Opiate addictionneuronal plasticity on brain reward system and emotional memory-related areas = Adicción a opiáceos : plasticidad neuronal en los circuitos neuronales de recompensa y en áreas de memoria emocional

Resumen

x ABSTRACT Opiate drugs, such as morphine, are a class of powerful analgesics that are used for treating many forms of acute and chronic pain. However, chronic use of opiates causes brain neuroadaptations that lead to undesirable effects, namely opiate addiction that is a significant medical and public health problem. Increasing evidence implicates various mechanisms of gene regulation (including epigenetic, molecular, cellular and circuit level effects) in the changes that drugs of abuse induce in the brain, indicating a potential therapeutic strategy for addiction therapy. We have focused the present study on several neurocircuits involved in opiate addiction: i) the hypothalamic-pituitary-adrenal (HPA) axis and extrahypothalamic stress systems; ii) the dopaminergic reward pathways; and iii) aversive memories-related neuronal circuits. 1. The first objective of the present study was designed after considering the above. We examined the role of glucocorticoids (GCs) in regulation of ?FosB expression in specific populations of the brain stress system during morphine dependence. For that, expression of ?FosB was measured in control (sham-operated) and adrenalectomized (ADX) rats that were made dependent on morphine. 2. Second objective: Given the important implications of dopamine (DA) neurotransmission in addiction disorders, we have focused this part of our study on identifying the DA markers that are altered in association with acute and chronic morphine exposure, as well as with morphine withdrawal in the ventral tegmental area (VTA) and nucleus accumbens [NAc(medial shell)]. For that, we have determined i) the expression of miR-133b and Argonaute (Ago2) in VTA; ii) tyrosine hydroxylase (TH) content and activity; and iii) dopaminergic activity (DA turnover and TH activation) in the mesolimbic system. 3. The third objective was to study: i) Nurr1 and Pitx3 mRNAs and proteins changes as well as the expression of Ago2 and TH mRNA and protein levels in specific region of the mesolimbic system; ii) dopaminergic activity in the NAc; iii) quantitative co-localization of Nurr1 and Pitx3 in the VTA TH-positive neurons; and iv) the plasticity changes in VTA DA neurons subpopulations in response to morphine, morphine dependence and morphine withdrawal. 4. Fourth Objective: The long-lasting effects of opiate withdrawal on Nurr1 and Pitx3 expression prompted us to investigate expression of other genes and proteins that are involved in the regulation of DA function, some of which represent putative targets of Nurr1 and Pitx3. For that, rats were exposed to acute and chronic morphine administration as well as to morphine withdrawal and analized: i) the expression of dopamine transporter (Dat)-DAT, vesicular monoamine transporter 2 (Vmat2)-VMAT2, dopamine receptor 2 (Drd2)-DRD2 and DRD1 in VTA/NAc(shell), dysfunction of which is causally linked to addiction; ii) the co-localization of Nurr1 and/or Pitx3 with TH-positive neurons in the VTA as well as the percentage of DA neurons expressing Nurr1 and Pitx3; and iii) the possible correlation between Nurr1/Pitx3 expression and DA markers levels in the VTA and/or NAc(shell). 5. Fifth Objective: There is increasing evidence that drugs of abuse produce alterations in CNS immunology. Thus, it has been proposed that astrocytes contribute to the synaptic plasticity during the development of drug addiction by the synthesis and release of substances, such as cytokines. The present study was focused on: i) identifying whether the expression of pleiotrophin (PTN), midkine (MK), receptor protein tyrosine phosphatase ?/? (RPTP?/?) and their intracellular signaling pathways are altered in association with acute and chronic morphine exposure as well as with morphine withdrawal in the VTA and NAc; ii) the possible activation of astrocytes; iii) identify those cell subpopulations that produced and secreted PTN and/or MK and those that expressed RPTP?/? in response to morphine administration or morphine withdrawal. The conclusions from the present work are: 1. Present work provides evidence that GCs are critically involved in FosB/?FosB accumulation in the brain stress system after chronic morphine exposure, which might result in lasting changes of gene expression pattern in stress-related areas. 2. Although no significant changes of miR-133b levels are detected in the VTA, a role for Ago2 and specific miRs is hypothesized in regulating TH mRNA stability and/or translation in response to chronic morphine administration and naloxone-induced morphine withdrawal. Moreover, morphine dependence and withdrawal are associated with consistent alteration of transcription factors involved in the maintenance of dopaminergic neurons in the mesolimbic drug-reward pathway. 3. Morphine dependence and withdrawal are associated with consistent alteration of most of the DA markers (DAT, VMAT2, DRD2) in the mesolimbic drug-reward pathway which correlated with alteration of transcription factors involved in the maintenance of dopaminergic neurons (Nurr1 and Pitx3). Moeover, the correlations between DA markers and Nurr1/Pitx3 add evidence onto previous results. 4. Because of the glial activation in the VTA during acute and chronic morphine administration besides the enhancement in MK and PTN mRNAs, we propose a role for these cytokines in mediating, at least in part, the trophic adaptations that are observed during drug addiction.