Ghrelin and its role on memory

Introduction

Ghrelin is a lately discovered hormone that is accepted to play a function in the guideline of consuming and body weight. Ghrelin is produced in the fundus of the stomach and is part of a class of compounds known as gut peptides. Other gut peptides include peptide YY and glucagon-like-peptide-1 (GLP-1). Ghrelin is the natural ligand (substrate) for the growth hormone secretagogue receptor. (Carlini, 65-73)

Ghrelin levels fluctuate throughout the day and peak before a person begins to eat. After a meal is consumed, ghrelin levels decrease. This is part of the proposed mechanism for ghrelin's role in eating in which ghrelin travels from the stomach to the brain. Once in the brain, ghrelin sends its message through a series of other pathways to initiate eating. These include the activation of neuropeptide Y (NPY) and agouti-related protein (AgRP), compounds located within the brain that stimulate eating. (Moran et al, 635-641)

People who are underweight have elevated ghrelin levels (to stimulate food intake), while overweight and obese people have decreased ghrelin levels (to inhibit food intake). In one study, when ghrelin was infused into both animals and people, food intake increased, and when ghrelin antagonists were infused, food intake decreased. (Bourtchouladze et al, 739-743)

Weight loss by dietary restriction increases plasma ghrelin levels. Paradoxically, patients who underwent gastric bypass surgery, which is associated with large amounts of weight loss, had very low ghrelin levels and lacked the circadian pattern associated with the ghrelin. This may be the result of the gastric bypass surgery because the part of the stomach that makes ghrelin is no longer fully functioning. Taken altogether, this hormone may be one reason why it is difficult to maintain weight loss by dietary restriction. (Gil-Campos et al, 365-374)

Ghrelin may be related to leptin and insulin, two other hormones that affect eating and body weight. Leptin and ghrelin oppose each other in action and circadian rhythm (i.e., ghrelin is high before a meal and leptin is low before a meal and vice versa). However, while leptin is not the primary regulator of ghrelin levels, leptin was demonstrated to decrease ghrelin levels in the blood. Insulin also opposes ghrelin in function, but it was demonstrated that insulin only suppressed ghrelin levels at superphysiological levels. Given its role in eating and body weight, ghrelin may have significant potential as a pharmaceutical. Currently, ghrelin is being studied to be a pharmaceutical agent as either an agonist to increase patients' appetites (i.e., cancer patients) or as an antagonist to decrease patients' appetite (i.e., obesity). (Murphy ety al, 629-634)

Ghrelin, relationship with memory

Ghrelin (Ghr) is a 28 amino acid peptide that induces robust feeding responses in different experimental models and. Although Ghr arrives from both peripheral (stomach) and centered sources, its hyperphagic properties, to a large span, arise from activity at the mind level. Ghrelin receptors (GHS-R) are expressed in several hypothalamic nucleus and other areas such as the hippocampus, substantia nigra, ventral tegmental area, and dorsal and median raphe nucleus. (Barros et al, 183-192)

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