Characterization of dopaminergic receptor sites in bovine pineal gland

In addition to beta-adrenergic receptor agonists, L-dopa and dopamine have been also shown to activate the production of melatonin and its synthesizing enzyme, serotonin N-acetyltransferase. In an attempt to characterize dopaminergic receptor sites, bovine pineal synaptosomes were prepared by differential centrifugation techniques. Washed disrupted synaptic membranes were used to study 3H-spiroperidol binding, using standard membrane-binding techniques. Association of 3H-spiroperidol to pineal membranes was very rapid, reaching equilibrium within 2 min and remaining stable for 20 min. Dissociation was also rapid with a t 1/2 of 3 min. Analysis of saturation studies (0.035 to 20 nM spiroperidol, 16 concentrations) using the LIGAND program indicated the presence of two binding sites with KDS (dissociation equilibrium constant) of 0.18 nM and 2.1 nM. The Bmax's (receptor density) of the sites were 37 and 630 fmoles/mg protein respectively. The IC50S of haloperidol, cis-flupenthixol, and chlorpromazine were 8, 12, and 80 nM, respectively, while those of pipamperone, cyproheptadine, and cinanserin were 60, 400, and 1500 nM. These and other data indicate that the most abundant site is a dopamine D2 receptor while the less abundant site may be a serotonin receptor. The function of dopamine and dopaminergic neurons in bovine pineal gland is not known and has not been established.