Trigeminal Subnucleus Oralis Research Articles

Effects of pre-emptive local anaesthesia on tooth pulp deafferentation-induced neuroplastic changes in cat trigeminal brainstem neurones

Previous work has shown increased excitability of brainstem neurones in the trigeminal (V) subnucleus oralis following the deafferentation produced by tooth-pulp removal. The present study was designed to determine if the changes in oralis neuronal properties seen in cats 1–2 wk after the deafferentation could be blocked by local anaesthesia of the Vth nerve proximal to the sites of pulp injury just before the actual deafferentation. The response properties of neurones recorded in V subnucleus oralis were determined in anaesthetized cats. One or two weeks before neuronal recording, the pulps of the posterior mandibular teeth were removed under local mandibular anaesthesia in one group of cats (group A) and without local anaesthesia in a second group (group B); a third group (group C) had no pulp removal but received local anaesthesia. Consistent with the earlier data, there was a significantly increased incidence of neurones having an enlarged mechanoreceptive field, spontaneous activity and habituating tap sensitivity in group B compared to group C, but no significant differences were found between the two deafferented groups (A and B). As local anaesthesia did not prevent the development of pulp deafferentation-induced changes in the oralis neurones, it is unlikely that an afferent barrage of impulses induced by the deafferentation procedure was responsible for the neuroplastic changes that subsequently developed in the V subnucleus oralis.

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 μl of 60 nmol/μl 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 terminals of orofacial primary afferents and NADPH-diaphorase activity in the trigemino–solitary complex of rats

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) activity and the central terminal fields of branches of the mandibular and chorda tympani nerves were visualized histochemically at the same time using transganglionic transport of wheat germ agglutinin conjugated with horseradish peroxidase. The blue NADPH-d-positive neurons comprised a sparse network in the dorsomedial spinal trigeminal subnucleus oralis and a dense one in the rostral lateral division of the nucleus of the solitary tract. In the subnucleus caudalis, most labeled neurons were in the superficial zone, and smaller numbers were in the magnocellular zone. The NADPH-d-positive neurons in the subnucleus oralis and the nucleus of the solitary tract overlapped mostly with the transganglionically labeled terminal field from the lingual nerve, partly with the terminal field from the inferior alveolar and chorda tympani nerves, and rarely with the terminal field from the mental nerve. The NADPH-d-positive neurons in the dorsomedial paratrigeminal nucleus and subnucleus caudalis overlapped mostly with the terminal field from the lingual nerve, partly with the terminal field from the inferior alveolar and mental nerves and never with the terminal field from the chorda tympani. A statistically significant reduction in the number of NADPH-d-positive neurons was seen bilaterally in subnucleus oralis and the nucleus of the solitary tract when the lingual nerve was transected. Inflammatory insults to the lingual nerve or tooth pulps significantly increased the number of NADPH-d-positive neurons in subnucleus oralis, the nucleus of the solitary tract, and subnucleus caudalis. These results show that the NO/cyclic GMP system in the trigeminal and solitary nuclei is differentially regulated trans-synaptically by trigeminal afferents depending on the nucleus and sensory modality.

Neuroplastic effects of neonatal capsaicin on neurons in adult rat trigeminal nucleus principalis and subnucleus oralis.

1. The effects of C-fiber depletion induced by neonatal capsaicin treatment on the functional properties of low-threshold mechanoreceptive (LTM) neurons in the rat trigeminal (V) subnucleus oralis and nucleus principalis were examined. Neonatal rats were injected with capsaicin within 48 h of birth. The mechanoreceptive field (RF) and response properties of 184 oralis LTM neurons and 185 principalis LTM neurons were studied in adult capsaicin-treated rats. These properties were compared with those of 200 oralis LTM neurons and 253 principalis LTM neurons from untreated or vehicle-treated (control) adult rats. 2. The effectiveness of neonatal capsaicin in depleting C fibers was tested by determining the plasma extravasation of Evans blue dye that was induced in the hindlimb skin by the cutaneous application of the C-fiber excitant and inflammatory irritant mustard oil. The amount of extravasation in capsaicin-treated rats was significantly less than that of control rats. 3. Neonatal capsaicin treatment was associated with significant increases in neuronal RF size and in the percentage of neurons with convergent inputs from more than one type of peripheral tissue (e.g., nonsinus hair, vibrissae, glabrous skin/mucosa, subcutaneous structures such as joint or muscle, periodontal ligament) in both subnucleus oralis and nucleus principalis. In subnucleus oralis, neonatal capsaicin treatment produced a significant increase in the percentage of neurons with a RF involving both V1 and V2 divisions, and a significant decrease in the percentage of neurons with a RF restricted to the V1 division. Analogous changes were not observed in nucleus principalis, although for principalis vibrissa-sensitive neurons, neonatal capsaicin treatment was associated with significant increases in the total number of vibrissae per neuronal RF and in the maximal length of the vibrissal row (i.e., the number of vibrissae in the longest row of vibrissae, stimulation of which was effective in activating a given neuron). 4. Neonatal capsaicin treatment did not significantly affect other oralis or principalis neuronal properties, including the percentage of neurons exhibiting spontaneous activity or abnormal response properties (such as habituating tap sensitivity, discontinuous RF, or mixed adaptation properties within the RF). 5. The changes in the functional properties of oralis and principalis LTM neurons induced by neonatal capsaicin treatment are consistent with those previously reported at other levels of the rodent CNS. They provide additional support to the view that C fibers may have an important role in shaping the functional properties of central LTM somatosensory neurons.

1647 A cortico-tecto-bulbar pathway to the parvicellular reticular formation in the rat: combined anterograde and retrograde tracing study

This study has revealed direct projections from the dorsal peduncular cortex (DP) in the medial prefrontal cortex (mPfC) to the trigeminal brainstem sensory nuclear complex and other lower brainstem areas in rats. We first examined the distribution of mPfC neurons projecting directly to the medullary dorsal horn (trigeminal subnucleus caudalis [Vc]) and trigeminal subnucleus oralis (Vo) which are known to receive direct projections from the lateral prefrontal cortex (insular cortex). After injections of the retrograde tracer Fluorogold (FG) into the rostro-dorsomedial part of laminae I/II of Vc (rdm-I/II-Vc), many neurons were labeled bilaterally (with an ipsilateral predominance) in the rostrocaudal middle level of DP (mid-DP) and not in other mPfC areas. After FG injections into the lateral and caudal parts of laminae I/II of Vc, or the Vo, no neurons were labeled in the mPfC. We then examined projections from the mid-DP by using the anterograde tracer biotinylated dextranamine (BDA). After BDA injections into the mid-DP, many axons and terminals were labeled bilaterally (with an ipsilateral predominance) in the rdm-I/II-Vc, periaqueductal gray and solitary tract nucleus, and ipsilaterally in the parabrachial nucleus and trigeminal mesencephalic nucleus. In addition, the connections of the mid-DP with the insular cortex were examined. Many BDA-labeled axons and terminals from the mid-DP were also found ipsilaterally in the caudalmost level of the granular and dysgranular insular cortex (GI/DI). After BDA injections into the caudalmost GI/DI, many axons and terminals were labeled ipsilaterally in the mid-DP. The projections from the mid-DP to the rdm-I/II-Vc and other brainstem nuclei suggest that mid-DP neurons may regulate intraoral and perioral sensory processing (including nociceptive processing) of rdm-I/II-Vc neurons directly or indirectly through the brainstem nuclei. The reciprocal connections between the mid-DP and caudalmost GI/DI suggest that this regulation may involve mid-DP interactions with the caudalmost GI/DI neurons.

1922 Vestibular association and commissural connections in the albino rat an anterograde tracing study using biocytin

We have recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces central sensitization that is reflected in changes in spontaneous activity, mechanoreceptive field (RF) size, mechanical activation threshold, and responses to graded mechanical stimuli applied to the neuronal RF in trigeminal brainstem subnucleus caudalis and subnucleus oralis. The aim of this study was to test whether central sensitization can be induced in nociceptive neurons of the posterior thalamus by MO application to the pulp. Single unit neuronal activity was recorded in the ventroposterior medial nucleus (VPM) or posterior nuclear group (PO) of the thalamus in anesthetized rats, and nociceptive neurons were classified as wide dynamic range (WDR) or nociceptive-specific (NS). MO application to the pulp was studied in 47 thalamic nociceptive neurons and found to excite over 50% of the 35 VPM neurons tested and to produce significant long-lasting (over 40 min) increases in spontaneous activity, cutaneous pinch RF size and responses to graded mechanical stimuli, and a decrease in threshold in the 29 NS neurons tested; a smaller but statistically significant increase in mean spontaneous firing rate and decrease in activation threshold occurred following MO in the six WDR neurons tested. Vehicle application to the pulp did not produce any significant changes in six VPM NS neurons tested. MO application to the pulp produced pronounced increases in spontaneous activity, pinch RF size, and responses to mechanical stimuli, and a decrease in threshold in three of the six PO neurons. In conclusion, application of the inflammatory irritant MO to the tooth pulp results in central sensitization of thalamic nociceptive neurons and this neuronal hyperexcitability likely contributes to the behavioral consequences of peripheral inflammation manifesting as pain referral, hyperalgesia and allodynia.

1923 Propriospinal afferents to the substantia grisea centralis of the spinal cord in the rat

We have recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces central sensitization that is reflected in changes in spontaneous activity, mechanoreceptive field (RF) size, mechanical activation threshold, and responses to graded mechanical stimuli applied to the neuronal RF in trigeminal brainstem subnucleus caudalis and subnucleus oralis. The aim of this study was to test whether central sensitization can be induced in nociceptive neurons of the posterior thalamus by MO application to the pulp. Single unit neuronal activity was recorded in the ventroposterior medial nucleus (VPM) or posterior nuclear group (PO) of the thalamus in anesthetized rats, and nociceptive neurons were classified as wide dynamic range (WDR) or nociceptive-specific (NS). MO application to the pulp was studied in 47 thalamic nociceptive neurons and found to excite over 50% of the 35 VPM neurons tested and to produce significant long-lasting (over 40 min) increases in spontaneous activity, cutaneous pinch RF size and responses to graded mechanical stimuli, and a decrease in threshold in the 29 NS neurons tested; a smaller but statistically significant increase in mean spontaneous firing rate and decrease in activation threshold occurred following MO in the six WDR neurons tested. Vehicle application to the pulp did not produce any significant changes in six VPM NS neurons tested. MO application to the pulp produced pronounced increases in spontaneous activity, pinch RF size, and responses to mechanical stimuli, and a decrease in threshold in three of the six PO neurons. In conclusion, application of the inflammatory irritant MO to the tooth pulp results in central sensitization of thalamic nociceptive neurons and this neuronal hyperexcitability likely contributes to the behavioral consequences of peripheral inflammation manifesting as pain referral, hyperalgesia and allodynia.

Projection of jaw‐muscle spindle afferents to the caudal brainstem in rats demonstrated using intracellular biotinamide

Intracellular staining with biotinamide was used to study the axonal projection and synaptic morphology of rat jaw-muscle spindle afferents. Intracellular recordings in the mesencephalic trigeminal nucleus (Vme) were identified as spindle afferent responses by their increased firing during stretching of the jaw-elevator muscles. Biotinamide-stained axon collaterals with boutons were found in the trigeminal motor nucleus (Vmo), Vme, the region dorsal to Vmo including the supratrigeminal region, the dorsomedial portion of the trigeminal principal sensory nucleus, and the dorsomedial part of the rostral spinal trigeminal subnucleus oralis. Additional, previously undescribed projections of jaw-muscle spindle afferents were found to the dorsomedial portion of the caudal spinal trigeminal subnucleus oralis (Vodm), the dorsomedial part of the spinal trigeminal subnucleus interpolaris (Vidm), the caudal parvicellular reticular formation, laminae IV and V of the spinal trigeminal subnucleus caudalis (Vc), and the dorsal division of the medullary reticular field. Labeled spindle boutons in Vodm formed predominately axodendritic synapses. Some of these boutons received presynaptic inputs from unlabeled P-type boutons containing clear, spherical, or flattened vesicles. In Vidm, labeled collaterals and boutons were densely clustered into glomerular-like structures. Labeled boutons in Vidm made axodendritic, axosomatic, and axoaxonic synapses and received synaptic contacts from unlabeled boutons containing clear, spherical, or flat and pleomorphic vesicles. Unlabeled presynaptic boutons in Vidm occasionally contained dense core vesicles. Labeled boutons in Vc mainly formed synaptic contacts with large diameter dendrites. This projection of jaw-muscle spindle afferents to caudal brainstem regions may play a significant role in masticatory-muscle stretch reflexes and in the integration of trigeminal proprioceptive information and its transmission to higher centers.

Inputs from identified jaw-muscle spindle afferents to trigeminothalamic neurons in the rat: a double-labeling study using retrograde HRP and intracellular biotinamide.

Projections from physiologically identified jaw-muscle spindle afferents onto trigeminothalamic neurons were studied in the rat. Trigeminothalamic neurons were identified by means of retrograde transport of horseradish peroxidase from the ventroposteromedial nucleus of the thalamus. Labeled neurons were found contralaterally in the supratrigeminal region (Vsup), the trigeminal principal sensory nucleus, the ventrolateral part of the trigeminal subnucleus oralis, the spinal trigeminal subnuclei interpolaris and caudalis, the reticular formation, and an area ventral to the trigeminal motor nucleus (Vmo) and medial to the trigeminal principal sensory nucleus (AVM). Jaw-muscle spindle afferents were physiologically identified by their increased firing during stretching of the jaw muscles and intracellularly injected with biotinamide. Axon collaterals and boutons from jaw-muscle spindle afferents were found in Vmo; Vsup; the dorsomedial part of the trigeminal principal sensory nucleus (Vpdm); the dorsomedial part of the spinal trigeminal subnuclei oralis, interpolaris (Vidm) and caudalis; the parvicellular reticular formation (PCRt); and the mesencephalic trigeminal nucleus. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, PCRt, and AVM were associated with axon collaterals and boutons from intracellularly stained jaw-muscle spindle afferents. Trigeminothalamic neurons in Vsup, Vpdm, Vidm, and PCRt were closely apposed by one to 14 intracellularly labeled boutons from jaw-muscle spindle afferents, suggesting a powerful input to some trigeminothalamic neurons. These data demonstrate that muscle length and velocity feedback from jaw-muscle spindle afferents is projected to the contralateral thalamus via multiple regions of the trigeminal system and implicates these pathways in the projection of trigeminal proprioceptive information to the cerebral cortex.

Antinociceptive effect of morphine microinjections into the spinal trigeminal subnucleus oralis.

We analysed the effects of morphine microinjections (0.2 microliter) into the subnucleus oralis (SNO), i.e. the rostral division of the spinal trigeminal nucleus, on the formalin test adapted for orofacial pain. Duration of rubbing following an injection (50 microliters) of formalin solution (1.5%) in the right upper lip of the rat was measured. Compared to microinjections of saline (NaCl, 0.9%) used as control, 90 and 180 nmol microliter-1 of morphine solution in the SNO significantly curtailed rubbing. This effect was dose-dependent, reversed by naloxone, and also site-specific to the SNO since no effect was observed after microinjection of morphine into areas adjacent to the SNO. These results are further evidence for the involvement of the SNO in perioral nociceptive mechanisms.

Tooth Pulp Deafferentation Is Not Associated with Changes in Primary Afferent Depolarization of Facial Afferent Endings in the Brain Stem

Previous studies have demonstrated that tooth pulp deafferentation is associated with statistically significant alterations in the low-threshold facial mechanoreceptive field properties of brain stem neurons in trigeminal (V) subnucleus oralis. A loss of spinal afferent-induced presynaptic inhibition as a consequence of a decrease in primary afferent depolarization (PAD) following spinal nerve deafferentation has been invoked as a mechanism underlying deafferentation-induced somatosensory neuroplasticity. Therefore, this study was initiated to determine if these pulp deafferentation-induced neuroplastic changes could be accounted for by an alteration in PAD of low-threshold facial afferent endings in subnucleus oralis of anesthetized rats. In control (unoperated) rats ( n = 7) and rats ( n = 7) that had undergone mandibular pulp deafferentation 6-10 days previously, antidromic compound action potentials evoked by test stimulation in V subnucleus oralis were recorded in branches of the infraorbital (IO) and supraorbital (SO) nerves, and conditioning stimuli were applied to some of the same nerves. PAD of the afferent endings in oralis of these nerve branches was documented in all animals, and there was no significant difference between the two groups in the incidence or any of the other features of PAD. The features of the PAD were consistent with those described in several previous studies of normal animals. These findings indicate that the reported deafferentation-induced loss of spinal presynaptic regulatory mechanisms cannot be entrapolated to all forms of deafferentation injury and that the mechanoreceptive field changes that can occur in central V somatosensory neurons as a result of tooth pulp deafferentation may not reflect an alteration in PAD.

Response characteristics of lamb pontine neurons to stimulation of the oral cavity and epiglottis with different sensory modalities.

1. To better understand sensory information processing in pontine neurons that receive afferent fiber terminations from oral cavity and upper airway receptors, we investigated the response characteristics of single neurons to stimulation of the oral cavity and epiglottis with different stimulus modalities. These response characteristics were then compared with previously recorded response properties of neurons located in other brain stem regions that receive oral cavity and upper airway sensory inputs. 2. Receptive field sizes of pontine neurons were mapped, and responses to mechanical, thermal, and chemical stimuli were determined. A total of 47 neurons were isolated and most neurons were located near the dorsomedial border of the rostral trigeminal subnucleus oralis and caudal principal trigeminal nucleus. The likelihood that a particular stimulus modality would elicit a response was somewhat dependent on a neuron's location. Neurons that responded to chemical stimuli were always located outside the trigeminal nucleus, whereas neurons that responded exclusively to mechanical or thermal stimuli were more frequently located in the trigeminal nucleus. Receptive fields were mapped for 45 of the 47 neurons. Forty-three of the neurons had a single ipsilateral receptive field and > 80% of the receptive fields were > 100 mm2. The majority of neurons responded to only one of the three stimulus modalities. The remaining neurons were multimodal and the combination of stimulus modalities most frequently observed was mechanical and chemical. 3. Mechanical stimuli were the most effective of the three stimulus modalities, eliciting responses in > 65% of the neurons. Neurons that responded to mechanical stimuli were generally rapidly adapting and a moving stimulus was more effective than a punctate stimulus. Mechanosensitive neurons that also responded to chemical stimuli exhibited larger mean response frequencies than mechanosensitive neurons that did not respond to chemical stimuli. Chemical stimuli elicited responses in about half the neurons. A greater percentage of neurons with receptive fields on the epiglottis than neurons with oral cavity receptive fields responded to chemical stimuli. The effectiveness of a chemical stimulus was dependent on a neuron's receptive field. NH4Cl was the most effective stimulus for neurons with receptive fields located in the oral cavity, whereas KCl was more effective for neurons with receptive fields on the epiglottis. Thermal stimuli were relatively ineffective whatever the location of a neuron's receptive field. The majority of neurons showed an increase in response frequency to cooling the receptive field and in all thermosensitive neurons the response was restricted to the dynamic phase of thermal stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Location, morphology, and central projections of mesencephalic trigeminal neurons innervating rat masticatory muscles studied by axonal transport of choleragenoid‐horseradish peroxidase

Retrograde and transganglionic transport of horseradish peroxidase conjugated to the B-fragment of cholera toxin (B-HRP) was used to study the location, morphology, and central projections of mesencephalic trigeminal (Me5) neurons innervating rat masticatory muscles. Labeled Me5 cell bodies were found throughout the Me5 nucleus from a level slightly caudal to the trigeminal motor nucleus to the level of the superior colliculus 5 mm further rostrally. Occasionally, labeled Me5 cells were observed in the anterior medullary velum, in the cerebellum, and in the brainstem contralateral to the B-HRP injection. The vast majority of the labeled Me5 cells were pseudounipolar, but multipolar cells were also found. Extensive central projections from labeled Me5 cells could be seen extending from the nucleus of Darkschewitsch rostrally to the C2 segment caudally. Small but consistent projections from Me5 neurons were observed in nuclear islands among the incoming Me5 root fibers. Trigeminal and hypoglossal motor nuclei received direct projections from Me5 cells, but not the facial motor nucleus. The most prominent Me5 projections appeared in the brainstem reticular formation, including the supratrigeminal nucleus. Smaller projections also extended into the main sensory trigeminal nucleus, trigeminal subnucleus oralis, and the nucleus of the solitary tract.

Effects of Tooth Pulp Deafferentation on Brainstem Neurons of the Rat Trigeminal Subnucleus Oralis

The effects of tooth pulp deafferentation on brainstem neuronal properties were examined in the rat trigeminal (V) subnucleus oralis. Deafferentation was produced by removal of the coronal tooth pulp of all left mandibular molars. Neurons in the subnucleus oralis were then electrophysiologically characterized in chloralose/urethane-anesthetized rats at a single postoperative time. The mechanoreceptive field and response properties of low-threshold mechanoreceptive (LTM) neurons in rats studied at postoperative times of 3-4 days, 7-13 days, and 28-40 days were compared to those in control (unoperated) rats. Functional changes in oralis LTM neurons were observed in all tooth-pulp-deafferented rats, but most statistically significant changes were apparent only at the 7- to 13-day postoperative period. In 7- to 13-day pulp-deafferented rats, there was a significant increase in the incidence of neurons with a two-divisional mechanoreceptive field, accompanied by a significant decrease in the incidence of neurons with a maxillary mechanoreceptive field. This group of rats also showed a significant increase in the incidence of neurons with a mechanoreceptive field involving both mandibular and maxillary divisions. For neurons that could be activated by light mechanical stimulation of one or more mandibular or maxillary teeth, the 7- to 13-day pulp-deafferented rats showed a significant increase in the incidence of neurons with such periodontal mechanosensitive inputs involving both mandibular and maxillary divisions. There was also a significant increase in the incidence of spontaneously active neurons in this group of rats. For neurons with a mechanoreceptive field involving mystacial vibrissae, there was a significant increase in all three groups of pulp-deafferented rats in the maximal width of the vibrissal row (i.e., the number of vibrissae in the longest horizontal row of vibrissae, stimulation of which was effective in activating a given neuron). No significant differences were found between groups in the incidence of neurons with a mandibular mechanoreceptive field or in the proportions of neurons with a mechanoreceptive field located in each of several defined orofacial regions. There was also no significant difference between groups in the mean latency to electrical stimulation of the neuronal mechanoreceptive field, or in the proportions of rapidly adapting (RA) and slowly adapting (SA) neurons. These results show many similarities with the functional changes of oralis LTM neurons in the cat following tooth pulp deafferentation, and indicate that the rat may serve as a very useful model for V brainstem neuroplasticity induced by tooth pulp deafferentation.

Response characteristics of tooth pulp-driven postsynaptic neurons in the spinal trigeminal subnucleus oralis of the cat.

Neuronal activity in the spinal trigeminal subnucleus oralis in response to electrical tooth stimulation was recorded in the anaesthetized cat in order to compare the electrophysiological characteristics of the oralis neurons with those of subnucleus caudalis and interpolaris neurons recorded in previous studies. The most sensitive oralis neurons had lower thresholds and shorter latencies than the most sensitive caudalis and interpolaris neurons. The thresholds of the oralis neurons were lower and their strength-duration curves flatter than those depicting liminal dental pain in man but similar to those depicting liminal jaw reflexes in the cat. Noxious conditioning stimulus elevated the threshold of only 1 of 10 neurons tested. The converging input from the skin and oral mucosa was from low-threshold mechanoreceptors. The results indicate that the response properties of the subnucleus oralis neurons differ significantly from those of other spinal subnuclei. Human pain thresholds cannot be explained by the liminal response properties of oralis neurons. These neurons might be important in the mediation of liminal reflex events evoked by dental stimuli.

Synaptic connections of a low-threshold mechanoreceptive primary neuron within the trigeminal subnucleus oralis

The central axon of a primary afferent neuron that responded to indentation of the glabrous skin of the lower lip in a slowly adapting fashion was intra-axonally injected with horseradish peroxidase. The labeled terminal within the subnucleus oralis was examined electron microscopically. The labeled ending had a pale axoplasm and contained clear spherical synaptic vesicles. The labeled ending formed a synaptic triad with a dendrite and an unlabeled axonal ending with pleomorphic vesicles (a mixture of oval, flattened and dense core vesicles). The labeled primary ending was presynaptic only to the dendrite, while the unlabeled ending was presynaptic to both the dendrite and the labeled primary ending.

Responses of trigeminal subnucleus oralis nociceptive neurones to subcutaneous formalin in the rat

Extracellular recordings of 33 single nociceptive neurones of the trigeminal subnucleus oralis (SNO) were made in rats under halothane nitrous oxide anaesthesia. These neurones were tested for their responses to a s.c. injection of formalin in their receptive field. Such a chemical noxious stimulation is known to induce a biphasic response of nociceptive dorsal horn neurones, the second period of which would be due to inflammation. Twenty-three neurones were characterized as nociceptive non-specific (NnS) and 10 as nociceptive specific neurones (NS). Formalin activated both SNO NS and NnS neurones, but, when they responded, NS neurones ( n = 5) showed only the first phase of activity while NnS neurones showed either one ( n = 13) or two phases ( n = 6). Biphasic responses were most often observed for NnS neurones with Aδ- and C-fibre inputs. These results indicate that the time-course similarity between the behavioural and the neuronal responses to formalin exists only for some SNO convergent neurones and that therefore the SNO does not seem to be very involved in the inflammatory component of the pain caused by formalin.

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. These findings provide support for recent evidence suggesting that V subnucleus oralis is an important element in orofacial pain processing, and further indicate that oralis may be involved particularly in intraoral and perioral nociceptive mechanisms.

Effects of one- or two-stage deafferentation of mandibular and maxillary tooth pulps on the functional properties of trigeminal brainstem neurons

We have recently demonstrated that deafferentation of the adult cat's maxillary or mandibular posterior tooth pulps results in statistically significant changes in mechanoreceptive field and response properties of low-threshold mechanoreceptive (LTM) brainstem neurons in trigeminal (V) subnucleus oralis. These effects were however reversible, and the statistically significant changes were apparent only for 1–2 weeks after the deafferentation procedure. The aim of this study was to examine the effects of a more extensive deafferentation involving both maxillary and mandibular pulps. In accordance with our earlier study, the pulps of the teeth were deafferented and, in a ‘blind’ design, the physiological properties of oralis LTM neurons were studied in each of these animals and compared with those from control, unoperated cats. Statistically significant changes in mechanoreceptive field and response properties were produced in these animals with both maxillary and mandibular quadrants deafferented and were similar to those documented in our earlier study involving deafferentation of only one quadrant. However, as well as occurring at 1–2 weeks after the deafferentation procedures, the changes were still apparent at 4 weeks following the deafferentation. In addition, we also documented that changes could be produced by a two-stage deafferentation procedure mandibular pulps and then, 3 weeks later, the maxillary pulps. These findings thus demonstrate that an extensive deafferentation procedure involving mandibular as well as maxillary tooth pulps can produce statistically significant changes in the physiological properties of V brainstem neurons of adult cats that may last at least 4 weeks postoperatively.

Cell Structure and Response Properties in the Trigeminal Subnucleus Oralis

Extra- and intracellular recording, electrical stimulation, receptive field mapping, and horseradish peroxidase injection techniques were used to study the structure of functionally identified neurons in trigeminal (V) brainstem subnucleus oralis of the rat. Of 15 heavily labeled cells located within oralis, 4 were local-circuit neurons with receptive fields restricted to either an incisor, guard hairs, one vibrissa, or deep facial tissue (nociceptors). Their morphologies were highly varied, with expansive and spiny dendritic trees and recurrent and intersubnuclear axon collaterals. Oralis local-circuit neurons therefore most closely resembled non-vibrissa-sensitive local-circuit cells in adjacent subnucleus interpolaris. Six other stained cells projected to contralateral thalamus, and two others projected to ipsilateral cerebellum. They typically had intramodality convergent receptive fields (i.e., spanning more than one receptor organ, such as multiple vibrissae or teeth) with widespread dendritic trees, and were therefore indistinguishable from similarly projecting cells in interpolaris. Two other cells projected to the ipsilateral spinal cord, as well as other V brainstem subnuclei. One of these responded to high-threshold mechanical stimulation of teeth; the other was discharged by deflection of one mystacial vibrissa. Their dendrites were very widespread and ended in spiny and bulbous appendages. Local axon collaterals were also extensive. The remaining oralis cell had two axons, one projecting to the thalamus, the other to the spinal cord. Its receptive field expressed convergence from multiple receptor organs, including vibrissae, guard hairs, and skin. Its somadendritic morphology was similar to that of oralis cells projecting only to thalamus. We conclude that, with some exceptions, the extensive dendritic trees, axon branching, convergence, and functional diversity of oralis cells approximate those described previously for functionally equivalent neurons in interpolaris (Jacquin et al., 1989a,b). Such anatomical and physiological properties are rarely seen, however, in nucleus principalis (Jacquin et al., 1988a). The structure and function of three atypical principalis cells with structural and functional characteristics typical of oralis cells are also described. It is argued that such cells are rostrally displaced oralis cells.