Migraine, perhaps the most common neurological condition, is a debilitating disorder characterized by episodic pain and other symptoms (i.e., nausea, vomiting, photophobia or phonophobia with or without aura). While the pathophysiology of this disorder remains unknown, an important theory suggests that migraine may occur as a consequence of cortical spreading depression (CSD). Migraine, and perhaps CSD events, may be precipitated by a variety of "triggers" including stress and provocative chemical agents such as donors of nitric oxide (NO). Migraine is often treated with triptan drugs which are effective in some patients but which are also associated with an increase in the incidence of headache frequency, i.e., medication overuse headache. We reasoned that transformation of episodic headaches into MOH, and perhaps CM, by triptans may result, in part, from medication-induced neural adaptations that enhance responses to stimuli that can "trigger" migraine attacks (i.e., "migraine triggers"). For this reason, we hypothesize that a period of triptan exposure results in persistent hypersensitivity to migraine triggers that are associated with increased cortical spreading depression (CSD) events and activation of primary afferent fibers that innervate the dura.
Three specific aims test this hypothesis. First, are enhanced behavioral and neurochemical responses observed in rats pre-exposed to a period of triptan treatment accompanied by spontaneous (i.e., non-evoked) or increased CSD events following an evoking stimulus? (Aim 1);second, do the persistent pronociceptive changes elicited in dural primary afferent neurons by triptans result in subsequent activation and potentially increased excitability of dural afferent neurons in response to stimuli known to promote migraine attack in humans or after a CSD event? (Aim 2);and third, what are the roles of nNOS and CGRP in promoting enhanced sensitivity to migraine triggers, and/or CSD events and excitation of dural afferents? (Aim 3). A significant problem in preclinical efforts to study migraine mechanisms and pain is that animal models of headache generally require induction of tissue injury whereas migraineurs suffer from pain in the absence of injury (i.e., "dysfunctional pain"). Our approach mimics the human state by inducing neural adaptations using medications without causing tissue injury. Our goal is to use the triptan-induced sensitized state to investigate possible mechanisms underlying MOH and by which episodic pain may transform into CM. Critically, the events that may promote enhanced pain signaling following challenge of rats with latent sensitization with migraine triggers are also likely to yield insights into mechanisms relevant to pathophysiological states of underlying migraine. The data from the proposed experiments will result in mechanistic insights that can be tested clinically.
Migraine is one of the most common neurological disorders. This condition is often treated with triptan drugs. However, overuse of these mechanisms may elicit transformation of episodic migraine to chronic migraine. The mechanisms underlying "chronification of pain" are not known but may be related to changes in threshold to respond to triggers of migraine. Understanding such mechanisms may give insight into underlying pathophysiology of medication overuse headache and chronic migraine and of migraine pathophysiology.
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