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Galanin
A 30-amino acid neuropeptide widely distributed in the nervous system, involved in pain modulation, feeding behavior, cognition, mood regulation, and neuroendocrine function.
Overview
Galanin is a 30-amino acid neuropeptide (29 amino acids in humans) discovered in 1983 by Viktor Mutt and colleagues in porcine intestinal extracts. Its name derives from its N-terminal glycine and C-terminal alanine residues. Galanin is widely distributed throughout the central and peripheral nervous systems, coexisting with several classical neurotransmitters including acetylcholine, serotonin, norepinephrine, and GABA in various neuronal populations.
Galanin acts through three G-protein coupled receptor subtypes: GalR1, GalR2, and GalR3. GalR1 and GalR3 are primarily inhibitory (Gi/Go-coupled), while GalR2 can couple to both inhibitory and excitatory pathways (Gq/G11). This receptor diversity allows galanin to exert varied and sometimes opposing effects depending on the tissue and receptor expression pattern. In the central nervous system, galanin generally acts as an inhibitory neuromodulator, reducing the release of other neurotransmitters.
In pain processing, galanin has a complex dual role. In normal conditions, galanin release from sensory neurons modulates nociceptive transmission. After nerve injury, galanin expression is dramatically upregulated in dorsal root ganglion neurons, where it appears to have both pronociceptive (via GalR2) and antinociceptive (via GalR1) effects depending on the specific pain model and site of action. This complexity has made galanin receptor-targeting drug development challenging but potentially rewarding.
Galanin is also implicated in cognition and Alzheimer's disease. Galanin-containing fibers hyperinnervate the basal forebrain cholinergic nuclei in Alzheimer's disease, and galanin inhibits acetylcholine release in the hippocampus. While this has been interpreted as galanin contributing to cognitive decline, some evidence suggests galanin may also have neuroprotective effects. Galanin's roles extend to feeding behavior (it stimulates fat intake), neuroendocrine regulation (modulating GH, prolactin, and insulin release), seizure modulation (anticonvulsant effects), and mood regulation.