Agonist-induced desensitization and phosphorylation of human 5-HT1A receptor expressed in Sf9 insect cells

The human 5-HT1A receptor was expressed in Sf9 insect cells to examine desensitization as manifested by agonist-induced uncoupling from G proteins and second messengers. New binding sites were detected after infection of cells with the 5-HT1A receptor-bearing baculovirus. 5-HT1A receptor agonists caused inhibition of cAMP accumulation that could be attenuated by specific receptor antagonists. Brief pretreatment with 5-HT resulted in (1) an uncoupling of receptor from G proteins as evidenced by a loss of high-affinity agonist binding sites and a diminished ability of the receptor to increase incorporation of AA-GTP into endogenous Go alpha-like G proteins, (2) a decreased ability of the receptor to inhibit cAMP accumulation, and (3) increased phosphorylation of the 5-HT1A receptor on serine and threonine residues. Phosphorylation occurred in the presence of a number of cyclic nucleotide dependent kinase inhibitors, and desensitization of the cAMP response occurred in the presence of H-7 and also in cells with prolonged exposure to PMA. Both phosphorylation and desensitization were markedly attenuated by 100 nM and 1 microM heparin and demonstrated similar time courses and concentration-response relationships. Those results demonstrate a close association between agonist-induced desensitization and phosphorylation of the 5-HT1A receptor in Sf9 cells through a pathway that mainly does not involve protein kinase A or C and might involve a G protein-linked receptor kinase.