Forebrain dopamine neurons project down to a brainstem region controlling locomotion

Olfactory Projections to Locomotor Control Centers in the Sea Lamprey

Although olfaction is well known to guide animal behavior, the neural circuits underlying the motor responses elicited by olfactory inputs are poorly understood. In the sea lamprey, anatomical evidence shows that olfactory inputs project to the posterior tuberculum (PT), a structure containing dopaminergic (DA) neurons homologous to the mammalian ventral tegmental area and the substantia nigra pars compacta. Olfactory inputs travel directly from the medial olfactory bulb (medOB) or indirectly through the main olfactory bulb and the lateral pallium (LPal). Here, we characterized the transmission of olfactory inputs to the PT in the sea lamprey, Petromyzon marinus. Abundant projections from the medOB were observed close to DA neurons of the PT. Moreover, electrophysiological experiments revealed that PT neurons are activated by both the medOB and LPal, and calcium imaging indicated that the olfactory signal is then relayed to the mesencephalic locomotor region to initiate locomotion. In semi-intact preparations, stimulation of the medOB and LPal induced locomotion that was tightly associated with neural activity in the PT. Moreover, PT neurons were active throughout spontaneously occurring locomotor bouts. Altogether, our observations suggest that the medOB and LPal convey olfactory inputs to DA neurons of the PT, which in turn activate the brainstem motor command system to elicit locomotion.

Dopamine-sensitive neurons in the mesencephalic locomotor region control locomotion initiation, stop, and turns

The locomotor role of dopaminergic neurons is traditionally attributed to their ascending projections to the basal ganglia, which project to the mesencephalic locomotor region (MLR). In addition, descending dopaminergic projections to the MLR are present from basal vertebrates to mammals. However, the neurons targeted in the MLR and their behavioral role are unknown in mammals. Here, we identify genetically defined MLR cells that express D1 or D2 receptors and control different motor behaviors in mice. In the cuneiform nucleus, D1-expressing neurons promote locomotion, while D2-expressing neurons stop locomotion. In the pedunculopontine nucleus, D1-expressing neurons promote locomotion, while D2-expressing neurons evoke ipsilateral turns. Using RNAscope, we show that MLR dopamine-sensitive neurons comprise a combination of glutamatergic, GABAergic, and cholinergic neurons, suggesting that different neurotransmitter-based cell types work together to control distinct behavioral modules. Altogether, our study uncovers behaviorally relevant cell types in the mammalian MLR based on the expression of dopaminergic receptors.

Grid cells: the missing link in understanding Parkinson's disease?

The mechanisms underlying Parkinson's disease (PD) are complex and not fully understood, and the box-and-arrow model among other current models present significant challenges. This paper explores the potential role of the allocentric brain and especially its grid cells in several PD motor symptoms, including bradykinesia, kinesia paradoxa, freezing of gait, the bottleneck phenomenon, and their dependency on cueing. It is argued that central hubs, like the locus coeruleus and the pedunculopontine nucleus, often narrowly interpreted in the context of PD, play an equally important role in governing the allocentric brain as the basal ganglia. Consequently, the motor and secondary motor (e.g., spatially related) symptoms of PD linked with dopamine depletion may be more closely tied to erroneous computation by grid cells than to the basal ganglia alone. Because grid cells and their associated central hubs introduce both spatial and temporal information to the brain influencing velocity perception they may cause bradykinesia or hyperkinesia as well. In summary, PD motor symptoms may primarily be an allocentric disturbance resulting from virtual faulty computation by grid cells revealed by dopamine depletion in PD.

Revisiting the two rhythm generators for respiration in lampreys

In lampreys, respiration consists of a fast and a slow rhythm. This study was aimed at characterizing both anatomically and physiologically the brainstem regions involved in generating the two rhythms. The fast rhythm generator has been located by us and others in the rostral hindbrain, rostro-lateral to the trigeminal motor nucleus. More recently, this was challenged by researchers reporting that the fast rhythm generator was located more rostrally and dorsomedially, in a region corresponding to the mesencephalic locomotor region. These contradictory observations made us re-examine the location of the fast rhythm generator using anatomical lesions and physiological recordings. We now confirm that the fast respiratory rhythm generator is in the rostro-lateral hindbrain as originally described. The slow rhythm generator has received less attention. Previous studies suggested that it was composed of bilateral, interconnected rhythm generating regions located in the caudal hindbrain, with ascending projections to the fast rhythm generator. We used anatomical and physiological approaches to locate neurons that could be part of this slow rhythm generator. Combinations of unilateral injections of anatomical tracers, one in the fast rhythm generator area and another in the lateral tegmentum of the caudal hindbrain, were performed to label candidate neurons on the non-injected side of the lateral tegmentum. We found a population of neurons extending from the facial to the caudal vagal motor nuclei, with no clear clustering in the cell distribution. We examined the effects of stimulating different portions of the labeled population on the respiratory activity. The rostro-caudal extent of the population was arbitrarily divided in three portions that were each stimulated electrically or chemically. Stimulation of either of the three sites triggered bursts of discharge characteristic of the slow rhythm, whereas inactivating any of them stopped the slow rhythm. Substance P injected locally in the lateral tegmentum accelerated the slow respiratory rhythm in a caudal hindbrain preparation. Our results show that the fast respiratory rhythm generator consists mostly of a population of neurons rostro-lateral to the trigeminal motor nucleus, whereas the slow rhythm generator is distributed in the lateral tegmentum of the caudal hindbrain.

The Basal Ganglia and Mesencephalic Locomotor Region Connectivity Matrix

Although classically considered a relay station for basal ganglia (BG) output, the anatomy, connectivity, and function of the mesencephalic locomotor region (MLR) were redefined during the last two decades. In striking opposition to what was initially thought, MLR and BG are actually reciprocally and intimately interconnected. New viral-based, optogenetic, and mapping technologies revealed that cholinergic, glutamatergic, and GABAergic neurons coexist in this structure, which, in addition to extending descending projections, send long-range ascending fibers to the BG. These MLR projections to the BG convey motor and non-motor information to specific synaptic targets throughout different nuclei. Moreover, MLR efferent fibers originate from precise neuronal subpopulations located in particular MLR subregions, defining independent anatomo-functional subcircuits involved in particular aspects of animal behavior such as fast locomotion, explorative locomotion, posture, forelimb- related movements, speed, reinforcement, among others. In this review, we revised the literature produced during the last decade linking MLR and BG. We conclude that the classic framework considering the MLR as a homogeneous output structure passively receiving input from the BG needs to be revisited. We propose instead that the multiple subcircuits embedded in this region should be taken as independent entities that convey relevant and specific ascending information to the BG and, thus, actively participate in the execution and tuning of behavior.

Dopamine control of downstream motor centers

The role of dopamine in the control of movement is traditionally associated with ascending projections to the basal ganglia. However, more recently descending dopaminergic pathways projecting to downstream brainstem motor circuits were discovered. In lampreys, salamanders, and rodents, these include projections to the downstream Mesencephalic Locomotor Region (MLR), a brainstem region controlling locomotion. Such descending dopaminergic projections could prime brainstem networks controlling movement. Other descending dopaminergic projections have been shown to reach reticulospinal cells involved in the control of locomotion. In addition, dopamine directly modulates the activity of interneurons and motoneurons. Beyond locomotion, dopaminergic inputs modulate visuomotor transformations within the optic tectum (mammalian superior colliculus). Loss of descending dopaminergic inputs will likely contribute to pathological conditions such as in Parkinson's disease.

Locomotor pattern generation and descending control: a historical perspective

The ability to generate and control locomotor movements depends on complex interactions between many areas of the nervous system, the musculoskeletal system, and the environment. How the nervous system manages to accomplish this task has been the subject of investigation for more than a century. In vertebrates, locomotion is generated by neural networks located in the spinal cord referred to as central pattern generators. Descending inputs from the brain stem initiate, maintain, and stop locomotion as well as control speed and direction. Sensory inputs adapt locomotor programs to the environmental conditions. This review presents a comparative and historical overview of some of the neural mechanisms underlying the control of locomotion in vertebrates. We have put an emphasis on spinal mechanisms and descending control.

Brainstem neural mechanisms controlling locomotion with special reference to basal vertebrates

Over the last 60 years, the basic neural circuitry responsible for the supraspinal control of locomotion has progressively been uncovered. Initially, significant progress was made in identifying the different supraspinal structures controlling locomotion in mammals as well as some of the underlying mechanisms. It became clear, however, that the complexity of the mammalian central nervous system (CNS) prevented researchers from characterizing the detailed cellular mechanisms involved and that animal models with a simpler nervous system were needed. Basal vertebrate species such as lampreys, xenopus embryos, and zebrafish became models of choice. More recently, optogenetic approaches have considerably revived interest in mammalian models. The mesencephalic locomotor region (MLR) is an important brainstem region known to control locomotion in all vertebrate species examined to date. It controls locomotion through intermediary cells in the hindbrain, the reticulospinal neurons (RSNs). The MLR comprises populations of cholinergic and glutamatergic neurons and their specific contribution to the control of locomotion is not fully resolved yet. Moreover, the downward projections from the MLR to RSNs is still not fully understood. Reporting on discoveries made in different animal models, this review article focuses on the MLR, its projections to RSNs, and the contribution of these neural elements to the control of locomotion. Excellent and detailed reviews on the brainstem control of locomotion have been recently published with emphasis on mammalian species. The present review article focuses on findings made in basal vertebrates such as the lamprey, to help direct new research in mammals, including humans.

Effect of selective cyclooxygenase inhibitors on animal behaviour and monoaminergic systems of the rat brain

The long-term effects of cyclooxygenase 1 and 2 (COX-1/2) inhibitors are usually tested in terms of the periphery of the organism. Therefore, we studied the effects of SC560 (selective COX-1 inhibitor) and celecoxib (selective COX-2 inhibitor) on the activity of brain monoaminergic systems and animal behaviour. Additionally, we tested the effect of these inhibitors during inflammation. We have observed that long-term administration of celecoxib reduces the activity of the noradrenergic system, increases the activity of dopaminergic and serotonergic systems, increases locomotor activity, and enhances the exploratory behaviour of rats. Administration of SC560 also increases the activity of dopaminergic and serotonergic systems but reduces locomotor activity and impairs the exploratory behaviour of rats. The mechanism responsible for decreased activity of the noradrenergic system may be related to the weakening of activity of the positive feedback loop between the paraventricular nucleus and coeruleus locus. We suggest that the effect of used inhibitors on the dopaminergic system is associated with a possible increase in anandamide concentration and its effect on dopamine reuptake in synaptic clefts. It also appears that cyclooxygenase peroxidase activity may play a role in this process. In turn, changes in the activity of the serotonergic system may be related to the activity of indoleamine-2,3-dioxygenase, which decreases because of the decreased concentration of pro-inflammatory compounds. We believe that behavioural changes induced by COX inhibitors are the result of the modified activity of monoaminergic CNS systems in the brainstem, hypothalamus, and medial prefrontal cortex.

Cholinergic Modulation of Locomotor Circuits in Vertebrates

Locomotion is a basic motor act essential for survival. Amongst other things, it allows animals to move in their environment to seek food, escape predators, or seek mates for reproduction. The neural mechanisms involved in the control of locomotion have been examined in many vertebrate species and a clearer picture is progressively emerging. The basic muscle synergies responsible for propulsion are generated by neural networks located in the spinal cord. In turn, descending supraspinal inputs are responsible for starting, maintaining, and stopping locomotion as well as for steering and controlling speed. Several neurotransmitter systems play a crucial role in modulating the neural activity during locomotion. For instance, cholinergic inputs act both at the spinal and supraspinal levels and the underlying mechanisms are the focus of the present review. Much information gained on supraspinal cholinergic modulation of locomotion was obtained from the lamprey model. Nicotinic cholinergic inputs increase the level of excitation of brainstem descending command neurons, the reticulospinal neurons (RSNs), whereas muscarinic inputs activate a select group of hindbrain neurons that project to the RSNs to boost their level of excitation. Muscarinic inputs also reduce the transmission of sensory inputs in the brainstem, a phenomenon that could help in sustaining goal directed locomotion. In the spinal cord, intrinsic cholinergic inputs strongly modulate the activity of interneurons and motoneurons to control the locomotor output. Altogether, the present review underlines the importance of the cholinergic inputs in the modulation of locomotor activity in vertebrates.

From retina to motoneurons: A substrate for visuomotor transformation in salamanders

The transformation of visual input into motor output is essential to approach a target or avoid a predator. In salamanders, visually guided orientation behaviors have been extensively studied during prey capture. However, the neural circuitry involved is not resolved. Using salamander brain preparations, calcium imaging and tracing experiments, we describe a neural substrate through which retinal input is transformed into spinal motor output. We found that retina stimulation evoked responses in reticulospinal neurons of the middle reticular nucleus, known to control steering movements in salamanders. Microinjection of glutamatergic antagonists in the optic tectum (superior colliculus in mammals) decreased the reticulospinal responses. Using tracing, we found that retina projected to the dorsal layers of the contralateral tectum, where the dendrites of neurons projecting to the middle reticular nucleus were located. In slices, stimulation of the tectal dorsal layers evoked glutamatergic responses in deep tectal neurons retrogradely labeled from the middle reticular nucleus. We then examined how tectum activation translated into spinal motor output. Tectum stimulation evoked motoneuronal responses, which were decreased by microinjections of glutamatergic antagonists in the contralateral middle reticular nucleus. Reticulospinal fibers anterogradely labeled from tracer injection in the middle reticular nucleus were preferentially distributed in proximity with the dendrites of ipsilateral motoneurons. Our work establishes a neural substrate linking visual and motor centers in salamanders. This retino‐tecto‐reticulo‐spinal circuitry is well positioned to control orienting behaviors. Our study bridges the gap between the behavioral studies and the neural mechanisms involved in the transformation of visual input into motor output in salamanders.

The Mesencephalic Locomotor Region: Beyond Locomotor Control

The mesencephalic locomotor region (MLR) was discovered several decades ago in the cat. It was functionally defined based on the ability of low threshold electrical stimuli within a region comprising the cuneiform and pedunculopontine nucleus to evoke locomotion. Since then, similar regions have been found in diverse vertebrate species, including the lamprey, skate, rodent, pig, monkey, and human. The MLR, while often viewed under the lens of locomotion, is involved in diverse processes involving the autonomic nervous system, respiratory system, and the state-dependent activation of motor systems. This review will discuss the pedunculopontine nucleus and cuneiform nucleus that comprises the MLR and examine their respective connectomes from both an anatomical and functional angle. From a functional perspective, the MLR primes the cardiovascular and respiratory systems before the locomotor activity occurs. Inputs from a variety of higher structures, and direct outputs to the monoaminergic nuclei, allow the MLR to be able to respond appropriately to state-dependent locomotion. These state-dependent effects are roughly divided into escape and exploratory behavior, and the MLR also can reinforce the selection of these locomotor behaviors through projections to adjacent structures such as the periaqueductal gray or to limbic and cortical regions. Findings from the rat, mouse, pig, and cat will be discussed to highlight similarities and differences among diverse species.

The neural bases of vertebrate motor behaviour through the lens of evolution

The primary driver of the evolution of the vertebrate nervous system has been the necessity to move, along with the requirement of controlling the plethora of motor behavioural repertoires seen among the vast and diverse vertebrate species. Understanding the neural basis of motor control through the perspective of evolution, mandates thorough examinations of the nervous systems of species in critical phylogenetic positions. We present here, a broad review of studies on the neural motor infrastructure of the lamprey, a basal and ancient vertebrate, which enjoys a unique phylogenetic position as being an extant representative of the earliest group of vertebrates. From the central pattern generators in the spinal cord to the microcircuits of the pallial cortex, work on the lamprey brain over the years, has provided detailed insights into the basic organization (a bauplan) of the ancestral vertebrate brain, and narrates a compelling account of common ancestry of fundamental aspects of the neural bases for motion control, maintained through half a billion years of vertebrate evolution. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

Optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus controls locomotion in a mouse model of Parkinson's disease

In Parkinson's disease (PD), the loss of midbrain dopaminergic cells results in severe locomotor deficits, such as gait freezing and akinesia. Growing evidence indicates that these deficits can be attributed to the decreased activity in the mesencephalic locomotor region (MLR), a brainstem region controlling locomotion. Clinicians are exploring the deep brain stimulation of the MLR as a treatment option to improve locomotor function. The results are variable, from modest to promising. However, within the MLR, clinicians have targeted the pedunculopontine nucleus exclusively, while leaving the cuneiform nucleus unexplored. To our knowledge, the effects of cuneiform nucleus stimulation have never been determined in parkinsonian conditions in any animal model. Here, we addressed this issue in a mouse model of PD, based on the bilateral striatal injection of 6-hydroxydopamine, which damaged the nigrostriatal pathway and decreased locomotor activity. We show that selective optogenetic stimulation of glutamatergic neurons in the cuneiform nucleus in mice expressing channelrhodopsin in a Cre-dependent manner in Vglut2-positive neurons (Vglut2-ChR2-EYFP mice) increased the number of locomotor initiations, increased the time spent in locomotion, and controlled locomotor speed. Using deep learning-based movement analysis, we found that the limb kinematics of optogenetic-evoked locomotion in pathological conditions were largely similar to those recorded in intact animals. Our work identifies the glutamatergic neurons of the cuneiform nucleus as a potentially clinically relevant target to improve locomotor activity in parkinsonian conditions. Our study should open avenues to develop the targeted stimulation of these neurons using deep brain stimulation, pharmacotherapy, or optogenetics.

Olfactory-induced locomotion in lampreys

The olfactory system allows animals to navigate in their environment to feed, mate, and escape predators. It is well established that odorant exposure or electrical stimulation of the olfactory system induces stereotyped motor responses in fishes. However, the neural circuitry responsible for the olfactomotor transformations is only beginning to be unraveled. A neural substrate eliciting motor responses to olfactory inputs was identified in the lamprey, a basal vertebrate used extensively to examine the neural mechanisms underlying sensorimotor transformations. Two pathways were discovered from the olfactory organ in the periphery to the brainstem motor nuclei responsible for controlling swimming. The first pathway originates from sensory neurons located in the accessory olfactory organ and reaches a single population of projection neurons in the medial olfactory bulb, which, in turn, transmit the olfactory signals to the posterior tuberculum and then to downstream brainstem locomotor centers. A second pathway originates from the main olfactory epithelium and reaches the main olfactory bulb, the neurons of which project to the pallium/cortex. The olfactory signals are then conveyed to the posterior tuberculum and then to brainstem locomotor centers. Olfactomotor behavior can adapt, and studies were aimed at defining the underlying neural mechanisms. Modulation of bulbar neural activity by GABAergic, dopaminergic, and serotoninergic inputs is likely to provide strong control over the hardwired circuits to produce appropriate motor behavior in response to olfactory cues. This review summarizes current knowledge relative to the neural circuitry producing olfactomotor behavior in lampreys and their modulatory mechanisms.

The Dopaminergic Control of Movement-Evolutionary Considerations

Dopamine is likely the most studied modulatory neurotransmitter, in great part due to characteristic motor deficits in Parkinson's disease that arise after the degeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNc). The SNc, together with the ventral tegmental area (VTA), play a key role modulating motor responses through the basal ganglia. In contrast to the large amount of existing literature addressing the mammalian dopaminergic system, comparatively little is known in other vertebrate groups. However, in the last several years, numerous studies have been carried out in basal vertebrates, allowing a better understanding of the evolution of the dopaminergic system, especially the SNc/VTA. We provide an overview of existing research in basal vertebrates, mainly focusing on lampreys, belonging to the oldest group of extant vertebrates. The lamprey dopaminergic system and its role in modulating motor responses have been characterized in significant detail, both anatomically and functionally, providing the basis for understanding the evolution of the SNc/VTA in vertebrates. When considered alongside results from other early vertebrates, data in lampreys show that the key role of the SNc/VTA dopaminergic neurons modulating motor responses through the basal ganglia was already well developed early in vertebrate evolution.

L-DOPA and 5-HTP modulation of air-stepping in newborn rats

KEY POINTS

In newborn rats, L-DOPA increases the occurrence of air-stepping activity without affecting movement characteristics. L-DOPA administration increases the spinal content of dopamine in a dose-dependent manner. Injection of 5-HTP increases the spinal serotonin content but does not trigger air-stepping. 5-HTP counteracts the pro-locomotor action of L-DOPA. Less dopamine and serotonin are synthesized when L-DOPA and 5-HTP are administered as a cocktail.

ABSTRACT

The catecholamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), is a well-established pharmacological agent for promoting locomotor action in vertebrates, including triggering air-stepping activities in the neonatal rat. Serotonin is also a well-known neuromodulator of the rodent spinal locomotor networks. Here, using kinematic analysis, we compared locomotor-related activities expressed by newborn rats in response to varying doses of L-DOPA and the serotonin precursor 5-hydroxytryptophan (5-HTP) administered separately or in combination. L-DOPA alone triggered episodes of air-stepping in a dose-dependent manner (25-100 mg/kg), notably determining the duration of locomotor episodes, but without affecting step cycle frequency or amplitude. In contrast, 5-HTP (25-150 mg/kg) was ineffective in instigating air-stepping, but altered episode durations of L-DOPA-induced air-stepping, and decreased locomotor cycle frequency. High performance liquid chromatography revealed that L-DOPA, which was undetectable in control conditions, accumulated in a dose-dependent manner in the lumbar spinal cord 30 min after its administration. This was paralleled by an increase in dopamine levels, whereas the spinal content of noradrenaline and serotonin remained unaffected. In the same way, the spinal levels of serotonin increased in parallel with the dose of 5-HTP without affecting the levels of dopamine and noradrenaline. When both precursors are administrated, they counteract each other for the production of serotonin and dopamine. Our data thus indicate for the first time that both L-DOPA and 5-HTP exert opposing neuromodulatory actions on air-stepping behaviour in the developing rat, and we speculate that competition for the production of dopamine and serotonin occurs when they are administered as a cocktail.

The Lamprey Forebrain - Evolutionary Implications

The forebrain plays a critical role in a broad range of neural processes encompassing sensory integration and initiation/selection of behaviour. The forebrain functions through an interaction between different cortical areas, the thalamus, the basal ganglia with the dopamine system, and the habenulae. The ambition here is to compare the mammalian forebrain with that of the lamprey representing the oldest now living group of vertebrates, by a review of earlier studies. We show that the lamprey dorsal pallium has a motor, a somatosensory, and a visual area with retinotopic representation. The lamprey pallium was previously thought to be largely olfactory. There is also a detailed similarity between the lamprey and mammals with regard to other forebrain structures like the basal ganglia in which the general organisation, connectivity, transmitters and their receptors, neuropeptides, and expression of ion channels are virtually identical. These initially unexpected results allow for the possibility that many aspects of the basic design of the vertebrate forebrain had evolved before the lamprey diverged from the evolutionary line leading to mammals. Based on a detailed comparison between the mammalian forebrain and that of the lamprey and with due consideration of data from other vertebrate groups, we propose a compelling account of a pan-vertebrate schema for basic forebrain structures, suggesting a common ancestry of over half a billion years of vertebrate evolution.

Freely Behaving Mice Can Brake and Turn During Optogenetic Stimulation of the Mesencephalic Locomotor Region

A key function of the mesencephalic locomotor region (MLR) is to control the speed of forward symmetrical locomotor movements. However, the ability of freely moving mammals to integrate environmental cues to brake and turn during MLR stimulation is poorly documented. Here, we investigated whether freely behaving mice could brake or turn, based on environmental cues during MLR stimulation. We photostimulated the cuneiform nucleus (part of the MLR) in mice expressing channelrhodopsin in Vglut2-positive neurons in a Cre-dependent manner (Vglut2-ChR2-EYFP) using optogenetics. We detected locomotor movements using deep learning. We used patch-clamp recordings to validate the functional expression of channelrhodopsin and neuroanatomy to visualize the stimulation sites. In the linear corridor, gait diagram and limb kinematics were similar during spontaneous and optogenetic-evoked locomotion. In the open-field arena, optogenetic stimulation of the MLR evoked locomotion, and increasing laser power increased locomotor speed. Mice could brake and make sharp turns (~90°) when approaching a corner during MLR stimulation in the open-field arena. The speed during the turn was scaled with the speed before the turn, and with the turn angle. Patch-clamp recordings in Vglut2-ChR2-EYFP mice show that blue light evoked short-latency spiking in MLR neurons. Our results strengthen the idea that different brainstem neurons convey braking/turning and MLR speed commands in mammals. Our study also shows that Vglut2-positive neurons of the cuneiform nucleus are a relevant target to increase locomotor activity without impeding the ability to brake and turn when approaching obstacles, thus ensuring smooth and adaptable navigation. Our observations may have clinical relevance since cuneiform nucleus stimulation is increasingly considered to improve locomotion function in pathological states such as Parkinson's disease, spinal cord injury, or stroke.

Serotonergic Modulation of Locomotor Activity From Basal Vertebrates to Mammals

During the last 50 years, the serotonergic (5-HT) system was reported to exert a complex modulation of locomotor activity. Here, we focus on two key factors that likely contribute to such complexity. First, locomotion is modulated directly and indirectly by 5-HT neurons. The locomotor circuitry is directly innervated by 5-HT neurons in the caudal brainstem and spinal cord. Also, indirect control of locomotor activity results from ascending projections of 5-HT cells in the rostral brainstem that innervate multiple brain centers involved in motor action planning. Second, each approach used to manipulate the 5-HT system likely engages different 5-HT-dependent mechanisms. This includes the recruitment of different 5-HT receptors, which can have excitatory or inhibitory effects on cell activity. These receptors can be located far or close to the 5-HT release sites, making their activation dependent on the level of 5-HT released. Here we review the activity of different 5-HT nuclei during locomotor activity, and the locomotor effects of 5-HT precursors, exogenous 5-HT, selective 5-HT reuptake inhibitors (SSRI), electrical or chemical stimulation of 5-HT neurons, genetic deletions, optogenetic and chemogenetic manipulations. We highlight both the coherent and controversial aspects of 5-HT modulation of locomotor activity from basal vertebrates to mammals. This mini review may hopefully inspire future studies aiming at dissecting the complex effects of 5-HT on locomotor function.

Descending Dopaminergic Inputs to Reticulospinal Neurons Promote Locomotor Movements

Meso-diencephalic dopaminergic neurons are known to modulate locomotor behaviors through their ascending projections to the basal ganglia, which in turn project to the mesencephalic locomotor region, known to control locomotion in vertebrates. In addition to their ascending projections, dopaminergic neurons were found to increase locomotor movements through direct descending projections to the mesencephalic locomotor region and spinal cord. Intriguingly, fibers expressing tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis, were also observed around reticulospinal neurons of lampreys. We now examined the origin and the role of this innervation. Using immunofluorescence and tracing experiments, we found that fibers positive for dopamine innervate reticulospinal neurons in the four reticular nuclei of lampreys. We identified the dopaminergic source using tracer injections in reticular nuclei, which retrogradely labeled dopaminergic neurons in a caudal diencephalic nucleus (posterior tuberculum [PT]). Using voltammetry in brain preparations isolated in vitro, we found that PT stimulation evoked dopamine release in all four reticular nuclei, but not in the spinal cord. In semi-intact preparations where the brain is accessible and the body moves, PT stimulation evoked swimming, and injection of a D1 receptor antagonist within the middle rhombencephalic reticular nucleus was sufficient to decrease reticulospinal activity and PT-evoked swimming. Our study reveals that dopaminergic neurons have access to command neurons that integrate sensory and descending inputs to activate spinal locomotor neurons. As such, our findings strengthen the idea that dopamine can modulate locomotor behavior both via ascending projections to the basal ganglia and through descending projections to brainstem motor circuits.SIGNIFICANCE STATEMENT Meso-diencephalic dopaminergic neurons play a key role in modulating locomotion by releasing dopamine in the basal ganglia, spinal networks, and the mesencephalic locomotor region, a brainstem region that controls locomotion in a graded fashion. Here, we report in lampreys that dopaminergic neurons release dopamine in the four reticular nuclei where reticulospinal neurons are located. Reticulospinal neurons integrate sensory and descending suprareticular inputs to control spinal interneurons and motoneurons. By directly modulating the activity of reticulospinal neurons, meso-diencephalic dopaminergic neurons control the very last instructions sent by the brain to spinal locomotor circuits. Our study reports on a new direct descending dopaminergic projection to reticulospinal neurons that modulates locomotor behavior.

The Translesional Spinal Network and Its Reorganization after Spinal Cord Injury

Evidence from preclinical and clinical research suggest that neuromodulation technologies can facilitate the sublesional spinal networks, isolated from supraspinal commands after spinal cord injury (SCI), by reestablishing the levels of excitability and enabling descending motor signals via residual connections. Herein, we evaluate available evidence that sublesional and supralesional spinal circuits could form a translesional spinal network after SCI. We further discuss evidence of translesional network reorganization after SCI in the presence of sensory inputs during motor training. In this review, we evaluate potential mechanisms that underlie translesional circuitry reorganization during neuromodulation and rehabilitation in order to enable motor functions after SCI. We discuss the potential of neuromodulation technologies to engage various components that comprise the translesional network, their functional recovery after SCI, and the implications of the concept of translesional network in development of future neuromodulation, rehabilitation, and neuroprosthetics technologies.

Heterogeneous expression of dopaminergic markers and Vglut2 in mouse mesodiencephalic dopaminergic nuclei A8-A13

Co-transmission of glutamate by brain dopaminergic (DA) neurons was recently proposed as a potential factor influencing cell survival in models of Parkinson's disease. Intriguingly, brain DA nuclei are differentially affected in Parkinson's disease. Whether this is associated with different patterns of co-expression of the glutamatergic phenotype along the rostrocaudal brain axis is unknown in mammals. We hypothesized that, as in zebrafish, the glutamatergic phenotype is present preferentially in the caudal mesodiencephalic DA nuclei. Here, we used in mice a cell fate mapping strategy based on reporter protein expression (ZsGreen) consecutive to previous expression of the vesicular glutamate transporter 2 (Vglut2) gene, coupled with immunofluorescence experiments against tyrosine hydroxylase (TH) or dopamine transporter (DAT). We found three expression patterns in DA cells, organized along the rostrocaudal brain axis. The first pattern (TH-positive, DAT-positive, ZsGreen-positive) was found in A8-A10. The second pattern (TH-positive, DAT-negative, ZsGreen-positive) was found in A11. The third pattern (TH-positive, DAT-negative, ZsGreen-negative) was found in A12-A13. These patterns should help to refine the establishment of the homology of DA nuclei between vertebrate species. Our results also uncover that Vglut2 is expressed at some point during cell lifetime in DA nuclei known to degenerate in Parkinson's disease and largely absent from those that are preserved, suggesting that co-expression of the glutamatergic phenotype in DA cells influences their survival in Parkinson's disease.

Deletion of α-Synuclein in Prrx1-positive cells causes partial loss of function in the central nervous system (CNS) but does not affect ovariectomy induced bone loss

α-Synuclein is a small 140 amino acid polypeptide encoded by the Snca gene that is highly expressed in neural tissue, but it is also found in osteoblasts, erythroblasts, macrophages, and adipose tissue. Previously, using co-expression network analysis we found that Snca was a key regulator of skeletal homeostasis, and its deletion partially prevented bone loss after ovariectomy (OVX). Here we tested the hypothesis that Snca deletion in mesenchymal progenitors using the Prrx1Cre (Prrx1, Paired-related homeobox 1) limb enhancer would protect bone mass after OVX. Prrx1Cre;Snca^fl/fl and littermate controls (Snca^fl/fl) were sham operated or ovariectomized (OVX) at 8 weeks of age and sacrificed at 20 weeks. Independently, eight-week female and male Prrx1Cre;Snca^fl/fl mice and littermate controls were administered a high fat (60% fat) or low fat (10% fat) diet for 15 weeks. Bone loss was not prevented in either genotype after ovariectomy, but the Prrx1Cre;Snca^fl/fl. mice were partially protected from weight gain after OVX and high fat diet (HFD). Serum catecholamine levels were lower in the Prrx1Cre;Snca^fl/fl both on a low fat diet (LFD) and HFD versus fl/fl controls. Importantly, mutant mice exhibited a number of physical and behavioral phenotypes that were associated with conditional deletion of Snca in several brain regions. Cells labeled with Prrx1 were noted throughout the central nervous system (CNS). These data support earlier preliminary reports of Prrx1 expression in neural progenitors, and raise a cautionary note about the evaluation of skeletal and body composition phenotypes when using this Cre driver to study osteoprogenitor development.

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Motor Control: Swim Harder, Faster, Stronger

A new study provides evidence in zebrafish that dopamine increases the activity of motor neurons in the spinal cord, and this translates into faster swimming bouts in response to visual stimulation.

Neuromodulatory Selection of Motor Neuron Recruitment Patterns in a Visuomotor Behavior Increases Speed

Animals generate locomotion at different speeds to suit their behavioral needs. Spinal circuits generate locomotion at these varying speeds by sequential activation of different spinal interneurons and motor neurons. Larval zebrafish can generate slow swims for prey capture and exploration by activation of secondary motor neurons and much faster and vigorous swims during escape and struggle via additional activation of primary motor neurons. Neuromodulators are known to alter the motor output of spinal circuits, but their precise role in speed regulation is not well understood. Here, in the context of optomotor response (OMR), an innate evoked locomotor behavior, we show that dopamine (DA) provides an additional layer to regulation of swim speed in larval zebrafish. Activation of D1-like receptors increases swim speed during OMR in free-swimming larvae. By analyzing tail bend kinematics in head-restrained larvae, we show that the increase in speed is actuated by larger tail bends. Whole-cell patch-clamp recordings from motor neurons reveal that, during OMR, typically only secondary motor neurons are active, whereas primary motor neurons are quiescent. Activation of D1-like receptors increases intrinsic excitability and excitatory synaptic drive in primary and secondary motor neurons. These actions result in greater recruitment of motor neurons during OMR. Our findings provide an example of neuromodulatory reconfiguration of spinal motor neuron speed modules where members are selectively recruited and motor drive is increased to effect changes in locomotor speed. VIDEO ABSTRACT.

Dopaminergic modulation of olfactory-evoked motor output in sea lampreys (Petromyzon marinus L.)

Detection of chemical cues is important to guide locomotion in association with feeding and sexual behavior. Two neural pathways responsible for odor-evoked locomotion have been characterized in the sea lamprey (Petromyzon marinus L.), a basal vertebrate. There is a medial pathway originating in the medial olfactory bulb (OB) and a lateral pathway originating from the rest of the OB. These olfactomotor pathways are present throughout the life cycle of lampreys, but olfactory-driven behaviors differ according to the developmental stage. Among possible mechanisms, dopaminergic (DA) modulation in the OB might explain the behavioral changes. Here, we examined DA modulation of olfactory transmission in lampreys. Immunofluorescence against DA revealed immunoreactivity in the OB that was denser in the medial part (medOB), where processes were observed close to primary olfactory afferents and projection neurons. Dopaminergic neurons labeled by tracer injections in the medOB were located in the OB, the posterior tuberculum, and the dorsal hypothalamic nucleus, suggesting the presence of both intrinsic and extrinsic DA innervation. Electrical stimulation of the olfactory nerve in an in vitro whole-brain preparation elicited synaptic responses in reticulospinal cells that were modulated by DA. Local injection of DA agonists in the medOB decreased the reticulospinal cell responses whereas the D2 receptor antagonist raclopride increased the response amplitude. These observations suggest that DA in the medOB could modulate odor-evoked locomotion. Altogether, these results show the presence of a DA innervation within the medOB that may play a role in modulating olfactory inputs to the motor command system of lampreys.

Morning and Evening Circadian Pacemakers Independently Drive Premotor Centers via a Specific Dopamine Relay

Many animals exhibit morning and evening peaks of locomotor behavior. In Drosophila, two corresponding circadian neural oscillators-M (morning) cells and E (evening) cells-exhibit a corresponding morning or evening neural activity peak. Yet we know little of the neural circuitry by which distinct circadian oscillators produce specific outputs to precisely control behavioral episodes. Here, we show that ring neurons of the ellipsoid body (EB-RNs) display spontaneous morning and evening neural activity peaks in vivo: these peaks coincide with the bouts of locomotor activity and result from independent activation by M and E pacemakers. Further, M and E cells regulate EB-RNs via identified PPM3 dopaminergic neurons, which project to the EB and are normally co-active with EB-RNs. These in vivo findings establish the fundamental elements of a circadian neuronal output pathway: distinct circadian oscillators independently drive a common pre-motor center through the agency of specific dopaminergic interneurons.

A Brainstem Neural Substrate for Stopping Locomotion

Locomotion occurs sporadically and needs to be started, maintained, and stopped. The neural substrate underlying the activation of locomotion is partly known, but little is known about mechanisms involved in termination of locomotion. Recently, reticulospinal neurons (stop cells) were found to play a crucial role in stopping locomotion in the lamprey: their activation halts ongoing locomotion and their inactivation slows down the termination process. Intracellular recordings of these cells revealed a distinct activity pattern, with a burst of action potentials at the beginning of a locomotor bout and one at the end (termination burst). The termination burst was shown to be time linked to the end of locomotion, but the mechanisms by which it is triggered have remained unknown. We studied this in larval sea lampreys (Petromyzon marinus; the sex of the animals was not taken into account). We found that the mesencephalic locomotor region (MLR), which is known to initiate and control locomotion, stops ongoing locomotion by providing synaptic inputs that trigger the termination burst in stop cells. When locomotion is elicited by MLR stimulation, a second MLR stimulation stops the locomotor bout if it is of lower intensity than the initial stimulation. This occurs for MLR-induced, sensory-evoked, and spontaneous locomotion. Furthermore, we show that glutamatergic and, most likely, monosynaptic projections from the MLR activate stop cells during locomotion. Therefore, activation of the MLR not only initiates locomotion, but can also control the end of a locomotor bout. These results provide new insights onto the neural mechanisms responsible for stopping locomotion.SIGNIFICANCE STATEMENT The mesencephalic locomotor region (MLR) is a brainstem region well known to initiate and control locomotion. Since its discovery in cats in the 1960s, the MLR has been identified in all vertebrate species tested from lampreys to humans. We now demonstrate that stimulation of the MLR not only activates locomotion, but can also stop it. This is achieved through a descending glutamatergic signal, most likely monosynaptic, from the MLR to the reticular formation that activates reticulospinal stop cells. Together, our findings have uncovered a neural mechanism for stopping locomotion and bring new insights into the function of the MLR.

Reciprocal interaction between monoaminergic systems and the pedunculopontine nucleus: Implication in the mechanism of L-DOPA

The pedunculopontine nucleus (PPN) is part of the mesencephalic locomotor region (MLR) and has been involved in the control of gait, posture, locomotion, sleep, and arousal. It likely participates in some motor and non-motor symptoms of Parkinson's disease and is regularly proposed as a surgical target to ameliorate gait, posture and sleep disorders in Parkinsonian patients. The PPN overlaps with the monoaminergic systems including dopamine, serotonin and noradrenaline in the modulation of the above-mentioned functions. All these systems are involved in Parkinson's disease and the mechanism of the anti-Parkinsonian agents, mostly L-DOPA. This suggests that PPN interacts with monoaminergic neurons and vice versa. Some evidence indicates that the PPN sends cholinergic, glutamatergic and even gabaergic inputs to mesencephalic dopaminergic cells, with the data regarding serotonergic or noradrenergic cells being less well known. Similarly, the control exerted by the PPN on dopaminergic neurons, is multiple and complex, and more extensively explored than the other monoaminergic systems. The data on the influence of monoaminergic systems on PPN neuron activity are rather scarce. While there is evidence that the PPN influences the therapeutic response of L-DOPA, it is still difficult to discerne the reciprocal action of the PPN and monoaminergic systems in this action. Additional data are required to better understand the functional organization of monoaminergic inputs to the MLR including the PPN to get a clearer picture of their interaction.

Parallel descending dopaminergic connectivity of A13 cells to the brainstem locomotor centers

The mesencephalic locomotor region (MLR) is an important integrative area for the initiation and modulation of locomotion. Recently it has been realized that dopamine (DA) projections from the substantia nigra pars compacta project to the MLR. Here we explore DA projections from an area of the medial zona incerta (ZI) known for its role in motor control onto the MLR. We provide evidence that dopaminergic (DAergic) A13 neurons have connectivity to the cuneiform nucleus (CnF) and pedunculopontine tegmental nucleus (PPTg) of the MLR. No ascending connectivity to the dorsolateral striatum was observed. On the other hand, DAergic A13 projections to the medullary reticular formation (MRF) and the lumbar spinal cord were sparse. A small number of non-DAergic neurons within the medial ZI projected to the lumbar spinal cord. We then characterized the DA A13 cells and report that these cells differ from canonical DA neurons since they lack the Dopamine Transporter (DAT). The lack of DAT expression, and possibly the lack of a dopamine reuptake mechanism, points to a longer time of action compared to typical dopamine neurons. Collectively our data suggest a parallel descending DAergic pathway from the A13 neurons of the medial ZI to the MLR, which we expect is important for modulating movement.

Midbrain circuits that set locomotor speed and gait selection

Locomotion is a fundamental motor function common to the animal kingdom. It is executed episodically and adapted to behavioural needs including exploration, requiring slow locomotion, and escaping behaviour, necessitating faster speeds. The control of these functions originates in brainstem structures although the neuronal substrate(s) supporting them are debated. Here, we show in mice that speed/gait selection are controlled by glutamatergic excitatory neurons (GlutNs) segregated in two distinct midbrain nuclei: the Cuneiform Nucleus (CnF) and the Pedunculopontine Nucleus (PPN). GlutNs in each of those two regions are sufficient for controlling slower alternating locomotor behavior but only GlutNs in the CnF are necessary for high-speed synchronous locomotion. Additionally, PPN- and CnF-GlutNs activation dynamics and their input and output connectivity matrices support explorative and escape locomotion, respectively. Our results identify dual regions in the midbrain that act in common to select context dependent locomotor behaviours.

Integration of Descending Command Systems for the Generation of Context-Specific Locomotor Behaviors

Over the past decade there has been a renaissance in our understanding of spinal cord circuits; new technologies are beginning to provide key insights into descending circuits which project onto spinal cord central pattern generators. By integrating work from both the locomotor and animal behavioral fields, we can now examine context-specific control of locomotion, with an emphasis on descending modulation arising from various regions of the brainstem. Here we examine approach and avoidance behaviors and the circuits that lead to the production and arrest of locomotion.

Nigral Glutamatergic Neurons Control the Speed of Locomotion

The mesencephalic locomotor region (MLR) plays a crucial role in locomotor control. In vertebrates, stimulation of the MLR at increasing intensities elicits locomotion of growing speed. This effect has been presumed to result from higher brain inputs activating the MLR like a dimmer switch. Here, we show in lampreys (Petromyzon marinus) of either sex that incremental stimulation of a region homologous to the mammalian substantia nigra pars compacta (SNc) evokes increasing activation of MLR cells with a graded increase in the frequency of locomotor movements. Neurons co-storing glutamate and dopamine were found to project from the primal SNc to the MLR. Blockade of glutamatergic transmission largely diminished MLR cell responses and locomotion. Local blockade of D₁ receptors in the MLR decreased locomotor frequency, but did not disrupt the SNc-evoked graded control of locomotion. Our findings revealed the presence of a glutamatergic input to the MLR originating from the primal SNc that evokes graded locomotor movements.SIGNIFICANCE STATEMENT The mesencephalic locomotor region (MLR) plays a crucial role in the control of locomotion. It projects downward to reticulospinal neurons that in turn activate the spinal locomotor networks. Increasing the intensity of MLR stimulation produces a growing activation of reticulospinal cells and a progressive increase in the speed of locomotor movements. Since the discovery of the MLR some 50 years ago, it has been presumed that higher brain regions activate the MLR in a graded fashion, but this has not been confirmed yet. Here, using a combination of techniques from cell to behavior, we provide evidence of a new glutamatergic pathway activating the MLR in a graded fashion, and consequently evoking a progressive increase in locomotor output.

Restricted co-localization of glutamate and dopamine in neurons of the adult sea lamprey brain

Co-localization of dopamine with other classical neurotransmitters in the same neuron is a common phenomenon in the brain of vertebrates. In mammals, some dopaminergic neurons of the ventral tegmental area and the hypothalamus have a glutamatergic co-phenotype. However, information on the presence of this type of dopaminergic neurons in other vertebrate groups is very scant. Here, we aimed to provide new insights on the evolution of this neuronal co-phenotype by studying the presence of a dual dopaminergic/glutamatergic neuron phenotype in the central nervous system of lampreys. Double immunofluorescence experiments for dopamine and glutamate in adult sea lampreys revealed co-localization of both neurotransmitters in some neurons of the preoptic nucleus, the nucleus of the postoptic commissure, the dorsal hypothalamus and in cerebrospinal fluid-contacting cells of the caudal rhombencephalon and rostral spinal cord. Moreover, co-localization of dopamine and glutamate was found in dopaminergic fibres in a few brain regions including the lateral pallium, striatum, and the preoptic and postoptic areas but not in the brainstem. Our results suggest that the presence of neurons with a dopaminergic/glutamatergic co-phenotype is a primitive character shared by jawless and jawed vertebrates. However, important differences in the distribution of these neurons and fibres were noted among the few vertebrates investigated to date. This study offers an anatomical basis for further work on the role of glutamate in dopaminergic neurons.

Dopamine and the Brainstem Locomotor Networks: From Lamprey to Human

In vertebrates, dopamine neurons are classically known to modulate locomotion via their ascending projections to the basal ganglia that project to brainstem locomotor networks. An increased dopaminergic tone is associated with increase in locomotor activity. In pathological conditions where dopamine cells are lost, such as in Parkinson's disease, locomotor deficits are traditionally associated with the reduced ascending dopaminergic input to the basal ganglia. However, a descending dopaminergic pathway originating from the substantia nigra pars compacta was recently discovered. It innervates the mesencephalic locomotor region (MLR) from basal vertebrates to mammals. This pathway was shown to increase locomotor output in lampreys, and could very well play an important role in mammals. Here, we provide a detailed account on the newly found dopaminergic pathway in lamprey, salamander, rat, monkey, and human. In lampreys and salamanders, dopamine release in the MLR is associated with the activation of reticulospinal neurons that carry the locomotor command to the spinal cord. Dopamine release in the MLR potentiates locomotor movements through a D1-receptor mechanism in lampreys. In rats, stimulation of the substantia nigra pars compacta elicited dopamine release in the pedunculopontine nucleus, a known part of the MLR. In a monkey model of Parkinson's disease, a reduced dopaminergic innervation of the brainstem locomotor networks was reported. Dopaminergic fibers are also present in human pedunculopontine nucleus. We discuss the conserved locomotor role of this pathway from lamprey to mammals, and the hypothesis that this pathway could play a role in the locomotor deficits reported in Parkinson's disease.

The neural control of respiration in lampreys

This review focuses on past and recent findings that have contributed to characterize the neural networks controlling respiration in the lamprey, a basal vertebrate. As in other vertebrates, respiration in lampreys is generated centrally in the brainstem. It is characterized by the presence of a fast and a slow respiratory rhythm. The anatomical and the basic physiological properties of the neural networks underlying the generation of the fast rhythm have been more thoroughly investigated; less is known about the generation of the slow respiratory rhythm. Comparative aspects with respiratory generators in other vertebrates as well as the mechanisms of modulation of respiration in association with locomotion are discussed.