2-[I-125]IODOMELATONIN BINDING-SITES IN THE BOVINE HIPPOCAMPUS ARE NOT SENSITIVE TO GUANINE-NUCLEOTIDES

The discrete distribution and pharmacological characteristics of melat onin binding sites in the bovine hippocampus were determined. Autoradi ography revealed the presence of melatonin binding sites in the stratu m lacunosum-molecularis of the hippocampus (CA1), stratum molecularis of the subiculum and in the enthorhinal cortex. Analysis of the kineti c parameters demonstrated that the binding was stable and reversible, represented by a single class high affinity binding sites (k(d) 40 pM, B-max = 3.9 fmol/mg protein). However, 2-iodomelatonin and 2-bromomel atonin inhibited 2-[I-125]iodomelatonin binding in a biphasic manner. The presence of 4 mM CaCl2 did not cause changes in the affinity const ant values. Finally, experiments performed with GTP(G)amma S revealed that binding affinity was not decreased even with high concentrations of the nucleotide. These findings show that 2-[I-125]iodomelatonin bin ding sites in the bovine parahippocampal-hipocampal region possess som e binding features not common to melatonin receptors described so far; moreover they seem not to be linked to a regulatory G-protein.