Substance use disorders, memory and neural plasticityDopamine D3 receptor as a potential therapeutical target to avoid drug relapses. Epigenetic and neurobiological mechanisms

Abstract

Substance use disorders (SUDs) are a series of behavioural outcomes that arise after sustained drug consumption, despite its negative consequences, in order to avoid withdrawal syndrome. In this psychiatric condition, the mesocorticolimbic structures in the brain play a major role in overseeing the reinforcing effects of the drug of abuse. Memory-related nuclei, such as the amygdalar complex, the hippocampus and the infralimbic cortex (IL), receive dopaminergic inputs from the ventral tegmental area, where drugs of abuse exert their effects in the brain. Therefore, there is a connection between memory and SUDs, as drug-paired cues and contexts can trigger drug-seeking behaviours. Despite sustained efforts, currently, extinction-based therapies are ineffective since drug memories show remarkable resistance, lasting years after consumption. Recent literature has pointed out the potential role of the dopamine type 3 receptor (D3R) in the processing of these memories. In this doctoral dissertation, we have first explored some of the neurobiological events underlying the retrieval of aversive morphine withdrawal memories and their extinction in nuclei involved in emotional (basolateral amygdala, BLA) and declarative (hippocampus) memories. We assessed the activity of the mTOR pathway and their downstream synaptic protein targets Arc, Homer1 and GluN1. Also, since epigenetic marks have been shown to be stable and gained attention as an underlying mechanism in drug-related memories, we studied the levels of the histone 4 acetylated in the lysine 5 (H4K5ac) and its coupled activator transcription factor, phospho Brd4 (p-Brd4), as well as the expression of some target genes and a chromatin remodeller component. Secondly, in line with previous investigations of our laboratory, we tested the effects of selective D3R-antagonism upon tail-pinch and immobilisation-induced reinstatement of cocaine-seeking behaviours, as well as its molecular impact on D3R-coupled pathways in the BLA and dentate gyrus (DG). Additionally, studied the impact of D3R antagonists on the extinction of morphine withdrawal-induced drug seeking behaviours. Then, we evaluated D3R blockade and impact on activation of glia in several mesocorticolimbic areas. To accomplish the aims of this thesis, we used a combined approach of behavioural rodent models (conditioned place aversion -CPA- and preference -CPP- paradigms) and molecular techniques (western blot, radioimmunoassay, quantitative polymerase chain reaction and immunofluorescence). Our results provide evidence for the regulation of the activity of the mTOR pathway in the DG upon the recall of aversive memories, which was accompanied with variations in the expression of synaptic molecules during the retrieval and extinction of opiate-withdrawal memories. Secondly, we report increased levels of p-Brd4 and H4K5ac in BLA and hippocampal areas, that return to control levels after extinction procedures. However, targeted genes remained unchanged, suggesting a more complex regulation mechanism underlying transcription induction. Moreover, our work supports the use of D3R antagonists as a pharmacological tool to prevent relapse in cocaine-seeking behaviours induced by physiological and psychological stressors and reflect a complex and distinct modulation of D3R-coupled pathways depending on the type of stress in BLA and DG. Furthermore, this project endorses the efficacy of D3R antagonism for accelerating the extinction of drug-seeking behaviours associated with opiates negative reinforcement, but through motivational regulation instead of enhanced cognition. Successful behavioural training alone significantly reduced D3R expression in the IL and dorsal striatum, hence indicating that this receptor participates in the extinction of morphine withdrawal memories. Moreover, the decreased levels of microglial activation markers and their morphological alterations after D3R blockade reveal that neuroinflammatory mechanisms take part in D3R effects.