Rats that are predisposed to excessive obesity show reduced (leptin-induced) thermoregulation even in the preobese state

Abstract

Both feeding behavior and thermogenesis are regulated by leptin. The sensitivity to leptin's anorexigenic effects on chow diet was previously shown to predict the development of diet-induced obesity. In this study, we determined whether the sensitivity to leptin's anorexigenic effects correlates with leptin's thermogenic response, and if this response is exerted at the level of the dorsomedial hypothalamus (DMH), a brain area that plays an important role in thermoregulation. Based on the feeding response to injected leptin on a chow diet, rats were divided into leptin-sensitive (LS) and leptin-resistant (LR) groups. The effects of leptin on core body, brown adipose tissue (BAT) and tail temperature were compared after intravenous versus intra-DMH leptin administration. After intravenous leptin injection, LS rats increased their BAT thermogenesis and reduced heat loss via the tail, resulting in a modest increase in core body temperature. The induction of these thermoregulatory mechanisms with intra-DMH leptin was smaller, but in the same direction as with intravenous leptin administration. In contrast, LR rats did not show any thermogenic response to either intravenous or intra-DMH leptin. These differences in the thermogenic response to leptin were associated with a 1°C lower BAT temperature and reduced UCP1 expression in LR rats under ad libitum feeding. The preexisting sensitivity to the anorexigenic effects of leptin, a predictor for obesity, correlates with the sensitivity to the thermoregulatory effects of leptin, which appears to be exerted, at least in part, at the level of the DMH.

Keywords: Brown adipose tissue; leptin; leptin sensitivity; tail temperature; thermogenesis.

Figure 1

Animal model and experimental design. (A) Rats underwent surgery to implant: 1) A catheter in the jugular vein; 2) Local cannulas bilaterally above the DMH; 3) An intraabdominal telemetric transmitter with probes in the liver and brown adipose tissue (BAT). An infrared/thermosensitive camera was used to measure the tail base temperature. The region of interest for the tail base is indicated. (B) The thermogenic response to leptin was tested both via systemic injections through the jugular catheter (250 ug, i.v), and local infusions in the DMH (bilateral, 300 ng/300 nL/60sec). Rats were food restricted prior to injections to lower their body temperature. (C) Example of the anatomical verification of correctly placed DMH cannulas.

Figure 2

Individual 1–24 h leptin sensitivity in LS and LR rats fed a chow diet. Rats were divided into those showing a reduction in food intake during the first hour after intravenous leptin injection (leptin sensitive, LS) and those that did not change or increased their food intake with intravenous leptin at 1 h food intake (leptin resistant, LR). (A) Experimental design. Leptin sensitivity was measured by cumulative food intake after leptin injection normalized to vehicle food intake. Average leptin sensitivity of two tests is shown. Leptin sensitivity (B) at group level and (C) individual level; a heat plot of the relative level of sensitivity is shown at 1–24 h food intake for each individual rat (i.e., each row). The heat plot indicates the relative degree of leptin sensitivity at a particular time point in comparison with the other time points in the row. Data are shown as mean ± SEM; n = 10–11 per group

Figure 3

Comparison of core body and BAT temperature between LS and LR rats. (A) Core body (liver) temperature, BAT temperature, and locomotor activity during ad libitum feeding, food restriction (10 grams of chow overnight for 2 days), and refeeding in LS versus LR rats. For statistics, see Tables S2 and S3. Data are shown as mean ± SEM. The dotted lines show the SEM. N = 3–5 for LS rats and n = 5–6 for LR rats. (B) Maximal BAT temperature in the light and dark phase in LS versus LR rats during ad libitum feeding. For each rat, the average of the five highest BAT temperatures during ad libitum feeding was taken during the light phase and dark phase, respectively. Flight = 2.377, P = 0.084; Fdark = 2.203, P = 0.091. Data are shown as mean ± SEM; n = 3 for LS rats and n = 5 for LR rats. #P = 0.08‐0.09 in LS versus LR rats. The shaded areas indicate the dark phase. (C) BAT UCP1 mRNA expression (2^‐(∆ cycle threshold) with Hmbs as reference gene) in LS versus LR rats. Data are shown as mean ± SEM; n = 4 per group; t = 2.260, *P = 0.033.

Figure 4

Comparison of leptin regulation of thermogenesis between LS and LR rats after intravenous leptin administration. (A) Continuous and (B) average change in core body (liver) temperature after intravenous leptin/vehicle injection in LS (n = 5) versus LR rats (n = 9), in the presence and absence of food. Without food: Ftreatment*responder=1.441, P = 0.107; with food: Ftreatment*responder=2.945, P = 0.112. (C) Continuous and (D) average change in BAT temperature after intravenous leptin/vehicle injection in LS (n = 3) versus LR rats (n = 6), in the presence and absence of food. Without food: Ftreatment*responder=4.218, P = 0.079. Post hoc in LS and LR separately: LS t = −3.474, P = 0.037; LR t = 1.052, P = 0.171. With food: Ftreatment*responder=7.961, P = 0.026. Post hoc in LS and LR separately: LS t = −2.632, P = 0.06; LR t = 0.760, P = 0.240. (E) Change in tail base temperature and representative temperature images after intravenous leptin/vehicle injection in LS (n = 9) versus LR (n = 11) rats in the absence of food. Thermal images were taken shortly before injection, and at 60 and 120 min. postinjection. (F) Average change in tail base temperature. Ftreatment*responder=3.718, P = 0.074. Post hoc in LS and LR separately: LS t = 2.086, P = 0.041; LR t = −0.791, P = 0.226. Data are shown as mean ± SEM. The dotted lines show the SEM. *P < 0.05, #P = 0.06 for leptin versus vehicle. The shaded areas indicate measurements in the presence of food.

Figure 5

Comparison of leptin regulation of thermogenesis between LS and LR rats after intra‐DMH leptin administration. (A) Continuous and (B) average change in core body (liver) temperature after intra‐DMH leptin/vehicle injection in LS (n = 6) versus LR rats (n = 7), in the presence and absence of food. Without food: Ftreatment*responder=1.973, P = 0.186; with food: Ftreatment*responder=0.389, P = 0.544. (C) Continuous and (D) average change in BAT temperature after intra‐DMH leptin/vehicle injection in LS (n = 3) versus LR rats (n = 4), in the presence and absence of food. Without food: Ftreatment*responder=2.214, P = 0.187; with food: Ftreatment*responder=0.009, P = 0.928. (E) Change in tail base temperature and representative temperature images after intra‐DMH leptin/vehicle injection in LS (n = 7) versus LR (n = 9) rats in the absence of food. Thermal images were taken shortly before injection, and at 60 and 120 min. postinjection. (F) Average change in tail base temperature. Ftreatment*responder=0.273, P = 0.608. (G) Correlation between core body temperature before injection and delta change in core body temperature 0–2 h following injection of leptin (orange) or vehicle (blue). LS, R2 = 0.488, P = 0.012, and LR, R2 = 0.042, P = 0.445. Data are shown as mean ± SEM. The dotted lines show the SEM. #P = 0.07 for leptin versus vehicle. The shaded areas indicate measurements in the presence of food.