The endogenous
opioid system is expressed throughout the brain reinforcement circuitry, and plays a major role in reward processing, mood control and the development of addiction. This neuromodulator system is composed of three
receptors,
mu, delta and kappa,
interacting with a family of
opioid peptides derived from POMC (尾-endorphin), preproenkephalin (pEnk) and preprodynorphin (pDyn) precursors. Knockout mice targeting each gene of the
opioid system have been created almost two decades ago. Extending classical pharmacology, these
mutant mice represent unique tools to tease apart the specific role of each
opioid receptor and peptide
in聽vivo, and a powerful approach to understand how the
opioid system modulates behavioral effects of drugs of abuse. The present review summarizes these studies, with a focus on major drugs of abuse including morphine/heroin, cannabinoids, psychosti
mulants, nicotine or alcohol. Genetic data, altogether, set the
mu receptor as the primary target for morphine and heroin. In addition, this
receptor is essential to mediate rewarding properties of non-
opioid drugs of abuse, with a demonstrated implication of 尾-endorphin for cocaine and nicotine. Delta
receptor activity reduces levels of anxiety and depressive-like behaviors, and facilitates morphine-context association. pEnk is involved in these processes and delta/pEnk signaling likely regulates alcohol intake. The kappa
receptor mainly interacts with pDyn peptides to limit drug reward, and mediate dysphoric effects of cannabinoids and nicotine. Kappa/dynorphin activity also increases sensitivity to cocaine reward under stressful conditions. The
opioid system remains a prime candidate to develop successful therapies in addicted individuals, and understanding
opioid-mediated processes at systems level, through emerging genetic and imaging technologies, represents the next challenging goal and a promising avenue in addiction research.
This article is part of a Special Issue entitled 鈥楴IDA 40th Anniversary Issue鈥?