Neuropeptides and substance P
Neuropeptides, such as substance P, are the third class of neurotransmitter: chains of amino acids joined by peptide bonds, with slower and longer lasting effects.
Remember
Neuropeptides such as substance P are made in the cell body (which has the protein-making machinery), carried along the axon to the nerve terminal and released with high-frequency firing, giving slower, longer-lasting effects.
Memory aid
peptides are made in the head office (the cell body) and shipped to the branch, whereas the small transmitters are made at the branch
Use it to recall where each kind of transmitter is made: peptides at the cell body, small transmitters at the branch.
Alongside the amino acids and the biogenic amines (such as dopamine and noradrenaline), there is a third class of neurotransmitter. Neuropeptides are that third class. Examples are substance P, somatostatin and cholecystokinin. Somatostatin and cholecystokinin act in the gut. Each is a chain of amino acids joined by peptide bonds.
How they are made distinguishes them from the small classic transmitters. A neuropeptide starts as a larger precursor protein (a big molecule that is cut into its active form inside vesicles) in the cell body, because peptides need the cell body's protein-making machinery. It is then carried along the axon to the nerve terminal. Small transmitters are instead made at the terminal from local precursors.
Neuropeptides are stored in dense-core vesicles, stores that let go of them only with high-frequency firing. They act on G-protein coupled receptors (receptors that give slower effects), so the response is slower and lasts longer.
Substance P is a neuropeptide that sensory neurones release to transmit pain signals into the spinal cord, and it also influences gut function. Neuropeptides are not limited to the nervous system, because they often act as hormones elsewhere in the body.
Key facts
- Class
- Peptides are the third group of transmitters, alongside the amino acids and the biogenic amines.
- Made
- Neuropeptides are first made in the cell body as larger precursors, then carried along the axon in vesicles.
- Release
- Dense-core vesicles let go of them only with high-frequency firing, giving slower, longer-lasting effects.
- Receptors
- They act on G-protein coupled receptors, which give slower effects than the ligand-gated ion channels behind fast synaptic transmission.
- Pain
- Sensory neurones release substance P, and it transmits pain signals into the spinal cord and brainstem.
- Beyond the nervous system
- Neuropeptides are not limited to the nervous system, as they often act as hormones elsewhere, and somatostatin and cholecystokinin act in the gut.
Exam traps
- Neuropeptides are not confined to the central nervous system, because they also act in the wider body, for example substance P, somatostatin and cholecystokinin in the gut.
- Rapid reuptake into the neurone, which clears a transmitter from the synapse, is how amino acid and biogenic amine transmitters are cleared, so it does not define a peptide transmitter.
- Fast synaptic transmission relies on ligand-gated ion channels, and neuropeptides do not mainly work that way: they act on G-protein coupled receptors, which give slower effects.
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