SELECTIVE BETA-ANTAGONISTS ARE EQUALLY AND HIGHLY POTENT AT 5-HT SITES IN THE RAT HIPPOCAMPUS

Serotonin (5-hydroxytryptamine, 5-HT) and various tryptamine-related drugs were equi-potent to known .beta.-antagonists in competition experiments of 125Iodo-cyanopindolol binding in the rat hippocampus. IC50 values for all the tryptamine related drugs (5,7-DHT, 5-MT, 5-MEO, DMT) were very similar to those obtained for (-)-propranolol, (.+-.)-cyanopindolol, zinterol and atenolol and were all in the nanomolar range. Saturation experiments demonstrated that in the rat hippocampus, a subpopulation of serotonin recognition sites comprised 50% of 125I-CYP binding. The KD was 140 .+-. 30 pM and the Bmax was 71 .+-. 7 fmole/mg protein. This suggests that 125I-CYP binding studies for the quantitation of .beta.-adrenergic receptors should be re-evaluated and caution should be exercised in the choice of the displacing agent for the definition of non-specific binding. (.+-.)-[125Iodo]cyanopindolol (I-CYP) has been used as a radioligand which binds with high affinity and specificity to .beta.-adrenoceptors (Engel, Hoyer, Berthold and Wagner, 1981). The reported low dissociation constant (27-40 pM) of 125I-CYP for .beta.-adrenoceptors in various tissues, in combination with it''s high specific radioactivity (2175 Ci mmol-1) allowed binding studies to be carried out with low protein and ligand concentrations. These factors have established 125I-CYP as the choice ligand for the quantitation of .beta.-adrenoceptors in our laboratory (Edwards and Henn, 1985). Although other .beta.-antagonists, in particular 125 Iodohydroxybenzylpindolol, had been reported to bind to multiple sites other than .beta.-adrenoceptors (Willcocks and Nahorski, 1983), the specificity of 125I-CYP had not been questioned until a recent autoradiographic study (Pazos, Engel and Palacios, 1985). In that study, 125I-CYP binding in specific rat brain regions was blocked by various serotonergic drugs. Radioligand binding assays and detailed competition experiments, can be used to separate the components of ligand binding (Whitaker et al., 1983). With this method we have characterized the binding of 125Iodo-cyanopindolol in the rat hippocampus. Our results show that in addition to .beta.-adrenergic receptors, 125I-CYP is an equally potent label for 5-HT1 receptors.