D-2L, D-2S, and D-3 dopamine receptors stably transfected into NG108-15 cells couple to a voltage-dependent potassium current via distinct G protein mechanisms

The D-2-like dopamine (DA) receptor family has continued to expand and now includes the D-2-short (D-2S) and DB-long (D-2L) receptor isoforms and the D-3 and D-4 receptors. The D-2 receptor isoforms differ in length by 29 amino acids within the third cytoplasmic loop, a region of the receptor believed to be important for G protein coupling. This observation has led to the hypothesis that the two isoforms of the D-2 receptor may utilize different signal transduction pathways when present in the same cell. The D-2 and D-3 receptors, although mostly different, show some common amino acid sequences within the third cytoplasmic loop. Thus, it is possible that the D-2 and D-3 receptors may employ similar signal transduction pathways. To test these hypotheses directly, NG108-15 neuroblastoma-glioma hybrid cells were stably transfected to express either the Das, D-2L, Or D-3 DA receptors. All transfected but not untransfected NG108-15 cells demonstrated a dose-dependent reduction in the peak whole-cell potassium (K+) current in response to receptor activation by DA or the DA receptor agonists quinpirole (QUIN) and apomorphine (APO). The modulation of K+ current by D-2S receptor stimulation was prevented by pretreatment of the cells with cholera toxin (20 mu g/ml for 18 h), whereas pertussis toxin pretreatment (500 ng/ml for 4 h) completely blocked the effects of D-2L and D-3 receptor activation. These observations suggest that the signal transduction mechanisms involved in coupling the two isoforms of the D-2 receptor to the K+ current are different, whereas the D-2L and D-3 receptor coupling mechanisms may be similar. In direct support of this hypothesis, it was observed that the intracellular application of a polyclonal antibody that is specific for the G(O alpha) subunit completely blocked the ability of D-2L and D-3 receptors to modulate outward K+ currents. In contrast, the D-2S-mediated modulation of K+ currents was blocked by intracellular application of an antibody recognizing G(S alpha) but not G(O alpha). These findings demonstrate that D-2S and D-2L receptors are able to couple to a common effector in a cell via two G protein pathways. (C) 1996 Wiley-Liss, Inc.