Identification of binding sites for SR 46349B, a 5-hydroxytryptamine2 receptor antagonist, in rodent brain.

SR 46349B belongs to a new class of compounds (propenone oxime ether derivative) that inhibit 5-hydroxytryptamine (HT)2 receptors in vitro and in vivo. (3H) SR 46349B has been shown to bind with high affinity (Kd = 1.20 nM) to a single class of sites in rat prefrontal cortical membranes. The maximum binding capacity (Bmax = 0.262 pmol/mg of protein) is similar to that found for other classes of 5-HT2 receptor antagonists. Although the highest density of specific (3H) SR 46349B binding was found in cortex tissue, specific binding was also detectable in other brain areas. Among various receptor or channel ligands [including alpha or beta adrenergic, dopamine (D1 or D2), histamine (H1 or H2), 5-HT subclasses (5-HT1, 5-HT3), muscarinic and Na+ and Ca2+ channel blockers] only 5-HT2 receptor effectors were able to displace (3H) SR 46349B. In addition, the type of inhibition exerted by known 5-HT2 receptor antagonists such as ketanserin and ritanserin was investigated by saturation studies. In vivo, (3H) SR 46349B bound predominantly in mouse brain regions containing 5-HT2 receptors. This binding was displaced by SR 46349B, ketanserin and ritanserin following oral administration. From these results we suggest that SR 46349B in its tritiated form is a useful tool to label the 5-HT2 receptor in vitro and in vivo.

Characterization of the binding sites of [3H]SR 48968, a potent nonpeptide radioligand antagonist of the neurokinin-2 receptor.

[3H]SR 48968, a radiolabeled nonpeptide antagonist of NK-2 receptor, has been tested in ligand-receptor binding assays using rat duodenum, hamster urinary bladder and guinea pig ileum membranes. [3H]SR 48968 bound to a single class of high affinity binding sites. Its affinity was slightly species-dependent. Its binding was inhibited by neurokinins, following the rank order of potency NKA > NKB > SP. It was also inhibited by peptide antagonists of NK-2 receptor (MEN 10,376, L 659,877), but their relative potencies were highly species-dependent. Unlabeled SR 48968, but not its R-enantiomer (SR 48965), also potently inhibited its binding. These data show that [3H]SR 48968 potently binds to NK-2 receptor and therefore is a useful tool to study NK-2 receptor.