Ghrelin: The Hormone Of Hunger
Everybody has experienced the discomfort of an empty stomach at some point in their lives: however, not everyone knows why this common symptom for hunger occurs and how it has come to be. Feeding overall must satisfy a variety of the body’s requirements, providing enough energy to the body’s systems to maintain growth, survival, and reproductive success. The process by which feeding is regulated incorporates a variety of physical, chemical and nutritional signalling inputs to ensure that the body’s healthy homeostatic levels are maintained. The chemical compound responsible for this feeling of hunger and which is involved in an intricate signalling pathway around the body is the hormone ghrelin.
Ghrelin is primarily produced in the fundus of the stomach and secreted into the bloodstream when content levels in the stomach are low. Although it is commonly known as “the hunger hormone” ghrelin actually does much more than simply telling one’s brain when one is hungry, and its cascading effects are critical in order for the energy homeostasis processes to function and ultimately for organism survival. The structure and functions of ghrelin have been observed most frequently and in the most detail among mammals, but it is not exclusive to these warm-blooded vertebrates: in fact, the presence of ghrelin‘s amino acid sequence has been discovered among other non-mammalian vertebrates - birds, fish, amphibians, and reptiles have all been identified with the hormone - as well as homologous hormones and receptors within the digestive systems of invertebrate species of nematodes and arthropods. This suggests that these mechanisms for monitoring food intake and regulating energy storage date back prior to the emergence and subsequent radiation of metazoan animal life.
In mammals during times of scarcity when nutrient intake is low, high levels of ghrelin secretion triggers receptors in the hypothalamus to increase body weight and fat mass: this stored fat can then be used as fuel for the body at a later time, making the signalling process critical when available food sources were often few and far between. However, it was more recently discovered that the garden warbler, a type of migratory bird, relies on ghrelin as an indicator for migration. As the bird consumes forest mast in preparation for the journey, ghrelin levels in the blood increase until they reach what is thought to be a threshold, triggering the initiation of migration; as ghrelin levels decrease they signal the warbler to continue their migratory path until they reach their final destination.
The presence and cascading reactions of ghrelin and its receptors are integral to the lifestyle of organisms from all periods and radiations of life. Current and future research into the broad reach of ghrelin‘s effects have generated exciting results that may prove useful in fully understanding the feeling of hunger and its side effects, including obesity. Lack of ghrelin in mutant mice has been linked to increased anxiety levels when exposed to a variety of stressors, and mice with synthetically higher levels of ghrelin provided antidepressant-like responses when exposed to high-stress conditions, meaning that ghrelin may act as an acute, natural adaptation to counter the effects of stress in an environment.
Ghrelin has also gained notoriety more recently for being a key hormone affecting those with eating disorders: ghrelin levels are found to be higher in individuals with anorexia nervosa and lower in cases of obesity, leading some to believe that ghrelin injections may be a solution towards obesity and genetically-linked metabolic disorders. Although researches regarding the medical possibilities with ghrelin are relatively recent, its significance in the development and survival of vertebrates is without a doubt pertinent in any discussion about the evolution of survival hormones in vertebrates.
