Homeostasis: The Role Of Ghrelin
Homeostasis is the tendency for the body to maintain a relatively constant internal environment, which is crucial to sustain life as both humans and many other organisms operate on a narrow range of optimal conditions. Homeostasis is achieved through the collaboration of the various systems and hormones within the body (Englehart, 2018). Ghrelin is an important feeding peptide hormone which is primarily synthesized by an organism’s gastrointestinal endocrine cells. Ghrelin is largely produced and secreted during periods of negative energy balance, for example, during times of stress, anxiety, or depression (Lutter, 2008, 1). Overall, ghrelin is significant as studies suggest that the hormone plays a substantial role in helping the body achieve and maintain homeostasis by regulating the depressive symptoms associated with stress, hippocampal neurogenesis, and spine synapse density in the hippocampus.
To begin, ghrelin helps achieve and maintain homeostasis by regulating the depressive symptoms associated with stress and anxiety. This is evident as studies suggest levels of circulating ghrelin within mice increasing after undergoing certain tasks. These tasks include the chronic defeat stress procedure (CSDS), an examination that induces behavioural deficits such as depression, social avoidance, and anxiety within the test subjects. Although these behavioural issues can be reversed with the use of antidepressant treatments, the results indicate levels of ghrelin within mice elevate in order to naturally combat the acute stress and depressive symptoms associated with the CSDS procedure. To elaborate, the results specify the ghrelin hormone works to regulate or re-establish a stable state of conditions, homeostasis, within the mice during periods of negative energy balance. Further research also showcases that the stimulation of ghrelin signalling pathways is a homeostatic adaption to help cope with anxiety or stress. This is evident as when determining the purpose of increased ghrelin levels in the formation of the depressive symptoms between two mice genotypes, mice with genetic blockade of ghrelin (Ghsr-/-) tend to exhibit greater behavioural deficits such as social avoidance and depression in comparison to their wild-type counterpart. This occurs as the CSDS procedure significantly increases the appetite of the wild-type mice, signifying that the stimulation of the ghrelin signalling pathways is a homeostatic adaption in order to help an individual cope with stress or anxiety by regulating a stable state of conditions within the body, homeostasis (Lutter, 2008, 2).
Furthermore, ghrelin plays an important role in helping the body achieve and maintain homeostasis by regulating hippocampal neurogenesis. Neurogenesis is a process that produces functional neurons from progenitor cells. The cells which generate the functional neurons, progenitor cells, are similar to stem cells in the sense they are yet to be specialized or differentiated. Overall, hippocampal neurogenesis is crucial as it serves an important role in the learning, memory, and cognitive ability of an individual. Although several factors such as age, growth factors, adrenal steroids, seizures, and excitatory input are known to affect both the progenitor cells and the neurogenesis process, studies showcase ghrelin directly impacts the hippocampal neurogenesis by regulating learning, memory, and cognitive processes (Moon, 2009, 1). This is evident as when determining the role of ghrelin in regulating hippocampal neurogenesis between two mice genotypes, mice administered with synthetic ghrelin or saline tend to exhibit greater retention in comparison to their wild-type counterparts when undergoing certain memory demanding tests such as the T-maze foot-shock avoidance task (Diano, 2006, 3). The T-maze foot-shock avoidance task is one of the few examinations which tests the memory of the test subjects by placing them in a maze in which they are exposed to two-goal boxes, one being “correct” and the other “incorrect”, and the purpose of the examination is for the test subject to go to the “correct” goal box after completing the first trial run. However, after being in the maze for over five seconds, a foot-shock is applied to encourage the test subject to approach the “correct” goal box despite experiencing the shocking sensation on their feet. Although the time between each trial run can be altered, the ultimate goal of the T-maze foot-shock avoidance task is to test the retentive capabilities of the test subjects (Diano, 2006, 7). Therefore, the results from the T-maze experiment further acknowledge the presence of ghrelin helping achieve and maintain homeostasis as during times of negative energy balance (e.g. shock), the ghrelin hormone works to regulate hippocampal neurogenesis activity (e.g. memory) to bring back the test subject within the stable state of conditions, homeostasis, in order to reach their end goal or complete their task (Moon, 2009, 6).
Lastly, ghrelin plays a major role in achieving and maintaining homeostasis through controlling spine synapse density within the hippocampus, a structure located towards the middle of the temporal lobe. This is evident as studies suggest mice administered with either synthetic ghrelin or saline tend to exhibit rapid remodeling of synaptic neurons within the hippocampal area. This, indicating the hormone, ghrelin, is responsible for changing the spinal synapse density of the hippocampus, which may help regulate or improve the cognitive function in the area of the brain. When examining the density of axo-spinal synapses in the CA1 region of the hippocampus and hippocampal tissues within mice, results showcase spine synapse density within the ghrelin administered mice being significantly higher in comparison to their vehicle-treated or wild-type counterparts. Once again, signifying ghrelin is substantial in the sense that the hormone is able to alter the spine synapse density of a crucial region in the brain in order to help regulate or maintain a stable state of conditions, homeostasis, allowing for normal or in this case an increase in cognitive function of the hippocampus (Diano, 2006, 2).
In conclusion, results showcase a clear correlation between the hormone, ghrelin, helping the body achieve and maintain homeostasis, allowing the body to achieve a stable state of conditions. However, further research must follow to better understand how ghrelin impacts the other regions of the brain and possibly treat individuals with impaired learning, memory loss or processing, or conditions in which the hormone is altered with such as anorexia nervosa (Moon, 2009, 6).