Postnatal electrical and morphological abnormalities in lumbar motoneurons from transgenic mouse models of amyotrophic lateral sclerosis.

Antidromically identified lumbar motoneurons intracellularly recorded in the entire brainstem/spinal cord preparation isolated from SOD1(G85R) postnatal mice (P3-P10) were labelled with neurobiotin and fully reconstructed in 3D from serial sections in order to analyse their morphology. This staining procedure revealed differences between WT and SOD1(G85R) dendritic trees for most metric and topologic parameters analyzed. A highly complex morphology of SOD1(G85R) motoneurons dendrites (increased number of branching points and terminations) was found and the dendritic trees were longer compared to the WT motoneurons. These morphological changes observed in P8-P9 motoneurons mice occurred concomitantly with a decrease in the input resistance and gain. During electrophysiological recordings, four patterns of discharge were observed in response to ramp stimulations, that were equally distributed in WT and SOD1(G85R) motoneurons. In slice preparation, whole cell patch-clamp recordings made from developing motoneurons in SOD1(G85R) and double transgenic SOD1(G93A)/Hb9-eGFP mice showed that Riluzole, a blocker of persistent inward sodium conductance, altered the repetitive firing in a similar way for the 2 strains. These results show that the SOD1 mutations linked to familial ALS alter the development of the electrical and morphological properties of lumbar motoneurons.

How shunting inhibition affects the discharge of lumbar motoneurones: a dynamic clamp study in anaesthetized cats.

In the present work, dynamic clamp was used to inject a current that mimicked tonic synaptic activity in the soma of cat lumbar motoneurones with a microelectrode. The reversal potential of this current could be set at the resting potential so as to prevent membrane depolarization or hyperpolarization. The only effect of the dynamic clamp was then to elicit a constant and calibrated increase of the motoneurone input conductance. The effect of the resulting shunt was investigated on repetitive discharges elicited by current pulses. Shunting inhibition reduced very substantially the firing frequency in the primary range without changing the slope of the current-frequency curves. The shift of the I-f curve was proportional to the conductance increase imposed by the dynamic clamp and depended on an intrinsic property of the motoneurone that we called the shunt potential. The shunt potential ranged between 11 and 37 mV above the resting potential, indicating that the sensitivity of motoneurones to shunting inhibition was quite variable. The shunt potential was always near or above the action potential voltage threshold. A theoretical model allowed us to interpret these experimental results. The shunt potential was shown to be a weighted time average of membrane voltage. The weighting factor is the phase response function of the neurone that peaks at the end of the interspike interval. The shunt potential indicates whether mixed synaptic inputs have an excitatory or inhibitory effect on the ongoing discharge of the motoneurone.

Control of the temporal fidelity of synaptic transmission by a presynaptic high voltage-activated transient K current.

The type of K(+) channels controlling the waveform of the presynaptic spike and synaptic transmission were examined in the lamprey spinal cord. Reticulospinal neuron somata displayed a transient K(+) current with a high voltage-activation and inactivation. This current was selectively blocked by catechol at 100 microM. Reticulospinal axons also displayed a high voltage-activated fast K(+) current sensitive to catechol. The function of this presynaptic high voltage-activated fast K(+) current in controlling synaptic transmission was investigated by using paired intracellular recordings from reticulospinal axons and their targets. Blockade of this current by catechol (100 microM) prolonged the presynaptic spike elicited by a single stimulus leading to a potentiation of the postsynaptic EPSP. Calcium imaging of reticulospinal axons showed an increase in presynaptic calcium transients after blockade of the presynaptic K(+) current by catechol. During high frequency firing, catechol revealed an activity-dependent decrease in the spike duration, which resulted in a depression of synaptic transmission. These results suggest that the presynaptic high voltage-activated transient K(+) current acts to optimize the temporal fidelity of synaptic transmission by minimizing activity-dependent changes in the presynaptic spike waveform and calcium dynamics.

Cooperation of muscle and cutaneous afferents in the feedback of contraction to peroneal motoneurons.

Peroneal motoneurons were recorded intracellularly in anesthetized cats during sustained submaximal contractions of peroneus brevis muscle (PB) elicited by repetitive electrical stimulation of motor axons in the distal portion of cut ventral root filaments. Mechanical stimulation of the territory innervated by the superficial peroneal nerve (SP) was applied during contraction to assess the influence of afferents from this territory on the contraction-induced excitation of motoneurons. In 21 peroneal motoneurons in which PB contraction evoked excitatory potentials, a stimulation engaging mechanoreceptors located in the skin around toes was found to either enhance (in 12 motoneurons) or reduce (in 9 motoneurons) the contraction-induced excitatory potentials. Among positive effects, six showed simple summation of the responses to each individual stimulus, suggesting a convergence of afferent pathways on motoneurons. In six other motoneurons, complex interactions were observed, as may result from convergence at a premotoneuronal level. Among negative effects, a single instance was observed of inhibitory facilitation, as may result from convergence of cutaneous and muscular, possibly Ib, afferents on inhibitory interneurons. Several pathways, mediating either facilitory or inhibitory influences, are available for cooperation of muscle and cutaneous input, allowing flexibility of motoneuron activation in different tasks.

Effects on peroneal motoneurons of cutaneous afferents activated by mechanical or electrical stimulations.

The postsynaptic potentials elicited in peroneal motoneurons by either mechanical stimulation of cutaneous areas innervated by the superficial peroneal nerve (SP) or repetitive electrical stimulation of SP were compared in anesthetized cats. After denervation of the foot sparing only the territory of SP terminal branches, reproducible mechanical stimulations were applied by pressure on the plantar surface of the toes via a plastic disk attached to a servo-length device, causing a mild compression of toes. This stimulus evoked small but consistent postsynaptic potentials in every peroneal motoneuron. Weak stimuli elicited only excitatory postsynaptic potentials (EPSPs), whereas increase in stimulation strength allowed distinction of three patterns of response. In about one half of the sample, mechanical stimulation or trains of 20/s electric pulses at strengths up to six times the threshold of the most excitable fibers in the nerve evoked only EPSPs. Responses to electrical stimulation appeared with 3-7 ms central latencies, suggesting oligosynaptic pathways. In another, smaller fraction of the sample, inhibitory postsynaptic potentials (IPSPs) appeared with an increase of stimulation strength, and the last fraction showed a mixed pattern of excitation and inhibition. In 24 of 32 motoneurons where electrical and mechanical effects could be compared, the responses were similar, and in 6 others, they changed from pure excitation on mechanical stimulation to mixed on electrical stimulation. With both kinds of stimulation, stronger stimulations were required to evoke inhibitory postsynaptic potentials (IPSPs), which appeared at longer central latencies than EPSPs, indicating longer interneuronal pathways. The similarity of responses to mechanical and electrical stimulation in a majority of peroneal motoneurons suggests that the effects of commonly used electrical stimulation are good predictors of the responses of peroneal motoneurons to natural skin stimulation. The different types of responses to cutaneous afferents from SP territory reflect a complex connectivity allowing modulations of cutaneous reflex responses in various postures and gaits.

Flexible processing of sensory information induced by axo-axonic synapses on afferent fibers.

Recent experiments indicate that afferent information is processed in the intraspinal arborisation of mammalian group I fibres. During muscle contraction, Ib inputs arising from tendon organs are filtered out by presynaptic inhibition after their entry in the spinal cord. This paper reviews the mechanisms by which GABAergic axo-axonic synapses, i.e., the morphological substrate of presynaptic inhibition, exert this filtering effect. Using confocal microscopy, axo-axonic synapses were demonstrated on segmental Ib collaterals. Most synapses were located on short preterminal and terminal branches. Using a simple compartmental model of myelinated axon, the primary afferent depolarisation (PAD), generated by such synapses, was predicted to reduce the amplitude of incoming action potentials by inactivating the sodium current, and this prediction was experimentally verified. A further theoretical work, relying on cable theory, suggests that the electrotonic structure of collaterals and the distribution of axo-axonic synapses allow large PADs (about 10 mV) to develop on some distal branches, which is likely to result in a substantial presynaptic inhibition. In addition, the electrotonic structure of group I collaterals is likely to prevent PAD from spreading to the whole arborisation. Such a non-uniform diffusion of the PAD accounts for differential presynaptic inhibition in intraspinal branches of the same fibre. Altogether, our experimental and theoretical works suggest that axo-axonic synapses can control the selective funnelling of sensory information toward relevant targets specified according to the motor task.

Indications for GABA-immunoreactive axo-axonic contacts on the intraspinal arborization of a Ib fiber in cat: a confocal microscope study.

Confocal microscopy was used to detect GABA-immunoreactive axo-axonic appositions, indicating possible synaptic contacts, on Ib fiber terminals in the lumbosacral spinal cord. A Ib fiber from posterior biceps-semitendinosus muscles was labeled by intra-axonal ejection of tetramethylrhodamine dextran (red), and serial sections of S1-L7 spinal cord segments were processed for GABA immunocytochemistry revealed by fluorescein isothiocynate (green). Appositions between GABA-immunoreactive structures and the labeled fiber appeared as yellow spots because of the presence of both fluorochromes in small volumes (0.3 * 0.3 * 0.5 micrometer(3)) of tissue. These spots were identified as probable axo-axonic contacts when: (1) they were observed in two to four serial confocal planes, indicating that they did not occur by chance; and (2) their sizes, shapes, and locations were similar to those of axo-axonic contacts found on Ia terminals, known to bear presynaptic boutons, and resembled the axo-axonic synapses described in electron microscope studies of Ib boutons in Clarke's column. A total of 59 presumed axo-axonic contacts was observed on two Ib collaterals, representing an estimated 20% of the total complement. In a three-dimensional reconstruction of one collateral, they were mostly located in terminal positions, and some branches bore more contacts than others. Such differential distribution could not result from chance appositions between GABAergic structures and Ib arborization and further supported the identification of axo-axonic contacts. Segmental Ib collaterals bear axo-axonic synapses that might ensure differential funneling of information toward different targets.

Reduction of presynaptic action potentials by PAD: model and experimental study.

A compartmental model of myelinated nerve fiber was used to show that primary afferent depolarization (PAD), as elicited by axo-axonic synapses, reduces the amplitude of propagating action potentials primarily by interfering with ionic current responsible for the spike regeneration. This reduction adds to the effect of the synaptic shunt, increases with the PAD amplitude, and occurs at significant distances from the synaptic zone. PAD transiently enhances the sodium current activation, which partly accounts for the PAD-induced fiber hyperexcitability, and enhances sodium inactivation on a slower time course, thus reducing the amplitude of action potentials. In vivo, intraaxonal recordings from the intraspinal portion of group I afferent fibers were carried out to verify that depolarizations reduced the amplitude of propagating action potentials as predicted by the model. This article suggests PAD might play a major role in presynaptic inhibition.

Effect of laser irradiation on latency of herpes simplex virus in a mouse model.

Low power Laser irradiation is used in medical practice for different indications. Several firms which commercialise Laser claim that Laser irradiation may be beneficial towards Herpes virus pathogenicity. We observe that, in the ear experimental model of HSV latency [3], repeated exposure to infrared Laser radiation of cervical ganglia following HSV inoculation appears to specifically hinder the establishment of virus latency in mouse.