This study was examined whether or not the orphanin FQ (OFQ)-stimulated [³⁵S]GTPγS activity interact with DAMGO in the whole brain of mice.

ICR mice (male, n = 20, 20-25 g) were euthanized for the membrane preparations. In the agonist-stimulated [³⁵S]GTPγS binding dose-response curves by OFQ, Ro-64-6198 and DAMGO, the EC₅₀ (effective concentration 50, nM) and maximum stimulation (% over basal) were determined in the presence or absence of J-113397 (10 nM), a NOP (nociceptin-opioid peptide) receptor antagonist. OFQ (1 µM), Ro-64-6198 (10 µM), DAMGO (10 µM) and their combination cocktail were used to determine the interaction between the NOP and MOP (µ-opioid peptide) receptor.

The values of EC₅₀ and maximum stimulation of [³⁵S]GTPγS binding were as follows: OFQ (9.2 ± 0.2 nM/17.9 ± 0.1%), Ro-64-6198 (143.5 ± 0.5 nM/18.1 ± 0.4%), and DAMGO (680.6 ± 0.7 nM/18.1 ± 0.5%). J-113397 produced a 8.7 and 7.1 fold rightward shifting in the OFQ and Ro-64-6198-stimulated [³⁵S]GTPγS binding dose-response curve respectively, but not in the DAMGO. OFQ combined with DAMGO-stimulated [³⁵S]GTPγS binding had an additive effect, but not in the OFQ combined with Ro-64-6198.

OFQ, Ro-64-6198 and DAMGO-stimulated [³⁵S]GTPγS binding in the brain of mice has receptor selectivity. The [³⁵S]GTPγS stimulation of OFQ and DAMGO had an additive effect rather than an anti-opioid effect on the level of intracellular signal transduction through agonist-stimulated [³⁵S]GTPγS bindings.