GABAergic and glycinergic inputs modulate rhythmogenic mechanisms in the lamprey respiratory network

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 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'.

Pre-treatment with Tandospirone attenuates fentanyl-induced respiratory depression without affecting the analgesic effects of fentanyl in rodents

Opioid analgesics are widely used to treat acute, postoperative, and chronic pain. However, opioid receptor activation can result in severe respiratory depression. In this study, we demonstrated that Tandospirone (TS), a selective serotonin-1A receptor partial agonist, is effective against opioid-induced respiratory depression. Fentanyl was used to establish a respiratory depression model in rodents. We observed the effects of TS on respiratory depression in rats by using plethysmographic recordings and arterial oxygen saturation. In addition, we evaluated the effects of TS on fentanyl-induced sedation and analgesia by using the loss of righting reflex (LORR) and hot-plate tests, respectively. Rats (n = 5) were treated with TS or saline 5 min prior to fentanyl administration. TS [2 mg/kg, intravenous (i.v.)] dose-dependently attenuated fentanyl-induced respiratory depression versus saline + fentanyl group. Furthermore, pre-treatment with TS (2 mg/kg, i.v.) increased arterial oxygen saturation to 76.5 ± 2.0% at 5 min after fentanyl injection, compared with 35.9 ± 2.5% in saline pre-treated rats (P < 0.001), whereas the time to induction of LORR (P > 0.99) and duration of LORR (P = 0.95) did not differ between the "TS + fentanyl" and "saline + fentanyl" group. The antinociceptive effect of fentanyl was not affected by the administration of TS (P = 0.99) in mice (n = 10). In conclusion, we found that TS, a novel non-benzodiazepine anxiolytic/antidepressant drug, could attenuate severe fentanyl-induced respiratory depression and did not affect the analgesic/sedative effect of fentanyl. The clinical application of TS could significantly improve pain management.

The lamprey respiratory network: Some evolutionary aspects

Breathing is a complex behaviour that involves rhythm generating networks. In this review, we examine the main characteristics of respiratory rhythm generation in vertebrates and, in particular, we describe the main results of our studies on the role of neural mechanisms involved in the neuromodulation of the lamprey respiration. The lamprey respiratory rhythm generator is located in the paratrigeminal respiratory group (pTRG) and shows similarities with the mammalian preBötzinger complex. In fact, within the pTRG a major role is played by glutamate, but also GABA and glycine display important contributions. In addition, neuromodulatory influences are exerted by opioids, substance P, acetylcholine and serotonin. Both structures respond to exogenous ATP with a biphasic response and astrocytes there located strongly contribute to the modulation of the respiratory pattern. The results emphasize that some important characteristics of the respiratory rhythm generating network are, to a great extent, maintained throughout evolution.

Vertebrate Evolution Conserves Hindbrain Circuits despite Diverse Feeding and Breathing Modes

Feeding and breathing are two functions vital to the survival of all vertebrate species. Throughout the evolution, vertebrates living in different environments have evolved drastically different modes of feeding and breathing through using diversified orofacial and pharyngeal (oropharyngeal) muscles. The oropharyngeal structures are controlled by hindbrain neural circuits. The developing hindbrain shares strikingly conserved organizations and gene expression patterns across vertebrates, thus begs the question of how a highly conserved hindbrain generates circuits subserving diverse feeding/breathing patterns. In this review, we summarize major modes of feeding and breathing and principles underlying their coordination in many vertebrate species. We provide a hypothesis for the existence of a common hindbrain circuit at the phylotypic embryonic stage controlling oropharyngeal movements that is shared across vertebrate species; and reconfiguration and repurposing of this conserved circuit give rise to more complex behaviors in adult higher vertebrates.

Key role of 5-HT1A receptors in the modulation of the neuronal network underlying the respiratory rhythm generation in lampreys

In mammals, 5-HTexcitatory respiratory effects imply 5-HT_1A receptor-mediated disinhibition of pre-Bötzinger complex neurons. In the lamprey, 5-HT_1A receptors are involved in the neural control of locomotion, but their role in the respiratory regulation, particularly at the level of the putative respiratory rhythm generator, the paratrigeminal respiratory group (pTRG), is not known. We here investigate the respiratory function of inhibitory 5-HT_1A receptors within the pTRG of the isolated brainstem of the adult lamprey. The 5-HT_1A receptor agonists either bath applied or microinjected into the pTRG did not cause significant effects. However, the selective 5-HT_1A receptor antagonist (S)-WAY 100135 bath applied or microinjected into the pTRG induced depressing respiratory effects or even apnoea, thus revealing that 5-HT exerts a 5-HT_1A receptor-mediated potent tonic influence on respiration and contributes to maintain baseline levels of respiratory activity. Microinjections of strychnine or bicuculline, either alone or in combination, into the pTRG prevented (S)-WAY 100135-induced apnoea. In addition, immunohistochemical studies corroborate the present findings suggesting that 5-HT_1A receptors are widely expressed in close apposition to the soma of glycine-immunoreactive cells located within the pTRG region. The results show that in the lamprey respiratory network, 5-HT exerts a tonic influence on respiration by a potent inhibitory control on both GABAergic and glycinergic mechanisms. The observed disinhibitory effects resemble the excitatory respiratory modulation exerted by 5-HT_1A receptor-mediated inhibition of glycinergic and/or GABAergic neurons present in mammals, supporting the notion that some features of the neuronal network subserving respiratory rhythm generation are highly conserved throughout phylogeny.

Central control of air breathing in fishes

The diversity of sites and surfaces that are utilized for gas transfer from air to blood in fish is remarkable. While a few species do utilize their gills for gas exchange in air, this is a rare occurrence and most air-breathing fish utilize other surfaces including air-breathing organs and lungs. At present almost nothing is known about the central sites that initiate and regulate air breathing although hypotheses can be put forward based on our rudimentary understanding of the sites involved in water breathing in lampreys and teleost fishes, and those involved in air breathing in pre-metamorphic anuran ampibians. The pumps involved in producing both water and air breathing in fishes are highly conserved, a buccal pump, assisted by pharyngeal and/or parabranchial/opercular pumps, produce both forms of ventilation. What varies between species are the manner in which air breaths are produced (in two versus four phases), and the 'valving' involved in producing water flow over the gills versus air flow in and out of air-breathing organs. The latter suggests that a major step in the evolution of air breathing was the evolution of the mechanisms that control the flow of the respiratory medium. The neural matrix that underlies the co-ordination of the pump and the valving events remains enigmatic and in much need of further research.

Oscillatory motor patterning is impaired in neurofibromatosis type 1: a behavioural, EEG and fMRI study

Background

Neurofibromatosis type1 (NF1) is associated with a broad range of behavioural deficits, and an imbalance between excitatory and inhibitory neurotransmission has been postulated in this disorder. Inhibition is involved in the control of frequency and stability of motor rhythms. Therefore, we aimed to explore the link between behavioural motor control, brain rhythms and brain activity, as assessed by EEG and fMRI in NF1.

Methods

We studied a cohort of 21 participants with NF1 and 20 age- and gender-matched healthy controls, with a finger-tapping task requiring pacing at distinct frequencies during EEG and fMRI scans.

Results

We found that task performance was significantly different between NF1 and controls, the latter showing higher tapping time precision. The time-frequency patterns at the beta sub-band (20–26 Hz) mirrored the behavioural modulations, with similar cyclic synchronization/desynchronization patterns for both groups. fMRI results showed a higher recruitment of the extrapyramidal motor system (putamen, cerebellum and red nucleus) in the control group during the fastest pacing condition.

Conclusions

The present study demonstrated impaired precision in rhythmic pacing behaviour in NF1 as compared with controls. We found a decreased recruitment of the cerebellum, a structure where inhibitory interneurons are essential regulators of rhythmic synchronization, and in deep brain regions pivotally involved in motor pacing. Our findings shed light into the neural underpinnings of motor timing deficits in NF1.

ATP and astrocytes play a prominent role in the control of the respiratory pattern generator in the lamprey

KEY POINTS

The paratrigeminal respiratory group (pTRG) is responsible for the respiratory pattern generation in the lamprey. The role of ATP and astrocytes, known to control respiratory activity in mammals, was investigated in the lamprey respiratory network. ATP microinjected into the pTRG induces a biphasic response consisting of marked increases in respiratory frequency mediated by P2X receptors followed by a decrease in the respiratory motor output due to the ATP metabolite adenosine. We provide evidence that astrocytes are involved in the genesis of the normal respiratory pattern, ATP-induced responses and acidification-induced increases of the respiratory activity. The function of astrocytes in rhythmic networks appears to be phylogenetically conserved.

ABSTRACT

The role of ATP and astrocytes in respiratory rhythm modulation has been recently investigated in neonatal rodents. However, no information on the role of ATP and astrocytes within the respiratory network of the lamprey is available, particularly within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator. To address these issues, the present study was carried out on isolated brainstems of the adult lamprey. Bath application of ATP caused marked increases in respiratory frequency followed by decreases in the respiratory motor output, mediated by the ATP metabolite adenosine at the level of the pTRG. Bath applications and microinjections of agonists and antagonists of purinergic receptors showed that ATP increased respiratory activity through an action on pTRG P2X receptors. To disclose the respiratory role of astrocytes, we used bath application of the gliotoxin aminoadipic acid, which dramatically depressed the respiratory motor output that, however, promptly recovered following glutamine application. Furthermore, the excitatory responses to ATP-γ-S (a non-hydrolysable ATP analogue), but not to substance P, microinjected into the pTRG, were abolished. Finally, we also demonstrated that acidification-induced increases in respiratory activity were ATP-independent, but mediated by the astrocytes' glutamate-glutamine cycle. The results show for the first time that ATP and especially astrocytes strongly contribute to the modulation of the lamprey respiratory pattern. Their role in the modulation or maintenance of rhythmic neuronal activities appears to be phylogenetically conserved.

Lamprey: a model for vertebrate evolutionary research

Lampreys belong to the superclass Cyclostomata and represent the most ancient group of vertebrates. Existing for over 360 million years, they are known as living fossils due to their many evolutionally conserved features. They are not only a keystone species for studying the origin and evolution of vertebrates, but also one of the best models for researching vertebrate embryonic development and organ differentiation. From the perspective of genetic information, the lamprey genome remains primitive compared with that of other higher vertebrates, and possesses abundant functional genes. Through scientific and technological progress, scientists have conducted in-depth studies on the nervous, endocrine, and immune systems of lampreys. Such research has significance for understanding and revealing the origin and evolution of vertebrates, and could contribute to a greater understanding of human diseases and treatments. This review presents the current progress and significance of lamprey research.

Cloning of the GABAB Receptor Subunits B1 and B2 and their Expression in the Central Nervous System of the Adult Sea Lamprey

In vertebrates, γ-aminobutyric acid (GABA) is the main inhibitory transmitter in the central nervous system (CNS) acting through ionotropic (GABA_A) and metabotropic (GABA_B) receptors. The GABA_B receptor produces a slow inhibition since it activates second messenger systems through the binding and activation of guanine nucleotide-binding proteins [G-protein-coupled receptors (GPCRs)]. Lampreys are a key reference to understand molecular evolution in vertebrates. The importance of the GABA_B receptor for the modulation of the circuits controlling locomotion and other behaviors has been shown in pharmacological/physiological studies in lampreys. However, there is no data about the sequence of the GABA_B subunits or their expression in the CNS of lampreys. Our aim was to identify the sea lamprey GABA_B1 and GABA_B2 transcripts and study their expression in the CNS of adults. We cloned two partial sequences corresponding to the GABA_B1 and GABA_B2 cDNAs of the sea lamprey as confirmed by sequence analysis and comparison with known GABA_B sequences of other vertebrates. In phylogenetic analyses, the sea lamprey GABA_B sequences clustered together with GABA_Bs sequences of vertebrates and emerged as an outgroup to all gnathostome sequences. We observed a broad and overlapping expression of both transcripts in the entire CNS. Expression was mainly observed in neuronal somas of the periventricular regions including the identified reticulospinal cells. No expression was observed in identifiable fibers. Comparison of our results with those reported in other vertebrates indicates that a broad and overlapping expression of the GABA_B subunits in the CNS is a conserved character shared by agnathans and gnathostomes.

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.

State-Dependent Modulation of Locomotion by GABAergic Spinal Sensory Neurons

The cerebrospinal fluid (CSF) constitutes an interface through which chemical cues can reach and modulate the activity of neurons located at the epithelial boundary within the entire nervous system. Here, we investigate the role and functional connectivity of a class of GABAergic sensory neurons contacting the CSF in the vertebrate spinal cord and referred to as CSF-cNs. The remote activation of CSF-cNs was shown to trigger delayed slow locomotion in the zebrafish larva, suggesting that these cells modulate components of locomotor central pattern generators (CPGs). Combining anatomy, electrophysiology, and optogenetics in vivo, we show that CSF-cNs form active GABAergic synapses onto V0-v glutamatergic interneurons, an essential component of locomotor CPGs. We confirmed that activating CSF-cNs at rest induced delayed slow locomotion in the fictive preparation. In contrast, the activation of CSF-cNs promptly inhibited ongoing slow locomotion. Moreover, selective activation of rostral CSF-cNs during ongoing activity disrupted rostrocaudal propagation of descending excitation along the spinal cord, indicating that CSF-cNs primarily act at the premotor level. Altogether, our results demonstrate how a spinal GABAergic sensory neuron can tune the excitability of locomotor CPGs in a state-dependent manner by projecting onto essential components of the excitatory premotor pool.

Diving into the mammalian swamp of respiratory rhythm generation with the bullfrog

All vertebrates produce some form of respiratory rhythm, whether to pump water over gills or ventilate lungs. Yet despite the critical importance of ventilation for survival, the architecture of the respiratory central pattern generator has not been resolved. In frogs and mammals, there is increasing evidence for multiple burst-generating regions in the ventral respiratory group. These regions work together to produce the respiratory rhythm. However, each region appears to be pivotally important to a different phase of the motor act. Regions also exhibit differing rhythmogenic capabilities when isolated and have different CO2 sensitivity and pharmacological profiles. Interestingly, in both frogs and rats the regions with the most robust rhythmogenic capabilities when isolated are located in rhombomeres 7/8. In addition, rhombomeres 4/5 in both clades are critical for controlling phases of the motor pattern most strongly modulated by CO2 (expiration in mammals, and recruitment of lung bursts in frogs). These key signatures may indicate that these cell clusters arose in a common ancestor at least 400 million years ago.

Three brainstem areas involved in respiratory rhythm generation in bullfrogs

For most multiphasic motor patterns, rhythm and pattern are produced by the same circuit elements. For respiration, however, these functions have long been assumed to occur separately. In frogs, the ventilatory motor pattern produced by the isolated brainstem consists of buccal and biphasic lung bursts. Previously, two discrete necessary and sufficient sites for lung and buccal bursts were identified. Here we identify a third site, the Priming Area, important for and having neuronal activity correlated with the first phase of biphasic lung bursts. As each site is important for burst generation of a separate phase, we suggest each major phase of ventilation is produced by an anatomically distinct part of an extensive brainstem network. Embedding of discrete circuit elements producing major phases of respiration within an extensive rhythmogenic brainstem network may be a shared architectural characteristic of vertebrates.

ABSTRACT

Ventilation in mammals consists of at least three distinct phases: inspiration, post-inspiration and late-expiration. While distinct brainstem rhythm generating and pattern forming networks have long been assumed, recent data suggest the mammalian brainstem contains two coupled neuronal oscillators: one for inspiration and the other for active expiration. However, whether additional burst generating ability is required for generating other phases of ventilation in mammals is controversial. To investigate brainstem circuit architectures capable of producing multiphasic ventilatory rhythms, we utilized the isolated frog brainstem. This preparation produces two types of ventilatory motor patterns, buccal and lung bursts. Lung bursts can be divided into two phases, priming and powerstroke. Previously we identified two putative oscillators, the Buccal and Lung Areas. The Lung Area produces the lung powerstroke and the Buccal Area produces buccal bursts and - we assumed - the priming phase of lung bursts. However, here we identify an additional brainstem region that generates the priming phase. This Priming Area extends rostral and caudal of the Lung Area and is distinct from the Buccal Area. Using AMPA microinjections and reversible synaptic blockade, we demonstrate selective excitation and ablation (respectively) of priming phase activity. We also demonstrate that the Priming Area contains neurons active selectively during the priming phase. Thus, we propose that three distinct neuronal components generate the multiphase respiratory motor pattern produced by the frog brainstem: the buccal, priming and powerstroke burst generators. This raises the possibility that a similar multi-burst generator architecture mediates the three distinct phases of ventilation in mammals.

Neural mechanisms underlying respiratory rhythm generation in the lamprey

The isolated brainstem of the adult lamprey spontaneously generates respiratory activity. The paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator, has been anatomically and functionally characterized. It is sensitive to opioids, neurokinins and acetylcholine. Excitatory amino acids, but not GABA and glycine, play a crucial role in the respiratory rhythmogenesis. These results are corroborated by immunohistochemical data. While only GABA exerts an important modulatory control on the pTRG, both GABA and glycine markedly influence the respiratory frequency via neurons projecting from the vagal motoneuron region to the pTRG. Noticeably, the removal of GABAergic transmission within the pTRG causes the resumption of rhythmic activity during apnea induced by blockade of glutamatergic transmission. The same result is obtained by microinjections of substance P or nicotine into the pTRG during apnea. The results prompted us to present some considerations on the phylogenesis of respiratory pattern generation. They may also encourage comparative studies on the basic mechanisms underlying respiratory rhythmogenesis of vertebrates.