Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.     

Isolated Bovine Spinal Motoneurons Have Specific Ganglioside Antigens Recognized by Sera from Patients with Motor Neuron Disease and Motor Neuropathy

The gangliosides GM1 and GD1b have recently been reported to be potential target antigens in human motor neuron disease (MND) or motor neuropathy. The mechanism for selective motoneuron and motor nerve impairment by the antibodies directed against these gangliosides, however, is not fully understood. We recently investigated the ganglioside composition of isolated bovine spinal motoneurons and found that the ganglioside pattern of the isolated motoneurons was extremely complex. GM1, GD1a, GD1b, and GT1b, which are major ganglioside components of CNS tissues, were only minor species in motoneurons. Among the various ganglioside species in motoneurons, several were immunoreactive to sera from patients with MND and motor neuropathy. One of these gangliosides was purified from bovine spinal cord and characterized as N-glycolylneuraminic acid-containing GM1 [GM1(NeuGc)] by compositional analysis, fast atom bombardment mass spectra, and the use of specific antibodies. Among seven sera with anti-GM1 antibody activities, five sera reacted with GM1(NeuGc) and two did not. Two other gangliosides, which were recognized by another patient's serum, appeared to be specific for motoneurons. We conclude that motoneurons contained, in addition to the known ganglioside antigens GM1 and GD1b, other specific ganglioside antigens that could be recognized by sera from patients with MND and motor neuropathy.     

人脊髓前角运动神经元190KD蛋白的初步研究

本研究采用ＳＤＳ凝胶电泳方法从人脊神经前根中分离出人脊髓前角运动神经元特有的蛋白—１９０ＫＤ。将该蛋白作为抗原,免疫ＢＡＬＢ／ｃ小鼠,经杂交瘤技术,获得了抗１９０ＫＤ蛋白的单克隆抗体。免疫细胞化学检测表明,１９０ＫＤ单抗与脊髓灰质前角神经元、前根及肌支发生阳性反应。实验结果提示,１９０ＫＤ蛋白分布在脊髓运动神经元胞体及脊神经的前根和肌支纤维中。     

Responses of efferent octopaminergic thoracic unpaired median neurons in the locust to visual and mechanosensory signals

Insect thoracic ganglia contain efferent octopaminergic unpaired median neurons (UM neurons) located in the midline, projecting bilaterally and modulating neuromuscular transmission, muscle contraction kinetics, sensory sensitivity and muscle metabolism. In locusts, these neurons are located dorsally or ventrally (DUM- or VUM-neurons) and divided into functionally different sub-populations activated during different motor tasks. This study addresses the responsiveness of locust thoracic DUM neurons to various sensory stimuli. Two classes of sense organs, cuticular exteroreceptor mechanosensilla (tactile hairs and campaniform sensilla), and photoreceptors (compound eyes and ocelli) elicited excitatory reflex responses. Chordotonal organ joint receptors caused no responses. The tympanal organ (Müller's organ) elicited weak excitatory responses most likely via generally increased network activity due to increased arousal. Vibratory stimuli to the hind leg subgenual organ never elicited responses. Whereas DUM neurons innervating wing muscles are not very responsive to sensory stimulation, those innervating leg and other muscles are very responsive to stimulation of exteroreceptors and hardly responsive to stimulation of proprioceptors. After cutting both cervical connectives all mechanosensory excitation is lost, even for sensory inputs from the abdomen. This suggests that, in contrast to motor neurons, the sensory inputs to octopaminergic efferent neuromodulatory cells are pre-processed in the suboesophageal ganglion.     

Activation of Phosphatidylinositol 3-Kinase, but Not Extracellular-Regulated Kinases, Is Necessary to Mediate Brain-Derived Neurotrophic Factor-Induced Motoneuron Survival

Chick embryo spinal cord motoneurons develop a trophic response to some neurotrophins when they are maintained in culture in the presence of muscle extract. Thus, after 2 days in culture, brain-derived neurotrophic factor (BDNF) promotes motoneuron survival. In the present study we have analyzed the intracellular pathways that may be involved in the BDNF-induced motoneuron survival. We have observed that BDNF activated the extracellular-regulated kinase (ERK) mitogen-activated protein (MAP) kinase and the phosphatidylinositol (PI) 3-kinase pathways. To examine the contribution of these pathways to the survival effect triggered by BDNF, we used PD 98059, a specific inhibitor of MAP kinase kinase, and LY 294002, a selective inhibitor of PI 3-kinase. PD 98059, at doses that significantly reduced the phosphorylation of ERKs, did not show any prominent effect on neuronal survival. However, LY 294002 at doses that inhibited the phosphorylation of Akt, a down-stream element of the PI 3-kinase, completely abolished the motoneuron survival effects of BDNF. Moreover, cell death triggered by LY 294002 treatment exhibited features similar to those observed after muscle extract deprivation. Our results suggest that the PI 3-kinase pathway plays an important role in the survival effect triggered by BDNF on motoneurons, whereas activation of the ERK MAP kinase pathway is not relevant.     

Androgen regulation of axon growth and neurite extension in motoneurons

Androgens act on the CNS to affect motor function through interaction with a widespread distribution of intracellular androgen receptors (AR). This review highlights our work on androgens and process outgrowth in motoneurons, both in vitro and in vivo. The actions of androgens on motoneurons involve the generation of novel neuronal interactions that are mediated by the induction of androgen-dependent neurite or axonal outgrowth. Here, we summarize the experimental evidence for the androgenic regulation of the extension and regeneration of motoneuron neurites in vitro using cultured immortalized motoneurons, and axons in vivo using the hamster facial nerve crush paradigm. We place particular emphasis on the relevance of these effects to SBMA and peripheral nerve injuries.     

Probing the neuromodulatory gain control system in sports and exercise sciences

The monoaminergic bulbospinal pathways from the brainstem are central to motor functions by regulating the gains of spinal motoneurons and represent, in that respect, probably the primary control system for motoneuron excitability. Yet, the efficiency of this system is few, if not never, assessed in the fields of sports and exercise sciences. In this review paper, we propose a methodological approach intended to assess how this neuromodulatory system affects motoneuron excitability. This approach is based on the use of tendon vibration which can, in certain circumstances, induce the generation of the so-called tonic vibration reflex through the stimulation of muscle spindles. Force and EMG responses to tendon vibration are indeed indicative of how this descending system modulates the gain of the ionotropic inputs from Ia afferents and thus of the strength of the monoaminergic drive. After a brief presentation of the neuromodulatory system and of the mechanisms involved in the generation of the tonic vibration reflex, we address some important methodological considerations regarding the use of the TVR to probe this neuromodulatory gain control system. Hopefully, this paper will encourage sports and exercise scientists to investigate this system.     

Monosynaptic connections mediate resistance reflex in crayfish (Procambarus clarkii) walking legs

Summary In crustacean walking legs, the coxo-basipodite chordotonal organ (CB) composed of about 50 sensory cells, evokes a resistance reflex in the levator (Lev) and depressor (Dep) muscles responsible for the movements of the coxo-basipodite joint where it is located. Mechanical stimulation of the CB strand and electrical stimulation of its sensory nerve have been performed along with systematic intracellular recordings from CB terminals (CB T) and levator (Lev) or depressor (Dep) motoneurons (MNs) in order to study their connections. Measurements of conduction times in the CB nerve demonstrated different pools of sensory fibres, the fastest of which reach the ganglion in 2.5 ms. During imposed movements to the CB strand, intracellularly recorded Lev or Dep MN display EPSPs that are correlated to spikes in the CB nerve, their delays are incompatible with a polysynaptic pathway. Systematic stimulation of the CB nerve demonstrates that about 4 to 8 CB fibres are connected with each Lev or Dep MN. Classical tests for monosynaptic connections indicate that EPSPs occurring between 3 ms and 6 ms correspond mainly to monosynaptic connections with CB T, whereas IPSPs (the latencies of which are above 12 ms) are polysynaptic. In spite of the high selectivity of the CB T onto MNs, eight simultaneous intracellular recordings of coupled CB T and MN (out of more than 300 MNs penetrated) have allowed a direct measurement of synaptic delays (less than 1 ms). The functional significance of these results is discussed in relation to the proprioceptive control of locomotor movements.Abbreviations CB Coxo-basipodite chordotonal organ - CB n CB sensory nerve - CB T CB sensory terminal - Dep depressor - Lev levator - MN motoneuron     

Neuronal firing properties and swimming motor patterns in young tadpoles of four amphibians: Xenopus, Rana, Bufo and Triturus

We have compared intrinsic firing properties of motoneurons with the way they fire during locomotion in young tadpoles of four species of amphibian. Xenopus motoneurons have the highest current threshold for spiking; most fire a single spike to depolarising current steps; all fire reliably once per cycle during fictive swimming. Xenopus motoneurons recorded with Cs⁺-filled microlelectrodes fire repetitively to current but still fire only once per swimming cycle. Rana, Bufo and Triturus motoneurons have lower current thresholds; most fire bursts of spikes to suprathreshold current but most do not fire reliably during swimming and most still fire only once (if at all) per cycle. We conclude that neuronal firing patterns during locomotion cannot reliably be predicted from intrinsic firing properties, and suggest the composition and form of the underlying synaptic input is more important. We also measured cycle period, ventral root burst duration, and longitudinal delay during fictive swimming. These basic swimming parameters range from relatively long in Rana to relatively short in Xenopus. By discounting differences in neuronal firing properties between the four species, we can start to relate differences in fictive swimming to differences in synaptic drive, particularly the strong electrotonic input seen only in Xenopus. Accepted: 27 January 1997