HPLC-purified 2-[ 125I]iodomelatonin labels multiple binding sites in hamster brain

Binding of 2-[ 125I]iodomelatonin in hamster brain synaptosomal membranes at 0° C is rapid, saturable, reversible and sensitive to heat and trypsin treatment. Computer resolution of curvilinear Scatchard plots yielded high- and low-affinity components as follows: Kd 1 = 0.32 ± 0.14 nM, Bmax 1 = 5.6 ± 1.7 fmol/mg protein and Kd 2 = 10.5 ± 3.2 nM, Bmax 2 = 123 ± 33 fmol/mg protein (n = 3). Competition experiments indicated that 2-iodomelatonin and prazosin are the most potent inhibitors of high-affinity binding. Unlike prazosin, several α-adrenergic agents and various neurotransmitters were ineffective. These findings suggest that prazosin may be a potent antagonist at a unique, non-α-adrenergic, high-affinity binding site for melatonin.