GABA(B) modulation of GABA(A) and glycine receptor-mediated synaptic currents in hypoglossal motoneurons

Development of γ-aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on the rat genioglossal motoneurons

Detailed information about the development of excitatory and inhibitory synapses on the genioglossal (GG) motoneuron may help to understand the mechanism of fine control of GG motoneuron firing and the coordinated tongue movement during postnatal development. For this, we investigated the development of γ-aminobutyric acid (GABA)-immunopositive (GABA +), glycine + (Gly +), and glutamate + (Glut +) axon terminals (boutons) on the somata of rat GG motoneurons at a postnatal day 2 (P2), P6 and P18 by retrograde labeling of GG motoneurons with horseradish peroxidase, electron microscopic postembedding immunogold staining with GABA, Gly, and Glut antisera, and quantitative analysis. The number of boutons per GG motoneuron somata and the mean length of bouton apposition, measures of bouton size and synaptic covering percentage, were significantly increased from P2/P6 to P18. The number and fraction of GABA + only boutons of all boutons decreased significantly, whereas those of Gly + only boutons increased significantly from P2/P6 to P18, suggesting developmental switch from GABAergic to glycinergic synaptic transmission. The fraction of mixed GABA +/Gly + boutons of all boutons was the highest among inhibitory bouton types throughout the postnatal development. The fractions of excitatory and inhibitory boutons of all boutons remained unchanged during postnatal development. These findings reveal a distinct developmental pattern of inhibitory synapses on the GG motoneurons different from that on spinal or trigeminal motoneurons, which may have an important role in the regulation of the precise and coordinated movements of the tongue during the maturation of the oral motor system.

Role of GirK Channels in Long-Term Potentiation of Synaptic Inhibition in an In Vivo Mouse Model of Early Amyloid-β Pathology

Imbalances of excitatory/inhibitory synaptic transmission occur early in the pathogenesis of Alzheimer’s disease (AD), leading to hippocampal hyperexcitability and causing synaptic, network, and cognitive dysfunctions. G-protein-gated potassium (GirK) channels play a key role in the control of neuronal excitability, contributing to inhibitory signaling. Here, we evaluate the relationship between GirK channel activity and inhibitory hippocampal functionality in vivo. In a non-transgenic mouse model of AD, field postsynaptic potentials (fPSPs) from the CA3–CA1 synapse in the dorsal hippocampus were recorded in freely moving mice. Intracerebroventricular (ICV) injections of amyloid-β (Aβ) or GirK channel modulators impaired ionotropic (GABA_A-mediated fPSPs) and metabotropic (GirK-mediated fPSPs) inhibitory signaling and disrupted the potentiation of synaptic inhibition. However, the activation of GirK channels prevented Aβ-induced changes in GABA_A components. Our data shows, for the first time, the presence of long-term potentiation (LTP) for both the GABA_A and GirK-mediated inhibitory postsynaptic responses in vivo. In addition, our results support the importance of an accurate level of GirK-dependent signaling for dorsal hippocampal performance in early amyloid pathology models by controlling the excess of excitation that disrupts synaptic plasticity processes.

Increased tongue use enhances 5-HT2C receptor immunostaining in hypoglossal motor nucleus

Hypoglossal (XII) motoneurons are activated by type 2 receptors for serotonin (5-HT). This activation is especially strong during wakefulness which facilitates diverse motor functions of the tongue, including the maintenance of upper airway patency in obstructive sleep apnea (OSA) patients. We tested whether 5-HT₂ receptor levels in the XII nucleus vary with intensity of tongue use. Three groups of rats were housed overnight under conditions of increasing oromotor activity: W-water available ad lib; S-sweetened water to stimulate drinking; S + O-sweetened water + oil applied on fur to increase grooming. After the exposures, immunostaining for 5-HT_2C, but not 5-HT_2A, receptors was higher in the XII nucleus in S + O than in W rats (65 ± 1.8 (SE) vs. 60 ± 2.0 arbitrary units; p = 0.008). In the medullary raphé obscurus region, the percentage of c-Fos-positive 5-HT cells was 13% higher (p = 0.03) in S + O than in W rats. The positive feedback between tongue use and 5-HT_2C receptor immunostaining reveals a novel mechanism potentially relevant for OSA and neuromuscular disorders.

Heterogeneous Signaling at GABA and Glycine Co-releasing Terminals

The corelease of several neurotransmitters from a single synaptic vesicle has been observed at many central synapses. Nevertheless, the signaling synergy offered by cotransmission and the mechanisms that maintain the optimal release and detection of neurotransmitters at mixed synapses remain poorly understood, thus limiting our ability to interpret changes in synaptic signaling and identify molecules important for plasticity. In the brainstem and spinal cord, GABA and glycine cotransmission is facilitated by a shared vesicular transporter VIAAT (also named VGAT), and occurs at many immature inhibitory synapses. As sensory and motor networks mature, GABA/glycine cotransmission is generally replaced by either pure glycinergic or GABAergic transmission, and the functional role for the continued corelease of GABA and glycine is unclear. Whether or not, and how, the GABA/glycine content is balanced in VIAAT-expressing vesicles from the same terminal, and how loading variability effects the strength of inhibitory transmission is not known. Here, we use a combination of loose-patch (LP) and whole-cell (WC) electrophysiology in cultured spinal neurons of GlyT2:eGFP mice to sample miniature inhibitory post synaptic currents (mIPSCs) that originate from individual GABA/glycine co-releasing synapses and develop a modeling approach to illustrate the gradual change in mIPSC phenotypes as glycine replaces GABA in vesicles. As a consistent GABA/glycine balance is predicted if VIAAT has access to both amino-acids, we test whether vesicle exocytosis from a single terminal evokes a homogeneous population of mixed mIPSCs. We recorded mIPSCs from 18 individual synapses and detected glycine-only mIPSCs in 4/18 synapses sampled. The rest (14/18) were co-releasing synapses that had a significant proportion of mixed GABA/glycine mIPSCs with a characteristic biphasic decay. The majority (9/14) of co-releasing synapses did not have a homogenous phenotype, but instead signaled with a combination of mixed and pure mIPSCs, suggesting that there is variability in the loading and/or storage of GABA and glycine at the level of individual vesicles. Our modeling predicts that when glycine replaces GABA in synaptic vesicles, the redistribution between the peak amplitude and charge transfer of mIPSCs acts to maintain the strength of inhibition while increasing the temporal precision of signaling.

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.

Efferent Vestibular Neurons Show Homogenous Discharge Output But Heterogeneous Synaptic Input Profile In Vitro

Despite the importance of our sense of balance we still know remarkably little about the central control of the peripheral balance system. While previous work has shown that activation of the efferent vestibular system results in modulation of afferent vestibular neuron discharge, the intrinsic and synaptic properties of efferent neurons themselves are largely unknown. Here we substantiate the location of the efferent vestibular nucleus (EVN) in the mouse, before characterizing the input and output properties of EVN neurons in vitro. We made transverse serial sections through the brainstem of 4-week-old mice, and performed immunohistochemistry for calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT), both expressed in the EVN of other species. We also injected fluorogold into the posterior canal and retrogradely labelled neurons in the EVN of ChAT:: tdTomato mice expressing tdTomato in all cholinergic neurons. As expected the EVN lies dorsolateral to the genu of the facial nerve (CNVII). We then made whole-cell current-, and voltage-clamp recordings from visually identified EVN neurons. In current-clamp, EVN neurons display a homogeneous discharge pattern. This is characterized by a high frequency burst of action potentials at the onset of a depolarizing stimulus and the offset of a hyperpolarizing stimulus that is mediated by T-type calcium channels. In voltage-clamp, EVN neurons receive either exclusively excitatory or inhibitory inputs, or a combination of both. Despite this heterogeneous mixture of inputs, we show that synaptic inputs onto EVN neurons are predominantly excitatory. Together these findings suggest that the inputs onto EVN neurons, and more specifically the origin of these inputs may underlie EVN neuron function.

Postsynaptic inhibition of hypoglossal motoneurons produces atonia of the genioglossal muscle during rapid eye movement sleep

STUDY OBJECTIVES

Hypoglossal motoneurons were recorded intracellularly to determine whether postsynaptic inhibition or disfacilitation was responsible for atonia of the lingual muscles during rapid eye movement (REM) sleep.

DESIGN

Intracellular records were obtained of the action potentials and subthreshold membrane potential activity of antidromically identified hypoglossal motoneurons in cats during wakefulness, nonrapid eye movement (NREM) sleep, and REM sleep. A cuff electrode was placed around the hypoglossal nerve to antidromically activate hypoglossal motoneurons. The state-dependent changes in membrane potential, spontaneous discharge, postsynaptic potentials, and rheobase of hypoglossal motoneurons were determined.

ANALYSES AND RESULTS

During quiet wakefulness and NREM sleep, hypoglossal motoneurons exhibited spontaneous repetitive discharge. In the transition from NREM sleep to REM sleep, repetitive discharge ceased and the membrane potential began to hyperpolarize; maximal hyperpolarization (10.5 mV) persisted throughout REM sleep. During REM sleep there was a significant increase in rheobase, which was accompanied by barrages of large-amplitude inhibitory postsynaptic potentials (IPSPs), which were reversed following the intracellular injection of chloride ions. The latter result indicates that they were mediated by glycine; IPSPs were not present during wakefulness or NREM sleep.

CONCLUSIONS

We conclude that hypoglossal motoneurons are postsynaptically inhibited during naturally occurring REM sleep; no evidence of disfacilitation was observed. The data also indicate that glycine receptor-mediated postsynaptic inhibition of hypoglossal motoneurons is crucial in promoting atonia of the lingual muscles during REM sleep.

Adrenoreceptor modulation of oromotor pathways in the rat medulla

Regulation of feeding behavior involves the integration of multiple physiological and neurological pathways that control both nutrient-seeking and consummatory behaviors. The consummatory phase of ingestion includes stereotyped oromotor movements of the tongue and jaw that are controlled through brain stem pathways. These pathways encompass not only cranial nerve sensory and motor nuclei for processing feeding-related afferent signals and supplying the oromotor musculature but also reticular neurons for orchestrating ingestion and coordinating it with other behaviors that utilize the same musculature. Based on decerebrate studies, this circuit should be sensitive to satiety mechanisms mediated centrally by A2 noradrenergic neurons in the caudal nucleus of the solitary tract (cNST) that are potently activated during satiety. Because the first observable phase of satiety is inhibition of oromotor movements, we hypothesized that norepinephrine (NE) would act to inhibit prehypoglossal neurons in the medullary reticular formation. Using patch-clamp electrophysiology of retrogradely labeled prehypoglossal neurons and calcium imaging to test this hypothesis, we demonstrate that norepinephrine can influence both pre- and postsynaptic properties of reticular neurons through both α1- and α2-adrenoreceptors. The α1-adrenoreceptor agonist phenylephrine (PE) activated an inward current in the presence of TTX and increased the frequency of both inhibitory and excitatory miniature postsynaptic currents. The α2-adrenoreceptor agonist dexmedetomidine (DMT) inhibited cNST-evoked excitatory currents as well as spontaneous and miniature excitatory currents through presynaptic mechanisms. The diversity of adrenoreceptor modulation of these prehypoglossal neurons may reflect their role in a multifunctional circuit coordinating both ingestive and respiratory lingual function.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Identification of the transmitter and receptor mechanisms responsible for REM sleep paralysis

During REM sleep the CNS is intensely active, but the skeletal motor system is paradoxically forced into a state of muscle paralysis. The mechanisms that trigger REM sleep paralysis are a matter of intense debate. Two competing theories argue that it is caused by either active inhibition or reduced excitation of somatic motoneuron activity. Here, we identify the transmitter and receptor mechanisms that function to silence skeletal muscles during REM sleep. We used behavioral, electrophysiological, receptor pharmacology and neuroanatomical approaches to determine how trigeminal motoneurons and masseter muscles are switched off during REM sleep in rats. We show that a powerful GABA and glycine drive triggers REM paralysis by switching off motoneuron activity. This drive inhibits motoneurons by targeting both metabotropic GABA(B) and ionotropic GABA(A)/glycine receptors. REM paralysis is only reversed when motoneurons are cut off from GABA(B), GABA(A) and glycine receptor-mediated inhibition. Neither metabotropic nor ionotropic receptor mechanisms alone are sufficient for generating REM paralysis. These results demonstrate that multiple receptor mechanisms trigger REM sleep paralysis. Breakdown in normal REM inhibition may underlie common sleep motor pathologies such as REM sleep behavior disorder.

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.

Excitatory-inhibitory imbalance in hypoglossal neurons during the critical period of postnatal development in the rat

Hypoglossal motoneurons (HMs) innervate tongue muscles and are critical in maintaining patency of the upper airway during respiration. Abnormalities in HMs have been implicated in sudden infant death syndrome (SIDS) and obstructive sleep apnoea. Previously, we found a critical period in respiratory network development in rats around postnatal day (P) 12-13, when abrupt neurochemical, metabolic and physiological changes occurred. To test our hypothesis that an imbalance between inhibitory and excitatory synaptic transmission exists during the critical period, whole-cell patch-clamp recordings of HMs were done in brainstem slices of rats daily from P0 to P16. The results indicated that: (1) the amplitude and charge transfer of miniature excitatory postsynaptic currents (mEPSCs) were significantly reduced at P12-13; (2) the amplitude, mean frequency and charge transfer of miniature inhibitory postsynaptic currents (mIPSCs) were significantly increased at P12-13; (3) the kinetics (rise time and decay time) of both mEPSCs and mIPSCs accelerated with age; (4) the amplitude and frequency of spontaneous EPSCs were significantly reduced at P12-13, whereas those of spontaneous IPSCs were significantly increased at P12-13; and (5) both glycine and GABA contributed to mIPSCs. However, GABAergic currents fluctuated within a narrow range during the first three postnatal weeks, whereas glycinergic ones exhibited age-dependent changes comparable to those of total mIPSCs, indicating a reversal in dominance from GABA to glycine with development. Thus, our results provide strong electrophysiological evidence for an excitatory-inhibitory imbalance in HMs during the critical period of postnatal development in rats that may have significant implications for SIDS.

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.

Progesterone and allopregnanolone enhance the miniature synaptic release of glycine in the rat hypoglossal nucleus

It is well known that progesterone is synthesised and metabolised within the nervous system, and that one of its metabolites, allopregnanolone, potentiates the activity of GABA receptor anionic channels and modulates GABAergic neurotransmission. Progesterone is now under clinical trial for its neuroprotective properties, but its possible effects on neurotransmission have not yet been fully explored. The present study investigated acute effects of progesterone on the other major type of synaptic inhibition, glycinergic neurotransmission. Spontaneous glycinergic miniature currents were recorded in hypoglossal motoneurons, using the whole-cell patch-clamp technique in rat brainstem slices. A 20-min superfusion with progesterone (1 mum) triggered an increase in the frequency of glycinergic miniatures, whereas no effect of progesterone was observed after block with finasteride (5 mum) of 5alpha -reductase, the first enzymatic step leading from progesterone to allopregnanolone. The effect of progesterone could be mimicked by superfusion with allopregnanolone (0.3 mum), whereas no effect was induced by epiallopregnanolone. Thus, progesterone can increase the synaptic miniature release of glycine and this effect appears to be indirect, resulting from its metabolism into 5alpha-reduced derivatives, in particular into allopregnanolone. A low concentration of an exogenous GABA(A) agonist can also increase the frequency of inhibitory miniature currents in hypoglossal motoneurons. Thus, the effects of progesterone and allopregnanolone on glycine release can be at least partly explained by the potentiation of the activity of depolarizing presynaptic GABA receptor channels. The increase in the tonic synaptic release of a major inhibitory neurotransmitter should reduce the excitability of the neurons and contribute to their protection against excitotoxicity.

Oestradiol rapidly enhances spontaneous glycinergic synaptic inhibition of hypoglossal motoneurones

Whereas oestradiol is well-known to facilitate excitatory glutamatergic synaptic transmission, its effects on fast inhibitory neurotransmission are not as well established. Possible acute modulation of the spontaneous glycinergic synaptic activity by oestradiol was investigated in voltage-clamped hypoglossal motoneurones by whole-cell patch-clamp recording in rat brainstem slices. The spontaneous glycinergic synaptic activity was continuously recorded in each neurone under control conditions, during 12-20 min of perfusion with 17beta-oestradiol and during washing. When oestradiol was diluted in ethanol, the control solution contained the same amount of ethanol. At 100 nM, oestradiol markedly increased the frequency of the total spontaneous glycinergic activity. Similar experiments were performed after blockade of action potentials by tetrodotoxin, aiming to isolate miniature glycinergic synaptic currents. Oestradiol increased the frequency of glycinergic miniatures in most slices, in some cases within less than 1 min. In some slices, oestradiol also favoured the occurrence of glycinergic miniatures of large amplitude. These effects were slowly reversible during washing. At 1 nm, oestradiol still increased the frequency of glycinergic miniatures. The results were confirmed in the absence of ethanol by using water-soluble cyclodextrin-encapsulated oestradiol. In these experiments, the control solution contained the same amount of (2-hydroxypropyl)-beta-cyclodextrin as the oestradiol-containing solution. In addition, prolonged control recordings were performed without applying oestradiol to check the stability of the glycinergic synaptic activity during prolonged whole-cell recordings. The results show, for the first time, that, within a few minutes, oestradiol can enhance the spontaneous synaptic release of a major inhibitory transmitter, glycine.

Genioglossus premotoneurons and the negative pressure reflex in rats

Reflex increases in genioglossus (GG) muscle activity in response to negative pharyngeal pressure are important for maintenance of upper airway patency in humans. However, little is known of the central circuitry that mediates this negative pressure reflex (NPR). We used two approaches to determine which GG premotoneurons relay negative pressure-related information to the hypoglossal motor nucleus. First, to identify GG premotoneurons, we injected pseudorabies virus (PRV152) into the GG muscle. We found that medullary GG premotoneurons were concentrated mainly in the reticular formation adjacent to the hypoglossal motor nucleus. Second, in order to determine whether these perihypoglossal neurons were involved in the NPR, we quantified GG EMG responses to negative pressure applied to the isolated upper airway in anaesthetized rats before and after microinjection of muscimol (9 nl; 0.25 mM), a GABA-A receptor agonist, into the perihypoglossal premotor field. Pressures as low as -4 cm H(2)O increased inspiratory phase-related GG activity. The NPR was abolished following bilateral injections of muscimol into the perihypoglossal premotor field at and up to 500 mum rostral to the obex. Muscimol in this location also increased the amplitude of basal, unstimulated phasic GG activity. By contrast, inhibition of neurons caudal to the obex decreased phasic GG activity but had no impact on the NPR. These results suggest that perihypoglossal GG premotoneurons near the obex mediate the NPR and those caudal to the obex are important mediators of respiratory-related GG activity but are not involved in the NPR.

Inhibitory Synaptic Transmission Differs in Mouse Type A and B Medial Vestibular Nucleus Neurons In Vitro

Fast inhibitory synaptic transmission in the medial vestibular nucleus (MVN) is mediated by GABAA receptors (GABAARs) and glycine receptors (GlyRs). To assess their relative contribution to inhibition in the MVN, we recorded miniature inhibitory postsynaptic currents (mIPSCs) in physiologically characterized type A and type B MVN neurons. Transverse brain stem slices were prepared from mice (3–8 wk old), and whole cell patch-clamp recordings were obtained from visualized MVN neurons (CsCl internal; Vm = –70 mV; 23°C). In 81 MVN neurons, 69% received exclusively GABAAergic inputs, 6% exclusively glycinergic inputs, and 25% received both types of mIPSCs. The mean amplitude of GABAAR-mediated mIPSCs was smaller than those mediated by GlyRs (22.6 ± 1.8 vs. 35.3 ± 5.3 pA). The rise time and decay time constants of GABAAR- versus GlyR-mediated mIPSCs were slower (1.3 ± 0.1 vs. 0.9 ± 0.1 ms and 10.5 ± 0.3 vs. 4.7 ± 0.3 ms, respectively). Comparison of type A ( n = 20) and type B ( n = 32) neurons showed that type A neurons received almost exclusively GABAAergic inhibitory inputs, whereas type B neurons received GABAAergic inputs, glycinergic inputs, or both. Intracellular labeling in a subset of MVN neurons showed that morphology was not related to a MVN neuron's inhibitory profile ( n = 15), or whether it was classified as type A or B ( n = 29). Together, these findings indicate that both GABA and glycine contribute to inhibitory synaptic processing in MVN neurons, although GABA dominates and there is a difference in the distribution of GABAA and Gly receptors between type A and type B MVN neurons.

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.

Postnatal development differentially affects voltage-activated calcium currents in respiratory rhythmic versus nonrhythmic neurons of the pre-Bötzinger complex

The mammalian respiratory network reorganizes during early postnatal life. We characterized the postnatal developmental changes of calcium currents in neurons of the pre-Bötzinger complex (pBC), the presumed site for respiratory rhythm generation. The pBC contains not only respiratory rhythmic (R) but also nonrhythmic neurons (nR). Both types of neurons express low- and high-voltage-activated (LVA and HVA) calcium currents. This raises the interesting issue: do calcium currents of the two co-localized neuron types have similar developmental profiles? To address this issue, we used the whole cell patch-clamp technique to compare in transverse slices of mice LVA and HVA calcium current amplitudes of the two neuron populations (R and nR) during the first and second postnatal week (P0-P16). The amplitude of HVA currents did not significantly change in R pBC-neurons (P0-P16), but it significantly increased in nR pBC-neurons during P8-P16. The dehydropyridine (DHP)-sensitive current amplitudes did not significantly change during the early postnatal development, suggesting that the observed amplitude changes in nR pBC-neurons are caused by (DHP) insensitive calcium currents. The ratio between HVA calcium current amplitudes dramatically changed during early postnatal development: At P0-P3, current amplitudes were significantly larger in R pBC-neurons, whereas at P8-P16, current amplitudes were significantly larger in nR pBC-neurons. Our results suggest that calcium currents in pBC neurons are differentially altered during postnatal development and that R pBC-neurons have fully expressed calcium currents early during postnatal development. This may be critical for stable respiratory rhythm generation in the underlying rhythm generating network.