Dissertations / Theses on the topic 'Trigeminal nucleus'

The mesencephalic trigeminal nucleus (MTN) forms part of the monosynaptic trigeminal circuit and is essential for eating and suckling in mammals. Little is known about how the MTN forms. For this thesis I aimed to elucidate the molecular and cellular basis of MTN development. I also aimed to investigate the role of the Fgf and Wnt signalling pathways in MTN development. The zebrafish was used as a model organism to investigate these aims. Putative MTN cells in zebrafish larvae were retrogradely labelled by applying Dil to the adductor mandibulae, a jaw closing muscle. Labelled axons projected from muscles via the trigeminal ganglion to cell bodies in the dorsal anterior mesencephalon, suggesting that the MTN does innervate jaw muscles in teleosts, contrary to previous studies. Molecular characterisation of the MTN in zebrafish revealed a similar expression profile as the mammalian MTN. To investigate whether MTN neurons are neural crest-derived, the neural crest was ablated, which resulted in an increase in MTN number. This suggests that the neural crest may play an inhibitory role in MTN development contradictory to previous studies in chick that suggested MTN neurons are derived from the neural crest. The role of the Fgf and Wnt signalling pathways was investigated by analysis of mutants and drug treatments where the pathways had been genetically or chemically manipulated. Down-regulation of Fgf signalling showed an increase in MTN neuron numbers, suggesting that Fgf signalling from the midbrain/hindbrain boundary inhibits development of the MTN in the midbrain. When the Wnt pathway was up-regulated there was also an increase in MTN neuron number. Based on the results from these experiments a model is proposed, in which Fgf signalling regulates the formation of MTN neurons in a spatial and temporal manner, and Wnt signals from the dorsal roof plate induce the proliferation of MTN precursor cells.

Pharmacology
Ph.D.
Amylin, or islet amyloid polypeptide is a 37-amino acid member of the calcitonin peptide family. Amylin role in the brainstem and its function in regulating heart rates is unknown. The diving reflex is a powerful autonomic reflex, however no neuropeptides have been described to modulate its function. In this thesis study, amylin expression in the brainstem involving pathways between the trigeminal ganglion and the nucleus ambiguus was visualized and characterized using immunohistochemistry. Its functional role in slowing heart rate and also its involvement in the diving reflex were elucidated using stereotaxic microinjection, whole-cel patch-clamp, and a rat diving model. Immunohistochemical and tract tracing studies in rats revealed amylin expression in trigeminal ganglion cells, which also contained vesicular glutamate transporter 2 positive. With respect to the brainstem, amylin containing fibers were discovered in spinal trigeminal tracts. These fibers curved dorsally toward choline acetyltransferase immunoreactive neurons of the nucleus ambiguus, suggesting that amylin may synapse to parasympathetic preganglionic neurons in the nucleus ambiguus. Microinjection of fluorogold to the nucleus ambiguus retrogradely labeled a population of trigeminal ganglion neurons; some of which also contained amylin. In urethane-anesthetized rats, stereotaxic microinjections of amylin to the nucleus ambiguus caused a dose-dependent bradycardia that was reversibly attenuated by microinjections of the selective amylin receptor antagonist, salmon calcitonin (8-32) (sCT (8-32)) or AC187, and abolished by bilateral vagotomy. In an anesthetized rat diving model, diving bradycardia was attenuated by glutamate receptor antagonists CNQX and AP5, and was further suppressed by AC187. Whole-cel patch-clamp recordings from cardiac preganglionic vagal neurons revealed that amylin depolarizes neurons while decreasing conductance. Amylin also resulted in a reduction in whole cell currents, consistent with the decrease in conductance. Amylin is also found to increase excitability of neurons. In the presence of TTX, spontaneous currents in cardiac preganglionic vagal neurons were observed to decrease in frequency in response to amylin while amplitude remained constant, signifying that amylin reduces presynaptic activity at cardiac preganglionic vagal neurons. Finally, evoked synaptic currents revealed that amylin decreases evoked currents, further demonstrating that amylin depolarization and increase in excitability of cardiac preganglionic vagal neurons is also associated with simultaneous inhibition of presynaptic transmission. Our study has demonstrated for the first time that the bradycardia elicited by the diving reflex is mediated by amylin from trigeminal ganglion cells projecting to cardiac preganglionic neurons in the nucleus ambiguus. Additionally, amylin results in the depolarization and increased excitability of cardiac preganglionic vagal neurons while inhibiting presynaptic transmission.
Temple University--Theses

1. The quantitative and somatotopic aspects of periodontal/gingival and pulpal afferent connections of the mandibular and maxillary incisors, canines and molar teeth to the mesencephalic nucleus of trigeminal nerve and the trigeminal ganglion have been investigated in the vervet monkey and olive baboon using horseradish peroxidase (HRP) retrograde axonal tracing method. 2. It has been demonstrated that the periodontal proprioceptive afferent neurons of incisors, canines and molars are found predominantly in the ipsilateral caudal part of the trigeminal mesencephalic nucleus extending from the level of inferior colliculi to the trigeminal motor nucleus in pons. The incisors have significantly more mesencephalic neural connections than canines and molars. No HRP labelled pulpal mesencephalic neurons have been observed. Faintly labelled neurons have been observed bilaterally, presumably in the supratrigeminal nuclei. 3. It has been shown that the incisors and canines have a large and preponderantly ipsilateral representation in the trigeminal ganglion compared to the molars which have a sparse ipsilateral representation. The discrete periodontal/gingival and pulpal HRP labelled afferent neurons innervating mandibular teeth are found in the postero-lateral aspect of the ganglion and those of the maxillary teeth are found in the middle, along the dorso-ventral extent of the ganglion. 4. Present study shows that about 10% to 15% of the mesencephalic neurons (unilaterally) and 0.32% to 0.58&37 of trigeminal ganglion neurons have afferent connections with the periodontium of incisors, canines and molars in the monkey and baboon. The stereological analysis and cell counts in stratified serial paraffin wax sections has shown that there are bilaterally 1379-2674 and 1620-2816 mesencephalic neurons; 98073-10178 and 137250-153555 ganglion neurons in the monkey and baboon respectively. 5. The periodontal proprioceptive mesencephalic afferent connections of the anterior and posterior teeth suggest that they are involved in the modulation of the reflex effects on the jaw-opening and jaw-closing motor neurons and are thus important in the regulation of masticatory jaw movements. Moreover, a cluster of mesencephalic neurons may form a functional unit for synchronizing jaw movements during mastication. The numerous trigeminal ganglion afferent connections of the anterior teeth suggest that they have a major sensory role particularly in perception of the food bolus. Furthermore, the afferent connections of the anterior teeth may serve to regulate the jaw movement by providing anterior guidance during the occlusal phase of chewing. It is concluded that the connections of teeth to the ipsilateral trigeminal mesencephalic nucleus and ganglion; the connections to the interneurons of the supratrigeminal and the sensory nuclei are involved in the reflex modulation and bilateral control of jaw movements and the perception of stimulation of the teeth.

O núcleo motor do trigêmeo (Mo5) está cercado por um anel de neurônios pré-motores localizados na região h. Estudos demonstram que neurônios que inervam o Mo5 estão distribuídos no tronco encefálico e no prosencéfalo. Após implante de traçador retrógrado no Mo5, verificamos células retrogradamente marcadas no núcleo mesencefálico do trigêmeo (Me5), na região h e em núcleos prosencefálicos como o central da amígdala (CeA), a área hipotalâmica lateral (LH) e o parasubtalâmico (PSTh). Para confirmação, realizamos injeção de traçador anterógrado e investigamos, também, a neuroquímica das projeções. Neurônios do CeA que se projetam para o Mo5 recebem inervação de fibras imunorreativas ao fator liberador de corticotrofina (CFR-ir) e/ou à tirosina hidroxilase (TH-ir); alguns neurônios da LH que se projetam para o Mo5 são imunorreativos à orexina (ORX) e alguns neurônios do PSTh que se projetam para o Mo5 são innervados por fibras TH-ir. O Me5 recebe grande inervação do CeA e moderada da LH e do PSTh, possuindo grande aferência de fibras imunorreativas ao CRF, ORX e TH
The trigeminal motor nucleus (Mo5) is surrounded by a ring of premotor neurons defined as the h region. Studies have shown that neurons innervating the Mo5 are located in brainstem and in forebrain nuclei. Through the injection of the retrograde tracer cholera toxin b subunit/CTb in the Mo5, we found retrograde labeled neurons in the brainstem including the h region and the mesencephalic trigeminal nucleus (Me5), and in forebrain nuclei such as the central nucleus of amygdala (CeA), the lateral hypothalamic area (LH) and the parasubthalamic nucleus (PSTh). As control, we injected the anterograde tracer biotin dextran amine and found that these areas project direct or indirectly via the h region or the Me5 to the Mo5. Some CeA neurons that project to the Mo5 receive corticotrophin releasing factor (CRF) and tyrosine hydroxylase (TH) innervation, some LH neurons that project to Mo5 express orexin, and PSTh neurons that project to the Mo5 receive TH innervation. The Me5 is also innervated by CeA, LH and PSTh neurons and by CRF, orexin and TH immunoreactive fibers

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Pacientes com distrofia muscular de Duchenne apresentam alteracoes no sistema nervoso central (SNC). Mudancas no SNC tambem ocorrem nos camundongos distroficos (mdx), incluindo perda de fibras rubro-espinais. Para examinar se outras vias tambem sao reduzidas no mdx, propusemo-nos a estudar a organizacao topografica das vias trigemino-rubrais e quantificar os neuronios do Complexo Trigeminal que se projetam para o nucleo Rubro em camundongos C57BL10 (normais) e distroficos (mdx) de diferentes idades. Para tanto, os animais foram submetidos a cirurgia estereotaxica para as injecoes dos tracadores de transporte retrogrado Fluorogold, Rodamina e Fluoresceina, bilateralmente, no nucleo Rubro. Sete dias depois, os animais foram sacrificados sob anestesia atraves de perfusao transcardiaca e os encefalos foram congelados em meio de embebicao proprio no uso do aparelho criostato e, destes encefalos foram realizados cortes seriados na espessura de 35 ƒÊm. A analise foi realizada em microscopico de epifluorescencia. Foram contados os neuronios do subnucleo oral do nucleo espinal do nervo trigemeo em camundongos normais e distroficos de 3, 6 e 12 meses de idade. No Sistema Intersticial, foram contados todos os neuronios marcados ao longo de sua extensao. Nossos resultados mostraram que existe uma organizacao topografica na projecao dos neuronios do Complexo Trigeminal e do Sistema Intersticial para o nucleo Rubro, em camundongos. Todos os nucleos sensoriais do Complexo Trigeminal apresentaram intensa marcacao bilateral, com predominio contralateral quando o sitio de injecao foi na regiao magnocelular do nucleo Rubro; os neuronios apresentaram pouca ou nenhuma marcacao quando o sitio atingiu a região parvocelular e, quando o sítio de injeção atingiu a região intermediária do núcleo a qual abrange suas partes magnocelular e parvocelular, a marcação retrógrada foi mais intensa só quando o foco do sítio atingiu mais a região magnocelular. O núcleo motor do Complexo Trigeminal não foi marcado em nenhuma das situações. No Sistema Intersticial, foram marcados os neurônios apenas quando o sítio de injeção abrangeu a região magnocelular do núcleo Rubro. Também foi verificado que no Complexo Trigeminal essa organização é semelhante em ambos os grupos normais e distróficos. Os resultados mostraram uma redução de 50% no número de neurônios do Complexo Trigeminal no mdx com a idade de 3 meses. Essa redução neuronal se manteve nos camundongos mdx nos grupos de 6 e 12 meses, não ocorrendo progressão desta perda com a idade. Além disso, o grupo de camundongos C57BL10 (normais) também não apresentou perda neuronal com a idade. Concluímos que a diferença observada no complexo trigeminal no grupo de 3 meses já está estabelecida e que não é progressiva com o avanço da idade; portanto, é bem provável que os camundongos mdx já nasçam com essa redução ou que a mesma ocorra logo nas primeiras semanas após o nascimento.
TEDE
BV UNIFESP: Teses e dissertações

Sudden perinatal death including SIDS is a rare but lethal syndrome and there is no symptom of this disorder until the fatal outcome has occurred. Epidemiological, genetic, molecular and pathological studies conducted so far give us some possible explanations about it but are inadequate to explain it completely. Brainstem etiology is a mostly accepted hypothesis to induce sudden perinatal death. We investigated the immunohistochemical expression of substance P (SP) in the brainstems of 56 subjects aged from 17 gestational weeks to 10 postnatal months, died of unknown (sudden unexplained perinatal deaths and SIDS) and known causes (controls). The goals of this study were to obtain basic information about the expression of the SP during the first phases of human nervous system development; to evaluate whether there are altered manifestations of this neuromodulator in victims of sudden death; to verify the correlation with maternal cigarette smoking and to see the evolutionary aspects of SP gene (TAC1) through computational analysis. Immunohistochemistry demonstrated SP-immunoreactivity in correspondence of the caudal trigeminal nucleus area, with progressive increase in density of positive fibers of the corresponding tract from fetal life to first postnatal months. So, we first delineated the structure of the human trigeminal nucleus, so far little investigated, and provided essential data on its morphologic and functional development. Nevertheless, a negativity or low SP-positivity of the tract fibres was detectable in a wide subset of SIDS and, conversely, high SP-expression in a wide subset of sudden fetal deaths. Therefore we postulate, on the basis of these results, the functional importance of the SP in the early phases of central nervous system development and in the regulation of autonomic functions. Besides, the observation of a significant correlation between sudden unexplained death, altered SP staining and maternal smoking, prompted us to suppose a close relation between smoking absorption in utero and decrease of the functional activity of the trigeminal nucleus, leading to sudden death during pregnancy and in the first months of life. Computational analysis suggests that SP encoding gene (TAC1) is a singleton, appeared in vertebrates and is more prone to induce neuropathologies along with its interactors, if mutated or functionally altered, as it is located mostly in brain.

An underlying disorder in the migraine condition is an apparent subclinical sensitization of the trigemino cervical nucleus (TCN) indicated, for example, by an interictal deficient habituation of the nociceptive blink reflex (nBR). This has ramifications for tension-type headache (TTH), as there is considerable support for a pathogenesis of TTH which overlaps with that of migraine. The aim of this thesis was to investigate the upper cervical (C1-3) afferents as a potential sensitising source of the TCN in migraine and TTH, thereby addressing the hypothesis that upper cervical afferents evoke sensitisation of the TCN in migraine and TTH. Firstly, manual examination of the upper cervical (atlanto-occipital and C2-3) joints was performed in 20 migraineurs, 14 TTH patients and 14 controls. The reproduction of customary head pain in 100 and 95 per cent of TTH patients and migraineurs, respectively, supports a role of the upper cervical spine in primary headache, perhaps involving sensitization of the TCN. The second study employed the nociceptive blink reflex (nBR) to assess processing of trigeminal nociceptive information during reproduction and resolution of customary head pain (as the examination technique was sustained) in 15 migraineurs interictally. Reproduction and resolution of head pain was repeated over four 90 second trials; each trial was separated by 30 seconds. Migraineurs reported significant lessening of reproduction and increasing resolution of customary head pain over the four trials. In parallel was a significant increase in latency and decrease in amplitude of the nBR. The desensitizing effect of this examination technique on head pain implies that modulation of cervical afferent information may benefit migraineurs during manual cervical reproduction of customary head pain. Whiplash of the neck is considered a musculo skeletal event and subsequent headache implies involvement of upper cervical (C1-3) afferents. The symptomatic profile of chronic whiplash associated headache (CWAH) mirrors that of primary headache, inviting speculation that CWAH shares a pathophysiology similar to that of primary headache. This prompted us to assess trigeminal nociceptive processing in CWAH patients in the third study. The symptomatic profile of 22 CWAH patients confirmed previous studies demonstrating similar profiles to primary headache. Furthermore, when compared to controls (n=25), CWAH patients had significant photophobia and allodynia. In addition, analysis of the nBR revealed hyperexcitability in central nociceptive pathways in CWAH patients, thus reinforcing the hypothesis that CWAH could be driven by central sensitization from upper cervical afferents. Together, these studies support the view that upper cervical (C1-3) nociceptive information may contribute to sensitising the TCN in primary headache. Thus, therapeutic strategies that aim to alleviate aberrant discharge of cervical afferents may play a role in the management of primary headache.

The role of dopamine in directly modulating somatic motoneuron excitability and hence muscle tone is unknown. We investigated whether dopamine influences the trigeminal motor pool (MoV) that innervates the masseter and tensor palatini muscles, both of which function to maintain upper airway patency. We hypothesized that dopamine facilitates motor outflow at the MoV. We focally applied apomorphine (nonspecific dopamine receptor agonist) at the MoV in anaesthetized rats. We also applied receptor-specific agonists and antagonists to determine the receptor subtype mediating dopaminergic mechanisms of action. We demonstrated that dopaminergic transmission at the MoV potently increased motor outflow via the D1-like receptor and facilitated masseter and tensor palatini muscle tone. It is unknown whether endogenous dopamine release on to airway motoneurons influences their activity to regulate muscle tone in natural sleep-wake behaviours. This issue warrants investigation because the neurochemical basis of upper airway motor dysfunction (e.g. obstructive sleep apnea) remains poorly characterized.

碩士
中國醫藥大學
醫學研究所
92
The trigeminal motor nucleus (Mo5) contains the cell bodies of the motoneurons innervating the jaw muscles to control the rhythmical jaw movements (RJMs). Noradrenergic (NAergic) neurons located in the pontine area, A5 and A7, project to the Mo5 and have an effect on the modulation of RJMs. The aim of the study was to investigate: 1) the ultrastructure of NAergic varicosities in the Mo5, 2) the postnatal development of the NAergic neurons in A5/A7 areas, 3) the expression of GABAB receptors on A5/A7 NAergic neurons during the postnatal ages. NA varicosities in the Mo5 in adult rats were detected by immunohistochemistry for tyrosine hydroxylase (TH: the enzyme to convert tyrosine to DOPA). The ultrastructure of synaptic contacts, postsynaptic elements and contents formed by TH-immunoreactive (-ir) boutons were analyzed. It was found that there were 40% of NA varicosities not making synaptic contacts with any neuronal elements, and 60% of NA varicosities forming symmetrical or asymmetrical synaptic contacts with soma or dendrite. The postnatal development of the NAergic neurons in A5/A7 areas was investigated by immunohistochemistry for dopamine-b-hydroxylase (DBH: the synthesis enzyme of NA). The adult pattern of area of A5/A7 neurons and density of NAergic fibers in Mo5 were attained by P10-20. The expression of GABAB receptors on the A5/A7 neurons was not changed at the postnatal development by immunofluorescence analysis In conclusions, the modulation of A5/A7 neurons by GABA system was constituted at the stage before birth. The adult pattern of density of NAergic fibers raised from A5/A7 neurons were attained by P10-15. The evidence of ultrastructure suggested that noradrenaline acts on the Mo5 neurons via diffusing from non-synaptic varicosities partially to exert its influence on a large number of neurons. However, partial NAergic varicosities exert their role in modulation of specific neuronal targets via a specialized synaptic mechanism.

La communication entre les neurones est fondée sur leur capacité à changer leur patron de décharge pour l’encodage de différents messages. Pour plusieurs fonctions vitales, comme la respiration et la mastication, les neurones doivent pouvoir générer des patrons d’activité répétitifs, et les groupes de neurones responsables de ces décharges rythmiques sont des générateurs de patron central (GPC). En dépit de recherches soutenues, les mécanismes précis qui sous-tendent la rythmogénèse dans les GPCs ne sont pas bien définis. Le plus souvent, la potentielle contribution des astrocytes demeure grandement inexplorée, même si ces cellules sont aujourd’hui connues pour leur implication dans la modulation synaptique neuronale. Pour nos travaux, le noyau sensoriel principal du trijumeau (NVsnpr) a été pris comme modèle à cause de son rôle central dans les mouvements rythmiques de la mastication. Dans ce noyau, des travaux antérieurs ont montré que la décharge en bouffées rythmiques est déclenchée dans les neurones lorsque la concentration de calcium extracellulaire ([Ca2+]e) est artificiellement baissée. Nous fondant sur cette observation, notre première hypothèse a postulé que la baisse de la [Ca2+]e pouvait survenir de façon physiologique en lien avec des stimulations sensorielles pertinentes. Deuxièmement, parce que les astrocytes ont été impliqués dans le tamponnage et l’homéostasie d’ions extracellulaires comme le K+, nous avons postulé que ces cellules pouvaient jouer un rôle équivalent dans le contrôle de la [Ca2+]e. Nos résultats montrent que les astrocytes peuvent réguler la [Ca2+]e et ainsi contrôler la capacité des neurones à changer leur patron de décharge. Premièrement, en stimulant les afférences sensorielles au NVsnpr, nous avons montré que des baisses physiologiques de la [Ca2+]e sont observées en parallèle à l’apparition de bouffées rythmiques neuronales. Deuxièmement, nous avons démontré que les astrocytes répondent aux mêmes stimuli qui induisent l’activité rythmique neuronale, et que leur blocage avec un chélateur de Ca2+ empêche les neurones de générer un patron de décharge en bouffées rythmiques. Cette habilité est rétablie en rajoutant la S100β, une protéine astrocytaire liant le Ca2+, dans le milieu extracellulaire, alors que l’anticorps anti-S100β empêche l’activité rythmique. Ces résultats indiquent que les astrocytes régulent une propriété neuronale fondamentale : la capacité à changer de patron de décharge. Ainsi, les GPCs dépendraient des fonctions intégrées des astrocytes et des neurones. Ces découvertes pourraient avoir des implications transposables à plusieurs autres circuits neuronaux dont la fonction dépend de l’induction d’activité rythmique.
Communication between neurons rests on their capacity to change their firing pattern to encode different messages. For several vital functions, such as respiration and mastication, neurons need to generate a repetitive firing pattern, and the groups of neurons responsible for these rhythmic discharges are called central pattern generator (CPG). Despite intense research in this field, the exact mechanisms underlying rhythmogenesis in CPGs are not completely defined. In most instances, the potential contribution of astrocytes is largely unexplored, even though these cells are now well known to be involved in neuronal synaptic modulation. In our work, the trigeminal main sensory nucleus (NVsnpr) was used as a model owing to its central role in the rhythmic movement of mastication. Previous work have shown that rhythmic bursting discharge is triggered in NVsnpr neurons when extracellular calcium concentration ([Ca2+]e) is artificially decreased. Based on this observation, our first hypothesis postulated that the reduction of [Ca2+]e could also happen physiologically in relation to relevant sensory stimulation. Secondly, because astrocytes have been involved in the buffering and the homeostasis of extracellular ions like potassium, we have postulated that these cells could also play a role in the control of [Ca2+]e. The results presented in this thesis show that astrocytes can regulate [Ca2+]e and thus control the ability of neurons to change their firing pattern. First, we showed that stimulation of sensory afferent fibers to the NVsnpr induced neuronal rhythmic bursting and in parallel reduction of [Ca2+]e . Secondly, we have demonstrated that astrocytes respond to the same sensory stimuli that induce neuronal rhythmic activity, and their blockade with a Ca2+ chelator prevents generation of neuronal rhythmic bursting. This ability is restored by adding S100β, an astrocytic Ca2+-binding protein, to the extracellular space, while the application of an anti- S100β antibody prevents generation of rhythmic activity. These results indicate that astrocytes regulate a fundamental neuronal property: that is the capacity to change their firing pattern. Thus, CPG functions result from integrated neuronal and glial activities. These findings may have broad implications for many other neural networks whose functions depend on the generation of rhythmic activity.

Le mouvement masticatoire est généré et coordonné par un générateur de patron central (GPC) situé au niveau du pont. Plusieurs résultats antérieurs de notre laboratoire soutiennent que le réseau de neurones à l’origine de la rythmogénèse est situé dans le noyau sensoriel principal du nerf trijumeau (NVsnpr). Ces mêmes expériences révèlent que des diminutions de la concentration calcique extracellulaire ([Ca2+]e) tiennent une place importante dans la génération des bouffées de décharges des neurones de cette région. Notre laboratoire tente de vérifier si la contribution des astrocytes à l’homéostasie de la concentration calcique extracellulaire est impliquée dans la genèse du rythme. Cette étude a pour but la caractérisation spatiale du syncytium astrocytaire au sein du NVsnpr dorsal et l’étude de l’effet de la [Ca2+]e sur les propriétés astrocytaires électrophysiologiques et de connectivité. Nous avons utilisés pour ce faire la technique d’enregistrement par patch-clamp sur une préparation en tranche de tronc cérébral de rat. Nous démontrons ici que la diminution de la [Ca2+]e n’affecte pas les propriétés électrophysiologiques astrocytaires, mais induit une augmentation de la taille du syncytium. De plus, nous établissons l’existence au sein du NVsnpr dorsal d’une organisation anatomofonctionnelle du réseau astrocytaire calquée sur l’organisation neuronale.
The masticatory movement is generated and coordinated by a central pattern generator (CPG) located in the pons. Previous results from our laboratory suggest that the neural network responsible for its rythmogenesis is located in the trigeminal main sensory nucleus (NVsnpr). Moreover, results indicate that in this region, decrease in extracellular calcium concentration ([Ca2+]e) plays an important role in genarating burst. One of our laboratory's goal is to assess if the contribution of astrocytes to the extracellular calcium concentration homeostasis is involved in the genesis of the mastication rhythm. With this study, we characterized the astrocyte syncytium within the NVsnpr and measured the effect of [Ca2+]e on the astrocytes electrophysiology and their networks. A patch-clamp recording technique in conjunction with a rat brain stem slice preparation was used. We demonstrate that a decrease in [Ca2+]e does not affect the electrophysiological properties of astrocytes but induces an increase in the size of the syncytium. We also report the existence, within the dorsal NVsnpr, of an anatomofunctional organization between neurons and astrocytes.

We investigated the electrophysiological properties of neurons in the nucleus principalis trigemini (PrV), using whole-cell recordings in in vitro slice preparations of brainstem. We identified three groups mainly by their firing properties: type 1 neurons were spontaneously active and able to discharge action potentials in doublets or bursts; type 2 neurons, depolarized by current pulses, fired action potentials in a nonadapting (tonic) pattern; and the less commonly encountered type 3 neurons also fired in such patterns but with biphasic afterhyperpolarizations. Neurobiotin staining and reconstruction did not reveal significant morphological differences between types 1 and 2 neurons which were multipolar, with dendritic trees distributed mainly along one axis. Type 3 neurons had more expansive and circular dendritic arborizations. Hyperpolarization beyond -75 mV or down to the K⁺ reversal aotential due to current pulse injection, resulted in an inward rectification which was expressed as a sag in the voltage responses of types 1 and type 2 neurons. A rebound subthreshold depolarization or spike burst was evident on termination of a pulse. In type 1 neurons, the application of Cs⁺ (2 mM), a blocker of a hyperpolarization-activated cation current (l[sub H]), eliminated the voltage sag and the dependence of the rebound spike-latency on membrane voltage, but did not alter the main features of the rebound response. We attribute the inward rectification to the activation of an l[sub H]-like current. Depolarization by current pulse injection into type 1 neurons, hyperpolarized with DC to prevent firing, occasionally evoked "plateau potentials". This feature, not observed in types 2 or 3 neurons, consisted of an initial oscillatory burst of 3 or 4 spikes that decreased in amplitude, and then plateaued for a variable duration, followed by an abrupt repolarization. We always observed these voltage shapes on depolarizing current pulse injection during perfusion with Ca²⁺ free media, with or without the Ca²⁺-channel antagonists, Co²⁺ or Cd²⁺, and during external tetraethylammonium (TEA) application. An analysis of the depolarizing voltage responses evoked by current pulses in type 1 neurons during blockade of persistent and transient Na+ conductances with TTX (600 nM) and K⁺ conductances with TEA (10 mM) and 4-aminopyridine (4-AP; 0.5 mM), revealed the presence of inward rectification. This had a peak activation near the plateau itself and was completely blocked by Ni²⁺ (600 μM). These observations are consistent with the activation of a transient 2+-conductance. Hence, we propose that a Ca²⁺-dependent K⁺ conductance mechanism controls the generation of the plateau potential. The application of TTX, as low as 0.6 nM, increased the latency to onset and decreased the duration of the plateau potential, without greatly affecting action potentials. In a concentration-dependent manner, TTX enhanced the negative slope of the plateau, as it descended towards an abrupt terminal repolarization. Higher concentrations of TTX (e.g., 60 nM for 6 min) abolished the plateau potential before completely blocking action potential genesis. Low [Na⁺]-perfusion, however, simultaneously reduced the amplitudes of plateau potentials and fast spikes. Evidently, small changes in a persistent Na⁺ conductance can produce marked changes in firing behavior of type 1 neurons. A long-lasting hyperpolarization followed current pulses producing the plateau potential. Indeed, subthreshold or suprathreshold depolarization in Ca²⁺ free ACSF with Co²⁺ (1 mM) also evoked a hyperpolarization at the offset of the current pulse. This hyperpolarization was blocked by TTX (5 nM and 300 nM) and varied with changes in the duration of the plateau potential. A semiquantitative analysis revealed that the magnitude of the hyperpolarization depended on the neuronal depolarization. We conclude, therefore, that Na⁺ entry during a depolarization can increase a K⁺ conductance in type 1 neurons. From our studies, we conclude that plateau potentials represent the contributions of persistent and transient Na⁺ conductances, high threshold Ca²⁺-dependent rectification, as well as Ca²⁺- and Na⁺-dependent K⁺ conductances. The ability to fire bursts as part of Na+-dependent plateaus is an unusual property in neurons of primary sensory nuclei. In nucleus principalis trigemini, burst responses to mechanical stimuli represent a normal output of neurons that likely are subject to intra- and extracellular messenger regulation.