Insular cortex dopamine 1 and 2 receptors in methamphetamine conditioned place preference and aversion: Age and sex differences

Rodent studies have proposed that adolescent susceptibility to substance use is at least partly due to adolescents experiencing reduced aversive effects of drugs compared to adults. We thus investigated methamphetamine (meth) conditioned place preference/aversion (CPP/CPA) in adolescent and adult mice in both sexes using a high dose of meth (3 mg/kg) or saline as controls. Mice tagged with green-fluorescent protein (GFP) at Drd1a or Drd2 were used so that dopamine receptor 1 (D1) and 2 (D2) expression within the insular cortex (insula) could be quantified. There are sex differences in how the density of D1+ and D2+ cells in the insula changes across adolescence that may be related to drug-seeking behaviors. Immunohistochemistry followed by stereology were used to quantify the density of cells with c-Fos and/or GFP in the insula. Unexpectedly, mice showed huge variability in behaviors including CPA, CPP, or no preference or aversion. Females were less likely to show CPP compared to males, but no age differences in behavior were observed. Conditioning with meth increased the number of D2 + cells co-labelled with c-Fos in adults but not in adolescents. D1:D2 ratio also sex- and age-dependently changed due to meth compared to saline. These findings suggest that reduced aversion to meth is unlikely an explanation for adolescent vulnerability to meth use. Sex- and age-specific expressions of insula D1 and D2 are changed by meth injections, which has implications for subsequent meth use

Investigating the role of CREB and CBP in addiction using conditional knockout mouse models

© 2012 Dr. Heather Bronwyn MadsenDrug addiction is a chronic, relapsing brain disease which represents an enormous social and economic burden to society. The transition from casual to compulsive drug use and the enduring propensity to relapse is thought to be underpinned by long-lasting neuroadaptations within specific brain circuitry. The transcription factor cAMP response element-binding protein (CREB) has been identified as a key molecular substrate involved in drug-induced plasticity. The first broad aim of this thesis was therefore to further elucidate the role of CREB signalling within specific brain reward regions in addiction-related behaviours. To this end, mice were generated with a postnatal deletion of CREB targeted to the striatum, a brain structure critically involved in reward-related learning. While striatal CREB deletion did not appear to alter the reinforcing properties of cocaine, enhanced expression of locomotor sensitization to cocaine was revealed following chronic treatment, in addition to increased amphetamine-induced stereotypies. However in the absence of striatal CREB, upregulation of the related transcription factor cAMP responsive element modulator (CREM) was observed, indicating possible redundancy amongst this family of transcription factors. In support of this, striatal deletion of CREB-binding protein (CBP), a coactivator recruited by both CREB and CREM, resulted in an even more pronounced sensitivity to psychostimulants. This suggests that CREM, acting via CBP, is able to partially compensate in the absence of CREB. In contrast, mice with a postnatal deletion of CBP directed to midbrain dopamine neurons were indistinguishable from controls on a number of addiction-related measures. The second main aim of this thesis was to investigate possible brain nuclei involved in cue-induced morphine- vs. sucrose-seeking in a mouse model of relapse using the neuronal activity marker Fos. This resulted in the identification of putative circuitry common to morphine- and sucrose-seeking as well as circuitry specific to each reinforcer. Structures activated in both relapse groups included the anterior cingulate, nucleus accumbens shell, basolateral amygdala, substantia nigra, ventral tegmental area, anterior periaqueductal gray and locus coeruleus. Structures activated only in morphine-seeking mice included the orbitofrontal cortex, nucleus accumbens core, ventral pallidum, bed nucleus of the stria terminalis, central nucleus of the amygdala and hippocampus. The dorsal raphe was the only structure found to be specifically activated in sucrose-seeking mice. These findings broadly support a cortico-striatal limbic circuit driving opiate-seeking behaviour, and additional circuitry potentially relevant to reward-seeking was identified. In summary, this thesis has contributed to a growing body of literature investigating the role of CREB signalling upon behavioural responses to drugs of abuse. Deletion of CREB/CBP from the striatum confers increased sensitivity to the locomotor activating properties of psychostimulants, without influencing the reinforcing properties of cocaine. In contrast, deletion of CBP from midbrain dopamine neurons does not appear to affect behavioural responses to cocaine under the tested paradigms. Finally, findings from the relapse study have revealed a number of structures not previously explored in the context of reward-seeking that are worthy of further investigation, and concordance with data from human imaging studies supports this model as a relevant tool for studying relapse-like behaviour in mice