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Inside the Hypothalamus: POMC, AgRP & the Melanocortin Switch

Where do leptin and ghrelin actually act? In a tiny first-order circuit of two opposing neuron types — POMC and AgRP — that push the melanocortin system toward “stop eating” or “keep eating.” This is the control room of appetite.

Two neurons, opposite votes

Deep in the hypothalamus sit the hypothalamic feeding centers. The key node is a region called the arcuate nucleus, which has a leaky blood barrier so it can sample blood-borne hormones directly. There, two opposing populations of neurons cast competing votes on appetite.

  1. POMC neurons are the “stop eating” party. When active, they release a melanocortin peptide (α-MSH) that suppresses appetite and raises energy expenditure.
  2. AgRP neurons are the “keep eating” party. When active, they powerfully drive hunger — and they also release AgRP, a neuropeptide that blocks the melanocortin signal.
  3. The two compete at downstream melanocortin receptors. The net balance of α-MSH (push) versus AgRP (block) sets the appetite output.

How leptin and ghrelin cast their votes

Now the hormones from the last guide click into place. Leptin, the fullness signal, activates POMC neurons and inhibits AgRP neurons — both pushing toward “stop eating.” Ghrelin, the hunger signal, does the reverse: it activates AgRP neurons, driving the urge to eat. So the same circuit reads both the long-term fat gauge and the short-term meal alarm.

The first-order appetite circuit

  LEPTIN (fat full) ──(+)──> POMC ──> α-MSH ──┐
                    ──(−)──> AgRP             │
                                              ▼
  GHRELIN (hungry)  ──(+)──> AgRP ──> AgRP ─> MC4 receptor
                                       (blocks)   │
                                                  ▼
           more α-MSH  →  appetite DOWN, burn UP
           more AgRP   →  appetite UP,   burn DOWN
Leptin and ghrelin tilt the POMC/AgRP balance onto the melanocortin receptor.

The melanocortin system and why it matters

The α-MSH and AgRP signals converge on receptors called melanocortin receptors, especially MC4R — the heart of the melanocortin system. Think of MC4R as a dial: α-MSH turns appetite down, AgRP turns it back up. This single dial integrates the votes and hands a verdict to the rest of the brain.

Understanding this circuit reframes appetite entirely. Hunger is not a flaw of character — it is the readout of a measurable, drug-targetable control system. Newer obesity drugs that engage MC4R-related pathways act precisely here.