Partial agonist behaviour depends upon the level of nociceptin/orphanin FQ receptor expression: studies using the ecdysone‐inducible mammalian expression system

Partial agonism is primarily dependent upon receptor density and coupling efficiency. As these parameters are tissue/model dependent, intrinsic activity in different tissues can vary. We have utilised the ecdysone‐inducible expression system containing the human nociceptin/orphanin FQ (N/OFQ) peptide receptor (hNOP) expressed in Chinese hamster ovary cells (CHO INDhNOP ) to examine the activity of a range of partial agonists in receptor binding, GTP γ 35 S binding and inhibition of adenylyl cyclase studies. Incubation of CHO INDhNOP cells with ponasterone A (PON) induced hNOP expression ([leucyl‐ 3 H]N/OFQ binding) of 24, 68, 191 and 1101 fmol mg −1 protein at 1, 2, 5 and 10 μ M PON, respectively. At 191 fmol mg −1 , protein hNOP pharmacology was identical to that reported for other traditional expression systems. pEC 50 values for GTP γ 35 S binding ranged from 7.23 to 7.72 (2–10 μ M PON) for the partial agonist [Phe 1 ψ (CH 2 –NH)Gly 2 ]N/OFQ(1–13)–NH 2 ([F/G]N/OFQ(1–13)–NH 2 ) and 8.12–8.60 (1–10 μ M PON) for N/OFQ(1–13)–NH 2 and E max values (stimulation factor relative to basal) ranged from 1.51 to 3.21 (2–10 μ M PON) for [F/G]N/OFQ(1–13)–NH 2 and 1.28–6.95 (1–10 μ M ) for N/OFQ(1–13)–NH 2 . Intrinsic activity of [F/G]N/OFQ(1–13)–NH 2 relative to N/OFQ(1–13)–NH 2 was 0.3–0.5. [F/G]N/OFQ(1–13)–NH 2 did not stimulate GTP γ 35 S binding at 1 μ M PON, but competitively antagonised the effects of N/OFQ(1–13)–NH 2 with a p K B =7.62. pEC 50 values for cAMP inhibition ranged from 8.26 to 8.32 (2–10 μ M PON) for [F/G]N/OFQ(1–13)–NH 2 and 9.42–10.35 for N/OFQ(1–13)–NH 2 and E max values (% inhibition) ranged from 19.6 to 83.2 for [F/G]N/OFQ(1–13)–NH 2 and 40.9–86.0 for N/OFQ(1–13)–NH 2 . The intrinsic activity of [F/G]N/OFQ(1–13)–NH 2 relative to N/OFQ(1–13)–NH 2 was 0.48–0.97. In the same cellular environment with receptor density as the only variable, we show that the profile of [F/G]N/OFQ(1–13)–NH 2 can be manipulated to encompass full and partial agonism along with antagonism.British Journal of Pharmacology (2003) 140 , 61–70. doi: 10.1038/sj.bjp.0705401