Novel radioligand for the purification of the serotonin/sub 1A/ receptor

Novel phenylpiperazine derivatives were developed in order to reversibly and irreversibly label the serotonin/sub 1a/ (5-HT/sub 1a/) binding site in the brain. 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)piperazine (PAPP) acts as a serotonin agonist in vivo and binds with high affinity and selectivity to the 5-HT/sub 1a/ site in the rat brain in vitro. Evidence establishing that [3H]PAPP reversibly radiolabels the 5-HT/sub 1a/ binding site in the rat brain is as follows: (1) [3H]PAPP reversibly labels the same maximal number of 5-HT/sub 1a/ sites in rat cortical membranes as [3H]5-HT with a K/sub d/ of 1.6 nM; (2) the affinity of a series of serotonergic drugs for the [3H]PAPP sites correlates with the affinity of these drugs at the 5-HT/sub 1a/ binding site as measured by inhibition of [3H]5-HT binding; (3) GTP and divalent cations modulates the [3H]PAPP sites in the same way as the 5-HT/sub 1a/ binding sites; (4) the regional distribution of [3H]PAPP binding site density correlates with the regional distribution of 5-HT/sub 1a/ sites in the rat brain. 5-HT/sub 1a/ and 5-HT/sub 1b/ binding sites were solubilized from bovine brain cortex using a detergent mixture of 0.1% Nonidet P-40 and 0.3% digitonin. The solubilized binding sites retained characteristics of the membrane bound binding sites including the ability to be modulated by the GTP. The solubilization procedure and PAPP affinity column can now be used together for the purification of the 5-HT/sub 1a/ receptor site