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-HT_1B) and serotonin-ID (5-HT_1D) 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-HT_1B 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-HT_1B and 5-HT_1D binding sites.