GALANIN, PORCINE

Structure

GAL is a 29-aa C-terminally amidated peptide, apart from human GAL, which has 20 aa and no amidated N-terminus. The N-terminal 1–15 aa are highly conserved.  Human proGAL: Mr 13,302.1, pI 6.84. Soluble in water and physiological saline solution.

Gene, mRNA, and precursor

The GAL gene is located on human chromosome 11q13.2, rat chromosome 1q42, and mouse chromosome 19A. The peptide precursor of GAL is encoded by a single-copy gene consisting of six small exons spanning about 6 kb of genomic DNA. Regarding its structure, a sequence of GAL is followed by the signal peptide, and the remainder is called the GAL message-associated peptide (GMAP). GAL is widely expressed in the central and peripheral nervous systems in many mammalian species. In the brain, GAL is synthesized in the dorsal raphe nucleus, locus coeruleus, rostral ventrolateral medulla, central nucleus of the amygdala, paraventricular nucleus, and supraoptic nucleus. GAL is also found in the spinal cord and gut.

Synthesis and release

GAL colocalizes in vasopressin neurons and increases during salt loading, suggesting its influence on osmotically stimulated vasopressin release. GAL gene expression in magnocellular neurons is increased by estrogen.

Receptors

GAL receptors have three subtypes, referred to as GAL receptor types 1, 2, and 3 (GALR1, GALR2, and GALR3).6 The human GALR1 gene contains three exons and is translated into a 349-aa protein. The homology between species is 93% for rat and human GALR1. The expression of GALR1 is regulated by cAMP through the transcription factor CREB. Human GALR2 has 92% sequence identity to rat GALR2, although there is a 15-aa extension of the C-terminal end in human GALR2. The GALR2 gene is expressed more ubiquitously compared with that of GALR1, as it is found in several peripheral tissues, including the pituitary gland, gastrointestinal tract, skeletal muscle, heart, kidney, uterus, ovary, and testis, in addition to the central nervous system. The Galr3 transcript was first isolated from rat hypothalamic cDNA libraries. Human GALR3 consists of 368 aa and shares 36% identity with human GALR1, 58% with human GALR2, and approximately 90% with rat GALR3.

Agonists and Antagonists

M617 (GALR1), M1153, M1145 (GALR2). M35 and galantide (nonselective GALR antagonists), RWJ-57408 (GALR1), M871 (GALR2), and SNAP37889 and SNAP398299 (GALR3).

Biological functions

In rats, GALR1 is prominently distributed in the hypothalamus, amygdala, hippocampus, thalamus, brainstem, spinal cord, dorsal root ganglion (DRG), gut, heart, lung, kidney, muscle, adipocytes, and testis. GALR2 mainly exists in the cortex, hypothalamus, hippocampus, amygdala, cerebellum, DRG, heart, liver, lung, kidney, intestine, uterus, ovary, stomach, pancreas, and testis. GALR3 is found in the hypothalamus, dorsal raphe nucleus, locus coeruleus, and amygdala. GAL is thought to regulate numerous physiological processes in the adult mammalian nervous system, including sleep/wake regulation, energy and osmotic homeostasis, reproduction, nociception, and cognition.

Clinical implications

GAL increases food intake and body weight via GALR1. Intranasal administration is an effective route for the delivery of GAL-related agents into the brain by use of various cyclodextrins. Thus, the intranasal administration of a GALR1 antagonist offers an attractive approach to combat obesity. Clinical studies indicated that the GAL concentration is correlative to the morbidity of type 2 diabetes mellitus in humans. In addition, some clinical studies have shown that the activation of the GAL pathway is effective in treating type 2 diabetes.

Description

GAL is expressed in the brain and peripheral organs, and has diverse physiological actions, including energy homeostasis, reproduction, nociception, and cognition. GAL was isolated in 1983 from the porcine intestine by Tatemoto and colleagues. The GAL peptide has been purified from the chicken, alligator, and fish species, including trout, tuna, bowfin, dogfish, and sturgeon.

Biological Functions

Galanin is colocalized with acetylcholine, 5-HT , and NE in neurons or in brain regions implicated in cognitive and affective behavior, suggesting a possible role in the regulation of 5-HT and NA neurotransmission in depressive states and during the course of antidepressant therapy. Three galanin receptor subtypes have been cloned and studied, but little is known about their specific contributions to behavioral processes. In the CNS, galanin inhibits acetylcholine release, suggesting a possible role for galanin in cholinergic dysfunction; inhibits neurotransmitter release and neuronal firing rate; and inhibits signal transduction by inhibition of phosphatidyl inositol hydrolysis, leading to symptoms of depression. Thus, blocking the inhibitory effects of galanin on monoamine neurotransmitters with galanin receptor antagonists would be predicted to mimic or augment the action of the other monoamine classes of antidepressants."

Clinical Use

Since its discovery in 1983, the neuropeptide galanin has been found to be involved in a wide range of functions, including pain sensation, sexual activity, feeding, and learning and memory. Galanin is widely distributed in the central and peripheral nervous systems and in the endocrine system, and it acts as a inhibitory neuromodulator of NE and 5-HT in the brain. The 29- to 30-amino-acid sequence of galanin is conserved (almost 90% among species), indicating the importance of the molecule among species.