Title

 

Authors

S. Molinari

Doctorate School in Pharmacology and Molecular Oncology
Dept. of Pharmacology and National Institute of Neuroscience - University of Ferrara, Italy

 

Abstract

Nociceptin/orphanin FQ (N/OFQ) evokes antinociceptive effects at spinal level via selective activation of the N/OFQ peptide (NOP) receptor both in rodents and in non human primates. Recent studies in monkeys demonstrated that N/OFQ and morphine elicit synergistic antinociceptive actions when given intrathecally suggesting mixed NOP and mu opioid (MOP) receptor agonists as innovative spinal analgesics. In order to identify such molecules SAR studies were conduced on N/OFQ and N/OFQ(1-13)NH₂by substituting Phe¹ with Tyr and Dmt (to increase activity at MOP). Standard and substituted peptides were evaluated in calcium mobilization experiments performed with cells expressing the human NOP or MOP receptors and the chimeric protein Galphaqi5. From these studies [Dmt¹]N/OFQ(1-13)NH₂ was selected as the most potent (pEC₅₀ on NOP 8.94) and least selective (approx 30 fold over MOP) compound. The mixed NOP/MOP agonist activity of [Dmt¹]N/OFQ(1-13)NH₂ was confirmed in vitro in the following series of experiments i) receptor binding and ii) [³⁵S]GTPgS binding at human recombinant receptors, iii) [³⁵S]GTPgS binding at native rat spinal cord receptors, and iv) at native guinea pig receptors inhibiting neurogenic contractions in the ileum. In vivo in the mouse tail withdrawal assay morphine and N/OFQ injected intrathecally evoked similar antinociceptive effects. [Dmt¹]N/OFQ(1-13)NH₂ was also able to elicit a robust antinociceptive effect being more potent than N/OFQ (by 30 fold) and morphine (by 3 fold). Collectively these results demonstrate that [Dmt¹]N/OFQ(1-13)NH₂ behaves as a mixed NOP/MOP agonist and substantiate the suggestion that such mixed compounds are worthy of further development as innovative spinal analgesics.