Distinct subtypes of metabotropic glutamate receptors mediate differential actions on excitability of spinal respiratory motoneurons

The phrenic neuromuscular system

The phrenic neuromuscular system consists of the phrenic motor nucleus in the mid-cervical spinal cord, the phrenic nerve, and the diaphragm muscle. This motor system helps sustain breathing throughout life, while also contributing to posture, coughing, swallowing, and speaking. The phrenic nerve contains primarily efferent phrenic axons and afferent axons from diaphragm sensory receptors but is also a conduit for autonomic fibers. On a breath-by-breath basis, rhythmic (inspiratory) depolarization of phrenic motoneurons occurs due to excitatory bulbospinal synaptic pathways. Further, a complex propriospinal network innervates phrenic motoneurons and may serve to coordinate postural, locomotor, and respiratory movements. The phrenic neuromuscular system is impacted in a wide range of neuromuscular diseases and injuries. Contemporary research is focused on understanding how neuromuscular plasticity occurs in the phrenic neuromuscular system and using this information to optimize treatments and rehabilitation strategies to improve breathing and related behaviors.

Spontaneous respiratory plasticity following unilateral high cervical spinal cord injury in behaving rats

Unilateral cervical C2 hemisection (C2Hx) is a classic model of spinal cord injury (SCI) for studying respiratory dysfunction and plasticity. However, most previous studies were performed under anesthesia, which significantly alters respiratory network. Therefore, the goal of this work was to assess spontaneous diaphragm recovery post-C2Hx in awake, freely behaving animals. Adult rats were chronically implanted with diaphragm EMG electrodes and recorded during 8 weeks post-C2Hx. Our results reveal that ipsilateral diaphragm activity partially recovers within days post-injury and reaches pre-injury amplitude in a few weeks. However, the full extent of spontaneous ipsilateral recovery is significantly attenuated by anesthesia (ketamine/xylazine, isoflurane, and urethane). This suggests that the observed recovery may be attributed in part to activation of NMDA receptors which are suppressed by anesthesia. Despite spontaneous recovery in awake animals, ipsilateral hemidiaphragm dysfunction still persists: i) Inspiratory bursts during basal (slow) breathing exhibit an altered pattern, ii) the amplitude of sighs - or augmented breaths - is significantly decreased, and iii) the injured hemidiaphragm exhibits spontaneous events of hyperexcitation. The results from this study offer an under-appreciated insight into spontaneous diaphragm activity and recovery following high cervical spinal cord injury in awake animals.

Motoneuron glutamatergic receptor expression following recovery from cervical spinal hemisection

Cervical spinal hemisection at C2 (SH) removes premotor drive to phrenic motoneurons located in segments C3-C5 in rats. Spontaneous recovery of ipsilateral diaphragm muscle activity is associated with increased phrenic motoneuron expression of glutamatergic N-methyl-D-aspartate (NMDA) receptors and decreased expression of α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptors. Glutamatergic receptor expression is regulated by tropomyosin-related kinase receptor subtype B (TrkB) signaling in various neuronal systems, and increased TrkB receptor expression in phrenic motoneurons enhances recovery post-SH. Accordingly, we hypothesize that recovery of ipsilateral diaphragm muscle activity post-SH, whether spontaneous or enhanced by adenoassociated virus (AAV)-mediated upregulation of TrkB receptor expression, is associated with increased expression of glutamatergic NMDA receptors in phrenic motoneurons. Adult male Sprague-Dawley rats underwent diaphragm electromyography electrode implantation and SH surgery. Rats were injected intrapleurally with AAV expressing TrkB or GFP 3 weeks before SH. At 14 days post-SH, the proportion of animals displaying recovery of ipsilateral diaphragm activity increased in AAV-TrkB-treated (9/9) compared with untreated (3/5) or AAV-GFP-treated (4/10; P < 0.027) animals. Phrenic motoneuron NMDA NR1 subunit mRNA expression was approximately fourfold greater in AAV-TrkB- vs. AAV-GFP-treated SH animals (P < 0.004) and in animals displaying recovery vs. those not recovering (P < 0.005). Phrenic motoneuron AMPA glutamate receptor 2 (GluR2) subunit mRNA expression decreased after SH, and, albeit increased in animals displaying recovery vs. those not recovering, levels remained lower than control. We conclude that increased phrenic motoneuron expression of glutamatergic NMDA receptors is associated with spontaneous recovery after SH and enhanced recovery after AAV-TrkB treatment. J. Comp. Neurol. 525:1192-1205, 2017. © 2016 Wiley Periodicals, Inc.

Functional recovery after cervical spinal cord injury: Role of neurotrophin and glutamatergic signaling in phrenic motoneurons

Cervical spinal cord injury (SCI) interrupts descending neural drive to phrenic motoneurons causing diaphragm muscle (DIAm) paralysis. Recent studies using a well-established model of SCI, unilateral spinal hemisection of the C2 segment of the cervical spinal cord (SH), provide novel information regarding the molecular and cellular mechanisms of functional recovery after SCI. Over time post-SH, gradual recovery of rhythmic ipsilateral DIAm activity occurs. Recovery of ipsilateral DIAm electromyogram (EMG) activity following SH is enhanced by increasing brain-derived neurotrophic factor (BDNF) in the region of the phrenic motoneuron pool. Delivery of exogenous BDNF either via intrathecal infusion or via mesenchymal stem cells engineered to release BDNF similarly enhance recovery. Conversely, recovery after SH is blunted by quenching endogenous BDNF with the fusion-protein TrkB-Fc in the region of the phrenic motoneuron pool or by selective inhibition of TrkB kinase activity using a chemical-genetic approach in TrkB(F616A) mice. Furthermore, the importance of BDNF signaling via TrkB receptors at phrenic motoneurons is highlighted by the blunting of recovery by siRNA-mediated downregulation of TrkB receptor expression in phrenic motoneurons and by the enhancement of recovery evident following virally-induced increases in TrkB expression specifically in phrenic motoneurons. BDNF/TrkB signaling regulates synaptic plasticity in various neuronal systems, including glutamatergic pathways. Glutamatergic neurotransmission constitutes the main inspiratory-related, excitatory drive to motoneurons, and following SH, spontaneous neuroplasticity is associated with increased expression of ionotropic N-methyl-d-aspartate (NMDA) receptors in phrenic motoneurons. Evidence for the role of BDNF/TrkB and glutamatergic signaling in recovery of DIAm activity following cervical SCI is reviewed.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Phrenic motoneuron expression of serotonergic and glutamatergic receptors following upper cervical spinal cord injury

Following cervical spinal cord injury at C(2) (SH hemisection model) there is progressive recovery of phrenic activity. Neuroplasticity in the postsynaptic expression of neurotransmitter receptors may contribute to functional recovery. Phrenic motoneurons express multiple serotonergic (5-HTR) and glutamatergic (GluR) receptors, but the timing and possible role of these different neurotransmitter receptor subtypes in the neuroplasticity following SH are not clear. The current study was designed to test the hypothesis that there is an increased expression of serotonergic and glutamatergic neurotransmitter receptors within phrenic motoneurons after SH. In adult male rats, phrenic motoneurons were labeled retrogradely by intrapleural injection of Alexa 488-conjugated cholera toxin B. In thin (10μm) frozen sections of the spinal cord, fluorescently-labeled phrenic motoneurons were visualized for laser capture microdissection (LCM). Using quantitative real-time RT-PCR in LCM samples, the time course of changes in 5-HTR and GluR mRNA expression was determined in phrenic motoneurons up to 21 days post-SH. Expression of 5-HTR subtypes 1b, 2a and 2c and GluR subtypes AMPA, NMDA, mGluR1 and mGluR5 was evident in phrenic motoneurons from control and SH rats. Phrenic motoneuron expression of 5-HTR2a increased ~8-fold (relative to control) at 14 days post-SH, whereas NMDA expression increased ~16-fold by 21-days post-SH. There were no other significant changes in receptor expression at any time post-SH. This is the first study to systematically document changes in motoneuron expression of multiple neurotransmitter receptors involved in regulation of motoneuron excitability. By providing information on the neuroplasticity of receptors expressed in a motoneuron pool that is inactivated by a higher-level spinal cord injury, appropriate pharmacological targets can be identified to alter motoneuron excitability.

Activation of group I metabotropic glutamate receptors modulates locomotor-related motoneuron output in mice

Fast glutamatergic transmission via ionotropic receptors is critical for the generation of locomotion by spinal motor networks. In addition, glutamate can act via metabotropic glutamate receptors (mGluRs) to modulate the timing of ongoing locomotor activity. In the present study, we investigated whether mGluRs also modulate the intensity of motor output generated by spinal motor networks. Application of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) reduced the amplitude and increased the frequency of locomotor-related motoneuron output recorded from the lumbar ventral roots of isolated mouse spinal cord preparations. Whole cell patch-clamp recordings of spinal motoneurons revealed multiple mechanisms by which group I mGluRs modulate motoneuron output. Although DHPG depolarized the resting membrane potential and reduced the voltage threshold for action potential generation, the activation of group I mGluRs had a net inhibitory effect on motoneuron output that appeared to reflect the modulation of fast, inactivating Na(+) currents and action potential parameters. In addition, group I mGluR activation decreased the amplitude of locomotor-related excitatory input to motoneurons. Analyses of miniature excitatory postsynaptic currents indicated that mGluRs modulate synaptic drive to motoneurons via both pre- and postsynaptic mechanisms. These data highlight group I mGluRs as a potentially important source of neuromodulation within the spinal cord that, in addition to modulating components of the central pattern generator for locomotion, can modulate the intensity of motoneuron output during motor behavior. Given that group I mGluR activation reduces motoneuron excitability, mGluRs may provide negative feedback control of motoneuron output, particularly during high levels of glutamatergic stimulation.

Facilitation and Inhibition of Tibialis Anterior Responses to Corticospinal Stimulation After Maximal Voluntary Contractions

The corticospinal pathway is the major pathway controlling human voluntary movements. After strong voluntary contractions, the efficacy of corticospinal transmission to elbow flexors is reduced for ∼90 s, and this limits motoneuronal output. This reduction may reflect activity-dependent changes at cortico-motoneuronal synapses. We investigated whether similar changes occur in a leg muscle, tibialis anterior (TA). Electrical stimuli over high thoracic vertebrae activated corticospinal axons to evoke an EMG response in TA (TMEP). Stimuli were delivered before and after short 10-s and prolonged 1-min maximal contractions (MVCs) of ankle dorsiflexors. In two studies, stimuli were given with the muscle relaxed. In other studies, stimuli were given during weak contraction. After a 10-s MVC ( study 1, n = 10), TMEPs increased immediately to 349 ± 335% (mean ± SD) of control values. By 1 min after contraction, TMEPs decreased to 38 ± 28% of control and remained depressed for >10 min. Facilitation (191 ± 133% control) and depression (18 ± 22% control) occurred over the same time course after the 1-min MVC ( study 2, n = 10). When tested during weak contraction ( study 3, n = 10), TMEPs showed less facilitation (131 ± 41% control) and less depression (67 ± 21% control) and responses returned to baseline over ∼15 min. In contrast to TMEPs, H-reflexes in TA were little changed after a 10-s MVC ( study 4, n = 7). Our findings reveal an immediate facilitation and subsequent longer-lasting depression in corticospinal transmission to TA, which originate at a premotoneuronal site. This behavior differs markedly from that in elbow flexor muscles and suggests that activity-dependent changes in the motor pathway may be muscle specific.

Role of neurotrophins in recovery of phrenic motor function following spinal cord injury

Many individuals who sustain a cervical spinal cord injury are unable to maintain adequate ventilation due to diaphragm muscle paralysis. These patients become dependent on mechanical ventilators and this situation is associated with ongoing problems with pulmonary clearance, infections, and lung injury leading to significant morbidity and reduced life expectancy. Therefore, functional recovery of rhythmic phrenic activity and the ability to generate expulsive forces would dramatically affect the quality of life of patients with cervical spinal cord injury. Neurotrophins are very promising in that they have been shown to play an important role in modulating functional neuroplasticity. Specifically, brain-derived neurotrophic factor (BDNF) acting via the tropomyosin-related kinase receptor type B (TrkB) has been implicated in neuroplasticity following spinal cord injury. Our central hypothesis is that functional recovery of rhythmic phrenic activity after cervical spinal cord injury is enhanced by an increase in BDNF/TrkB signaling in phrenic motoneurons, providing a novel therapeutic target for patients.

Pattern-specific synaptic mechanisms in a multifunctional network. II. Intrinsic modulation by metabotropic glutamate receptors

The in vitro respiratory network contained in the transverse brain stem slice of mice simultaneously generates fast (approximately 15 min(-1)) and slow ( approximately 0.5 min(-1)) rhythmic activities corresponding to fictive eupnea ("normal" breathing) and fictive sighs. We show that these two activity patterns are differentially controlled through the modulatory actions of metabotropic glutamate receptors (mGluRs). Sighs were selectively inhibited by agonists of the group III mGluRs according to a pharmacological profile most consistent with activation of mGluR8. Sighs were also blocked by the supposedly inactive L-isomer of the widely used N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonopentanoic acid (L-AP5, 5 microM), an effect that was abolished in the presence of group III mGluR antagonists. Excitatory postsynaptic potentials (EPSPs) were recorded in pre-Bötzinger Complex neurons after stimulation of the contralateral ventral respiratory group (VRG); evoked EPSP amplitude was variably reduced after bath application of the group III agonist L-serine-O-phosphate (L-SOP), with an average reduction of 15%. Therefore although group III mGluRs do play a role in regulating synapse strength, this seems to be only a minor factor in the regulation of synapses made by midline-crossing axons. Intrinsic modulation of the respiratory central pattern generator by mGluRs appears to be an essential component of the multifunctionality that characterizes this network.

Characterization of long-term potentiation of primary afferent transmission at trigeminal synapses of juvenile rats: essential role of subtype 5 metabotropic glutamate receptors

Recent work has demonstrated that a brief high-frequency conditioning stimulation to the primary afferent nerve fibers can induce a long-term potentiation (LTP) of synaptic transmission in neurons in the superficial layer of the trigeminal caudal nucleus; however, the cellular and molecular mechanisms underlying this synaptic potentiation remain unclear. Using both extracellular field potential and whole-cell patch-clamp recordings in brainstem parasagital slices of juvenile rat with the mandibular nerve attached, we show here that the induction of trigeminal primary afferent LTP: (1) does not require the activation of ionotropic glutamate receptors; (2) is dependent on extracellular Ca(2+) and the release of Ca(2+) from intracellular stores; (3) is specifically prevented by the metabotropic glutamate receptor subtype 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)pyridine but not the mGluR1 antagonist LY367385, group II mGluR antagonist LY341495 or group III mGluR antagonist MAP4; (4) is mimicked by the bath-applied group I mGluR agonist (S)-3,5-dihydroxyphenylglycine and mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine; (5) requires the activation of phospholipase C (PLC) and protein kinase C (PKC); and (6) is concomitantly with a decrease in paired-pulse depression. These results demonstrate that the activation of mGluR5 and in turn triggering a PLC/PKC-dependent signaling cascade may contribute to the induction of LTP of primary afferent synaptic transmission in the superficial layer of trigeminal caudal nucleus of juvenile rats. This may be relevant to the processing of nociceptive information.

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.

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

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

Induction of long-term potentiation of C fibre-evoked spinal field potentials requires recruitment of group I, but not group II/III metabotropic glutamate receptors

In superficial layers of the lumbar spinal dorsal horn, N-methyl-D-aspartate-dependent long-term potentiation (LTP) of C fibre-evoked field potentials, a synaptic model of central sensitisation and hyperalgesia, ensues the application of electrical high-frequency, high-intensity conditioning stimulation to the sciatic nerve. In order to investigate the putative involvement of the G protein-coupled metabotropic glutamate receptors (mGluRs) in the induction of this form of LTP, we applied a series of mGluR antagonists exhibiting distinct group-specific activity profiles to the spinal lumbar enlargement, prior to conditioning stimulation. The group I (mGluR1/5) and group II (mGluR2/3) mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine or the selective mGluR1/5 antagonist (S)-4-carboxyphenylglycine consistently impaired the development of spinal LTP. However, potentiation occurred in the presence of the inactive enantiomer (R)-alpha-methyl-4-carboxyphenylglycine. LTP proved insensitive to the selective mGluR2/3 antagonists (2S)-alpha-ethylglutamic acid and LY341495, either spinally or intravenously delivered. LTP could also be induced in the presence of the selective group III (mGluR4/mGluR6-mGluR8) mGluR antagonist (RS)-alpha-methylserine-O-phosphate. However, none of the mGluR-active compounds alone noticeably altered the amplitudes of C fibre-evoked field potentials in the absence of conditioning stimulation. These findings suggest that the induction of LTP of C fibre-evoked field potentials in the spinal dorsal horn by high-frequency, high-intensity stimulation of afferent C fibres requires a group-specific mGluR recruitment, activation of mGluR1/5 but not that of mGluR4/6-8 and mGluR2/3 being a requisite step.

Distinct subtypes of group I metabotropic glutamate receptors on rat spinal neurons mediate complex facilitatory and inhibitory effects

While group I glutamate metabotropic (mGlu) receptors show discrete neuronal distribution in the neonatal rat spinal cord, the functional role of their distinct receptor subtypes remains uncertain. Intracellular recording from lumbar motoneurons together with extracellular recording of ventral root (VR) responses was used to investigate the differential contribution by mGlu receptor subtypes to cell excitability and network activity. The group I agonist DHPG evoked motoneuron depolarization (via the AIDA or CPCCOEt-sensitive mGlu receptor subtype 1) mainly at network level and generated sustained, network-dependent oscillations (via the MPEP-sensitive mGlu receptor subtype 5). DHPG also decreased the peak amplitude of synaptic responses induced by dorsal root stimuli, an effect unrelated to depolarization and dependent on glycinergic transmission. Synaptic responses were insensitive to AIDA or MPEP. The present results can be explained by assuming excitation of discrete classes of interneurons by group I mGlu receptor activity. Thus, the cellular distribution of those mGlu receptors at strategic circuit connections may determine the functional outcome of the network in terms of excitation or inhibition. Even if there was insufficient activation by endogenous glutamate of mGlu receptors during synaptic activity evoked by DR stimuli, it is apparent that such receptors are important pharmacological targets for powerful and rapid up- or down-regulation of spinal signal processing at network level, providing a rationale for the proposed use of mGlu receptor agonists in a variety of spinal pathological conditions.

Pre and postsynaptic effects of metabotropic glutamate receptor activation on neonatal rat hypoglossal motoneurons

Activation of metabotropic glutamate receptors by (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) produced pre- and postsynaptic effects on hypoglossal motoneurons (HMs) patch clamped in thin slices from neonatal rat brainstem. t-ACPD depolarized HMs (with parallel increase in R(in)), induced membrane oscillations, and depressed glutamatergic transmission. However, t-ACPD neither changed the discharge pattern of HMs induced by current pulses nor modulated the spike afterhyperpolarization. The only apparent alteration in firing was the delayed onset of the second spike in a train. The present results provide new evidence that activation of metabotropic glutamate receptors could influence HM excitability by changing the cell responsiveness to synaptic inputs rather than via obvious alterations in the spike firing properties.

Group I and II metabotropic glutamate receptors modulate respiratory activity in the lamprey

The respiratory role of group I and II metabotropic glutamate receptors (mGluRs) was investigated in the in vitro lamprey brainstem preparation by analysing changes in respiratory activity induced by bath application of specific agonists (10 and 25 micro m) and antagonists (500 micro m). Respiratory responses reached their maximum within 10 or 15 min, without any obvious further changes during application periods of 60 min The broad-spectrum mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) and the specific group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) caused dose-dependent increases in respiratory frequency and peak vagal activity. DHPG at higher concentrations (50-100 micro m) did not cause any further consistent change in respiratory variables. Two different agonists acting on group II mGluRs, i.e. (2S,3S,4S)-CCG/(2S,1'S,2'S)-2-(carboxycyclopropyl)-glycine (L-CCG-I) and (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV), were employed. At 10 micro m DCG-IV increased respiratory frequency, whilst L-CCG-I did not produce any significant effect. Both drugs increased respiratory frequency and peak vagal amplitude at 25 micro m. The nonselective group I and II antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG) as well as the specific group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) elicited reversible reductions in respiratory frequency. Blockade of group II mGluRs by (2S)-alpha-ethylglutamic acid (EGLU) increased both frequency and peak amplitude of vagal bursts. The results indicate that both group I and II mGluRs have an important modulatory role in the control of the lamprey pattern of breathing, probably through an action on the rhythm generating mechanisms. They support the view that these receptors are activated under physiological conditions and differentially affect the frequency and intensity of respiratory bursts.

Localization of metabotropic glutamate receptors 2/3 on primary afferent axons in the rat

The goal of the present study is to determine the relationship of metabotropic glutamate receptors 2/3 (mGluR2/3) to dorsal root ganglion cells, peripheral primary afferent fibers in digital nerves and central primary afferent fibers in the spinal cord. We demonstrate that approximately 40% of L4 and L5 dorsal root ganglion cells contain mGluR2/3-like immunoreactivity. These mGluR2/3-positive cells are small in diameter (23 microm) and 76% stain for the isolectin Griffonia simplicifolia (I-B4), while 67% of I-B4 cells have mGluR2/3-like immunoreactivity. Electron microscopic analyses of mGluR2/3-like immunoreactivity in axons in digital nerves indicate that 32% of unmyelinated and 28% of myelinated axons are labeled. In the lumbar dorsal horn, mGluR2/3-like immunoreactivity is localized preferentially in lamina IIi with lighter staining in laminae III and IV. The dense mGluR2/3-like immunoreactivity in lamina IIi is consistent with the localization of these receptors in I-B4-labeled dorsal root ganglion cells. Elimination of primary afferent input following unilateral dorsal rhizotomies significantly decreases the mGluR2/3-like immunoreactivity density in the dorsal horn although some residual staining does remain, suggesting that many but not all of these receptors are located on primary afferent processes. The finding that mGluR2/3s are located on peripheral sensory axons suggests that they are involved in peripheral sensory transduction and can modulate transmission of sensory input before it reaches the spinal cord. This offers the possibility of altering sensory input, particularly noxious input, at a site that would avoid CNS side effects. Since many but not all of these receptors are located on primary afferent terminals, these receptors may also influence primary afferent transmission in the dorsal horn through presynaptic mechanisms and glutamatergic transmission in general through both presynaptic and postsynaptic mechanisms. Since these receptors are concentrated in lamina IIi and also largely co-localized with I-B4, they may have considerable influence on nociceptive processing by what are considered to be non-peptidergic primary afferent neurons.

TASK-1 is a highly modulated pH-sensitive 'leak' K(+) channel expressed in brainstem respiratory neurons

Central respiratory chemoreceptors adjust respiratory drive in a homeostatic response to alterations in brain pH and/or P(CO(2)). Multiple brainstem sites are proposed as neural substrates for central chemoreception, but molecular substrates that underlie chemosensitivity in respiratory neurons have not been identified. In rat brainstem neurons expressing transcripts for TASK-1, a two-pore domain K(+) channel, we characterized K(+) currents with kinetic and voltage-dependent properties identical to cloned rat TASK-1 currents. Native currents were sensitive to acid and alkaline shifts in the same physiological pH range as TASK-1 (pK approximately 7.4), and native and cloned pH-sensitive currents were modulated similarly by neurotransmitters and inhalational anesthetics. This pH-sensitive TASK-1 channel is an attractive candidate to mediate chemoreception because it is functionally expressed in respiratory-related neurons, including airway motoneurons and putative chemoreceptor neurons of locus coeruleus (LC). Inhibition of TASK-1 channels by extracellular acidosis can depolarize and increase excitability in those cells, thereby contributing to chemoreceptor function in LC neurons and directly enhancing respiratory motoneuronal output.

Involvement of metabotropic glutamate receptors in excitatory amino acid and GABA release following spinal cord injury in rat

Spinal cord injury (SCI) leads to an increase in extracellular excitatory amino acid (EAA) concentrations resulting in glutamate receptor-mediated excitotoxic events. The glutamate receptors include ionotropic (iGluRs) and metabotropic (mGluR) receptors. Of the three groups of mGluRs, group-I activation can initiate intracellular pathways that lead to further transmitter release. Groups II and III mGluRs function mainly as autoreceptors to regulate neurotransmitter release. In an effort to examine the role of mGluRs in the increase in EAAs following SCI, we administered AIDA, a potent group-I mGluR antagonist immediately after injury. To determine subtype specific roles of the group-I mGluRs, we evaluated EAA release following LY 367385 (mGluR1 antagonist) and MPEP (mGluR5 antagonist) administration. To evaluate group-II and -III mGluRs we administered APDC (group-II agonist) and L-AP4 (group-III agonist) immediately following injury; additionally, we initiated treatment with CPPG (group-II/-III antagonist) and LY 341495 (group-II antagonist) 5 min prior to injury. Subjects were adult male Sprague-Dawley rats (225-250 g), impact injured at T10 with an NYU impactor (12.5 mm drop). Agents were injected into the epicenter of injury, amino acids where collected by microdialysis fibers inserted 0.5 mm caudal from the edge of the impact region and quantified by HPLC. Treatment with AIDA significantly decreased extracellular EAA and GABA concentrations. MPEP reduced EAA concentrations without affecting GABA. Combining LY 367385 and MPEP resulted in a decrease in EAA and GABA concentrations greater than either agent alone. L-AP4 decreased EAA levels, while treatment with LY 341495 increased EAA levels. These results suggest that mGluRs play an important role in EAA toxicity following SCI.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

Studying rhythmogenesis of breathing: comparison of in vivo and in vitro models

In all mammalian species, breathing is controlled by a neuronal network within the lower brainstem. A component known as the ventral respiratory group produces rhythmic activity, which is transmitted to spinal motoneurons to produce a periodic contraction of respiratory muscles. A dispute about the mechanisms of 'normal' respiratory rhythm generation arose from the differences between experimental preparations that have been used to dissect the process. It is, therefore, essential to compare the various experimental approaches and to discuss the differences between experimental data. We conclude that the various preparations all have great value, but that they define different operational conditions of the network, including maturation of neurons and synaptic processes. We have taken note of these in formulating a 'maturational network-burster model' for rhythm generation that includes most features of the existing models of respiratory rhythm generation.

Intraspinal amino acid neurotransmitter activities are involved in the generation of rhythmic sympathetic nerve discharge in newborn rat spinal cord

Endogenous neurotransmitter activities underlying the sympathetic nerve discharge (SND) generated by newborn rat spinal cord in vitro were investigated using glutamatergic, glycinergic, and GABAergic antagonists. Under control conditions, the SND power spectrum had two major frequency components: synchronous bursting SND (bSND) with power dominant at < 0.1 Hz and quasiperiodic SND (qSND) oscillating at 1-2 Hz. Using high Mg2+ solution (12-24 mM) to block Ca2+-dependent synaptic transmission reversibly abolished SND. An interruption of glutamatergic neurotransmission by CNQX (non-NMDA receptor blocker) or L-AP4 (reducing the synaptic release of glutamate) failed to affect qSND, but consistently reduced bSND. Application of kynurenate, a broad-spectrum ionotropic glutamate receptor blocker, only caused an unstable SND but did not reduce SND. In contrast, strychnine (Stry, glycine receptor antagonist) consistently reduced qSND in a dose-dependent manner. Bicuculline (Bic, GABA(A) receptor antagonist) induced a synchronous bSND of irregular rhythm, which could be further regularized by adding Stry. Bic-induced bSND was reversibly abolished by CNQX or L-AP4. In conclusion, intraspinal glycinergic, GABAergic, and glutamatergic activities are involved in the generation of the spinal cord-derived SND in newborn rats. Intraspinal GABAergic interneurons may tonically inhibit the glutamatergic bursting neurons that generate a synchronous bSND. Activities of these glutamatergic bursting neurons may also be modulated by intraspinal glycinergic interneurons.

Groups II and III metabotropic glutamate receptors differentially modulate brief and prolonged nociception in primate STT cells

The heterogeneous family of G-protein-coupled metabotropic glutamate receptors (mGluRs) provides excitatory and inhibitory controls of synaptic transmission and neuronal excitability in the nervous system. Eight mGluR subtypes have been cloned and are classified in three subgroups. Group I mGluRs can stimulate phosphoinositide hydrolysis and activate protein kinase C whereas group II (mGluR2 and 3) and group III (mGluR4, 6, 7, and 8) mGluRs share the ability to inhibit cAMP formation. The present study examined the roles of groups II and III mGluRs in the processing of brief nociceptive information and capsaicin-induced central sensitization of primate spinothalamic tract (STT) cells in vivo. In 11 anesthetized male monkeys (Macaca fascicularis), extracellular recordings were made from 21 STT cells in the lumbar dorsal horn. Responses to brief (15 s) cutaneous stimuli of innocuous (brush), marginally and distinctly noxious (press and pinch, respectively) intensity were recorded before, during, and after the infusion of group II and group III mGluR agonists into the dorsal horn by microdialysis. Different concentrations were applied for at least 20 min each (at 5 microliter/min) to obtain cumulative concentration-response relationships. Values in this paper refer to the drug concentrations in the microdialysis fibers; actual concentrations in the tissue are about three orders of magnitude lower. The agonists were also applied at 10-25 min after intradermal capsaicin injection. The group II agonists (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG1, 1 microM-10 mM, n = 6) and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4, 6-dicarboxylate (LY379268; 1 microM-10 mM, n = 6) had no significant effects on the responses to brief cutaneous mechanical stimuli (brush, press, pinch) or on ongoing background activity. In contrast, the group III agonist L(+)-2-amino-4-phosphonobutyric acid (LAP4, 0. 1 microM-10 mM, n = 6) inhibited the responses to cutaneous mechanical stimuli in a concentration-dependent manner, having a stronger effect on brush responses than on responses to press and pinch. LAP4 did not change background discharges significantly. Intradermal injections of capsaicin increased ongoing background activity and sensitized the STT cells to cutaneous mechanical stimuli (ongoing activity > brush > press > pinch). When given as posttreatment, the group II agonists LCCG1 (100 microM, n = 5) and LY379268 (100 microM, n = 6) and the group III agonist LAP4 (100 microM, n = 6) reversed the capsaicin-induced sensitization. After washout of the agonists, the central sensitization resumed. Our data suggest that, while activation of both group II and group III mGluRs can reverse capsaicin-induced central sensitization, it is the actions of group II mGluRs in particular that undergo significant functional changes during central sensitization because they modulate responses of sensitized STT cells but have no effect under control conditions.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

TASK-1, a two-pore domain K+ channel, is modulated by multiple neurotransmitters in motoneurons

Inhibition of "leak" potassium (K+) channels is a widespread CNS mechanism by which transmitters induce slow excitation. We show that TASK-1, a two pore domain K+ channel, provides a prominent leak K+ current and target for neurotransmitter modulation in hypoglossal motoneurons (HMs). TASK-1 mRNA is present at high levels in motoneurons, including HMs, which express a K+ current with pH- and voltage-dependent properties virtually identical to those of the cloned channel. This pH-sensitive K+ channel was fully inhibited by serotonin, norepinephrine, substance P, thyrotropin-releasing hormone, and 3,5-dihydroxyphenylglycine, a group I metabotropic glutamate receptor agonist. The neurotransmitter effect was entirely reconstituted in HEK 293 cells coexpressing TASK-1 and the TRH-R1 receptor. Given its expression patterns and the widespread prevalence of this neuromodulatory mechanism, TASK-1 also likely supports this action in other CNS neurons.