FUNCTIONAL AND RADIOLIGAND BINDING CHARACTERIZATION OF RAT 5-HT6 RECEPTORS STABLY EXPRESSED IN HEK293 CELLS

We have stably expressed the rat 5-HT6 receptor in HEK293 cells at a d ensity of >2 pmol/mg protein, as determined in equilibrium binding stu dies with [H-3]-LSD and [H-3]-5-HT and compared the affinity of a rang e of compounds in competition binding experiments with either [H-3]-LS D or [H-3]-5-HT as radioligand. A variety of tryptamine derivatives we re tested and showed a significantly higher affinity when the 5-HT6 re ceptor was labelled with [H-3]-5-HT, whereas ergoline compounds and se veral antagonists had higher affinities when [H-3]-LSD was used as rad ioligand. Subsequently we examined the ability of LSD, 5-HT and a numb er of tryptamine derivatives to stimulate cAMP accumulation in order t o determine their agonist potency and efficacy. We observed the follow ing rank order of potency: LSD > omega-N-methyl-5-HT approximate to bu fotenine approximate to 5-methoxytryptamine > 5-HT > 2-methyl-5-HT app roximate to 5-benzyloxytryptamine approximate to tryptamine > 5-carbox amidotryptamine much greater than 5-HTQ. LSD, lisuride, 2-methyl-5-HT, tryptamine and 5-benzyloxytryptamine behaved as partial agonists rela tive to 5-HT. The rank order of potency of the tryptamine compounds co rrelated well with their affinities determined in binding assays. In a ddition, we have tested a number of antagonists in this system (rank o rder of potency: methiothepin, clozapine, mianserin and ritanserin). T his characterization of the pharmacological properties of recombinant 5-HT6 receptor will facilitate the identification of 5-HT6 receptor-me diated responses in physiological systems. (C) 1997 Elsevier Science L td.