Activation of peripheral NOP receptors reduces periorbital mechanical allodynia evoked by CGRP in mice.

Background and purpose: Migraine is a neurovascular disorder largely mediated by calcitonin gene-related peptide (CGRP). This study explores the role of the nociceptin/orphanin FQ (N/OFQ)-N/OFQ receptor (NOP) system in CGRP-induced periorbital mechanical allodynia (PMA) in mice.

Experimental approach: Male or female wild type (NOP(+/+)) and NOP receptor knockout (NOP(-/-)) mice and CD-1 mice were used. The brain penetrant, AT-403, and the peripherally restricted, UFP-112, NOP agonists were tested for PMA prevention. To identify a potential site of action at the cellular level, the ability of N/OFQ to signal at membrane and endosomal level in NOP-expressing HEK293 cells, and to inhibit the increase in cyclic adenosine monophosphate (cAMP) induced by CGRP in human Schwann cells (hSCs) was investigated.

Key results: CGRP-induced PMA was comparable in NOP(+/+) and NOP(-/-) mice. AT-403 and UFP-112 equally reduced CGRP-evoked PMA in CD-1 mice. In NOP-expressing cells, activation of NOP resulted in the internalisation and movement of NOP away from the plasma membrane marker CAAX and to early endosomes marker Rab5a. N/OFQ stimulated G_αi recruitment to NOP at the plasma membrane and from the endosomal compartment. N/OFQ attenuated cAMP increase elicited by CGRP in hSCs.

Conclusions and implications: The peripherally restricted NOP agonist showed efficacy similar to the brain-penetrant compound, indicating that peripheral NOP activation is sufficient to alleviate CGRP-evoked PMA. Despite NOP ability to halt Gα_i recruitment and cAMP increase in cells, further studies are required to confirm that SCs are the cellular site where N/OFQ operates to attenuate the CGRP pro-migraine action.