Supra- and juxtatrigeminal inhibitory premotor neurons with bifurcating axons projecting to masseter motoneurons on both sides

Supratrigeminal Bilaterally Projecting Neurons Maintain Basal Tone and Enable Bilateral Phasic Activation of Jaw-Closing Muscles

Anatomical studies have identified brainstem neurons that project bilaterally to left and right oromotor pools, which could potentially mediate bilateral muscle coordination. We use retrograde lentiviruses combined with a split-intein-mediated split-Cre-recombinase system in mice to isolate, characterize, and manipulate a population of neurons projecting to both the left and right jaw-closing trigeminal motoneurons. We find that these bilaterally projecting premotor neurons (BPNs) reside primarily in the supratrigeminal nucleus (SupV) and the parvicellular and intermediate reticular regions dorsal to the facial motor nucleus. These BPNs also project to multiple midbrain and brainstem targets implicated in orofacial sensorimotor control, and consist of a mix of glutamatergic, GABAergic, and glycinergic neurons, which can drive both excitatory and inhibitory inputs to trigeminal motoneurons when optogenetically activated in slice. Silencing BPNs with tetanus toxin light chain (TeNT) increases bilateral masseter activation during chewing, an effect driven by the expression of TeNT in SupV BPNs. Acute unilateral optogenetic inhibition of SupV BPNs identifies a group of tonically active neurons that function to lower masseter muscle tone, whereas unilateral optogenetic activation of SupV BPNs is sufficient to induce bilateral masseter activation both during resting state and during chewing. These results provide evidence for SupV BPNs in tonically modulating jaw-closing muscle tone and in mediating bilateral jaw closing.

SIGNIFICANCE STATEMENT

We developed a method that combines retrograde lentiviruses with the split-intein-split-Cre system in mice to isolate, characterize, and manipulate neurons that project to both left and right jaw-closing motoneurons. We show that these bilaterally projecting premotor neurons (BPNs) reside primarily in the supratrigeminal nucleus and the rostral parvicellular and intermediate reticular nuclei. BPNs consist of both excitatory and inhibitory populations, and also project to multiple brainstem nuclei implicated in orofacial sensorimotor control. Manipulation of the supratrigeminal BPNs during natural jaw-closing behavior reveals a dual role for these neurons in eliciting phasic muscle activation and in maintaining basal muscle tone. The retrograde lentivirus carrying the split-intein-split-Cre system can be applied to study any neurons with bifurcating axons innervating two brain regions.

Monosynaptic premotor circuit tracing reveals neural substrates for oro-motor coordination

Feeding behaviors require intricately coordinated activation among the muscles of the jaw, tongue, and face, but the neural anatomical substrates underlying such coordination remain unclear. In this study, we investigate whether the premotor circuitry of jaw and tongue motoneurons contain elements for coordination. Using a modified monosynaptic rabies virus-based transsynaptic tracing strategy, we systematically mapped premotor neurons for the jaw-closing masseter muscle and the tongue-protruding genioglossus muscle. The maps revealed that the two groups of premotor neurons are distributed in regions implicated in rhythmogenesis, descending motor control, and sensory feedback. Importantly, we discovered several premotor connection configurations that are ideally suited for coordinating bilaterally symmetric jaw movements, and for enabling co-activation of specific jaw, tongue, and facial muscles. Our findings suggest that shared premotor neurons that form specific multi-target connections with selected motoneurons are a simple and general solution to the problem of orofacial coordination.DOI: http://dx.doi.org/10.7554/eLife.02511.001.

Electrophysiological and morphological properties of rat supratrigeminal premotor neurons targeting the trigeminal motor nucleus

The electrophysiological and morphological characteristics of premotor neurons in the supratrigeminal region (SupV) targeting the trigeminal motor nucleus (MoV) were examined in neonatal rat brain stem slice preparations with Ca(2+) imaging, whole cell recordings, and intracellular biocytin labeling. First, we screened SupV neurons that showed a rapid rise in intracellular free Ca(2+) concentration ([Ca(2+)]i) after single-pulse electrical stimulation of the ipsilateral MoV. Subsequent whole cell recordings were generated from the screened SupV neurons, and their antidromic responses to MoV stimulation were confirmed. We divided the antidromically activated premotor neurons into two groups according to their discharge patterns during the steady state in response to 1-s depolarizing current pulses: those firing at a frequency higher (HF neurons, n = 19) or lower (LF neurons, n = 17) than 33 Hz. In addition, HF neurons had a narrower action potential and a larger afterhyperpolarization than LF neurons. Intracellular labeling revealed that the axons of all HF neurons (6/6) and half of the LF neurons (4/9) entered the MoV from its dorsomedial aspect, whereas the axons of the remaining LF neurons (5/9) entered the MoV from its dorsolateral aspect. Furthermore, the dendrites of three HF neurons penetrated into the principal sensory trigeminal nucleus (Vp), whereas the dendrites of all LF neurons were confined within the SupV. These results suggest that the types of SupV premotor neurons targeting the MoV with different firing properties have different dendritic and axonal morphologies, and these SupV neuron classes may play unique roles in diverse oral motor behaviors, such as suckling and mastication.

Jaw muscle spindle afferents coordinate multiple orofacial motoneurons via common premotor neurons in rats: an electrophysiological and anatomical study

Jaw muscle spindle afferents (JMSA) in the mesencephalic trigeminal nucleus (Vme) project to the parvocellular reticular nucleus (PCRt) and dorsomedial spinal trigeminal nucleus (dm-Vsp). A number of premotor neurons that project to the trigeminal motor nucleus (Vmo), facial nucleus (VII) and hypoglossal nucleus (XII) are also located in the PCRt and dm-Vsp. In this study, we examined whether these premotor neurons serve as common relay pool for relaying JMSA to multiple orofacial motoneurons. JMSA inputs to the PCRt and dm-Vsp neurons were verified by recording extracellular responses to electrical stimulation of the caudal Vme or masseter nerve, mechanical stimulation of jaw muscles and jaw opening. After recording, biocytin in recording electrode was inotophorized into recording sites. Biocytin-Iabeled fibers traveled to the Vmo, VII, XII, and the nucleus ambiguus (Amb). Labeled boutons were seen in close apposition with Nissl-stained motoneurons in the Vmo, VII, XII and Amb. In addition, an anterograde tracer (biotinylated dextran amine) was iontophorized into the caudal Vme, and a retrograde tracer (Cholera toxin B subunit) was delivered into either the VII or Xll to identify VII and XII premotor neurons that receive JMSA input. Contacts between labeled Vme neuronal boutons and premotor neurons were observed in the PCRt and adjacent dm-Vsp. Confocal microscopic observations confirmed close contacts between Vme boutons and VII and XII premotor neurons. This study provides evidence that JMSA may coordinate activities of multiple orofacial motor nuclei, including Vmo, VII, XII and Amb in the brainstem via a common premotor neuron pool.

The trigeminal circuits responsible for chewing

Mastication is a vital function that ensures that ingested food is broken down into pieces and prepared for digestion. This review outlines the masticatory behavior in terms of the muscle activation patterns and jaw movements and gives an overview of the organization and function of the trigeminal neuronal circuits that are known to take part in the generation and control of oro-facial motor functions. The basic pattern of rhythmic jaw movements produced during mastication is generated by a Central Pattern Generator (CPG) located in the pons and medulla. Neurons within the CPG have intrinsic properties that produce a rhythmic activity, but the output of these neurons is modified by inputs that descend from the higher centers of the brain, and by feedback from sensory receptors, in order to constantly adapt the movement to the food properties.

Ultrastructural analysis of glutamate-, GABA-, and glycine-immunopositive boutons from supratrigeminal premotoneurons in the rat trigeminal motor nucleus

The supratrigeminal region (Vsup) is important for coordination of smooth jaw movement. However, little is known about the synaptic connections of the Vsup premotoneurons with the trigeminal motor neurons. In the present study, we examined axon terminals of Vsup premotoneurons in the contralateral trigeminal motor nucleus (Vmo) by a combination of anterograde tracing with cholera toxin B-horseradish peroxidase (CTB-HRP), postembedding immunohistochemistry for the amino acid transmitters glutamate, GABA, and glycine, and electron microscopy. Tracer injections resulted in anterograde labeling of axon terminals of the Vsup premotoneurons in the motor trigeminal nucleus (Vmo). The labeled boutons in Vmo exhibited immunoreactivity for glutamate, GABA, or glycine: glutamate-immunopositive boutons (69%) were more frequently observed than GABA- or glycine-immunopositive boutons (19% and 12%, respectively). Although most labeled boutons (97%) made synaptic contacts with a single postsynaptic dendrite, a few glutamate-immunopositive boutons (3%) showed synaptic contact with two dendrites. No labeled boutons participated in axoaxonic synaptic contacts. Most labeled boutons (78%) were presynaptic to dendritic shafts, and the remaining 22% were presynaptic to somata or primary dendrites. A large proportion of GABA- or glycine-immunopositive boutons (40%) were presynaptic to somata or primary dendrites, whereas most glutamate-immunopositive boutons (86%) were presynaptic to dendritic shafts. These results indicate that axon terminals of Vsup premotoneurons show simple synaptic connection with Vmo neurons. This may provide the anatomical basis for the neural information processing responsible for jaw movement control.

Neuronal circuitry and synaptic organization of trigeminal proprioceptive afferents mediating tongue movement and jaw-tongue coordination via hypoglossal premotor neurons

The neural framework and synaptic organization of trigeminal proprioceptive afferent-mediated jaw-tongue coordination were studied in rats using multiple electrophysiological and neuroanatomical approaches. Electrostimulation of the masseter nerve evoked short-latency responses (5.86 +/- 2.59 ms) in hypoglossal premotor pools including the parvocellular (PCRt) and intermediate (IRt) reticular nuclei and the dorsomedial part of the spinal trigeminal nucleus oralis (Vodm) and interpolaris (Vidm). Biocytin-labelled axon terminals from these areas traveled into the hypoglossal nucleus (XII) and contacted motoneurons. Double labelling of biotinylated dextran amine (BDA) tracing and cholera toxin B (CTB) transport demonstrated that labelled axons and terminals from the mesencephalic trigeminal nucleus (Vme) overlapped with XII premotor neurons in the alpha division and in PCRt, IRt, Vodm and Vidm. Confocal microscopic observations revealed that Vme terminals closely contacted XII premotor neurons. Dual labelling of intracellular neurobiotin staining of jaw-muscle spindle afferents (JMSAs) combined with horseradish peroxidase (HRP) retrograde transport revealed that 498 JMSA boutons apposed to 146 HRP-labelled premotor neurons. Electron microscopic observations demonstrated that 127 JMSA boutons made both axodendritic (68%) and axosomatic (32%) synapses with XII premotor neurons. Eighty-three per cent of synapses were asymmetric and the rest (17%) were symmetric. Thirty-nine per cent of JMSA boutons received presynaptic contacts from P-type terminals. Varieties of synaptic organizations were found. These results provide evidence that trigeminal proprioceptive afferents mediate jaw-tongue coordination through XII premotor neurons. Ultrastructural findings demonstrated that synapses between JMSA boutons and XII premotor neurons are predominantly excitatory, and synaptic transmission to XII motoneurons is modified on XII premotor neurons by presynaptic mechanisms. These frameworks and synaptic organizations are most probably the neural substrate for trigeminal proprioceptive afferent-mediated jaw-tongue coordination.

Intrinsic membrane properties and morphological characteristics of interneurons in the rat supratrigeminal region

The membrane properties and morphological features of interneurons in the supratrigeminal area (SupV) were studied in rat brain slices using whole-cell patch clamp recording techniques. We classified three morphological types of neurons as fusiform, pyramidal, and multipolar and four physiological types of neurons according to their discharge pattern in response to a 1-sec depolarizing current pulse from -80 mV. Single-spike neurons responded with a single spike, phasic neurons showed an initial burst of spikes and were silent during the remainder of the stimulus, delayed-firing (DF) neurons exhibited a slow depolarization and delay to initial spike onset, and tonic (T) neurons showed maintained a discharge throughout the stimulus pulse. In a subpopulation of neurons (10%), membrane depolarization to around -44 mV produced a rhythmic burst discharge (RB) that was associated with voltage-dependent subthreshold membrane oscillations. Both these phenomena were blocked by the sodium channel blocker riluzole at a concentration that did not affect the fast transient spike. Low doses of 4-AP, which blocks low-threshold K+ currents, transformed bursting into low-frequency tonic discharge. In contrast, bursting occurred with exposure to cadium, a calcium-channel blocker. This suggests that persistent sodium currents and low-threshold K+ currents have a role in intrinsic burst generation. Importantly, RB cells were most often associated with multipolar neurons that exhibited either a DF or a T discharge. Thus, the SupV contains a variety of physiological cell types with unique morphologies and discharge characteristics. Intrinsic bursting neurons form a unique group in this region. .

The location of brainstem neurons with bilateral projections to the motor nuclei of jaw openers in the cat

Symmetrical motor output is the rule in the masticatory system. We examined morphologically how this functional symmetry might be reflected in the organization of premotor neurons that could mediate excitation of jaw-opener motoneurons. Premotor neurons projecting bilaterally to jaw-opener motoneurons by way of axon collaterals were identified by retrograde dual-labeling with cholera toxin B-conjugated fluorescein isothiocyanate (CTb-FITC) and tetramethylrhodamine (TMR). In each cat, CTb-FITC and TMR were injected into the digastric motoneuron pools, respectively, on the left and right sides. In three animals, 69-147 neurons were labeled with both tracers, comprising approximately 44% of all retrogradely labeled cells. Double-labeled cells were located bilaterally in the trigeminal oral nucleus (Vo) and the adjacent reticular formation (RF), the former containing a larger number of cells. Neurons labeled with only one tracer were also distributed bilaterally in the Vo and RF. The results indicated that the bilaterally projecting premoter neurons identified mainly in the Vo and RF represent neuronal substrates for the symmetry that characterizes most jaw movements.

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for these physiological observations. The transynaptic retrograde tracer pseudorabies virus-Bartha was injected into multiple sites of the lower third of the superficial layer of the masseter muscle in rats, a subset of which underwent a sympathectomy prior to virus injections, and the animals were euthanized 24-120 h later. Labeled masseteric motoneurons were first found in the ipsilateral trigeminal motor nucleus following a 24-h postinoculation period; subsequent to 72-h survival times, the number of infected motoneurons increased, and at > or =96 h many of these cells showed signs of cytopathic changes. Following 72-h survival times, a few transynaptically labeled neurons appeared bilaterally in the medial vestibular nucleus (MVe) and the caudal prepositus hypoglossi (PH) and in the ipsilateral spinal vestibular nucleus (SpVe). At survival times of 96-120 h, labeled neurons were consistently observed bilaterally in all vestibular nuclei (VN), although the highest concentration of infected cells was located in the caudal part of the MVe, the SpVe, and the caudal portion of PH. The distribution and density of labeling in the VN and PH were similar in sympathectomized and nonsympathectomized rats. These anatomical data provide the first direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVe, PH, and SpVe appear to play a predominant integrative role in producing vestibulo-trigeminal responses.

Objective evaluation of iatrogenic lingual nerve injuries using the jaw-opening reflex

The extent of reflex inhibition of masseteric electromyographic activity, after an electrical stimulus applied to lingual mucosa, was used as a test of the ability of the lingual nerve to conduct nerve impulses and this was compared with the results of standard clinical tests. Two groups of subjects were assessed: healthy subjects (n=10) and patients with lingual nerve injuries (n=17). The patients were tested 8-9 weeks after their injury and retested 6 months later when they were retrospectively allocated to either a temporary injury or a permanent injury group. The group measure of reflex inhibition after stimulation of the tongue on the opposite side to the injury was no different from the same measure in controls, whereas two-point discrimination did differ. Group measures of inhibition and of subjective function after stimulation on the side of the injury were significantly different from controls whereas light touch and two-point discrimination were not. There was good agreement between quantified masseteric inhibition and subjective function, but it was not possible at 8-9 weeks after the injury to differentiate between those that would recover and those that would be permanent.

Efferent and afferent connections of GABAergic neurons in the supratrigeminal and the intertrigeminal regions

It has been reported in the cat and rat that inhibitory premotor neurons, which send their axons to motoneurons of the trigeminal motor nucleus (Vm) are distributed in the reticular regions around the Vm, especially in the supratrigeminal region (Vsup) and the intertrigeminal region (Vint). In the present study, we examined neuronal connections of GABAergic neurons in the Vsup and Vint in the mouse by utilizing the adult heterozygous GAD67-GFP knock-in mouse, in which green fluorescence protein (GFP) is expressed in GABAergic neurons under the control of the endogenous GAD (GAD67) gene promoter [Yanagawa, Y., Kaneko, K., Kanbara, N., Totsuka, M., Yagi, T., Obata, K., 2001. Development of mouse expressing GFP in GABAergic neurons. Neurosci. Res. Suppl. 25, S77; Tamamaki, N., Yanagawa, Y., Tomioka, R., Miyazaki, J.-I., Obata, K., Kaneko, T., 2003. Green fluorescent protein expression and colocalization with calretinin, parvalbumin and somatostatin in the GAD67-GFP knock-in mouse. J. Comp. Neurol. 467, 60-79]. The connections were examined light- and electron-microscopically by combining the anterograde or the retrograde tract-tracing method with the immunohistochemical method for GFP. The data indicated that the Vsup and Vint of the mouse contained GABAergic neurons, which received projection fibers from the marginal layer of the nucleus of the spinal tract of the trigeminal nerve (Vc) on the ipsilateral side and sent their axons to the Vm on the contralateral side. Some of these GABAergic neurons may represent Vm-premotor neurons that receive nociceptive input from the Vc to elicit jaw-opening reflex by inhibiting jaw-closing Vm-motoneurons.

Convergence of selected inputs from sensory afferents to trigeminal premotor neurons with possible projections to masseter motoneurons in the rabbit

Peripheral input convergence on trigeminal premotor neurons in the vicinity of trigeminal motor nucleus has been investigated. Thirty neurons were identified by their antidromic responses to microstimulation of the masseteric subnucleus of trigeminal motor nucleus (NVmot-mass). Peripheral receptive fields were found in the buccal mucosae, periodontal ligaments, palate, tongue and vibrissae for 16 neurons located in the intertrigeminal area (NVint), supratrigeminal area (NVs), main sensory trigeminal nucleus (NVsnpr) and subnucleus gamma of the oral nucleus of the spinal trigeminal tract (NVspo-gamma). Eleven neurons in the NVint, NVs and NVspo-gamma responded to passive jaw opening: nine neurons were activated and two were inhibited. None of the neurons responded to both the orofacial mechanical stimulation and passive jaw opening. Forty-six percent of neurons (13 out of 28 tested) received inputs from the inferior alveolar nerve (IAN) and 53% of neurons (8 out of 15 tested) received inputs from the infraorbital nerve (ION). Out of 15 neurons tested for inputs from the IAN and ION, 7 neurons in the NVsnpr and NVspo-gamma received input from both. Sixteen percent of neurons (4 out of 25) received inputs from the masseteric nerve (MassN). None of the neurons with inputs from IAN and/or ION also received inputs from the MassN. We suggest that trigeminal premotor interneurons with projections to the NVmot-mass fall into two broad categories, those with inputs from the IAN and/or ION and those with inputs from the MassN, possibly muscle spindle afferents, and no neuron receiving inputs from both.

The supratrigeminal region of the rat sends GABA/glycine-cocontaining axon terminals to the motor trigeminal nucleus on the contralateral side

The supratrigeminal region (STR), a reticular zone capping the motor trigeminal nucleus (Tm), contains gamma-aminobutyric acid (GABA)ergic and glycinergic neurons which send axons to the contralateral Tm (J. Comp. Neurol. 373 (1996) 498). In the present study we observed that some single synaptic terminals upon Tm motoneurons showed immunoreactivities (IRs) for both glutamic acid decarboxylase (GAD) and glycine transporter 2 (GlyT2). After injecting biotinylated dextran amine (BDA) into the STR, we further observed in the Tm contralateral to the BDA injection that some BDA-labeled axon terminals in close contact with Tm motoneurons showed both GAD- and GlyT2-IRs. Thus, the STR was indicated to send GABAergic/glycinergic axon terminals contralaterally to Tm motoneurons.

Properties and interconnections of trigeminal interneurons of the lateral pontine reticular formation in the rat

Numerous evidence suggests that interneurons located in the lateral tegmentum at the level of the trigeminal motor nucleus contribute importantly to the circuitry involved in mastication. However, the question of whether these neurons participate actively to genesis of the rhythmic motor pattern or simply relay it to trigeminal motoneurons remains open. To answer this question, intracellular recordings were performed in an in vitro slice preparation comprising interneurons of the peritrigeminal area (PeriV) surrounding the trigeminal motor nucleus (NVmt) and the parvocellular reticular formation ventral and caudal to it (PCRt). Intracellular and extracellular injections of anterograde tracers were also used to examine the local connections established by these neurons. In 97% of recordings, electrical stimulation of adjacent areas evoked a postsynaptic potential (PSP). These PSPs were primarily excitatory, but inhibitory and biphasic responses were also induced. Most occurred at latencies longer than those required for monosynaptic transmission and were considered to involve oligosynaptic pathways. Both the anatomical and physiological findings show that all divisions of PeriV and PCRt are extensively interconnected. Most responses followed high-frequency stimulation (50 Hz) and showed little variability in latency indicating that the network reliably distributes inputs across all areas. In all neurons but one, excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs) were also elicited by stimulation of NVmt, suggesting the existence of excitatory and inhibitory interneurons within the motor nucleus. In a number of cases, these PSPs were reproduced by local injection of glutamate in lieu of the electrical stimulation. All EPSPs induced by stimulation of PeriV, PCRt, or NVmt were sensitive to ionotropic glutamate receptor antagonists 6-cyano-7-dinitroquinoxaline and D,L-2-amino-5-phosphonovaleric acid, while IPSPs were blocked by bicuculline and strychnine, antagonists of GABA(A) and glycine receptors. Examination of PeriV and PCRt intrinsic properties indicate that they form a fairly uniform network. Three types of neurons were identified on the basis of their firing adaptation properties. These types were not associated with particular regions. Only 5% of all neurons showed bursting behavior. Our results do not support the hypothesis that neurons of PeriV and PCRt participate actively to rhythm generation, but suggest instead that they are driven by rhythmical synaptic inputs. The organization of the network allows for rapid distribution of this rhythmic input across premotoneuron groups.

Axonal projections and synapses from the supratrigeminal region to hypoglossal motoneurons in the rat

Neural circuits from the supratrigeminal region (Vsup) to the hypoglossal motor nucleus were studied in rats using anterograde and retrograde neuroanatomical tracing methodologies. Iontophoretic injection of 10% biotinylated dextran amine (BDA) unilaterally into the Vsup anterogradely labeled axons and axon terminals bilaterally in the hypoglossal nucleus (XII) as well as other regions of the brainstem. In the ipsilateral XII, the highest density of BDA labeling was found in the dorsal compartment and the ventromedial subcompartment of the ventral compartment, where BDA labeling formed a dense, patchy distribution. Microinjection of 20% horseradish peroxidase (HRP) ipsilaterally or bilaterally into the tongue resulted in retrograde labeling of XII motoneurons confined to the dorsal and ventral compartments of the hypoglossal motor nucleus. Under light microscopical examination, BDA-labeled terminals were observed closely apposing the somata and primary dendrites of HRP-labeled hypoglossal motoneurons. Two hundred and sixty-five of these BDA-labeled terminals were examined at the ultrastructural level. One hundred and twelve BDA-labeled axon terminals were observed synapsing with either the somata (39%, 44/112) or the large or medium-size dendrites (61%, 68/112) of retrogradely labeled hypoglossal motoneurons. Axon terminals containing spherical vesicles (S-type) formed asymmetric synapses with HRP-labeled hypoglossal motoneuron dendrites. In contrast to this, F(F)-type axon terminals, containing flattened vesicles, formed symmetric synapses with both the somata and dendrites of HRP-labeled hypoglossal motoneurons with a preponderance of the contacts on their somata. Axon terminals containing pleomorphic vesicles (F(P)-type) were noted forming both symmetric and asymmetric synapses with HRP-labeled hypoglossal motoneuron somata and dendrites. The present study provides anatomical evidence of neuronal projections and synaptic connections from the supratrigeminal region to hypoglossal motoneurons. These data suggest that the supratrigeminal region, as one of the premotor neuronal pools of the hypoglossal nucleus, may coordinate and modulate the activity of tongue muscles during oral motor behaviors.

Influence of periodontal receptors on the jaw-jerk reflex amplitude in man

To study the role of periodontal mechanoreceptors in the modulation of the human jaw-jerk reflex, 60 reflexes were elicited in each of six healthy individuals under three different conditions: (a) with unilateral tooth support on the ipsilateral side; (b) with support on the contralateral side; and (c) with support and local anesthesia of the periodontal tissues on the ipsilateral side. During all conditions, background muscle activity and mandibular displacement were kept as constant as possible. Using on-surface EMG and the amplitude of the first peak of the biphasic jaw-jerk potential as the outcome variable, a condition-dependent reflex amplitude was found for the visually controlled right anterior temporalis muscle. Specifically, the 'ipsilateral support plus anesthesia' condition yielded higher amplitudes than the 'ipsilateral support' condition. Highest amplitudes were found for the 'contralateral support' condition. It was concluded that periodontal mechanoreceptors on the ipsilateral side have an inhibitory effect on the jaw-jerk reflex amplitude in the anterior temporalis muscle.

Projections from the caudal spinal trigeminal nucleus to commissural interneurons in the supratrigeminal region: an electron microscope study in the rat

Electron microscopic double-labeling study in the rat indicated that projection fibers from the caudal spinal trigeminal nucleus (Vc) were distributed ipsilaterally within the supratrigeminal region (STR) capping the trigeminal motor nucleus (Tm) and made synaptic contact with neurons projecting to the contralateral Tm. Nociceptive inputs to the Vc may reflexly control, via interneurons in the STR, the activities of Tm neurons innervating the masticatory, tensor tympani, and/or tensor veli palatine muscles.

Effects of chemical stimulation of masseter muscle nociceptors on trigeminal motoneuron and interneuron activities during fictive mastication in the rabbit

An electrophysiological study was carried out in sixteen decerebrate and paralyzed New Zealand rabbits to determine how a bolus injection of a nociceptor stimulant (hypertonic saline, 5%) into the masseter muscle influences the activity of the trigeminal motor circuitry during fictive jaw movements. Hypodermic needles connected to a syringe held in a computer-controlled infusion pump were inserted into the anterior deep layer of either the right or the left masseter. Twenty-three infusions of 50, 70 or 80 microl saline were made in fourteen animals at constant rates over 1 min. Eight control infusions of normal saline (0.9%) were made in a subpopulation of five animals in an identical manner. Fictive jaw movements were evoked before and after the infusions by repetitive electrical stimulation of the corticobulbar tract. Effects were assessed by extracellular microelectrode recordings made from the digastric motoneuron pool and from putative last-order interneurons in the oral subnucleus of the spinal trigeminal tract and adjacent structures. In comparison with pre-infusion control cycles, nociceptor stimulation caused significant slowing of the rhythm and a reduction of the area of the digastric motoneuron bursts in the majority of the animals (12/14). The decrease in cycle frequency was due almost entirely to a lengthening of the time between the digastric bursts. Changes usually began 1-2 min after the infusion and returned to pre-infusion values within 10-15 min. No significant effects were seen when isotonic saline was applied. Recordings were obtained from nine interneurons, eight of which had low threshold mechanosensitive receptive fields. One neuron was, in addition, excited by pinch. Eight were not active in the absence of motor activity and this did not change when hypertonic saline was applied. However, once fictive movements began, all started to fire rhythmic bursts of spikes. In five cases, there was a significant post-infusion increase in spike frequency, and three showed decreases. Seven showed significant post-infusion changes in mean phase and/or concentration of their firing within the movement cycle. Changes in the preferred phase of interneuronal firing were significantly correlated to changes in the phase of offset of the digastric burst. The present results provide evidence that the stimulation of nociceptors in a muscle slows the frequency of rhythmical movements in the absence of sensory feedback. They confirm that infusions into one muscle affect the output of its antagonist. The results also suggest that neurons in the oral subnucleus of the spinal trigeminal tract and adjacent reticular formation appear to participate in programming these changes in motor output.

GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: a double labeling study in the rat

The distribution of GABAergic and glycinergic premotor neurons projecting to the trigeminal motor nucleus (Vm) was examined in the lower brainstem of the rat by a double labeling method combining retrograde axonal tracing with immunofluorescence histochemistry. After injection of the fluorescent retrograde tracer, tetramethylrhodamine dextran amine (TRDA), into the Vm unilaterally, neurons labeled with TRDA were seen ipsilaterally in the mesencephalic trigeminal nucleus, and bilaterally in the parabrachial region, the supratrigeminal and intertrigeminal regions, the reticular formation just medial to the Vm, the principal sensory and spinal trigeminal nuclei, the pontine and medullary reticular formation, especially the parvicellular part of the medullary reticular formation, the alpha part of the gigantocellular reticular nucleus, and the medullary raphe nuclei. Some of these neurons labeled with TRDA were found to display glutamic acid decarboxylase (the enzyme involved in GABA synthesis)-like or glycine-like immunoreactivity. Such double-labeled neurons were seen mainly in the supratrigeminal region, the reticular region adjacent to the medial border of the Vm, and the dorsal part of the lateral reticular formation of the medulla oblongata; a number of them were further scattered in the intertrigeminal region, the alpha part of the gigantocellular reticular nucleus, the nucleus raphe magnus, the principal sensory trigeminal nucleus, and the interpolar subnucleus of the spinal trigeminal nucleus. These neurons were considered to be inhibitory (GABAergic or glycinergic) neurons sending their axons to motoneurons in the Vm, or to local interneurons within and around the Vm.

Transneuronal transport of intracellularly injected biotinamide in primary afferent axons

Transneuronal transport of biotinamide was observed following intracellular injection of biotinamide into rat jaw-muscle spindle afferent axons. Microelectrodes were advanced into the mesencephalic nucleus of the trigeminal nerve where jaw-muscle spindle afferent axons were identified by their increased firing during stretching of the jaw-elevator muscles. Biotinamide (Neurobiotin) was then injected into individual axons and the animals were maintained under anesthesia for 2-6 h. The animals were then killed via an overdose of anesthetic and the brainstem was processed histochemically. Biotinamide-filled axon collaterals and terminals were readily visible in the trigeminal motor nucleus, the trigeminal sensory nuclei, and adjacent reticular formation. In addition to these intracellularly stained axons, two to five neurons per animal (total of 36 in eight rats) were observed with a homogeneous gray reaction product distributed throughout their somata, proximal, and secondary dendrites. These neurons ranged in size from small (8-20 mu m, n - 26) to medium-sized (<30 mu m, n = 10) and were closely apposed by numerous (up to 20) biotinamide-stained spindle afferent boutons. Most of these neurons (n = 22) were located in the dorsomedial portion of the spinal trigeminal subnucleus interpolaris (Vi) 2.5-4.5 mm caudal to the intra-axonal injection site. Electron microscopic analysis in two rats suggests that the transneuronal biotinamide labeling occurred predominantly through asymmetric, axodendritic synapses between biotinamide-filled axon terminals and Vi neuronal dendrites. Although recent in vitro studies have reported that biotinamide permeates through gap junctions, in this study we found no evidence of biotinamide traversing the gap junctions which exist between trigeminal mesencephalic nucleus (Vme) neuronal somata. These results demonstrate that biotinamide can occasionally be transneuronally transported presumably via synapses; further information is needed to explain the seemingly sporadic nature of this transport.

Premotor neurons for trigeminal motor nucleus neurons innervating the jaw-closing and jaw-opening muscles: differential distribution in the lower brainstem of the rat

The distribution of premotor neurons for trigeminal motor nucleus neurons innervating the jaw-closing and jaw-opening muscles was examined in the lower brainstem of the rat by using retrograde and anterograde labeling techniques. First, Fluorogold, a fluorescent retrograde tracer, was injected into the dorsolateral or ventromedial division of the trigeminal motor nucleus, each of which contains motoneurons innervating the jaw-closing or jaw-opening muscles, respectively. Second, Phaseolus vulgaris-leucoagglutinin, an anterograde tracer, was injected into each of the lower brainstem sites, where clusters of retrogradely labeled premotor neurons had been seen in the first set of experiments. Third, after injection of the anterograde tracer into a lower brainstem site, followed by injection of the retrograde tracer cholera toxin B subunit into a masticatory muscle, termination of anterogradely labeled axons onto retrogradely labeled motoneurons was confirmed with the aid of a confocal laser-scanning microscope. It was found that the premotor neurons distributed in the mesencephalic trigeminal nucleus, medial part of the parabrachial region, supratrigeminal region, and dorsal parts of the principal sensory, oral spinal and interpolar spinal trigeminal nuclei project preferentially to the dorsolateral division of the trigeminal motor nucleus, whereas those in the lateral part of the parabrachial region, intermediate parts of the principal sensory, oral spinal and interpolar spinal trigeminal nuclei, and alpha part of the gigantocellular reticular nucleus project preferentially to the ventromedial division of the trigeminal motor nucleus. The dorsal and lateral parts of the medullary reticular formation and the medullary raphe nuclei contain premotor neurons of both types. Group k motoneurons, a cluster of trigeminal motoneurons that innervate the tensor tympani muscle, receive projection fibers predominantly from the dorsolateral part of the oral pontine reticular formation.