Contribution of Persistent Sodium Current to Locomotor Pattern Generation in Neonatal Rats

2007 ◽  
Vol 98 (2) ◽  
pp. 613-628 ◽  
Author(s):  
Sabrina Tazerart ◽  
Jean-Charles Viemari ◽  
Pascal Darbon ◽  
Laurent Vinay ◽  
Frédéric Brocard
Keyword(s):  
Spinal Cord ◽  
Sodium Current ◽  
Input Resistance ◽  
Firing Frequency ◽  
Pattern Generation ◽  
Neonatal Rat ◽  
Persistent Sodium Current ◽  
Locomotor Pattern ◽  
Functional Features

The persistent sodium current ( INaP) is known to play a role in rhythm generation in different systems. Here, we investigated its contribution to locomotor pattern generation in the neonatal rat spinal cord. The locomotor network is mainly located in the ventromedial gray matter of upper lumbar segments. By means of whole cell recordings in slices, we characterized membrane and INaP biophysical properties of interneurons located in this area. Compared with motoneurons, interneurons were more excitable, because of higher input resistance and membrane time constant, and displayed lower firing frequency arising from broader spikes and longer AHPs. Ramp voltage-clamp protocols revealed a riluzole- or TTX-sensitive inward current, presumably INaP, three times smaller in interneurons than in motoneurons. However, in contrast to motoneurons, INaP mediated a prolonged plateau potential in interneurons after reducing K+ and Ca2+ currents. We further used in vitro isolated spinal cord preparations to investigate the contribution of INaP to locomotor pattern. Application of riluzole (10 μM) to the whole spinal cord or to the upper lumbar segments disturbed fictive locomotion, whereas application of riluzole over the caudal lumbar segments had no effect. The effects of riluzole appeared to arise from a specific blockade of INaP because action potential waveform, dorsal root–evoked potentials, and miniature excitatory postsynaptic currents were not affected. This study provides new functional features of ventromedial interneurons, with the first description of INaP-mediated plateau potentials, and new insights into the operation of the locomotor network with a critical implication of INaP in stabilizing the locomotor pattern.

Extracellular K+ Induces Locomotor-Like Patterns in the Rat Spinal Cord In Vitro: Comparison With NMDA or 5-HT Induced Activity

1998 ◽  
Vol 79 (5) ◽  
pp. 2643-2652 ◽  
Author(s):  
E. Bracci ◽  
M. Beato ◽  
A. Nistri
Keyword(s):  
Spinal Cord ◽  
Threshold Concentration ◽  
Pattern Generation ◽  
Neonatal Rat ◽  
Rat Spinal Cord ◽  
Motor Patterns ◽  
Pattern Frequency ◽  
High K ◽  
In Vitro Comparison

Bracci, E., M. Beato, and A. Nistri. Extracellular K+ induces locomotor-like patterns in the rat spinal cord in vitro: comparison with NMDA or 5-HT induced activity. J. Neurophysiol. 79: 2643–2652, 1998. Bath-application of increasing concentrations of extracellular K+ elicited alternating motor patterns recorded from pairs of various lumbar ventral roots of the neonatal rat (0–2 days old) spinal cord in vitro. The threshold concentration of K+ for this effect was 7.9 ± 0.8 mM (mean ± SD). The suprathreshold concentration range useful to evoke persistent motor patterns (lasting ≥10 min) was very narrow (∼1 mM) as further increments elicited only rhythmic activity lasting from 20 s to a few minutes. On average, the fastest period of rhythmic patterns was 1.1 ± 0.3 s. Intracellular recording from lumbar motoneurons showed that raised extracellular K+ elicited membrane potential oscillations with superimposed repetitive firing. In the presence of N-methyl-d-aspartate (NMDA) or non-NMDA receptor blockers [ R(−)-2-amino-phosphonovaleric acid or 6-cyano-7-nitroquinoxaline-2,3-dione, respectively] extracellular K+ increases could still induce motor patterns although the threshold concentration was raised. Serotonin (5-HT) also induced alternating motor patterns (threshold 15 ± 7 μM) that were consistently slower than those induced by high K+ or NMDA. Ritanserin (1 μM) prevented the locomotor-like activity of 5-HT but not that of high K+ provided the concentration of the latter was further increased. Subthreshold concentrations of K+ became effective in the presence of subthreshold doses of 5-HT or NMDA, indicating mutual facilitation between these substances. The fastest pattern frequency was observed by raising K+ or by adding NMDA. In the presence of 5-HT, the pattern frequency was never as fast even if NMDA (or high K+) was coapplied. Furthermore, application of 5-HT significantly slowed down the K+- or NMDA-induced rhythm, an effect strongly potentiated in the presence of ritanserin. It is suggested that the operation of the spinal locomotor network was activated by rises in extracellular K+, which presumably led to a broad increase in neuronal excitability. Whenever the efficiency of excitatory synaptic transmission was diminished (for example by glutamate receptor antagonism), a larger concentration of K+ was required to evoke locomotor-like patterns. The complex effect (comprising stimulation and inhibition) of 5-HT on alternating pattern generation appeared to result from a dual action of this substance on the spinal locomotor network.

scholarly journals Calpain fosters the hyperexcitability of motoneurons after spinal cord injury and leads to spasticity

eLife ◽  
2019 ◽  
Vol 8 ◽  
Author(s):  
Vanessa Plantier ◽  
Irene Sanchez-Brualla ◽  
Nejada Dingu ◽  
Cécile Brocard ◽  
Sylvie Liabeuf ◽  
...  
Keyword(s):  
Spinal Cord ◽  
Spinal Cord Injury ◽  
Sodium Current ◽  
Neonatal Rat ◽  
Down Regulation ◽  
Involuntary Muscle ◽  
Cord Injury ◽  
Nav Channels ◽  
Lumbar Spinal

Up-regulation of the persistent sodium current (INaP) and down-regulation of the potassium/chloride extruder KCC2 lead to spasticity after spinal cord injury (SCI). We here identified calpain as the driver of the up- and down-regulation of INaP and KCC2, respectively, in neonatal rat lumbar motoneurons. Few days after SCI, neonatal rats developed behavioral signs of spasticity with the emergence of both hyperreflexia and abnormal involuntary muscle contractions on hindlimbs. At the same time, in vitro isolated lumbar spinal cords became hyperreflexive and displayed numerous spontaneous motor outputs. Calpain-I expression paralleled with a proteolysis of voltage-gated sodium (Nav) channels and KCC2. Acute inhibition of calpains reduced this proteolysis, restored the motoneuronal expression of Nav and KCC2, normalized INaP and KCC2 function, and curtailed spasticity. In sum, by up- and down-regulating INaP and KCC2, the calpain-mediated proteolysis of Nav and KCC2 drives the hyperexcitability of motoneurons which leads to spasticity after SCI.

Neuronal Bursting Induced by NK3 Receptor Activation in the Neonatal Rat Spinal Cord In Vitro

2001 ◽  
Vol 86 (6) ◽  
pp. 2939-2950 ◽  
Author(s):  
Cristina Marchetti ◽  
Andrea Nistri
Keyword(s):  
Spinal Cord ◽  
Membrane Potential ◽  
Dorsal Root ◽  
Extracellular Recording ◽  
Rhythmic Activity ◽  
Input Resistance ◽  
Receptor Activation ◽  
Neonatal Rat ◽  
Rat Spinal Cord

Intracellular recording from lumbar motoneurons and extracellular recording from ventral roots of the neonatal rat isolated spinal cord were used to study the mechanisms responsible for the excitation mediated by NK3 tachykinin receptors. The selective NK3 agonists senktide or [MePhe7]neurokinin B induced a slow depolarization with superimposed oscillations (mean period ± SD was 2.8 ± 0.8 s) that, in the majority of cases, showed left-right alternation at segmental level and were synchronous between L2 and L5 of the same side. During agonist wash out (5–20 min) a delayed form of hyperexcitability emerged consisting of bursts lasting 8 ± 2 s (average interburst interval 55 ± 21 s) with superimposed oscillations usually with homosegmental alternation and heterosegmental synchronicity. Such bursting was accompanied by depression of GABAergic dorsal root potentials evoked by dorsal root stimulation and of the recurrent inhibitory postsynaptic potential recorded from motoneurons. Despite bursting, motoneuron membrane potential returned to baseline while input resistance was increased. Bursts were a network-dependent phenomenon triggered by previous NK3 receptor activation because bursting was suppressed by glutamate receptor antagonists and was insensitive to motoneuron membrane potential or subsequent application of an NK3 receptor antagonist. NK3 receptors operated synergistically with N-methyl-d-aspartate (NMDA) and 5-hydroxytryptamine (5-HT) to trigger fully alternating locomotor-like rhythms while NK3 receptor antagonism disrupted the same rhythm. In summary, in the neonatal rat spinal cord NK3 receptors could trigger rhythmic activity predominantly with alternation at segmental level but with synchronous coupling between ipsilateral motor pools. NK3receptor activation could also facilitate fictive locomotor patterns induced by NMDA and 5-HT.

scholarly journals Regional Distribution of the Locomotor Pattern-Generating Network in the Neonatal Rat Spinal Cord

1997 ◽  
Vol 77 (1) ◽  
pp. 247-259 ◽  
Author(s):  
K. C. Cowley ◽  
B. J. Schmidt
Keyword(s):  
Spinal Cord ◽  
Regional Distribution ◽  
Rhythmic Activity ◽  
Neonatal Rat ◽  
Lumbar Region ◽  
Rat Spinal Cord ◽  
Locomotor Pattern ◽  
Thoracic Cord ◽  
Lumbar Cord

Cowley, K. C. and B. J. Schmidt. Regional distribution of the locomotor pattern-generating network in the neonatal rat spinal cord. J. Neurophysiol. 77: 247–259, 1997. The regional distribution of spinal cord networks producing locomotor-like, as well as non-locomotor-like, activity was studied with the use of an in vitro neonatal rat preparation. Rhythmic activity was induced by bath application of either serotonin (5-HT), acetylcholine (ACh), N-methyl-d,l-aspartate (NMA), or combined 5-HT/NMA, and was monitored via hindlimb flexor (peroneal) and extensor (tibial) electroneurograms (ENGs) or ventral root recordings. In some experiments, synchronous patterns were produced by the addition of inhibitory amino acid (IAA) receptor antagonists. Selective application of 5-HT to cervical and thoracic cord regions induced rhythmic activity in these segments but failed to evoke hindlimb ENG discharge. Exposure of the isolated lumbar region to 5-HT produced tonic activity only. Application of 5-HT to the whole cord produced locomotor-like activity in hindlimb ENGs that persisted after midsagittal section of the spinal cord from the conus to the thoracolumbar junction. In other experiments, transverse hemisection of the rostral lumbar cord during whole cord exposure to 5-HT abolished rhythmic activity in ipsilateral hindlimb ENGs, suggesting that under these conditions rhythmic activity on one side of the lumbar cord was insufficient to maintain rhythmic activity on the contralateral side. Selective application of NMA or ACh to cervical and/or thoracic cord regions evoked rhythmic activity in these supralumbar segments, as well as rhythmic, but non-locomotor-like, activity in the lumbar region. In contrast to the effect of 5-HT, both NMA and ACh evoked rhythmic activity when applied solely to the lumbar region, and the side-to-side alternation produced by whole cord ACh application was uncoupled by midsagittal lesions of the lumbar region. In the presence of IAA antagonists, the side-to-side coupling of bilaterally synchronous rhythms was maintained despite extensive midsagittal lesions leaving all but one or two segments of either cervical, thoracic, or lumbar cord bilaterally intact, and rhythmic activity could be maintained even in single isolated hemisegments. The effects of 5-HT/NMA were similar to those observed with the use of 5-HT alone, although 5-HT/NMA induced rhythmic activity in hindlimb ENGs when applied selectively to supralumbar regions. The results suggest that 1) a 5-HT-sensitive oscillatory network, capable of producing a locomotor-like pattern of activity, is distributed throughout the supralumbar region of the spinal cord and mediates descending rhythmic drive to lumbar motor centers; 2) NMA- and ACh-sensitive rhythmogenic elements are distributed throughout the spinal cord, including the lumbar region; and 3) the spinal cord contains an extensive propriospinal network of reciprocal inhibitory and excitatory connections characterized by redundantly organized side-to-side projections.

Whole Cell Recordings of Lumbar Motoneurons During Locomotor-Like Activity in the In Vitro Neonatal Rat Spinal Cord

1998 ◽  
Vol 79 (2) ◽  
pp. 743-752 ◽  
Author(s):  
S. Hochman ◽  
B. J. Schmidt
Keyword(s):  
Spinal Cord ◽  
Membrane Potential ◽  
Rhythmic Activity ◽  
Input Resistance ◽  
Membrane Depolarization ◽  
Neonatal Rat ◽  
Rat Spinal Cord ◽  
Whole Cell ◽  
Whole Cell Recordings

Hochman, S. and B. J. Schmidt. Whole cell recordings of lumbar motoneurons during locomotor-like activity in the in vitro neonatal rat spinal cord. J. Neurophysiol. 79: 743–752, 1998. Whole cell current- and voltage-clamp recordings were obtained from lumbar motoneurons in the isolated neonatal rat spinal cord to characterize the behavior of motoneurons during neurochemically induced locomotor-like activity. Bath application of serotonin (10–100 μM) in combination with N-methyl-d-aspartate (1–12 μM) initially produced tonic membrane depolarization (mean = 26 mV), increased input resistance, decreased rheobase, and increased spike inactivation in response to depolarizing current pulse injections. After the initial tonic depolarization, rhythmic fluctuations of the motoneuron membrane potential (locomotor drive potentials; LDPs) developed that were modulated phasically in association with ventral root discharge. The peak and trough voltage levels of the LDP fluctuated above and below the membrane potential recorded immediately before the onset of rhythmic activity. Similarly, firing frequency was modulated above and below prelocomotion firing rates (in those motoneurons that displayed neurochemically induced tonic firing immediately before the onset of rhythmic activity). These observations are consistent with an alternation between phasic excitatory and inhibitory synaptic drives. The amplitude of LDPs and rhythmic excitatory drive current increased with membrane depolarization from −80 to −40 mV and then decreased with further depolarization, thus displaying nonlinear voltage-dependence. Faster frequency, small amplitude voltage fluctuations were observed superimposed on the depolarized phase of LDPs. In some motoneurons, the trajectory of these superimposed fluctuations was consistent with a synaptic origin, whereas in other cells, the regular sinusoidal appearance of the fluctuations and the occurrence of superimposed plateau potentials were more compatible with the activation of an intrinsic membrane property. One motoneuron displayed exclusively excitatory phasic drive, and another motoneuron was characterized by inhibitory phasic drive alone, during rhythmic activity. These findings are compatible with the concept of a central pattern generator that is capable of delivering both excitatory and inhibitory drive to motoneurons during locomotion. The data also suggest that the rhythmic excitatory and inhibitory outputs of the hypothetical half-center model can be dissociated and operate in isolation.

scholarly journals 5-HT2 Receptor Activation Facilitates a Persistent Sodium Current and Repetitive Firing in Spinal Motoneurons of Rats With and Without Chronic Spinal Cord Injury

2006 ◽  
Vol 96 (3) ◽  
pp. 1158-1170 ◽  
Author(s):  
P. J. Harvey ◽  
X. Li ◽  
Y. Li ◽  
D. J. Bennett
Keyword(s):  
Spinal Cord ◽  
Sodium Current ◽  
Receptor Activation ◽  
Repetitive Firing ◽  
Persistent Sodium Current ◽  
Spinal Motoneurons ◽  
Spinal Transection ◽  
Acute Spinal Rats ◽  
Chronic Spinal Rats

We examined the modulation of persistent inward currents (PICs) by serotonin (5-HT) in spinal motoneurons of normal and chronic spinal rats. PICs are composed of both a TTX-sensitive persistent sodium current (Na PIC) and a nimodipine-sensitive persistent calcium current (Ca PIC), and we focused on quantifying the Na PIC (and its action on the total PIC), which is known to be critical in enabling repetitive firing. Intracellular recordings were made from motoneurons of the whole sacrocaudal spinal cord of normal adult rats after the cord was acutely transected at the S2 spinal level (acute spinal rat condition), removed from the animal, and then maintained in vitro. In vitro motoneuron recordings were likewise made from rats that had a sacral spinal transection 2 mo previously (chronic spinal rats). In motoneurons from acute spinal rats, moderately high doses of 5-HT (≥10 μM), or the 5-HT2 receptor agonist DOI (≥30 μM), significantly increased the total PIC, hyperpolarized the PIC onset voltage, and hyperpolarized the spike threshold, whereas lower doses had no effect. Both 5-HT and DOI specifically increased the Na PIC portion of the total PIC (tested with nimodipine blocking the Ca PIC). Additionally, 5-HT, but not DOI, depolarized the resting membrane potential ( Vm) and increased the input resistance ( Rm) in a dose-dependent manner. Therefore 5-HT2 receptor activation facilitated the Na PIC, whereas other 5-HT receptors modulated Vm and Rm. Motoneurons of chronic spinal rats responded to 5-HT and DOI in the same way, but with larger responses and at much lower doses (0.3–1 μM), thus exhibiting a 30-fold supersensitivity to 5-HT. Specifically the Na PIC was supersensitive to 5-HT2 receptor activation with DOI. Also, Rm and Vm were supersensitive to 5-HT. Consistent with the known critical role of the Na PIC in repetitive firing, enhancement of the Na PIC by DOI or 5-HT facilitated the repetitive firing evoked by steady current injection and enabled repetitive firing in a subpopulation of motoneurons of acute spinal rats that were initially unable to produce sustained repetitive firing. We suggest that after spinal transection, residual endogenous spinal sources of 5-HT help facilitate the Na PIC and repetitive firing. With chronic injury, the developed 5-HT supersensitivity more than compensates for lost brain stem 5-HT, so that the Na PIC is large and motoneurons are very excitable, thus contributing to spasticity.

The in vitro neonatal rat spinal cord preparation: a new insight into mammalian locomotor mechanisms

2004 ◽  
Vol 190 (5) ◽  
pp. 343-357 ◽  
Author(s):  
F. Clarac ◽  
E. Pearlstein ◽  
J. F. Pflieger ◽  
L. Vinay
Keyword(s):  
Spinal Cord ◽  
Neonatal Rat ◽  
Rat Spinal Cord ◽  
Insight Into

Sodium Currents in Neurons From the Rostroventrolateral Medulla of the Rat

2003 ◽  
Vol 90 (3) ◽  
pp. 1635-1642 ◽  
Author(s):  
Ilya A. Rybak ◽  
Krzysztof Ptak ◽  
Natalia A. Shevtsova ◽  
Donald R. McCrimmon
Keyword(s):  
Sodium Channels ◽  
Sodium Current ◽  
Cell Voltage ◽  
Kinetic Properties ◽  
Persistent Sodium Current ◽  
Sodium Currents ◽  
Bötzinger Complex ◽  
Window Current ◽  
Bursting Activity

Rapidly inactivating and persistent sodium currents have been characterized in acutely dissociated neurons from the area of rostroventrolateral medulla that included the pre-Bötzinger Complex. As demonstrated in many studies in vitro, this area can generate endogenous rhythmic bursting activity. Experiments were performed on neonate and young rats (P1-15). Neurons were investigated using the whole cell voltage-clamp technique. Standard activation and inactivation protocols were used to characterize the steady-state and kinetic properties of the rapidly inactivating sodium current. Slow depolarizing ramp protocols were used to characterize the noninactivating sodium current. The “window” component of the rapidly inactivating sodium current was calculated using mathematical modeling. The persistent sodium current was revealed by subtraction of the window current from the total noninactivating sodium current. Our results provide evidence of the presence of persistent sodium currents in neurons of the rat rostroventrolateral medulla and determine voltage-gated characteristics of activation and inactivation of rapidly inactivating and persistent sodium channels in these neurons.

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