[3H]LY303870, a Novel Nonpeptide Radioligand for the NK-1 Receptor |
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Authors: | D R Gehlert D A Schober P A Hipskind B D Gitter J J Howbert |
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Institution: | CNS Research, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, Indiana, U.S.A. |
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Abstract: | Abstract: We synthesized a potent and selective antagonist radioligand for the neurokinin (NK)-1 receptor and characterized its binding to guinea pig striatal membranes. ( R ) - N - 2 - Acetyl3H3](2 - methoxyphenyl) - methyl]amino] - 1 - (1 H - indol - 3 - ylmethyl)ethyl]1,4' - bipiperidine]-1'-acetamide (3H]LY303870) binds to a single class of sites with an equilibrium K D of 0.22 n M and a B max of 723 fmol/mg of protein. Unlabeled LY303870 potently inhibited the binding with an IC50 of 0.56 n M , whereas the less active ( S )-enantiomer (LY306155) was substantially less potent. The nonpeptide NK-1 antagonists (±)-CP96,345 and (±)-RP 67580 had IC50 values of 0.74 and 49 n M , respectively. Substance P (SP) was also a potent inhibitor with with an IC50 of 3.1 n M . The inhibition by SP could be separated into two components: a high-affinity component with a K i of 0.53 n M and a lower-affinity component with a K i of 155 n M . Addition of 100 µ M guanylyl 5'-imidodiphosphate Gpp(NH)p] in the incubation increased the relative amount of the low-affinity agonist state of the receptor. Consistent with the antagonist properties of LY303870, the dissociation rate of 3H]LY303870 was not changed by the presence of 100 µ M Gpp(NH)p. The distribution of 3H]LY303870 binding sites in the guinea pig brain closely matched the distribution of NK-1 receptors labeled by 3H]SP. Therefore, 3H]LY303870 is a potent and selective antagonist radioligand for NK-1 receptors in guinea pig brain. In addition, regulation of NK-1 agonist affinity by guanine nucleotides is similar to that seen for monoaminergic receptors. |
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Keywords: | Substance P Autoradiography Guinea pig Pain Migraine |
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