Rubiscolin‑6 rapidly suppresses the postprandial motility of the gastric antrum and subsequently increases food intake via δ‑opioid receptors in mice

Abstract

Rubiscolin‑6 is a food‑derived opioid peptide found in Spinacia oleracea that has anti‑nociceptive, memory‑enhancing, anxiolytic‑like and anti‑depressant effects. Rubiscolin‑6 has been reported to have two opposing effects on food intake. Food intake is closely connected to gut motility; however, to the best of our knowledge, the effect of rubiscolin‑6 on gut motility has not been reported. The present study aimed to investigate the effect of rubiscolin‑6 on postprandial motility of the gastric antrum in conscious mice. A catheter was implanted in the gastric antrum of male C57BL/6J mice. Manometric measurements were performed in fasted male mice and chow was then provided to assess motility in the fed state. Rubiscolin‑6, the δ‑opioid receptor antagonist naltrindole, a mixture of rubiscolin‑6 and naltrindole, or vehicle was administered intraperitoneally 30 min after eating. The percentage motor index (%MI) was then calculated. Cumulative food intake was measured in both ad libitum‑fed and overnight‑fasted mice. The %MI was significantly lower in mice treated with rubiscolin‑6 compared with that in the other groups, but normalized by treatment with the rubiscolin‑6/naltrindole mixture. The decrease in %MI induced by rubiscolin‑6 remained for 1 h after administration. Cumulative food intake was significantly higher 4 and 6 h after rubiscolin‑6 administration in ad libitum‑fed mice but was normalized by the rubiscolin‑6/naltrindole mixture. Food intake 30 min after rubiscolin‑6 administration was normal, but was higher in mice treated with the rubiscolin‑6/naltrindole mixture. Thus, rubiscolin‑6 may have a rapid effect to reduce postprandial antral motility and may subsequently increase food intake after this inhibitory effect disappears. These effects were revealed to be mediated through δ‑opioid receptors. The orexigenic effect of rubiscolin‑6 may be applicable to the treatment of anorexia and cachexia.

Keywords: eating behavior; manometric method; postprandial motility; stomach; δ‑opioid receptor agonist.

Figure 1.

Effects of rubiscolin-6 and a mixture of rubiscolin-6 and a δ-opioid receptor antagonist, naltrindole, on the antral motor activity in the postprandial state of mice. Representative antral motility traces from mice that underwent intraperitoneal (ip) administration of vehicle (Aa), 0.3 mg/kg rubiscolin-6 (Ab), 1 mg/kg naltrindole (Ba), or a mixture of 0.3 mg/kg rubscolin-6 and 1 mg/kg naltrindole (Bb) are shown. (C) Effects of the ip administration of rubiscolin-6 and a mixture of rubiscolin-6 and naltrindole on the change in the percentage motor index (%MI) over the subsequent 20 min. Values are mean ± SEM (n=4-7). ^**P<0.01. %MI, percentage motor index.

Figure 2.

Effects of rubiscolin-6 and a mixture of rubiscolin-6 and a δ-opioid receptor antagonist, naltrindole, on the food intake of mice. (A) Cumulative food intake of ad libitum-fed mice that underwent intraperitoneal (ip) administration of vehicle, 0.3 mg/kg rubiscolin-6, 1 mg/kg naltrindole, or a mixture of 0.3 mg/kg rubscolin-6 and 1 mg/kg naltrindole (n=11-13). (B) Food intake over 30 min of ad libitum-fed mice that underwent ip administration of vehicle, 0.3 mg/kg rubiscolin-6, 1 mg/kg naltrindole, or a mixture of 0.3 mg/kg rubscolin-6 and 1 mg/kg naltrindole (n=13-18). (C) Cumulative food intake of fasted mice that underwent ip administration of vehicle, 0.3 mg/kg rubiscolin-6, 1 mg/kg naltrindole, or a mixture of 0.3 mg/kg rubscolin-6 and 1 mg/kg naltrindole (n=8). Values are mean ± SEM. ^*P<0.05, ^**P<0.01.

Figure 3.

Changes in the percentage motor index (%MI) in the antrum in the periods 0-20, 20-40, 40-60, 60-80 and 80-100 min after administration of vehicle or rubiscolin-6. Values are the means ± SEM (n=4). ^*P<0.05, ^**P<0.01. %MI, percentage motor index.

Figure 4.

Schematic diagram of the relationship between the inhibitory effect of rubiscolin-6 on antral motility and its positive effect on food intake.