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R Dubuc

Publications and source records attributed to R Dubuc.

At least 19 recordsLinked to original sources
PubMed1992

Phasic modulation of transmission from vestibular inputs to reticulospinal neurons during fictive locomotion in lampreys.

The aim of this study was to determine whether the transmission from sensory inputs to reticulospinal neurons is modulated during fictive locomotion in lampreys. Reticulospinal neurons play a key role in the control of locomotion; modulation of sensory transmission to these neurons might be of importance for the adaptation of the control they exert during locomotion. In this series of experiments, intracellular synaptic responses of reticulospinal neurons of the posterior rhombencephalic reticular nucleus elicited by electrical stimulation of vestibular nerves on each side were studied during fictive locomotion induced by 50 microM N-methyl-D-aspartate (NMDA). Interestingly, shortly after NMDA had reached the bath and much before locomotor discharges were apparent in the recorded ventral roots, there was a significant depression of the synaptic transmission from vestibular nerves. The effect was reversed by washing out the NMDA and persisted in the isolated brainstem after spinal transection at the first segmental level. As locomotor discharges appeared in the ventral roots, synaptic responses elicited by vestibular nerve stimulation showed a clear phasic modulation of their amplitude during the locomotor cycle. Responses to stimulation of the ipsilateral vestibular nerve were smaller during the ipsilateral burst discharge than during the contralateral activity, whilst responses to stimulation of the contralateral vestibular nerve were minimal during contralateral activity and maximal during ipsilateral activity. This opposite pattern of modulation observed in the same reticulospinal neuron suggests that the phasic modulation of vestibular transmission is not due to changes in the membrane properties of the reticulospinal cell but is produced at a pre-reticular level.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Phasic modulation of vestibulospinal neuron activity during fictive locomotion in lampreys.

This study was aimed at characterizing the activity of vestibulospinal neurons recorded intracellularly during fictive locomotion in lampreys. The majority (78%) of identified vestibulospinal neurons showed rhythmic fluctuations of their membrane potential correlated with locomotor discharges recorded in pairs of rostral ventral roots. Of the rhythmically modulated vestibulospinal cells, most (72%) were maximally depolarized during ipsilateral ventral root discharges and showed a minimum during contralateral activity. Other cells (20%) showed an opposite pattern, that is their peak of depolarization occurred during contralateral activity. Finally, a third category of cells (8%) showed a more complex pattern of activity. Two waves of depolarization could occur per locomotor cycle, one during each burst discharge. The pattern of fluctuation recorded in vestibulospinal neurons appears to be related to the side of the spinal cord onto which the cells are projecting.

Animals↗
PubMed1992

5-Hydroxytryptamine modulates spike frequency regulation in reticulospinal neurons involved in the control of locomotion in lamprey.

To investigate the effects of 5-hydroxytryptamine (5-HT) on reticulospinal neurons involved in the initiation and control of locomotion in lamprey, 5-HT (10 mM) was locally pressure ejected on the dorsal surface of the brainstem during intracellular recordings from identified reticulospinal neurons in the in vitro brainstem-spinal cord preparation. 5-HT induced a reduction of the late afterhyperpolarization (AHP) following the spike due to a reduction of a Ca(2+)-activated K+ current. In addition, 5-HT caused a resting membrane hyperpolarization in a proportion of these cells. Due to the 5-HT induced reduction of the AHP, reticulospinal cells, including those that became hyperpolarized by an application of 5-HT, discharged at a higher rate after 5-HT as a response to the same excitatory drive.

Animals↗
PubMed1992

Evidence that the masticatory muscles receive a direct innervation from cell group k in the rabbit.

These experiments have shown that a group of neurons lateral to the trigeminal motor nucleus innervates the muscles of mastication. The work began to describe the location of digastric last-order interneurons, using the technique of transneuronal labeling with wheatgerm agglutinin-conjugated horseradish peroxide injected into the left digastric muscle of rabbits under general anaesthesia. Four to eight days later, the animals were killed with an overdose of anaesthetic and perfused. Coronal sections of the frozen brainstem were cut at 20 microns thickness and processed for peroxidase activity. Motoneurons in the ventral and caudal divisions of the trigeminal motor nucleus were labeled in all animals as expected. An additional population of neurons located ventrolaterally to the motor nucleus in cell group k were also found to be labeled if the survival time was five days or more. In an attempt to determine whether cell group k neurons were labeled transynaptically, two series of control experiments were carried out. In the first, crystals of fluorescein- and rhodamine-conjugated dextran amines and horseradish peroxidase were applied directly to central ends of cut digastric nerves. In the second, central ends of cut digastric nerves were enclosed in cuffs containing 40-60% horseradish peroxidase solutions. Again, neurons in both the trigeminal motor nucleus and cell group k were labeled suggesting that neurons within cell group k project to the digastric muscle. Similar experiments using dextran amines and wheatgerm peroxidase were carried out on the masseter muscle. Motoneurons in the dorsomedial and rostral half of the trigeminal motor nucleus, as well as primary afferent cell bodies in the mesencephalic nucleus of the trigeminal nerve, were labeled in all experiments. In addition, a population of neurons in cell group k, dorsal to those associated with the digastric muscle, were found to contain each one of the reaction products. Since it is thought that only the wheatgerm agglutinin-conjugated horseradish peroxidase transferred from one neuron to another, we conclude that cell group k neurons provide an additional innervation to the digastric and masseter muscles.

Animals↗
PubMed1991

A new population of neurons with crossed axons in the lamprey spinal cord.

Neurons with contralateral, rostrally and caudally projecting axons were studied in whole mounts of lamprey spinal cord using retrograde labelling techniques with fluorescent dextran-amines, cobalt-lysine or horseradish peroxidase. A previously unknown large population (180-300 cells per hemisegment) of small (less than 25 microns) cells with contralateral projecting axons is described. Their axons extend over less than 5 segments rostrally or caudally. The number of these cells per segment was relatively constant in the rostral half of the spinal cord, but increased significantly in the caudal half. In comparison, medium-sized cells with contralateral axons corresponding to previously identified premotor interneurons were far less numerous (14-21 per hemisegment) and their axons extended more than 5 segments. Contralaterally projecting edge cells (intraspinal stretch receptor neurons) with principal rostral or caudal axons plus short collaterals in the other direction were distributed throughout the length of the spinal cord, whereas large and giant cells with a varied morphology were found in the caudal half.

Animals↗
PubMed1990

Organization of the six motor nuclei innervating the ocular muscles in lamprey.

The topography of motoneurons supplying each of the six ocular muscles of the lamprey, Lampetra fluviatilis, was studied by selective application of HRP to the cut nerves of identified muscles. In addition, the distributions of motoneuron populations to both eyes were studied simultaneously with fluorescein and rhodamine coupled dextran-amines (FDA and RDA) applied to cut ocular muscle nerves of either side. The motoneuron pool of the caudal oblique muscle is represented bilaterally in the trochlear (N IV) motor nucleus. The dorsal rectus muscle is innervated from a contralateral group of oculomotor (N III) motoneurons and the remaining four muscles exclusively from the ipsilateral side (N III and N VI). The inferior and posterior rectus muscles are both innervated by the abducens nerve. In contrast to all jawed vertebrates, only three eye muscles (the dorsal rectus, rostral rectus, and rostral oblique) are innervated by the oculomotor nerve in lampreys (N III). Lampreys have a motor nucleus similar to the accessory abducens nucleus previously described only in tetrapods. They lack the muscle homologous to the nasal rectus muscle of elasmobranchs and the medial rectus muscle of osteognathostomes. The distribution of the dendrites of different groups of motoneurons was studied and is considered in relation to inputs from tectum and the different cranial nerves.

Animals↗
PubMed1989

The effects of 4-aminopyridine on the cat spinal cord: rhythmic antidromic discharges recorded from the dorsal roots.

In a previous paper, we have reported that 4-aminopyridine (4-AP, i.v., 10 mg/kg) induces in decerebrate spinal and paralyzed cats, a sustained rhythmic activity (2.5-8.5 Hz) in various muscle nerves. We describe here that similar discharges are recorded from the proximal stump of cut cutaneous nerves. The latter rhythmic activity arises from intense antidromic discharges in the dorsal roots. The rhythmic discharges are recorded from dorsal roots of both spinal cord enlargements as well as from thoracic roots. The rhythmic activity is highly synchronous among adjacent dorsal roots. Bilateral activity is also highly cross-correlated, but may be dissociated by unilateral stimulation of one dorsal root. It is not yet possible to determine the precise site where the antidromic discharges recorded from the dorsal roots are generated. 4-AP could act directly at the terminal level of the primary afferents or could activate interneurons impinging upon the terminals.

4-Aminopyridine↗
PubMed1989

Unitary discharges in dorsal and ventral roots after the administration of 4-aminopyridine in the cat.

The administration of 4-aminopyridine (4-AP) in decerebrate paralyzed cats induces centrifugal rhythmic discharges in both ventral and dorsal roots. This study describes the mode of discharge of individual primary afferents as well as some ventral root fibers. Several patterns of antidromic discharge have been observed in primary afferents after the administration of 4-AP. A large proportion of the units (n = 96; 53%) showed rhythmic bursts of discharge related (n = 41) or not (n = 55) to the ongoing rhythmic activity in the peripheral nerves. Other units (n = 86; 47%) discharged either tonically, sporadically or had no antidromic activity at all. The conduction velocity of the non-bursting units was significantly higher (89.7 +/- 18.4 m/s) than that of the bursting units (70.6 +/- 15.4 m/s; P less than 0.01). Ventral roots showed rhythmic activity although less intense than that of the dorsal roots. As in dorsal roots, some fibers showed a rhythmical pattern of discharge related to the mass activity recorded from whole dorsal roots or peripheral nerves, while other units were not related. It is concluded that bursting activity which occurs in peripheral nerves after the administration of 4-AP is mainly the result of the antidromic activation of medium to small size primary afferent fibers.

4-Aminopyridine↗
PubMed1989

Phasic modulation of reticulospinal neurones during fictive locomotion and other types of spinal motor activity in lamprey.

The intracellular activity of different types of reticulospinal neurones was studied during fictive locomotion and other types of spinal motor activity in an in vitro preparation of the lamprey brainstem-spinal cord. The examined neurones included large Müller cells of the rhombencephalic and mesencephalic reticular formation, the Mauthner cell, and neurones in the posterior rhombencephalic reticular nucleus with different sizes and conduction velocities. During bouts of fictive swimming initiated spontaneously or by stimulation of the trigeminal nerve or spinal cord, the Müller cells were depolarized and fired action potentials. Bulbar Müller cells in addition showed a phasic modulation of membrane potential with excitation in phase with ipsilateral motoneurones of the rostral spinal cord. The Mauthner cell was depolarized in phase with contralateral motoneurones. Many neurones in the posterior rhombencephalic reticular nucleus showed modulation in phase with ipsilateral motoneurones during fictive swimming. Such oscillations were observed in both fast-conducting neurones, located mainly in the medial part of the nucleus, and slower conducting cells with a more lateral distribution. All examined reticulospinal neurones showed a strong coupling also with other types of spinal motor activity, such as slow alternating bursting and synchronous bilateral ventral root bursts, but the reticulospinal activity had no correlation with respiratory activity recorded from the Xth nerve. The consequences of a phasic reticulospinal activity during locomotion are discussed.

Action Potentials↗
PubMed1989

The role of spinal cord inputs in modulating the activity of reticulospinal neurons during fictive locomotion in the lamprey.

Lamprey reticulospinal neurons are rhythmically modulated during fictive swimming. The present study examines the possibility that this modulation may originate from the spinal cord locomotor networks rather than from the brainstem. To test this, the in vitro preparation of the lamprey brainstem-spinal cord was separated into two compartments which could be exposed to different chemical environments. Locomotor activity was induced pharmacologically in the caudal spinal cord compartment and reticulospinal (RS) neurons from the posterior rhombencephalic reticular nucleus (PRRN) were recorded intracellularly in the rostral compartment containing normal lamprey Ringer. Under these conditions, the membrane potential of RS neurons showed clear rhythmic oscillations which are correlated with the ongoing locomotor activity in the caudal spinal cord bath, although no locomotor discharges were present in the ventral roots of the rostral bath. Such oscillations were not present in the absence of locomotion. These results indicate that the spinal cord locomotor networks can contribute to the rhythmic oscillations which occur in RS neurons during fictive locomotion. Moreover, the latter oscillations of membrane potential are due to both phasic excitation and Cl- -dependent inhibition in the opposite phase.

Action Potentials↗
PubMed1989

Efferents to the labyrinth of the river lamprey (Lampetra fluviatilis) as revealed with retrograde tracing techniques.

The cells of origin of centrifugal fibres to the labyrinth were examined in a jawless vertebrate, the lamprey, with several retrograde tracers (horseradish peroxidase (HRP), fluorescein isothiocyanate (FITC) and rhodamine coupled dextran-amines (FDA and RDA), and cobalt-lysine) that had been applied to the eighth nerve. With all 3 techniques, up to 20 cells were consistently labelled ipsilaterally, adjacent to the Mauthner cell; none were labelled on the contralateral side. The axons of the efferent cells emanate either from lateral dendrites or the perikaryon and course laterally towards the ipsilateral eighth nerve root. The dendrites ramify widely in the basal plate and may thus receive diverse inputs. The same tracers were applied to the lateral-line nerve and afferent fibres were labelled but no efferent cells, thus confirming previous reports. These data suggest that lampreys have an efferent innervation to the labyrinth but not to the lateral line.

Animals↗
PubMed1988

Rhythmic fluctuations of dorsal root potentials and antidromic discharges of primary afferents during fictive locomotion in the cat.

1. This study examines rhythmical activity of primary afferents occurring during "fictive" locomotion in decorticate paralyzed cats. Oscillations of the dorsal root potential (DRP) at the frequency of the locomotor rhythm have been observed at the lumbosacral and cervical levels. In addition, rhythmic antidromic discharges of primary afferent units have been recorded from the proximal stumps of cut dorsal root filaments. A detailed study of the relationships between the DRP fluctuations, the antidromic discharges, and the locomotor activity monitored by recording extensor and flexor muscle nerves is presented. 2. Typical DRP recordings from both lumbosacral and cervical levels show two negative waves (N1 and N2) separated by positive troughs (P1 and P2) in each locomotor cycle. Linear regression analyses indicate that the first negative wave (which generally has the largest amplitude) is related to the flexor activity whereas the second is related to the extensor activity. The relative amplitude of the two negative waves may vary without apparent concomitant changes in the recorded flexor or extensor motor nerves. The positive troughs occur respectively close to the period of transition between flexor and extensor activities and between extensor and flexor activities. 3. DRPs of similar period and amplitude can be observed in different ipsilateral roots recorded simultaneously. The DRPs recorded bilaterally from the same segment have the same periodicity but are out-of-phase. Point-to-point variations of amplitude in bilaterally recorded roots are not correlated. This suggests that the polarization of primary afferents on one side is mainly related to the locomotor events on that side. DRPs have been recorded in cats spinalized at Th13 and injected with nialamide and l-DOPA. This suggests that although the supraspinal contribution may be important, at least part of the DRPs may result from locomotor activity within the spinal cord itself. 4. A salient finding in our experiments was that of rhythmic antidromic unit discharges in the proximal stump of cut dorsal root filaments. Of the 194 units recorded, 19% (37/194) discharged in distinct bursts occurring at fixed times in the locomotor cycle. The majority of the units discharged either one burst during the period of flexor or extensor activity or one burst during one of the two periods of transition. Three units discharged two bursts per locomotor cycle. The frequency of the antidromic discharges of some units in one limb were also found to be modulated by stimulation of the skin or passive manipulation of the limbs.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1986

The effects of 4-aminopyridine on the spinal cord: rhythmic discharges recorded from the peripheral nerves.

The effects of an intravenous injection (20 mg/kg) of 4-aminopyridine (4-AP) were initially investigated in acute low spinal cats (Th 13), in which L-DOPA had induced fictive locomotion after paralysis. 4-AP first accelerated the locomotor rhythm and could also change markedly the pattern of activation of some muscle nerves. Shortly after, the locomotor activity was replaced by synchronous rhythmic discharges (2.5-8.5 Hz) in flexor and extensor muscle nerves of the same limb girdle. Similar rhythmic activity was recorded after 4-AP alone (5-20 mg/kg) in the acute decerebrate spinal cat. Whilst the mean rate of the rhythmic activity could differ in the two limb girdles, discharges generated in one girdle appeared to be strongly influenced by those generated in the other. After a complete section of the spinal cord (Th13), the activity persisted in both the rostral and caudal segments although the interactions between the two disappeared. The persistence of the rhythmic activity caudal to the section underscores its spinal origin. In the chronic spinal rat, such rhythmic activity could still be induced in the lumbo-sacral cord despite degeneration of descending pathways. It appears that large doses of 4-AP exert potent effects on the spinal cord which can override other patterns of activity and synchronize the electrical activity of many neuronal elements.

4-Aminopyridine↗
PubMed1986

Discharge patterns of reticulospinal and other reticular neurons in chronic, unrestrained cats walking on a treadmill.

Recordings were made from single units in the medullary reticular formation (MRF) between AP-4.2 and AP-12.9 and from the midline to 3.7 mm lateral in chronically prepared, unrestrained cats walking on a treadmill. Recordings were made with rigid microelectrodes held in a microdrive, and reticulospinal neurons were identified by antidromic stimulation of their axons through microwires chronically implanted into the spinal cord at the L2 level. Electromyograms (EMGs) were recorded from flexor and extensor muscles of the fore- and hindlimbs as well as from back and neck muscles. In total, 295 cells were recorded from 40 penetrations in 4 cats; 252 of these cells were recorded from the more medial regions of the reticular formation encompassing the gigantocellular, magnocellular, and lateral tegmental fields; 38.5% of these (97/252) were antidromically identified from the spinal cord. The remaining 43 neurons (43/295) were recorded from a more lateral and ventral position. These medial and ventrolateral groups of neurons differed not only in position but also in aspects of their discharge during locomotion. Rank-ordered raster displays, triggered from the onset of each recorded muscle, were used to correlate neuronal and muscular activity. The discharge rate of 31% of the reticulospinal neurons (30/97) was modulated once or twice in each step cycle and was strictly related to one or more of the recorded EMGs (EMG-related neurons) on the basis of the pattern of discharge. The discharge of 33/97 (34%) of the neurons was modulated at the periodicity of the locomotor rhythm but could not be correlated with any of the recorded EMGs (locomotor-related cells), whereas the remaining 34/97 neurons (35%) were either silent, fired tonically, or were not related to the locomotor pattern (unrelated cells). Of the EMG-related neurons 27% were related to flexor muscles and the remaining 63% to extensor muscle activity. The discharge pattern of all except two of the flexor-related neurons was correlated with hindlimb muscle activity, whereas that of the extensor-related neurons was correlated almost equally with fore- and hindlimb muscles. Correlations were found with muscles lying both ipsilaterally and contralaterally to the site of the recordings. Although the locomotor-related neurons showed no preferential relation with any of the recorded EMGs, a comparison of the depth of modulation of their discharge measured from postevent histograms suggested that more of these cells were related to the forelimb than to the hindlimb.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Rhythmic antidromic discharges of single primary afferents recorded in cut dorsal root filaments during locomotion in the cat.

Single units recorded in the proximal stump of cut dorsal root filaments were found to antidromically discharge rhythmically during fictive locomotion in decorticate and paralyzed cats. Some units fired throughout the period of flexor or extensor nerve activity, whereas other units discharged near the transitional phases. Similar findings were made in acutely spinalized and paralyzed cats injected with L-DOPA, as well as in non-paralyzed decorticate cats walking on a treadmill. These results suggest that different types of primary afferents may be depolarized cyclically at different specific time in the step cycle by the central pattern generator for locomotion, and that this central control of the primary afferents may be involved in the modulation of the reflex transmission observed during locomotion.

Animals↗