Online

Syed, Arsalan

Pharmacology

2015

PhD

Migraine is one of the most prevalent contributors to the global burden of mental and neurological disorders. It is a complex episodic condition that presents as intense recurrent unilateral headaches lasting hours to days that can be accompanied by nausea, photophobia, phonophobia and other neurological symptoms. The causes of migraine appear multifactorial and are not fully understood. However, activation of the trigeminovascular system and sphenopalatine parasympathetic neurons and the resulting vasodilation of meningeal arteries have been associated with the development of migraine pain. Recently, the neurotransmitter and neurotrophic peptide pituitary adenylate cyclase activating polypeptide (PACAP) has been implicated in this migraine headache pathway. The effects of PACAP parallel those of other migraine inducing agents and notably PACAP induces vasodilation of the MMA concurrent with the genesis of migraine headache when administered to human subjects. The mechanisms by which PACAP induces dilation are presently unclear. The objective of this present work was to elucidate the signaling pathways linking PACAP to MMA dilation. To achieve this objective, we developed an ex vivo approach to study isolated MMA at physiologically relevant intravascular pressure. Using this preparation we found that PACAP dilates MMA at picomolar concentrations via PAC1 receptors. Further, in MMA, PACAP-induced dilation is mediated exclusively though activation of KATP channels. While investigating the mechanisms of PACAP-induced dilation of MMA we discovered that basal KATP channel activity influences MMA diameter. Inhibition of KATP channels with glibenclamide or PNU37883 at physiological intravascular pressure resulted in a vasoconstriction of ≈ 20 %. Also consistent with basal KATP activity, glibenclamide induced a membrane potential depolarization of ≈ 14 mV. Further, in MMA loaded with the ratiometric Ca2+ indicator, Fura-2-AM, glibenclamide-induced MMA constriction was correlated with a simultaneous increase in the ratio of 340 nm/380 nm excited fura-2 fluorescence, consistent with an increase in intracellular Ca2+. Vascular smooth muscle KATP channels can be phosphorylated and activated by PKA, resulting in membrane potential hyperpolarization. KT5720, a PKA inhibitor, induced a constriction in MMA similar to that of glibenclamide (≈ 25 %). Additional treatment with glibenclamide did not induce further constriction suggesting that PKA activity may underlie tonic KATP channel activation. Together these results suggest that tonic PKA activity underlies basal KATP channel activity and together play a key role in regulation of MMA diameter. In summary, results presented in this dissertation suggest that picomolar PACAP-induced dilation of MMA is via activation of the PAC1-Hop1 receptor splice variant and KATP channel activation. Furthermore, KATP channels are also involved in tonic regulation of MMA diameter due to basal PKA activity. These unique features of the MMA provide additional insight into potential therapeutic targets in the development of treatments for migraine.