Agonist Activity of the delta-antagonists TIPP and TIPP-psi in cellular models expressing endogenous or transfected delta-opioid receptors.

A new class of highly selective delta-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-psi (H-Tyr-Tic[CH2NH]-Phe-Phe-OH). Surprisingly, both TIPP and TIPP-psi demonstrated inhibition of adenylyl cyclase activity in GH3 cells transfected with delta-opioid receptors (GH3DORT), an effect normally observed by agonists. The agonist activity was delta-selective, because no inhibition occurred in wild-type GH3 or GH3MOR (mu-opioid receptor) cells. Both TIPP and TIPP-psi exhibited concentration-dependent inhibition of adenylyl cyclase activity; however, TIPP-psi 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-psi, indicating the involvement of G(i)alpha/G(o)alpha G-proteins. Other delta-antagonists, naltriben, naloxone, and ICI 174864, attenuated the inhibition of adenylyl cyclase activity mediated by TIPP. Coadministration of TIPP with the selective delta-agonist [D-Pen2,5]enkephalin resulted in an additive interaction. Both TIPP and TIPP-psi exhibited significant inhibition of adenylyl cyclase activity in different GH3DORT clones expressing a 28-fold range of delta-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) delta-opioid receptors, with densities ranging from 0.12 to 6.67 pmol/mg. These results suggest that compounds previously thought to be purely delta-opioid receptor antagonists also demonstrate agonist activity in several in vitro models.