Epigenetic mechanisms underlying opiate dependence.

Addiction to drugs is characterized by craving and compulsive drug-seeking behaviour and is often a chronic relapsing disorder. One of the most challenging questions in addiction research relates to understanding how the brain modifies its structure and function in response to drug exposure. The persistence of drug addiction may be analogous to learning and memory formation and suggests enduring changes in gene expression and neuronal structure and function. In this research program, we plan to investigate the cellular and molecular adaptations that may underlie persistent changes in brain function induced by chronic exposure to morphine. A major goal is to determine whether morphine administration and withdrawal triggers changes in the structure of chromatin in brain regions that are critically involved in drug-dependent behaviours (ventral tegmental area, nucleus accumbens, prefrontal cortex, lateral septal nucleus and hippocampus). The underlying rationale for this investigation is that remodelling of chromatin may drive persistent changes in gene expression, which may ultimately result in modifications of brain structure and function. As a next step, we will investigate whether the ERK signalling pathway, which has an important role in synaptic plasticity and drug addiction, is activated by morphine, and whether this intracellular cascade is required for opiate-induced chromatin remodelling. We will also try to identify possible target genes regulated by chronic morphine. We will focus in particular on the neurotrophin BDNF (brain-derived neurotrophic factor), which plays a key role in synaptic plasticity and in cellular events thought to underlie learning and memory. Importantly, the BDNF gene is regulated by activity-dependent mechanisms that rely on chromatin modifications at specific promoters and which depend on activation of the ERK pathway. Furthermore, recent studies have implicated this neurotrophin in cocaine addiction. Therefore, we will evaluate whether both acute and chronic exposure to morphine and subsequent withdrawal alter the expression levels of BDNF and its precursor proBDNF, as well as their receptors, TrkB and p75NTR. Because BDNF plays an important role in hippocampal-dependent learning, we will also investigate what are the effects of chronic treatment with morphine and withdrawal on spatial learning abilities and memory retention. Finally, given that relapse to drug is a major problem in the treatment of opiate dependence, we propose that long-lived modifications in chromatin structure and gene expression might be a crucial mechanism for morphine-induced plasticity. Therefore, a major goal of this project will be to determine whether the changes in chromatin remodelling and gene expression are stable and persist long after morphine administration is terminated. To pursue these goals, we will use a multidiscliplinary approach that spans from immunofluorescence and confocal microscopy to biochemical, molecular and behavioural studies. The results will be relevant for understanding the molecular and cellular adaptations that underlie the long-term effects associated with opiate dependence.