Characterization of central melatonin receptors using ¹²⁵I-melatonin

The binding of ¹²⁵I-melatonin, a potent analog of melatonin, to rat brain synaptosomal preparations was investigated. ¹²⁵I-melatonin bound with high affinity (K_d = 38 nM) to a single class of sites (B_max = 81 fmol mg protein). Kinetic studies indicated that binding was time-dependent and reversible. Specific ¹²⁵I-melatonin binding was inhibited by melatonin, and was unaffected by other structurally related compounds including serotonin. Binding of ¹²⁵I-melatonin was greatly reduced if the synaptosomal preparations were pretreated by heat or trypsin but was unaffected by freeze-thawing. These results suggest that ¹²⁵I-melatonin may serve as a valuable probe for studying melatonin receptors.