Identification of serotonin 5HT2 receptors in bovine pineal gland

The concentration of serotonin in the pineal gland is extremely high, which prompted speculation that in addition to serving as a precursor of melatonin, serotonin may have an independent function of its own. By using [3H]-spiperone as a ligand, and ketanserine as a selective serotonin 5HT2 receptor antagonist, we have identified 5HT2 receptor in the bovine pineal gland, revealing a single population of binding sites with a dissociation equilibrium constant (Kd) value of 1.26 +/- 0.41 nM and a receptor density (Bmax) value of 193 +/- 38.85 fmol/mg protein. In displacement experiments, the concentrations of the drugs required to inhibit 50% of the specific binding of [3H]-spiperone in descending order of potency were methysergide greater than ritanserin greater than pirenperone greater than pipamperone greater than ketanserin greater than cyproheptadine greater than M-trifluoromethylphenyl-piperazine greater than prazosin greater than 5-methoxy-N-N-dimethyltryptamine hydrogen oxalate greater than 1-(3-chlorophenol) piperazine greater than serotonin. In the rat pineal gland, [3H]-spiperone revealed a low affinity serotonin binding site with a Kd value of 25.77 +/- 10.7 nM and a Bmax value of 1244 +/- 472 fmol/mg protein. The results of these studies are interpreted to indicate that the bovine pineal gland possess serotonin 5HT2 receptor. However, the rat pineal gland possess a serotoninergic binding site of unknown nature.