The binding profiles of SA4503 (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride), a novel sigma receptor ligand, to sigma 1 and sigma 2 receptor subtypes in guinea pig and rat brain membranes were evaluated. SA4503 showed a high affinity for the sigma 1 receptor subtype labeled by (+)-[3H]pentazocine (IC50 = 17.4 +/- 1.9 nM), while it had about 100-fold less affinity for the sigma 2 receptor subtype labeled by [3H]1,3-di(2-tolyl)guanidine ([3H]DTG) in the presence of 200 nM (+)-pentazocine. SA4503 showed little affinity for 36 other receptors, ion channels and second messenger systems. The inhibition curves of SA4503 for (+)-[3H]pentazocine binding were shifted to the right in the presence of guanosine 5'-o-(3-thiotriphosphate) (GTP gamma S), as similar to those of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-3-PPP) and (+)-pentazocine, sigma 1 receptor agonists. SA4503 significantly increased the KD value, but did not affect the Bmax value for specific (+)-[3H]pentazocine binding. These results indicated that SA4503 is a potent and selective agonist for the sigma 1 receptor subtype in the brain. In addition, SA4503 inhibited specific (+)-[3H]pentazocine binding in a competitive manner.