15 years of genetic approaches in vivo for addiction research: Opioid receptor and peptide gene knockout in mouse models of drug abuse.

Milestone discoveries in opioid researchOpium is extracted from poppy seeds (Papaver somniferum) and consumed for several thousand years to relieve pain and produce euphoria. Morphine, the most active alkaloid extracted from opium, was the first opioid to be isolated (1805). Opiates act on the nervous system, where they specifically activate receptors (1973), which are normally stimulated by a family of endogenous neurotransmitters, β-endorphin, enkephalins and dynorphins (1975). Several opioid receptors subtypes were further described based on receptor pharmacology (1976). Gene cloning occurred in early 80’s for peptide precursors (1979) and early 90’s for opioid receptors (1992). Opioid receptors genes (Oprm1, Oprd1 and Oprk encoding mu-, delta- and kappa-opioid receptor; pomc, pEnk and pDyn encoding peptide precursors) were targeted in mice by homologous recombination, and mice lacking the mu receptor and enkephalins were available first (1996). Recently, refinement of in vivo targeted mutagenesis techniques led to the first conditional knockout mouse for the opioid system, with a delta receptor deletion restricted to primary afferent nociceptive neurons (2011). The 3D crystal structure of all three receptors was elucidated very recently (2012). OR: opioid receptor, KO: knockout mouse, cKO: conditional knockout mouse. Detailed references are in the text.

Involvement of opioid receptors in drug rewardThe scheme summarizes data from receptor KO mice and highlights the role of each receptor in drug reward. The mu opioid receptor mediates rewarding properties of both opioid and non-opioid drugs of abuse. With the exception of nicotine, the delta receptor does not seem involved in drug reward. The kappa receptor mediates dysphoric effects of THC and favors cocaine reward after stress (red lines). The role of delta and kappa receptor in alcohol intake is under investigation (see text). Circles indicate euphoria (red/orange), no effect (white) or dysphoria (blue); n.d: not determined in receptor KO mice.

Distinct roles of opioid receptors and peptides in addiction-related effects of drugs of abuseThe upper left scheme summarizes known roles of opioid receptors in brain functions related to hedonic homeostasis and mood (from (Lutz and Kieffer, 2012). In the five other panels, we propose mechanisms implicating opioid receptors and/or peptides in addiction liability of each class of drugs of abuse, as inferred from both receptor and peptide knockout mouse data reviewed here. “Reward” and “drug-context association” refer to CPP data, “aversive effects” to CPA data, “motivation for the drug” to SA experiments, and “dependence” to scores of physical withdrawal under antagonist treatment. Data from locomotor studies are not included (see summary in Table 6). Opiates: peptide KO mice show paradoxical (β-end/reward, pEnk/withdrawal) or no (pDyn/withdrawal) phenotype. THC: β-end KO mice not tested; cocaine: pEnk KO mice not tested; nicotine: β-end KO mice tested for reward but not withdrawal; alcohol: β-end KO mice show contrasting phenotypes and pEnk show a phenotype under stress.Altogether, data from peptide KO mice, combined with those from receptor KO mice, concur to substantiate involvement of a kappa/dynorphin system in dysphoric states associated to drugs of abuse, although this may not apply to alcohol. Data also suggest a role for mu/Bend signaling in cocaine and nicotine reward, and implication of delta/pEnk signaling to regulate alcohol intake.