Welcome back, to my Coursera class and I'm about to introduce the second topic of second week which is Appetite Control. In particular, I'm going to focus on hunger. So appetite control is very, very important in terms of regulating whole body energy homeostasis. So energy balance in the human body relies on the relationship between the amount of food you intake and amount of energy you use. So, because of this simple balanced relationship, the any amount of food you consume daily or per year over time, depends on one's appetite and activity of the rewarding system. So again, I want to focus on the importance of energy homeostasis. That means maintenance of simply speaking weight. So, energy consumption, I'm sorry about that, food consumption and energy expenditure supposed to be well balanced. But the idea is very important. So how human body, we can control or regulate appetite? So in other words, what's the definition of hunger? Or satiety? So hunger is a motivation of identifying and consuming food. And throughout those complex behavior changes, we can initiate feeding responses. And on the other hand, after this hunger is relieved by consuming food, we can feel satiety. Satiety means a feeling or a featureless condition of being full after consuming food. So how this hunger and satiety can be sensed and can be regulated. And this physiologic event is mediated by the neural action. I mean brain, throughout our brain. So as I explained, there is a specific portion, region of your brain called hypothalamus. There are collections of neuron circuits governing this hunger and satiety control. So, as you can see this diagram, you're looking at hypothalamic area. And, there are two critical neural circuits, depending on the type of neuropeptide, HRP neuron, POMC neuron. Those two neurons are key neural circuits in terms of regulating your hunger or satiety farther to a sympathetic nervous system based energy expenditure control. Right. So the idea is when the levels of hormones or nutrient are fluctuating depending upon different types of physiologic condition. Some examples, fasting or very prolonged fasting or after taking meals like under the satiety condition. And those hormones and nutrients, you don't need to remember those names right now. So my point is, those hormonal actions or nutrient levels before or after feeding affects, sometimes minor sometimes positive, the neural activities of those two circuits. And finally, throughout those secondary neural circuits inside your brain, your ingesting behavior, sometimes food seeking behavior or sometimes you just avoid eating once you sense satiety. And, autonomous sympathetic actions can farther regulate other metabolic organs and their functions in terms of maintaining energy balance. So, the topic and the title of the second session for second week is about hunger hormone. Just assume that you just woke up in the morning after like overnight fasting or right before lunchtime, you'll feel hunger, right? And, you are very ready to ingest food. So that type of ingestive feeding behavior is controlled by hormone called ghrelin. And this ghrelin hormone has been identified by a group of Japanese scientists and this is in crystal structure of ghrelin hormone. Just the total number of amino acid is only 28, very short hormone and farther lipid modify. This is an important point. The thing is, this ghrelin is synthesized and secreted from the gastric, I mean stomach epithelial cells and released into the local circulatory system and finally, arrive at the brain, hypothalamic region. And the main function of ghrelin is very clear, stimulating appetite. So, I'm going to introduce a human, even rare human genetic disease which is involved in the ghrelin dysfunction. This disease is called the Prader-Willi syndrome. And this disease, one of the key feature of this disease is hyperphagia. Hyperphagia means the eating activity is very, very high. So, that means a chronic feeling of hunger. So in particular, the patient cannot control their feeding behaviors. So excessive eating and accompanied obesity cause life-threatening childhood obesity. So obviously, the elevation of blood levels of biologically active ghrelin hormone can be found in this Prader-Willi Syndrome patient. What about ghrelin and obesity? This is a little bit complicated and still an active area of investigation. So, you can imagine obese people, probably you imagine that probably the higher levels of ghrelin can be related to the onset of obesity. But, ghrelin levels seems to be rather lower in people with higher body weight compared to lean people. This is little bit controversial, right? So what about the other clinical context? So, gastric bypass or a bariatric surgery. So simply speaking, physically remove a part of the stomach, in-particular to very acutely treat the obese people. The thing is, after this gastric bypass or bariatric surgery, the ghrelin levels are greatly, greatly suppressed. And the patient had this gastric bypass surgery exhibit a very nice clinical symptoms and they indeed lose the appetite. And people believe that the huge reduction of ghrelin levels is the primary factor for that clinical outcomes after gastric bypass. This is so-called a summary slide showing that brain and peripheral organ interaction. This is a very simplified cartoon. I'm highlighting this hormone ghrelin over here. So there are two types of hormones being synthesized and secreted, and acting upon the brain region, in particular hypothalamic region. The key area for feeding behavior. So in particular, upon the energy low condition, fasting condition, gastric epithelial cells produce ghrelin and this ghrelin act upon hypothalamic region and induce orexigenic behavior. Orexigenic means food seeking behavior. So once the subject consume food when the nutrient level and energy level stort back to normal level. And then, other hormones like insulin and leptin from pancreas or from adipose tissue synthesize and release, and these case, rather inhibit that hypothalamic orexigenic neural circuits and rather turn on anorexigenic responses. That means stop feeding. And this is how our body's feeding behavior can be regulated by multiple different types of metabolic organs via different types of hormones. Please remember that in terms of boosting your feeding behavior in response to fasting or low energy condition is the hormone called ghrelin, secreted from your stomach.
