Evidence for rhombomeric organization of multiple respiratory oscillators in the bullfrog brainstem

Activation of respiratory-related bursting in an isolated medullary section from adult bullfrogs

Breathing is generated by a rhythmic neural circuit in the brainstem, which contains conserved elements across vertebrate groups. In adult frogs, the 'lung area' located in the reticularis parvocellularis is thought to represent the core rhythm generator for breathing. Although this region is necessary for breathing-related motor output, whether it functions as an endogenous oscillator when isolated from other brainstem centers is not clear. Therefore, we generated thick brainstem sections that encompass the lung area to determine whether it can generate breathing-related motor output in a highly reduced preparation. Brainstem sections did not produce activity. However, subsaturating block of glycine receptors reliably led to the emergence of rhythmic motor output that was further enhanced by blockade of GABAA receptors. Output occurred in singlets and multi-burst episodes resembling the intact network. However, burst frequency was slower and individual bursts had longer durations than those produced by the intact preparation. In addition, burst frequency was reduced by noradrenaline and μ-opioids, and increased by serotonin, as observed in the intact network and in vivo. These results suggest that the lung area can be activated to produce rhythmic respiratory-related motor output in a reduced brainstem section and provide new insights into respiratory rhythm generation in adult amphibians. First, clustering breaths into episodes can occur within the rhythm-generating network without long-range input from structures such as the pons. Second, local inhibition near, or within, the rhythmogenic center may need to be overridden to express the respiratory rhythm.

Evolution of vertebrate respiratory central rhythm generators

Tracing the evolution of the central rhythm generators associated with ventilation in vertebrates is hindered by a lack of information surrounding key transitions. To begin with, central rhythm generation has been studied in detail in only a few species from four vertebrate groups, lamprey, anuran amphibians, turtles, and mammals (primarily rodents). Secondly, there is a lack of information regarding the transition from water breathing fish to air breathing amniotes (reptiles, birds, and mammals). Specifically, the respiratory rhythm generators of fish appear to be single oscillators capable of generating both phases of the respiratory cycle (expansion and compression) and projecting to motoneurons in cranial nerves innervating bucco-pharyngeal muscles. In the amniotes we find oscillators capable of independently generating separate phases of the respiratory cycle (expiration and inspiration) and projecting to pre-motoneurons in the ventrolateral medulla that in turn project to spinal motoneurons innervating thoracic and abdominal muscles (reptiles, birds, and mammals). Studies of the one group of amphibians that lie at this transition (the anurans), raise intriguing possibilities but, for a variety of reasons that we explore, also raise unanswered questions. In this review we summarize what is known about the rhythm generating circuits associated with breathing that arise from the different rhombomeric segments in each of the different vertebrate classes. Assuming oscillating circuits form in every pair of rhombomeres in every vertebrate during development, we trace what appears to be the evolutionary fate of each and highlight the questions that remain to be answered to properly understand the evolutionary transitions in vertebrate central respiratory rhythm generation.

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Neural network model of an amphibian ventilatory central pattern generator

The neuronal multiunit model presented here is a formal model of the central pattern generator (CPG) of the amphibian ventilatory neural network, inspired by experimental data from Pelophylax ridibundus. The kernel of the CPG consists of three pacemakers and two follower neurons (buccal and lung respectively). This kernel is connected to a chain of excitatory and inhibitory neurons organized in loops. Simulations are performed with Izhikevich-type neurons. When driven by the buccal follower, the excitatory neurons transmit and reorganize the follower activity pattern along the chain, and when driven by the lung follower, the excitatory and inhibitory neurons of the chain fire in synchrony. The additive effects of synaptic inputs from the pacemakers on the buccal follower account for (1) the low frequency buccal rhythm, (2) the intra-burst high frequency oscillations, and (3) the episodic lung activity. Chemosensitivity to acidosis is implemented by an increase in the firing frequency of one of the pacemakers. This frequency increase leads to both a decrease in the buccal burst frequency and an increase in the lung episode frequency. The rhythmogenic properties of the model are robust against synaptic noise and pacemaker jitter. To validate the rhythm and pattern genesis of this formal CPG, neurograms were built from simulated motoneuron activity, and compared with experimental neurograms. The basic principles of our model account for several experimental observations, and we suggest that these principles may be generic for amphibian ventilation.

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.

The rostral medulla of bullfrog tadpoles contains critical lung rhythmogenic and chemosensitive regions across metamorphosis

The development of amphibian breathing provides insight into vertebrate respiratory control mechanisms. Neural oscillators in the rostral and caudal medulla drive ventilation in amphibians, and previous reports describe ventilatory oscillators and CO₂ sensitive regions arise during different stages of amphibian metamorphosis. However, inconsistent findings have been enigmatic, and make comparisons to potential mammalian counterparts challenging. In the current study we assessed amphibian central CO₂ responsiveness and respiratory rhythm generation during two different developmental stages. Whole-nerve recordings of respiratory burst activity in cranial and spinal nerves were made from intact or transected brainstems isolated from tadpoles during early or late stages of metamorphosis. Brainstems were transected at the level of the trigeminal nerve, removing rostral structures including the nucleus isthmi, midbrain, and locus coeruleus, or transected at the level of the glossopharyngeal nerve, removing the putative buccal oscillator and caudal medulla. Removal of caudal structures stimulated the frequency of lung ventilatory bursts and revealed a hypercapnic response in normally unresponsive preparations derived from early stage tadpoles. In preparations derived from late stage tadpoles, removal of rostral or caudal structures reduced lung burst frequency, while CO₂ responsiveness was retained. Our results illustrate that structures within the rostral medulla are capable of sensing CO₂ throughout metamorphic development. Similarly, the region controlling lung ventilation appears to be contained in the rostral medulla throughout metamorphosis. This work offers insight into the consistency of rhythmic respiratory and chemosensitive capacities during metamorphosis.

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.