Tropisetron modulates peripheral and central serotonin/insulin levels via insulin and nuclear factor kappa B/receptor for advanced glycation end products signalling to regulate type-2 diabetes in rats

Abstract

Despite its known central effect, 5% of serotonin is found centrally, while around 95% is found peripherally. Serotonin is stored and co-released with insulin upon pancreatic islets stimulation by glucose. This fact raises the curiosity regarding its possible role in diabetes. Hence, in this study, we assessed the possible modulatory effects of tropisetron, a 5-HT3 receptor antagonist, on type 2 diabetes mellitus models in rats. The rats were allocated into two groups: normal and diabetic. The latter group was treated with metformin (500 mg kg^-1, p.o.), tropisetron (1 and 2 mg kg^-1, i.p.), and a combination of metformin and tropisetron (1 mg kg^-1). The different treatment regimens corrected glucose and lipid homeostasis manifested by the decrease in serum levels of glucose, fructosamine, homeostasis model of insulin resistance, triglycerides, total cholesterol, free fatty acid, as well as receptor for advanced glycation end products. Additionally, the treatments elevated levels of insulin, serotonin, and homeostasis model of β-cell function. On the molecular level, treatments corrected the altered insulin signaling cascade (phosphorylated insulin receptor substrate 1, phosphorylated protein kinase B, and glucose transporter 4), and inhibited β-catenin and phosphorylated nuclear factor kappa B p65 in the assessed soleus skeletal muscle. A similar pattern was duplicated in the hippocampus. This study provided evidence for the role of tropisetron on type 2 diabetes mellitus via modulating the insulin signaling cascade (insulin, phosphorylated insulin receptor substrate 1, phosphorylated protein kinase B, and glucose transporter 4), improving lipid/glucose profile, decreasing inflammatory markers (receptor for advanced glycation end products, and phosphorylated nuclear factor kappa B p65), as well as increasing 5-HT and reducing β-catenin.

Fig. 1. Effect of 13 weeks of high fat fructose diet (HFFD) intake on IPGTT in rats. Values are expressed as mean ± S.E of 6 rats as compared to normal control group (*) (unpaired t-test), P < 0.05. Line graph showing the effect of high fat fructose diet + insulin (HFFD/insulin) on glucose tolerance test. The rats on HFFD/insulin exhibited an upward shift in the curve along the 2 h period, indicating a state of insulin resistance.

Fig. 2. Effect of diabetes and 15 day administration of Met (500 mg kg⁻¹), Trop₁, Trop₂ or Met + Trop₁ on serum (a) glucose, (b) fructosamine, (c) HOMA-IR, and (d) HOMA-β in rats. Statistical analysis was performed using one-way ANOVA, followed by Tukey's post hoc test (P < 0.05). Values are expressed as mean ± S.E of (6 rats) and a comparison was obtained for normal control (*), diabetic control (#), and Met ($) treated groups. Met: metformin (500 mg kg⁻¹, p.o.); Trop₁: tropisetron (1 mg kg⁻¹, i.p); Trop₂: tropisetron (2 mg kg⁻¹, i.p).

Fig. 3. Effect of diabetes and 15 day administration of Met (500 mg kg⁻¹), Trop₁, Trop₂ or Met + Trop₁ on serum (a and b) and hippocampal (c and d) contents of insulin and 5-HT in rats. Statistical analysis was performed using one-way ANOVA, followed by Tukey's post hoc test (P < 0.05). Values are expressed as mean ± S.E of (6 rats) and a comparison was obtained for normal control (*), diabetic control (#), Met ($) and Trop₁ treated (@) groups. Met: metformin (500 mg kg⁻¹, p.o.); Trop₁: tropisetron (1 mg kg⁻¹, i.p); Trop₂: tropisetron (2 mg kg⁻¹, i.p).

Fig. 4. Effect of diabetes and 15 day administration of Met (500 mg kg⁻¹), Trop₁, Trop₂ or Met + Trop₁ on BW (a) serum TG (b), TC (c) and (d) FFAs in rats. Statistical analysis was performed using one-way ANOVA, followed by Tukey's post hoc test (P < 0.05). Values are expressed as mean ± S.E of (6 rats) and a comparison was obtained for normal control (*), diabetic control (#), Met ($) and Trop₁ treated (@) groups. Met: metformin (500 mg kg⁻¹, p.o.); Trop₁: tropisetron (1 mg kg⁻¹, i.p); Trop₂: tropisetron (2 mg kg⁻¹, i.p).

Fig. 5. Effect of diabetes and 15 day administration of Met (500 mg kg⁻¹), Trop₁, Trop₂ or Met + Trop₁ on serum (a) RAGE, muscular and hippocampal (b and c) β-catenin, muscular and hippocampal (d and e) contents of p-NF-κB in rats. Statistical analysis was performed using one-way ANOVA, followed by Tukey's post hoc test (P < 0.05). Values are expressed as mean ± S.E of (6 rats) and a comparison was obtained for normal control (*), diabetic control (#), Met ($) and Trop₁ treated (@) groups. Met: metformin (500 mg kg⁻¹, p.o.); Trop₁: tropisetron (1 mg kg⁻¹, i.p); Trop₂: tropisetron (2 mg kg⁻¹, i.p).

Fig. 6. Effect of diabetes and 15 day administration of Met (500 mg kg⁻¹), Trop₁, Trop₂ or Met + Trop₁ on muscular content of (a) pIRS1, (b) p-AKT, (c) GLUT4, and hippocampal content of (d) p-AKT, and (e) GLUT4 in rats. Statistical analysis was performed using one-way ANOVA, followed by Tukey's post hoc test (P < 0.05). Values are expressed as mean ± S.E of (6 rats) and a comparison was obtained for normal control (*), diabetic control (#), Met ($) and Trop₁ treated (@) groups. Met: metformin (500 mg kg⁻¹, p.o.); Trop₁: tropisetron (1 mg kg⁻¹, i.p); Trop₂: tropisetron (2 mg kg⁻¹, i.p).