Therapeutic implications of cortical spreading depression models in migraine

Migraine is among the most common and disabling neurological diseases in the world. Cortical spreading depression (CSD) is a wave of near-complete depolarization of neurons and glial cells that slowly propagates along the cortex creating the perception of aura. Evidence suggests that CSD can trigger migraine headache. Experimental models of CSD have been considered highly translational as they recapitulate migraine-related phenomena and have been validated for screening migraine therapeutics. Here we outline the essential components of validated experimental models of CSD and provide a comprehensive review of potential modulators and targets against CSD. We further focus on novel interventions that have been recently shown to suppress CSD susceptibility that may lead to therapeutic targets in migraine.

Novel Therapeutic Targets Against Spreading Depression

Migraine is among the most prevalent and disabling neurological diseases in the world. Cortical spreading depression (SD) is an intense wave of neuronal and glial depolarization underlying migraine aura, and a headache trigger, which has been used as an experimental platform for drug screening in migraine. Here, we provide an overview of novel therapeutic targets that show promise to suppress SD, such as acid-sensing ion channels, casein kinase Iδ, P2X7-pannexin 1 complex, and neuromodulation, and outline the experimental models and essential quality measures for rigorous and reproducible efficacy testing.

Cortical spreading depression as a target for anti-migraine agents

Spreading depression (SD) is a slowly propagating wave of neuronal and glial depolarization lasting a few minutes, that can develop within the cerebral cortex or other brain areas after electrical, mechanical or chemical depolarizing stimulations. Cortical SD (CSD) is considered the neurophysiological correlate of migraine aura. It is characterized by massive increases in both extracellular K⁺ and glutamate, as well as rises in intracellular Na⁺ and Ca²⁺. These ionic shifts produce slow direct current (DC) potential shifts that can be recorded extracellularly. Moreover, CSD is associated with changes in cortical parenchymal blood flow. CSD has been shown to be a common therapeutic target for currently prescribed migraine prophylactic drugs. Yet, no effects have been observed for the antiepileptic drugs carbamazepine and oxcarbazepine, consistent with their lack of efficacy on migraine. Some molecules of interest for migraine have been tested for their effect on CSD. Specifically, blocking CSD may play an enabling role for novel benzopyran derivative tonabersat in preventing migraine with aura. Additionally, calcitonin gene-related peptide (CGRP) antagonists have been recently reported to inhibit CSD, suggesting the contribution of CGRP receptor activation to the initiation and maintenance of CSD not only at the classic vascular sites, but also at a central neuronal level. Understanding what may be lying behind this contribution, would add further insights into the mechanisms of actions for "gepants", which may be pivotal for the effectiveness of these drugs as anti-migraine agents. CSD models are useful tools for testing current and novel prophylactic drugs, providing knowledge on mechanisms of action relevant for migraine.

Pharmacological targeting of spreading depression in migraine

Migraine, particularly with aura, is a genetically heterogeneous disorder of ion channels, pumps or transporters associated with increased cortical excitability. Spreading depression, as one reflection of hyperexcitability, is the electrophysiological event underlying aura symptoms and a trigger for headache. Endogenous (e.g., genes and hormones) and exogenous factors (e.g., drugs) modulating migraine susceptibility have also been shown to modulate spreading depression susceptibility concordantly, suggesting that spreading depression can be a relevant therapeutic target in migraine. In support of this, several migraine prophylactic drugs used in clinical practice have been shown to suppress spreading depression susceptibility as a probable mechanism of action, despite belonging to widely different pharmacological classes. Hence, susceptibility to spreading depression can be a useful preclinical model with good positive and negative predictive value for drug screening.

Spreading depression: from serendipity to targeted therapy in migraine prophylaxis

Despite the relatively well-characterized headache mechanisms in migraine, upstream events triggering individual attacks are poorly understood. This lack of mechanistic insight has hampered a rational approach to prophylactic drug discovery. Unlike targeted abortive and analgesic interventions, mainstream migraine prophylaxis has been largely based on serendipitous observations (e.g. propranolol) and presumed class effects (e.g. anticonvulsants). Recent studies suggest that spreading depression is the final common pathophysiological target for several established or investigational migraine prophylactic drugs. Building on these observations, spreading depression can now be explored for its predictive utility as a preclinical drug screening paradigm in migraine prophylaxis.

Long-term effect of a single dose of flunarizine in Huntington's disease

We report a preliminary pilot study on the clinical efficacy of flunarizine (FNZ), a calcium-entry blocker that causes extrapyramidal side-effects, in 10 patients with Huntington's disease (HD). FNZ (20 mg) administered by the sublingual route resulted in a decrease in choreic movements and improved dexterity in performing several tests. These effects lasted for at least 7 days after a single dose. Therefore, FNZ seems to exert the same effect as a long-acting neuroleptic agent in our HD patients.Copyright Lippincott-Raven Publishers

Neuroprotective therapy for argon-laser induced retinal injury

Laser photocoagulation treatment of the central retina is often complicated by an immediate side effect of visual impairment, caused by the unavoidable laser-induced destruction of the normal tissue lying adjacent to the lesion and not affected directly by the laser beam. Furthermore, accidental laser injuries are at present untreatable. A neuroprotective therapy for salvaging the normal tissue might enhance the benefit obtained from treatment and allow safe perifoveal photocoagulation. We have developed a rat model for studying the efficacy of putative neuroprotective compounds in ameliorating laser-induced retinal damage. Four compounds were evaluated: the corticosteroid methylprednisolone, the glutamate-receptor blocker MK-801, the anti-oxidant enzyme superoxide dismutase, and the calcium-overload antagonist flunarizine. The study was carried out in two steps: in the first, the histopathological development of retinal laser injuries was studied. Argon laser lesions were inflicted in the retinas of 18 pigmented rats. The animals were killed after 3, 20 or 60 days and their retinal lesions were evaluated under the light microscope. The laser injury mainly involved the outer layers of the retina, where it destroyed significant numbers of photoreceptor cells. Over time, evidence of two major histopathological processes was observed: traction of adjacent normal retinal cells into the central area of the lesion forming an internal retinal bulging, and a retinal pigmented epithelial proliferative reaction associated with subretinal neovascularization and invasion of the retinal lesion site by phagocytes. The neuroprotective effects of each of the four compounds were verified in a second step of the study. For each drug tested, 12 rats were irradiated with argon laser inflictions: six of them received the tested agent while the other six were treated with the corresponding vehicle. Twenty days after laser exposure, the rats were killed and their lesions were subjected to image-analysis morphometry. The extent of retinal destruction was assessed by measuring the lesion diameter and the amount of photoreceptor cell loss in the outer nuclear layer. Methylprednisolone and MK-801 were shown to ameliorate laser-induced retinal damage, whereas both superoxide dismutase and flunarizine were ineffective. Furthermore, MK-801 diminished the proliferative reaction of the retinal pigment epithelial cells. On the basis of our results we suggest that the pigmented rat model is suitable for studying and screening various compounds for their neuroprotective efficacy in treating retinal laser injury. We further suggest that glutamate might play a key role in mediating retinal injury induced by laser irradiation.

Photochemical stroke model: flunarizine prevents sensorimotor deficits after neocortical infarcts in rats

We produced unilateral photochemical infarcts in the hindlimb sensorimotor neocortex of 186 rats by intravenous injection of the fluorescein derivative rose bengal and focal illumination of the intact skull surface. Infarcted rats showed specific, long-lasting deficits in tactile and proprioceptive placing reactions of the contralateral limbs, mostly the hindlimb. Placing deficits were most prominent during transition to immobility and/or when independent limb movements were required. Administration of flunarizine, a Class IV calcium antagonist, 30 minutes after infarction resulted in marked sparing of sensorimotor function in 30 rats. In contrast to 20 vehicle-treated rats, which remained deficient for at least 21 days, 15 (75%) of the rats treated with 1.25 mg/kg i.v. flunarizine showed normal placing on Day 1 after infarction, whereas the remaining five (25%) recovered within 5 days. Oral treatment of 10 rats with 40 mg/kg flunarizine was also effective. Neocortical infarct volume and thalamic gliosis, assessed 21 days after infarction, did not differ between 30 flunarizine- and 30 vehicle-treated rats. However, when 4-hour-old infarcts were measured in 16 rats, posttreatment with intravenous flunarizine reduced infarct size by 31%. In combination with appropriate behavioral analyses, photochemical thrombosis may constitute a relevant stroke model, in which flunarizine preserved behavioral function during a critical period, corresponding to the spread of ischemic damage.

Long-term neurological assessment of the post-resuscitative effects of flunarizine, verapamil and nimodipine in a new model of global complete ischaemia

In anesthetized rats, global complete ischaemia lasting for 9 min was induced by controlled hydraulic compression of the chest. A neurological score, based on cranial and spinal reflexes, postural tone, gait, movement and limb placement, was determined at 2 hr and 1, 2, 3, 7, 14, 21 and 28 days after resuscitation. Three doses of three calcium antagonists, flunarizine, verapamil and nimodipine and their respective solvents, were given intravenously during the resuscitation. The total neurological score was significantly better than solvent with 0.16 and 0.63 mg/kg of flunarizine and 0.04 and 0.16 mg/kg of verapamil; it was significantly better with solvent (10% ethanol) than with 0.04 and 0.16 mg/kg of nimodipine. The deficiency in tactile placing reactions of the hindpaws was the most resistant to therapy. This non-invasive model of global ischaemia in rats seems useful for the evaluation of drugs, since it requires minimal anesthesia and allows assessment of neurological recovery over an extended period of time.

Effects of flunarizine on induced nystagmus and cochlear blood flow

The effects of flunarizine on induced nystagmus and cochlear blood flow were compared with those of cinnarizine and diphenidol. Flunarizine significantly inhibited caloric (cool water)-induced nystagmus frequency and duration of nystagmus in rabbits at 5 mg/kg i.v., whereas cinnarizine and diphenidol only slightly decreased the frequency of nystagmus at 5 mg/kg, i.v. As for optokinetic stimuli-induced nystagmus in rabbits, flunarizine significantly decreased the amplitude of nystagmus at 2.5 mg/kg i.v., and cinnarizine and diphenidol inhibited nystagmus at 5 mg/kg, i.v. Flunarizine had no effect on nystagmus induced by electrical stimulation of the lateral geniculate body in rabbits at doses up to 5 mg/kg, i.v. Flunarizine increased the cochlear blood flow in anesthetized guinea pigs dose-dependently (0.312-1.25 mg/kg i.v.) On the other hand, cinnarizine (0.625-2.5 mg/kg i.v.) and diphenidol (0.625-2.5 mg/kg i.v.) increased cochlear blood flow, but the duration of action of both cinnarizine and diphenidol was shorter than that of flunarizine at the same dose. As stated above, flunarizine inhibited nystagmus experimentally induced by caloric or optokinetic stimuli. Increased cochlear blood flow suggested that the enhancement of vestibular blood flow might play an important role in the treatment of vestibular dysfunctions with this drug.