Supraspinal Mechanisms of Intestinal Hypersensitivity.

Gut inflammation or injury causes intestinal hypersensitivity (IHS) and hyperalgesia, which can persist after the initiating pathology resolves, are often referred to somatic regions and exacerbated by psychological stress, anxiety or depression, suggesting the involvement of both the spinal cord and the brain. The supraspinal mechanisms of IHS remain to be fully elucidated, however, over the last decades the series of intestinal pathology-associated neuroplastic changes in the brain has been revealed, being potentially responsible for the phenomenon. This paper reviews current clinical and experimental data, including the authors' own findings, on these functional, structural, and neurochemical/molecular changes within cortical, subcortical and brainstem regions processing and modulating sensory signals from the gut. As concluded in the review, IHS can develop and maintain due to the bowel inflammation/injury-induced persistent hyperexcitability of viscerosensory brainstem and thalamic nuclei and sensitization of hypothalamic, amygdala, hippocampal, anterior insular, and anterior cingulate cortical areas implicated in the neuroendocrine, emotional and cognitive modulation of visceral sensation and pain. An additional contribution may come from the pathology-triggered dysfunction of the brainstem structures inhibiting nociception. The mechanism underlying IHS-associated regional hyperexcitability is enhanced NMDA-, AMPA- and group I metabotropic receptor-mediated glutamatergic neurotransmission in association with altered neuropeptide Y, corticotropin-releasing factor, and cannabinoid 1 receptor signaling. These alterations are at least partially mediated by brain microglia and local production of cytokines, especially tumor necrosis factor α. Studying the IHS-related brain neuroplasticity in greater depth may enable the development of new therapeutic approaches against chronic abdominal pain in inflammatory bowel disease.

The Buspirone-dependent Abdominal Pain Transmission Within the Nucleus Tractus Solitarius in the Rat.

Buspirone, a partial agonist of the 5-HT1aR, due to potential antinociceptive properties can be useful for abdominal pain treatment in IBS patients. Pain-related effects of buspirone can be mediated by the 5-HT1aRs, located within the nucleus tractus solitarius. The 5-HT1aR involvement in pain transmission within the NTS is unclear. The objective of our study was to evaluate the involvement of the 5-HT1aR in abdominal pain transmission within the NTS. Using a model of abdominal pain on urethane-anesthetized rats, two types of NTS pain-related neurons responding to the noxious colorectal distension (CRD) with excitatory and inhibitory sustained patterns of evoked activity were revealed. Buspirone (1.0-4.0 mg kg-1, iv) has complex time- and dose-depended action on the CRD-induced NTS neuron responses. Buspirone inhibits the responses of the excitatory neurons and inverts the responses of the inhibitory pain-related neurons but at a dose of 4.0 buspirone, the effect on NTS pain-related neurons attenuates. The inhibitory effect of buspirone on the CRD-evoked responses of the excitatory NTS neurons is completely blocked by an intra-cerebroventricular administration of buspirone agonist WAY100,635. The inhibitory responses do not change by this agonist. The inhibitory action of buspirone is mediated by supraspinal 5-HT1a receptors however, its excitatory effect on inhibitory neurons does not dependents on these receptors. We proposed that the NTS pain-related neurons could be involved in anti- or pronociceptive effects of buspirone on abdominal pain.

Colitis-induced alterations in response properties of visceral nociceptive neurons in the rat caudal medulla oblongata and their modulation by 5-HT3 receptor blockade.

There is considerable clinical and experimental evidence that intestinal inflammation is associated with altered visceral nociceptive processing in the spinal cord and brain, but the underlying neuronal mechanisms, especially acting at the supraspinal level, remain unclear. Considering that the caudal ventrolateral medulla (CVLM) and the nucleus tractus solitarius (NTS) are the first sites for supraspinal processing of visceral pain signals, in the present study we evaluated the experimental colitis-induced changes in response properties of CVLM and NTS medullary neurons to noxious colorectal distension (CRD) in urethane-anesthetized adult male Wistar rats. To determine if gut inflammation alters the 5-HT3 receptor-dependent modulation of visceral pain-related CVLM and NTS cells, we examined the effects of intravenously administered selective 5-HT3 antagonist granisetron on ongoing and CRD-evoked activity of CVLM and NTS neurons in healthy control and colitic animals. In the absence of colonic pathology, the CVLM neurons were more excited by noxious CRD that the NTS cells, which demonstrated a greater tendency to be inhibited by the stimulation. The difference was eliminated after the development of colitis due to the increase in the proportion of CRD-excited neurons in both medullary regions associated with enhanced magnitude of the neuronal nociceptive responses. Intravenous granisetron (1 or 2 mg/kg) produced the dose-dependent suppression of the ongoing and evoked firing of CRD-excited cells within both the CVLM and NTS in normal conditions as well as was able to substantially reduce excitability of the caudal medullary neurons in the presence of colonic inflammation, arguing for the potential efficacy of the 5-HT3 receptor blockade with granisetron against both acute and inflammatory abdominal pain. Taken together, the data obtained can contribute to a deeper understanding of supraspinal serotonergic mechanisms responsible for the persistence of visceral hypersensitivity and hyperalgesia triggered by colonic inflammation.

Blockade of 5-HT3 receptors with granisetron does not affect trigeminothalamic nociceptive transmission in rats: Implication for migraine.

One way to expand the existing range of anti-migraine drugs seems to be the search for pharmacological agents with anti-cephalalgic properties among medicines approved for clinical indications other than migraine. Numerous experimental and clinical data imply that selective serotonin 5-HT3 receptor antagonists can be considered as potential anti-migraine agents. Therefore, the objective of our work was to examine the impact of selective 5-HT3 receptor blockade with granisetron on migraine-related nociceptive transmission within the spinal trigeminal nucleus (STN) and the ventroposteromedial nucleus of the thalamus (VPM). Using an electrophysiological model of trigemino-durovascular nociception in anaesthetised male Wistar rats, we evaluated the effects of intravenous administration of granisetron on ongoing firing and dural electrical stimulation-evoked responses of the spinal trigeminal and thalamic cells. Granisetron did not substantially affect responses of the STN and VPM neurons to electrical stimulation of the dura mater as well as did not cause steady changes in ongoing firing of the spinal trigeminal cells. The results obtained argue against the use of 5-HT3 receptor antagonists for treating migraine. These data also lead to the conclusion that in the absence of sustained sensitisation of neurons along the trigemino-thalamo-cortical pathway the role of 5-HT3 receptor-dependent mechanisms in serotonergic modulation of trigeminovascular nociceptive transmission can hardly be considered crucial.

Inhibitory effect of high-frequency greater occipital nerve electrical stimulation on trigeminovascular nociceptive processing in rats.

Electrical stimulation of the greater occipital nerve (GON) has recently shown promise as an effective non-pharmacological prophylactic therapy for drug-resistant chronic primary headaches, but the neurobiological mechanisms underlying its anticephalgic action are not elucidated. Considering that the spinal trigeminal nucleus (STN) is a key segmental structure playing a prominent role in pathophysiology of headaches, in the present study we evaluated the effects of GON electrical stimulation on ongoing and evoked firing of the dura-sensitive STN neurons. The experiments were carried out on urethane/chloralose-anesthetized, paralyzed and artificially ventilated male Wistar rats. Extracellular recordings were made from 11 neurons within the caudal part of the STN that received convergent input from the ipsilateral facial cutaneous receptive fields, dura mater and GON. In each experiment, five various combinations of the GON stimulation frequency (50, 75, 100 Hz) and intensity (1, 3, 6 V) were tested successively in 10 min interval. At all parameter sets, preconditioning GON stimulation (250 ms train of pulses applied before each recording) produced suppression of both the ongoing activity of the STN neurons and their responses to electrical stimulation of the dura mater. The inhibitory effect depended mostly on the GON stimulation intensity, being maximally pronounced when a stimulus of 6 V was applied. Thus, the GON stimulation-induced inhibition of trigeminovascular nociceptive processing at the level of STN has been demonstrated for the first time. The data obtained can contribute to a deeper understanding of neurophysiological mechanisms underlying the therapeutic efficacy of GON stimulation in primary headaches.

The inhibitory effect of granisetron on ventrolateral medulla neuron responses to colorectal distension in rats.

Irritable bowel syndrome (IBS) is one of the most widespread functional gastrointestinal disorders characterized by abdominal pain. A key pathophysiological mechanism of abdominal pain is associated with disturbances of serotonergic transmission in feedback control loops of endogenous pain modulation in which the ventrolateral medulla (VLM) plays an important role. The receptors to serotonin (5-HT), and particularly the serotonin 3 (5-HT3) receptors have been extensively used as a potential target for abdominal pain treatment of IBS patients due to antinociceptive features of the 5-HT3 receptor antagonists. The precise mechanisms underlying the antinociceptive action of these antagonists remain unclear. The main objective of our study was to evaluate the involvement of the 5-HT3 receptors in abdominal pain transmission within the VLM. Experiments were carried out on urethane-anaesthetized rats using the animal model of abdominal pain. Noxious colorectal distension (CRD) with a pressure of 80mmHg induced a significant increase in VLM neuron-evoked activity and depressor reactions (171.1±12.7% and 64±1.8% to baseline, accordingly). Selective blockade of the 5-HT3 receptors with granisetron at doses of 1.0 or 2.0mg/kg (i.v) resulted in long-lasting (90min) dose-dependent inhibition of VLM neuron-evoked activity and depressor reactions. When brainstem dorsal surface applications of granisetron (10 or 20µM) were used, the changes were more pronounced. These results suggest involvement of the 5-HT3 receptors in abdominal pain transmission within the VLM, which will be discussed in relation to the central antinociceptive effect of granisetron.

Effects of intravenous metamizole on ongoing and evoked activity of dura-sensitive thalamic neurons in rats.

Migraine and tension-type headache (TTH) are the most common forms of primary headaches. A general key mechanism underlying development of both the diseases is the trigeminal system activation associated with the ascending nociceptive transmission via the trigemino-thalamo-cortical pathway. The ventroposteromedial (VPM) nucleus is a key thalamic structure, receiving afferent inflow from the craniofacial region; it holds the third-order neurons responsible for conveying sensory information from the extra- and intracranial nociceptors to the cortex. The VPM is currently seen as a therapeutic target for various antimigraine medications, which is shown to reduce the VPM neuronal excitability. A non-opioid analgesic metamizole is widely used in some countries for acute treatment of migraine or TTH. However, the precise mechanisms underlying anticephalgic action of metamizole remain unclear. The objective of our study performed in the rat model of trigemino-durovascular nociception was to evaluate the effects of intravenously administered metamizole on ongoing and evoked firing of the dura-sensitive VPM neurons. The experiments were carried out on rats under urethane-chloralose anesthesia. Cumulative administration of metamizole (thrice-repeated intravenous infusion of 150 mg/kg performed 30 min apart) in 56% of cases produced a suppression of both the ongoing activity of the thalamic VPM neurons and their responses to dural electrical stimulation. Although the inhibitory effect was prevailing, a number of VPM neurons were indifferent to the administration of metamizole. These data suggest that one of the main components of neural mechanism underlying anticephalgic action of metamizole is suppression of the thalamo-cortical nociceptive transmission associated with trigemino-vascular activation.

Intravenous valproate inhibits ongoing and evoked activity of dura-sensitive thalamic neurons in rats.

Valproate is widely used for migraine treatments, although precise mechanisms of its anticephalgic action are poorly understood. Migraine attacks are thought to occur due to trigemino-vascular system activation, which in turn, stimulates nociceptive transmission in trigemino-thalamo-cortical pathway. The ventroposteromedial (VPM) nucleus of the thalamus is considered to play a prominent role in neurobiology of headaches by serving as the highest subcortical relay for conveying nociceptive information from intra- and extra-cranial structures to the cortex. While it has been demonstrated that valproate can modulate trigemino-vascular nociceptive neurotransmission in the VPM, its effects have been investigated using only intrathalamic ejection of the compound in pentobarbitone sodium anesthetized rats. The objective of our study was to evaluate the effects of intravenously administered valproate on both ongoing firing of the VPM neurons and their activity induced by electrical stimulation of the dura mater. The experiments were performed on rats under nonbarbiturate anesthesia. To define the dose-dependent properties and longevity of the studied effects of valproate, two distinguished dosing regiments were used: bolus (single infusion at a dose of 300 mg/kg) and cumulative (thrice-repeated administration of 100mg/kg performed 30 min apart). Intravenous administration of valproate produced the dose-dependent suppression of both the ongoing activity of the thalamic VPM neurons and their responses to electrical stimulation of the dura mater. This effect was fast-developing (within 5 min) and short-lasting (no longer than 30 min). These data suggest that intravenous administration of valproate could produce a reduction of the thalamo-cortical nociceptive transmission associated with trigemino-vascular activation.

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

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

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

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