Inhibition of trigeminal neurones after intravenous administration of naratriptan through an action at 5-hydroxy-tryptamine (5-HT(1B/1D)) receptors

Functional neuroimaging of primary headache disorders

Until recently, primary headache disorders such as migraine and cluster headache were considered to be vascular in origin. However, advances in neuroimaging techniques, such as positron emission tomography, single photon emission computerized tomography, and functional magnetic resonance imaging, have augmented the growing clinical evidence that these headaches are primarily driven from the brain. This review covers functional imaging studies in migraine, cluster headache, rarer headache syndromes, and experimental head pain. Together with newer techniques, such as voxel-based morphometry and magnetic resonance spectrometry, functional imaging continues to play a role in elucidating and targeting the neural substrates in each of the primary headache syndromes.

Naratriptan: a review

Even though naratriptan and sumatriptan are both 5-HT1B/5-HT1D receptor agonists, the biological and pharmacokinetic profile of naratriptan differs significantly from that of sumatriptan. With a plasma half-life of 6 h, very high oral bioavailability of 63-74% and higher lipophilicity than sumatriptan, naratriptan exhibits a distinct clinical therapeutic profile. The similar tolerability to placebo, prolonged efficacy for 24 h or more and very low headache recurrence rate make naratriptan an attractive option in the treatment of acute migraine.

Calcitonin Gene-Related Peptide Antagonists as Treatments of Migraine and Other Primary Headaches

Calcitonin gene-related peptide (CGRP) is a potent neuromodulator that is expressed in the trigeminovascular system and is released into the cranial circulation in various primary headaches. CGRP is released in migraine, cluster headache and paroxysmal hemicrania. The blockade of its release is associated with the successful treatment of acute migraine and cluster headache. CGRP receptor blockade has recently been shown to be an effective acute anti-migraine strategy and is non-vasoconstricting in terms of the mechanism of action. The prospect of a non-vasoconstricting therapy for acute migraine offers a real opportunity to patients, and perhaps more importantly, provides a therapeutic rationale to reinforce migraine as a neurological disorder.

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.

Nitrergic and glutamatergic neuronal mechanisms at the trigeminovascular first-order synapse

Nitric oxide (NO) donors such as glyceryl trinitrate cause headache, which suggests involvement of NO in trigeminovascular sensory processing. Sensory transmission at first-order synapses is believed to involve glutamate and the question arises as to whether it is also involved in trigeminovascular sensation and whether it might interact with nitrergic mechanisms. We investigated these questions at the first central synapse in the trigeminovascular sensory system of the cat. Neuronal action potentials in the trigeminal nucleus were recorded while the superior sagittal sinus (SSS) or facial receptive field (RF) were stimulated electrically. Drugs, including the neuronal excitant glutamate, were applied to neurons via microiontophoresis. Results were obtained from 152 neurons activated with A-delta latencies by SSS stimulation and by glutamate. The NO donor S-nitrosoglutathione (SNOG, 50 nA) was applied iontophoretically to 41 neurons during SSS stimulation and 13 neurons during pulsatile glutamate ejection. Responses to both modes of stimulation were enhanced by SNOG; the proportion of neurons enhanced was 56% to SSS stimulation and 59% to glutamate. The inhibitor of nitric oxide synthase (NOS), N(omega)-propyl-L-arginine (p-ARG, 50 nA) was applied iontophoretically to 17 neurons during stimulation of SSS and to 10 neurons during pulsatile glutamate ejection. Responses to both stimuli were suppressed by p-ARG: The proportion of neurons suppressed were: to SSS stimulation 59% and to glutamate 80%. Microiontophoretic ejection of eletriptan (50 nA) reversibly suppressed responses of neurons to SSS stimulation, to RF electrical stimulation and to pulsatile iontophoretic application of glutamate. This suppression of responses was antagonised by the concurrent local iontophoretic application of the 5-HT1B/1D receptor antagonist GR127935 or by concurrent iontophoretic application of the selective 5-HT1D receptor antagonist BRL155732. These results suggest that glutamatergic mechanisms are important in sensory transmission in the trigeminovascular system and that they can be modulated by nitrergic and serotonergic mechanisms.

Analgesic triptan action in an animal model of intracranial pain: a race against the development of central sensitization

We have shown that the development of cutaneous allodynia (exaggerated skin sensitivity) during migraine is detrimental to the anti-migraine action of the 5HT(IB/ID) receptor agonists known is triptans. Because cutaneous allodynia is a manifestation of sensitization of central trigeminovascular neurons, we examined whether triptan treatment can intercept such sensitization before its initiation or after its establishment in our rat model for cutaneous allodynia induced by intracranial pain. Single-unit recordings were obtained from spinal trigeminal neurons that proved to received convergent inputs from the dura and facial skin. The effects of sumatriptan (300 microg/kg i.v.) on central sensitization induced by topical application of inflammatory soup (IS) on the dura were determined when the drug was administered either 2 h after IS (late intervention) or at the same time as IS (early intervention). Late sumatriptan intervention counteracted two aspects of central sensitization: dural receptive fields, which initially expanded by IS, shrunk back after treatment; neuronal response threshold to dural indentation, which initially decreased after IS, increased after sumatriptan. On the other hand, late sumatriptan intervention did not reverse other aspects of central sensitization: spontaneous firing rate and neuronal response magnitude to skin brushing which initially increased after IS, remained elevated after sumatriptan; response threshold to heating of the skin, which initially dropped after IS, remained low after sumatriptan. Early sumatriptan intervention effectively blocked the development of all aspects of central sensitization expected to be induced 2 h after IS application: dural receptive fields did not expand; neuronal response threshold to dural indentation and skin stimulation did not decrease; spontaneous firing rate did not increase. The early treatment results suggest that triptan action provides a powerful means of preventing the initiation of central sensitization triggered by chemical stimulation of meningeal nociceptors. The late treatment results suggest that triptan action is insufficient to counteract an already established central sensitization. Thus, triptan action appears to be exerted directly on peripheral rather than central trigeminovascular neurons.

Functional neuroimaging of primary headache disorders

Until recently, primary headache disorders such as migraine and cluster headache were considered to be vascular in origin. However, advances in neuroimaging techniques, such as positron emission tomography, single photon emission computerized tomography, and functional magnetic resonance imaging, have augmented the growing clinical evidence that these headaches are primarily driven from the brain. This review covers functional imaging studies in migraine, cluster headache, rarer headache syndromes, and experimental head pain. Together with newer techniques, such as voxel-based morphometry and magnetic resonance spectrometry, functional imaging continues to play a role in elucidating and targeting the neural substrates in each of the primary headache syndromes.

Activation of 5-HT1B/1D receptor in the periaqueductal gray inhibits nociception

It is considered that the site of action of the abortive antimigraine compounds acting at serotonin, 5-HT(1B/1D,) receptors (triptans) is the trigeminovascular system. We tested whether there is a non-trigeminal site of action. The 5-HT(1B/1D) agonist, naratriptan, was microinjected into the ventrolateral periaqueductal gray (vlPAG), and activity in the trigeminal nucleus caudalis (TNC) was monitored. Recordings were made from 20 nociceptive neurons in the dorsal horn of the TNC that received convergent input from the dura mater and face. Responses of neurons to dural, facial cutaneous and corneal stimulation were studied before and after injection of naratriptan. Naratriptan decreased the excitability to electrical stimulation of the dura mater as the A-fiber response decreased by 24 +/- 4.1% (p < 0.001) and the C-fiber response decreased by 42 +/- 8.2% (p < 0.001). Spontaneous activity was decreased by 38 +/- 7.5% (p < 0.001). After injection, the mechanical thresholds of the dura mater increased from (n = 14, p < 0.01). Responses to stimulation of the face and cornea were not altered by injection of naratriptan. These results suggest that 5-HT(1B/1D) receptor activation in the vlPAG activates descending pain-modulating pathways that inhibit dural, but not facial and corneal nociceptive input. These findings have implications for the understanding of the action of triptans in migraine and cluster headache, suggesting that brain loci other than the trigeminal nucleus may play a role in the clinical action of triptans.

Effect of the adenosine A1 receptor agonist GR79236 on trigeminal nociception with blink reflex recordings in healthy human subjects

GR79236 is a highly potent and selective adenosine A1 receptor agonist that has analgesic and anti-inflammatory actions in humans and animals. In animal models it inhibits trigeminal nerve firing and calcitonin gene-related peptide release which play a pivotal role in migraine pathophysiology. Thus GR79236 may have therapeutic potential in migraine. Although there are no validated human models of migraine, the trigeminal nociceptive pathways may be studied with a novel electrode to elicit nociception-specific blink reflex responses. Twelve healthy female volunteers were randomized in a double-blind, placebo-controlled, cross-over trial to investigate the effect of GR79236 on trigeminal nociceptive pathways, as measured by the blink reflex. A secondary objective was to compare the use of two types of electrode, the standard (SE) and nociception-specific electrodes (NE), to investigate human trigeminal pharmacology. Blink reflexes were elicited with SE and NE before and 30 min after GR79236 (10 microg/kg i.v.) or placebo. The median area under the curve of repeated sweeps of the R2 component of the blink reflex was analysed using analysis of covariance with baseline as covariate. Using NE, GR79236 produced a non-significant reduction of the ipsilateral R2 compared with placebo (P = 0.097) and a significant reduction contralaterally (P = 0.008). No significant changes were observed using SE. There were no significant adverse events. The results suggest that NE is more sensitive than SE to detect pharmacological effects in the trigeminal nociceptive system. Furthermore, the adenosine A1 receptor agonist GR79236 inhibits trigeminal nociception in humans. These results support a possible therapeutic role for GR79236 in primary headache disorders.

Donitriptan selectively decreases jugular venous oxygen saturation in the anesthetized pig: further insights into its mechanism of action relevant to headache relief

The effects of donitriptan on systemic arterial-jugular venous oxygen saturation difference were evaluated in pentobarbitone-anesthetized pigs. Oxygen and carbon dioxide partial pressures in systemic arterial and jugular venous blood as well as hemoglobin oxygen saturation were determined by conventional blood gas analysis. Vehicle (40% polyethyleneglycol in saline, n = 9) or donitriptan (0.01, 0.04, 0.16, 0.63, 2.5, 10, and 40 microg/kg, n = 7) were cumulatively infused over 15 min/dose. The involvement of 5-hydroxytryptamine(1B) (5-HT(1B)) receptors was assessed in the presence of the 5-HT(1B/1D) receptor antagonist, GR 127935. Donitriptan decreased markedly and dose dependently jugular venous oxygen saturation [ED(50) 0.5 (0.3-1.1) microg/kg], in parallel with increases in carotid vascular resistance [ED(50) 0.9 (0.7-1.1) microg/kg]. Since arterial oxygen saturation and partial pressure remained unchanged, donitriptan significantly increased arteriovenous oxygen saturation difference from 0.63 microg/kg (maximal variation: 57 +/- 18%, P < 0.05 compared with vehicle). Unexpectedly, donitriptan from 2.5 microg/kg induced marked and significant increases in carbon dioxide partial pressure (pVCO(2)) in venous blood (maximal increase 18.8 +/- 5.7%; P < 0.05 compared with vehicle). Pretreatment with GR 127935 (0.63 mg/kg, n = 5) abolished the fall in venous oxygen saturation and the increase in carotid vascular resistance and reduced the increases in pVCO(2) induced by donitriptan. The results demonstrate that donitriptan, via 5-HT(1B) receptor activation, decreases the oxygen saturation of venous blood draining the head, concomitantly with cranial vasoconstriction. Since donitriptan also increased pVCO(2), an effect upon cerebral oxygen consumption and metabolism is suggested in addition to cranial vasoconstriction, which may be relevant to its headache-relieving effects.

Tolerability of the triptans: clinical implications

The triptans represent a relatively new class of compounds effective in the treatment of migraine. The safety and tolerability of these drugs have been extensively investigated since the first triptan (sumatriptan) became commercially available. A report on a very large population of patients tested during clinical trials and in postmarketing studies, confirms that these drugs are safe and well tolerated when correctly used. Adverse events are frequently reported, but are usually mild and only a few patients discontinue therapy because of them. These adverse events include, in particular, the so-called 'triptan symptoms' (tingling, sensation of warmth, etc.). The exact mechanism of chest symptoms reported by 20% of patients with migraine treated with triptans remains unclear, but are exceptionally related to a cardiac mechanism. CNS adverse events (i.e. somnolence) are also reported, but it is a matter of debate whether they are related to the pharmacological properties (i.e. lipophilicity) of the drug or are symptoms of the disease itself. The potential risk for drug overuse must be taken into account when the triptans are given to patients with a high frequency of migraine attacks. Clinical interaction of triptans with other drugs metabolised in the liver may theoretically influence the incidence of adverse events, but there is little evidence to support this assumption. There is no evidence of a teratogenic risk of triptans in pregnant women taking these drugs.

Evidence for serotonin (5-HT)1B, 5-HT1D and 5-HT1F receptor inhibitory effects on trigeminal neurons with craniovascular input

Development of serotonin (5HT(1B/1D)) agonists for the acute attack of migraine resulted in considerable interest in their action. The superior sagittal sinus (SSS) was isolated in alpha-chloralose (60 mg/kg, i.p. and 20 mg/kg i.v.i. supplementary 2 hourly) anaesthetised cats. The SSS was stimulated electrically (100 V, 250 micros duration, 0.3 Hz) and neurons of the trigeminocervical complex monitored using electrophysiological methods. To test 5-HT(1B) receptor-mediated activity common carotid blood flow (CCF) was monitored with a transonic flow probe placed around the vessel. Naratriptan (5-HT(1B/1D/1F) receptor agonist) and alniditan (5-HT(1B/1D) receptor agonist) produced reductions in carotid blood flow of 38+/-5% and 42+/-6%, respectively. These effects were attenuated by the 5-HT(1B) receptor antagonist SB224289 (P<0.05). LY344864 (5-HT(1F) receptor agonist) had no effect on CCF. Naratriptan inhibited SSS-evoked activity (61+/-7%), an effect partially inhibited by the 5-HT(1B) receptor antagonist SB224289 (30+/-5%), or by the 5-HT(1D) receptor antagonist BRL-15572 (37+/-6%). There remained an inhibitory effect of naratriptan after both 5-HT(1B) and 5-HT(1D) receptor blockade (22+/-5%). Alniditan inhibited SSS-evoked trigeminal activity (53+/-6%), an effect abolished after 5-HT(1B) and 5-HT(1D) receptor blockade. LY344864 (5-HT(1F) receptor agonist) inhibited SSS-evoked trigeminal activity (28+/-5%), an effect unaltered by either SB224289 or BRL-15572. It can be concluded that there are inhibitory 5-HT(1B), 5-HT(1D) and 5-HT(1F) receptors in the trigeminocervical complex of the cat. 5-HT(1B) receptor-mediated inhibition is the most potent of the three in terms of inhibition of trigeminovascular nociceptive traffic.

Molecular Mechanisms of Migraine

Migraine is a common complex disorder that affects a large portion of the population and thus incurs a substantial economic burden on society. The disorder is characterized by recurrent headaches that are unilateral and usually accompanied by nausea, vomiting, photophobia, and phonophobia. The range of clinical characteristics is broad and there is evidence of comorbidity with other neurological diseases, complicating both the diagnosis and management of the disorder. Although the class of drugs known as the triptans (serotonin 5-HT(1B/1D) agonists) has been shown to be effective in treating a significant number of patients with migraine, treatment may in the future be further enhanced by identifying drugs that selectively target molecular mechanisms causing susceptibility to the disease.Genetically, migraine is a complex familial disorder in which the severity and susceptibility of individuals is most likely governed by several genes that may be different among families. Identification of the genomic variants involved in genetic predisposition to migraine should facilitate the development of more effective diagnostic and therapeutic applications. Genetic profiling, combined with our knowledge of therapeutic response to drugs, should enable the development of specific, individually-tailored treatment.

The antimigraine 5-HT 1B/1D receptor agonists, sumatriptan, zolmitriptan and dihydroergotamine, attenuate pain-related behaviour in a rat model of trigeminal neuropathic pain

1. Peripheral lesion to the trigeminal nerve may induce severe pain states. Several lines of evidence have suggested that the antimigraine effect of the triptans with 5-HT(1B/1D) receptor agonist properties may result from inhibition of nociceptive transmission in the spinal nucleus of the trigeminal nerve by these drugs. On this basis, we have assessed the potential antinociceptive effects of sumatriptan and zolmitriptan, compared to dihydroergotamine (DHE), in a rat model of trigeminal neuropathic pain. 2. Chronic constriction injury was produced by two loose ligatures of the infraorbital nerve on the right side. Responsiveness to von Frey filament stimulation of the vibrissal pad was used to evaluate allodynia. 3. Two weeks after ligatures, rats with a chronic constriction of the right infraorbital nerve displayed bilateral mechanical hyper-responsiveness to von Frey filament stimulation of the vibrissal pad with a mean threshold of 0.38+/-0.04 g on the injured side and of 0.43+/-0.04 g on the contralateral (left) side (versus > or =12.5 g on both sides in the same rats prior to nerve constriction injury). 4. Sumatriptan at a clinically relevant dose (100 microg kg(-1), s.c.) led to a significant reduction of the mechanical allodynia-like behaviour on both the injured and the contralateral sides (peak-effects 6.3+/-1.1 g and 4.4+/-0.7 g, respectively). A more pronounced effect was obtained with zolmitriptan (100 microg kg(-1), s.c.) (peak-effects: 7.4+/-0.9 g and 3.2+/-1.3 g) whereas DHE (50-100 microg kg(-1), i.v.) was less active (peak-effect approximately 1.5 g). 5. Subcutaneous pretreatment with the 5-HT(1B/1D) receptor antagonist, GR 127935 (3 mg kg(-1)), prevented the anti-allodynia-like effects of triptans and DHE. Pretreatment with the 5-HT(1A) receptor antagonist, WAY 100635 (2 mg kg(-1), s.c.), did not alter the effect of triptans but significantly enhanced that of DHE (peak effect 4.3+/-0.5 g). 6. In a rat model of peripheral neuropathic pain, which consisted of a unilateral loose constriction of the sciatic nerve, neither sumatriptan (50-300 microg kg(-1)) nor zolmitriptan (50-300 microg kg(-1)) modified the thresholds for paw withdrawal and vocalization in response to noxious mechanical stimulation. 7. These results support the rationale for exploring the clinical efficacy of brain penetrant 5-HT(1B/1D) receptor agonists as analgesics to reduce certain types of trigeminal neuropathic pain in humans.

Persistence of attacks of cluster headache after trigeminal nerve root section

Cluster headache is a strictly unilateral headache that occurs in association with cranial autonomic features. We report a patient with a trigeminal nerve section who continued to have attacks. A 59-year-old man described a 14-year history of left-sided episodes of excruciating pain centred on the retro-orbital and orbital regions. These episodes lasted 1-4 h, recurring 2-3 times daily. The attacks were associated with ipsilateral ptosis, conjunctival injection, lacrimation, rhinorrhoea and facial flushing. From 1986 to 1988, he had trials of medications without any benefit. In February 1988, he had complete surgical section of the left trigeminal sensory root that shortened the attacks in length for 1 month without change in their frequency or character. In April 1988, he had further surgical exploration and the root was found to be completely excised; post-operatively, there was no change in the symptoms. From 1988 to 1999, he had a number of medications, including verapamil and indomethacin, all of which were ineffective. Prednisolone 30 mg orally daily rendered the patient completely pain free. Sumatriptan 100 mg orally and 6 mg subcutaneously aborted the attack after approximately 45 and 15 min, respectively. He was completely anaesthetic over the entire left trigeminal distribution. Left corneal reflex was absent. Motor function of the left trigeminal nerve was preserved. Neurological and physical examination was otherwise normal. MRI scan showed a marked reduction in the calibre of the left trigeminal nerve from the nerve root exit zone in the pons to Meckel's cave. An ECG-gated three-dimensional multislab MRI inflow angiogram was performed. No dilatation was observed in the left internal carotid artery during the cluster attack. Blink reflexes were elicited with a standard electrode and stimulus. Stimulation of the left supraorbital nerve produced neither ipsilateral nor contralateral blink reflex response. Stimulation of the right supraorbital nerve produced an ipsilateral response with a mean R2 onset latency of 36 ms and a contralateral response with a mean R2 onset latency of 32 ms. Lack of ipsilateral vessel dilatation makes the role of vascular factors in the initiation of cluster attacks questionable. With complete section of the left trigeminal sensory root the brain would perceive neither vasodilatation nor a peripheral neural inflammatory process; however, the patient continued to have an excellent response to sumatriptan. The case illustrates that cluster headache may be generated primarily from within the brain, and that triptans may have anti-headache effects through an entirely central mechanism.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

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.

Effects of donitriptan on carotid haemodynamics and cardiac output distribution in anaesthetized pigs

We investigated the effects of donitriptan, which possesses a uniquely high affinity and efficacy at 5-HT1B/1D receptors, on carotid and systemic haemodynamics in anaes thetized pigs. Donitriptan (0.16-100 microg kg(-1), i.v.) dose-dependently decreased total carotid blood flow and vascular conductance (maximum response: -25 +/- 3%). This effect was entirely due to a selective reduction in the cephalic arteriovenous anastomotic fraction (maximum response: - 63 +/- 3%; ED50%: 92 +/- 31 nmol/kg); the nutrient vascular conductance increased. Donitriptan did not decrease vascular conductances in or blood flow to a number of organs, including the heart and kidneys; in fact, vascular conductances in the skin, brain and skeletal muscles increased. Cardiac output was slightly decreased by donitriptan, but this effect was confined to peripheral arteriovenous anastomoses. The haemodynamic effects of donitriptan were substantially reduced by the 5-HT1B/1D receptor antagonist GR127935. These results show that donitriptan selectively constricts arteriovenous anastomoses via 5-HT1B receptor activation. The drug should be able to abort migraine headaches and it is unlikely to compromize blood flow to vital organs.

Eletriptan

Eletriptan (Relpax, Pfizer) is one of a group of anti-migraine medications commonly referred to as 'triptans'. It is a potent serotonin agonist at the 5-HT(1B/1D) receptor and is indicated for the acute treatment of migraine headaches. Eletriptan is administered orally. It is rapidly absorbed and has a bioavailability of 50% compared to 14% for sumatriptan. The relatively high lipophilicity of eletriptan compared to sumatriptan may explain its faster oral absorption and shorter time to onset of action. Results from comparative studies between oral eletriptan and sumatriptan indicate that eletriptan 80 mg was superior to sumatriptan 100 mg in onset of action, headache response rate, pain free response rate and relief of associated migraine symptoms at the 1 or 2 h time intervals. Although there was a modest increase in adverse events with eletriptan 80 mg than with sumatriptan 100 mg, eletriptan received a high patient acceptability rating (84%).

Serotonergic effects and extracellular brain levels of eletriptan, zolmitriptan and sumatriptan in rat brain

In vivo microdialysis was used to assess the central serotonergic effects and extracellular brain levels of the 5-HT(1B/1D) receptor agonists eletriptan, zolmitriptan and sumatriptan in rats after intravenous and intracerebral administration, while their binding affinities and functional potencies were determined at 5-HT(1B), 5-HT(1D) and 5-HT(1A) receptors. In vitro studies showed that all three triptans are high affinity, full agonists at 5-HT(1B/1D) receptors, but that sumatriptan is functionally less potent as a 5-HT(1B/1D) agonist than zolmitriptan and eletriptan. Local intracortical perfusion with the compounds via the dialysis probe decreased cortical 5-HT (5-hydroxytryptamine, serotonin) release with ED(50) values of approximately 0.1 microM for eletriptan and zolmitriptan and 0.5 microM for sumatriptan. At 3.2 mg/kg i.v., both eletriptan and zolmitriptan decreased 5-HT levels by about 35%, while sumatriptan had no effect, despite the fact that maximal sumatriptan concentrations in cortical dialysates were higher (8.8 nM at 20 min) than those of zolmitriptan (5.9 nM at 20 min) and eletriptan (2.6 nM at 40 min). The observation that eletriptan and zolmitriptan produce almost identical central serotonergic effects, after intracerebral as well as after systemic administration, is in agreement with their comparable functional 5-HT(1B/1D) receptor agonist potencies and their free levels in cortical dialysates after 3.2 mg/kg i.v. On the other hand, the lack of central serotonergic effects of 3.2 mg/kg i.v. sumatriptan is likely due to its weaker functional 5-HT(1B/1D) receptor agonist potency than eletriptan and zolmitriptan, rather than lower brain levels, consistent with sumatriptan's fivefold lower potency after intracerebral administration.

4991W93, a potent blocker of neurogenic plasma protein extravasation, inhibits trigeminal neurons at 5-hydroxytryptamine (5-HT1B/1D) agonist doses

Triptans share the pharmacological profile of being 5-hydroxytryptamine (5-HT1B/1D) agonists and having potent anti-migraine activity. The conformationally restricted zolmitriptan analogue 4991W93 was developed as a potent, and at low doses, specific, non-vasconstrictor inhibitor of neurogenic dural plasma protein extravasation. Here, we sought to study the effect of 4991W93 at plasma protein extravasation blocking and at 5-HT(1B/1D) agonist doses. Nociceptive cells with firing latencies consistent with Adelta fibres were recorded in the dorsal horn region of the trigeminal nucleus caudalis after electrical stimulation of the sagittal sinus. Both evoked (13 units) and free running (6 units) activity in cells linked to sagittal sinus stimulation were inhibited by 4991W93 delivered microiontophoretically or by intravenous administration at 10 microg/kg or 100 microg/kg, but not 0.1 microg/kg. When applied iontophoretically, 4991W93 did not appear to have an additive effect over a 5-HT(1B/1D) agonist effective concentration of zolmitriptan. These data suggest that 4991W93 is only effective at modulating the trigeminocervical complex at 5-HT(1B/1D) agonist doses. To account for neurogenic dural plasma protein extravasation blockade in animal studies, 4991W93 might have non-5-HT(1B/1D)-based pharmacological targets that are yet to be described.

Molecular cloning, pharmacological properties and tissue distribution of the porcine 5-HT(1B) receptor

Using a combination of RT - PCR and inverse-PCR techniques, we amplified, cloned and sequenced a full-length porcine 5-HT(1B) receptor cDNA derived from porcine cerebral cortex. Sequence analysis revealed 1170 bp encoding an open reading frame of 390 amino acids showing a 95% similarity with the human 5-HT(1B) receptor. The recombinant porcine 5-HT(1B) cDNA was expressed in monkey Cos-7 cells and its pharmacological profile was determined by radioligand binding assay using [(3)H]-GR125743. The affinities of several agonists (L694247>ergotamine > or =5-carboxamidotryptamine=dihydroergotamine=5-HT>CP122638=zolmitriptan>sumatriptan) and putative antagonists (GR127935>methiothepin>SB224289>>ritanserin>ketanserin > or =BRL15572) correlated highly with those described for the recombinant human 5-HT(1B) receptor. In membranes obtained from cells co-expressing the porcine 5-HT(1B) receptor and a mutant G(alphao)Cys(351)Ile protein, 5-HT and zolmitriptan increased, while the 5-HT(1B) receptor antagonist SB224289 decreased basal [(35)S]-GTPgammaS binding, thus showing inverse agonism. The potency of zolmitriptan in the [(35)S]-GTPgammaS binding assay (pEC(50): 7.64+/-0.04) agreed with its affinity in displacing the antagonist [(3)H]-GR125743 (pK(i): 7.36+/-0.07). The 5-HT(1B) receptor mRNA was observed by RT-PCR in several blood vessels, cerebral cortex, cerebellum and trigeminal ganglion. In situ hybridization performed in frontal cerebral cortex sections revealed the expression of 5-HT(1B) receptor mRNA in pyramidal cells. In conclusion, we have cloned and established the amino acid sequence, ligand binding profile and location of the porcine 5-HT(1B) receptor. This information may be useful in exploring the role of 5-HT(1B) receptor in pathophysiological processes relevant for novel drug discovery in diseases such as migraine.

Meningeal nociception: electrophysiological studies related to headache and referred pain

Headaches, which are usually referred to characteristic sites of the skull, are believed to involve meningeal nociceptors located in the dura mater encephali. Animal experiments show that these meningeal nociceptors are polymodal and usually highly sensitive to mechanical stimulation. These properties are also characteristic for the second order neurons in the spinal trigeminal nucleus, most of which receive convergent input from facial receptive sites. Sensitization of primary and secondary neurons by chemical irritants to mechanical stimuli may be an important mechanism in the generation of headaches. The convergent input from extracranial structures, which seems to be differentially organized in rodents and man, may explain the typical features of referred headache. Targets for analgesics used in the therapy of headaches (non-steroidal antiinflammatory drugs, 5-HT(1) receptor agonists) are probably meningeal nociceptors and different sites of the central trigeminal nociceptive and antinociceptive pathways.

Modulation of neuronal activity in the nucleus raphé magnus by the 5-HT(1)-receptor agonist naratriptan in rat

Modulatory effects of the new antimigraine drug naratriptan, a 5-HT(1)-receptor-agonist, on neurons of the nucleus raphé magnus were examined in rat by extracellular recordings. In the nucleus raphé magnus neuronal activity decreased in on-cells and increased in off-cells after intravenous administration of naratriptan. The modulatory effects of naratriptan were similar to the well-known effects of morphine on neurons in the nucleus raphé magnus. The results of this study suggest central actions of naratriptan and may point to an involvement of the endogenous pain control system in the antinociceptive effects of the 5-HT(1)-receptor-agonist.

Triptans in migraine: a comparative review of pharmacology, pharmacokinetics and efficacy

Triptans are a new class of compounds developed for the treatment of migraine attacks. The first of the class, sumatriptan, and the newer triptans (zolmitriptan, naratriptan, rizatriptan, eletriptan, almotriptan and frovatriptan) display high agonist activity at mainly the serotonin 5-HT1B and 5-HT1D receptor subtypes. As expected for a class of compounds developed for affinity at a specific receptor, there are minor pharmacodynamic differences between the triptans. Sumatriptan has a low oral bioavailability (14%) and all the newer triptans have an improved oral bioavailability and for one, risatriptan, the rate of absorption is faster. The half-lives of naratriptan, eletriptan and, in particular, frovatriptan (26 to 30h) are longer than that of sumatriptan (2h). These pharmacokinetic improvements of the newer triptans so far seem to have only resulted in minor differences in their efficacy in migraine. Double-blind, randomised clinical trials (RCTs) comparing the different triptans and triptans with other medication should ideally be the basis for judging their place in migraine therapy. In only 15 of the 83 reported RCTs were 2 triptans compared, and in 11 trials triptans were compared with other drugs. Therefore, in all placebo-controlled randomised clinical trials, the relative efficacy of the triptans was also judged by calculating the therapeutic gain (i.e. percentage response for active minus percentage response for placebo). The mean therapeutic gain with subcutaneous sumatriptan 6mg (51%) was more than that for all other dosage forms of triptans (oral sumatriptan 100mg 32%; oral sumatriptan 50mg 29%: intranasal sumatriptan 20mg 30%; rectal sumatriptan 25mg 31%; oral zolmitriptan 2.5mg 32%; oral rizatriptan 10mg 37%; oral eletriptan 40mg 37%; oral almotriptan 12.5mg 26%). Compared with oral sumatriptan 100mg (32%), the mean therapeutic gain was higher with oral eletriptan 80mg (42%) but lower with oral naratriptan 2.5mg (22%) or oral frovatriptan 2.5mg (16%). The few direct comparative randomised clinical trials with oral triptans reveal the same picture. Recurrence of headache within 24 hours after an initial successful response occurs in 30 to 40% of sumatriptan-treated patients. Apart from naratriptan, which has a tendency towards less recurrence, there appears to be no consistent difference in recurrence rates between the newer triptans and sumatriptan. Rizatriptan with its shorter time to maximum concentration (tmax) tended to produce a quicker onset of headache relief than sumatriptan and zolmitriptan. The place of triptans compared with non-triptan drugs in migraine therapy remains to be established and further RCTs are required.

Glutamatergic transmission in the trigeminal nucleus assessed with local blood flow

Stimulation of the superior sagittal sinus in humans is pain-producing and in experimental animals leads to excitation of neurons in the caudal trigeminal nucleus and dorsal horns of the C(1/)C(2) cervical spinal cord: the trigeminocervical complex. Neuronal excitation is generally associated with an increase in local blood flow due to flow/metabolism coupling and we have used local blood flow in the trigeminocervical complex to examine the role of N-methyl-D-aspartate (NMDA)-mediated transmission in these neurons. Cats were anaesthetised with alpha-chloralose (60 mg/kg, ip; supplements 20 mg/kg iv) after surgical preparation under halothane (0.5-3%). Animals were paralysed with gallamine triethiodide to prevent possible movement artefact distorting the laser Doppler signals. The superior sagittal sinus was isolated for electrical stimulation (150 V; 250 microsec duration; 0.5, 1, 2, 5, 10 and 20 Hz) and the dorsal surface of the spinal cord exposed at the C(2) level. Blood flow was recorded from the region over the trigeminocervical complex by careful placement of a laser Doppler flow probe. Flow was recorded continuously by an online collection programme and NMDA-mediated transmission modulated by intravenous administration of MK-801 (0.4, 1 and 4 mg/kg, iv) at the stimulation frequency of 5 Hz. Stimulation of the superior sagittal sinus produced a stimulus-locked, frequency-dependent increase in blood flow in the region of the trigeminocervical complex. The mean maximum response was 39+/-4% at 20 Hz. MK-801 had no effect on the resting flow signal but markedly attenuated the SSS-evoked response in a dose-dependent manner. The mean maximum response after 4 mg/kg MK-801 was 13+/-2%. NMDA-mediated transmission is likely to be involved in nociceptive trigeminovascular transmission within the trigeminocervical complex and offers a possible target for both acute and preventative treatment of migraine.

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.

Current and emerging second-generation triptans in acute migraine therapy: a comparative review

Sterile neurogenic inflammation within cephalic tissue, involving vasodilation and plasma protein extravasation, has been proposed as a pathophysiological mechanism in acute migraine. The action of 5-hydroxytryptamine (5-HT1B/1D) agonists--so-called triptans--on receptors located in meningeal arteries (5-HT1B) and trigeminovascular fiber endings (5-HT1D) has an inhibitory effect on this neurogenic inflammation. Recently, a series of second-generation 5-HT1B/1D agonists (almotriptan, eletriptan, frovatriptan, naratriptan, rizatriptan, and zolmitriptan) have been developed and are reviewed in this article. Their in vitro pharmacological properties, pharmacokinetics, clinical efficacy, drug interactions, and adverse effects are evaluated and compared to the golden standard in the treatment of acute migraine, sumatriptan.

Auditory evoked potentials in the assessment of central nervous system effects of antimigraine drugs

Because the "intensity dependence" of cortical auditory evoked potentials (IDAP) is under serotonergic control, it can be used to assess central antimigraine effects of 5HT1B/1D agonists. We measured IDAP before and 2 h after naratriptan (5 mg, n = 19) and zolmitriptan (5 mg, n = 19) in healthy volunteers. IDAP was expressed as the amplitude-stimulus intensity function ("ASF slope"). Naratriptan tended to increase ASF slope (mean difference 0.23 +/- 0.62 microV/10 dB, p = 0.06) while zolmitriptan (0.08 +/- 0.95 microV/10 dB, p = 0.35) did not. We assessed the suitability of IDAP for measuring central antimigraine drug effects using repeatability data (see companion paper). We calculated the trade-off between the size of the expected drug effects (ASF slope difference) and the necessary sample size. Because of poor repeatability 36 to 80 subjects are required to detect ASF slope changes in the 0.25-0.5 microV/10 dB range. These data can be used to design trials using IDAP.

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.

Pharmacological aspects of experimental headache models in relation to acute antimigraine therapy

The last decade has witnessed a tremendous progress in the acute therapy of migraine, with sumatriptan, belonging to a new class of drugs, now known as 5-HT(1B/1D/1F) receptor agonists, leading the way. The undoubted success of sumatriptan stimulated the development of new triptans as well as other suitable pharmacological tools and experimental models to probe into complex migraine mechanisms. In this review, we discuss the main experimental models for migraine, against the background of the disease pathophysiology and 5-HT receptors considered most important for migraine therapy. We believe that the use of these migraine models will provide even better treatment for migraine patients in the next millennium.

Naratriptan: an alternative for migraine

OBJECTIVE

To critically evaluate the literature regarding naratriptan's clinical pharmacology, efficacy, safety, and indications.

DATA SOURCE

A MEDLINE search was conducted for the period from January 1990 to June 1998. Key words used included naratriptan, triptan, serotonin agonists, migraine, and migraine therapy. In addition, pertinent references cited in articles obtained from MEDLINE and product information for triptans were reviewed.

STUDY SELECTION AND DATA EXTRACTION

All original and review articles and abstracts pertaining to naratriptan were reviewed, as were product information extracts. Clinical trials of naratriptan were critically reviewed and compared with pertinent clinical trials of other oral triptans.

DATA SYNTHESIS

The treatment of migraine has been dramatically improved with the use of sumatriptan, other triptans, and serotonin-receptor subtype 1B and 1D agonists. Drawbacks to these medications, however, have included poorly tolerated adverse effects and the recurrence of the migraine. Naratriptan has been recently approved for acute oral migraine therapy. In two Phase III trials of naratriptan compared with placebo, relief at four hours was obtained in 60% and 68% of patients using the 2.5-mg dose, with recurrence of headache in 24 hours in 27% and 28% of patients. The data on migraine recurrence were similar to those of other oral triptans; the efficacy of naratriptan at two hours was not specifically analyzed. Adverse effects of naratriptan were similar to placebo, and its tolerability seemed superior compared with studies of other oral triptans.

CONCLUSIONS

Naratriptan is a promising new oral therapy for acute migraine; it may successfully treat patients who poorly tolerate other triptan therapies or have longer duration migraine headaches.

Trigeminovascular nociceptive transmission involves N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate receptors

Interest in the fundamental mechanisms underlying headache, particularly the pathophysiology of migraine and cluster headache, has lead to the study of the physiology and pharmacology of the trigeminovascular system and its central ramifications. Cats were anaesthetized (60 mg/kg alpha-chloralose, i.p., along with halothane for all surgical procedures) and prepared for physiological monitoring. The animals were placed in a stereotaxic frame and ventilated. A midline craniotomy and C2 laminectomy were performed for access to the superior sagittal sinus and C2 dorsal horn, respectively. The sinus was isolated from the underlying cortex and stimulated electrically after the animals had been paralysed with gallamine (6 mg/kg, i.v.). Units linked to stimulation were recorded with a tungsten-in-glass microelectrode placed in the most caudal part of the trigeminal nucleus, the trigeminocervical complex. Signals from the neurons were amplified, filtered and passed to a microcomputer, where post-stimulus histograms were constructed on-line to analyse the responses to stimulation. Units responded to sagittal sinus stimulation with a typical latency of 8-10 ms. All units studied had a probability of firing of 0.6 or greater. Intravenous injection of the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (4 mg/kg, i.v.), resulted in a substantial and prolonged blockade of firing of units in the trigeminocervical complex. Similarly, administration of the non-N-methyl-D-aspartate excitatory amino acid receptor blocker, GYKI 52466, lead to a dose-dependent inhibition of trigeminovascular-evoked responses in the trigeminocervical complex. These data demonstrate the participation of both N-methyl-D-aspartate- and non-N-methyl-D-aspartate-mediated mechanisms in transmission within the trigeminocervical complex, and suggest a clear preclinical role of glutamatergic mechanisms in primary headache syndromes, such as migraine and cluster headache.

Fos expression in the trigeminocervical complex of the cat after stimulation of the superior sagittal sinus is reduced by L-NAME

Primary neurovascular headaches, such as migraine and cluster headache probably involve activation of trigeminovascular pain structures projecting to the trigeminocervical complex of neurons in the caudal brain stem and upper cervical spinal cord. It has recently been demonstrated that blockade of the synthesis of nitric oxide (NO) by an NO synthesis inhibitor can abort acute migraine attacks and thus it is of interest to determine whether there is an influence of NO generation on trigeminocervical neurons. Cats were anaesthetised with alpha-chloralose (60 mg/kg, i.t.). supplemental 20 mg/kg, intravenously (i.v.)) and halothane for surgery (0.5-3% by inhalation). A circular midline craniotomy was performed to isolate the superior sagittal sinus (SSS) for electrical stimulation (0.3 Hz, 150 V, 250 micros duration for 2 h). Two groups were compared, one stimulated after administration of vehicle and the other stimulated after administration of N(G)-nitro-L-arginine methylester (L-NAME: 100 mg/kg, i.v.). After stimulation of the SSS Fos immunoreactivity was observed in lamina I/IIo of the trigeminal nucleus caudalis and dorsal horns of C1 and C2 to a median total of 136 cells (range 122-146). After L-NAME treatment Fos expression was significantly reduced to 40 cells (24-54; P < 0.02). In conclusion, inhibition of NO synthesis L-NAME markedly reduces Fos expression in the trigeminocervical complex of the cat. These data taken together with the clinical observations of the effect of NO synthesis blockade in migraine suggest a role for NO generation in mediating nociceptive transmission in acute migraine.

Dural vasodilation causes a sensitization of rat caudal trigeminal neurones in vivo that is blocked by a 5-HT1B/1D agonist

1. Migraine headache pain is thought to result from an abnormal distention of intracranial, extracerebral blood vessels and the consequent activation of the trigeminal nervous system. Migraine is also often accompanied by extracranial sensory disturbances from facial tissues. These experiments investigate whether meningeal dilation produces central sensitization of neurones that receive convergent input from the face. 2. Single unit extracellular activity was recorded from the trigeminal nucleus caudalis of anaesthetized rats in response to either noxious stimulation of the dura mater, innocuous stimulation of the vibrissae or to a transient dilation of the meningeal vascular bed. 3. Rat alpha-CGRP (calcitonin gene-related peptide; 1 microg kg(-1), i.v.) caused a dilation of the middle meningeal artery and facilitated vibrissal responses by 36+/-7%. 4. The 5-HT1B/1D agonist, L-741,604 (3 mg kg(-1), i.v.), inhibited responses to noxious stimulation of the dura mater (16+/-7% of control) and, in a separate group of animals, blocked the CGRP-evoked facilitation of vibrissal responses. 5. L-741,604 (3 mg kg(-1), i.v.) also inhibited responses to innocuous stimulation of the vibrissa (14+/-10% of control) with neurones that received convergent input from the face and from the dura mater, but not with cells that received input only from the face (70+/-12% of control). 6. These data show that dilation of meningeal blood vessels causes a sensitization of central trigeminal neurones and a facilitation of facial sensory processing which was blocked by activation of pre-synaptic 5-HT1B/1D receptors. 7. Sustained dural blood vessel dilation during migraine may cause a sensitization of trigeminal neurones. This may underlie some of the symptoms of migraine, such as the headache pain and the extracranial allodynia. Inhibition of this central sensitization may therefore offer a novel strategy for the development of acute and/or prophylactic anti-migraine therapies.

The effects of 5-HT1A, 5-HT1B and 5-HT1D receptor agonists on trigeminal nociceptive neurotransmission in anaesthetized rats

Pre-clinical studies have suggested that one mechanism of antimigraine action of the 'triptan' 5-HT1B/1D receptor agonists may be through inhibition of central nociceptive transmission in the trigeminal dorsal horn. In anaesthetized rats, the 5-HT1B/1D receptor agonist, zolmitriptan (up to 3 mg kg(-1), i.v.), inhibited the action potential discharge of single trigeminal neurones to noxious electrical stimulation of the middle meningeal artery. In contrast, the selective 5-HT1B receptor agonist, CP-93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one), and the 5-HT1A receptor selective agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) had no effect in this assay at up to 3 mg kg(-1), i.v.. Brain penetrant, triptan 5-HT1B/1D receptor agonists may therefore mediate their central trigeminal anti-nociceptive action in the rat via 5-HT1D, but not 5-HT1B or 5-HT1A, receptors.

Serotonin inhibits trigeminal nucleus activity evoked by craniovascular stimulation through a 5HT1B/1D receptor: a central action in migraine?

The development of serotonin (5HT1B/1D) agonists as treatments for the acute attack of migraine has resulted in considerable interest in their mechanism of action and, to some extent, renewed interest in the role of serotonin (5-hydroxytryptamine; 5HT) in the disorder. The initial synthesis of this class of compounds was predicated on the clinical observation that intravenous 5HT terminated acute attacks of migraine. In this study the superior sagittal sinus was isolated in the alpha-chloralose (60 mg/kg i.p. and 20 mg/kg i.v. injection supplementary 2 hourly) anesthetized cat. The sinus was stimulated electrically (120V, 250 microsec duration, 0.3 Hz), and neurons of the trigeminocervical complex in the dorsal C2 spinal cord were monitored using electrophysiological methods. After baseline recordings in each animal, 5HT (15 microg/kg/min) was infused for 5 minutes in the presence of either vehicle (group A) or the 5HT1B/1D antagonist GR127935 (100 microg/kg i.v. injection; group B). The baseline probability of cell firing after sagittal sinus stimulation was 0.61 +/- 0.1 at a latency to the fastest peak of 11.1 +/- 0.4 msec. In group A, 5HT infusion alone had a small effect of increasing mean blood pressure (12 +/- 3 mm Hg), which in itself did not alter cell firing. In group A, 5HT alone had an inhibitory effect on evoked trigeminal activity, which developed 15 to 20 minutes after commencement of the infusion. The inhibition of cell firing lasted for 20 minutes, after which the activity returned to baseline. In group B, the combination of 5HT and GR127935 had no effect on trigeminal cell firing, although the small hypertensive effect was still present. These data indicate that 5HT inhibits evoked trigeminal nucleus firing via the 5HT1B/1D receptor at which GR127935 is an antagonist. It is likely that some part of the effect of 5HT in migraine relates to inhibition of trigeminal nucleus activity, just as it is likely that some part of the effect of the triptans is also mediated at this central site and may be complementary to their nonneuronal actions. Moreover, the data highlight the case for describing this class of headache as neurovascular headaches rather than vascular headaches, to recognize the implicit contribution of the trigeminovascular system to their pathophysiology.

Comparison of more and less lipophilic serotonin (5HT1B/1D) agonists in a model of trigeminovascular nociception in cat

The trigeminovascular system consists of bipolar neurons innervating pain-producing intracranial structures, such as the superior sagittal sinus (SSS), and projecting to the medullary and upper cervical dorsal horn second order neurons. Zolmitriptan is a newly developed 5HT1B/1D receptor agonist with both peripheral and central sites of action in the trigeminovascular system due to greater lipophilicity relative to the more hydrophilic antimigraine compound sumatriptan. Given that we have seen electrophysiological and autoradiographic binding data to suggest that the compound may inhibit activity at second-order neurons this study was designed to examine whether such an effect could be demonstrated in a population of trigeminal neurons using Fos immunohistochemistry. Cats were anesthetised with alpha-chloralose (60 mg/kg intraperitoneal then 20 mg/kg intravenous maintenance) with all surgery being conducted using halothane (1-3%). The animals were prepared for physiological monitoring, including blood pressure, heart rate, rectal temperature, and end-expiratory CO2. They were intubated, ventilated, and paralyzed with gallamine triethiodide (6 mg/kg i.v.). A midline craniotomy was performed to expose the sinus for electrical stimulation using hook electrodes. Twenty-four hours after completion of the surgical procedures the animal was ready for treatment. Vehicle, sumatriptan (85 micrograms/kg), or zolmitriptan (30 micrograms/kg) was administered and the SSS was stimulated (250 microseconds, 100 V at 0.3 Hz) for 1 h. Following an additional 1 h the animal was perfused and immunohistochemistry was used to detect the protein product of the immediate early gene c-Fos. We compared the dorsal horns of the medulla (trigeminal nucleus caudalis) and the C1 and C2 cervical spinal cords in control animals with those receiving zolmitriptan or sumatriptan. We noted a significant reduction in Fos expression after treatment with zolmitriptan but no effect with sumatriptan. Given that zolmitriptan accesses central neurons and that the method of stimulation we have employed would bypass peripheral trigeminal mechanisms it is likely that the reduction in second-order trigeminal neuronal activity was due to a direct inhibitory effect of the compound on those cells. These neurons form a possible site for the treatment of acute attacks of migraine.