Neuropeptides · Grey Matter

Neuropeptides are short chains of amino acids that neurons release as slow, widespread signals (endorphins, substance P, oxytocin, orexin, neuropeptide Y and about a hundred others), and they tune whole circuits to a state such as pain relief, hunger, bonding or wakefulness.

Neuropeptides. Neuropeptides are short chains of amino acids that neurons release as slow, widespread signals (endorphins, substance P, oxytocin, orexin, neuropeptide Y and about a hundred others), and they tune whole circuits to a state such as pain relief, hunger, bonding or wakefulness.

They differ from small transmitters at every step. A peptide is cut from a larger precursor protein in the cell body and shipped down the axon already packed, in larger dense-core vesicles, so it cannot be refilled at the terminal. Those vesicles sit away from the release sites and need a stronger, more widespread calcium rise to fuse, which comes with high-frequency or burst firing. Released peptides are not taken back up; they diffuse over micrometres to millimetres and act on metabotropic receptors, often far from the release site (volume transmission), until peptidases break them down.

They are released alongside a classical transmitter. A neuron that releases glutamate or GABA at every spike may add a peptide only when it fires hard, so the peptide reports the intensity of activity.

Their effects are slow and long. Acting through G proteins at low concentrations, they change excitability and behaviour for seconds to hours.

Several are drug targets. Opioid drugs act on the receptors of endorphins and enkephalins; the migraine drugs that block CGRP act on a peptide released by trigeminal sensory fibres.

A neuropeptide is a message reserved for loud activity.

Because it needs bursts to be released and spreads widely, it tells a whole region that something sustained is happening.

Questions: Why are neuropeptides released only when a neuron fires hard? Peptides are stored in large dense-core vesicles that sit away from the active zone, outside the tight calcium clouds around the release-site channels. A single spike raises calcium enough to fuse the small vesicles docked next to the channels but not these distant ones; high-frequency firing or bursts let calcium build up throughout the terminal, and only then do dense-core vesicles fuse, often outside the synapse. Since peptides are made in the cell body and shipped down the axon, their supply at the terminal is also limited. The result is a two-level code: the small transmitter reports every spike, the peptide reports sustained, intense activity.