Endogenous Opioids, Opioid Receptors, and Incentive Processes

Abstract

Opioid receptors and endogenous opioid peptides play a role in a wide variety of behavioral and physiological processes. Four types of opioid receptors have been identified: mu (MOR), delta (DOR), kappa (KOP), and opioid-receptor-like receptor (ORL-1). A variety of endogenous opioids have been identified, including β-endorphin, met-enkephalin, dynorphins, and nociceptin, among others. Whereas endogenous opioids are relatively nonselective, several receptor-selective compounds are available for research purposes (e.g., DAMGO, DPDPE, and U50,488H selectively agonize MOR, DOR, and KOR, respectively). This has permitted researchers to identify the role played by specific opioid receptors in incentive processes. A brief overview of opioid receptors and neuropeptides is followed by a review of recent research on their role in modulating absolute and relative incentive value. Incentive value refers to the hedonic dimension of stimuli such as food and fluids, i.e., to their appetitive or aversive value. Opioid release is involved in two major functions related to incentive value. First, they influence food palatability and so-called liking orofacial responses. Second, they are implicated in the behavioral adjustment to situations involving unexpected incentive devaluations. Opioid function in these situations is surprisingly selective. Thus, for example, different opioid receptors affect behavior at different points in the process of adjustment to reward devaluation.