Identification and biochemical characterization of the human brain galanin receptor

Human galanin (hGal) is an important neuro-modulator present in the brain, gastrointestinal system and the hypothalamo-pituitary axis. A specific receptor for hGal has been identified in various areas in human brain. A single class of high affinity binding sites was found on plasma membranes of the amygdala (Kd 0.23 nM, Bmax 44 fmol/mg), the hypothalamus (Kd 0.20 nM, Bmax 25 fmol/mg) and the cortex cerebri (Kd 0.11 nM, Bmax 8.2 fmol/mg). Other brain areas, i.e. cerebellum, thalamus or pons, expressed binding sites of identical high affinity in lower quantities (Bmax < 3 fmol/mg). Specific binding of 125I-labelled hGal was found to be reversible, time- and temperature-dependent and inhibited by Ca2+, Na+ and K+ ions at a concentration of 5 mM. Non-hydrolysable guanosine nucleotides potently reduced specific binding of 125-I-labelled hGal by more than 80%. Synthetic hGal analogues substituted in the N-terminal region exhibited strongly reduced binding affinity for the hGal receptor. Using 3-[(3-cholamidopropyl) dimethylammonio]-2-hydroxy-1-propanesulphonate, hGal receptors were successfully solubilized from human cortical membranes, exhibiting no significant loss of binding affinity. Affinity cross-linking to 125I-labelled hGal revealed a labelled band of approximately 60 kDa sensitive to unlabelled Gal. This putative hGal receptor is glycosylated since its molecular size was reduced after treatment with endoglycosidase F. Receptors bound to 125I-labelled hGal could be specifically adsorbed to wheat germ agglutinin and ricinus communis agglutinin, suggesting that receptor glycosylation involves N-acetyl glucosamine and galactose respectively.