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[ 3 H]LY303870, a Novel Nonpeptide Radioligand for the NK‐1 Receptor
Author(s) -
Gehlert D. R.,
Schober D. A.,
Hipskind P. A.,
Gitter B. D.,
Howbert J. J.
Publication year - 1996
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1996.66031095.x
Subject(s) - radioligand , receptor , dissociation constant , stereochemistry , agonist , chemistry , antagonist , ligand (biochemistry) , radioligand assay , receptor antagonist , biochemistry
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 ‐ [Acetyl[ 3 H 3 ][(2 ‐ methoxyphenyl) ‐ methyl]amino] ‐ 1 ‐ (1 H ‐ indol ‐ 3 ‐ ylmethyl)ethyl][1,4′ ‐ bipiperidine]‐1′‐acetamide ([ 3 H]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 IC 50 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 IC 50 values of 0.74 and 49 n M , respectively. Substance P (SP) was also a potent inhibitor with with an IC 50 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 [ 3 H]LY303870 was not changed by the presence of 100 µ M Gpp(NH)p. The distribution of [ 3 H]LY303870 binding sites in the guinea pig brain closely matched the distribution of NK‐1 receptors labeled by [ 3 H]SP. Therefore, [ 3 H]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.