Suppression of abnormally increased excitability of monosynaptic spinal reflex arcs by riluzole

We studied the effects of a neuroprotector, riluzole, on the evoked mass activity of spinal neuronal mechanisms and on action potentials (APs) recorded from the sciatic nerve in intact rats and rats with the manifestations of postdenervational and 4-aminopyridine (4-AP)-induced hyperreflexia, as well as in animals in the superreflexia state (induced by combined action of denervation and 4-AP). We measured the parameters of monosynaptic reflex discharges (monosynaptic reflexes, MRs) recorded from the ventral root (VR), of the spinal dorsal surface potential (DSPs), and of mass APs evoked in afferent and efferent fibers of the SN before and 10, 30, 60, and 120 min after injection of riluzole. It was found that in intact animals riluzole significantly (by 60–70%) decreased the amplitude of VR MRs and those of the afferent peak and N₁ component of DSPs. Riluzole exerted smaller suppressive effects on mass APs in the afferent fibers of the SN; the effect on APs in the SN efferent fibers was the minimum (a 4 to 5% decrease). Under conditions of increased sensitivity of the motoneuronal postsynaptic membrane to the transmitter (postdenervational hyperreflexia) and an increased release of glutamate from presynaptic elements (4-AP-induced hyperreflexia), as well as under superreflexia conditions, the dynamics of suppression of the evoked spinal activity by riluzole showed relatively moderate differences from those in intact animals. Under the above conditions, riluzole in the same manner decreased the amplitude of VR MRs. In the superreflexia state, the agent blocked the development of additional components of these dramatically increased potentials (in the above state, their amplitude increased by nearly nine times, on average, and this resulted in the generation of such components). We believe that the inhibitory effect of riluzole on glutamatergic neurotransmission in the spinal cord is based, first of all, on blocking of excitation in afferent presynaptic terminals. The possibility to use riluzole for correction of abnormally increased hyperexcitability of the spinal neuronal systems is discussed. Neirofiziologiya/Neurophysiology, Vol. 37, Nos. 5/6, pp. 416–423, September–December, 2005.     

Dynamics of post-denervational modifications of spinal reflex activity in albino rats

In experiments on rats, we studied the characteristics of reflex discharges in the ventral root (VR) L ₅; the discharges were evoked by stimulation of segmental (peripheral nerve or dorsal root, DR) and suprasegmental vestibular (stimulation of the round window of the labyrinth) inputs. Potentials were recorded within different time intervals (from 1 to 150 days) after transection of the sciatic nerve (SN); measures preventing regeneration of its fibers were used. Modifications of the segmental responses related to post-denervational changes included four phases: (i) latent period, (ii) post-denervational spinal hyperreflexia (PDSH), (iii) partial suppression of monosynaptic discharges (MDs) in the VR, and (iv) complete disappearance of VR MDs resulting from late post-denervational changes. The latency of post-denervational modifications was about 18–48 h after the moment of transection of the SN. Within the PDSH phase, modifications were the greatest 3 to 5 days after transection; these changes could be more adequately estimated in the case of stimulation of the DR on the side of transection and not under conditions of stimulation of the central segment of the transected SN per se. Within this phase, the amplitudes of VR MDs and responses to vestibular stimulation were augmented two to three and four to five times, as compared with the respective indices in intact animals. From the 7th to 10th day after the nerve transection, the amplitude of VR MDs progressively dropped, and on about the 20th day these discharges practically disappeared, while polysynaptic components of segmental responses were preserved. Vestibular responses within this period were, as earlier, considerably facilitated. On the 60th and 150th days (within the phase of late post-denervational modifications) there were no VR MDs after stimulation of segmental inputs, and polysynaptic responses were exclusively observed. The amplitude of discharges evoked by vestibular stimulation became lower than in the PDSH state but remained significantly higher than the control values of this parameter. Probable mechanisms of post-denervational modifications of the evoked spinal activity within different time intervals after transection of the SN are discussed. Neirofiziologiya/Neurophysiology, Vol. 39, No. 1, pp. 37–46, January–February, 2007.     

Two Methods of Modeling of Spinal Superreflexia in Rats

We studied the relations between conditions of induction and quantitative characteristics of abnormally amplified monosynaptic reflex discharges (MSD) in ventral roots (VR) observed in two experimental situations: (i) 5 days after simultaneously performed denervation (transection of the sciatic nerve) and spinalization at the L ₁ level, and (ii) 5 days after preliminary denervation and with systemic injection of 4-aminopyridine (4-AP) in the course of the acute experiment. In both situations, the amplitude of the MSD conducted via the VR was close to the threshold of excitation of fibers in this root or even exceeded this value (a superreflexia phenomenon). Under both (i) and (ii) conditions, we observed generation of the second component of MSD in the VR, which was probably related to transition of excitation from excited to “silent” fibers in the VR. The latter of the above variants of induction superreflexia (5 days after denervation and with the effect of 4-AP in the acute experiment) is preferred because there is practically no death of the experimental animals in the course of the chronic experiment, there are no negative post-spinalization changes in the spinal cord, and the possibility of supraspinal activation of motoneurons is preserved. Neirofiziologiya/Neurophysiology, Vol. 37, No. 2, pp. 157–163, March–April, 2005.     

Mechanisms of stepping rhythm formation in decerebrated and chronic spinal cats under epidural stimulation

A study was made of the stepping pattern formation in decerebrated and in chronic spinal cats during epidural stimulation (ES). The hindlimb stepping performance depended on the parameters of ES and afferent input. At non-optimal ES parameters, no stepping was induced, only muscle reflexes followed the stimulation rhythm. Optimized ES (3–5 Hz, 50–100 μA for decerebrated and 20–30 Hz, 150–250 μA for spinal cats) evoked coordinated stepping movements at a natural rate (0.8–1 Hz) accompanied by electromyographic burst activity of the corresponding muscles. In decerebrated cats, the bursts are formed owing to modulation of early responses and the late polysynaptic activity. In chronic spinal cats, this process is mainly due to amplitude modulation of the early responses. Formation of the stepping pattern in decerebrated cats involves spinal interneurons responsible for the polysynaptic activity, which allows its correction based on processing the afferent signals. Activation of this system in chronic spinal cats can be realized by afferent stimulation alone, without ES.     

Manipulations of spinal cord excitability evoke developmentally-dependent compensatory changes in the lamprey spinal cord

We have examined homeostatic or compensatory plasticity evoked by tonic changes in spinal cord excitability in the lamprey, a model system for investigating spinal cord function. In larval animals, reducing excitability by incubating in tetrodotoxin or the glutamate receptor antagonists CNQX or CNQX/AP5 for 20–48 h resulted in a diverse set of cellular and synaptic changes that together were consistent with an increase in spinal cord excitability. Similar changes occurred to a tonic increase in excitation evoked by incubating in high potassium physiological solution (i.e. responses were unidirectional). We also examined developmental influences on these effects. In animals developing from the larval to adult form effects were reduced or absent, suggesting that at this stage the spinal cord was more tolerant of changes in activity levels. Responses had returned in adult animals, but they were now bi-directional (i.e. opposite effects were evoked by an increase or decrease in excitability). The spinal cord can thus monitor and adapt cellular and synaptic properties to tonic changes in excitability levels. This should be considered in analyses of spinal cord plasticity and injury.     

A Possible Mechanism of Blocking of Limbic Motor Seizure Reactions Induced by Activation of the Thalamic Reticular Nucleus

Experiments were directed toward elucidation of the role of the thalamic reticular nucleus (R) in the modulation of generalized seizure reactions under kindling conditions and of the mechanisms mediating the effects of stimulation of the above nucleus on seizure activity. It was shown that activation of the thalamic R in rats limits generalization of the seizure reactions both in the course of development of seizures of limbic genesis (evoked by stimulation of the hippocampus) and under conditions of the existence of a pre-formed epileptic nidus. Tetanic stimulation of the R in cats under conditions of acute experiments induced significant facilitation of IPSPs in thalamo-cortical neurons of the ventrolateral thalamic nucleus. This effect is rather long-lasting and may be considered a mechanism providing blocking of generalized seizures under kindling conditions. Neirofiziologiya/Neurophysiology, Vol. 37, No. 4, pp. 352–361, July–August, 2005.     

Influence of vibration on the excitability of spinal α-motoneurons under static conditions and during evoked stepping in humans

The excitability of spinal α-motoneurons in healthy humans was investigated with vibrostimulation (20–60 Hz) applied to different groups of muscles both under stationary conditions and during vibration-evoked stepping movements with leg suspension. In 15 subjects, the H-reflex amplitude was compared under the conditions of vibration of the left leg quadriceps femoris (QFM) or biceps femoris (BFM) muscle, as well as under the conditions of vibration of the contralateral, motionless leg QFM muscle in three spatial positions of the body: upright, supine, and lying on the side with the left leg suspended. Under dynamic conditions, the H-reflex value was compared during evoked and voluntary steppings at eight intervals of the step cycle. In all body positions, the vibration of each ipsilateral leg muscle caused a significant H-reflex suppression, this suppression being more prominent under the air-stepping conditions. The vibration of the contralateral leg QFM had weak influence on the H-reflex amplitude. In seven subjects, the vibration of the ipsilateral and contralateral leg muscles generated stepping movements. During vibration-evoked air-stepping, the H-reflex had different amplitudes in different phases of the step cycle. At the same time, the differences between responses under voluntary and involuntary stepping conditions were revealed only in the step cycle phase corresponding to the stance phase. Thus, the different degrees of the H-reflex suppression by vibration in different spatial positions of the body seem to depend on the summary afferent inflows to the spinal cord interneurons involved in the regulation of locomotion and posture. Apparently, an increase in the spinal cord neuronal excitability, which is necessary for activating locomotor automatism under the leg unloading conditions, occurs during evoked air-stepping in the swing phase.     

Neuronal activity of the lateral vestibular nucleus of the guinea pig evoked by tilting the animal about the longitudinal axis during locomotion

In experiments on decerebrate guinea pigs, the impulse activity of neurons of the lateral vestibular nucleus evoked by tilting the animal about the longitudinal axis was investigated under conditions of spontaneous and mesencephalon stimulation-evoked locomotor activity. In most investigated neurons, locomotor activity led to changes in their responses to adequate vestibular stimulation. The dominant reaction was intensification of such responses, which was observed in almost all vestibulospinal neurons and in 2/3 of cells not having descending projections. Responses were suppressed only in 1/4 of the neurons not projecting to the spinal cord. The changes in the evoked responses had an amplitude character; the lag of the changes in the discharge frequency relative to the acceleration that caused them was constant. It is suggested that intensification of dynamic reactions of vestibular neurons during locomotion provides maintenance of the animal's equilibrium during movements in space by various gaits and along different trajectories.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 541–549, September–October, 1991.     

Effects of Near-Threshold Stimulation of the Vestibular Apparatus on Postural Responses Evoked by Displacements of the Visualized Surrounding

In healthy subjects in the relaxed upward stance and perceiving a virtual visual environment (VVE), we recorded postural reactions to isolated visual and vestibular stimulations or their combinations. Lateral displacements of the visualized virtual scene were used as visual stimuli. The vestibular apparatus was stimulated by application of near-threshold galvanic current pulses to the proc. mastoidei of the temporal bones. Isolated VVE shifts evoked mild, nonetheless clear, body tilts readily distinguished in separate trials; at the same time, postural effects of isolated vestibular stimulation could be detected only after averaging of several trials synchronized with respect to the beginning of stimulation. Under conditions of simultaneous combined presentation of visual and vestibular stimuli, the direction of the resulting postural responses always corresponded to the direction of responses induced by VVE shifts. The contribution of an afferent volley from the vestibular organ depended on the coincidence/mismatch of the direction of motor response evoked by such a volley with the direction of response to visual stimulation. When both types of stimulations evoked unidirectional body tilts, postural responses were facilitated, and the resulting effect was greater than that of simple summation of the reactions to isolated actions of the above stimuli. In the case where isolated galvanic stimulation evoked a response opposite with respect to that induced by visual stimulation, the combined action of these stimuli of different modalities evoked postural responses identical in their magnitude, direction, and shape to those evoked by isolated visual stimulation. The above findings allow us to conclude that the effects of visual afferent input on the vertical posture under conditions of our experiments clearly dominate. In general, these results confirm the statement that neuronal structures involved in integrative processing of different afferent volleys preferably select certain type of afferentation carrying more significant or more detailed information on displacements (including oscillations) of the body in space.     

氯胺酮对单足致炎大鼠脊髓背角神经元活动的影响

在大鼠脊髓背角用细胞外记录技术共记录到３２个单位。角叉菜胶一侧足底注射致炎后,电刺激该侧足底内外侧神经激动其中Ａ、Ｃ纤维时,脊髓背角神经元诱发放电数均显著增加;静脉注射ＮＭＤＡ受体拮抗剂氯胺酮后,Ａ、Ｃ纤维刺激诱发的放电反应均显著下降甚至消失。致炎后脊髓背角深层单位出现Ｗｉｎｄｕｐ现象,静脉注射氯胺酮后该现象减轻消失。结果提示：角叉菜胶致炎导致脊髓背角神经元兴奋性升高和Ｗｉｎｄｕｐ;ＮＭＤＡ受体参     

Reflex responses of the spinal cord under conditions of the spinal “superreflexia” evoked by pharmacological agents increasing its excitability

We studied monosynaptic reflex discharges (MRD) recorded from the ventral roots (VR) of rats subjected to systemic administration with thyroliberin, thyroxin, or 4-aminopyridine (4-AP). Under such conditions, in some of the experiments the MRD amplitude reached values sufficient to excite non-active VR fibers. In these cases, immediately after the MRD peak had been reached, abnormally increased responses (AIR) developed, whose amplitude was 2–2.5 times higher than the amplitude of highly facilitated MRD. Proofs are presented that AIR reflect excitation of “neighboring” VR fibers, which were not involved in the reflex response, and MRD plays the role of a “stimulus” exciting these fibers. Therefore, we demonstrate the possibility of transmission of excitation from “active” to “silent” fibers within a nerve trunk under conditions of the development of “superreflexia”. This state can be considered a model of excitation spreading via a non-synaptic pathway under conditions of hyperexcitability of the CNS structures, in seizure states in particular. neirofiziologiya/Neurophysiology, Vol. 32, No. 2, pp. 120–127, March–April, 2000.     

Pain and neurotransmitters

1. To study physiological roles of substance P (SP), gamma-aminobutyric acid (GABA), enkephalins and other endogenous substances, we developed several kinds of isolated spinal cord preparations of newborn rats. 2. In these preparations, various slow responses of spinal neurons evoked by stimulation of primary afferent C fibers were depressed by a tachykinin antagonist, spantide. These results together with many other lines of evidence suggest that SP and neurokinin A serve as pain transmitters in a subpopulation of primary afferent C fibers. 3. Some C-fiber responses in various isolated spinal cord preparations were depressed by GABA, muscimol, and opioid peptides. In contrast, bicuculline (GABA antagonist) and naloxone (opioid antagonist) potentiated the "tail pinch potential," i.e., a nociceptive response of the ventral root evoked by pinch stimulation of the tail in isolated spinal cord-tail preparation of the newborn rat. The latter results support the hypothesis that some primary afferents activate inhibitory spinal interneurons which release GABA and enkephalins as transmitters to modulate pain inputs.     

Convergence of reticulo-, vestibulo-, and rubrospinal influences on cervical spinal motoneurons in cats

Intracellular recordings were made of synaptic responses of 93 motoneurons in the cervical region of the cat spinal cord to stimulation of the medial longitudinal bundle, the brain-stem reticular formation, the lateral vestibular nucleus of Deiters, and the red nucleus. In response to stimulation of the medial longitudinal bundle and the vestibular nucleus responses in the motoneurons of the distal groups of muscles of the forelimb were predominantly excitatory, whereas in motoneurons of the proximal extensor muscles they were predominantly inhibitory. During stimulation of the red nucleus, excitatory and inhibitory responses were recorded in almost equal numbers of cells regardless of their functional class. Monosynaptic EPSPs appeared in one-fifth of motoneurons in response to stimulation of the medial longitudinal bundle and, in a few cases, to stimulation of the vestibular and red nuclei. Otherwise, during stimulation of these structures polysynaptic responses were recorded in the motoneurons. In 62% of cases postsynaptic potentials arising in response to stimulation of the various suprasegmental structures tested were identical in direction in the same motoneurons. A mutually facilitatory effect was observed during stimulation of different suprasegmental inputs. The results are evidence that interaction between influences of the structures tested takes place largely at the level of spinal interneurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 4, pp. 391–399, July–August, 1978.     

Locomotion induced by epidural stimulation in a decerebrated cat after spinal cord injury

The effect of partial and complete spinal cord transection (Th7–Th8) on locomotor activity evoked in decerebrated cats by electrical epidural stimulation (segment L5, 80–100 μA, 0.5 ms at 5 Hz) has been investigated. Transection of dorsal columns did not substantially influence the locomotion. Disruption of the ventral spinal quadrant resulted in deterioration and instability of the locomotor rhythm. Injury to lateral or medial descending motor systems led to redistribution of the tone in antagonist muscles. Locomotion could be evoked by epidural stimulation within 20 h after complete transection of the spinal cord. The restoration of polysynaptic components in EMG responses correlated with recovery of the stepping function. The data obtained confirm that initiation of locomotion under epidural stimulation is caused by direct action on intraspinal systems responsible for locomotor regulation. With intact or partially injured spinal cord, this effect is under the influence of supraspinal motor systems correcting and stabilizing the evoked locomotor pattern.     

Contribution of antidromic efferent-fiber excitation to mass spinal potentials in the cat

The contribution of antidromic excitation of motoneurons to cord dorsum potentials (CDP) was studied in the spinal cord of anesthetized cats. It was shown that stimulation of ventral roots (VR) or peripheral nerves following deafferentiation of a number of segments by crosscutting of dorsal roots on the dorsal surface evokes appreciable positive-negative CDP (VR-CDP). Under intact conditions, VR effects of antidromic stimulation of efferent fibers brings appreciable input to the initial "fast" CDP component (the "afferent" peak); input values for the main mixed nerves of the hindlimb are presented. After conditioning stimulation of a mixed nerve, VR-CDP undergo inhibition with two maximums, associated with blocking of the effects of antidromic excitation of efferents by orthodromic mono- and polysynaptic reflex discharges of motoneurons. The hypothesis that intactness of efferents in nerves under stimulation can be determined from an analysis of initial CDP components is stated.Scientific-Research Institute of Biology, Dnepropetrovsk State University, Dnepropetrovsk. Translated from Neirofiziologiya, Vol. 23, No. 6, pp. 655–661, November–December, 1991.     

Effect of noradrenalin and serotonin on synaptic transmission in the rat spinal cord

Experiments on superfused isolated spinal cord preparations from rats aged 8–13 days showed that noradrenal in and serotonin have only a weak effect on monosynaptic reflex discharges but a substantial effect on polysynaptic motoneuronal discharges: noradrenalin potentiates whereas serotonin inhibits them. Both amines inhibit dorsal root potentials evoked by stimulation of high-threshold afferents. Potentiation of polysynaptic motoneuronal discharges induced by noradrenalin is connected with hyperpolarization of high-threshold afferents due to inhibition of the function of neurons in the substantia gelatinosa, and with increased excitability of interneurons participating in the generation of motoneuronal discharges. Serotonin inhibits polysynaptic motoneuronal discharges through its direct depolarizing effect on terminals of high-threshold afferents and depression of interneuron activity responsible for these discharges. Adrenergic and serotonin receptors, mediating these effects of noradrenalin and serotonin, were subjected to pharmacologic analysis.A. M. Gor'kii Donetsk Medical Institute. Translated from Neirofiziologiya, Vol. 14, No. 3, pp. 241–247, May–June, 1982.     

"Phenomenon" of vestibular compensation]

The role of spinal afferentation from the lower part of the body in establishing compensation of the consequences of the vestibular function abaissement was studied in experiments on guinea pigs. The ligation of the spinal cord at the level of thoracic segements performed under local anesthesia neither produced appreciable effect on the compensatory development at simultaneous or subsequent destruction of the labyrinth nor destroyed it in the preliminarily labyrinthectomized animals. The ligation of the spinal cord in the labyrinthectomized animals under ether or chloroform anesthesia was accompanied by a strong disorder of the compensation. The above substances also provoked analogous decompensation in the unilaterally labyrinthectomized animals with an intact spinal cord. The results obtained indicate that the reported disorder of the vestibular compensation induced by ligation of the spinal cord under ether anesthesia is consequent on an immediate effect of inhalation anesthetics on the compensatory mechanisms rather than is resultant of abolishing spinal afferentation from the lower part of the body.     

Morphofunctional characteristics of the rat distal spinal cord after its complete experimental transection with subsequent animal treadmill training

Motor activity of rats was studied after experimental complete transection of the spinal cord at lower thoracic level. Treadmill training 1 day after the surgery was shown to lead to the appearance of movements in hindlimbs and restoration of the body weight support function. According to our data, the key moment in initiation of locomotor movements is stimulation of foot. Morphoimmunohistochemical investigation of the lumbar enlargement (study of proliferating cell nuclear protein, synaptophysin, and glial fibrillary acidic protein immunohistochemistry) revealed a rearrangement of motoneurons, interneurons, and the afferent chain in the distal part of the transected spinal cord. In the trained animals, there was observed the normal structure of motoneurons and the appearance of aggregates of the synaptophysin-immunoreactive structures lost after the surgery.     

Inhibitory effects of reticulospinal fibers of the lateral funiculus on neurons of thoracic spinal reflex pathways

Experiments on anesthetized cats with partial transection of the spinal cord showed that reticulo-spinal fibers in the ventral part of the lateral funiculus participate in the inhibition of polysynaptic reflexes evoked by stimulation of the ipsi- and contralateral reticular formation. The reticulo-fugal wave in the ventrolateral funiculus evoked comparatively short (up to 70 msec) IPSPs in some motoneurons of the internal intercostal nerve investigated and at the same time evoked prolonged (up to 500 msec) inhibition of IPSPs caused by activation of high-threshold segmental afferents. This wave also led to the appearance of IPSPs in 14 of 91 (15.5 %) thoracic spinal interneurons studied. The duration of these IPSPs did not exceed 100 msec; meanwhile, segment excitatory responses of 21 of 43 interneurons remained partly suppressed for 120–500 msec. It is concluded that the inhibitory action of the lateral reticulo-spinal system on segmental reflexes is due to several synaptic mechanisms, some of them unconnected with hyperpolarization of spinal neurons. The possible types of mechanisms of this inhibition are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 162–172, March–April, 1978.     

Peculiarities of spinal hyperreflexia after combined synchronous transection of the sciatic nerve and chordotomy in rats

Parameters of the reflex discharges evoked by spinal dorsal root stimulation were measured in rats with the sciatic nerve and spinal cord (at low thorasic level) transected five days earlier. Monosynaptic discharges in the ventral roots were found to increase after the operation; the degree of increase was significantly higher as compared with that observed after isolated transections of the spinal cord or the nerve. The combined lesion of the nerve and spinal cord could result in the appearance of high-amplitude reflex discharge components, probably of a polysynaptic nature. We concluded, from the comparison of modifications of reflex discharges, that the mechanisms underlying spinal hyperreflexia after nerve or spinal cord lesions differ considerably from each other.Neirofiziologiya/Neurophysiology, Vol. 26, No. 3, pp. 197–202, May–June, 1994.