Agonist-induced desensitization of D2 dopamine receptors in human Y-79 retinoblastoma cells

Abstract

Y-79 human retinoblastoma cells can be induced to express significant quantities of functional D2 dopamine receptors after attachment and differentiation with sodium butyrate. In membranes prepared from differentiated Y-79 cells, the D2 dopaminergic antagonist [3H] methylspiperone exhibits a KD of 77 pm and a Bmax of 60 fmol/mg of protein, whereas the antagonist [125I]iodosulpride reveals a KD of 0.77 nM and a Bmax of 40 fmol/mg of protein. Dopamine also induces a pharmacologically specific, pertussis toxin-sensitive, dose-dependent inhibition of forskolin-stimulated adenylyl cyclase activity, with an EC50 of 2 microM and a maximal response at 100 microM (approximately 50% enzyme inhibition). Pretreatment of the cells with dopamine results in a diminution in the subsequent ability of dopamine to inhibit adenylyl cyclase activity. This effect is time dependent, reaching maximal desensitization after approximately 24 hr. The dopamine dose-response curve for inducing desensitization exhibits an EC50 of approximately 2-3 microM and a maximal response at approximately 0.1-1 mM, similar to that for inhibiting adenylyl cyclase activity. After maximal desensitization, the EC50 for dopamine-induced inhibition of adenylyl cyclase activity is increased greater than 20 fold (lower affinity) and the maximum inhibition is decreased to approximately 15%, representing an approximately 70% desensitization. The agonist-induced desensitization is pharmacologically specific, inasmuch as preincubation of the cells with the dopaminergic agonists epinine and (+-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene or the D2-selective agonist N-0434 also results in desensitization of dopamine-induced inhibition of enzyme activity, whereas preincubation with the D1-selective agonist SKF-38393 or with the nondopaminergic agonists isoproterenol and serotonin results in little or no desensitization. Preincubation of the cells with dopamine also promotes a time-dependent increase (approximately 3-fold) in the KD for [3H]methylspiperone, with no change in its Bmax. In contrast, after dopamine preincubation, the KD for [125I]iodosulpride is unchanged, whereas its Bmax is reduced by approximately 50% upon maximum desensitization. In addition, agonist pretreatment promotes a functional uncoupling of the D2 receptor, as suggested by a loss of high affinity agonist binding observed in radioligand competition binding assays after desensitization. Upon removal of agonist, the cellular D2 receptor binding activity and functional response recover to control levels within a 24-hr period. These results suggest that prolonged exposure of cells to dopaminergic agonists initiates a desensitization process involving a functional uncoupling of the D2 dopamine receptor as well as a loss of its ligand binding activity.