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 its high specific radioactivity (2175 Ci mmole-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).(ABSTRACT TRUNCATED AT 250 WORDS)