Biochemical and Pharmacological Characterization of Serotonin‐O‐Carboxymethylglycyl[125I]Iodotyrosinamide5 a New Radioiodinated Probe for 5‐HT1B and 5‐HT1D Binding Sites

Abstract: There is a lack of radioactive probes, particularly radioiodinated probes, for the direct labeling of serotonin‐1B (5‐HT1B) and serotonin‐ID (5‐HT1D) binding sites. Serotonin‐0‐carboxymethylglycyltyrosinamide (S‐CM‐GTNH2) was shown previously to be specific for these two subtypes; we, therefore, linked a 125I to its tyrosine residue. Biochemical and pharmacological properties of S‐CM‐G[125I]TNH2‐binding sites were studied by quantitative au‐toradiography on rat and guinea pig brain sections. S‐CM‐G[I25I]TNH2 binding is saturable and reversible with a KD value of 1.3 nM in the rat and 6.4 nM in the guinea pig. Binding is heterogeneous, paralleling the anatomical distribution of 5‐HT1B sites in the rat and of 5‐HTD sites in the guinea pig. The binding of 0.02 nM S‐CM‐G[125I]TNH2 was inhibited by low concentrations of 5‐HT, S‐CM‐GTNH2, CGS 12066 B, 5‐methoxytryptamine, and tryptamine in both species. Propranolol inhibited the radioligand binding with a greater affinity in the rat than in the guinea pig. Conversely, 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin inhibited S‐CM‐G[125I]TNH2 binding with a greater affinity in the guinea pig than in the rat. Other competitors, specific for 5‐HT,c, 5‐HT2, 5‐HT3, and adrenergic receptors, inhibited S‐CM‐G[125I]TNH2 binding in rat and guinea pig substantia nigra and in other labeled structures known to contain these receptors, but only at high concentrations. S‐CM‐G[I25I]TNH2 is then a useful new probe for the direct study of 5‐HT1B and 5‐HT1D binding sites.