The Journal of Neurophysiology Vol. 83 No. 5 May 2000, pp. 2691-2698
Copyright ©2000 by the American Physiological Society

Cloned -Opioid Receptors in GH₃ Cells Inhibit Spontaneous Ca²⁺ Oscillations and Prolactin Release Through K_IR Channel Activation

 ¹Department of Physiology, Cornell University, New York, New York 10021;  ²Department of Neurology, University of California Los Angeles School of Medicine, Los Angeles, California 90095;  ³Department of Pharmacology, The George Washington University, Washington, DC 20037; and  ⁴Department of Psychiatry, University of California Los Angeles School of Medicine, Los Angeles, California 90095

Piros, Elemer T., Rew C. Charles, Lei Song, Chris J. Evans, and Tim G. Hales. Cloned -Opioid Receptors in GH₃ Cells Inhibit Spontaneous Ca²⁺ Oscillations and Prolactin Release Through K_IR Channel Activation. J. Neurophysiol. 83: 2691-2698, 2000. Opioid receptors can couple to K⁺ and Ca²⁺ channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH₃ cells exhibit spontaneous oscillations of intracellular Ca²⁺ concentration ([Ca²⁺]_i) and prolactin release. Activation of cloned -opioid receptors stably expressed in GH₃ cells inhibits both spontaneous Ca²⁺ signaling and basal prolactin release. The objective of this study was to examine a possible role for K⁺ channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective  receptor agonist [D-Pen², D-Pen²]enkephalin (DPDPE) inhibited [Ca²⁺]_i oscillations in GH₃ cells expressing both µ and  receptors (GH₃MORDOR cells) but had no effect on control GH₃ cells or cells expressing µ receptors alone (GH₃MOR cells). The inhibition of [Ca²⁺]_i oscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca²⁺ stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH₃MORDOR cells with an IC₅₀ of 4 nM. DPDPE increased inward K⁺ current recorded from GH₃MORDOR cells but had no significant effect on K⁺ currents recorded from control GH₃ cells or GH₃MOR cells. The µ receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K⁺ currents recorded from GH₃MOR and GH₃MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K⁺ current was inwardly rectifying and was inhibited by Ba²⁺ but not TEA. DPDPE had no effect on delayed rectifier-, Ca²⁺-, and voltage-activated or A-type K⁺ currents, recorded from GH₃MORDOR cells. Ba²⁺ attenuated the inhibition of [Ca²⁺]_i and prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of K_IR channels in these actions of the opioid.