Biomedical subjects

Ariane Ménard

Publications and source records attributed to Ariane Ménard.

6 recordsLinked to original sources

PubMed2007

Descending GABAergic projections to the mesencephalic locomotor region in the lamprey Petromyzon marinus.

The mesencephalic locomotor region (MLR) plays a significant role in the control of locomotion in all vertebrate species investigated. Forebrain neurons are likely to modulate MLR activity, but little is known about their inputs. Descending GABAergic projections to the MLR were identified by double-labeling neurons using Neurobiotin injected into the MLR combined with immunofluorescence against GABA. Several GABAergic projections to the MLR were identified in the telencephalon and diencephalon. The most abundant GABAergic projection to the MLR came from the caudal portion of the medial pallium, a region that may have similarities with the amygdala of higher vertebrates. A small population of GABAergic cells projecting to the MLR was found in the striatum and the ventral portion of the lateral pallium, which could respectively correspond to the input and output components of the basal ganglia thought to be involved in the selection of motor programs. Other GABAergic projections were found to come from the thalamus and the hypothalamus, which could take part in the motivational aspect of motor behavior in lampreys. Electrophysiological experiments were also carried out to examine the effects of GABA agonists and antagonists injected into the MLR in a semi-intact lamprey preparation. The GABA agonist inhibited locomotion, whereas the GABA antagonist initiated it. These results suggest that the GABAergic projections to the MLR modulate the activity of MLR neurons, which would be inhibited by GABA at rest.

Afferent Pathways↗

PubMed2006

Afferents of the lamprey optic tectum with special reference to the GABA input: combined tracing and immunohistochemical study.

The optic tectum in the lamprey midbrain, homologue of the superior colliculus in mammals, is important for eye movement control and orienting responses. There is, however, only limited information regarding the afferent input to the optic tectum except for that from the eyes. The objective of this study was to define specifically the gamma-aminobutyric acid (GABA)-ergic projections to the optic tectum in the river lamprey (Lampetra fluviatilis) and also to describe the tectal afferent input in general. The origin of afferents to the optic tectum was studied by using the neuronal tracer neurobiotin. Injection of neurobiotin into the optic tectum resulted in retrograde labelling of cell groups in all major subdivisions of the brain. The main areas shown to project to the optic tectum were the following: the caudoventral part of the medial pallium, the area of the ventral thalamus and dorsal thalamus, the nucleus of the posterior commissure, the torus semicircularis, the mesencephalic M5 nucleus of Schober, the mesencephalic reticular area, the ishtmic area, and the octavolateral nuclei. GABAergic projections to the optic tectum were identified by combining neurobiotin tracing and GABA immunohistochemistry. On the basis of these double-labelling experiments, it was shown that the optic tectum receives a GABAergic input from the caudoventral part of the medial pallium, the dorsal and ventral thalamus, the nucleus of M5, and the torus semicircularis. The afferent input to the optic tectum in the lamprey brain is similar to that described for other vertebrate species, which is of particular interest considering its position in phylogeny.

Afferent Pathways↗

PubMed2005

Mechanisms for selection of basic motor programs--roles for the striatum and pallidum.

The nervous system contains a toolbox of motor programs in the brainstem and spinal cord--that is, neuronal networks designed to handle the basic motor repertoire required for survival, including locomotion, posture, eye movements, breathing, chewing, swallowing and expression of emotions. The neural mechanisms responsible for selecting which motor program should be recruited at a given instant are the focus of this review. Motor programs are kept under tonic inhibition by GABAergic pallidal neurons (the output nuclei of the basal ganglia). The motor programs can be relieved from pallidal inhibition through activation of striatal neurons at the input stage of the basal ganglia. It is argued that the striatum has a prominent role in selecting which motor program should be called into action.

Animals↗

PubMed2003

Spinal cats on the treadmill: changes in load pathways.

Treadmill training and clonidine, an alpha-2 noradrenergic agonist, have been shown to improve locomotion after spinal cord injury. We speculate that transmission in load pathways, which are involved in body support during stance, is specifically modified by training. This was evaluated by comparing two groups of spinal cats; one group (n = 11) was trained to walk until full-weight-bearing (3-4 weeks), and the other (shams; n = 7) was not. During an acute experiment, changes in group I pathways, monosynaptic excitation, disynaptic inhibition, and polysynaptic excitation were investigated by measuring the response amplitude in extensor motoneurons before and after clonidine injection. Monosynaptic excitation was not modified by clonidine but was decreased significantly by training. Disynaptic inhibition was significantly decreased by clonidine in both groups, but more significantly in trained cats, and significantly reduced by training after clonidine. Also, clonidine could reverse group IB inhibition into polysynaptic excitation in both groups but more frequently in trained cats. We also investigated whether fictive stepping revealed additional changes. In trained cats, the phase-dependent modulation of all three responses was similar to patterns reported previously, but in shams, modulation of monosynaptic and polysynaptic responses was not. Overall, training appears to decrease monosynaptic excitation and enhance the effects of clonidine in the reduction of disynaptic inhibition and reversal to polysynaptic excitation. Because it is believed that polysynaptic excitatory group I pathways transmit locomotor drive to extensor motoneurons, we suggest that the latter changes would facilitate the recruitment of extensor muscles for recovering weight-bearing during stepping.

Adrenergic alpha-Agonists↗

PubMed2003

Modulation of monosynaptic transmission by presynaptic inhibition during fictive locomotion in the cat.

The effect of multisensory inputs onto the presynaptic inhibitory pathways affecting IA terminals was studied during fictive locomotion in decerebrated cats. The effect was evaluated from changes in amplitude of the monosynaptic excitatory postsynaptic potential (EPSP) measured in lumbosacral motoneurones. Responses were grouped and averaged according to their timing within the step cycle divided into five bins. Presynaptic inhibition was evoked by stimulating group I afferents from the posterior biceps-semitendinosus (PBSt) muscles and one of three cutaneous nerves: superficial peroneal (SP), sural and saphenous. Statistical analysis was applied to compare (1) EPSPs conditioned by PBSt input alone and those conditioned by the combined PBSt and cutaneous inputs, and (2) each bin dividing the step cycle to disclose phase-dependent changes. Results from 19 motoneurones showed that: (1) there was a significant phase-dependent modulation in EPSP amplitude (by 25%) with the maximum usually occurring during the depolarized phase; (2) PBSt alone reduced the EPSP amplitude (by 21%) in 3.2 bins on average; (3) combined PBSt and cutaneous stimuli further modified (up or down) the EPSP amplitude in half the trials but only in one to two bins; and (4) the most efficient cutaneous nerve (SP) usually decreased the PBSt-evoked reduction in EPSP size. Minimal changes in membrane input resistance suggest that the EPSP modifications were mostly due to presynaptic inhibition. Results indicate that muscle afferents can induce an important phase-dependent presynaptic inhibition of monosynaptic transmission and that concomitant activation of cutaneous afferents can alter this inhibition but only for a restricted part of the step cycle.

Action Potentials↗

PubMed2002

Sensory integration in presynaptic inhibitory pathways during fictive locomotion in the cat.

The aim of this study is to understand how sensory inputs of different modalities are integrated into spinal cord pathways controlling presynaptic inhibition during locomotion. Primary afferent depolarization (PAD), an estimate of presynaptic inhibition, was recorded intra-axonally in group I afferents (n = 31) from seven hindlimb muscles in L(6)-S(1) segments during fictive locomotion in the decerebrate cat. PADs were evoked by stimulating alternatively low-threshold afferents from a flexor nerve, a cutaneous nerve and a combination of both. The fictive step cycle was divided in five bins and PADs were averaged in each bin and their amplitude compared. PADs evoked by muscle stimuli alone showed a significant phase-dependent modulation in 20/31 group I afferents. In 12/20 afferents, the cutaneous stimuli alone evoked a phase-dependent modulation of primary afferent hyperpolarization (PAH, n = 9) or of PADs (n = 3). Combining the two sensory modalities showed that cutaneous volleys could significantly modify the amplitude of PADs evoked by muscle stimuli in at least one part (bin) of the step cycle in 17/31 (55%) of group I afferents. The most common effect (13/17) was a decrease in the PAD amplitude by 35% on average, whereas it was increased by 17% on average in the others (4/17). Moreover, in 8/13 afferents, the PAD reduction was obtained in 4/5 bins i.e., for most of the duration of the step cycle. These effects were seen in group I afferents from all seven muscles. On the other hand, we found that different cutaneous nerves had quite different efficacy; the superficial peroneal (SP) being the most efficient (85% of trials) followed by Saphenous (60%) and caudal sural (44%) nerves. The results indicate that cutaneous interneurons may act, in part, by modulating the transmission in PAD pathways activated by group I muscle afferents. We conclude that cutaneous input, especially from the skin area on the dorsum of the paw (SP), could subtract presynaptic inhibition in some group I afferents during perturbations of stepping (e.g., hitting an obstacle) and could thus adjust the influence of proprioceptive feedback onto motoneuronal excitability.

Animals↗