The complex interplay of heroin, reward and gene expression

Open Access
- Author:
- Mcfalls, Ashley Jane
- Graduate Program:
- Neuroscience
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 01, 2017
- Committee Members:
- Kent Eugene Vrana, Dissertation Advisor/Co-Advisor
Kent Eugene Vrana, Committee Chair/Co-Chair
Andras Hajnal, Committee Member
Patricia Sue Grigson-Kennedy, Committee Member
Kirsteen Nairn Browning, Outside Member - Keywords:
- gene expression
heroin
reward devaluation
laterality
saccharin
self administration
environmental enrichment - Abstract:
- Addiction is a disease of brain reward circuitry whereby systems which evolved to respond to “life-giving” rewards (such as food and social relationships) are now primed to bring about the ultimate goal of seeking and acquiring more drug. Moreover, individual propensities to becoming addicted to a given drug vary greatly, a phenomenon that we now know finds its root in individual internal and external environments. Therefore, the primary goal of this thesis was to examine the intersection of gene-expression, environment and individual susceptibilities to addiction-like behavior. In Chapter 2, we utilize a model of reward devaluation and heroin self-administration. When the opportunity to self-administer heroin is preceded by the opportunity to ingest a sweet saccharin solution, an outbred group of Sprague-Dawley rats splits into two distinct behavioral phenotypes: Those that show a preference for saccharin and low/moderate intake of heroin (small saccharin suppressors; SS) and those that avoid the saccharin cue and escalate heroin intake (large saccharin suppressors; LS). We examined the gene expression of the corticotropin releasing factor (CRF) pathway in reward areas (medial prefrontal cortex; mPFC, hippocampus, ventral tegmental area; VTA and nucleus accumbens; NAc) of these two different phenotypes and found that LS demonstrated greater gene expression of the CRF signaling pathway across reward regions. DNA methylation analysis revealed that the LS group had decreased methylation of cytosines in the CRFbp gene promoter. Chapter 3 explored whether saccharin suppression, brought about by passive infusion of heroin, would recapitulate the gene expression findings of the self-administration model. Overall, gene expression differences were markedly different from those reported in Chapter 2, indicating that passive infusion and self-administration bring about different molecular effects. Chapter 4 investigated gene expression differences between the mPFC transcriptome of SS and LS rats. Genes found to be differentially expressed between groups included those involved in schizophrenia and dopamine signaling, signal transduction, development and synaptic plasticity. The occurrence of several failed confirmatory qRT-PCR experiments led us to the hypothesis that the two mPFC hemispheres may not always have the same levels of gene expression. Chapter 5 explored this question through an RNA-Seq study on the left and right mPFC of drug-naïve male rats. 2.6% of expressed genes were found to differ between the left and right mPFC. Ingenuity Pathway Analysis revealed these differentially expressed genes to be involved in pathways such as cell signaling and cell morphology. Given the reports of functional laterality of the mPFC in the stress response, we asked whether the CRF pathway would be differentially expressed in reward regions in response to the passive heroin infusion model (which likely contains a stress component). CRF binding protein and CRFR2 were differentially expressed between left and right hippocampus between groups, CRF was differentially expressed between left and right VTA. Lastly, in Chapter 6, we show that environmental enrichment attenuates drug-seeking and motivation and is accompanied by expression differences of the early response genes EGR1 and EGR2. Bisulfite amplicon sequencing revealed site-specific methylation differences for EGR1 and EGR2 in the mPFC and NAc. Taken together, we show that individual propensities to drug-taking behavior are tied to differential gene expression and regulation and can be influenced by environmental factors. Moreover, we show that consideration should be given to hemisphere when investigating neurogenetic phenomena.