S. Grillner

Phasic gain control of reflexes from the dorsum of the paw during spinal locomotion

In chronic spinal cats walking with their hindlimbs on a treadmill belt, tactile stimuli were applied to the dorsum of the paw during various phases of the step cycle. A stimulation during the swing phase evoked a flexion response with a concomitant crossed extension, whereas in stance it induced an increased ipsilateral extension. EMG-recordings show short latency reflex responses in flexors and extensors, respectively. The responses are organized such that latencies of knee muscles are shorter than those of ankle and hip muscles. The movements induced by the stimulations appear to be very meaningful during normal conditions in compensating for any unpredicted obstacle disturbing the movement of the paw during locomotion. Responses during forward flexion and during the support phase are well adapted to the ongoing locomotor activity and do not influence the interlimb coordination whereas a stimulation when the foot approaches the ground after the end of flexion disturbs the regular alternating pattern. Different possible mechanisms underlying this phase-dependent reflex reversal are discussed.

N-methyl-d-aspartate (NMDA), kainate and quisqualate receptors and the generation of fictive locomotion in the lamprey spinal cord

The motor pattern underlying swimming can be elicited in an in vitro preparation of the lamprey spinal cord by applying excitatory amino acids in the bath activating N-methyl-D-aspartate (NMDA) receptors and kainate receptors, but not quisqualate receptors. L-DOPA exerts a weak rythmogenic effect due to an action on kainate receptors. The kainate-induced rhythm is unchanged when a NMDA receptor antagonist is applied (2APV) and the N-methyl-aspartate-induced fictive locomotion can occur when kainate receptors are blocked (PDA). The burst frequency of the NMA-induced activity (dose range 30-5000 microM) is wide and ranges from 0.05-0.1 Hz up to 2.5-4 Hz, while the kainate-induced activity (dose range 7-30 microM) ranges from 0.5-1 Hz up to 4-8 Hz. This frequency range overlaps largely with that of the intact swimming animal. The findings further consolidate that NMDA receptors are efficient and demonstrates that kainate can also be effective in inducing fictive locomotion, and also that activation of either receptor type is sufficient. It has previously been shown that fictive locomotion elicited via sensory stimuli is depressed by NMDA and kainate receptor antagonists. It is suggested that these effects, presumably via aspartate and/or glutamate actions, are exerted on the input stage of interneuronal network.

Galanin induces a hyperpolarization of norepinephrine-containing locus coeruleus neurons in the brainstem slice

Galanin applied in the bath or by micropipette directly on to locus coeruleus neurons in an in vitro slice preparation caused a hyperpolarization accompanied by a small decrease in membrane resistance. Immunohistochemical staining of intracellularly filled neurons indicated that the effect of galanin was exerted on norepinephrine neurons of the locus coeruleus. The galanin effect was variable in amplitude and duration and often showed desensitization, with subsequent applications producing a smaller response. When cells were exposed to tetrodotoxin or tetrodotoxin/low calcium media, the galanin response was still present. Under voltage clamp galanin application caused a net outward current that did not reverse in normal potassium concentrations; however, by increasing extracellular potassium concentrations the net outward current was reversed and the reversal potential shifted to a less negative potential. The response to galanin was identical when either KCl or KAc was used as the intracellular electrode solution. Tetraethylammonium chloride significantly reduced or abolished the response to galanin in most cells, although in a few cells the galanin response was not affected. Glibenclamide, a blocker of ATP-sensitive potassium channels, did not affect the galanin hyperpolarization. In addition, diazoxide had no effect on the membrane properties of locus coeruleus neurons. These results demonstrate that galanin exerts its inhibitory effect in the locus coeruleus via an increase in K+ conductance; however, not via the pancreatic type of ATP-sensitive K+ channels. Cryostat sections of the locus coeruleus incubated in 125I-labeled galanin revealed binding sites in the locus coeruleus at all levels. Sections of the locus coeruleus processed for ultrastructural immunocytochemistry showed galanin immunoreactivity in many neuronal somata and dendritic processes within the nucleus, confirming earlier evidence for the coexistence of galanin and noradrenaline in locus coeruleus neurons. Galanin-immunoreactive soma and dendrites in the locus coeruleus less frequently received galanin-immunoreactive synapses of axonal origin. These findings suggest that endogenous galanin in the locus coeruleus is mainly released from noradrenaline galanin somata and/or dendrites to act on autoreceptors or on receptors on adjacent neurons.

Galanin-5-hydroxytryptamine interactions: electrophysiological, immunohistochemical and in situ hybridization studies on rat dorsal raphe neurons with a note on galanin R1 and R2 receptors.

Galaninergic mechanisms related to 5-hydroxytryptamine neurons in the dorsal raphe nucleus of the rat were analysed using electrophysiology, immunohistochemistry and in situ hybridization. Galanin caused a dose-dependent hyperpolarization accompanied by a decrease in membrane resistance in most 5-hydroxytryptamine-sensitive dorsal raphe neurons. The galanin-induced outward current reversed at about - 105 mV and shifted to a more positive potential with increasing extracellular potassium concentrations. The 5-hydroxytryptamine-induced outward current was enhanced and prolonged by preincubation with a low concentration of galanin (1-10 nM). The immunohistochemical analysis showed (i) generally low levels of galanin in the 5-hydroxytryptamine cell bodies, (ii) moderate numbers of galanin-positive nerve endings around the 5-hydroxytryptamine cell bodies, (iii) presence of galanin-like immunoreactivity in 5-hydroxytryptamine-positive dendrites and (iv) galanin-positive, 5-hydroxytryptamine-negative boutons making synaptic contact with 5-hydroxytryptamine-positive dendrites. The in situ hybridization results suggest that the galanin receptor present in the galanin/5-hydroxytryptamine neurons is not of the recently cloned galanin-R1 type. Taken together these results indicate that galanin exerts an inhibitory effect via an increase in K+ conductance in 5-hydroxytryptamine neurons by acting on a postsynaptic receptor. In addition, galanin at low, possibly physiological concentrations enhances the inhibitory effect of 5-hydroxytryptamine at the cell soma level. We propose that galanin primarily is released from adjacent galanin boutons lacking 5-hydroxytryptamine and also from soma and dendrites of galanin/5-hydroxytryptamine dorsal raphe neurons. Galanin may thus be involved in the manifold functions hitherto ascribed to ascending 5-hydroxytryptamine neurons, for example in mood regulation.

Activation of NMDA receptors elecits fictive locomotion and bistable membrane properties in the lamprey spinal cord

The motor pattern underlying locomotion in the lamprey can be elicited in the spinal cord in vitro by applying excitatory amino acids that activate NMDA receptors. When this is done oscillatory membrane potentials phase-linked with the locomotory rhythm can be recorded in different types of neurones. In some spinal neurones large amplitude oscillation continues after elimination of synaptic input with application of TTX. This oscillatory pacemaker-like activity is dependent on an activation of NMDA receptors, and is probably important in the generation of locomotion.

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.

Transmitters, membrane properties and network circuitry in the control of locomotion in lamprey

Abstract The lamprey brainstem and spinal cord can be maintained in vitro . It is a simple vertebrate preparation with comparatively few neurones. The neural correlates of different patterns of behaviour can be elicited in this in-vitro preparation. The subject of this review is the neuronal organization underlying locomotion, and, in particular, the role of different types of interneurones and their transmitters and mode of synaptic interaction. Excitatory amino acids, glycine, GABA, 5-HT, tachykinins and CCK have been implied as putative transmitters. The activation of one type of excitatory amino acid receptor, the NMDA receptor, can elicit TTX-resistant pacemaker-like membrane, potential oscillations. 5-HT can exert indirectly a potentiating effect via a depression of the postspike after-hyperpolarization (Ca 2+ -dependent potassium channels).

Diencephalic projection to reticulospinal neurons involved in the initiation of locomotion in adult lampreys Lampetra fluviatilis

Morphological and electrophysiological techniques were used to characterize a diencephalic projection from the ventral thalamus to reticulospinal neurons and its role in initiating rhythmic locomotor activity in the spinal cord of adult lampreys (Lampetra fluviatilis). Injection of fluorescein‐coupled dextran amine (FDA) into the rhombencephalic reticular nuclei labeled neurons in the ventral thalamus region on both the ipsilateral side and the contralateral side. Injection of FDA into the ventral thalamus labeled axonal projections in all reticular nuclei, but no direct projections were found to the spinal cord. Extracellular stimulation of the ventral thalamus elicited monosynaptic excitatory postsynaptic potentials (EPSPs), polysynaptic EPSPs, and inhibitory postsynaptic potentials (IPSPs) in reticulospinal neurons in the posterior (prrn) and middle (mrrn) rhombencephalic reticular nuclei. The monosynaptic EPSPs were blocked by the glutamate antagonist kynurenic acid and can be considered glutamatergic. The monosynaptic EPSPs were potentiated (up to 12 minutes) following a brief high‐frequency stimulation. Stimulation of the ventral thalamus induced rhythmic firing of reticulospinal neurons and elicited rhythmic burst activity in the spinal ventral roots. The projections from the ventral thalamus to the reticulospinal neurons in the prrn and mrrn thus provide excitatory inputs to the reticulospinal neurons, which, in turn, can activate the spinal circuits underlying locomotion. Also, the input nuclei to the ventral thalamus were labeled following injection of FDA into this nucleus. Labeled cells were found in the olfactory bulb, pallial areas, striatum, preoptic nucleus, hypothalamus, dorsal thalamus, optic tectum, and dorsal isthmic gray. The ventral thalamus, therefore, receives inputs from several different regions in the brain and controls the level of excitability in reticulospinal neurons. J. Comp. Neurol. 389:603–616, 1997.

Reticulospinal neurones activate excitatory amino acid receptors

Paired intracellular recordings were used to study the monosynaptic excitatory postsynaptic potentials (EPSP) in lamprey motoneurones evoked by stimulation of single reticulospinal Müller and Mauthner cells. The chemical component of the synaptic potentials was depressed by both application of the non-selective excitatory amino acid antagonists kynurenic acid and cis-2,3-piperidine dicarboxylate. The N-methyl-D-aspartate (NMDA) antagonists Mg2+ and 2-amino-5-phosphonovalerate caused a selective depression of a late component of the EPSP. Thus, fast-conducting reticulospinal neurones appear to release an excitatory amino acid acting at both NMDA and non-NMDA receptors.

The ionic mechanisms underlying N-methyl-d-aspartate receptor-induced, tetrodotoxin-resistant membrane potential oscillations in lamprey neurons active during locomotion

Activation of N-methyl-D-aspartate (NMDA) receptors can induce tetrodotoxin (TTX)-resistant membrane potential oscillations as well as fictive locomotion in the in vitro preparation of the lamprey spinal cord. The ionic basis of these oscillations were investigated in the presence of N-methyl-D,L-aspartate and TTX. Addition of blocking agents (2-amino-5-phosphonovalerate and tetraethylammonium (TEA)) and selective removal or substitution of certain ions (Mg2+, Ca2+, Na+, Ba2+) were used in the analysis of the oscillations. The depolarizing phase of the oscillation requires Na+ ions but not Ca2+ ions. The depolarization becomes larger if TEA is administered in the bath, which presumably is due to a blockade of potassium (K+) channels activated during the depolarizing phase. The repolarization appears to depend on a Ca2+ entry, which presumably acts indirectly by an activation of Ca2+-dependent K+ channels. Together with the NMDA-induced voltage dependence, this will bring the membrane potential back down to a hyperpolarized level.