Novel Melatonin Binding Site on the TrkB Receptor
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Melatonin (5-methoxy-N-acetyltryptamine) is primarily synthesized in the pineal gland and retina with high levels at night. Melatonin modulates physiological functions through activation of MT 1 and MT 2 melatonin receptors and exerts neuroprotective effects as an antioxidant. N-acetylserotonin (NAS), the precursor of melatonin, was recently reported to trigger TrkB receptor phosphorylation in a BDNF, MT 1 and MT 2 receptors, and MT 3 independent manner (Jang et al., 2010a). However, Sung-Wuk Jang (2009) tested selective MT 3 ligands binding to the TrkB receptor in primary culture of mouse cortical neurons at submaximal effective concentrations (Jang et al., 2010a). Here we tested the hypothesis that NAS is binding to T48-TrkB at a site with the kinetic and pharmacological characteristics reported for the MT 3 binding site. We therefore assessed and compared 2-[ 125 I]-iodomelatonin binding to membranes of T48-TrkB, SN56 (parent cells), to establish similarities and differences on binding kinetics, pharmacology, affinities and binding capacity with the MT 3 binding site and the CHO-hMT 1 and CHO-hMT 2 melatonin receptors. Specific 2-[ 125 I]-iodomelatonin binding to T48-TrkB, but not SN56 membranes, increased with temperature, was saturable, and showed high affinity. These binding characteristics are similar to those displayed by binding of this radioligand to either hMT 1 or hMT 2 melatonin receptors. Statistically significant correlations were found between binding affinities determined in competition studies for T48-TrkBand either hMT 1 or MT 2 receptors. 2-[ 125 I]-iodomelatonin binding to T48-TrkB cell membranes is irreversible setting up a major difference that distinguished this receptor from the MT 1 and MT 2 receptors. Taken together our data showed that the binding characteristics of T48-TrkB are different from those of the MT 3 binding site, and also appear to be distinct from the MT 1 and MT 2 receptor sites. We conclude that we have uncovered a novel melatonin binding site on the TrkB receptor.