Pharmacologic evidence for 5-HT1A receptors associated with human retinal pigment epithelial cells in culture.

PURPOSE The authors investigate the possible presence of 5-hydroxytryptamine (5-HT1) type serotonin receptors negatively coupled to adenylate cyclase activity in cultured human retinal pigment epithelial (RPE) cells. METHODS Adenylate cyclase activity was assessed by the determination of cellular adenosine 3':5' cyclic monophosphate (cAMP) levels and the effects of serotonin on both basal and forskolin-stimulated cAMP levels studied. RESULTS Serotonin at 100 microM had no effect on the basal levels of cAMP in cultured human RPE cells, but attenuated by 43.6% the stimulation in cAMP production induced by forskolin (5 microM). This effect was dose dependent for serotonin with half-maximal inhibition (EC50) occurring at approximately 1.5 x 10(-9) M. The 5-HT1 receptor agonist 8-hydroxy [2-di-n-propylamino] tetralin (8-OH DPAT), buspirone, 5-carboxyamidotryptamine, and RU24969 mimicked the inhibitory effect of serotonin in a dose-dependent manner. The actions of serotonin and 8-OH DPAT (10 microM) were dose-dependently attenuated by the serotonergic antagonists spiroxatrine, propranolol, and spiperone. Pretreatment of RPE cell cultures with pertussis toxin abolished the serotonin-induced reduction of forskolin-elevated cAMP levels. Stimulation of cAMP production by the beta-adrenoceptor agonist isoproterenol at 0.1 microM, but not at 10 microM or 100 microM, also was attenuated by serotonin (100 microM), whereas cAMP production induced by the adenosine receptor agonist 5'-[N-ethyl]-carboxamidoadenosine (NECA) at 1 microM, 10 microM, and 100 microM was unaffected. Serotonin and 8-OH DPAT dose-dependently inhibited isoproterenol-stimulated (0.1 microM) cAMP production with EC50 values of approximately 10 microM, and pertussis toxin pretreatment partially blocked these effects. CONCLUSIONS Cultured human RPE cells possess 5-HT1A receptors negatively coupled to cAMP production through a pertussis toxin-sensitive G protein. These receptors show differential effects on forskolin-, isoproterenol-, and NECA-stimulated cAMP production, which may reflect a unique spatial distribution of receptor proteins or the phenotypic heterogeneity of RPE cells that is the result of or that is preserved in culture.