How does the opioid system work?

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).

mu-opioid-receptor

The finding that morphine's 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

• a loss of morphine-induced analgesia, reward, and dependence
• increased sensitivity to painful stimuli
• reduced reward to non-opioid drugs of abuse and
• altered emotional responses

delta-opioid-receptor

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.

• The opioid system consists of three G protein-coupled receptors, mu-, delta-, and kappa, which are stimulated by a family of endogenous opioid peptides.
• mu-opioid receptors are a key molecular switch triggering brain reward systems and potentially initiating addictive behaviors. The lack of mu-receptors abolishes the analgesic effect of morphine, as well as place-preference activity and physical dependence. This receptor therefore mediates therapeutic (analgesia) and adverse (addiction) activities of morphine, suggesting that further development of morphine-like compounds may necessarily lead to addictive analgesics.
• Studies of mutant mice also suggest a role for mu-opioid receptors in diseases characterized by deficits in attachment behavior, such as autism or reactive attachment disorder. The data also highlight mice lacking mu-opioid receptors as a useful animal model to evaluate the consequences of deficits in the affiliative system during development and adulthood.
• The rewarding properties of both opioid, as well as non-opioid drugs of abuse (cannabinoids, ethanol and nicotine, natural reinforcers) are abolished in the mu-receptor knockout mice. Blocking the mu-receptor may build a valuable approach for the treatment for drug abuse.
• Beyond the rewarding aspect of drug consumption, pharmacological studies have also suggested a role for this receptor in the maintenance of drug use, as well as craving and relapse. As a consequence, expanding our understanding of mu-receptor function should greatly help to further our knowledge of the general mechanisms that underlie addiction.
• Opiate addicts, who mainly abuse the mu-opioid agonist heroin, present a high incidence of depressive disorders that seem to contribute to the maintenance of the addictive state. Also, the treatment of chronic pain states frequently includes antidepressant therapy. Therefore, in addition to their potential analgesic activity, delta-agonists may be useful in improving emotional states and, more generally, may be considered in the future as an alternative therapy to alleviate affective disorders.