Galanin

Galanin is a 29-amino acid neuropeptide in most mammals (30 amino acids in humans due to a C-terminal extension) that is widely distributed in both the central nervous system and peripheral tissues. It was first isolated from porcine intestine in 1983 by Tatemoto et al.

**Alzheimer's Disease connection:** The most clinically relevant aspect of galanin is its role in Alzheimer's disease (AD). In the basal forebrain, galanin-positive fibers from the hypothalamus hyper-innervate cholinergic neurons as they degenerate in AD — galanin fiber density increases 2-5 fold in correlation with cholinergic neuron loss. This hyperinnervation is believed to further suppress acetylcholine release via GALR1, worsening the cholinergic deficit that contributes to memory impairment in AD. GALR1 antagonism has been proposed as a strategy to improve cholinergic function in early AD.

**Pain research:** Galanin expression in dorsal root ganglion neurons is dramatically upregulated after nerve injury (5-10 fold). This injury-induced galanin plays a complex role: GALR1 activation is inhibitory (analgesic), while GALR2 may facilitate certain aspects of neuropathic pain. Spinal galanin from primary afferents modulates nociception, and intrathecal galanin produces analgesia in multiple pain models.

**Serotonin/depression connection:** Galanin co-localizes with serotonin in dorsal raphe neurons. GALR1 activation inhibits serotonergic firing. Chronic stress elevates galanin expression in the raphe, potentially dampening serotonin transmission — a mechanism proposed to link chronic stress to depression. GALR2 agonism may have antidepressant effects (stimulates BDNF, promotes neurogenesis).