Photoaffinity labeling of the 5-hydroxytryptamine 1A receptor in rat hippocampus

1-[2-(4-Azidophenyl)ethyl]-4-(3-trifluoromethylphenyl)piperazine (p-azido-PAPP) inhibits [3H]5-hydroxytryptamine [( 3H]5-HT) binding to 5-HT1A and 5-HT1B sites in rat brain with equilibrium dissociation constants (KD) of 0.9 nM and 230 nM, respectively. [3H]p-Azido-PAPP was synthesized and its reversible and irreversible binding properties to the hippocampal 5-HT1A site characterized. [3H]p-Azido-PAPP labeled a single class of sites in rat hippocampal membranes with a KD of 1 nM and a maximal binding density of 370 fmol/mg protein. The pharmacological profile of [3H]p-azido-PAPP binding was consistent with the radioligand's selective interaction with the 5-HT1A receptor. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membranes preincubated with [3H]p-azido-PAPP and irradiated showed a major band of incorporation of radioactivity at approximately 55,000 daltons. This incorporation could be blocked when membranes were incubated with 1 microM of several agents that have high affinity for 5-HT1A sites [5-HT, 8-hydroxy-2-(di-n-propylamino)tetraline, TVX Q 7821, spiperone, buspirone, d-lysergic acid diethylamide, metergoline]. The results indicate that on photolysis [3H]p-azido-PAPP irreversibly labels a polypeptide that is, or is a subunit of, the 5-HT1A receptor in rat hippocampus.