Abstract Spontaneous voltage oscillations in neuronal ensembles play a critical role in memory formation and storage. Although oscillatory activities arising endogenously within central pattern-generating (CPG) networks underlie many rhythmic motor behaviors, the contribution of such autonomous signals to motor learning and memory remains poorly understood. Previously, we found that the buccal CPG network driving food-seeking behavior in Aplysia contains a subset of electrically-coupled neurons that produces spontaneous, variable-amplitude voltage oscillations instigating infrequent and irregular cycles of patterned motor output. This pattern-initiating activity originates from organelle-derived, inositol triphosphate (IP3) receptor-dependent calcium oscillations in a pair of identified decision-making neurons (B63) within the CPG subset (Bédécarrats et al., 2021). Here, we show that the cycle frequency of this spontaneous pacemaker mechanism based on intracellular calcium store release is persistently increased by operant reward-learning, and in association with increased B63 excitability, constitutes a fundamental memory trace for accelerated and stereotyped rhythmic food-seeking movements. eLife Assessment Slow voltage oscillations in neuronal ensembles can contribute to learning and memory. This study identifies long-lasting plasticity in the dynamics of oscillatory intracellular calcium store release in key neurons of Aplysia ’s food-seeking network, serving as a sub-cellular substrate for operant-reward learning that leads to the rhythmic expression of motor output producing compulsive-like behavior.

Hans Straka died in the morning of December 11, 2022 at his home in Munich, unexpected and much too early. He was a dedicated biologist, loved the mountains and was connected to home (Oberammergau, active participant in the Passion Play). His scientific journey took him from Munich via Paris and New York back to Munich and his many academic accomplishments ranged from a membership of the Editorial board of the Journal of Neurophysiology and of the Journal of Neuroscience. He was associate editor for Frontiers in Neuro-otology and for the volume "The Senses" he edited the part on Vestibular Function in 2020. In 2009 he became Professor of Systemic Neurosciences at the Department of Biology in Munich. Apart from his many academic accomplishments, however, Hans was a close friend to those of us who were fortunate enough to get to know him better.

Hans Straka died in the morning of December 11, 2022 at his home in Munich, unexpected and much too early. He was a dedicated biologist, loved the mountains and was connected to home (Oberammergau, active participant in the Passion Play). His scientific journey took him from Munich via Paris and New York back to Munich and his many academic accomplishments ranged from a membership of the Editorial board of the Journal of Neurophysiology and of the Journal of Neuroscience. He was associate editor for Frontiers in Neuro-otology and for the volume "The Senses" he edited the part on Vestibular Function in 2020. In 2009 he became Professor of Systemic Neurosciences at the Department of Biology in Munich. Apart from his many academic accomplishments, however, Hans was a close friend to those of us who were fortunate enough to get to know him better.

The transition from larval to adult locomotion in the anuran, Xenopus laevis, involves a dramatic switch from axial to appendicular swimming including intermediate stages when the tail and hindlimbs co-exist and contribute to propulsion. Hatchling tadpole swimming is generated by an axial central pattern generator (CPG) which matures rapidly during early larval life. During metamorphosis, the developing limbs are controlled by a de novo appendicular CPG driven initially by the axial system before segregating to allow both systems to operate together or independently. Neuromodulation plays important roles throughout, but key modulators switch their effects from early inhibitory influences to facilitating locomotion. Temperature affects the construction and operation of locomotor networks and global changes in environmental temperature place aquatic poikilotherms, like amphibians, at risk. The locomotor control strategy of anurans differs from other amphibian groups such as salamanders, where evolution has acted upon the thyroid hormone pathway to sculpt different developmental outcomes.

Neural replicas of the spinal motor commands that drive locomotion have become increasingly recognized as an intrinsic neural mechanism for producing gaze-stabilizing eye movements that counteract the perturbing effects of self-generated head/body motion. By pre-empting reactive signaling by motion-detecting vestibular sensors, such locomotor efference copies (ECs) provide estimates of the sensory consequences of behavioral action. Initially demonstrated in amphibian larvae during spontaneous fictive swimming in deafferented in vitro preparations, direct evidence for a contribution of locomotor ECs to gaze stabilization now extends to the ancestral lamprey and to tetrapod adult frogs and mice. Supporting behavioral evidence also exists for other mammals, including humans, therefore further indicating the mechanism's conservation during vertebrate evolution. The relationship between feedforward ECs and vestibular sensory feedback in ocular movement control is variable, ranging from additive to the former supplanting the latter, depending on vestibular sensing ability, and the intensity and regularity of rhythmic locomotor movements.

Motivated behaviors such as feeding depend on the functional properties of decision neurons to provide the flexibility required for behavioral adaptation. Here, we analyzed the ionic basis of the endogenous membrane properties of an identified decision neuron (B63) that drive radula biting cycles underlying food-seeking behavior in Aplysia. Each spontaneous bite cycle arises from the irregular triggering of a plateau-like potential and resultant bursting by rhythmic subthreshold oscillations in B63's membrane potential. In isolated buccal ganglion preparations, and after synaptic isolation, the expression of B63's plateau potentials persisted after removal of extracellular calcium, but was completely suppressed in a tetrodotoxin (TTX)- containing bath solution, thereby indicating the contribution of a transmembrane Na+ influx. Potassium outward efflux through tetraethylammonium (TEA)- and calcium-sensitive channels was found to contribute to each plateau's active termination. This intrinsic plateauing capability, in contrast to B63's membrane potential oscillation, was blocked by the calcium-activated non-specific cationic current (I-CAN) blocker flufenamic acid (FFA). Conversely, the SERCA blocker cyclopianozic acid (CPA), which abolished the neuron's oscillation, did not prevent the expression of experimentally evoked plateau potentials. These results therefore indicate that the dynamic properties of the decision neuron B63 rely on two distinct mechanisms involving different sub-populations of ionic conductances.

Central circuitry of the vestibular nuclei integrates sensory inputs in the adaptive control of motor behaviors such as posture, locomotion, and gaze stabilization. Thus far, such circuits have been mostly examined at mature stages, whereas their emergence and early development have remained poorly described. Here, we focused on the perinatal period of murine development, from embryonic day E14.5 to post-natal day P5, to investigate the ontogeny of two functionally distinct vestibular neuronal groups, neurons projecting to the spinal cord via the lateral vestibulospinal tract (LVST) and commissural neurons of the medial vestibular nucleus that cross the midline to the contralateral nucleus. Using transgenic mice and retrograde labeling, we found that network-constitutive GABAergic and glycinergic neurons are already established in the two vestibular groups at embryonic stages. Although incapable of repetitive firing at E14.5, neurons of both groups can generate spike trains from E15.5 onward and diverge into previously established A or B subtypes according to the absence (A) or presence (B) of a two-stage spike after hyperpolarization. Investigation of several voltage-dependent membrane properties indicated that solely LVST neurons undergo significant maturational changes in their electrophysiological characteristics during perinatal development. The proportions of A vs B subtypes also evolve in both groups, with type A neurons remaining predominant at all stages, and type B commissural neurons appearing only post-natally. Together, our results indicate that vestibular neurons acquire their distinct morpho-functional identities after E14.5 and that the early maturation of membrane properties does not emerge uniformly in the different functional subpopulations of vestibulo-motor pathways.

Vertebrate locomotion presents a major challenge for maintaining visual acuity due to head movements resulting from the intimate biomechanical coupling with the propulsive musculoskeletal system. Retinal image stabilization has been traditionally ascribed to the transformation of motion-related sensory feedback into counteracting ocular motor commands. However, extensive exploration of spontaneously active semi-intact and isolated brain/spinal cord preparations of the amphibian Xenopus laevis, have revealed that efference copies (ECs) of the spinal motor program that generates axial- or limb-based propulsion directly drive compensatory eye movements. During fictive locomotion in larvae, ascending ECs from rostral spinal central pattern generating (CPG) circuitry are relayed through a defined ascending pathway to the mid- and hindbrain ocular motor nuclei to produce conjugate eye rotations during tail-based undulatory swimming in the intact animal. In post-metamorphic adult frogs, this spinal rhythmic command switches to a bilaterally-synchronous burst pattern that is appropriate for generating convergent eye movements required for maintaining image stability during limb kick-based rectilinear forward propulsion. The transition between these two fundamentally different coupling patterns is underpinned by the emergence of altered trajectories in spino-ocular motor coupling pathways that occur gradually during metamorphosis, providing a goal-specific, morpho-functional plasticity that ensures retinal image stability irrespective of locomotor mode. Although the functional impact of predictive ECs produced by the locomotory CPG matches the spatio-temporal specificity of reactive sensory-motor responses, rather than contributing additively to image stabilization, horizontal vestibulo-ocular reflexes (VORs) are selectively suppressed during intense locomotor CPG activity. This is achieved at least in part by an EC-mediated attenuation of mechano-electrical encoding at the vestibular sensory periphery. Thus, locomotor ECs and their potential suppressive impact on vestibular sensory-motor processing, both of which have now been reported in other vertebrates including humans, appear to play an important role in the maintenance of stable vision during active body displacements.

Microglia, brain-resident macrophages, play key roles during prenatal development in defining neural circuitry function, including ensuring proper synaptic wiring and maintaining homeostasis. Mammalian breathing rhythmogenesis arises from interacting brainstem neural networks that are assembled during embryonic development, but the specific role of microglia in this process remains unknown. Here, we investigated the anatomical and functional consequences of respiratory circuit formation in the absence of microglia. We first established the normal distribution of microglia within the wild-type (WT, Pu.1 +/+ ) mouse brainstem at embryonic ages when the respiratory networks are known to emerge (embryonic day (E) 14.5 for the parafacial respiratory group (epF) and E16.5 for the preBötzinger complex (preBötC)). In transgenic mice depleted of microglia (Pu.1 -/- mutant), we performed anatomical staining, calcium imaging and electrophysiological recordings of neuronal activities in vitro to assess the status of these circuits at their respective times of functional emergence. Spontaneous respiratory-related activity recorded from reduced in vitro preparations showed an abnormally slow rhythm frequency expressed by the epF at E14.5, the preBötC at E16.5 and in the phrenic motor nerves from E16.5 onwards. These deficits were associated with a reduced number of active epF neurons, defects in commissural projections that couple the bilateral preBötC half-centers, and an accompanying decrease in their functional coordination. These abnormalities probably contribute to eventual neonatal death, since plethysmography revealed that E18.5 Pu.1 -/- embryos are unable to sustain breathing activity ex utero . Our results thus point to a crucial contribution of microglia in the proper establishment of the central respiratory command during embryonic development.

Opioids are a mainstay of pain management but can induce unwanted effects, including analgesic tolerance and paradoxical hyperalgesia, either of which leads to increased pain. Clinically, however, the relationship between these two phenomena remains elusive. By evaluating changes in mechanical nociceptive threshold in male rats, we found that in contrast to a purely analgesic control response to a single subcutaneous administration of fentanyl (25 μg/kg), in rats subjected to inflammatory pain 2 weeks previously (Day0), the same test dose (D13) induced a bi-phasic response: initial decreased analgesia (tolerance) followed by hyperalgesia lasting several hours. Both the tolerance and hyperalgesia were further enhanced in rats that had additionally received fentanyl on D0. The dose-response profiles (5 fg to 50 μg/kg) of pain- and opioid-experienced rats were very different from pain/drug-naive rats. At ultra-low fentanyl doses (<5 ng/kg and <500 ng/kg for naïve control and pain/drug-experienced rats, respectively), solely hyperalgesia was observed in all cases. At higher doses, which now produced analgesia alone in naive rats, reduced analgesia (tolerance) coupled with hyperalgesia occurred in pain/fentanyl-experienced rats, with both phases increasing with dose. Transcriptomic and pharmacological data revealed that an overactivation of the spinal N-methyl-D-aspartate receptor-inducible NO synthase cascade plays a critical role in both acute tolerance and hyperalgesia, and together with the finding that the magnitudes of analgesia and associated hyperalgesia are negatively correlated, is indicative of closely related phenomena. Finally, a polyamine deficient diet prevented inducible NO synthase transcript upregulation, restored fentanyl's analgesic efficacy and suppressed the emergence of hyperalgesia.

Key points Stimulation of hindlimb afferent fibres can both stabilize and increase the activity of fore- and hindlimb motoneurons during fictive locomotion. The increase in motoneuron activity is at least partially due to the production of doublets of action potentials in a subpopulation of motoneurons. These results were obtained using an in vitro brainstem/spinal cord preparation of neonatal rat. Quadrupedal locomotion relies on a dynamic coordination between central pattern generators (CPGs) located in the cervical and lumbar spinal cord, and controlling the fore- and hindlimbs, respectively. It is assumed that this CPG interaction is achieved through separate closed-loop processes involving propriospinal and sensory pathways. However, the functional consequences of a concomitant involvement of these different influences on the degree of coordination between the fore- and hindlimb CPGs is still largely unknown. Using an in vitro brainstem/spinal cord preparation of neonatal rat, we found that rhythmic, bilaterally alternating stimulation of hindlimb sensory input pathways elicited coordinated hindlimb and forelimb CPG activity. During pharmacologically induced fictive locomotion, lumbar dorsal root (DR) stimulation entrained and stabilized an ongoing cervico-lumbar locomotor-like rhythm and increased the amplitude of both lumbar and cervical ventral root bursting. The increase in cervical burst amplitudes was correlated with the occurrence of doublet action potential firing in a subpopulation of motoneurons, enabling the latter to transition between low and high frequency discharge according to the intensity of DR stimulation. Moreover, our data revealed that propriospinal and sensory pathways act synergistically to strengthen cervico-lumbar interactions. Indeed, split-bath experiments showed that fully coordinated cervico-lumbar fictive locomotion was induced by combining pharmacological stimulation of either the lumbar or cervical CPGs with lumbar DR stimulation. This study thus highlights the powerful interactions between sensory and propriospinal pathways which serve to ensure the coupling of the fore- and hindlimb CPGs for effective quadrupedal locomotion.

The expression of motivated behaviors depends on both external and internally arising neural stimuli, yet the intrinsic releasing mechanisms for such variably occurring behaviors remain elusive. In isolated nervous system preparations of Aplysia, we have found that irregularly expressed cycles of motor output underlying food-seeking behavior arise from regular membrane potential oscillations of varying magnitude in an identified pair of interneurons (B63) in the bilateral buccal ganglia. This rhythmic signal, which is specific to the B63 cells, is generated by organelle-derived intracellular calcium fluxes that activate voltage-independent plasma membrane channels. The resulting voltage oscillation spreads throughout a subset of gap junction-coupled buccal network neurons and by triggering plateau potential-mediated bursts in B63, can initiate motor output driving food-seeking action. Thus, an atypical neuronal pacemaker mechanism, based on rhythmic intracellular calcium store release and intercellular propagation, can act as an autonomous intrinsic releaser for the occurrence of a motivated behavior.

Frog metamorphosis includes a complete switch in the animal's locomotor strategy from larval undulatory, tail-based swimming to rhythmic hindlimb-kick propulsion in the young adult. At critical stages during this dramatic behavioral transition, both locomotor systems are present and functional, implying a progressive and dynamic remodeling of underlying spinal motor circuitry as limbs are added and the tail regresses. This process thus provides an attractive model for studying mechanisms of neural network assembly/disassembly during locomotor system development. Novel insights have been facilitated by the deployment of isolated brainstem/spinal cord preparations from the frog Xenopus, in vitro preparations that spontaneously generate the motor output rhythms driving swimming in vivo. As in vertebrates generally, the spinal locomotor circuits of Xenopus are potently modulated by biogenic amines and nitric oxide. This chapter reviews current knowledge on metamorphosis and the contrasting and changing roles of neuromodulators, both in the immediate adaptive control of axial and limb network output and in long-term circuit development.

John Simmers works with the CNRS as Emeritus Director in the Institut de Neurosciences Cognitives et Intégratives d’Aquitaine at the University of Bordeaux, France. He was born and raised in New Zealand, but most of his career has been spent in France. After obtaining first degrees (BSc, MSc) in Zoology at the University of Otago in Dunedin in 1973 and 1975, respectively, he did his PhD in Neurophysiology at the University of Bristol in the UK, before moving to the Bassin d’Arcachon in southwest France in 1981, first as a postdoc and subsequently as a permanent researcher for the French CNRS. His work focuses on the neural basis of short-term and long-term plasticity of rhythmic motor systems, with the principle objectives of trying to relate cellular, synaptic and neural network physiology to adaptive behavior. In addition to the particularity of studying different motor behaviors in a variety of animals, he maintains that his primary claim to scientific fame is that, at 6 feet 6 inches, he is arguably the tallest neurobiologist on the planet. What brought you to neurobiology in the first place? I was an extremely late starter, coming to neurobiology in the most unlikely and convoluted way imaginable. Thinking back on high school days and the stimulation of an excellent biology teacher, Steve Gillanders, I realize that the seeds of a much later germinating love for biology were sown at that time. I also spent summer holidays working on my family’s sheep farm and that always kept me close to nature and animal behavior. I was fortunate to have parents who were supportive of whatever academic directions I wanted to follow — my mother especially, who, mainly because of growing up in a rather conservative and isolated farming district where academic achievement (especially for females) was not a high priority, had received little secondary education and had made it a primary parental mission to ensure that her offspring did not follow the same route. My first two years at university were fairly catastrophic, with my interests turning more to the extracurricular side of student activities than to the real reason for being on campus in the first place. In those two years, if there had been majors in billiards, beer and boyish behavior, I would have already gained a first-class honors degree. However, with persistence and patience, mostly from my parents and the university’s student progress assessment committee, I extracted the proverbial digit, rekindled some semblance of work ethic and, under the guidance of my research project supervisor, John Pilkington, went on to complete bachelor’s and master’s degrees in Zoology. But I still wasn’t there yet. It was already vogue in those days for new graduates to take a gap year to supposedly recharge batteries and reflect on future career directions. Well, I went for the gulf year version in deciding with my new wife to worry about the future later and take off abroad on the kiwi overseas experience. In moving to London, the plan was to overwinter in casual jobs and explore Europe in the interim. It was after two years of this nomadic and insouciant routine that academia returned to my life. By an incredible stroke of luck, Brian Bush from the Physiology Department at Bristol University contacted me after reading my MSc thesis, which I had sent to him in a long-shot attempt to sell myself. For reasons that still escape me, Brian, bless him, chose to rescue me from the dungeon, both metaphorically and literally, given that I was laboring in the bowels of London as a miner in the new Jubilee Line section of the London Underground at the time, 100 feet below Trafalgar Square. I expressly mention this job, which was physically tough and unrelenting, because it contributed enormously to kick-starting where I am today. Most of the blokes I worked with in the tunnel were hard men, although as inherently smart as anyone I have met. The point of difference was simply opportunity — I had been given (and was in the process of throwing away) the gift of an advanced education, whereas they had languished on the opposite side of the ledger, without much support or schooling, and with a resultant lack of career hope or ambition. I was becoming increasingly aware of this divide and appreciative of the high-number cards my kiwi upbringing had dealt me. I guess the bottom line to this is that, when the ‘what-do-I-really-want-to-do-with-the-rest-of-my-life’ penny finally drops, then don’t hesitate, just go for it. The dawn may be interminably long to break, but if the stars of desire, luck and circumstances align then it’s never too late. I had almost missed the boat once, was in the process of missing it again, and I decided that enough was enough. So, after moving to Bristol and falling under the never-ending support and encouragement of Brian Bush, I finally entered the wondrous realm of neurobiology. Brian was interested in some of my master’s thesis work on the rhythmic movements of bailer-like appendages (the splendidly named scaphognathites) that irrigate and thus help oxygenate the gills of crabs. Rhythmically active pacemaker neurons that function without themselves producing action potentials were thought to generate the motor patterns that drive these movements. In connecting with Brian’s own interest in crustacean non-spiking neurons, my PhD project aimed to characterize these atypical oscillatory cells and scaphognathite neural network activity with isolated CNS preparations and cellular electrophysiology. What were the further, major stepping stones in your career? My belated discovery of neurobiology was followed by a series of pivotal, early circumstances, again mostly related to chance. Toward the end of my PhD, I met a French neurobiologist, François Clarac, at an annual meeting for the Society of Experimental Biology. As well as rugby, we discovered a mutual interest in the neural basis of rhythmic behavior (in his case, locomotor movements in crustaceans), and François persuaded me to join him as a postdoc for 18 months in southwest France on the magnificent Bay of Arcachon at a comparative neurobiology laboratory, a satellite of the University of Bordeaux. A key factor in this decision was my wife Isobel’s acceptance to postpone a budding career in teaching. Such a professional self-sacrifice from a partner cannot be understated — she took a career hit (in truth, a demolition) for me and I will be eternally grateful. After my postdoc with François, the next, critical step was my introduction to one of the most important systems for understanding the neural fundamentals of rhythmic behavior, the stomatogastric ganglion (STG) system, which controls food processing movements in the crustacean foregut. Working on this iconic system marked the point when I became seriously passionate about neurobiology, and I still feel privileged to have been a member of the STG research family. In the 1970s, Maurice Moulins in Arcachon (along with his close friend Allen Selverston in San Diego) met Don Maynard, who was the first to realize the potential advantages of the STG system as a simpler and tractable model for studying motor control. Maurice had become particularly interested in the functional dynamics of the STG rhythmogenic networks and his ‘cheval de bataille’, which I too soon mounted, was to understand this flexibility in terms of identified neuromodulatory and sensory influences on the membrane and synaptic properties of circuit neurons. The final, early stepping stone under Maurice’s encouragement was my recruitment as a junior researcher into the French CNRS. This also involved a major family decision (by now we had two young kids) because it meant that our lives were about to permanently forsake the culture and language of Shakespeare for those of Molière. But we embraced this transition and France has been good to us. I feel indebted to my adopted country and especially to the CNRS for giving me a career opportunity that, at my relatively advanced age at the time, would probably not have been obtainable in the Anglo-Saxon academic system. Who are your scientific heroes? My choice in heroes is twofold. The first category definitely embodies the wonderful animals that I have been fortunate to study. Here again, my research has been atypical in that it has spread to a variety of model systems, as opposed to the more conventional tendency of being ‘known’ for work on a single organism. Once hooked on the neurobiology of rhythmic movement generally, my research has ranged across the respiratory system of crabs and embryonic mice, the foregut and feeding rhythms, respectively, of lobsters and the mollusk Aplysia, the locomotor systems of crayfish, neonatal rats, and postembryonic tadpoles, and the neural development of locomotion and its control of gaze in metamorphosing frogs. Each of these biological gems and their behaviors have divulged an array of intriguing secrets about motor control, so to my mind these guys are the real heroes. My second hero category encompasses the scientists I have been so lucky to work with over the years. I am firmly of the belief that you do better science if you actually like and get on well with the people who share your research bed. In addition to the previously mentioned Brian Bush, François Clarac and Maurice Moulins, my associates who became far more than just collaborators include former PhD students Denis Combes, Muriel Thoby-Brisson (both now in Bordeaux) and Stefan Clemens (Greenville, North Carolina), as well as David Macmillan (Melbourne), Hans Straka (Munich) and my long-standing great mate Keith Sillar in St Andrews. I have been truly fortunate to have worked alongside these super-talented people, who probably also deem themselves heroic in putting up with me! How does the push toward more applied science affect your work? Having just attended the biennial conference of the International Society for Neuroethology in Brisbane, and as always with this wonderful meeting, I was struck by the diversity and richness of animal models and neurobiological questions that researchers are addressing, whether these concern song production in birds or fish, the navigation skills of flying insects, or the maternal assiduity of a poisonous frog. But an ever recurring concern amongst neurobiologists, especially those in the early stages of their careers, is the need to convince both their future institutional employers and research funding agencies of the value and importance of their work in this increasingly restrictive world of human navel-gazing. Of course, fathoming issues related to human health and disease is both desirable and understandable. But gaining an understanding of how the nervous system operates is an all-embracing task that extends far beyond elucidating the specific good or bad workings of the human brain. After all, it is the advancement of scientific knowledge on a broad front that enables upward reaching fingers of important breakthroughs to emerge. Indeed, the ionic bases of the action and bursting potentials, electrical coupling, mechanisms of neural circuit neuromodulation and the cellular correlates of learning and memory readily come to mind as examples of neural mechanisms that were first discovered in invertebrates. So, my research philosophy has stuck stubbornly to the analogy that, to understand how a combustion engine (aka the brain) works, it can be more instructive to dismantle the simpler and more accessible lawn mower than to peer confoundedly beneath the bonnet of a Formula One car! Has your approach to research evolved over the years? When I started my career in the 1980s, research seemed to be fun — more like organized play for adults. We were under pressure to produce, but it was a period when investigators were looking far and wide for suitable experimental preparations for answering a particular, interesting question in neuroscience. The strategy then, and that I believe is still valid today, was to try and dissociate general principles of nervous system function from idiosyncratic features by making comparisons across different species. Today, with the advent of impact factors and H indices, fundamental research seems to have become much less fun, being increasingly driven by bibliometrics and the dictates of funding agencies through a pressure to concentrate efforts on humans and a few genetically modifiable models — no names mentioned, although a clue to one such system lies in the above image, which is also testament to the extent to which the transgenic approach can go so horribly wrong — with only a few recalcitrant labs persisting with work on other animals. However, the presence of close to 500 neuroethologists at the recent Brisbane ISN meeting would suggest that the problem is not numbers. Perhaps we neurobiologists engaged in basic research are failing to convince the wider public that the study of different species, such as insects, snails, crabs and frogs, will still allow us to uncover principles that might not arise from the current push to solve everything with a handful of preferred model systems. What are the challenges facing young neurobiologists today? There are several huge challenges that currently confront young scientists. Firstly, the increasing number of PhD graduates on the market means an increasingly intense competition for tenured faculty positions and a delay in job security. Unfortunately, this can often lead to talented and potentially excellent, young researchers turning away from science to pursue alternative careers. Secondly, biological research is becoming increasingly enamored with ‘big data’. But big science requires big money (with a commensurate intensifying of competition for funding) and leads to a further narrowing of research perspectives to topics that are deemed ‘hot’ or the most medically relevant. Here again, science ultimately benefits most from diversity and thankfully this view is still championed by major journals, such as Current Biology! Thirdly, with increasing competition for funding, young scientists who finally make it are faced as much with the time-consuming business of trying to convince funding agencies as doing actual research. Indeed, the amount of acquired money has become as (or even more) critical for success as having exciting new research ideas, doing the necessary experimental work and writing papers.

In vertebrates, functional motoneurons are defined as differentiated neurons that are connected to a central premotor network and activate peripheral muscle using acetylcholine. Generally, motoneurons and muscles develop simultaneously during embryogenesis. However, during Xenopus metamorphosis, developing limb motoneurons must reach their target muscles through the already established larval cholinergic axial neuromuscular system. Here, we demonstrate that at metamorphosis onset, spinal neurons retrogradely labeled from the emerging hindlimbs initially express neither choline acetyltransferase nor vesicular acetylcholine transporter. Nevertheless, they are positive for the motoneuronal transcription factor Islet1/2 and exhibit intrinsic and axial locomotor-driven electrophysiological activity. Moreover, the early appendicular motoneurons activate developing limb muscles via nicotinic antagonist-resistant, glutamate antagonist-sensitive, neuromuscular synapses. Coincidently, the hindlimb muscles transiently express glutamate, but not nicotinic receptors. Subsequently, both pre- and postsynaptic neuromuscular partners switch definitively to typical cholinergic transmitter signaling. Thus, our results demonstrate a novel context-dependent re-specification of neurotransmitter phenotype during neuromuscular system development.

Adaptive behavior relies on complex neural processing in multiple interacting networks of both motor and sensory systems. One such interaction employs intrinsic neuronal signals, so-called 'corollary discharge' or 'efference copy', that may be used to predict the sensory consequences of a specific behavioral action, thereby enabling self-generated (reafferent) sensory information and extrinsic (exafferent) sensory inflow to be dissociated. Here, by using well-established examples, we seek to identify the distinguishing features of corollary discharge and efference copy within the framework of predictive motor-to-sensory system coordination. We then extend the more general concept of predictive signaling by showing how neural replicas of a particular motor command not only inform sensory pathways in order to gate reafferent stimulation, but can also be used to directly coordinate distinct and otherwise independent behaviors to the original motor task. Moreover, this motor-to-motor pairing may additionally extend to a gating of sensory input to either or both of the coupled systems. The employment of predictive internal signaling in such motor systems coupling and remote sensory input control thus adds to our understanding of how an organism's central nervous system is able to coordinate the activity of multiple and generally disparate motor and sensory circuits in the production of effective behavior.

In central respiratory circuitry, synaptic excitation is responsible for synchronizing neuronal activity in the different respiratory rhythm phases, whereas chloride-mediated inhibition is important for shaping the respiratory pattern itself. The potassium chloride cotransporter KCC2, which serves to maintain low intraneuronal Cl- concentration and thus render chloride-mediated synaptic signaling inhibitory, exists in two isoforms, KCC2a and KCC2b. KCC2 is essential for functional breathing motor control at birth, but the specific contribution of the KCC2a isoform remains unknown. Here, to address this issue, we investigated the respiratory phenotype of mice deficient for KCC2a. In vivo plethysmographic recordings revealed that KCC2a-deficient pups at P0 transiently express an abnormally low breathing rate and a high occurrence of apneas. Immunostainings confirmed that KCC2a is normally expressed in the brainstem neuronal groups involved in breathing (pre-Bötzinger complex, parafacial respiratory group, hypoglossus nucleus) and is absent in these regions in the KCC2a-/- mutant. However, in variously reduced in vitro medullary preparations, spontaneous rhythmic respiratory activity is similar to that expressed in wild-type preparations, as is hypoglossal motor output, and no respiratory pauses are detected, suggesting that the rhythm-generating networks are not intrinsically affected in mutants at P0. In contrast, inhibitory neuromodulatory influences exerted by the pons on respiratory rhythmogenesis are stronger in the mutant, thereby explaining the breathing anomalies observed in vivo. Thus, our results indicate that the KCC2a isoform is important for establishing proper breathing behavior at the time of birth, but by acting at sites that are extrinsic to the central respiratory networks themselves.

The central command for breathing arises mainly from two interconnected rhythmogenic hindbrain networks, the parafacial respiratory group (pFRG or epF at embryonic stages) and the preBötzinger complex (preBötC), which are comprised of a limited number of neurons located in confined regions of the ventral medulla. In rodents, both networks become active toward the end of gestation but little is known about the signaling pathways involved in their anatomical and functional establishment during embryogenesis. During embryonic development, epF and preBötC neurons migrate from their territories of origin to their final positions in ventral brainstem areas. Planar Cell Polarity (PCP) signaling, including the molecule Scrib, is known to control the developmental migration of several hindbrain neuronal groups. Accordingly, a homozygous mutation of Scrib leads to severe disruption of hindbrain anatomy and function. Here, we aimed to determine whether Scrib is also involved in the prenatal development of the hindbrain nuclei controlling breathing. We combined immunostaining, calcium imaging and electrophysiological recordings of neuronal activity in isolated in vitro preparations. In the Scrib mutant, despite severe neural tube defects, epF and preBötC neurons settled at their expected hindbrain positions. Furthermore, both networks remained capable of generating rhythmically organized, respiratory-related activities and exhibited normal sensitivity to pharmacological agents known to modify respiratory circuit function. Thus Scrib is not required for the proper migration of epF and preBötC neurons during mouse embryogenesis. Our findings thus further illustrate the robustness and specificity of the developmental processes involved in the establishment of hindbrain respiratory circuits.

During swimming in the amphibian ITALIC! Xenopus laevis, efference copies of rhythmic locomotor commands produced by the spinal central pattern generator (CPG) can drive extraocular motor output appropriate for producing image-stabilizing eye movements to offset the disruptive effects of self-motion. During metamorphosis, ITALIC! X. laevisremodels its locomotor strategy from larval tail-based undulatory movements to bilaterally synchronous hindlimb kicking in the adult. This change in propulsive mode results in head/body motion with entirely different dynamics, necessitating a concomitant switch in compensatory ocular movements from conjugate left-right rotations to non-conjugate convergence during the linear forward acceleration produced during each kick cycle. Here, using semi-intact or isolated brainstem/spinal cord preparations at intermediate metamorphic stages, we monitored bilateral eye motion along with extraocular, spinal axial and limb motor nerve activity during episodes of spontaneous fictive swimming. Our results show a progressive transition in spinal efference copy control of extraocular motor output that remains adapted to offsetting visual disturbances during the combinatorial expression of bimodal propulsion when functional larval and adult locomotor systems co-exist within the same animal. In stages at metamorphic climax, spino-extraocular motor coupling, which previously derived from axial locomotor circuitry alone, can originate from both axial and ITALIC! de novohindlimb CPGs, although the latter's influence becomes progressively more dominant and eventually exclusive as metamorphosis terminates with tail resorption. Thus, adaptive interactions between locomotor and extraocular motor circuitry allows CPG-driven efference copy signaling to continuously match the changing spatio-temporal requirements for visual image stabilization throughout the transitional period when one propulsive mechanism emerges and replaces another.