Rikke Holm Rasmussen:
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Migraine, a prevalent primary headache disorder impacting 15% of the global population, typically manifests as a moderate to severe unilateral headache lasting 4-72 hours. Multiple tissue structures are implicated in migraine, encompassing the trigeminovascular system, which comprises the trigeminal ganglia, its central projections, afferent neurons communicating with the meninges, and the meningeal arteries. Additionally, higher brain structures and arteries within the head are recognized as contributors to the overall complexity of migraine. Despite extensive research, the exact pathophysiology of migraine remains elusive and so does the impact of the environment. The aim of this PhD thesis was first to dissect the signaling pathways and examine site of action in mouse models of provoked migraine. Secondly, we examined how chemical pollutants found in the environment could interact with the migraine-associated signaling.
Several methods were used in this dissertation ranging from calcium imaging of human cell lines to in vivo behavioral studies in mice. In a calcium imaging–based screen, new transient receptor potential ankyrin 1 (TRPA1) agonists were identified from a library of known environmental pollutants using human embryonic kidney (HEK) cells expressing TRPA1 and transient receptor potential vanilloid 1 (TRPV1). Whole-cell patch clamp was used to validated TRPA1 and neuronal activation and ex vivo tissue preparations of the trigeminal ganglion (TG) and trigeminal nucleus caudalis (TNC) to examine calcitonin gene-related peptide (CGRP) release. Myography was employed to determine vasoactive properties and in vivo von Frey measurements of withdrawal thresholds to detect cutaneous sensitivity. Results from in silico modeling and other in vitro methods were used to support the main data. Genetically altered mice and application of chemical blockers were used to dissect signaling mechanisms.
In the first paper, we provided a detailed protocol and a step-by-step video guide for the “ex vivo CGRP release from the trigeminovascular system” method. In the second paper, we found that nitroglycerin (GTN)- induced sensitivity was dependent on TRPA1. Levcromakalim and cilostazol-induced hypersensitivity were dependent on CGRP, without causing direct CGRP release from the TG or TNC, suggesting a complex mechanistic interplay. The third study showed that deletion of the ATP-sensitive potassium channel (KATP) subunit Kir6.1 in smooth muscle cells prevented levcromakalim and GTN-induced hypersensitivity. Deletion of Kir6.1 also impaired the vascular response to levcromakalim, and collectively this emphasized the importance of the vasculature in these models. Finally in the last paper, we showed a mechanistic relation between environmental pollutants and migraine-associated signaling. 16 of 52 screened chemicals could activate TRPA1 and the pesticide pentachlorophenol was linked to migraine-associated signaling as is caused release of CGRP, dilated blood vessels, and induced cutaneous hypersensitivity in mice.
The studies included in this dissertation range over several aspects of migraine research. We provided new knowledge on the signaling pathways and involvement of the vasculature in mouse models of provoked migraine mimicking the humane provocation models. Furthermore, we contributed to a less studied area of migraine research investigating how environmental pollutants can interact with migraine-associated signaling.