AGONIST-INDUCED DESENSITIZATION AND LOSS OF HIGH-AFFINITY BINDING-SITES OF STABLY EXPRESSED HUMAN 5-HT1A RECEPTORS

Exposure of HeLa cells stably expressing cloned human 5-hydroxytryptam ine (S-HT)(1A) receptors (HA7cells) to the agonist 8-hydroxy-2-(di-N-p ropylamino)-tetralin (8-OH-DPAT) results in a loss of high-affinity bi nding sites and a desensitization of receptor-adenylate cyclase coupli ng, as measured by 5-HT1A-mediated inhibition of forskolin-stimulated adenylate cyclase activity. These responses can also be observed after exposure to forskolin, which activates cyclic AMP-dependent protein k inase A or after treatment with known activators of protein kinase C ( PKC) such as phorbol-12-myristate 13-acetate (PMA). The responses elic ited by exposure to 8-OH-DPAT or PMA can be blocked completely by inhi bitors of PKC and also by 24-hr exposure to PMA. Preincubation of HA7 cells with 8-OH-DPAT also stimulates hydrolysis of inositol phospholip ids and the production of arachidonic acid. Inhibition of phospholipas e A(2) With quinacrine or by removal of extracellular Ca++ blocks the agonist-mediated loss of 5-HT1A receptor binding sites. These data dem onstrate that agonist-induced down regulation of the 5-HT1A receptor o ccurs after stimulation of both the PKC and phospholipase A(2) signali ng pathways, both of which may activate PKC. The subsequent response i s a loss of high-affinity ligand binding sites and functional receptor coupling to adenylate cyclase.