Treadmill locomotion and aversive effects induced by electrical stimulation of the mesencephalic locomotor region in the rat

Inhibitory medial zona incerta pathway drives exploratory behavior by inhibiting glutamatergic cuneiform neurons

The cuneiform nucleus (CnF) regulates locomotor activity, which is canonically viewed as being primarily involved in initiating locomotion and regulating speed. Recent research shows greater context dependency in the locomotor functions of this nucleus. Glutamatergic neurons, which contain vesicular glutamate transporter 2 (vGLUT2), regulate context-dependent locomotor speed in the CnF and play a role in defensive behavior. Here, we identify projections from the medial zona incerta (mZI) to CnF vGLUT2 neurons that promote exploratory behavior. Using fiber photometry recordings in male mice, we find that mZI gamma-aminobutyric acid (GABA) neurons increase activity during periods of exploration. Activation of mZI GABAergic neurons is associated with reduced spiking of CnF neurons. Additionally, activating both retrogradely labeled mZI-CnF GABAergic projection neurons and their terminals in the CnF increase exploratory behavior. Inhibiting CnF vGLUT2 neuronal activity also increases exploratory behavior. These findings provide evidence for the context-dependent dynamic regulation of CnF vGLUT2 neurons, with the mZI-CnF circuit shaping exploratory behavior.

Differential Anaesthetic Effects following Microinjection of Thiopentone and Propofol into the Pons of Adult Rats: A Pilot Study

Identifying the central nervous system sites of action of anaesthetics is important for understanding the link between their molecular actions and clinical effects. The aim of the present pilot study was to compare the anaesthetic effect of bilateral microinjections of propofol and thiopentone (both 200 μg/μl, in Intralipid and 0.9% saline respectively) into a recently discovered anaesthetic-sensitive region in the rat brainstem, the “mesopontine tegmental anaesthetic area” (MPTA). Microinjections (1 μl per side) were made into the MPTA of fifteen male Sprague-Dawley rats. The effect of each agent on spontaneous behaviour, postural control and nociceptive responsiveness was subjectively assessed according to established criteria. The main finding was that thiopentone induced an “anaesthesia-like” state, including complete atonia and loss of righting ability, in 20% of the subjects. Overall, thiopentone significantly reduced postural control and had a moderate antinociceptive effect compared to saline microinjections (P<0.01 and 0.05, respectively, Wilcoxon test). In contrast, propofol did not induce “anaesthesia” in any animal tested, although a similar antinociceptive effect to that of thiopentone was observed (P<0.05, Wilcoxon test). In summary, propofol and thiopentone have different effects when microinjected into the MPTA. While both agents reduced reflex withdrawal to a nociceptive stimulus, only thiopentone induced an “anaesthesia-like” state.

Distinct Contributions of Mesencephalic Locomotor Region Nuclei to Locomotor Control in the Freely Behaving Mouse

The mesencephalic locomotor region (MLR) has been initially identified as a supraspinal center capable of initiating and modulating locomotion. Whereas its functional contribution to locomotion has been widely documented throughout the phylogeny from the lamprey to humans, there is still debate about its exact organization. Combining kinematic and electrophysiological recordings in mouse genetics, our study reveals that glutamatergic neurons of the cuneiform nucleus initiate locomotion and induce running gaits, whereas glutamatergic and cholinergic neurons of the pedunculopontine nucleus modulate locomotor pattern and rhythm, contributing to slow-walking gaits. By initiating, modulating, and accelerating locomotion, our study identifies and characterizes distinct neuronal populations of this functional region important to locomotor command.

Brainstem control of locomotion and muscle tone with special reference to the role of the mesopontine tegmentum and medullary reticulospinal systems

The lateral part of the mesopontine tegmentum contains functionally important structures involved in the control of posture and gait. Specifically, the mesencephalic locomotor region, which may consist of the cuneiform nucleus and pedunculopontine tegmental nucleus (PPN), occupies the interest with respect to the pathophysiology of posture-gait disorders. The purpose of this article is to review the mechanisms involved in the control of postural muscle tone and locomotion by the mesopontine tegmentum and the pontomedullary reticulospinal system. To make interpretation and discussion more robust, the above issue is considered largely based on our findings in the experiments using decerebrate cat preparations in addition to the results in animal experimentations and clinical investigations in other laboratories. Our investigations revealed the presence of functional topographical organizations with respect to the regulation of postural muscle tone and locomotion in both the mesopontine tegmentum and the pontomedullary reticulospinal system. These organizations were modified by neurotransmitter systems, particularly the cholinergic PPN projection to the pontine reticular formation. Because efferents from the forebrain structures as well as the cerebellum converge to the mesencephalic and pontomedullary reticular formation, changes in these organizations may be involved in the appropriate regulation of posture-gait synergy depending on the behavioral context. On the other hand, abnormal signals from the higher motor centers may produce dysfunction of the mesencephalic-reticulospinal system. Here we highlight the significance of elucidating the mechanisms of the mesencephalic-reticulospinal control of posture and locomotion so that thorough understanding of the pathophysiological mechanisms of posture-gait disorders can be made.

Evaluating the Evidence Surrounding Pontine Cholinergic Involvement in REM Sleep Generation

Rapid eye movement (REM) sleep - characterized by vivid dreaming, motor paralysis, and heightened neural activity - is one of the fundamental states of the mammalian central nervous system. Initial theories of REM sleep generation posited that induction of the state required activation of the "pontine REM sleep generator" by cholinergic inputs. Here, we review and evaluate the evidence surrounding cholinergic involvement in REM sleep generation. We submit that: (i) the capacity of pontine cholinergic neurotransmission to generate REM sleep has been firmly established by gain-of-function experiments, (ii) the function of endogenous cholinergic input to REM sleep generating sites cannot be determined by gain-of-function experiments; rather, loss-of-function studies are required, (iii) loss-of-function studies show that endogenous cholinergic input to the PTF is not required for REM sleep generation, and (iv) cholinergic input to the pontine REM sleep generating sites serve an accessory role in REM sleep generation: reinforcing non-REM-to-REM sleep transitions making them quicker and less likely to fail.

Endogenous cholinergic input to the pontine REM sleep generator is not required for REM sleep to occur

Initial theories of rapid eye movement (REM) sleep generation posited that induction of the state required activation of the pontine subceruleus (SubC) by cholinergic inputs. Although the capacity of cholinergic neurotransmission to contribute to REM sleep generation has been established, the role of cholinergic inputs in the generation of REM sleep is ultimately undetermined as the critical test of this hypothesis (local blockade of SubC acetylcholine receptors) has not been rigorously performed. We used bilateral microdialysis in freely behaving rats (n = 32), instrumented for electroencephalographic and electromyographic recording, to locally manipulate neurotransmission in the SubC with select drugs. As predicted, combined microperfusion of D-AP5 (glutamate receptor antagonist) and muscimol (GABAA receptor agonist) in the SubC virtually eliminated REM sleep. However, REM sleep was not reduced by scopolamine microperfusion in this same region, at a concentration capable of blocking the effects of cholinergic receptor stimulation. This result suggests that transmission of REM sleep drive to the SubC is acetylcholine-independent. Although SubC cholinergic inputs are not majorly involved in REM sleep generation, they may perform a minor function in the reinforcement of transitions into REM sleep, as evidenced by increases in non-REM-to-REM sleep transition duration and failure rate during cholinergic receptor blockade. Cholinergic receptor antagonism also attenuated the normal increase in hippocampal θ oscillations that characterize REM sleep. Using computational modeling, we show that our in vivo results are consistent with a mutually excitatory interaction between the SubC and cholinergic neurons where, importantly, cholinergic neuron activation is gated by SubC activity.

Basic concepts of activity-based interventions for improved recovery of motor function after spinal cord injury

Spinal cord injury (SCI) is a devastating condition that affects a large number of individuals. Historically, the recovery process after an SCI has been slow and with limited success. Recently, a number of advances have been made in the strategies used for rehabilitation, resulting in marked improved recovery, even after a complete SCI. Several rehabilitative interventions, that is, assisted motor training, spinal cord epidural stimulation, and/or administration of pharmacologic agents, alone or in combination, have produced remarkable recovery in motor function in both humans and animals. The success with each of these interventions appears to be related to the fact that the spinal cord is smart, in that it can use ensembles of sensory information to generate appropriate motor responses without input from supraspinal centers, a property commonly referred to as central pattern generation. This ability of the spinal cord reflects a level of automaticity, that is, the ability of the neural circuitry of the spinal cord to interpret complex sensory information and to make appropriate decisions to generate successful postural and locomotor tasks. Herein, we provide a brief review of some of the neurophysiologic rationale for the success of these interventions.

Local-distributed integration by a novel neuron ensures rapid initiation of animal locomotion

Animals are adapted to respond quickly to threats in their environment. In many invertebrate and some vertebrate species, the evolutionary pressures have resulted in rapidly conducting giant axons, which allow short response times. Although neural circuits mediating escape behavior are identified in several species, little attention has been paid to this behavior in the medicinal leech, a model organism whose neuronal circuits are well known. We present data that suggest an alternative to giant axons for the rapid initiation of locomotion. A novel individual neuron, cell E21, appears to be one mediator of this short-latency action in the leech. In isolated nerve cord and semi-intact preparations, cell E21 excitation initiates and extends swimming and reduces the cycle period. The soma of this cell is located caudally, but its axon extends nearly the entire length of the nerve cord. We found that cell E21 fires impulses following local sensory inputs anywhere along the body and makes excitatory synapses onto the gating cells that drive swimming behavior. These distributed input-output sites minimize the distance information travels to initiate swimming behavior, thus minimizing the latency between sensory input and motor output. We propose that this single cell E21 functions to rapidly initiate or modulate locomotion through its distributed synaptic connections.

Rapid eye movement sleep induction by microinjection of the GABA-A antagonist bicuculline into the dorsal subcoeruleus area of the rat

In cats, rapid eye movement sleep (REMS) can be induced rapidly and reliably by injections of the cholinergic agonist carbachol into the anterodorsal pontine tegmentum, also recognized as the perilocus coeruleus alpha, and designated the REMS Induction Zone (RIZ). In rats, the RIZ has been ascribed to a much larger and more ventral region within the entire oral pontine reticular formation. However, carbachol injections throughout this area produce only small, unreliable, and long latency REMS enhancements. The present study investigated whether REMS induction in the rat is possible by microinjection into the dorsal subcoeruleus nucleus (SubCD), a region with similarities to the cat RIZ. In freely moving unanaesthetized rats, microinjection of the GABA-A antagonist bicuculline significantly increased the amount and reduced the latency to REMS during a 2-h recording in the mid-light period. However, at effective doses, bicuculline usually also produced intermittent ipsiversive circling behavior that disrupted REMS maintenance. Attempts at eliminating this side-effect by: (i) coinjection of bicuculline with the NMDA antagonist, APV, (ii) lower bicuculline doses, or (iii) injection of the GABA-B antagonist, phaclofen, were unsuccessful. Other drugs injected into this area did not induce REMS; these included carbachol, the acetylcholinesterase inhibitor neostigmine, the glutamate agonist kainate, and vasopressin. In the rat, the SubCD is a highly sensitive region for both REMS induction and locomotor effects.

Brainstem regions with neuronal activity patterns correlated with priming of locomotor stepping in the anesthetized rat

Locomotor stimulation in the perifornical hypothalamus produces a transient facilitation of subsequent locomotion, a priming effect, such that stepping to a second train of stimulation occurs with a shorter latency of onset and increased amplitude. Neurons responsible for the initiation of this facilitated stepping presumably respond to locomotor stimulation with a similar priming effect, i.e. either a shorter latency or a larger change in activity rate. This study used anesthetized rats (urethane, 800mg/kg) to compare brainstem regions in terms of the relative rates of occurrence of single neurons that showed both specific responses to locomotor stimulation and also priming effects. Specific responses were characterized by a progressive increase in activity prior to the first step (a Type I pattern). In that they co-varied in time with the increased probability of stepping onset, Type I responses were more specific than Type II responses, which peaked early in the stimulation train several seconds before the onset of stepping. Regions with high proportions of neurons showing Type I responses and priming effects included the anterior dorsal tegmentum lateral to the central gray, the oral pontine reticular nucleus and the medial gigantocellular nucleus. Few Type I neurons showed a modulation of activity related to the step cycle. Type I primed neurons were uncommon in the cuneiform and the pedunculopontine regions, but neurons showing other patterns (decreases and antidromic responses) were relatively prevalent there. The ventral tegmental area was generally unresponsive. The results indicate that stepping elicited by perifornical stimulation in the anesthetized rat is mediated by circuits that differ at midbrain levels from the circuits implicated in other types of locomotion. Two regions, the anterior dorsal tegmentum and the oral pontine reticular nucleus, warrant further attention to determine their possible roles in the initiation of locomotion.

Initiation of locomotion in mammals

Several "locomotor regions" of the mammalian brain stem can be stimulated, either electrically or chemically, to induce locomotion. Active cells labeled with c-fos within the mesencephalic locomotor region (MLR) have been found in the periaqueductal gray, the cuneiform nucleus, the pedunculopontine nucleus, and the locus coeruleus. Different subsets of these nuclei appear to be activated during locomotion produced in different behavioral contexts. The locomotor nuclei can be classified into areas associated with exploratory, appetitive, and defensive locomotion, in accordance with the proposal of Sinnamon (1993, Prog. Neurobiol. 41: 323-344). The interpretation of lesion studies designed to reveal areas of the brain essential for locomotion must be based on knowledge of the nuclei which become active in the specific locomotor task being tested. An argument is put forward in favor of the continued use of the term "mesencephalic locomotor region."

Involvement of the spinoparabrachial pathway in inflammatory nociceptive processes: a c-Fos protein study in the awake rat

The effect of graded inflammatory stimuli (intraplantar-carrageenan, 0.2, 1, and 6 mg/150 microl) on paw edema and c-Fos protein expression at two levels of the spinoparabrachial pathway, the spinal cord and parabrachial area (PB), were studied. The present study, in awake rats, is an extension of previous study (Bester et al. [1997] J. Comp. Neurol. 383:439-458) which evaluated, in anesthetized rats, the effect of graded cutaneous heat stimulation on c-Fos-expression at the same levels. At the spinal level, the c-Fos-protein-like-immunoreactive (c-Fos-LI) neurons were located primarily in superficial laminae ipsilateral to intraplantar carrageenan. The number of c-Fos-LI neurons increased dose dependently (r = 0.973, n = 24) for carrageenan, from a number close to zero for the saline injection. At the PB level, c-Fos was predominantly expressed contralateral to intraplantar carrageenan. c-Fos-LI neurons were located primarily around the pontomesencephalic junction in (i) a restricted pontine area, centered in the lateral crescent, and including an adjacent part of the outer portion of the external lateral subnucleus, and (ii) the mesencephalic superior lateral subnuclei. The number of c-Fos-LI neurons in the PB area was correlated with that in the superficial laminae (r = 0.935, n = 24) and with the paw edema (r = 0.931, n = 24). No significant changes in c-Fos expression were observed in the nucleus of the solitary tract and ventrolateral medulla. The close correlation between c-Fos expression at both the spinal and PB levels and inflammatory edema provides further evidence for the involvement of spinoparabrachial pathway in inflammatory nociceptive processes. The present results are congruent with the existence of electrophysiologically demonstrated spinoparabrachio-amygdaloid and -hypothalamic nociceptive pathways.

C-Fos immunohistochemical localization of neurons in the mesencephalic locomotor region in the rat brain

The projection from the limbic system via the subpallidal region to the mesencephalic locomotor region is implicated in limbic-motor integration. The goal of this study was to visualize neurons of the mesencephalic locomotor region which are active during locomotor activity induced by the disinhibition of the subpallidal region. The subpallidal region was disinhibited by picrotoxin, which antagonizes the effects of GABA. The unilateral injection of picrotoxin into the subpallidal region caused a significant increase in locomotor activity. Active tegmental neurons were subsequently visualized by immunocytochemical staining of c-Fos protein. There were significantly more immunostained neurons in the picrotoxin-injected animals than in the saline-treated rats. Heavily stained neuronal nuclei, prevailing on the brain side ipsilateral to the injection of picrotoxin, were localized within a narrow strip of tissue which stretched from the ventrolateral periaqueductal gray (including the dorsal raphe), the cuneiform nucleus, through the region of the dorsal tegmental bundle to the pedunculopontine nucleus. There were 3.5 times more immunostained neurons in the cuneiform/pedunculopontine region and 2.5 times more stained neurons in the periaqueductal region of the picrotoxin-injected rats, as compared to the saline group. This strip of immunostained cells represents neurons which are involved in the initiation and maintenance of locomotor activity due to subpallidal activation (predominantly pedunculopontine and cuneiform nuclei), as well as neurons possibly involved in the inhibition of locomotor activity (ventrolateral periaqueductal gray) and other feedback regulations. This study will help identify the neuronal pool involved in coupling the motivational commands with the locomotor system for execution of behaviour.

Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat

The organization of efferent projections from the spinal cervical enlargement to the parabrachial (PB) area and the periaqueductal gray (PAG) was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which project in different ways to the PB area and PAG. First, the superficial laminae (I, II) showed a very dense projection, with a clear contralateral dominance at the coronal level where the inferior colliculus merges with the pons, to a restricted "superficial" portion of the PB area, namely the lateral crescent area, the dorsal lateral, the superior lateral (PBsl), and the outer portion of the external lateral PB subnuclei. Less dense projections were observed in the Kölliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subnuclei and the outer adjacent regions; in particular, no, or very little, labeling was found in the cuneiform nucleus. The PB area appeared to be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thalamus and markedly less in other sites such as the ventrolateral medulla, the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a very restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent portion of the PBsl, the KF, and the lateral quadrant of the PAG. These results suggest that signals carried by neurons from lamina I-II converge on a restricted superficial portion of the PB area and the ventral part of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in nociception.

Defensive reactions evoked by activation of NMDA receptors in distinct sites of the inferior colliculus

The inferior colliculus (IC) is primarily involved in conveying auditory information to higher cortical structures. Recently we have shown that this structure may also be part of a brain system commanding defensive behaviour. There is evidence that the neural substrates responsible for defensive behaviour in the inferior colliculus are regulated by GABAergic, serotonergic and opioid mechanisms and that these substrates may also be depressed by benzodiazepines as part of their anxiolytic action. Here we present evidence for the involvement of excitatory amino acids in the IC in the expression of defensive reactions. Microinjections of NMDA (5-40 nmol)--an excitatory amino acid--into the ventrolateral division of the central nucleus of the IC of rats placed inside a circular arena induced aversive reactions, characterized by running, rearing, and jumping. This hyperactivity was interspersed by immobility states which often progressed to convulsive seizures. These reactions were inhibited by the NMDA specific antagonist AP7 previously microinjected into the IC. It is suggested that NMDA receptor mediated mechanisms are called into play during the display of the defensive behaviour.

Wild running and switch-off behavior elicited by electrical stimulation of the inferior colliculus: effect of anticonvulsant drugs

The behavioral and motivational effects of electrical stimulation of the inferior colliculus (IC) were investigated. Electrical stimulations of either the dorsal part or ventral part of the IC both elicited wild running (WR). Nevertheless, the ventral part was found more sensitive than the dorsal part, as lower intensities were needed to elicit WR. Moreover, WR differed depending on the part of the IC stimulated. It stopped as soon as the stimulation was switched off when the ventral IC was stimulated, whereas it further persisted in a poststimulus WR when the dorsal IC was stimulated. This poststimulus WR was abolished by anticonvulsant drugs such as diazepam, phenytoin or sodium valproate. In an operant escape conditioning paradigm (switch-off test), only stimulation of the ventral IC readily sustained switch-off learning. Dorsal IC stimulations did not, possibly because of the poststimulus enduring effects of the stimulation, as evidenced by poststimulus WR. Indeed, the anticonvulsant drugs which abolished this poststimulus WR also permitted switch-off of dorsal IC stimulations. It is concluded that electrical stimulations of the IC (dorsal or ventral) elicit aversive effects and that WR elicited either by ventral or dorsal stimulation may represent the overt expression of these aversive effects.