Early and transient increase in spontaneous synaptic inputs to the rat facial motoneurons after axotomy in isolated brainstem slices of rats

Normal Development and Pathology of Motoneurons: Anatomy, Electrophysiological Properties, Firing Patterns and Circuit Connectivity

This chapter will provide an introduction into motoneuron anatomy, electrophysiological properties, firing patterns focusing on development and also describing several pathological conditions that affect mononeurons. It starts with a historical retrospective describing the early landmark work into motoneurons. The next section lays out the various types of motoneurons (alpha, beta, and gamma) and their subclasses (fast-twitch fatigable, fast-twitch fatigue-resistant, and slow-twitch fatigue resistant), highlighting the functional relevance of this classification scheme. The third section describes the development of motoneurons' passive and active electrophysiological properties. This section also defines the major terms one uses in describing how a neuron functions electrophysiologically. The electrophysiological aspects of a neuron is critical to understanding how it behaves within a circuit and contributes to behavior since the firing of an action potential is how neurons communicate with each other and with muscles. The electrophysiological changes of motoneurons over development underlies how their function changes over the lifetime of an organism. After describing the properties of individual motoneurons, the chapter then turns to revealing how motoneurons interact within complex neural circuits, with other motoneurons as well as sensory neurons, and how these circuits change over development. Finally, this chapter ends with highlighting some recent advances made in motoneuron pathology, focusing on spinal muscular atrophy, amyotrophic lateral sclerosis, and axotomy.

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

Differential involvement of vesicular and glial glutamate transporters around spinal α-motoneurons in the pathogenesis of SOD1G93A mouse model of amyotrophic lateral sclerosis

From a view point of the glutamate excitotoxicity theory, several studies have suggested that abnormal glutamate homeostasis via dysfunction of glial glutamate transporter-1 (GLT-1) may underlie neurodegeneration in amyotrophic lateral sclerosis (ALS). However, the detailed role of GLT-1 in the pathogenies of ALS remains controversial. To assess this issue, here we elucidated structural alterations associated with dysregulation of glutamate homeostasis using SOD1^G93A mice, a genetic model of familial ALS. We first examined the viability of α-motoneurons in the lumbar spinal cord of SOD1^G93A mice. Measurement of the soma size and density indicated that α-motoneurons might be intact at 9weeks of age (presymptomatic stage), then soma shrinkage began at 15weeks of age (progressive stage), and finally neuronal density declined at 21weeks of age (end stage). Next, we carried out the line profile analysis, and found that the coverage of α-motoneurons by GLT-1-positive (GLT-1⁺) astrocytic processes was decreased only at 21weeks of age, while the reduction of coverage of α-motoneurons by synaptophysin-positive (SYP⁺) presynaptic terminals began at 15weeks of age. Interestingly, the coverage of α-motoneurons by VGluT2⁺ presynaptic terminals was transiently increased at 9weeks of age, and then gradually decreased towards 21weeks of age. On the other hand, there were no time-dependent alterations in the coverage of α-motoneurons by GABAergic presynaptic terminals. These findings suggest that VGluT2 and GLT-1 may be differentially involved in the pathogenesis of ALS via abnormal glutamate homeostasis at the presymptomatic stage and end stage of disease, respectively.

Facilitation of distinct inhibitory synaptic inputs by chemical anoxia in neurons in the oculomotor, facial and hypoglossal motor nuclei of the rat

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the selective loss of motor neurons in the brainstem and spinal cord. Clinical studies have indicated that there is a distinct region-dependent difference in the vulnerability of motor neurons. For example, the motor neurons in the facial and hypoglossal nuclei are more susceptible to neuronal death than those in the oculomotor nucleus. To understand the mechanism underlying the differential susceptibility to cell death of the neurons in different motor nuclei, we compared the effects of chemical anoxia on the membrane currents and postsynaptic currents in different motor nuclei. The membrane currents were recorded from neurons in the oculomotor, facial and hypoglossal nuclei in brain slices of juvenile Wistar rats by using whole-cell recording in the presence of tetrodotoxin that prevents action potential-dependent synaptic transmission. NaCN consistently induced an inward current and a significant increase in the frequency of spontaneous synaptic inputs in neurons from these three nuclei. However, this increase in the synaptic input frequency was abolished by strychnine, a glycine receptor antagonist, but not by picrotoxin in neurons from the hypoglossal and facial nuclei, whereas that in neurons from the oculomotor nucleus was abolished by picrotoxin, but not by strychnine. Blocking ionotropic glutamate receptors did not significantly affect the NaCN-induced release facilitation in any of the three motor nuclei. These results suggest that anoxia selectively facilitates glycine release in the hypoglossal and facial nuclei and GABA release in the oculomotor nucleus. The region-dependent differences in the neurotransmitters involved in the anoxia-triggered release facilitation might provide a basis for the selective vulnerability of motor neurons in the neurodegeneration associated with ALS.

Orexin-a modulates firing of rat rostral ventromedial medulla neurons: an in vitro study

The rostral ventromedial medulla (RVM) acts a key role in the descending inhibitory pain modulation. Neuropeptide orexin-A (ORXA) is confined to thousands of neurons in the lateral hypothalamus (LH). While RVM gets the orexinergic projections, the orexin receptors are also expressed in this structure. The aim of this study was to specify the cellular effects of ORXA on RVM neurons in vitro by using the whole cell patch-clamp recording. RVM neurons were classified into three types based on their electrophysiological characteristics. Type 1 neurons exhibited an irregular spontaneous activity which was interrupted by periods of pause in 25% of recorded neurons. Type 2 neurons did not show any spontaneous baseline activity (53.8% of recorded neurons). Type 3 neurons fired repetitively without interruption (51.2% of recorded neurons). ORXA had either inhibitory or excitatory effects on 53.8% (7/13) of type 1 neurons. ORXA excited 46.4% (13/28) of type 2 neurons and 27.3% (3/11) of type 3 neurons. The excitatory effect of ORXA observed in type 2 neurons was suppressed by an orexin 1 receptor (OXR1) antagonist, SB-334867. Briefly, we hypothesized that the ORXA mediated excitation and/or inhibition in RVM neurons might work as a mechanism to modulate pain processing by orexinergic neurons.

Retrograde response in axotomized motoneurons: nitric oxide as a key player in triggering reversion toward a dedifferentiated phenotype

The adult brain retains a considerable capacity to functionally reorganize its circuits, which mainly relies on the prevalence of three basic processes that confer plastic potential: synaptic plasticity, plastic changes in intrinsic excitability and, in certain central nervous system (CNS) regions, also neurogenesis. Experimental models of peripheral nerve injury have provided a useful paradigm for studying injury-induced mechanisms of central plasticity. In particular, axotomy of somatic motoneurons triggers a robust retrograde reaction in the CNS, characterized by the expression of plastic changes affecting motoneurons, their synaptic inputs and surrounding glia. Axotomized motoneurons undergo a reprograming of their gene expression and biosynthetic machineries which produce cell components required for axonal regrowth and lead them to resume a functionally dedifferentiated phenotype characterized by the removal of afferent synaptic contacts, atrophy of dendritic arbors and an enhanced somato-dendritic excitability. Although experimental research has provided valuable clues to unravel many basic aspects of this central response, we are still lacking detailed information on the cellular/molecular mechanisms underlying its expression. It becomes clear, however, that the state-switch must be orchestrated by motoneuron-derived signals produced under the direction of the re-activated growth program. Our group has identified the highly reactive gas nitric oxide (NO) as one of these signals, by providing robust evidence for its key role to induce synapse elimination and increases in intrinsic excitability following motor axon damage. We have elucidated operational principles of the NO-triggered downstream transduction pathways mediating each of these changes. Our findings further demonstrate that de novo NO synthesis is not only "necessary" but also "sufficient" to promote the expression of at least some of the features that reflect reversion toward a dedifferentiated state in axotomized adult motoneurons.

Facial nerve schwannoma

PURPOSE OF REVIEW

The purpose of this review is to summarize the current literature on facial nerve schwannoma and make practical recommendations based on best practices for the management of this difficult but benign neoplasm.

RECENT FINDINGS

Facial nerve schwannoma can be asymptomatic or can present with progressive or acute facial nerve palsy. Associated otological symptoms such as conductive and/or sensorineural hearing loss can occur. The tumor is usually slow-growing and can involve multiple segments of the nerve. Radiographic imaging and facial nerve electrical testing can be helpful in treatment planning. Options for management can include observation, decompression, stripping, resection with grafting, and possibly radiotherapy. Future adjunctive therapies to improve facial nerve function may include electrical stimulation, steroid hormones, and possibly stem cell therapy.

SUMMARY

Treatment of facial schwannoma is individualized based on patient symptoms, history, and clinicoradiographic evaluation. Not all patients require surgery. As the tumor can involve multiple segments of the nerve, the surgeon attempting removal should be familiar with modern neurotological surgical techniques. Ongoing translational research will hopefully allow us to decrease facial nerve morbidity in these patients.

Reduced synaptic activity precedes synaptic stripping in vagal motoneurons after axotomy

Activated microglia, which spread on the motor neurons following nerve injury, engage in the displacement of detached afferent synaptic boutons from the surface of regenerating motor neurons. This phenomenon is known as "synaptic stripping." The present study attempted to examine whether changes in the synaptic inputs after motor nerve injury correlated with the microglial attachment to the dorsal motor neurons of the vagus (DMV). DMV neurons in Wistar rats could survive after nerve injury, whereas most of injured DMV neurons in the C57BL/6 mice died. At 2 days after nerve injury, a significant decrease was observed in the frequencies of both spontaneous and miniature EPSCs and IPSCs recorded from DMV neurons in the slice preparation but not from the mechanically dissociated neurons in the Wistar rats. At this stage, no direct apposition of microglia on the injured neurons was observed. High-K(+) stimulation restored their frequencies to control levels. Furthermore, PPADS and DPCPX, antagonists of P2 and adenosine receptors, respectively, also stimulated the recovery of their frequencies. In contrast, no significant change was detected in the spontaneous EPSCs frequency recorded from the severely injured DMV neurons in the slice preparation of the C57BL/6 mice. These observations strongly suggest that presynaptic inhibition through glia-derived ATP and adenosine, thus precedes synaptic stripping in regenerating DMV neurons following nerve injury.

Plastic changes of synapses and excitatory neurotransmitter receptors in facial nucleus following facial-facial anastomosis

The remodeling process of synapses and neurotransmitter receptors of facial nucleus were observed. Models were set up by facial-facial anastomosis in rat. At post-surgery day (PSD) 0, 7, 21 and 60, synaptophysin (p38), NMDA receptor subunit 2A and AMPA receptor subunit 2 (GluR2) were observed by immunohistochemical method and semi-quantitative RT-PCR, respectively. Meanwhile, the synaptic structure of the facial motorneurons was observed under a transmission electron microscope (TEM). The intensity of p38 immunoreactivity was decreased, reaching the lowest value at PSD day 7, and then increased slightly at PSD 21. Ultrastructurally, the number of synapses in nucleus of the operational side decreased, which was consistent with the change in P38 immunoreactivity. NMDAR2A mRNA was down-regulated significantly in facial nucleus after the operation (P<0.05), whereas AMPAR2 mRNA levels remained unchanged (P>0.05). The synapses innervation and the expression of NMDAR2A and AMPAR2 mRNA in facial nucleus might be modified to suit for the new motor tasks following facial-facial anastomosis, and influenced facial nerve regeneration and recovery.

NMDA receptor-independent synaptic plasticity in the central amygdala in the rat model of neuropathic pain

Neurons in the latero-capsular part of the central nucleus of the amygdala (CeA), a region now called the "nociceptive amygdala", receive predominantly nociceptive information from the dorsal horn through afferent pathways relayed at the nucleus parabrachialis (PB). Excitatory synaptic transmission between the PB afferents and these neurons is reported to become potentiated within a few hours of the establishment of arthritic or visceral pain, making it a possible mechanism linking chronic pain and unpleasant negative emotional experiences. However, it remains unknown whether such synaptic potentiation is consolidated or becomes adaptively extinct in the longer-lasting form of chronic pain, such as neuropathic pain, an as yet serious and frequent type of pain of important clinical concern. To address this issue, we recorded postsynaptic currents in CeA neurons evoked by PB tract stimulation in acute brain slices from young rats with neuropathic pain, as evaluated by tactile allodynic responses, following unilateral spinal nerve ligature made 1 week earlier. CeA neurons contralateral to the nerve ligation showed significantly larger-amplitude postsynaptic currents than those in the ipsilateral CeA and sham- and non-operated groups. The degree of synaptic potentiation, as compared between two sides, was positively correlated to that of tactile allodynia responses. In addition, blockade of NMDA receptors did not affect this potentiation. We conclude that potentiation of the PB-CeA synapse is consolidated in long-lasting neuropathic pain but that this potentiation results from a molecular mechanism distinct from that in arthritic and visceral pain.

Short-term reorganization of input-deprived motor vibrissae representation following motor disconnection in adult rats

It has been proposed that abnormal vibrissae input to the motor cortex (M1) mediates short-term cortical reorganization after facial nerve lesion. To test this hypothesis, we cut first the infraorbital nerve (ION cut) and then the facial nerve (VII cut) in order to evaluate M1 reorganization without any aberrant, facial-nerve-lesion-induced sensory feedback. In each animal, M1 output was assessed in both hemispheres by mapping movements induced by intracortical microstimulation. M1 output was compared in different types of peripheral manipulations: (i) contralateral intact vibrissal pad (intact hemispheres), (ii) contralateral VII cut (VII hemispheres), (iii) contralateral ION cut (ION hemispheres), (iv) contralateral VII cut after contralateral ION cut (ION + VII hemispheres), (v) contralateral pad botulinum-toxin-injected after ION cut (ION + BTX hemispheres). Right and left hemispheres in untouched animals were the reference for normal M1 map (control hemispheres). Findings demonstrated that: (1) in ION hemispheres, the mean size of the vibrissae representation was not significantly different from those in intact and control hemispheres; (2) reorganization of the vibrissae movement representation clearly emerged only in hemispheres where the contralateral vibrissae pad had undergone motor output disconnection (VII cut hemispheres); (3) the persistent loss of vibrissae input did not change the M1 reorganization pattern during the first 48 h after motor paralysis (ION + VII cut and ION + BTX hemispheres). Thus, after motor paralysis, vibrissa input does not provide the gating signal necessary to trigger M1 reorganization.