Source of Satiety signalling

{{Infobox concept | name = Satiety signalling | subtitle = Appetite regulation | Satiation = Termination of a meal in progress | Satiety = Suppression of intake between meals | Principal relay = Nucleus of the solitary tract, area postrema }} '''Satiety signalling''' comprises the peripheral signals and central circuits that end a meal and delay the next one. The literature distinguishes ''satiation'' — the process terminating an eating episode — from ''satiety'', the inter-meal suppression of appetite; the distinction matters because different signals dominate each.{{r|woods2009}} Peripheral signals include gastric distension relayed by vagal mechanoreceptors, nutrient-sensing hormones from the small intestine such as cholecystokinin, [[Glucagon-like peptide-1|GLP-1]], peptide YY and [[Amylin|amylin]], and longer-term adiposity signals such as leptin and insulin. These converge on the caudal brainstem and on hypothalamic circuits including the [[Arcuate nucleus|arcuate nucleus]].{{r|woods2009}} Pharmacological exploitation of this system is what [[GLP-1 receptor agonist|GLP-1 receptor agonists]] do. They produce a sustained supraphysiological signal in a system evolved for episodic signalling, and the great majority of the weight loss they produce is attributable to reduced energy intake rather than to increased expenditure.{{r|drucker2018}} == Peripheral signals == Gastric distension is the most immediate satiation signal, relayed by vagal afferents in the stomach wall. It is volume-dependent rather than calorie-dependent, which is why delayed [[Gastric emptying|gastric emptying]] produces satiation out of proportion to the nutrient consumed.{{r|woods2009}} Intestinal hormones add nutrient specificity. Cholecystokinin is released from duodenal I cells in response to fat and protein and acts largely within a meal. GLP-1 and peptide YY are released from more distal [[Enteroendocrine L cell|L cells]] and act over a longer window. Amylin, co-secreted with insulin from the pancreas, acts at the area postrema.{{r|holst2007}} Adiposity signals set the background against which meal-related signals are read. Leptin and insulin circulate in proportion to fat mass and modulate the sensitivity of the hypothalamic circuits to meal-related input rather than terminating meals themselves.{{r|schwartz2000}} == Central integration == The nucleus of the solitary tract in the caudal brainstem receives vagal afferent input and is adjacent to the area postrema, a circumventricular organ lacking a blood-brain barrier and therefore able to sample circulating peptides directly. Both structures express the [[GLP-1 receptor]].{{r|schwartz2000}} The [[Arcuate nucleus|arcuate nucleus]] of the hypothalamus contains two opposing neuronal populations — those expressing pro-opiomelanocortin, which suppress intake, and those expressing agouti-related peptide and neuropeptide Y, which promote it. The arcuate lies adjacent to the median eminence, where fenestrated capillaries allow access to circulating signals. Brainstem and hypothalamic circuits are interconnected and partly redundant. Brainstem circuits alone are sufficient for meal termination in decerebrate animal preparations, whereas hypothalamic circuits carry the longer-term adiposity signal — one reason acute satiation and chronic body-weight regulation can be dissociated pharmacologically.{{r|woods2009}} == Why pharmacological agonism differs from physiology == Endogenous GLP-1 is secreted episodically, is degraded within minutes by [[Dipeptidyl peptidase-4|DPP-4]], and acts substantially through vagal afferents in the intestinal wall before reaching the systemic circulation. A weekly agonist produces a continuous circulating concentration far above the physiological range and acts predominantly at central sites reachable from the blood.{{r|drucker2018}} This is why the pharmacology does not simply amplify the physiology. The nausea that accompanies agonist therapy reflects area postrema engagement that endogenous secretion does not produce at comparable intensity, and the sustained reduction in food intake reflects continuous rather than meal-linked signalling. It is also why reduced intake, not increased expenditure, dominates the weight effect: energy expenditure falls with weight loss as it does with any caloric deficit, and no incretin agonist has been shown to raise it. The glucagon component of [[Retatrutide|triple agonists]] is the deliberate attempt to add an expenditure arm.{{r|schwartz2000}} == References == {{reflist}} <ref name="woods2009">Woods SC. "The control of food intake: behavioral versus molecular perspectives." ''Cell Metabolism'' 9(6):489–498 (2009). DOI:10.1016/j.cmet.2009.04.007. PMID 19490904.</ref> <ref name="schwartz2000">Schwartz MW, Woods SC, Porte D, Seeley RJ, Baskin DG. "Central nervous system control of food intake." ''Nature'' 404(6778):661–671 (2000). DOI:10.1038/35007534. PMID 10766253.</ref> <ref name="drucker2018">Drucker DJ. "Mechanisms of action and therapeutic application of glucagon-like peptide-1." ''Cell Metabolism'' 27(4):740–756 (2018). PMID 29617641.</ref> <ref name="holst2007">Holst JJ. "The physiology of glucagon-like peptide 1." ''Physiological Reviews'' 87(4):1409–1439 (2007). PMID 17928588.</ref> == See also == * [[Arcuate nucleus]] * [[Gastric emptying]] * [[Glucagon-like peptide-1]] * [[Amylin]] * [[GLP-1 receptor agonist]] {{DEFAULTSORT:Satiety signalling}} [[Category:Appetite regulation]] [[Category:Incretin biology]] [[Category:Gastrointestinal physiology]]