The interactions between locomotion and respiration

The synthetic cathinones MDPHP and MDPV: Comparison of the acute effects in mice, in silico ADMET profiles and clinical reports

The 3,4-methylenedioxy-alpha-pyrrolidinohexanophenone (MDPHP) is a synthetic cathinone closely related to 3,4-methylenedioxypyrovalerone (MDPV), one of the most common synthetic cathinones present in the "bath salts". MDPHP has recently gained attention due to increasing seizures and involvement in human intoxications which occurred in Europe and Italy in the last years, but currently there is a lack of information about its pharmaco-toxicological effects. With the aim at filling this gap, the present study is endeavoured to (i) evaluate the effects of acute administration of MDPHP (0.01-20 mg/kg; i.p.) on behaviour, cardiorespiratory and cardiovascular parameters in CD-1 male mice, comparing them to those observed after administration of MDPV; (ii) predict the ADMET profile of the two analogues using the Plus ADMET Predictor®; (iii) present clinical data related to MDPHP and MDPV-induced intoxications recorded between 2011 and 2023 by the Pavia Poison Control Centre (PCC) - National Toxicology Information Centre (Istituti Clinici Scientifici Maugeri, IRCCS Pavia, Italy). Our results substantiated that MDPHP and MDPV similarly affect sensorimotor and behavioural responses in mice, importantly increased locomotion and induced aggressive behaviour, and, at higher dosage, increased heart rate and blood pressure. These findings are in line with those observed in humans, revealing severe toxidromes typically characterized by Central Nervous System (CNS) alterations (behavioural/neuropsychiatric symptoms), including psychomotor agitation and aggressiveness, cardiovascular and respiratory disorders (e.g. tachycardia, hypertension, dyspnoea), and other peripheral symptoms (e.g. hyperthermia, acidosis, rhabdomyolysis).

Neuropeptide Modulation Enables Biphasic Internetwork Coordination via a Dual-Network Neuron

Linked rhythmic behaviors, such as respiration/locomotion or swallowing/chewing, often require coordination for proper function. Despite its prevalence, the cellular mechanisms controlling coordination of the underlying neural networks remain undetermined in most systems. We use the stomatogastric nervous system of the crab Cancer borealis to investigate mechanisms of internetwork coordination, due to its small, well-characterized feeding-related networks (gastric mill [chewing, ∼0.1 Hz]; pyloric [filtering food, ∼1 Hz]). Here, we investigate coordination between these networks during the Gly1-SIFamide neuropeptide modulatory state. Gly1-SIFamide activates a unique triphasic gastric mill rhythm in which the typically pyloric-only LPG neuron generates dual pyloric-plus gastric mill-timed oscillations. Additionally, the pyloric rhythm exhibits shorter cycles during gastric mill rhythm-timed LPG bursts, and longer cycles during IC, or IC plus LG gastric mill neuron bursts. Photoinactivation revealed that LPG is necessary to shorten pyloric cycle period, likely through its rectified electrical coupling to pyloric pacemaker neurons. Hyperpolarizing current injections demonstrated that although LG bursting enables IC bursts, only gastric mill rhythm bursts in IC are necessary to prolong the pyloric cycle period. Surprisingly, LPG photoinactivation also eliminated prolonged pyloric cycles, without changing IC firing frequency or gastric mill burst duration, suggesting that pyloric cycles are prolonged via IC synaptic inhibition of LPG, which indirectly slows the pyloric pacemakers via electrical coupling. Thus, the same dual-network neuron directly conveys excitation from its endogenous bursting and indirectly funnels synaptic inhibition to enable one network to alternately decrease and increase the cycle period of a related network.

Motor-Respiratory Coupling Improves Endurance Performance during Rhythmic Isometric Handgrip Exercise

PURPOSE

This study aimed to evaluate whether motor-respiratory coupling exists in rhythmic isometric handgrip exercises and its effect on endurance performance. In addition, the mechanism underlying observed effects was to be investigated if higher motor-respiratory coupling rate could enhance endurance performance.

METHODS

Eleven subjects completed three rhythmic isometric handgrip trials to task failure in a randomized manner. After one pretraining session to determine personal grip frequency, one trial was performed without respiration requirement (CON), and two trials were performed with inspiration-motor coupling (IMC) or expiration-motor coupling. Changes in maximal voluntary contraction (MVC) and EMG were used to measure neuromuscular fatigue. Force data during test were used to assess exercise intensity. Another 10 subjects completed electrical stimulation-induced finger flexion and extension during normal inspiration, normal expiration, fast inspiration, fast expiration, and breath holding. Force changes of different breathing conditions were compared.

RESULTS

Normalized exercise time to exhaustion was significantly longer in IMC (1.27 ± 0.23) compared with expiration-motor coupling (0.82 ± 0.18) and CON (0.91 ± 0.18, P < 0.001). ΔMVC, grip frequency, force, and EMG indices were not different among conditions (all P > 0.05). Electrical stimulation-induced finger extensor force was significant higher during fast inspiration (1.11 ± 0.09) than normal respiration (1.00 ± 0.05) and fast expiration (0.94 ± 0.08, P < 0.05).

CONCLUSIONS

IMC is an effective way to improve endurance performance of rhythmic handgrip exercise. This is likely due to a reduction in the energy consumption of motion control, as evidenced by similar peripheral fatigue in different conditions and modulation of corticospinal excitability by respiration.

Pedunculopontine Chx10+ neurons control global motor arrest in mice

Arrest of ongoing movements is an integral part of executing motor programs. Behavioral arrest may happen upon termination of a variety of goal-directed movements or as a global motor arrest either in the context of fear or in response to salient environmental cues. The neuronal circuits that bridge with the executive motor circuits to implement a global motor arrest are poorly understood. We report the discovery that the activation of glutamatergic Chx10-derived neurons in the pedunculopontine nucleus (PPN) in mice arrests all ongoing movements while simultaneously causing apnea and bradycardia. This global motor arrest has a pause-and-play pattern with an instantaneous interruption of movement followed by a short-latency continuation from where it was paused. Mice naturally perform arrest bouts with the same combination of motor and autonomic features. The Chx10-PPN-evoked arrest is different to ventrolateral periaqueductal gray-induced freezing. Our study defines a motor command that induces a global motor arrest, which may be recruited in response to salient environmental cues to allow for a preparatory or arousal state, and identifies a locomotor-opposing role for rostrally biased glutamatergic neurons in the PPN.

Upregulation of breathing rate during running exercise by central locomotor circuits in mice

While respiratory adaptation to exercise is compulsory to cope with the increased metabolic demand, the neural signals at stake remain poorly identified. Using neural circuit tracing and activity interference strategies in mice, we uncover here two systems by which the central locomotor network can enable respiratory augmentation in relation to running activity. One originates in the mesencephalic locomotor region (MLR), a conserved locomotor controller. Through direct projections onto the neurons of the preBötzinger complex that generate the inspiratory rhythm, the MLR can trigger a moderate increase of respiratory frequency, prior to, or even in the absence of, locomotion. The other is the lumbar enlargement of the spinal cord containing the hindlimb motor circuits. When activated, and through projections onto the retrotrapezoid nucleus (RTN), it also potently upregulates breathing rate. On top of identifying critical underpinnings for respiratory hyperpnea, these data also expand the functional implication of cell types and pathways that are typically regarded as "locomotor" or "respiratory" related.

Neurogenic mechanisms for locomotor-respiratory coordination in mammals

Central motor rhythm-generating networks controlling different functions are generally considered to operate mostly independently from one another, each controlling the specific behavioral task to which it is assigned. However, under certain physiological circumstances, central pattern generators (CPGs) can exhibit strong uni- or bidirectional interactions that render them closely inter-dependent. One of the best illustrations of such an inter-CPG interaction is the functional relationship that may occur between rhythmic locomotor and respiratory functions. It is well known that in vertebrates, lung ventilatory rates accelerate at the onset of physical exercise in order to satisfy the accompanying rapid increase in metabolism. Part of this acceleration is sustained by a coupling between locomotion and ventilation, which most often results in a periodic drive of the respiratory cycle by the locomotor rhythm. In terrestrial vertebrates, the likely physiological significance of this coordination is that it serves to reduce the mechanical interference between the two motor systems, thereby producing an energetic benefit and ultimately, enabling sustained aerobic activity. Several decades of studies have shown that locomotor-respiratory coupling is present in most species, independent of the mode of locomotion employed. The present article aims to review and discuss mechanisms engaged in shaping locomotor-respiratory coupling (LRC), with an emphasis on the role of sensory feedback inputs, the direct influences between CPG networks themselves, and finally on spinal cellular candidates that are potentially involved in the coupling of these two vital motor functions.

Absent phasing of respiratory and locomotor rhythms in running mice

Examining whether and how the rhythms of limb and breathing movements interact is highly informative about the mechanistic origin of hyperpnoea during running exercise. However, studies have failed to reveal regularities. In particular, whether breathing frequency is inherently proportional to limb velocity and imposed by a synchronization of breaths to strides is still unclear. Here, we examined respiratory changes during running in the resourceful mouse model. We show that, for a wide range of trotting speeds on a treadmill, respiratory rate increases to a fixed and stable value irrespective of trotting velocities. Respiratory rate was yet further increased during escape-like running and most particularly at gallop. However, we found no temporal coordination of breaths to strides at any speed, intensity, or gait. Our work thus highlights that exercise hyperpnoea can operate, at least in mice and in the presently examined running regimes, without phasic constraints from limb movements.

Modulation of respiratory network activity by forelimb and hindlimb locomotor generators

Early at the onset of exercise, breathing rate accelerates in order to anticipate the increasing metabolic demand resulting from the extra effort produced. Accordingly, the respiratory neural networks are the target of various input signals originating either centrally or peripherally. For example, during locomotion, the activation of muscle sensory afferents is able to entrain and thereby increase the frequency of spontaneous respiratory rhythmogenesis. Moreover, the lumbar spinal networks engaged in generating hindlimb locomotor rhythms are also capable of activating the medullary respiratory generators through an ascending excitatory command. However, in the context of quadrupedal locomotion, the influence of other spinal cord regions, such as cervical and thoracic segments, remains unknown. Using isolated brainstem-spinal cord preparations from neonatal rats and mice, we show that cervicothoracic circuitry may also contribute to locomotion-induced acceleration of respiratory cycle frequency. As previously observed for the hindlimb CPGs, the pharmacological activation of forelimb locomotor networks produces episodes of fictive locomotion that in turn increase the ongoing respiratory rhythm. Thoracic neuronal circuitry may also participate indirectly in this modulation via the activation of both cervical and lumbar CPG neurons. Furthermore, using light stimulation of CHR2-expressing glutamatergic neurons, we found that the modulation of the respiratory rate during locomotion involves lumbar glutamatergic circuitry. Our results demonstrate that during locomotion, the respiratory rhythm-generating networks receive excitatory ascending inputs from the spinal circuits responsible for generating and coordinating fore- and hindlimb movements. This constitutes a distributed central mechanism that contributes to matching breathing rate to the speed of locomotion.

Entrainment of chewing rhythm by gait speed during treadmill walking in humans

It remains unclear whether the rhythmic processes of chewing and gait synchronize during concurrent execution in humans. To evaluate the entrainment of chewing rhythm by gait speed, we measured electromyography from the masseter and tibialis anterior muscles during chewing at a habitual rhythm while walking on a linear treadmill in 12 healthy volunteers. Vertical movement of the head was also measured using an accelerometer. Each 5-min session included gait tasks using a treadmill at three speeds: Auto: the participant's self-selected gait speed, High: Auto × 1.3, and Low: Auto ÷ 1.3. Electromyography from the masseter muscles were also measured during chewing while stationary (Chew-Only). Chewing rhythm during walking was the same as that for head movement, occurring at twice the speed of the walking rhythm, in nine participants (Low), eight participants (Auto), and eight participants (High). For these participants, chewing rhythm in the Auto and High conditions differed significantly from that in the Chew-Only condition. Significant differences in chewing rhythm were also observed among gait speeds　(Low vs. Auto vs. High). Our findings demonstrate that entrainment of habitual chewing rhythm to gait speed is a significant phenomenon, and that the dominant ratio of chewing-walking-head movement rhythms is 2:1:2.

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

Resetting the Respiratory Rhythm with a Spinal Central Pattern Generator

There is evidence that a variety of central and afferent stimuli, including swallowing, can produce phase resetting in the respiratory rhythmicity. Also, there are reports about the intrinsic linkage between locomotion and respiration. However, little is known about the interaction between the central pattern generators (CPGs) for scratching and respiration. The present study aims to examine whether the activation of scratching CPG produces phase resetting of the respiratory rhythm. We employed decerebrate cats to apply brief tactile stimuli to the pinna during the inspiratory-expiratory transition. We observed that those stimuli to the pinna not eliciting fictive scratching did not reset the respiratory rhythm. However, when the pinna stimuli elicited fictive scratching, then the respiratory rhythm exhibited a significant phase resetting. We also found interneurons in the medulla oblongata exhibiting phase resetting related to scratching-CPG episodes. This second finding suggests that this type of resetting involves brainstem components of the respiratory CPG. These results shed new light on the resetting action from a spinal CPG on the respiratory rhythm.

Modulation of Respiratory System by Limb Muscle Afferents in Intact and Injured Spinal Cord

Breathing constantly adapts to environmental, metabolic or behavioral changes by responding to different sensory information, including afferent feedback from muscles. Importantly, not just respiratory muscle feedback influences respiratory activity. Afferent sensory information from rhythmically moving limbs has also been shown to play an essential role in the breathing. The present review will discuss the neuronal mechanisms of respiratory modulation by activation of peripheral muscles that usually occurs during locomotion or exercise. An understanding of these mechanisms and finding the most effective approaches to regulate respiratory motor output by stimulation of limb muscles could be extremely beneficial for people with respiratory dysfunctions. Specific attention in the present review is given to the muscle stimulation to treat respiratory deficits following cervical spinal cord injury.

Swimming rhythm generation in the caudal hindbrain of the lamprey

The spinal cord has been well established as the site of generation of the locomotor rhythm in vertebrates, but studies have suggested that the caudal hindbrain in larval fish and amphibians can also generate locomotor rhythms. Here, we investigated whether the caudal hindbrain of the adult lamprey ( Petromyzon marinus and Ichthyomyzon unicuspis) has the ability to generate the swimming rhythm. The hindbrain-spinal cord transition zone of the lamprey contains a bilateral column of somatic motoneurons that project via the spino-occipital (S-O) nerves to several muscles of the head. In the brainstem-spinal cord-muscle preparation, these muscles were found to burst and contract rhythmically with a left-right alternation when swimming activity was evoked with a brief electrical stimulation of the spinal cord. In the absence of muscles, the isolated brainstem-spinal cord preparation also produced alternating left-right bursts in S-O nerves (i.e., fictive swimming), and the S-O nerve bursts preceded the bursts occurring in the first ipsilateral spinal ventral root. After physical isolation of the S-O region using transverse cuts of the nervous system, the S-O nerves still exhibited rhythmic bursting with left-right alternation when glutamate was added to the bathing solution. We conclude that the S-O region of the lamprey contains a swimming rhythm generator that produces the leading motor nerve bursts of each swimming cycle, which then propagate down the spinal cord to produce forward swimming. The S-O region of the hindbrain-spinal cord transition zone may play a role in regulating speed, turning, and head orientation during swimming in lamprey. NEW & NOTEWORTHY Although it has been well established that locomotor rhythm generation occurs in the spinal cord of vertebrates, it was unknown whether the hindbrain of the adult vertebrate nervous system can also generate the locomotor rhythm. Here, we show that the isolated hindbrain-spinal cord transition zone of adult lamprey can generate the swimming rhythm. In addition, the swimming bursts of the hindbrain lead the bursts occurring in the first segment of the spinal cord.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

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.

Inhibitory control of ascending glutamatergic projections to the lamprey respiratory rhythm generator

Neurons within the vagal motoneuron region of the lamprey have been shown to modulate respiratory activity via ascending excitatory projections to the paratrigeminal respiratory group (pTRG), the proposed respiratory rhythm generator. The present study was performed on in vitro brainstem preparations of the lamprey to provide a characterization of ascending projections within the whole respiratory motoneuron column with regard to the distribution of neurons projecting to the pTRG and related neurochemical markers. Injections of Neurobiotin were performed into the pTRG and the presence of glutamate, GABA and glycine immunoreactivity was investigated by double-labeling experiments. Interestingly, retrogradely labeled neurons were found not only in the vagal region, but also in the facial and glossopharyngeal motoneuron regions. They were also present within the sensory octavolateral area (OLA). The results show for the first time that neurons projecting to the pTRG are immunoreactive for glutamate, surrounded by GABA-immunoreactive structures and associated with the presence of glycinergic cells. Consistently, GABAA or glycine receptor blockade within the investigated regions increased the respiratory frequency. Furthermore, microinjections of agonists and antagonists of ionotropic glutamate receptors and of the GABAA receptor agonist muscimol showed that OLA neurons do not contribute to respiratory rhythm generation. The results provide evidence that glutamatergic ascending pathways to the pTRG are subject to a potent inhibitory control and suggest that disinhibition is one important mechanism subserving their function. The general characteristics of inhibitory control involved in rhythmic activities, such as respiration, appear to be highly conserved throughout vertebrate evolution.

Responses to increasing exercise upon reaching the anaerobic threshold, and their control by the central nervous system

The anaerobic threshold (AT) has been one of the most studied of all physiological variables. Many authors have proposed the use of several markers to determine the moment at with the AT is reached. The present work discusses the physiological responses made to exercise - the measurement of which indicates the point at which the AT is reached - and how these responses might be controlled by the central nervous system. The detection of the AT having been reached is a sign for the central nervous system (CNS) to respond via an increase in efferent activity via the peripheral nervous system (PNS). An increase in CNS and PNS activities are related to changes in ventilation, cardiovascular function, and gland and muscle function. The directing action of the central command (CC) allows for the coordination of the autonomous and motor systems, suggesting that the AT can be identified in the many ways: changes in lactate, ventilation, plasma catecholamines, heart rate (HR), salivary amylase and muscular electrical activity. This change in response could be indicative that the organism would face failure if the exercise load continued to increase. To avoid this, the CC manages the efferent signals that show the organism that it is running out of homeostatic potential.

Remote control of respiratory neural network by spinal locomotor generators

During exercise and locomotion, breathing rate rapidly increases to meet the suddenly enhanced oxygen demand. The extent to which direct central interactions between the spinal networks controlling locomotion and the brainstem networks controlling breathing are involved in this rhythm modulation remains unknown. Here, we show that in isolated neonatal rat brainstem-spinal cord preparations, the increase in respiratory rate observed during fictive locomotion is associated with an increase in the excitability of pre-inspiratory neurons of the parafacial respiratory group (pFRG/Pre-I). In addition, this locomotion-induced respiratory rhythm modulation is prevented both by bilateral lesion of the pFRG region and by blockade of neurokinin 1 receptors in the brainstem. Thus, our results assign pFRG/Pre-I neurons a new role as elements of a previously undescribed pathway involved in the functional interaction between respiratory and locomotor networks, an interaction that also involves a substance P-dependent modulating mechanism requiring the activation of neurokinin 1 receptors. This neurogenic mechanism may take an active part in the increased respiratory rhythmicity produced at the onset and during episodes of locomotion in mammals.

Modulation of circuit feedback specifies motor circuit output

Bidirectional communication (i.e., feedforward and feedback pathways) between functional levels is common in neural systems, but in most systems little is known regarding the function and modifiability of the feedback pathway. We are exploring this issue in the crab (Cancer borealis) stomatogastric nervous system by examining bidirectional communication between projection neurons and their target central pattern generator (CPG) circuit neurons. Specifically, we addressed the question of whether the peptidergic post-oesophageal commissure (POC) neurons trigger a specific gastric mill (chewing) motor pattern in the stomatogastric ganglion solely by activating projection neurons, or by additionally altering the strength of CPG feedback to these projection neurons. The POC-triggered gastric mill rhythm is shaped by feedback inhibition onto projection neurons from a CPG neuron. Here, we establish that POC stimulation triggers a long-lasting enhancement of feedback-mediated IPSC/Ps in the projection neurons, which persists for the same duration as POC-gastric mill rhythms. This strengthened CPG feedback appears to result from presynaptic modulation, because it also occurs in other projection neurons whose activity does not change after POC stimulation. To determine the function of this strengthened feedback synapse, we compared the influence of dynamic-clamp-injected feedback IPSPs of pre- and post-POC amplitude into a pivotal projection neuron after POC stimulation. Only the post-POC amplitude IPSPs elicited the POC-triggered activity pattern in this projection neuron and enabled full expression of the POC-gastric mill rhythm. Thus, the strength of CPG feedback to projection neurons is modifiable and can be instrumental to motor pattern selection.

Coordination of distinct motor structures through remote axonal coupling of projection interneurons

Complex behaviors often require coordinated movements of dissimilar motor structures. The underlying neural mechanisms are poorly understood. We investigated cycle-by-cycle coordination of two dissimilar feeding structures in Aplysia californica: the external lips and the internal radula. During feeding, the lips open while the radula protracts. Lip and radula motoneurons are located in the cerebral and buccal ganglia, respectively, and radula motoneurons are controlled by a well characterized buccal central pattern generator (CPG). Here, we examined whether the three electrically coupled lip motoneurons C15/16/17 are controlled by the buccal CPG or by a previously postulated cerebral CPG. Two buccal-cerebral projection interneurons, B34 and B63, which are part of the buccal CPG and mediate radula protraction, monosynaptically excite C15/16/17. Recordings from the B34 axon in the cerebral ganglion demonstrate its direct electrical coupling with C15/16/17, eliminating the need for a cerebral CPG. Moreover, when the multifunctional buccal CPG generates multiple forms of motor programs due to the activation of two inputs, the command-like neuron CBI-2 and the esophageal nerve (EN), C15/16 exhibit activity patterns that are distinct from C17. These distinct activity patterns result from combined activity of B34 and B63 and their differential excitation of C15/16 versus C17. In more general terms, we identified neuronal mechanisms that allow a single CPG to coordinate the phasing and activity of remotely located motoneurons innervating distinct structures that participate in the production of different motor outputs. We also demonstrated that axodendritic electrical coupling by projection interneurons plays a pivotal role in coordinating activity of these remotely located neurons.

Specific neural substrate linking respiration to locomotion

When animals move, respiration increases to adapt for increased energy demands; the underlying mechanisms are still not understood. We investigated the neural substrates underlying the respiratory changes in relation to movement in lampreys. We showed that respiration increases following stimulation of the mesencephalic locomotor region (MLR) in an in vitro isolated preparation, an effect that persists in the absence of the spinal cord and caudal brainstem. By using electrophysiological and anatomical techniques, including whole-cell patch recordings, we identified a subset of neurons located in the dorsal MLR that send direct inputs to neurons in the respiratory generator. In semi-intact preparations, blockade of this region with 6-cyano-7-nitroquinoxaline-2,3-dione and (2R)-amino-5-phosphonovaleric acid greatly reduced the respiratory increases without affecting the locomotor movements. These results show that neurons in the respiratory generator receive direct glutamatergic connections from the MLR and that a subpopulation of MLR neurons plays a key role in the respiratory changes linked to movement.

Identification of a cholinergic modulatory and rhythmogenic mechanism within the lamprey respiratory network

Acetylcholine (ACh) is well known to be involved in the control of breathing. However, no information is available on the role of ACh receptors (AChRs) within the lamprey respiratory network. The present study was performed on in vitro brainstem preparations of adult lampreys to investigate whether ACh affects respiratory activity possibly through an action on the paratrigeminal respiratory group (pTRG) that has been identified as an essential component of the respiratory network. Respiratory activity was monitored as vagal motor output. Bath application of 100 μM physostigmine or 1 μM nicotine increased respiratory frequency, while bath application of 100 μM D-tubocurarine or 0.25 μM α-bungarotoxin reduced respiratory frequency and increased the duration of vagal bursts. Since these effects were mimicked by microinjections of the same drugs into the pTRG, ACh proved to influence respiratory activity by acting on α7 nicotinic AChRs located within the pTRG. During apnea caused by partial blockade of ionotropic glutamate receptors at the level of the pTRG, bath application of bicuculline and strychnine restored the respiratory rhythm, although at reduced frequency. Similar results were obtained by the concurrent removal of both fast synaptic excitatory and inhibitory transmission. Blockade of pTRG α7 nicotinic AChRs suppressed this respiratory activity, thus indicating that pTRG neurons expressing these receptors contribute to respiratory rhythm generation. Together, these findings identify a novel cholinergic modulatory and possibly subsidiary rhythmogenic mechanism within the respiratory network of the adult lamprey and encourage further studies on the respiratory role of cholinergic receptors in different animal species.

The same core rhythm generator underlies different rhythmic motor patterns

Rhythmically active motor circuits can generate different activity patterns in response to different inputs. In most systems, however, it is not known whether the same neurons generate the underlying rhythm for each different pattern. Thus far, information regarding the degree of conservation of rhythm generator neurons is limited to a few pacemaker-driven circuits, in most of which the core rhythm generator is unchanged across different output patterns. We are addressing this issue in the network-driven, gastric mill (chewing) circuit in the crab stomatogastric nervous system. We first establish that distinct gastric mill motor patterns are triggered by separate stimulation of two extrinsic input pathways, the ventral cardiac neurons (VCNs) and postoesophageal commissure (POC) neurons. A prominent feature that distinguishes these gastric mill motor patterns is the LG (lateral gastric) protractor motor neuron activity pattern, which is tonic during the VCN rhythm and exhibits fast rhythmic bursting during the POC rhythm. These two motor patterns also differed in their cycle period and some motor neuron phase relationships, duty cycles, and burst durations. Despite the POC and VCN motor patterns being distinct, rhythm generation during each motor pattern required the activity of the same two, reciprocally inhibitory gastric mill neurons [LG, Int1 (interneuron 1)]. Specifically, reversibly hyperpolarizing LG or Int1, but no other gastric mill neuron, delayed the start of the next gastric mill cycle until after the imposed hyperpolarization. Thus, the same circuit neurons can comprise the core rhythm generator during different versions of a network-driven rhythmic motor pattern.