Properties of neurons in the trigeminal nucleus caudalis responding to noxious dural and facial stimulation

Can upper cervical manual therapy affect the blink reflex in subjects with migraine and neck pain?

BACKGROUND

Neck pain is a common complaint among migraineurs possibly due to the anatomic connections between cervical and trigeminal afferents in the trigeminocervical complex (TCC). Manual therapy (MT) is used in the management of headache disorders, with demonstrable neurophysiological effects. The blink reflex (BR) is one method of analyzing neurophysiological effects in headache patients. The purpose of this study was to investigate the effect of upper cervical spine MT on BR in subjects with migraine and neck pain.

METHODS & MATERIALS

Twenty subjects were assigned to a medication plus MT (MedMT) group (n = 10) and medication plus sham MT (sham MT) group (n = 10). After random assignment, all patients underwent testing for the BR (R1, R2, R2c responses). Then, subjects in group MedMT and group sham MT received either 4 sessions of MT or sham MT to the upper cervical spine. After completion of the intervention, BR testing was repeated.

RESULTS

There were no significant differences in both side R1 latency between group MT and group sham MT (P > 0.050). For both sides, R2 latencies were significantly prolonged in MedMT group compared with sham MT group (P < 0.050). Subjects in MedMT group showed significant prolongation in right and left R2c latency compared with sham MT group (P < 0.050).

DISCUSSION

The present study demonstrated that upper cervical MT affected trigeminal nociceptive neurotransmission in subjects with migraine and neck pain as reflected by changes in the BR. The increase in BR late response latencies of BR indicates an inhibitory effect of upper cervical spine MT on the TCC in these subjects. Trial Registration: The trial design was registered at the Iranian Registry of Clinical Trials (IRCT ID: IRCT20160621028567N2, url: https://www.irct.ir/) before the first patient was enrolled.

The cervical spine in tension type headache

INTRODUCTION

The concept that headaches may originate in the cervical spine has been discussed over decades and is still a matter of debate. The cervical spine has been traditionally linked to cervicogenic headache; however, current evidence supports the presence of cervical musculoskeletal dysfunctions also in tension-type headache.

PURPOSE

This position paper discusses the most updated clinical and evidence-based data about the cervical spine in tension-type headache.

IMPLICATIONS

Subjects with tension-type headache exhibit concomitant neck pain, cervical spine sensitivity, forward head posture, limited cervical range of motion, positive flexion-rotation test and also cervical motor control disturbances. In addition, the referred pain elicited by manual examination of the upper cervical joints and muscle trigger points reproduces the pain pattern in tension-type headache. Current data supports that the cervical spine can be also involved in tension-type headache, and not just in cervicogenic headache. Several physical therapies including upper cervical spine mobilization or manipulation, soft tissue interventions (including dry needling) and exercises targeting the cervical spine are proposed for managing tension-type headache; however, the effectiveness of these interventions depends on a proper clinical reasoning since not all will be equally effective for all individuals with tension-type headache. Based on current evidence, we propose to use the terms cervical "component" and cervical "source" when discussing about headache. In such a scenario, in cervicogenic headache the neck can be the cause (source) of the headache whereas in tension-type headache the neck will have a component on the pain pattern, but it will be not the cause since it is a primary headache.

Migraine headache pathophysiology

In both episodic and chronic migraine, headache is the most disabling symptom that requires medical care. The migraine headache is the most well-studied symptom of migraine pathophysiology. The trigeminal system and the central processing of sensory information transmitted by the trigeminal system are of considerable importance in the pathophysiology of migraine headache. Glutamate is the main neurotransmitter that drives activation of the ascending trigeminal and trigeminothalamic pathways. The neuropeptide, calcitonin gene-related peptide (CGRP) that is released by the trigeminal system, plays a crucial role in the neurobiology of headache. Peripheral and central sensitizations associated with trigeminal sensory processing are neurobiologic states that contribute to both the development of headache during a migraine attack and the maintenance of chronic migraine.

Different processing of meningeal and cutaneous pain information in the spinal trigeminal nucleus caudalis

Introduction

Within superficial trigeminal nucleus caudalis (Sp5C) (laminae I/II), meningeal primary afferents project exclusively to lamina I, whereas nociceptive cutaneous ones distribute in both lamina I and outer lamina II. Whether such a relative absence of meningeal inputs to lamina II represents a fundamental difference from cutaneous pathways in the central processing of sensory information is still unknown.

Methods

We recorded extracellular field potentials in the superficial Sp5C of anesthetised rats evoked by electrically stimulating the dura mater, to selectively assess the synaptic transmission between meningeal primary afferents and second-order Sp5C neurons, the first synapse in trigeminovascular pathways. We tested the effect of systemic morphine and local glycinergic and GABAAergic disinhibition.

Results

Meningeal stimulation evokes two negative field potentials in superficial Sp5C. The conduction velocities of the activated primary afferents are within the Aδ- and C-fibre ranges. Systemic morphine specifically suppresses meningeal C-fibre-evoked field potentials, and this effect is reversed by systemic naloxone. Segmental glycinergic or GABAAergic disinhibition strongly potentiates meningeal C-fibre-evoked field potentials but not Aδ-fibre ones. Interestingly, the same segmental disinhibition conversely potentiates cutaneous Aδ-fibre-evoked field potentials and suppresses C-fibre ones.

Conclusion

These findings reveal that the different anatomical organization of meningeal and cutaneous inputs into superficial Sp5C is associated with a different central processing of meningeal and cutaneous pain information within Sp5C. Moreover, they suggest that the potentiation upon local disinhibition of the first synapse in trigeminovascular pathways may contribute to the generation of headache pain.

Meningeal afferent signaling and the pathophysiology of migraine

Migraine is the most common neurological disorder. Attacks are complex and consist of multiple phases but are most commonly characterized by intense, unilateral, throbbing headache. The pathophysiology contributing to migraine is poorly understood and the disorder is not well managed with currently available therapeutics, often rendering patients disabled during attacks. The mechanisms most likely to contribute to the pain phase of migraine require activation of trigeminal afferent signaling from the cranial meninges and subsequent relay of nociceptive information into the central nervous system in a region of the dorsal brainstem known as the trigeminal nucleus caudalis. Events leading to activation of meningeal afferents are unclear, but nerve endings within this tissue are mechanosensitive and also express a variety of ion channels including acid-sensing ion channels and transient receptor-potential channels. These properties may provide clues into the pathophysiology of migraine by suggesting that decreased extracellular pH and environmental irritant exposure in the meninges contributes to headache. Neuroplasticity is also likely to play a role in migraine given that attacks are triggered by routine events that are typically nonnoxious in healthy patients and clear evidence of sensitization occurs during an attack. Where and how plasticity develops is also not clear but may include events directly on the afferents and/or within the TNC. Among the mediators potentially contributing to plasticity, calcitonin gene-related peptide has received the most attention within the migraine field but other mechanisms may also contribute. Ultimately, greater understanding of the molecules and mechanisms contributing to migraine will undoubtedly lead to better therapeutics and relief for the large number of patients across the globe who suffer from this highly disabling neurological disorder.

Intravenous dextromethorphan/quinidine inhibits activity of dura-sensitive spinal trigeminal neurons in rats

BACKGROUND

Migraine is a chronic neurological disorder characterized by episodes of throbbing headaches. Practically all medications currently used in migraine prophylaxis have a number of substantial disadvantages and use limitations. Therefore, the further search for principally new prophylactic antimigraine agents remains an important task. The objective of our study was to evaluate the effects of a fixed combination of dextromethorphan hydrobromide and quinidine sulphate (DM/Q) on activity of the spinal trigeminal neurons in an electrophysiological model of trigemino-durovascular nociception.

METHODS

The study was performed in 15 male Wistar rats, which were anaesthetized with urethane/α-chloralose and paralysed using pipecuronium bromide. The effects of cumulative intravenous infusions of DM/Q (three steps performed 30 min apart, 15/7.5 mg/kg of DM/Q in 0.5 mL of isotonic saline per step) on ongoing and dural electrical stimulation-induced neuronal activities were tested in a group of eight rats over 90 min. Other seven animals received cumulative infusion of equal volumes of saline and served as control.

RESULTS

Cumulative administration of DM/Q produced steady suppression of both the ongoing activity of the spinal trigeminal neurons and their responses to electrical stimulation of the dura mater.

CONCLUSIONS

It is evident that the observed DM/Q-induced suppression of trigeminal neuron excitability can lead to a reduction in nociceptive transmission from meninges to higher centres of the brain. Since the same mechanism is believed to underlie the pharmacodynamics of many well-known antimigraine drugs, results of the present study enable us to anticipate the potential efficacy of DM/Q in migraine.

Therapeutic options for cervicogenic headache

The term cervicogenic headache (CeH) describes a syndrome originating from the cervical spine. There are a variety of therapeutic approaches used for the management of CeH, but scientific evidence of their effectiveness is scarce. No medication drug has proven to be effective. The evidence for greater occipital nerve blocks, cervical nerve blockades, facet joint injections and surgical procedures is limited. Several physical therapy interventions are proposed for CeH, with spinal manipulation and soft tissue interventions being the most commonly used. However, the lack of solid evidence of positive effects and risks of serious complications for spinal manipulation should be considered in favor of other physical therapy interventions associated with less risk. The inconsistent results in the literature can be related to the fact that maybe not all therapeutic interventions are appropriate for all patients with CeH or maybe not all patients with CeH will benefit from particular interventions.

Paraventricular hypothalamic regulation of trigeminovascular mechanisms involved in headaches

While functional imaging and deep brain stimulation studies point to a pivotal role of the hypothalamus in the pathophysiology of migraine and trigeminal autonomic cephalalgias, the circuitry and the mechanisms underlying the modulation of medullary trigeminovascular (Sp5C) neurons have not been fully identified. We investigated the existence of a direct anatomo-functional relationship between hypothalamic excitability disturbances and modifications of the activities of Sp5C neurons in the rat. Anterograde and retrograde neuronal anatomical tracing, intrahypothalamic microinjections, extracellular single-unit recordings of Sp5C neurons, and behavioral trials were used in this study. We found that neurons of the paraventricular nucleus of the hypothalamus (PVN) send descending projections to the superior salivatory nucleus, a region that gives rise to parasympathetic outflow to cephalic and ocular/nasal structures. PVN cells project also to laminae I and outer II of the Sp5C. Microinjections of the GABAA agonist muscimol into PVN inhibit both basal and meningeal-evoked activities of Sp5C neurons. Such inhibitions were reduced in acutely restrained stressed rats. GABAA antagonist gabazine infusions into the PVN facilitate meningeal-evoked responses of Sp5C neurons. PVN injections of the neuropeptide pituitary adenylate cyclase activating peptide (PACAP38) enhance Sp5C basal activities, whereas the antagonist PACAP6-38 depresses all types of Sp5C activities. 5-HT1B/D receptor agonist naratriptan infusion confined to the PVN depresses both basal and meningeal-evoked Sp5C activities. Our findings suggest that paraventricular hypothalamic neurons directly control both spontaneous and evoked activities of Sp5C neurons and could act either as modulators or triggers of migraine and/or trigeminal autonomic cephalalgias by integrating nociceptive, autonomic, and stress processing mechanisms.

Spinal trigeminal neurons demonstrate an increase in responses to dural electrical stimulation in the orofacial formalin test

Primary headaches are often associated with pain in the maxillofacial region commonly classified under the term “orofacial pain” (OFP). In turn, long-lasting OFP can trigger and perpetuate headache as an independent entity, which is able to persist after the resolution of the main disorder. A close association between OFP and headache complicates their cause and effect definition and leads to misdiagnosis. The precise mechanisms underlying this phenomenon are poorly understood, partly because of the deficiency of research-related findings. We combined the animal models of OFP and headache—the orofacial formalin test and the model of trigeminovascular nociception—to investigate the neurophysiological mechanisms underlying their comorbidity. In anesthetized rats, the ongoing activity of single convergent neurons in the spinal trigeminal nucleus was recorded in parallel to their responses to the electrical stimulation of the dura mater before and after the injection of formalin into their cutaneous receptive fields. Subcutaneous formalin resulted not only in the biphasic increase in the ongoing activity, but also in an enhancement of neuronal responses to dural electrical stimulation, which had similar time profile. These results demonstrated that under tonic pain in the orofacial region a nociceptive signaling from the dura mater to convergent trigeminal neurons is significantly enhanced apparently because of the development of central sensitization; this may contribute to the comorbidity of OFP and headache.

Esmolol modulates inhibitory neurotransmission in the substantia gelatinosa of the spinal trigeminal nucleus of the rat

Background

β₁-adrenaline receptor antagonists are often used to avoid circulatory complications during anesthesia in patients with cardiovascular diseases. Of these drugs, esmolol, a short-acting β antagonist, is also reported to exert antinociceptive and anesthetic sparing effects. This study was designed to identify the central mechanism underlying the antinociceptive effect of esmolol.

Methods

Wistar rats (7-21 d, 17-50 g) were anesthetized with ketamine (100-150 mg/kg) or isoflurane (5%) and decapitated. Horizontal slices (400-μm thick) of the lower brainstem containing the substantia gelatinosa (SG) of the caudal part of the spinal trigeminal nucleus (Sp5c), in which the nociceptive primary afferents form the first intracranial synapses, were made with a vibrating slicer. The miniature inhibitory and excitatory postsynaptic currents (mIPSCs and mEPSCs, respectively) were simultaneously recorded from visually identified SG neurons of the Sp5c in the presence of tetrodotoxin (1 μM). Additionally, mIPSCs were recorded during pharmacological isolation of GABA- and glycine-mediated mIPSCs with kynurenic acid (1 mM).

Results

Esmolol (500 μM) significantly and selectively increased the mIPSC frequency (to 214.2% ± 34.2% of the control, mean ± SEM, n = 35; P < 0.001), but not that of mEPSCs, without changing their amplitude. The increase in mIPSC frequency with esmolol was not affected by prior activation of β receptors with isoproterenol (100 μM) but it was significantly attenuated by removal of extracellular Ca²⁺.

Conclusions

These data suggest that esmolol modulates inhibitory transmitter release in the Sp5c through a mechanism involving Ca²⁺-entry but in a β₁-adrenoceptor-independent manner. The present results suggest that the facilitation of inhibitory transmitter release in the central nociceptive network underlies, at least in part, the antinociceptive effect of esmolol.

Animal models of headache: from bedside to bench and back to bedside

In recent years bench-based studies have greatly enhanced our understanding of headache pathophysiology, while facilitating the development of new headache medicines. At present, established animal models of headache utilize activation of pain-producing cranial structures, which for a complex syndrome, such as migraine, leaves many dimensions of the syndrome unstudied. The focus on modeling the central nociceptive mechanisms and the complexity of sensory phenomena that accompany migraine may offer new approaches for the development of new therapeutics. Given the complexity of the primary headaches, multiple approaches and techniques need to be employed. As an example, recently a model for trigeminal autonomic cephalalgias has been tested successfully, while by contrast, a satisfactory model of tension-type headache has been elusive. Moreover, although useful in many regards, migraine models are yet to provide a more complete picture of the disorder.

Neurophysiological markers of central sensitisation in the trigeminal pathway and their modulation by the cyclo-oxygenase inhibitor ketorolac

Central sensitisation is a key mechanism of migraine; understanding its modulation by anti-migraine drugs is essential for rationalising treatment. We used an animal model of central trigeminal sensitisation to investigate neuronal responses to dural electrical stimulation as a putative electrophysiological marker of sensitisation and its modulation by ketorolac. In anaesthetised rats, responses of single convergent wide-dynamic range neurons of the spinal trigeminal nucleus to dural electrical simulation were recorded in parallel to their ongoing activity and responses to facial mechanical stimulation before and after a short-term dural application of an IS. Both ongoing activity and responses to dural electrical stimuli were enhanced by the inflammatory challenge, whereas neuronal thresholds to mechanical skin stimulation were reduced ( p < .05, N = 12). Intravenous ketorolac (2 mg/kg, N = 6) reduced ongoing activity and responses to dural electrical stimulation, and increased mechanical thresholds versus vehicle controls ( p < .05, N = 6). We conclude that neuronal responses to dural electrical stimulation can serve as a suitable marker which together with admitted electrophysiological signs can objectively detect central trigeminal sensitisation and its modulation by anti-migraine treatments in this preclinical model of migraine.

Differential activation of the human trigeminal nuclear complex by noxious and non-noxious orofacial stimulation

There is good evidence from animal studies for segregation in the processing of non-nociceptive and nociceptive information within the trigeminal brainstem sensory nuclear complex. However, it remains unknown whether a similar segregation occurs in humans, and a recent tract tracing study suggests that this segregation may not exist. We used functional magnetic resonance imaging (fMRI) to define and compare activity patterns of the trigeminal brainstem nuclear complex during non-noxious and noxious cutaneous and non-noxious and noxious muscle orofacial stimulation in humans. We found that during cutaneous pain, signal intensity increased within the entire rostrocaudal extent of the spinal trigeminal nucleus (SpV), encompassing the ipsilateral oralis (SpVo), interpolaris (SpVi) and caudalis (SpVc) subdivisions. In contrast, muscle pain did not activate SpVi, but instead activated a discrete region of the ipsilateral SpVo and SpVc. Further, muscle noxious stimulation activated a region of the ipsilateral lateral pons in the region of the trigeminal principal sensory nucleus (Vp). Innocuous orofacial stimulation (lip brushing) also evoked a significant increase in signal intensity in the ipsilateral Vp; however, non-noxious muscle stimulation showed no increase in signal in this area. The data reveal that orofacial cutaneous and muscle nociceptive information and innocuous cutaneous stimulation are differentially represented within the trigeminal nuclear complex. It is well established that cutaneous and muscle noxious stimuli evoke different perceptual, behavioural and cardiovascular changes. We speculate that the differential activation evoked by cutaneous and muscle noxious stimuli within the trigeminal sensory complex may contribute to the neural basis for these differences.

Neurons of the dopaminergic/calcitonin gene-related peptide A11 cell group modulate neuronal firing in the trigeminocervical complex: an electrophysiological and immunohistochemical study

Activation of spinal trigeminal afferents innervating the cranial vasculature is likely to play a role in migraine, although some parts of the clinical presentation may have a dopaminergic basis. The A11 nucleus, located in the posterior hypothalamus, provides the only known source of descending dopaminergic innervation for the spinal gray matter. Extracellular recordings were made in the trigeminocervical complex (TCC) in response to electrical stimulation of the dura mater. Receptive fields were characterized by mechanical noxious and innocuous stimulation of the ipsilateral ophthalmic dermatome. Stimulation of the A11 significantly inhibited peri-middle meningeal artery dural and noxious pinch evoked firing of neurons in the TCC. This inhibition was reversed by the D(2) receptor antagonist eticlopride. Lesioning of the A11 significantly facilitated dural and noxious pinch and innocuous brush evoked firing from the TCC. In previous work using immunohistofluorescence, it was shown that D(1) and D(2) receptors were found in the rat TCC, and here we report, in addition, that D(4) and D(5) dopamine receptors are also present, whereas D(3) receptors are not. No dopamine receptors were present in the A11 nucleus itself. However, the A11 does contain dopamine and calcitonin gene-related peptide (CGRP) and, by this combination, is distinct from the neighboring CGRPergic subparafascicular nucleus. Exploration of dopaminergic influences and mechanisms in migraine may open up an almost untapped opportunity to pursue potential new therapeutic options for the disorder.

Electrophysiological and morphological properties of neurons in the substantia gelatinosa of the mouse trigeminal subnucleus caudalis

The excitability of the second order neurons within the trigeminal subnucleus caudalis underlies pain perception and processing in migraine and trigeminal neuralgia. These neurons were studied with whole-cell patch-clamp technique in slices from mouse brain stem. Electrical and morphological characteristics of 56 neurons were determined. Four categories were distinguished from electrophysiological properties: tonic (39%), phasic (34%), delayed (16%) and single spiking (11%). These categories did not show distinct morphological properties. Neurons had tetrodotoxin-sensitive sodium currents that activated and inactivated within milliseconds. They also showed a high voltage-activated, slowly inactivating calcium current: up to half of this current was blocked by omega-conotoxin GVIA (1microM) and omega-agatoxin IVA (100-300 nM), but it was not affected by nifedipine (10microM). Exogenously applied capsaicin (1microM) and alphabetamethylene-5'-adenosine triphosphate (100microM) elicited large amplitude, spontaneous excitatory postsynaptic currents that were blocked by capsazepine (10microM) and 5-[(3-phenoxybenzyl)-(1,2,3,4-tetrahydro-naphthalen-1-yl)-carbamoyl]-benzene-1,2,4-tricarboxylic acid (A-317491: 10microM), respectively. Thus, neurons of the mouse trigeminal subnucleus caudalis substantia gelatinosa exhibit N-type and P/Q-type voltage-gated calcium channels, and receive presynaptic afferents that express TRPV1 and P2X(2/3) receptors. These results suggest possible therapeutic interventions, and serve as a basis for the characterization of cellular changes that may underlie trigeminal neuropathic pain.

Pain remapping in migraine: a novel characteristic following trigeminal nerve injury

The location of pain during the headache phase of migraine varies between individuals as well as between attacks in some individuals. We have observed a "remapping" or a change in the location of migraine pain following injury to the trigeminal system that is a novel characteristic to migraine and has not been described in other trigeminal pain syndromes of the head, neck, and face. Recognition of this clinical feature implies that the pathophysiology of migraine is impressionable and may be why diagnosis and treatment are often delayed.

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.

Dorsal horn NK1-expressing neurons control windup of downstream trigeminal nociceptive neurons

Windup is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons to repetitive stimulation of primary afferent nociceptive C-fibers. Superficial dorsal horn neurokinin 1 receptor (NK1R)-expressing neurons were recently shown to regulate sensitization of WDR nociceptive neurons through activation of a defined spino-bulbo-spinal loop. However, the windup of WDR nociceptive neurons was not regulated through this loop. In the present study, we sought to identify the alternative circuit activated by dorsal horn NK1Rs that mediates WDR neuron windup. As a model we used the rat spinal trigeminal nucleus, in which the subnucleus oralis (Sp5O) contains a pool of WDR neurons that receive their nociceptive C-input indirectly via interneurons located in the medullary dorsal horn (MDH). First, we found that intravenous injection of NK1R antagonists (SR140333 and RP67580) produced a reversible inhibition of Sp5O WDR neuron windup. Second, we anatomically identified in the MDH lamina III a subpopulation of NK1R-expressing local interneurons that relay nociceptive information from the MDH to downstream Sp5O neurons. Third, using microinjections of NK1R antagonists during in vivo electrophysiological recordings from Sp5O WDR neurons, we showed that WDR neuron windup depends on activation of NK1Rs located in the MDH laminae I-III. We conclude that, in contrast to central sensitization that is controlled by a spino-bulbo-spinal loop, Sp5O WDR neuron windup is regulated through a local circuit activated by MDH lamina III NK1Rs.

Ongoing activity in trigeminal wide-dynamic range neurons is driven from the periphery

Ongoing activity of spinal trigeminal neurons is observed under various conditions and suggested to be responsible for ongoing headache. It can be spontaneous, i.e. arising intrinsically from the neuron, or the product of descending influences from other central neurons, or maintained by ongoing afferent input. The aim of the present study was to examine if ongoing activity of neurons in different subnuclei of the spinal trigeminal nucleus is driven from peripheral afferent input. Experiments were performed in Wistar rats anesthetized with isoflurane or Nembutal/urethane. Ongoing activity of single wide-dynamic range (WDR) neurons was recorded with carbon fiber glass microelectrodes in two subnuclei of the spinal trigeminal nucleus: oral (Sp5O) and caudal (Sp5C). Peripheral receptive fields were evaluated using von Frey filaments. Sp5O neurons received peripheral input from facial areas innervated by the mandibular branch of the trigeminal nerve. Units in Sp5C had receptive fields in the surgically exposed dura mater and in facial areas innervated by the ophthalmic and maxillary branch of the trigeminal nerve. Saline or the local anesthetic lidocaine was locally applied onto the exposed dura mater or microinjected into V3 (for Sp5O units) or V1/V2 (for Sp5C units) divisions of the trigeminal ganglion via the infraorbital channel. Local application of lidocaine onto the exposed dura caused mechanical insensitivity of dural receptive fields but not significant decrease in ongoing activity. Microinjection of lidocaine but not saline into the trigeminal ganglion was followed by a substantial decrease in both the receptive field size and the activity of the recorded WDR units. Mechanical insensitivity of receptive fields after trigeminal ganglion blockade was accompanied by the disappearance of ongoing activity. We conclude that the ongoing activity of WDR neurons in the spinal trigeminal nucleus, which may be indicative for processes of sensitization, is driven remotely by ongoing afferent input.

Animal models of migraine: looking at the component parts of a complex disorder

Animal models of human disease have been extremely helpful both in advancing the understanding of brain disorders and in developing new therapeutic approaches. Models for studying headache mechanisms, particularly those directed at migraine, have been developed and exploited efficiently in the last decade, leading to better understanding of the potential mechanisms of the disorder and of the action for antimigraine treatments. Model systems employed have focused on the pain-producing cranial structures, the large vessels and dura mater, in order to provide reproducible physiological measures that could be subject to pharmacological exploration. A wide range of methods using both in vivo and in vitro approaches are now employed; these range from manipulation of the mouse genome in order to produce animals with human disease-producing mutations, through sensitive immunohistochemical methods to vascular, neurovascular and electrophysiological studies. No one model system in experimental animals can explain all the features of migraine; however, the systems available have begun to offer ways to dissect migraine's component parts to allow a better understanding of the problem and the development of new treatment strategies.

Phosphodiesterase 3 and 5 and cyclic nucleotide-gated ion channel expression in rat trigeminovascular system

Activation of the trigeminovascular pain signalling system appears involved in migraine pathophysiology. However, the molecular mechanisms are only partially known. Stimulation of cAMP and cGMP production as well as inhibition of their breakdown induce migraine-like headache. Additionally, migraine may be associated with mutations in ion channels. The aim of the present study was to describe the expression of phosphodiesterase 3 (PDE3) and 5 (PDE5) and cyclic nucleotide-gated ion channels (CNG) in cerebral arteries, meninges, and the trigeminal ganglion. mRNA for PDE and CNG was determined in the rat middle cerebral artery, basilar artery, trigeminal ganglion, and dura mater using real-time PCR. PDE and CNG proteins were identified using Western blot. For comparison, rat aorta and mesenteric artery were analysed. PDE3A, PDE3B, and PDE5A mRNA were detected in all tissues examined except for PDE3A mRNA in dura mater and the trigeminal ganglion. PDE5A and PDE3A protein expression was present in both cerebral and peripheral arteries, whereas PDE3B protein was present only in the cerebral arteries. The CNGA4 and B1 subunit mRNAs were detected in cerebral arteries and CNGA2 also in the mesenteric artery. CNGA2 and A3 proteins were found in cerebral arteries and dura and CNGA1, CNGA2 and CNGA3 in the trigeminal ganglion. In conclusion, PDE3A, PDE3B, PDE5A, and five CNG subunits were expressed in several components of the trigeminovascular system of the rat. This suggests that modulation of cAMP and cGMP levels by PDE and activation of CNG may play a role in trigeminovascular pain signalling leading to migraine headache.

Migraine, Allodynia, Sensitisation and All of That

Migraine is the most common form of disabling primary headache. One common and often troublesome feature of the disorder is an abnormal sensory state where normally innocuous stimuli are felt as painful: allodynia. This occurs in about two-thirds of patients and manifests as common complaints, such as pain when touching the hair. The neurophysiological correlate of allodynia is sensitisation, an increased afferent barrage for an unchanged peripheral stimulus. Sensitisation may be peripheral, central or disinhibitory. The potential mechanisms of each of these and their possible manipulation by treatments of the acute attack are considered.