The reward value of sucrose in leptin-deficient obese mice

Abstract

Leptin-deficient patients report higher "liking" ratings for food, and leptin replacement therapy normalizes these ratings even before weight loss is achieved. Since animals cannot report their ratings, we studied the relationship between leptin and food reward in leptin-deficient ob/ob mice using a optogenetic assay that quantifies the reward value of sucrose. In this assay, mice chose between one sipper dispensing the artificial sweetener sucralose coupled to optogenetic activation of dopaminergic (DA) neurons, and another sipper dispensing sucrose. We found that the reward value of sucrose was high under a state of leptin deficiency, as well as at a dose of leptin that does not suppress food intake (12.5 ng/h). Treatment with higher doses of leptin decreased the reward value of sucrose before weight loss was achieved (100 ng/h), as seen in leptin-deficient patients. These results phenocopy in mice the behavior of leptin-deficient patients.

Keywords: Food preference; Leptin; Obesity; Optogenetics; Reward value; Sucrose.

Figure A1

Food intake and body weight throughout leptin treatment: (a) percent change of food intake relative to the previous measurements, starting at pre-treatment on day 0; (b) percent change of body weight on the previous measurements, starting at pre-treatment on day 0.

Figure 1

Tissue-specific expression of channelrhodopsin in DA neurons of ob/ob mice and cranial fiber optic implant. (a–c) AAV-DIO–ChR2-mCherry injection into the VTA of ob/ob;Dat-cre mice led to ChR2-mCherry expression in neurons colocalizing with tyrosine hydroxylase (TH), a marker for DA neurons (statistics imbedded in text, scale bars represent 100 µm). (d and e) Optical fibers were implanted above the VTA (red inset box in (d)) of ob/ob;Dat-cre mice.

Figure 2

Leptin withdrawal protocol in implanted ob/ob Dat-cre mice. (a) This experimental design, in which the low dose was given before cranial surgery, was intended to improve implant stability, postoperative recovery and survival of ob/ob Dat-cre mice, minimizing the number of animals to be genotyped, implanted and euthanized. Mice had 14 days of leptin treatment before and after surgery and were behaviorally assayed before changing osmotic pumps. (b) Serum leptin levels were consistent with the dose regimen. Only the threshold dose induced serum leptin levels that were different from background serum leptin levels of untreated ob/ob mice (statistics imbedded in text). Bars are mean±SEM, ^⁎⁎⁎⁎p<0.000022.

Figure 3

Subthreshold leptin treatment did not suppress the reward value of sucrose in ob/ob mice. (a) A subthreshold dose had no significant effect either on both eating behavior or on (b) body weight (statistics imbedded in text). (c) ob/ob;Chr2+, but not ob/ob;Chr2− mice, were sensitive to the rewarding effect of optogenetic activation of DA neurons during a regimen of leptin treatment of 12.5 ng per gram of body weight. ob/ob;Chr2+, but not ob/ob;Chr2− mice, preferred to lick the sipper coupled to laser stimulation of DA neurons. (d) Both ob/ob;Chr2+ and control ob/ob;Chr2− mice preferred sucrose to sucralose coupled to laser stimulation of DA neurons (statistics imbedded in text). Bars are mean±SEM, ^⁎⁎p<0.0014.

Figure 4

Threshold leptin treatment decreased the reward value of sucrose in ob/ob mice before weight loss. (a and b) A threshold dose of 100 ng per gram of body weight had an acute and significant effect on daily food intake, even before body weight decreases (statistics imbedded in text). (c) ob/ob;Chr2+ mice, but not ob/ob;Chr2− mice, were sensitive to the rewarding effect of optogenetic activation of DA neurons during a regimen of leptin treatment of 100 ng per gram of body weight. ob/ob;Chr2+, but not ob/ob;Chr2− mice, prefer to lick the sipper coupled to laser stimulation of DA neurons. (d) Control ob/ob;Chr2− mice preferred sucrose to sucralose coupled to laser stimulation of DA neurons, but ob/ob;Chr2+ mice preferred the reverse (statistics imbedded in text). Bars are mean±SEM, ^⁎p<0.031, ^⁎⁎⁎⁎p<0.00000023.

Figure 5

Leptin deficiency upregulates the reward value of sucrose. (a and b) Leptin deficiency by treatment withdrawal rapidly reverses changes in daily food intake (statistics imbedded in text). (c) ob/ob;Chr2+ mice, but not ob/ob;Chr2− mice, were sensitive to the rewarding effect of optogenetic activation of DA neurons after leptin treatment withdrawal. ob/ob;Chr2+, but not ob/ob;Chr2− mice, preferred to lick the sipper coupled to laser stimulation of DA neurons. (d) Both ob/ob;Chr2+ and control ob/ob;Chr2− mice preferred sucrose to sucralose coupled to laser stimulation of DA neurons 48 h after leptin treatment withdrawal (statistics imbedded in text). Bars are mean±SEM, ^⁎⁎p<0.0012.