Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

The legacies of A. O. Dennis Willows and Peter A. Getting: neuroscience research using Tritonia

This review was inspired by a January 2024 conference held at Friday Harbor Laboratories, WA, honoring the pioneering work of A.O. Dennis Willows, who initiated research on the sea slug Tritonia diomedea (now T. exsulans). A chance discovery while he was a student at a summer course there has, over the years, led to many insights into the roles of identified neurons in neural circuits and their influence on behavior. Among Dennis's trainees was Peter Getting, whose later groundbreaking work on central pattern generators profoundly influenced the field and included one of the earliest uses of realistic modeling for understanding neural circuits. Research on Tritonia has led to key conceptual advances in polymorphic or multifunctional neural networks, intrinsic neuromodulation, and the evolution of neural circuits. It also has enhanced our understanding of geomagnetic sensing, learning and memory mechanisms, prepulse inhibition, and even drug-induced hallucinations. Although the community of researchers studying Tritonia has never been large, its contributions to neuroscience have been substantial, underscoring the importance of examining a diverse array of animal species rather than focusing on a small number of standard model organisms.

NMJ-related diseases beyond the congenital myasthenic syndromes

Neuromuscular junctions (NMJs) are a special type of chemical synapse that transmits electrical stimuli from motor neurons (MNs) to their innervating skeletal muscle to induce a motor response. They are an ideal model for the study of synapses, given their manageable size and easy accessibility. Alterations in their morphology or function lead to neuromuscular disorders, such as the congenital myasthenic syndromes, which are caused by mutations in proteins located in the NMJ. In this review, we highlight novel potential candidate genes that may cause or modify NMJs-related pathologies in humans by exploring the phenotypes of hundreds of mouse models available in the literature. We also underscore the fact that NMJs may differ between species, muscles or even sexes. Hence the importance of choosing a good model organism for the study of NMJ-related diseases: only taking into account the specific features of the mammalian NMJ, experimental results would be efficiently translated to the clinic.

Invertebrate neurons as a simple model to study the hyperexcitable state of epileptic disorders in single cells, monosynaptic connections, and polysynaptic circuits

Epilepsy is a neurological disorder characterized by a hyperexcitable state in neurons from different brain regions. Much is unknown about epilepsy and seizures development, depicting a growing field of research. Animal models have provided important clues about the underlying mechanisms of seizure-generating neuronal circuits. Mammalian complexity still makes it difficult to define some principles of nervous system function, and non-mammalian models have played pivotal roles depending on the research question at hand. Mollusks and the Helix land snail have been used to study epileptic-like behavior in neurons. Neurons from these organisms confer advantages as single-cell identification, isolation, and culture, either as single cells or as physiological relevant monosynaptic or polysynaptic circuits, together with amenability to different protocols and treatments. This review's purpose consists in presenting relevant papers in order to gain a better understanding of Helix neurons, their characteristics, uses, and capabilities for studying the fundamental mechanisms of epileptic disorders and their treatment, to facilitate their more expansive use in epilepsy research.

An Interesting Molecule: γ-Aminobutyric Acid. What Can We Learn from Hydra Polyps?

Neuronal excitability is controlled primarily by γ-aminobutyric acid (GABA) in the central and peripheral nervous systems of vertebrate as well as invertebrate organisms. Besides its recognized neurotransmitter functions, GABA also plays a fundamental role in neurogenesis and synaptogenesis during embryonic development. In addition, GABAergic mechanisms are also involved in disorders of various peripheral tissues, ranging from diabetes to hypothyroidism to inflammatory responses. The discovery of the molecule and the history of its biosynthetic pathways in vertebrate and invertebrate phyla are summarized here. The occurrence and distribution of GABA, GABA-synthesizing enzymes, and receptors to GABA in the freshwater polyp Hydra vulgaris (Cnidaria: Hydrozoa), endowed with an early evolved nervous system, are discussed in relation to possible interactions with the microbiota, a stable component of Hydra polyps; their contribution to the evolution of nervous systems through microbe-neuronal interactions is proposed.

Cyclic Guanosine Monophosphate Modulates Locomotor Acceleration Induced by Nitric Oxide but not Serotonin in Clione limacina Central Pattern Generator Swim Interneurons

Both nitric oxide (NO) and serotonin (5HT) mediate swim acceleration in the marine mollusk, Clione  limacina. In this study, we examine the role that the second messenger, cyclic guanosine monophosphate (cGMP), plays in mediating NO and 5HT-induced swim acceleration. We observed that the application of an analog of cGMP or an activator of soluble guanylyl cyclase (sGC) increased fictive locomotor speed recorded from Pd-7 interneurons of the animal's locomotor central pattern generator. Moreover, inhibition of sGC decreased fictive locomotor speed. These results suggest that basal levels of cGMP are important for slow swimming and that increased production of cGMP mediates swim acceleration in Clione. Because NO has its effect through cGMP signaling and because we show herein that cGMP produces cellular changes in Clione swim interneurons that are consistent with cellular changes produced by 5HT application, we hypothesize that both NO and 5HT function via a common signal transduction pathway that involves cGMP. Our results show that cGMP mediates NO-induced but not 5HT-induced swim acceleration in Clione.

Arthropod spatial cognition

The feats of arthropods, and of the well-studied insects and crustaceans in particular, have fascinated scientists and laymen alike for centuries. Arthropods show a diverse repertoire of cognitive feats, of often unexpected sophistication. Despite their smaller brains and resulting lower neuronal capacity, the cognitive abilities of arthropods are comparable to, or may even exceed, those of vertebrates, depending on the species compared. Miniature brains often provide parsimonious but smart solutions for complex behaviours or ecologically relevant problems. This makes arthropods inspiring subjects for basic research, bionics, and robotics. Investigations of arthropod spatial cognition have originally concentrated on the honeybee, an animal domesticated for several thousand years. Bees are easy to keep and handle, making this species amenable to experimental study. However, there are an estimated 5–10 million arthropod species worldwide, with a broad diversity of lifestyles, ecology, and cognitive abilities. This high diversity provides ample opportunity for comparative analyses. Comparative study, rather than focusing on single model species, is well suited to scrutinise the link between ecological niche, lifestyle, and cognitive competence. It also allows the discovery of general concepts that are transferable between distantly related groups of organisms. With species diversity and a comparative approach in mind, this special issue compiles four review articles and ten original research reports from a spectrum of arthropod species. These contributions range from the well-studied hymenopterans, and ants in particular, to chelicerates and crustaceans. They thus present a broad spectrum of glimpses into current research on arthropod spatial cognition, and together they cogently emphasise the merits of research into arthropod cognitive achievements.

Comparative anatomy of the mammalian neuromuscular junction

The neuromuscular junction (NMJ)—a synapse formed between lower motor neuron and skeletal muscle fibre—represents a major focus of both basic neuroscience research and clinical neuroscience research. Although the NMJ is known to play an important role in many neurodegenerative conditions affecting humans, the vast majority of anatomical and physiological data concerning the NMJ come from lower mammalian (e.g. rodent) animal models. However, recent findings have demonstrated major differences between the cellular anatomy and molecular anatomy of human and rodent NMJs. Therefore, we undertook a comparative morphometric analysis of the NMJ across several larger mammalian species in order to generate baseline inter‐species anatomical reference data for the NMJ and to identify animal models that better represent the morphology of the human NMJ in vivo. Using a standardized morphometric platform (‘NMJ‐morph’), we analysed 5,385 individual NMJs from lower/pelvic limb muscles (EDL, soleus and peronei) of 6 mammalian species (mouse, cat, dog, sheep, pig and human). There was marked heterogeneity of NMJ morphology both within and between species, with no overall relationship found between NMJ morphology and muscle fibre diameter or body size. Mice had the largest NMJs on the smallest muscle fibres; cats had the smallest NMJs on the largest muscle fibres. Of all the species examined, the sheep NMJ had the most closely matched morphology to that found in humans. Taken together, we present a series of comprehensive baseline morphometric data for the mammalian NMJ and suggest that ovine models are likely to best represent the human NMJ in health and disease.

The brain of a nocturnal migratory insect, the Australian Bogong moth

Every year, millions of Australian Bogong moths (Agrotis infusa) complete an astonishing journey: In Spring, they migrate over 1,000 km from their breeding grounds to the alpine regions of the Snowy Mountains, where they endure the hot summer in the cool climate of alpine caves. In autumn, the moths return to their breeding grounds, where they mate, lay eggs and die. These moths can use visual cues in combination with the geomagnetic field to guide their flight, but how these cues are processed and integrated into the brain to drive migratory behavior is unknown. To generate an access point for functional studies, we provide a detailed description of the Bogong moth's brain. Based on immunohistochemical stainings against synapsin and serotonin (5HT), we describe the overall layout as well as the fine structure of all major neuropils, including the regions that have previously been implicated in compass-based navigation. The resulting average brain atlas consists of 3D reconstructions of 25 separate neuropils, comprising the most detailed account of a moth brain to date. Our results show that the Bogong moth brain follows the typical lepidopteran ground pattern, with no major specializations that can be attributed to their spectacular migratory lifestyle. These findings suggest that migratory behavior does not require widespread modifications of brain structure, but might be achievable via small adjustments of neural circuitry in key brain areas. Locating these subtle changes will be a challenging task for the future, for which our study provides an essential anatomical framework.

Carbon/PEEK nails: a case-control study of 22 cases

BACKGROUND

Interest around carbon/PEEK plates and nails has been raising. The elastic modulus close to the bone, the high load-carrying capacity and radiolucency make CFR/PEEK materials a potential breakthrough. In the literature, there are abundant data about CFR/PEEK plates in the treatment of proximal humerus, distal radius and distal fibula fractures. In patients affected by bone metastasis, CFR/PEEK nails were proved effective and safe with 12 months of follow-up. Very little is known about performances of CFR/PEEK nails in patients affected by other pathologies.

PURPOSES

The aim of the study was to evaluate safety and efficacy of CFR/PEEK nails in the treatment of various pathological conditions. It was also investigated whatever radiolucency of this nails could lead to a more objective evaluation of bone callus or disease site.

PATIENTS AND METHODS

In the study group were included 20 patients (22 bone segments) who underwent CFR/PEEK nail implantation (eight humerus, one tibia, nine femur and four knee arthrodesis). They were affected by pathological fractures, and in four cases, they required an arthrodesis of the knee. They were retrospectively evaluated considering nail failures and bone callus or disease progression (RUSH scores). Mean follow-up time was 11 months (min 6.8-max 20.3). In the control group were included patients treated with titanium nails in the same institution for the same pathologies. An interclass correlation coefficient (ICC) analysis was performed in both groups considering RUSH scores by two expert surgeon from two institution to assess whether radiolucency could lead to a more objective evaluation of disease or bone callus site.

RESULTS

The ICC of mean values between RUSH scores was 0.882 (IC 95%: 0.702-0.953) in the CFR/PEEK group, while it was 0.778 (IC 95%: 0.41-0.91) in the titanium group. Observers' evaluation showed a significantly higher obscuration by titanium nails than by CFR/PEEK nails. No osteosynthesis failures were reported in both groups.

CONCLUSIONS

Our results confirm the safety of CFR/PEEK nails in the short-medium term. The radiolucency of these materials led our observers to perform more objective evaluations of bone callus formation or disease progression compared to the titanium group given the higher ICC.

LEVEL OF EVIDENCE

III Case-control therapeutic study.

Feedforward discharges couple the singing central pattern generator and ventilation central pattern generator in the cricket abdominal central nervous system

We investigated the central nervous coordination between singing motor activity and abdominal ventilatory pumping in crickets. Fictive singing, with sensory feedback removed, was elicited by eserine-microinjection into the brain, and the motor activity underlying singing and abdominal ventilation was recorded with extracellular electrodes. During singing, expiratory abdominal muscle activity is tightly phase coupled to the chirping pattern. Occasional temporary desynchronization of the two motor patterns indicate discrete central pattern generator (CPG) networks that can operate independently. Intracellular recordings revealed a sub-threshold depolarization in phase with the ventilatory cycle in a singing-CPG interneuron, and in a ventilation-CPG interneuron an excitatory input in phase with each syllable of the chirps. Inhibitory synaptic inputs coupled to the syllables of the singing motor pattern were present in another ventilatory interneuron, which is not part of the ventilation-CPG. Our recordings suggest that the two centrally generated motor patterns are coordinated by reciprocal feedforward discharges from the singing-CPG to the ventilation-CPG and vice versa. Consequently, expiratory contraction of the abdomen usually occurs in phase with the chirps and ventilation accelerates during singing due to entrainment by the faster chirp cycle.

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.

Identification of Immediate Early Genes in the Nervous System of Snail Helix lucorum

Immediate early genes (IEGs) are useful markers of neuronal activation and essential components of neuronal response. While studies of gastropods have provided many insights into the basic learning and memory mechanisms, the genome-wide assessment of IEGs has been mainly restricted to vertebrates. In this study, we identified IEGs in the terrestrial snail Helix lucorum In the absence of the genome, we conducted de novo transcriptome assembly using reads with short and intermediate lengths cumulatively covering more than 98 billion nucleotides. Based on this assembly, we identified 37 proteins corresponding to contigs differentially expressed (DE) in either the parietal ganglia (PaG) or two giant interneurons located within the PaG of the snail in response to the neuronal stimulation. These proteins included homologues of well-known mammalian IEGs, such as c-jun/jund, C/EBP, c-fos/fosl2, and Egr1, as well as homologues of genes not yet implicated in the neuronal response.

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

Characterization of locomotor response to psychostimulants in the parthenogenetic marbled crayfish (Procambarus fallax forma virginalis): A promising model for studying the neural and molecular mechanisms of drug addiction

Although scientific research using mammalian models has made great strides in uncovering the enigmatic neural and molecular mechanisms orchestrating the state of drug addiction, a complete understanding has thus far eluded researchers. The complexity of the task has led to the use of invertebrate model systems to complement the research of drug-induced reward in mammalian systems. Invertebrates, such as crayfish, offer excellent model systems to help reveal the underlying mechanisms of drug addiction as they retain the ancestral neural reward circuit that is evolutionarily conserved across taxa, and they possess relatively few, large neurons, laid out in an accessible, modularly organized nervous system. Crayfish offer the benefits of delineated developmental life stages, a large body size suitable for a variety of experimental methods, and stereotyped behaviors. Unique among crayfish is the parthenogenetic marbled crayfish (Procambarus fallax forma virginalis), a species of asexually reproducing, genetically identical clones. With the benefits of reduced individual variation, high fecundity, and easy lab husbandry, the marbled crayfish would make a particularly powerful addition to the animal model repertoire. Here we characterize the locomotor response of juvenile P. f. f. virginalis exposed to the psychostimulant, d-amphetamine sulfate. Custom video-tracking software was used to record the movement patterns of juveniles exposed to water infused with varying concentrations of d-amphetamine sulfate. ANOVA demonstrated that crayfish locomotion was significantly impacted by drug concentration. These psychostimulant effects provide the foundation of P. f. f. virginalis as a model for parsing the neural and molecular mechanisms of drug addiction.

An Argument for Amphetamine-Induced Hallucinations in an Invertebrate

Hallucinations – compelling perceptions of stimuli that aren’t really there – occur in many psychiatric and neurological disorders, and are triggered by certain drugs of abuse. Despite their clinical importance, the neuronal mechanisms giving rise to hallucinations are poorly understood, in large part due to the absence of animal models in which they can be induced, confirmed to be endogenously generated, and objectively analyzed. In humans, amphetamine (AMPH) and related psychostimulants taken in large or repeated doses can induce hallucinations. Here we present evidence for such phenomena in the marine mollusk Tritonia diomedea. Animals injected with AMPH were found to sporadically launch spontaneous escape swims in the absence of eliciting stimuli. Deafferented isolated brains exposed to AMPH, where real stimuli could play no role, generated sporadic, spontaneous swim motor programs. A neurophysiological search of the swim network traced the origin of these drug-induced spontaneous motor programs to spontaneous bursts of firing in the S-cells, the CNS afferent neurons that normally inform the animal of skin contact with its predators and trigger the animal’s escape swim. Further investigation identified AMPH-induced enhanced excitability and plateau potential properties in the S-cells. Taken together, these observations support an argument that Tritonia’s spontaneous AMPH-induced swims are triggered by false perceptions of predator contact – i.e., hallucinations—and illuminate potential cellular mechanisms for such phenomena.

Crayfish Self-Administer Amphetamine in a Spatially Contingent Task

Natural reward is an essential element of any organism’s ability to adapt to environmental variation. Its underlying circuits and mechanisms guide the learning process as they help associate an event, or cue, with the perception of an outcome’s value. More generally, natural reward serves as the fundamental generator of all motivated behavior. Addictive plant alkaloids are able to activate this circuitry in taxa ranging from planaria to humans. With modularly organized nervous systems and confirmed vulnerabilities to human drugs of abuse, crayfish have recently emerged as a compelling model for the study of the addiction cycle, including psychostimulant effects, sensitization, withdrawal, reinstatement, and drug reward in conditioned place preference paradigms. Here we extend this work with the demonstration of a spatially contingent, operant drug self-administration paradigm for amphetamine. When the animal enters a quadrant of the arena with a particular textured substrate, a computer-based control system delivers amphetamine through an indwelling fine-bore cannula. Resulting reward strength, dose-response, and the time course of operant conditioning were assessed. Individuals experiencing the drug contingent on their behavior, displayed enhanced rates of operant responses compared to that of their yoked (non-contingent) counterparts. Application of amphetamine near the supra-esophageal ganglion elicited stronger and more robust increases in operant responding than did systemic infusions. This work demonstrates automated implementation of a spatially contingent self-administration paradigm in crayfish, which provides a powerful tool to explore comparative perspectives in drug-sensitive reward, the mechanisms of learning underlying the addictive cycle, and phylogenetically conserved vulnerabilities to psychostimulant compounds.

The world of the identified or digital neuron

In general, neurons in insects and many other invertebrate groups are individually recognizable, enabling us to assign an index number to specific neurons in a manner which is rarely possible in a vertebrate brain. This endows many studies on insect nervous systems with the opportunity to document neurons with great precision, so that in favourable cases we can return to the same neuron or neuron type repeatedly so as to recognize many separate morphological classes. The visual system of the fly's compound eye particularly provides clear examples of the accuracy of neuron wiring, allowing numerical comparisons between representatives of the same cell type, and estimates of the accuracy of their wiring.

Six-legged walking in insects: how CPGs, peripheral feedback, and descending signals generate coordinated and adaptive motor rhythms

Walking is a rhythmic locomotor behavior of legged animals, and its underlying mechanisms have been the subject of neurobiological research for more than 100 years. In this article, we review relevant historical aspects and contemporary studies in this field of research with a particular focus on the role of central pattern generating networks (CPGs) and their contribution to the generation of six-legged walking in insects. Aspects of importance are the generation of single-leg stepping, the generation of interleg coordination, and how descending signals influence walking. We first review how CPGs interact with sensory signals from the leg in the generation of leg stepping. Next, we summarize how these interactions are modified in the generation of motor flexibility for forward and backward walking, curve walking, and speed changes. We then review the present state of knowledge with regard to the role of CPGs in intersegmental coordination and how CPGs might be involved in mediating descending influences from the brain for the initiation, maintenance, modification, and cessation of the motor output for walking. Throughout, we aim to specifically address gaps in knowledge, and we describe potential future avenues and approaches, conceptual and methodological, with the latter emphasizing in particular options arising from the advent of neurogenetic approaches to this field of research and its combination with traditional approaches.

Relational learning in honeybees (Apis mellifera): Oddity and nonoddity discrimination

Honeybee learning is surprisingly similar to vertebrate learning and one implication is that the basic associative learning principles are also similar. This research extends the work to more complex cognitive phenomena. Forager bees were trained individually to visit a laboratory window for sucrose. On each training trial for all experiments, bees found three stimuli, two identical and one different. In Experiments 1 and 2, stimuli were three-dimensional two-color patterns, and in Experiments 3 and 4, stimuli were two-color patterns displayed on a computer monitor. Training was trial-unique, that is, a different triad of stimuli was presented on each trial. In Experiments 1 and 3, choice of odd was rewarded and choice of nonodd was punished. In Experiments 2 and 4, choice of nonodd was rewarded and choice of odd was punished. On every trial, the initial choice was recorded and correction permitted. Honeybees learned to choose the odd stimulus in Experiments 1 and 3 and the nonodd stimuli in Experiments 2 and 4. The results provide compelling evidence of oddity and nonoddity learning, often interpreted as relational learning in vertebrates. Whether the mechanism of such learning in honeybees is similar to that of vertebrate species remains to be determined.

Recent developments in VSD imaging of small neuronal networks

Voltage-sensitive dye (VSD) imaging is a powerful technique that can provide, in single experiments, a large-scale view of network activity unobtainable with traditional sharp electrode recording methods. Here we review recent work using VSDs to study small networks and highlight several results from this approach. Topics covered include circuit mapping, network multifunctionality, the network basis of decision making, and the presence of variably participating neurons in networks. Analytical tools being developed and applied to large-scale VSD imaging data sets are discussed, and the future prospects for this exciting field are considered.

Inhibitory motoneurons in arthropod motor control: organisation, function, evolution

Miniaturisation of somatic cells in animals is limited, for reasons ranging from the accommodation of organelles to surface-to-volume ratio. Consequently, muscle and nerve cells vary in diameters by about two orders of magnitude, in animals covering 12 orders of magnitude in body mass. Small animals thus have to control their behaviour with few muscle fibres and neurons. Hexapod leg muscles, for instance, may consist of a single to a few 100 fibres, and they are controlled by one to, rarely, 19 motoneurons. A typical mammal has thousands of fibres per muscle supplied by hundreds of motoneurons for comparable behavioural performances. Arthopods—crustaceans, hexapods, spiders, and their kin—are on average much smaller than vertebrates, and they possess inhibitory motoneurons for a motor control strategy that allows a broad performance spectrum despite necessarily small cell numbers. This arthropod motor control strategy is reviewed from functional and evolutionary perspectives and its components are described with a focus on inhibitory motoneurons. Inhibitory motoneurons are particularly interesting for a number of reasons: evolutionary and phylogenetic comparison of functional specialisations, evolutionary and developmental origin and diversification, and muscle fibre recruitment strategies.

Preclinical evidence supporting the clinical development of central pattern generator-modulating therapies for chronic spinal cord-injured patients

Ambulation or walking is one of the main gaits of locomotion. In terrestrial animals, it may be defined as a series of rhythmic and bilaterally coordinated movement of the limbs which creates a forward movement of the body. This applies regardless of the number of limbs-from arthropods with six or more limbs to bipedal primates. These fundamental similarities among species may explain why comparable neural systems and cellular properties have been found, thus far, to control in similar ways locomotor rhythm generation in most animal models. The aim of this article is to provide a comprehensive review of the known structural and functional features associated with central nervous system (CNS) networks that are involved in the control of ambulation and other stereotyped motor patterns-specifically Central Pattern Generators (CPGs) that produce basic rhythmic patterned outputs for locomotion, micturition, ejaculation, and defecation. Although there is compelling evidence of their existence in humans, CPGs have been most studied in reduced models including in vitro isolated preparations, genetically-engineered mice and spinal cord-transected animals. Compared with other structures of the CNS, the spinal cord is generally considered as being well-preserved phylogenetically. As such, most animal models of spinal cord-injured (SCI) should be considered as valuable tools for the development of novel pharmacological strategies aimed at modulating spinal activity and restoring corresponding functions in chronic SCI patients.

Control of breathing in invertebrate model systems

The invertebrates have adopted a myriad of breathing strategies to facilitate the extraction of adequate quantities of oxygen from their surrounding environments. Their respiratory structures can take a wide variety of forms, including integumentary surfaces, lungs, gills, tracheal systems, and even parallel combinations of these same gas exchange structures. Like their vertebrate counterparts, the invertebrates have evolved elaborate control strategies to regulate their breathing activity. Our goal in this article is to present the reader with a description of what is known regarding the control of breathing in some of the specific invertebrate species that have been used as model systems to study different mechanistic aspects of the control of breathing. We will examine how several species have been used to study fundamental principles of respiratory rhythm generation, central and peripheral chemosensory modulation of breathing, and plasticity in the control of breathing. We will also present the reader with an overview of some of the behavioral and neuronal adaptability that has been extensively documented in these animals. By presenting explicit invertebrate species as model organisms, we will illustrate mechanistic principles that form the neuronal foundation of respiratory control, and moreover appear likely to be conserved across not only invertebrates, but vertebrate species as well.

Characterization of locomotor-related spike activity in protocerebrum of freely walking cricket

To characterize the neural elements involved in the higher-order control of spontaneous walking in insects, we recorded extracellular spike activity in the protocerebrum of freely walking crickets (Gryllus bimaculatus). Locomotor behavior was simultaneously recorded using a newly developed motion tracking system. We focused on spike units that altered their firing patterns during walking. According to their activity patterns with reference to walking bouts, these locomotor-related spike units were classified into the following four types: continuously activated unit during walking (type 1); continuously inhibited unit during walking (type 2); transiently activated unit at the onset of walking (type 3); and transiently activated unit at the termination of walking (type 4). The type 1 unit was the most dominant group (25 out of 33 units), whereas only a few units each were recorded for types 2-4. Some of the locomotor-related units tended to change firing pattern before the onset or termination of walking bouts. Spike activity in some type 1 units was found to be closely correlated with walking speed. When firing timing was compared between unit pairs, their temporal relationships (synchronization/desynchronization) altered, depending on the behavioral state (standing/walking). Mechanical stimuli applied to the body surface elicited excitatory responses in the majority of the units. Histological observations revealed that the recorded sites were concentrated near or within the mushroom body and central complex in the protocerebrum.

Diverse in- and output polarities and high complexity of local synaptic and non-synaptic signaling within a chemically defined class of peptidergic Drosophila neurons

Peptidergic neurons are not easily integrated into current connectomics concepts, since their peptide messages can be distributed via non-synaptic paracrine signaling or volume transmission. Moreover, the polarity of peptidergic interneurons in terms of in- and out-put sites can be hard to predict and is very little explored. We describe in detail the morphology and the subcellular distribution of fluorescent vesicle/dendrite markers in CCAP neurons (N_CCAP), a well defined set of peptidergic neurons in the Drosophila larva. N_CCAP can be divided into five morphologically distinct subsets. In contrast to other subsets, serial homologous interneurons in the ventral ganglion show a mixed localization of in- and output markers along ventral neurites that defy a classification as dendritic or axonal compartments. Ultrastructurally, these neurites contain both pre- and postsynaptic sites preferably at varicosities. A significant portion of the synaptic events are due to reciprocal synapses. Peptides are mostly non-synaptically or parasynaptically released, and dense-core vesicles and synaptic vesicle pools are typically well separated. The responsiveness of the N_CCAP to ecdysis-triggering hormone may be at least partly dependent on a tonic synaptic inhibition, and is independent of ecdysteroids. Our results reveal a remarkable variety and complexity of local synaptic circuitry within a chemically defined set of peptidergic neurons. Synaptic transmitter signaling as well as peptidergic paracrine signaling and volume transmission from varicosities can be main signaling modes of peptidergic interneurons depending on the subcellular region. The possibility of region-specific variable signaling modes should be taken into account in connectomic studies that aim to dissect the circuitry underlying insect behavior and physiology, in which peptidergic neurons act as important regulators.

APIS-a novel approach for conditioning honey bees

Honey bees perform robustly in different conditioning paradigms. This makes them excellent candidates for studying mechanisms of learning and memory at both an individual and a population level. Here we introduce a novel method of honey bee conditioning: APIS, the Automatic Performance Index System. In an enclosed walking arena where the interior is covered with an electric grid, presentation of odors from either end can be combined with weak electric shocks to form aversive associations. To quantify behavioral responses, we continuously monitor the movement of the bee by an automatic tracking system. We found that escapes from one side to the other, changes in velocity as well as distance and time spent away from the punished odor are suitable parameters to describe the bee's learning capabilities. Our data show that in a short-term memory test the response rate for the conditioned stimulus (CS) in APIS correlates well with response rate obtained from conventional Proboscis Extension Response (PER)-conditioning. Additionally, we discovered that bees modulate their behavior to aversively learned odors by reducing their rate, speed and magnitude of escapes and that both generalization and extinction seem to be different between appetitive and aversive stimuli. The advantages of this automatic system make it ideal for assessing learning rates in a standardized and convenient way, and its flexibility adds to the toolbox for studying honey bee behavior.

Central pattern generator for locomotion: anatomical, physiological, and pathophysiological considerations

This article provides a perspective on major innovations over the past century in research on the spinal cord and, specifically, on specialized spinal circuits involved in the control of rhythmic locomotor pattern generation and modulation. Pioneers such as Charles Sherrington and Thomas Graham Brown have conducted experiments in the early twentieth century that changed our views of the neural control of locomotion. Their seminal work supported subsequently by several decades of evidence has led to the conclusion that walking, flying, and swimming are largely controlled by a network of spinal neurons generally referred to as the central pattern generator (CPG) for locomotion. It has been subsequently demonstrated across all vertebrate species examined, from lampreys to humans, that this CPG is capable, under some conditions, to self-produce, even in absence of descending or peripheral inputs, basic rhythmic, and coordinated locomotor movements. Recent evidence suggests, in turn, that plasticity changes of some CPG elements may contribute to the development of specific pathophysiological conditions associated with impaired locomotion or spontaneous locomotor-like movements. This article constitutes a comprehensive review summarizing key findings on the CPG as well as on its potential role in Restless Leg Syndrome, Periodic Leg Movement, and Alternating Leg Muscle Activation. Special attention will be paid to the role of the CPG in a recently identified, and uniquely different neurological disorder, called the Uner Tan Syndrome.

Axonal conduction block as a novel mechanism of prepulse inhibition

In prepulse inhibition (PPI), the startle response to a strong, unexpected stimulus is diminished if shortly preceded by the onset of a different stimulus. Because deficits in this inhibitory gating process are a hallmark feature of schizophrenia and certain other psychiatric disorders, the mechanisms underlying PPI are of significant interest. We previously used the invertebrate model system Tritonia diomedea to identify the first cellular mechanism for PPI--presynaptic inhibition of transmitter release from the afferent neurons (S-cells) mediating the startle response. Here, we report the involvement of a second, more powerful PPI mechanism in Tritonia: prepulse-elicited conduction block of action potentials traveling in the startle pathway caused by identified inhibitory interneurons activated by the prepulse. This example of axo-axonic conduction block--neurons in one pathway inhibiting the propagation of action potentials in another--represents a novel and potent mechanism of sensory gating in prepulse inhibition.

Cellular basis for singing motor pattern generation in the field cricket (Gryllus bimaculatus DeGeer)

The singing behavior of male crickets allows analyzing a central pattern generator (CPG) that was shaped by sexual selection for reliable production of species-specific communication signals. After localizing the essential ganglia for singing in Gryllus bimaculatus, we now studied the calling song CPG at the cellular level. Fictive singing was initiated by pharmacological brain stimulation. The motor pattern underlying syllables and chirps was recorded as alternating spike bursts of wing-opener and wing-closer motoneurons in a truncated wing nerve; it precisely reflected the natural calling song. During fictive singing, we intracellularly recorded and stained interneurons in thoracic and abdominal ganglia and tested their impact on the song pattern by intracellular current injections. We identified three interneurons of the metathoracic and first unfused abdominal ganglion that rhythmically de- and hyperpolarized in phase with the syllable pattern and spiked strictly before the wing-opener motoneurons. Depolarizing current injection in two of these opener interneurons caused additional rhythmic singing activity, which reliably reset the ongoing chirp rhythm. The closely intermeshing arborizations of the singing interneurons revealed the dorsal midline neuropiles of the metathoracic and three most anterior abdominal neuromeres as the anatomical location of singing pattern generation. In the same neuropiles, we also recorded several closer interneurons that rhythmically hyper- and depolarized in the syllable rhythm and spiked strictly before the wing-closer motoneurons. Some of them received pronounced inhibition at the beginning of each chirp. Hyperpolarizing current injection in the dendrite revealed postinhibitory rebound depolarization as one functional mechanism of central pattern generation in singing crickets.

Decision Making and Behavioral Choice during Predator Avoidance

One of the most important decisions animals have to make is how to respond to an attack from a potential predator. The response must be prompt and appropriate to ensure survival. Invertebrates have been important models in studying the underlying neurobiology of the escape response due to their accessible nervous systems and easily quantifiable behavioral output. Moreover, invertebrates provide opportunities for investigating these processes at a level of analysis not available in most other organisms. Recently, there has been a renewed focus in understanding how value-based calculations are made on the level of the nervous system, i.e., when decisions are made under conflicting circumstances, and the most desirable choice must be selected by weighing the costs and benefits for each behavioral choice. This article reviews samples from the current literature on anti-predator decision making in invertebrates, from single neurons to complex behaviors. Recent progress in understanding the mechanisms underlying value-based behavioral decisions is also discussed.

Immunolocalization of choline acetyltransferase of common type in the central brain mass of Octopus vulgaris

Acetylcholine, the first neurotransmitter to be identified in the vertebrate frog, is widely distributed among the animal kingdom. The presence of a large amount of acetylcholine in the nervous system of cephalopods is well known from several biochemical and physiological studies. However, little is known about the precise distribution of cholinergic structures due to a lack of a suitable histochemical technique for detecting acetylcholine. The most reliable method to visualize the cholinergic neurons is the immunohistochemical localization of the enzyme choline acetyltransferase, the synthetic enzyme of acetylcholine. Following our previous study on the distribution patterns of cholinergic neurons in the Octopus vulgaris visual system, using a novel antibody that recognizes choline acetyltransferase of the common type (cChAT), now we extend our investigation on the octopus central brain mass. When applied on sections of octopus central ganglia, immunoreactivity for cChAT was detected in cell bodies of all central brain mass lobes with the notable exception of the subfrontal and subvertical lobes. Positive varicosed nerves fibers where observed in the neuropil of all central brain mass lobes.

The role of dopamine and serotonin in conditioned food aversion learning in the honeybee

For most animals, eating entails the risk of being poisoned. Learning how to identify foods with toxins is an important mechanism that reduces the risk of poisoning. While conditioned food aversions have been studied in vertebrates for over 50 years, the neural circuits underlying this form of learning have been difficult to elucidate because of their complexity. Insects, such as fruit flies and honeybees, are important models for the study of the neural mechanisms of learning and memory, but conditioned food aversions have not yet been reported from either species. My collaborators and I recently established that the honeybee has the ability to learn to avoid odors associated with toxins in food using two independent neural pathways. In these experiments, we found that honeybees can learn to associate scents with toxins that they can pre-ingestively detect using their proboscis. This form of learning is primarily mediated by the neurotransmitter, dopamine. We also found a second mechanism: bees can learn to avoid odors associated with the malaise caused by ingesting toxins. This form of learning is mediated by serotonin. Our data are the first to show that two different mechanisms account for conditioned food aversions in insects.

Single synapse information coding in intraburst spike patterns of central pattern generator motor neurons

Burst firing is ubiquitous in nervous systems and has been intensively studied in central pattern generators (CPGs). Previous works have described subtle intraburst spike patterns (IBSPs) that, despite being traditionally neglected for their lack of relation to CPG motor function, were shown to be cell-type specific and sensitive to CPG connectivity. Here we address this matter by investigating how a bursting motor neuron expresses information about other neurons in the network. We performed experiments on the crustacean stomatogastric pyloric CPG, both in control conditions and interacting in real-time with computer model neurons. The sensitivity of postsynaptic to presynaptic IBSPs was inferred by computing their average mutual information along each neuron burst. We found that details of input patterns are nonlinearly and inhomogeneously coded through a single synapse into the fine IBSPs structure of the postsynaptic neuron following burst. In this way, motor neurons are able to use different time scales to convey two types of information simultaneously: muscle contraction (related to bursting rhythm) and the behavior of other CPG neurons (at a much shorter timescale by using IBSPs as information carriers). Moreover, the analysis revealed that the coding mechanism described takes part in a previously unsuspected information pathway from a CPG motor neuron to a nerve that projects to sensory brain areas, thus providing evidence of the general physiological role of information coding through IBSPs in the regulation of neuronal firing patterns in remote circuits by the CNS.

Neural circuits controlling behavior and autonomic functions in medicinal leeches

In the study of the neural circuits underlying behavior and autonomic functions, the stereotyped and accessible nervous system of medicinal leeches, Hirudo sp., has been particularly informative. These leeches express well-defined behaviors and autonomic movements which are amenable to investigation at the circuit and neuronal levels. In this review, we discuss some of the best understood of these movements and the circuits which underlie them, focusing on swimming, crawling and heartbeat. We also discuss the rudiments of decision-making: the selection between generally mutually exclusive behaviors at the neuronal level.

High in situ repeatability of behaviour indicates animal personality in the beadlet anemone Actinia equina (Cnidaria)

'Animal personality' means that individuals differ from one another in either single behaviours or suites of related behaviours in a way that is consistent over time. It is usually assumed that such consistent individual differences in behaviour are driven by variation in how individuals respond to information about their environment, rather than by differences in external factors such as variation in microhabitat. Since behavioural variation is ubiquitous in nature we might expect 'animal personality' to be present in diverse taxa, including animals with relatively simple nervous systems. We investigated in situ startle responses in a sea anemone, Actinia equina, to determine whether personalities might be present in this example of an animal with a simple nervous system. We found very high levels of repeatability among individuals that were re-identified in the same locations over a three week sampling period. In a subset of the data, where we used tide-pool temperature measurements to control for a key element of variation in microhabitat, these high levels of repeatability remained. Although a range of other consistent differences in micro-habitat features could have contributed to consistent differences between the behaviour of individuals, these data suggest the presence of animal personality in A. equina. Rather than being restricted to certain groups, personality may be a general feature of animals and may be particularly pronounced in species with simple nervous systems.

The emergence of the "motoneuron concept": from the early 19th C to the beginning of the 20th C

This article addresses the emergence of the "motoneuron concept," i.e., the idea that this cell had properties of particular advantage for its control of muscle activation. The motor function of the ventral roots was established early in the 19th C and the term "motor cell," (or "motor nerve cell") was introduced shortly thereafter by Albrecht von Kölliker and some other histologists. They knew that motor cells were among the neurons with the largest soma in vertebrates and for this reason they were, and remained for many decades, the best and most studied neuronal model. The work of clinicians like Guillaume Duchenne de Boulogne and Jean-Martin Charcot on motor degenerative syndromes began before a clear description of motor cells was available, because it was initially more difficult to establish whether the deficits of paralysis and muscle weakness were due to neuronal or muscular lesions. Next, the pioneering physiologist, Charles Sherrington, who was influenced greatly by the anatomical contributions and speculations of Santiago Ramón y Cajal, used the term, "motor neuron," rather than motor cell for the neuron that he considered was functionally "the final common path" for providing command signals to the musculature. In the early 20th C he proposed that activation of a motor neuron resulted from the sum of its various excitatory and inhibitory CNS inputs. The contraction of motor neuron to "motoneuron(e)" was put into common usage by John Fulton (among possibly others) in 1926. The motoneuron concept is still evolving with new discoveries on the horizon.

Strategies for spinal cord repair after injury: a review of the literature and information

INTRODUCTION

Thanks to the Internet, we can now have access to more information about spinal cord repair. Spinal cord injured (SCI) patients request more information and hospitals offer specific spinal cord repair medical consultations.

OBJECTIVE

Provide practical and relevant elements to physicians and other healthcare professionals involved in the care of SCI patients in order to provide adequate answers to their questions.

METHOD

Our literature review was based on English and French publications indexed in PubMed and the main Internet websites dedicated to spinal cord repair.

RESULTS

A wide array of research possibilities including notions of anatomy, physiology, biology, anatomopathology and spinal cord imaging is available for the global care of the SCI patient. Prevention and repair strategies (regeneration, transplant, stem cells, gene therapy, biomaterials, using sublesional uninjured spinal tissue, electrical stimulation, brain/computer interface, etc.) for the injured spinal cord are under development. It is necessary to detail the studies conducted and define the limits of these new strategies and benchmark them to the realistic medical and rehabilitation care available to these patients.

CONCLUSION

Research is quickly progressing and clinical trials will be developed in the near future. They will have to answer to strict methodological and ethical guidelines. They will first be designed for a small number of patients. The results will probably be fragmented and progress will be made through different successive steps.

Do pacemakers drive the central pattern generator for locomotion in mammals?

Locomotor disorders profoundly impact quality of life of patients with spinal cord injury. Understanding the neuronal networks responsible for locomotion remains a major challenge for neuroscientists and a fundamental prerequisite to overcome motor deficits. Although neuronal circuitry governing swimming activities in lower vertebrates has been studied in great details, determinants of walking activities in mammals remain elusive. The manuscript reviews some of the principles relevant to the functional organization of the mammalian locomotor network and mainly focuses on mechanisms involved in rhythmogenesis. Based on recent publications supplemented with new experimental data, the authors will specifically discuss a new working hypothesis in which pacemakers, cells characterized by inherent oscillatory properties, might be functionally integrated in the locomotor network in mammals.

A hyperpolarization-activated inward current alters swim frequency of the pteropod mollusk Clione limacina

The pteropod mollusk, Clione limacina, exhibits behaviorally relevant swim speed changes that occur within the context of the animal's ecology. Modulation of C. limacina swimming speed involves changes that occur at the network and cellular levels. Intracellular recordings from interneurons of the swim central pattern generator show the presence of a sag potential that is indicative of the hyperpolarization-activated inward current (I(h)). Here we provide evidence that I(h) in primary swim interneurons plays a role in C. limacina swimming speed control and may be a modulatory target. Recordings from central pattern generator swim interneurons show that hyperpolarizing current injection produces a sag potential that lasts for the duration of the hyperpolarization, a characteristic of cells possessing I(h). Following the hyperpolarizing current injection, swim interneurons also exhibit postinhibitory rebound (PIR). Serotonin enhances the sag potential of C. limacina swim interneurons while the I(h) blocker, ZD7288, reduces the sag potential. Furthermore, a negative correlation was found between the amplitude of the sag potential and latency to PIR. Because latency to PIR was previously shown to influence swimming speed, we hypothesize that I(h) has an effect on swimming speed. The I(h) blocker, ZD7288, suppresses swimming in C. limacina and inhibits serotonin-induced acceleration, evidence that supports our hypothesis.

Readiness discharge for spontaneous initiation of walking in crayfish

Animals initiate behavior not only reflexively but also spontaneously in the absence of external stimuli. In vertebrates, electrophysiological data on the neuronal activity associated with the self-initiated voluntary behavior have accumulated extensively. In invertebrates, however, little is known about the neuronal basis of the spontaneous initiation of behavior. We investigated the spike activity of brain neurons at the time of spontaneous initiation of walking in the crayfish Procambarus clarkii and found neuronal signals indicative of readiness or preparatory activities in the vertebrate brain that precede the onset of voluntary actions. Those readiness discharge neurons became active >1 s before the initiation of walking regardless of stepping direction. They remained inactive at the onset of mechanical stimulus-evoked walking in which other descending units were recruited. These results suggest that the parallel descending mechanisms from the brain separately subserve the spontaneous and stimulus-evoked walking. Electrical stimulation of these different classes of neurons caused different types of walking. In addition, we found other descending units that represented different aspects of walking, including those units that showed a sustained activity increase throughout the walking bout depending on its stepping direction, as well as one veto unit for canceling out the output effect of the readiness discharge and three termination units for stopping the walking behavior. These findings suggest that the descending activities are modularized in parallel for spontaneous initiation, continuation, and termination of walking, constituting a sequentially hierarchical control.