The opioid system controls pain, reward and addictive behaviors. Opioids exert their pharmacological actions through three opioid receptors, mu, delta and kappa whose genes have been cloned (Oprm, Oprd1 and Oprk1, respectively). Opioid receptors in the brain are activated by a family of endogenous peptides like enkephalins, dynorphins and endorphin, which are released by neurons. Opioid receptors can also be activated exogenously by alkaloid opiates, the prototype of which is morphine, which remains the most valuable painkiller in contemporary medicine.
By acting at opioid receptors, opiates such as morphine or heroin (a close chemically synthesized derivative) are extremely potent pain-killers, but are also highly addictive drugs.
To understand how molecules act in the brain and control behavior one can manipulate genes encoding these molecules in complex organisms, such as the mouse, and explore the consequences of these targeted genetic manipulations on animal responses in vivo.
Today, genetically modified mouse models represent a state-of-the art approach towards understanding brain function.
The direct comparison of mice lacking each of the three opioid-receptor genes reveals that mu- and delta-opioid receptors act oppositely in regulating emotional reactivity. This highlights a novel aspect of mu- and delta-receptor interactions, which contrasts with the former commonly accepted idea that activation of mu- and delta-receptors produces similar biological effects (Traynor & Elliot, 1993).
The finding that morphines analgesic and addictive properties are abolished in mice lacking the mu-opioid receptor has unambiguously demonstrated that mu-receptors mediate both the therapeutic and the adverse activities of this compound (Matthes 1996). Importantly, a series of studies has shown that the reinforcing properties of alcohol, cannabinoids, and nicotine — each of which acts at a different receptor — are also strongly diminished in these mutant mice. The genetic approach therefore highlights mu-receptors as convergent molecular switches, which mediate reinforcement following direct (morphine) or indirect activation (non-opioid drugs of abuse; see Contet 2004).
Endogenous opioid binding to mu-receptors is furthermore hypothesized to mediate natural rewards and has been proposed to be the basis of infant attachment behavior (Moles 2004).
Mice lacking the mu-receptor gene show
Analysis showed an unexpected alteration of emotional reactivity in the delta-receptor knockout mice (Filliol et al 2000). The mutant mice demonstrated increased levels of anxiety, and a depressive-like behavior these findings have important implications on the field of opioid research und uncover the therapeutic potential for delta-agonists in the treatment of mood disorders.
The most recent findings are the direct visualization of an opioid receptor in the mouse brain. The combination of fluorescent genetically encoded proteins (green fluorescent protein GFP from the jellyfish (Aequora victoria) with mouse engineering provides a fascinating means to study dynamic biological processes in mammals. Fluorescent genetically encoded proteins are unique high-contrast, noninvasive molecular markers for live imaging in complex organisms and provide the exploration of the receptor localization and function in vivo.
Scherrer et al. have knocked enhanced green fluorescent protein (EGFP) into the opioid delta receptor gene and produced mice expressing a functional DOR-EGFP C-terminal fusion in place of the native DOR. After manipulation of the mouse genome mutant animals express a fluorescent functional version of the delta-receptor in place of the native receptor (knock-in mouse) (Scherrer et al. 2006). This is the first example of a G protein coupled receptor directly visible in vivo.
G protein-coupled receptors (GPCRs) are the largest family of membrane receptors and are therapeutically essential, representing targets for 50% of marketed drugs (Scherrer et al., 2006). mu-, delta- and kappa-opioid-receptors are GPCRs of the nervous system.
The DOR-EGFP mouse provides a unique approach to explore receptor localization and function in vivo. GPCR represent the largest and most versatile family of membrane receptors, and each member has a specific cellular life cycle. The EGFP-knocking approach could be extended to other GPCRs, particulary in the case of orphan receptors for which in vivo pharmacology is still in its infancy (Scherrer et al., 2006).
Altogether there have been identified genes encoding receptors from a complex neuromodulatory system, and developed gene targeting approaches to elucidate the function of these genes in the mammalian brain.
It was found that mu-receptors control reward, while delta-receptors regulate emotional responses and for the first time a genetic manipulation was pioneered to achieve functional imaging of opioid receptors in vivo.