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Magnetic Resonance Imaging (MRI) and immunohistochemical investigations of CNS activity in a novel preclinical migraine model

Final Report Summary - MRIGRAINE (Magnetic Resonance Imaging (MRI) and immunohistochemical investigations of CNS activity in a novel preclinical migraine model)

Migraine pain is a major public health problem, both in Europe and also globally, and as such presents a significant priority for research. The World Health Organization (WHO) ranks migraine 19th among all causes of years lived with disability with 14.7% of adults in Europe suffering from migraine. The cost of migraine in Europe is estimated at €27 billion annually due to the high prevalence of the disorder and substantial associated social, direct, and indirect costs. Unfortunately many of the available treatments are not very effective and come with a plethora of side effects. The cause of migraine is highly complex, involving multiple regions of the brain, and we do not yet have a complete understanding of the mechanisms underlying the disease.
In this project we combined a clinically-relevant model of migraine with magnetic resonance imaging (MRI), to further identify brain areas involved in the generation of migraine and its symptoms. We also looked at how changes in brain activity matched with the development of migraine-like symptoms and the presence of specific indicators of altered nervous system function. We used a novel clinically-relevant model of migraine (consisting of continuous infusion of sumatriptan) that elicits maladaptations that induce a ‘migrainous state’, in combination with MRI and immuno¬histochemical analysis. Our data showed that 6 days of sumatriptan infusion resulted in a decrease in withdrawal thresholds to a light touch stimulus (defined as development of cutaneous allodynia) applied to the periorbital region or hind paws. This developed within 2 days of starting the infusion, peaked at 6 days, and resolved 12 days after stopping triptan infusion (day 18). To look for changes in brain activity we used MRI imaging. Animals underwent MRI scan at days 0, 6 and 18 of sumatriptan (or saline for controls) exposure, to investigate which and how brain regions show structural and/or functional changes associated with the development of, and recovery from, allodynia.
A series of preliminary investigations were carried out to optimise the protocol for the MRI studies, from these studies the following methodology was established. Under isoflurane anaesthesia, high-spatial resolution structural MRI (7 Tesla) relaxometry was used to measure the T1, T2 and T2* signal relaxation time constants, thus allowing assessment of both gross (volumetric based) and subtle (lesion based) neuronal changes in response to sustained infusion of sumatriptan. Standard echo planar imaging was used for fast (TR=1s) image acquisition of BOLD and arterial spin labelling (ASL) functional MRI (fMRI) data. The former was used to assess both changes in resting state connectivity between experimental and control groups and the effect on the magnitude and temporal dynamics of an evoked neuronal response as a novel biomarker of facial allodynia. The latter ASL data was used to normalise the fMRI data to any gross changes in baseline perfusion that could result from the model. A magnet-compatible natural whisker blow stimulator (air puff), or fibre optic light, were used to elicit a neuronal response to a mechanical or visual stimulus, and hence BOLD fMRI response. We found that on day 6 of sumatriptan infusion, the cerebral blood flow in the grey matter structures was significantly reduced when compared to control animals. Furthermore on day 6 after sumatriptan administration, in response to a mechanical stimulation (air puff to the whisker pad region), animals showed a distinct, constant and unusually slow oscillation in the BOLD fMRI signal (standard GE-EPI sequence) only in areas of superficial cortex and thalamus (associated with pain pathways). We hypothesise that this oscillation is related to altered nociceptive processing following the stimulus onset as it was not observed in control animals and its frequency was not linked to any physiological measure. Consistent with human data showing that migraineurs are hyperresponsive and prone to aberrant cortical activity, these data suggest that sumatriptan treated animals show increased activity in the cortex. Importantly, at day 18 post sumatriptan administration animals showed increased activity in response to either whisker pad or bright light stimulation, in deeper regions of the brain including the brainstem and thalamus, regions known to be affected in migraine patients. Critically, sumatriptan treated animals show long-term (at day 18) altered neuronal connectivity over saline treated animals, with increased correlation between the brainstem, the thalamus and the cortex (using the thalamus as the seed region).
Changes in expression of specific indicators of altered excitability were investigated by immunohistochemistry. We found increased expression of both pp38 and pERK in the trigeminal nucleus caudalis and the trigeminal ganglia of sumatriptan-treated animals at day 6 (a time point in which animals showed cutaneous allodynia), and importantly also at day 18 when sumatriptan was eliminated from the system and the animals showed a return to baseline withdrawal thresholds (recovery form cutaneous allodynia). These results demonstrate altered and prolonged central and peripheral changes, which persist many days after the cessation of sumatriptan infusion. Importantly we also found that in the trigeminal nucleus caudalis the increased expression peaks in a region that is know to receive input from the ophthalmic division of the trigeminal nerve, which innervates the dura mater; indicating that the expression of these biomarkers is somatotopically appropriate with increased input from dural afferents and migraine pain.
This study (focused on advancing our understanding of migraine pain and its underlying mechanisms) showed that fMRI changes in this rodent model of migraine are similar to those observed in humans. These changes are correlated with the development of increased pain perception. Additionally, exposure to sumatriptan resulted in increased connectivity between the cerebral cortex and brain regions relevant to pain pathways. While the development of new therapies may be some years away, the results of this study are very important in the field of migraine and pain research. The development of this non-invasive preclinical model of migraine provides a novel translational tool available to researchers to further our understanding of the mechanisms underlying the development of the disease and to assess novel therapeutics. These results are of considerable socio-economic relevance. This methodology will enable the investigation and development of new anti-migraine treatments with better efficacy and side effect profiles, with great potential to provide substantially improved quality of life for patients and economic benefits to Europe.