Vol. 63, Issue 1, 89-95, January 2003

Coexpression of -Opioid Receptors with µ Receptors in GH₃ Cells Changes the Functional Response to µ Agonists from Inhibitory to Excitatory

Andrew C. Charles, Natalya Mostovskaya, Kathleen Asas, Christopher J. Evans, Megan L. Dankovich, and Tim G. Hales

Departments of Neurology (A.C., N.M., K.A.) and Psychiatry (C.E.), UCLA School of Medicine, Los Angeles, California; and Department of Pharmacology, the George Washington University, Washington, DC (M.L.D., T.G.H.)

GH₃ cells show spontaneous activity characterized by bursts of action potentials and oscillations in [Ca²⁺]_i. This activity is modulated by the activation of exogenously expressed opioid receptors. In GH₃ cells expressing only µ receptors (GH₃MOR cells), the µ receptor-specific ligand [D-Ala²,N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO) inhibited spontaneous Ca²⁺ signaling by the inhibition of voltage-gated Ca²⁺ channels, activation of inward-rectifying K⁺ channels, and inhibition of adenylyl cyclase. In contrast, in cells expressing both µ and  receptors (GH₃MORDOR cells), DAMGO had an excitatory effect on Ca²⁺ signaling that was mediated by phospholipase C and release of Ca²⁺ from intracellular stores. The excitatory effect of DAMGO was also inhibited by pretreatment with pertussis toxin. Despite the excitatory effect on Ca²⁺ signaling, DAMGO inhibited Ca²⁺ channels and activated inward-rectifying K⁺ channels in GH₃MORDOR cells, although to a lesser extent than in GH₃MOR cells. Long-term treatment with the  receptor-specific ligand [D-Pen²,D-Pen⁵]-enkephalin reduced the excitatory effect of DAMGO in the majority of GH₃MORDOR cells and restored the inhibitory response to DAMGO in some cells. The inhibitory effect of somatostatin on Ca²⁺ signaling was not different in GH₃MORDOR versus GH₃MOR cells. These results indicate that interaction between µ- and -opioid receptors causes a change in the functional response to µ ligands, possibly by the formation of a µ/ heterodimer with distinct functional properties.