Chronic Agonist Treatment Converts Antagonists into Inverse Agonists at δ-Opioid Receptors

In cellular models, chronic exposure to μ-opioid agonists converts antagonists into inverse agonists at μ-receptors. Such adaptations could contribute to the development of tolerance and/or dependence. To determine whether δ-receptors respond similarly, or whether this adaptation is unique for μ-receptors, this study examined the effects of prolonged agonist exposure on the intrinsic activity of several δ-opioid ligands in GH₃ cells expressing δ-receptors. In opioid naive cells, δ-receptors were constitutively active, and a series of δ-ligands displayed a range of intrinsic activities for G protein activation. Chronic treatment with the full δ-agonist [d-Pen^2,5]-enkephalin reduced the acute ability of [d-Pen^2,5]-enkephalin to stimulate and the full inverse agonistN,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174864) to inhibit G protein activation. In contrast, although naloxone and naltriben exhibited weak partial agonism in opioid naive cells, both ligands acted as full inverse agonists to produce concentration-dependent inhibition of guanosine 5′-O-(3-[³⁵S]thio)triphosphate binding after prolonged exposure to [d-Pen^2,5]-enkephalin or to the partial agonist morphine. This effect was reversed by a neutral δ-antagonist (N,N-bisallyl)-Tyr-Gly-Gly-ψ-(CH₂S)-Phe-Leu-OH (ICI-154129). Finally, as is also characteristic of inverse agonists, naloxone and naltriben demonstrated higher affinities for uncoupled δ-receptors in cells chronically treated with [d-Pen^2,5]-enkephalin, relative to opioid naive cells. Therefore, this relatively novel adaptation is shared by both μ- and δ-opioid receptors and therefore may serve as an important common mechanism involved the development of tolerance and/or dependence.