The mesencephalic locomotor region (MLR) in the rat

These studies demonstrate the presence of the MLR in the rat brain. Controlled locomotion on a treadmill could be induced by low level stimulation (< 50 μA) of an area in the posterior midbrain following a precollicular-prenigral brainstem transection. This area included the lateral part of the cuneiform nucleus and anterior as well as posterior portions of the pedunculopontine nucleus. In addition, the presence of a subthalamic locomotor region in the fields of Forel was determined in rats after prethalamic transections.     

Is the cuneiform nucleus a critical component of the mesencephalic locomotor region? An examination of the effects of excitotoxic lesions of the cuneiform nucleus on spontaneous and nucleus accumbens induced locomotion

The cuneiform nucleus and the pedunculopontine tegmental nucleus have both been suggested as possible sites for the mesencephalic locomotor region (MLR), an area from which controlled stepping on a treadmill can be elicited following electrical or chemical stimulation in a decerebrate animal. It has been shown that excitotoxic lesions of the pedunculopontine tegmental nucleus impair neither spontaneous locomotion nor locomotion induced by stimulation of the nucleus accumbens. Excitotoxic lesions of the cuneiform nucleus have not previously been investigated. Rats received either bilateral ibotenate or sham lesions of the cuneiform nucleus combined with bilateral implantation of guide cannulae aimed at the nucleus accumbens. On recovery from surgery spontaneous locomotion was tested, followed by accumbens-stimulated locomotion. For nucleus accumbens stimulation, each rat received bilateral microinjection of each of three doses of d-amphetamine (10.0, 20.0 and 30.0 μg) and a vehicle only injection. Locomotor activity was recorded following the injection. In comparison to the sham-lesioned group, the ibotenate-lesioned group showed no differences in either spontaneous or amphetamine-induced locomotor activity. These results suggest that, like the pedunculopontine tegmental nucleus, the cuneiform nucleus is not involved in the direct mediation of spontaneous or accumbens-induced locomotion, and thus is very unlikely to be the anatomical substrate of the MLR. The role of the cuneiform nucleus in other types of behavioural control is discussed.     

Effect of noradrenaline and 5-hydroxytryptamine depletion on locomotion in the cat

It has recently been hypothesized that stimulation of the mesencephalic locomotor region (MLR) can give rise to locomotion in mesencephalic cats due to activation of descending monoaminergic pathways to the spinal cord. This notion is based on the findings that monoamine agonists and precursors can induce hindlimb stepping in acute low spinal animals, and on the similarities between the effects of the noradrenaline (NA) precursor,l-DOPA, and stimulation of the MLR. The hypothesis that the descending monoamine systems comprose the only pathways which control the initiation of locomotion has been tested in the present study. NA was depleted from the CNS using intraspinal and intraventricular injections of 6-hydroxydopamine and i.v. injections of the NA synthesis inhibitor, α-methyltyrosine. Depletion of 5-hydroxytryptamine (5-HT) was achieved using intraventricular injections of 5,6-dihydroxytryptamine and i.p.p-chlorophenylalanine. These treatments did not abolish evoked locomotion in spite of substantial depletion of NA and 5-HT in the spinal cord and brain stem (maximal depletions of NA up to 14% of control in lumbar cord and 16% of control in pons; maximal depletions of 5-HT up to 19% of control in sacral cord and 25% of control in medulla). Combined depletion of NA and 5-HT did not abolish evoked locomotion in mesencephalic cats, although the treated animals displayed pronoounced ataxia prior to decerebration. Depletion of NA or 5-HT alone did not alter locomotion in otherwise intact animals. A previous report that phenoxybenzamine antagonizes the effects of MLR stimulation was not confirmed. The results therefore do not support the hypothesis that descending pathways containing monoamines are essential for locomotion evoked by brain stem stimulation.     

Activity in the mesencephalic locomotor region during locomotion

The activity of single neurons in the mesencephalic locomotor region (MLR) was recorded extracellularly in cats during spontaneous locomotion on a treadmill. Although stimulation of the MLR is required to induce locomotion on a treadmill after a precollicular-postmamillary brain stem transection in the cat, spontaneous locomotion may occur after a precollicular-premamillary transection. The activity of flexor and extensor muscles of each limb also was recorded by EMG. Nearly 50% of the MLR neurons exhibited rhythmic firing patterns during locomotion. In about one-half of those cells, unit firing patterns could be correlated with the EMG activity in one or more muscles by using spike-triggered averaging. Single MLR neurons were found to be correlated to EMG activity in a single limb, and others were related to the EMG from muscles in two limbs or in all four limbs. Passive movement or stoppage of the limb(s) did not abolish rhythmicity in these neurons. In addition, somatosensory stimulation did not appear to affect the firing patterns of MLR neurons. Averaged EMGs of correlated forelimb muscles revealed a postspike mean latency of 7.1 ms. These measurements agreed well with reports of a 1- to 1.5-ms delay in MLR projections to reticulospinal neurons and a 5- to 6-ms delay (postspike) in reticulospinal activity correlated to EMGs during locomotion. These findings suggest that (a) MLR neurons are rhythmically active during locomotion, (b) the activity of MLR neurons can be correlated with that of EMGs in one or more limbs, (c) rhythmicity in MLR neurons may be independent of phasic sensory input, and (d) the downstream influence of the MLR may be relayed, at least in part, via reticulospinal neurons.     

Autoradiographic demonstration of the projections from the mesencephalic locomotor region

An autoradiographic tracing technique was used to examine the projections of the classically defined mesencephalic locomotor region (MRL). Injections of [3H]proline and [3H]leucine were made into sites in the caudal mesencephalon which can be stimulated to produce locomotion. The injection sites were confined to the cuneiform nucleus (stereotaxic coordinates P2.0, L4.0, H-1.0). Descending projections were primarily ipsilateral to the gigantocellular and magnocellular reticular formation of the pons and medulla, the dorsal tegmental reticular nucleus, and the nucleus raphe magnus. Some sparse contralateral projections were also observed within the magnocellular and gigantocellular reticular formation. Direct axonal connections with the spinal cord were not consistently observed. Ascending projections were observed to the subthalamic nucleus, caudal hypothalamic nuclei, the centrum medianum nucleus of the thalamus, the ventral tegmental area of Tsai, the superior colliculus, and the periaqueductal gray region. The ascending projections were also ipsilateral, with sparse contralateral labeling confined to areas which received ipsilateral projections. Projections to the contralateral cuneiform nucleus were also consistently observed. The results, when compared to those of another study, suggest that the classical MLR is anatomically distinct from the more medial sites in the mesencephalon which can also induce locomotion.     

Activity of C3-C4 propriospinal neurons during fictitious forelimb locomotion in the cat

The activity of C3-C4 propriospinal neurons was recorded during fictitious forelimb locomotion in immobilized decerebrated cats with the spinal cord transected at the lower thoracic level. The discharge frequency of most neurons was rhythmically modulated in relation to the cycle of fictitious stepping in spite of the absence of any rhythmic signals from the limb receptors. Thus, the intraspinal mechanisms present a powerful input to the C3-C4 propriospinal neurons.     

Substance P-like immunoreactive fibres in the ventromedial mesencephalic tegmentum of rat

The distribution and morphology of the substance P-like immunoreactive (SP-IR) fibres and terminals in the rat ventromedial mesencephalic tegmentum (VMT) were studied using qualitative and quantitative immunohistochemical methods at light and electron microscopic levels. All five component nuclei of the VMT were examined and the size, number and density of immunoreactive terminals determined. The SP-IR fibres were distributed heterogeneously within the VMT. Under the electron microscope, SP-IR axon terminals contained both clear and dense-cored vesicles and made both symmetrical and asymmetrical synapses. The ultrastructure of the SP-IR terminals appeared to differ between nuclei. Small, clear vesicle terminals made symmetrical synaptic junctions with small calibre dendrites in the paranigral nucleus while large, clear and dense-cored vesicle terminals made asymmetrical junctions with somata and large calibre dendrites in the interfascicular nucleus. Quantitative differences between the VMT nuclei were also seen in the density of SP-IR terminals, the paranigral nucleus contained the highest density and the rostral linear nucleus the lowest. A comparison between the number of SP-IR terminals and the total number of axon terminals in the VMT reveals that the majority of all terminals in the paranigral nucleus were SP-IR, as well as the majority of axosomatic synapses in the interfascicular nucleus. These regional differences in the SP-IR innervation suggest that substance P and related peptides may perform several specific functions within the VMT and therefore have a more variable influence on this region than was previously thought.     

Chemical activation of the mesecephalic locomotor region

Electrical stimulation of the mesencephalic locomotor region (MLR) in the precollicular-postmammillary transected cat is known to induce controlled locomotion on a treadmill. We have been able to induce and block locomotion in this preparation by using localized infusions of transmitters and their agonists and antagonists. Infusions of the GABA antagonists bicuculline and picrotoxin into the MLR elicit locomotion at low concentration (5 mM). Applications of muscimol (5 mM) or GABA (0.5 M) were found to block chemically-induced locomotion, as well as electrically-elicited and spontaneous walking. Priming infusions of Diazepam amplified the blockage of locomotion by GABA. On the other hand, applications of strychnine (10 mM) were ineffective in inducing stepping, as were infusions of the excitatory agents glutamic acid, acetylcholine and norepinephrine. These findings suggest that the MLR is under inhibitory GABAergic input. The substantia nigra is the only known afferent to the MLR located posterior to the brainstem transection, and is a likely source for this input. A model is proposed to account for our results, as well as those of others, and it provides a working hypothesis for the neurochemical events occurring in brainstem centers which modulate locomotor events.     

Extra- and intracellular recordings from dorsal column postsynaptic spinomedullary neurons in the cat

Dorsal column postsynaptic (DCPS) spinomedullary neurons in the dorsal horn of spinal segments L6-S1 of adult cats anesthetized with sodium pentobarbital were identified by antidromic stimulation of cervical dorsal columns that were dissected free of, and electrically isolated from, the rest of the spinal cord. The neurons were categorized with respect to natural stimulation of their cutaneous receptive fields. An equal number of low-threshold mechanoreceptive and wide-dynamic-range neurons were found. No DCPS neurons could be classified as nociceptive-specific. All neurons received input from low-threshold mechanoreceptors with myelinated axons. There was no evidence that any neurons received monosynaptic input from unmyelinated, primary afferent fibers. The average conduction velocity of the antidromic responses was 45.7 m/s. Nearly half of the DCPS cells showed an antidromic spike followed by synaptically driven responses that were probably evoked by antidromic invasion into the intraspinal collaterals of A-beta primary afferent fibers that ascended the dorsal columns. Intracellularly recorded synaptic responses of DCPS neurons to dorsal column and receptive field stimulation usually consisted of an EPSP with overriding spike potentials followed by a prolonged IPSP whose amplitude decreased markedly as the stimulus frequency was increased in the range of 5 to 30 Hz. The results indicate that DCPS neurons constitute a projection system capable of signaling innocuous and tissue-damaging mechanical stimuli. The DCPS projection may play a role in the modulation of touch and pain perception.     

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

Extracellular recordings were made from 52 units in perifused 300 microns thick slices of rat hypothalamus taken parallel to the ventricular surface of the arcuate nucleus and kept at 37 degrees C in an organ bath. Addition of 8 nM adrenocorticotrophic hormone (ACTH) to the perfusate reversibly and rapidly inhibited 15 of the 38 units found within 300 microns of the ventricular surface (in the arcuate nucleus), and there were no comparable excitations. A prompt and reversible excitation of 6/14 units by ACTH was found when records were made from units found 300-600 microns lateral to the ventricle in the lateral arcuate region. It is suggested that ACTH may be an inhibitory transmitter in the arcuate nucleus and that this action may be an inhibition of ACTH-containing neurons.     

Initiation of locomotion from the mesencephalic locomotor region: Effects of selective brainstem lesions

The effects of selected brainstem lesions on controlled treadmill locomotion produced by stimulation of the mesencephalic locomotor region (MLR) in postmamillary cats were determined in these experiments. The importance for the initiation of locomotion of projections from the MLR to rostral brainstem structures, described in a preceding paper, were examined by selective lesioning or by adjusting the level of the decerebration. The role played by the lateral vestibulospinal tract (LVST) in the initiation of locomotion was examined by lesioning Deiters' nucleus bilaterally. Contrary to previous claims, the results of the present experiments show that areas of the brainstem rostral to the MLR are not required for the initiation of locomotion by MLR stimulation. This finding eliminates the ventral tegmental area of Tsai and the substantia nigra, both implicated in the initiation of locomotion, as required participants in MLR stimulated locomotion. Bilateral Deiters' nucleus (DN) lesions did not significantly affect the initiation of locomotion from the MLR, nor did such lesions alter in a systematic fashion the amplitude or timing of EMG activity in flexor or extensor muscles of the hindlimb during MLR evoked walking. Joint angle changes during the locomotor cycle were also essentially unaltered by DN lesions. The significance of these findings regarding the brainstem structures which must be involved in the initiation of locomotion are discussed.     

Association of the mesencephalic locomotor region with locomotor activity induced by injections of amphetamine into the nucleus accumbens

Injections of amphetamine into the nucleus accumbens increased locomotor activity of rats. Subsequent injections of procaine into the midbrain, in the region of the pedunculopontine nucleus, significantly reduced the amphetamine-induced locomotor activity. Control experiments showed that procaine injections into the contralateral pedunculopontine nucleus had little or no effect, as well as ipsilateral injections dorsal and ventral to the pedunculopontine nucleus. These findings suggest that release of dopamine from amphetamine injections into the accumbens gives rise to ipsilateral descending influences on the region of the pedunculopontine nucleus, a major component of the mesencephalic locomotor region. Descending influences from the nucleus accumbens to mesencephalic locomotor region may serve as a link for limbic-motor integration in behavioral response initiation.     

Parallel suppression of extensor muscle tone and respiration by stimulation of pontine dorsal tegmentum in decerebrate cat

This paper describes the pontine brainstem area responsible for the suppression of postural muscle tone as well as of respiration in acute precollicular-postmammillary decerebrate (mesencephalic) cats. Stimulation of the dorsal part of the pontine tegmentum (DTF) along the midline (P4-P7, H-5 to H-6) decreased the bilateral tone of the hindlimb extensor muscles and the diaphragmatic activity. Tonic discharges of the extensor muscles were suppressed by DTF stimulation and the suppression of muscle activity continued for more than 5 min after termination of the stimulation. In contrast, the suppression of the diaphragmatic activity, which resulted in apnea in some of the animals tested, resumed in spite of the continuation of the stimulation. However, the rebound augmentation of the diaphragmatic activity appeared immediately after the termination of the stimulation. The existence of such a rebound phenomenon suggested that the suppressive effects on the diaphragmatic activity persisted during the entire period of the stimulation. The recovery of respiratory movements during the stimulation led us to suggest that the strong respiratory drives emerge to overcome the exerted DTF-elicited suppressive effects on respiration. In the paralyzed and vagotomized animal, the DTF-elicited suppressive effects on phrenic neural discharges were minimal when the end-tidal pCO2 was set at a higher level than during spontaneous breathing.     

Anatomical study of spinobulbar neurons in lampreys

The present study was aimed at identifying spinal neurons ascending to the brainstem outside the dorsal columns in the lamprey. Two retrograde tracers (cobalt-lysine and horseradish peroxidase [HRP]) were injected in the brainstem or rostral spinal cord in vivo or in vitro. Labeled cells were distributed bilaterally with a contralateral dominance, along the whole rostrocaudal extent of the spinal cord. The density of cells markedly decreased rostrocaudally. Several classes of brainstem-projecting neurons were identified. Most cells with a short axon were small and formed columns, in the dorsolateral and ventrolateral gray matter, at the transition between the rhombencephalon and the spinal cord. Dorsal elongated cells were spindle shaped, located medially, in the first two spinal segments. Lateral elongated cells were medium to large size neurons, located in the intermediate and lateral gray matter, mainly contralateral to the injection site. Their axon emerging from the lateral part of the soma crossed the midline, ventral to the central canal. These cells were present throughout the rostral spinal cord. Cells were also labeled in the lateral white matter. Some of them had the typical dendritic arborizations of edge cells (intraspinal stretch receptor neurons) and were located in the most rostral segments, bilaterally. Other medium to large size neurons were identified dorsal and medial to most of the edge cells. We suggest that at least the group of lateral elongated cells exhibits rhythmic membrane potential oscillations during fictive locomotion. These cells may, together with the rostral edge cells, be responsible for the locomotor-related modulation of activity in reticulospinal and vestibulospinal neurons. J. Comp. Neurol. 397:475–492, 1998. © 1998 Wiley-Liss, Inc.     

Connections of the mesencephalic locomotor region (MLR) I. Substantia nigra afferents

These studies were designed to determine whether or not substantia nigra (SN) neurons project to the mesencephalic locomotor region (MLR). An attempt was made to activate SN neurons antidromically from the same site which induced locomotion on a treadmill following a precollicular-postmammillary transection in the same animal. Less than 10% of posterior SN neurons were activated antidromically from the physiologically-identified MLR. These results support previous anatomical findings describing a sparse projection from the SN to the MLR [5]. Locomotion on a treadmill was elicited at low current strengths (20–50 μA) from an area around the cuneiform nucleus in the posterior mesencephalon. This area included the lateral central gray, mesencephalic trigeminal root, dorsal brachium conjunctivum and nucleus tegmenti pedunculopontinus (NTPP) and perhaps anterodorsal locus coeruleus. Stimulation of the area just described induced a complete stepping cycle with a flexion phase and a three-part extension phase. Stimulation of the posterior SN produced spastic locomotion on a treadmill at higher current strengths (70 μA) in cats with a precollicular-postmammillary transection.     

A neuronal substrate for a state‐dependent modulation of sensory inputs in the brainstem

Central networks modulate sensory transmission during motor behavior. Sensory inputs may thus have distinct impacts according to the state of activity of the central networks. Using an in‐vitro isolated lamprey brainstem preparation, we investigated whether a brainstem locomotor center, the mesencephalic locomotor region (MLR), modulates sensory transmission. The synaptic responses of brainstem reticulospinal (RS) cells to electrical stimulation of the sensory trigeminal nerve were recorded before and after electrical stimulation of the MLR. The RS cell synaptic responses were significantly reduced by MLR stimulation and the reduction of the response increased with the stimulation intensity of the MLR. Bath perfusion of atropine prevented the depression of sensory transmission, indicating that muscarinic receptor activation is involved. Previous studies have shown that, upon stimulation of the MLR, behavioral activity switches from a resting state to an active‐locomotor state. Therefore, our results suggest that a state‐dependent modulation of sensory transmission to RS cells occurs in the behavioral context of locomotion and that muscarinic inputs from the MLR are involved.     

Anatomical and physiological study of brainstem nuclei relaying dorsal column inputs in lampreys

The course and sites of termination of dorsal column fibres in the lamprey brainstem are described along with their brainstem relays projecting to reticulospinal neurons. Dorsal column fibres ascend to the brainstem level where they intermingle with cells located in the alar plate close to the obex, a location that is analogous to that of the dorsal column nucleus in other vertebrates. Some dorsal column fibres continue further rostrally where they reach the octavolateralis and octavomotorii nuclei. Finally, a small contingent of fibres reach the cerebellum. Injections of cobalt-lysine into the posterior rhombencephalic reticular nucleus retrogradely label neurons within the dorsal column nucleus and within the octavolateralis and octavomotorii nuclei. Microstimulation of the dorsal column nucleus on either side elicits monosynaptic inhibitory responses in reticulospinal neurons while stimulation of octavolateralis and octavomotorius nuclei elicits excitation. By using intracellular recordings, it was shown that neurons within these alar plate nuclei receive monosynaptic inputs from the dorsal columns. It is thus proposed that disynaptic inputs from dorsal columns to reticulospinal neurons are relayed by these alar plate neurons: inhibition is relayed mainly by neurons in dorsal column nuclei and excitation by neurons in the octavolateralis and octavomotorii nuclei. © 1993 Wiley-Liss, Inc.     

Ventral mesencephalic tegmentum (VMT) controls electrocortical beta rhythms and associated attentive behaviour in the cat

When a cat is immobile, very alert and displaying behaviour suggesting focused attention toward a target in its environment, beta rhythms (ca. 40 Hz) develop in the fronto-parietal cortical areas. After bilateral electrolytic lesions of the ventral mesencephalic tegmentum (VMT), these beta rhythms are suppressed (while other cortical activities, with other behavioural correlates, persist), and at the same time, attentive immobility is no longer observed: the same experimental situation as in the control now elicits locomotor hyperactivity. Arguments are produced, favouring the hypothesis that both behavioural immobility and the accompanying thalamocortical beta rhythms are controlled through one of the dopaminergic system that originate from the VMT and are distinct from the nigrostriatal one.     

c-Fos expression in dopaminergic and GABAergic neurons of the ventral mesencephalic tegmentum after paradoxical sleep deprivation and recovery

Evidence suggests that dopaminergic neurons of the ventral mesencephalic tegmentum (VMT) could be important for paradoxical sleep (PS). Here, we examined whether dopamine (DA) and adjacent gamma-aminobutyric acid (GABA)-synthesizing neurons are active in association with PS recovery as compared to PS deprivation or control conditions in different groups of rats by using c-Fos expression as a reflection of neural activity, combined with dual immunostaining for tyrosine hydroxylase (TH) or glutamic acid decarboxylase (GAD). Numbers of TH+/c-Fos+ neurons in the substantia nigra (SN) were not significantly different across groups, whereas those in the ventral tegmental area (VTA) were significantly different and greatest in PS recovery. Numbers of GAD+/c-Fos+ neurons in both VTA and SN were greatest in PS recovery. Thus, DA neuronal activity does not appear to be suppressed by local GABAergic neuronal activity during PS but might be altered in pattern by this inhibitory as well as other excitatory, particularly cholinergic, inputs such as to allow DA VTA neurons to become maximally active during PS and thereby contribute to the unique physiological and cognitive aspects of that state.