Age related dopamine 1 and dopamine 2 receptor expression in addiction-related behaviours

© 2020 Ellen Rose CullityMethamphetamine (meth) is a significant social and public health concern worldwide, and a growing problem in Australia. One factor contributing to meth use disorder is the lasting memory of its rewarding experience, which can lead to persistent use in vulnerable individuals. Epidemiological data show that adolescence is a period of heightened vulnerability for developing meth use disorder. Furthermore, sex differences exist in numerous aspects of meth use motivations, behaviours, and consequences. Despite this, few studies have investigated age and sex effects of meth on brain and behaviour. A unified neural mechanism by which substances of abuse, including meth, produce their addictive properties is by increasing dopaminergic transmission throughout the mesocorticolimbic dopamine system. Although dopamine binds to and activates several subclasses of receptor in brain regions implicated in reward processing, the dopamine receptors 1 (D1) and 2 (D2) have been reported to be particularly important for drug-affected behaviour. Importantly, levels of D1 and D2 have been shown to fluctuate throughout development. D1 and D2 signalling may therefore be important mediators of adolescent vulnerability to substance use. However, there are numerous inconsistencies in the literature that describe developmental changes in D1 and D2 expression. Systematic characterisation of these changes is therefore critical for a full understanding of how changes to the dopamine system may affect susceptibility to meth (and other substance) use disorder. As such, the first aim of my thesis was to investigate the postnatal developmental trajectory of D1 and D2 in addiction-related brain regions. This was achieved using D1- and D2-green fluorescent protein (GFP) transgenic mice, starting from the juvenile period through to adulthood. The results showed region-specific changes in D1 and D2 expression occur across development, with the insular cortex (insula) showing the most dramatic changes. In particular, the density of D1 compared to D2 expressing neurons (D1:D2 ratio) in the insula substantially increased from adolescence to adulthood in males. Since substance use disorders are male dominant disorders which often have an adolescent onset, this reduced D1:D2 ratio may be relevant in understanding the neurobiological basis of substance use disorders. The second aim of my thesis was to investigate the role of insula D1 and D2 in potential age and sex differences in meth conditioned place preference (CPP) and aversion (CPA). Although no age or sex differences were observed in CPP to 0.1 or 3 mg/kg meth at a group level, analysis of individual data revealed females were less likely to form a place aversion compared to males, and adolescents formed more of a place preference to 0.1 mg/kg meth compared to adults. Conditioning with 3 mg/kg meth led to age differences in insula D1:D2 ratio in males, reduced age differences in insula D1:D2 ratio in females, and increased the activation of insula D2 expressing cells in adults compared to adolescents. Insula activity and expression of D1 and D2 did not correlate with place preference behaviour. Taken together, these findings suggest distinct sex differences in D1 and D2 expression across development in addiction-related cortical and striatal brain regions may underly age- and sex-associated vulnerability to meth-related behaviours