The effects of the TRPV1 receptor antagonist SB-705498 on trigeminovascular sensitisation and neurotransmission.

This report examines the effect of the transient receptor potential vanilloid 1 receptor antagonist SB-705498 on neurotransmission and inflammation-induced sensitisation in the trigeminovascular sensory system. A single-neuron electrophysiological animal model for neurovascular head pain was used to evaluate dural and facial noxious inputs and the effects of SB-705498 administered by intravenous (i.v.) injection. Electrical and mechanical stimulation of the dura mater and the facial skin activated second-order neurons in the trigeminal nucleus caudalis of cats, with A-delta latencies. Intravenous injection of SB-705498 (2 mg kg(-1)) produced a slowly developing and long-lasting suppression of responses to dural and skin stimulation. Maximum suppression occurred by 1 h and reached 41% for dura and 24% for skin. Intravenous injection of drug vehicle did not produce significant suppression of responses to stimulation of either dura or skin. Intravenous injection of SB-705498 produced a brief and small rise in blood pressure and dural blood flow, which both returned to normal before suppression of the responses to stimulation became manifest. Application of "inflammatory soup" to the dura mater produced a pronounced increase in dural blood flow and induced a slowly developing increase in the responses of neurons to both electrical and mechanical stimulations of their facial and dural receptive fields. This sensitisation reached a maximum in 60-90 min, at which time responses had risen to approximately twice that of control levels seen before the application of inflammatory soup. Intravenous injection of SB-705498 subsequent to the development of sensitisation produced a slowly developing, prolonged and statistically significant reversal of the sensitisation induced by inflammatory soup. Maximum reversal of sensitisation to electrical stimulation occurred by 150-180 min, when responses had fallen to, or below, control levels. At 70-85 min following injection of SB-705498, the responses of previously sensitised neurons to mechanical stimulation of dura mater and facial receptive field had also returned to near control levels. SB-705498 was also able to prevent the development of sensitisation; application of inflammatory soup to the dura mater induced a slowly developing increase in the responses of neurons to electrical stimulation of the skin and dura mater in cats which had received an i.v. injection of vehicle for SB-705498 but not in cats which had received the active drug. Blood levels of SB-705498 were maximal immediately following i.v. injection and declined over the following 2 h. Significant brain levels of SB-705498 were maintained for up to 9 h. These results suggest that SB-705498 may be an effective suppressant and reversal agent of the sensitisation to sensory input which follows inflammation in the trigeminovascular sensory distribution but may not be particularly useful in blocking primary pain processes such as migraine headache. SB-705498 could thus potentially prevent, modify or reverse the cutaneous trigeminal allodynia seen in certain migraine conditions, especially "transformed" migraine.

Cortico-NRM influences on trigeminal neuronal sensation.

We tested the idea that migraine triggers cause cortical activation, which disinhibits craniovascular sensation through the nucleus raphe magnus (NRM) and thus produces the headache of migraine. Stimulation of the dura mater and facial skin activated neurons in the NRM and the trigeminal nucleus. Stimulation of the NRM caused suppression of responses of trigeminal neurons to electrical and mechanical stimulation of the dura mater, but not of the skin. This suppression was antagonized by the iontophoretic application of the 5-HT(1B/1D) receptor antagonist GR127935 to trigeminal neurons. Migraine trigger factors were simulated by cortical spreading depression (CSD) and light flash. Activity of neurons in the NRM was inhibited by these stimuli. Multiple waves of CSD antagonized the inhibitory effect of NRM stimulation on responses of trigeminal neurons to dural mechanical stimulation but not to skin mechanical stimulation. The cortico-NRM-trigeminal neuraxis might provide a target for a more universally effective migraine prophylactic treatment.

Naratriptan has a selective inhibitory effect on trigeminovascular neurones at central 5-HT1A and 5-HT(1B/1D) receptors in the cat: implications for migraine therapy.

The triptans are agonists at serotonin (5-HT)1B/1D receptors; however, they are also active at 5-HT1A and 5-HT1F receptors. We conducted this series of experiments to further elucidate the site of action of naratriptan using a well-established animal model of trigeminovascular stimulation. Following electrical stimulation of the superior sagittal sinus of the cat, single cell responses (n=83) were recorded in the trigeminal nucleus caudalis. Most cells (91%) also responded to electrical and mechanical stimulation of cutaneous or mucosal facial receptive fields. The microiontophoretic application of naratriptan resulted in a significant suppression of the response to sagittal sinus stimulation (response suppressed by 47 +/- 4%, P<0.001). The effect of naratriptan was significantly attenuated by application of either the 5-HT(1B/1D) receptor antagonist GR-127935 (P<0.001) or the 5-HT1A antagonist WAY-100635 (P<0.05). The response of single cells to receptive field stimulation was also suppressed by microiontophoretic application of naratriptan, but by only 20 +/- 3%. Intravenous administration of naratriptan resulted in a similar selective suppression of sagittal sinus vs. receptive field responses in trigeminal neurones. These results indicate that naratriptan has a central effect in the trigeminovascular system, selectively inhibiting afferent activity in craniovascular neurones, via both 5-HT(1B/1D) and 5-HT1A receptors.

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.

5-HT(1A) and 5-HT(1B/1D) receptors are involved in the modulation of the trigeminovascular system of the cat: a microiontophoretic study.

Electrical stimulation of the superior sagittal sinus in the cat activated neurones in the trigeminal nucleus caudalis. The mean latency of these responses (10.1 ms) was consistent with activation of Adelta-fibres. Microiontophoretic ejection of either the selective serotonin(1A) (5-HT(1A)) agonist (+)8-OH-DPAT or the 5-HT(1B/1D) agonist alniditan resulted in the reversible suppression of the response to superior sagittal sinus stimulation of 29/46 and 18/20 trigeminal neurones, respectively. The response to sagittal sinus stimulation was suppressed by 39+/-5% (n=46) by (+)8-OH-DPAT and 65+/-5% (n=20) by alniditan. Microiontophoretic ejection of the selective 5-HT(1A) receptor antagonist WAY-100635 significantly antagonised the effect of (+)8-OH-DPAT (effect reduced by 30%, P<0.05). The ejection of GR-127935, a selective 5-HT(1B/1D), antagonist, significantly antagonised the effect of alniditan (effect reduced by 52%, P<0.02). In eight neurones the response to convergent facial receptive field stimulation was also tested in the presence of alniditan. Only 4/8 receptive field responses were suppressed by alniditan (compared to 8/8 sagittal sinus responses) and alniditan had significantly less quantitative effect on the response to receptive field stimulation than on the response to sagittal sinus stimulation in the same neurones (mean reduction 36+/-14% and 66+/-8%, respectively, P<0.05). These results suggest that pharmacological modulation of the trigeminovascular system can occur at the first central synapse and that, in addition to 5-HT(1B/1D) receptors, 5-HT(1A) receptors may be involved in the modulation of sensory neurotransmission in the trigeminovascular system.

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.

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.

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.

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.

Inhalational anesthetics inhibit spreading depression: relevance to migraine.

Cortical spreading depression (SD) has not been shown in the human neocortex by direct cortical recordings. However, animal studies suggest that cortical injury, such as that occurring during neurosurgical procedures, should result in the initiation of SD. It is possible that inhibition of SD by volatile anesthetic agents may partially explain the failure to observe SD in the human neocortex during surgery. This study examines the effect of the anesthetic agents alpha-chloralose, halothane, nitrous oxide and isoflurane on the initiation of cortical SD in the cat neocortex. SD was seen in 100% of cats anesthetized with alpha-chloralose (n = 15), in 3 of 7 (42%) animals anesthetized with isoflurane (p < 0.05, chi 2 with Yates correction) and none of the animals (n = 4, 6 hemispheric preparations) anesthetized with halothane (p < 0.005, chi 2 with Yates correction, halothane vs alpha-chloralose group). In all cases this inhibitory effect was reversible. In four animals the administration of nitrous oxide (66%) reduced the inspired concentration of isoflurane required to inhibit SD by 0.75%. This study suggests that halothane, and to a lesser extent isoflurane and nitrous oxide, protect against the initiation of cortical SD. This observation may partially explain why SD has not been demonstrated in human neocortex during surgery. Further studies are needed to determine if SD may occur under pathological conditions, such as during migraine with aura, where the cortex may be predisposed to SD.

Convergence of afferents from superior sagittal sinus and tooth pulp on cells in the thalamus of the cat.

We have previously shown convergence of craniovascular and tooth pulp afferents in the cervical spinal cord of cats. This study looked for similar convergence in the thalamus. Fifty-four thalamic cells with input from tooth pulp, superior sagittal sinus, or both, were identified. Twenty-nine cells with tooth pulp and superior sagittal sinus input were located in the ventrobasal complex of the intralaminar nuclei. Most of these 29 cells were also excited by cooling the contralateral tooth pulp, and 21 had receptive fields on the contralateral face or forelimb. Twenty cells excited by stimulation of superior sagittal sinus, and not tooth pulp, were found in several nuclei. The 5 cells excited by stimulation of tooth pulp, but not sagittal sinus, were restricted to the ventrobasal complex. The data confirm convergence from sagittal sinus, tooth pulp, and skin in the thalamus of anaesthetized cats.

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.

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.

Software-controlled testing for antidromic activation of single neurones.

This paper describes a technique which allows the determination of the shape of action potentials initiated spontaneously, orthodromically or antidromically, without the distorting effects on shape that may be produced by changes in latency of activation or by signal averaging techniques. Software-controlled stimulation allows precise control of parameters for antidromic collision testing. The method allows selective sampling of a single action potential from a rapidly discharging cell, from a population of similar cells or from a evoked action potential among spontaneous activity. It minimizes jitter in the antidromic peri-event histogram by presenting an antidromic stimulus at precise and reproducible delays following either a spontaneous or orthodromically evoked action potential.

Cortical spreading depression does not result in the release of calcitonin gene-related peptide into the external jugular vein of the cat: relevance to human migraine.

There is circumstantial evidence that cortical spreading depression (SD) may account for the scotoma and the "spreading cortical oligemia" seen during migraine with aura. It has been shown that calcitonin gene-related peptide (CGRP) is increased in blood taken from the external jugular vein (EJV) in humans during migraine and after stimulation of the trigeminal ganglion. To test the hypothesis that cortical SD may elevate the concentration of this vasoactive peptide in the EJV during migraine, we have measured its concentration in the external jugular vein of cats during cortical SD. This study demonstrates that SD has no effect on the concentration of CGRP either during the passage of a wave of spreading depression across the cortex or, 60 min later, during the period of post-SD cortical oligemia.

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.

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.

Cortical blood flow changes during spreading depression in cats.

Changes in cortical blood flow and cerebrovascular activity occurring during and after cortical spreading depression (CSD) were studied in alpha-chloralose-urethan-anesthetized cats. CSD was induced by superficial cortical pinprick, and laser-Doppler velocimetry (LDV) was used to measure cerebral blood flow (CBFLD). CSD resulted in a wave of cortical hyperemia during which there was a 215 +/- 48% peak increase in cortical blood flow that lasted for 2.7 +/- 0.4 min. This hyperemic phase was followed by prolonged cortical oligemia, with a reduction in flow of 20 +/- 4% at 1 h and 28 +/- 4% at 2 h. After CSD, cerebrovascular reactivity to the inhalation of CO2 was abolished and did not fully recover for at least 10 h. Spontaneous vasomotor activity in the cerebral microcirculation was significantly decreased after CSD, and autoregulation of cortical blood flow in response to hypotension was preserved. The abnormal cerebrovascular reactivity seen after CSD in the gyrencephalic cortex of the cat has possible significance for human migraine with aura.