While electrical disturbances like Cortical Spreading Depression (CSD) can initiate a migraine, the persistence of severe, throbbing headache pain requires sustained chemical activation of primary sensory neurons. This process is driven by neurogenic inflammation—a localized, non-infectious inflammatory cascade triggered by the release of vasoactive neuropeptides from perivascular trigeminal nerve endings. Understanding this chemical mediator pathway has yielded key therapeutic breakthroughs, including triptans and modern anti-CGRP therapies.
Antidromic Signal Propagation and Neuropeptide Release
The nerve endings of the ophthalmic branch ($V_1$) surrounding meningeal blood vessels feature specialized varicosities storing inflammatory neuropeptides.
When CSD or endogenous stressors stimulate these perivascular terminals, action potentials travel in two directions simultaneously:
Orthodromic Conduction: Signals propagate centrally through the trigeminal ganglion toward the trigeminocervical complex (TCC), alerting the brain to pain.
Antidromic Conduction: Signals travel backward down neighboring collateral branches, triggering the local release of stored neuropeptides directly into the walls of dural blood vessels.
Key Neuropeptide Mediators and Their Actions
Research confirms distinct roles for several key neuropeptides during an acute migraine attack:
| Neuropeptide | Physiological Mechanisms & Effects | Clinical Significance |
| Calcitonin Gene-Related Peptide (CGRP) | Promotes potent dilation of middle meningeal and cerebral arteries. Inhibits vascular smooth muscle contraction and drives sterile inflammation. | Levels in jugular venous blood rise sharply during migraine attacks. Target of triptans (which suppress CGRP release) and CGRP-specific drugs. |
| Substance P (SP) | Increases microvascular permeability, causing plasma protein extravasation (leakage of fluid into the meningeal interstitium). | Works synergistically with CGRP to promote local tissue edema. |
| Neurokinin A (NKA) | Co-released with Substance P, contributing to smooth muscle response and vascular permeability changes. | Contributes to inflammatory signaling alongside SP. |
| Neuropeptide Y (NPY) & Others | Marker of sympathetic vascular control. Circulating levels remain unchanged during acute migraine attacks. | Demonstrates that CGRP and SP are selectively elevated during acute attacks. |
Cascading Events in Neurogenic Inflammation
The release of CGRP and Substance P initiates a self-sustaining inflammatory cycle within the meninges:
Vasodilation & Plasma Extravasation: CGRP drives pronounced arterial vasodilation while Substance P increases vessel permeability, allowing plasma proteins to leak into surrounding meningeal tissue.
Mast Cell Degranulation: Released neuropeptides stimulate dural mast cells to degranulate, releasing secondary inflammatory mediators including histamine, bradykinin, prostaglandins (PGs), and pro-inflammatory cytokines.
Inflammatory Soup Formation: This combination of CGRP, SP, histamine, bradykinin, and prostaglandins forms an "inflammatory soup" that bathes the nearby trigeminal nerve endings.
Peripheral Sensitization: Lowering the Pain Threshold
Prolonged exposure to this inflammatory soup results in peripheral sensitization of first-order trigeminal neurons:
Mechanisms: Inflammatory mediators bind to receptors on nociceptive terminals, activating intracellular kinase cascades that phosphorylate membrane ion channels (such as $TRPV1$ and voltage-gated sodium channels).
Functional Consequence: The activation threshold of primary nociceptors drops, while their baseline firing rate increases.
Clinical Manifestation: Light vascular pulsations that normally pass unnoticed begin to trigger bursts of pain signals. This peripheral sensitization causes the characteristic throbbing or pulsating quality of migraine pain and its aggravation by minor physical movements.
Conclusion
Neurogenic inflammation and peripheral sensitization bridge initial neuronal activation with sustained, throbbing head pain. The elevated release of CGRP and Substance P highlights why inhibiting perivascular neuropeptide secretion remains a primary goal of acute and preventive migraine therapeutics.