Galanin receptor and its ligands in the rat hippocampus

Abstract

Receptors for the 29-amino-acid peptide, galanin, in membranes from the rat ventral hippocampus were examined using chloramine-T-iodinated porcine galanin as ligand. The equilibrium binding of¹²⁵I-galanin showed the presence of a high-affinity binding site (Kd=1.91 ± 40 nM). The concentration of the high-affinity-binding sites was 107 ± 15 fmol/mg membrane protein. The on rate constant was estimated to be 2.6 ± 0.1 M⁻¹ min⁻¹ at 37°C. The affinity of rat galanin (differing in three amino acid residues from the porcine protein) was equal to that of porcine galanin. The¹²⁵I-galanin-binding site is a trypsin-sensitive membrane protein, which is heat-denaturated at 60°C within 5 min. The effect of GTP and its analogs and of pertussis-toxin-catalyzed ADP-ribosylation on the binding of¹²⁵I-galanin suggest that the galanin receptor is coupled to an inhibitory G protein (G_i protein). ¹²⁷I-galanin was shown to be a ligand with affinity equal to that of galanin in displacing ¹²⁵I-galanin. The ¹²⁵I-galanin-binding site in the ventral hippocampus recognizes as a ligand the tryptic fragments 1–20 and 21–29 of rat galanin and the synthetic fragments 12–29, 18–29 and 21–29 of porcine galanin. None of these afforded full inhibition of the binding of fragment 1–29 of ¹²⁵I-galanin at a concentration of 1μM.