Brainstem nutrient sensing in the nucleus of the solitary tract inhibits feeding

Abstract

Direct detection of circulating nutrients by the central nervous system has been implicated in the regulation of energy balance, and the mediobasal hypothalamus is considered as the primary sensing site mediating these effects. Neurons sensitive to energyrelated signals have also been identified outside the hypothalamus, particularly within the caudomedial nucleus of the solitary tract (cmNTS) in brainstem, but the consequences of direct cmNTS nutrient detection on energy balance remain poorly characterized. Here we determined the behavioral and metabolic consequences of direct L-leucine detection by the cmNTS and investigated the intracellular signaling and neurochemical pathways implicated in cmNTS L-leucine sensing in rats. Our results support the distributed nature of central nutrient detection, evidence a role for the cmNTS S6K1 pathway in the regulation of meal size and body weight, and suggest that the cmNTS integrates direct cmNTS nutrient detection with gut-derived, descending forebrain, and adiposity signals of energy availability to regulate food intake.

Figure 1

NTS L-leucine administration reduces food intake and body weight. (A) Food intake, (B) 24h body weight change, (C) first meal size, (D) mean meal size and (E) cumulative meal number in rats following a bilateral injection of aCSF or L-leucine into the cmNTS. Data are means ± SEM (n=12). *: p<0.05, **: p<0.01.

Figure 2

NTS L-leucine activates NTS S6K1 this signaling and this activation is required for NTS L-leucine’s anorexigenic effects. (A) NTS S6K1 Thr389 phosphorylation (n=5) and (B) NTS pS6 immunoreactivity (n=4) in 24h fasted rats 30 min after a bilateral NTS aCSF or L-leucine injection. (C–E) Colocalization (right, orange) of DBH (C), TH (D) or POMC (E) (left, red) with pS6 (middle, green) in 24h fasted rats 30 min after a bilateral NTS L-leucine injection (n=4). (F) Food intake, first meal size and meal size following an icv injection of aCSF or SHU9119 combined with a bilateral injection of aCSF or L-leucine into the cmNTS. (G) Food intake, 24h body weight change, first meal size, mean meal size and cumulative meal number in rats following a bilateral injection of aCSF or L-leucine combined to a bilateral injection of DMSO or rapamycin into the cmNTS. Data are means ± SEM (n=12). *: p<0.05. scale bar: 100 μm.

Figure 3

Acute inhibition of NTS S6K1 signaling increases food intake. (A–B) DVC pS6 immunoreactivity in 24h fasted (A) and 30 min refed rats (B). (C) First meal latency, (D) first meal size, (E) food intake, (F) mean meal size and (G) cumulative meal number in rats following a bilateral injection of DMSO or rapamycin into the cmNTS. Data are means ± SEM (n=12). *: p<0.05.

Figure 4

NTS S6K1 is a physiologically relevant nutrient sensor regulating the control of energy balance (A) NTS S6K1 Thr389 phosphorylation in fasted rats bilaterally infected with the LACZ or CA S6K adenovirus in the NTS. (B) Food intake, (C) mean meal size, (D) cumulative meal number and (E) body weight in rats bilaterally infected with the LACZ or CA S6K adenoviruses in the NTS (n=6). (F) Fat mass and (G) fat free mass in rats 10 days following NTS bilateral infection with the LACZ or CA S6K adenovirus. Data are means ± SEM (n=6). *: p<0.05.

Figure 5

The NTS integrates ip CCK, NTS leptin and NTS MTII signals with L-leucine sensing to acutely decrease meal size. First meal size following NTS bilateral administration of an anorexigenic dose of L-leucine combined with (A) a subthreshold dose of ip CCK, (B) a subthreshold dose of NTS leptin, or (C) a subthreshold dose of NTS MTII. Data are means ± SEM (n=12). Means sharing a common letter are not significantly different (p>0.05).

Figure 6

Activation of NTS Erk_1/2 signaling is required for NTS L-leucine anorexigenic effect, and NTS Erk_1/2 acts as a molecular integrator of ip CCK and NTS L-leucine. (A) DVC Erk_1/2 Thr202/Tyr204 phosphorylation in 24h fasted 30 min following an NTS administration of aCSF or L-leucine (n=5). (B) Food intake, (C) 24h body weight change, (D) first meal size, and (E) mean meal size in rats following a bilateral injection of aCSF or L-leucine combined to a bilateral injection of DMSO:saline (1:1) or U0126 into the cmNTS (n=12). (F) DVC Erk_1/2 Thr202/Tyr204 phosphorylation in 24h fasted 30 min following an NTS administration of aCSF or L-leucine combined with an ip injection of saline or CCK. Data are means ± SEM (n=5). *: p<0.05.