Peptide Neurotransmitters

Peptide Neurotransmitters

1. Definition, scope & place among CNS transmitters

• Neuropeptides are short chains of amino acids that act as chemical messengers in the nervous system; CNS transmitters are classified by chemical structure into amino acids, acetylcholine, monoamines, neuropeptides, purines and gases — peptides are one of these structural categories (G&G 14e Ch.16, p.314, p.319).
• More than 100 chemical messengers have been identified in the brain; the number of recognised neuropeptides continues to grow and now spans dozens of families (Table 16–5) involved in functions ranging from analgesia to social behaviour, learning and memory (G&G 14e Ch.16, pp.309, 319).
• Defining functional stance: in the CNS, neuropeptides typically behave as modulators of neurotransmission rather than as direct agents of excitation or inhibition — i.e. they fine-tune the gain of fast amino-acid/monoamine signalling rather than carrying the primary fast signal themselves (G&G 14e Ch.16, p.319).
• "Vasoactive peptides" is the cardiovascular framing of the same molecules — peptides used by the body as signal molecules in the nervous system and for cell-to-cell communication, a subset of which have pronounced direct vascular effects (constrictor or dilator) (KDT 8e Ch.37, p.543).
• The same peptide is frequently both a central neurotransmitter/neuromodulator and a peripheral autonomic/paracrine signal — e.g. substance P, NPY, VIP and CGRP all act in brain and periphery (KDT 8e Ch.37, pp.548–555; G&G 14e Ch.16, p.319).
• A peptide qualifies as a central transmitter by the same general criteria used for autonomic transmitters: presence in presynaptic terminals; activity-dependent release in effective quantity; mimicry of pathway stimulation by exogenous application; block/mimicry by specific antagonists/agonists; and a defined termination mechanism (for peptides, enzymatic) (G&G 14e Ch.16, p.309).