Voltage-dependent inhibition of Ca2+ channels in GH3 cells by cloned mu- and delta-opioid receptors

To study cloned opioid receptor binding and modulation of both adenylyl cyclase and ion channel activity, we stably expressed mu- and delta-opioid receptors in the rodent pituitary-derived GH3 cell line. GH3 cells express G proteins and voltage-activated Ca2+ channels (predominantly of the L-type). Activation of cloned rat mu-opioid receptors expressed in GH3 cells (termed GH3MOR cells) inhibits L-type Ca2+ channel activity. GH3MOR cells, further transfected with mouse delta receptor cDNA (termed GH3MORDOR cells), bound both [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAMGO) and [D-Pen2,D-Pen5]enkephalin (DPDPE). These opioid ligands inhibited adenylyl cyclase activity (IC50 = 174 and 0.53 nM, respectively). This action of DAMGO and DPDPE was attenuated selectively by mu- and delta-opioid receptor-specific antagonists. Activation of both opioid receptors also led to inhibition of Ca2+ channel activity, measured with Ba2+ as the charge carrier using the whole-cell patch-clamp technique. Both DAMGO (1 microM) and DPDPE (1 microM) reversibly inhibited Ba2+ currents (by 17.0 +/- 1.4% and 20.7 +/- 1.3%, respectively) in GH3MORDOR cells. The inhibitory action of DPDPE was dose dependent (IC50 = 1.6 nM) and was attenuated by pretreatment with pertussis toxin (200 ng/ml) or by the inclusion of guanosine-5'-O-(2-thio)diphosphate (2 mM) in the recording electrode. Ba2+ current inhibitions by both DAMGO and DPDPE were completely reversed by depolarizing (to > 50 mV) prepulses in GH3MORDOR cells. In summary, cloned mu- and delta-opioid receptors expressed in GH3 cells voltage-dependently couple through Gi/G(o) proteins to L-type Ca2+ channels.