Abstract
Migraine involves brain hypersensitivity with episodic dysfunction triggered by behavioral or physiological stressors. During an acute migraine attack the trigeminal nerve is activated (peripheral sensitization). This leads to central sensitization with activation of the central pathways including the trigeminal nucleus caudalis, the trigemino-thalamic tract, and the thalamus. In episodic migraine the sensitization process ends with the individual act, but with chronic migraine central sensitization may continue interictally. Increased allostatic load, the consequence of chronic, repeated exposure to stressors, leads to central sensitization, lowering the threshold for future neuronal activation (hypervigilance). Ostensibly innocuous stressors are then sufficient to trigger an attack. Medications that reduce sensitization may help patients who are hypervigilant and help to balance allostatic load. Acute treatments and drugs for migraine prevention have traditionally been used to reduce attack duration and frequency. However, since many patients do not fully respond, an unmet treatment need remains. Calcitonin gene-related peptide (CGRP) is a vasoactive neuropeptide involved in nociception and in the sensitization of peripheral and central neurons of the trigeminovascular system, which is implicated in migraine pathophysiology. Elevated CGRP levels are associated with dysregulated signaling in the trigeminovascular system, leading to maladaptive responses to behavioral or physiological stressors. CGRP may, therefore, play a key role in the underlying pathophysiology of migraine. Increased understanding of the role of CGRP in migraine led to the development of small-molecule antagonists (gepants) and monoclonal antibodies (mAbs) that target either CGRP or the receptor (CGRP-R) to restore homeostasis, reducing the frequency, duration, and severity of attacks. In clinical trials, US Food and Drug Administration-approved anti-CGRP-R/CGRP mAbs were well tolerated and effective as preventive migraine treatments. Here, we explore the role of CGRP in migraine pathophysiology and the use of gepants or mAbs to suppress CGRP-R signaling via inhibition of the CGRP ligand or receptor.
PLAIN LANGUAGE SUMMARY
Migraine is a neurological disease affecting one in eight people. Symptoms include nausea and/or sensitivity to light and sound, and a throbbing headache. Although certain genes may increase the likelihood of migraine, environmental stimuli and molecules that increase the sensitivity of brain blood vessels and their innervations also play a role. During a migraine attack, nerves in the brain are activated, leading to increased sensitivity to stimuli, lowering the future threshold for activation, and making patients hypervigilant. Chronic, repeated exposure to certain stimuli can also lower this activation threshold, such that relatively innocuous stimuli can trigger an attack. Excessive use of certain migraine treatments can increase headache frequency over time and produce unwanted side effects; thus, selective agents are needed that specifically target the systems and pathways affected in migraine pathophysiology. Calcitonin gene-related peptide (CGRP), produced and released by nerve cells, is important in migraine pathophysiology. CGRP binds smooth muscle cell receptors, dilating blood vessels supplying the brain, and also binds to peripheral nerve cells that transmit pain signals to the spinal cord and brain. CGRP levels are elevated during a migraine attack. Medications targeting the CGRP pathway may decrease sensitivity and potentially normalize responses in hypervigilant patients. Two commercially available oral drugs that block CGRP receptors ('gepants') have reduced symptoms during migraine attacks in clinical trials. Four monoclonal antibodies (proteins that bind a specific molecule) have also been developed that target CGRP or the receptor and have been shown to significantly reduce the number of migraine days per month.