Neuropeptides are defined as peptides that are secreted by neurons and act as signaling molecules to neighboring cells. They are distinct from peptide hormones, which are secreted from endocrine cells and travel to distinct tissues, including the central nervous system, to evoke a response.

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Unlike neurotransmitters, which affect the excitability of other neurons, neuropeptides exhibit more diverse effects with prolonged action; they can affect gene expression, local blood flow, synaptogenesis and glial cell morphology, among other processes. These effects occur through their action at neuropeptide receptors, which are generally cell surface G-protein coupled receptors.

Neuropeptides are typically co-released with a primary neurotransmitter; for example the neurotransmitter acetylcholine coexists with Substance P and VIP. They differ to neurotransmitters in that they are produced from larger precursor proteins and are not recycled back into the neuron after secretion. Also, neuropeptides can be modified by extracellular peptidases, which can inactivate biological activity or increase the affinity of a peptide for a receptor while decreasing the affinity for a different receptor.

Neuropeptides are involved in energy expenditure, fluid retention, memory, pain, stress and anxiety and diabetes. Peptoid drugs act to block or mimic the action of neuropeptides and can be used clinically for the treatment of pain, anxiety disorders and drug abuse.