In Vitro Analyses of Spinach-Derived Opioid Peptides, Rubiscolins: Receptor Selectivity and Intracellular Activities through G Protein- and β-Arrestin-Mediated Pathways

Activated opioid receptors transmit internal signals through two major pathways: the G-protein-mediated pathway, which exerts analgesia, and the β-arrestin-mediated pathway, which leads to unfavorable side effects. Hence, G-protein-biased opioid agonists are preferable as opioid analgesics. Rubiscolins, the spinach-derived naturally occurring opioid peptides, are selective δ opioid receptor agonists, and their p.o. administration exhibits antinociceptive effects. Although the potency and effect of rubiscolins as G-protein-biased molecules are partially confirmed, their in vitro profiles remain unclear. We, therefore, evaluated the properties of rubiscolins, in detail, through several analyses, including the CellKey^TM assay, cADDis^® cAMP assay, and PathHunter^® β-arrestin recruitment assay, using cells stably expressing µ, δ, κ, or µ/δ heteromer opioid receptors. In the CellKey^TM assay, rubiscolins showed selective agonistic effects for δ opioid receptor and little agonistic or antagonistic effects for µ and κ opioid receptors. Furthermore, rubiscolins were found to be G-protein-biased δ opioid receptor agonists based on the results obtained in cADDis^® cAMP and PathHunter^® β-arrestin recruitment assays. Finally, we found, for the first time, that they are also partially agonistic for the µ/δ dimers. In conclusion, rubiscolins could serve as attractive seeds, as δ opioid receptor-specific agonists, for the development of novel opioid analgesics with reduced side effects.

Role of food-derived opioid peptides in the central nervous and gastrointestinal systems

Opioid receptors are widely distributed in central nervous system and peripheral tissues. Endogenous opioid receptor ligands are involved in many physiological processes. Exogenous peptides, derived from food proteins with gastrointestinal proteases, also exert opioid-like activities, and they include gluten exorphins (wheat), casomorphins (milk), rubiscolins (spinach), and soymorphins (soybean). Milk-derived opioid peptides play both agonistic and antagonistic roles, and most of the opioid peptides exert regulatory functions in the central nervous system, related to nociception, emotion and memory after oral, intracerebroventricular, or intraperitoneal administration. This indicates that the peptides may have crossed the blood-brain barrier or acted peripherally. Furthermore, some food-derived opioid peptides influence gastrointestinal functions such as gut motility, hormone release, appetite, mucus production, and local immunity. In healthy states, food-derived opioid peptides could benefit both the nervous and digestive systems, whereas in pathological conditions, the gastrointestinal permeability change and opioid excess may contribute to pathogenesis of some disorders. PRACTICAL APPLICATIONS: Opioid receptors are important biological targets for the treatment of multiple diseases. Traditional opiate compounds, such as alkaloids, are demonstrated to exert numerous side effects, thereby limiting their clinical effectiveness. It is thought that food-derived opioid peptides may be safer than the alkaloids, and therefore can be applied in functional food development. In this review, we summarized the already discovered food opioid peptides from different sources, and elaborated their physiological functions on the central nervous and gastrointestinal systems. These effects support further exploration of the opioid peptides as therapeutic agents or as functional food ingredient for human health promotion.