Figure 1.
Central OT infusion causes body weight loss independently from changes in food intake.
The measurements were performed over a 14-day experimental period (weeks 5 through 7 of a high fat diet): (A) Cumulative body weight changes; (B) cumulative food intake; (C) food efficiency ((body weight gain/cumulative food intake over the 2 week experimental period) x 100). Filled bars: i.c.v. saline–infused controls; open bars: i.c.v. OT-infused rats (1.6 nmol/d). Values are mean ± SEM of 6 to 7 rats/group. *P<0.05 compared to controls.
Figure 2.
Central OT infusion stimulates lipid metabolism.
The following analyses were performed on epididymal white adipose tissue (eWAT) of i.c.v. saline–infused controls (filled bars) and i.c.v. OT-infused rats (1.6 nmol/d; open bars): (A) mRNA expression of enzymes related to lipid metabolism; (B) Western blot analysis of HSL standardized to actin expression; (C) mRNA expression of PPAR-alpha and PPAR-alpha target genes; (D) Scd1 mRNA expression; (E) Oleoylethanolamide (OEA) content in eWAT. Values are mean ± SEM of 6 to 7 rats/group. *P<0.05, **P<0.01 compared to controls.
Table 1.
Effects of i.c.v. oxytocin (1.6 nmol/d) infusion on plasma glucose, insulin, leptin, FFA, glycerol, TG, oleoylethanolamide (OEA), palmitoylethanolamide (PEA), anandamide (AEA) and 2-arachidonoylglycerol (2-AG) levels.
Figure 3.
Dose-dependency of the effect of central OT infusion.
After 14-day treatments (weeks 5 to 7 of high fat diet, 45% fat), the following measurements were made in i.c.v. saline–infused controls (filled bars), i.c.v. OT-infused rats (16 nmol/d; open bars), and i.c.v. saline-infused pair-fed (PF) controls (hatched bars): (A) Delta body weight gain; (B) cumulative food intake; (C) plasma leptin levels; (D) respiratory exchange ratio (VO2: VCO2); (E) Scd1 mRNA expression; (F) OEA content in epididymal white adipose tissue (eWAT); and (G) mRNA expression of lipid metabolism-related enzymes in eWAT. Values are mean ± SEM of 6 to 7 animals/group. *P<0.05, **P<0.01, ***P<0.005, compared to controls.
Figure 4.
Central OT infusion induces hypothalamic OT synthesis and release into the bloodstream.
The following parameters were measured at the end of 14-day treatments with two doses of i.c.v. OT infusion: (A) Oxytocin expression (Oxt) in rat hypothalamus; (B) plasma OT levels in saline–infused controls (filled bars) and OT-infused rats (1.6 nmol/d, open bars). Values are mean ± SEM of 6 to 7 rats/group. *P<0.05 compared to controls; (C) Oxytocin expression (Oxt) in rat hypothalamus; (D) plasma OT levels in saline–infused controls (filled bars), OT-infused rats (16 nmol/d, open bars) and pair-fed (PF) controls (hatched bars). Values are mean ± SEM of 6 to 7 rats/group. *P<0.05 compared to controls.
Figure 5.
OT directly affects lipid metabolism.
(A) Lipid metabolism-related enzyme expression in differentiated 3T3-L1 adipocytes (24 h saline or 5 µM OT). Values are mean ± SEM of three independent experiments. *P<0.05, **P<0.01 compared to controls. (B–C) Epididymal fat pads from lean Wistar rats were incubated at 37°C in the presence of Krebs-Ringer-Hepes buffer containing 2% FA-free BSA and 0.1% glucose. After 4 h of incubation in the presence of either saline or OT (10 nM), the amount of (B) glycerol and (C) free fatty acid released in the medium was measured. Values are mean ± SEM of three independent experiments. (D–F) Measurements performed over the 14-day s.c. saline or OT treatment in lean rats fed a standard diet: (D) Cumulative body weight gain; (E) cumulative food intake; (F) changes in body composition between days 0 and 10 of treatment. Saline–infused controls (black circles, filled bars), OT-infused rats (50 nmol/d; white diamonds, open bars), and saline-infused PF controls (black triangles, hatched bars). Values are mean ± SEM of 6 to 7 rats/group. *P<0.05.
Figure 6.
Peripheral OT effects in HFD fed rats.
The measurements were performed over a 14-day experimental period (weeks 5 through 7 of a high fat diet) in s.c. saline–infused controls (black circles, filled bars), s.c. OT-infused rats (50 nmol/d; white diamonds, open bars), and s.c. saline-infused PF controls (black triangles, hatched bars): (A) Cumulative body weight gain; (B) cumulative food intake; (C) changes in body composition between days 0 and 10 of treatment. (D) Plasma OT levels; (E) NOPE and (F) OEA content in eWAT. Values are mean ± SEM of 7 to 8 rats/group. *P<0.05, **P<0.01 compared to controls.
Figure 7.
PPAR-alpha mediates peripheral OT effects.
(A) Cumulative body weight gain after 3 days of s.c. saline or OT treatment in PPAR-alpha KO and wild-type (WT) mice. (B) eWAT mRNA expression of PPAR-alpha target genes and Scd1 in PPAR-alpha KO and WT mice. Values are mean ± SEM of 5 animals/group. *P<0.05 compared to controls.
Figure 8.
Central and peripheral OT infusion protects against high fat diet-induced insulin resistance.
I.c.v. saline- (black circles) or OT- (1.6 nmol/d; white diamonds) infused rats received: glucose tolerance tests (1.5 g/kg) before (black triangles, dashed line; 3 weeks of HFD; n = 16 rats) or after infusions (7 weeks of HFD; 14-day i.c.v. infusions; n = 6 for each treatment group): (A) delta glucose and (B) delta insulin; One-way ANOVA: *P<0.05 compared to black triangles; ‡P<0.05 compared to black circles. (C–D) Euglycemic-hyperinsulinemic clamps performed at the end of 14-day treatments: (C) Glucose infusion rate (GIR) of i.c.v. saline- (filled bars) or OT- (1.6 nmol/d; open bars) infused rats. Values are mean ± SEM of 6 to 7 rats/group. *P<0.05 compared to controls. (D) GIR of s.c. saline–infused controls (filled bars), OT-infused rats (50 nmol/d; open bars), and saline-infused PF controls (hatched bars). Values are mean ± SEM of 6 to 7 rats/group. *P<0.05.
Figure 9.
Summary of the metabolic effects of oxytocin.
Upon chronic central (i.c.v.) or peripheral (s.c.) infusion into diet-induced obese rats, oxytocin (OT) increases triglyceride (TG) uptake, lipolysis, and fatty acid β-oxidation in adipose tissue. OT activates stearoyl-Coenzyme A desaturase 1 (Scd1) to produce the endocannabinoid oleoylethanolamide (OEA), a known ligand of PPAR-alpha. The action of OT on fatty acid β-oxidation is thus exerted by direct activation of PPAR-alpha target genes via the production of OEA. Red arrows indicate the direction (up or down) of regulation. Abbreviations: ACOX (acyl-CoA oxidase 1), ACC (acetyl-coenzyme A carboxylase alpha), ATGL (patatin-like phospholipase domain containing 2), DG (diglycerides), DGAT1 (diacylglycerol O-acyltransferase homolog 1), FA (fatty acid), FAS (fatty acid synthase), GLUT4 (glucose transporter-4), HD (enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase), HSL (hormone-sensitive lipase), LPL (lipoprotein lipase), MCAD (medium chain acyl-CoA dehydrogenase), MG (monoglycerides), MGL (monoglyceride lipase), NOPE (N-oleoyl-phosphatidylethanolamine), OA (oleic acid), SA (stearic acid), PPAR-α (peroxisome proliferator-activator receptor-alpha).