Adicción a cocaínaestudio del receptor dopaminérgico D3 (D3R) como posible diana farmacológica en la prevención de las recaídas en su consumo

Abstract

Drug addiction constitutes a serious public health problem at the international level. Among them, cocaine continues to be the most widely used illegal stimulant drug in Europe, with the highest prevalence in southern and western countries, as is the case in Spain. Addiction can be defined as a chronic and recurrent mental disorder characterized by the compulsive search for the drug. During long periods of abstinence there is a high risk of relapse in consumption, therefore, its prevention has become an object of study for the management of addiction due to the lack of effective therapies to avoid them. The importance of studying relapses is that they can be triggered by both stressful stimuli and by re-exposure to the drug itself. These same triggers are used to re-establish drug-associated behaviors in animal models. The Pavlovian paradigm of drug-induced conditioned place preference (CPP) is used to evaluate the rewarding properties of drugs of abuse. As a consequence, the Objectives of this work have been: - To evaluate the possible involvement of the dopaminergic receptor D3 (D3R) in the relapse in cocaine use, since accumulated evidence indicates that antagonists of these receptors seem to be very promising in attenuating cocaine reward and relapse in preclinical models of addiction. - To study if D3R is involved in the increase of plasma corticosterone levels during relapse in CPP. - To test the changes in the expression of DAT and D3R in the NAc shell during the relapse in the CPP caused by stress and by a priming of cocaine in mice conditioned to cocaine; in addition, we studied if the D3R blockade prevents the modifications those expression. - To investigate the changes in the mTOR cascade in BLA and DG in the relapse in CPP induced by cocaine priming and social stress. The methodology developed has been divided into two phases, first a behavioral study using the CPP has been carried out, and then a molecular study by processing the samples obtained (brain and plasma) to interpret the changes that occurred in the first phase. The different experimental groups of male mice of the C57BL/6 strain were conditioned with cocaine at a dose of 25 mg/kg for four days. On the fifth day, a post-conditioning test (Post-C) was performed and then, for 7-8 weeks, the CPP was extinguished (ext). Once the addictive behavior had ceased, the possible involvement of D3R in the restoration of cocaine-induced CPP was studied. For that, thirty minutes before the administration of a cocaine priming dose or a single stress episode (SD, immobilization or pain), we administered a single dose of the selective D3R antagonist, SB-277011-A. The doses administered in each group were a) 24 or 48 mg/kg, i.p. for the cocaine priming group, immobilization stress and pain stress; b) 12 or 24 mg/kg, i.p. for the SD stress group. Control animals received a vehicle injection (deionized water, 1 ml/kg, i.p.). In addition, plasma levels of corticosterone (as marker of stress) were quantified by radioimmunoassay (RIA) after relapse. The results showed that blocking D3R with SB-277011-A did not antagonize the relapse in CPP caused by cocaine-prime; by contrast, pretreatment with SB-277011-A inhibited the restoration of CPP caused by social stress. We have also observed that pretreatment with SB-277011-A inhibited the restoration of CPP caused by different types of physiological stress. At the molecular level, there were changes in the expression of DAT and D3R in the NAc shell after the relapse in CPP caused by cocaine priming and social stress. In addition, we have found that the Akt-mTOR signaling cascade could be coupled to D3R due to an increase in the expression of pAkt and pmTOR in the NAc after the relapse in CPP caused by cocaine priming and social stress. Relapse in CPP induced by cocaine priming or social stress modified D3R and DAT expression in BLA, but not in hippocampal DG. There was a decrease in mTOR activation in BLA and DG after reinstatement of CPP induced by cocaine priming or social stress. In conclusion, our results suggest that D3R antagonism could be a pharmacological target in the prevention of stress-induced relapse in the consumption of cocaine. Furthermore, present results are consistent with the hypothesis that D3R would play an important role in the reinstatement of cocaine-seeking.