Opioid Receptor

Opioid receptors are a group of G protein coupled receptors which are the binding sites of opioids. There are three types of opioid receptors: mu (μ), kappa (κ), and delta (δ). Opioids act on the brain and body by attaching to specific opioid receptors, which are found in the brain, spinal cord, and gastrointestinal tract. When opioids attach to certain opioid receptors, they can block the perception of pain. Opioid receptors are also defined by their blockage with naloxane. Agonists for mu receptors include morphine; enkephalin is an endogenous opioid and is selective for delta receptors; and dynorphin is another endogenous opioid which is selective for kappa receptor.

Opioid receptors are found in high concentration throughout the brain; they are found in presynaptic terminals where binding with opioids inhibit release of neurotransmitters. Other areas in the brain with a high concentration of opioids and opioid receptors include periaqueductal gray, nucleus raphe magnus, nucleus reticularis, paragiganticellularis, and dorsal horn of the spinal cord especially lamina II known as substantia gelatinosa. Although the main action of opioids on opioid receptors is pain control, many other effects have been reported. One such effect is an increase in food intake upon stimulation. Morphine is associated with an increased food intake particularly stimulating fat consumption. Other opioid agonists, including mu, delta, and kappa agonists, also increase food intake while in the short term, opioid antagonists decrease food consumption. Mu receptors appear to modulate the efficiency of energy storage during high-fat diets through the regulation of energy partitioning. Mu receptors enhance intake of high fat via their stimulation within the nucleus accumbens in the brain.

Endogenous opioid peptides are also known to be involved in the modulation of feeding behavior. Endogenous opioids within the ventral striatum of the brain may participate in the mechanisms controlling preferences for highly palatable foods, especially those rich in fat. Opioid antagonists decrease the intake of foods, especially those rich in sugar and fat, without affecting hunger and satiety. It is thus suggested that the use of antagonists may result in a decrease in diet-induced obesity. Studies performed in rats suggest antagonists of the opioid receptors increase metabolic energy consumption, and reduce weight in obese rats while maintaining muscle mass. Thus, the opioid system could serve as target for the control of feeding behavior and management of obesity. However, the search for an effective antagonist that has an overall benefit to the patient and has minimal side effects continues.