Forskolin-induced up-regulation and functional supersensitivity of dopamine D2long receptors expressed by Ltk- cells.

Mouse fibroblast Ltk- cells, stably expressing the human dopamine D2long receptor, were grown in the presence of forskolin (100 microM) for 4 or 16 h. The 16 h treatment resulted in a significant up-regulation of the dopamine D2long receptors by 43-96% as measured with [3H]raclopride with no change in the Kd value. A significant increase in the maximal inhibition of acute forskolin-stimulated cAMP accumulation by dopamine (0.1 nM-3 microM) was found both at 4 and 16 h. No such D2long-receptor-coupled response to dopamine could be detected in wild-type, untransfected, Ltk- cells with or without forskolin treatment. Furthermore, basal cAMP levels as well as the maximal response to acute forskolin stimulation decreased in the D2long receptor expressing cells with the treatment, by 33% and 23% respectively. The results indicate that persistent maintenance of high cAMP levels in transfected Ltk- cells may lead to adaptive quantitative and functional changes of the dopamine D2long receptor reminiscent of receptor supersensitivity induced by chronic antagonist treatment in vivo where the receptor targeted is inhibitorily coupled to adenylyl cyclase, as is the D2long receptor. This may provide a model for studying mechanisms underlying dopamine D2 receptor up-regulation and receptor supersensitivity, not readily induced in cell lines.