Cutting rostral trigeminal nuclear complex projections preferentially affects perioral nociception in the rat

Suppression of neurokinin-1 receptor in trigeminal ganglia attenuates central sensitization following inflammation

This study examined whether local application of a neurokinin-1 (NK1) receptor antagonist into the trigeminal ganglia (TRGs) modulates hyperexcitability of trigeminal spinal nucleus caudalis (SpVc) wide-dynamic range (WDR) neuron activity innervating both the temporomandibular joint (TMJ) region and facial skin following TMJ inflammation. Extracellular single unit recording combined with multibarrel electrodes was used. TMJ inflammation was induced by the injection of complete Freund's adjuvant (CFA). WDR neurons responding to electrical stimuli of the TMJ region and facial skin were recorded from the SpVc in anesthetized rats. The spontaneous and mechanical stimulation-induced discharge frequencies of WDR neurons were significantly larger in inflamed rats than in control rats. The spontaneous WDR activities were current-dependently decreased by local iontophoretic application of an NK1 receptor antagonist into the TRGs after 1 and 2 days of inflammation. The firing frequency of WDR neurons and threshold evoked by mechanical stimulation of facial skin returned to control levels by application of the NK1 receptor antagonist into TRGs after 1 day, but not 2 days, of inflammation. These results suggest that in the early stages of inflammation suppression of the NK1 receptor mechanism in TRGs may prevent central sensitization of SpVc nociceptive neurons.

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.

Neuroplasticity Induced by Tooth Pulp Stimulation in Trigeminal Subnucleus Oralis Involves NMDA Receptor Mechanisms

We have recently demonstrated that application of the mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces significant increases in jaw muscle electromyographic (EMG) activity and neuroplastic changes in trigeminal (V) subnucleus caudalis. Since subnucleus oralis (Vo) as well as caudalis receives projections from molar pulp afferents and is also an integral brain stem relay of afferent input from orofacial structures, we tested whether MO application to the exposed pulp induces neuroplastic changes in oralis neurons and whether microinjection of MK-801, a noncompetitive NMDA antagonist, into the Vo influences the pulp/MO-induced neuroplastic changes in chloralose/urethan-anesthetized rats. Single neuronal activity was recorded in Vo, and neurons classified as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), nociceptive-specific (NS), deep (D), or skin/mucosa and deep (S + D). The spontaneous activity, mechanoreceptive field (RF) size, mechanical threshold, and response to suprathreshold mechanical stimuli applied to the neuronal RF were assessed prior to and throughout a 40- to 60-min period after MO application to the maxillary molar pulp. In animals pretreated with saline microinjection (0.3 μl) into the Vo, MO application to the pulp produced a significant increase in spontaneous activity, expansion of the pinch or deep RF, decrease in the mechanical threshold, and increase in response to suprathreshold mechanical stimuli of the nociceptive (WDR, NS, and S + D) neurons except for those nociceptive neurons having their RF only in the intraoral region. The pulpal application of MO did not produce any significant neuroplastic changes in LTM neurons. Furthermore, in animals pretreated with MK-801 microinjection (3 μg/0.3 μl) into the Vo, MO application to the pulp did not produce any significant changes in the RF and response properties of nociceptive neurons. In other animals pretreated with saline (0.3 μl) or MK-801 (3 μg/0.3 μl) microinjected into the Vo, mineral oil application to the pulp did not produce any significant changes in RF and response properties of nociceptive neurons. These findings indicate that the application of MO to the tooth pulp can induce significant neuroplastic changes in oralis nociceptive neurons and that central NMDA receptor mechanisms may be involved in these neuroplastic changes.

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.

Effects of systemic morphine on the activity of convergent neurons of spinal trigeminal nucleus oralis in the rat

The spinal trigeminal nucleus oralis has been shown to relay nociceptive inputs mainly from the oral and perioral regions. In this study, we examined the effects of intravenous administration of morphine on C-fiber-evoked activities of spinal trigeminal nucleus oralis convergent neurons in halothane-anesthetized rats. Morphine depressed the C-fiber-evoked responses of spinal trigeminal nucleus oralis convergent neurons in a dose-related (3-12 mg/kg range) and naloxone-reversible fashion. The ED50 was 6.1 mg/kg, a dose similar to that found in the spinal horn. The observed strong depressive action of morphine on noxious-evoked activities of spinal trigeminal nucleus oralis neurons is consistent with our previous statement, based on electrophysiological studies, that this region plays an important role in the transmission of trigeminal nociceptive information. The effect of morphine on the spinal trigeminal nucleus oralis neurons is discussed in relation to its possible site and mechanism of action.

Trigeminal structure-function relationships: a reevaluation based on long-range staining of a large sample of brainstem a beta fibers

Prior studies suggest that some classes of thickly myelinated (A beta) afferents have distinct morphologies in the trigeminal (V) brainstem complex, and that single fibers have collaterals with different shapes in the four V subnuclei. However, these conclusions are based upon relatively few and incompletely stained fibers and limited statistical rigor. In the present study, 104 fibers were stained more completely with neurobiotin in rats to provide within-fiber intersubnucleus comparisons, and between-fiber intrasubnucleus comparisons, of collaterals associated with a vibrissa, guard hairs, hairy skin, glabrous skin, or oral structures. Collaterals from all functional categories had similar qualitative features and were distributed somatotopically in the transverse plane according to known maps. Fiber categories were not disproportionately represented at particular sites along the brainstem's rostrocaudal axis, although most fibers adhered to an onion-leaf topography in caudalis. Surprisingly few structure-function relationships were revealed by multivariate analysis of variance and post hoc group comparisons, as follows: Arbors were larger in caudalis than in any other subnucleus; collaterals were most numerous in interpolaris; vibrissa afferents had more collaterals than oral and guard hair afferents; and oral fibers had larger arbors than vibrissa or guard hair afferents in subnucleus oralis. Peripheral receptor association and response adaptation rate failed to predict arbor shapes and terminal bouton numbers in any V subnucleus. These data confirm that the locations of V primary afferent arbors are predicted by their receptive fields. However, collateral number and morphology are predicted only to a very limited extent by the V subnucleus and peripheral receptor affiliation--a conclusion that contrasts with those of most prior studies of somatosensory primary afferents.

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.

Tooth pulp primary neurons: cell size analysis, central connection, and carbonic anhydrase activity

Cell bodies and central terminals of trigeminal primary afferent neurons innervating the mandibular molar and incisor tooth pulps were labeled by injecting various neuroanatomical tracers into these tooth pulps. The cell bodies of major constituents of primary neurons innervating the tooth pulp, cornea, and cutaneous branch of the mylohyoid nerve were large (> or = 500 microns2, 65%), medium (300-400 microns2, 35%), and small (100-200 microns2, 34%), respectively. Those innervating the tooth pulp (tooth pulp cells) had the Nissl pattern characteristic of large light cells of the A type, while those innervating the cornea were small dark cells of the B type. Thirty percent of the tooth pulp cells exhibited histochemically demonstrable carbonic anhydrase activity. The transganglionic transport of HRP-WGA indicated marked concentration of central terminals of the tooth pulp primaries in the rostral subdivisions of the brain stem sensory trigeminal nuclear complex. In contrast, central terminals of the corneal primaries were concentrated in the medullary dorsal horn.

Properties of nociceptive and non-nociceptive neurons in trigeminal subnucleus oralis of the rat

Recent studies have provided evidence suggesting the involvement of rostral components of the V brainstem complex such as trigeminal (V) subnucleus oralis in orofacial pain mechanisms. Since there has been no detailed investigation of the possible existence of nociceptive oralis neurons in the rat to substantiate this recent evidence, the present study was initiated to determine if neurons responsive to noxious orofacial stimuli were present in subnucleus oralis and to characterize their functional properties. In anesthetized rats, recordings were made of the extracellular activity of single neurons functionally characterized as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR) or nociceptive-specific (NS) neurons. The 342 LTM neurons responded only to light mechanical stimulation of orofacial tissues. The mechanoreceptive field of the LTM neurons included the intraoral region in 28% and was localized to the adjacent perioral area in 65%. For 95% the field was localized within one V division. Responses evoked in LTM neurons by electrical stimulation of the orofacial mechanoreceptive field revealed A fiber afferent inputs but no activity that could be attributed to C fiber afferent inputs. The 72 nociceptive neurons included 52 WDR neurons which responded to light (e.g. tactile) as well as noxious (e.g. heavy pressure; pinch) mechanical stimulation of perioral cutaneous and intraoral structures, and 20 NS neurons which responded exclusively to noxious mechanical stimuli. They also differed from the LTM neurons in that 36% of the WDR and 20% of the NS neurons had a mechanoreceptive field involving more than one V division. However, in accordance with our findings for the LTM neurons, the majority of WDR and NS neurons had a mechanoreceptive field involving the intraoral and perioral representations of the mandibular and/or maxillary divisions; those neurons having a mandibular field which especially included intraoral structures predominated in the dorsomedial zone of subnucleus oralis whereas those with a perioral mechanoreceptive field which particularly involved the maxillary division were concentrated in the ventrolateral zone of oralis. In contrast to the LTM neurons, 57% of the WDR and 67% of the NS neurons showed evidence of electrically evoked C fiber as well as A fiber afferent inputs from their mechanoreceptive field. We also noted suppression of the electrically evoked responses by heating of the tail or pinching of the paw. This effect was considered to be a reflection of diffuse noxious inhibitory controls, and was seen in NS as well as WDR neurons; most, but not all, of these neurons received A fiber as well as C fiber orofacial afferent inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Applied psychophysiology and biofeedback of event-related potentials (brain waves): historical perspective, review, future directions

This paper reviews the efforts of workers in the 1960s-1980s to demonstrate voluntary control of exogenously evoked (event-related) potentials in visual, somatic sensory, and auditory systems in rats, cats, and humans. The first part of the paper reviews the conceptual foundation and development of the work--it actually arose from traditional sensory coding and neural correlates of behavior studies. The second part summarizes recent applications of the method in the area of pain control. In reviewing these matters, the major effort is directed at revealing how the ideas unfolded in very human, day-to-day, anecdotal terms. There is not much of an attempt to formally review the literature, which is cited for consultation elsewhere. In the same spirit, many possible future experiments are suggested by way of elucidating the key remaining questions in the area.

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.

Synaptic substrates for enkephalinergic and serotoninergic interactions with dental primary afferent terminals in trigeminal subnucleus interpolaris: an immunocytochemical study using peroxidase and colloidal gold

Pain processing in the trigeminal complex has been thought to reside primarily in the spinal subnucleus caudalis (Vc). However, trigeminal tractotomies eliminating primary afferent input to Vc and severance of secondary trigemino-thalamic fibers from Vc do not disturb pain perception from the central face and oral cavity. Furthermore, large numbers of neurons that are highly responsive to noxious stimuli and suppressed by inputs from the periaqueductal gray and raphe complex have been identified in subnuclei interpolaris (Vi) and oralis (Vo). Therefore, the purpose of this study was to assess the distribution and spatial arrangements of nociceptive modulatory transmitters with nociceptive afferents and trigemino-thalamic relay cells in the rostral portion of the spinal trigeminal nuclear complex. The dental pulp contains predominantly nociceptors that project to all three subdivisions of the trigeminal spinal complex. These projections were visualized by anterograde transganglionic transport of horseradish peroxidase or by degeneration following administration of toxic ricin to the pulp chambers. The spatial arrangements of dental primary afferents with enkephalinergic (ENK) and serotoninergic (5HT) inputs was then assessed by employing avidin-biotin peroxidase and protein-A colloidal gold double-labeling immunocytochemistry. Trigemino-thalamic relay cells were also labeled by retrograde transport of HRP after stereotaxic injections into the ventrobasal thalamus. ENK and 5HT immunoreactivity was found in the ventrolateral quadrant and lateral margin of Vi, together with the adjacent interstitial nucleus (IN). This activity extended from the caudal pole of Vi and the periobex region, where it was most dense, rostrally to a position approximately 2.9 mm from the Obex. Neither ENK nor 5HT immunoreactivity was observed in Vo. Primary dental afferents projected into the ventromedial quadrant of rostral Vi and were found in the ventrolateral quadrant and dorsal aspect of the subnucleus farther caudally. They appeared as simple boutons with single contacts or as larger, sometimes scalloped terminals that formed multiple contacts. Postsynaptic elements were usually small dendritic profiles, although relay cell somata rarely received primary afferent inputs. Many primary afferents entered areas of synaptic clustering and contacted enkephalinergic dendrites, some of which were also postsynaptic to serotoninergic synapses. Alternatively, primary afferents contacted unlabeled processes that were also postsynaptic to the enkephalinergic element to form a triad arrangement. The least common occurrence was axo-axonic contacts in which enkephalinergic synapses were presynaptic to primary afferents. Both enkephalinergic and serotoninergic synaptic categories displayed round vesicles and generally formed asymmetric junctions.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.