© Neuroscience-Net Volume 1, Article #10007

Received May 14, 1996 Accepted for Publication Auguest 20, 1996 Published September 13, 1996

Two isoforms of the D₂ dopamine (DA) receptor have been identified. They are derived via alternative splicing of a single gene and have been termed D₂-short (D_2S) and D₂-long (D_2L) to indicate that they vary by only a 29 amino acid sequence within the third cytoplasmic loop. In the present study, the role of changes in intracellular calcium concentrations ([Ca²⁺]i) in the coupling of D_2S and D_2L receptors to a whole-cell potassium (K⁺) current was examined in stably transfected NG108-15 cells. It was noted that changes in intracellular concentration of the calcium chelator BAPTA (0.1-2.0 mM) attenuated or blocked the ability of D_2S (as previously reported), but not D_2L, receptor stimulation to reduce whole-cell K⁺ current. Bath application of thapsigargin (10-50 micromolar), which are known to increase(s) [Ca²⁺]i did not alter the magnitude of this current in D_2L transfected cells. The ability of the D₂ dopamine receptor agonists quinpirole (QUIN) to mobilize intracellular calcium was measured in both D_2L and D_2S transfected cells using the fluorescent probe fura-2. Bath application of QUIN (2-50 micromolar) dose-dependently increased [Ca²⁺]i in both transfected groups (D_2S and D_2L) but not untransfected cells with levels reaching a maximal concentration of approximately 100 nM (from resting levels of approximately 40 nM). This effect of QUIN was completely abolished in both D_2S and D_2L transfected NG108-15 cells by pretreatment with ryanodine (10 or 20 micromolar), demonstrating that receptor activation induced a mobilization of calcium from intracellular stores. Pretreatment with pertussis toxin completely blocked the mobilization of [Ca²⁺]i in D_2L transfected NG108-15 cells while having no effect on D_2S transfected cells. These results provide further evidence that the two isoforms of the D₂ receptor can couple to common effectors in a cell (whole-cell K⁺ current and intracellular calcium release) via two distinct signal transduction pathways.