Responsiveness to Leptin Restored by Amylin in Diet Induced Obese (DIO) Rats: Magnitude and Mechanisms of Synergy.

Roth, Jonathan D.; Roland, Barbara; Cole, Rebecca; Coffey, Todd; Cronister, Carrie; Weyer, Christian; Baron, Alain; Parkes, David

Previously, we reported that single dose combination therapy with rat amylin (100 mcg/kg/d) and murine leptin (500 mcg/kg/d) elicited greater body weight (BW) loss in leptin resistant DIO rats than predicted by the sum of monotherapy-induced BW loss, a finding consistent with amylin restoration of responsiveness to leptin. Here, we used a 3*4 factorial design to define the interaction of a lower range of doses of amylin (0, 10, 50 mcg/kg/d) and leptin (0, 5, 25, 125 mcg/kg/d) for BW loss following 4 w of subcutaneous (SC) osmotic infusion in DIO rats. This design enabled a Response Surface Methodology (RSM) analysis and statistical testing for synergy. At 4 w, the RSM revealed a synergistic effect of the amylin+leptin combination (P<0.03), at doses lower than previously reported. The highest dose combination (amylin 50+leptin 125) induced a mean(±SEM) BW loss of -14.8(±3.2)% (vehicle corrected). Leptin resistance in DIO rodents has been attributed to impaired hypothalamic leptin signaling, chiefly within the arcuate nucleus (Arc). We hypothesized that amylin modulates hypothalamic sensitivity to leptin. Hypothalamic pSTAT-3 immunoreactive (IR) cells stimulated by leptin (15 mg/kg, IP) were quantified in DIO rats pre-treated with either vehicle, amylin (100 mcg/kg/d, SC) or food restriction by pair-feeding (PF) to amylin rats for 7d, and in lean rats maintained on low fat diet. Compared to lean controls, vehicle-treated DIO rats had reduced pSTAT-3 IR in both the Arc (-45%, p<0.0001) and the ventromedial hypothalamus (VMH; -45%, p<0.0001). Amylin p re-treatment for 7 d increased leptin-induced pSTAT-3 IR in the VMH relative to vehicle (by 39%; p=0.015) and PF controls (by 51%, p=0.004), but not in the Arc. These findings formally confirm BW loss synergy between amylin and leptin in DIO rats anti reveal a novel neural correlate for synergy, whereby amylin, via hindbrain signaling, upregulates leptin VMH signaling. Unexpectedly, restoration of pSTAT-3 signaling in Arc does not appear to be necessary for expression of this effect.

LEPTIN; AMYLIN; WEIGHT loss; BODY weight; RATS

Diabetes, 2007, Vol 56, pA72

0012-1797

Academic Journal