CHARACTERIZATION OF A 5-HYDROXYTRYPTAMINE RECEPTOR IN MOUSE NEUROBLASTOMA N18TG2 CELLS

Cyclic AMP formation was found to increase in mouse neuroblastoma N18T G2 cells exposed to 5-hydroxytryptamine (5-HT). This response was conc entration-dependent with an EC(50) value of 0.22 mu M. Tryptamine and other tryptamine-related compounds were also agonists in this assay wi th a rank order of potency of 5-methoxytryptamine > 5-HT > tryptamine > 2-methyl-5-HT > 5-carboxamidotryptamine >> alpha-methyl-5-HT. (D)-Ly sergic acid diethylamide and 2-bromo-lysergic acid diethylamide were p artial agonists in this system with maximal responses of 44 and 34%, r espectively, compared to 5-HT. 5-HT-stimulated cyclic AMP formation wa s inhibited by clozapine, mianserin and methiothepin with pA(2) values of 6.6, 6.5 and 6.4, respectively. Radioligand binding studies using [I-125]iodolysergic acid diethylamide revealed a binding site present at a density of 10.4 fmol/mg of protein, with an affinity for the liga nd of 1.18 nM. In competition studies this binding site displayed a ph armacology similar to that defined in studies of cyclic AMP formation. The pharmacological profile of this receptor, characterized by both r adioligand binding and functional coupling to adenylyl cyclase, does n ot correspond to that of any of the currently classified subtypes of 5 -HT receptor, but is similar to the 5-HT receptor cloned recently from rat striatum and referred to as the 5-HTs receptor. The N18TG2 cell l ine represents a useful model system in which this novel 5-HT receptor may be characterized fully.