Spinorphin Ameliorates Nitroglycerin‐Induced Migraine‐Like Responses and Trigeminovascular Neuroinflammation Potentially Through Opioid Receptor‐Mediated Mechanisms

ABSTRACT

Enhancement of endogenous opioid signaling through inhibition of enkephalin degradation has emerged as a potential strategy for pain control, yet its role in migraine remains unclear. This study investigated whether spinorphin, an endogenous inhibitor of multiple enkephalinases, attenuates trigeminovascular activation and migraine‐like responses in experimental migraine. Migraine‐like pain was induced in male Wistar rats by nitroglycerin. The effects of spinorphin were evaluated using periorbital and plantar von Frey tests, followed by assessment of c‐Fos levels in the trigeminal nucleus caudalis (TNC), calcitonin gene‐related peptide (CGRP) levels in the trigeminal ganglion (TG) and TNC, and meningeal mast cell degranulation. Opioid receptor involvement was examined using nalmefene. In complementary ex vivo hemiskull preparations, capsaicin‐evoked CGRP release was quantified, and the interaction between spinorphin and sumatriptan was investigated. Spinorphin dose‐dependently alleviated cephalic and extracephalic mechanical hypersensitivity, reduced c‐Fos levels, decreased CGRP concentrations in the TG and TNC, and inhibited meningeal mast cell degranulation. These protective effects were markedly attenuated by nalmefene, supporting the involvement of opioid receptor‐mediated mechanisms. In ex vivo preparations, spinorphin selectively reduced capsaicin‐induced CGRP levels in the superfusates without affecting basal CGRP levels, and this effect was abolished by nalmefene. Moreover, spinorphin enhanced the inhibitory effect of sumatriptan on capsaicin‐stimulated CGRP levels. Spinorphin attenuates behavioral, neurochemical, and neuroinflammatory features of experimental migraine, at least partly through mechanisms involving endogenous opioid signaling. These findings suggest enkephalinase inhibition as a promising therapeutic approach for migraine and support further investigation of spinorphin as a potential adjunct to established antimigraine therapies.