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 of I2'I-galanin showed the presence of a high-affinity binding site (Kd = 1.91 -t 0.40 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 f 0.1 M-' min-l 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. The '"I g galanin-binding site is a trypsin-sensitive membrane protein, which is heatdenaturated at 60°C within 5 min. The effect of GTP and its analogs and of pertussis-toxin-catalyzed ADPribosylation on the binding of 12'I-galanin suggest that the galanin receptor is coupled to an inhibitory G protein (Gi protein).'271-galanin was shown to be a ligand with affinity equal to that of galanin in displacing '251-galanin. The primary sequence of porcine galanin, a 29-aminoacid, C-terminal-amidated peptide, was determined in 1983 by Tatemoto et al. [l]. Within a short period a series of findings were published on the immunohistochemical mapping of the distribution of this peptide in the central [2 -41 and peripheral nervous system [S], using antisera raised against the porcine peptide. Autoradiography of 251-galanin binding indicated the presence of putative galanin receptors in several brain regions [6]. Among its numerous physiological and pharmacological actions it is noteworthy that galanin acts as a potent inhibitor of insulin release [7], of dopamine release from the median eminence [8] and of acetylcholine release from the ventral hippocampus [9]. This latter finding is of great interest, since galanin turned out to be a potent endogenous regulator of acetylcholine release in the hippocampus, a brain region where cholinergic deficit, such as that in Alzheimer's disease, is accompanied by a loss of cognitive function [lo].Interest in the biochemistry and pharmacology of galanin action is thus well motivated. The rat galanin cDNA was recently cloned and sequenced [II, 121 and the rat galanin sequence became available to us towards the end of this study.Meanwhile, the galanin receptor sites on hamster pancreatic 0-cell tumor [I31 and in rat brain membranes [14] wereCorrespondence to