Experimental and modeling studies of novel bursts induced by blocking na(+) pump and synaptic inhibition in the rat spinal cord

Contributions of h- and Na+/K+ Pump Currents to the Generation of Episodic and Continuous Rhythmic Activities

Developing spinal motor networks produce a diverse array of outputs, including episodic and continuous patterns of rhythmic activity. Variation in excitability state and neuromodulatory tone can facilitate transitions between episodic and continuous rhythms; however, the intrinsic mechanisms that govern these rhythms and their transitions are poorly understood. Here, we tested the capacity of a single central pattern generator (CPG) circuit with tunable properties to generate multiple outputs. To address this, we deployed a computational model composed of an inhibitory half-center oscillator (HCO). Following predictions of our computational model, we tested the contributions of key properties to the generation of an episodic rhythm produced by isolated spinal cords of the newborn mouse. The model recapitulates the diverse state-dependent rhythms evoked by dopamine. In the model, episodic bursting depended predominantly on the endogenous oscillatory properties of neurons, with Na⁺/K⁺ ATPase pump (I_Pump) and hyperpolarization-activated currents (I_h) playing key roles. Modulation of either I_Pump or I_h produced transitions between episodic and continuous rhythms and silence. As maximal activity of I_Pump decreased, the interepisode interval and period increased along with a reduction in episode duration. Decreasing maximal conductance of I_h decreased episode duration and increased interepisode interval. Pharmacological manipulations of I_h with ivabradine, and I_Pump with ouabain or monensin in isolated spinal cords produced findings consistent with the model. Our modeling and experimental results highlight key roles of I_h and I_Pump in producing episodic rhythms and provide insight into mechanisms that permit a single CPG to produce multiple patterns of rhythmicity.

Loss of inhibitory synapses causes locomotor network dysfunction of the rat spinal cord during prolonged maintenance in vitro

The isolated spinal cord of the neonatal rat is widely employed to clarify the basic mechanisms of network development or the early phase of degeneration after injury. Nevertheless, this preparation survives in Krebs solution up to 24 h only, making it desirable to explore approaches to extend its survival for longitudinal studies. The present report shows that culturing the spinal cord in oxygenated enriched Basal Medium Eagle (BME) provided excellent preservation of neurons (including motoneurons), glia and primary afferents (including dorsal root ganglia) for up to 72 h. Using DMEM medium was unsuccessful. Novel characteristics of spinal networks emerged with strong spontaneous activity, and deficit in fictive locomotion patterns with stereotypically slow cycles. Staining with markers for synaptic proteins synapsin 1 and synaptophysin showed thoroughly weaker signal after 3 days in vitro. Immunohistochemical staining of markers for glutamatergic and glycinergic neurons indicated significant reduction of the latter. Likewise, there was lower expression of the GABA-synthesizing enzyme GAD65. Thus, malfunction of locomotor networks appeared related to loss of inhibitory synapses. This phenomenon did not occur in analogous opossum preparations of the spinal cord kept in vitro. In conclusion, despite histological data suggesting that cultured spinal cords were undamaged (except for inhibitory biomarkers), electrophysiological data revealed important functional impairment. Thus, the downregulation of inhibitory synapses may account for the progressive hyperexcitability of rat spinal networks despite apparently normal histological appearance. Our observations may help to understand the basis of certain delayed effects of spinal injury like chronic pain and spasticity.

Control of Xenopus Tadpole Locomotion via Selective Expression of Ih in Excitatory Interneurons

Locomotion relies on the coordinated activity of rhythmic neurons in the hindbrain and spinal cord and depends critically on the intrinsic properties of excitatory interneurons. Therefore, understanding how ion channels sculpt the properties of these interneurons, and the consequences for circuit function and behavior, is an important task. The hyperpolarization-activated cation current, Ih, is known to play important roles in shaping neuronal properties and for rhythm generation in many neuronal networks. We show in stage 42 Xenopus laevis frog tadpoles that Ih is strongly expressed only in excitatory descending interneurons (dINs), an important ipsilaterally projecting population that drives swimming activity. The voltage-dependent HCN channel blocker ZD7288 completely abolished a prominent depolarizing sag potential in response to hyperpolarization, the hallmark of Ih, and hyperpolarized dINs. ZD7288 also affected dIN post-inhibitory rebound firing, upon which locomotor rhythm generation relies, and disrupted locomotor output. Block of Ih also unmasked an activity-dependent ultraslow afterhyperpolarization (usAHP) in dINs following swimming, mediated by a dynamic Na/K pump current. This usAHP, unmasked in dINs by ZD7288, resulted in suprathreshold stimuli failing to evoke swimming at short inter-swim intervals, indicating an important role for Ih in maintaining swim generation capacity and in setting the post-swim refractory period of the network. Collectively, our data suggest that the selective expression of Ih in dINs determines specific dIN properties that are important for rhythm generation and counteracts an activity-dependent usAHP to ensure that dINs can maintain coordinated swimming over a wide range of inter-swim intervals.

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Ih is strongly expressed in Xenopus locomotor-rhythm-generating dIN interneurons

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Ih is active at rest in dINs, contributing to their distinct electrical properties

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dINs normally lack a Na pump-dependent ultra-slow afterhyperpolarization (usAHP)

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Ih counterbalances dIN usAHPs to preserve tadpole rhythm generating capacity

Sodium pump regulation of locomotor control circuits

Sodium pumps are ubiquitously expressed membrane proteins that extrude three Na⁺ ions in exchange for two K⁺ ions, using ATP as an energy source. Recent studies have illuminated additional, dynamic roles for sodium pumps in regulating the excitability of neuronal networks in an activity-dependent fashion. We review their role in a novel form of short-term memory within rhythmic locomotor networks. The data we review derives mainly from recent studies on Xenopus tadpoles and neonatal mice. The role and underlying mechanisms of pump action broadly match previously published data from an invertebrate, the Drosophila larva. We therefore propose a highly conserved mechanism by which sodium pump activity increases following a bout of locomotion. This results in an ultraslow afterhyperpolarization (usAHP) of the membrane potential that lasts around 1 min, but which only occurs in around half the network neurons. This usAHP in turn alters network excitability so that network output is reduced in a locomotor interval-dependent manner. The pumps therefore confer on spinal locomotor networks a temporary memory trace of recent network performance.

α3Na+/K+-ATPase deficiency causes brain ventricle dilation and abrupt embryonic motility in zebrafish

Na(+)/K(+)-ATPases are transmembrane ion pumps that maintain ion gradients across the basolateral plasma membrane in all animal cells to facilitate essential biological functions. Mutations in the Na(+)/K(+)-ATPase α3 subunit gene (ATP1A3) cause rapid-onset dystonia-parkinsonism, a rare movement disorder characterized by sudden onset of dystonic spasms and slow movements. In the brain, ATP1A3 is principally expressed in neurons. In zebrafish, the transcripts of the two ATP1A3 orthologs, Atp1a3a and Atp1a3b, show distinct expression in the brain. Surprisingly, targeted knockdown of either Atp1a3a or Atp1a3b leads to brain ventricle dilation, a likely consequence of ion imbalances across the plasma membrane that cause accumulation of cerebrospinal fluid in the ventricle. The brain ventricle dilation is accompanied by a depolarization of spinal Rohon-Beard neurons in Atp1a3a knockdown embryos, suggesting impaired neuronal excitability. This is further supported by Atp1a3a or Atp1a3b knockdown results where altered responses to tactile stimuli as well as abnormal motility were observed. Finally, proteomic analysis identified several protein candidates highlighting proteome changes associated with the knockdown of Atp1a3a or Atp1a3b. Our data thus strongly support the role of α3Na(+)/K(+)-ATPase in zebrafish motility and brain development, associating for the first time the α3Na(+)/K(+)-ATPase deficiency with brain ventricle dilation.

Short-term memory of motor network performance via activity-dependent potentiation of Na+/K+ pump function

Brain networks memorize previous performance to adjust their output in light of past experience. These activity-dependent modifications generally result from changes in synaptic strengths or ionic conductances, and ion pumps have only rarely been demonstrated to play a dynamic role. Locomotor behavior is produced by central pattern generator (CPG) networks and modified by sensory and descending signals to allow for changes in movement frequency, intensity, and duration, but whether or how the CPG networks recall recent activity is largely unknown. In Xenopus frog tadpoles, swim bout duration correlates linearly with interswim interval, suggesting that the locomotor network retains a short-term memory of previous output. We discovered an ultraslow, minute-long afterhyperpolarization (usAHP) in network neurons following locomotor episodes. The usAHP is mediated by an activity- and sodium spike-dependent enhancement of electrogenic Na(+)/K(+) pump function. By integrating spike frequency over time and linking the membrane potential of spinal neurons to network performance, the usAHP plays a dynamic role in short-term motor memory. Because Na(+)/K(+) pumps are ubiquitously expressed in neurons of all animals and because sodium spikes inevitably accompany network activity, the usAHP may represent a phylogenetically conserved but largely overlooked mechanism for short-term memory of neural network function.

Mechanism for the universal pattern of activity in developing neuronal networks

Spontaneous episodic activity is a fundamental mode of operation of developing networks. Surprisingly, the duration of an episode of activity correlates with the length of the silent interval that precedes it, but not with the interval that follows. Here we use a modeling approach to explain this characteristic, but thus far unexplained, feature of developing networks. Because the correlation pattern is observed in networks with different structures and components, a satisfactory model needs to generate the right pattern of activity regardless of the details of network architecture or individual cell properties. We thus developed simple models incorporating excitatory coupling between heterogeneous neurons and activity-dependent synaptic depression. These models robustly generated episodic activity with the correct correlation pattern. The correlation pattern resulted from episodes being triggered at random levels of recovery from depression while they terminated around the same level of depression. To explain this fundamental difference between episode onset and termination, we used a mean field model, where only average activity and average level of recovery from synaptic depression are considered. In this model, episode onset is highly sensitive to inputs. Thus noise resulting from random coincidences in the spike times of individual neurons led to the high variability at episode onset and to the observed correlation pattern. This work further shows that networks with widely different architectures, different cell types, and different functions all operate according to the same general mechanism early in their development.

Spike integration and cellular memory in a rhythmic network from Na+/K+ pump current dynamics

The output of a neural circuit results from an interaction between the intrinsic properties of neurons within the circuit and the features of the synaptic connections between them. The plasticity of intrinsic properties has been primarily attributed to modification of ion channel function and/or number. In this study, we demonstrate a mechanism for intrinsic plasticity in rhythmically active Drosophila neurons that is not conductance-based. Larval motor neurons show a long lasting sodium-dependent afterhyperpolarization (AHP) following bursts of action potentials that is mediated by the electrogenic activity of Na⁺/K⁺ ATPase. This AHP persists for multiple seconds following volleys of action potentials and is able to function as a pattern-insensitive integrator of spike number that is independent of external calcium. This current also interacts with endogenous Shal K⁺ conductances to modulate spike timing for multiple seconds following rhythmic activity, providing a cellular memory of network activity on a behaviorally relevant time scale.

Differential modulation by tetraethylammonium of the processes underlying network bursting in the neonatal rat spinal cord in vitro

In the rat spinal cord in vitro, block of synaptic inhibition evokes persistent, regular disinhibited bursting which is a manifestation of the intrinsic network rhythmicity and is readily recorded from ventral roots. This model is advantageous to explore the network mechanisms controlling burst periodicity, and duration. We questioned the relative contribution of K+ conductances to spontaneous rhythmicity by investigating the effects of the broad K+ channel blocker tetraethylammonium (TEA). In TEA (10 mM) solution, bursts occurred at the same rate but became substantially longer, thus showing an unusual dissociation between mechanisms of burst periodicity and duration. In the presence of TEA, electrical stimulation of a single dorsal root or N-methyl-D-aspartate application (5 microM) could, however, fasten bursting associated with immediate decrease in burst length, thus demonstrating maintenance of short-term plasticity. Either riluzole (1 microM) or surgical sectioning that isolated a single spinal segment strongly depressed bursting which could, however, be revived by TEA. In the presence of TEA, the L-type channel blocker nifedipine (20 microM) made bursting faster and shorter. Our data are best explained by assuming that TEA increased network excitability to generate rhythmic bursting, an effect that was counteracted by intrinsic mechanisms, partly dependent on L-type channel activity, to retain standard periodicity. TEA-sensitive mechanisms were, nevertheless, an important process to regulate burst duration. Our results are consistent with the proposal of a hierarchical structural of the central pattern generator in which the circuits responsible for rhythmicity (the clock) drive the discharges of those creating the motor commands (pattern).

Riluzole-induced oscillations in spinal networks

We previously showed in dissociated cultures of fetal rat spinal cord that disinhibition-induced bursting is based on intrinsic spiking, network recruitment, and a network refractory period after the bursts. A persistent sodium current (I(NaP)) underlies intrinsic spiking, which, by recurrent excitation, generates the bursting activity. Although full blockade of I(NaP) with riluzole disrupts such bursting, the present study shows that partial blockade of I(NaP) with low doses of riluzole maintains bursting activity with unchanged burst rate and burst duration. More important, low doses of riluzole turned bursts composed of persistent activity into bursts composed of oscillatory activity at around 5 Hz. In a search for the mechanisms underlying the generation of such intraburst oscillations, we found that activity-dependent synaptic depression was not changed with low doses of riluzole. On the other hand, low doses of riluzole strongly increased spike-frequency adaptation and led to early depolarization block when bursts were simulated by injecting long current pulses into single neurons in the absence of fast synaptic transmission. Phenytoin is another I(NaP) blocker. When applied in doses that reduced intrinsic activity by 80-90%, as did low doses of riluzole, it had no effect either on spike-frequency adaptation or on depolarization block. Nor did phenytoin induce intraburst oscillations after disinhibition. A theoretical model incorporating a depolarization block mechanism could reproduce the generation of intraburst oscillations at the network level. From these findings we conclude that riluzole-induced intraburst oscillations are a network-driven phenomenon whose major accommodation mechanism is depolarization block arising from strong sodium channel inactivation.

The effects induced by the sulphonylurea glibenclamide on the neonatal rat spinal cord indicate a novel mechanism to control neuronal excitability and inhibitory neurotransmission

BACKGROUND AND PURPOSE

Using the neonatal rat spinal cord in vitro, we investigated the action of glibenclamide, a drug possessing dual pharmacological effects, namely block of K(ATP) channels and of the cystic fibrosis transmembrane conductance regulator (CFTR).

EXPERIMENTAL APPROACH

Intra- and extracellular recordings were performed on motoneurons and interneurons. RT-PCR and western immunoblotting were used to determine gene and protein expression.

KEY RESULTS

Glibenclamide (50 microM) facilitated mono- and polysynaptic reflexes, hyperpolarized motoneuron resting potential, increased action potential amplitude, decreased Renshaw cell-mediated recurrent inhibition, and increased network excitability by depressing GABA- and glycine-mediated transmission. The action of glibenclamide was mimicked by tolbutamide (500 microM) or the CFTR blocker diphenylamine-2,2-dicarboxylic acid (500 microM). The action of glibenclamide was independent from pharmacological inhibition of the Na(+)-K(+) pump with strophanthidin (4 microM) and was associated with a negative shift in the extrapolated reversal potential for CI(-) dependent synaptic inhibition. On interneurons, intracellularly-applied 8-bromo-cAMP elicited an inward current and resistance decrease; effects antagonized by the selective CFTR antagonist, CFTR(inh)-172 (5 microM). RT-PCR and western immunoblotting indicated strong expression of the CFTR in neonatal rat spinal cord.

CONCLUSIONS AND IMPLICATIONS

These data suggest the CFTR expressed in motoneurons and interneurons of the neonatal spinal cord is involved in the control of Cl(-) homeostasis and neuronal excitability. CFTR appeared to contribute to the relatively depolarized equilibrium potential for synaptic inhibition, an important process to control hyperexcitability and seizure-predisposition in neonates.

Differential control of active and silent phases in relaxation models of neuronal rhythms

Rhythmic bursting activity, found in many biological systems, serves a variety of important functions. Such activity is composed of episodes, or bursts (the active phase, AP) that are separated by quiescent periods (the silent phase, SP). Here, we use mean field, firing rate models of excitatory neural network activity to study how AP and SP durations depend on two critical network parameters that control network connectivity and cellular excitability. In these models, the AP and SP correspond to the network's underlying bistability on a fast time scale due to rapid recurrent excitatory connectivity. Activity switches between the AP and SP because of two types of slow negative feedback: synaptic depression-which has a divisive effect on the network input/output function, or cellular adaptation-a subtractive effect on the input/output function. We show that if a model incorporates the divisive process (regardless of the presence of the subtractive process), then increasing cellular excitability will speed up the activity, mostly by decreasing the silent phase. Reciprocally, if the subtractive process is present, increasing the excitatory connectivity will slow down the activity, mostly by lengthening the active phase. We also show that the model incorporating both slow processes is less sensitive to parameter variations than the models with only one process. Finally, we note that these network models are formally analogous to a type of cellular pacemaker and thus similar results apply to these cellular pacemakers.

An N-methyl-D-aspartate receptor mediated large, low-frequency, spontaneous excitatory postsynaptic current in neonatal rat spinal dorsal horn neurons

Examples of spontaneous oscillating neural activity contributing to both pathological and physiological states are abundant throughout the CNS. Here we report a spontaneous oscillating intermittent synaptic current located in lamina I of the neonatal rat spinal cord dorsal horn. The spontaneous oscillating intermittent synaptic current is characterized by its large amplitude, slow decay time, and low-frequency. We demonstrate that post-synaptic N-methyl-D-aspartate receptors (NMDARs) mediate the spontaneous oscillating intermittent synaptic current, as it is inhibited by magnesium, bath-applied d-2-amino-5-phosphonovalerate (APV), or intracellular MK-801. The NR2B subunit of the NMDAR appears important to this phenomenon, as the NR2B subunit selective NMDAR antagonist, alpha-(4-hydroxphenyl)-beta-methyl-4-benzyl-1-piperidineethanol tartrate (ifenprodil), also partially inhibited the spontaneous oscillating intermittent synaptic current. Inhibition of spontaneous glutamate release by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or the mu-opioid receptor agonist [D-Ala2, N-Me-Phe4, Gly5] enkephalin-ol (DAMGO) inhibited the spontaneous oscillating intermittent synaptic current frequency. Marked inhibition of spontaneous oscillating intermittent synaptic current frequency by tetrodotoxin (TTX), but not post-synaptic N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), suggests that the glutamate release important to the spontaneous oscillating intermittent synaptic current is dependent on active neural processes. Conversely, increasing dorsal horn synaptic glutamate release by GABAA or glycine inhibition increased spontaneous oscillating intermittent synaptic current frequency. Moreover, inhibiting glutamate transporters with threo-beta-benzyloxyaspartic acid (DL-TBOA) increased spontaneous oscillating intermittent synaptic current frequency and decay time. A possible functional role of this spontaneous NMDAR-mediated excitatory postsynaptic current in modulating nociceptive transmission within the spinal cord is discussed.

Glutamate uptake block triggers deadly rhythmic bursting of neonatal rat hypoglossal motoneurons

In the brain the extracellular concentration of glutamate is controlled by glial transporters that restrict the neurotransmitter action to synaptic sites and avoid excitotoxicity. Impaired transport of glutamate occurs in many cases of amyotrophic lateral sclerosis, a devastating motoneuron disease. Motoneurons of the brainstem nucleus hypoglossus are among the most vulnerable, giving early symptoms like slurred speech and dysphagia. However, the direct consequences of extracellular glutamate build-up, due to uptake block, on synaptic transmission and survival of hypoglossal motoneurons remain unclear and have been studied using the neonatal rat brainstem slice preparation as a model. Patch clamp recording from hypoglossal motoneurons showed that, in about one-third of these cells, inhibition of glutamate transport with the selective blocker dl-threo-beta-benzyloxyaspartate (TBOA; 50 mum) unexpectedly led to the emergence of rhythmic bursting consisting of inward currents of long duration with superimposed fast oscillations and synaptic events. Synaptic inhibition block facilitated bursting. Bursts had a reversal potential near 0 mV, and were blocked by tetrodotoxin, the gap junction blocker carbenoxolone, or antagonists of AMPA, NMDA or mGluR1 glutamate receptors. Intracellular Ca(2+) imaging showed bursts as synchronous discharges among motoneurons. Synergy of activation of distinct classes of glutamate receptor plus gap junctions were therefore essential for bursting. Ablating the lateral reticular formation preserved bursting, suggesting independence from propagated network activity within the brainstem. TBOA significantly increased the number of dead motoneurons, an effect prevented by the same agents that suppressed bursting. Bursting thus represents a novel hallmark of motoneuron dysfunction triggered by glutamate uptake block.

Myomodulin increases Ih and inhibits the NA/K pump to modulate bursting in leech heart interneurons

In the medicinal leech, a rhythmically active 14-interneuron network composes the central pattern generator for heartbeat. In two segmental ganglia, bilateral pairs of reciprocally inhibitory heart interneurons (oscillator interneurons) produce a rhythm of alternating bursts of action potentials that paces activity in the pattern-generating network. The neuropeptide myomodulin decreases the period of this bursting and increases the intraburst spike frequency when applied to isolated ganglia containing these oscillator interneurons. Myomodulin also decreases period, increases spike frequency, and increases the robustness of endogenous bursting in synaptically isolated (with bicuculline) oscillator interneurons. In voltage-clamp experiments using hyperpolarizing ramps, we identify an increase in membrane conductance elicited by myomodulin with the properties of a hyperpolarization-activated current. Voltage steps confirm that myomodulin indeed increases the maximum conductance of the hyperpolarization-activated current I(h). In similar experiments using Cs(+) to block I(h), we demonstrate that myomodulin also causes a steady offset in the ramp current that is not associated with an increase in conductance. This current offset is blocked by ouabain, indicating that myomodulin inhibits the Na/K pump. In current-clamp experiments, when I(h) is blocked with Cs(+), myomodulin decreases period and increases spike frequency of alternating bursting in synaptically connected oscillator interneurons, suggesting that inhibiting the Na/K pump modulates these burst characteristics. These observations indicate that myomodulin decreases period and increases spike frequency of endogenous bursting in synaptically isolated oscillator heart interneurons and alternating bursting of reciprocally inhibitory pairs of interneurons, at least in part, by increasing I(h) and by decreasing the Na/K pump.

The string method of burst identification in neuronal spike trains

The activity state of neuronal networks can be characterized by the spatial-temporal grouping of their action potentials given a sufficiently large simultaneous recording sample. A sequence of action potentials (spike train) often has high frequency spike episodes that are generally called bursts. However, bursts are difficult to quantify and require operational definitions that reflect the type of activity and the interest of the experimenter. This paper presents a simple method for defining bursts as strings of spikes with only two parameters: a minimum number of spikes per burst and a maximum interspike interval. These two values represent a simple parameterization that is adequate for the description of temporal grouping in spike trains. Because this method has a minimal computation time, it allows implementation of burst analysis in real-time, including statistical changes in burst variables, histograms of burst types, and patterns in combinations of burst variables.

Normal expression and inflammation-induced changes of Na and Na/K ATPase activity in spinal dorsal horn of the rat

We tested whether that peripheral inflammation induces changes in the spinal dorsal horn ATPase activity. Adult Sprague-Dawley rats were anesthetized (thiobarbital), the left hind paw (inflammation group; n = 15) was immersed in water at 60 degrees C for 60s, which induced a local inflammation. A control group (n = 12) was tested with water at room temperature. After 60 min of peripheral inflammation left (LDH) or right lumbar dorsal horn (RDH) were processed for total, Na/K, Na and remanent ATPase activities (nM P(i) (mgprotein)(-1) min(-1)). In control animals isoenzymatic activities were: Na (31.2%); Na/K (20.6%) and remanent (48.2%) from total ATPase activity. No LDH-RDH asymmetry was found. The inflammation group presented an ipsilateral increase of total ATPase activity in LDH (X+/-S.E.M.; 4798.9+/-601) over the RDH (3982.2+/-451; Delta+817; P<0.05). This is due to an increase in Na ATPase activity (1609.3+/-297) over RDH (1164.2+/-166; Delta+445; P<0.05). ATPase activities were increased in LDH from inflamed over the control group as follows: total (4798.9+/-601; Delta+840; P<0.05), Na/K (1298.1+/-301; Delta+483; P<0.05) and Na (1609.3+/-297; Delta+373; P<0.05). These increased ATPase activities, induced in a short time, can be considered a functional marker of nociceptive neuronal activity.

Modulation of rhythmic patterns and cumulative depolarization by group I metabotropic glutamate receptors in the neonatal rat spinal cord in vitro

The role of group I metabotropic glutamate receptors (mGluRs), and their subtypes 1 or 5, in rhythmic patterns generated by the neonatal rat spinal cord was investigated. Fictive locomotor patterns induced by N-methyl-d-aspartate + serotonin were slowed down by the subtype 1 antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) or 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) and unaffected by the subtype 5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). The group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) depolarized ventral roots and disrupted fictive locomotion, an effect blocked by AIDA (or CPCCOEt) and reversed by increasing the N-methyl-d-aspartate concentration. Cumulative depolarization induced by low frequency trains of dorsal root stimuli was attenuated by DHPG and unchanged by AIDA or MPEP while rhythmic patterns or motoneuron spike wind-up persisted. Disinhibited bursting induced by strychnine + bicuculline was accelerated by DHPG, slowed down by AIDA (which prevented the action of DHPG), unaffected by MPEP and counteracted by the selective group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine. The DHPG transformed regular bursting into arrhythmic bursting, a phenomenon also produced by the group II mGluR antagonist (2S)-alpha-ethylglutamic acid. These results indicate that, during fictive locomotion or disinhibited bursting, endogenous glutamate could activate discrete clusters of subtype 1 mGluRs to facilitate discharges. Diffuse activation by the exogenous agonist DHPG of group I mGluRs throughout spinal networks had an excitatory effect overshadowed by its much stronger depressant action due to concomitant facilitation of glycinergic transmission. Irregular disinhibited bursting caused by activation of subtype 1 receptors or block of group II receptors suggests that mGluRs could control not only the frequency but also the periodicity of bursting patterns, outlining novel mechanisms contributing to burst duration.

Role of group II and III metabotropic glutamate receptors in rhythmic patterns of the neonatal rat spinal cord in vitro

Electrophysiological recordings were used to explore the role of group II and III metabotropic glutamate receptors (mGluRs) in oscillatory patterns generated by the neonatal rat spinal cord in vitro. Neither the group II agonist DCG-IV (and the selective antagonist EGLU), nor the group III agonist L-AP4 (and its selective antagonist CPPG) had any effect on lumbar motoneuron membrane potential or input resistance. This observation suggests that motoneurons expressed no functional group II and III mGluRs and received no network-based, tonic influence mediated by them. DCG-IV or L-AP4 strongly depressed synaptic responses evoked by single dorsal root (DR) stimuli, an effect counteracted by their respective antagonist. EGLU or CPPG per se had no effect on synaptic responses, indicating no mGluR autoreceptor-dependent control of transmitter release. L-AP4 largely depressed cumulative depolarization, windup and associated oscillations, whereas synaptic depression induced by DCG-IV waned with repeated stimuli. L-AP4 slowed down fictive locomotor patterns and arrested disinhibited bursting, which could, however, be promptly restored by DR electrical stimulation. DCG-IV had no significant effect on fictive locomotion, but it blocked disinhibited bursting. EGLU facilitated bursting, suggesting that burst termination was partly controlled by group II mGluRs. All these effects were reversible on washout. It is concluded that activation of group II and III mGluRs differentially modulated rhythmic patterns recorded from motoneurons via network-dependent actions, which probably included decrease in the release of neurotransmitters at key circuit points.

Role of the electrogenic Na/K pump in disinhibition-induced bursting in cultured spinal networks

Disinhibition-induced bursting activity in cultures of fetal rat spinal cord is mainly controlled by intrinsic spiking with subsequent recurrent excitation of the network through glutamate synaptic transmission, and by autoregulation of neuronal excitability. Here we investigated the contribution of the electrogenic Na/K pump to the autoregulation of excitability using extracellular recordings by multielectrode arrays (MEAs) and intracellular whole cell recordings from spinal interneurons. The blockade of the electrogenic Na/K pump by strophanthidin led to an immediate and transient increase in the burst rate together with an increase in the asynchronous background activity. Later, the burst rate decreased to initial values and the bursts became shorter and smaller. In single neurons, we observed an immediate depolarization of the membrane during the interburst intervals concomitant with the rise in burst rate. This depolarization was more pronounced during disinhibition than during control, suggesting that the pump was more active. Later a decrease in burst rate was observed and, in some neurons, a complete cessation of firing. Most of the effects of strophanthidin could be reproduced by high K+-induced depolarization. During prolonged current injections, spinal interneurons exhibited spike frequency adaptation, which remained unaffected by strophanthidin. These results suggest that the electrogenic Na/K pump is responsible for the hyperpolarization and thus for the changes in excitability during the interburst intervals, although not for the spike frequency adaptation during the bursts.

Analysis of spontaneous bursting activity in random neural networks

Developing networks can generate bursting activity despite the absence of pacemaker cells and inhibitory synapses. Bursts are generated by positive feedback through excitatory connections and terminated by a slow depression of network excitability. Similar activity has been observed in cultures of spinal neurons containing inhibitory synapses. Is burst generation in these cultures operating according to the mechanism proposed for developing networks? To answer this question without perturbing the activity, we analyzed the burst pattern of individual neurons in active cultures. We observed a correlation between burst size and preceding interburst interval and a rapid rise and progressive decrease in firing rate during a burst. These findings are consistent with the mechanism of burst generation proposed for developing networks.

The Na,K-ATPase alpha 2 isoform is expressed in neurons, and its absence disrupts neuronal activity in newborn mice

Na,K-ATPase is an ion transporter that impacts neural and glial physiology by direct electrogenic activity and the modulation of ion gradients. Its three isoforms in brain have cell-type and development-specific expression patterns. Interestingly, our studies demonstrate that in late gestation, the alpha2 isoform is widely expressed in neurons, unlike in the adult brain, in which alpha2 has been shown to be expressed primarily in astrocytes. This unexpected distribution of alpha2 isoform expression in neurons is interesting in light of our examination of mice lacking the alpha2 isoform which fail to survive after birth. These animals showed no movement; however, defects in gross brain development, muscle contractility, neuromuscular transmission, and lung development were ruled out. Akinesia suggests a primary neuronal defect and electrophysiological recordings in the pre-Bötzinger complex, the brainstem breathing center, showed reduction of respiratory rhythm activity, with less regular and smaller population bursts. These data demonstrate that the Na,K-ATPase alpha2 isoform could be important in the modulation of neuronal activity in the neonate.