Inhibition of spinal or hypoglossal motoneurons of the newborn rat by glycine or GABA

Chronic, episodic nicotine exposure alters GABAergic synaptic transmission to hypoglossal motor neurons and genioglossus muscle function at a critical developmental age

Regulation of GABAergic signaling through nicotinic acetylcholine receptor (nAChR) activation is critical for neuronal development. Here, we test the hypothesis that chronic episodic developmental nicotine exposure (eDNE) disrupts GABAergic signaling, leading to dysfunction of hypoglossal motor neurons (XIIMNs), which innervate the tongue muscles. We studied control and eDNE pups at two developmentally vulnerable age ranges: postnatal days (P)1-5 and P10-12. The amplitude and frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs, mIPSCs) at baseline were not altered by eDNE at either age. In contrast, eDNE increased GABAAR-α1 receptor expression on XIIMNs and, in the older group, the postsynaptic response to muscimol (GABAA receptor agonist). Activation of nAChRs with exogenous nicotine increased the frequency of GABAergic sIPSCs in control and eDNE neurons at P1-5. By P10-12, acute nicotine increased sIPSC frequency in eDNE but not control neurons. In vivo experiments showed that the breathing-related activation of tongue muscles, which are innervated by XIIMNs, is reduced at P10-12. This effect was partially mitigated by subcutaneous muscimol, but only in the eDNE pups. Taken together, these data indicate that eDNE alters GABAergic transmission to XIIMNs at a critical developmental age, and this is expressed as reduced breathing-related drive to XIIMNs in vivo.NEW & NOTEWORTHY Here, we provide a thorough assessment of the effects of nicotine exposure on GABAergic synaptic transmission, from the cellular to the systems level. This work makes significant advances in our understanding of the impact of nicotine exposure during development on GABAergic neurotransmission within the respiratory network and the potential role this plays in the excitatory/inhibitory imbalance that is thought to be an important mechanism underlying neonatal breathing disorders, including sudden infant death syndrome.

GABAergic Mechanisms Can Redress the Tilted Balance between Excitation and Inhibition in Damaged Spinal Networks

Correct operation of neuronal networks depends on the interplay between synaptic excitation and inhibition processes leading to a dynamic state termed balanced network. In the spinal cord, balanced network activity is fundamental for the expression of locomotor patterns necessary for rhythmic activation of limb extensor and flexor muscles. After spinal cord lesion, paralysis ensues often followed by spasticity. These conditions imply that, below the damaged site, the state of balanced networks has been disrupted and that restoration might be attempted by modulating the excitability of sublesional spinal neurons. Because of the widespread expression of inhibitory GABAergic neurons in the spinal cord, their role in the early and late phases of spinal cord injury deserves full attention. Thus, an early surge in extracellular GABA might be involved in the onset of spinal shock while a relative deficit of GABAergic mechanisms may be a contributor to spasticity. We discuss the role of GABA A receptors at synaptic and extrasynaptic level to modulate network excitability and to offer a pharmacological target for symptom control. In particular, it is proposed that activation of GABA A receptors with synthetic GABA agonists may downregulate motoneuron hyperexcitability (due to enhanced persistent ionic currents) and, therefore, diminish spasticity. This approach might constitute a complementary strategy to regulate network excitability after injury so that reconstruction of damaged spinal networks with new materials or cell transplants might proceed more successfully.

Nitric oxide controls excitatory/inhibitory balance in the hypoglossal nucleus during early postnatal development

Synaptic remodeling during early postnatal development lies behind neuronal networks refinement and nervous system maturation. In particular, the respiratory system is immature at birth and is subjected to significant postnatal development. In this context, the excitatory/inhibitory balance dramatically changes in the respiratory-related hypoglossal nucleus (HN) during the 3 perinatal weeks. Since, development abnormalities of hypoglossal motor neurons (HMNs) are associated with sudden infant death syndrome and obstructive sleep apnea, deciphering molecular partners behind synaptic remodeling in the HN is of basic and clinical relevance. Interestingly, a transient expression of the neuronal isoform of nitric oxide (NO) synthase (NOS) occurs in HMNs at neonatal stage that disappears before postnatal day 21 (P21). NO, in turn, is a determining factor for synaptic refinement in several physiopathological conditions. Here, intracerebroventricular chronic administration (P7–P21) of the broad spectrum NOS inhibitor l-NAME (N(ω)-nitro-l-arginine methyl ester) differentially affected excitatory and inhibitory rearrangement during this neonatal interval in the rat. Whilst l-NAME led to a reduction in the number of excitatory structures, inhibitory synaptic puncta were increased at P21 in comparison to administration of the inactive stereoisomer d-NAME. Finally, l-NAME decreased levels of the phosphorylated form of myosin light chain in the nucleus, which is known to regulate the actomyosin contraction apparatus. These outcomes indicate that physiologically synthesized NO modulates excitatory/inhibitory balance during early postnatal development by acting as an anti-synaptotrophic and/or synaptotoxic factor for inhibitory synapses, and as a synaptotrophin for excitatory ones. The mechanism of action could rely on the modulation of the actomyosin contraction apparatus.

The etiological contribution of GABAergic plasticity to the pathogenesis of neuropathic pain

Neuropathic pain developing after peripheral or central nerve injury is the result of pathological changes generated through complex mechanisms. Disruption in the homeostasis of excitatory and inhibitory neurons within the central nervous system is a crucial factor in the formation of hyperalgesia or allodynia occurring with neuropathic pain. The central GABAergic pathway has received attention for its extensive distribution and function in neural circuits, including the generation and development of neuropathic pain. GABAergic inhibitory changes that occur in the interneurons along descending modulatory and nociceptive pathways in the central nervous system are believed to generate neuronal plasticity, such as synaptic plasticity or functional plasticity of the related genes or proteins, that is the foundation of persistent neuropathic pain. The primary GABAergic plasticity observed in neuropathic pain includes GABAergic synapse homo- and heterosynaptic plasticity, decreased synthesis of GABA, down-expression of glutamic acid decarboxylase and GABA transporter, abnormal expression of NKCC1 or KCC2, and disturbed function of GABA receptors. In this review, we describe possible mechanisms associated with GABAergic plasticity, such as central sensitization and GABAergic interneuron apoptosis, and the epigenetic etiologies of GABAergic plasticity in neuropathic pain. Moreover, we summarize potential therapeutic targets of GABAergic plasticity that may allow for successful relief of hyperalgesia from nerve injury. Finally, we compare the effects of the GABAergic system in neuropathic pain to other types of chronic pain to understand the contribution of GABAergic plasticity to neuropathic pain.

Meta-analysis of biological variables' impact on spinal motoneuron electrophysiology data

Experimental, methodological, and biological variables must be accounted for statistically to maximize accuracy and comparability of published neuroscience data. However, accounting for all variables is nigh impossible. Thus we aimed to identify particularly influential variables within published neurological data, from cat, rat, and mouse studies, via a robust statistical process. Our goal was to develop tools to improve rigor in the collection and analysis of data. We strictly constrained experimental and methodological variables and then assessed four key biological variables within motoneuron research: species, age, sex, and cell type. We quantified intraexperimental and interexperimental variances in 11 commonly reported electrophysiological properties of spinal motoneurons. We first assessed variances without accounting for biological variables and then reassessed them while accounting for all four variables. We next assessed variances with all possible combinations of these four variables. We concluded that some motoneuron properties have low intraexperimental, but high interexperimental, variance; that individual motoneuron properties are impacted differently by biological variables; and that some unexplained variances still remain. We report here the optimal combinations of biological variables to reduce interexperimental variance for all 11 parameters. We also rank each parameter by intra- and interexperimental consistency. We expect these results to assist with design of experimental and analytical methods, and to support accuracy in simulations. Furthermore, although demonstrated on spinal motoneuron electrophysiology literature, our approach is applicable to biological data from all fields of neuroscience. This approach represents an important aid to experimental design, comparison of reported data, and reduction of unexplained variance in neuroscience data.NEW & NOTEWORTHY Our meta-analysis shows the impact of species, age, sex, and cell type on lumbosacral motoneuron electrophysiological properties by thoroughly quantifying variances across literature for the first time. We quantify the variances of 11 motoneuron properties with consideration of biological variables, thus providing specific insights for motoneuron modelers and experimenters, and providing a general methodological template for the quantification of variance in neurological data with the consideration of any experimental, methodological, or biological variables of interest.

Modulation of extrasynaptic GABAergic receptor activity influences glutamate release and neuronal survival following excitotoxic damage to mouse spinal cord neurons

Excitotoxic levels of released glutamate trigger a cascade of deleterious cellular events leading to delayed neuronal death. This phenomenon implies extensive dysregulation in the balance between network excitation and inhibition. Our hypothesis was that enhancing network inhibition should prevent excitotoxicity and provide neuroprotection. To test this notion, we used mouse organotypic spinal slice cultures and explored if excitotoxicity caused by the potent glutamate analogue kainate was blocked by pharmacological increase in GABA_A receptor activity. To this end we monitored (with a biosensor) real-time glutamate release following 1 h kainate application and quantified neuronal survival 24 h later. Glutamate release evoked by kainate was strongly decreased by the allosteric GABA_A modulator midazolam (10 nM) or the GABA agonist THIP (10 μM), leading to neuroprotection. On the contrary, much higher glutamate release was induced by the GABA antagonist bicuculline (20 μM) that inhibits synaptic and extrasynaptic GABA_A receptors. Gabazine (20 μM), an antagonist of synaptic GABA_A receptors, had no effect on glutamate release or neuroprotection. No effect was observed with the glycine antagonist strychnine or the glycine agonist L-alanine. These findings indicate that enhancement of GABA receptor activity was an effective tool to counteract excitotoxic death in spinal networks. In view of the potent activity by THIP, preferentially acting on extrasynaptic GABA_A receptors, the present data imply a significant role for extrasynaptic GABA_A receptors in sparing spinal cord neurons from injury.

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.

Functional up-regulation of the M-current by retigabine contrasts hyperexcitability and excitotoxicity on rat hypoglossal motoneurons

KEY POINTS

Excessive neuronal excitability characterizes several neuropathological conditions, including neurodegenerative diseases such as amyotrophic lateral sclerosis. Hypoglossal motoneurons (HMs), which control tongue muscles, are extremely vulnerable to this disease and undergo damage and death when exposed to an excessive glutamate extracellular concentration that causes excitotoxicity. Our laboratory devised an in vitro model of excitotoxicity obtained by pharmacological blockade of glutamate transporters. In this paradigm, HMs display hyperexcitability, collective bursting and eventually cell death. The results of the present study show that pharmacological up-regulation of a K⁺ current (M-current), via application of the anti-convulsant retigabine, prevented all hallmarks of HM excitotoxicity, comprising bursting, generation of reactive oxygen species, expression of toxic markers and cell death. ○Our data may have translational value to develop new treatments against neurological diseases by using positive pharmacological modulators of the M-current.

ABSTRACT

Neuronal hyperexcitability is a symptom characterizing several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). In the ALS bulbar form, hypoglossal motoneurons (HMs) are an early target for neurodegeneration because of their high vulnerability to metabolic insults. In recent years, our laboratory has developed an in vitro model of a brainstem slice comprising the hypoglossal nucleus in which HM neurodegeneration is achieved by blocking glutamate clearance with dl-threo-β-benzyloxyaspartate (TBOA), thus leading to delayed excitotoxicity. During this process, HMs display a set of hallmarks such as hyperexcitability (and network bursting), reactive oxygen species (ROS) generation and, finally, cell death. The present study aimed to investigate whether blocking early hyperexcitability and bursting with the anti-convulsant drug retigabine was sufficient to achieve neuroprotection against excitotoxicity. Retigabine is a selective positive allosteric modulator of the M-current (I_M ), an endogenous mechanism that neurons (comprising HMs) express to dampen excitability. Retigabine (10 μm; co-applied with TBOA) contrasted ROS generation, release of endogenous toxic factors into the HM cytoplasm and excitotoxicity-induced HM death. Electrophysiological experiments showed that retigabine readily contrasted and arrested bursting evoked by TBOA administration. Because neuronal I_M subunits (Kv7.2, Kv7.3 and Kv7.5) were expressed in the hypoglossal nucleus and in functionally connected medullary nuclei, we suggest that they were responsible for the strong reduction in network excitability, a potent phenomenon for achieving neuroprotection against TBOA-induced excitotoxicity. The results of the present study may have translational value for testing novel positive pharmacological modulators of the I_M under pathological conditions (including neurodegenerative disorders) characterized by excessive neuronal excitability.

Developmental plasticity of GABAergic neurotransmission to brainstem motoneurons

KEY POINTS

Critical homeostatic behaviours such as suckling, swallowing and breathing depend on the precise control of tongue muscle activity. Perinatal nicotine exposure has multiple effects on baseline inhibitory GABAergic neurotransmission to hypoglossal motoneurons (XIIMNs), consistent with homeostatic compensations directed at maintaining normal motoneuron output. Developmental nicotine exposure (DNE) alters how GABAergic neurotransmission is modulated by acute activation of nicotinic acetylcholine receptors, which may provide insight into mechanisms by which nicotine exposure alters motor function under conditions that result in increased release of GABA, such as hypoxia, or endogenous acetylcholine, as occurs in the transition from NREM to REM sleep, or in response to exogenous nicotine.

ABSTRACT

Nicotinic acetylcholine receptor (nAChR) signalling regulates neuronal differentiation and synaptogenesis. Here we test the hypothesis that developmental nicotine exposure (DNE) disrupts the development of GABAergic synaptic transmission to hypoglossal motoneurons (XIIMNs). GABAergic spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) were recorded from XIIMNs in brainstem slices from control and DNE rat pups of either sex, 1-5 days old, at baseline and following acute stimulation of nAChRs with nicotine. At baseline, sIPSCs were less frequent and smaller in DNE cells (consistent with decreased action potential-mediated GABA release), and mIPSCs were more frequent (consistent with increased vesicular GABA release from presynaptic terminals). Acute nicotine challenge increased sIPSC frequency in both groups, though the increase was greater in DNE cells. Acute nicotine challenge did not change the frequency of mIPSCs in either group, though mIPSC amplitude increased significantly in DNE cells, but not control cells. Stimulation of postsynaptic GABA_A receptors with muscimol caused a significantly greater chloride current in DNE cells than in control cells. The increased quantal release of GABA, coupled with the rise in the strength of postsynaptic inhibition may be homeostatic adjustments to the decreased action-potential-mediated input from GABAergic interneurons. However, this will exaggerate synaptic inhibition under conditions where the release of GABA (e.g. hypoxia) or ACh (sleep-wake transitions) is increased. These findings reveal a mechanism that may explain why DNE is associated with deficits in the ability to respond appropriately to chemosensory stimuli or to changes in neuromodulation secondary to changes in central nervous system state.

Propofol Protects Rat Hypoglossal Motoneurons in an In Vitro Model of Excitotoxicity by Boosting GABAergic Inhibition and Reducing Oxidative Stress

In brainstem motor networks, hypoglossal motoneurons (HMs) play the physiological role of driving tongue contraction, an activity critical for inspiration, phonation, chewing and swallowing. HMs are an early target of neurodegenerative diseases like amyotrophic lateral sclerosis that, in its bulbar form, is manifested with initial dysphagia and dysarthria. One important pathogenetic component of this disease is the high level of extracellular glutamate due to uptake block that generates excitotoxicity. To understand the earliest phases of this condition we devised a model, the rat brainstem slice, in which block of glutamate uptake is associated with intense bursting of HMs, dysmetabolism and death. Since blocking bursting becomes a goal to prevent cell damage, the present report enquired whether boosting GABAergic inhibition could fulfill this aim and confer beneficial outcome. Propofol (0.5 µM) and midazolam (0.01 µM), two allosteric modulators of GABA_A receptors, were used at concentrations yielding analogous potentiation of GABA-mediated currents. Propofol also partly depressed NMDA receptor currents. Both drugs significantly shortened bursting episodes without changing single burst properties, their synchronicity, or their occurrence. Two hours later, propofol prevented the rise in reactive oxygen species (ROS) and, at 4 hours, it inhibited intracellular release of apoptosis-inducing factor (AIF) and prevented concomitant cell loss. Midazolam did not contrast ROS and AIF release. The present work provides experimental evidence for the neuroprotective action of a general anesthetic like propofol, which, in this case, may be achieved through a combination of boosted GABAergic inhibition and reduced ROS production.

Nicotinic receptor activation contrasts pathophysiological bursting and neurodegeneration evoked by glutamate uptake block on rat hypoglossal motoneurons

KEY POINTS

Impaired uptake of glutamate builds up the extracellular level of this excitatory transmitter to trigger rhythmic neuronal bursting and delayed cell death in the brainstem motor nucleus hypoglossus. This process is the expression of the excitotoxicity that underlies motoneuron degeneration in diseases such as amyotrophic lateral sclerosis affecting bulbar motoneurons. In a model of motoneuron excitotoxicity produced by pharmacological block of glutamate uptake in vitro, rhythmic bursting is suppressed by activation of neuronal nicotinic receptors with their conventional agonist nicotine. Emergence of bursting is facilitated by nicotinic receptor antagonists. Following excitotoxicity, nicotinic receptor activity decreases mitochondrial energy dysfunction, endoplasmic reticulum stress and production of toxic radicals. Globally, these phenomena synergize to provide motoneuron protection. Nicotinic receptors may represent a novel target to contrast pathological overactivity of brainstem motoneurons and therefore to prevent their metabolic distress and death.

ABSTRACT

Excitotoxicity is thought to be one of the early processes in the onset of amyotrophic lateral sclerosis (ALS) because high levels of glutamate have been detected in the cerebrospinal fluid of such patients due to dysfunctional uptake of this transmitter that gradually damages brainstem and spinal motoneurons. To explore potential mechanisms to arrest ALS onset, we used an established in vitro model of rat brainstem slice preparation in which excitotoxicity is induced by the glutamate uptake blocker dl-threo-β-benzyloxyaspartate (TBOA). Because certain brain neurons may be neuroprotected via activation of nicotinic acetylcholine receptors (nAChRs) by nicotine, we investigated if nicotine could arrest excitotoxic damage to highly ALS-vulnerable hypoglossal motoneurons (HMs). On 50% of patch-clamped HMs, TBOA induced intense network bursts that were inhibited by 1-10 μm nicotine, whereas nAChR antagonists facilitated burst emergence in non-burster cells. Furthermore, nicotine inhibited excitatory transmission and enhanced synaptic inhibition. Strong neuroprotection by nicotine prevented the HM loss observed after 4 h of TBOA exposure. This neuroprotective action was due to suppression of downstream effectors of neurotoxicity such as increased intracellular levels of reactive oxygen species, impaired energy metabolism and upregulated genes involved in endoplasmic reticulum (ER) stress. In addition, HMs surviving TBOA toxicity often expressed UDP-glucose glycoprotein glucosyltransferase, a key element in repair of misfolded proteins: this phenomenon was absent after nicotine application, indicative of ER stress prevention. Our results suggest nAChRs to be potential targets for inhibiting excitotoxic damage of motoneurons at an early stage of the neurodegenerative process.

Developmental changes in the morphology of mouse hypoglossal motor neurons

Hypoglossal motor neurons (XII MNs) innervate tongue muscles important in breathing, suckling and vocalization. Morphological properties of 103 XII MNs were studied using Neurobiotin™ filling in transverse brainstem slices from C57/Bl6 mice (n = 34) from embryonic day (E) 17 to postnatal day (P) 28. XII MNs from areas thought to innervate different tongue muscles showed similar morphology in most, but not all, features. Morphological properties of XII MNs were established prior to birth, not differing between E17-18 and P0. MN somatic volume gradually increased for the first 2 weeks post-birth. The complexity of dendritic branching and dendrite length of XII MNs increased throughout development (E17-P28). MNs in the ventromedial XII motor nucleus, likely to innervate the genioglossus, frequently (42 %) had dendrites crossing to the contralateral side at all ages, but their number declined with postnatal development. Unexpectedly, putative dendritic spines were found in all XII MNs at all ages, and were primarily localized to XII MN somata and primary dendrites at E18-P4, increased in distal dendrites by P5-P8, and were later predominantly found in distal dendrites. Dye-coupling between XII MNs was common from E18 to P7, but declined strongly with maturation after P7. Axon collaterals were found in 20 % (6 of 28) of XII MNs with filled axons; collaterals terminated widely outside and, in one case, within the XII motor nucleus. These results reveal new morphological features of mouse XII MNs, and suggest that dendritic projection patterns, spine density and distribution, and dye-coupling patterns show specific developmental changes in mice.

Emerging Roles of Filopodia and Dendritic Spines in Motoneuron Plasticity during Development and Disease

Motoneurons develop extensive dendritic trees for receiving excitatory and inhibitory synaptic inputs to perform a variety of complex motor tasks. At birth, the somatodendritic domains of mouse hypoglossal and lumbar motoneurons have dense filopodia and spines. Consistent with Vaughn's synaptotropic hypothesis, we propose a developmental unified-hybrid model implicating filopodia in motoneuron spinogenesis/synaptogenesis and dendritic growth and branching critical for circuit formation and synaptic plasticity at embryonic/prenatal/neonatal period. Filopodia density decreases and spine density initially increases until postnatal day 15 (P15) and then decreases by P30. Spine distribution shifts towards the distal dendrites, and spines become shorter (stubby), coinciding with decreases in frequency and increases in amplitude of excitatory postsynaptic currents with maturation. In transgenic mice, either overexpressing the mutated human Cu/Zn-superoxide dismutase (hSOD1^G93A) gene or deficient in GABAergic/glycinergic synaptic transmission (gephyrin, GAD-67, or VGAT gene knockout), hypoglossal motoneurons develop excitatory glutamatergic synaptic hyperactivity. Functional synaptic hyperactivity is associated with increased dendritic growth, branching, and increased spine and filopodia density, involving actin-based cytoskeletal and structural remodelling. Energy-dependent ionic pumps that maintain intracellular sodium/calcium homeostasis are chronically challenged by activity and selectively overwhelmed by hyperactivity which eventually causes sustained membrane depolarization leading to excitotoxicity, activating microglia to phagocytose degenerating neurons under neuropathological conditions.

To Break or to Brake Neuronal Network Accelerated by Ammonium Ions?

Purpose

The aim of present study was to investigate the effects of ammonium ions on in vitro neuronal network activity and to search alternative methods of acute ammonia neurotoxicity prevention.

Methods

Rat hippocampal neuronal and astrocytes co-cultures in vitro, fluorescent microscopy and perforated patch clamp were used to monitor the changes in intracellular Ca²⁺- and membrane potential produced by ammonium ions and various modulators in the cells implicated in neural networks.

Results

Low concentrations of NH₄Cl (0.1–4 mM) produce short temporal effects on network activity. Application of 5–8 mM NH₄Cl: invariably transforms diverse network firing regimen to identical burst patterns, characterized by substantial neuronal membrane depolarization at plateau phase of potential and high-amplitude Ca²⁺-oscillations; raises frequency and average for period of oscillations Ca²⁺-level in all cells implicated in network; results in the appearance of group of «run out» cells with high intracellular Ca²⁺ and steadily diminished amplitudes of oscillations; increases astrocyte Ca²⁺-signalling, characterized by the appearance of groups of cells with increased intracellular Ca²⁺-level and/or chaotic Ca²⁺-oscillations. Accelerated network activity may be suppressed by the blockade of NMDA or AMPA/kainate-receptors or by overactivation of AMPA/kainite-receptors. Ammonia still activate neuronal firing in the presence of GABA(A) receptors antagonist bicuculline, indicating that «disinhibition phenomenon» is not implicated in the mechanisms of networks acceleration. Network activity may also be slowed down by glycine, agonists of metabotropic inhibitory receptors, betaine, L-carnitine, L-arginine, etc.

Conclusions

Obtained results demonstrate that ammonium ions accelerate neuronal networks firing, implicating ionotropic glutamate receptors, having preserved the activities of group of inhibitory ionotropic and metabotropic receptors. This may mean, that ammonia neurotoxicity might be prevented by the activation of various inhibitory receptors (i.e. by the reinforcement of negative feedback control), instead of application of various enzyme inhibitors and receptor antagonists (breaking of neural, metabolic and signaling systems).

P2Y1 receptor-mediated potentiation of inspiratory motor output in neonatal rat in vitro

PreBötzinger complex inspiratory rhythm-generating networks are excited by metabotropic purinergic receptor subtype 1 (P2Y1R) activation. Despite this, and the fact that inspiratory MNs express P2Y1Rs, the role of P2Y1Rs in modulating motor output is not known for any MN pool. We used rhythmically active brainstem-spinal cord and medullary slice preparations from neonatal rats to investigate the effects of P2Y1R signalling on inspiratory output of phrenic and XII MNs that innervate diaphragm and airway muscles, respectively. MRS2365 (P2Y1R agonist, 0.1 mm) potentiated XII inspiratory burst amplitude by 60 ± 9%; 10-fold higher concentrations potentiated C4 burst amplitude by 25 ± 7%. In whole-cell voltage-clamped XII MNs, MRS2365 evoked small inward currents and potentiated spontaneous EPSCs and inspiratory synaptic currents, but these effects were absent in TTX at resting membrane potential. Voltage ramps revealed a persistent inward current (PIC) that was attenuated by: flufenamic acid (FFA), a blocker of the Ca(2+)-dependent non-selective cation current ICAN; high intracellular concentrations of BAPTA, which buffers Ca(2+) increases necessary for activation of ICAN; and 9-phenanthrol, a selective blocker of TRPM4 channels (candidate for ICAN). Real-time PCR analysis of mRNA extracted from XII punches and laser-microdissected XII MNs revealed the transcript for TRPM4. MRS2365 potentiated the PIC and this potentiation was blocked by FFA, which also blocked the MRS2365 potentiation of glutamate currents. These data suggest that XII MNs are more sensitive to P2Y1R modulation than phrenic MNs and that the P2Y1R potentiation of inspiratory output occurs in part via potentiation of TRPM4-mediated ICAN, which amplifies inspiratory inputs.

Bicuculline- and neurosteroid-sensitive tonic chloride current in rat hypoglossal motoneurons and atypical dual effect of SR95531

Hypoglossal motoneurons (HMs) are known to be under 'permanent' bicuculline-sensitive inhibition and to show 'transient' synaptic γ-aminobutyric acid (GABA)(A) and glycine inhibitory responses. The present paper describes a permanent bicuculline-sensitive current that should contribute to their tonic inhibition. This current was recorded in brainstem slices superfused without any exogenous agonist and remained detectable with tetrodotoxin. It could also be blocked by the other GABA(A) antagonists picrotoxin (PTX) and 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide) (SR95531; gabazine), but persisted in the presence of a specific blocker of α5-containing GABA(A) receptors. Addition of 2 μm 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), known to preferentially activate GABA(A) receptors devoid of a γ-subunit, induced a sustained anionic current that could be further enhanced by neurosteroids such as allopregnanolone (100 nm). Thus, HMs show a tonic inhibitory current carried by extrasynaptic γ-free GABA(A) receptors, highly sensitive to neurosteroids. A second result was obtained by using SR95531 at concentrations sufficiently high to rapidly block the tonic current above the chloride equilibrium potential (E(C) (l)). Surprisingly, below E(C) (l) , SR95531 (10-40 μm) activated a sustained inward current, associated with a conductance increase, and resistant to bicuculline or PTX (100 μm). Similarly, after blockade of the bicuculline-sensitive current, SR95531 activated an outward current above E(C) (l). The bicuculline-resistant anionic current activated by SR95531 could be blocked by a GABA(C) receptor antagonist. Thus, two types of inhibitory GABA receptors, belonging to the GABA(A) and GABA(C) families, are able to show a sustained activity in HMs and provide promising targets for neuroprotection under overexcitatory situations known to easily damage these particularly fragile neurons.

Statistical computer model analysis of the reciprocal and recurrent inhibitions of the Ia-EPSP in α-motoneurons

We simulate the inhibition of Ia-glutamatergic excitatory postsynaptic potential (EPSP) by preceding it with glycinergic recurrent (REN) and reciprocal (REC) inhibitory postsynaptic potentials (IPSPs). The inhibition is evaluated in the presence of voltage-dependent conductances of sodium, delayed rectifier potassium, and slow potassium in five α-motoneurons (MNs). We distribute the channels along the neuronal dendrites using, alternatively, a density function of exponential rise (ER), exponential decay (ED), or a step function (ST). We examine the change in EPSP amplitude, the rate of rise (RR), and the time integral (TI) due to inhibition. The results yield six major conclusions. First, the EPSP peak and the kinetics depending on the time interval are either amplified or depressed by the REC and REN shunting inhibitions. Second, the mean EPSP peak, its TI, and RR inhibition of ST, ER, and ED distributions turn out to be similar for analogous ranges of G. Third, for identical G, the large variations in the parameters' values can be attributed to the sodium conductance step (g(Na_step)) and the active dendritic area. We find that small g(Na_step) on a few dendrites maintains the EPSP peak, its TI, and RR inhibition similar to the passive state, but high g(Na_step) on many dendrites decrease the inhibition and sometimes generates even an excitatory effect. Fourth, the MN's input resistance does not alter the efficacy of EPSP inhibition. Fifth, the REC and REN inhibitions slightly change the EPSP peak and its RR. However, EPSP TI is depressed by the REN inhibition more than the REC inhibition. Finally, only an inhibitory effect shows up during the EPSP TI inhibition, while there are both inhibitory and excitatory impacts on the EPSP peak and its RR.

Development of synaptic transmission to respiratory motoneurons

Respiratory motoneurons provide the exclusive drive to respiratory muscles and therefore are a key relay between brainstem neural circuits that generate respiratory rhythm and respiratory muscles that control moment of gases into and out of the airways and lungs. This review is focused on postnatal development of fast ionotropic synaptic transmission to respiratory motoneurons, with a focus on hypoglossal motoneurons (HMs). Glutamatergic synaptic transmission to HMs involves activation of both non-NMDA and NMDA receptors and during the postnatal period co-activation of these receptors located at the same synapse may occur. Further, the relative role of each receptor type in inspiratory-phase motoneuron depolarization is dependent on the type of preparation used (in vitro versus in vivo; neonatal versus adult). Respiratory motoneurons receive both glycinergic and GABAergic inhibitory synaptic inputs. During inspiration phrenic and HMs receive concurrent excitatory and inhibitory synaptic inputs. During postnatal development in HMs GABAergic and glycinergic synaptic inputs have slow kinetics and are depolarizing and with postnatal development they become faster and hyperpolarizing. Additionally shunting inhibition may play an important role in synaptic processing by respiratory motoneurons.

Riluzole is a potent drug to protect neonatal rat hypoglossal motoneurons in vitro from excitotoxicity due to glutamate uptake block

Excitotoxic damage to motoneurons is thought to be an important contribution to the pathogenesis of amyotrophic lateral sclerosis (ALS), a slowly developing degeneration of motoneurons that, in most cases of sporadic occurrence, is associated with impaired glial glutamate uptake. Riluzole is the only drug licensed for symptomatic ALS treatment and is proposed to delay disease progression. As riluzole is administered only after full ALS manifestation, it is unclear if its early use might actually prevent motoneuron damage. We explored this issue by using, as a simple in vitro model, hypoglossal motoneurons (a primary target of ALS) of the neonatal rat brainstem slice preparation exposed to excitotoxic stress due to glutamate uptake block by DL-threo-β-benzyloxyaspartate (TBOA). TBOA evoked sustained network bursting, early (1 h) enhancement of the S100B immunostaining of gray matter astrocytes, and activated the motoneuronal stress ATF-3 transcription factor; 4 h later, loss (30%) of motoneuron staining ensued and pyknosis appeared. Riluzole (5 μM; applied 15 min after TBOA) inhibited bursting, decreased the frequency of spontaneous glutamatergic events, reversed changes in S100B immunostaining and prevented late loss of motoneuron staining. These results show that excitotoxicity induced by glutamate uptake block developed slowly, and was sensed by glia and motoneurons with delayed cell death. Our data provide novel evidence for the neuroprotective action of riluzole on motoneurons and glia when applied early after an excitotoxic stimulus.

Synchronization of presynaptic input to motor units of tongue, inspiratory intercostal, and diaphragm muscles

The respiratory central pattern generator distributes rhythmic excitatory input to phrenic, intercostal, and hypoglossal premotor neurons. The degree to which this input shapes motor neuron activity can vary across respiratory muscles and motor neuron pools. We evaluated the extent to which respiratory drive synchronizes the activation of motor unit pairs in tongue (genioglossus, hyoglossus) and chest-wall (diaphragm, external intercostals) muscles using coherence analysis. This is a frequency domain technique, which characterizes the frequency and relative strength of neural inputs that are common to each of the recorded motor units. We also examined coherence across the two tongue muscles, as our previous work shows that, despite being antagonists, they are strongly coactivated during the inspiratory phase, suggesting that excitatory input from the premotor neurons is distributed broadly throughout the hypoglossal motoneuron pool. All motor unit pairs showed highly correlated activity in the low-frequency range (1-8 Hz), reflecting the fundamental respiratory frequency and its harmonics. Coherence of motor unit pairs recorded either within or across the tongue muscles was similar, consistent with broadly distributed premotor input to the hypoglossal motoneuron pool. Interestingly, motor units from diaphragm and external intercostal muscles showed significantly higher coherence across the 10-20-Hz bandwidth than tongue-muscle units. We propose that the lower coherence in tongue-muscle motor units over this range reflects a larger constellation of presynaptic inputs, which collectively lead to a reduction in the coherence between hypoglossal motoneurons in this frequency band. This, in turn, may reflect the relative simplicity of the respiratory drive to the diaphragm and intercostal muscles, compared with the greater diversity of functions fulfilled by muscles of the tongue.

GAD67-GFP+ neurons in the Nucleus of Roller: a possible source of inhibitory input to hypoglossal motoneurons. I. Morphology and firing properties

In this study we examined the electrophysiological and morphological properties of inhibitory neurons located just ventrolateral to the hypoglossal motor (XII) nucleus in the Nucleus of Roller (NR). In vitro experiments were performed on medullary slices derived from postnatal day 5 (P5) to P15 GAD67-GFP knock-in mouse pups. on cell recordings from GFP+ cells in NR in rhythmic slices revealed that these neurons are spontaneously active, although their spiking activity does not exhibit inspiratory phase modulation. Morphologically, GFP+ cells were bi- or multipolar cells with small- to medium-sized cell bodies and small dendritic trees that were often oriented parallel to the border of the XII nucleus. GFP+ cells were classified as either tonic or phasic based on their firing responses to depolarizing step current stimulation in whole cell current clamp. Tonic GFP+ cells fired a regular train of action potentials (APs) throughout the duration of the pulse and often showed rebound spikes after a hyperpolarizing step. In contrast, phasic GFP+ neurons did not fire throughout the depolarizing current step but instead fired fewer than four APs at the onset of the pulse or fired multiple APs, but only after a marked delay. Phasic cells had a significantly smaller input resistance and shorter membrane time constant than tonic GFP+ cells. In addition, phasic GFP+ cells differed from tonic cells in the shape and time course of their spike afterpotentials, the minimum firing frequency at threshold current amplitude, and the slope of their current-frequency relationship. These results suggest that GABAergic neurons in the NR are morphologically and electrophysiologically heterogeneous cells that could provide tonic inhibitory synaptic input to HMs.

Postnatal development enhances the effects of cholinergic inputs on recruitment threshold and firing rate of rat oculomotor nucleus motoneurons

Changes in the electrophysiological and morphological characteristics of motoneurons (Mns) of the oculomotor nucleus during postnatal development have been reported, however synaptic modifications that take place concurrently with postnatal development in these Mns are yet to be elucidated. We investigated whether cholinergic inputs exert different effects on the recruitment threshold and firing rate of Mns during postnatal development. Rat oculomotor nucleus Mns were intracellularly recorded in brain slice preparations and separated in neonatal (4-7 postnatal days) and adult (20-30 postnatal days) age groups. Stimulation of the medial longitudinal fasciculus evoked a monosynaptic excitatory potential in Mns that was attenuated with atropine (1.5 μM, a muscarinic antagonist). Mns were silent at their resting membrane potential, and bath application of carbachol (10 μM, a cholinergic agonist) induced depolarization of the membrane potential and a sustained firing rate that were more pronounced in adult Mns. Pharmacological and immunohistochemical assays showed that these responses were attributable to muscarinic receptors located in the membrane of Mns. In addition, compared to control Mns, carbachol-exposed Mns exhibited a higher firing rate in response to the injection of the same amount of current, and a decrease in the current threshold required to achieve sustained firing. These latter effects were more pronounced in adult than in neonatal Mns. In conclusion, our findings suggest that cholinergic synaptic inputs are already present in neonatal Mns, and that the electrophysiological effects of such inputs on recruitment threshold and firing rate are enhanced with the postnatal development in oculomotor nucleus Mns. We propose that cholinergic input maturation could provide a greater dynamic range in adult Mns to encode the output necessary for graded muscle contraction.

Transient oxidative stress evokes early changes in the functional properties of neonatal rat hypoglossal motoneurons in vitro

Oxidative stress of motoneurons is believed to be an important contributor to neurodegeneration underlying the familial (and perhaps even the sporadic) form of amyotrophic lateral sclerosis (ALS). This concept has generated numerous rodent genetic models with inborn oxidative stress to mimic the clinical condition. ALS is, however, a predominantly sporadic disorder probably triggered by environmental causes. Thus, it is interesting to understand how wild-type motoneurons react to strong oxidative stress as this response might cast light on the presymptomatic disease stage. The present study used, as a model, hypoglossal motoneurons from the rat brainstem slice to investigate how hydrogen peroxide could affect synaptic transmission and intrinsic motoneuron excitability in relation to their survival. Hydrogen peroxide (1 mm; 30 min) induced inward current or membrane depolarization accompanied by an increase in input resistance, enhanced firing and depressed spontaneous synaptic events. Despite enhanced intracellular oxidative processes, there was no death of motoneurons, although most cells were immunopositive for activating transcription factor 3, a stress-related transcription factor. Voltage-clamp experiments indicated increased frequency of excitatory or inhibitory miniature events, and reduced voltage-gated persistent currents of motoneurons. The global effect of this transient oxidative challenge was to depress the input flow from the premotor interneurons to motoneurons that became more excitable due to a combination of enhanced input resistance and impaired spike afterhyperpolarization. Our data show previously unreported changes in motoneuron activity associated with cell distress caused by a transient oxidative insult.

A repertoire of rhythmic bursting produced by hypoglossal motoneurons in physiological and pathological conditions

The brainstem nucleus hypoglossus contains motoneurons that provide the exclusive motor nerve supply to the tongue. In addition to voluntary tongue movements, tongue muscles rhythmically contract during a wide range of physiological activities, such as respiration, swallowing, chewing and sucking. Hypoglossal motoneurons are destroyed early in amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease often associated with a deficit in the transport system of the neurotransmitter glutamate. The present study shows how periodic electrical discharges of motoneurons are mainly produced by a neuronal network that drives them into bursting mode via glutamatergic excitatory synapses. Burst activity is, however, modulated by the intrinsic properties of motoneurons that collectively synchronize their discharges via gap junctions to create 'group bursters'. When glial uptake of glutamate is blocked, a distinct form of pathological bursting spontaneously emerges and leads to motoneuron death. Conversely, H(2)O(2)-induced oxidative stress strongly increases motoneuron excitability without eliciting bursting. Riluzole (the only drug currently licensed for the treatment of ALS) suppresses bursting of hypoglossal motoneurons caused by blockage of glutamate uptake and limits motoneuron death. These findings highlight how different patterns of electrical oscillations of brainstem motoneurons underpin not only certain physiological activities, but also motoneuron death induced by glutamate transporter impairment.

GABAergic and glycinergic interneuron expression during spinal cord development: dynamic interplay between inhibition and excitation in the control of ventral network outputs

A key objective of neuroscience research is to understand the processes leading to mature neural circuitries in the central nervous system (CNS) that enable the control of different behaviours. During development, network-constitutive neurons undergo dramatic rearrangements, involving their intrinsic properties, such as the blend of ion channels governing their firing activity, and their synaptic interactions. The spinal cord is no exception to this rule; in fact, in the ventral horn the maturation of motor networks into functional circuits is a complex process where several mechanisms cooperate to achieve the development of motor control. Elucidating such a process is crucial in identifying neurons more vulnerable to degenerative or traumatic diseases or in developing new strategies aimed at rebuilding damaged tissue. The focus of this review is on recent advances in understanding the spatio-temporal expression of the glycinergic/GABAergic system and on the contribution of this system to early network function and to motor pattern transformation along with spinal maturation. During antenatal development, the operation of mammalian spinal networks strongly depends on the activity of glycinergic/GABAergic neurons, whose action is often excitatory until shortly before birth when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motor neurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA. At this stage of spinal maturation, the large majority of GABAergic neurons are located in the dorsal horn. We propose that elucidating the role of inhibitory systems in development will improve our knowledge on the processes regulating spinal cord maturation.

GABAA and strychnine-sensitive glycine receptors modulate N-methyl-D-aspartate-evoked acetylcholine release from rat spinal motoneurons: a possible role in neuroprotection

Increasing experimental and clinical evidence suggests that abnormal glutamate transmission might play a major role in a vast number of neurological disorders. As a measure of glutamatergic excitation, we have studied the acetylcholine (ACh) release induced by N-methyl-D-aspartate (NMDA) receptor stimulation in primary cultured rat ventral horn spinal neurons and we have evaluated the possibility to limit the consequences of the hyperactivation of glutamatergic receptors, by recruiting the inhibitory transmission mediated by GABA and glycine. For this purpose, we have exposed cell cultures, previously loaded with [(3)H]choline, to NMDA, which increased the spontaneous tritium efflux in a concentration-dependent manner. Tritium release is dependent upon external Ca(2+), tetrodotoxin, Cd(2+) ions and omega-conotoxin GVIA, but not on omega-conotoxin MVIIC nor nifedipine, suggesting the involvement of N-type voltage-sensitive calcium channels. NMDA-mediated [(3)H]ACh release was completely prevented by MK-801, 5,7-diclorokynurenic acid and ifenprodil, while it was strongly inhibited by a lower external pH, suggesting that the involved NMDA receptors contain NR1 and NR2B subunits. Muscimol inhibited NMDA-evoked [(3)H]ACh release and its effect was antagonized by SR95531 and potentiated by diazepam, indicating the involvement of benzodiazepine-sensitive GABA(A) receptors. Also glycine, via strychnine-sensitive receptors, inhibited the effect of NMDA. It is concluded that glutamate acts on the NMDA receptors situated on spinal motoneurons to evoke ACh release, which can be inhibited through the activation of GABA(A) and glycine receptors present on the same neurons. These data suggest that glutamatergic overload of receptors located onto spinal cord motoneurons might be decreased by activating GABA(A) and glycine receptors.

Dual personality of GABA/glycine-mediated depolarizations in immature spinal cord

The inhibitory action of glycine and GABA in adult neurons consists of both shunting incoming excitations and moving the membrane potential away from the action potential (AP) threshold. By contrast, in immature neurons, inhibitory postsynaptic potentials (IPSPs) are depolarizing; it is generally accepted that, despite their depolarizing action, these IPSPs are inhibitory because of the shunting action of the Cl(-) conductance increase. Here we investigated the integration of depolarizing IPSPs (dIPSPs) with excitatory inputs in the neonatal rodent spinal cord by means of both intracellular recordings from lumbar motoneurons and a simulation using the compartment model program "Neuron." We show that the ability of IPSPs to suppress suprathreshold excitatory events depends on E(Cl) and the location of inhibitory synapses. The depolarization outlasts the conductance changes and spreads electrotonically in the somatodendritic tree, whereas the shunting effect is restricted and local. As a consequence, dIPSPs facilitated AP generation by subthreshold excitatory events in the late phase of the response. The window of facilitation became wider as E(Cl) was more depolarized and started earlier as inhibitory synapses were moved away from the excitatory input. GAD65/67 immunohistochemistry demonstrated the existence of distal inhibitory synapses on motoneurons in the neonatal rodent spinal cord. This study demonstrates that small dIPSPs can either inhibit or facilitate excitatory inputs depending on timing and location. Our results raise the possibility that inhibitory synapses exert a facilitatory action on distant excitatory inputs and slight changes of E(Cl) may have important consequences for network processing.

An ex vivo preparation of mature mice spinal cord to study synaptic transmission on motoneurons

Mammalian spinal cord motoneurons are highly susceptible to chemical and mechanical disturbances, which imposes substantial difficulties for electrophysiological investigation in acute in vitro preparations. The aim of the present study was to establish an isolated spinal cord preparation from adult mice and to examine the synaptic activities of motoneurons in vitro. We removed the lumbo-sacral cord from the vertebral canal by hydraulic extrusion and maintained the isolated cord in vitro for extracellular recordings. Population spikes of motoneurons were evoked by electrical stimulation of dorsal roots (orthodromic) or ventral roots (antidromic) and these evoked responses could be continuously monitored for 5-6 h. The orthodromic population spikes were reversibly suppressed by the AMPA/kainate receptor antagonist 2,3-dihyro-6-nitro-7-sulfamoylbenzo quinoxaline (NBQX, 10 microM) but they persisted in the presence of the NMDA receptor antagonist D(-)-2-amino-5-phosphonovaleric acid (AP5, 50 microM). The antidromic population spikes exhibited evident paired pulse inhibition when evoked at inter-stimulus intervals of pound 6 ms. Histological examination revealed that basic morphological features of the lumbo-sacral motoneurons were preserved after 3-4 h of in vitro maintenance. This in vitro preparation is ideally suited for the electrophysiological study of synaptic transmission on adult mouse spinal motoneurons.

Developmental changes in distribution of γ-aminobutyric acid- and glycine-immunoreactive boutons on rat trigeminal motoneurons. I. Jaw-closing motoneurons

We have previously described the distribution pattern of inhibitory synapses on rat jaw-closing (JC) alpha- and gamma-motoneurons. In the present study, we investigated developmental changes in inhibitory synapses on JC motoneurons. We performed a quantitative ultrastructural analysis of putative inhibitory synaptic boutons on JC motoneuron somata by using postembedding immunogold labeling for GABA and glycine. In total, 206, 350, and 497 boutons contacting JC motoneuron somata were analyzed at postnatal days 2 (P2), 11 (P11) and 31 (P31), respectively. The size of the somata increased significantly during postnatal development. The size distribution was bimodal at P31. Mean length of the boutons and percentage of synaptic covering also increased during postnatal development, whereas bouton density did not differ significantly among the three age groups. Synaptic boutons on the somata of JC alpha-motoneurons could be classified into four types: boutons immunoreactive for 1) GABA only, 2) glycine only, 3) both GABA and glycine, and 4) neither GABA nor glycine. There was no developmental change in the proportion of putative inhibitory boutons to the total number of studied boutons. However, the glycine-only boutons increased significantly (15.1% to 27.3%), and the GABA-only boutons decreased significantly (17.7% to 2.6%) during the period from P11 to P31. Our ultrastructural data indicate that the inhibitory synaptic input to JC motoneurons is developmentally regulated and that there is a postnatal switch from GABA to glycine. The postnatal changes revealed in the present study could play an important role in the maturation of the oral motor system.

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.

Persistent rhythmic oscillations induced by nicotine on neonatal rat hypoglossal motoneurons in vitro

Patch-clamp recording from hypoglossal motoneurons in neonatal Wistar rat brainstem slices was used to investigate the electrophysiological effects of bath-applied nicotine (10 microm). While nicotine consistently evoked membrane depolarization (or inward current under voltage clamp), it also induced electrical oscillations (3-13 Hz; lasting for >/= 8.5 min) on 40% of motoneurons. Oscillations required activation of nicotinic receptors sensitive to dihydro-beta-erythroidine (0.5 microm) or methyllycaconitine (5 nm), and were accompanied by enhanced frequency of spontaneous glutamatergic events. The slight voltage dependence of oscillations and their block by the gap junction blocker, carbenoxolone, suggest they originate from electrically coupled neurons. Network nicotinic receptors desensitized more slowly than motoneuron ones, demonstrating that network receptors remained active longer to support heightened release of the endogenous glutamate necessary for enhancing the network excitability. The ionotropic glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and the group I metabotropic receptor antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), suppressed oscillations, while the NMDA receptor antagonist, d-amino-phosphonovaleriate (APV), produced minimal depression. Nicotine-evoked oscillations constrained spike firing at low rates, although motoneurons could still generate high-frequency trains of action potentials with unchanged gain for input depolarization. This is the first demonstration that persistent activation of nicotinic receptors could cause release of endogenous glutamate to evoke sustained oscillations in the theta frequency range. As this phenomenon likely represented a powerful process to coordinate motor output to tongue muscles, our results outline neuronal nicotinic acetylcholine receptors (nAChRs) as a novel target for pharmacological enhancement of motoneuron output in motor dysfunction.

Inhibitory postsynaptic potentials in lumbar motoneurons remain depolarizing after neonatal spinal cord transection in the rat

GABA and glycine are excitatory in the immature spinal cord and become inhibitory during development. The shift from depolarizing to hyperpolarizing inhibitory postsynaptic potentials (IPSPs) occurs during the perinatal period in the rat, a time window during which the projections from the brain stem reach the lumbar enlargement. In this study, we investigated the effects of suppressing influences of the brain on lumbar motoneurons during this critical period for the negative shift of the reversal potential of IPSPs (E(IPSP)). The spinal cord was transected at the thoracic level on the day of birth [postnatal day 0 (P0)]. E(IPSP), at P4-P7, was significantly more depolarized in cord-transected than in cord-intact animals (E(IPSP) above and below resting potential, respectively). E(IPSP) at P4-P7 in cord-transected animals was close to E(IPSP) at P0-P2. K-Cl cotransporter KCC2 immunohistochemistry revealed a developmental increase of staining in the area of lumbar motoneurons between P0 and P7 in cord-intact animals; this increase was not observed after spinal cord transection. The motoneurons recorded from cord-transected animals were less sensitive to the experimental manipulations aimed at testing the functionality of the KCC2 system, which is sensitive to [K(+)](o) and blocked by bumetanide. Although bumetanide significantly depolarized E(IPSP), the shift was less pronounced than in cord-intact animals. In addition, a reduction of [K(+)](o) affected E(IPSP) significantly only in cord-intact animals. Therefore influences from the brain stem may play an essential role in the maturation of inhibitory synaptic transmission, possibly by upregulating KCC2 and its functionality.

Developmental changes in the expression of GABAA receptor subunits alpha1, alpha2, and alpha3 in brain stem nuclei of rats

Gamma-aminobutyric acid (GABA)(A) receptor subunit switching is a suggested postnatal mechanism for changes in GABA transmission from depolarization to hyperpolarization. Previously, we found an apparent switch between GABA(A) alpha3 and alpha1 subunit expression in the rat pre-Bötzinger complex (PBC) on postnatal day (P) 12, a presumed peak critical period of respiratory nuclei development. The present study aimed at determining if GABA(A) subunit switching occurred in another respiratory nucleus, the ventrolateral subnucleus of the solitary tract nucleus (NTS(VL)), and in a non-respiratory cuneate nucleus (CN) of P0 to P21 rats. In both nuclei: (1) the expression of GABA(A) alpha1 subunit was relatively low at birth but increased with development; (2) that of GABA(A) alpha3 was relatively high at birth but declined with age; and (3) GABA(A) alpha2 remained relatively low and constant throughout development. However, the specific patterns differed between the two nuclei, but were similar between the NTS(VL) and the PBC. In the NTS(VL), GABA(A) alpha1 expression gradually increased from birth and peaked at P12, whereas that in the CN sharply rose from P7 and peaked at P10. GABA(A) alpha3 expression had a prominent decrease from P11 to P12 in the NTS(VL), whereas that in the CN only gradually declined from P10 to P12. The developmental trends of alpha1 and alpha3 in the NTS(VL) intersected close to P12, whereas those in the CN intersected at P10. Despite differences in timing, GABA(A) alpha subunit switching may be a common theme in the brain stem that may mediate different functional properties of GABA transmission.

Inhibitory synaptic transmission governs inspiratory motoneuron synchronization

Neurons within the intact respiratory network produce bursts of action potentials that cause inspiration or expiration. Within inspiratory bursts, activity is synchronized on a shorter timescale to generate clusters of action potentials that occur in a set frequency range and are called synchronous oscillations. We investigated how GABA and glycine modulate synchronous oscillations and respiratory rhythm during postnatal development. We recorded inspiratory activity from hypoglossal nerves using the in vitro rhythmically active mouse medullary slice preparation from P0-P11 mice. Average oscillation frequency increased with postnatal development, from 17 +/- 12 Hz in P0-P6 mice (n = 15) to 38 +/- 7 Hz in P7-P11 mice (n = 37) (P < 0.0001). Bath application of GABAA and GlyR antagonists significantly reduced oscillation power in neonates (P0-P6) and juveniles (P7-P10) and increased peak integrated activity in both age groups. To test whether elevating slice excitability is sufficient to reduce oscillation power, Substance P was bath applied alone. Substance P, although increasing peak integrated activity, had no significant effect on oscillation power. Prolonging the time course of GABAergic synaptic currents with zolpidem decreased the median oscillation frequency in P9-P10 mouse slices. These data demonstrate that oscillation frequency increases with postnatal development and that both GABAergic and glycinergic transmission contribute to synchronization of activity. Further, the time course of synaptic GABAergic currents is a determinant of oscillation frequency.

Activation and desensitization of neuronal nicotinic receptors modulate glutamatergic transmission on neonatal rat hypoglossal motoneurons

In the neonate the muscles of the tongue, which are exclusively innervated by the XII cranial nerves originating from the brainstem nucleus hypoglossus, must contract rhythmically in coincidence with breathing, suckling and swallowing. These motor commands are generated by hypoglossal motoneurons excited by glutamatergic inputs. Because in forebrain areas the efficiency of glutamatergic transmission is modulated by neuronal nicotinic receptors (nAChRs), the role and identity of nAChRs within the nucleus hypoglossus of the neonatal rat were explored using an in vitro brainstem slice preparation. This area expressed immunoreactivity for alpha4, alpha7 and beta2 nAChR subunits. Whole-cell patch-clamp recording from hypoglossal motoneurons showed lack of spontaneous cholinergic events mediated by nAChRs even in the presence of a cholinesterase inhibitor. However, pharmacological antagonism of alpha7- or beta2-containing receptors depressed glutamatergic currents arising either spontaneously or by electrical stimulation of the reticular formation. Hypoglossal motoneurons expressed functional nAChRs with characteristics of alpha4beta2 and alpha7 receptor subunits. Such receptors underwent fast desensitization (time constant of 200 ms) with full recovery within 1 min. Low (0.5 microm) concentration of nicotine first facilitated glutamatergic transmission on motoneurons and later depressed it through receptor desensitization. When 0.1 microm nicotine was used, only depression of synaptic transmission occurred, in keeping with the suggestion that nAChRs can be desensitized without prior activation. These results highlight the role of tonic nAChR activity in shaping excitatory inputs to hypoglossal motoneurons, and suggest that nAChR desensitization by ambient nicotine could contribute to disorders of tongue muscle movements.

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.

Glycinergic and GABAergic synaptic activity differentially regulate motoneuron survival and skeletal muscle innervation

GABAergic and glycinergic synaptic transmission is proposed to promote the maturation and refinement of the developing CNS. Here we provide morphological and functional evidence that glycinergic and GABAergic synapses control motoneuron development in a region-specific manner during programmed cell death. In gephyrin-deficient mice that lack all postsynaptic glycine receptor and some GABA(A) receptor clusters, there was increased spontaneous respiratory motor activity, reduced respiratory motoneuron survival, and decreased innervation of the diaphragm. In contrast, limb-innervating motoneurons showed decreased spontaneous activity, increased survival, and increased innervation of their target muscles. Both GABA and glycine increased limb-innervating motoneuron activity and decreased respiratory motoneuron activity in wild-type mice, but only glycine responses were abolished in gephyrin-deficient mice. Our results provide genetic evidence that the development of glycinergic and GABAergic synaptic inputs onto motoneurons plays an important role in the survival, axonal branching, and spontaneous activity of motoneurons in developing mammalian embryos.

Dual Ca2+ modulation of glycinergic synaptic currents in rodent hypoglossal motoneurones

Glycinergic synapses are implicated in the coordination of reflex responses, sensory signal processing and pain sensation. Their activity is pre- and postsynaptically regulated, although mechanisms are poorly understood. Using patch-clamp recording and Ca2+ imaging in hypoglossal motoneurones from rat and mouse brainstem slices, we address here the role of cytoplasmic Ca2+ (Ca(i)) in glycinergic synapse modulation. Ca2+ influx through voltage-gated or NMDA receptor channels caused powerful transient inhibition of glycinergic IPSCs. This effect was accompanied by an increase in both the failure rate and paired-pulse ratio, as well as a decrease in the frequency of mIPSCs, suggesting a presynaptic mechanism of depression. Inhibition was reduced by the cannabinoid receptor antagonist SR141716A and occluded by the agonist WIN55,212-2, indicating involvement of endocannabinoid retrograde signalling. Conversely, in the presence of SR141716A, glycinergic IPSCs were potentiated postsynaptically by glutamate or NMDA, displaying a Ca2(+)-dependent increase in amplitude and decay prolongation. Both presynaptic inhibition and postsynaptic potentiation were completely prevented by strong Ca(i) buffering (20 mm BAPTA). Our findings demonstrate two independent mechanisms by which Ca2+ modulates glycinergic synaptic transmission: (i) presynaptic inhibition of glycine release and (ii) postsynaptic potentiation of GlyR-mediated responses. This dual Ca2(+)-induced regulation might be important for feedback control of neurotransmission in a variety of glycinergic networks in mammalian nervous systems.

Activation of group I metabotropic glutamate receptors depresses recurrent inhibition of motoneurons in the neonatal rat spinal cord in vitro

This study examined whether activation of group I metabotropic glutamate receptors (mGluRs) could modulate synaptic inhibition of spinal motoneurons in the neonatal rat isolated spinal cord. Recurrent inhibitory postsynaptic potentials (IPSPs) generated by Renshaw cells were evoked via antidromic stimulation of motor axon collaterals and recorded intracellularly from lumbar motoneurons. The selective agonist of group I mGluRs DHPG (5 micromol L-1) depressed the recurrent IPSP, an effect prevented by the selective antagonist AIDA (500 micromol L-1). The depression by DHPG was use-independent and could be partly counteracted by increasing stimulus strength. Paired pulse depression observed at <or=50-ms intervals was blocked by DHPG in an AIDA-sensitive manner. These results suggest that, in the presence of DHPG, smaller recurrent IPSPs can contribute to the excitatory action of mGluR activation on spinal networks, including the generation of synchronous oscillations recorded from motoneurons.

Development of an Inhibitory Interneuronal Circuit in the Embryonic Spinal Cord

Locally projecting inhibitory interneurons play a crucial role in the patterning and timing of network activity. However, because of their relative inaccessibility, little is known about their development or incorporation into circuits. In this study, we characterized the functional onset, neurotransmitters, rostrocaudal spread, and funicular distribution of one such spinal interneuronal circuit during development. The R-interneuron is the avian homologue of the mammalian Renshaw cell. Both cell types receive input from motoneuron recurrent collaterals and make direct connections back onto motoneurons. By stimulating motoneurons projecting in a given ventral root and recording the response in adjacent ventral roots, we demonstrate that the R-interneuron circuit becomes functional between embryonic day 6 (E6) and E7. This ventral root response is observed at E11 and at E14 until it can no longer be detected at E16. Using bath-applied neurotransmitter receptor antagonists, we were able to demonstrate that the circuit is predominately nicotinic and GABAergic from E7.5 to E15. We also found a glutamatergic component to the pathway throughout this developmental period. The R-interneuron projects three or more segments both rostrally and caudally through the ventrolateral funiculus. The distribution of this circuit may become more locally focused between E7.5 and E15.

Modeling spontaneous activity in the developing spinal cord using activity-dependent variations of intracellular chloride

We investigated how spontaneous activity is generated in developing, hyperexcitable networks. We focused our study on the embryonic chick spinal cord, a preparation that exhibits rhythmic discharge on multiple timescales: slow episodes (lasting minutes) and faster intraepisode cycling (approximately 1 Hz frequency). For this purpose, we developed a mean field model of a recurrent network with slow chloride dynamics and a fast depression variable. We showed that the model, in addition to providing a biophysical mechanism for the slow dynamics, was able to account for the experimentally observed activity. The model made predictions on how interval and duration of episodes are affected when changing chloride-mediated synaptic transmission or chloride flux across cell membrane. These predictions guided experiments, and the model results were compared with experimental data obtained with electrophysiological recordings. We found agreement when transmission was affected through changes in synaptic conductance and good qualitative agreement when chloride flux was varied through changes in external chloride concentration or in the rate of the Na+-K+-2Cl- cotransporter. Furthermore, the model made predictions about the time course of intracellular chloride concentration and chloride reversal potential and how these are affected by changes in synaptic conductance. Based on the comparison between modeling and experimental results, we propose that chloride dynamics could be an important mechanism in rhythm generation in the developing chick spinal cord.

Pre- and postsynaptic modulation of glycinergic and gabaergic transmission by muscarinic receptors on rat hypoglossal motoneurons in vitro

The motor output of hypoglossal motoneurons to tongue muscles takes place in concert with the respiratory rhythm and is determined by the balance between excitatory glutamatergic transmission and inhibitory transmission mediated by glycine or GABA. The relative contribution by these transmitters is a phasic phenomenon modulated by other transmitters. We examined how metabotropic muscarinic receptors, widely expressed in the brainstem where they excite cranial motor nuclei, might influence synaptic activity mediated by GABA or glycine. For this purpose, using thin slices of the neonatal rat brainstem, we recorded (under whole-cell patch clamp) glycinergic or GABAergic responses from visually identified hypoglossal motoneurons after pharmacological block of glutamatergic transmission. Muscarine inhibited spontaneous and electrically induced events mediated by GABA or glycine. The amplitude of glycinergic miniature inhibitory postsynaptic currents was slightly reduced by muscarine, while GABAergic miniature inhibitory postsynaptic currents were unaffected. Motoneuron currents induced by focally applied GABA and glycine were depressed by muscarine with stronger reduction in glycine-mediated responses. Histochemical observations indicated the presence of M1, M2 and M5 subtypes of muscarinic receptors in the neonatal hypoglossal nucleus. These results suggest that muscarine potently depressed inhibitory neurotransmission on brainstem motoneurons, and that this action was exerted via preterminal and extrasynaptic receptors. Since the large reduction in inhibitory neurotransmission may contribute to overall excitation of brainstem motoneurons by muscarinic receptors, these data might help to understand the central components of action of antimuscarinic agents in preanesthetic medication or against motion sickness.

Metabotropic glutamate receptor activity induces a novel oscillatory pattern in neonatal rat hypoglossal motoneurones

Tongue muscles innervated by the hypoglossal nerves play a crucial role to ensure airway patency and milk suckling in the neonate. Using a slice preparation of the neonatal rat brain, we investigated the electrophysiological characteristics of hypoglossal motoneurones in the attempt to identify certain properties potentially capable of synchronizing motor commands to the tongue. Bath-applied DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), generated persistent, regular electrical oscillations (4-8 Hz) recorded from patch-clamped motoneurones. Under voltage clamp, oscillations were biphasic events, comprising large outward slow currents alternated with fast, repeated inward currents. Electrical oscillations had amplitude and period insensitive to cell membrane potential, and required intact glutamatergic transmission via AMPA receptors. Oscillations were mediated by subtype 1 receptors of group I mGluRs (mGluR1s), and were routinely observed during pharmacological block of glycinergic and GABAergic inhibition, although they could also be recorded in standard saline. Simultaneous recordings from pairs of motoneurones within the same hypoglossal nucleus demonstrated that oscillations were due to their strong electrical coupling and were blocked by the gap junction blocker carbenoxolone. Pacing of slow oscillations apparently depended on the operation of K(ATP) channels in view of the block by tolbutamide or glibenclamide. Under current clamp, oscillations generated more regular spike firing of motoneurones and facilitated glutamatergic excitatory inputs. These data suggest that neonatal motoneurones of the nucleus hypoglossus possess a formerly undisclosed ability to express synchronous electrical oscillations, unveiled by activation of mGluR1s.

Activation of group I metabotropic glutamate receptors enhances efficacy of glutamatergic inputs to neonatal rat hypoglossal motoneurons in vitro

Group I metabotropic glutamate receptors (mGluRs) are the main class of metabotropic receptors expressed in the hypoglossus nucleus. Their role in glutamatergic transmission was investigated using patch-clamp recording from motoneurons in a neonatal rat brainstem slice preparation. After pharmacological block of gamma-aminobutyric acid and glycine-mediated inhibition, under voltage-clamp, the selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) induced a motoneuron inward current by depressing a leak conductance, and strongly facilitated spontaneous glutamatergic synaptic currents. This effect was blocked by 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) and unaffected by 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP), indicating a role for subtype 1 mGluRs. The frequency but not the amplitude of miniature glutamatergic currents was also enhanced by DHPG. Currents elicited by puffer application of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the presence of tetrodotoxin were also unchanged, suggesting that DHPG facilitated release of glutamate. Glutamatergic currents evoked by electrically stimulating the dorsomedullary reticular column premotoneurons were, however, depressed by DHPG in a CPCCOEt-sensitive fashion. Neither CPCCOEt nor MPEP per se changed glutamatergic transmission. Under current-clamp, even if DHPG depressed excitatory postsynaptic potentials, motoneuron spike threshold and time to peak were reduced so that facilitation of synaptic potential/spike coupling became apparent. We propose a wiring diagram to account for the differential action by DHPG on spontaneous and evoked transmission, based on the discrete distribution of subtype 1 mGluRs on glutamatergic afferents. Although under standard recording conditions there was insufficient ambient glutamate to activate mGluRs, such receptors were a powerful target to upregulate excitatory synaptic transmission and enhance signalling by hypoglossal motoneurons to tongue muscles.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Development of GABAergic and glycinergic transmission in the neonatal rat dorsal horn

Cutaneous spinal sensory transmission appears to lack inhibitory control in the newborn spinal cord, but the properties of GABAergic and glycinergic synapses in the neonatal dorsal horn have not been characterized. Whole-cell patch-clamp recordings from rat superficial dorsal horn neurons in spinal cord slices at postnatal day 0 (P0) to P2, P6-P7, and P13-P14 revealed an age-dependent increase in the frequency of spontaneous IPSCs, which were abolished by the GABA(A) receptor (GABA(A)R) antagonist bicuculline between P0 and P7 but not at P14. GABA(A)R-mediated miniature IPSCs (mIPSCs), but not glycinergic mIPSCs, were present at birth, and GABA mIPSCs remained more frequent than glycine mIPSCs at all ages. Sciatic nerve stimulation resulted in IPSCs with both GABAergic and glycinergic components, although a larger contribution arose from GABA(A) receptors at all ages. In gramicidin perforated patch-clamp recordings, exogenous GABA applications produced depolarization in 40% of neurons at P0-P2, but the reversal potential of GABA-evoked currents (E(GABA)) was consistently more negative than action potential threshold at this age. By P6-P7, GABA evoked only membrane hyperpolarization. The GABA(B)R agonist baclofen elicited an outward current in all neurons with peak amplitudes observed by P6-P7 and abolished sciatic nerve-evoked monosynaptic glutamatergic EPSCs in all groups. The results show considerable postnatal development of inhibitory processing in the dorsal horn with GABAergic mechanisms initially dominant over glycinergic events. GABA(A)R-mediated depolarizations during the first postnatal week are likely to be important for the maturation of spinal networks but do not provide a major excitatory drive to the newborn dorsal horn.