Integrated effects of leptin in the forebrain and hindbrain of male rats

Abstract

Leptin receptors (ObRs) in the forebrain and hindbrain have been independently recognized as important mediators of leptin responses. It is unclear how leptin activity in these areas is integrated. We tested whether both forebrain and hindbrain ObRs have to be activated simultaneously to change energy balance and to maintain metabolic homeostasis. Previous studies used acute leptin injections in either the third ventricle (1-5 μg) or the fourth ventricle (3-10 μg); here we used 12-day infusions of low doses of leptin in one or both ventricles (0.1 μg/24 h in third, 0.6 μg/24 h in fourth). Male Sprague Dawley rats were fitted with third and fourth ventricle cannulas, and saline or leptin was infused from Alzet pumps for 6 or 12 days. Rats that received leptin into only the third or the fourth ventricle were not different from controls that received saline in both ventricles. By contrast, rats with low-dose leptin infusions into both the third and fourth ventricle showed a dramatic 60% reduction in food intake that was reversed on day 6, a 20% weight loss that stabilized on day 6, and a 50% decrease in body fat at day 12 despite the correction of food intake. They displayed normal activity and maintained energy expenditure despite weight loss, indicating inappropriately high thermogenesis that coincided with increased signal transducer and activator of transcription 3 (STAT3) phosphorylation in the brainstem. Altogether, these findings show that with low doses of leptin, chronic activation of both hypothalamic and brainstem ObRs is required to reduce body fat.

Figure 1.

Food intake, body weight, body fat, and lean body mass of rats from experiment 1 infused with saline (SS), low-dose leptin in either third or fourth ventricle (LS or SL), or low-dose leptin in both ventricles (LL) for 12 days. Data are means ± SEM for 9 rats. *, Significant difference between rats infused in both ventricles (LL) and all other groups (SS, LS, and SL) at P < .05. Data for body fat or lean body mass that do not share a common superscript are significantly different at P < .05.

Figure 2.

RER and energy expenditure of rats from experiment 1 infused with PBS (SS), low-dose leptin in either third or fourth ventricle (LS or SL), or low-dose leptin in both ventricles (LL) for 12 days. Data are means ± SEM for 9 rats. *, Significant difference between LL rats and all other groups at P < .05.

Figure 3.

Western blot analysis of total STAT3 expression, phosphorylation (p) of leptin signaling proteins STAT3 and ERK1 and -2 in hypothalamic and brainstem tissue of saline-infused (SS), third or fourth ventricle leptin-infused (LS or SL) or double ventricle leptin-infused rats (LL) for 12 days in experiment 1. Data are means ± SEM for 9 rats. Data for a specific parameter that do not share a common superscript are significantly different at P < .05.

Figure 4.

Food intake, Western blot analysis of UCP1 in brown fat, body fat, and lean body mass of rats from experiment 2 infused with saline, low-dose leptin in either third or fourth ventricles (LS or SL), or both ventricles (LL) for 6 or 12 days. Data are means ± SEM for 7 to 11 rats. *, Significant difference between saline and LL rats at P < .05. Values for body fat and lean body mass that do not share a common superscript are significantly different at P < .05.

Figure 5.

Western blot analysis of total STAT3 expression and phosphorylation (p) of leptin signaling proteins STAT3 and ERK1 and -2 in hypothalamic and brainstem tissue of saline-infused (SS6) and double ventricle leptin-infused rats (LL6) for 6 days in experiment 2. Data are means ± SEM for 8 rats. *, Significant difference between saline and LL rats at P < .05.