Pituitary Adenylate Cyclase-Activating Peptide (PACAP) Signaling and the Dark Side of Addiction.

While addiction to drugs of abuse represents a significant health problem worldwide, the behavioral and neural mechanisms that underlie addiction and relapse are largely unclear. The concept of the dark side of addiction, developed and explored by George Koob and colleagues, describes a systematic decrease in reward-related processing following drug self-administration and subsequent recruitment of anti-reward (i.e., stress) systems. Indeed, the activation of central nervous system (CNS) stress-response systems by drugs of abuse is contributory not only to mood and anxiety-related disorders but critical to both the maintenance of addiction and relapse following abstinence. In both human and animal studies, compounds that activate the bed nucleus of the stria terminalis (BNST) have roles in stress-related behaviors and addiction processes. The activation of pituitary adenylate cyclase-activating peptide (PACAP) systems in the BNST mediates many consequences of chronic stressor exposure that may engage in part downstream corticotropin-releasing hormone (CRH) signaling. Similar to footshock stress, the BNST administration of PACAP or the PAC1 receptor-specific agonist maxadilan can facilitate relapse following extinction of cocaine-seeking behavior. Further, in the same paradigm, the footshock-induced relapse could be attenuated following BNST pretreatment with PAC1 receptor antagonist PACAP6-38, implicating PACAP systems as critical components underlying stress-induced reinstatement. In congruence with previous work, the PAC1 receptor internalization and endosomal MEK/ERK signaling appear contributory mechanisms to the addiction processes. The studies offer new insights and approaches to addiction and relapse therapeutics.