Differential coupling of 5-HT1A receptors occupied by 5-HT or 8-OH-DPAT to adenylyl cyclase.

Human serotonin (5-hydroxytryptamine, 5-HT)-1A receptors have been transfected in NIH-3T3 cells, and their coupling to adenylyl cyclase was analysed depending on 1) the number of receptor expressed, 2) the experimental conditions used, 3) the nature of the agonists. Two monoclonal cell lines were used, expressing low (45 fmol/mg) and high (500 fmol/mg) levels of 5-HT1A receptor. Two methods were tested to study the negative coupling of the transfected 5-HT1A receptors to adenylyl cyclase: 1) measurement of cAMP production in intact cells, 2) measurement of adenylyl cyclase activity in vitro on membrane preparations. Studies on intact cells revealed that an increase in the receptor concentration was followed by 1) an increase in the efficacies of 5-HT, 5-CT (5-carboxamidotryptamine) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 2) a 2 to 3-fold increase in the potency of 5-CT and 8-OH-DPAT, but no change in the potency of 5-HT. In membrane preparations, 8-OH-DPAT dose-response curve was shifted leftwards when the receptor concentration became higher whereas the corresponding shift was smaller for 5-HT and absent for 5-CT. Surprisingly, on membrane preparations, 8-OH-DPAT was a partial agonist relative to 5-HT. The relative efficacy of 8-OH-DPAT was lower in the clone expressing the lowest level of receptor. This partial agonist behavior of 8-OH-DPAT could be modulated by the ionic conditions under which the adenylyl cyclase activity was measured.(ABSTRACT TRUNCATED AT 250 WORDS)