Migraine pathophysiology: neural basis of photophobia

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DESCRIPTION (provided by applicant): About 85% of people experiencing a migraine attack seek sanctuary in a dark environment in order to lessen headache intensification brought on by ambient light. The neural mechanism of this photophobic reaction remains a puzzle. Current views on the neural basis of migraine headache implicate trigeminal pain fibers in cranial dura mater, and central nociceptive neurons in the medullary dorsal horn, thalamus, and cortex. Our studies on dura-sensitive thalamic neurons as mediators of extracephalic allodynia led us, quite fortuitously, to evaluate if such neurons may play a role in migraine photophobia. In a clinical study leading up to this grant proposal, we learned that migraine photophobia occurs in blind persons (cone/rod degeneration) that perceive light (intact melanopsin photoreceptors), but not in migraineurs who are totally blind. We postulate that activity along migraine pain pathways may be modulated by converging signals transmitted from the retina to the brain through the optic nerve. In the rat thalamus, we identified dura-sensitive neurons whose ongoing activity was strongly modulated by light. Neural tract-tracing indicated that the cell bodies and dendrites of these thalamic neurons were apposed by many afferents of retinal origin, and that their own axons branched extensively into the primary somatosensory cortex. Here we will focus on this unique integration of meningeal nociception and retinal photoreception by thalamocortical neurons as a candidate mechanism for migraine photophobia. Study 1 will test the hypothesis that activity of dura/light-sensitive thalamic neurons is differentially modulated by classical photoreceptors (rods, cones) and melanopsinergic photoreceptors as it relates to qualitative and quantitative characteristics of migraine photophobia. Study 2 will test the hypothesis that photomodulation of dura-sensitive thalamic neurons is mediated by the optic nerve rather than by trigeminal innervation of the eye. Study 3 will test the hypothesis that cell bodies and dendrites of dura/light-sensitive neurons in the thalamus are apposed by axons of melanopsinergic retinal ganglion cells, providing a candidate neural substrate for incorporating retinal photoreception into a pathway of meningeal nociception in blind migraineurs with cone/rod degeneration. Study 4 will test the hypothesis that cell bodies and dendrites of dura/light-sensitive neurons in the thalamus are richly apposed by axons containing specific neuromodulating molecules as potential targets for pharmacological interception of migraine photophobia. Study 5 will test the hypothesis that thalamic neurons that integrate sensory information from the dura and retina project to cortical areas involved in the pain perception (e.g., somatosensory and insular cortices) or/and photoperception (visual cortices). Our working hypothesis that migraine headache can be exacerbated by non-image-forming retinal input converging upon dura-sensitive thalamocortical neurons represents a new concept in the field of migraine pathophysiology. As such, we submit this project as a new application that could potentially open a unique window into the biology of an adverse phenomenon described by millions of headache sufferers.