Vol. 298, Issue 1, 240-248, July 2001

Agonist Activity of the -Antagonists TIPP and TIPP- in Cellular Models Expressing Endogenous or Transfected -Opioid Receptors

Nancy A. Martin, Maguerite T. Terruso and Paul L. Prather

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

A new class of highly selective -opioid receptor antagonists has been recently developed, termed the TIP(P) peptides. Two prototypical compounds in this class are TIPP (H-Tyr-Tic-Phe-Phe-OH) and a derivative, TIPP- (H-Tyr-Tic[CH₂NH]-Phe-Phe-OH). Surprisingly, both TIPP and TIPP- demonstrated inhibition of adenylyl cyclase activity in GH₃ cells transfected with -opioid receptors (GH₃DORT), an effect normally observed by agonists. The agonist activity was -selective, because no inhibition occurred in wild-type GH₃ or GH₃MOR (µ-opioid receptor) cells. Both TIPP and TIPP- exhibited concentration-dependent inhibition of adenylyl cyclase activity; however, TIPP- was found to be less potent (IC₅₀ = 3.97 versus 0.162 nM) and less efficacious (I_max = 50% versus 70%) than TIPP. Pretreatment of cells with pertussis toxin attenuated the inhibition of maximally effective concentrations of TIPP and TIPP-, indicating the involvement of G_i/G_o G-proteins. Other -antagonists, naltriben, naloxone, and ICI 174864, attenuated the inhibition of adenylyl cyclase activity mediated by TIPP. Coadministration of TIPP with the selective -agonist [D-Pen^2,5]enkephalin resulted in an additive interaction. Both TIPP and TIPP- exhibited significant inhibition of adenylyl cyclase activity in different GH₃DORT clones expressing a 28-fold range of -opioid receptor densities, and in cell lines expressing endogenous (i.e., N1E115 and NG108-15) and transfected (i.e., Chinese hamster ovary-DOR and human embryonic kidney-DOR) -opioid receptors, with densities ranging from 0.12 to 6.67 pmol/mg. These results suggest that compounds previously thought to be purely -opioid receptor antagonists also demonstrate agonist activity in several in vitro models.