Serotonin Improves High Fat Diet Induced Obesity in Mice

There are two independent serotonin (5-HT) systems of organization: one in the central nervous system and the other in the periphery. 5-HT affects feeding behavior and obesity in the central nervous system. On the other hand, peripheral 5-HT also may play an important role in obesity, as it has been reported that 5-HT regulates glucose and lipid metabolism. Here we show that the intraperitoneal injection of 5-HT to mice inhibits weight gain, hyperglycemia and insulin resistance and completely prevented the enlargement of intra-abdominal adipocytes without having any effect on food intake when on a high fat diet, but not on a chow diet. 5-HT increased energy expenditure, O2 consumption and CO2 production. This novel metabolic effect of peripheral 5-HT is critically related to a shift in the profile of muscle fiber type from fast/glycolytic to slow/oxidative in soleus muscle. Additionally, 5-HT dramatically induced an increase in the mRNA expression of peroxisome proliferator-activated receptor coactivator 1α (PGC-1α)-b and PGC-1α-c in soleus muscle. The elevation of these gene mRNA expressions by 5-HT injection was inhibited by treatment with 5-HT receptor (5HTR) 2A or 7 antagonists. Our results demonstrate that peripheral 5-HT may play an important role in the relief of obesity and other metabolic disorders by accelerating energy consumption in skeletal muscle

Effect of 5-HT on Energy Metabolism.

<p>A, Energy expenditure, oxygen consumption, production of carbon dioxide and respiratory exchange ratio were measured for 24 h in each group mice at 21 to 23 weeks of age (n = 5–6). The acclimation time was 1 h. These values were normalized to body weight. B, The food intake of each group of mice was measured for 1 day at 17 weeks of age (n = 6). C, The rectal temperatures of each group mice at 22 weeks of age are indicated (n = 14). Data are the mean ± s.e. Data were analyzed by Student's t test (A) and two-way ANOVA (B). Columns with a different letter are significantly different (p<0.05). *p<0.05, **p<0.01 indicates significance in 5-HT-treated mice against PBS-treated mice.</p

Metabolic Effect of 5HT on a High Fat Diet.

<p>A, The concentrations of blood NEFA, cholesterol, triglyceride, glucose, insulin, leptin and adiponectin were measured in mice aged between 21 and 26 weeks of age (n = 7–12). B and C, Glucose tolerance (B) and insulin tolerance (C) tests were performed in each group of mice at 21 weeks of age (n = 7). Circles, Ch+PBS; filled circles, Ch+5HT; squares, F+PBS; filled squares, F+5HT. Data are means ± s.e. Data in (A) were analyzed by two-way ANOVA. Data in (B) and (C) were analyzed by Student's t test. Columns with a different letter are significantly different (p<0.05). *p<0.05, **p<0.01 indicates significance in F+5HT mice against F+PBS mice.</p

Effect of 5-HT on Adiposity of Mice Fed a High Fat Diet.

<p>A, Body weight of male C57BL/6 mice fed a chow diet (Ch) or a high fat diet (F) were measured over 26 weeks (n = 12 to 20). Mice were intraperitoneally injected with PBS or 5-HT (1 mg/mouse) twice a week. Open circles, chow plus PBS (Ch+PBS); filled circles, chow plus 5-HT (Ch+5HT); open squares, F diet plus PBS (F+PBS); filled squares, F diet plus 5-HT (F+5HT). B and C, Photographs are the gross appearance (B) and the abdominal cavity (C) of representative mice from each group at 26 weeks of age. D, The proportion of whole body fat (n = 5) and intraabdominal fat (n = 10) of each group of mice at 26 weeks of age. E, The adipose tissues of mice at 26 weeks of age were stained with hematoxylin and eosin. Bars in histological sections indicate 50 μm. Figs show the size distribution of adipocytes. Data are means ± s.d. Figs with a different superscript letter are significantly different (p<0.05). F, The change in body weight of C57BL/6 mice treated with several doses of 5-HT were measured over 22 weeks (n = 14 to 20). The body weight of mice at 22 weeks of age shows the concentration dependent effect of 5HT. Open circles, Ch+PBS; open squares, F+PBS; filled symbols, F+5HT at a concentration of 0.1 mg (▲), 0.5 mg (◆) and 1.0 mg/mouse (■). Red colored symbols show significance in F+5HT mice against F+PBS mice (p<0.05). Data are means ± s.e. All data were analyzed by two-way ANOVA without (F). Data in (F) was analyzed by one-way ANOVA. Columns with a different letter are significantly different (p<0.05). *p<0.05, **p<0.01 indicates significance in F+5HT mice against F+PBS mice.</p

Effect of 5-HT on mRNA expression in soleus muscle.

<p>The relative mRNA expression levels of Total PGC-1α, PGC-1α-a, PGC-1α-b, PGC-1α-c, D2, UCP3, PPARγ, NRF1, Cyt-c, COXIV, HSL and CPT-1b were analyzed in soleus muscle of each group of mice at 14 weeks of age by quantitative real-time RT-PCR (n = 6–8). Data are the mean ± s.e. Data were analyzed by two-way ANOVA. Columns with a different letter are significantly different (p<0.05).</p

5-HT elevates the proportion of slow type muscle fibers in the skeletal muscle of mice on the high fat diet.

<p>A, Mice fed a chow diet (Ch) or a high fat diet (F) were i.p. injected with PBS or 5-HT (1 mg/mouse) twice a week. After 8 weeks of treatment, the frozen sections of soleus and gastrocnemius muscles of 14 weeks of age mice were immunostained using anti-slow and anti-fast myosin heavy chain antibodies, specific markers of myofiber type-I and type-II, respectively (n = 5). S and G in (A) indicate the positions of soleus and gastrocnemius muscles, respectively. Bars in histological sections indicate 200 μm. B, The activities of oxidative enzyme in soleus and gastrocnemius muscles of mice at 14 weeks of age of each group were examined by enzymatic staining using NADH-tetrazolium reductase (NADH-TR) shown with a blue precipitate. Bars in histological sections indicate 50 μm. C, The proportions of slow, fast and intermediate type muscle fibers in soleus muscle of each group of mice were calculated by evaluating the type of all soleus muscle fiber in each immunostaining section in (A) (n = 5). D, Intracellular levels of NAD-to-NADH ratio were measured in soleus and gastrocnemius muscles of mice at 14 weeks of age (n = 5–7). Data are the mean ± s.e. Data were analyzed by two-way ANOVA. Columns with a different letter are significantly different (p<0.05).</p

Elevation of mRNA Expressions of PGC-1α-b and PGC-1α-c by 5HT.

<p>A, In 8 weeks of age mice, the mRNA expression levels of PGC-1α-b and PGC-1α-c were measured in soleus muscle and after i.p. injection of 1 mg 5-HT by real time PCR (n = 6–8). B, The relative mRNA expression levels of 5-HTR were measured in soleus muscle of mice at 8 weeks of age by using real time PCR (n = 6–8). C, At 30 min before the administration of 1 mg 5-HT, mice were injected with several 5-HTR antagonists: Methysergide (antagonist for 5-HTR1, 2 and 7, 0.1 mg/mouse), SB-269970 (5-HTR7, 0.6 mg), Ketanserin (5-HTR2A, 0.1 mg/mouse) and SB-204741 (5-HTR2B, 0.08 mg/mouse). The mRNA expressions of PGC-1α-b and PGC-1α-c in soleus muscle were determined at 120 min after 5-HT injection by real time PCR (n = 6–8). The mRNA expression levels in PBS-pretreated mice injected with 5-HT are designed as 100. Values in soleus muscle were normalized to those for 18S. Data are means ± s.e. Data were analyzed by Student's t test (A) and one-way ANOVA (C).*, P<0.05; **, P<0.01 relative to basal values at 0 min of PGC-1α-b. #, P<0.05; ##, P<0.01 relative to basal values at 0 min of PGC-1α-c. Columns with a different letter are significantly different (p<0.05).</p