A neural mechanism for exacerbation of headache by light. Nat Neurosci. 2010;13:239-245. [PMID: 20062053 PMCID: PMC2818758 DOI: 10.1038/nn.2475]

Migraine Mistakes: Error Awareness

Error awareness or detection is the conscious and subconscious processing to evaluate physiological signals that are different from a baseline or homeostatic level. Migraine is a unique neurological disorder in which there are repeated attacks interspersed by attack-free periods. These attacks are dynamic and multidimensional in the sense that sensory, affective, autonomic, and cognitive functions are altered and these changes evolve differently before (pre-ictal), during (ictal), and immediately after (post-ictal) an attack. Thus migraine serves as a model disease to understand how the brain monitors and react to the presence of errors.

Hypothalamic and basal ganglia projections to the posterior thalamus: possible role in modulation of migraine headache and photophobia

Migraine attacks are typically described as unilateral, throbbing pain that is usually accompanied by nausea, vomiting, and exaggerated sensitivities to light, noise and smell. The headache phase of a migraine attack is mediated by activation of the trigeminovascular pathway; a nociceptive pathway that originates in the meninges and carries pain signals through meningeal nociceptors to the spinal trigeminal nucleus and from there to the cortex through relay neurons in the thalamus. Recent studies in our lab have identified a population of trigeminovascular neurons in the posterior (Po) and lateral posterior (LP) thalamic nuclei that may be involved in the perception of whole-body allodynia (abnormal skin sensitivity) and photophobia (abnormal sensitivity to light) during migraine. The purpose of the current study was to identify sub-cortical areas that are in position to directly regulate the activity of these thalamic trigeminovascular neurons. Such process begins with anatomical mapping of neuronal projections to the posterior thalamus of the rat by performing discrete injections of the retrograde tracer Fluorogold into the Po/LP region. Such injections yielded retrogradely labeled neurons in the nucleus of the diagonal band of Broca, the dopaminergic cells group A11/A13, the ventromedial and ventral tuberomammillary nuclei of the hypothalamus. We also found that some of these neurons contain acetylcholine, dopamine, cholecystokinin and histamine, respectively. Accordingly, we speculate that these forebrain/hypothalamic projections to Po and LP may play a role in those migraine attacks triggered by disrupted sleep, skipping meals and emotional reactions.

Migraine headache triggered specifically by sunlight: report of 16 cases

Migraine headaches may take place due to various triggering factors. One or more triggering factors can be detected in a migraine patient. To our knowledge, a factor that is known to trigger migraine attacks in a patient does not cause headache each time the patient is exposed to it. Migraine headaches also can be experienced without these factors. Here, we describe a case series of 16 sunlight-induced migraine patients. Records of patients admitted to Firat University Faculty of Medicine Clinic of Neurology with a complaint of headache between January 2001 and June 2010 were scanned. Among those patients, the ones suffering headaches after being exposed to sunlight were examined comprehensively. Nine patients were female and 7 were male. Fourteen patients had the characteristics of migraine without aura, while 2 patients had the characteristics of migraine with aura. The mean times to headache onset after sunlight exposure were 5-10 min in summer and 60 min in winter. Migraine headaches can be triggered by many different causes. We view sunlight as a single triggering factor which should be questioned in migraine patients.

Blurring the boundaries of vision: novel functions of intrinsically photosensitive retinal ganglion cells

Mammalian vision consists of the classic image-forming pathway involving rod and cone photoreceptors interacting through a neural network within the retina before sending signals to the brain, and a non image-forming pathway that uses a photosensitive cell employing an alternative and evolutionary ancient phototransduction system and a direct connection to various centers in the brain. Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin, which is independently capable of photon detection while also receiving synaptic input from rod and cone photoreceptors via bipolar cells. These cells are the retinal sentry for subconscious visual processing that controls circadian photoentrainment and the pupillary light reflex. Classified as irradiance detectors, recent investigations have led to expanding roles for this specific cell type and its own neural pathways, some of which are blurring the boundaries between image-forming and non image-forming visual processes.

Towards dynamical network biomarkers in neuromodulation of episodic migraine

Computational methods have complemented experimental and clinical neurosciences and led to improvements in our understanding of the nervous systems in health and disease. In parallel, neuromodulation in form of electric and magnetic stimulation is gaining increasing acceptance in chronic and intractable diseases. In this paper, we firstly explore the relevant state of the art in fusion of both developments towards translational computational neuroscience. Then, we propose a strategy to employ the new theoretical concept of dynamical network biomarkers (DNB) in episodic manifestations of chronic disorders. In particular, as a first example, we introduce the use of computational models in migraine and illustrate on the basis of this example the potential of DNB as early-warning signals for neuromodulation in episodic migraine.

Small-molecule antagonists of melanopsin-mediated phototransduction

Melanopsin, expressed in a subset of retinal ganglion cells, mediates behavioral adaptation to ambient light and other non-image forming photic responses. This has raised the possibility that pharmacological manipulation of melanopsin can modulate several CNS responses including photophobia, sleep, circadian rhythms and neuroendocrine function. Here we describe the identification of a potent synthetic melanopsin antagonist with in vivo activity. Novel sulfonamide compounds inhibiting melanopsin (opsinamides) compete with retinal binding to melanopsin and inhibit its function without affecting rod/cone mediated responses. In vivo administration of opsinamides to mice specifically and reversibly modified melanopsin-dependent light responses including the pupillary light reflex and light aversion. The discovery of opsinamides raises the prospect of therapeutic control of the melanopsin phototransduction system to regulate light-dependent behavior and remediate pathological conditions.

Habituation and sensitization in primary headaches

The phenomena of habituation and sensitization are considered most useful for studying the neuronal substrates of information processing in the CNS. Both were studied in primary headaches, that are functional disorders of the brain characterized by an abnormal responsivity to any kind of incoming innocuous or painful stimuli and it's cycling pattern over time (interictal, pre-ictal, ictal). The present review summarizes available data on stimulus responsivity in primary headaches obtained with clinical neurophysiology. In migraine, the majority of electrophysiological studies between attacks have shown that, for a number of different sensory modalities, the brain is characterised by a lack of habituation of evoked responses to repeated stimuli. This abnormal processing of the incoming information reaches its maximum a few days before the beginning of an attack, and normalizes during the attack, at a time when sensitization may also manifest itself. An abnormal rhythmic activity between thalamus and cortex, namely thalamocortical dysrhythmia, may be the pathophysiological mechanism subtending abnormal information processing in migraine. In tension-type headache (TTH), only few signs of deficient habituation were observed only in subgroups of patients. By contrast, using grand-average responses indirect evidence for sensitization has been found in chronic TTH with increased nociceptive specific reflexes and evoked potentials. Generalized increased sensitivity to pain (lower thresholds and increased pain rating) and a dysfunction in supraspinal descending pain control systems may contribute to the development and/or maintenance of central sensitization in chronic TTH. Cluster headache patients are characterized during the bout and on the headache side by a pronounced lack of habituation of the brainstem blink reflex and a general sensitization of pain processing. A better insight into the nature of these ictal/interictal electrophysiological dysfunctions in primary headaches paves the way for novel therapeutic targets and may allow a better understanding of the mode of action of available therapies.

Identification of nonvisual photomotor response cells in the vertebrate hindbrain

Nonvisual photosensation enables animals to sense light without sight. However, the cellular and molecular mechanisms of nonvisual photobehaviors are poorly understood, especially in vertebrate animals. Here, we describe the photomotor response (PMR), a robust and reproducible series of motor behaviors in zebrafish that is elicited by visual wavelengths of light but does not require the eyes, pineal gland, or other canonical deep-brain photoreceptive organs. Unlike the relatively slow effects of canonical nonvisual pathways, motor circuits are strongly and quickly (seconds) recruited during the PMR behavior. We find that the hindbrain is both necessary and sufficient to drive these behaviors. Using in vivo calcium imaging, we identify a discrete set of neurons within the hindbrain whose responses to light mirror the PMR behavior. Pharmacological inhibition of the visual cycle blocks PMR behaviors, suggesting that opsin-based photoreceptors control this behavior. These data represent the first known light-sensing circuit in the vertebrate hindbrain.

Functional neuroimaging of migraine

This review summarizes the history of migraine imaging and key findings of studies on functional neuroimaging in migraine and describes how these data have changed our view of the disorder. Functional neuroimaging during migraine attacks and also interictally has initiated the description of "the migraine brain". These studies have led to the demonstration of cortical spreading depression in migraine with aura, the crucial role for the brainstem during migraine attacks, and cortical hypersensitivity in migraineurs modulated by the trigeminal pathway, explaining sensory sensitization such as photophobia and osmophobia.

Atypical resting-state functional connectivity of affective pain regions in chronic migraine

OBJECTIVE

Chronic migraineurs (CM) have painful intolerances to somatosensory, visual, olfactory, and auditory stimuli during and between migraine attacks. These intolerances are suggestive of atypical affective responses to potentially noxious stimuli. We hypothesized that atypical resting-state functional connectivity (rs-fc) of affective pain-processing brain regions may associate with these intolerances. This study compared rs-fc of affective pain-processing regions in CM with controls.

METHODS

Twelve minutes of resting-state blood oxygenation level-dependent data were collected from 20 interictal adult CM and 20 controls. Rs-fc between 5 affective regions (anterior cingulate cortex, right/left anterior insula, and right/left amygdala) with the rest of the brain was determined. Functional connections consistently differing between CM and controls were identified using summary analyses. Correlations between number of migraine years and the strengths of functional connections that consistently differed between CM and controls were calculated.

RESULTS

Functional connections with affective pain regions that differed in CM and controls included regions in anterior insula, amygdala, pulvinar, mediodorsal thalamus, middle temporal cortex, and periaqueductal gray. There were significant correlations between the number of years with CM and functional connectivity strength between the anterior insula with mediodorsal thalamus and anterior insula with periaqueductal gray.

CONCLUSION

CM is associated with interictal atypical rs-fc of affective pain regions with pain-facilitating and pain-inhibiting regions that participate in sensory-discriminative, cognitive, and integrative domains of the pain experience. Atypical rs-fc with affective pain regions may relate to aberrant affective pain processing and atypical affective responses to painful stimuli characteristic of CM.

Na,K-ATPase alpha isoforms at the blood-cerebrospinal fluid-trigeminal nerve and blood-retina interfaces in the rat

Background

Cerebrospinal fluid (CSF) sodium concentration increases during migraine attacks, and both CSF and vitreous humor sodium increase in the rat migraine model. The Na,K-ATPase is a probable source of these sodium fluxes. Since Na,K-ATPase isoforms have different locations and physiological roles, our objective was to establish which alpha isoforms are present at sites where sodium homeostasis is disrupted.

Methods

Specific Na,K-ATPase alpha isoforms were identified in rat tissues by immunohistochemistry at the blood-CSF barrier at the choroid plexus, at the blood-CSF-trigeminal barrier at the meninges, at the blood-retina barrier, and at the blood-aqueous barrier at the ciliary body. Calcitonin gene-related peptide (CGRP), occludin, or von Willibrand factor (vWF) were co-localized with Na,K-ATPase to identify trigeminal nociceptor fibers, tight junctions, and capillary endothelial cells respectively.

Results

The Na,K-ATPase alpha-2 isoform is located on capillaries and intensely at nociceptive trigeminal nerve fibers at the meningeal blood-CSF-trigeminal barrier. Alpha-1 and −3 are lightly expressed on the trigeminal nerve fibers but not at capillaries. Alpha-2 is expressed at the blood-retina barriers and, with alpha-1, at the ciliary body blood aqueous barrier. Intense apical membrane alpha-1 was associated with moderate cytoplasmic alpha-2 expression at the choroid plexus blood-CSF barrier.

Conclusion

Na,K-ATPase alpha isoforms are present at the meningeal, choroid plexus, and retinal barriers. Alpha-2 predominates at the capillary endothelial cells in the meninges and retinal ganglion cell layer.

Interictal cortical hyperresponsiveness in migraine is directly related to the presence of aura

OBJECTIVE

The objective of this study was to compare the interictal cortical response to a visual stimulus between migraine with aura (MWA), migraine without aura (MwoA), and control subjects.

METHODS

In a prospective case-control study, blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) was used to assess the response to a visual stimulus and arterial spin labeled perfusion MR to determine resting cerebral blood flow. A standardized questionnaire was used to assess interictal visual discomfort.

RESULTS

Seventy-five subjects (25 MWA, 25 MwoA, and 25 controls) were studied. BOLD fMRI response to visual stimulation within primary visual cortex was greater in MWA (3.09 ± 0.15%) compared to MwoA (2.36 ± 0.13%, P  = 0.0008) and control subjects (2.47 ± 0.11%, P  = 0.002); responses were also greater in the lateral geniculate nuclei in MWA. No difference was found between MwoA and control groups. Whole brain analysis showed that increased activation in MWA was confined to the occipital pole. Regional resting cerebral blood flow did not differ between groups. MWA and MwoA subjects had significantly greater levels of interictal visual discomfort compared to controls ( P  = 0.008 and P  = 0.005, respectively), but this did not correlate with BOLD response.

CONCLUSIONS

Despite similar interictal symptoms of visual discomfort, only MWA subjects have cortical hyperresponsiveness to visual stimulus, suggesting a direct connection between cortical hyperresponsiveness and aura itself.

Modulation of CGRP-induced light aversion in wild-type mice by a 5-HT(1B/D) agonist

The neuropeptide calcitonin gene-related peptide (CGRP) plays a critical role in the pathophysiology of migraine. We have focused on the role of CGRP in photophobia, which is a common migraine symptom. We previously used an operant-based assay to show that CGRP-sensitized transgenic (nestin/hRAMP1), but not control, mice exhibited light aversion in response to an intracerebroventricular CGRP injection. A key question was whether the transgenic phenotype was due to overexpression of the CGRP receptor at endogenous or novel expression sites. We reasoned that if endogenous receptor sites were sufficient for light-aversive behavior, then wild-type mice should also show the phenotype when given a sufficiently strong stimulus. In this study, we report that mice with normal levels of endogenous CGRP receptors demonstrate light avoidance following CGRP administration. This phenotype required the combination of two factors: higher light intensity and habituation to the testing chamber. Control tests confirmed that light aversion was dependent on coincident exposure to CGRP and light and cannot be fully explained by increased anxiety. Furthermore, CGRP reduced locomotion only in the dark, not in the light. Coadministration of rizatriptan, a 5-HT(1B/D) agonist anti-migraine drug, attenuated the effects of exogenous CGRP on light aversion and motility. This suggests that triptans can act by mechanisms that are distinct from inhibition of CGRP release. Thus, we demonstrate that activation of endogenous CGRP receptors is sufficient to elicit light aversion in mice, which can be modulated by a drug commonly used to treat migraine.

The evolution of a migraine attack - a review of recent evidence

A migraine attack is an extraordinarily complex brain event that takes place over hours to days. This review focuses on recent human studies that shed light on the evolution of a migraine attack. It begins with a constellation of premonitory symptoms that are associated with activation of the hypothalamus and may involve the neurotransmitter dopamine. Even in the premonitory phase, patients experience sensitivity to sensory stimuli, indicating that central sensitization is a primary phenomenon. The migraine attack progresses to a phase that in some patients includes aura, which involves changes in cortical function, blood flow, and neurovascular coupling. The aura phase overlaps with the headache phase, which is associated with further changes in blood flow and function of the brainstem, thalamus, hypothalamus, and cortex. Serotonin receptors, nitric oxide, calcitonin gene-related peptide, pituitary adenylate cyclase-activating polypeptide, and prostanoids are demonstrated specific chemical mediators of migraine based on therapeutic and triggered migraine studies. A number of migraine symptoms persist beyond resolution of headache into a postdromal phase, accompanied by persistent blood flow changes in several brain regions. Although these phases of migraine have substantial temporal, neurochemical, and anatomical overlap, each represents an important window onto the pathophysiology of migraine as well as a target for therapeutic intervention. A comprehensive approach to migraine requires an understanding of the entire range of mechanisms and resultant symptoms that occur throughout the evolution of an attack.

Pathophysiology of migraine

Migraine is a collection of perplexing neurological conditions in which the brain and its associated tissues have been implicated as major players during an attack. Once considered exclusively a disorder of blood vessels, compelling evidence has led to the realization that migraine represents a highly choreographed interaction between major inputs from both the peripheral and central nervous systems, with the trigeminovascular system and the cerebral cortex among the main players. Advances in in vivo and in vitro technologies have informed us about the significance to migraine of events such as cortical spreading depression and activation of the trigeminovascular system and its constituent neuropeptides, as well as about the importance of neuronal and glial ion channels and transporters that contribute to the putative cortical excitatory/inhibitory imbalance that renders migraineurs susceptible to an attack. This review focuses on emerging concepts that drive the science of migraine in both a mechanistic direction and a therapeutic direction.

The locus of flicker adaptation in the migraine visual system: a dichoptic study

BACKGROUND

Flickering light has been shown to sensitize the migraine visual system at high stimulus contrast while elevating thresholds at low contrast. The present study employs a dichoptic psychophysical paradigm to ask whether the abnormal adaptation to flicker in migraine occurs before or after the binocular combination of inputs from the two eyes in the visual cortex.

METHODS

Following adaptation to high contrast flicker presented to one eye only, flicker contrast increment thresholds were measured in each eye separately using dichoptic viewing.

RESULTS

Modest interocular transfer of adaptation was seen in both migraine and control groups at low contrast. Sensitization at high contrast in migraine relative to control participants was seen in the adapted eye only, and an unanticipated threshold elevation occurred in the non-adapted eye. Migraineurs also showed significantly lower aversion thresholds to full field flicker than control participants, but aversion scores and increment thresholds were not correlated.

CONCLUSIONS

The results are simulated with a three-stage neural model of adaptation that points to strong adaptation at monocular sites prior to binocular combination, and weaker adaptation at the level of cortical binocular neurons. The sensitization at high contrast in migraine is proposed to result from stronger adaptation of inhibitory neurons, which act as a monocular normalization pool.

Light evokes melanopsin-dependent vocalization and neural activation associated with aversive experience in neonatal mice

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are the only functional photoreceptive cells in the eye of newborn mice. Through postnatal day 9, in the absence of functional rods and cones, these ipRGCs mediate a robust avoidance behavior to a light source, termed negative phototaxis. To determine whether this behavior is associated with an aversive experience in neonatal mice, we characterized light-induced vocalizations and patterns of neuronal activation in regions of the brain involved in the processing of aversive and painful stimuli. Light evoked distinct melanopsin-dependent ultrasonic vocalizations identical to those emitted under stressful conditions, such as isolation from the litter. In contrast, light did not evoke the broad-spectrum calls elicited by acute mechanical pain. Using markers of neuronal activation, we found that light induced the immediate-early gene product Fos in the posterior thalamus, a brain region associated with the enhancement of responses to mechanical stimulation of the dura by light, and thought to be the basis for migrainous photophobia. Additionally, light induced the phosphorylation of extracellular-related kinase (pERK) in neurons of the central amygdala, an intracellular signal associated with the processing of the aversive aspects of pain. However, light did not activate Fos expression in the spinal trigeminal nucleus caudalis, the primary receptive field for painful stimulation to the head. We conclude that these light-evoked vocalizations and the distinct pattern of brain activation in neonatal mice are consistent with a melanopsin-dependent neural pathway involved in processing light as an aversive but not acutely painful stimulus.

Primary headache disorders and neuro-ophthalmologic manifestations

Headache is an extraordinarily common complaint presenting to medical practitioners in all arenas and specialties, particularly primary care physicians, neurologists, and ophthalmologists. A wide variety of headache disorders may manifest with a myriad of neuro-ophthalmologic symptoms, including orbital pain, disturbances of vision, aura, photophobia, lacrimation, conjunctival injection, ptosis, and other manifestations. The differential diagnosis in these patients is broad and includes both secondary, or symptomatic, and primary headache disorders. Awareness of the headache patterns and associated symptoms of these various disorders is essential to achieve the correct diagnosis. This paper reviews the primary headache disorders that prominently feature neuro-ophthalmologic manifestations, including migraine, the trigeminal autonomic cephalalgias, and hemicrania continua. Migraine variants with prominent neuro-ophthalmologic symptoms including aura without headache, basilar-type migraine, retinal migraine, and ophthalmoplegic migraine are also reviewed. This paper focuses particularly on the symptomatology of these primary headache disorders, but also discusses their epidemiology, clinical features, and treatment.

The role of intrinsically photosensitive retinal ganglion cells in nonimage-forming responses to light

Light exerts many effects on behavior and physiology. These effects can be characterized as either image-forming or nonimage-forming (NIF) visual processes. Image-forming vision refers to the process of detecting objects and organisms in the environment and distinguishing their physical characteristics, such as size, shape, and direction of motion. NIF vision, in contrast, refers to effects of light that are independent of fine spatiotemporal vision. NIF effects are many and varied, ranging from modulation of basal physiology, such as heart rate and body temperature, to changes in higher functions, such as mood and cognitive performance. In mammals, many NIF effects of light are dependent upon the inner retinal photopigment melanopsin and the cells in which melanopsin is expressed, the intrinsically photosensitive retinal ganglion cells (ipRGCs). The ipRGCs project broadly throughout the brain. Many of these projections terminate in areas known to mediate NIF effects, while others terminate in regions whose link to photoreception remains to be established. Additionally, the presence of ipRGC projections to areas of the brain with no known link to photoreception suggests the existence of additional ipRGC-mediated NIF effects. This review summarizes the known NIF effects of light and the role of melanopsin and ipRGCs in driving these effects, with an eye toward stimulating further investigation of the many and varied effects of light on physiology and behavior.

Trigeminal interpolaris/caudalis transition neurons mediate reflex lacrimation evoked by bright light in the rat

Abnormal sensitivity to bright light can cause discomfort or pain and evoke protective reflexes such as lacrimation. Although the trigeminal nerve is probably involved, the mechanism linking luminance to somatic sensory nerve activity remains uncertain. This study determined the effect of bright light on second-order ocular neurons at the ventral trigeminal interpolaris/caudalis transition (Vi/Vc) region, a major termination zone for trigeminal sensory fibers that innervate the eye. Most Vi/Vc neurons (80.9%) identified by responses to mechanical stimulation of the ocular surface also encoded bright light intensity. Light-evoked neural activity displayed a long latency to activation (> 10 s) and required transmission through the trigeminal root ganglion. Light-evoked neural activity was inhibited by intravitreal injection of phenylephrine or l-N(G) -nitro-arginine methyl ester (L-NAME), suggesting a mechanism coupled to vascular events within the eye. Laser Doppler flowmetry revealed rapid light-evoked increases in ocular blood flow that occurred prior to the increase in Vi/Vc neural activity. Synaptic blockade of the Vi/Vc region by cobalt chloride prevented light-evoked increases in tear volume, whereas blockade at the more caudal spinomedullary junction (Vc/C1) had no effect. In summary, Vi/Vc neurons encoded bright light intensity and were inhibited by drugs that alter blood flow to the eye. These results support the hypothesis that light-responsive neurons at the Vi/Vc transition region are critical for ocular-specific functions such as reflex lacrimation, whereas neurons at the caudal Vc/C1 junction region probably serve other aspects of ocular nociception.

Migraine changes the brain: neuroimaging makes its mark

PURPOSE OF REVIEW

This review summarizes key findings of the current literature on functional neuroimaging in migraine and describes how these studies have changed our view of the disorder.

RECENT FINDINGS

Recent studies have started not only to investigate the global cerebral activation pattern during migraine attacks, but to address specific aspects of migraine attacks such as photophobia, osmophobia as well as pain perception with the aim of disentangling the underlying mechanisms. There is also more and more evidence that the migraine brain is abnormal even outside of attacks and that repeated attacks are leading to functional and structural alterations in the brain, which may in turn drive the transformation of migraine to its chronic form. Some new results are pinpointing toward a potential role of interesting new brain areas in migraine pathophysiology such as the temporal cortex or the basal ganglia.

SUMMARY

Neuroimaging studies are beginning to shed light on the mechanisms underlying the development and evolution of migraine and its specific symptoms. Future studies have the potential to also improve our understanding of established and upcoming treatment approaches and to monitor treatment effects in an objective and noninvasive way.

Dynamic tilt thresholds are reduced in vestibular migraine

Vestibular symptoms caused by migraine, referred to as vestibular migraine, are a frequently diagnosed but poorly understood entity. Based on recent evidence that normal subjects generate vestibular-mediated percepts of head motion and reflexive eye movements using different mechanisms, we hypothesized that percepts of head motion may be abnormal in vestibular migraine. We therefore measured motion detection thresholds in patients with vestibular migraine, migraine patients with no history of vestibular symptoms, and normal subjects using the following paradigms: roll rotation while supine (dynamically activating the semicircular canals); quasi-static roll tilt (statically activating the otolith organs); and dynamic roll tilt (dynamically activating the canals and otoliths). Thresholds were determined while patients were asymptomatic using a staircase paradigm, whereby the peak acceleration of the motion was decreased or increased based on correct or incorrect reports of movement direction. We found a dramatic reduction in motion thresholds in vestibular migraine compared to normal and migraine subjects in the dynamic roll tilt paradigm, but normal thresholds in the roll rotation and quasi-static roll tilt paradigms. These results suggest that patients with vestibular migraine may have enhanced perceptual sensitivity (e.g. increased signal-to-noise ratio) for head motions that dynamically modulate canal and otolith inputs together.

Photochemical properties of mammalian melanopsin

Melanopsin is the photoreceptor molecule of intrinsically photosensitive retinal ganglion cells, which serve as the input for various nonvisual behavior and physiological functions fundamental to organisms. The retina, therefore, possess a melanopsin-based nonvisual system in addition to the visual system based on the classical visual photoreceptor molecules. To elucidate the molecular properties of melanopsin, we have exogenously expressed mouse melanopsin in cultured cells. We were able to obtain large amounts of purified mouse melanopsin and conducted a comprehensive spectroscopic study of its photochemical properties. Melanopsin has an absorption maximum at 467 nm, and it converts to a meta intermediate having an absorption maximum at 476 nm. The melanopsin photoreaction is similar to that of squid rhodopsin, exhibiting bistability that results in a photosteady mixture of a resting state (melanopsin containing 11-cis-retinal) and an excited state (metamelanopsin containing all-trans-retinal) upon sustained irradiation. The absorption coefficient of melanopsin is 33000 ± 1000 M(-1) cm(-1), and its quantum yield of isomerization is 0.52; these values are also typical of invertebrate bistable pigments. Thus, the nonvisual system in the retina relies on a type of photoreceptor molecule different from that of the visual system. Additionally, we found a new state of melanopsin, containing 7-cis-retinal (extramelanopsin), which forms readily upon long-wavelength irradiation (yellow to red light) and photoconverts to metamelanopsin with short-wavelength (blue light) irradiation. Although it is unclear whether extramelanopsin would have any physiological role, it could potentially allow wavelength-dependent regulation of melanopsin functions.

Migraine: a disorder of brain excitatory-inhibitory balance?

Migraine is a common disabling brain disorder whose key manifestations are recurrent attacks of unilateral headache and interictal hypersensitivity to sensory stimuli. Migraine arises from a primary brain dysfunction that leads to episodic activation and sensitization of the trigeminovascular pain pathway and as a consequence to headache. Major open issues concern the molecular and cellular mechanisms of the primary brain dysfunction(s) and of migraine pain. We review here our current understanding of these mechanisms, focusing on recent advances regarding migraine genetics, headache mechanisms, and the primary brain dysfunction(s) underlying migraine onset and susceptibility to cortical spreading depression, the neurophysiological correlate of migraine aura. We also discuss insights obtained from the functional analysis of familial hemiplegic migraine mouse models.

Serotonin and CGRP in migraine

Migraine is defined as recurrent attack of headache that are commonly unilateral and accompanied by gastrointestinal and visual disorders. Migraine is more prevalent in females than males with a ratio of 3:1. It is primarily a complex neurovascular disorder involving local vasodilation of intracranial, extracerebral blood vessels and simultaneous stimulation of surrounding trigeminal sensory nervous pain pathway that results in headache. The activation of 'trigeminovascular system' causes release of various vasodilators, especially calcitonin gene-related peptide (CGRP) that induces pain response. At the same time, decreased levels of neurotransmitter, serotonin have been observed in migraineurs. Serotonin receptors have been found on the trigeminal nerve and cranial vessels and their agonists especially triptans prove effective in migraine treatment. It has been found that triptans act on trigeminovascular system and bring the elevated serum levels of key molecules like calcitonin gene related peptide (CGRP) to normal. Currently CGRP receptor antagonists, olcegepant and telcagepant are under consideration for antimigraine therapeutics. It has been observed that varying levels of ovarian hormones especially estrogen influence serotonin neurotransmission system and CGRP levels making women more predisposed to migraine attacks. This review provides comprehensive information about the role of serotonin and CGRP in migraine, specifically the menstrual migraine.

Glare's causes, consequences, and clinical challenges after a century of ophthalmic study

PURPOSE

To provide a multidisciplinary synthesis of scientific information on disability, discomfort, dazzling, and scotomatic (photostress) glare.

DESIGN

Perspective.

METHODS

Analysis and integration of relevant historical and contemporary publications on glare in ophthalmology, illumination engineering, neurology, and other relevant disciplines.

RESULTS

Disability glare is caused by scattered intraocular light (straylight) not useful for vision. Straylight casts a veiling luminance on the retina, reducing image contrast and impairing vision. In common environments, glare and target illumination sources have the same or similar spectra. Colored spectacle or intraocular lens filters reduce both proportionately, so they do not increase retinal image contrast or decrease disability glare. Discomfort glare is caused by situational illumination too intense or variable. Dazzling glare occurs when high illuminances are spread across the retina. Neurophysiological research is clarifying how discomfort and dazzling glare depend on different retinal photoreceptors and nociceptive brain pathways involving the trigeminal ganglion and thalamus. Photostress is caused by excessive local retinal photopigment bleaching uncommon in ordinary situations. Optical glare countermeasures are available for daytime driving but not oncoming automobile headlights at night. Filters that decrease daytime discomfort or dazzling glare also reduce nighttime mesopic and scotopic sensitivity.

CONCLUSIONS

Glare is problematic for patients and clinicians despite a century of scientific research. Advances in understanding glare have been hampered by its complex, multidisciplinary nature and limited interdisciplinary communication. We provide one pathway through the forest of glare nomenclature and mechanisms. Improved diagnostic and therapeutic methodologies await continuing progress in understanding glare.

Diverse types of ganglion cell photoreceptors in the mammalian retina

Photoreceptors carry out the first step in vision by capturing light and transducing it into electrical signals. Rod and cone photoreceptors efficiently translate photon capture into electrical signals by light activation of opsin-type photopigments. Until recently, the central dogma was that, for mammals, all phototransduction occurred in rods and cones. However, the recent discovery of a novel photoreceptor type in the inner retina has fundamentally challenged this view. These retinal ganglion cells are intrinsically photosensitive and mediate a broad range of physiological responses such as photoentrainment of the circadian clock, light regulation of sleep, pupillary light reflex, and light suppression of melatonin secretion. Intrinsically photosensitive retinal ganglion cells express melanopsin, a novel opsin-based signaling mechanism reminiscent of that found in invertebrate rhabdomeric photoreceptors. Melanopsin-expressing retinal ganglion cells convey environmental irradiance information directly to brain centers such as the hypothalamus, preoptic nucleus, and lateral geniculate nucleus. Initial studies suggested that these melanopsin-expressing photoreceptors were an anatomically and functionally homogeneous population. However, over the past decade or so, it has become apparent that these photoreceptors are distinguishable as individual subtypes on the basis of their morphology, molecular markers, functional properties, and efferent projections. These results have provided a novel classification scheme with five melanopsin photoreceptor subtypes in the mammalian retina, each presumably with differential input and output properties. In this review, we summarize the evidence for the structural and functional diversity of melanopsin photoreceptor subtypes and current controversies in the field.

The corneal pain system. Part I: the missing piece of the dry eye puzzle

The traditional model of dry eye disease based on tear deficiency has presented us with many unanswered questions. Recent studies support the notion that dry eye-like symptoms represent non-specific corneal pain and provide new insights into the mechanisms that sustain the integrity of the optical tear layer. Thus, this enigmatic disease can be viewed with a new perspective, which involves the dysfunctional corneal pain system as a central pathogenetic feature of a series of disorders collectively known today as dry eye.

Effects of selective-wavelength block filters on pupillary light reflex under red and blue light stimuli

PURPOSE

To investigate at which wavelength melanopsin-containing retinal ganglion cells (mRGCs) depolarize and how they affect pupillary constriction induced by light stimulation in humans.

METHODS

The pupil light reflex was evaluated for 30 normal subjects by use of an infrared pupillometer. Blue light stimulation (470 nm) and red light stimulation (635 nm) of 100 cd/m(2) were selected. Selective-wavelength block filters which can selectively remove the wavelengths 440 and 470 nm were used. Visual tests were also performed to observe the effects of the filters on visual acuity, color vision, and contrast sensitivity.

RESULTS

The pupil transiently constricts and then settles toward a steady-state diameter when stimulated with the light. When the 470-nm-block filter was worn, the sustained phase of pupillary constriction, thought to be mediated by the mRGCs, was not stable but there was no effect on the initial phase of pupillary constriction under blue light stimulation. Visual acuity, color vision, and contrast sensitivity were not affected by the 470-nm-block filter.

CONCLUSIONS

These results suggest that the mRGC in humans may respond to 470-nm-wavelength light at 100 cd/m(2), and there is a possibility of affecting the sustained phase of the light reflex without changing visual performance.

Calcitonin gene-related peptide in migraine: intersection of peripheral inflammation and central modulation

Over the past two decades, a convergence of basic and clinical evidence has established the neuropeptide calcitonin-gene-related peptide (CGRP) as a key player in migraine. Although CGRP is a recognised neuromodulator of nociception, its mechanism of action in migraine remains elusive. In this review, we present evidence that led us to propose that CGRP is well poised to enhance neurotransmission in migraine by both peripheral and central mechanisms. In the periphery, it is thought that local release of CGRP from the nerve endings of meningeal nociceptors following their initial activation by cortical spreading depression is critical for the induction of vasodilation, plasma protein extravasation, neurogenic inflammation and the consequential sensitisation of meningeal nociceptors. Mechanistically, we propose that CGRP release can give rise to a positive-feedback loop involved in localised increased synthesis and release of CGRP from neurons and a CGRP-like peptide called procalcitonin from trigeminal ganglion glia. Within the brain, the wide distribution of CGRP and CGRP receptors provides numerous possible targets for CGRP to act as a neuromodulator.

Melanopsin-positive intrinsically photosensitive retinal ganglion cells: from form to function

Melanopsin imparts an intrinsic photosensitivity to a subclass of retinal ganglion cells (ipRGCs). Generally thought of as irradiance detectors, ipRGCs target numerous brain regions involved in non-image-forming vision. ipRGCs integrate their intrinsic, melanopsin-mediated light information with rod/cone signals relayed via synaptic connections to influence light-dependent behaviors. Early observations indicated diversity among these cells and recently several specific subtypes have been identified. These subtypes differ in morphological and physiological form, controlling separate functions that range from biological rhythm via circadian photoentrainment, to protective behavioral responses including pupil constriction and light avoidance, and even image-forming vision. In this Mini-Symposium review, we will discuss some recent findings that highlight the diversity in both form and function of these recently discovered atypical photoreceptors.

Neurological diseases and pain

Chronic pain is a frequent component of many neurological disorders, affecting 20-40% of patients for many primary neurological diseases. These diseases result from a wide range of pathophysiologies including traumatic injury to the central nervous system, neurodegeneration and neuroinflammation, and exploring the aetiology of pain in these disorders is an opportunity to achieve new insight into pain processing. Whether pain originates in the central or peripheral nervous system, it frequently becomes centralized through maladaptive responses within the central nervous system that can profoundly alter brain systems and thereby behaviour (e.g. depression). Chronic pain should thus be considered a brain disease in which alterations in neural networks affect multiple aspects of brain function, structure and chemistry. The study and treatment of this disease is greatly complicated by the lack of objective measures for either the symptoms or the underlying mechanisms of chronic pain. In pain associated with neurological disease, it is sometimes difficult to obtain even a subjective evaluation of pain, as is the case for patients in a vegetative state or end-stage Alzheimer's disease. It is critical that neurologists become more involved in chronic pain treatment and research (already significant in the fields of migraine and peripheral neuropathies). To achieve this goal, greater efforts are needed to enhance training for neurologists in pain treatment and promote greater interest in the field. This review describes examples of pain in different neurological diseases including primary neurological pain conditions, discusses the therapeutic potential of brain-targeted therapies and highlights the need for objective measures of pain.

Cortical projections of functionally identified thalamic trigeminovascular neurons: implications for migraine headache and its associated symptoms

This study identifies massive axonal arbors of trigeminovascular (dura-sensitive) thalamic neurons in multiple cortical areas and proposes a novel framework for conceptualizing migraine headache and its associated symptoms. Individual dura-sensitive neurons identified and characterized electrophysiologically in first-order and higher-order relay thalamic nuclei were juxtacellularly filled with an anterograde tracer that labeled their cell bodies and processes. First-order neurons located in the ventral posteromedial nucleus projected mainly to trigeminal areas of primary (S1) as well as secondary (S2) somatosensory and insular cortices. Higher-order neurons located in the posterior (Po), lateral posterior (LP), and lateral dorsal (LD) nuclei projected to trigeminal and extra-trigeminal areas of S1 and S2, as well as parietal association, retrosplenial, auditory, ectorhinal, motor, and visual cortices. Axonal arbors spread at various densities across most layers of the different cortical areas. Such parallel network of thalamocortical projections may play different roles in the transmission of nociceptive signals from the meninges to the cortex. The findings that individual dura-sensitive Po, LP, and LD neurons project to many functionally distinct and anatomically remote cortical areas extend current thinking on projection patterns of high-order thalamic neurons and position them to relay nociceptive information directly rather than indirectly from one cortical area to another. Such extensive input to diverse cortical areas that are involved in regulation of affect, motor function, visual and auditory perception, spatial orientation, memory retrieval, and olfaction may explain some of the common disturbances in neurological functions during migraine.

Light-induced trigeminal sensitization without central visual pathways: another mechanism for photophobia

PURPOSE

The authors investigated whether trigeminal sensitization occurs in response to bright light with the retina disconnected from the rest of the central nervous system by optic nerve section.

METHODS

In urethane-anesthetized rats, trigeminal reflex blinks were evoked with air puff stimuli directed at the cornea in darkness and at three different light intensities. After normative data were collected, the optic nerve was lesioned and the rats were retested. In an alert rat, reflex blinks were evoked by stimulation of the supraorbital branch of the trigeminal nerve in the dark and in the light.

RESULTS

A 9.1 × 10(3) μW/cm(2) and a 15.1 × 10(3) μW/cm(2) light significantly enhanced the magnitude of reflex blinks relative to blinks evoked by the same trigeminal stimulus when the rats were in the dark. In addition, rats exhibited a significant increase in spontaneous blinking in the light relative to the blink rate in darkness. After lesioning of the optic nerve, the 15.1 × 10(3) μW/cm(2) light still significantly increased the magnitude of trigeminal reflex blinks.

CONCLUSIONS

Bright lights increase trigeminal reflex blink amplitude and the rate of spontaneous blinking in rodents. Light can modify trigeminal activity without involving the central visual system.

Topiramate in the treatment of migraine: A kainate (glutamate) receptor antagonist within the trigeminothalamic pathway

Background: The development of new agents for the preventive treatment of migraine is the greatest unmet need in the therapeutics of primary headaches. Topiramate, an anticonvulsant drug, is an effective anti-migraine preventive whose mechanism of action is not fully elucidated. Since glutamate plays a major role in migraine pathophysiology, the potential action of topiramate through glutamatergic mechanisms is of considerable interest.

Methods: Recordings of neurons in the trigeminocervical complex (TCC) and the ventroposteromedial thalamic nucleus (VPM) of anesthetized rats were made using electrophysiological techniques. The effects of intravenous or microiontophorezed topiramate on trigeminovascular activation of second- and third-order neurons in the trigeminothalamic pathway were characterized. The potential interactions of topiramate with the ionotropic glutamate receptors were studied using microiontophoresis.

Results: Both intravenous and microiontophorized topiramate significantly inhibited trigeminovascular activity in the TCC and VPM. In both nuclei microiontophoretic application of topiramate significantly attenuated kainate receptor-evoked firing but had no effect on N-methyl-d-aspartic acid or α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor activation.

Conclusion: The data demonstrate for the first time that topiramate modulates trigeminovascular transmission within the trigeminothalamic pathway with the kainate receptor being a potential target. Understanding the mechanism of action of topiramate may help in the design of new medications for migraine prevention, with the data pointing to glutamate-kainate receptors as a fruitful target to pursue.

Advances in understanding the mechanisms of migraine-type photophobia

PURPOSE OF REVIEW

Historically, photophobia was studied in patients and attempts to explain the underlying mechanisms have been speculative. Efforts to understand better the neural substrate of photophobia paved a way to the development of different animal models and the publication of several articles (all in 2010) on the mechanism by which light exacerbates migraine headache.

RECENT FINDINGS

Observations made in blind migraine patients devoid of any visual perception and blind migraine patients capable of detecting light have led to the discovery of a novel retino-thalamo-cortical pathway that carries photic signal from the retina to thalamic trigeminovascular neurons believed to play a critical role in the perception of headache intensity during migraine. Evidence for modulation of the trigeminovascular pathway by light and identification of the pathway through which photic signals converge on the nociceptive pathway that mediates migraine headache provide first set of scientific data on the mechanism by which light intensifies migraine headache.

SUMMARY

The findings provide a neural substrate for migraine-type photophobia. This may lead to identification and development of molecular targets for selective prevention of photophobia during migraine.