NMDA receptor-mediated oscillatory activity in the neonatal rat spinal cord is serotonin dependent

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischaemic (HI) injury in utero (at 70%-83% gestation) are born with muscle stiffness, hyperreflexia and, as recently discovered, increased 5-HT in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to the application of α-methyl 5-HT (a 5-HT1 /5-HT2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with increased amplitude and hyperpolarization of persistent inward currents and hyperpolarized threshold voltage for action potentials, whereas control MNs did not exhibit any of these responses. Although 5-HT similarly modulated MN properties of HI motor-unaffected and motor-affected kits, it affected sag/hyperpolarization-activated cation current (Ih ) and spike frequency adaptation only in HI motor-affected MNs. To further explore the differential sensitivity of MNs to 5-HT, we performed immunostaining for inhibitory 5-HT1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT1A receptor compared to age-matched control MNs. This suggests that HI MNs may lack a normal mechanism of central fatigue, mediated by 5-HT1A receptors. Altered expression of other 5-HT receptors (including 5-HT2 ) likely also contributes to the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI-affected rabbits can cause MN hyperexcitability, muscle stiffness and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP. KEY POINTS: We used whole cell patch clamp electrophysiology to test the responsivity of spinal motoneurons (MNs) from neonatal control and hypoxia-ischaemia (HI) rabbits to 5-HT, which is elevated in the spinal cord after prenatal HI injury. HI rabbit MNs showed a more robust excitatory response to 5-HT than control rabbit MNs, including hyperpolarization of the persistent inward current and threshold voltage for action potentials. Although most MN properties of HI motor-unaffected and motor-affected kits responded similarly to 5-HT, 5-HT caused larger sag/hyperpolarization-activated cation current (Ih ) and altered repetitive firing patterns only in HI motor-affected MNs. Immunostaining revealed that fewer lumbar MNs expressed inhibitory 5-HT1A receptors in HI rabbits compared to controls, which could account for the more robust excitatory response of HI MNs to 5-HT. These results suggest that elevated 5-HT after prenatal HI injury could trigger a cascade of events that lead to muscle stiffness and altered motor unit development.

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischemic (HI) injury in utero (at 70-80% gestation) are born with muscle stiffness, hyperreflexia, and, as recently discovered, increased serotonin (5-HT) in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to application of α-methyl 5-HT (a 5-HT 1 /5-HT 2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with hyperpolarization of persistent inward currents and threshold voltage for action potentials, reduced maximum firing rate, and an altered pattern of spike frequency adaptation while control MNs did not exhibit any of these responses. To further explore the differential sensitivity of MNs to 5-HT, we performed immunohistochemistry for inhibitory 5-HT 1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT 1A receptor compared to age-matched controls. This suggests many HI MNs lack a normal mechanism of central fatigue mediated by 5-HT 1A receptors. Other 5-HT receptors (including 5-HT 2 ) are likely responsible for the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI rabbits can cause MN hyperexcitability, muscle stiffness, and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP.

Key points

After prenatal hypoxia-ischemia (HI), neonatal rabbits that show hypertonia are known to have higher levels of spinal serotoninWe tested responsivity of spinal motoneurons (MNs) in neonatal control and HI rabbits to serotonin using whole cell patch clampMNs from HI rabbits showed a more robust excitatory response to serotonin than control MNs, including hyperpolarization of the persistent inward current and threshold for action potentials, larger post-inhibitory rebound, and less spike frequency adaptation Based on immunohistochemistry of lumbar MNs, fewer HI MNs express inhibitory 5HT 1A receptors than control MNs, which could account for the more robust excitatory response of HI MNs. These results suggest that after HI injury, the increased serotonin could trigger a cascade of events leading to muscle stiffness and altered motor unit development.

Recruitment of Motoneurons

Beginning about half a century ago, the rules that determine how motor units are recruited during movement have been deduced. These classical experiments led to the formulation of the 'size principle'. It is now clear that motoneuronal size is not the only indicator of recruitment order. In fact, motoneuronal passive, active and synaptic conductances are carefully tuned to achieve sequential recruitment. More recent studies, over the last decade or so, show that the premotor circuitry is also functionally specialized and differentially recruited. Modular sub networks of interneurons and their post-synaptic motoneurons have been shown to drive movements with varying intensities. In addition, these modular networks are under the influence of neuromodulators, which are capable of acting upon multiple motor and premotor targets, thereby altering behavioral outcomes. We discuss the recruitment patterns of motoneurons in light of these new and exciting studies.

Serotonergic Modulation of Locomotor Activity From Basal Vertebrates to Mammals

During the last 50 years, the serotonergic (5-HT) system was reported to exert a complex modulation of locomotor activity. Here, we focus on two key factors that likely contribute to such complexity. First, locomotion is modulated directly and indirectly by 5-HT neurons. The locomotor circuitry is directly innervated by 5-HT neurons in the caudal brainstem and spinal cord. Also, indirect control of locomotor activity results from ascending projections of 5-HT cells in the rostral brainstem that innervate multiple brain centers involved in motor action planning. Second, each approach used to manipulate the 5-HT system likely engages different 5-HT-dependent mechanisms. This includes the recruitment of different 5-HT receptors, which can have excitatory or inhibitory effects on cell activity. These receptors can be located far or close to the 5-HT release sites, making their activation dependent on the level of 5-HT released. Here we review the activity of different 5-HT nuclei during locomotor activity, and the locomotor effects of 5-HT precursors, exogenous 5-HT, selective 5-HT reuptake inhibitors (SSRI), electrical or chemical stimulation of 5-HT neurons, genetic deletions, optogenetic and chemogenetic manipulations. We highlight both the coherent and controversial aspects of 5-HT modulation of locomotor activity from basal vertebrates to mammals. This mini review may hopefully inspire future studies aiming at dissecting the complex effects of 5-HT on locomotor function.

Morphological and electrophysiological properties of serotonin neurons with NMDA modulation in the mesencephalic locomotor region of neonatal ePet-EYFP mice

The mesencephalic locomotor region (MLR) is an essential area for initiation of locomotion. Its functional roles and circuits underlying locomotion have been studied intensively in many species. Studies suggest that cuneiform nucleus and pedunculopontine nucleus (PPN) are two core regions in the MLR for locomotion. However, it remains unclear about cellular components and morphological and intrinsic membrane properties of the neurons in these regions, especially the serotonergic neurons. Using neonatal ePet-EYFP transgenic mice and immunofluorescent technique, we demonstrated existence of 5-HT neurons in the MLR and discovered that 5-HT neurons distributed mainly in the caudal PPN. 5-HT neurons were heterogeneous in MLR and had three types of firing pattern (single spike, phasic and tonic) and two subtypes of morphology (pyramidal and stellate). We measured parameters of 5-HT neurons (n = 35) including resting membrane potential (- 69.2 ± 4.2 mV), input resistance (1410.1 ± 616.9 MΩ), membrane capacitance (36.4 ± 14.9 pF), time constant (49.7 ± 19.4 ms), voltage threshold (- 32.1 ± 7.4 mV), rheobase (21.3 ± 12.4 pA), action potential amplitude (58.9 ± 12.8 mV) and half-width (4.7 ± 1.1 ms), afterhyperpolarization amplitude (23.6 ± 10.4 mV) and half-decay (331.6 ± 157.7 ms). 5-HT neurons were intrinsically different from adjacent non-5-HT neurons and less excitable than them. Hyperpolarization-activated inward currents and persistent inward currents were recorded in 5-HT neurons. NMDA increased excitability of 5-HT neurons, especially the tonic-firing neurons, accompanied with depolarization of membrane potential, hyperpolarization of voltage threshold, reduction of afterhyperpolarization half-decay, and left-shift of frequency-current relationship. This study provided insight into the distribution and properties of 5-HT neurons in the MLR and interaction between serotonergic and glutamatergic modulations.

Modulation of Rhythmic Activity in Mammalian Spinal Networks Is Dependent on Excitability State

Neuromodulators play an important role in activating rhythmically active motor networks; however, what remains unclear are the network interactions whereby neuromodulators recruit spinal motor networks to produce rhythmic activity. Evidence from invertebrate systems has demonstrated that the effect of neuromodulators depends on the pre-existing state of the network. We explored how network excitation state affects the ability of dopamine to evoke rhythmic locomotor activity in the neonatal mouse isolated spinal cord. We found that dopamine can evoke unique patterns of motor activity that are dependent on the excitability state of motor networks. Different patterns of motor activity ranging from tonic, nonrhythmic activity to multirhythmic, nonlocomotor activity to locomotor activity were produced by altering global motor network excitability through manipulations of the extracellular potassium and bath NMDA concentration. A similar effect was observed when network excitation was manipulated during an unstable multirhythm evoked by a low concentration (15 µm) of 5-HT, suggesting that our results are not neuromodulator specific. Our data show in vertebrate systems that modulation is a two-way street and that modulatory actions are largely influenced by the network state. The level of network excitation can account for variability between preparations and is an additional factor to be considered when circuit elements are removed from the network.

Sensory Activation of Command Cells for Locomotion and Modulatory Mechanisms: Lessons from Lampreys

Sensorimotor transformation is one of the most fundamental and ubiquitous functions of the central nervous system (CNS). Although the general organization of the locomotor neural circuitry is relatively well understood, less is known about its activation by sensory inputs and its modulation. Utilizing the lamprey model, a detailed understanding of sensorimotor integration in vertebrates is emerging. In this article, we explore how the vertebrate CNS integrates sensory signals to generate motor behavior by examining the pathways and neural mechanisms involved in the transformation of cutaneous and olfactory inputs into motor output in the lamprey. We then review how 5-hydroxytryptamine (5-HT) acts on these systems by modulating both sensory inputs and motor output. A comprehensive review of this fundamental topic should provide a useful framework in the fields of motor control, sensorimotor integration and neuromodulation.

The role of the serotonergic system in locomotor recovery after spinal cord injury

Serotonin (5-HT), a monoamine neurotransmitter synthesized in various populations of brainstem neurons, plays an important role in modulating the activity of spinal networks involved in vertebrate locomotion. Following spinal cord injury (SCI) there is a disruption of descending serotonergic projections to spinal motor areas, which results in a subsequent depletion in 5-HT, the dysregulation of 5-HT transporters as well as the elevated expression, super-sensitivity and/or constitutive auto-activation of specific 5-HT receptors. These changes in the serotonergic system can produce varying degrees of locomotor dysfunction through to paralysis. To date, various approaches targeting the different components of the serotonergic system have been employed to restore limb coordination and improve locomotor function in experimental models of SCI. These strategies have included pharmacological modulation of serotonergic receptors, through the administration of specific 5-HT receptor agonists, or by elevating the 5-HT precursor 5-hydroxytryptophan, which produces a global activation of all classes of 5-HT receptors. Stimulation of these receptors leads to the activation of the locomotor central pattern generator (CPG) below the site of injury to facilitate or improve the quality and frequency of movements, particularly when used in concert with the activation of other monoaminergic systems or coupled with electrical stimulation. Another approach has been to employ cell therapeutics to replace the loss of descending serotonergic input to the CPG, either through transplanted fetal brainstem 5-HT neurons at the site of injury that can supply 5-HT to below the level of the lesion or by other cell types to provide a substrate at the injury site for encouraging serotonergic axon regrowth across the lesion to the caudal spinal cord for restoring locomotion.

Adult spinal V2a interneurons show increased excitability and serotonin-dependent bistability

In mice, most studies of the organization of the spinal central pattern generator (CPG) for locomotion, and its component neuron classes, have been performed on neonatal [postnatal day (P)2-P4] animals. While the neonatal spinal cord can generate a basic locomotor pattern, it is often argued that the CPG network is in an immature form whose detailed properties mature with postnatal development. Here, we compare intrinsic properties and serotonergic modulation of the V2a class of excitatory spinal interneurons in behaviorally mature (older than P43) mice to those in neonatal mice. Using perforated patch recordings from genetically tagged V2a interneurons, we revealed an age-dependent increase in excitability. The input resistance increased, the rheobase values decreased, and the relation between injected current and firing frequency (F/I plot) showed higher excitability in the adult neurons, with almost all neurons firing tonically during a current step. The adult action potential (AP) properties became narrower and taller, and the AP threshold hyperpolarized. While in neonates the AP afterhyperpolarization was monophasic, most adult V2a interneurons showed a biphasic afterhyperpolarization. Serotonin increased excitability and depolarized most neonatal and adult V2a interneurons. However, in ∼30% of adult V2a interneurons, serotonin additionally elicited spontaneous intrinsic membrane potential bistability, resulting in alternations between hyperpolarized and depolarized states with a dramatically decreased membrane input resistance and facilitation of evoked plateau potentials. This was never seen in younger animals. Our findings indicate a significant postnatal development of the properties of locomotor-related V2a interneurons, which could alter their interpretation of synaptic inputs in the locomotor CPG.

Endogenous release of 5-HT modulates the plateau phase of NMDA-induced membrane potential oscillations in lamprey spinal neurons

The lamprey central nervous system has been used extensively as a model system for investigating the networks underlying vertebrate motor behavior. The locomotor networks can be activated by application of glutamate agonists, such as N-methyl-D-aspartic acid (NMDA), to the isolated spinal cord preparation. Many spinal neurons are capable of generating pacemaker-like membrane potential oscillations upon activation of NMDA receptors. These oscillations rely on the voltage-dependent properties of NMDA receptors in interaction with voltage-dependent potassium and calcium-dependent potassium (K(Ca)) channels, as well as low voltage-activated calcium channels. Upon membrane depolarization, influx of calcium will activate K(Ca) channels, which in turn, will contribute to repolarization and termination of the depolarized phase. The appearance of the NMDA-induced oscillations varies markedly between spinal cord preparations; they may either have a pronounced, depolarized plateau phase or be characterized by a short-lasting depolarization lasting approximately 200-300 ms without a plateau. Both types of oscillations increase in frequency with increased concentrations of NMDA. Here, we characterize these two types of membrane potential oscillations and show that they depend on the level of endogenous release of 5-HT in the spinal cord preparations. In the lamprey, 5-HT acts to block voltage-dependent calcium channels and will thereby modulate the activity of K(Ca) channels. When 5-HT antagonists were administered, the plateau-like oscillations were converted to the second type of oscillations lacking a plateau phase. Conversely, plateau-like oscillations can be induced or prolonged by 5-HT agonists. These properties are most likely of significance for the modulatory action of 5-HT on the spinal networks for locomotion.

Neural correlates of motor vigour and motor urgency during exercise

This article reviews the brain structures and neural circuitry underlying the motor system as it pertains to endurance exercise. Some obvious phenomena that occur during endurance racing events that need to be explained neurophysiologically are variable pacing strategies, the end spurt, motivation and the rating of perceived exertion. Understanding the above phenomena physiologically is problematic due to the sheer complexity of obtaining real-time brain measurements during exercise. In those rare instances where brain measurements have been made during exercise, the measurements have usually been limited to the sensory and motor cortices; or the exercise itself was limited to small muscle groups. Without discounting the crucial importance of the primary motor cortex in the execution of voluntary movement, it is surprising that very few exercise studies pay any attention to the complex and dynamic organization of motor action in relation to the subcortical nuclei, given that they are essential for the execution of normal movement patterns. In addition, the findings from laboratory-based exercise performance trials are hampered by the absence of objective measures of the motivational state of subjects. In this review we propose that some of the above phenomena may be explained by distinguishing between voluntary, vigorous and urgent motor behaviours during exercise, given that different CNS structures and neurotransmitters are involved in the execution of these different motor behaviours.

TTX-resistant NMDA receptor-mediated membrane potential oscillations in neonatal mouse Hb9 interneurons

Conditional neuronal membrane potential oscillations have been identified as a potential mechanism to help support or generate rhythmogenesis in neural circuits. A genetically identified population of ventromedial interneurons, called Hb9, in the mouse spinal cord has been shown to generate TTX-resistant membrane potential oscillations in the presence of NMDA, serotonin and dopamine, but these oscillatory properties are not well characterized. Hb9 interneurons are rhythmically active during fictive locomotor-like behavior. In this study, we report that exogenous N-Methyl-D-Aspartic acid (NMDA) application is sufficient to produce membrane potential oscillations in Hb9 interneurons. In contrast, exogenous serotonin and dopamine application, alone or in combination, are not sufficient. The properties of NMDA-induced oscillations vary among the Hb9 interneuron population; their frequency and amplitude increase with increasing NMDA concentration. NMDA does not modulate the T-type calcium current (I_Ca(T)), which is thought to be important in generating locomotor-like activity, in Hb9 neurons. These results suggest that NMDA receptor activation is sufficient for the generation of TTX-resistant NMDA-induced membrane potential oscillations in Hb9 interneurons.

Postnatal emergence of serotonin-induced plateau potentials in commissural interneurons of the mouse spinal cord

Most studies of the mouse hindlimb locomotor network have used neonatal (P0-5) mice. In this study, we examine the postnatal development of intrinsic properties and serotonergic modulation of intersegmental commissural interneurons (CINs) from the neonatal period (P0-3) to the time the animals bear weight (P8-10) and begin to show adult walking (P14-16). CINs show an increase in excitability with age, associated with a decrease in action potential halfwidth and appearance of a fast component to the afterhyperpolarization at P14-16. Serotonin (5-HT) depolarizes and increases the excitability of most CINs at all ages. The major developmental difference is that serotonin can induce plateau potential capability in P14-16 CINs, but not at younger ages. These plateau potentials are abolished by nifedipine, suggesting that they are mediated by an L-type calcium current, I(Ca(L)). Voltage-clamp analysis demonstrates that 5-HT increases a nifedipine-sensitive voltage-activated calcium current, I(Ca(V)), in P14-16 CINs but does not increase I(Ca(V)) in P8-10 CINs. These results, together with earlier work on 5-HT effects on neonatal CINs, suggest that 5-HT increases the excitability of CINs at all ages studied, but by opposite effects on calcium currents, decreasing N- and P/Q-type calcium currents and, indirectly, calcium-activated potassium current, at P0-3 but increasing I(Ca(L)) at P14-16.

Opposing modulatory effects of D1- and D2-like receptor activation on a spinal central pattern generator

The role of dopamine in regulating spinal cord function is receiving increasing attention, but its actions on spinal motor networks responsible for rhythmic behaviors remain poorly understood. Here, we have explored the modulatory influence of dopamine on locomotory central pattern generator (CPG) circuitry in the spinal cord of premetamorphic Xenopus laevis tadpoles. Bath application of exogenous dopamine to isolated brain stem-spinal cords exerted divergent dose-dependent effects on spontaneous episodic patterns of locomotory-related activity recorded extracellularly from spinal ventral roots. At low concentration (2 μM), dopamine reduced the occurrence of bursts and fictive swim episodes and increased episode cycle periods. In contrast, at high concentration (50 μM) dopamine reversed its actions on fictive swimming, now increasing both burst and swim episode occurrences while reducing episode periods. The low-dopamine effects were mimicked by the D2-like receptor agonists bromocriptine and quinpirole, whereas the D1-like receptor agonist SKF 38393 reproduced the effects of high dopamine. Furthermore, the motor response to the D1-like antagonist SCH 23390 resembled that to the D2 agonists, whereas the D2-like antagonist raclopride mimicked the effects of the D1 agonist. Together, these findings indicate that dopamine plays an important role in modulating spinal locomotor activity. Moreover, the transmitter's opposing influences on the same target CPG are likely to be accomplished by a specific, concentration-dependent recruitment of independent D2- and D1-like receptor signaling pathways that differentially mediate inhibitory and excitatory actions.

Serotonin modulates multiple calcium current subtypes in commissural interneurons of the neonatal mouse

Calcium currents are critical to the intrinsic properties of neurons and the networks that contain them. These currents make attractive targets for neuromodulation. Here, we examine the serotonergic modulation of specific calcium current subtypes in neonatal (P0-5) intersegmental commissural interneurons (CINs), members of the hindlimb locomotor central pattern generator in the mouse spinal cord. Previous work in our lab showed that serotonin (5-HT) excited CINs in part by reducing a calcium current and thus indirectly reducing the calcium-activated potassium current (Diaz-Rios et al. 2007). We have determined which calcium currents are targets of serotonin modulation. Utilizing whole cell voltage clamp and toxins to specific calcium current subtypes, we found that N- and P/Q-type currents comprise over 60% of the overall calcium current. Blockade of each of these subtypes alone with either ω-conotoxin GVIA or ω-agatoxin TK was unable to occlude 5-HT's reduction of the calcium current. However, coapplication of both blockers together fully occluded 5-HT's reduction of the calcium current. Thus, 5-HT decreases both N- and P/Q-type calcium current to excite neonatal CINs.

Neuromodulation of Vertebrate Locomotor Control Networks

Vertebrate locomotion must be adaptable in light of changing environmental, organismal, and developmental demands. Much of the underlying flexibility in the output of central pattern generating (CPG) networks of the spinal cord and brain stem is endowed by neuromodulation. This review provides a synthesis of current knowledge on the way that various neuromodulators modify the properties of and connections between CPG neurons to sculpt CPG network output during locomotion.

Generation of locomotion rhythms without inhibition in vertebrates: the search for pacemaker neurons

Locomotion rhythms are thought to be generated by neurons in the central-pattern-generator (CPG) circuit in the spinal cord. Synaptic connections in the CPG and pacemaker properties in certain CPG neurons, both may contribute to generation of the rhythms. In the half-center model proposed by Graham Brown a century ago, reciprocal inhibition plays a critical role. However, in all vertebrate preparations examined, rhythmic motor bursts can be induced when inhibition is blocked in the spinal cord. Without inhibition, neuronal pacemaker properties may become more important in generation of the rhythms. Pacemaker properties have been found in motoneurons and some premotor interneurons in different vertebrates and they can be dependent on N-Methyl-d-aspartate (NMDA) receptors (NMDAR) or rely on other ionic currents like persistent inward currents. In the swimming circuit of the hatchling Xenopus tadpole, there is substantial evidence that emergent network properties can give rise to swimming rhythms. During fictive swimming, excitatory interneurons (dINs) in the caudal hindbrain fire earliest on each swimming cycle and their spikes drive the firing of other CPG neurons. Regenerative dIN firing itself relies on reciprocal inhibition and background excitation. We now find that the activation of NMDARs can change dINs from firing singly at rest to current injection to firing repetitively at swimming frequencies. When action potentials are blocked, some intrinsic membrane potential oscillations at about 10 Hz are revealed, which may underlie repetitive dIN firing during NMDAR activation. In confirmation of this, dIN repetitive firing persists in NMDA when synaptic transmission is blocked by Cd(2+). When inhibition is blocked, only dINs and motoneurons are functional in the spinal circuit. We propose that the conditional intrinsic NMDAR-dependent pacemaker firing of dINs can drive the production of swimming-like rhythms without the participation of inhibitory neurotransmission.

Contribution of 5-HT to locomotion - the paradox of Pet-1(-/-) mice

Serotonin (5-HT) plays a critical role in locomotor pattern generation by modulating the rhythm and the coordinations. Pet-1, a transcription factor selectively expressed in the raphe nuclei, controls the differentiation of 5-HT neurons. Surprisingly, inactivation of Pet-1 (Pet-1(-/-) mice) that causes a 70% reduction in the number of 5-HT-positive neurons in the raphe does not impair locomotion in adult mice. The goal of the present study was to investigate the operation of the locomotor central pattern generator (CPG) in neonatal Pet-1(-/-) mice. We first confirmed, by means of immunohistochemistry, that there is a marked reduction of 5-HT innervation in the lumbar spinal cord of Pet-1(-/-) mice. Fictive locomotion was induced in the in vitro neonatal mouse spinal cord preparation by bath application of N-methyl-d,l-Aspartate (NMA) alone or together with dopamine and 5-HT. A locomotor pattern characterized by left-right and flexor-extensor alternations was observed in both conditions. Increasing the concentration of 5-HT from 0.5 to 5 μm impaired the pattern in Pet-1(-/-) mice. We tested the role of endogenous 5-HT in the NMA-induced fictive locomotion. Application of 5-HT(2) or 5-HT(7) receptor antagonists affected the NMA-induced fictive locomotion in both heterozygous and homozygous mice although the effects were weaker in the latter strain. This may be, at least partly, explained by the reduced expression of 5-HT(2A) R as observed by means of immunohistochemistry. These results suggest that compensatory mechanisms take place in Pet-1(-/-) mice that make locomotion less dependent upon 5-HT.

Locomotion after spinal cord injury depends on constitutive activity in serotonin receptors

Following spinal cord injury (SCI) neurons caudal to the injury are capable of rhythmic locomotor-related activity that can form the basis for substantial functional recovery of stepping despite the loss of crucial brain stem-derived neuromodulators like serotonin (5-HT). Here we investigated the contribution of constitutive 5-HT(2) receptor activity (activity in the absence of 5-HT) to locomotion after SCI. We used a staggered hemisection injury model in rats to study this because these rats showed a robust recovery of locomotor function and yet a loss of most descending axons. Immunolabeling for 5-HT showed little remaining 5-HT below the injury, and locomotor ability was not correlated with the amount of residual 5-HT. Furthermore, blocking 5-HT(2) receptors with an intrathecal (IT) application of the neutral antagonist SB242084 did not affect locomotion (locomotor score and kinematics were unaffected), further indicating that residual 5-HT below the injury did not contribute to generation of locomotion. As a positive control, we found that the same application of SB242084 completely antagonized the muscle activity induced by exogenous application of the 5-HT(2) receptor agonists alpha-methyl-5-HT (IT). In contrast, blocking constitutive 5-HT(2) receptor activity with the potent inverse agonist SB206553 (IT) severely impaired stepping as assessed with kinematic recordings, eliminating most hindlimb weight support and overall reducing the locomotor score in both hind legs. However, even in the most severely impaired animals, rhythmic sweeping movements of the hindlimb feet were still visible during forelimb locomotion, suggesting that SB206553 did not completely eliminate locomotor drive to the motoneurons or motoneuron excitability. The same application of SB206553 had no affect on stepping in normal rats. Thus while normal rats can compensate for loss of 5-HT(2) receptor activity, after severe spinal cord injury rats require constitutive activity in these 5-HT(2) receptors to produce locomotion.

Glutamatergic mechanisms for speed control and network operation in the rodent locomotor CpG

Locomotion is a fundamental motor act that, to a large degree, is controlled by central pattern-generating (CPG) networks in the spinal cord. Glutamate is thought to be responsible for most of the excitatory input to and the excitatory activity within the locomotor CPG. However, previous studies in mammals have produced conflicting results regarding the necessity and role of the different ionotropic glutamate receptors (GluRs) in the CPG function. Here, we use electrophysiological and pharmacological techniques in the in vitro neonatal mouse lumbar spinal cord to investigate the role of a broad range of ionotropic GluRs in the control of locomotor speed and intrinsic locomotor network function. We show that non-NMDA (non-NMDARs) and NMDA receptor (NMDAR) systems may independently mediate locomotor-like activity and that these receptors set different speeds of locomotor-like activity through mechanisms acting at various network levels. AMPA and kainate receptors are necessary for generating the highest locomotor frequencies. For coordination, NMDARs are more important than non-NMDARs for conveying the rhythmic signal from the network to the motor neurons during long-lasting and steady locomotor activity. This study reveals that a diversity of ionotropic GluRs tunes the network to perform at different locomotor speeds and provides multiple levels for potential regulation and plasticity.

Serotonin controls the maturation of the GABA phenotype in the ventral spinal cord via 5-HT1b receptors

Serotonin (5-hydroxytryptamine or 5-HT) is a pleiotropic neurotransmitter known to play a crucial modulating role during the construction of brain circuits. Descending bulbo-spinal 5-HT fibers, coming from the caudal medullary cell groups of the raphe nuclei, progressively invade the mouse spinal cord and arrive at lumbar segments at E15.5 when the number of ventral GABA immunoreactive (GABA-ir) interneurons reaches its maximum. We thus raised the question of a possible interaction between these two neurotransmitter systems and investigated the effect of 5-HT descending inputs on the maturation of the GABA phenotype in ventral spinal interneurons. Using a quantitative anatomical study performed on acute and cultured embryonic mouse spinal cord, we found that the GABAergic neuronal population matured according to a similar rostro-caudal gradient both in utero and in organotypic culture. We showed that 5-HT delayed the maturation of the GABA phenotype in lumbar but not brachial interneurons. Using pharmacological treatments and mice lacking 5-HT(1B) or 5-HT(1A), we demonstrated that the 5-HT repressing effect on the GABAergic phenotype was specifically attributed to 5-HT(1B) receptors.

Electrophysiological characterization of V2a interneurons and their locomotor-related activity in the neonatal mouse spinal cord

The V2a class of Chx10-expressing interneurons has been implicated in frequency-dependent control of left-right phase during locomotion in the mouse. We have used the Chx10::CFP mouse line to further investigate the properties and locomotion-related activity of V2a interneurons in the isolated neonatal spinal cord. V2a interneurons can be divided into three classes, based on their tonic, phasic, or delayed-onset responses to step depolarization. Electrical coupling is found only between neurons of same class and helps to synchronize neuronal activity within the class. Serotonin (5-HT) excites isolated tonic V2a interneurons by depolarizing the neurons and increasing their membrane input resistance, with no significant effects on action potential properties, a mechanism distinct from 5-HT excitation of commissural interneurons. During NMDA-/5-HT-induced locomotor-like activity, patch-clamp recordings and two-photon calcium imaging experiments show that approximately half of V2a interneurons fire rhythmically with ventral root-recorded motor activity; the rhythmic V2a interneurons fired during one half of the cycle, in phase with either the ipsilateral or the contralateral L2 ventral root bursts. The percentage of rhythmically firing V2a interneurons increases during higher-frequency fictive locomotion, and they become significantly more rhythmic in their firing during the locomotor cycle; this may help to explain the frequency-dependent shift in left-right coupling in Chx10::DTA mice, which lack these neurons. Our results together with data from the accompanying paper (Dougherty and Kiehn, 2009) reinforce earlier proposals that the V2a interneurons are components of the hindlimb central pattern generator, helping to organize left-right locomotor coordination in the neonatal mouse spinal cord.

Serotonergic modulation of locomotion in zebrafish: endogenous release and synaptic mechanisms

Serotonin (5-HT) plays an important role in shaping the activity of the spinal networks underlying locomotion in many vertebrate preparations. At larval stages in zebrafish, 5-HT does not change the frequency of spontaneous swimming; and it only decreases the quiescent period between consecutive swimming episodes. However, it is not known whether 5-HT exerts similar actions on the locomotor network at later developmental stages. For this, the effect of 5-HT on the fictive locomotor pattern of juvenile and adult zebrafish was analyzed. Bath-application of 5-HT (1-20 mum) reduced the frequency of the NMDA-induced locomotor rhythm. Blocking removal from the synaptic cleft with the reuptake inhibitor citalopram had similar effects, suggesting that endogenous serotonin is modulating the locomotor pattern. One target for this modulation was the mid-cycle inhibition during locomotion because the IPSPs recorded in spinal neurons during the hyperpolarized phase were increased both in amplitude and occurrence by 5-HT. Similar results were obtained for IPSCs recorded in spinal neurons clamped at the reversal potential of excitatory currents (0 mV). 5-HT also slows down the rising phase of the excitatory drive recorded in spinal cord neurons when glycinergic inhibition is blocked. These results suggest that the decrease in the locomotor burst frequency induced by 5-HT is mediated by a potentiation of mid-cycle inhibition combined with a delayed onset of the subsequent depolarization.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

Locomotor-activated neurons of the cat. I. Serotonergic innervation and co-localization of 5-HT7, 5-HT2A, and 5-HT1A receptors in the thoraco-lumbar spinal cord

Monoamines are strong modulators and/or activators of spinal locomotor networks. Thus monoaminergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the serotonergic innervation of locomotor-activated neurons within the thoraco-lumbar spinal cord following induction of hindlimb locomotion. This was determined by immunohistochemical co-localization of serotonin (5-HT) fibers or 5-HT(7)/5-HT2A/5-HT1A receptors with cells expressing the activity-dependent marker c-fos. Experiments were performed on paralyzed, decerebrate cats in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. Abundant c-fos immunoreactive cells were observed in laminae VII and VIII throughout the thoraco-lumbar segments of locomotor animals. Control sections from the same segments showed significantly fewer labeled neurons, mostly within the dorsal horn. Multiple serotonergic boutons were found in close apposition to the majority (80-100%) of locomotor cells, which were most abundant in lumbar segments L3-7. 5-HT7 receptor immunoreactivity was observed on cells across the thoraco-lumbar segments (T7-L7), in a dorsoventral gradient. Most locomotor-activated cells co-localized with 5-HT7, 5-HT2A, and 5-HT1A receptors, with largest numbers in laminae VII and VIII. Co-localization of c-fos and 5-HT7 receptor was highest in the L5-L7 segments (>90%) and decreased rostrally (to approximately 50%) due to the absence of receptors on cells within the intermediolateral nucleus. In contrast, 60-80 and 35-80% of c-fos immunoreactive cells stained positive for 5-HT2A and 5-HT1A receptors, respectively, with no rostrocaudal gradient. These results indicate that serotonergic modulation of locomotion likely involves 5-HT(7)/5-HT2A/5-HT1A receptors located on the soma and proximal dendrites of serotonergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments.

Control of sympathetic, respiratory and somatomotor outflow by an intraspinal pattern generator

1. Sympathetic and somatic motor outflow results from the summation of excitatory and inhibitory inputs arising from intra- and supra-spinal origins. Here we determined the contribution of intra- and supra-spinal GABAergic inputs, utilizing GABA-A receptors, in producing sympathetic and somatic motor outflow. 2. Spinal GABA-A receptor blockade, with bicuculline or picrotoxin injected intrathecally at T9, increased the level and lability of arterial pressure, sympathetic (splanchnic and cervical sympathetic) and motor (phrenic) nerve activity. Bursts of activity occurring irregularly, at low frequency were seen in all nerves. 3. C1 spinal transection abolished phrenic nerve activity and reduced sympathetic nerve activities and arterial pressure. Intrathecal bicuculline-induced bursting in sympathetic and motor (phrenic, sciatic and brachial) nerves was similar to that seen prior to C1 transection. Thus supraspinal control of sympathetic and somatomotor outflow is not dependent on GABA-A receptors. 4. Bicuculline-induced effects on phrenic nerve activity were eliminated after C8 spinal cord transection and regular phrenic rhythm resumed indicating that bicuculline was not acting directly on phrenic motoneurons. 5. Bicuculline evoked similar bursting characteristics in both sympathetic and motor nerves attributable to increased excitability of spinal cord neurons. The bursting patterns evoked were often coincident in sympathetic and motor nerves suggesting a common site of origin. 6. These data suggest there is intraspinal coupling between multiple sympathetic and motor outflows in the adult rat spinal cord in vivo. Cervicothoracic spinal cord generator/s perhaps in the form of interneuronal networks, utilizing GABA-A and glutamate receptors, can simultaneously drive functionally independent nerves.

Serotonin Modulates Dendritic Calcium Influx in Commissural Interneurons in the Mouse Spinal Locomotor Network

Commissural interneurons (CINs) help to coordinate left–right alternating bursting activity during fictive locomotion in the neonatal mouse spinal cord. Serotonin (5-HT) plays an active role in the induction of fictive locomotion in the isolated spinal cord, but the cellular targets and mechanisms of its actions are relatively unknown. We investigated the possible role of serotonin in modifying dendritic calcium currents, using a combination of two-photon microscopy and patch-clamp recordings, in identified CINs in the upper lumbar region. Dendritic calcium responses to applied somatic voltage-clamp steps were measured using fluorescent calcium indicator imaging. Serotonin evoked significant reductions in voltage-dependent dendritic calcium influx in about 40% of the dendritic sites studied, with no detectable effect in the remaining sites. We also detected differential effects of serotonin in different dendritic sites of the same neuron; serotonin could decrease voltage-sensitive calcium influx at one site, with no effect at a nearby site. Voltage-clamp studies confirmed that serotonin reduces the voltage-dependent calcium current in CINs. Current-clamp experiments showed that the serotonin-evoked decreases in dendritic calcium influx were coupled with increases in neuronal excitability; we discuss possible mechanisms by which these two seemingly opposing results can be reconciled. This research demonstrates that dendritic calcium currents are targets of serotonin modulation in a group of spinal interneurons that are components of the mouse locomotor network.

Strategies for delineating spinal locomotor rhythm-generating networks and the possible role of Hb9 interneurones in rhythmogenesis

Despite significant advances in our understanding of pattern generation in invertebrates and lower vertebrates, there have been barriers to the application of the principles learned to the definition of networks underlying mammalian locomotion. Major difficulties have arisen in identifying spinal interneurones in preparations which allow study of neuronal intrinsic properties and the role of identified interneurones in locomotor networks. Recent genetic technologies in which selective expression of fluorescent proteins in specific populations of mouse spinal neurones have provided new avenues of investigation. In this review, we focus on the generation of locomotor rhythm and outline criteria that rhythm-generating neurones might be expected to fulfill. We then examine the extent to which a recently identified population of spinal interneurones, Hb9 interneurones, fulfill these criteria. Finally, we suggest that Hb9 interneurones could be involved in an asymmetric model of locomotor rhythmogenesis through projections of electrotonically coupled rhythm-generating modules to flexor pattern formation half-centres. The principles learned from studying this population of interneurones have led to strategies to systematically evaluate neurones that may be involved in locomotor rhythmogenesis.

Serotonergic modulation of synaptic transmission and action potential firing in frog motoneurons

Frog spinal neurons receive a prominent innervation from the bulbar serotonergic nuclear complex. We used an isolated spinal cord preparation to examine the effect of serotonin (5-hydroxytryptamine, 5-HT) receptor activation on segmental and descending monosynaptic excitatory inputs to frog lumbar motoneurons. Bath-application of 5-HT (0.05 mM) caused a significant reduction in the peak amplitude of segmental EPSP elicited by dorsal root (DR) stimulation (P < 0.05). Contrasting to DR evoked responses 5-HT did not affect the descending monosynaptic EPSP conditioned by ventrolateral column (VLC) stimulation. Recording of the VLC induced EPSP-spike (E-S) field response within the ventral horn motor nucleus disclosed a substantial enhancement in the population discharge of motoneurons upon 5-HT application (P < 0.05). These data suggest the potential importance of serotonergic receptors in motor integration and gaining of motor output in the frog spinal cord.

Intrinsic and functional differences among commissural interneurons during fictive locomotion and serotonergic modulation in the neonatal mouse

Commissural interneurons (CINs) send their axons across the midline to innervate contralateral targets and have been implicated in the coordination of left-right limb movements during locomotion. In the neonatal mouse spinal cord, we studied the firing properties and responses to serotonin (5-HT) of two classes of CINs: those whose axons turn caudally after crossing the midline (dCINs) and those whose axons bifurcate after crossing the midline (adCINs). During NMDA and 5-HT-induced locomotor-like activity, a majority of lumbar (L2) dCINs fired rhythmically with ventral root-recorded motor activity, although their firing phase was widely distributed throughout the locomotor cycle. In contrast, none of the adCINs fired rhythmically during fictive locomotion. We studied the baseline firing and membrane properties, and responses to current injection, in dCINs and adCINs that had been partially isolated by blockade of rapid synaptic transmission (with antagonists to glutamate, GABA, and glycine). No significant baseline differences were found between the cell types. In contrast, 5-HT significantly increased the excitability of the isolated dCINs by depolarizing the membrane potential, reducing the postspike afterhyperpolarization amplitude and decreasing the action potential threshold. None of these parameters were affected by 5-HT in adCINs. These results, together with our recent study of a third class of CINs, the aCINs whose axons ascend after crossing the midline (Zhong et al., 2006), suggest that dCINs and aCINs, but not adCINs, are excited by 5-HT and are rhythmically active during fictive locomotion. Thus, they may play important roles in the coordination of left-right movements during fictive locomotion.

Serotonin modulates the properties of ascending commissural interneurons in the neonatal mouse spinal cord

The interneuron populations that constitute the central pattern generator (CPG) for locomotion in the mammalian spinal cord are not well understood. We studied the properties of a set of commissural interneurons whose axons cross and ascend in the contralateral cord (aCINs) in the neonatal mouse. During N-methyl-D-aspartate (NMDA) and 5-HT-induced fictive locomotion, a majority of lumbar (L2) aCINs examined were rhythmically active; most of them fired in phase with the ipsilateral motoneuron pool, but some fired in phase with contralateral motoneurons. 5-HT plays a critical role in enabling the locomotor CPG to function. We found that 5-HT increased the excitability of aCINs by depolarizing the membrane potential, reducing the postspike afterhyperpolarization amplitude, broadening the action potential, and decreasing the action potential threshold. Serotonin had no significant effect on the input resistance and sag amplitude of aCINs. These results support the hypothesis that aCINs play important roles in coordinating left-right movements during fictive locomotion and thus may be component neurons in the locomotor CPG in neonatal mice.

Conditional intrinsic voltage oscillations in mature vertebrate neurons undergo specific changes in culture

Although intrinsic neuronal properties in invertebrates are well known to undergo specific adaptive changes in culture, long-term adaptation of similar properties in mature vertebrate neurons remain poorly understood. To investigate this, we used an organotypic slice preparation from the spinal cord of adult turtles maintainable for several weeks in culture conditions. N-methyl-D-aspartate (NMDA)-induced-tetrodotoxin (TTX)-resistant voltage oscillations in motoneurons were approximately 10 times faster in culture than in acute preparations. Oscillations in culture were abolished by NMDA receptor antagonists or by high extracellular Mg2+ concentrations. However, in contrast with results from motoneurons in the acute slice, NMDA-induced oscillations in culture did not depend on CaV1.3 channel activation as they still remained after nifedipine application. Other CaV1.3 channel-mediated properties such as metabotropic receptor-induced oscillations and plateau potentials failed to be induced in culture. This study shows that changes specifically affecting CaV1.3 channel contribution to intrinsic oscillatory property expression may occur in culture. The results contribute also to understanding further the potential for plasticity of mature vertebrate neurons.

Is NMDA receptor activation essential for the production of locomotor-like activity in the neonatal rat spinal cord?

Previous work has established that in vitro bath application of N-methyl-D-aspartic acid (NMDA) promotes locomotor activity in a variety of vertebrate preparations including the neonatal rat spinal cord. In addition, NMDA receptor activation gives rise to active membrane properties that are postulated to contribute to the generation or stabilization of locomotor rhythm. However, earlier studies yielded conflicting evidence as to whether NMDA receptors are essential in this role. Therefore in this study, we examined the effect of NMDA receptor blockade, using D-2-amino-5-phosphono-valeric acid (AP5), on locomotor-like activity in the in vitro neonatal rat spinal cord. Locomotor-like activity was induced using 5-hydroxytryptamine (5-HT), acetylcholine, combined 5-HT and NMDA receptor activation, increased K(+) concentration, or electrical stimulation of the brain stem and monitored using suction electrode recordings of left and right lumbar ventral root discharge. We also studied the effect on locomotor capacity of selectively suppressing NMDA receptor-mediated active membrane properties; this was achieved by removing Mg(2+) ions from the bath, which in turn abolishes voltage-sensitive blockade of the NMDA receptor channel. The results show that, although NMDA receptor activation may seem essential for locomotor network operation under some experimental conditions, locomotor-like rhythms can nevertheless be generated in the presence of AP5 if spinal cord circuitry is exposed to appropriate levels of non-NMDA receptor-dependent excitation. Therefore neither NMDA receptor-mediated nonlinear membrane properties nor NMDA receptor activation in general is universally essential for locomotor network activation in the in vitro neonatal rat spinal cord.

Conditional rhythmicity of ventral spinal interneurons defined by expression of the Hb9 homeodomain protein

The properties of mammalian spinal interneurons that underlie rhythmic locomotor networks remain poorly described. Using postnatal transgenic mice in which expression of green fluorescent protein is driven by the promoter for the homeodomain transcription factor Hb9, as well as Hb9-lacZ knock-in mice, we describe a novel population of glutamatergic interneurons located adjacent to the ventral commissure from cervical to midlumbar spinal cord levels. Hb9+ interneurons exhibit strong postinhibitory rebound and demonstrate pronounced membrane potential oscillations in response to chemical stimuli that induce locomotor activity. These data provide a molecular and physiological delineation of a small population of ventral spinal interneurons that exhibit homogeneous electrophysiological features, the properties of which suggest that they are candidate locomotor rhythm-generating interneurons.

Stimulation of the parapyramidal region of the neonatal rat brain stem produces locomotor-like activity involving spinal 5-HT7 and 5-HT2A receptors

Locomotion can be induced in rodents by direct application 5-hydroxytryptamine (5-HT) onto the spinal cord. Previous studies suggest important roles for 5-HT7 and 5-HT2A receptors in the locomotor effects of 5-HT. Here we show for the first time that activation of a discrete population of 5-HT neurons in the rodent brain stem produces locomotion and that the evoked locomotion requires 5-HT7 and 5-HT2A receptors. Cells localized in the parapyramidal region (PPR) of the mid-medulla produced locomotor-like activity as a result of either electrical or chemical stimulation, and PPR-evoked locomotor-like activity was blocked by antagonists to 5-HT2A and 5-HT7 receptors located on separate populations of neurons concentrated in different rostro-caudal regions. 5-HT7 receptor antagonists blocked locomotor-like activity when applied above the L3 segment; 5-HT2A receptor antagonists blocked locomotor-like activity only when applied below the L2 segment. 5-HT7 receptor antagonists decreased step cycle duration, consistent with an action on neurons involved in the rhythm-generating function of the central pattern generator (CPG) for locomotion. 5-HT2A antagonists reduced the amplitude of ventral root activity with only small effects on step cycle duration, suggesting an action directly on cells involved in the output stage of the pattern generator for locomotion, including motoneurons and premotor cells. Experiments with selective antagonists show that dopaminergic (D1, D2) and noradrenergic (alpha1, alpha2) receptors are not critical for PPR-evoked locomotor-like activity.

Serotonin refines the locomotor-related alternations in thein vitroneonatal rat spinal cord

Serotonergic projections from raphe nuclei arrive in the lumbar enlargement of the spinal cord during the late fetal period in the rat, a time window during which the locomotor-related left/right and flexor/extensor coordinations switch from synchrony to alternation. The goal of the present study was to investigate the role played by serotonin (5-HT) in modulating the left/right and flexor/extensor alternations. Fictive locomotion was induced by bath application of N-methyl-D,L-aspartate (NMA) in the in vitro neonatal rat spinal cord preparation. By means of cross-correlation analysis we demonstrate that 5-HT, when added to NMA, improves left/right and flexor/extensor (recorded from the 3rd and 5th lumbar ventral roots, respectively) alternations. This effect was partly reproduced by activation of 5-HT(2A/2C) receptors. We then tested the contribution of endogenous 5-HT to NMA-induced fictive locomotion. Reducing the functional importance of endogenous 5-HT, either by inhibiting its synthesis with daily injections of p-chloro-phenylalanine (PCPA), starting on the day of birth, or by application of ketanserin (a 5-HT(2) receptor antagonist) or SB269970 (a 5-HT(7) receptor antagonist), disorganized the NMA-induced locomotor pattern. This pattern was restored in PCPA-treated animals by adding 5-HT to the bath. Blocking 5-HT(7) receptors disorganized the locomotor-like rhythm even in the absence of electrical activity in the brain stem, suggesting that NMA applied to the spinal cord does not cause 5-HT release by activating a spino-raphe-spinal loop. These results demonstrate that 5-HT is critical in improving the locomotor-related alternations in the neonatal rat.

Intrathecal application of cyproheptadine impairs locomotion in intact rats

In intact adult rats, cyproheptadine, a 5-HT2 antagonist, administered intrathecally at the midlumbar segments was found to impair hindlimb locomotor movements during overground locomotion. These effects were dose-dependent; they varied from transient complete hindlimb paraplegia seen at doses of 300 microg/20 microl, to short-lasting trunk instability at doses of 100 microg/20 microl. After the return of overground locomotion, transient abduction of one of the hindlimbs was observed in some animals. These findings demonstrate that the blockade of 5-HT2 receptors affects locomotion in intact rats. Our results provide support for the hypothesis of serotonergic involvement in rat locomotion, which, so far, has been based mainly on the effects of 5-HT2 agonists on the recovery of locomotion in spinal rats.

Modulation of Locomotor Activity by Multiple 5-HT and Dopaminergic Receptor Subtypes in the Neonatal Mouse Spinal Cord

Recently, it has been shown that bath-applied 5-HT can elicit fictive locomotion from perinatal mouse preparations. Since 5-HT acts on multiple receptor subtypes, the focus of this study was to examine which receptor families contribute to the genesis and modulation of locomotor activity. Blockade of 5-HT2 (ketanserin or N-desmethylclozapine) or 5-HT7 receptors (SB-269970) could reversibly block or modulate the locomotor-like pattern. A 5-HT2 agonist (α-methyl-5-HT) was shown to be capable of activating the rhythm. Bath application of 5-HT7 agonists (5-CT) generally led to a tonic increase in neurogram discharge, accompanied by bouts of rhythmic activity. Blockade of dopaminergic receptors {D1 [ R-(+)-SCH-23390 or LE 300]/D2 [(±)-sulpiride or L-741,626] } could reversibly disrupt the rhythm and most effectively did so when the D1 and D2 antagonists were added together. Conversely, 5-HT2 and D1/D2 agonists can interact to evoke locomotor activity. Overall, our data show that, in the neonatal mouse preparation, 5-HT evoked locomotion is partly dependent on activation of 5-HT2, 5-HT7, and dopaminergic receptor subtypes.

The excitability of lumbar motoneurones in the neonatal rat is increased by a hyperpolarization of their voltage threshold for activation by descending serotonergic fibres

Previous work has shown there is an increase in motoneurone excitability produced by hyperpolarization of the threshold potential at which an action potential is elicited (Vth) at the onset, and throughout brainstem-induced fictive locomotion in the decerebrate cat. This represents a transient facilitation in the membrane potential for activation dependent on the presence of fictive locomotion. The present study tests the hypothesis that a similar neuromodulatory mechanism facilitating neuronal recruitment also exists in the neonatal rat, and the endogenous pathway mediating the Vth hyperpolarization can be activated by electrical stimulation of the neonatal brainstem. Isolated brainstem-spinal cord preparations from 1- to 5-day-old neonatal rats, and whole-cell recording techniques were used to examine the patterns of ventral root (VR) activity produced, and the effect of electrical stimulation of the ventromedial medulla on lumbar spinal neurones. Hyperpolarization of Vth was seen in 10/11 (range -2 to -18 mV) neurones recorded during locomotor-like VR activity, and appeared analogous to the locomotor-dependent Vth hyperpolarization previously described in the cat. However, in the present study, Vth hyperpolarization was also seen during electrical brainstem stimulation that evoked alternating, rhythmic, or tonic VR activity, or failed to evoke VR activity. Thirty-six of 71 neurones were antidromically identified as lumbar motoneurones and 33/36 showed a hyperpolarization of Vth (-2 to -14 mV) during electrical brainstem stimulation. Of the unidentified lumbar ventral horn neurones, 31/35 also showed hyperpolarization of Vth (-2 to -20 mV) during brainstem stimulation. The hyperpolarization of Vth and VR activity induced by brainstem stimulation was reversibly blocked by cooling of the cervical cord, indicating it is mediated by descending fibres, and application of the serotonergic antagonist ketanserin to the spinal cord was effectively able to block the brainstem-evoked hyperpolarization of Vth. These results demonstrate a previously unknown action of the endogenous descending serotonergic system to facilitate spinal motoneuronal recruitment and firing by inducing a hyperpolarization of Vth. This modulatory process can be examined in the neonatal rat brainstem-spinal cord preparation without the requirement for ongoing locomotor activity.

Monoamines increase the excitability of spinal neurones in the neonatal rat by hyperpolarizing the threshold for action potential production

During fictive locomotion in the adult decerebrate cat, motoneurone excitability is increased by a hyperpolarization of the threshold potential at which an action potential is elicited (V(th)). This lowering of V(th) occurs at the onset of fictive locomotion, is evident for the first action potential elicited and is presumably caused by a neuromodulatory process. The present study tests the hypothesis that the monoamines serotonin (5-HT) and noradrenaline (NA) can hyperpolarize neuronal V(th). The neonatal rat isolated spinal cord preparation and whole-cell recording techniques were used to examine the effects of bath-applied 5-HT and NA on the V(th) of spinal ventral horn neurones. In the majority of lumbar ventral horn neurones, 5-HT (13/26) and NA (10/16) induced a hyperpolarization of V(th) ranging from -2 to -8 mV. 5-HT and NA had similar effects on V(th) for individual neurones. This hyperpolarization of V(th) was not due to a reduction of an accommodative process, and could be seen without changes in membrane potential or membrane resistance. These data reveal a previously unknown action of 5-HT and NA, hyperpolarization of V(th) of spinal neurones, a process that would facilitate both neuronal recruitment and firing.

Role of NMDA receptor activation in serotonin agonist-induced air-stepping in paraplegic mice

STUDY DESIGN

Experimental laboratory investigation of the effects of serotonergic and glutamatergic drugs in early paraplegic mice.

OBJECTIVES

To examine whether NMDA and 5-HT receptors synergistically participate to generate basic stepping movements in paraplegic mice.

SETTING

Laval University Medical Center, Quebec, Canada.

METHODS

Adult mice completely spinalized at the low-thoracic level 1 week earlier were suspended in harnesses for experiments. Acute drug-induced effects were examined on hindlimb movements filmed with a digital video camera. Detailed kinematic analyses included stick diagrams reconstructions of hindlimb movements and analysis of bilateral coordination, angular excursion, stepping amplitude and frequency.

RESULTS

A single treatment with the 5-HT2 agonist quipazine (>0.7 mg/kg, i.p.) induced episodes of air-stepping movements in the hindlimbs of paraplegic mice. In contrast, injection of the glutamatergic agonist NMDA (1-45 mg/kg i.p.) failed to induce rhythmicity, although nonlocomotor rhythmic movements were observed with higher doses (45-60 mg/kg i.p.). Subthreshold doses of NMDA (22-30 mg/kg) could induce episodes of hindlimb air-stepping if combined with subthreshold doses of quipazine (0.3-0.7 mg/kg). Air-stepping was entirely blocked by administration of the selective NMDA antagonist MK-801.

CONCLUSION

A single treatment with quipazine can trigger episodes of locomotor-like movements in early chronic spinal mice. Even though NMDA alone could not generate bilaterally coordinated air-stepping, NMDA receptor activation was nonetheless critical for spinal locomotor rhythmogenesis induced by 5-HT agonists in awake behaving animals.

Serotonin patterns locomotor network activity in the developing zebrafish by modulating quiescent periods

Developing neural networks follow common trends such as expression of spontaneous, recurring activity patterns, and appearance of neuromodulation. How these processes integrate to yield mature, behaviorally relevant activity patterns is largely unknown. We examined the integration of serotonergic neuromodulation and its role in the functional organization of the accessible locomotor network in developing zebrafish at behavioral and cellular levels. Locally restricted populations of serotonergic neurons and their projections appeared in the hindbrain and spinal cord of larvae after hatching (approximately day 2). However, 5-HT affected the swimming pattern only from day 4 on, when sustained spontaneous swimming appeared. 5-HT and its agonist quipazine increased motor output by reducing intervals of inactivity, observed behaviorally (by high-speed video) and in recordings from spinal neurons during fictive swimming (by whole-cell current clamp). 5-HT and quipazine had little effect on the properties of the activity periods, such as the duration of swim episodes and swim frequency. Further, neuronal input resistance, rheobasic current, and resting potential were not affected significantly. The 5-HT antagonists methysergide and ketanserin decreased motor output by prolonging the periods of inactivity with little effect on the active swim episode or neuronal properties. Our results suggest that 5-HT neuromodulation is integrated early in development of the locomotor network to increase its output by reducing periods of inactivity with little effect on the activity periods, which in contrast are the main targets of 5-HT neuromodulation in neonatal and adult preparations.

Effects of intrathecal glutamatergic drugs on locomotion. II. NMDA and AP-5 in intact and late spinal cats

In a previous article, we have shown that, in cats, intrathecal injections of N-methyl-D-aspartate (NMDA) in the first few days after spinalization at T13 do not induce locomotion as in many other spinal preparations. This is in contrast to alpha-2 noradrenergic receptor stimulation, which can trigger locomotion at this early stage. However, it is known that spinal cats do recover spontaneous locomotion in the absence of descending noradrenergic pathways and that the spinal pattern generator must then depend on other neurotransmitters still present in the cord such as excitatory amino acids. In the present paper, therefore we look at the effects of intrathecal NMDA, a glutamatergic agonist, and 2-amino-5-phosphonovaleric acid (AP-5), an NMDA receptor blocker, in both intact and late spinal cats. Low doses of NMDA had no major effect on the locomotor pattern in both intact and late spinal cats. Larger doses of NMDA in the chronic spinal cat initially produced an increase in the general excitability followed by more regular locomotion. AP-5 in intact cats caused a decrease in the amplitude of the flexion reflex and induced a bilateral foot drag as well as some decrease in weight support but it did not prevent locomotion. However, in late spinal cats, the same dose of AP-5 blocked locomotion completely. These results indicate that NMDA receptors may be critical for the spontaneous expression of spinal locomotion. It is proposed that the basic locomotor rhythmicity in cats is NMDA-dependent and that normally this glutamatergic mechanism is modulated by other neurotransmitters, such as 5-HT and NA.

Involvement of the GABAergic system for Ptychodiscus brevis toxin-induced depression of synaptic transmission elicited in isolated spinal cord from neonatal rats

The involvement of inhibitory transmitters for Ptychodiscus brevis toxin (PbTx)-induced depression of spinal synaptic transmission in neonatal rats was investigated. Stimulation of a dorsal root evoked monosynaptic reflex (MSR) and polysynaptic reflex (PSR) potentials in the segmental ventral root. The PbTx depressed the reflexes in a concentration-dependent manner and the depression was blocked by GABA(A) antagonist, bicuculline (1 microM). GABA also produced depression of the reflexes in a concentration-dependent manner. Simultaneous application of submaximal concentrations of PbTx (28 microM) and GABA (30 microM) enhanced the depression (>75%). In contrast, PbTx alone (28 microM) depressed the MSR and the PSR by 33 and 47%, respectively, and GABA (30 microM) alone depressed the reflexes by 30%. The N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphono-pentanoic acid (10 microM), blocked the PbTx-induced depression of MSR and also the enhancement of GABA response by PbTx. A glycine receptor antagonist, strychnine (1 microM), failed to block the depression by the toxin up to 28 microM; however, the depression was attenuated significantly at 84 microM of the toxin. The results indicate that PbTx depressed the spinal reflexes via GABA(A) receptors. Furthermore, the potentiation of GABAergic action by PbTx requires the N-methyl-D-aspartate dependent mechanism.

Reversible disorganization of the locomotor pattern after neonatal spinal cord transection in the rat

The central pattern generators (CPGs) for locomotion, located in the lumbar spinal cord, are functional at birth in the rat. Their maturation occurs during the last few days preceding birth, a period during which the first projections from the brainstem start to reach the lumbar enlargement of the spinal cord. The goal of the present study was to investigate the effect of suppressing inputs from supraspinal structures on the CPGs, shortly after their formation. The spinal cord was transected at the thoracic level at birth [postnatal day 0 (P0)]. We examined during the first postnatal week the capacity of the CPGs to produce rhythmic motor activity in two complementary experimental conditions. Left and right ankle extensor muscles were recorded in vivo during airstepping, and lumbar ventral roots were recorded in vitro during pharmacologically evoked fictive locomotion. Mechanical stimulation of the tail elicited long-lasting sequences of airstepping in the spinal neonates and only a few steps in sham-operated rats. In vitro experiments made simultaneously on spinal and sham animals confirmed the increased excitability of the CPGs after spinalization. A left-right alternating locomotor pattern was observed at P1-P3. Both types of experiments showed that the pattern was disorganized at P6-P7, and that the left-right alternation was lost. Alternation was restored after the activation of serotonergic 5-HT(2) receptors in vivo. These results suggest that descending pathways, in particular serotonergic projections, control the strength of reciprocal inhibition and therefore shape the locomotor pattern in the neonatal rat.

Development and serotonergic modulation of NMDA bursting in rat trigeminal motoneurons

The development of N-methyl-D-aspartate (NMDA)-induced burst discharge in rat trigeminal motoneurons (TMNs) between postnatal days P1 and P10 was examined using whole cell patch-clamp recording methods in brain slices. Bath application of NMDA (50 microM) induced a Mg(2+)-dependent rhythmical bursting activity starting around P8. Prior to the onset of bursting, the membrane potential depolarized and the input resistance increased. Hyperpolarization of the membrane potential with extrinsic current demonstrated a narrow window of membrane potential where maintained rhythmical burst discharge was evident. In P1-P4 neurons, NMDA application produced membrane depolarization and a minimal change in input resistance, but no burst activity at any membrane potential. Voltage-clamp analysis indicated that the bursting activity was related to the presence or absence of a voltage-dependent Mg(2+) block and induction of a negative slope conductance (NSC) region in the I(NMDA)-V relationship. Regardless of age, reduction of extracellular Mg(2+) from 1 mM to 30 microM enhanced I(NMDA) at voltages negative to -60 mV. However, in 1 mM Mg(2+), P1-P4 neurons were devoid of a prominent NSC region compared with P8-P10 neurons, suggesting that the efficacy of depolarization in unblocking the NMDA receptors increased with age. NMDA bursting was not dependent on calcium influx through voltage-gated calcium channels (VGCC) but did require a minimal concentration of Ca(2+) in the bath. Intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid application suppressed burst discharge completely, suggesting that intracellular Ca(2+) directly, or via second-messenger systems, regulates NMDA receptor activity and bursting. Interestingly, NMDA bursting could be induced in P1-P4 neurons by simultaneous bath application of serotonin (5-HT, 10 microM), which by itself did not produce bursting, suggesting an "enabling" role for 5-HT. Voltage-clamp analysis demonstrated that the NMDA/5-HT bursting resulted from induction of an NSC in the I-V relationship of total membrane current. 5-HT by itself produced no such effect. The mechanisms for this effect were due to an enhancement of the NSC region of the I(NMDA)-V relationship and reduction of a presumed leak current by 5-HT. These data indicate that NMDA bursting in trigeminal motoneurons is developmentally regulated and subject to neuromessenger modulation. Control of the Mg(2+) sensitivity of the NMDA receptor and voltage-dependent block by neuromessengers could be an effective means to control the efficacy of glutamatergic synaptic drive to motoneurons during rhythmical oral-motor activity at early postnatal ages.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

NMDA receptor-dependent periodic oscillations in cultured spinal cord networks

Cultured spinal cord networks grown on microelectrode arrays display complex patterns of spontaneous burst and spike activity. During disinhibition with bicuculline and strychnine, synchronized burst patterns routinely emerge. However, the variability of both intra- and interculture burst periods and durations are typically large under these conditions. As a further step in simplification of synaptic interactions, we blocked excitatory AMPA synapses with 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzoquinoxaline-7-sulphonamide (NBQX), resulting in network activity mediated through the N-methyl-D-aspartate (NMDA) receptor (NMDA(ONLY)). This activity was APV sensitive. The oscillation under NMDA(ONLY) conditions at 37 degrees C was characterized by a period of 2.9 +/- 0.3 s (16 separate cultures). More than 98% of all neurons recorded participated in this highly rhythmic activity. The temporal coefficients of variation, reflecting the rhythmic nature of the oscillation, were 3.7, 4.7, and 4.9% for burst rate, burst duration, and interburst interval, respectively [mean coefficients of variation (CVs) for 16 cultures]. The oscillation persisted for at least 12 h without change (maximum observation time). Once established, it was not perturbed by agents that block mGlu receptors, GABA(B) receptors, cholinergic receptors, purinergic receptors, tachykinin receptors, serotonin (5-HT) receptors, dopamine receptors, electrical synapses, burst afterhyperpolarization, NMDA receptor desensitization, or the hyperpolarization-activated current. However, the oscillation was destroyed by bath application of NMDA (20-50 microM). These results suggest a presynaptic mechanism underlying this periodic rhythm that is solely dependent on the NMDA synapse. When the AMPA/kainate synapse was the sole driving force (n = 6), the resulting burst patterns showed much higher variability and did not develop the highly periodic, synchronized nature of the NMDA(ONLY) activity. Network size or age did not appear to influence the reliability of expression of the NMDA(ONLY) activity pattern. For this reason, we suggest that the NMDA(ONLY) condition unmasks a fundamental rhythmogenic mechanism of possible functional importance during periods of NMDA receptor-dominated activity, such as embryonic and early postnatal development.