- Title
- Molecular correlates of dopamine signalling in addiction vulnerability
- Creator
- Brown, Amanda Louise
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2011
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The work presented in this thesis takes a unique approach to extend our understanding of the molecular signatures within the dopamine signalling system that are associated with the development of addiction-like behaviours and relapse to drug-seeking. This study integrates a self-administration animal model, which closely recapitulates behaviours observed in human addiction, with targeted gene expression analysis. By adapting procedures from Deroche-Gamonet et al. (2004), the model employed here characterises individual behavioural differences to determine whether animals are addiction and relapse-vulnerable or –resilient. Dysregulation of synaptic plasticity and its associated processes are thought to be involved in long-term relapse susceptibility. Therefore, using these behaviourally phenotyped animals, the expression of a selected set of synaptic plasticity-related genes was assessed in the ventral and dorsal striatum, and in midbrain dopamine neurons, which project to these striatal subregions. The striatum and midbrain are brain regions critically implicated in the development of addiction and relapse. The first part of the studies focussed on the striatum. Within this region, it was found that transcript levels for dopamine receptors, and several genes encoding proteins that affect or are involved in the synaptic plasticity processes of long-term potentiation and depression, were significantly lower in addiction and relapse-vulnerable animals compared to -resilient controls. These synaptic plasticity related transcripts included: ionotropic α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) and group I metabotropic glutamate receptors, protein kinase C and cGMP-dependent protein kinase II. The mammalian target of rapamycin (mTOR) and the activity-regulated cytoskeletal (Arc) genes, which encode proteins involved in the regulation of AMPA receptor subunit composition, also displayed reduced expression in the ventral and dorsal striatum of addiction and relapse-vulnerable animals. Furthermore, expression of the transcript encoding the upstream regulator of mTOR, phosphatidylinositol 3 kinase (PI3K), was also decreased. In the second major part of the studies, the midbrain dopamine neurons, which provide dopaminergic input to both the dorsal and ventral striatum, were the focus. To allow examination of the molecular profile of synaptic plasticity-related transcripts specifically within these dopamine neurons, a novel immuno-laser-microdissection method had to be developed xi and validated, that made possible the identification and extraction of dopamine neurons from surrounding brain tissue. This high-salt buffer protocol yielded a sample that not only retained its RNA integrity, a prerequisite to allow meaningful interpretation of genomic investigations, but it also contained an enriched and relatively pure population of dopamine neurons. This methodology was then applied to midbrain cryosections obtained from rats phenotyped for addiction and relapse-vulnerability. Distinct profiles were observed following qPCR based gene expression analysis, when addiction and relapse-vulnerable animals were compared to –resilient animals. Most notable, was a change in the expression of the transcript encoding for brain derived neurotrophic factor, BDNF, a known addiction-related signalling molecule. Within VTA dopamine neurons, BDNF gene expression was significantly lower, whereas within SN dopamine neurons, it was significantly higher in vulnerable rats relative to resilient controls. Additionally, transcripts encoding for AKT - a key modulator within the PI3K-AKT-mTOR signalling pathway that is implicated in addiction, and for the dopamine D2 receptor, were significantly lower in the SN dopamine neurons of the addiction and relapse-vulnerable rats. Remarkably, despite indistinguishable cocaine consumption between relapse-vulnerable and –resilient rats, addiction-like behaviours were only evident in the –vulnerable group. Also, altered molecular profiles were identified two months after the start of cocaine abstinence, suggesting that they likely reflect persistent neuroadaptive changes rather than direct actions of the drug. Furthermore, the studies presented in this thesis are the first to utilize a model that behaviourally phenotyped animals as relapse-vulnerable or -resilient to examine the long-lasting expression of synaptic plasticity-related genes in both the ventral and dorsal striatum and, additionally, in specifically isolated VTA and SN dopamine neurons using immuno-laser-microdissection. Importantly, it should be noted that these data demonstrating altered gene expression within the dorsal striatum and SN dopamine neurons support emerging evidence of a crucial role for the dorsal striatum in the compulsive drug-taking behaviours and habitual components of addiction. By employing an approach that incorporates individual differences to elucidate underlying neural correlates, the work presented in this thesis is an important addition to the field as it endeavours to further our understanding of the neurobiology of addiction and relapse, and provide novel targets for future and potentially more efficacious pharmacotherapies.
- Subject
- dopamine signalling; addiction; relapse; gene expression; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/932650
- Identifier
- uon:11414
- Rights
- Copyright 2011 Amanda Louise Brown
- Language
- eng
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