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 [H-3]-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 (B(max)) 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 [H-3]-spiperone in descending order of potency were methysergide > ritanserin > pirenperone > pipamperone > ketanserin > cyproheptadine > M-trifluoromethylphenyl-piperazine > prazosin > 5-methoxy-N-N-dimethyltryptamine hydrogen oxalate > 1-(3-chlorophenol) piperazine > serotonin. In the rat pineal gland, [H-3]-spiperone revealed a low affinity serotonin binding site with a Kd value of 25.77 +/- 10.7 nM and a B(max) 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.