The role of putative excitatory amino acid neurotransmitters in the initiation of locomotion in the lamprey spinal cord. I. The effects of excitatory amino acid antagonists

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.

The Pharmacology of Vertebrate Spinal Central Pattern Generators

Central pattern generators are networks of neurons capable of generating an output pattern of spike activity in a relatively stereotyped, rhythmic pattern that has been found to underlie vital functions like respiration and locomotion. The central pattern generator for locomotion in vertebrates seems to share some basic building blocks. Activation and excitation of activity is driven by descending, sensory, and intraspinal glutamatergic neurons. NMDA receptor activation may also lead to the activation of oscillatory properties in individual neurons that depend on an array of ion channels situated in those neurons. Coordination across joints or the midline of the animal is driven primarily by glycinergic inhibition. In addition to these processes, numerous modulatory mechanisms alter the function of the central pattern generator. These include metabotropic amino acid receptors activated by rhythmic release of glutamate and GABA as well as monoamines, ACh, and peptides. Function and stability of the central pattern generator is also critically dependent on the array of ion channels found in neurons that compose these oscillators, including Ca2+and voltage-gated K+channels and Ca2+channels.

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.

A synaptic mechanism for network synchrony

Within neural networks, synchronization of activity is dependent upon the synaptic connectivity of embedded microcircuits and the intrinsic membrane properties of their constituent neurons. Synaptic integration, dendritic Ca²⁺ signaling, and non-linear interactions are crucial cellular attributes that dictate single neuron computation, but their roles promoting synchrony and the generation of network oscillations are not well understood, especially within the context of a defined behavior. In this regard, the lamprey spinal central pattern generator (CPG) stands out as a well-characterized, conserved vertebrate model of a neural network (Smith et al., 2013a), which produces synchronized oscillations in which neural elements from the systems to cellular level that control rhythmic locomotion have been determined. We review the current evidence for the synaptic basis of oscillation generation with a particular emphasis on the linkage between synaptic communication and its cellular coupling to membrane processes that control oscillatory behavior of neurons within the locomotor network. We seek to relate dendritic function found in many vertebrate systems to the accessible lamprey central nervous system in which the relationship between neural network activity and behavior is well understood. This enables us to address how Ca²⁺ signaling in spinal neuron dendrites orchestrate oscillations that drive network behavior.

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.

Neuronal substrates for state-dependent changes in coordination between motoneuron pools during fictive locomotion in the lamprey spinal cord

Locomotion relies on a precisely timed activation of sets of motoneurons. A fundamental question is how this is achieved. In the lamprey, fin and myotomal motoneurons located on the same side of the spinal cord display alternating activity during straight swimming. The neural mechanism underlying this alternation is studied here during fictive locomotion induced by superfusion with NMDA, or locomotor bursting induced by electrical stimulation. If the spinal cord is split longitudinally, each hemicord still displays rhythmic locomotor related burst activity, but now fin and myotomal motoneurons become active in-phase. The out-of-phase activation of fin motoneurons persists only when at least three segments are left intact in the rostral part of the spinal cord. Proper coordination of fin motoneurons thus requires input from contralateral rostral segments. We show that commissural excitatory interneurons with long descending axons, previously reported to be active in phase with their ipsilateral myotomal motoneurons, provide monosynaptic excitation to contralateral fin motoneurons. Together, these results strongly indicate that, although myotomal motoneurons receive their phasic excitation from ipsilateral excitatory interneurons, fin motoneurons are mainly driven from the contralateral segmental network during bilateral locomotor activity. However, during unilateral bursting, fin and myotomal motoneurons instead receive a common input, which is apparently masked during normal fictive swimming. The spinal organization thus also provides circuitry for different patterns of coordination, i.e., alternation or coactivation of the two pools of motoneurons, which may subserve different forms of locomotor behavior.

Activity of fin muscles and fin motoneurons during swimming motor pattern in the lamprey

Coordination of motoneuron activity is a fundamental prerequisite for the generation of functional locomotor patterns. We investigate the neural mechanisms that coordinate activity of motoneuron pools in the vertebrate spinal cord with differing phases of activity in the locomotor cycle in a simple motor system, the lamprey swimming network. In the region of dorsal fins the lamprey spinal cord contains two groups of motoneurons: the myotomal motoneurons that innervate the trunk muscles; and the fin motoneurons controlling muscle fibres of the dorsal fins. We investigated the activity of fin muscles during swimming in vivo and that of fin motoneurons during fictive swimming in vitro. During swimming in vivo with cycle periods of 4-8 Hz, fin muscle activity covered a broad portion of the cycle, with the peak of activity out-of-phase to the ipsilateral myotomal muscles. During fictive swimming evoked by N-methyl-d-aspartate in the isolated spinal cord, fin motoneurons expressed similar out-of-phase activity. The phase relationship of the synaptic drive to fin motoneurons was examined by recording their activity intracellular during fictive swimming. Three different forms of membrane potential oscillation with different time courses in the locomotor cycle could be distinguished. Sagittal lesions of the spinal cord in the segment where fin motoneurons are recorded and up to one segment rostral and caudal from it did not influence the out-of-phase activity pattern of the motoneurons. Our results indicate that coordination of fin motoneuron activity with the locomotor activity of myotomal motoneurons does not depend on intrasegmental contralateral premotor elements.

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.

Activation of NMDA receptors is required for the initiation and maintenance of walking-like activity in the mudpuppy (Necturus Maculatus)

We hypothesized that blocking the activation of N-methyl-D-aspartate (NMDA) receptors prevents the initiation of walking-like activity and abolishes the ongoing rhythmic activity in the spinal cord-forelimb preparation from the mudpuppy. Robust walking-like movements of the limb and rhythmic alternating elbow flexor-extensor EMG pattern characteristic of walking were elicited when continuous perfusion of the spinal cord with solution containing D-glutamate. The frequency of the walking-like activity was dose-dependent on the concentration of D-glutamate in the bath over a range of 0.2 to 0.9 mmol/L. Elevation of potassium concentrations failed to induce walking-like activity. Application of the selective antagonist 2-amino-5-phosphonovalerate (AP-5) produced dose-dependent block of the initiation and maintenance of walking-like activity induced by D-glutamate. Complete block of the activity was achieved when the concentration of AP-5 reached 20 micromol/L. Furthermore, application of L-701,324 (a selective antagonist of the strychnine-insensitive glycine site of NMDA receptor) (1-10 micromol/L) also resulted in complete block of the walking-like activity. In contrast, application of the non-NMDA receptor antagonist 6-cyno-7-nitroquinoxaline-2,3-dione (CNQX) (1-50 micromol/L) induced a dose-dependent inhibition of the burst frequency but failed to result in a complete block. Only at concentration as high as 100 micromol/L, did CNQX cause complete block of the rhythmic activity, presumably through nonspecific action on the strychnine-insensitive glycine site of NMDA receptors. These results suggest that activation of NMDA receptors is required for the initiation and maintenance of walking-like activity. Operation of non-NMDA receptors plays a powerful role in the modulation of the walking-like activity in the mudpuppy.

Pharmacological characterization of ionotropic glutamate receptors in the zebrafish olfactory bulb

The distribution of N-methyl-D-aspartate- (NMDA) and kainic acid- (KA) sensitive ionotropic glutamate receptors (iGluR) in the zebrafish olfactory bulb was assessed using an activity-dependent labeling method. Olfactory bulbs were incubated with an ion channel permeant probe, agmatine (AGB), and iGluR agonists in vitro, and the labeled neurons containing AGB were visualized immunocytochemically. Preparations exposed to 250 microM KA in the presence of a NMDA receptor antagonist (D-2-amino-5-phosphono-valeric acid) and an alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist (sym 2206), revealed KA receptor-mediated labeling of approximately 60-70% of mitral cells, juxtaglomerular cells, tyrosine hydroxylase-positive cells and granule cells. A higher proportion of ventral olfactory bulb neurons were KA-sensitive. Application of 333 microM NMDA in the presence of an AMPA/KA receptor antagonist (6-cyano-7-nitroquinoxaline-2,3-dione) resulted in NMDA receptor-mediated labeling of almost all neurons. The concentrations eliciting 50% of the maximal response (effective concentration: EC(50)s) for NMDA-stimulated labeling of different cell types were not significantly different and ranged from 148 microM to 162 microM. These results suggest that while NMDA receptors with similar binding affinities are widely distributed in the neurons of the zebrafish olfactory bulb, KA receptors are heterogeneously expressed among these cells and may serve unique roles in different regions of the olfactory bulb.

Odor-stimulated glutamatergic neurotransmission in the zebrafish olfactory bulb

The role of glutamate as a neurotransmitter in the zebrafish olfactory bulb (OB) was established by examining neuronal activation following 1). glutamate receptor agonist stimulation of isolated olfactory bulbs and 2). odorant stimulation of intact fish. Four groups of neurons (mitral cells, projection neurons; granule cells, juxtaglomerular cells, and tyrosine hydroxylase-containing cells; interneurons) were identified on the basis of cell size, cell location, ionotropic glutamate receptor (iGluR) agonist/odorant sensitivity, and glutamate, gamma-aminobutyric acid (GABA), and tyrosine hydroxylase immunoreactivity. Immunoreactive glutamate levels were highest in olfactory sensory neurons (OSNs) and mitral cells, the putative glutamatergic neurons. The sensitivity of bulbar neurons to iGluR agonists and odorants was established using a cationic channel permeant probe, agmatine (AGB). Agmatine that permeated agonist- or odor-activated iGluRs was fixed in place with glutaraldehyde and detected immunohistochemically. N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA)/kainic acid (KA) iGluR agonists and odorants (glutamine, taurocholic acid) stimulated activity-dependent labeling of bulbar neurons, which was blocked with a mixture of the iGluR antagonists, D-2-amino-5-phosphono-valeric acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). The AMPA/KA antagonist CNQX completely blocked glutamine-stimulated AGB labeling of granule cells and tyrosine hydroxylase-containing cells, suggesting that, in these cell types, AMPA/KA receptor activation is essential for NMDA receptor activation. However, blocking AMPA/KA receptor activity failed to eliminate AGB labeling of mitral cells or juxtaglomerular cells. Collectively, these findings indicate that glutamate is the primary excitatory neurotransmitter in the zebrafish OB and that iGluR subtypes function heterogeneously in the bulbar neurons.

Effects of intrathecal glutamatergic drugs on locomotion I. NMDA in short-term spinal cats

Excitatory amino acids (EAA) have been reported to induce fictive locomotion in different in vitro and in vivo preparations in a variety of species through their actions on both N-methyl-D-aspartate (NMDA), and non-NMDA receptors. NMDA-induced intrinsic membrane properties such as intrinsic motoneuronal membrane oscillations and plateau potentials have been suggested to play a role in the generation of locomotion. There is, however, no information on the ability of NMDA in triggering spinal locomotion in awake behaving animals. Because most of the previous work on the induction of locomotion has concentrated on monoaminergic drugs, mainly noradrenergic drugs, the aim of this study is to examine the potential of NMDA in initiating locomotion in chronic spinal cats within the first week after spinalization. Five cats chronically implanted with an intrathecal cannula and electromyographic (EMG) electrodes were used. EMG activity synchronized to video images of the hindlimbs were recorded. The results show that during the early posttransection period (within the 1st week postspinalization), NMDA did not trigger robust locomotion as did noradrenergic drugs. The predominant effects of NMDA were a general hyperexcitability reflected by fast tremor, toe fanning, and an increase in small alternating hindlimb movements with no foot placement nor weight support. During the intermediate phase posttransection (6-8 days), when the cats were able to make some rudimentary steps with foot placement, NMDA significantly enhanced the locomotor performance, which lasted for 24-72 h postinjection. NMDA was also found to increase the excitability of the cutaneous reflex transmission only in early spinal cats. One possible hypothesis for the ineffectiveness of NMDA in triggering locomotion in early spinal cats could be attributed to the widespread activation of NMDA receptors on various neuronal elements involved in the transmission of afferent pathways that in turn may interfere with the expression of locomotion. The marked effects of NMDA in intermediate-spinal cats suggest that NMDA receptors play an important role in locomotion perhaps through its role on intrinsic membrane properties of neurons in shaping and amplifying spinal neuronal transmission or by augmenting the sensory afferent inputs. The long-term effects mediated by NMDA receptors have been reported in the literature and may involve mechanisms such as induction of long-term potentiation or interactions with neuropeptides. The effects of NMDA injection in intact cats and long-term chronic spinal cats will be addressed in a forthcoming companion paper.

Presynaptic GABAA and GABAB Receptor-mediated Phasic Modulation in Axons of Spinal Motor Interneurons

The lamprey spinal cord has been utilized to investigate the role of presynaptic inhibition in the control of the spinal motor system. Axons of the lamprey spinal cord are comparatively large because of their lack of myelination. Axons impaled with microelectrodes demonstrate depolarizing responses to the application of GABAA and GABAB receptor agonists, muscimol and baclofen. These depolarizing effects are counteracted by the specific GABAA and GABAB receptor antagonists, bicuculline and phaclofen. GABAA receptor activation leads to a gating of Cl- channels on the axons. However, the ionic mechanism leading to axonal depolarization following GABAB receptor activation is unknown. After initiation of fictive locomotion, these axons demonstrate oscillations in axonal membrane potential related to the locomotor cycle. During ficitive locomotion they depolarize in phase with the bursting of the ipsilateral ventral root of the same segment. These axonal membrane potential oscillations are due to a phasic GABAA and GABAB receptor-mediated gating of ion channels on the axonal membrane. Fictive locomotion in the lamprey spinal cord is largely unaffected by antagonism of one or other GABA receptor subtype alone, but is severely disrupted by simultaneous antagonism of both subtypes. In conclusions, we demonstrate, for the first time, an agonist-gated depolarization of a vertebrate presynaptic element measured by direct impalement of the axon under study. We also demonstrate that GABA-mediated presynaptic inhibition occurs in axons of spinal interneurons. It is not limited to the primary afferents as has previously been believed.

Interactions between multiple rhythm generators produce complex patterns of oscillation in the developing rat spinal cord

Neural networks capable of generating coordinated rhythmic activity form at early stages of development in the spinal cord. In this study, voltage-imaging techniques were used to examine the spatiotemporal pattern of rhythmic activity in transverse slices of lumbar spinal cord from embryonic and neonatal rats. Real-time images were recorded in slices stained with the voltage-sensitive fluorescent dye RH414 using a 464-element photodiode array. Fluorescence signals showed spontaneous voltage oscillations with a frequency of 3 Hz. Simultaneous recordings of fluorescence and extracellular field potential demonstrated that the two signals oscillated with the same frequency and had a distinct phase relationship, indicating that the fluorescence changes represented changes in transmembrane potentials. The oscillations were reversibly blocked by cobalt (1 mM), indicating a dependence on Ca(2+) influx through voltage-gated Ca(2+) channels. Extracellular field potentials revealed oscillations with the same frequency in both stained and unstained slices. Oscillations were apparent throughout a slice, although their amplitudes varied in different regions. The largest amplitude oscillations were produced in the lateral regions. To examine the spatial organization of rhythm-generating networks, slices were cut into halves and quarters. Each fragment continued to oscillate with the same frequency as intact slices but with smaller amplitudes. This finding suggested that rhythm-generating networks were widely distributed and did not depend on long-range projections. In slices from neonatal rats, the oscillations exhibited a complex spatiotemporal pattern, with depolarizations alternating between mirror locations in the right and left sides of the cord. Furthermore, within each side depolarizations alternated between the lateral and medial regions. This medial-lateral pattern was preserved in hemisected slices, indicating that pathways intrinsic to each side coordinated this activity. A different pattern of oscillation was observed in slices from embryos with synchronous 3-Hz oscillations occurring in limited regions. Our study demonstrated that rhythm generators were distributed throughout transverse sections of the lumbar spinal cord and exhibited a complex spatiotemporal pattern of coordinated rhythmic activity.

Synaptic drive to motoneurons during fictive swimming in the developing zebrafish

The development of swimming behavior and the correlated activity patterns recorded in motoneurons during fictive swimming in paralyzed zebrafish larvae were examined and compared. Larvae were studied from when they hatch (after 2 days) and are first capable of locomotion to when they are active swimmers capable of capturing prey (after 4 days). High-speed (500 Hz) video imaging was used to make a basic behavioral characterization of swimming. At hatching and up to day 3, the larvae swam infrequently and in an undirected fashion. They displayed sustained bursts of contractions ('burst swimming') at an average frequency of 60-70 Hz that lasted from several seconds to a minute in duration. By day 4 the swimming had matured to a more frequent and less erratic "beat-and-glide" mode, with slower (approximately 35 Hz) beats of contractions for approximately 200 ms alternating with glides that were twice as long, lasting from just a few cycles to several minutes overall. In whole cell current-clamp recordings, motoneurons displayed similar excitatory synaptic activity and firing patterns, corresponding to either fictive burst swimming (day 2-3) or beat-and-glide swimming (day 4). The resting potentials were similar at all stages (about -70 mV) and the motoneurons were depolarized (to about -40 mV) with generally non-overshooting action potentials during fictive swimming. The frequency of sustained inputs during fictive burst swimming and of repetitive inputs during fictive beat-and glide swimming corresponded to the behavioral contraction patterns. Fictive swimming activity patterns were eliminated by application of glutamate antagonists (kynurenic acid or 6-cyano-7-nitroquinoxalene-2,3-dione and DL-2-amino-5-phosphonovaleric acid) and were modified but maintained in the presence of the glycinergic antagonist strychnine. The corresponding synaptic currents underlying the synaptic drive to motoneurons during fictive swimming could be isolated under voltage clamp and consisted of cationic [glutamatergic postsynaptic currents (PSCs)] and anionic inputs (glycinergic PSCs). Either sustained or interrupted patterns of PSCs were observed during fictive burst or beat-and-glide swimming, respectively. During beat-and-glide swimming, a tonic inward current and rhythmic glutamatergic PSCs (approximately 35 Hz) were observed. In contrast, bursts of glycinergic PSCs occurred at a higher frequency, resulting in a more tonic pattern with little evidence for synchronized activity. We conclude that a rhythmic glutamatergic synaptic drive underlies swimming and that a tonic, shunting glycinergic input acts to more closely match the membrane time constant to the fast synaptic drive.

Slow dorsal-ventral rhythm generator in the lamprey spinal cord

In the isolated lamprey spinal cord, a very slow rhythm (0.03-0.11 Hz), superimposed on fast N-methyl-D-aspartate (NMDA)-induced locomotor activity (0.26-2.98 Hz), could be induced by a blockade of GABA(A) or glycine receptors or by administration of (1 s, 3 s)-l-aminocyclopentane-1,3-dicarboxylic acid a metabotropic glutamate receptor agonist. Ventral root branches supplying dorsal and ventral myotomes were exposed bilaterally to study the motor pattern in detail. The slow rhythm was expressed in two main forms: 1) a dorsal-ventral reciprocal pattern was the most common (18 of 24 preparations), in which bilateral dorsal branches were synchronous and alternated with the ventral branches, in two additional cases a diagonal dorsal-ventral reciprocal pattern with alternation between the left (or right) dorsal and the right (or left) ventral branches was observed; 2) synchronous bursting in all branches was encountered in four cases. In contrast, the fast locomotor rhythm occurred always in a left-right reciprocal pattern. Thus when the slow rhythm appeared in a dorsal-ventral reciprocal pattern, fast rhythms would simultaneously display left-right alternation. A longitudinal midline section of the spinal cord during ongoing slow bursting abolished the reciprocal pattern between ipsilateral dorsal and ventral branches but a synchronous burst activity could still remain. The fast swimming rhythm did not recover after the midline section. These results suggest that in addition to the network generating the swimming rhythm in the lamprey spinal cord, there is also a network providing slow reciprocal alternation between dorsal and ventral parts of the myotome. During steering, a selective activation of dorsal and ventral myotomes is required and the neural network generating the slow rhythm may represent activity in the spinal machinery used for steering.

The intrinsic function of a motor system--from ion channels to networks and behavior

The forebrain, brainstem and spinal cord contribution to the control of locomotion is reviewed in this article. The lamprey is used as an experimental model since it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre- and postsynaptic modulation. This experimental model is bridging the gap between the molecular and cellular level to the network and behavioral level.

The role of putative glutamatergic neurons and their connections in the locomotor central pattern generator of the mollusk, Clione limacina

In the pteropod mollusk Clione limacina, locomotor rhythm is produced by the central pattern generator (CPG), due mainly to the activity of interneurons of groups 7 (active in the phase of the dorsal flexion of the wings) and 8 (active in the phase of the ventral flexion). Each of these groups excites the neurons active in the same phase of the locomotor cycle, and inhibits the neurons of the opposite phase. In this work, the nature of connections formed by group 7 interneurons was studied. Riluzole (2-amino-6-trifluoro-methoxybenzothiazole), which is known to inhibit the presynaptic release of glutamate, suppressed the action of the type 7 interneurons onto the follower neurons of the same and of the antagonistic phase of the locomotor cycle. The main pattern of rhythmic activity of CPG with alternation of two phases could be maintained after suppression of inhibitory connections from group 7 interneurons to antagonistic neurons. This suggests redundancy of the mechanisms controlling swimming rhythm generation, which ensures the reliable operation of the system.

The NMDA antagonist, MK-801, alters L-DOPA-induced air-stepping in neonatal rats

Administration of L-DOPA (sc) to neonatal rats suspended in harnesses induces coordinated stepping of all four limbs (diagonal progression; L-DOPA-induced air-stepping) by 5 days of age. Because NMDA also induces locomotion in several species, NMDA receptor activation may be required for L-DOPA to elicit coordinated air-stepping. The purpose of the present experiment was to determine if the NMDA receptor antagonist, MK-801, would block L-DOPA-induced air-stepping in developing rats. Neonatal rats administered MK-801 alone rarely air-stepped with the forelimbs or hindlimbs in a coordinated fashion, whereas those treated with L-DOPA alone primarily stepped with all four limbs using a diagonal progression pattern during the session. In contrast, the number of limbs that stepped during the session was gradually altered in 5- to 20-day-old rats treated with MK-801 + L-DOPA. Gaits of those rats progressed from diagonal progression to extension of the forelimbs beneath the chin with hindlimb alternation, to forelimb extension without hindlimb activity. Twenty-day-olds treated with MK-801 + L-DOPA subsequently became completely inactive when the forelimbs dropped from their elevated position beneath the chin. In addition to the sequence just described, 15-day-old rats treated with the lowest concentration of MK-801 + L-DOPA occasionally stepped with one pair of homolateral limbs or stepped with the hindlimbs in near synchrony while the forelimbs either stepped in alternation, were extended beneath the chin or groomed the face. Because limb participation during L-DOPA-induced air-stepping was altered in neonatal rats pretreated with MK-801, NMDA receptor activation may be important for locomotor coordination (gait).

Neuropeptide-mediated facilitation and inhibition of sensory inputs and spinal cord reflexes in the lamprey

The effects of neuromodulators present in the dorsal horn [tachykinins, neuropeptide Y (NPY), bombesin, and GABAB agonists] were studied on reflex responses evoked by cutaneous stimulation in the lamprey. Reflex responses were elicited in an isolated spinal cord preparation by electrical stimulation of the attached tail fin. To be able to separate modulator-induced effects at the sensory level from that at the motor or premotor level, the spinal cord was separated into three pools with Vaseline barriers. The caudal pool contained the tail fin. Neuromodulators were added to this pool to modulate sensory inputs evoked by tail fin stimulation. The middle pool contained high divalent cation or low calcium Ringer to block polysynaptic transmission and thus limit the input to the rostral pool to that from ascending axons that project through the middle pool. Ascending inputs and reflex responses were monitored by making intracellular recordings from motor neurons and extracellular recordings from ventral roots in the rostral pool. The tachykinin neuropeptide substance P, which has previously been shown to potentiate sensory input at the cellular and synaptic levels, facilitated tail fin-evoked synaptic inputs to neurons in the rostral pool and concentration dependently facilitated rostral ventral root activity. Substance P also facilitated the modulatory effects of tail fin stimulation on ongoing locomotor activity in the rostral pool. In contrast, NPY and the GABAB receptor agonist baclofen, both of which have presynaptic inhibitory effects on sensory afferents, reduced the strength of ascending inputs and rostral ventral root responses. We also examined the effects of the neuropeptide bombesin, which is present in sensory axons, at the cellular, synaptic, and reflex levels. As with substance P, bombesin increased tail fin stimulation-evoked inputs and ventral root responses in the rostral pool. These effects were associated with the increased excitability of slowly adapting mechanosensory neurons and the potentiation of glutamatergic synaptic inputs to spinobulbar neurons. These results show the possible behavioral relevance of neuropeptide-mediated modulation of sensory inputs at the cellular and synaptic levels. Given that the types and locations of neuropeptides in the dorsal spinal cord of the lamprey show strong homologies to that of higher vertebrates, these results are presumably relevant to other vertebrate systems.

Vertebrate locomotion--a lamprey perspective

The forebrain, brain stem, and spinal cord contribution to the control of locomotion is reviewed in this chapter. The lamprey is used as an experimental model because it allows a detailed cellular analysis of the neuronal network underlying locomotion. The focus is on cellular mechanisms that are important for the pattern generation, as well as different types of pre- and postsynaptic modulation. Neuropeptides target different cellular and synaptic mechanisms and cause long-lasting changes (> 24 h) in network function.

Substance P modulates NMDA responses and causes long-term protein synthesis-dependent modulation of the lamprey locomotor network

Tachykinin immunoreactivity is found in a ventromedial spinal plexus in the lamprey. Neurons in this plexus project bilaterally and are thus in a position to modulate locomotor networks on both sides of the spinal cord. We have examined the effects of the tachykinin substance P on NMDA-evoked locomotor activity. Brief (10 min) application of tachykinin neuropeptides results in a prolonged concentration-dependent (>24 hr) modulation of locomotor activity, shown by the increased burst frequency and more regular burst activity. These effects are blocked by the tachykinin antagonist spantide II. There are at least two phases to the burst frequency modulation. An initial phase (approximately 2 hr) is associated with the protein kinase C-dependent potentiation of cellular responses to NMDA. The long-lasting phase (>2 hr) appears to be protein synthesis-dependent, with protein synthesis inhibitors causing the increased burst frequency to recover after washing for 2-3 hr. The modulation of the burst regularity is caused by a separate effect of tachykinins, because unlike the burst frequency modulation it does not require the modulation of NMDA receptors for its induction and is blocked by H8, an inhibitor of cAMP- and cGMP-dependent protein kinases. The effects of substance P were mimicked by the dopamine D2 receptor antagonist eticlopride. The effects of eticlopride were blocked by the tachykinin antagonist spantide II, suggesting that eticlopride may endogenously release tachykinins. Because locomotor activity in vitro corresponds to that during swimming in intact animals, we suggest that endogenously released tachykinins will result in prolonged modulation of locomotor behavior.

Genesis of spontaneous rhythmic motor patterns in the lumbosacral spinal cord of neonate mouse

The isolated spinal cord of the neonatal mouse spontaneously generates two different motor patterns of continuous rhythmic bursting: one in lumbar ventral roots in earliest postnatal preparations (P0-2) and another at the sacral level at later postnatal times (P3-5). Lumbar rhythmic motor discharges clearly alternate on contralateral roots and are in a frequency range (approximately 1 Hz) usually described for locomotor-like activity, while sacral motor sequences include mixed synchrony and irregular bilateral alternation that differ from typical locomotor-like activity. A significant decrease in the frequency and increase in the duration of spontaneous rhythmic bursts occur between lumbar and sacral motor patterns. In quiescent preparations from both postnatal periods, perfusion with Mg(2+)-free medium systematically induces a rhythmic activity at both lumbar and sacral level. Temporal characteristics of motor patterns under Mg(2+)-free medium are similar to spontaneous rhythms. Activating NMDA receptor channels by diminishing their Mg2+ block appears to be an efficient way of decreasing the threshold for genesis of the spinal rhythm in mouse. Bath application of NMDA and non-NMDA receptor antagonists blocks Mg(2+)-free-induced rhythmic activities in an irreversible or reversible manner, respectively. The effects of Mg(2+)-free medium and of glutamate antagonists provide evidence for the excitatory amino acid (EAA) dependence of both rhythmic motor patterns. Finally, the possibility that the recording of two different motor patterns may reflect a rostrocaudal developmental process is discussed.

Modulation of burst frequency by calcium-dependent potassium channels in the lamprey locomotor system: dependence of the activity level

It is crucial to determine the effects on the network level of a modulation of intrinsic membrane properties. The role calcium-dependent potassium channels, KCa, in the lamprey locomotor system has been investigated extensively. Earlier experimental studies have shown that apamin, which affects one type of KCa, increases the cycle duration of the locomotor network, due to effects on the burst termination. The effects of apamin were here larger when the network had a low level of activity (burst frequency 0.5 to 1 Hz) as compared to a higher rate (> 2 Hz). By using a previously developed simulation model based on the lamprey locomotor network, we show that the model could account for the frequency dependence of the apamin modulation, if only the KCa conductance activated by Ca2+ entering during the action potential was altered and not the KCa conductance activated by Ca2+ entering through NMDA channels. The present simulation model of the spinal network in the lamprey can thus account for earlier experimental results with apamin on the network and cellular level that previously appeared enigmatic.

Role of sensory-evoked NMDA plateau potentials in the initiation of locomotion

Reticulospinal (RS) neurons constitute the main descending motor system of lampreys. This study reports on natural conditions whereby N-methyl-D-aspartate (NMDA)-mediated plateau potentials were elicited and associated with the onset of locomotion. Reticulospinal neurons responded in a linear fashion to mild skin stimulation. With stronger stimuli, large depolarizing plateaus with spiking activity were elicited and were accompanied by swimming movements. Calcium imaging revealed sustained intracellular calcium rise upon sensory stimulation. Blocking NMDA receptors on RS neurons prevented the plateau potentials as well as the associated rise in intracellular calcium. Thus, the activation of NMDA receptors mediates a switch from sensory-reception mode to a motor command mode in RS neurons.

Effects of acetylcholine and glutamate on isolated neurons of locomotory network of Clione

In Clione limacina, locomotory rhythm is produced in the central pattern generator by reciprocal activity of two groups of interneurons. Dorsal (D) and ventral (V) phase interneurons activate neurons of the same phase and inhibit neurons of the opposite phase. Which neurotransmitters are used by these interneurons is not clear. In this study, identified follower neurons to V and D interneurons were isolated, and their responses to the local application of potential neurotransmitters were examined. Acetylcholine exerted inhibitory action on the isolated D-phase neurons and excitatory action on V-phase neurons. Glutamate produced excitation in D-phase neurons, and inhibition in V-phase neurons. These results suggest that acetylcholine is the neurotransmitter of D-phase interneurons, while glutamate might be the neurotransmitter of V-phase interneurons.

Motor control of the jamming avoidance response of Apteronotus leptorhynchus: evolutionary changes of a behavior and its neuronal substrates

The two closely related gymnotiform fishes, Apteronotus and Eigenmannia, share many similar communication and electrolocation behaviors that require modulation of the frequency of their electric organ discharges. The premotor linkages between their electrosensory system and their medullary pacemaker nucleus, which controls the repetition rate of their electric organ discharges, appear to function differently, however. In the context of the jamming avoidance response, Eigenmannia can raise or lower its electric organ discharge frequency from its resting level. A normally quiescent input from the diencephalic pre-pacemaker nucleus can be recruited to raise the electric organ discharge frequency above the resting level. Another normally active input, from the sublemniscal pre-pacemaker nucleus, can be inhibited to lower the electric organ discharge frequency below the resting level (Metzner 1993). In contrast, during a jamming avoidance response, Apteronotus cannot lower its electric organ discharge frequency below the resting level. The sublemniscal pre-pacemaker is normally completely inhibited and release of this inhibition allows the electric organ discharge frequency to rise during the jamming avoidance response. Further inhibition of this nucleus cannot lower the electric organ discharge frequency below the resting level. Lesions of the diencephalic pre-pacemaker do not affect performance of the jamming avoidance response. Thus, in Apteronotus, the sublemniscal pre-pacemaker alone controls the changes of the electric organ discharge frequency during the jamming avoidance response.

Rostrocaudal distribution of 5-HT innervation in the lamprey spinal cord and differential effects of 5-HT on fictive locomotion

5-hydroxytryptamine (5-HT) is known to modulate the locomotion generator network in the lamprey spinal cord, but little is known about the pattern of 5-HT innervation along the spinal cord. The distribution of 5-HT-immunoreactive (5-HT-ir) cells and fibers, as well as the effects of 5-HT on the locomotor network in the rostral and caudal parts of the spinal cord were compared in two lamprey species, Lampetra fluviatilis and Petromyzon marinus. Intraspinal 5-HT cells form a very dense ventromedial plexus in which the dendrites of neurons forming the locomotor network are distributed. The number of 5-HT cells and varicosities in this plexus decreases in the fin area (segments 70-90), and then increases somewhat in the most caudal segments. The descending 5-HT fibers from the rhombencephalon are located in the lateral and ventral columns, and their numbers gradually decrease to around 50% in the tail part of the spinal cord. In contrast, the number of 5-HT-ir axons in the dorsal column remains the same along the spinal cord. Bath application of both N-methyl-D-aspartic acid (NMDA, 20-250 microM) and D-glutamate (250-1000 microM) was used to induce fictive locomotion in the isolated spinal cord. Bath application of 5-HT (1 microM) reduced the burst frequency in the presence of NMDA. The 5-HT effect was, however, significantly greater in the rostral as compared to the caudal part. With D-glutamate, the 5-HT effects was instead more pronounced in the caudal spinal cord. To account for this difference in 5-HT effects on NMDA- and D-glutamate-induced fictive locomotion, the cellular effect of D-glutamate was further investigated. It activates not only NMDA, but also alpha amino-3-hydroxy-5-methyl-4-isoxyl propionate (AMPA)/kainate and metabotropic glutamate receptors. In contrast to NMDA, D-glutamate did not elicit tetrodotoxin (TTX)-resistant membrane potential oscillations. This difference in action between NMDA (selective NMDA receptor agonist) and D-glutamate (mixed agonist) may partially account for the differences in effect of 5-HT on the locomotor pattern.

Role of excitatory amino acids in brainstem activation of spinal locomotor networks in larval lamprey

An in vitro brain/spinal cord preparation from larval lamprey was used to determine the role of excitatory amino acid (EAA) receptors in the descending activation of spinal locomotor networks. The general EAA receptor blockers KYN, PDA, and DGG completely blocked locomotor activity initiated from the brainstem. The NMDA receptor blocker APV and the non-NMDA receptor blocker DNQX usually attenuated but did not block locomotor activity. Relatively long and short cycle times were attenuated about equally by APV or DNQX, and therefore the attenuation was not cycle time dependent. Receptor blockers for EAAs attenuated locomotor activity, but often with little or no change in the cycle time of burst activity. Although both NMDA and non-NMDA receptors for EAAs are important for the descending initiation of locomotor activity in the lamprey, it is unclear whether these receptors are concentrated in areas of the spinal locomotor networks that control cycle time.

Pattern generation

Significant advances have been made in understanding the cellular mechanisms for pattern generation in both invertebrate and vertebrate preparations. In a number of preparations, slow neuromodulators have been shown not only to modify network function, but to be intimately involved in development and/or normal function of the neural network and its associated behavior. The mechanisms underlying coordination between multiple pattern-generating networks, including switching of neurons from one network to another, are now being studied. Several new quantitative models of network function have been developed, and modeling is now an important component of research in this field.

Activation of the central pattern generators for locomotion by serotonin and excitatory amino acids in neonatal rat

1. The role of serotonin (5-HT) and excitatory amino-acids (EAAs) in the activation of the neural networks (i.e. the central pattern generators) that organize locomotion in mammals was investigated in an isolated brainstem-spinal cord preparation from the newborn rat. 2. The neuroactive substances were bath applied and the activity of fictive locomotion was recorded in the ventral roots. 3. Serotonin initiated an alternating pattern of right and left action potential bursts. The period of this rhythm was dose dependent, i.e. it decreased from around 10 s at 2 x 10(-5) M to 5 s at 10(-4) M. The effects of serotonin were blocked by a 5-HT1 antagonist (propranolol) and by 5-HT2 antagonists (ketanserin, cyproheptadine, mianserin). 5-HT3 antagonists were ineffective. The effects of methoxytryptamine, a non-selective 5-HT agonist, mimicked the 5-HT effects. 4. The endogenous EAAs, glutamate and aspartate, also triggered an alternating rhythmic pattern. Their effects were blocked by 2-amino-5-phosphonovaleric acid (AP-5; a N-methyl-D-aspartate (NMDA) receptor blocker) and 6,7-dinitro-quinoxaline-2,3-dione (a non-NMDA receptor blocker). 5. Several EAA agonists (N-methyl-D,L-aspartate (NMA) and kainate) initiated rhythmic activity. The period of the induced rhythm (from 3 to 1 s) was similar with both of these substances but in a range of concentrations which was ten times lower in the case of kainate (10(-6) to 5 x 10(-6) M) than in that of NMA (10(-5) to 4 x 10(-5) M). alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate and quisqualate occasionally triggered some episodes of fictive locomotion with a threshold at 6 x 10(-7) and 10(-5) M, respectively.

Two distinct inputs to an avian song nucleus activate different glutamate receptor subtypes on individual neurons

Although neural circuits mediating various simple behaviors have been delineated, those generating more complex behaviors are less well described. The discrete structure of avian song control nuclei promises that circuits controlling complex behaviors, such as birdsong, can also be understood. To this end, we developed an in vitro brain slice preparation containing the robust nucleus of the archistriatum (RA), a forebrain song control nucleus, and its inputs from two other song nuclei, the caudal nucleus of the ventral hyperstriatum (HVc) and the lateral part of the magnocellular nucleus of the anterior neostriatum (L-MAN). Using intracellular recordings, we examined the pharmacological properties of the synapses made on RA neurons by L-MAN and HVc axons. Electrical stimulation of the L-MAN and the HVc fiber tracts evoked excitatory postsynaptic potentials (EPSPs) from >70% of RA neurons when slices were prepared from male birds of 40-90 days of age, suggesting that many individual RA neurons receive excitatory input from L-MAN and HVc axons. The "L-MAN" EPSPs were blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid (D-APV) as well as the broad-spectrum glutamate receptor antagonist kynurenic acid but were relatively unaffected by the non-NMDA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, "HVc" EP-SPs were relatively insensitive to D-APV but almost completely abolished by CNQX. These experiments suggest that L-MAN and HVc axons provide pharmacologically distinct types of excitatory input to many of the same RA neurons.

Spontaneous and NMDA evoked motor rhythms in the neonatal mouse spinal cord: an in vitro study with comparisons to in situ activity

The current paper presents our initial efforts to establish an in vitro spinal preparation for investigating locomotor pattern generation in mice. We have characterized the step cycle timing from EMG activity in the gastrocnemius (G) and tibialis anterior (TA) muscles of freely moving intact adult as well as neonatal mice and then compared those data with rhythmic EMG activity in an isolated spinal cord-hindlimb preparation. The motor output during the first four days of life was evaluated in an effort to identify the optimal post-partum period for in vitro locomotor studies. The in vitro pattern generating capabilities of the lumbosacral region were tested in both nonhemisected and hemisected preparations. Spontaneous as well as NMDA evoked in vitro activity in the antagonist set of hindlimb muscles included sequences of: 1) synchronous bursting; 2) mixed synchrony and alternation; and/or 3) irregular alternations. The alternating bursting observed in vitro was more often an alternation of sequences rather than a cycle-to-cycle phasing between G and TA muscles. In summary, while there was evidence of reciprocal inhibition in neonates, the circuitry for cycle-to-cycle alternation between antagonists was found to be labile.

Control of locomotion in vitro: II. Chemical stimulation

Previous studies have described the presence of alternating activity induced in left and right ventral roots of the neonate rat in vitro brainstem-spinal cord preparation, following application of certain neuroactive substances to the bathing solution. The present findings show the presence of chemically induced, adult-like coordinated airstepping demonstrated by electromyographic recordings in the hindlimb-attached in vitro brainstem-spinal cord preparation. Analysis of muscular activity demonstrated alternation between antagonists of one limb and between agonists of different limbs, as well as a proximodistal delay in agonists active at different joints of the same limb. Neuroactive agents were applied independently to either the brainstem or spinal cord bath. The substances surveyed in the present studies included some of those used previously, as well as additional compounds: bicuculline and picrotoxin (gamma-aminobutyric acid-ergic antagonists), N-methyl-D-aspartic acid (excitatory amino acid agonist), substance P, acetylcholine, carbachol (cholinergic agonist), and serotonin. Application of these substances to the brainstem bath produced rhythmic airstepping. Application of dopamine, aspartate, glutamate, and N-methyl-D-aspartic acid to the spinal cord bath also produced rhythmic airstepping, while application of acetylcholine produced tonic, long-lasting co-contractions. These findings reveal the presence of several neurochemical systems in the central nervous system that can be activated at birth to induce coordinated airstepping in the neonate rat in vitro brainstem-spinal cord preparation.

Induction of locomotion in spinal tadpoles by excitatory amino acids and their agonists

Bath application of the excitatory amino acids L-aspartate and/or L-glutamate or their agonists N-methyl-D,L-aspartate and/or kainate elicited swimming movements in spinal tadpoles. Swimming cycles induced by the amino acids were in the frequency range of natural movements, and could be evoked after sectioning all dorsal roots in the exposed spinal segments. Locomotion was only elicited by L-aspartate or L-glutamate at low concentrations when the bath medium was rapidly circulated over the exposed surface of the spinal cord, and was of much shorter duration than the agonist-induced movements. These results indicate some differences between the actions of L-aspartate and L-glutamate and their agonists on the tadpole spinal cord.

CNQX and DNQX block non-NMDA synaptic transmission but not NMDA-evoked locomotion in lamprey spinal cord

The motor pattern underlying locomotion in the lamprey is activated and maintained by excitatory amino acid neurotransmission. The quinoxalinediones 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) are potent and selective antagonists of non-N-methyl-D-aspartate (NMDA) receptors in the mammalian central nervous system. In the lamprey, these compounds are now shown to block fast excitatory synaptic potentials elicited in neurones of the spinal ventral horn. They selectively antagonise responses to the application of selective kainate and quisqualate receptor agonists (kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxalone (AMPA)) but do not influence NMDA receptor-mediated responses. Additionally, it is shown that the activation of NMDA receptors is sufficient to elicit and maintain fictive locomotion after blockade of non-NMDA receptors with either DNQX or CNQX. Conversely, activation of quisqualate receptors with AMPA, but not quisqualate leads to fictive locomotion with properties much like that activated by kainate.

Activation of N-methyl-D-aspartate (NMDA) receptors augments repolarizing responses in lamprey spinal neurons

Current- and voltage-clamp techniques were used to analyze the mechanisms underlying the repolarization during N-methyl-D-aspartate (NMDA)-induced, tetrodotoxin-resistant pacemaker-like oscillations in lamprey spinal neurons. Long-lasting depolarizing current pulses (15-40 mV, 50-400 ms, tetrodotoxin and tetraethylammonium present) were followed by hyperpolarizing afterpotentials even when NMDA receptors were blocked, but they were markedly enhanced by application of N-methyl-D,L-aspartate (NM(DL)A). The afterpotentials were depressed by replacing Ca2+ with Ba2+. During voltage-clamp NM(DL)A enhanced a Ba2+-sensitive outward tail current following voltage steps of 15-40 mV. The outward current remained after injection of Cl-, as did the NMDA-induced membrane potential oscillations observed under current-clamp. These results suggest that the repolarization during NMDA-induced oscillations is due to Ca2+ entry both via NMDA-gated channels and conventional voltage-gated Ca2+ channels, leading to an activation of Ca2+-dependent K+ channels. The afterhyperpolarization following single action potentials, which is also due to Ca2+-dependent K+ channels, was not significantly altered by NMDA receptor activation, suggesting a different location of the Ca2+ entry during the two conditions in relation to the location of the activated Ca2+-dependent K+ channels.

The effect of an uptake inhibitor (dihydrokainate) on endogenous excitatory amino acids in the lamprey spinal cord as revealed by microdialysis

Microdialysis was utilized to test the effects of the uptake inhibitor dihydrokainate (DHK) on endogenous amino acid levels in the lamprey spinal cord in vitro. The level of L-glutamate increased markedly (165%) in the presence of DHK, whereas the level of the glutamate precursor L-glutamine decreased (53%). Since DHK can potentiate or evoke fictive locomotion in the lamprey spinal cord, it is suggested that L-glutamate is released by neurons which take part in the activation of the spinal locomotor network.

Simulation of the segmental burst generating network for locomotion in lamprey

Recently a segmental network of inhibitory and excitatory interneurones, which are active during locomotion, has been described in the lamprey, a lower vertebrate. The interactions between the different neurones were established by paired intracellular recordings. A computer simulation of the segmental network has been performed, which shows that with the established neuronal connectivity rhythmic alternating burst activity can be generated within the upper part of the normal physiological range of locomotion. Three neurones of each kind were used (altogether 18 neurones). As shown previously the lower frequency range used in locomotion most likely depends on an activation of voltage-dependent N-methyl-D-aspartate (NMDA) receptors, which could, however, not be simulated with the present neuronal models.

Excitatory amino acid-evoked membrane currents and excitatory synaptic transmission in lamprey reticulospinal neurons

The characteristics of excitatory amino acid-evoked currents and of excitatory synaptic events have been examined in lamprey Müller neurons using voltage clamp and current clamp recording techniques. Application of glutamate evoked depolarizations associated with a decrease in input resistance. The reversal potential of the responses was -35 mV. Under voltage clamp conditions, a series of excitatory amino acid agonists evoked inward currents associated with little or no increase in baseline current noise. The order of potency of the excitatory amino acid agonists was quisqualate greater than kainate greater than glutamate greater than aspartate, while N-methyl-D-aspartic acid (NMDA) was inactive. Inward currents evoked by glutamate, as well as by kainate and quisqualate were attenuated reversibly by 1 mM kynurenic acid (KYN). In contrast, glutamate-evoked currents were not affected by 100 microM D(-)-2-amino-5-phosphonovaleric acid (APV), a selective NMDA antagonist. Spontaneously occurring and stimulus-evoked excitatory postsynaptic events were antagonized reversibly by 1 mM KYN. At this concentration, KYN had no effect on membrane potential, input resistance, or excitability of the cells. In contrast, excitatory postsynaptic currents were unaffected by APV. It is concluded that both glutamate responses and excitatory synaptic transmission in lamprey Müller neurons are mediated by non-NMDA-type receptors and that these receptors are associated with ionic channels with a low elementary conductance. The combined pharmacological and biophysical characteristics of these responses are therefore different from those previously reported in other preparations. Spontaneous (but not stimulus-evoked) inhibitory synaptic events in Müller neurons were blocked reversibly by 1 mM KYN but not by 100 microM APV, suggesting that excitation of interneurons inhibitory to Müller cells was also mediated by non-NMDA receptors.

The effects of excitatory amino acids and their antagonists on the generation of motor activity in the isolated chick spinal cord

We have investigated the action of excitatory amino acids and their antagonists on spontaneous motor activity produced by an isolated preparation of the chick lumbosacral cord. Bath application of N-methyl-DL-aspartic acid (NMDA) or D-glutamate increased the occurrence and duration of spontaneous episodes of motor activity. Both NMDA-induced and spontaneous activity were reversibly inhibited by several excitatory amino acid antagonists including 2-amino-5-phosphono valeric acid and gamma-D-glutamyl glycine in a dose-dependent manner. These results suggest that motor activity in the chick spinal cord may be regulated by the release of endogenous excitatory amino acids from spinal interneurons.

In vitro CNS preparations: unique approaches to the study of command and pattern generation systems in motor control

In vitro preparations of the nervous system, which were originally developed for investigations of invertebrate neural networks, have recently gained popularity for studying locomotor networks in the vertebrate nervous system. The nervous system is removed from the animal and maintained in a physiological bathing solution. These preparations can be induced to generate locomotor patterns in a number of ways and offer several unique advantages. The function of the motor networks can be manipulated by alterating the composition of the bath, and immobilization of the preparations, either by cutting ventral roots or bath application of curare, greatly facilitates intracellular recordings. In vitro preparations offer the opportunity to acquire the detailed information necessary for understanding vertebrate motor networks at the cellular level. This article reviews some of the many applications of in vitro preparations for studying vertebrate locomotor control using examples derived mainly from work on the lamprey.

In vitro brainstem-spinal cord preparations for study of motor systems for mammalian respiration and locomotion

Recently developed in vitro preparations of the brainstem-spinal cord from neonatal rat suitable for investigation of motor control systems for mammalian locomotion and respiration are described. The preparations remain viable for extended periods under standard in vitro conditions and generate rhythmic motor patterns for locomotion and respiration. The methodology of the preparations and characteristics of the motor output patterns are described. The preparations retain functional circuitry for major components of the motor control systems, including brainstem respiratory and spinal locomotor pattern generating networks, brainstem locomotor command regions, descending bulbospinal and ascending spinal pathways, and mechanosensory afferent input systems. They therefore offer potential for investigation of diverse aspects of the mammalian respiratory and locomotor control systems.