A key component of the opioid system is its capacity to regulate pain, reward, and addiction. A variety of opioids exert their pharmacological effects by binding directly to their opioid receptors, mu, delta, and kappa, the genes for which have been cloned (Oprm, Oprd1, An endogenous family of peptides released by neurons activates opioid receptors in the brain. It is also possible to activate opioid receptors exogenously with alkaloid opiates, the prototype of which is morphine, which accounts for the majority of painkillers in modern

Psychedelic opiates such as morphine and heroin (another chemically synthesized derivative) are extremely potent painkillers, but also highly addictive. Brigitte Kieffer is presenting new methods that now allow us to understand how molecules affect brain functions. The theory is that by manipulating genes encoding molecules that act in the brain and control behavior, one can examine the effects of these manipulations in complex organisms, such as the mouse, and see how those genetic manipulations affect animal behavior. Molecularly modified mice provide a state-of-the-art approach to understanding brain function today. In direct comparison to mice lacking one of the three opioid receptor genes, it can be seen that mu- and delta-opioid receptors offer opposite effects In contrast with the previous commonly accepted view, activation of mu- and delta-receptors causes similar biological effects (Traynor & Elliot, 1993), this new study brings to light an interesting aspect of the interaction between mu- and delta-receptors.


In the study demonstrating that morphine has no analgesic or addictive properties in mice lacking high mu-opioid receptor levels, it has been demonstrated that mu-receptors are both involved in its therapeutic as well as its adverse effects 

Further, endogenous opioid ligands for mu-receptors have been proposed to be involved in the modulation of natural rewards and as the basis for infant attachment

Mice lacking the mu-receptor gene show:

There is a loss of the analgesic, rewarding, and compulsive effects of morphine. Pain perception increases with increased sensitivity. The reward for abuse of non-opioid drugs of abuse is reduced and emotional responses are altered.


Researchers in 2000 found an unexpected change in emotional reactivity in mice missing delta receptors (Filliol et al.). It is important to note that mutant mice displayed depressive-like behavior and high levels of anxiety – these findings have important implications for the field of opioid research as well as unveiling the therapeutic potential for delta-agonists in treating Recent studies in mice show the presence of an opioid receptor directly visible in the A combination of fluorescent genetically encoded proteins (green fluorescent protein GFP from the jellyfish (Aequora victoria) with mouse engineering allows us to study dynamic biological processes in mammals in an incredibly powerful manner. Molecular markers that exhibit fluorescent properties under high contrast are essential to the study of receptor localization and function in vivo. They are unique, noninvasive molecular markers for live imaging of complex organisms. The author The team knocked in enhanced green fluorescent protein (EGFP) into the opioid delta receptor gene to make mice with a functional DOR-EGFP C-terminal fusion in place of the native AMR receptor. As a result of manipulating the protein sequence in the mouse genome, mutant animals express a fluorescent functional version of the delta-receptor in place of the native receptor (Knocked-in The first Gprotein coupled receptor that can be directly visualized in vivo has been identified.

The G protein-coupled receptor family (GPCRs) represents the largest class of receptors on membranes, and represents a target for 50% of drugs on the market These GPCRs are found throughout the nervous system, and are known as mu, delta, and kappa opioid receptors. It offers a unique opportunity to study receptor localization and function in vivo using the DOR-EGFP mouse model. Located on the surface of a cell, GPCRs are the largest and most diverse family of membrane receptors. Each member of the family is responsible for regulating a specific cellular process. EGFP knockdown could also be extended to other GPCRs, especially in the case of orphan receptors for which in vivo pharmacology is still at a very early stage The discovery of genes encoding receptors from a complex neuromodulatory system as well as the development of gene targeting strategies to study their function in mammalian brain have led to a better understanding of these genes and how they function. Researchers have discovered that mu receptors regulate reward, while delta receptors regulate emotional responses. For the first time, functional imaging of opioid receptors has been accomplished in vivo as a result of genetic manipulation.


A family of endogenous opioid peptides inhibits the opioid system by stimulating its three G protein-coupled receptors, mu, delta, and kappa. A mu-opioid receptor is a critical molecular switch triggering reward systems in the brain and potentially initiating addictive behavior. By eliminating mu receptors, morphine loses its analgesic effects, as well as its activity for place preference and physical dependency. Consequently, this receptor is involved in both therapeutic (analgesia) and harmful (addiction) effects of morphine, suggesting that the development of more morphine-like compounds might lead to the development of A role for mu-opioid receptors in diseases with attachment deficits, such as autism or reactive attachment disorder, has also been suggested by studies of mutant mice. Data showed not only that mice lacking mu-opioid receptors are a useful animal model for evaluating the outcomes of deficits in the affiliative system during development and adulthood, but also that these mice also exhibit less anxiety. It appears that mu-receptor knockout mice lose the rewarding properties of both opioid and non-opioid drugs of abuse (cannabinoids, alcohol, nicotine), which may lead to a valuable approach to the treatment for drug addiction.

Aside from the pleasure-enhancement aspect of drug consumption, pharmacological studies suggest that this receptor might be involved in maintaining drug use, craving, and relapse after drug use. We should therefore be able to learn more about the general mechanisms that underlie addiction by deepening our understanding of mu-receptor function. It has been observed that opioid addicts, who use heroin as their main mu-opioid agonist, suffer from depression a high amount of the time, which adds to Additionally, antidepressant therapy is frequently used in the treatment of chronic pain states. Thus, delta-agonists may be able to provide an additional benefit, in addition to their analgesic capacities, improving emotional wellness and, more generally, serving as an alternative therapy that may alleviate affective disorders in the future.