G protein-coupled opioid receptor activity

Definition

Target type: molecularfunction

Combining with an opioid (any narcotic derived from or resembling opium), and transmitting the signal across the membrane by activating an associated G-protein. [GOC:ai, GOC:bf, PMID:20494127]

G protein-coupled opioid receptor activity encompasses the intricate molecular processes by which opioid receptors, a class of transmembrane proteins, mediate their physiological effects in response to the binding of opioid ligands. Opioid receptors belong to the G protein-coupled receptor (GPCR) superfamily, characterized by their seven transmembrane domains and their coupling to intracellular signaling pathways through heterotrimeric G proteins. Upon ligand binding, opioid receptors undergo conformational changes that trigger the activation of G proteins, leading to the generation of intracellular second messengers and ultimately, the modulation of downstream cellular processes.

Opioid receptors exhibit a high degree of selectivity for different opioid ligands, such as morphine, heroin, endorphins, and enkephalins. This selectivity is achieved through specific amino acid residues within the ligand-binding pocket of the receptor, which interact with the chemical structure of the opioid molecule.

The binding of an opioid ligand to its receptor initiates a cascade of events involving G protein activation. The activated G protein, composed of alpha, beta, and gamma subunits, dissociates into its subunits. The alpha subunit, carrying a bound guanosine triphosphate (GTP) molecule, detaches from the beta-gamma dimer and interacts with downstream effector proteins, such as adenylyl cyclase, phospholipase C, and ion channels.

The activation of these effector proteins results in the production of intracellular second messengers, such as cyclic adenosine monophosphate (cAMP), diacylglycerol (DAG), and inositol trisphosphate (IP3). These second messengers propagate the signal downstream, leading to the modulation of various cellular processes, including pain perception, mood regulation, reward circuitry, and respiratory function.

The activation of opioid receptors can lead to both stimulatory and inhibitory effects on downstream signaling pathways, depending on the specific receptor subtype and the G protein involved. For example, some opioid receptors activate G proteins that inhibit adenylyl cyclase, leading to a decrease in cAMP levels. Other receptors activate G proteins that stimulate phospholipase C, resulting in the production of DAG and IP3.

The diversity of opioid receptor subtypes, their distinct ligand affinities, and their coupling to different G proteins contribute to the wide range of physiological effects mediated by opioids. These effects include analgesia, sedation, euphoria, respiratory depression, constipation, and dependence.

In summary, G protein-coupled opioid receptor activity involves the complex interplay of ligand binding, receptor activation, G protein signaling, and the modulation of downstream effector proteins, ultimately leading to a diverse array of physiological responses.'
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Proteins (4)

Compounds (388)