Vol. 301, Issue 2, 661-671, May 2002

Agonist, Antagonist, and Inverse Agonist Characteristics of TIPP (H-Tyr-Tic-Phe-Phe-OH), a Selective -Opioid Receptor Ligand

Nancy A. Martin, Michael B. Ruckle, Stephanie L. VanHoof and Paul L. Prather

Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock, Arkansas

Recent evidence indicates that the well established -opioid antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH) also displays agonist activity in several cellular models. Therefore, it is possible that TIPP, and structurally related compounds, might represent a novel class of opioid agonists exhibiting unique characteristics. The purpose of this study was to examine the properties of TIPP at selected points of the signal transduction pathway (i.e., receptor binding, G-protein activation, and effector regulation) in GH₃DORT cells (GH₃ cells expressing -opioid receptors) and compare them with that of an established -opioid agonist, [D-Pen²,D-Pen⁵]-enkephalin (DPDPE). DPDPE exhibited properties of an agonist in all assays. In contrast, TIPP demonstrated characteristics of an agonist, antagonist, or inverse agonist, depending on the step in the signal transduction cascade examined and the assay conditions employed. In receptor binding assays, the addition of guanine nucleotides and sodium ions increased the affinity of TIPP for -opioid receptors in both membrane preparations and digitonin-permeabilized cells, which is characteristic of an inverse agonist. In assays measuring G-protein activation, TIPP failed to stimulate guanosine 5'-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPS) binding in membrane preparations, which is consistent with an antagonist profile. However, when using cells semi-permeabilized with digitonin, TIPP exhibited properties of an agonist, producing concentration-dependent, antagonist-reversible stimulation of [³⁵S]GTPS binding. Finally, in assays examining regulation of the intracellular effector adenylyl cyclase, TIPP exhibited characteristics of an agonist, producing inhibition of enzyme activity in both membrane preparations and whole cells. Therefore, although DPDPE and TIPP act similarly as agonists to regulate the intracellular effector adenylyl cyclase, they demonstrate significant differences in the signal transduction cascade preceding this final point of convergence.