Peptide Neurotransmitters

Introduction

• Neuropeptides are short amino-acid chains that act as chemical messengers in the nervous system; among the structural classes of CNS transmitter (amino acids, acetylcholine, monoamines, neuropeptides, purines, gases) they are one major category. More than 100 brain messengers are known, with dozens of neuropeptide families spanning analgesia, feeding, mood, learning and social behaviour.
• Defining functional stance — In the CNS neuropeptides typically act as modulators of neurotransmission rather than as direct excitatory/inhibitory agents — they fine-tune the gain of fast amino-acid/monoamine signalling rather than carrying the primary fast signal.
• One molecule, two worlds — The same peptide is frequently both a central transmitter/neuromodulator and a peripheral autonomic/paracrine signal — substance P, NPY, VIP and CGRP all act in brain and periphery; "vasoactive peptides" is the cardiovascular framing of the same molecules.
• Transmitter criteria — A peptide qualifies as a central transmitter by the usual criteria — presence in presynaptic terminals, activity-dependent release in effective quantity, mimicry of pathway stimulation by exogenous peptide, block/mimicry by specific antagonists/agonists, and a defined termination mechanism (for peptides, enzymatic).