Blockade of the cerebral aqueduct in rats provides evidence of antagonistic leptin responses in the forebrain and hindbrain

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

Low-dose leptin infusion in the fourth ventricle of rats enhances the response to third-ventricle leptin injection

We previously reported that low-dose leptin infusions into the third or fourth ventricle that do not affect energy balance when given independently cause rapid weight loss when given simultaneously. Therefore, we tested whether hindbrain leptin enhances the response to forebrain leptin or whether forebrain leptin enhances the response to hindbrain leptin. Rats received fourth-ventricle infusions of saline or 0.01, 0.1, 0.3, or 0.6 μg leptin/day for 13 days. On days 9 and 13, 0.1 μg leptin was injected into the third ventricle. The injection inhibited food intake for 36 h in saline-infused rats but for 60 h in those infused with 0.6 μg leptin/day. Leptin injection increased intrascapular brown fat temperature in leptin-infused, but not saline-infused, rats. In a separate experiment, rats received third-ventricle infusions of saline or 0.005, 0.01, 0.05, or 0.1 μg leptin/day and fourth-ventricle injections of 1.0 μg leptin on days 9 and 13 Leptin injection inhibited food intake, respiratory exchange ratio, and 14-h food intake in rats infused with saline or the two lowest doses of leptin. There was no effect with higher-dose leptin infusions because food intake, body fat, and lean mass were already inhibited. These data suggest that activation of leptin receptors in the hindbrain enhances the response to third-ventricle leptin, whereas activation of forebrain leptin receptors does not enhance the response to fourth-ventricle leptin, consistent with our previous finding that weight loss in rats treated with fourth-ventricle leptin is associated with indirect activation of hypothalamic STAT3.

Leptin in the hindbrain facilitates phosphorylation of STAT3 in the hypothalamus

Leptin receptors (ObRs) in the forebrain and hindbrain have been independently recognized as important mediators of leptin responses. We recently used low-dose leptin infusions to show that chronic activation of both hypothalamic and hindbrain ObRs is required to reduce body fat. The objective of the present study was to identify the brain nuclei that are selectively activated in rats that received chronic infusion of leptin in both the forebrain and hindbrain. Either saline or leptin was infused into third and fourth ventricles (0.1 μg/24 h in the third ventricle and 0.6 μg/24 h in the fourth ventricle) of male Sprague-Dawley rats for 6 days using Alzet pumps. Rats infused with leptin into both ventricles (LL rats) showed a significant increase in phosphorylated (p)STAT3 immunoreactivity in the arcuate nucleus, ventromedial hypothalamus, dorsomedial hypothalamus, and posterior hypothalamus compared with other groups. No differences in pSTAT3 immunoreactivity were observed in midbrain or hindbrain nuclei despite a sixfold higher infusion of leptin into the fourth ventricle than the third ventricle. ΔFosB immunoreactivity, a marker of chronic neuronal activation, showed that multiple brain nuclei were chronically activated due to the process of infusion, but only the arcuate nucleus, ventromedial hypothalamus, dorsomedial hypothalamus, and ventral tuberomamillary nucleus showed a significant increase in LL rats compared with other groups. These data demonstrate that low-dose leptin in the hindbrain increases pSTAT3 in areas of the hypothalamus known to respond to leptin, supporting the hypothesis that leptin-induced weight loss requires an integrated response from both the hindbrain and forebrain.