Neuronal responses in rostral trigeminal brain-stem nuclei of macaque monkeys after chronic trigeminal tractotomy

Peripheral Purinergic Modulation in Pediatric Orofacial Inflammatory Pain Affects Brainstem Nitroxidergic System: A Translational Research

Physiology of orofacial pain pathways embraces primary afferent neurons, pathologic changes in the trigeminal ganglion, brainstem nociceptive neurons, and higher brain function regulating orofacial nociception. The goal of this study was to investigate the nitroxidergic system alteration at brainstem level (spinal trigeminal nucleus), and the role of peripheral P2 purinergic receptors in an experimental mouse model of pediatric inflammatory orofacial pain, to increase knowledge and supply information concerning orofacial pain in children and adolescents, like pediatric dentists and pathologists, as well as oro-maxillo-facial surgeons, may be asked to participate in the treatment of these patients. The experimental animals were treated subcutaneously in the perioral region with pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), a P2 receptor antagonist, 30 minutes before formalin injection. The pain-related behavior and the nitroxidergic system alterations in the spinal trigeminal nucleus using immunohistochemistry and western blotting analysis have been evaluated. The local administration of PPADS decreased the face-rubbing activity and the expression of both neuronal and inducible nitric oxide (NO) synthase isoforms in the spinal trigeminal nucleus. These results underline a relationship between orofacial inflammatory pain and nitroxidergic system in the spinal trigeminal nucleus and suggest a role of peripheral P2 receptors in trigeminal pain transmission influencing NO production at central level. In this way, orofacial pain physiology should be elucidated and applied to clinical practice in the future.

Effect of electrical stimulation of the internal capsule on nociceptive neurons responding to orofacial stimuli in the medullary dorsal horn of the rat

Internal capsule (IC) stimulation has been used clinically to alleviate central pain. However, the neuronal mechanisms underlying pain relief by IC stimulation are poorly understood. In order to elucidate the analgesic mechanism, the effect of IC conditioning stimulation on nociceptive neurons in the rat medullary dorsal horn was investigated in the present study. Rats were anaesthetized with N(2)O-O(2) (2:1) and 0.5% halothane and were immobilized with pancuronium bromide. Two kinds of nociceptive neurons, wide dynamic range (WDR) and nociceptive specific (NS) neurons, responding to noxious stimulations of the face and oral structures were recorded in the trigeminal caudal nucleus and the medial reticular subnuclei. A test stimulus with a single rectangular pulse (2ms in duration, 5-70V) was applied to the centre of the receptive field. Responses in 55.9% of the WDR neurons and in 60% of the NS neurons were inhibited by conditioning stimuli to the ipsilateral IC with trains of 33 pulses (100-300microA) at 330Hz. The percents of peak inhibitory effects on WDR neurons and NS neurons were 78.1+/-25.0% (n=19) and 89.0+/-13.6% (n=3), respectively. The inhibitory effect continued for conditioning-test intervals of up to 500ms. Effective sites for conditioning stimulation were concentrated in the lateral side of the IC. These findings suggest that modulation of nociceptive transmission by IC stimulation occurs at second-order neurons via a presynaptic phenomenon by corticofugal fibers in the IC.

Synaptic organization of tooth pulp afferent terminals in the rat trigeminal sensory nuclei

Previous studies provide evidence that a structure/function correlation exists in the distinct zones of the trigeminal sensory nuclei. To evaluate this relationship, we examined the ultrastructure of afferent terminals from the tooth pulp in the rat trigeminal sensory nuclei: the principalis (Vp), the dorsomedial part of oral nucleus (Vdm), and the superficial layers of caudalis (Vc), by using transganglionic transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). A total of 93 labeled boutons were serially sectioned, in which some sections were incubated with gamma-aminobutyric acid (GABA) antiserum. Almost all labeled boutons formed asymmetric contact with nonprimary dendrites, in which more than half of labeled boutons in the Vc made synapses with their spines. The labeled boutons could be divided into two types on the basis of numbers of dense-cored vesicles (DCVs) in a boutons: S-type and DCV-type. Almost all labeled boutons in the Vp and Vdm were S-type, whereas two types were distributed evenly in the Vc. In contrast to DCV-type boutons, the S-type was frequently postsynaptic to unlabeled axon terminals containing a mixture of round, oval, and flattened vesicles (p-endings) and forming symmetrical synapses. Most p-endings examined were immunoreactive to GABA. The frequency of axoaxonic contacts was higher for labeled boutons in the Vp than in the Vdm and Vc. These results suggest that the three structures of trigeminal sensory nuclei serve distinct functions in nociceptive processing.

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.

Differential effects of trigeminal tractotomy on Adelta- and C-fiber-mediated nociceptive responses

In this study we have tested in the rat, whether trigeminal tractotomy, which deprives the spinal trigeminal nucleus caudalis (Sp5C) of its trigeminal inputs, affected differentially nociceptive responses mediated by C- vs. Adelta-nociceptors from oral and perioral regions. Tractotomy had no effect on the threshold of the jaw opening reflex, induced by incisive pulp stimulation (Adelta-fiber-mediated), but blocked the formalin response (mainly C-fiber-mediated). These results suggest that nociceptive responses mediated by trigeminal C-fibers completely depend on the integrity of the Sp5C, while intraoral sensations triggered Adelta-fibers (especially of dental origin) are primarily processed in the rostral part of the spinal trigeminal nucleus.

Stimulus-function, wind-up and modulation by diffuse noxious inhibitory controls of responses of convergent neurons of the spinal trigeminal nucleus oralis

Extracellular unitary recordings were made from 53 spinal trigeminal nucleus oralis (Sp5O) convergent neurons in halothane-anaesthetized rats. The neurons had an ipsilateral receptive field including mainly oral or perioral regions. They responded to percutaneous electrical stimulation with two peaks of activation. The first had a short latency (4.3 +/- 0.3 ms) and low threshold (0.35 +/- 0.04 mA), whereas the second had a longer latency (68.1 +/- 3.4 ms) and higher threshold (7.3 +/- 0.5 mA). Intracutaneous injection of capsaicin (0.1%) produced a strong and rapid reduction of the long-latency responses of Sp5O convergent neurons with little effect on the short-latency responses. In most cases (73%), the long-latency responses exhibited a wind-up phenomenon during repetitive (0.66 Hz) suprathreshold electrical stimulation. These results suggest that C-fibres mediate the long-latency response of Sp5O convergent neurons. Regarding the C-fibre-evoked responses, a linear relationship between the intensity of the applied current and the magnitude of the response was found within the one to three times threshold range. The Sp5O convergent neurons also encoded the intensity of mechanical stimuli applied to the skin or mucosa in the 5-50 g ranges. The evoked activity of Sp5O convergent neurons could be suppressed by noxious heat applied to the tail (52 degrees C) and long-lasting poststimulus effects followed this. These findings show that convergent neurons in the Sp5O resemble those in the deep laminae of the spinal dorsal horn and spinal trigeminal nucleus caudalis, and further support that the Sp5O plays a part in the processing of nociceptive information from the orofacial region.

Effects of lesions in the trigeminal oralis and caudalis subnuclei on different orofacial nociceptive responses in the rat

Behavioural responses to two different orofacial noxious stimulations were analysed following lesion of spinal trigeminal subnucleus oralis (Sp5O) in the rat. Lesions were obtained by intranuclear microinjections of quinolinic acid (0.4 microliter of 60 nmol/microliter solution). The control groups received microinjection of saline. Noxious stimulation was a subcutaneous injection of formalin into the upper lip or electrical stimulation of the tooth pulp. The measured behavioural responses were duration of rubbing induced by the formalin injection and thresholds of the jaw-opening reflex (JOR), head rotation (HR) and face rubbing (FR) evoked by the pulp stimulation. In addition, formalin injection was also performed in two groups of rats that had received intranuclear injection of quinolinic acid or saline into rostral subnucleus caudalis (Sp5C). Rubbing duration was not significantly modified by the lesion of Sp5O, whereas a significant decrease occurred after the lesion of rostral Sp5C. After the lesion of Sp5O, an increase in the threshold of JOR was observed whereas the thresholds of HR and FR were not significantly modified. These results suggest that Sp5O is not necessary for the processing and relay of nociceptive inputs triggered by intense stimulations of oral and perioral areas. However further experiments are needed to reconcile these results with the relevant data obtained from cell recording experiments which indicate the existence, in Sp5O, of neuronal activities related to the sensory discriminative aspect of intense nociception.

Central projection of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreactive trigeminal primary neurons in the rat

Substance P (SP) is implicated in transmission of primary afferent nociceptive signals. In primary neurons, SP is colocalized with calcitonin gene-related peptide (CGRP), which is another neuropeptide marker for small to medium primary neurons. CGRP coreleased with SP augments the postsynaptic effect of SP and thereby modulates the nociceptive transmission. This study demonstrates the distribution of CGRP-like immunoreactivity (-ir) and SP-ir in the lower brainstem of normal rats and after trigeminal rhizotomy or tractotomy at the level of subnucleus interpolaris (Vi). By comparing the results obtained from normal and deafferented rats, we analyzed the central projection of trigeminal primary nociceptors. The CGRP-immunoreactive (-ir) trigeminal primaries projected to the entire rostrocaudal extent of the spinal trigeminal nucleus, the principal nucleus (PrV), the paratrigeminal nucleus (paraV), and the lateral subnucleus of solitary tract nucleus (STN) on the ipsilateral side. The trigeminal primaries projecting to the spinal trigeminal nucleus, paraV and STN also contained SP-ir. The ipsilateral trigeminal primaries were the exclusive source of CGRP-ir terminals in the PrV, the Vi and the dorsomedial nucleus within the subnucleus oralis (Vo). The medullary dorsal horn (MDH) and the lateral edge of Vo received convergent CGRP-ir projection from the ipsilateral trigeminal primaries and other neurons. The glossopharyngeal and vagal primaries are candidates for the source of CGRP-ir projection to the Vo and the MDH, while the dorsal root axons supply the MDH with CGRP-ir terminals. In addition, contralateral primary neurons crossing the midline appear to contain CGRP and to terminate in the MDH.

Light microscopic localization of calcitonin gene-related peptide in the normal feline trigeminal system and following retrogasserian rhizotomy

Calcitonin gene-related peptide (CGRP) is a neuropeptide that has been implicated in the transmission and modulation of primary afferent nociceptive stimuli. In this study, we describe the light microscopic distribution of CGRP immunoreactivity (IR) within the feline trigeminal ganglion and trigeminal nucleus of normal adult subjects and in subjects 10 and 30 days following complete retrogasserian rhizotomy. Within the trigeminal ganglion of normal subjects, cell bodies and fibers showed CGRP-IR, whereas immunoreactive fibers were rare in the central root region. Within the normal spinal trigeminal and main sensory nuclei, CGRP-IR was seen to form a reproducible pattern that varied between the different nuclei. Following rhizotomy, most, but not all, of the CGRP-IR was lost from the spinal trigeminal and main sensory nuclei, except in regions where the upper cervical roots and cranial nerves VII, IX and X project into the trigeminal nucleus. The pattern seen at 10 days contained more CGRP-IR than that seen at 30 days and suggests that degenerating fibers still show CGRP-IR. In contrast to the decrease seen in the nuclei after rhizotomy, examination of the central root that was still attached to the trigeminal ganglion showed an increase in CGRP-IR within fibers, some of which ended in growth conelike enlargements. Rhizotomy induced a dramatic increase in CGRP-IR within trigeminal motoneurons and their fibers, which was strongest 10 days after rhizotomy and weaker at 30 days, which was still stronger than normal. These results indicate that the majority of CGRP-IR found in the trigeminal nucleus originates from trigeminal primary afferents and that an upregulation of CGRP-IR occurs in trigeminal motoneurons and in regenerating fibers in the part of the central root that was still attached to the ganglion. In addition, the persistence of CGRP-IR fibers in the trigeminal nucleus provides one possible explanation for the preservation of pain in humans following trigeminal rhizotomy.

Ultrasonic lesion of the trigeminal nucleus caudalis for deafferentation facial pain

The deafferentation facial pain syndrome, caused by physical, herpetic or tumourous lesions of the trigeminal sensory root, was successfully managed with ultrasonic trigeminal nucleotomy. The article presents this new surgical technique and its results in 14 patients suffering from severe facial pain and dysaesthesias.

Physiological properties and morphological characteristics of cutaneous and mucosal mechanical nociceptive neurons with A-delta peripheral axons in the trigeminal ganglia of crotaline snakes

Primary A-delta nociceptive neurons in the trigeminal ganglia of immobilized crotaline snakes were examined by intrasomal recording and injection of horseradish peroxidase in vivo. Thirty-four neurons supplying the oral mucosa or facial skin were identified as A-delta nociceptive neurons which responded exclusively to noxious mechanical stimuli and had a peripheral conduction velocity ranging from 2.6 to 15.4 m/s. These neurons were subdivided into a fast-conducting type (FC-type) and a slowly conducting type (SC-type). Neurons of both types had a receptive field limited to a single spot which responded to pin prick stimulus with a threshold of more than 5 g. The FC-type neurons had a narrow spike followed by a shorter after-hyperpolarization. In contrast, SC-type neurons exhibited a broad spike with a hump on the falling phase and a longer after-hyperpolarization. The diameters of the stem, central and peripheral axons of the FC-type neurons were significantly thicker than those of the SC-type neurons, but there was no statistical difference in the soma size of the two types. Central axons of both types of neurons were thinner than their stem and peripheral axons. Dichotomizing fibers of peripheral axons were observed within the ganglion on 3 neurons. Central axons of the FC-type neurons terminated ipsilaterally in the nucleus principalis, the subnucleus oralis, interpolaris and caudalis and the interstitial nucleus, whereas those of the SC-type neurons generally projected only to the caudal half of the subnucleus interpolaris, subnucleus caudalis and interstitial nucleus ipsilaterally. The present data showed for the first time the physiological and morphological heterogeneity of the primary trigeminal A-delta nociceptive neurons and revealed that the trigeminal nucleus principalis and all the subdivisions of the trigeminal descending nucleus are involved in nociception as relay nuclei, but the subnucleus caudalis and the caudal half subnucleus interpolaris are the essential relay sites of the primary nociceptive afferents supplying the oral mucosa and facial skin. The interstitial nucleus also appears to play an important role in orofacial nociception.

Trigeminal ganglion neurons which project by way of axon collaterals to both the caudal spinal trigeminal and the principal sensory trigeminal nuclei

Employing a combination of fluorescent retrograde double labeling and immunofluorescence histochemistry, we found that some single neurons in the trigeminal ganglion of the rat projected by way of axon collaterals both to the caudal spinal trigeminal nucleus and to the principal sensory trigeminal nucleus, and that about 40% or 57% of these neurons showed respectively substance P- or calcitonin gene-related peptide-like immunoreactivity.

Oral nociceptive activity in the rat superior colliculus

Single units were recorded using extracellular glass microelectrodes in all laminae of the superior colliculus of the rat under halothane nitrous oxide anaesthesia. Fifty-one units were encountered which responded to a low intensity mechanical stimulus applied to a contralateral or bilateral field located in the oral sphere (intraoral 11, perioral 16), on the face (29) or on the rest of the body (21). Sixteen units responded to a jaw movement. Sixty-one cells were recorded which were preferentially (10) or only (51) activated (30) or inhibited (21) by noxious stimuli. Contralateral or bilateral mechanoreceptive fields located in intraoral (34) and perioral (35) areas were frequent. There is therefore a high incidence of the nociceptive representation of the mouth in the superior colliculus. The other functional properties of the nociceptive units were similar to those reported in other studies. From the subsequent histological localization of the recorded units, it appeared that the nociceptive projections from the intraoral and perioral regions to the superior colliculus reach the lateral part of the intermediate and deep layers of the superior colliculus.

Intersubnuclear connections within the rat trigeminal brainstem complex

Prior intracellular recording and labeling experiments have documented local-circuit and projection neurons in the spinal trigeminal (V) nucleus with axons that arborize in more rostral and caudal spinal trigeminal subnuclei and nucleus principalis. Anterograde tracing studies were therefore carried out to assess the origin, extent, distribution, and morphology of such intersubnuclear axons in the rat trigeminal brainstem nuclear complex (TBNC). Phaseolus vulgaris leucoagglutinin (PHA-L) was used as the anterograde marker because of its high sensitivity and the morphological detail provided. Injections restricted to TBNC subnucleus caudalis resulted in dense terminal labeling in each of the more rostral ipsilateral subnuclei. Subnucleus interpolaris projected ipsilaterally and heavily to magnocellular portions of subnucleus caudalis, as well as subnucleus oralis and nucleus principalis. Nucleus principalis, on the other hand, had only a sparse projection to each of the caudal ipsilateral subnuclei. Intersubnuclear axons most frequently traveled in the deep bundles within the TBNC, the V spinal tract, and the reticular formation. They gave rise to a number of circumscribed, highly branched arbors with many boutons of the terminal and en passant types. Retrograde single- or multiple-labeling experiments assessed the cells giving rise to TBNC intersubnuclear collaterals. Horseradish peroxidase (HRP) and/or fluorescent tracer injections into the thalamus, colliculus, cerebellum, nucleus principalis, and/or subnucleus caudalis revealed large numbers of neurons in subnuclei caudalis, interpolaris, and oralis projecting to the region of nucleus principalis. Cells projecting to more caudal spinal trigeminal regions were most numerous in subnuclei interpolaris and oralis. Some cells in lamina V of subnucleus caudalis and in subnuclei interpolaris and oralis projected to thalamus and/or colliculus, as well as other TBNC subnuclei. Such collateral projections were rare in nucleus principalis and more superficial laminae of subnucleus caudalis. TBNC cells labeled by cerebellar injections were not double-labeled by tracer injections into the thalamus, colliculus, or TBNC. These findings lend generality to currently available data obtained with intracellular recording and HRP labeling methods, and suggest that most intersubnuclear axons originate in TBNC local-circuit neurons, though some originate in cells that project to midbrain and/or diencephalon.

Effects of tractotomy on nociceptive reactions induced by tooth pulp stimulation in the rat

The effects of a trigeminal tractotomy on nociceptive reactions induced by electrical stimulation of an inferior incisor were studied in freely moving rats. Behavioral tests were based on the observation of three reactions: jaw opening reflex, face scratching with forepaws, and head rotation toward the stimulated side. These reactions appear successively when the stimulation intensity increases. The results were evaluated by comparing a test group with a sham-operated group. Both groups were prepared surgically in the same way but only the former had undergone tractotomy. The results were as follows: The threshold of the jaw opening reflex triggered by stimulation of the tooth pulp ipsilateral to the tractotomy was not modified; however, the threshold of the face-scratching and the head rotation reactions rose (P less than or equal to 0.05) after tractotomy. These data suggest that the rostral part of the trigeminal sensory complex is involved in oral nociception. However, the modification of the face-scratching and head rotation thresholds reveals that the subnucleus caudalis also participates in this function when a certain level of pain is reached. The thresholds of the three nociceptive reactions evoked by stimulation of the contralateral tooth pulp were significantly lower in the test group than those in the sham-operated group (P less than or equal to 0.001), which is interpreted as being indicative of an modulating effect of the subnucleus caudalis on the contralateral trigeminal sensory complex. This effect was probably revealed by the use of a sham-operated group as a control.

Effect of trigeminal subnucleus caudalis cold block on the cerebrovascular-evoked responses of rostral trigeminal complex neurons

The technique of reversible cold block was used to explore the possibility that the trigeminal subnucleus caudalis (Vc) influences the responses of rostral trigeminal brainstem nuclear complex (TBNC) neurons to stimulation of the cerebrovasculature. Reversible cold block of Vc was found to abolish totally the responses of many rostral TBNC neurons to stimulation of the cerebrovasculature. The remaining neurons were not affected by the cold block. These data suggest that some rostral TBNC neurons may receive an indirect input from the cerebrovasculature via Vc while other rostral TBNC neurons receive a direct input from the cerebrovasculature.

The rostral part of the trigeminal sensory complex is involved in orofacial nociception

Single units responsive to noxious mechanical stimulation of orofacial receptive fields were recorded within the ventrobasal complex of the rat thalamus. The induced activities were compared before and after deafferentation of the subnucleus caudalis by a trigeminal tractotomy performed at the obex level. The receptive fields activated by noxious stimulation were classified as 'oral' when included in the oral, perioral or paranasal areas, and as 'facial' when included in facial regions distant from the oral cavity. After tractotomy, the unit responses to noxious stimulation of an oral field remained unchanged in 8 cases, decreased in 3 cases, and were suppressed in 4 cases. For units responding to noxious stimulation of a facial field, the responses were suppressed in 8 cases, decreased in two cases and remained unchanged in two other cases. So it appears that the rostral part of the trigeminal sensory complex (1) receives nociceptive afferents mainly from the oral and perioral areas and (2) is a relay in ascending pathways which convey painful sensations.