Lumbar Motoneurons Research Articles

Altered synaptic and electrical properties of lumbar motoneurons in the neurological glial mutant taiep rat.

Maturation and differentiation of electrical properties of neurons and synaptic transmission are modulated by neuronal interaction. In vitro experiments have shown that these processes also seem to be regulated by signals from non-neuronal elements such as glial cells. It is not known, however, whether glial alterations in intact neural networks may also affect the maturation of electrical properties and synaptic transmission during development. We used the taiep rat, a neurological mutant with a progressive demyelination and astrogliosis, as an experimental model to study the postnatal development of motoneurons in an altered glial environment. Using the patch-clamp technique, we made intracellular recording from motoneurons of Rexed's lamina IX in spinal cord slices of neonatal rats (postnatal day P4-P10). The electrical properties of normal motoneurons changed significantly with age, showing decreasing input resistance (R(in)) and increasing membrane capacity (C(m)). The rheobase increased with age, accompanied by an increase of the amplitude and a decrease of the duration of action potentials (APs). In contrast, mutant neurons showed no age-dependent changes of R(in), C(m), or AP characteristics. After blocking inhibitory transmission, intralaminar bipolar stimulation elicited, in both control and taiep motoneurons, fast glutamatergic excitatory postsynaptic potentials (EPSPs). Two types of taiep motoneurons were identified according to the temporal patterns of synaptic responses; (1). taiep(SYN) neurons, which showed no significant differences to control motoneurons, and (2). taiep(ASYN) neurons, in which the initial EPSP was followed by a variable number of delayed, asynchronous EPSP responses (for up to 300 ms). All these electrophysiological findings suggest that the mutation in taiep rats interfered with the development of the electrical properties of neurons and with the maturation of synaptic transmission, probably due to alterations in the neuron-glia interactions.

Passive exercise and fetal spinal cord transplant both help to restore motoneuronal properties after spinal cord transection in rats

Spinal cord transection influences the properties of motoneurons and muscles below the lesion, but the effects of interventions that conserve muscle mass of the paralyzed limbs on these motoneuronal changes are unknown. We examined the electrophysiological properties of rat lumbar motoneurons following spinal cord transection, and the effects of two interventions shown previously to significantly attenuate the associated hindlimb muscle atrophy. Adult rats receiving a complete thoracic spinal cord transection (T-10) were divided into three groups receiving: (1) no further treatment; (2) passive cycling exercise for 5 days/week; or (3) acute transplantation of fetal spinal cord tissue. Intracellular recording of motoneurons was carried out 4-5 weeks following transection. Transection led to a significant change in the rhythmic firing patterns of motoneurons in response to injected currents, as well as a decrease in the resting membrane potential and spike trigger level. Transplants of fetal tissue and cycling exercise each attenuated these changes, the latter having a stronger effect on maintenance of motoneuron properties, coinciding with the reported maintenance of structural and biochemical features of hindlimb muscles. The mechanisms by which these distinct treatments affect motoneuron properties remain to be uncovered, but these changes in motoneuron excitability are consistent with influences on ion conductances at or near the initial segment. The results may support a therapeutic role for passive limb manipulation and transplant of stem cells in slowing the deleterious responses of motoneurons to spinal cord injury, such that they remain more viable for subsequent alternative strategies.

Cardiotrophin-Like Cytokine/Cytokine-Like Factor 1 is an Essential Trophic Factor for Lumbar and Facial MotoneuronsIn Vivo

The ciliary neurotrophic factor alpha-receptor (CNTFRalpha) is required for motoneuron survival during development, but the relevant ligand(s) has not been determined. One candidate is the heterodimer formed by cardiotrophin-like cytokine (CLC) and cytokine-like factor 1 (CLF). CLC/CLF binds to CNTFRalpha and enhances the survival of developing motoneurons in vitro; whether this novel trophic factor plays a role in neural development in vivo has not been tested. We examined motor and sensory neurons in embryonic chicks treated with CLC and in mice with a targeted deletion of the clf gene. Treatment with CLC increased the number of lumbar spinal cord motoneurons that survived the cell death period in chicks. However, this effect was regionally specific, because brachial and thoracic motoneurons were unaffected. Similarly, newborn clf-/- mice exhibited a significant reduction in lumbar motoneurons, with no change in the brachial or thoracic cord. Clf deletion also affected brainstem motor nuclei in a regionally specific manner; the number of motoneurons in the facial but not hypoglossal nucleus was significantly reduced. Sensory neurons of the dorsal root ganglia were not affected by either CLC treatment or clf gene deletion. Finally, mRNA for both clc and clf was found in skeletal muscle fibers of embryonic mice during the motoneuron cell death period. These findings support the view that CLC/CLF is a target-derived factor required for the survival of specific pools of motoneurons. The in vivo actions of CLC and CLF can account for many of the effects of CNTFRalpha on developing motoneurons.

Gravity influences the development of inputs from the brain to lumbar motoneurons in the rat.

We investigated the influence of gravity on the maturation of electrical properties of lumbar motoneurons and the development of their inputs from ventral descending pathways, which are important for the control of posture and locomotion. Using electrophysiological approaches in the in vitro brain stem-spinal cord preparation of neonatal rats born and reared in hypergravity field we demonstrate that: (1) the postnatal development of descending inputs to lumbar enlargement was reduced in animals submitted to hypergravity; (2) similar developmental pattern of basic electrical properties observed between motoneurons of hypergravity and control animals could not account for the changes in descending inputs. We concluded that gravity was critical to shape development of the supraspinal afferents in the lumbar spinal cord throughout the postnatal period.

Spinal reflex excitability changes after cervical and lumbar spinal manipulation: a comparative study

Background context: Spinal manipulation (SM) is a commonly employed nonoperative treatment modality in the management of patients with neck, low back or pelvic pain. One basic physiologic response to SM is a transient decrease in motoneuron activity as assessed using the Hoffmann reflex (H-reflex) technique. Previous research from our laboratory indicates that both SM with a high-velocity, low-amplitude thrust and mobilization without thrust produced a profound but transient attenuation of motoneuronal activity of the lumbosacral spine in asymptomatic subjects. To date, effects of cervical SM procedures on the excitability cervical motoneuron pools are unknown. Purpose: The objective of this research was to a gain a more complete understanding of the physiologic effects of SM procedures on motoneuron activity, by comparing the effects of regional SM on cervical and lumbar motoneuron pool excitability. Study design/setting: Maximal H-reflex amplitudes were recorded before and after SM in both the cervical and lumbar regions of asymptomatic subjects in two successive experimental sessions. Patient sample: Asymptomatic, young healthy volunteers were used in this study. Outcome measures: Changes in flexor carpi radialis and gastrocnemius H-reflex amplitudes before and after SM procedures. Methods: H-reflexes recorded form the tibial and median nerves were evaluated before and after lumbar and cervical SM, respectively. Results: Both Lumbar and cervical SM produced a transient but significant attenuation of motoneuron excitability. The attenuation of the tibial nerve H-reflex amplitude was proportionately greater than that of the median nerve, which occurred after cervical SM. Conclusions: SM procedures lead to transient suppression of motoneuron excitability, as assessed by the H-reflex technique. Lumbar spine SM appears to lead to greater attenuation of motoneuron activity compared with that of the cervical region. Thus, these two distinct regions of the spine may possess different responsiveness levels to spinal manipulative therapy.

Mechanism for Activation of Locomotor Centers in the Spinal Cord by Stimulation of the Mesencephalic Locomotor Region

The synaptic pathways of mesencephalic locomotor region (MLR)-evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) recorded from lumbar motoneurons of unanesthetized decerebrate cats during fictive locomotion were analyzed prior to, during, and after cold block of the medial reticular formation (MedRF) or the low thoracic ventral funiculus (VF). As others have shown, electrical stimulation of the MLR typically evoked short-latency excitatory or mixed excitatory/inhibitory PSPs in flexor and extensor motoneurons. The bulbospinal conduction velocities averaged approximately 88 m/s (range: 62-145 m/s) and segmental latencies for EPSPs ranged from 1.2 to 10.9 ms. The histogram of segmental latencies showed three peaks, suggesting di-, tri-, and polysynaptic linkages. Segmental latencies for IPSPs suggested trisynaptic or polysynaptic transmission. Most EPSPs (69/77) were significantly larger during the depolarized phase of the intracellular locomotor drive potential (LDP), and most IPSPs (35/46) were larger during the corresponding hyperpolarized phase. Bilateral cooling of the MedRF reversibly abolished locomotion of both hindlimbs as measured from the electroneurogram (ENG) activity of muscle nerves and simultaneously abolished or diminished the motoneuron PSPs and LDPs. Unilateral cooling of the VF blocked locomotion ipsilaterally and diminished it contralaterally with concomitant loss or decrease the motoneuron PSPs and LDPs. Relative to the side of motoneuron recording, cooling of the ipsilateral VF sometimes uncovered longer-latency EPSPs, whereas cooling of the contralateral VF abolished longer-latency EPSPs. It is concluded that MLR stimulation activates a pathway that relays in the MedRF and descends bilaterally in the VF to contact spinal interneurons that project to motoneurons. Local segmental pathways that activate or inhibit motoneurons during MLR-evoked fictive locomotion appear to be both ipsilateral and contralateral.

Effect of 17β-estradiol on gene expression in lumbar spinal cord following sciatic nerve crush injury in ovariectomized mice

Previously, we observed that estrogen treatment enhances regeneration of the sciatic nerve after crush injury [Brain Res. 943 (2002) 283]. In this research, we studied expression of estrogen receptors and effects of estrogen on gene expression in the lumbar spinal cord, following sciatic nerve crush injury. Using the Atlas Mouse 1.2 Array, changes in the expression of 267 of 1176 genes were registered 4 days after nerve injury. Those genes that exhibited a change in signal intensity ratios of 2-fold or greater were selected as up-regulated (42) or down-regulated (21). In estrogen treated mice, we have observed up-regulation of the genes known to control apoptosis, cell proliferation, and growth, which might account for the positive effects of estrogen on the regeneration of motor neurons. Immunohistochemical staining revealed estrogen receptor-α and estrogen receptor-β localized in the nucleus and cytoplasm of lumbar motor neurons, and in the regenerating neurites of the sciatic nerve. Expression of estrogen receptor-α and estrogen receptor-β mRNA in lumbar spinal cord was shown by traditional RT-PCR. Using real-time quantitative RT-PCR, we demonstrated increased expression of estrogen receptors-α and -β mRNA on the injured side of the lumbar spinal cord. Western blot analysis showed the accumulation of ERs in regenerating sciatic nerve, and revealed a 40% increase of activated ERK1/2 in estrogen treated mice, compared to placebo. Our findings indicate that: (i) axotomized motor neurons increase expression of estrogen receptors-α and -β mRNA, (ii) estrogen mediates the expression of genes which accelerate the growth and maturation of axons, and (iii) estrogen receptors are transported from the perikaryon into regenerating neurites, and estrogen promotes regeneration locally through the non-genomic ERK-activated signaling pathway.

Expression of K Ca channels in identified populations of developing vertebrate neurons: role of neurotrophic factors and activity

Changes in the intrinsic spike discharge properties in one neuronal population can alter the functions and even the formation of an entire neuronal network. Therefore it is important to understand the factors that regulate acquisition of a mature electrophysiological phenotype. Here we focus on large-conductance K Ca channels, which shape the pattern of repetitive discharge and which are therefore likely to play a role in the refinement of neural networks during development. In the parasympathetic ciliary ganglion of chick, the developmental expression of K Ca channels coincides with stages at which ciliary cells form synapses with target tissues. Moreover, K Ca expression requires formation of synapses with target tissues, and with afferent preganglionic inputs. The trophic effect of targets is mediated by TGFβ1, whereas the effect of the preganglionic input is mediated by an isoform of β-neuregulin-1. These trophic factors act synergistically, and this appears to be a normal feature of their actions in vivo. The acute effects of TGFβ1 entail translocation of preexisting K Ca channels from intracellular stores to the plasma membrane. This requires activation of the signaling enzymes Ras, Erk MAP kinase and PI3 kinase. TGFβ1 also causes a more sustained increase in K Ca channels (i.e. for up to 2 weeks) that requires synthesis of new channel proteins. Inductive regulation of K Ca expression is also observed in CNS cells that form more complex networks. In lumbar motoneurons, the largest changes in K Ca expression coincide with the elimination of synapses with hindlimb targets. Interactions with target tissues play a key role in regulation of motoneuron K Ca expression, and this trophic effect of target muscle is mediated by GDNF or a closely related factor. In addition, K Ca expression in motoneurons is dependent on ongoing electrical activity both in vivo and in vitro. This provides an additional mechanism for use-dependent refinement of neural networks during embryonic development.

Neuronal organization of the stellate ganglion in mammals during postnatal ontogenesis

The effects of antagonists of excitatory amino acids (AP-5, kynurenate, and CNQX) on PSP recorded intracellularly in lumbar motoneurons of a preparation of the isolated spinal cord in the frog, Rana ridibunda, in response to activation of three different synaptic inputs (stimulation of DR, RF, and VC or LC) were analyzed. It is shown that the effects of the antagonists were non-uniform in different motoneurons. Inputs of suprasegmental and sensomotor projections substantially differed from each other. A considerable amount of DC-PSP resistant to kynurenate and CNQX was found, whereas the latter regularly inhibited DR-PSP in the same cell. The disynaptic, as judged by its latency, plane-shaped component was always relatively more stable to kynurenate as compared with other components. Unlike kynurenate that inhibited the early and late components, CNQF selectively depressed the early components of DR-PSP.

Analysis of Combined Action of Antagonists of Excitatory and Inhibitory Amino Acids on Motoneuron Postsynaptic Potentials of the Frog Rana ridibunda

The effect of blockers of excitatory and inhibitory amino acid receptors on postsynaptic potentials (PSP) evoked by activation of three synaptic inputs of the lumbar motoneuron (stimulation of the dorsal root, reticular formation, ventral and lateral columns) was studied on preparation of the isolated spinal cord of the frog Rana ridibunda. It has been shown that sensitivity of PSP to antagonists differs in different motoneurons, in the same motoneuron at activation of different inputs, and in the same input in different PSP components. It has been found that many descendent (DC) PSPs resistant to kynurenate or CNQX [1] were inhibited by blockers of inhibitory receptors. In this case the early component of DC-PSP varied considerably by amplitude and changed its polarity from positive to negative on the background of a low transmembrane depolarizing current. These changes were absent under conditions of replacement of chlorine ion by sulfate in the perfusion solution or treatment of the spinal cord with a blocker of inhibitory amino acids. All this allows suggesting that these DC-PSPs or their components were inhibitory. A part of PSPs resistant to kynurenate and CNQX were also resistant to the blockers of inhibitory amino acids (strychnine, picrotoxin, and bicuculline). In some cases, as a result of treatment with convulsants, the same blockers of excitatory receptors inhibited the initially resistant PSPs.

Glial Cell Line-Derived Neurotrophic Factor and Target-Dependent Regulation of Large-Conductance KCaChannels in Developing Chick Lumbar Motoneurons

The functional expression of large-conductance Ca2+-activated K+ (K(Ca)) channels in lumbar motoneurons (LMNs) of the developing chick embryo is regulated in part by interactions with striated muscle target tissues. Here we show that the functional expression of K(Ca) channels in LMNs developing in vitro can be stimulated by application of a skeletal muscle extract (MEX) or by coculture with hindlimb myotubes. A similar stimulation of K(Ca) channels in vitro can be produced by the trophic factors glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor but not by neurotrophin (NT)-3 or NT-4. The actions of MEX and hindlimb myotubes are blocked by a GDNF-neutralizing antiserum. Moreover, injection of this same antiserum into the embryonic hindlimb reduced the functional expression of K(Ca) channels in vivo to levels seen in LMNs deprived of interactions with the hindlimb. The effects of GDNF on K(Ca) channel expression in LMNs require 24 hr of continuous exposure to reach maximum and are blocked by the translation inhibitor anisomycin, indicating the need for synthesis of new proteins. GDNF actions are also blocked by the farnesyl transferase inhibitor manumycin, suggesting a role for Ras in the actions of GDNF. Finally, the actions of GDNF are inhibited by PP2, an inhibitor of Src family tyrosine kinases, and by LY29003, an inhibitor of phosphatidylinositol 3 kinases, but not by PD98059, an inhibitor of the Erk signaling cascade. None of these treatments alter expression of voltage-activated Ca2+ channels. Thus, the actions of GDNF on LMN K(Ca) channel expression appear to use a transduction pathway similar to that used for regulation of apoptosis.

Developmental changes in short-term synaptic depression in the neonatal mouse spinal cord.

We examined age-dependent changes in short-term synaptic depression of monosynaptic excitatory postsynaptic potentials (EPSPs) recorded in lumbar motoneurons in hemisected spinal cords of neonatal Swiss-Webster mice between postnatal day 2 (P2) and 12 (P12). We used four paradigms that sample the input-output dependence on stimulation history in different but complementary ways: 1) paired-pulse depression; 2) steady-state depression during constant frequency trains; 3) modulation during irregular stimulation sequences; and 4) recovery after high-frequency conditioning trains. Paired-pulse synaptic depression declined more than steady-state depression during 10-pulse trains at frequencies from 0.125 to 8 Hz in this age range. Depression during sequences of irregular stimulations that more closely mimic physiological activation also declined with postnatal age. On the other hand, the overall rate of synaptic recovery after a 4-Hz conditioning train exhibited surprisingly little change between P2 and P12. Control experiments indicated that these observations depend primarily, if not exclusively, on changes in presynaptic transmitter release. The data were examined using quantitative models that incorporate factors that have been suggested to exist at more specialized central synapses. The model that best predicted the observations included two presynaptic compartments that are depleted during activation, plus two superimposed processes that enhance transmitter release by different mechanisms. One of the latter produced rapidly-decaying enhancement of transmitter release fraction. The other mechanism indirectly enhanced the rate of renewal of one of the depleted presynaptic compartments. This model successfully predicted the constant frequency and irregular sequence data from all age groups, as well as the recovery curves following short, high-frequency tetani. The results suggest that a reduction in release fraction accounts for much of the decline in synaptic depression during early postnatal development, although changes in both enhancement processes also contribute. The time constants of resource renewal showed surprisingly little change through the first 12 days of postnatal life.

Lumbar motoneuron fate in a mouse model of amyotrophic lateral sclerosis.

Onuf's nucleus, a collection of motoneurons within the spinal cord, is often spared in the neurodegenerative disorder amyotrophic lateral sclerosis. To assess whether these cells survive in a rodent model of this disease, motoneurons were counted in the spinal nucleus of the bulbocavernosus (an homologous structure to Onuf's), as well as in two other cell groups at the same level of the spinal cord, the dorsolateral nucleus and the retrodorsolateral nucleus. In mice displaying signs of neurodegeneration, both the dorsolateral and retrodorsolateral nuclei displayed significant motoneuron loss compared to controls; this cell loss was particularly exaggerated in the retrodorsolateral nucleus of animals displaying a rapid disease progression. However, no significant decline in motoneuron number was observed in the spinal nucleus of the bulbocavernosus, and the perineal muscle bulbocavernosus, which is innervated by this nucleus, appeared to be unaffected. This was in stark contrast to the thigh muscles, which displayed significant atrophy. Overall, these data indicate that the spinal nucleus of the bulbocavernosus is spared from degeneration in an animal model of amyotrophic lateral sclerosis, paralleling observations in patients suffering from this disease. Further study of this resistance to motoneuron loss may provide useful insights into the pathophysiology of the degenerative process.

Frequency of occurrence of the F wave in distal flexor muscles as a function of hypnotic susceptibility and hypnosis

The aim of the experiment was to assess whether the membrane excitability of flexor cervical and/or lumbar motoneurons is related to hypnotic susceptibility (measured with the Stanford Hypnotic Susceptibility Scale, Form A) and hypnosis. During the experimental sessions, hypnotized subjects were given only suggestions of relaxation (neutral hypnosis) while awake subjects were given instructions to be quiet and relaxed (simple relaxation). F waves were recorded from the abductor digiti minimi and abductor hallucis of both sides after electrical stimulation of the ulnar or tibial nerve, respectively, and were used as an index of motoneuron membrane excitability in three groups of subjects: highly susceptible under neutral hypnosis, highly susceptible and non-susceptible during simple relaxation sessions. In lower limbs, there was no difference among the groups in the frequency of occurrence of F waves. In upper limbs, hypnosis selectively reduced F frequency of occurrence in flexor motoneurons on the right side. This reduction persisted after the end of hypnosis and also occurred during the last period of relaxation in highly susceptible non-hypnotized subjects. Results support the idea that hypnotic phenomena develop along a continuum in which some trait differences can be more easily revealed by the induction of hypnosis.

Effects of pharmacologic androgen treatment duration on glucocorticoid receptor α immunoreactivity of lumbosacral motor neurons in the male rat

It has previously been suggested that anabolic-androgenic steroids may affect neuromuscular function through their potential action as glucocorticoid receptor antagonists. Alternatively, androgens may regulate the sensitivity of neuromuscular systems to glucocorticoids by modulating GR levels. The purpose of this study was to determine the effects of chronic pharmacologic testosterone treatment of gonadally intact male rats on glucocorticoid receptor α immunoreactivity (GRα-IR) of motor neurons in the lumbosacral spinal cord. Circulating testosterone levels were chronically increased by subcutaneous Silastic™ capsules containing crystalline testosterone propionate (TP) for 7, 14, and 28 days. Age-matched sham-operated gonadally intact males served as controls. Relative cytoplasmic and nuclear GRα-IR of motor neurons located in the lateral motor column of spinal cord segments L 3 and L 4 (L Lat; innervating rat hindlimb muscles) and the spinal nucleus of the bulbocavernosus (SNB; innervating the external anal sphincter, bulbocavernosus and levator ani muscles) was measured densitometrically. TP treatment duration had a significant impact on the mean GRα levels of both cellular compartments regardless of motor column (two-way ANOVA, P<0.001). The mean nuclear GRα-IR of lumbar motor neurons was significantly reduced after 7 days (OD: 0.239±0.013 S.E.M.; P<0.016) and 14 days (OD: 0.196±0.013; P<0.001) from the GRα-IR levels observed among the control group (OD: 0.296±0.012) by 20 and 40%, respectively. Interestingly, nuclear GRα-IR levels were similar to control levels after 28 days of TP treatment (OD: 0.307±0.010). Treatment-dependent changes in cytoplasmic GRα-IR paralleled the observed changes in nuclear GRα-IR. These data suggest that pharmacologic testosterone treatment effects on motor neuron gene expression may be mediated by testosterone-induced temporal fluctuations of GRα-dependent gene regulation.

Transmission through the dorsal spinocerebellar and spinoreticular tracts: wakefulness versus thiopental anesthesia.

Most of what is known regarding the actions of injectable barbiturate anesthetics on the activity of lumbar sensory neurons arises from experiments performed in acute animal preparations that are exposed to invasive surgery and neural depression caused by coadministered inhalational anesthetics. Other parameters such as cortical synchronization and motor ouflow are typically not monitored, and, therefore, anesthetic actions on multiple cellular systems have not been quantitatively compared. The activities of antidromically identified dorsal spinocerebellar and spinoreticular tract neurons, neck motoneurons, and cortical neurons were monitored extracellularly before, during, and following recovery from the anesthetic state induced by thiopental in intact, chronically instrumented animal preparations. Intravenous administration of 15 mg/kg, but not 5 mg/kg, of thiopental to awake cats induced general anesthesia that was characterized by 5-10 min of cortical synchronization, reflected as large-amplitude slow-wave events and neck muscle atonia. However, even though the animal behaviorally began to reemerge from the anesthetic state after this 5-10-min period, neck muscle (neck motoneuron) activity recovered more slowly and remained significantly suppressed for up to 23 min after thiopental administration. The spontaneous activity of both dorsal spinocerebellar and spinoreticular tract neurons was maximally suppressed 5 min after administration but remained significantly attenuated for up to 17 min after injection. Peripheral nerve and glutamate-evoked responses of dorsal spinocerebellar and spinoreticular tract neurons were particularly sensitive to thiopental administration and remained suppressed for up to 20 min after injection. These results demonstrate that thiopental administration is associated with a prolonged blockade of motoneuron output and sensory transmission through the dorsal spinocerebellar and spinoreticular tracts that exceeds the duration of general anesthesia. Further, the blockade of glutamate-evoked neuronal responses indicates that these effects are due, in part, to a local action of the drug in the spinal cord. The authors suggest that this combination of lumbar sensory and motoneuron inhibition underlies the prolonged impairment of reflex coordination observed when thiopental is used clinically.

BCL-2 and BAX protect adult mice from lethal Sindbis virus infection but do not protect spinal cord motor neurons or prevent paralysis.

Cellular proteins that regulate apoptotic cell death can modulate the outcome of Sindbis virus (SV) encephalitis in mice. Both endogenous and overexpressed BCL-2 and BAX proteins protect newborn mice from fatal SV infection by blocking apoptosis in infected neurons. To determine the effects of these cellular factors on the course of infection in older animals, a more neurovirulent SV vector (dsNSV) was constructed from a viral strain that causes both prominent spinal cord infection with hind-limb paralysis and death in weanling mice. This vector has allowed assessment of the effects of BCL-2 and BAX on both mortality and paralysis in these hosts. Similar to newborn hosts, weanling mice infected with dsNSV encoding BCL-2 or BAX survived better than animals infected with control viruses. This finding indicates that BCL-2 and BAX both protect neurons that mediate host survival. Neither cellular factor, however, could suppress the development of hind-limb paralysis or prevent the degeneration of motor neurons in the lumbar spinal cord. Infection of BAX knockout mice with dsNSV demonstrated that endogenous BAX also enhances the survival of animals but has no effect on paralysis. These findings for the spinal cord are consistent with earlier data showing that dying lumbar motor neurons do not exhibit an apoptotic morphology. Thus, divergent cell death pathways are activated in different target populations of neurons during neurovirulent SV infection of weanling mice.

A modelling study of locomotion-induced hyperpolarization of voltage threshold in cat lumbar motoneurones.

During fictive locomotion the excitability of adult cat lumbar motoneurones is increased by a reduction (a mean hyperpolarization of approximately 6.0 mV) of voltage threshold (Vth) for action potential (AP) initiation that is accompanied by only small changes in AP height and width. Further examination of the experimental data in the present study confirms that Vth lowering is present to a similar degree in both the hyperpolarized and depolarized portions of the locomotor step cycle. This indicates that Vth reduction is a modulation of motoneurone membrane currents throughout the locomotor state rather than being related to the phasic synaptic input within the locomotor cycle. Potential ionic mechanisms of this locomotor-state-dependent increase in excitability were examined using three five-compartment models of the motoneurone innervating slow, fast fatigue resistant and fast fatigable muscle fibres. Passive and active membrane conductances were set to produce input resistance, rheobase, afterhyperpolarization (AHP) and membrane time constant values similar to those measured in adult cat motoneurones in non-locomoting conditions. The parameters of 10 membrane conductances were then individually altered in an attempt to replicate the hyperpolarization of Vth that occurs in decerebrate cats during fictive locomotion. The goal was to find conductance changes that could produce a greater than 3 mV hyperpolarization of Vth with only small changes in AP height (< 3 mV) and width (< 1.2 ms). Vth reduction without large changes in AP shape could be produced either by increasing fast sodium current or by reducing delayed rectifier potassium current. The most effective Vth reductions were achieved by either increasing the conductance of fast sodium channels or by hyperpolarizing the voltage dependency of their activation. These changes were particularly effective when localized to the initial segment. Reducing the conductance of delayed rectifier channels or depolarizing their activation produced similar but smaller changes in Vth. Changes in current underlying the AHP, the persistent Na(+) current, three Ca(2+) currents, the "h" mixed cation current, the "A" potassium current and the leak current were either ineffective in reducing Vth or also produced gross changes in the AP. It is suggested that the increased excitability of motoneurones during locomotion could be readily accomplished by hyperpolarizing the voltage dependency of fast sodium channels in the axon hillock by a hitherto unknown neuromodulatory action.

Expression of glutamate receptor subtypes in the spinal cord of control and mnd mice, a model of motor neuron disorder.

We studied the expression and distribution of glutamate receptor subtypes in the spinal cord of mnd mice, a model of motor neuron disorders and neuronal ceroid lipofuscinosis, and control mice using immunocytochemistry and in situ hybridization. The constitutive subunit of the NMDA ionotropic glutamate receptor, NMDAR1, was expressed in all neurons of the grey matter and was not modified in the spinal cord of mnd mice in either its normal or phosphorylated form. The immunoreactivity of GluR2, but not its mRNA, was increased mainly in the substantia gelatinosa both in presymptomatic and in 8-month-old symptomatic mice, suggesting compensatory changes aimed at reducing the Ca2+ permeability of the receptor channel. In spinal cord of mnd mice, mRNA, and protein levels of GluR3 were low only at the symptomatic stage, possibly as a consequence of motor neuron dysfunction. This was not due to motoneuron degeneration, because the number of choline acetyltransferase (ChAT) immunopositive lumbar motor neurons and the ChAT activity in the spinal cord and hind leg muscles of symptomatic mnd mice were no different from control mice. GluR4 mRNA was increased throughout the grey matter, presumably in relation to the marked microglia activation reported in the grey matter of the lumbar spinal cord in mnd mice. These changes in ionotropic glutamate receptors may alter glutamatergic neurotransmission and play some role in the pathology of mnd mice.

The reciprocal change of neurotrophin-4 and glial cell line-derived neurotrophic factor protein in the muscles, spinal cord and cerebellum of the dy mouse.

Laminin alpha2 (merosin)-deficient congenital muscular dystrophy (CMD) patients show progressive muscle fiber necrosis and ineffective muscle regeneration, probably due to a lower formation of multinucleated myotubes due to an adhesion defect of myoblasts to each other. Some recent studies found that CMD patients have a white matter disorder and cerebellum atrophy. In the spinal cord of dy mice, a model of CMD, inducible nitric oxide synthase (iNOS) was markedly expressed. Using Western blotting and immunohistochemical analyses, we investigated the levels of neurotrophin-4 (NT-4), brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor (GDNF) and ciliary neurotrophic factor (CNTF) in the central nervous system and skeletal muscles of dy mice. In the dy mice, the microtubule-associated protein-2 (MAP-2) protein level was markedly decreased in the Purkinje and granule cells of the cerebellum, and in lumbar motoneurons of the spinal cord. The motoneurons and axons of dy mice possessed lower expressions of phosphorylated tau. The amount of NT-4 was markedly lower in the cerebellum, spinal cord and hindlimb muscles of dy mice. In dy mice, GDNF was markedly enhanced in the Purkinje and granule cells of the cerebellum, in many lumbar motoneurons, and in the regenerating atrophied fibers. The CNTF protein level did not differ in the hindlimb muscles between the normal and dy mice. Therefore, GDNF could act to inhibit the death of Purkinje and granular neurons, and motoneurons, and to promote the remodeling of the neuromuscular junction of atrophied muscle fibers of dy mice. Furthermore, dy mice include neurogenic abnormalities in the cerebellum and spinal cord along with myogenic disorder of muscle fibers.