The stimulating effect of ghrelin on gastric motility and firing activity of gastric-distension-sensitive hippocampal neurons and its underlying regulation by the hypothalamus

Higher-Order Inputs Involved in Appetite Control

The understanding of the neural control of appetite sheds light on the pathogenesis of eating disorders such as anorexia nervosa and obesity. Both diseases are a result of maladaptive eating behaviors (overeating or undereating) and are associated with life-threatening health problems. The fine regulation of appetite involves genetic, physiological, and environmental factors, which are detected and integrated in the brain by specific neuronal populations. For centuries, the hypothalamus has been the center of attention in the scientific community as a key regulator of appetite. The hypothalamus receives and sends axonal projections to several other brain regions that are important for the integration of sensory and emotional information. These connections ensure that appropriate behavioral decisions are made depending on the individual's emotional state and environment. Thus, the mechanisms by which higher-order brain regions integrate exteroceptive information to coordinate feeding is of great importance. In this review, we will focus on the functional and anatomical projections connecting the hypothalamus to the limbic system and higher-order brain centers in the cortex. We will also address the mechanisms by which specific neuronal populations located in higher-order centers regulate appetite and how maladaptive eating behaviors might arise from altered connections among cortical and subcortical areas with the hypothalamus.

"Sibling" battle or harmony: crosstalk between nesfatin-1 and ghrelin

Ghrelin was first identified as an endogenous ligand of the growth hormone secretagogue receptor (GHSR) in 1999, with the function of stimulating the release of growth hormone (GH), while nesfatin-1 was identified in 2006. Both peptides are secreted by the same kind of endocrine cells, X/A-like cells in the stomach. Compared with ghrelin, nesfatin-1 exerts opposite effects on energy metabolism, glucose metabolism, gastrointestinal functions and regulation of blood pressure, but exerts similar effects on anti-inflammation and neuroprotection. Up to now, nesfatin-1 remains as an orphan ligand because its receptor has not been identified. Several studies have shown the effects of nesfatin-1 are dependent on the receptor of ghrelin. We herein compare the effects of nesfatin-1 and ghrelin in several aspects and explore the possibility of their interactions.

Ghrelin and Orexin Interact to Increase Meal Size Through a Descending Hippocampus to Hindbrain Signaling Pathway

BACKGROUND

Memory and cognitive processes influence the amount of food consumed during a meal, yet the neurobiological mechanisms mediating these effects are poorly understood. The hippocampus (HPC) has recently emerged as a brain region that integrates feeding-relevant biological signals with learning and memory processes to regulate feeding. We investigated whether the gut-derived hormone ghrelin acts in the ventral HPC (vHPC) to increase meal size through interactions with gut-derived satiation signaling.

METHODS

Interactions between vHPC ghrelin signaling, gut-derived satiation signaling, feeding, and interoceptive discrimination learning were assessed via rodent behavioral neuropharmacological approaches. Downstream neural pathways were identified using transsynaptic virus-based tracing strategies.

RESULTS

vHPC ghrelin signaling counteracted the food intake-reducing effects produced by various peripheral biological satiation signals, including cholecystokinin, exendin-4 (a glucagon-like peptide-1 receptor agonist), amylin, and mechanical distension of the stomach. Furthermore, vHPC ghrelin signaling produced interoceptive cues that generalized to a perceived state of energy deficit, thereby providing a potential mechanism for the attenuation of satiation processing. Neuroanatomical tracing identified a multiorder connection from vHPC neurons to lateral hypothalamic area orexin (hypocretin)-producing neurons that project to the laterodorsal tegmental nucleus in the hindbrain. Lastly, vHPC ghrelin signaling increased spontaneous meal size via downstream orexin receptor signaling in the laterodorsal tegmental nucleus.

CONCLUSIONS

vHPC ghrelin signaling increases meal size by counteracting the efficacy of various gut-derived satiation signals. These effects occur via downstream orexin signaling to the hindbrain laterodorsal tegmental nucleus, thereby highlighting a novel hippocampus-hypothalamus-hindbrain pathway regulating meal size control.

Ghrelin fiber projections from the hypothalamic arcuate nucleus into the dorsal vagal complex and the regulation of glycolipid metabolism

OBJECTIVES

This study aimed to explore the involvement of the ghrelin pathway from the arcuate nucleus (ARC) to the dorsal vagal complex (DVC) and to determine its role in the regulation of glycolipid metabolism.

METHODS

The protein and mRNA expression of ghrelin and growth hormone (GH) secretagogue receptor type 1a (GHSR-1a) were measured using immunohistochemistry and the polymerase chain reaction (PCR) method, respectively. Ghrelin fiber projections arising from the ARC and projecting into the DVC were investigated using retrograde tracing, combined with fluorescence immunohistochemical staining. The effects of electrical stimulation (ES) of the ARC on ghrelin-responsive, glucose-sensitive DVC neurons, glycolipid metabolism, and liver lipid enzymes were determined using electrical physiological method, biochemical analysis, quantitative real-time PCR (qRT-PCR) and Western blot analysis.

RESULTS

GHSR-1a was expressed in the DVC neurons. Ghrelin fibers originating from the ARC projected into the DVC. ES of the ARC-activated the ghrelin-responsive glucose-excited (GE) and glucose-inhibited (GI) neurons in the DVC. ES of the ARC significantly elevated the serum triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and glucose levels; it reduced the serum high-density lipoprotein (HDLC) and insulin levels. Moreover, ES of the ARC increased liver acetyl-CoA carboxylase-1 (ACC-1) and decreased carnitine palmitoyltransferase-1 (CPT-1) expression, resulting in lipid accumulation in the liver. All the aforementioned effects were partially blocked by pretreatment with the ghrelin receptor antagonist [D-Lys-3]-GHRP-6 in the DVC and were reduced by vagotomy. ES of the ARC increased agouti-related protein (AgRP)/neuropeptide Y (NPY) expression in the ARC and ghrelin expression in the DVC.

CONCLUSION

Ghrelin fiber projections arising from the ARC and projecting into the DVC play a role in the regulation of afferent glucose metabolism and glycolipid metabolism via the ghrelin receptor GHSR-1a in the DVC.

Sucrose ingestion induces glutamate AMPA receptor phosphorylation in dorsal hippocampal neurons: Increased sucrose experience prevents this effect

Evidence suggests that meal-related memory influences later eating behavior. Memory can serve as a powerful mechanism for controlling eating behavior because it provides a record of recent intake that likely outlasts most physiological signals generated by ingestion. Dorsal (dHC) and ventral hippocampal (vHC) neurons are critical for memory, and we demonstrated previously that they limit energy intake during the postprandial period. If dHC or vHC neurons control intake through a process that requires memory, then ingestion should increase events necessary for synaptic plasticity in dHC and vHC during the postprandial period. To test this, we determined whether ingesting a sucrose solution induced posttranslational events critical for hippocampal synaptic plasticity: phosphorylation of AMPAR GluA1 subunits at 1) serine 831 (pSer831) and 2) serine 845 (pSer845). We also examined whether increasing the amount of previous experience with the sucrose solution, which would be expected to decrease the mnemonic demand involved in an ingestion bout, would also attenuate sucrose-induced phosphorylation. Quantitative immunoblotting of dHC and vHC membrane fractions demonstrated that sucrose ingestion increased postprandial pSer831 in dHC but not vHC. Increased previous sucrose experience prevented sucrose-induced dHC pSer831. Sucrose ingestion did not affect pSer845 in either dHC or vHC. Thus, the present findings show that ingestion activates a postranslational event necessary for synaptic plasticity in an experience-dependent manner, which is consistent with the hypothesis that dHC neurons form a memory of a meal during the postprandial period.

Postmeal Optogenetic Inhibition of Dorsal or Ventral Hippocampal Pyramidal Neurons Increases Future Intake

Memory of a recently eaten meal can serve as a powerful mechanism for controlling future eating behavior because it provides a record of intake that likely outlasts most physiological signals generated by the meal. In support, impairing the encoding of a meal in humans increases the amount ingested at the next eating episode. However, the brain regions that mediate the inhibitory effects of memory on future intake are unknown. In the present study, we tested the hypothesis that dorsal hippocampal (dHC) and ventral hippocampal (vHC) glutamatergic pyramidal neurons play a critical role in the inhibition of energy intake during the postprandial period by optogenetically inhibiting these neurons at specific times relative to a meal. Male Sprague Dawley rats were given viral vectors containing CaMKIIα-eArchT3.0-eYFP or CaMKIIα-GFP and fiber optic probes into dHC of one hemisphere and vHC of the other. Compared to intake on a day in which illumination was not given, inhibition of dHC or vHC glutamatergic neurons after the end of a chow, sucrose, or saccharin meal accelerated the onset of the next meal and increased the amount consumed during that next meal when the neurons were no longer inhibited. Inhibition given during a meal did not affect the amount consumed during that meal or the next one but did hasten meal initiation. These data show that dHC and vHC glutamatergic neuronal activity during the postprandial period is critical for limiting subsequent ingestion and suggest that these neurons inhibit future intake by consolidating the memory of the preceding meal.

Circulating motilin, ghrelin, and GLP-1 and their correlations with gastric slow waves in patients with chronic kidney disease

Patients with chronic kidney disease (CKD) commonly complain upper gastrointestinal (GI) symptoms, especially anorexia. Hemodialysis (HD) has been noted to improve GI symptoms; however, the underlying mechanisms are unclear. This study was designed 1) to study effects of HD on GI symptoms and gastric slow waves; and 2) to investigate possible roles of ghrelin and glucagon-like peptide-1 (GLP-1): the study recruited 13 healthy controls, 20 CKD patients without HD (CKD group), and 18 CKD patients with HD (HD group). Dyspeptic symptoms, autonomic functions, gastric slow waves, and plasma level of ghrelin and GLP-1 were analyzed. First, the CKD patients with HD showed markedly lower scores of anorexia (0.6 ± 0.2 vs. 3.2 ± 0.4, P < 0.001) compared with patients without HD. Second, the CKD group but not HD group showed a significant reduction (25.6%) in the percentage of normal gastric slow waves, compared with controls. Third, the CKD group exhibited a significantly lower ghrelin level compared with the HD group (26.8 ± 0.9 vs. 34.1 ± 2.3 ng/l, P < 0.02) and a higher GLP-1 level (29.4 ± 2.8 vs. 20.0 ± 2.1 pmol/l, P < 0.05) compared with controls. Moreover, the percentage of normal slow waves was positively correlated with ghrelin (r = 0.385, P = 0.019) but negatively correlated with GLP-1 (r = -0.558, P < 0.001) in all CKD patients. Hemodialysis improves upper GI symptoms and gastric slow waves in CKD patients. An increase in ghrelin and a decrease in GLP-1 might be involved in the HD-induced improvement in gastric slow waves.

Cognitive control of meal onset and meal size: Role of dorsal hippocampal-dependent episodic memory

There is a large gap in our understanding of how top-down cognitive processes, such as memory, influence energy intake. Similarly, there is limited knowledge regarding how the brain controls the timing of meals and meal frequency. Understanding how cognition influences ingestive behavior and how the brain controls meal frequency will provide a more complete explanation of the neural mechanisms that regulate energy intake and may also increase our knowledge of the factors that contribute to diet-induced obesity. We hypothesize that dorsal hippocampal neurons, which are critical for memory of personal experiences (i.e., episodic memory), form a memory of a meal, inhibit meal onset during the period following a meal, and limit the amount ingested at the next meal. In support, we describe evidence from human research suggesting that episodic memory of a meal inhibits intake and review data from human and non-human animals showing that impaired hippocampal function is associated with increased intake. We then describe evidence from our laboratory showing that inactivation of dorsal hippocampal neurons decreases the interval between sucrose meals and increases intake at the next meal. We also describe our evidence suggesting that sweet orosensation is sufficient to induce synaptic plasticity in dorsal hippocampal neurons and raise the possibility that impaired dorsal hippocampal function and episodic memory deficits contribute to the development and/or maintenance of diet-induced obesity. Finally, we raise some critical questions that need to be addressed in future research.

Gastrointestinal hormones and gut motility

PURPOSE OF REVIEW

To summarize the recent findings.

RECENT FINDINGS

Studies of changes in the plasma levels confirm the earlier concepts, but offer little proof of causal effect. It is increasingly realized that peptides produced in the gut have a paracrine role or an indirect effect via the gut-brain axis. Interest in prokinetic peptide agonists remains high despite the failure of two candidate drugs, but relamorelin and camicinal offer new hope.

SUMMARY

We review the original studies published since January 2013 on peptides produced in the gut and with an effect on gastrointestinal motility.

Ghrelin: ghrelin as a regulatory Peptide in growth hormone secretion

BACKGROUND

Ghrelin is a type of growth hormone (GH) secretagogue that stimulates the release of GH. It is a first hormone linking gastrointestinal-pituitary axis.

OBJECTIVE

This review highlights the interaction of ghrelin with GHRH and somatostatin to regulate the secretion of GH and intends to explore the possible physiological role of the ghrelin-pituitary-GH axis linkage system.

OBSERVATION

Ghrelin is highly conserved among species and is classified into octanoylated (C8:0), decanoylated (C10:0), decenoylated (C10:1) and nonacylated,ghrelin. Acylated ghrelin is the major active form of human ghrelin. The primary production site of ghrelin is the stomach, and it interacts with stomach ghrelin as well as hypothalamic GHRH and somatostatin in the regulation of pituitary GH secretion. Ghrelin stimulate GH release through the GHS receptor to increase intracellular Ca2+ ([Ca2+] levels via IP3 signal transduction pathway. Ghrelin is a specific endogenous ligand for the GHS receptor and provides a definitive proof of the occurance of a GHS-GHS receptor signalling system in the regulation of GH secretion.

CONCLUSION

Studies suggests that ghrelin is a powerful pharmacological agent that exerts a potent, time-dependent stimulation of pulsatile secretion of GH.