Cloned delta-opioid receptors in GH(3) cells inhibit spontaneous Ca2+ oscillations and prolactin release through K-IR channel activation

Opioid receptors can couple to K+ and Ca2+ channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in th e regulation of neurotransmitter and hormone release from excitable cells. GH(3) cells exhibit spontaneous oscillations of intracellular Ca2+ concentr ation ([Ca2+](i)) and prolactin release. Activation of cloned delta-opioid receptors stably expressed in GH(3) cells inhibits both spontaneous Ca2+ si gnaling and basal prolactin release. The objective of this study was to exa mine a possible role for K+ channels in these processes using the patch-cla mp technique, fluorescence imaging, and a sensitive ELISA for prolactin. Th e selective delta receptor agonist [D-Pen(2), D-Pen(2)]enkephalin (DPDPE) i nhibited [Ca2+](i) oscillations in GH(3) cells expressing both mu and delta receptors (GH(3)MORDOR cells) but had no effect on control GH(3) cells or cells expressing mu receptors alone (GH(3)MOR cells). The inhibition of [Ca 2+](i) oscillations by DPDPE was unaffected by thapsigargin pretreatment, s uggesting that this effect is independent of inositol 1,4,5-triphosphate-se nsitive Ca2+ stores. DPDPE caused a concentration-dependent inhibition of p rolactin release from GH(3)MORDOR cells with an IC50 of 4 nM. DPDPE increas ed inward K+ current recorded from GH(3)MORDOR cells but had no significant effect on K+ currents recorded from control GH(3) cells or GH(3)MOR cells. The mu receptor agonist morphine also had no effect on currents recorded f rom control cells but activated inward K+ currents recorded from GH(3)MOR a nd GH(3)MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K+ current was inwardly rectifyin g and was inhibited by Ba2+ but not TEA. DPDPE had no effect on delayed rec tifier-, Ca2+-, and voltage-activated or A-type K+ currents, recorded from GH(3)MORDOR cells. Ba2+ attenuated the inhibition of [Ca2+](i) and prolacti n release by DPDPE, whereas TEA had no effect, consistent with an involveme nt of K-IR channels in these actions of the opioid.