontal cortex and nucleus accumbens) and neuropharmacology - examining effects at dopaminergic receptor sub-types - will tell us where targeted brain interventions should be applied (in principle, when they become available). For example, using selective regulation of gene expression in particular signalling pathways, as well as non-invasive treatments such as rTMS.

CHARITIES: The University of Nottingham has established links with the charitable sector and their representatives are invited to engagement events, such as 'Healthy Brains at Every Age' which was recently hosted by the School of Psychology in collaboration with Cambridge Cognition. Various mental health charities and other stakeholders will be invited to outreach and engagement events arranged over the course of the current project (see also Pathways to Impact).

HEALTH CARE PROFESSIONALS: The Nottingham University Hospital NHS Trust provides further points of contact to discuss the translational relevance of our findings.

MANAGED ANIMALS: Evidence-based welfare is fundamental to best practice. The proposed behavioural tests measure fundamental mechanisms of animal cognition, performance of which will be related to measures of neurochemical function. The rat is a good model system for a wide range of mammalian species.

SCHOOLS: Outreach activities are an invaluable part of our widening participation programme, to attract the best university applicants to our BSc courses, irrespective of background and prior expectations. Additionally, Understanding Animal Research school visits allow the benefits of essential animal research to be communicated to school children. The experiences provided by our outreach activities help prospective undergraduates make informed decisions about what to study at university. Young people also appreciate the opportunity to discuss the use of animals in research in relation to evidence, and to have their specific questions answered. Feedback suggests that this kind of activity is particularly effective when they have the opportunity to engage with younger researchers.