Leptin regulates energy balance and motivation through action at distinct neural circuits

Abstract

Background: Overconsumption of calorically dense foods contributes substantially to the current obesity epidemic. The adiposity hormone leptin has been identified as a potential modulator of reward-induced feeding. The current study asked whether leptin signaling within the lateral hypothalamus (LH) and midbrain is involved in effort-based responding for food rewards and/or the modulation of mesolimbic dopamine.

Methods: The contribution of endogenous leptin signaling for food motivation and mesolimbic dopamine tone was examined after viral-mediated reduction of the leptin receptor within LH and midbrain neurons in male rats.

Results: Knockdown of leptin receptors selectively in the LH caused increased body weight, caloric consumption, and body fat in rats maintained on a calorically dense diet. Knockdown of leptin receptors selectively in midbrain augmented progressive ratio responding for sucrose and restored high-fat, diet-induced suppression of dopamine content in the nucleus accumbens.

Conclusions: In summary, endogenous leptin signaling in the hypothalamus restrains the overconsumption of calorically dense foods and the consequent increase in body mass, whereas leptin action in the midbrain regulates effort-based responding for food rewards and mesolimbic dopamine tone. These data highlight the ability of leptin to regulate overconsumption of palatable foods and food motivation through pathways that mediate energy homeostasis and reward, respectively.

Figure 1. Leptin receptor knockdown in the LH and midbrain

(A and B) Low-power immunofluorescent analysis depicting the extent of lenti-viral infection, with identification of EGFP-expressing neurons in the (A) LH and (B) midbrain brain nuclei that express the LepR. EGFP-expressing neurons were present in the LH, lateral VTA, and substantia nigra (SNM and SN). Scale Bar: 200 μm. (C) Representative qPCR analysis revealed a significant reduction in LepR mRNA in LH/Midbrain micropunched tissue for LepR shRNA-treated rats compared to Scrambled control rats. **= (p< 0.0024575 & p<0.0016894). Error bars represent +/− s.e.m. (D and E) High-power immunofluorescent analysis illustrating the extent of (D) orexin and (E) dopamine neuronal infection in LepR-treated rats. Both orexin (D) and Tyrosine Hxdroxylase (TH) (E) are immunostained red. Scale Bar: 200 μm. Abbreviations: AHP: Anterior Hypothalamic Area, Posterior Part, AHC: Anterior Hypothalamic Area, Central Part, F: Fornix, LH: Lateral Hypothalamic Area, PN:Paranigral Nucleus, VTA: Ventral Tegmental Area, SNM: Substantia Nigra Medial Part, SN: Substantia Nigra.

Figure 2. Reduction of lepr in the LH Increases Body Weight, Caloric Consumption and Body Fat

(A) Body weight change in LepR and Scrambled treated rats maintained on either chow or high fat diet (HFD). *= (p< 0.0015379) indicates significant differences between Scrambled and LepR-treated rats maintained on HFD. (B) Caloric consumption in LepR and Scrambled treated rats maintained on either chow or HFD. *= (p< 0.000117) indicates significant differences between Scrambled and LepR-treated rats maintained on HFD. (C) NMR body fat analysis in LepR and Scrambled treated rats after thirty days of exposure to chow or HFD. *= (p< 0.000433) relative to chow, **= (p<0.0031673) indicates significant differences between LepR and Scrambled-treated rats maintained on HFD.

Figure 3. Reduction of LepR in the LH does not affect PR responding or mesolimbic dopamine

(A) Fixed and (B) Progressive Ratio (PR) break point responding in LepR and Scrambled treated rats maintained on chow. (C) Basal dopamine levels in the nucleus accumbens (NAcc) from LepR and Scrambled treated rats maintained on chow or HFD. *= (p< 0.017784) indicates significant differences between Scrambled treated rats maintained on Chow or HFD *= (p< 0.039399) indicates significant differences between LepR treated rats maintained on Chow or HFD. Error bars represent +/− s.e.m.

Figure 4. Leptin receptor knockdown in midbrain neurons does not affect body weight or caloric consumption

(A) Body weight change in LepR and Scrambled treated rats maintained on either chow or high fat diet (HFD). (B) Caloric consumption in LepR and Scrambled treated rats maintained on either chow or HFD. *= (p< 0.0009431) indicates significant differences between Scrambled and LepR-treated rats maintained on HFD after four days of diet exposure.

Figure 5. Leptin receptor knockdown in midbrain neurons augments effort-based responding for food rewards

(A) Fixed and (B) Progressive Ratio (PR) break point responding in LepR and Scrambled treated rats maintained on chow. *= (p< 0.0022121). (C) Basal dopamine levels in the nucleus accumbens (NAcc) from LepR and Scrambled treated rats maintained on chow or HFD. *= (p< 0.003456) indicates significant differences between Scrambled treated rats maintained on chow or HFD. **= (p< 0.009563) indicates significant differences between Scrambled and LepR-treated rats maintained on HFD. Error bars represent +/− s.e.m. (D) PR break point responding in rats injected bilaterally with 500ng of leptin. Error bars represent +/− s.e.m. (E) Double-label fluorescent immunostaining of TH and pSTAT3 taken 1 h after a 500 ng dose of saline or leptin (F) administered directly into the midbrain. Scale Bar: 200μm. TH immunostaining is green, pSTAT3 staining is red.