Functional and radioligand binding characterization of rat 5-HT sub(6) receptors stably expressed in HEK293 cells

We have stably expressed the rat 5-HT sub(6) receptor in HEK293 cells at a density of >2 pmol/mg protein, as determined in equilibrium binding studies with [ super(3)H]-LSD and [ super(3)H]-5-HT and compared the affinity of a range of compounds in competition binding experiments with either [ super(3)H]-LSD or [ super(3)H]-5-HT as radioligand. A variety of tryptamine derivatives were tested and showed a significantly higher affinity when the 5-HT sub(6) receptor was labelled with [ super(3)H]-5-HT, whereas ergoline compounds and several antagonists had higher affinities when [ super(3)H]-LSD was used as radioligand. Subsequently we examined the ability of LSD, 5-HT and a number of tryptamine derivatives to stimulate cAMP accumulation in order to determine their agonist potency and efficacy. We observed the following rank order of potency: LSD > omega -N-methyl-5-HT approximately bufotenine approximately 5-methoxytryptamine > 5-HT > 2-methyl-5-HT approximately 5-benzyloxytryptamine approximately tryptamine > 5-carboxamidotryptamine approximately 5-HTQ. LSD, lisuride, 2-methyl-5-HT, tryptamine and 5-benzyloxytryptamine behaved as partial agonists relative to 5-HT. The rank order of potency of the tryptamine compounds correlated well with their affinities determined in binding assays. In addition, we have tested a number of antagonists in this system (rank order of potency: methiothepin, clozapine, mianserin and ritanserin). This characterization of the pharmacological properties of recombinant 5-HT sub(6) receptor will facilitate the identification of 5-HT sub(6) receptor-mediated responses in physiological systems.