Noradrenergic modulation of intrinsic and synaptic properties of lumbar motoneurons in the neonatal rat spinal cord

Isoproterenol modulates expiratory activities in the brainstem spinal cord preparation in neonatal mice in vitro

Motor behaviors such as breathing required temporal coordination of different muscle groups to insured efficient ventilation and provide oxygen to the body. This action is the result of interactions between neural networks located within the brainstem. Inspiration and expiration depend at least in part on interactions between two separate oscillators: inspiration is driven by a neural network located in the preBötzinger complex (PreBötC) and active expiration is driven by a network in the parafacial respiratory group (pFRG). Neurons of the pFRG are silent at rest and become active when the respiratory drive increased. This study investigated the temporal coordination between the brainstem respiratory network and the lumbar spinal network that generates spontaneous activities that is different of the induced fictive locomotion. The remaining question is how these activities coordinate early during the development. Results of this study show that brainstem networks contribute to the temporal coordination of the lumbar spontaneous activity during inspiration since lumbar motor activity occurs exclusively during the expiratory time. This study also investigated the role of the β-noradrenergic modulation on the respiratory activities. β-noradrenergic receptors activation increased the frequency of the double bursts and increased expiratory activity at the lumbar level. These results suggest interactions between brainstem and spinal networks and reveal a descending drive that may contribute to the coordination of the respiratory and lumbar spontaneous activities.

Developmentally Regulated Modulation of Lumbar Motoneurons by Metabotropic Glutamate Receptors: A Cellular and Behavioral Analysis in Newborn Mice

The present study explores the impact of metabotropic glutamate receptor (mGluR) activation on activity-dependent synaptic plasticity (ADSP) and the intrinsic membrane properties of lumbar motoneurons (MNs) using a combination of biochemical, pharmacological, electrophysiological and behavioral techniques. Using spinal cord slices from C57BL/6JRJ mice at two developmental stages, 1-3 and 8-12 postnatal days (P1-P3; P8-P12, respectively), we found that ADSP expressed at glutamatergic synapses between axons conveyed in the ventrolateral funiculus (VLF) and MNs, involved mGluR activation. Using specific agonists of the three groups of mGluRs, we observed that mGluR stimulation causes subtype-specific and developmentally regulated modulation of the ADSP and synaptic transmission at VLF-MN synapses as well as the intrinsic membrane properties of MNs. RT-qPCR analysis revealed a downregulation of mGluR gene expression with age in the ventral part of the lumbar spinal cord. Interestingly, the selective harvest by laser microdissection of MNs innervating the Gastrocnemius and Tibialis anterior muscles unraveled that the level of Grm2 expression is higher in Tibialis MNs compared to Gastrocnemius MNs suggesting a specific mGluR gene expression profile in these two MN pools. Finally, we assessed the functional impact of mGluR modulation on electrically induced bouts of fictive locomotion in the isolated spinal cord preparation of P1-P3 mice, and in vivo during spontaneous episodes of swimming activity in both P1-P3 and P8-P12 mouse pups. We observed that the mGluR agonists induced distinct and specific effects on the motor burst amplitudes and period of the locomotor rhythms tested and that their actions are function of the developmental stage of the animals. Altogether our data show that the metabotropic glutamatergic system exerts a complex neuromodulation in the developing spinal lumbar motor networks and provide new insights into the expression and modulation of ADSP in MNs.

Ia EPSPs in rat spinal motoneurons are potentiated after a 5-week whole-body vibration

Whole-body vibration (WBV) is often applied as an alternative method for strength training or to prevent muscle force decrease. Previous studies indicated that WBV induced: 1) changes in the contractile parameters predominantly of fast motor units, 2) higher motoneuron excitability, and 3) higher motoneuron firing rates at lower stimulus intensities compared with the control. In this study, we evaluated the influence of WBV on Ia monosynaptic input from muscle spindles because the tonic vibration reflex is responsible for the enhancement of muscle activity observed after WBV. The aim was to answer the question of whether repeated activation of muscle spindles during WBV may result in altered synaptic excitation of motoneurons. WBV was performed on adult male Wistar rats, 5 days per week, for 5 weeks, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Fast-type medial gastrocnemius motoneurons were investigated intracellularly in deeply anesthetized animals in the experimental (n=7, 34 motoneurons) and control (n=7, 32 motoneurons) groups. Monosynaptic Ia EPSPs were evoked by electrical stimulation of afferent fibers from the synergistic lateral gastrocnemius and soleus muscles. Data were analyzed using a mixed linear model. WBV induced an increase of the mean EPSP amplitude by 28% (P=0.025), correlated with the resting membrane potential and input resistance, and a shortening of the mean EPSP rise time by 11% (P=0.012). The potentiation of synaptic excitation of motoneurons indicates that WBV may support rehabilitation or training processes aimed at increasing muscle strength on the basis of increased motoneuronal drive.

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the degeneration of both upper and lower motor neurons. Despite decades of research, we still to date lack a cure or disease modifying treatment, emphasizing the need for a much-improved insight into disease mechanisms and cell type vulnerability. Altered neuronal excitability is a common phenomenon reported in ALS patients, as well as in animal models of the disease, but the cellular and circuit processes involved, as well as the causal relevance of those observations to molecular alterations and final cell death, remain poorly understood. Here, we review evidence from clinical studies, cell type-specific electrophysiology, genetic manipulations and molecular characterizations in animal models and culture experiments, which argue for a causal involvement of complex alterations of structure, function and connectivity of different neuronal subtypes within the cortical and spinal cord motor circuitries. We also summarize the current knowledge regarding the detrimental role of astrocytes and reassess the frequently proposed hypothesis of glutamate-mediated excitotoxicity with respect to changes in neuronal excitability. Together, these findings suggest multifaceted cell type-, brain area- and disease stage- specific disturbances of the excitation/inhibition balance as a cardinal aspect of ALS pathophysiology.

Activation of α-adrenoceptors depresses synaptic transmission of myelinated afferents and inhibits pathways mediating primary afferent depolarization (PAD) in the in vitro mouse spinal cord

Somatosensory afferent transmission strength is controlled by several presynaptic mechanisms that reduce transmitter release at the spinal cord level. We focused this investigation on the role of α-adrenoceptors in modulating sensory transmission in low-threshold myelinated afferents and in pathways mediating primary afferent depolarization (PAD) of neonatal mouse spinal cord. We hypothesized that the activation of α-adrenoceptors depresses low threshold-evoked synaptic transmission and inhibits pathways mediating PAD. Extracellular field potentials (EFPs) recorded in the deep dorsal horn assessed adrenergic modulation of population monosynaptic transmission, while dorsal root potentials (DRPs) recorded at root entry zone assessed adrenergic modulation of PAD. We found that noradrenaline (NA) and the α1-adrenoceptor agonists phenylephrine and cirazoline depressed synaptic transmission (by 15, 14 and 22%, respectively). DRPs were also depressed by NA, phenylephrine and cirazoline (by 62, 30, and 64%, respectively), and by the α2-adrenoceptor agonist clonidine, although to a lower extent (20%). We conclude that NA depresses monosynaptic transmission of myelinated afferents onto deep dorsal horn neurons via α1-adrenoceptors and inhibits interneuronal pathways mediating PAD through the activation of α1- and α2-adrenoceptors. The functional significance of these modulatory actions in shaping cutaneous and muscle sensory information during motor behaviors requires further study.

Noradrenergic effects in rat sacral autonomic nucleus using in vitro slice patch-clamp recordings

Noradrenergic modulation has been frequently discussed in the context of neural activities that are related to pelvic organs. The sacral preganglionic nucleus (SPN) is a spinal nucleus containing parasympathetic preganglionic neurons that send fibers to pelvic nerves. In spite of the abundant presence of noradrenergic fibers around the SPN, the effects of noradrenaline (NA) remain obscure. To explore this issue, NA (50 μM) was applied to parasympathetic preganglionic neurons in the SPN during whole-cell patch clamp recording. The SPN was labeled with the retrograde tracer, DiI. These neurons demonstrated two classes of firing patterns (delayed and regular) in terms of initiation of firing. Independent of these firing patterns, NA induced inward (56%) or outward (32%) currents in labeled SPN neurons. Phenylephrine, an α1 receptor agonist, induced an inward current, and clonidine, an α2 receptor agonist, induced an outward current, indicating the existence of both α1 and α2 adrenoreceptors in DiI-labeled SPN neurons. NA also modulated synaptic currents according to the firing patterns. In delayed firing neurons, NA inhibited both spontaneous excitatory post-synaptic currents (sEPSCs) and spontaneous inhibitory post-synaptic currents (sIPSCs). Hence, NA facilitated sEPSCs and sIPSCs in about a half of regular firing neurons. Bath application of phenylephrine facilitated sEPSCs and sIPSCs, and clonidine inhibited them. These results support the hypothesis of multiple effects of NA in the SPN, and may suggest functional differences among SPN neurons.

Origin of thoracic spinal network activity during locomotor-like activity in the neonatal rat

Effective quadrupedal locomotor behaviors require the coordination of many muscles in the limbs, back, neck, and tail. Because of the spinal motoneuronal somatotopic organization, motor coordination implies interactions among distant spinal networks. Here, we investigated some of the interactions between the lumbar locomotor networks that control limb movements and the thoracic networks that control the axial muscles involved in trunk movement. For this purpose, we used an in vitro isolated newborn rat spinal cord (from T2 to sacrococcygeal) preparation. Using extracellular ventral root recordings, we showed that, while the thoracic cord possesses an intrinsic rhythmogenic capacity, the lumbar circuits, if they are rhythmically active, will entrain the rhythmicity of the thoracic circuitry. However, if the lumbar circuits are rhythmically active, these latter circuits will entrain the rhythmicity of the thoracic circuitry. Blocking the synaptic transmission in some thoracic areas revealed that the lumbar locomotor network could trigger locomotor bursting in distant thoracic segments through short and long propriospinal pathways. Patch-clamp recordings revealed that 72% of the thoracic motoneurons (locomotor-driven motoneurons) expressed membrane potential oscillations and spiking activity coordinated with the locomotor activity expressed by the lumbar cord. A biphasic excitatory (glutamatergic)/inhibitory (glycinergic) synaptic drive was recorded in thoracic locomotor-driven motoneurons. Finally, we found evidence that part of this locomotor drive involved a monosynaptic component coming directly from the lumbar locomotor network. We conclude that the lumbar locomotor network plays a central role in the generation of locomotor outputs in the thoracic cord by acting at both the premotoneuronal and motoneuronal levels.

Monoaminergic control of spinal locomotor networks in SOD1G93A newborn mice

Mutations in the gene that encodes Cu/Zn-superoxide dismutase (SOD1) are the cause of approximately 20% of familial forms of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by the progressive loss of motor neurons. While ALS symptoms appear in adulthood, spinal motoneurons exhibit functional alterations as early as the embryonic and postnatal stages in the murine model of ALS, the SOD1 mice. Monoaminergic - i.e., dopaminergic (DA), serotoninergic (5-HT), and noradrenergic (NA) - pathways powerfully control spinal networks and contribute significantly to their embryonic and postnatal maturation. Alterations in monoaminergic neuromodulation during development could therefore lead to impairments in the motoneuronal physiology. In this study, we sought to determine whether the monoaminergic spinal systems are modified in the early stages of development in SOD1 mice. Using a post-mortem analysis by high performance liquid chromatography (HPLC), monoaminergic neuromodulators and their metabolites were quantified in the lumbar spinal cord of SOD1 and wild-type (WT) mice aged one postnatal day (P1) and P10. This analysis underscores an increased content of DA in the SOD1 lumbar spinal cord compared to that of WT mice but failed to reveal any modification of the other monoaminergic contents. In a next step, we compared the efficiency of the monoaminergic compounds in triggering and modulating fictive locomotion in WT and SOD1 mice. This study was performed in P1-P3 SOD1 mice and age-matched control littermates using extracellular recordings from the lumbar ventral roots in the in vitro isolated spinal cord preparation. This analysis revealed that the spinal networks of SOD1(G93A) mice could generate normal locomotor activity in the presence of NMA-5-HT. Interestingly, we also observed that SOD1 spinal networks have an increased sensitivity to NA compared to WT spinal circuits but exhibited similar DA responses.

β-Noradrenergic receptor activation specifically modulates the generation of sighs in vivo and in vitro

The pre-Bötzinger complex (preBötC), an area that is critical for generating breathing (eupnea), gasps and sighs is continuously modulated by catecholamines. These amines and the generation of sighs have also been implicated in the regulation of arousal. Here we studied the catecholaminergic modulation of sighs not only in anesthetized freely breathing mice (in vivo), but also in medullary slice preparations that contain the preBötC and that generate fictive eupneic and sigh rhythms in vitro. We demonstrate that activating β-noradrenergic receptors (β-NR) specifically increases the frequency of sighs, while eupnea remains unaffected both in vitro and in vivo. β-NR activation specifically increased the frequency of intrinsically bursting pacemaker neurons that rely on persistent sodium current (I(Nap)). By contrast, all parameters of bursting pacemakers that rely on the non-specific cation current (I(CAN)) remained unaffected. Moreover, riluzole, which blocks bursting in I(Nap) pacemakers abolished sighs altogether, while flufenamic acid (FFA) which blocks the I(CAN) current did not alter the sigh-increasing effect caused by β-NR. Our results suggest that the selective β-NR action of sighs may result from the modulation of I(Nap) pacemaker activity and that disturbances in noradrenergic system may contribute to abnormal arousal response. The β-NR action on the preBötC may be an important mechanism in modulating behaviors that are specifically associated with sighs, such as the regulation of the early events leading to the arousal response.

Alterations of cAMP-dependent signaling in dystrophic skeletal muscle

Autonomic regulation processes in striated muscles are largely mediated by cAMP/PKA-signaling. In order to achieve specificity of signaling its spatial-temporal compartmentation plays a critical role. We discuss here how specificity of cAMP/PKA-signaling can be achieved in skeletal muscle by spatio-temporal compartmentation. While a microdomain containing PKA type I in the region of the neuromuscular junction (NMJ) is important for postsynaptic, activity-dependent stabilization of the nicotinic acetylcholine receptor (AChR), PKA type I and II microdomains in the sarcomeric part of skeletal muscle are likely to play different roles, including the regulation of muscle homeostasis. These microdomains are due to specific A-kinase anchoring proteins, like rapsyn and myospryn. Importantly, recent evidence indicates that compartmentation of the cAMP/PKA-dependent signaling pathway and pharmacological activation of cAMP production are aberrant in different skeletal muscles disorders. Thus, we discuss here their potential as targets for palliative treatment of certain forms of dystrophy and myasthenia. Under physiological conditions, the neuropeptide, α-calcitonin-related peptide, as well as catecholamines are the most-mentioned natural triggers for activating cAMP/PKA signaling in skeletal muscle. While the precise domains and functions of these first messengers are still under investigation, agonists of β₂-adrenoceptors clearly exhibit anabolic activity under normal conditions and reduce protein degradation during atrophic periods. Past and recent studies suggest direct sympathetic innervation of skeletal muscle fibers. In summary, the organization and roles of cAMP-dependent signaling in skeletal muscle are increasingly understood, revealing crucial functions in processes like nerve-muscle interaction and muscle trophicity.

β adrenergic receptor modulation of neurotransmission to cardiac vagal neurons in the nucleus ambiguus

β-adrenergic receptors are a class of G protein-coupled receptors that have essential roles in regulating heart rate, blood pressure, and other cardiorespiratory functions. Although the role of β adrenergic receptors in the peripheral nervous system is well characterized, very little is known about their role in the central nervous system despite being localized in many brain regions involved in autonomic activity and regulation. Since parasympathetic activity to the heart is dominated by cardiac vagal neurons (CVNs) originating in the nucleus ambiguus (NA), β adrenergic receptors localized in the NA represent a potential target for modulating cardiac vagal activity and heart rate. This study tests the hypothesis that activation of β adrenergic receptors alters the membrane properties and synaptic neurotransmission to CVNs. CVNs were identified in brainstem slices, and membrane properties and synaptic events were recorded using the whole-cell voltage-clamp technique. The nonselective β agonist isoproterenol significantly decreased inhibitory GABAergic and glycinergic as well as excitatory glutamatergic neurotransmission to CVNs. In addition, the β(1)-selective receptor agonist dobutamine, but not β(2) or β(3) receptor agonists, significantly decreased inhibitory GABAergic and glycinergic and excitatory glutamatergic neurotransmission to CVNs. These decreases in neurotransmission to CVNs persisted in the presence of tetrodotoxin (TTX). These results provide a mechanism by which activation of adrenergic receptors in the brainstem can alter parasympathetic activity to the heart. Likely physiological roles for this adrenergic receptor activation are coordination of parasympathetic-sympathetic activity and β receptor-mediated increases in heart rate upon arousal.

Locomotor-activated neurons of the cat. II. Noradrenergic innervation and colocalization with NEα 1a or NEα 2b receptors in the thoraco-lumbar spinal cord

Norepinephrine (NE) is a strong modulator and/or activator of spinal locomotor networks. Thus noradrenergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the noradrenergic innervation of functionally related, locomotor-activated neurons within the thoraco-lumbar spinal cord. This was accomplished by immunohistochemical colocalization of noradrenergic fibers using dopamine-β-hydroxylase or NEα(1A) and NEα(2B) receptors with cells expressing the c-fos gene activity-dependent marker Fos. Experiments were performed on paralyzed, precollicular-postmamillary decerebrate cats, in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. The majority of Fos labeled neurons, especially abundant in laminae VII and VIII throughout the thoraco-lumbar (T13-L7) region of locomotor animals, showed close contacts with multiple noradrenergic boutons. A small percentage (10-40%) of Fos neurons in the T7-L7 segments showed colocalization with NEα(1A) receptors. In contrast, NEα(2B) receptor immunoreactivity was observed in 70-90% of Fos cells, with no obvious rostrocaudal gradient. In comparison with results obtained from our previous study on the same animals, a significantly smaller proportion of Fos labeled neurons were innervated by noradrenergic than serotonergic fibers, with significant differences observed for laminae VII and VIII in some segments. In lamina VII of the lumbar segments, the degree of monoaminergic receptor subtype/Fos colocalization examined statistically generally fell into the following order: NEα(2B) = 5-HT(2A) ≥ 5-HT(7) = 5-HT(1A) > NEα(1A). These results suggest that noradrenergic modulation of locomotion involves NEα(1A)/NEα(2B) receptors on noradrenergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments. Further study of the functional role of these receptors in locomotion is warranted.

Properties of urethral rhabdosphincter motoneurons and their regulation by noradrenaline

The urethral rhabdosphincter (URS), commonly known as the external urethral sphincter, facilitates urinary continence by constricting the urethra. Striated muscle fibres in the urethral rhabdosphincter are innervated by Onuf's nuclei motoneurons in the spinal cord. Although noradrenaline (NA) reuptake inhibitors are shown to increase URS tone preventing urinary leakage in incontinent patients, whether or how NA affects URS motoneurons is unknown. Properties of dye-labelled URS motoneurons were investigated by whole-cell patch-clamp recordings in isolated spinal cord slices prepared from neonatal female rats. As previously shown for adult sphincter motoneurons, neonatal URS motoneurons are more depolarized and possess higher input resistance than other spinal α-motoneurons. These distinct properties make URS motoneurons more excitable than other α-motoneurons. Moreover, bath application of noradrenaline (NA) significantly depolarizes URS motoneurons and in many cases evokes action potentials. NA also significantly increases input resistance and reduces rheobase. These changes are reversed with wash, are largely blocked by the α(1)-adrenoceptor-selective antagonist prazosin, and are mimicked by the α(1)-adrenoceptor-selective agonist phenylephrine. In addition, NA significantly reduces the amplitude of the afterhyperpolarization and increases action potential frequency. Both the increase in action potential frequency and the reduction in afterhyperpolarization are occluded by apamin, a small-conductance calcium-activated potassium (SK(Ca)) channel blocker. In conclusion, NA effectively increases the excitability of URS motoneurons through multiple mechanisms. The NA-induced increase in excitability of urethral rhabdosphincter motoneurons could be a key mechanism by which NA reuptake inhibitors improve stress urinary incontinence.