Involvement of nNOS in the antinociceptive activity of melatonin in inflammatory pain at the level of sensory neurons.

OBJECTIVE: The efficacy of melatonin as an analgesic agent has been well documented in animals and humans. However, the underlying mechanisms by which melatonin exerts antinociceptive effects on inflammatory pain are poorly understood. Here, we investigated the potential of melatonin to ameliorate inflammatory pain. MATERIALS AND METHODS: In vitro, ND7/23 neurons were treated with capsaicin. We used PCR and Western blot analyses to detect the expression of neuronal nitric oxide synthase (nNOS) in response to melatonin. Orofacial inflammatory pain was induced by 4% formalin administration on the right whisker pad of Sprague Dawley (SD) rats. The analgesic effect of melatonin was evaluated using mechanical threshold analyses. The expression level of nNOS in the trigeminal ganglion (TG) and trigeminal nucleus caudalis (Vc) neurons was assessed by RNAscope and immunohistochemistry. RESULTS: In vitro, capsaicin upregulated the expression of nNOS, which was dose-dependently reversed by melatonin pretreatment (p < 0.001). In a rat model of orofacial inflammatory pain, melatonin pretreatment significantly attenuated mechanical allodynia in both the acute and chronic phases (p < 0.05). Furthermore, melatonin decreased the formalin-evoked elevated nNOS mRNA and protein levels in the TG and Vc neurons in the acute and chronic phases (p < 0.05). CONCLUSIONS: Taken together, these results suggest that nNOS may play an active role in both peripheral and central processing of nociceptive information following orofacial inflammatory pain induction. The regulatory effect of melatonin on nNOS in inflammatory pain may have potential implications for the development of novel analgesic strategies.

Electro-acupuncture inhibits C-fiber-evoked WDR neuronal activity of the trigeminocervical complex: Neurophysiological hypothesis of a complementary therapy for acute migraine modeled rats.

INTRODUCTION: Acupuncture has become a relevant complementary and alternative treatment for acute migraine; however, the neurophysiological mechanism (C-fibers) underlying this effect remains unclear. C-fibers play a crucial role for diffuse noxious inhibitory controls (DNIC) at wide dynamic range (WDR) neurons in the trigeminocervical complex (TCC) in migraine attacks, and we supposed that this may be the mechanism of acupuncture analgesia. This study aimed to examine the neurophysiology of acupuncture intervention in an acute migraine rat model. METHODS: Inflammatory soup (IS) or saline was injected into the dura mater to establish a migraine and control model in rats. To explore the neurobiological mechanism of acupuncture for migraine, we implemented electro-acupuncture (EA), non-electric-stimulation acupuncture, and no-acupuncture in IS and saline injected rats, and recorded the single-cell extraneural neurophysiology of the atlas (C1) spinal dorsal horn neurons in the TCC. RESULTS: Our research shows that electro-acupuncture at GB8 (Shuaigu), located in the periorbital region receptive field of the trigeminal nerve, may rapidly reduce the C-fiber evoked WDR neuronal discharges of the TCC within 60 s. DISCUSSION: This study provides pioneering evidence of a potential neurobiological mechanism for the analgesic effect on migraine attacks achieved by electro-acupuncture intervention via DNIC. The data indicates that EA may become a crucial supplementary and alternative therapy for migraineurs that failed to respond to acute medications, e.g., fremanezumab, which achieves its analgesic effect via modulating Aσ-fibers, not C-fibers.

Chronic dry eye induced corneal hypersensitivity, neuroinflammatory responses, and synaptic plasticity in the mouse trigeminal brainstem.

BACKGROUND: Dry eye disease (DED) is a multifactorial disease associated with ocular surface inflammation, pain, and nerve abnormalities. We studied the peripheral and central neuroinflammatory responses that occur during persistent DED using molecular, cellular, behavioral, and electrophysiological approaches. METHODS: A mouse model of DED was obtained by unilateral excision of the extraorbital lachrymal gland (ELG) and Harderian gland (HG) of adult female C57BL/6 mice. In vivo tests were conducted at 7, 14, and 21 days (d) after surgery. Tear production was measured by a phenol red test and corneal alterations and inflammation were assessed by fluorescein staining and in vivo confocal microscopy. Corneal nerve morphology was evaluated by nerve staining. Mechanical corneal sensitivity was monitored using von Frey filaments. Multi-unit extracellular recording of ciliary nerve fiber activity was used to monitor spontaneous corneal nerve activity. RT-qPCR and immunostaining were used to determine RNA and protein levels at d21. RESULTS: We observed a marked reduction of tear production and the development of corneal inflammation at d7, d14, and d21 post-surgery in DED animals. Chronic DE induced a reduction of intraepithelial corneal nerve terminals. Behavioral and electrophysiological studies showed that the DED animals developed time-dependent mechanical corneal hypersensitivity accompanied by increased spontaneous ciliary nerve fiber electrical activity. Consistent with these findings, DED mice exhibited central presynaptic plasticity, demonstrated by a higher Piccolo immunoreactivity in the ipsilateral trigeminal brainstem sensory complex (TBSC). At d21 post-surgery, mRNA levels of pro-inflammatory (IL-6 and IL-1ß), astrocyte (GFAP), and oxidative (iNOS2 and NOX4) markers increased significantly in the ipsilateral trigeminal ganglion (TG). This correlated with an increase in Iba1, GFAP, and ATF3 immunostaining in the ipsilateral TG of DED animals. Furthermore, pro-inflammatory cytokines (IL-6, TNFα, IL-1ß, and CCL2), iNOS2, neuronal (ATF3 and FOS), and microglial (CD68 and Itgam) markers were also upregulated in the TBSC of DED animals at d21, along with increased immunoreactivity against GFAP and Iba1. CONCLUSIONS: Overall, these data highlight peripheral sensitization and neuroinflammatory responses that participate in the development and maintenance of dry eye-related pain. This model may be useful to identify new analgesic molecules to alleviate ocular pain.

Altered activity in the nucleus raphe magnus underlies cortical hyperexcitability and facilitates trigeminal nociception in a rat model of medication overuse headache.

BACKGROUND: The pathogenesis of medication overuse headache (MOH) involves hyperexcitability of cortical and trigeminal neurons. Derangement of the brainstem modulating system, especially raphe nuclei may contribute to this hyperexcitability. The present study aimed to investigate the involvement of the nucleus raphe magnus (NRM) in the development of cortical and trigeminal hyperexcitability in a rat model of MOH. RESULTS: Chronic treatment with acetaminophen increased the frequency of cortical spreading depression (CSD) and the number of c-Fos-immunoreactive (Fos-IR) neurons in the trigeminal nucleus caudalis (TNC). In the control group, muscimol microinjected into the NRM increased significantly the frequency of CSD-evoked direct current shift and Fos-IR neurons in the TNC. This facilitating effect was not found in rats with chronic acetaminophen exposure. In a model of migraine induced by intravenous systemic infusion of nitroglycerin (NTG), rats with chronic exposure to acetaminophen exhibited significantly more frequent neuronal firing in the TNC and greater Fos-IR than those without the acetaminophen treatment. Muscimol microinjection increased neuronal firing in the TNC in control rats, but not in acetaminophen-treated rats. The number of Fos-IR cells in TNC was not changed significantly. CONCLUSION: Chronic exposure to acetaminophen alters the function of the NRM contributing to cortical hyperexcitability and facilitating trigeminal nociception.

Effect of Paeonia lactiflora, a traditional Chinese herb, on migraines based on clinical application and animal behavior analyses.

BACKGROUND: Paeonia lactiflora (PL) was widely used for pain relief, but its effects on migraine headaches remain unclear. PURPOSE: The aim of the present study was to investigate the effects of PL on migraine headaches. METHODS: First, we found that PL was frequently used in Taiwan for headache treatment based on data from Taiwan's National Health Insurance Research Database. Migraine was induced through the intraperitoneal injection (i.p.) of nitroglycerin (NTG, 10 mg/kg) in rats. Pretreatment with PL was administered orally 30 min prior to the NTG i.p. Migraine headache behavior was observed by video-recordings. Finally, the rats were sacrificed and brain was removed for immunohistochemistry staining analysis. RESULTS: The frequency and total time spent rearing up and sniffing in exploratory behavior, and walking in locomotor behavior, were reduced in the NTG group compared with the control group (all p <  0.001). This reduction could be ameliorated by pretreatment with PL 1.0 g/kg (all p <  0.05). Total time spent in the light chamber was lower in the NTG group compared with the control group (p <  0.05); this could be ameliorated by pretreatment with 1.0 g/kg PL (p <  0.05). The rats in the NTG group spent longer time on the smooth surface than those in the control group (p <  0.001); this could be shortened by pretreatment with 0.5 and 1.0 g/kg PL (both p <  0.01). The traveling distance of rats in the NTG group was shorter than in the control group (p <  0.001); rats given 1.0 g/kg PL had a longer traveling distance than those in the NTG group (p <  0.01). Both c-fos and CGRP immunoreactive cells increased in the TNC in the NTG group compared with that of the control group (both p <  0.001); this increased could be reduced by pretreatment with PL 0.5 and 1.0 g/kg (both p <  0.05). CONCLUSION: Pretreatment with PL ameliorated migraine headache behaviors in the NTG-induced migraine rat model, suggesting pretreatment with PL is beneficial for migraine headache treatment. This effect of PL is related to the decrease of c-fos and CGRP in the TNC. However, still there are too many methodological limitations which need to be overcome in further experiments to support the data.

Advanced visual network and cerebellar hyperresponsiveness to trigeminal nociception in migraine with aura.

BACKGROUND: Despite the growing body of advanced studies investigating the neuronal correlates of pain processing in patients with migraine without aura (MwoA), only few similar studies have been conducted in patients with migraine with aura (MwA). Therefore, we aimed to explore the functional brain response to trigeminal noxious heat stimulation in patients with MwA. METHODS: Seventeen patients with MwA and 15 age- and sex-matched healthy controls (HC) underwent whole-brain blood oxygen level-dependent (BOLD) fMRI during trigeminal noxious heat stimulation. To examine the specificity of any observed differences between patients with MwA and HC, the functional response of neural pathways to trigeminal noxious heat stimulation in patients with MwA was compared with 18 patients with MwoA. Secondary analyses investigated the correlations between BOLD signal changes and clinical parameters of migraine severity. RESULTS: We observed a robust cortical and subcortical pattern of BOLD response to trigeminal noxious heat stimulation across all participants. Patients with MwA showed a significantly increased activity in higher cortical areas known to be part of a distributed network involved in advanced visual processing, including lingual gyrus, inferior parietal lobule, inferior frontal gyrus and medial frontal gyrus. Moreover, a significantly greater cerebellar activation was observed in patients with MwA when compared with both patients with MwA and HC. Interestingly, no correlations were found between migraine severity parameters and magnitude of BOLD response in patients with MwA. CONCLUSION: Our findings, characterized by abnormal visual pathway response to trigeminal noxious heat stimulation, support the role of a functional integration between visual and trigeminal pain networks in the pathophysiological mechanisms underlying migraine with aura. Moreover, they expand the concept of "neurolimbic-pain network" as a model of MwoA including both limbic dysfunction and cortical dys-excitability. Indeed, we suggest a model of "neurolimbic-visual-pain network" in MwA patients, characterized by dysfunctional correlations between pain-modulating circuits not only with the cortical limbic areas but with advanced visual areas as well. Furthermore, the abnormal cerebellar response to trigeminal noxious heat stimulation may suggest a dysfunctional cerebellar inhibitory control on thalamic sensory gating, impinging on the advanced visual processing cortical areas in patients with MwA.

Brain structure and function related to headache: Brainstem structure and function in headache.

OBJECTIVE: To review and discuss the literature relevant to the role of brainstem structure and function in headache. BACKGROUND: Primary headache disorders, such as migraine and cluster headache, are considered disorders of the brain. As well as head-related pain, these headache disorders are also associated with other neurological symptoms, such as those related to sensory, homeostatic, autonomic, cognitive and affective processing that can all occur before, during or even after headache has ceased. Many imaging studies demonstrate activation in brainstem areas that appear specifically associated with headache disorders, especially migraine, which may be related to the mechanisms of many of these symptoms. This is further supported by preclinical studies, which demonstrate that modulation of specific brainstem nuclei alters sensory processing relevant to these symptoms, including headache, cranial autonomic responses and homeostatic mechanisms. REVIEW FOCUS: This review will specifically focus on the role of brainstem structures relevant to primary headaches, including medullary, pontine, and midbrain, and describe their functional role and how they relate to mechanisms of primary headaches, especially migraine.

Restoration of the nasopharyngeal response after bilateral sectioning of the anterior ethmoidal nerve in the rat.

In response to stimulation of the nasal passages with volatile ammonia vapors, the nasopharyngeal reflex produces parasympathetically mediated bradycardia, sympathetically mediated increased peripheral vascular tone, and apnea. The anterior ethmoidal nerve (AEN), which innervates the anterior nasal mucosa, is thought to be primarily responsible for providing the sensory afferent signals that initiate these protective reflexes, as bilateral sectioning causes an attenuation of this response. However, recent evidence has shown cardiovascular responses to nasal stimulation with ammonia vapors are fully intact 9 days after bilateral AEN sectioning, and are similar to control animals without bilaterally sectioned AENs. To investigate this restoration of the nasopharyngeal response, we recorded the cardiorespiratory responses to nasal stimulation with ammonia vapors immediately after, and 3 and 9 days after, bilateral AEN sectioning. We also processed brainstem tissue for Fos to determine how the restoration of the nasopharyngeal response would affect the activity of neurons in the medullary dorsal horn (MDH), the part of the ventral spinal trigeminal nucleus caudalis region that receives primary afferent signals from the nose and nasal passages. We found 3 days after bilateral AEN sectioning the cardiorespiratory responses to nasal stimulation are partially restored. The bradycardic response to nasal stimulation is significantly more intense 3 days after AEN sectioning compared to Acute AEN sectioning. Surprisingly, 3 days after AEN sectioning the number of Fos-positive neurons within MDH decreased, even though the cardiorespiratory responses to nasal stimulation intensified. Collectively these findings indicate that, besides the AEN, there are alternate sensory pathways that can activate neurons within the trigeminal nucleus in response to nasal stimulation. The findings further suggest trigeminal neuronal plasticity involving these alternate sensory pathways occurs in as few as 3 days after bilateral AEN sectioning. Finally, activation of even a significantly reduced number of MDH neurons is sufficient to initiate the nasopharyngeal response.

Protein Kinase C γ Contributes to Central Sensitization in a Rat Model of Chronic Migraine.

Protein kinase C γ (PKCγ) is a critical regulator of central sensitization and is widely recognized to be involved in the pathogenesis of chronic migraine (CM). However, the function of PKCγ in CM remains unknown. This study investigated the role of PKCγ on pathogenesis of CM. We repeated infusions of inflammatory soup (IS) on the intact dura of conscious rats to model recurrent trigeminovascular or dural nociceptor activation assumed to occur in patients with CM. The von Frey test was then used to detect changes in pain threshold. QT-PCR, western blotting, and double immunofluorescence staining were performed to detect the expression and location of PKCγ in the trigeminal nucleus caudalis (TNC) and the expressions of calcitonin gene-related peptide (CGRP), c-Fos, and phosphorylation level of GluR1 subunit at serine 831. Chelerythrine chloride (CHE) and phorbol 12-myristate 13-acetate (PMA) were administrated to investigate the role of PKCγ in central sensitization. We found that repeated infusions of IS induced mechanical allodynia. PKCγ was significantly increased in TNC after CM. Furthermore, inhibition of PKCγ by CHE relieved allodynia and reduced the expression of CGRP and c-Fos. Activation of PKCγ by PMA aggravated allodynia and increased the expression of CGRP and c-Fos. In addition, inhibition of PKCγ reduced the phosphorylation level of GluR1; in contrast, activation of PKCγ increased the phosphorylation level of GluR1. These results suggest PKCγ-induced GluR1 phosphorylation might participate in central sensitization in a rat model of CM. We suggest that PKCγ is a potential therapeutic target for CM.

Frequency-Dependent Habituation Deficit of the Nociceptive Blink Reflex in Aura With Migraine Headache. Can Migraine Aura Modulate Trigeminal Excitability?

OBJECTIVE: To study the influence of the migraine aura on the trigeminal nociception, we investigated the habituation of the nociceptive blink reflex (nBR) R2 responses in aura with migraine headache (AwMH) and comparatively in migraine without aura (MWoA) and healthy subjects (HS). BACKGROUND: A clear deficit of habituation in trigeminal nociceptive responses has been documented in MWoA; however, similar data in MWA are lacking. METHODS: Seventeen AwMH, 29 MWoA, and 30 HS were enrolled and a nonrandomized clinical neurophysiological study examining nBR habituation by clinical diagnosis was devised. We delivered a series of 26 electrical stimuli, at different stimulation frequencies (SF) (0.05, 0.1, 0.2, 0.3, 0.5, and 1 Hz), subsequently subdivided in five blocks of five responses for each SF. The mean area values of the second to the fifth block expressed as the percentage of the mean area value of the first block were taken as an index of habituation for each SF. RESULTS: A significant lower mean percentage decrease of the R2 area across all blocks was found at 1, 0.5, 0.3, and 0.2 Hz SF in MWoA and at 0.3 and 0.2 Hz SF in AwMH, when compared to HS. In the most representative fifth block of responses, we found in MWoA vs HS at 1 Hz, 57.0 ± 27.8 vs 30.6 ± 12.0; at 0.5 Hz, 54.8 ± 26.1 vs 32.51 ± 17.7; at 0.3 Hz, 44.7 ± 21.6 vs 27.6 ± 13.2; at 0.2 Hz, 61.3 ± 29.5 vs 32.6 ± 18.0, and in AwMH vs HS at 0.3 Hz, 52.7 ± 24.7 vs 27.6 ± 13.2; at 0.2 Hz, 69.3 ± 38.6 vs 32.6 ± 18.0 as mean ± SD of the R2 area percentage of the first block, respectively. Interestingly, AwMH subjects did not show differences in mean percentage decrease of the R2 area at 1 and 0.5 Hz SF when compared to HS. No differences between groups were found at 0.1 and 0.05 Hz SF. CONCLUSIONS: We demonstrated in AwMH a deficit of habituation of the nBR R2 responses after repeated stimulations, although less pronounced than that observed in MWoA of comparable clinical severity. We hypothesize that AwMH and MWoA share some pathogenetic aspects, and also that migraine aura physiopathology may play a modulating role on the excitability of the nociceptive trigeminal pathways.

Afferent Fiber Remodeling in the Somatosensory Thalamus of Mice as a Neural Basis of Somatotopic Reorganization in the Brain and Ectopic Mechanical Hypersensitivity after Peripheral Sensory Nerve Injury.

Plastic changes in the CNS in response to peripheral sensory nerve injury are a series of complex processes, ranging from local circuit remodeling to somatotopic reorganization. However, the link between circuit remodeling and somatotopic reorganization remains unclear. We have previously reported that transection of the primary whisker sensory nerve causes the abnormal rewiring of lemniscal fibers (sensory afferents) on a neuron in the mouse whisker sensory thalamus (V2 VPM). In the present study, using transgenic mice whose lemniscal fibers originate from the whisker sensory principle trigeminal nucleus (PrV2) are specifically labeled, we identified that the transection induced retraction of PrV2-originating lemniscal fibers and invasion of those not originating from PrV2 in the V2 VPM. This anatomical remodeling with somatotopic reorganization was highly correlated with the rewiring of lemniscal fibers. Origins of the non-PrV2-origin lemniscal fibers in the V2 VPM included the mandibular subregion of trigeminal nuclei and the dorsal column nuclei (DCNs), which normally represent body parts other than whiskers. The transection also resulted in ectopic receptive fields of V2 VPM neurons and extraterritorial pain behavior on the uninjured mandibular region of the face. The anatomical remodeling, emergence of ectopic receptive fields, and extraterritorial pain behavior all concomitantly developed within a week and lasted more than three months after the transection. Our findings, thus, indicate a strong linkage between these plastic changes after peripheral sensory nerve injury, which may provide a neural circuit basis underlying large-scale reorganization of somatotopic representation and abnormal ectopic sensations.

Vagus nerve stimulation suppresses acute noxious activation of trigeminocervical neurons in animal models of primary headache.

Vagus nerve stimulation (VNS) has been reported to be effective in the abortive treatment of both migraine and cluster headache. Using validated animal models of acute dural-intracranial (migraine-like) and trigeminal-autonomic (cluster-like) head pain we tested whether VNS suppresses ongoing and nociceptive-evoked firing of trigeminocervical neurons to explain its abortive effects in migraine and cluster headache. Unilateral VNS was applied invasively via hook electrodes placed on the vagus nerve. A single dose of ipsilateral or contralateral VNS, to trigeminal recording and dural-stimulating side, suppressed ongoing spontaneous and noxious dural-evoked trigeminocervical neuronal firing. This effect was dose-dependent, with two doses of ipsilateral VNS prolonging suppression of ongoing spontaneous firing (maximally by ~60%) for up to three hours, and dural-evoked (Aδ-fiber; by ~22%, C-fiber: by ~55%) responses for at least two hours. Statistically, there was no difference between ipsilateral and contralateral groups. Two doses of VNS also suppressed superior salivatory nucleus-evoked trigeminocervical neuronal responses (maximally by ~22%) for 2.5h, to model nociceptive activation of the trigeminal-autonomic pathway. VNS had no effect on normal somatosensory cutaneous facial responses throughout. These studies provide a mechanistic rationale for the observed benefits of VNS in the abortive treatment of migraine and cluster headache. In addition, they further validate these preclinical models as suitable approaches to optimize therapeutic efficacy, and provide an opportunity to hypothesize and dissect the neurobiological mechanisms of VNS in the treatment of primary headaches.

The effects of unilateral lesion of the tuberomammillary nucleus E2 sub-region on nocturnal feeding and related behaviors in mice.

AIMS: Bilateral lesions of the mesencephalic trigeminal sensory nucleus (Me5), which receives histaminergic neurons from the tuberomammillary nucleus (TMN), alter nocturnal feeding and related behaviors in mice, concomitant with a decrease in orexin mRNA level in the perifornical area (PFA) during the dark phase. Therefore, we investigated the neuronal input to the TMN from the Me5, as well as the effects of TMN lesions on the circadian profiles of feeding and related behaviors. MAIN METHODS: We examined the presence of neurons projecting from the Me5 to the TMN by direct injection of a retrograde tracer, Fluorogold, into the TMN E2 sub-region (TMN-E2). We also assessed feeding, drinking, and locomotion for 24h using an automated feeding behavior measurement apparatus, and analyzed the hypothalamic orexin mRNA levels in both TMN-lesion and sham-operated mice. KEY FINDINGS: The presence of neuronal projections from the Me5 to the TMN-E2 was confirmed. A decrease in food and water intake and locomotion during the latter half of the dark phase was delayed in TMN-lesion but not sham-operation mice. Further, orexin mRNA expression levels were higher in both the PFA and lateral hypothalamus area (LHA) in TMN-E2-lesion mice relative to control mice, during the early half of the dark phase compared with the light phase. SIGNIFICANCE: Our results suggest that histaminergic neurons in the TMN-E2 receive signals from the Me5 that modulate a switch from dark to light phase feeding and related behaviors, which in turn may be regulated by orexin neurons in the PFA and/or LHA.

The Role of the Trigemino Cervical Complex in Chronic Whiplash Associated Headache: A Cross Sectional Study.

OBJECTIVE: To investigate signs of central sensitization in a cohort of patients with chronic whiplash associated headache (CWAH). BACKGROUND: Central sensitization is one of the mechanisms leading to chronicity of primary headache, and thus might contribute to CWAH. However, the pathophysiological mechanism of CWAH is poorly understood and whether it is simply an expression of the primary headache or has a distinct pathogenesis remains unclear. Thus, the factors involved in the genesis of CWAH require further investigation. METHODS: Twenty-two patients with CWAH (20 females, 2 males; age 25-50 years, mean age 36.3 years) and 25 asymptomatic participants (13 females, 12 males; age 18-50 years, mean age 35.6 years) rated glare and light-induced discomfort in response to light from an ophthalmoscope. Hyperalgesia evoked by a pressure algometer was assessed bilaterally on the forehead, temples, occipital base, and the middle phalanx of the third finger. The number, latency, area under the curve, and recovery cycle of nociceptive blink reflexes elicited by a supraorbital electrical stimulus were also recorded. RESULTS: Eight and 6 CWAH patients had migrainous and tension-type headache (TTH) profiles, respectively; the remainder had features attributable to both migraine and TTH. Patients in the whiplash group reported significantly greater light-induced pain than controls (8.48 ± .35 vs 6.66 ± .43 on a 0-10 scale; P = .001). The CWAH patients reported significantly lower pressure pain thresholds at all sites. For stimuli delivered at 20 second intervals, whiplash patients were more responsive than controls (4.8 ± .6 blinks vs 3.0 ± .6 blinks in a block of 10 stimuli; P = .036). While R2 latencies and the area under the curve for the 20 second interval trials were comparable in both groups, there was a significant reduction of the area under the curve from the first to the second of the 2-second interval trials only in controls (99 ± 8% of baseline in whiplash patients vs 68 ± 7% in controls; P = .009). The recovery cycle was comparable for both groups. CONCLUSIONS: Our results corroborate previous findings of mechanical hypersensitivity and photophobia in CWAH patients. The neurophysiological data provide further evidence for hyperexcitability in central nociceptive pathways, and endorse the hypothesis that CWAH may be driven by central sensitization.

Physiological brainstem mechanisms of trigeminal nociception: An fMRI study at 3T.

The brainstem is a major site of processing and modulation of nociceptive input and plays a key role in the pathophysiology of various headache disorders. However, human imaging studies on brainstem function following trigeminal nociceptive stimulation are scarce as brainstem specific imaging approaches have to address multiple challenges such as magnetic field inhomogeneities and an enhanced level of physiological noise. In this study we used a viable protocol for brainstem fMRI of standardized trigeminal nociceptive stimulation to achieve detailed insight into physiological brainstem mechanisms of trigeminal nociception. We conducted a study of 21 healthy participants using a nociceptive ammonia stimulation of the left nasal mucosa with an optimized MR acquisition protocol for high resolution brainstem echoplanar imaging in combination with two different noise correction techniques. Significant BOLD responses to noxious ammonia stimulation were observed in areas typically involved in trigeminal nociceptive processing such as the spinal trigeminal nuclei (sTN), thalamus, secondary somatosensory cortex, insular cortex and cerebellum as well as in a pain modulating network including the periaqueductal gray area, hypothalamus (HT), locus coeruleus and cuneiform nucleus (CNF). Activations of the left CNF were positively correlated with pain intensity ratings. Employing psychophysiological interaction (PPI) analysis we found enhanced functional connectivity of the sTN with the contralateral sTN and HT following trigeminal nociception. We also observed enhanced functional connectivity of the CNF with the RVM during painful stimulation thus implying an important role of these two brainstem regions in central pain processing. The chosen approach to study trigeminal nociception with high-resolution fMRI offers new insight into human pain processing and might thus lead to a better understanding of headache pathophysiology.

Neonatal sensory nerve injury-induced synaptic plasticity in the trigeminal principal sensory nucleus.

Sensory deprivation studies in neonatal mammals, such as monocular eye closure, whisker trimming, and chemical blockade of the olfactory epithelium have revealed the importance of sensory inputs in brain wiring during distinct critical periods. But very few studies have paid attention to the effects of neonatal peripheral sensory nerve damage on synaptic wiring of the central nervous system (CNS) circuits. Peripheral somatosensory nerves differ from other special sensory afferents in that they are more prone to crush or severance because of their locations in the body. Unlike the visual and auditory afferents, these nerves show regenerative capabilities after damage. Uniquely, damage to a somatosensory peripheral nerve does not only block activity incoming from the sensory receptors but also mediates injury-induced neuro- and glial chemical signals to the brain through the uninjured central axons of the primary sensory neurons. These chemical signals can have both far more and longer lasting effects than sensory blockade alone. Here we review studies which focus on the consequences of neonatal peripheral sensory nerve damage in the principal sensory nucleus of the brainstem trigeminal complex.

Inhibition of FAAH reduces nitroglycerin-induced migraine-like pain and trigeminal neuronal hyperactivity in mice.

There is evidence to suggest that a dysregulation of endocannabinoid signaling may contribute to the etiology and pathophysiology of migraine. Thus, patients suffering from chronic migraine or medication overuse headache showed alterations in the activity of the arachidonoylethanolamide (AEA) degrading enzyme fatty acid amide hydrolase (FAAH) and a specific AEA membrane transporter, alongside with changes in AEA levels. The precise role of different endocannabinoid system components is, however, not clear. We have therefore investigated mice with a genetic deletion of the two main cannabinoid receptors CB1 and CB2, or the main endocannabinoid degrading enzymes, FAAH and monoacylglycerol lipase (MAGL), which degrades 2-arachidonoylglycerol (2-AG), in a nitroglycerine-induced animal model of migraine. We found that nitroglycerin-induced mechanical allodynia and neuronal activation of the trigeminal nucleus were completely abolished in FAAH-deficient mice. To validate these results, we used two structurally different FAAH inhibitors, URB597 and PF3945. Both inhibitors also dose-dependently blocked nitroglycerin-induced hyperalgesia and the activation of trigeminal neurons. The effects of the genetic deletion of pharmacological blockade of FAAH are mediated by CB1 receptors, because they were completely disrupted with the CB1 antagonist rimonabant. These results identify FAAH as a target for migraine pharmacotherapy.

Animal models of chronic migraine.

Many animal models of migraine have been described. Some of them have been useful in the development of new therapies. All of them have their shortcomings. Animal models of chronic migraine have been relatively less frequently described. Whether a rigid distinction between episodic and chronic migraine is useful when their underlying pathophysiology is likely to be the same and that migraine frequency probably depends on complex polygenic influences remains to be determined. Any model of chronic migraine must reflect the chronicity of the disorder and be reliable and validated with pharmacological interventions. Future animal models of chronic migraine are likely to involve recurrent activation of the trigeminal nociceptive system. Valid models would provide a means for investigating pathophysiological mechanism of the transformation from episodic to chronic migraine and may also be used to test the efficacy of potential preventive medications.

Central effects of acetylsalicylic acid on trigeminal-nociceptive stimuli.

BACKGROUND: Acetylsalicylic acid is one of the most used analgesics to treat an acute migraine attack. Next to the inhibitory effects on peripheral prostaglandin synthesis, central mechanisms of action have also been discussed. METHODS: Using a standardized model for trigeminal-nociceptive stimulation during fMRI scanning, we investigated the effect of acetylsalicylic acid on acute pain compared to saline in 22 healthy volunteers in a double-blind within-subject design. Painful stimulation was applied using gaseous ammonia and presented in a pseudo-randomized order with several control stimuli. All participants were instructed to rate the intensity and unpleasantness of every stimulus on a VAS scale. Based on previous results, we hypothesized to find an effect of ASA on central pain processing structures like the ACC, SI and SII as well as the trigeminal nuclei and the hypothalamus. RESULTS: Even though we did not find any differences in pain ratings between saline and ASA, we observed decreased BOLD signal changes in response to trigemino-nociceptive stimulation in the ACC and SII after administration of ASA compared to saline. This finding is in line with earlier imaging results investigating the effect of ASA on acute pain. Contrary to earlier findings from animal studies, we could not find an effect of ASA on the trigeminal nuclei in the brainstem or within the hypothalamic area. CONCLUSION: Taken together our study replicates earlier findings of an attenuating effect of ASA on pain processing structures, which adds further evidence to a possibly central mechanism of action of ASA.