Review
2 February 2026
Institute of Molecular Life Sciences, Centre of Excellence of the Hungarian Academy of Sciences, HUN-REN Research Centre for Natural Sciences, 1117 Budapest, Hungary
Department of Physiology, Semmelweis University, 1094 Budapest, Hungary
Author to whom correspondence should be addressed.
The G protein-coupled galanin receptors include three different subtypes: galanin receptor 1, 2 and 3 (GalR1, GalR2, GalR3). The neuropeptide galanin is the principal natural agonist of the galanin receptors, the so-called galaninergic system. Galanin-like peptide and spexin have also been identified as natural ligands of the galanin receptors. Galanin receptors are widely expressed in the brain; however, they can be found in other tissues, such as the skeletal muscle, the heart, and the gastrointestinal tract. The galaninergic system regulates diverse biological processes, including feeding behavior, neuroprotection, learning, memory, cardiovascular and renal function, and nociception. Its dysregulation is associated with various diseases, such as Alzheimer’s disease, diabetes mellitus, epilepsy, depression, and cancer. The stimulation of GalR1 and GalR3 leads to the Gαi/o-type G protein-mediated inhibition of cyclic AMP/protein kinase A, whereas GalR2 stimulation initiates phospholipase C activation via Gαq/11-type G proteins. A galanin-activated β-arrestin-dependent pathway has also been described for GalR2. In this review, we summarize the recent advances concerning galanin receptor signaling, including both the G protein-dependent and -independent pathways. A better understanding of the complex interplay of the signaling molecules, receptors, and various signaling pathways is crucial for the future development of specific agonists with therapeutic potential.
G protein-coupled receptors (GPCRs) are the largest family of signaling proteins; they are among the most common therapeutic targets [1]. Most classification systems of GPCRs are based on their structural similarity. Galanin receptors are identified as class A GPCRs [2,3]. The galanin receptors include three different subtypes in humans: galanin receptor 1, 2 and 3 (GalR1, GalR2, GalR3, respectively) [4,5,6]. All GPCRs share a highly conserved structure composed of an extracellular N-terminus, seven transmembrane α-helices (TMs), three intracellular loops (ICLs), three extracellular loops (ECLs), and an intracellular C-terminus [7]. The homology among the galanin receptors is only 36–54% [3]; the transmembrane helices show the highest similarity, whereas the terminal regions differ substantially.
The principal endogenous agonist of the galaninergic system is galanin, a 30-amino acid neuropeptide in humans, which was discovered in the early 1980s [13]. After the cloning of Gal gene, a further peptide product was found to be encoded by it, the galanin message-associated peptide (GMAP), which is 61-amino acid long in humans [14]. Later, other endogenous peptides were found such as galanin-like peptide (GALP) [15], alarin [16], and more recently, spexin [17]. A second form of spexin, SPX2, was identified in non-mammalian species, but this form was not detected in humans [17].
These peptides are derived from a prepro-protein, for example, the proteolytic cleavages of Gal result in the galanin and GMAP. The N-terminus of galanin, the first 19 amino acids, is highly conserved throughout evolution, while its C-terminal part is much less conserved. The 60-amino acid GALP is derived from another prepro-peptide; however, its 9-21-amino acid region is identical to the first 13 amino acids of galanin. A further transcript is derived from GALP, the 25-amino acid-long alarin, but due to alternative splicing, its sequence is different. It is also considered part of the galaninergic system; nevertheless, it does not bind to galanin receptors. So far, no receptor is known for either alarin or GMAP. It has also been suggested that although GALP binds to galanin receptors, it may regulate its actions through yet unknown receptors instead of GalR1-3 [18].
Several aspects of the galaninergic system have been reviewed since their discovery, especially the characterization of their G protein-dependent signaling pathways and their roles in various physiological and pathological processes. A milestone in the research field has been the recent determination of galanin receptor structures by cryo-electron microscopy (cryo-EM) [9,10,11]. The purpose of this review is to summarize the recent advances concerning galanin receptor signaling, including their G protein and β-arrestin-dependent pathways, and the progress in the structural insight into their signaling, and outline some future open questions and challenges.
The distribution of galanin and other peptides’ mRNA, as well as that of the galanin receptors, has been characterized in various species and reviewed in detail (see [3] and references therein). However, significant differences were observed even among primates let alone among mammals [19]. Therefore, normalized gene expression values taken from the Human Protein Atlas were used to represent the distribution of the galanin receptor mRNA, as well as that of the endogenous agonists. These values are reported as transcripts per million (nTPM) units. Data concerning the protein levels often cannot be validated (for example, in the case of GalR1 and GalR3) due to the questionable specificity of the available antibodies. For GalR2, both the RNA and the protein expression data are available; however, the consistency between antibody staining and RNA expression is rather low.
The highest GalR1 mRNA levels can be found in endocrine tissues and various parts of the brain, such as cerebral cortex, hypothalamus, and amygdala. In contrast, the expressions of GalR2 and GalR3 are more widespread; they have been observed in peripheral tissues as well. GalR2 mRNA is highly expressed in the gastrointestinal tract, muscle tissue, and bone marrow, although it can also be detected in the hypothalamus. GalR3 mRNA was detected in various brain areas, and its expression is also relatively high in the pancreas, skeletal muscle, and liver.
The localization of galanin and the three subtypes of galanin receptors was studied in the locus coeruleus (LC) and the dorsal raphe nucleus (DRN) in human postmortem brain [20]. The LC is of special interest, since it plays crucial roles in the modulation of several behavioral and physiological processes, as well as in depression, anxiety, and neurodegeneration under pathological conditions [21,22]. The LC consists of predominantly noradrenergic neurons, while the DRN comprises mainly 5-hydroxytryptamine (5-HT; serotonin) neurons. In this study, galanin and GalR3 mRNA were detected in many noradrenergic LC neurons [20]. An overlap between GalR3 and serotonin neurons was also detected in the DRN, but galanin was not detected in 5-HT-DRN neurons. The major galanin receptor in these neurons is GalR3, while GalR2 and GalR1 are expressed at low levels, if at all. However, in the forebrain, the most abundant expression was found for GalR1, while GalR3 was likely not expressed [20]. Other studies have also found that galanin is highly expressed in the LC in norepinephrine neurons, and it is expressed in some GABAergic neurons as well [22,23].
In the Human Protein Atlas, the highest galanin mRNA levels can be found in the pituitary gland, followed by those in the hypothalamus, skin, colon, and appendix. Protein expression data are also available for galanin in the Human Protein Atlas. Galanin is mainly expressed in the pituitary gland, hypothalamus, adrenal gland, and the gastrointestinal tract at protein level, indicating a broad agreement between transcriptomic and protein-level datasets at the tissue level. However, for neuropeptides such as galanin, protein localization may extend beyond sites of mRNA expression due to axonal transport and vesicular storage. An early study was carried out to investigate the distribution of galanin-like immunoreactivity in the human brain [24]. The immunoreactive cells were mainly restricted to the basal nucleus of Meynert and to various areas of the hypothalamus; a more widespread fiber staining was observed in the hypothalamus, the diagonal band, the septum, amygdala, hippocampus, and cortex. The authors noted that galanin seems to have a relatively widespread distribution in the human brain, although not as extensive as that described for the rat brain [24].
The highest spexin mRNA levels were observed in adipose tissue, kidney, pancreas, thyroid gland, and placenta. Its expression was studied in human tissues, and higher expression was found in several tissues (adrenal gland, skin, stomach, small intestine, liver, thyroid, pancreatic islets, visceral fat, lung, colon, and kidney) as compared to expression in muscle and connective tissues [25]. Spexin is also widely expressed in the brain, including the hypothalamus [26]. GALP is expressed mainly in the hypothalamic arcuate nucleus and the posterior pituitary [27].
Class A (rhodopsin family) GPCRs are characterized by a relatively short N-terminal extracellular domain, their binding pocket is usually deep and narrow [28,29]. A common binding pocket involving residues of TM2, TM3, TM6, TM7 and ECL2 was suggested for the peptide agonists [29]. In class A GPCRs, agonist binding induces several conformational changes [30,31]. Upon activation, the rotation and outward displacement of TM6 accompanied by inward movement of TM5 and TM7 as well as the rotation and upward movement of TM3 lead to the formation of a cavity, this intracellular surface can interact with the various transducers, such as G proteins and β-arrestins. The conformational changes stem from the rearrangement of the hydrophobic receptor core and characteristics motifs, such as the toggle switch Trp 6.48 (in the Cys/Ser/Thr 6.47-Trp 6.48-Phe 6.50 motif), the DRY (Glu/Asp 3.49-Arg 3.50-Tyr/Trp 3.51) and the NPxxY (Asn 7.49-Pro 7.50-XX-Tyr 7.53) motifs (superscripts denote Ballesteros–Weinstein numbering) [30,31,32].
After the discovery of galanin receptors, alanine scanning mutagenesis studies gave insight into the structural aspects of their ligand binding [33,34]. The N-terminal part of galanin is highly conserved across species, and the binding affinity of galanin (1–16) is comparable to that of the full-length agonist. Trp2, Asn5, Tyr9, Leu10, Leu11, and the free N-terminal amino group of galanin were found to be important pharmacophores, and its Trp2 and Tyr9 are especially critical for ligand binding [33,35]. Early studies identified important residues involved in the formation of the binding pocket of GalR1, such as His264 6.52 and Phe282 7.32, which interact with Tyr9 and Trp2 of galanin, respectively [33,34]. An important difference between GalR1 and GalR2 is that, although both bind galanin with high affinity, only GalR2 displays high affinity for the N-terminally truncated galanin (2–30) form.
An interesting aspect of the galanin receptor subtypes is ligand selectivity. All three receptors bind galanin, which displays high affinity for GalR1 and GalR2, but lower affinity for GalR3 [17], meanwhile only GalR2 and GalR3 can be stimulated by spexin. There is a synthetic, spexin-based GalR2 agonist, SG2A. Similarly to spexin, SG2A induces GalR2 signaling via Gαq protein, but it does not stimulate GalR3. Using site-directed mutagenesis and domain swapping between GalR2 and GalR3, Lee and co-workers provided an explanation for the ligand selectivity of SG2A [36]. When four residues in spexin were substituted with residues derived from the corresponding positions in galanin (Asn5, Ala7, Leu11/Phe11, and Pro13), this quadruple mutant displayed full GalR2 specificity. They also identified specific amino acids within TM3, TM5, and ECL3 of GalR2 that promote favorable interaction with SG2A as well as residues in GalR3 that inhibit the interaction [36].
More detailed studies have been carried out only recently, and three publications have been published almost at the same time. The receptor structures determined by cryo-EM are available for GalR1 (galanin-bound structures 7WQ3 [11], 7XJJ [9]) and GalR2 (galanin-bound structures 7XBD [10], 7WQ4 [11], 7XJK [9]; spexin-bound receptor 7XJL [9])
