Inhibition of the Nociceptive R2 Blink Reflex after Supraorbital or Index Finger Stimulation is Normal in Migraine Without Aura Between Attacks

In order to explore possible interictal brainstem dysfunctions in migraine, we have studied the R2 component of the nociceptive specific blink reflex (nBR) after conditioning by supraorbital or index finger stimuli in 14 untreated migraine without aura patients (MO) between attacks and in 15 healthy volunteers. We determined the R2 recovery curve at increasing inter-stimulus intervals between 50 and 600 ms. The nBR was conditioned by a paired supraorbital stimulus and, in another session, by an ipsilateral electrical shock delivered to the index finger. The R2 nBR recovery curves were normal in MO patients for both the supraorbital and peripheral conditioning. These results do not favour persistent interictal sensitization in the spinal trigeminal sensory system. They also suggest that the control exerted by descending brainstem pathways on medullary R2 interneurones is normal in migraine between attacks.

Trigemino-hypoglossal somatic reflex in the pharmacological studies of nociception in orofacial area

Disorders involving the orofacial area represent a major medical and social problem. They are a consequence of central nociceptive processes associated with stimulation of the trigeminal nerve nucleus. A rat model of trigeminal pain, utilizing tongue jerks evoked by electrical tooth pulp stimulation during perfusion of the cerebral ventricles with various neuropeptide solutions, can be used in the pharmacological studies of nociception in orofacial area. The investigated neuropeptides diffuse through the cerebroventricular lining producing an analgesic effect either directly, through the trigemino-hypoglossal reflex arc neurons or indirectly through the periaqueductal central gray, raphe nuclei or locus coeruleus neurons. The aim of this review is to present the effect of pharmacological activity of various neuropeptides affecting the transmission of the sensory information from the orofacial area on the example of trigemino-hypoglossal reflex in rats.

Characterization of pruriceptive trigeminothalamic tract neurons in rats

Rodent models of facial itch and pain provide a valuable tool for distinguishing between behaviors related to each sensation. In rats, pruritogens applied to the face elicit scratching using the hindlimb while algogens elicit wiping using the forelimb. We wished to determine the role of trigeminothalamic tract (VTT) neurons in carrying information regarding facial itch and pain to the forebrain. We have characterized responses to facially applied pruritogens (serotonin, BAM8-22, chloroquine, histamine, capsaicin, and cowhage) and noxious stimuli in 104 VTT neurons recorded from anesthetized rats. Each VTT neuron had a mechanically sensitive cutaneous receptive field on the ipsilateral face. All pruriceptive VTT neurons also responded to noxious mechanical and/or thermal stimulation. Over half of VTT neurons responsive to noxious stimuli also responded to at least one pruritogen. Each tested pruritogen, with the exception of cowhage, produced an increase in discharge rate in a subset of VTT neurons. The response to each pruritogen was characterized, including maximum discharge rate, response duration, and spike timing dynamics. Pruriceptive VTT neurons were recorded from throughout superficial and deep layers of the spinal trigeminal nucleus and were shown to project via antidromic mapping to the ventroposterior medial nucleus or posterior thalamic nuclei. These results indicate that pruriceptive VTT neurons are a subset of polymodal nociceptive VTT neurons and characterize a system conducive to future experiments regarding the similarities and differences between facial itch and pain.

Transition to persistent orofacial pain after nerve injury involves supraspinal serotonin mechanisms

The orofacial region is a major focus of chronic neuropathic pain conditions characterized by primary hyperalgesia at the site of injury and secondary hyperalgesia outside the injured zone. We have used a rat model of injury to the maxillary branch (V2) of the trigeminal nerve to produce constant and long-lasting primary hyperalgesia in the V2 territory and secondary hyperalgesia in territories innervated by the mandibular branch (V3). Our findings indicate that the induction of primary and secondary hyperalgesia depended on peripheral input from the injured nerve. In contrast, the maintenance of secondary hyperalgesia depended on central mechanisms. The centralization of the secondary hyperalgesia involved descending 5-HT drive from the rostral ventromedial medulla and the contribution of 5-HT3 receptors in the trigeminal nucleus caudalis (Vc), the homolog of the spinal dorsal horn. Electrophysiological studies further indicate that after nerve injury spontaneous responses and enhanced poststimulus discharges in Vc nociresponsive neurons were time-dependent on descending 5-HT drive and peripheral input. The induction phase of secondary hyperalgesia involved central sensitization mechanisms in Vc neurons that were dependent on peripheral input, whereas the maintenance phase of secondary hyperalgesia involved central sensitization in Vc neurons conducted by a delayed descending 5-HT drive and a persistence of peripheral inputs. Our results are the first to show that the maintenance of secondary hyperalgesia and underlying central sensitization associated with persistent pain depend on a transition to supraspinal mechanisms involving the serotonin system in rostral ventromedial medulla-dorsal horn circuits.

Nociceptive afferents to the premotor neurons that send axons simultaneously to the facial and hypoglossal motoneurons by means of axon collaterals

It is well known that the brainstem premotor neurons of the facial nucleus and hypoglossal nucleus coordinate orofacial nociceptive reflex (ONR) responses. However, whether the brainstem PNs receive the nociceptive projection directly from the caudal spinal trigeminal nucleus is still kept unclear. Our present study focuses on the distribution of premotor neurons in the ONR pathways of rats and the collateral projection of the premotor neurons which are involved in the brainstem local pathways of the orofacial nociceptive reflexes of rat. Retrograde tracer Fluoro-gold (FG) or FG/tetramethylrhodamine-dextran amine (TMR-DA) were injected into the VII or/and XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the caudal spinal trigeminal nucleus (Vc). The tracing studies indicated that FG-labeled neurons receiving BDA-labeled fibers from the Vc were mainly distributed bilaterally in the parvicellular reticular formation (PCRt), dorsal and ventral medullary reticular formation (MdD, MdV), supratrigeminal nucleus (Vsup) and parabrachial nucleus (PBN) with an ipsilateral dominance. Some FG/TMR-DA double-labeled premotor neurons, which were observed bilaterally in the PCRt, MdD, dorsal part of the MdV, peri-motor nucleus regions, contacted with BDA-labeled axonal terminals and expressed c-fos protein-like immunoreactivity which induced by subcutaneous injection of formalin into the lip. After retrograde tracer wheat germ agglutinated horseradish peroxidase (WGA-HRP) was injected into VII or XII and BDA into Vc, electron microscopic study revealed that some BDA-labeled axonal terminals made mainly asymmetric synapses on the dendritic and somatic profiles of WGA-HRP-labeled premotor neurons. These data indicate that some premotor neurons could integrate the orofacial nociceptive input from the Vc and transfer these signals simultaneously to different brainstem motonuclei by axonal collaterals.

Distribution of trigeminothalamic and spinothalamic lamina I terminations in the cat

The distribution in the thalamus of terminal projections from lamina I neurons of the trigeminal, cervical, and lumbosacral dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat. Iontophoretic injections were guided by single- and multi-unit physiological recordings. The injections in particular cases were essentially restricted to lamina I, whereas in others they spread across laminae I-III or laminae I-V. The trigemino- and spinothalamic (TSTT) terminations were identified immunohistochemically. In all cases, regardless of the level of the injections, terminal fibers were consistently distributed in three main locations: the submedial nucleus; the ventral aspect of the basal ventral medial nucleus and ventral posterior nuclei; and, the dorsomedial aspect of the ventral posterior medial nucleus. The terminal fields in the submedial nucleus and the ventral aspect of the ventral posterior group were topographically organized. Terminations along the ventral aspect of the ventral posterior group extended posterolaterally into the caudal part of the posterior nucleus and anteromedially into the ventromedial part of the ventral lateral nucleus. In several cases with trigeminal lamina I injections, a terminal labeling patch was observed within the core of the ventral posterior medial nucleus. In cases with spinal lamina I injections, terminations were also consistently found in the lateral habenula, the parafascicular nucleus, and the nucleus reuniens. Isolated terminal fibers were occasionally seen in the zona incerta, the dorsomedial hypothalamus, and other locations. These anatomical observations extend prior studies of TSTT projections and identify lamina I projection targets that are important for nociceptive, thermoreceptive, and homeostatic processing in the cat. The findings are consistent with evidence from physiological (single-unit and antidromic mapping) and behavioral studies. The novel identification of spinal lamina I input to the lateral habenula could be significant for homeostatic behaviors.

Laterodorsal nucleus of the thalamus: A processor of somatosensory inputs

The laterodorsal (LD) nucleus of the thalamus has been considered a "higher order" nucleus that provides inputs to limbic cortical areas. Although its functions are largely unknown, it is often considered to be involved in spatial learning and memory. Here we provide evidence that LD is part of a hitherto unknown pathway for processing somatosensory information. Juxtacellular and extracellular recordings from LD neurons reveal that they respond to vibrissa stimulation with short latency (median = 7 ms) and large magnitude responses (median = 1.2 spikes/stimulus). Most neurons (62%) had large receptive fields, responding to six and more individual vibrissae. Electrical stimulation of the trigeminal nucleus interpolaris (SpVi) evoked short latency responses (median = 3.8 ms) in vibrissa-responsive LD neurons. Labeling produced by anterograde and retrograde neuroanatomical tracers confirmed that LD neurons receive direct inputs from SpVi. Electrophysiological and neuroanatomical analyses revealed also that LD projects upon the cingulate and retrosplenial cortex, but has only sparse projections to the barrel cortex. These findings suggest that LD is part of a novel processing stream involved in spatial orientation and learning related to somatosensory cues.

Pharmacological analysis of excitatory and inhibitory synaptic transmission in horizontal brainstem slices preserving three subnuclei of spinal trigeminal nucleus

Spinal trigeminal nucleus (Vsp) consists of three subnuclei: oralis (Vo), interpolaris (Vi) and caudalis (Vc). Previous anatomical studies using antero-/retro-grade tracers have suggested that intersubnuclear ascending/descending synaptic transmissions exist between subnuclei. However, pharmacological properties of the intersubnuclear synaptic transmission have not been studied yet. Since three subnuclei are located in Vsp along rostro-caudal axis, it will be necessary to prepare horizontal brainstem slices to perform pharmacological analysis of the intersubnuclear synaptic transmission. We here show horizontal brainstem slices retaining three subnuclei, and that, using blind whole-cell recordings in the slices, synaptic transmission may be abundantly retained between subnuclei in the horizontal slices, except for the transmission from Vo to Vc. Finally, pharmacological analysis shows that excitatory and inhibitory synaptic responses, respectively, are mediated by AMPA and NMDA receptors and by GABA(A) and glycine receptors, with a differential contribution to the synaptic responses between subnuclei. We therefore conclude that horizontal brainstem slices will be a useful preparation for studies on intersubnuclear synaptic transmission, modulation and plasticity between subnuclei, as well as, further, other brainstem nuclei.

Impairment of the discrimination of the direction of single-whisker stimulation induced by the lemniscal pathway lesion

In the rodent somatosensory system, stimulus information received by the whiskers is relayed to the barrel cortex via two parallel pathways, the lemniscal pathway and the paralemniscal pathway. The lemniscal pathway includes the principal trigeminal nucleus (Pr5) and the ventral posteromedial thalamic nucleus (VPm). The paralemniscal pathway includes the spinal trigeminal subnucleus interpolaris (Sp5i) and the medial division of posterior thalamic nucleus (POm). The purpose of this study was to investigate the roles of those pathways in perceptions of the direction of the single-whisker stimulation in the rat. Rats were trained to perform a go/no-go task that required the discrimination of forward or backward stimulation applied to their single whisker. When a selective lesion was made in VPm or Pr5, error rate for the task performance increased significantly. In contrast, when a selective lesion was made in POm or Sp5i, we found no significant change in performance. These results suggest that the lemniscal pathway plays more important roles in a discrimination of stimulus direction applied to the single whisker.

Selective distribution and function of primary afferent nociceptive inputs from deep muscle tissue to the brainstem trigeminal transition zone

Orofacial injury activates two distinct regions in the spinal trigeminal complex, the subnuclei interpolaris/caudalis (Vi/Vc) transition zone and the laminated Vc, or medullary dorsal horn (MDH). Studies suggest that the Vi/Vc transition zone plays an important role in processing orofacial deep input. To test this hypothesis, we employed a double-tracing strategy to compare central projections of primary afferent neurons that innervate the masseter muscle and the overlying skin. Different tracers were injected either centrally (Fluoro-Gold: ventral Vi/Vc, or MDH) or peripherally (wheat germ agglutinin-conjugated horseradish peroxidase or cholera toxin B: masseter or overlying skin) in the same rat. Trigeminal ganglion tissue sections were processed for single or double immunohistochemistry. The double labeling of ganglion neurons indicates their site of peripheral and central innervations. A population of small to medium-sized neurons was doubly labeled after injections of the tracers into the masseter-Vi/Vc, masseter-MDH, or the skin-MDH. However, only a few double-labeled neurons were occasionally observed after injections of the tracers into the skin-Vi/Vc. Injection of an N-methyl-D-aspartate receptor antagonist, AP-5, into the Vi/Vc and MDH attenuated masseter inflammatory hyperalgesia. In contrast, hyperalgesia after inflammation of the skin overlying the masseter was attenuated by injection of AP-5 into the MDH but not Vi/Vc. These results indicate that while both masseter and cutaneous inputs project to the MDH, masseter afferents provide an additional input to the Vi/Vc. These findings provide further evidence to support a role of the trigeminal transition zone in response to orofacial deep injury.

Topographic organization of the projections from the interstitial system of the spinal trigeminal tract to the parabrachial nucleus in the rat

Neurons in the paratrigeminal nucleus are known to project to the parabrachial region, but both these areas are heterogeneous, and the subnuclei that account for these connections are not known. To characterize better these projections, we injected small amounts of fluorogold or latex beads labeled with rhodamine or fluorescein into the parabrachial area in the rat and evaluated the retrograde transport of tracer to the paratrigeminal nucleus and neighboring regions. The results show that the rostral part of the paratrigeminal nucleus projects to the medial subnucleus of the parabrachial nucleus. The intermediary part of the paratrigeminal nucleus projects to both the external lateral and to the external medial subnuclei of the parabrachial nucleus. The caudal part of the paratrigeminal nucleus projects to the ventral lateral subnucleus of the parabrachial nucleus. The dorsal paramarginal nucleus projects to the external lateral and the extreme lateral subnuclei of the parabrachial nucleus. Lamina I and II of the spinal trigeminal nucleus also project to the external lateral and the extreme lateral subnuclei of the parabrachial nucleus. In conclusion, the rostral, intermediate, and caudal parts of the paratrigeminal nucleus and the dorsal paramarginal nucleus each have clearly different projection patterns and presumably have different functions.

Convergence of spinal trigeminal and cochlear nucleus projections in the inferior colliculus of the guinea pig

In addition to ascending auditory inputs, the external cortex of the inferior colliculus (ICX) receives prominent somatosensory inputs. To elucidate the extent of interaction between auditory and somatosensory representations at the level of IC, we explored the dual projections from the cochlear nucleus (CN) and the spinal trigeminal nucleus (Sp5) to the inferior colliculus (IC) in the guinea pig, using both retrograde and anterograde tracing techniques. Injections of retrograde tracers into ICX resulted in cell-labeling primarily in the contralateral DCN and pars interpolaris and caudalis of Sp5. Labeled cells in DCN were either fusiform or multipolar cells, whereas those in Sp5 varied in size and shape. Injections of anterograde tracers into either CN or Sp5 resulted in terminal labeling in ICX primarily on the contralateral side. Most projection fibers from Sp5 terminated in a laminar pattern from ventromedial to dorsolateral within the ventrolateral ICX, the ventral border of IC, and the ventromedial edge of IC (collectively termed "the ventrolateral border region of IC," ICXV). Less dense anterograde labeling was observed in lateral and rostral ICX. Injecting different tracers into both Sp5 and CN confirmed the overlapping areas of convergent projections from Sp5 and CN in IC: The most intense dual labeling was seen in the ICXV, and less intense dual labeling was also observed in the rostral part of ICX. This convergence of projection fibers from CN and Sp5 provides an anatomical substrate for multimodal integration in the IC.

The nonpeptide calcitonin gene-related peptide receptor antagonist BIBN4096BS lowers the activity of neurons with meningeal input in the rat spinal trigeminal nucleus

Calcitonin gene-related peptide (CGRP) has been suggested to play a major role in the pathogenesis of migraines and other primary headaches. CGRP may be involved in the control of neuronal activity in the spinal trigeminal nucleus (STN), which integrates nociceptive afferent inputs from trigeminal tissues, including intracranial afferents. The activity of STN neurons is thought to reflect the activity of central trigeminal nociceptive pathways causing facial pain and headaches in humans. In a rat model of meningeal nociception, single neuronal activity in the STN was recorded. All units had receptive fields located in the exposed parietal dura mater. Heat and cold stimuli were repetitively applied to the dura in a fixed pattern of ramps and steps. The nonpeptide CGRP receptor antagonist BIBN4096BS was topically applied onto the exposed dura or infused intravenously. BIBN4096BS (300 microg/kg, i.v.) reduced spontaneous activity by approximately 30%, the additional dose of 900 microg/kg intravenously by approximately 50% of the initial activity, whereas saline had no effect. The activity evoked by heat ramps was also reduced after BIBN4096BS (900 microg/kg, i.v.) by approximately 50%. Topical administration of BIBN4096BS (1 mm) did not significantly change the spontaneous neuronal activity within 15 min. We conclude that the endogenous release of CGRP significantly contributes to the maintenance of spontaneous activity in STN neurons. Blockade of CGRP receptors, possibly at central and peripheral sites, may therefore be an effective way to decrease nociceptive transmission. This may offer a new therapeutic strategy for the treatment of facial pain and primary headaches.

Activation of the trigeminal medullary dorsal horn during voluntary diving in rats

Fos immunohistochemistry was used to indicate whether activation of trigeminal neurons occurs in voluntarily diving rats. In rats trained to dive underwater, significant increases in Fos labeling were found within the ventral superficial MDH and paratrigeminal nucleus, 100-150 microm caudal to the obex compared to control rats. The conclusion is that the ventral superficial MDH is the initial brainstem afferent relay of diving response in voluntarily diving rats.

Parasympathetic Mediated Pupillary Dilation Elicited by Lingual Nerve Stimulation in Cats

PURPOSE

To determine the autonomic efferent nerve pathways for the reflex pupillary dilation elicited by somatic stimulation in cats.

METHODS

Cats anesthetized with a mixture of alpha-chloralose (50 mg/kg) and urethane (100 mg/kg) were intubated and paralyzed by intravenous injection of pancuronium bromide. The central cut end of the lingual nerve (LN) was stimulated electrically to simulate somatic stimulation, and 1 microL of lidocaine (2%) was microinjected into the Vsp or the EW nucleus to determine its effect on the pupillary dilation induced by LN stimulation. The effect of electrically stimulating the Vsp or sectioning the superior cervical sympathetic nerve (CSN) on the pupillary response was also examined.

RESULTS

Stimulation of the LN or the trigeminal spinal nucleus (Vsp) evoked pupillary dilation in a frequency- and intensity-dependent manner. These responses were not affected by sectioning the ipsilateral or both CSNs. The pupillary responses were markedly suppressed by microinjecting lidocaine into the ipsilateral Vsp or the Edinger-Westphal (EW) nucleus, but not by injection into the contralateral Vsp.

CONCLUSIONS

These results indicate that the Vsp and EW nucleus act as bulbar relay centers for pupillary dilation elicited by LN stimulation and suggest that the efferent arc of the response is a parasympathetic pathway. The contralateral pupillary dilation appears to be mediated, at least in part, by fibers projecting from the Vsp to the contralateral EW nucleus.

Both oral and caudal parts of the spinal trigeminal nucleus project to the somatosensory thalamus in the rat

Recent evidence has been accumulated that not only spinal trigeminal nucleus caudalis (Sp5C) neurons but also spinal trigeminal nucleus oralis (Sp5O) neurons respond to noxious stimuli. It is unknown, however, whether Sp5O neurons project to supratrigeminal structures implicated in the sensory processing of orofacial nociceptive information. This study used retrograde tracing with Fluorogold in rats to investigate and compare the projections from the Sp5O and Sp5C to two major thalamic nuclei that relay ascending somatosensory information to the primary somatic sensory cortex: the ventroposteromedial thalamic nucleus (VPM) and the posterior thalamic nuclear group (Po). Results not only confirmed the existence of contralateral projections from the Sp5C to the VPM and Po, with retrogradely labelled neurons displaying a specific distribution in laminae I, III and V, they also showed consistent and similar numbers of retrogradely labelled cell bodies in the contralateral Sp5O. In addition, a topographic distribution of VPM projections from Sp5C and Sp5O was found: neurons in the dorsomedial parts of Sp5O and Sp5C projected to the medial VPM, neurons in the ventrolateral Sp5O and Sp5C projected to the lateral VPM, and neurons in intermediate parts of Sp5O and Sp5C projected to the intermediate VPM. All together, these data suggest that not only the Sp5C, but also the Sp5O relay somatosensory orofacial information from the brainstem to the thalamus. Furthermore, trigemino-VPM pathways conserve the somatotopic distribution of primary afferents found in each subnucleus. These results thus improve our understanding of trigeminal somatosensory processing and help to direct future electrophysiological investigations.

Bilateral projection of functionally characterized trigeminal oralis neurons to trigeminal motoneurons in cats

Intracellular Neurobiotin-injections were used to label functionally identified neurons in the rostro-dorsomedial part of the trigeminal oral nucleus (Vo.r) in the cat. The labeled Vo.r neurons with the mechanoreceptive field in oral tissues innervated bilaterally either jaw-opening motoneurons or jaw-closing motoneurons. This result suggests that Vo.r neurons play an important role in sensory-motor reflexes responsible for coordination of bilaterally symmetrical jaw movements.

Projections from the spinal trigeminal nucleus to the cochlear nucleus in the rat

The integration of information across sensory modalities enables sound to be processed in the context of position, movement, and object identity. Inputs to the granule cell domain (GCD) of the cochlear nucleus have been shown to arise from somatosensory brain stem structures, but the nature of the projection from the spinal trigeminal nucleus is unknown. In the present study, we labeled spinal trigeminal neurons projecting to the cochlear nucleus using the retrograde tracer, Fast Blue, and mapped their distribution. In a second set of experiments, we injected the anterograde tracer biotinylated dextran amine into the spinal trigeminal nucleus and studied the resulting anterograde projections with light and electron microscopy. Spinal trigeminal neurons were distributed primarily in pars caudalis and interpolaris and provided inputs to the cochlear nucleus. Their axons gave rise to small (1-3 microm in diameter) en passant swellings and terminal boutons in the GCD and deep layers of the dorsal cochlear nucleus. Less frequently, larger (3-15 microm in diameter) lobulated endings known as mossy fibers were distributed within the GCD. Ventrally placed injections had an additional projection into the anteroventral cochlear nucleus, whereas dorsally placed injections had an additional projection into the posteroventral cochlear nucleus. All endings were filled with round synaptic vesicles and formed asymmetric specializations with postsynaptic targets, implying that they are excitatory in nature. The postsynaptic targets of these terminals included dendrites of granule cells. These projections provide a structural substrate for somatosensory information to influence auditory processing at the earliest level of the central auditory pathways.

The contribution of the principal and spinal trigeminal nuclei to the receptive field properties of thalamic VPM neurons in the rat

Primary sensory information from neurons innervating whisker follicles on one side of a rat's face is relayed primarily through two subnuclei of the brainstem trigeminal complex to the contralateral thalamus. The present experiments were undertaken to separate the contribution of the principal trigeminal nucleus (PrV) from that of the spinal trigeminal nucleus (SpV) to whisker evoked responses in the ventral posterior medial (VPM) nucleus in the adult rat thalamus. Extracellular single-unit responses of VPM neurons to controlled stimulation of the contralateral whiskers under urethane anesthesia were quantified in terms of receptive field size, modal latency, response probability and response magnitude. The SpV contribution to VPM cell responses was isolated by making kainic acid lesions of the PrV. The PrV contribution was ascertained by cutting the trigeminothalamic axons arising from SpV just before they cross the midline. After destruction of the PrV, the SpV pathway alone produced large receptive fields (mean: 9.04 whiskers) and long latency (mean: 11.07 ms) responses from VPM neurons. In contrast, PrV input alone (SpV disconnected) generated small receptive fields (mean: 1.06 whiskers) and shorter latency (mean: 6.74 ms) responses. With both pathways intact the average receptive field size was 2.4 whiskers and peak (modal) response latency was 7.33 ms. The responses with both pathways intact were significantly different from either pathway operating in isolation. Response probability and magnitude followed the same trend. We conclude that normal responses of individual VPM neurons represent the integration of input activity transmitted through both PrV and SpV pathways.

A novel class of neurons at the trigeminal subnucleus interpolaris/caudalis transition region monitors ocular surface fluid status and modulates tear production

Reflex tears are produced by many conditions, one of which is drying of the ocular surface. Although peripheral neural control of the lacrimal gland is well established, the afferent pathways and properties of central premotor neurons necessary for this reflex are not known. Male rats under barbiturate anesthesia were used to determine whether neurons at the ventral trigeminal subnucleus interpolaris- caudalis (Vi/Vc) transition or the trigeminal subnucleus caudalis-cervical cord (Vc/C1) junction region in the lower brainstem were necessary for tears evoked by noxious chemical stimulation (CO2 pulses) or drying of the ocular surface. Both the Vi/Vc transition and Vc/C1 junction regions receive a dense direct projection from corneal nociceptors. Synaptic blockade of the Vi/Vc transition, but not the Vc/C1 junction, by the GABA(A) receptor agonist muscimol inhibited CO2-evoked tears. Glutamate excitation of the Vi/Vc transition, but not the Vc/C1 junction, increased tear volume. Single units recorded at the Vi/Vc transition, but not at the Vc/C1 junction, were inhibited by wetting and excited by drying the ocular surface. Nearly all moisture-sensitive Vi/Vc units displayed an initial inhibitory phase to noxious concentrations of CO2 followed by delayed excitation and displayed an inhibitory surround receptive field from periorbital facial skin. Drying of the ocular surface produced many Fos-positive neurons at the Vi/Vc transition, but not at the Vc/C1 junction. This is the first report of a unique class of moisture-sensitive neurons that exist only at the ventral Vi/Vc transition, and not at more caudal portions of Vc, that may underlie fluid homeostasis of the ocular surface.

Kynurenate in the pontine reticular formation inhibits acoustic and trigeminal nucleus-evoked startle, but not vestibular nucleus-evoked startle

The startle reflex is elicited by acoustic, trigeminal or vestibular stimulation, or by combinations of these stimuli. Acoustic startle is mediated largely by ibotenate-sensitive neurons in the ventrocaudal pontine reticular formation (PnC). In these studies we tested whether startle elicited by stimulation of different modalities is affected by infusion of the non-selective glutamate antagonist, kynurenate, into the PnC. In awake rats, startle responses evoked by either acoustic or spinal trigeminal nucleus stimulation were inhibited by kynurenate, but not saline, infusions, with the most effective placements nearest PnC. In chloral hydrate-anesthetized rats, kynurenate in the PnC reduced trigeminal nucleus-evoked hindlimb EMG responses, but not vestibular nucleus-evoked startle. Kynurenate in the vestibular nucleus had no effect on trigeminal nucleus-evoked startle. These results indicate that trigeminal nucleus stimulation evokes startle largely through glutamate receptors in the PnC, similarly to acoustic startle, but vestibular nucleus-evoked startle is mediated through other pathways, such as the vestibulospinal tract.

Barosensitive neurons in the rat tractus solitarius and paratrigeminal nucleus: a new model for medullary, cardiovascular reflex regulation

The nucleus of the solitary tract (NTS), a termination site for primary afferent fibers from baroreceptors and other peripheral cardiovascular receptors, contains blood pressure-sensitive neurons, some of which have rhythmic activity locked to the cardiac cycle, making them key components of the central pathway for cardiovascular regulation. The paratrigeminal nucleus (Pa5), a small collection of medullary neurons in the dorsal lateral spinal trigeminal tract, like the NTS, receives primary somatosensory inputs of glossopharyngeal, vagal, and other nerves. Recent studies show that the Pa5 has efferent connections to the rostroventrolateral reticular nucleus (RVL), NTS, and ambiguus nucleus, suggesting that its structure may play a role in the baroreceptor reflex modulation. In the present study, simultaneous recording from multiple single neurons in freely behaving rats challenged with i.v. phenylephrine administration, showed that 83% of NTS units and 72% of Pa5 units were baroreceptor sensitive. Whereas most of the baroreceptor-sensitive NTS and Pa5 neurons (86 and 61%, respectively) increased firing rate during the ascending phase of the pressor response, about 16% of Pa5 and NTS baroreceptor-sensitive neurons had a decreased firing rate. On one hand, the decrease in firing rate occurred during the ascending phase of the pressor response, indicating sensitivity to rapid changes in arterial pressure. On the other hand, the increases in neuron activity in the Pa5 or NTS occurred during the entire pressor response to phenylephrine. Cross-correlational analysis showed that 71% of Pa5 and 93% of NTS baroreceptor-activated neurons possessed phasic discharge patterns locked to the cardiac cycle. These findings suggest that the Pa5, like the NTS, acts as a terminal for primary afferents in the medullary-baroreflex or cardiorespiratory-reflex pathways.Key words: cardiovascular reflexes, baroreflex response, arterial blood pressure, multiple single unit recording.

Bradykinin microinjection in the paratrigeminal nucleus triggers neuronal discharge in the rat rostroventrolateral reticular nucleus

A small collection of neurons in the dorsal lateral medulla, the paratrigeminal nucleus (Pa5), projects directly to the rostroventrolateral reticular nucleus (RVL). Bradykinin (BK) microinjections in the Pa5 produce marked pressor responses. Also, the Pa5 is believed to be a component of the neuronal substrates of the somatosensory response and the baroreflex arc. Considering the developing interest in the functional physiology of the Pa5, the present study was designed to characterize RVL neuronal activity in response to BK microinjections in the Pa5 as well as to phenylephrine-induced blood pressure increases in freely behaving rats. Of the 46 discriminated RVL neurons, 82% responded with a 180% mean increase in firing rate after BK application to the paratrigeminal nucleus, before the onset of the blood pressure increase. Thirty (79%) of the RVL BK-excited neurons were baroreceptor-inhibited units that responded with a 30% decrease in firing rate in response to a phenylephrine-produced increase of blood pressure. Twenty-seven (71%) units of the latter population displayed cardiac-cycle-locked rhythmic activity. The findings demonstrate a BK-stimulated functional connection between the Pa5 and RVL that may represent the neural pathway in the BK-mediated pressor response. This pathway may be relevant to baroreflex mechanisms since it relates to cardiovascular pressure-sensitive neurons.Key words: bradykinin, arterial blood pressure, ensemble neuron recording, RVL, baroreflex.

Bidirectional modulation of windup by NMDA receptors in the rat spinal trigeminal nucleus

Activation of afferent nociceptive pathways is subject to activity-dependent plasticity, which may manifest as windup, a progressive increase in the response of dorsal horn nociceptive neurons to repeated stimuli. At the cellular level, N-methyl-d-aspartate (NMDA) receptor activation by glutamate released from nociceptive C-afferent terminals is currently thought to generate windup. Most of the wide dynamic range nociceptive neurons that display windup, however, do not receive direct C-fibre input. It is thus unknown where the NMDA mechanisms for windup operate. Here, using the Sprague-Dawley rat trigeminal system as a model, we anatomically identify a subpopulation of interneurons that relay nociceptive information from the superficial dorsal horn where C-fibres terminate, to downstream wide dynamic range nociceptive neurons. Using in vivo electrophysiological recordings, we show that at the end of this pathway, windup was reduced (24 +/- 6%, n = 7) by the NMDA receptor antagonist AP-5 (2.0 fmol) and enhanced (62 +/- 19%, n = 12) by NMDA (1 nmol). In contrast, microinjections of AP-5 (1.0 fmol) within the superficial laminae increased windup (83 +/- 44%, n = 9), whereas NMDA dose dependently decreased windup (n = 19). These results indicate that NMDA receptor function at the segmental level depends on their precise location in nociceptive neural networks. While some NMDA receptors actually amplify pain information, the new evidence for NMDA dependent inhibition of windup we show here indicates that, simultaneously, others act in the opposite direction. Working together, the two mechanisms may provide a fine tuning of gain in pain.

Organization of parabrachial projections from the spinal trigeminal nucleus oralis: An anterograde tracing study in the rat

In recent years, we have accumulated data showing that the spinal trigeminal nucleus oralis (Sp5O) contributes to the processing of somatosensory inputs from the orofacial region. Although the parabrachial area (PB) represents the main brainstem relay for autonomic, nociceptive, and gustatory afferents, few data are available regarding the topographical distribution of the efferent projections from the Sp5O to the PB. We have addressed this question with the rat, by using the anterograde tracer Phaseolus vulgaris leucoagglutinin. A dense trigeminoparabrachial pathway from the Sp5O toward, predominantly, the ipsilateral PB was revealed. Projections come mainly from the dorsal part of the Sp5O that was found to innervate densely the medial, external medial, and ventral lateral subnuclei. In contrast, the ventral part of the Sp5O projected almost exclusively to an as yet not formally described region, located dorsally and laterally to the lateral tip of the brachium conjunctivum, close to the Kölliker-Fuse nucleus. These results suggest that distinct regions within the Sp5O may be involved in the processing of gustatory and nociceptive information.

[Response of the spinal trigeminal nucleus neurons to electric stimulation of the rat dura mater]

Aseptic inflammation of tissues surrounding large meningeal blood vessels, e.g. the superior sagittal sinus, underlies pathogenesis of migraine. This inflammation develops due to antidromic activation of sensory trigeminal nerve endings and is followed by changes in responses of the spinal nucleus of the trigeminal nerve neurons to electrical stimulation of the superior sagittal sinus. However, characteristics of these reactions are still unclear. In experiments ou urethane-anesthetized rats, responses of 387 neurons of the spinal nucleus of the trigeminal nerve to electrical stimulation of the superior sagittal sinus, were recorded. It was tial discharge with the latency 7 to 19 ms (11.4 +/- 0.17 ms) and a subsequent long-lasting discharge with the latency 20 to 50 ms (34.2 +/- 0.8 ms). It is presumed that the first phase reflects orthodromic activation of prevascular A delta and C-fibers of the trigeminal nerve while the second phase is connected with activation of meningeal C-fibers which have low conduction velocity, and/or with a secondary activation of perivascular sensory endings of trigeminal nerve by releasing algogenic and vasoactive substances. These changes could be used as an indicator of efficacy of some antimigraine substances in animal experiments.

Organization of diencephalic projections from the spinal trigeminal nucleus oralis: An anterograde tracing study in the rat

The organization of the efferent projections from the spinal trigeminal nucleus oralis (Sp5O) to the diencephalon was studied in the rat using the anterograde tracer Phaseolus vulgaris leucoagglutinin. The present study confirms the existence of trigemino-thalamic pathways originating from the Sp5O and details their distribution. The main diencephalic targets of the Sp5O are the ventral posteromedial thalamic nucleus (VPM), the posterior thalamic nuclei (Po) and the ventral part of the zona incerta (ZIv), contralaterally, and the parvicellular part of the ventral posterior thalamic nucleus (VPpc), bilaterally. The distribution of these projections varies according to the dorso-ventral location of the injection sites: the dorsal part of the Sp5O projects to the medial part of the VPM and the Po, and to the caudal part of the ZIv, as well as to the VPpc. The ventral part of the Sp5O projects to the lateral part of the VPM and the Po and to the rostral part of the ZIv. These results suggest that the trigemino-diencephalic pathways originating from the Sp5O are involved in the processing of gustatory and somatosensory information.

Synthesis of multiwhisker-receptive fields in subcortical stations of the vibrissa system

This study addresses the origins of multiwhisker-receptive fields of neurons in the thalamic ventral posterior medial (VPM) nucleus of the rat. We sought to determine whether multiwhisker-receptive field synthesis occurs in VPM through convergent projections from the principalis (PrV) and interpolaris (SpVi) nuclei, or in PrV by intersubnuclear projections from the spinal trigeminal complex. We tested these hypotheses by recording whisker-evoked responses in PrV and VPM before and after electrolytic lesion of the SpVi in lightly anesthetized rats. Before the lesion PrV cells responded, on average, to 3.2 +/- 1.2 whiskers but responsiveness was reduced to 1.07 +/- 0.31 whisker after the lesion. A similar reduction of receptive field size was observed in VPM, where neurons responded, on average, to 2.94 +/- 0.95 whiskers before the lesion and to 1.05 +/- 0.22 whisker after the lesion. Thus one can conclude that intersubnuclear projections mediate surround whisker-receptive fields in PrV, and therefore in VPM. However, it has previously been shown that parasagittal brain stem transection, which severed ascending projections from SpVi, but left intersubnuclear connections intact, rendered VPM cells monowhisker responsive. We wondered whether midline brain stem lesion modified receptive field properties in SpVi. In normal rats SpVi cells responded, on average, to 7.52 +/- 4.25 whiskers, but responsiveness was dramatically reduced to 1.47 +/- 1.07 whisker after the lesion. Together these results indicate that the synthesis of surround receptive fields in subcortical stations relies almost exclusively on intersubnuclear projections from the spinal trigeminal complex to the PrV.

Synaptology of trigemino- and spinothalamic lamina I terminations in the posterior ventral medial nucleus of the macaque

We used the electron microscope to examine lamina I trigemino- and spinothalamic (TSTT) terminations in the posterior part of the ventral medial nucleus (VMpo) of the macaque thalamus. Lamina I terminations were identified by anterograde labeling with biotinylated dextran, and 109 boutons on 38 terminal fibers were closely studied in series of ultrathin sections. Five unlabeled terminal boutons of similar appearance were also examined in detail. Three-dimensional, volume-rendered computer models were reconstructed from complete series of serial sections for 29 boutons on 10 labeled terminal fibers and one unlabeled terminal fiber. In addition, postembedding immunogold staining for GABA was obtained in alternate sections through 23 boutons. Lamina I TSTT terminations in VMpo generally have several large boutons (mean length = 2.16 microm, mean width = 1.29 microm) that are densely packed with vesicles and make asymmetric synaptic contacts on low-order dendrites of VMpo neurons (mean diameter 1.45 microm). They are closely associated with GABAergic presynaptic dendrites (PSDs), and nearly all form classic triadic arrangements (28 of 29 reconstructed boutons). Consecutive boutons on individual terminal fibers make multiple contacts with a single postsynaptic dendrite and can show evidence of progressive complexity. Dendritic appendages that enwrap and invaginate the terminal bouton constitute additional anatomic evidence for secure, high-fidelity synaptic transfer. These observations provide direct ultrastructural evidence supporting the hypothesis that VMpo is a lamina I TSTT thalamocortical relay nucleus in primates that subserves pain, temperature, itch, and other sensations related to the physiological condition of the body.

Baroreceptor-sensitive neurons in the rat paratrigeminal nucleus

The paratrigeminal nucleus (Pa5) is a small collection of medullary neurons localized in the dorsal lateral spinal trigeminal tract. Electrophysiological and anatomical studies showed functional Pa5 efferent connections to the rostroventrolateral reticular nucleus (RVL) and the nucleus of the solitary tract (NTS), both well-studied components of the baroreflex arch. Similarly to the NTS, the main site for termination of cardiovascular peripheral afferents, the Pa5 receives primary sensory inputs of glossopharyngeal and vagus nerves, which suggests that the Pa5 may play a role in the baroreceptor reflex modulation. Simultaneous recording from multiple single neurons in 10 freely behaving rats showed that 37% of recorded Pa5 neurons altered firing rates (35% increased and 2% decreased) during the peak arterial blood pressure response to i.v. phenylephrine. Forty two percent of the 84 identified Pa5 baroreceptor-excited neurons showed high correlation to cardiac cycle denoting the synchronous phasicity to fast changes of blood pressure. Autocorrelation analysis revealed that 48 pressure-sensitive and 55 nonpressure-sensitive neurons have periodical activities which were not directly linked to cardiac cycle. We suggest that the Pa5, a yet unknown component of the baroreflex pathway, may relay baroreceptor information to the NTS and by passing other components of the baroreceptor reflex arch, directly to sympathetic premotor neurons in the RVL.

Identification of lingual motor control circuits using two strains of pseudorabies virus

First-order interneurons that project to hypoglossal motoneurons are distributed within reticular formation subdivisions in the pons and medulla in areas thought to control licking, swallowing, chewing, and respiration. Movement of the tongue in each of these functions is achieved by the coordinated action of both intrinsic and extrinsic lingual muscles. Interneuron populations that project to these different lingual motoneuronal pools appear to be largely overlapping in the reticular formation. Because of the functional coupling between intrinsic and extrinsic muscles during most tongue movements, one might predict that individual pre-hypoglossal interneurons project to multiple motoneuronal pools. To test this hypothesis, one strain of pseudorabies virus was injected into the styloglossus muscle (an extrinsic lingual muscle) and a second strain of pseudorabies virus was injected into the intrinsic lingual muscles of the anterior tongue in the same preparation. Rats were perfused with fixative 84-96 h later, and dual-labeling immunohistochemistry was performed to reveal populations of single- and double-labeled brainstem neurons. Motoneurons innervating the different lingual muscles were spatially segregated within the hypoglossal motor nucleus, and no double-labeled motoneurons were observed. In contrast, pre-hypoglossal neurons projecting to each lingual motoneuron pool were highly overlapping in the reticular formation, and many were double-labeled. These observations suggest that coactivation of lingual muscles can be achieved, at least in part, through divergent projections of first-order interneurons to anatomically and functionally distinct pools of lingual motoneurons in the hypoglossal nucleus.

[Calbindin D-28k-containing neurons receiving visceral and somatic nociceptive information in interstitial nucleus of the spinal trigeminal tract project to the parabrachial nuclei in the rat]

The calbindin D-28k (CB)-containing neurons in the interstitial nucleus of the spinal trigeminal tract (INV) that receive visceral and orofacial somatic nociceptive information and emanate projections to the parabrachial nuclei (PB) were investigated by the triple-labeled methods of fluorogold (FG) retrograde tracing combined with Fos and CB proteins immunofluorescence histochemistry in the rat. The results showed (1) in the perioral stimulation group, a large number of FG-retrograde labeled and Fos-immunoreactive neurons were found in the paratrigeminal nucleus (PaV) and the dorsal paramarginal nucleus (PaMd) of the INV ipsilateral to FG and formalin injection made to the PB and lips, respectively, while a lot of CB-immunoreactive neurons were distributed in the INV bilaterally; (2) a majority of the FG-retrograde labeled neurons (77.3%) were double-labeled with CB, and 40.7% of them were double-labeled with Fos; about 38.5% of FG/CB double-labeled neurons were FG/CB/Fos triple-labeled in the INV; and (3) in the upper alimentary tract stimulation group, the distribution and the numbers of FG-retrograde labeled, CB-immunoreactive neurons and FG/CB double-labeled neurons in the INV were similar to those of the perioral stimulation group as described above, except that the Fos immunoreactive neurons were distributed in the INV bilaterally, approximately 41.9% of the FG-retrograde labeled neurons were FG/Fos double-labeled, and over half (52.0%) of those double-labeled neurons were FG/CB/Fos triple-labeled. The results indicate that a part of CB-containing neurons in the INV receive orofacial somatic and visceral nociceptive information and that these neurons sent projections directly to the PB. The CB-containing neurons might play an important role in the transmission of the peripheral nociceptive information from INV to PB.

Reduction in parasympathetic reflex vasodilatation following stereotaxic ear-bar insertion: importance of reduced afferent input

As in our previous report, when cats were fitted with stereotaxic ear-bars 'type A' animals (26 out of 41) still exhibited a parasympathetic reflex lip blood flow (LBF) increase in response to lingual nerve stimulation, while in 'type B' animals (the remaining 15) it was greatly reduced or abolished. We compared (in both magnitude and in their sensitivity to hexamethonium, 10 mg/kg, i.v.) the LBF responses evoked by electrical stimulation of various sites within the reflex arc (lingual nerve, trigeminal ganglion, spinal trigeminal nucleus (Vsp)) in type A and type B animals to examine where the suppressive effect of ear-bar insertion might be exerted (using artificially ventilated, cervically vago-sympathectomized cats deeply anesthetized with alpha-chloralose and urethane). After ear-bar insertion: (a) in type A animals, stimulation of both lingual nerve and Vsp evoked a similar, hexamethonium-sensitive LBF increase; (b) in type B animals (in which lingual-nerve stimulation evoked no LBF increase), Vsp stimulation evoked a hexamethonium-sensitive LBF increase; (c) in both type A and type B animals, trigeminal ganglion stimulation consistently elicited an LBF increase (abolished by hexamethonium in type A, but reduced by only 50% in type B). These results suggest (i) that abolition of the lingual nerve-induced parasympathetic reflex vasodilatation by ear-bar insertion is due to reduced afferent traffic (in peripheral trigeminal or facial nerves) rather than to a damaged efferent output, and (ii) this effect in type B animals seems somehow to allow an antidromic (hexamethonium-insensitive) vasodilatation to occur on trigeminal ganglion stimulation.

Simultaneous measurement of parasympathetic reflex vasodilator and arterial blood pressure responses in the cat

We measured the changes in lower lip blood flow and systemic arterial blood pressure evoked by lingual nerve or trigeminal spinal nucleus (Vsp) stimulation to gain an insight into the brainstem integration of sympathetic and parasympathetic responses to nociceptive stimulation. We used artificially ventilated, cervically vago-sympathectomized cats deeply anesthetized with alpha-chloralose and urethane. A lip blood flow increase occurred in an intensity- and frequency-dependent manner following electrical stimulation of Vsp or lingual nerve regardless of whether systemic arterial blood pressure increased or decreased. In contrast, there was no apparent optimal frequency for the changes in systemic arterial blood pressure elicited by electrical stimulation of Vsp or lingual nerve. No relationship was found between the amplitude of the lip blood flow increase and that of the systemic arterial blood pressure change. Microinjection of lidocaine or kainic acid into the Vsp evoked, respectively, reversible and irreversible inhibition of the lip blood flow increase and systemic arterial blood pressure change evoked by lingual nerve stimulation. When microinjected unilaterally directly into the ipsilateral Vsp, the GABA agonist muscimol abolished both lingual nerve-evoked effects (increase in lip blood flow and changes in systemic arterial blood pressure) without changing basal systemic arterial blood pressure, suggesting the presence in the Vsp of GABA receptors serving to modulate both the parasympathetically mediated lip blood flow increase and the sympathetically mediated systemic arterial blood pressure change. Lidocaine microinjection into the salivatory nucleus caused a significant attenuation of the lingual nerve-induced blood flow increase, but had no effect on the lingual nerve-induced systemic arterial blood pressure change. Thus, the neural pathway mediating the lingual nerve-induced lip blood flow increase seems to be simple, requiring a minimum of four neurons: trigeminal afferent-Vsp-parasympathetic pre-ganglionic neurons with cell body located in the inferior salivatory nucleus-otic postganglionic neuron. On the other hand, the pathway underlying the evoked systemic arterial blood pressure changes, presumably mediated via altered sympathetic activity, seems to be more complicated and could be affected by more numerous factors.

Somatotopic activation in the human trigeminal pain pathway

Functional magnetic resonance imaging was used to image pain-associated activity in three levels of the neuraxis: the medullary dorsal horn, thalamus, and primary somatosensory cortex. In nine subjects, noxious thermal stimuli (46 degrees C) were applied to the facial skin at sites within the three divisions of the trigeminal nerve (V1, V2, and V3) and also to the ipsilateral thumb. Anatomical and functional data were acquired to capture activation across the spinothalamocortical pathway in each individual. Significant activation was observed in the ipsilateral spinal trigeminal nucleus within the medulla and lower pons in response to at least one of the three facial stimuli in all applicable data sets. Activation from the three facial stimulation sites exhibited a somatotopic organization along the longitudinal (rostrocaudal) axis of the brain stem that was consistent with the classically described "onion skin" pattern of sensory deficits observed in patients after trigeminal tractotomy. In the thalamus, activation was observed in the contralateral side involving the ventroposteromedial and dorsomedial nuclei after stimulation of the face and in the ventroposterolateral and dorsomedial nuclei after stimulation of the thumb. Activation in the primary somatosensory cortex displayed a laminar sequence that resembled the trigeminal nucleus, with V2 more rostral, V1 caudal, and V3 medial, abutting the region of cortical activation observed for the thumb. These results represent the first simultaneous imaging of pain-associated activation at three levels of the neuraxis in individual subjects. This approach will be useful for exploring central correlates of plasticity in models of experimental and clinical pain.

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.

Volume expansion suppresses the tooth-pulp evoked jaw-opening reflex related activity of trigeminal neurons in rats

The aim of the present study is to clarify whether physiological stimulation of vagal afferents modulates the activity of the trigeminal spinal nucleus oralis (TSNO) neurons related to the tooth-pulp (TP)-evoked jaw-opening reflex (JOR) in pentobarbital-anesthetized rats. The activity of TSNO neurons and the amplitude of digastric electromyogram (dEMG) increased proportionally during 1.0-3.5 times the threshold for JOR. The amplitude of the dEMG of 14 out of 17 rats was suppressed by physiological stimulation of vagal afferents after intravenous infusion of Ficoll. Out of 23, 18 TSNO unit activities in 14 rats were also suppressed by Ficoll infusion. This suppressive effect of unit and dEMG activities returned to the control level within 25 min. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.) the suppressive effect of Ficoll infusion on the activity of TSNO neurons (5/7) was significantly attenuated compared to the control (p < 0.01). The inhibition TSNO neuronal and dEMG activities by Ficoll infusion was volume-dependent in a range of 5-10% of total blood volume. Furthermore, right vagus nerve ligation greatly inhibited the suppressive effect of Ficoll-induced TSNO activity. These results therefore suggest that low-pressure cardiopulmonary baroreceptors whose afferents travel in the vagus nerve inhibit the pulpal nociceptive transmission.

Trigeminal collaterals in the nasal epithelium and olfactory bulb: a potential route for direct modulation of olfactory information by trigeminal stimuli

The nasal epithelium is richly invested with peptidergic (substance P and calcitonin gene-related peptide [CGRP]) trigeminal polymodal nociceptors, which respond to numerous odorants as well as irritants. Peptidergic trigeminal sensory fibers also enter the glomerular layer of the olfactory bulb. To test whether the trigeminal fibers in the olfactory bulb are collaterals of the epithelial trigeminal fibers, we utilized dual retrograde labeling techniques in rats to identify the trigeminal ganglion cells innervating each of these territories. Nuclear Yellow was injected into the dorsal nasal epithelium, and True Blue was injected into the olfactory bulb of the same side. Following a survival period of 3-7 days, the trigeminal ganglion contained double-labeled, small (11.8 x 8.0 microm), ellipsoid ganglion cells within the ethmoid nerve region of the ganglion. Tracer injections into the spinal trigeminal complex established that these branched trigeminal ganglion cells also extended an axon into the brainstem. These results indicate that some trigeminal ganglion cells with sensory endings in the nasal epithelium also have branches reaching directly into both the olfactory bulb and the spinal trigeminal complex. These trigeminal ganglion cells are unique among primary sensory neurons in having two branches entering the central nervous system at widely distant points. Furthermore, the collateral innervation of the epithelium and bulb may provide an avenue whereby nasal irritants could affect processing of coincident olfactory stimuli.

Activation of spinobulbar lamina I neurons by static muscle contraction

Spinal lamina I neurons are selectively activated by small-diameter somatic afferents, and they project to brain stem sites that are critical for homeostatic control. Because small-diameter afferent activity evoked by contraction of skeletal muscle reflexly elicits exercise-related cardiorespiratory activation, we tested whether spinobulbar lamina I cells respond to muscle contraction. Spinobulbar lamina I neurons were identified in chloralose-anesthetized cats by antidromic activation from the ipsilateral caudal ventrolateral medulla. Static contractions of the ipsilateral triceps surae muscle were evoked by tibial nerve stimulation using parameters that avoid afferent activation, and arterial blood pressure responses were recorded. Recordings were maintained from 13 of 17 L(7) lamina I spinobulbar neurons during static muscle contraction, and 5 of these neurons were excited. Three were selectively activated only by muscle afferents and did not have a cutaneous receptive field. Spinobulbar lamina I neurons activated by muscle contraction provide an ascending link for the reflex cardiorespiratory adjustments that accompany muscular work. This study provides an important first step in elucidating an ascending afferent pathway for somato-autonomic reflexes.

Potential role of medullary raphe-spinal neurons in cutaneous vasoconstriction: an in vivo electrophysiological study

In rabbits, raphe magnus/pallidus neurons form a link in the CNS pathway regulating changes in cutaneous blood flow elicited by nociceptive stimulation and activation of the central nucleus of the amygdala. To characterize relevant raphe-spinal neurons, we performed extracellular recordings from the rostral medullary raphe nuclei in anesthetized, paralyzed, mechanically ventilated rabbits. All studied neurons were antidromically activated from the dorsolateral funiculus of the spinal cord (C(8)-T(2)). Of 129 studied neurons, 40% were silent. The remaining neurons discharged spontaneously at 0.3-29 Hz. Nociceptive stimulation (lip squeeze with pliers) excited 63 (49%), inhibited 9 (7%), and did not affect 57 (44%) neurons. The same stimulation also elicited falls in ear pinna blood flow. In neurons activated by the stimulation, the increase in discharge preceded the fall in flow. Electrical stimulation of the spinal trigeminal tract excited 61/63 nociception-activated neurons [onset latencies range: 6-75 ms, mean: 28 +/- 3 (SE) ms], inhibited 9/9 nociception-inhibited neurons (onset latencies range: 9-85 ms, mean: 32 +/- 10 ms), and failed to affect 55/57 neurons insensitive to nociceptive stimulation. Neurons insensitive to nociceptive/trigeminal stimulation were also insensitive to nonnociceptive tactile stimulation and to electrical stimulation of the amygdala. They were either silent (32/45) or discharged regularly at low frequencies. They possessed long-duration action potentials (1.26 +/- 0.08 ms) and slow-conducting axons (6.0 +/- 0.5 m/s). These neurons may be serotonergic raphe-spinal cells. They do not appear to be involved in nociceptive-related cutaneous vascular control. Of the 63 neurons sensitive to nociceptive and trigeminal tract stimulation, 35 also responded to tactile stimulation (wide receptive field). These neurons possessed short action potentials (0.80 +/- 0.03 ms) and fast-conducting axons (30.3 +/- 3.1 m/s). In this subpopulation, electrical stimulation of the amygdala activated nearly all neurons tested (10/12), with a mean onset latency of 34 +/- 3 ms. The remaining 28 neurons sensitive to nociceptive and trigeminal stimulation did not respond to tactile stimuli and were mainly unaffected by amygdala stimulation. It may be that fast-conducting raphe-spinal neurons, with wide multimodal receptive fields and with input from the central nucleus of the amygdala, constitute the bulbo-spinal link in the CNS pathway regulating cutaneous blood flow in response to nociceptive and alerting stimuli.

Involvement of trigeminal spinal nucleus in parasympathetic reflex vasodilatation in cat lower lip

We examined whether the trigeminal spinal nucleus (Vsp) forms part of the central mechanism by which electrical stimulation of the central cut end of the lingual nerve (LN) evokes parasympathetic reflex vasodilatation in the lower lip in artificially ventilated, cervically vagosympathectomized cats deeply anesthetized with alpha-chloralose and urethane. For this purpose, we made microinjections within the brain stem to produce nonselective, reversible local anesthesia (lidocaine) or soma-selective, irreversible neurotoxic damage (kainic acid). Local anesthesia of Vsp by microinjection of lidocaine (2%; 1 microl/site) reversibly and significantly reduced the ipsilateral-LN-evoked parasympathetic reflex vasodilatation. Unilateral microinjection of kainic acid (10 mM/site; 1 microl) into Vsp ipsilateral to the stimulated LN led to an irreversible reduction in the reflex vasodilatation but had no effect on the vasodilatation elicited by stimulation of the contralateral LN. Such microinjection of kainic acid into Vsp had no effect on the vasodilatation evoked by electrical stimulation of the ipsilateral inferior salivatory nucleus. Electrical stimulation of Vsp elicited a blood flow increase in the lower lip in an intensity- and frequency-dependent manner, regardless of whether systemic arterial blood pressure rose or fell. Hexamethonium (1.0 mg/kg iv) significantly reduced the vasodilator responses elicited by electrical stimulation of the central cut end of LN or of Vsp, each to a similar degree. After hexamethonium, both vasodilator responses showed time-dependent recovery. These results strongly suggest that Vsp is an important bulbar relay for LN-evoked parasympathetic reflex vasodilatation in the cat lower lip.

Induction of Fos protein-like immunoreactivity in the trigeminal spinal nucleus caudalis and upper cervical cord following noxious and non-noxious mechanical stimulation of the whisker pad of the rat with an inferior alveolar nerve transection

After transection of the inferior alveolar nerve (IAN: the third branch of the trigeminal nerve), the whisker pad area, which is innervated by the second branch of the trigeminal nerve, showed hypersensitivity to mechanical stimulation. Two days after IAN transection, the threshold intensity for escape behavior to mechanical stimulation of the ipsilateral whisker pad area was less than 1.0 g, a sign of allodynia, and returned to the preoperative level (preoperative threshold: 52.0 g) at 32 days after surgery. This decrement of escape threshold lasted for more than 3 weeks. The whisker pad area contralateral to the IAN transection also showed a decrease in escape threshold to non-noxious mechanical stimulation as compared with sham-operated rats. However, the change in threshold intensity for the side contralateral to transection was not as pronounced as that on the ipsilateral side. Fos protein-like immunoreactive (LI) cells were observed in the superficial laminae but not dominant in deeper laminae of the trigeminal spinal nucleus caudalis (Vc) and the first segment of the spinal cord (C1) after non-noxious mechanical stimulation of the whisker pad area in the rats with IAN transection. Fos protein-LI cells were expressed bilaterally in the Vc and C1, but were more numerous on the ipsilateral side to transection than on the contralateral side. The largest number of Fos protein-LI cells was observed at 2400 microm caudal from the trigeminal subnucleus interporalis (Vi)-Vc border both in ipsilateral and contralateral sides. The number of Fos protein-LI cells increased after application of 1, 4, and 16 g stimuli as compared to rats without mechanical stimulation. Furthermore, an extensively greater number of Fos protein-LI cells were expressed both in superficial and deep laminae of the bilateral Vc and C1 of the spinal cord after subcutaneous injection of mustard oil into the whisker pad. Fos protein expression after mustard oil injection was much stronger than that observed after any mechanical stimulation in the rats with IAN transection. These data suggest that the change in the numbers and spatial arrangement of nociceptive neurons in the Vc and C1 after IAN transection reflect the development of mechanical hyperalgesia in the area adjacent to the IAN innervated region.

Raphe magnus/pallidus neurons regulate tail but not mesenteric arterial blood flow in rats

In urethane-anesthetized rats with body temperature maintained at 39-40 degrees C, electrical stimulation of raphe magnus/pallidus/parapyramidal region within 0.5 mm of the ventral medullary surface reduced arterial blood flow to the tail cutaneous bed (measured with a chronically implanted Doppler ultrasonic flowmeter) from 28+/-5 to 6+/-1 cm/s (P<0.01), without changing mesenteric arterial blood flow, and with only small, variable changes in arterial pressure. Injection of bicuculline (50 pmol in 50 nl) at the same site reduced tail flow from 19+/-2 to 3+/-1 cm/s (P<0.01), again without significantly changing mesenteric flow, but with a moderate increase in arterial pressure. When the rat was cooled to reduce basal tail blood flow, injection of muscimol (1 nmol in 100 nl) or GABA (100 nmol in 100 nl) into the raphe site restored tail blood flow to 93+/-4% of the pre-cooling level. These recordings are the first reported direct measurements of rat tail blood flow changes elicited by alteration of neuronal function in the brainstem. The rostral medullary raphe controls the tail cutaneous vascular bed in a relatively selective manner. Our findings add to evidence that raphe magnus/pallidus/parapyramidal neurons are involved in regulating cutaneous blood flow in response to changes in body temperature in the rat.

Systemic morphine reduces the wind-up of trigeminal nociceptive neurons

We assessed the effects of intravenous morphine on the wind-up of nociceptive neurons of the spinal trigeminal nucleus oralis (Sp5O). Extracellular recordings of Sp5O nociceptive convergent neurons were performed in intact halothane-anesthetized rats. Wind-up of C-fiber-evoked responses was elicited by repetitive electrical stimulation (train of 16 shocks, 0.66 Hz) of their receptive field at C-fiber intensity (3 times the threshold). Wind-up was tested for its sensitivity to morphine (6 mg/kg,i.v.), and the specificity of the effects was verified with naloxone (0.4 mg/kg, i.v.). Nineteen convergent neurons displaying wind-up were recorded. Morphine reduced the wind-up of all but one. In five cases, notwithstanding a reduced wind-up, the neuronal response evoked by the first stimulus in the train (initial input) was unexpectedly increased. Naloxone always antagonized morphine inhibitory effects on the wind-up. When administered systemically, morphine reduced the wind-up of trigeminal nociceptive neurons. This inhibitory effect occurred independently of morphine's ability to affect the initial C-fiber-evoked input. Our findings support the idea that systemic morphine probably blocks wind-up by acting at opioid receptors located postsynaptically to nociceptive primary afferents.

Depletion of substance P, neurokinin A and calcitonin gene-related peptide from the contralateral and ipsilateral caudal trigeminal nucleus following unilateral electrical stimulation of the trigeminal ganglion; a possible neurophysiological and neuroanatomical link to generalized head pain

Primary trigeminal neurons of the trigeminal ganglion (TG) innervate major parts of the face and head, including the dura. Electrical stimulation of the TG at specific parameters, can activate its nociceptive neurons and may serve as an experimental pain model. Markowitz [J. Neurosci. 7 (1987) 4129] reported that electrical stimulation of the trigeminal ganglion (TG) causes extravasation of plasma proteins from venules of the trigeminally innervated domain possibly due to the release of vasoactive substances. Neurogenic inflammation (vasodilatation, plasma protein extravasation, release of vasoactive peptides) in dura may serve as one of the possible pathomechanisms underlying vascular head pain [Moskowitz, Ann. Neurol. 16 (1984) 157]. We performed a unilateral electrical stimulation (7.5 Hz, 5 ms, 0.8-1.4 mA for 5 min) of the TG in rat, to induce a neurogenic inflammation in the peripheral trigeminal domain including the dura, looking for calcitonin gene related peptide (CGRP), substance P (SP) and neurokinin A (NKA) immunoreactivity (IR) in the caudal trigeminal nucleus (CTN) into which massive central trigeminal processes terminate. Here, we show patchy depletion(s) of CGRP-, SP- and NKA-IRs in the contralateral CTN of the rat in addition to their ipsilateral depletion. Such depletion is due to the release of these neuropeptides in the CTN leading to the activation of bilateral trigeminal nociceptive pathway. These data afford the possibility that under specific frequencies (which may roughly correlate to the intensity of the painful stimulus) and/or specific intensities (may correlate to specific areas of the peripheral trigeminal domain) of stimulation, activation of one side of the TG may activate bilateral trigeminal nociceptive pathway leading to the perception of an ill localized/generalized pain or headache rather than a unilateral one.

Suppressive effects of oxcarbazepine on tooth pulp-evoked potentials recorded at the trigeminal spinal tract nucleus in cats

1. The purpose of the present study was to evaluate the antinociceptive effect of oxcarbazepine, a keto derivitive of carbamazepine (an anticonvulsant), in an animal model. To evoke a nociceptive response, we electrically stimulated the maxillary canine tooth pulp (MCTP) in anaesthetized (allobarbital-urethane), spontaneously breathing cats. 2. The evoked potentials were recorded from the superficial layers of the caudal part of the trigeminal spinal tract nucleus (5ST). We examined a slow component with a large amplitude (the P3 component) in evoked compound potentials; its mean conduction velocity was 1.7 m/s, suggesting a response mediated by C-fibres. 3. To confirm that the P3 component was related to pain sensation, we used morphine, a most efficacious antinociceptive agent, in the present study. The P3 component was significantly suppressed by intravenous administration of morphine (3 mg/kg) and was also suppressed by microinjection of morphine (2 microg) into the recording site of the 5ST. These results suggest that the P3 component is involved in the transmission of nociceptive information. 4. We compared the effect of oxcarbazepine with mexiletine; both are known to block neuronal Na+ channels. Intravenous administration of mexiletine suppressed the P3 component at a dose of 5 mg/kg. Microinjection of mexiletine (10 microg) into the recording site of the 5ST tended to suppress the P3 component, but this effect was not significant. 5. Intravenous administration of oxcarbazepine (1-10 mg/kg) caused a dose-dependent inhibition of the P3 component, which was significantly suppressed at 10 mg/kg oxcarbazepine. Intravenous administration of 10,11-dihydro-10-hydroxy-5H-dibenz[b,f]azepine-5-carboxamide (MHD), a metabolite of oxcarbazepine, at doses of 3-30 mg/kg caused a dose-dependent inhibition of the P3 component. Oxcarbazepine was not available for the microinjection study because it is not water soluble. We used MHD for the microinjection study instead of oxcarbazepine, because MHD can be dissolved in water up to 3 mg/mL. Microinjections of MHD (6 microg) into the recording site of the 5ST suppressed the P3 component. These results indicate that oxcarbazepine has an antinociceptive action.

Possible role of retrotrapezoid nucleus and parapyramidal area in the respiratory response to anoxia: an in vitro study in neonatal rat

The brainstem-spinal cord preparation from neonatal rat has been used in several reports to evaluate the central effect of low oxygen level on the respiratory network. We demonstrate that bilateral lesion of retrotrapezoid nucleus and parapyramidal area unmasks an early reinforcement of the respiratory output in response to anoxia. This suggests that neurons in both areas might trigger or relay a central depressive influence of hypoxia on the respiratory network.

Change in mechanical receptive field properties induced by GABA(A) receptor activation in the trigeminal spinal nucleus caudalis neurons in rats

The purpose of the present study was to characterize the effect of a local GABAergic inhibitory mechanism on the mechanical receptive field properties of trigeminal spinal nucleus caudalis (SpVc) neurons by iontophoretic application of a gamma-aminobutyric acidA (GABA(A))-antagonist and -agonist. A total of 24 SpVc neurons that responded to orofacial mechanical stimulation were extracellularly recorded by means of multibarrel microelectrodes in urethane-anesthetized rats. The GABA(A) antagonist bicuculline (30 nA, 5 min) enhanced the activities of SpVc neurons (20/24) induced by both touch/pressure and pinch stimuli and also lowered the mechanical stimulation threshold (touch/pressure). Spontaneous discharges in these neurons (20/24) were significantly increased after bicuculline application. Eighteen out of 24 SpVc neurons showed signs of expansion of the receptive field size after iontophoretic application of bicuculline. These changes showed a current-dependent manner and were reversed in approximately 15-20 min. Iontophoretic application of the GABA(A) agonist muscimol induced a current-related inhibition of neuronal activity elicited by touch/pressure and pinch stimuli as well as a decrease in the size of receptive fields. The facilitation of evoked responses and receptive field expansion of SpVc neuron induced by bicuculline application were blocked by coapplication of muscimol (50 nA, 5 min). These results suggest that a local mechanism acting via GABA(A) receptors normally exerts a tonic inhibition of mechanoreceptive transmission in the trigeminal spinal nucleus neurons and this effect may limit responsiveness and size of receptive fields.

Parallel streams for the relay of vibrissal information through thalamic barreloids

This study investigated the organization of a vibrissal pathway that arises from the interpolar division of the spinal trigeminal complex (SP5i), transits through the ventral posterior medial nucleus (VPM), and innervates the somatosensory cortical areas in the rat. Using Fluoro-Gold and biotinylated dextran amine, respectively, as retrograde and anterograde tracers, the following organization plan was disclosed. The SP5i projection arises from a population of small-sized neurons that selectively innervate the ventral lateral part of VPM. In cytochrome oxidase-stained material, this region does not display any barreloid arrangement, but Fluoro-Gold injections in single barrel columns labeled rods of cells that extend caudally into the ventral lateral division of VPM. Thus, on the basis of retrograde labeling, barreloids were divided into core and tail compartments, which correspond to the rod segments running across the dorsal and ventral lateral parts of VPM, respectively. Double-labeling experiments revealed that SP5i afferents innervate the tail of barreloids. The anterograde labeling of thalamocortical axons show that most "core cells" project to a single barrel column, whereas some "tail cells" give rise to branching axons that innervate the second somatosensory area and the dysgranular zone of the barrel field. Injections that straddled the transition zone between the core and tail regions disclosed cells projecting to a single barrel column and to the surrounding dysgranular zone. These results suggest that the projection of "barreloids cells" to the granular and/or dysgranular zones relates to the class of prethalamic input(s) they receive.