Cervical spinal cord neurons receiving sensory input from the cranial vasculature

Effect of cortical spreading depression on basal and evoked traffic in the trigeminovascular sensory system

AIM

To use an animal model to test whether migraine pain arises peripherally or centrally.

METHODS

We monitored the spontaneous and evoked activity of second-order trigeminovascular neurons in rats to test whether traffic increased following a potential migraine trigger (cortical spreading depression, CSD) and by what mechanism any such change was mediated.

RESULTS

Neurons (n = 33) responded to stimulation of the dura mater and facial skin with A-δ latencies. They were spontaneously active with a discharge rate of 6.1 ± 6.4 discharges s(-1). Injection of 10 µg lignocaine into the trigeminal ganglion produced a fully reversible reduction of the spontaneous discharge rate of neurons. Neuronal discharge rate returned to normal by 90 min. Lignocaine reduced the evoked responses of neurons to dural stimulation to 37% and to facial skin stimulation to 53% of control. Induction of CSD by cortical injection of KCl increased the spontaneous discharge rate of neurons from 2.9 to 16.3 discharges s(-1) at 20 min post CSD. Injection of 10 µg lignocaine into the trigeminal ganglion at this time failed to arrest or reverse this increase. Injection of lignocaine prior to the initiation of CSD failed to prevent the subsequent development of CSD-induced increases in discharge rates.

CONCLUSIONS

These results suggest that there is a continuous baseline traffic in primary trigeminovascular fibres and that CSD does not act to increase this traffic by a peripheral action alone - rather, it must produce some of its effect by a mechanism intrinsic to the central nervous system. Thus the pain of migraine may not always be the result of peripheral sensory stimulation, but may also arise by a central mechanism.

Tracing neural connections to pain pathways with relevance to primary headaches

Background: Symptoms associated with primary headaches are linked to cranial vascular activity and to the central nervous system (CNS).

Review: The central projections of sensory nerves from three cranial vessels are described in order to further understand pain mechanisms involved in primary headaches. Tracers that label small and large calibre primary afferent fibres revealed similar distributions for the central terminations of sensory nerves in the superficial temporal artery, superior sagittal sinus and middle meningeal artery. The sensory nerve fibres from the vessels pass through both the trigeminal and rostral cervical spinal nerves and terminate in the ventrolateral part of the C1-C3 dorsal horns and the caudal and interpolar divisions of the spinal trigeminal nucleus. The C-fibre terminations were located mainly in the superficial layers (Rexed laminae I and II), and the Aδ-fibres terminated in the deep layers (laminae III and IV). The rostral projections from the ventrolateral C1-C2 dorsal horn revealed terminations in the medial and lateral parabrachial nuclei, the cuneiform nucleus, the periaqueductal gray, the deep mesencephalic nucleus, the thalamic posterior nuclear group and its triangular part, and the thalamic ventral posteromedial nucleus. The terminations in the pons and midbrain were predominately bilateral, whereas those in the thalamus were confined to the contralateral side.

Conclusions: The observations, done in rats with the understanding that similar trigeminovascular organization exists in man, reveal vascular projections into the brainstem and some aspects of the central regions putatively involved in the central processing of noxious craniovascular signals.

Preclinical neuropharmacology of naratriptan

The basic CNS neuropharmacology of naratriptan is reviewed here. Naratriptan is a second-generation triptan antimigraine drug, developed at a time when CNS activity was thought not to be relevant to its therapeutic effect in migraine. It was, however, developed to be a more lipid-soluble, more readily absorbed and less readily metabolized variant on preexisting triptans and these variations conferred on it a higher CNS profile. Naratriptan is a 5-HT(1B/1D) receptor agonist with a highly selective action on migraine pain and nausea, without significant effect on other pain or even other trigeminal pain. Probable sites of therapeutic action of naratriptan include any or all of: the cranial vasculature; the peripheral terminations of trigeminovascular sensory nerves; the first-order synapses of the trigeminovascular sensory system; the descending pain control system; and the nuclei of the thalamus. Naratriptan may prevent painful dilatation of intracranial vessels or reverse such painful dilatation. Naratriptan can prevent the release of sensory peptides and inhibit painful neurogenic vasodilatation of intracranial blood vessels. At the first order synapse of the trigeminal sensory system, naratriptan can selectively suppress neurotransmission from sensory fibers from dural and vascular tissue, while sparing transmission from other trigeminal fibers, probably through inhibition of neuropeptide transmitter release. In the periaqueductal gray matter and in the nucleus raphe magnus, naratriptan selectively activates inhibitory neurons which project to the trigeminal nucleus and spinal cord and which exert inhibitory influences on trigeminovascular sensory input. Naratriptan has also a therapeutic effect on the nausea of migraine, possibly exerting its action at the level of the nucleus tractus solitarius via the same mechanisms by which it inhibits trigeminovascular nociceptive input. The incidence of naratriptan-induced adverse effects in the CNS is low and it is not an analgesic for pain other than that of vascular headache. In patients receiving selective serotonin uptake inhibitors (SSRIs) naratriptan may cause serotonin syndrome-like behavioral side effects. The mechanism of action involved in the production of behavioral and other CNS side effects of naratriptan is unknown.

Axon diameters and intradural trajectories of the dural innervation in the rat

Neurophysiological studies have characterized the sensory responses of primary afferent nociceptors that innervate the intracranial dura. The present study used anatomical methods to examine in greater detail the axonal trajectories within the dura, as well as the axonal size distribution of the dural innervation. Immunostaining for CGRP in dural wholemounts revealed a network of fibers extending across the entire dura, with an especially dense plexus running along the borders of the transverse and superior sagittal sinuses. The plexus along the caudal border of the transverse sinus partially overlapped the dural area that shows the greatest density of mast cells. Visualization of axon bundles by DiI application in formalin-fixed tissue revealed two separate systems of fibers in the dura that could be distinguished by the orientation of their trajectories: one that runs parallel to the middle meningeal artery (MMA), and another with a more or less orthogonal orientation that runs rostromedially from the transverse sinus across the MMA. Axons traversed large distances across the dura, but the majority of the branching and arborization was usually concentrated in the distal part of the trajectory. In separate animals, measurement of myelinated axon diameters with electron microscopy showed that approximately one-third of the myelinated axons in the nerves supplying the dura (nervus spinosus and tentorial nerves) could be classified as A-beta, since they were comparable in size to the majority of axons in the trochlear nerve and the upper end of the size range in the trigeminal nerve (i.e., > 5 microm).

The 5-hydroxytryptamine1B/1D/1F receptor agonists eletriptan and naratriptan inhibit trigeminovascular input to the nucleus tractus solitarius in the cat

Migraine pain arises in the trigeminovascular system and is often associated with nausea and sometimes with vomiting. In this study, an in vivo cat model of trigeminovascular stimulation was used to determine first whether there is a functional connection between the trigeminovascular system and the nucleus tractus solitarius (NTS), which is involved in regulating vomiting, and second whether anti-migraine drugs have any effect on such a connection. Chloralose-anaesthetised cats (n=16) were prepared for single neuron recording. The superior sagittal sinus (SSS) was isolated and stimulated electrically. The brainstem near the obex was exposed and a metal microelectrode equipped with six glass barrels for microiontophoresis was placed in the NTS. Recordings were made from 44 NTS neurons which responded to SSS stimulation with A-delta latencies. Iontophoretic ejection (50 nA) of eletriptan or naratriptan suppressed the response in 75% (15/20) and 78% (11/14) of cells and caused an average suppression of cell firing of 42+/-5% (n=20) and 54+/-8% (n=14), respectively. This suppression could be antagonized by the concurrent ejection (20-50 nA) of the 5-HT(1B/1D) receptor antagonist GR127935. We conclude that activation of the trigeminovascular system excites cells in the NTS that can be inhibited by eletriptan and naratriptan through activation of 5-HT(1B/1D) receptors. It is possible that in patients having a migraine attack trigeminovascular activation triggers nausea and vomiting, and that the alleviation of these symptoms by anti-migraine compounds may be via an action at 5-HT(1B/1D) receptors in the NTS.

Spinal and cortical spreading depression enhance spinal cord activity

Cortical spreading depression (CSD) has been suggested to underlie some neurological disorders such as migraine. Despite the intensity with which many investigators have studied SD in the brain, only a few studies have aimed to identify SD in the spinal cord. Here we described the main characteristic features of SD in the spinal cord induced by different methods including various spinal cord injury models and demonstrated that SD enhances the spinal cord activity following a transient suppressive period. These findings suggest that SD may play a role in the mechanisms of spinal neurogenic shock, spinal cord injury, and pain. Furthermore, we studied the effect of CSD on the neuronal activity of the spinal cord. CSD was induced via cortical pinprick injury or KCl injection in the somatosensory cortex. CSD did not propagate into the cervical spinal cord. However, intracellular recordings of the neurons in the dorsal horn of C2 segment, ipsilateral to the hemisphere in which CSD was evoked, showed a transient suppression of spontaneous burst discharges, followed by a significant enhancement of the neuronal activity. This indicates a link between a putative cause of the neurological symptoms and the subsequent pain of migraine.

Convergence of cervical and trigeminal sensory afferents

Cranial nociceptive perception shows a distinct topographic distribution, with the trigeminal nerve receiving sensory information from the anterior portions of the head, the greater occipital nerve, and branches of the upper cervical roots in the posterior regions. However, this distribution is not respected during headache attacks, even if the etiology of the headache is specific for only one nerve. Nociceptive information from the trigeminal and cervical territories activates the neurons in the trigeminal nucleus caudalis that extend to the C2 spinal segment and lateral cervical nucleus in the dorsolateral cervical area. These neurons are classified as multimodal because they receive sensory information from more than one afferent type. Clinically, trigeminal activation produces symptoms in the trigeminal and cervical territory and cervical activation produces symptoms in the cervical and trigeminal territory. The overlap between the trigeminal nerve and cervical is known as a convergence mechanism. For some time, convergence mechanisms were thought to be secondary to clinical observations. However, animal studies and clinical evidence have expanded our knowledge of convergence mechanisms. In this paper, the role of convergence mechanisms in nociceptive physiology, physiopathology of the headaches, clinical diagnosis, and therapeutic conduct are reviewed.

Mechanical response properties of A and C primary afferent neurons innervating the rat intracranial dura

The intracranial dura receives a small-fiber sensory innervation from the trigeminal ganglion that is thought to be involved in some types of headaches, including migraine. Mechanical response properties of dural afferent neurons were examined to investigate variation across the population in the properties of threshold, slope, adaptation, and incidence of mechanosensitivity. Dural afferent neurons were recorded in the trigeminal ganglion of urethan-anesthetized rats and were identified by their constant-latency response to dural shock. Neurons were classified as fast A (>5 m/s), slow A (5 >or= conduction velocity (CV) >or= 1.5 m/s), or C (<1.5 m/s), based on response latency to dural shock. Mechanical receptive fields were identified by stroking or indenting the outer surface of the dura. Stimulus-response curves were obtained from responses to 2-s constant-force indenting stimuli of graded intensities delivered to the dural receptive field with a servo force-controlled mechanical stimulator. The slow A population had the highest percentage of mechanosensitive units (97%) as well as the highest slopes and the lowest thresholds. Thus by all three criteria, the slow As had the highest mechanosensitivity. Conversely, the fast A population had the lowest mechanosensitivity in that it had the lowest percentage of mechanosensitive units (66%), the lowest slopes, and the highest thresholds. The C population was intermediate with respect to all three properties but was much more similar to the slow As than to the fast As. All three fiber classes showed a negative correlation between slope and threshold. The majority of neurons showed a slowly adapting response to a maintained 2-s stimulus. Adapting neurons could be subdivided based on whether the fitted exponential curve decayed to zero or to a nonzero plateau; the latter group contained the most sensitive neurons in that they had the lowest thresholds and highest slopes. Nonadapting neurons generally had lower initial firing rates than adapting neurons. Fast A neurons exhibited greater and more rapid adaptation than slow A and C neurons. Neurons with the lowest slopes, regardless of CV, had relatively rapid adaptation. The more slowly conducting portion of the C population was distinguished from the other C neurons by a number of properties: more mechanically insensitive neurons, higher thresholds, and more nonadapting neurons. These differences in mechanical response properties may be related in part to differences in membrane currents involved in impulse generation that have been described in subpopulations of dorsal root ganglion cells.

Suppression by eletriptan of the activation of trigeminovascular sensory neurons by glyceryl trinitrate

The effect of intracarotid arterial infusions of glyceryl trinitrate (GTN), a substance known to precipitate vascular headache, on the spontaneous activity of trigeminal neurons with craniovascular input was studied in cats. Cats were anaesthetised with alpha-chloralose, immobilised and artificially ventilated. The superior sagittal sinus (SSS) was isolated and stimulated electrically. Facial receptive fields (RF) were also stimulated. Single neurons were recorded from the trigeminal nucleus caudalis with a metal microelectrode equipped with six glass barrels for microiontophoresis. Infusions of GTN were administered via a catheter inserted retrogradely into the common carotid artery through the lingual artery. Infusions of GTN (mean rate 19+/-7, range 5-100 microg kg(-1) min(-1), in a volume of 2 ml min(-1)) increased the spontaneous discharge rate of second-order neurons which received dural and facial sensory input to 429+/-80% of control. Iontophoretic application of the 5-HT(1B/1D) receptor agonist eletriptan (50 nA) at the peak of the response decreased the discharge rate of neurons towards pre-GTN control levels. In the presence of continuous iontophoretic application of the 5-HT(1B/1D) receptor antagonist GR127935, the decrease in discharge rate caused by eletriptan was antagonised. We conclude (1) that GTN activates craniovascular sensory pathways at a site at, or peripheral to, the second-order neuron and that such an action may account for at least the acute-onset headache induced by GTN and (2) that the antimigraine agent eletriptan is able to selectively suppress noxious sensory information from the dura, induced by GTN, via an action at 5-HT(1B/1D) receptors.

The role of 5-HT1B and 5-HT1D receptors in the selective inhibitory effect of naratriptan on trigeminovascular neurons

The importance of 5-HT(1B) and 5-HT(1D) receptors in the actions of the anti-migraine drug naratriptan was investigated using the relatively selective 5-HT(1) receptor ligands SB224289 and BRL15572. Electrical stimulation of the superior sagittal sinus (SSS) in cats activated neurones in the trigeminal nucleus caudalis. Facial receptive fields (RF) were also electrically stimulated to activate the same neurones. Responses of these neurones to SSS stimulation were suppressed by iontophoretic application of naratriptan (5-50 nA). There were two distinct populations of neurones in the nucleus--those in deeper laminae in which the responses to SSS and RF stimulation were equally suppressed by naratriptan ('non-selective') and more superficial neurones in which only the SSS responses were suppressed by naratriptan ('selective'). Concurrent micro-iontophoretic application (50 nA) of the 5-HT(1D) antagonist BRL15572 antagonised the suppression by naratriptan of the response of 'selective' cells to SSS stimulation. Iontophoretic application of SB224289 (50 nA), a 5-HT(1B) antagonist, antagonised the suppression by naratriptan of responses of 'non-selective' cells to RF stimulation and, to a lesser extent, also antagonised the suppression of responses to SSS stimulation. Intravenous administration of SB224289 antagonised the suppression only of RF responses of "non-selective" neurons by naratriptan and intravenous administration of BRL15572 antagonised the suppression only of SSS responses of "selective" neurons by naratriptan. These results suggest that the response of nucleus caudalis neurons to stimulation of the sagittal sinus can be modulated by both 5-HT(1B) and 5-HT(1D) receptor activation, with the 5-HT(1D) receptors perhaps playing a greater role. The response to RF stimulation is more influenced by 5-HT(1B) receptor modulation with 5-HT(1D) receptors being less important. Therefore, this suggests that selective 5-HT(1D) agonists may be able to target the neuronal population, which is selectively involved in the transmission of dural inputs. We conclude that the central terminals of trigeminal primary afferent fibres contain 5-HT(1B) and 5-HT(1D) receptors. Primary afferents from the dura mater may predominantly express 5-HT(1D) receptors, while facial afferents may predominantly express 5-HT(1B) receptors. Activation of 5-HT(1D) receptors in particular may be important in the anti-migraine effect of naratriptan.

Physiology of meningeal innervation: aspects and consequences of chemosensitivity of meningeal nociceptors

Up to now, the cause of most types of headaches is unknown. Why headache starts or why it fades away during hours or a few days is still a mystery. This phenomenon makes headache unique compared to other pain states. For long it has been known that during headache sensory structures in the meninges are activated. But it was not until the last two decades that scientists investigated the physiology of the sensory innervation of the meninges. Animal models and in vitro preparations have been developed to get access to the meninges and to determine the response properties of meningeal afferents. Although animals hardly can tell their pain, blood pressure measurements and observations of behaviour in two models of headache suggest that such animal models are valid and may add remarkable information to our understanding of human headache. Since chemicals and endogenous inflammatory mediators may alter sensory thresholds and responsiveness of neurons, they are putative key molecules in triggering pathophysiological sensory processing. This review briefly summarizes what is known about the chemosensitivity of meningeal innervation.

Activation of trigeminovascular neurons by glyceryl trinitrate

The effect of intra-carotid arterial infusions of glyceryl trinitrate (GTN), a substance known to precipitate headache, including migraine, upon the spontaneous activity of trigeminal neurons with craniovascular input was studied in cats. Second-order craniovascular neurons which received sensory input from the superior sagittal sinus were recorded in the trigeminal nucleus caudalis. Infusions of GTN were administered via a catheter inserted retrogradely into the common carotid artery through the lingual artery. Infusions of GTN (100 microg kg(-1) min(-1) in a volume of 2 ml min(-1)) increased the mean basal discharge rate of all second-order neurons to 239+/-47% of control. GTN produced a fall in mean blood pressure, but there was no correlation between this fall and the changes in discharge rate. GTN infusions sensitised neurons to the effects of electrical stimulation of the superior sagittal sinus, but not to subsequent GTN infusions. Infusions of similar volumes of vehicle did not alter the discharge rate of neurons. We conclude that GTN activates craniovascular sensory pathways at a site at, or peripheral to, the second-order neuron and that such an action may account for at least the acute-onset headache induced by GTN.

Serotonin infusions inhibit sensory input from the dural vasculature

Intravenous infusions of serotonin (5-hydroxtryptamine creatinine sulphate, 5HT, 50-300 microg/kg/min) in cats reversibly inhibited the responses of cervical spinal cord neurons to electrical stimulation of the superior sagittal sinus. Inhibition developed over 20-30 min and resolved over the same time course, suggesting a dependence on accumulation of 5HT in the central nervous system. Inhibition was suppressed by prior intravenous injection of the 5HT antagonists methysergide (1 mg/kg) and methiothepin (1 mg/kg). Infusions of 5HT (50 microg/kg/min) caused a rise in whole blood levels of 5HT by a factor of 1.5 of control values. 5HT levels in platelet-free plasma rose by a factor of 50. Levels of 5HT and 5 hydroxyindole acetic acid released into the cerebrospinal fluid rose significantly. The results suggest that earlier clinical observations that 5HT infusions can ameliorate the pain of migraine may not have been due to cranial vasoconstriction alone, but could have involved a central action of 5HT.

Effect of cortical spreading depression on activity of trigeminovascular sensory neurons

The effect of cortical spreading depression, a proposed initiating event for migraine pain, on cortical blood flow (laser Doppler method) and on the spontaneous firing rate and stimulus-evoked responses of trigemino-cervical neurons with craniovascular input was studied in 17 neurons in 8 cats anesthetized with chloralose. Cortical spreading depression, induced via cortical pinprick injury, produced an initial wave of cortical hyperemia (243+/-57% of control) and a later and smaller phase of oligemia (96+/-4% of control). Neither the basal discharge rate (6.7+/-1.7 sec(-1)) nor the evoked responses to electrical stimulation of the superior sagittal sinus (4.1+/-0.8 discharges per stimulus) of upper cervical spinal cord neurons was altered over periods of up to 2 h following one, two, or three waves of spreading cortical depression. We conclude that a small number of episodes of cortical spreading depression is not capable of activating C2 cervical spinal cord craniovascular sensory neurons in the cat.

Convergence of trigeminal input with visceral and phrenic inputs on primate C1-C2 spinothalamic tract neurons

Trigeminal, spinal and vagal afferent fibers overlap in C1-C2 segments. We hypothesized that trigeminal input from the superior sagittal sinus (SSS) can excite C1-C2 spinothalamic tract (STT) neurons receiving thoracic visceral or phrenic inputs. Effects of SSS stimulation were evenly divided among cells responding to each nerve stimulus; magnitude of responses to ipsilateral vagal input was greater in neurons excited by SSS input. Somatic fields of 80% of neurons responding to SSS stimulation included face areas innervated by the trigeminal nerve, whereas somatic fields of 89% of neurons unaffected by SSS stimulation were located only on areas innervated by cervical spinal nerves. Results are consistent with the idea that pain referred to trigeminal areas could originate in thoracic organs.

Stimulation of the middle meningeal artery leads to Fos expression in the trigeminocervical nucleus: a comparative study of monkey and cat

The pain of a migraine attack is often described as unilateral, with a throbbing or pulsating quality. The middle meningeal artery (MMA) is the largest artery supplying the dura mater, is paired, and pain-producing in humans. This artery, or its branches, and other large intracranial extracerebral vessels have been implicated in the pathophysiology of migraine by theories suggesting neurogenic inflammation or cranial vasodilatation, or both, as explanations for the pain of migraine. Having previously studied in detail the distribution of the second order neurons that are involved in the transmission of nociceptive signals from intracranial venous sinuses, we sought to compare the distribution of second order neurons from a pain-producing intracranial artery in both monkey and cat. By electrically stimulating the middle meningeal artery in these species and using immunohistochemical detection of the proto-oncogene Fos as a marker of neuronal activation, we have mapped the sites of the central trigeminal neurons which may be involved in transmission of nociception from intracranial extracerebral arteries. Ten cats and 3 monkeys were anaesthetised with alpha-chloralose and the middle meningeal artery was isolated following a temporal craniotomy. The animals were maintained under stable anaesthesia for 24 h to allow Fos expression due to the initial surgery to dissipate. Following the rest period, the vessel was carefully lifted onto hook electrodes, and then left alone in control animals (cat n = 3), or stimulated (cat n = 6, monkey n = 3). Stimulation of the left middle meningeal artery evoked Fos expression in the trigeminocervical nucleus, consisting of the dorsal horn of the caudal medulla and upper 2 divisions of the cervical spinal cord, on both the ipsilateral and contralateral sides. Cats had larger amounts of Fos expressed on the ipsilateral than on the contralateral side. Fos expression in the caudal nucleus tractus solitarius and its caudal extension in lamina X of the spinal cord was seen bilaterally in response to middle meningeal artery stimulation. This study demonstrates a comparable anatomical distribution of Fos activation between cat and monkey and, when compared with previous studies, between this arterial structure and the superior sagittal sinus. These data add to the overall picture of the trigeminovascular innervation of the intracranial pain-producing vessels showing marked anatomical overlap which is consistent with the often poorly localised pain of migraine.

Vascular head pain selectively activates ventrolateral periaqueductal gray in the cat

Electrical stimulation of the superior sagittal sinus is an experimental model of migraine which activates neurons within the upper cervical spinal cord. The ventrolateral periaqueductal gray has been proposed as an integrative centre for the autonomic and behavioural responses to deep pain and also receives significant inputs from the upper cervical spinal cord. The noxious-stimulation evoked expression of the immediate-early gene c-fos, was used to determine if sagittal sinus stimulation activates neurons of the ventrolateral periaqueductal gray. The superior sagittal sinus was stimulated in anesthetised cats and Fos-protein was detected in coronal brain sections using standard avidin-biotin immunohistochemistry. A pattern of Fos-positive cells restricted to the caudal ventrolateral periaqueductal gray was revealed suggesting that this region may mediate the pattern of somatic and autonomic responses characteristic of migraine.

Headache and the cervical spine: a critical review

Headache related to the cervical spine is often misdiagnosed and treated inadequately because of confusing and varying terminology. Primary headaches such as tension-type headache and migraine are incorrectly categorized as "cervicogenic" merely because of their occipital localization. Cervicogenic headache as described by Sjaastad presents as a unilateral headache of fluctuating intensity increased by movement of the head and typically radiates from occipital to frontal regions. Definition, pathophysiology; differential diagnoses and therapy of cervicogenic headache are demonstrated. Ipsilateral blockades of the C2 root and/or greater occipital nerve allow a differentiation between cervicogenic headache and primary headache syndromes such as migraine or tension-type headache. Neither pharmacological nor surgical or chiropractic procedures lead to a significant improvement or remission of cervicogenic headache. Pains of various anatomical regions possibly join into a common anatomical pathway, then present as cervicogenic headache, which should therefore be understood as a homogeneous but also unspecific pattern of reaction.

Convergence of occipital nerve and superior sagittal sinus input in the cervical spinal cord of the cat

Co-existence of facial and occipital pain may occur in occipital neuralgia, migraine and cluster headache; suggesting convergence of trigeminal and cervical afferents. Such convergence has been shown in humans and other animals, but the site and extent of this are uncertain. In anaesthetized adult cats, the superior sagittal sinus and occipital nerve were stimulated electrically, and extracellular recordings made in the dorsolateral area of the upper cervical cord using glass-coated tungsten electrodes. Of 49 units in 10 cats, 33 (67%) had input from the superior sagittal sinus and the occipital nerve. Thirteen (27%) had superior sagittal sinus input and 3 (6%) had occipital nerve input. Convergent receptive fields were identified mechanically in 7 units. These experiments in cats show convergent input from occipital nerve and superior sagittal sinus on dorsolateral area units in two-thirds of cases studied. This experimental site of trigeminocervical convergence may relate to referral of pain in occipital neuralgia and other headaches.

Responses of the dural circulation to electrical stimulation of the trigeminal ganglion in the cat

1. In cats anaesthetized with alpha-chloralose, electrical stimulation (ES) of the trigeminal ganglion produced a fall in blood pressure, a predominantly ipsilateral dilatation in the common carotid vascular bed and bilateral dilatation of the middle meningeal vascular bed. Section of the trigeminal root abolished these responses. 2. Dilatation in the middle meningeal artery was not affected by section of the cervical sympathetic trunk nor by the section of the seventh cranial nerve trunk. The dilator response was abolished by section of the spinal cord at the C3 level and by intravenous administration of bretylium (10 mg/kg) or phentolamine (5 mg/kg). The response was significantly reduced by the prior administration of papaverine (10 mg/kg). 3. Functional adrenalectomy by means of a snare placed around the nerves and blood vessels supplying the adrenal glands significantly reduced the response. Electrical stimulation of the trigeminal ganglion was accompanied by a fall in circulating levels of noradrenaline and serotonin. 4. We conclude that ES of the trigeminal ganglion produces dilatation in the middle meningeal artery partly by autoregulation during the trigeminal depressor response and partly by a reduction in the circulating levels of noradrenaline. It differs from the dilatation seen in the general carotid circulation and the cortical microcirculation, which is mediated by parasympathetic nerves. There is no evidence that antidromic release of neuropeptides from sensory nerve endings in the dura plays a part in the dilatation.

Recordings from brain stem neurons responding to chemical stimulation of the subarachnoid space

The subarachnoid space at the base of the skull was perfused continuously with artificial cerebrospinal fluid in anesthetized rats. A combination of inflammatory mediators consisting of histamine, bradykinin, serotonin, and prostaglandin E2 (10(-5) M) at pH of 6.1 was introduced into the flow for defined periods to stimulate meningeal primary afferents. Secondary neurons in the caudal nucleus of the trigeminal brain stem were searched by electrical stimulation of the cornea. Of the units receiving oligosynaptic input from the cornea, 44% were excited by stimulation of the meninges with inflammatory mediators. Most of these units had small receptive fields including cornea and the periorbital region, and their responsiveness was restricted to stimuli of noxious intensity. Three types of responses to stimulation of the meninges with algogenic agents were encountered: responses that did not outlast the stimulus period, responses outlasting the stimulus period for several minutes, and oscillating response patterns containing periods of enhanced and suppressed activity. The response pattern of a unit was reproducible, however, upon repetitive stimulation at 20-min intervals; the response magnitude showed tachyphylaxis upon stimulus repetition. The preparation presented mimics pathophysiolocial states normally accompanied by headache, e.g., subarachnoidal bleeding. Responsiveness of neurons in the caudal nucleus of the trigeminal brain stem to inflammatory mediators may play a role in the generation and maintenance of headache, e.g., migraine.

On the origin of headaches

Headache is one of the most common types of pain, but its causes remain poorly understood. The long-standing idea that some headaches, particularly migraine, might be caused by cerebral or cranial vasodilation has failed to find support in recent studies. Alternative hypotheses have focused on other processes that might be capable of activating or sensitizing sensory nerve fibres that innervate the blood vessels of the intracranial meninges.

Sensitization of meningeal sensory neurons and the origin of headaches

The headaches that accompany certain intracranial pathologies (such as meningitis, subarachnoid haemorrhage and tumour) have been considered to result from mechanical or chemical stimulation of pain-sensitive structures of the intracranial meninges. Although the recurrent headache of migraine is of unknown origin and is not accompanied by an identifiable pathology, it shares with intracranial headaches features that suggest an exaggerated intracranial mechanosensitivity (worsening of the pain by coughing, breath-holding or sudden head movement). One possible basis for such symptoms would be a sensitization of meningeal afferents to mechanical stimuli. Previous studies of neuronal responses to meningeal stimulation have focused primarily on cells in the central portion of the trigeminal pathway, and have not investigated the possible occurrence of sensitization. We have recorded the activity of primary afferent neurons in the rat trigeminal ganglion that innervate the dural venous sinuses. Chemical stimulation of their dural receptive fields with inflammatory mediators both directly excited the neurons and enhanced their mechanical sensitivity, such that they were strongly activated by mechanical stimuli that initially had evoked little or no response. These properties of meningeal afferents (chemosensitivity and sensitization) may contribute to the intracranial mechanical hypersensitivity that is characteristic of some types of clinically occurring headaches, and may also contribute to the throbbing pain of migraine.

Third cervical nerve root and ganglion compression: clinical syndrome, surgical anatomy, and pathological findings

OBJECTIVE

The aim of this study is to determine whether C3 nerve root and ganglion compression occurs and, if so, to provide a preliminary description of the associated clinical syndrome and surgical pathological findings.

METHODS

The normal anatomy of the C2-C3 foramen was studied bilaterally in five fresh cadaver dissections and on 10 normal vertebral angiograms. Six patients were selected whose C3 dermatome sensory deficits resolved after C2-C3 facetectomy for C3 nerve root and ganglion decompression.

RESULTS

Patients with C3 nerve root and ganglion compression presented with radiating pain, dysesthesias, and numbness referred to the C3 pain dermatome: the scalp area behind and over the ear, the pinna, and the angle of the mandible. At physical examination, the presence of analgesia/dense hypalgesia in the C3 pain dermatome established the diagnosis. Imaging studies were suggestive but inconclusive. Surgical pathological findings showed the C3 nerve root and medial portion of the ganglion flattened by C2-C3 facet and uncovertebral joint spurs and the lateral part of the ganglion stretched and flattened by C2-C3 arthrosis and the C2-C3 vertebral loop. Decompression was obtained by a complete facetectomy. Complications required four operations: recurrence, contralateral C3 root decompression, bilateral C2 nerve root decompression, and C2-C3 fusion.

CONCLUSION

C3 nerve root and ganglion compression, although uncommon, does occur. It presents with radiating pain, dysesthesias, numbness, and a C3 dermatome sensory deficit. The associated clinical syndrome resolves after facetectomy and C3 root and ganglion decompression.

Trigemino-cervical reflexes in normal subjects

Trigemino-cervical reflexes, recorded from the semispinalis capitis muscle (SCM) in the posterior neck, were studied in 35 healthy volunteers, in response to electrical stimulation of the supraorbital trigeminal nerve and glabellar tapping. Simultaneous responses evoked from the ipsilateral orbicularis oculi muscle (OOM) were also recorded i.e. blink reflexes. Electrical stimulation of the supraorbital nerve elicited a reflex response with a latency of about 50 ms from the ipsilateral SCM which was called C3. An early reflex response, which sometimes had two components with latencies of 18 ms and 35 ms, was elicited with glabellar taps. They were called C1 and C2 respectively. When C1 and C2 were elicited with usual glabellar taps, C3 was suppressed. With electrical stimulation, suppression of C1 and C2 was noted, though C3 could easily be obtained. Electrophysiological characteristics of C1 (and C2) were compatible with an oligosynaptic, innocuous reflex, whereas C3 seemed to be multisynaptic and nociceptive in nature. A negative interaction between these two reflexes was observed.

Effect of antimigraine drugs on dural blood flows and resistances and the responses to trigeminal stimulation

The effects of 2 antimigraine drugs sumatriptan and dihydroergotamine on dilatation of the middle meningeal artery elicited by stimulation of the trigeminal ganglion at the entry point of the first and second divisions was investigated in cats. Carotid and middle meningeal arterial blood flows and resistances were measured in 9 cats anesthetised with chloralose. Electrical stimulation of either trigeminal ganglion produced a frequency-dependent decrease in resistance of the carotid artery ipsilaterally and the middle meningeal artery bilaterally. The intravenous injection of sumatriptan increased carotid and meningeal vascular resistance, but this response was not prolonged. The intravenous injection of dihydroergotamine produced a larger and more prolonged vasoconstriction in these 2 beds than did sumatriptan. Dihydroergotamine, but not sumatriptan, blocked some components of the vascular response induced by stimulation of the trigeminal ganglion. Dihydroergotamine and sumatriptan have a different spectrum of activity on cranial circulatory beds and neither of them is able to reduce trigeminal-induced vasodilatation by blocking antidromic activation of trigeminal nerve fibres in cats at the doses used in these experiments.

Outcome of surgical decompression of the second cervical root for cervicogenic headache

OBJECTIVE

There is limited information on the surgical treatment of cervicogenic headache (CH). The objective of this study is to analyze the utility of microsurgical decompression of the second cervical (C2) root and ganglion as a treatment for CH.

METHODS

Thirty-nine sequential C2 root and ganglion decompressions, performed for the treatment of CH on 35 patients at the Dartmouth-Hitchcock Medical Center during a 70-month period, were analyzed retrospectively. Preoperative factors and intraoperative findings were evaluated with respect to outcome.

RESULTS

At a mean follow-up of 21 months, 12 patients (13 sides) were pain free, and 15 were adequately improved (18 sides). Treatment eventually failed in eight patients. No patient, however, was worse after surgical intervention. There was no major morbidity or mortality associated with the operation. The eight patients with unsatisfactory outcome were evaluated for a possible second operation, and four underwent it. One patient of the four is pain free after 28 months, and two gained adequate improvement at 3 and 12 months. The fourth patient required a third operation but has achieved adequate relief at 6 months. Thus, the overall success rate (either pain free or with adequate improvement) was 90%. No specific prognostic factors could be established, other than the accepted diagnostic criteria and successful anesthetic blockade of the C2 root and ganglion.

CONCLUSION

The results suggest that microsurgical decompression of the C2 root and ganglion has some utility in treating CH. The accepted diagnostic criteria and success of anesthetic blockade of C2 should identify the subset of patients with CH predominantly caused by C2 root or ganglion effect at this level, which may favor surgical treatment.

Reactions of the middle meningeal artery of the cat to neural and humoral stimulation

The physiology and pharmacology of the middle meningeal artery was investigated in cats in order to determine whether this artery was subject to normal neural and humoral control mechanisms. Carotid and middle meningeal arterial blood flows and resistances were measured in 16 cats anaesthetized with chloralose. The cervical sympathetic nerves were stimulated electrically. Stimulation of the cervical sympathetic nerves pre-ganglionically reduced blood flow in the middle meningeal artery by producing vasoconstriction in its resistance bed. The vasoconstriction was mediated via catecholamine-containing nerves, as it was abolished by prior intravenous administration of bretylium. Intravenous injections of noradrenaline or adrenaline also produced vasoconstriction in the middle meningeal arterial bed. 5-Hydroxytryptamine (5HT), on the other hand, produced a dilatation in the middle meningeal artery. We conclude that neurally or humorally released catecholamines can provide a plausible mechanism for vasoconstriction in the middle meningeal artery. The dilator effect of 5HT contrasts with the constrictor effect of the 5HT1-like receptor agonist sumatriptan and suggests a complex 5HT receptor pharmacology for the artery.

Convergence of afferents from superior sagittal sinus and tooth pulp on cells in the upper cervical spinal cord of the cat

Units in the dorsolateral area of the upper cervical cord respond to craniovascular stimulation. This study examined tooth pulp responses in this area in cats. Eleven of 21 units tested in the dorsolateral area had convergent inputs from superior sagittal sinus and tooth pulp; while 10 units had sagittal sinus, but not tooth pulp, input. Mean response latency to tooth pulp stimulation (25.8 ms) was significantly longer than to superior sagittal sinus stimulation (9.8 ms). Half of the units had cutaneous receptive fields; and in five units, action potentials could be evoked by electrical stimulation in the posterior complex of the thalamus.

Cortical spreading depression reduces dural blood flow--a possible mechanism for migraine pain?

These experiments were designed to investigate a possible mechanism linking the phenomenon of cortical spreading depression with activation of the trigeminal sensory system in migraine. Blood flow in the cortex and middle meningeal artery was measured in cats before and during propagation of a wave of cortical spreading depression, initiated by cortical pin-prick. This caused a transient propagated increase in cortical blood flow. Cortical spreading depression was accompanied by a decrease in blood flow in the middle meningeal artery, sometimes to very low levels. The results suggest that the pain of migraine could arise from dural ischemia induced by cortical spreading depression.

Anatomical properties of brainstem trigeminal neurons that respond to electrical stimulation of dural blood vessels

Single unit recording studies in anesthetized cats have identified a population of neurons in the brainstem trigeminal complex that can be activated by stimulation of major dural blood vessels. Such dura-responsive neurons exhibit response properties that are appropriate for a role in the mediation of vascular head pain in that they typically exhibit nociceptive facial receptive fields whose periorbital distribution is similar to the region of referred pain evoked by dural stimulation in humans. In the present study, intracellular labelling with horseradish peroxidase was used to examine the anatomical characteristics of brainstem trigeminal neurons that respond to dural stimulation. A total of 17 neurons was labelled that responded to electrical stimulation of dural sites overlying the superior sagittal sinus or middle meningeal artery. Fourteen of these neurons also responded to electrical stimulation of the cornea. The neurons in this sample were located in the rostral two-thirds of the trigeminal nucleus caudalis and the caudalmost part of the nucleus interpolaris. Within caudalis, the neurons were located in the deeper part of the nucleus, primarily lamina V, and were concentrated ventrolaterally. The dendritic arborizations of the dura-responsive neurons typically exhibited a dorsolateral-to-ventromedial orientation and did not extend into the superficial laminae of caudalis. Dura-responsive neurons had axonal collaterals and boutons in the nucleus caudalis, nucleus interpolaris, the infratrigeminal region ventral to nucleus interpolaris, the nucleus of the solitary tract, and the medullary reticular formation. The axonal boutons within the trigeminal complex exhibited a ventrolateral distribution which largely overlapped the distribution of the somata. The results are consistent with previous evidence that dura-responsive brainstem trigeminal neurons may have a role in the mediation of dural vascular head pain and also indicate that such neurons may contribute to nociceptive processing within the dorsal horn.

Expression of c-Fos-like immunoreactivity in the caudal medulla and upper cervical spinal cord following stimulation of the superior sagittal sinus in the cat

Migraine is an episodic vascular headache with a well-recognized clinical picture but a poorly understood pathogenesis. Stimulation of a pain-sensitive trigeminally innervated intracranial structure, the superior sagittal sinus (SSS), was undertaken to map the higher-order neurons potentially involved in the processing of vascular head pain. The animals were prepared for stimulation by exposure of the sinus and then maintained under alpha-chloralose anaesthesia for 24 h before SSS stimulation, perfusion and immunohistochemical processing for the detection of Fos protein. Examination of the medulla and upper cervical cord revealed marked increases in Fos-like immunoreactivity in laminae I and IIo of the trigeminal nucleus caudalis and the dorsal horn of the upper cervical spinal cord. In addition, Fos-like immunoreactivity was observed in lamina X of the upper cervical spinal cord, in the commissural and medial nuclei of the solitary tract and in the nucleus retroambigualis. The use of immunohistochemical detection of Fos has allowed visualization of several populations of neurons likely to be involved in the central neural processing of vascular headache syndromes, particularly migraine.

The spinal cord processing of input from the superior sagittal sinus: pathway and modulation by ergot alkaloids

The effects of ergot alkaloids on field potentials and unit responses produced in the upper cervical spinal cord by stimulation of the superior sagittal sinus (SSS) were examined in 57 anesthetized cats. Electrical stimulation of the SSS produced field potentials and single-unit responses at latencies of 5-20 ms. Field potentials were abolished by section of the first division of the trigeminal nerve but were unaffected or increased by section of the upper cervical nerves. Field potentials were reduced or abolished by intravenous injection of ergotamine or dihydroergotamine (DHE). The evoked response of 41 units (34.4%) were suppressed by either i.v. or iontophoretic administration of ergotamine, DHE or ergometrine. The results suggest that ergot alkaloids exert an effect at a spinal cord relay centre which receives trigeminally mediated input from cranial blood vessels.

Activation of the trigeminovascular system by mechanical distension of the superior sagittal sinus in the cat

Distension of dural sinuses in man produces migraine-like pain. In eight alpha-chloralose anaesthetized cats mechanical distension of the superior sagittal sinus with a small intraluminal device was used to activate single units in the dorsolateral C2 spinal cord. Units in this region have been shown to respond to electrical stimulation of the superior sagittal sinus in the cat model. Linked responses to mechanical dilatation could only be obtained with very rapid stretching stimuli or high amplitudes of distension of the vessel. Lower thresholds for transduction of distension in the vessel wall may depend on transferral to the dura or biochemical or neural pre-sensitization of the superior sagittal sinus. These data are consistent with the view that migraine is not primarily a vascular disorder but requires at least humoral or neural facilitation.

Craniovascular nociceptive pathways relay in the upper cervical spinal cord in the cat

Units in the dorsolateral area of the upper cervical cord and the ventroposteromedial nucleus of the thalamus respond to stimulation of cranial vessels. To study the physiological role of the upper cervical cord in craniovascular transmission, we used a cryoprobe to interrupt reversibly neural transmission through the cord while recording in the thalamus. Twenty-one of 47 thalamic units tested showed reversible diminution in their response to superior sagittal sinus stimulation during cervical cord cooling. In contrast, receptive field responses and spontaneous thalamic activity were unaffected. These data suggest offt the cervical cord relays craniovascular nociceptive afferents.