Migraine is a complex neurological disorder that affects approximately 38 million Americans. For over 25 years, the neuropeptide calcitonin gene-related peptide (CGRP) has been implicated in the pathogenesis of migraine. In fact, several pharmaceutical companies are tailoring treatments to antagonize CGRP actions. However, due to the complexity of migraine, exactly how and where CGRP acts to contribute to migraine have remained controversial: whereas several studies suggest that CGRP acts in the central nervous system (CNS) in this context, others have indicated a role in the periphery. Central nervous system sites of action include the trigeminal nucleus and several higher brain regions, and peripheral sites include the vasculature and dural mast cells in the meninges. Among the sites of CGRP action, the trigeminal nerve, which is the major somatosensory structure of the face, is of particular interest because it bridges the CNS and the periphery.
Migraine is generally thought to involve abnormal signaling in the trigeminovascular system, and about 50% of trigeminal neurons have CGRP immunoreactivity. Although the notion that CGRP has a central site of action in relation to migraine had gained ground over the past decade, the recent discovery that monoclonal antibodies against CGRP can prevent migraine attacks has resurrected the possibility that a peripheral site of action is involved as well. Clarification of the sites of CGRP action in migraine will be crucial to developing an understanding of mechanisms that underlie migraine so that future treatments can be rationally designed.
One diagnostic criterion for migraine is photophobia, a painful and often debilitating response to non-noxious levels of light. Our laboratory previously developed a preclinical model of migraine in which the light-aversive behavior of mice is used as a surrogate of photophobia. Specifically, mice were sensitized to CGRP by introducing a nestin/hRAMP1 transgene. In these mice versus control littermates, light aversion in response to central (intracerebroventricular, ICV) injection of CGRP was enhanced in dim light. In wild-type mice, CGRP (ICV) also elicited aversion to very bright light; this did not occur in vehicle-treated mice. Additionally, I have shown that CGRP injected peripherally (intraperitoneal, IP) can induce significant light aversion in wild-type mice. I have begun to identify the sites of action outside of the central nervous system, using four lines of transgenic mice with different patterns of overexpression of CGRP receptors: global hRAMP1 mice (expression in all tissues), nestin/hRAMP1 mice (expression only in nervous tissue), tagln/hRAMP1 (expression only in smooth muscle cells), and tek2/hRAMP1 (expression in endothelial cells). As predicted, in the global hRAMP mice light aversion, in response, to IP-injected CGRP was enhanced. However, in nestin/hRAMP1 mice, only ICV-injected, and not IP-injected, CGRP induced enhanced light aversion. This finding suggests that peripheral CGRP activates neural pathways involved in light aversion, but by an indirect mechanism.
To determine where in the periphery CGRP is acting, a pharmacological and genetic approach was taken. Since CGRP is one of the most potent vasodilators in the body, it is well positioned to have vascular effects that induce light aversive behavior. This hypothesis was based on findings that 1) intravenous administration of CGRP in human subjects can cause migraine pain, and 2) perivascular CGRP can sensitize the trigeminal nerve, which could alter synaptic transmission to the central nervous system and 3) CGRP monoclonal antibodies are effective in clinical trial and likely do not cross the blood brain barrier. Thus, there is a mechanism by which CGRP in the periphery can sensitize the trigeminal nerve and alter sensory perception, leading to photophobia.
The role of the vasculature in migraine, specifically vasodilation, has been controversial and now the consensus is that it is neither necessary nor sufficient. First, I wanted to test the role of vasodilation in this model. I pharmacologically inhibited CGRP-induced vasodilation using two vasoconstrictors, phenylephrine and endothelin-1. Blocking CGRP-induced vasodilation partially attenuates the light aversive response. Moreover, mice that overexpress the CGRP receptor in smooth muscle, but not endothelial, cells exhibit enhanced light aversion indicating a role for vascular actions of CGRP in this preclinical model of migraine. These results present clear evidence that CGRP has actions on the vasculature to induce light aversion. Additionally, the inability of blocking vasodilation to completely rescue the light aversion suggests that the vasculature may not be the only peripheral target of CGRP in migraine pathophysiology.
This work improves the understanding of peripheral CGRP actions in migraine and raises awareness that contribution of the vasculature in migraine should not be ignored. The identification of sites of CGRP action in regions inside and outside of the CNS could lead to improved and more successful therapeutics for migraine.