Neurotransmitter
A chemical messenger released at synapses to transmit signals between neurons. Classical neurotransmitters include acetylcholine, dopamine, serotonin, GABA, and glutamate. Neuropeptides (oxytocin, substance P, endorphins) function as co-transmitters alongside classical neurotransmitters.
Technical Context
Neurotransmitter categories: amino acids (glutamate — primary excitatory; GABA — primary inhibitory; glycine — inhibitory in spinal cord), monoamines (dopamine — reward/motor; serotonin/5-HT — mood/sleep; norepinephrine — arousal/attention; histamine — wakefulness), acetylcholine (muscle activation, cognition), purines (ATP, adenosine), and neuropeptides (>100 identified — oxytocin, vasopressin, endorphins, substance P, NPY, CGRP, somatostatin, CCK, VIP). Neuropeptide neurotransmission differs from classical: neuropeptides are synthesised in the cell body (not the terminal), stored in large dense-core vesicles (not small synaptic vesicles), released from extrasynaptic sites (volume transmission, not point-to-point), act on GPCRs (slower, modulatory), and have no reuptake mechanism (terminated by enzymatic degradation). Understanding neurotransmitter systems provides context for how peptide drugs interact with neural circuits.