Motoneuron output regulated by ionic channels: a modeling study of motoneuron frequency-current relationships during fictive locomotion

Exercise-induced adaptation of neurons in the vertebrate locomotor system

Vertebrate neurons are highly dynamic cells that undergo several alterations in their functioning and physiologies in adaptation to various external stimuli. In particular, how these neurons respond to physical exercise has long been an area of active research. Studies of the vertebrate locomotor system's adaptability suggest multiple mechanisms are involved in the regulation of neuronal activity and properties during exercise. In this brief review, we highlight recent results and insights from the field with a focus on the following mechanisms: (a) alterations in neuronal excitability during acute exercise; (b) alterations in neuronal excitability after chronic exercise; (c) exercise-induced changes in neuronal membrane properties via modulation of ion channel activity; (d) exercise-enhanced dendritic plasticity; and (e) exercise-induced alterations in neuronal gene expression and protein synthesis. Our hope is to update the community with a cellular and molecular understanding of the recent mechanisms underlying the adaptability of the vertebrate locomotor system in response to both acute and chronic physical exercise.

Signaling pathways regulated by natural active ingredients in the fight against exercise fatigue-a review

Exercise fatigue is a normal protective mechanism of the body. However, long-term fatigue hinders normal metabolism and exercise capacity. The generation and recovery from exercise fatigue involves alterations in multiple signaling pathways, mainly AMPK, PI3K/Akt, Nrf2/ARE, NF-κB, PINK1/Parkin, and BDNF/TrkB, as well as MAPK signaling pathways that mediate energy supply, reduction of metabolites, oxidative stress homeostasis, muscle fiber type switching, and central protective effects. In recent studies, a rich variety of natural active ingredients have been identified in traditional Chinese medicines and plant extracts with anti-fatigue effects, opening up the field of research in new anti-fatigue drugs. In this review we give an overview of the signaling pathways associated with the activity of natural food active ingredients against exercise fatigue. Such a comprehensive review is necessary to understand the potential of these materials as preventive measures and treatments of exercise fatigue. We expect the findings highlighted and discussed here will help guide the development of new health products and provide a theoretical and scientific basis for future research on exercise fatigue.

The effects of membrane potential oscillations on the excitability of rat hypoglossal motoneurons

Oscillations in membrane potential induced by synaptic inputs and intrinsic ion channel activity play a role in regulating neuronal excitability, but the precise mechanisms underlying their contributions remain largely unknown. Here we used electrophysiological and modeling approaches to investigate the effects of Gaussian white noise injected currents on the membrane properties and discharge characteristics of hypoglossal (HG) motoneurons in P16-21 day old rats. We found that the noise-induced membrane potential oscillations facilitated spike initiation by hyperpolarizing the cells’ voltage threshold by 3.1 ± 1.0 mV and reducing the recruitment current for the tonic discharges by 0.26 ± 0.1 nA, on average (n = 59). Further analysis revealed that the noise reduced both recruitment and decruitment currents by 0.26 ± 0.13 and 0.33 ± 0.1 nA, respectively, and prolonged the repetitive firing. The noise also increased the slopes of frequency-current (F-I) relationships by 1.1 ± 0.2 Hz/nA. To investigate the potential mechanisms underlying these findings, we constructed a series of HG motoneuron models based on their electrophysiological properties. The models consisted of five compartments endowed with transient sodium (NaT), delayed-rectify potassium [K(DR)], persistent sodium (NaP), calcium-activated potassium [K(AHP)], L-type calcium (CaL) and H-current channels. In general, all our experimental results could be well fitted by the models, however, a modification of standard Hodgkin-Huxley kinetics was required to reproduce the changes in the F-I relationships and the prolonged discharge firing. This modification, corresponding to the noise generated by the stochastic flicker of voltage-gated ion channels (channel flicker, CF), was an adjustable sinusoidal function added to kinetics of the channels that increased their sensitivity to subthreshold membrane potential oscillations. Models with CF added to NaP and CaL channels mimicked the noise-induced alterations of membrane properties, whereas models with CF added to NaT and K(DR) were particularly effective in reproducing the noise-induced changes for repetitive firing observed in the real motoneurons. Further analysis indicated that the modified channel kinetics enhanced NaP- and CaL-mediated inward currents thus increasing the excitability and output of HG motoneurons, whereas they produced relatively small changes in NaT and K(DR), thus balancing these two currents and triggering variability of repetitive firing. This study provided insight into the types of membrane channel mechanisms that might underlie oscillation-induced alterations of neuronal excitability and motor output in rat HG motoneurons.

Chronic exercise increases excitability of lamina X neurons through enhancement of persistent inward currents and dendritic development in mice

KEY POINTS

Chronic exercise alters adaptability of spinal motor system in rodents. Multiple mechanisms are responsible for the adaptation, including regulation of neuronal excitability and change in dendritic morphology. Spinal interneurons in lamina X are a cluster of heterogeneous neurons playing multifunctional roles in the spinal cord, especially in regulating locomotor activity. Chronic exercise in juvenile mice increased excitability of these interneurons and facilitated dendritic development. Mechanisms underlying these changes remain unknown. Lamina X neurons expressed persistent inward currents (PICs) composed of calcium (Ca-PIC) and sodium (Na-PIC) components. The exercise-increased excitability of lamina X neurons was mediated by enhancing Ca-PIC and Na-PIC components and facilitating dendritic length. Na-PIC contributed more to lowering of PIC onset and Ca-PIC to increase of PIC amplitude. This study unveiled novel morphological and ionic mechanisms underlying adaptation of lamina X neurons in rodents during chronic exercise.

ABSTRACT

Chronic exercise has been shown to enhance excitability of spinal interneurons in rodents. However, the mechanisms underlying this enhancement remain unclear. In this study we investigated adaptability of lamina X neurons with three-week treadmill exercise in mice of P21-P24. Whole-cell path-clamp recording was performed on the interneurons from slices of T12-L4. The experimental results included: (1) Treadmill exercise reduced rheobase by 7.4±2.2 pA (control: 11.3±6.1 pA, n = 12; exercise: 3.8±4.6 pA, n = 13; P = 0.002) and hyperpolarized voltage threshold by 7.1±1.5 mV (control: -36.6±4.6 mV, exercise: -43.7±2.7 mV; P = 0.001). (2) Exercise enhanced persistent inward currents (PICs) with increase of amplitude (control: 140.6±56.3 pA, n = 25; exercise: 225.9±62.5 pA, n = 17; P = 0.001) and hyperpolarization of onset (control: -50.3±3.6 mV, exercise: -56.5±5.5 mV; P = 0.001). (3) PICs consisted of dihydropyridine-sensitive calcium (Ca-PIC) and tetrodotoxin-sensitive sodium (Na-PIC) components. Exercise increased amplitude of both components but hyperpolarized onset of Na-PIC only. (4) Exercise reduced derecruitment current of repetitive firing evoked by current bi-ramp and prolonged firing in falling phase of the bi-ramp. The derecruitment reduction was eliminated by bath application of 3 μM riluzole or 25 μM nimodipine, suggesting that both Na-PIC and Ca-PIC contributed to the exercise-prolonged hysteresis of firing. (5) Exercise facilitated dendritic development with significant increase in dendritic length by 285.1±113 μm (control: 457.8±171.8 μm, n = 12; exercise: 742.9±357 μm, n = 14; P = 0.019). We concluded that three-week treadmill exercise increased excitability of lamina X interneurons through enhancement of PICs and increase of dendritic length. This study provided insight into cellular and channel mechanisms underlying adaptation of the spinal motor system in exercise. Abstract figure legend A. B6 mice were randomly divided into control group and exercise group. Control group mice remained sedentary in the cage; exercise group mice completed 60 min treadmill runs each day (6 days/week) for a period of 3 weeks. B. Whole-cell patch clamp recordings were made from lumbar lamina X neurons after three-weeks exercise. C. Exercise facilitated development of dendrites of lamina X neurons. D. Exercise enhanced persistent inward currents. E. Exercise increased excitability of lamina X neurons by hyperpolarizing voltage threshold for action potential generation. This article is protected by copyright. All rights reserved.

Locomotor Pattern and Force Generation Modulated by Ionic Channels: A Computational Study of Spinal Networks Underlying Locomotion

Locomotion is a fundamental movement in vertebrates produced by spinal networks known as central pattern generators (CPG). During fictive locomotion cat lumbar motoneurons (MNs) exhibit changes in membrane properties, including hyperpolarization of voltage threshold, reduction of afterhyperpolarization and input resistance, and amplification of nonlinear membrane properties. Both modeling and electrophysiological studies suggest that these changes can be produced by upregulating voltage-gated sodium channel (VGSC), persistent sodium (NaP), or L-type calcium channel (LTCC) or downregulating delayed-rectifier potassium (K(DR)) or calcium-dependent potassium channel (KCa) in spinal MNs. Further studies implicate that these channel modulations increase motor output and facilitate MN recruitment. However, it remains unknown how the channel modulation of CPG networks or MN pools affects the rhythmic generation of locomotion and force production of skeletal muscle during locomotion. In order to investigate this issue, we built a two-level CPG model composed of excitatory interneuron pools (Exc-INs), coupled reciprocally with inhibitory interneuron pools (Inh-INs), and projected to the flexor-extensor MN pools innervating skeletal muscles. Each pool consisted of 100 neurons with membrane properties based on cat spinal neurons. VGSC, K(DR), NaP, KCa, LTCC, and H-current channels were included in the model. Simulation results showed that (1) upregulating VGSC, NaP, or LTCC or downregulating KCa in MNs increased discharge rate and recruitment of MNs, thus facilitating locomotor pattern formation, increased amplitude of electroneurogram (ENG) bursting, and enhanced force generation of skeletal muscles. (2) The same channel modulation in Exc-INs increased the firing frequency of the Exc-INs, facilitated rhythmic generation, and increased flexor-extensor durations of step cycles. (3) Contrarily, downregulation of NaP or LTCC in MNs or Exc-INs or both CPG (Exc-INs and Inh-INs) and MNs disrupted locomotor pattern and reduced or even blocked the ENG bursting of MNs and force generation of skeletal muscles. (4) Pharmacological experiments showed that bath application of 25 μM nimodipine or 2 μM riluzole completely blocked fictive locomotion in isolated rat spinal cord, consistent with simulation results. We concluded that upregulation of VGSC, NaP, or LTCC or downregulation of KCa facilitated rhythmic generation and force production during walking, with NaP and LTCC playing an essential role.

KCNQ Current Contributes to Inspiratory Burst Termination in the Pre-Bötzinger Complex of Neonatal Rats in vitro

The pre-Bötzinger complex (preBötC) of the ventral medulla generates the mammalian inspiratory breathing rhythm. When isolated in explants and deprived of synaptic inhibition, the preBötC continues to generate inspiratory-related rhythm. Mechanisms underlying burst generation have been investigated for decades, but cellular and synaptic mechanisms responsible for burst termination have received less attention. KCNQ-mediated K+ currents contribute to burst termination in other systems, and their transcripts are expressed in preBötC neurons. Therefore, we tested the hypothesis that KCNQ channels also contribute to burst termination in the preBötC. We recorded KCNQ-like currents in preBötC inspiratory neurons in neonatal rat slices that retain respiratory rhythmicity. Blocking KCNQ channels with XE991 or linopirdine (applied via superfusion or locally) increased inspiratory burst duration by 2- to 3-fold. By contrast, activation of KCNQ with retigabine decreased inspiratory burst duration by ~35%. These data from reduced preparations suggest that the KCNQ current in preBötC neurons contributes to inspiratory burst termination.

Serotonergic Modulation of Persistent Inward Currents in Serotonergic Neurons of Medulla in ePet-EYFP Mice

Serotonergic (5-HT) neurons in the medulla play multiple functional roles associated with many symptoms and motor activities. The descending serotonergic pathway from medulla is essential for initiating locomotion. However, the ionic properties of 5-HT neurons in the medulla remain unclear. Using whole-cell patch-clamp technique, we studied the biophysical and modulatory properties of persistent inward currents (PICs) in 5-HT neurons of medulla in ePet-EYFP transgenic mice (P3–P6). PICs were recorded by a family of voltage bi-ramps (10-s duration, 40-mV peak step), and the ascending and descending PICs were mirrored to analyze the PIC hysteresis. PICs were found in 77% of 5-HT neurons (198/258) with no significant difference between parapyramidal region (n = 107) and midline raphe nuclei (MRN) (n = 91) in either PIC onset (−47.4 ± 10 mV and −48.7 ± 7 mV; P = 0.44) or PIC amplitude (226.9 ± 138 pA and 259.2 ± 141 pA; P = 0.29). Ninety-six percentage (191/198) of the 5-HT neurons displayed counterclockwise hysteresis and four percentage (7/198) exhibited the clockwise hysteresis. The composite PICs could be differentiated as calcium component (Ca_PIC) by bath application of nimodipine (25 μM), sodium component (Na_PIC) by tetrodotoxin (TTX, 2 μM), and TTX- and dihydropyridine-resistance component (TDR_PIC) by TTX and nimodipine. Ca_PIC, Na_PIC and TDR_PIC all contributed to upregulation of excitability of 5-HT neurons. 5-HT (15 μM) enhanced the PICs, including a 26% increase in amplitude of the compound currents of Ca_PIC and TDR_PIC (P < 0.001, n = 9), 3.6 ± 5 mV hyperpolarization of Na_PIC and TDR_PIC onset (P < 0.05, n = 12), 30% increase in amplitude of TDR_PIC (P < 0.01), and 2.0 ± 3 mV hyperpolarization of TDR_PIC onset (P < 0.05, n = 18). 5-HT also facilitated repetitive firing of 5-HT neurons through modulation of composite PIC, Na_PIC and TDR_PIC, and Ca_PIC and TDR_PIC, respectively. In particular, the high voltage-activated TDR_PIC facilitated the repetitive firing in higher membrane potential, and this facilitation could be amplified by 5-HT. Morphological data analysis indicated that the dendrites of 5-HT neurons possessed dense spherical varicosities intensively crossing 5-HT neurons in medulla. We characterized the PICs in 5-HT neurons and unveiled the mechanism underlying upregulation of excitability of 5-HT neurons through serotonergic modulation of PICs. This study provided insight into channel mechanisms responsible for the serotonergic modulation of serotonergic neurons in brainstem.

Three-Week Treadmill Exercise Enhances Persistent Inward Currents, Facilitates Dendritic Plasticity, and Upregulates the Excitability of Dorsal Raphe Serotonin Neurons in ePet-EYFP Mice

Exercise plays a key role in preventing or treating mental or motor disorders caused by dysfunction of the serotonergic system. However, the electrophysiological and ionic channel mechanisms underlying these effects remain unclear. In this study, we investigated the effects of 3-week treadmill exercise on the electrophysiological and channel properties of dorsal raphe nucleus (DRN). Serotonin (5-HT) neurons in ePet-EYFP mice, using whole-cell patch clamp recording. Treadmill exercise was induced in ePet-EYFP mice of P21–24 for 3 weeks, and whole-cell patch clamp recording was performed on EYFP-positive 5-HT neurons from DRN slices of P42–45 mice. Experiment data showed that 5-HT neurons in the DRN were a heterogeneous population with multiple firing patterns (single firing, phasic firing, and tonic firing). Persistent inward currents (PICs) with multiple patterns were expressed in 5-HT neurons and composed of Cav1.3 (Ca-PIC) and sodium (Na-PIC) components. Exercise hyperpolarized the voltage threshold for action potential (AP) by 3.1 ± 1.0 mV (control: n = 14, exercise: n = 18, p = 0.005) and increased the AP amplitude by 6.7 ± 3.0 mV (p = 0.031) and firing frequency by more than 22% especially within a range of current stimulation stronger than 70 pA. A 3-week treadmill exercise was sufficient to hyperpolarize PIC onset by 2.6 ± 1.3 mV (control: −53.4 ± 4.7 mV, n = 28; exercise: −56.0 ± 4.7 mV, n = 25, p = 0.050) and increase the PIC amplitude by 28% (control: 193.6 ± 81.8 pA; exercise: 248.5 ± 105.4 pA, p = 0.038). Furthermore, exercise hyperpolarized Na-PIC onset by 3.8 ± 1.8 mV (control: n = 8, exercise: n = 9, p = 0.049) and increased the Ca-PIC amplitude by 23% (p = 0.013). The exercise-induced enhancement of the PIC amplitude was mainly mediated by Ca-PIC and hyperpolarization of PIC onset by Na-PIC. Moreover, exercise facilitated dendritic plasticity, which was shown as the increased number of branch points by 1.5 ± 0.5 (p = 0.009) and dendritic branches by 2.1 ± 0.6 (n = 20, p = 0.001) and length by 732.0 ± 100.1 μm (p < 0.001) especially within the range of 50–200 μm from the soma. Functional analysis suggested that treadmill exercise enhanced Na-PIC for facilitation of spike initiation and Ca-PIC for regulation of repetitive firing. We concluded that PICs broadly existed in DRN 5-HT neurons and could influence serotonergic neurotransmission in juvenile mice and that 3-week treadmill exercise induced synaptic adaptations, enhanced PICs, and thus upregulated the excitability of the 5-HT neurons.

A modeling study of spinal motoneuron recruitment regulated by ionic channels during fictive locomotion

During fictive locomotion cat lumbar motoneurons exhibit changes in membrane proprieties including a decrease in voltage threshold (V_th), afterhyperpolarization (AHP) and input resistance (R_in) and an increase in non-linear membrane property. The impact of these changes on the motoneuron recruitment remains unknown. Using modeling approach we investigated the channel mechanism regulating the motoneuron recruitment. Three types of motoneuron pools including slow (S), fatigue-resistant (FR) and fast-fatigable (FF) motoneurons were constructed based on the membrane proprieties of cat lumbar motoneurons. The transient sodium (NaT), persistent sodium (NaP), delayed-rectifier potassium [K(DR)], Ca²⁺-dependent K⁺ [K(AHP)] and L-type calcium (Ca_L) channels were included in the models. Simulation results showed that (1) Strengthening synaptic inputs increased the number of recruitments in all three types of motoneurons following the size principle. (2) Increasing NaT or NaP or decreasing K(DR) or K(AHP) lowered rheobase of spike generation thus increased recruitment of motoneuron pools. (3) Decreasing R_in reduced recruitment in all three types of motoneurons. (4) The FF-type motoneuron pool, followed by FR- and S-type, were the most sensitive to increase of synaptic inputs, reduction of R_in, upregulation of NaT and NaP, and downregulation of K(DR) and K(AHP). (5) Increasing Ca_L enhanced overall discharge rate of motoneuron pools with little effect on the recruitment. Simulation results suggested that modulation of ionic channels altered the output of motoneuron pools with either modulating the number of recruited motoneurons or regulating the overall discharge rate of motoneuron pools. Multiple channels contributed to the recruitment of motoneurons with interaction of excitatory and inhibitory synaptic inputs during walking.

Sequential activation of multiple persistent inward currents induces staircase currents in serotonergic neurons of medulla in ePet-EYFP mice

Persistent inward currents (PICs) are widely reported in rodent spinal neurons. A distinctive pattern observed recently is staircase-like PICs induced by voltage ramp in serotonergic neurons of mouse medulla. The mechanism underlying this pattern of PICs is unclear. Combining electrophysiological, pharmacological, and computational approaches, we investigated the staircase PICs in serotonergic neurons of medulla in ePet-EYFP transgenic mice (postnatal days 1-7). Staircase PICs induced by 10-s voltage biramps were observed in 70% of serotonergic neurons (n = 73). Staircase PICs activated at -48.8 ± 5 mV and consisted of two components, with the first PIC of 45.8 ± 51 pA and the second PIC of 197.3 ± 126 pA (n = 51). Staircase PICs were also composed of low-voltage-activated sodium PIC (Na-PIC; onset -46.2 ± 5 mV, n = 34), high-voltage-activated calcium PIC (Ca-PIC; onset -29.3 ± 6 mV, n = 23), and high-voltage-activated tetrodotoxin (TTX)- and dihydropyridine-resistant sodium PIC (TDR-PIC; onset -16.8 ± 4 mV, n = 28). Serotonergic neurons expressing Na-PIC, Ca-PIC, and TDR-PIC were evenly distributed in medulla. Bath application of 1-2 μM TTX blocked the first PIC and decreased the second PIC by 36% (n = 23, P < 0.05). Nimodipine (25 μM) reduced the second PIC by 38% (n = 34, P < 0.001) without altering the first PIC. TTX and nimodipine removed the first PIC and reduced the second PIC by 59% (n = 28, P < 0.01). A modeling study mimicked the staircase PICs and verified experimental conclusions that sequential activation of Na-PIC, Ca-PIC, and TDR-PIC in order of voltage thresholds induced staircase PICs in serotonergic neurons. Further experimental results suggested that the multiple components of staircase PICs play functional roles in regulating excitability of serotonergic neurons in medulla.NEW & NOTEWORTHY Staircase persistent inward currents (PICs) are mediated by activation of L-type calcium channels in dendrites of mouse spinal motoneurons. A novel mechanism is explored in this study. Here we report that the staircase PICs are mediated by sequentially activating sodium and calcium PICs in serotonergic neurons of mouse medulla.

Morphological and electrophysiological properties of serotonin neurons with NMDA modulation in the mesencephalic locomotor region of neonatal ePet-EYFP mice

The mesencephalic locomotor region (MLR) is an essential area for initiation of locomotion. Its functional roles and circuits underlying locomotion have been studied intensively in many species. Studies suggest that cuneiform nucleus and pedunculopontine nucleus (PPN) are two core regions in the MLR for locomotion. However, it remains unclear about cellular components and morphological and intrinsic membrane properties of the neurons in these regions, especially the serotonergic neurons. Using neonatal ePet-EYFP transgenic mice and immunofluorescent technique, we demonstrated existence of 5-HT neurons in the MLR and discovered that 5-HT neurons distributed mainly in the caudal PPN. 5-HT neurons were heterogeneous in MLR and had three types of firing pattern (single spike, phasic and tonic) and two subtypes of morphology (pyramidal and stellate). We measured parameters of 5-HT neurons (n = 35) including resting membrane potential (- 69.2 ± 4.2 mV), input resistance (1410.1 ± 616.9 MΩ), membrane capacitance (36.4 ± 14.9 pF), time constant (49.7 ± 19.4 ms), voltage threshold (- 32.1 ± 7.4 mV), rheobase (21.3 ± 12.4 pA), action potential amplitude (58.9 ± 12.8 mV) and half-width (4.7 ± 1.1 ms), afterhyperpolarization amplitude (23.6 ± 10.4 mV) and half-decay (331.6 ± 157.7 ms). 5-HT neurons were intrinsically different from adjacent non-5-HT neurons and less excitable than them. Hyperpolarization-activated inward currents and persistent inward currents were recorded in 5-HT neurons. NMDA increased excitability of 5-HT neurons, especially the tonic-firing neurons, accompanied with depolarization of membrane potential, hyperpolarization of voltage threshold, reduction of afterhyperpolarization half-decay, and left-shift of frequency-current relationship. This study provided insight into the distribution and properties of 5-HT neurons in the MLR and interaction between serotonergic and glutamatergic modulations.

Three-week treadmill training changes the electrophysiological properties of spinal interneurons in the mice

It was shown in previous studies that endurance training enhanced excitability of rat spinal motoneurons. However, the influence of the training on the spinal interneurons remains unclear. In this study, we investigated the training effects on spinal interneurons in dorsal and ventromedial area in mice (P42-P50). The electrophysiological properties of the interneurons were recorded from spinal cord slices (T13-L6) by whole-cell patch-clamp recording. The interneurons could be classified into three types based on their response to step currents: single spike (type 1), phasic firing (type 2), and tonic firing (type 3) in both control and trained mice. Interneurons collected from control mice possessed rheobase of 11.3 ± 6.0 pA and voltage threshold (V_th) of - 37.3 ± 4.7 mV. Treadmill training reduced the rheobase by 4.8 ± 1.5 pA and V_th by 3.1 ± 1.2 mV (P < 0.05). Furthermore, the training effects were dependent on the distribution and types of the interneurons. Treadmill training hyperpolarized V_th and decreased rheobase in ventromedial interneurons, while the significant change was observed only in the action potation height of the interneurons in dorsal horn. Treadmill training also hyperpolarized V_th and increased input resistance in type 3 interneurons, but none of these changes was shown in type 1 and 2 interneurons. Bath application of 5-HT (10-20 μM) increased the neuronal excitability in both control and trained mice. Serotonin had similar effect on membrane properties of the interneurons collected from both groups. This study suggested that treadmill training increased excitability of spinal interneurons of the mice and thus would make the spinal motor system easier to generate locomotion.