Hunger and Satiety Regulation

Hypothalamic Nuclei - Brain's Appetite Command

• Central Integrator: Hypothalamus, especially Arcuate Nucleus (ARC), processes peripheral hunger/satiety signals.
• Key Nuclei & Roles:
  • Arcuate Nucleus (ARC): Dual neuron populations.
    • Orexigenic: NPY/AgRP → ↑ appetite. Project to LHA.
    • Anorexigenic: POMC/CART (α-MSH) → ↓ appetite. Project to VMH.
  • Lateral Hypothalamic Area (LHA): "Hunger center".
    • Stimulated by NPY/AgRP. Produces orexin, MCH.
    • 📌 LHA lesion → Lean (aphagia).
  • Ventromedial Nucleus (VMH): "Satiety center".
    • Stimulated by α-MSH.
    • 📌 VMH lesion → Very Massive (hyperphagia).
• Brainstem Link: Nucleus of Solitary Tract (NTS) relays gut satiety signals (CCK, GLP-1) via vagus nerve to hypothalamus.

⭐ Lesions in the VMH lead to hyperphagia and obesity, while LHA lesions cause aphagia and weight loss. This highlights their critical roles as satiety and hunger centers, respectively.

Gut-Brain Axis Hormones - Belly's Broadcast

GI signals to hypothalamus; regulate meal start/end.

• Orexigenic (Stimulate Eating):
  • Ghrelin: 📌 'Grrr-helin' for hunger.
    • Source: Stomach (P/D1 cells).
    • Action: ↑ Hunger, stimulates NPY/AgRP.
    • Levels: ↑ pre-meal, ↓ post-meal.
• Anorexigenic (Inhibit Eating):
  • Cholecystokinin (CCK):
    • Source: Duodenum/Jejunum (I-cells); fats, proteins.
    • Action: ↓ Intake, ↑ satiety (vagal).
  • Peptide YY (PYY$_{3-36}$):
    • Source: Ileum/Colon (L-cells); fats.
    • Action: ↓ Intake, inhibits NPY/AgRP.
  • Glucagon-Like Peptide-1 (GLP-1):
    • Source: Ileum/Colon (L-cells).
    • Action: ↓ Intake, ↑ satiety, incretin. (POMC/CART).
  • Oxyntomodulin (OXM):
    • Source: L-cells (with GLP-1/PYY).
    • Action: ↓ Intake.
  • Amylin:
    • Source: Pancreatic β-cells (with insulin).
    • Action: ↓ Intake, slows gastric emptying.

⭐ Ghrelin is the only major orexigenic gut hormone; others (CCK, PYY, GLP-1, OXM, Amylin) are anorexigenic.

Adiposity & Pancreatic Signals - Long-Term Regulators

• Leptin ("Satiety Hormone"):
  • Source: Adipose tissue; proportional to fat mass.
  • Action: ↓Appetite (anorexigenic), ↑Energy expenditure.
  • Mechanism: Hypothalamic (ARC nucleus - ↑POMC/CART, ↓NPY/AgRP).
  • Deficiency/Resistance: Leads to hyperphagia, obesity.
• Insulin:
  • Source: Pancreatic β-cells.
  • Action: Anorexigenic signal to CNS (long-term adiposity).
  • Mechanism: Acts on hypothalamus; transport across BBB.
• Amylin (IAPP):
  • Source: Pancreatic β-cells (co-secreted with insulin).
  • Action: ↓Gastric emptying, ↓Glucagon secretion, ↑Satiety.
  • Mechanism: Acts on hindbrain (Area Postrema, NTS).

⭐ Leptin resistance, not deficiency, is the more common scenario in human obesity, characterized by high leptin levels but reduced central response.

Key Neurotransmitters & Dysregulation - Chemical Messengers & Misfires

Central neurotransmitters balance hunger/satiety. Dysregulation causes obesity/eating disorders.

• Orexigenic (Appetite Stimulating):
  • NPY (Neuropeptide Y): Potent stimulator from ARC.
  • AgRP (Agouti-Related Peptide): Co-released with NPY; antagonizes MC3/4R.
  • Endocannabinoids (e.g., Anandamide, 2-AG): ↑ appetite (CB1R). 📌 "Munchies".
• Anorexigenic (Appetite Suppressing):
  • POMC/α-MSH: ARC neurons make POMC → α-MSH. Acts on MC4R.
  • CART (Cocaine- and Amphetamine-Regulated Transcript): Co-expressed with POMC; anorexigenic.
  • Serotonin (5-HT): ↑ satiety, ↓ meal size (5-HT2C R).
  • Dopamine: Complex role: food reward, motivation, feeding.

Dysregulation: Imbalances: Obesity (leptin resistance, MC4R mutations), Eating disorders (serotonin/dopamine pathway alterations).

⭐ α-MSH (from POMC) is key anorexigenic via MC4R. MC4R mutations: most common monogenic cause of human obesity.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypothalamus is central: Lateral (LH) for hunger, Ventromedial (VMH) for satiety.
• Ghrelin (stomach) is the primary orexigenic hormone, stimulating appetite.
• Leptin (adipose tissue) is a key anorexigenic hormone, signaling long-term satiety.
• Insulin, PYY 3-36, and GLP-1 are important anorexigenic signals reducing food intake.
• Cholecystokinin (CCK) from the duodenum promotes short-term satiety.
• Arcuate nucleus (ARC) integrates signals via NPY/AgRP (hunger) and POMC/CART (satiety) neurons_