Rhythmic motor activity and interlimb co-ordination in the developing pouch young of a wallaby (Macropus eugenii)

Alpha-pine self-emulsifying nano formulation attenuates rotenone and trichloroethylene-induced dopaminergic loss

AIM

The current investigation's goals are to pharmacologically evaluate the neurotherapeutic role of the bioactive compound Alpha Pinene (ALP)-loaded Self-emulsifying nano-formulation (SENF) in neurotoxin (Rotenone and the Industrial Solvent Trichloroethylene)- induced dopaminergic loss. It is believed that these models simulate important aspects of the molecular pathogenesis of Parkinson's disease.

MATERIAL AND METHODS

The ALP-nano-formulation's anti-Parkinson's activity was compared to ALP suspension in Wistar rats after rotenone and trichloro ethylene-induced dopaminergic loss. Neurobehavioral and motor performances were measured on the 14th, 21st, and 28th day in the rotenone model. However, in the trichloroethylene model, it was measured from the 4th to the 8th week.

RESULTS

Significant neurobehavioral improvement has been found in ALP-SENF treated animals then untreated and animals treated with plain ALP suspension. Furthermore, biochemical tests reveal marked expression of catalase, glutathione, and superoxide dismutase, which significantly combat the (Oxidative stress) OS-induced neurodegeneration.

CONCLUSION

The antioxidant effect of ALP-SENF likely includes free radicals neutralization and the activation of enzymes associated with antioxidant activity, leading to the enhancement of neurobehavioral abnormalities caused by rotenone and trichloroethylene.

Influence of the vestibular system on the neonatal motor behaviors in the gray short-tailed opossum (Monodelphis domestica)

Marsupials are born very immature yet must be sufficiently autonomous to crawl on the mother's belly, find a teat and attach to it to pursue their development. Sensory inputs are necessary to guide the newborn to a teat and induce attachment. The vestibular system, which perceives gravity and head movements, is one of the senses proposed to guide newborns towards the teats but there are conflicting observations about its functionality at birth (postnatal day (P) 0). To test if the vestibular system of opossum newborns is functional and can influence locomotion, we used two approaches. First, we stimulated the vestibular apparatus in in vitro preparations from opossums aged from P1 to P12 and recorded motor responses: at all ages studied, mechanical pressures applied on the vestibular organs induced spinal roots activity whereas head tilts did not induce forelimb muscle contractions. Second, using immunofluorescence, we assessed the presence of Piezo2, a protein involved in mechanotransduction in vestibular hair cells. Piezo2 labeling was scant in the utricular macula at birth, but observed in all vestibular organs at P7, its intensity increasing up to P14; it seemed to stay the same at P21. Our results indicate that neural pathways from the labyrinth to the spinal cord are already in place around birth but that the vestibular organs are too immature to influence motor activity before the end of the second postnatal week in the opossum. It may be the rule in marsupial species that the vestibular system becomes functional only after birth.

Non-uniform temporal scaling of developmental processes in the mammalian cortex

The time that it takes the brain to develop is highly variable across animals. Although staging systems equate major developmental milestones between mammalian species, it remains unclear how distinct processes of cortical development scale within these timeframes. Here, we compare the timing of cortical development in two mammals of similar size but different developmental pace: eutherian mice and marsupial fat-tailed dunnarts. Our results reveal that the temporal relationship between cell birth and laminar specification aligns to equivalent stages between these species, but that migration and axon extension do not scale uniformly according to the developmental stages, and are relatively more advanced in dunnarts. We identify a lack of basal intermediate progenitor cells in dunnarts that likely contributes in part to this timing difference. These findings demonstrate temporal limitations and differential plasticity of cortical developmental processes between similarly sized Therians and provide insight into subtle temporal changes that may have contributed to the early diversification of the mammalian brain.

Influence of Temperature on Motor Behaviors in Newborn Opossums (Monodelphis domestica): An In Vitro Study

External thermosensation is crucial to regulate animal behavior and homeostasis, but the development of the mammalian thermosensory system is not well known. We investigated whether temperature could play a role in the control of movements in a mammalian model born very immature, the opossum (Monodelphis domestica). Like other marsupials, at birth the opossum performs alternate and rhythmic movements with its forelimbs (FLs) to reach a teat where it attaches in order to continue its development. It was shown that FL movements can be induced by mechanical stimulation of the snout in in vitro preparations of newborns consisting of the neuraxis with skin and FLs intact. In the present study, we used puff ejections of cold, neutral (bath temperature) and hot liquid directed toward the snout to induce FL responses in such preparations. Either the responses were visually observed under a microscope or triceps muscle activity was recorded. Cold liquid systematically induced FL movements and triceps contractions, but neutral and hot temperatures were less potent to do so. Sections of the trigeminal nerves and removal of the facial skin diminished responses to cold and nearly abolished those to hot and neutral stimulations. Transient receptor potential melastatin 8 (TRPM8) being the major cold receptor cation channel in adult mammals, we employed immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR) to test for its expression, but found that it is not expressed before 13 postnatal days. Overall our results indicate that cold thermosensation exerts a strong influence on motor behaviors in newborn opossums.

Quantitative Analysis of the Maturation of the Main and Accessory Olfactory Systems in Monotremes and Metatherians in Comparison to Rodents and Humans

We have used an unbiased statistical approach to compare the pace of development in the main and accessory olfactory systems in monotremes and metatherians with that in rodents and humans. We hypothesized that if metatherians and monotremes, which are born at small body size, use olfaction to locate the pouch and/or teat/milk field, then olfactory structures should reach structural maturity in metatherians and monotremes at a smaller size than eutherians like humans and rodents. The achievement of key structural milestones in the development of the main and accessory olfactory systems (11 and 7 milestones, respectively) was scored for 354 specimens and compared against a measure of general somatic growth (body length). We used a statistical approach adapted from Kaplan-Meier analysis to determine median body length at which structural milestones were achieved, and to test the differences for statistical significance (Braslow statistic). The laboratory mouse achieved most main olfactory milestones at a smaller body size than all the metatherians and the monotremes, although the dasyurids (which are born at only 5.0 mm) and mouse achieved accessory olfactory milestones at similar body length. All other metatherians and monotremes reached olfactory milestones at body lengths similar to or larger than the laboratory rat. We therefore reject the hypothesis that metatherians and monotremes as a group exhibit advanced development of the olfactory pathways relative to body size. The findings suggest that, if olfaction is used by metatherians and monotremes at birth, it is achieved with only a rudimentary system without structurally mature central components. Anat Rec, 301:1258-1275, 2018. © 2018 Wiley Periodicals, Inc.

A bipedal mammalian model for spinal cord injury research: The tammar wallaby

Background: Most animal studies of spinal cord injury are conducted in quadrupeds, usually rodents. It is unclear to what extent functional results from such studies can be translated to bipedal species such as humans because bipedal and quadrupedal locomotion involve very different patterns of spinal control of muscle coordination. Bipedalism requires upright trunk stability and coordinated postural muscle control; it has been suggested that peripheral sensory input is less important in humans than quadrupeds for recovery of locomotion following spinal injury.

Methods: We used an Australian macropod marsupial, the tammar wallaby (Macropuseugenii), because tammars exhibit an upright trunk posture, human-like alternating hindlimb movement when swimming and bipedal over-ground locomotion. Regulation of their muscle movements is more similar to humans than quadrupeds. At different postnatal (P) days (P7–60) tammars received a complete mid-thoracic spinal cord transection. Morphological repair, as well as functional use of hind limbs, was studied up to the time of their pouch exit.

Results: Growth of axons across the lesion restored supraspinal innervation in animals injured up to 3 weeks of age but not in animals injured after 6 weeks of age. At initial pouch exit (P180), the young injured at P7-21 were able to hop on their hind limbs similar to age-matched controls and to swim albeit with a different stroke. Those animals injured at P40-45 appeared to be incapable of normal use of hind limbs even while still in the pouch.

Conclusions: Data indicate that the characteristic over-ground locomotion of tammars provides a model in which regrowth of supraspinal connections across the site of injury can be studied in a bipedal animal. Forelimb weight-bearing motion and peripheral sensory input appear not to compensate for lack of hindlimb control, as occurs in quadrupeds. Tammars may be a more appropriate model for studies of therapeutic interventions relevant to humans.

Central control of interlimb coordination and speed-dependent gait expression in quadrupeds

KEY POINTS

Quadrupeds express different gaits depending on speed of locomotion. Central pattern generators (one per limb) within the spinal cord generate locomotor oscillations and control limb movements. Neural interactions between these generators define interlimb coordination and gait. We present a computational model of spinal circuits representing four rhythm generators with left-right excitatory and inhibitory commissural and fore-hind inhibitory interactions within the cord. Increasing brainstem drive to all rhythm generators and excitatory commissural interneurons induces an increasing frequency of locomotor oscillations accompanied by speed-dependent gait changes from walk to trot and to gallop and bound. The model closely reproduces and suggests explanations for multiple experimental data, including speed-dependent gait transitions in intact mice and changes in gait expression in mutants lacking certain types of commissural interneurons. The model suggests the possible circuit organization in the spinal cord and proposes predictions that can be tested experimentally.

ABSTRACT

As speed of locomotion is increasing, most quadrupeds, including mice, demonstrate sequential gait transitions from walk to trot and to gallop and bound. The neural mechanisms underlying these transitions are poorly understood. We propose that the speed-dependent expression of different gaits results from speed-dependent changes in the interactions between spinal circuits controlling different limbs and interlimb coordination. As a result, the expression of each gait depends on (1) left-right interactions within the spinal cord mediated by different commissural interneurons (CINs), (2) fore-hind interactions on each side of the spinal cord and (3) brainstem drives to rhythm-generating circuits and CIN pathways. We developed a computational model of spinal circuits consisting of four rhythm generators (RGs) with bilateral left-right interactions mediated by V0 CINs (V0_D and V0_V sub-types) providing left-right alternation, and conditional V3 CINs promoting left-right synchronization. Fore and hind RGs mutually inhibited each other. We demonstrate that linearly increasing excitatory drives to the RGs and V3 CINs can produce a progressive increase in the locomotor speed accompanied by sequential changes of gaits from walk to trot and to gallop and bound. The model closely reproduces and suggests explanations for the speed-dependent gait expression observed in vivo in intact mice and in mutants lacking V0_V or all V0 CINs. Specifically, trot is not expressed after removal of V0_V CINs, and only bound is expressed after removal of all V0 CINs. The model provides important insights into the organization of spinal circuits and neural control of locomotion.

Survival implications of the development of behavioural responsiveness and awareness in different groups of mammalian young

This paper focuses on the development of behaviours that are critical for the survival of newborn and juvenile mammals of veterinary and wider biological interest. It provides an updated, integrated and comparative analysis of how postnatal maturation of sensory, motor and perceptual capacities support and constrain behavioural interactions between mammalian young and the mother, any littermates and the environment. Young that are neurologically exceptionally immature, moderately immature and mature at birth are compared, and include, for example, marsupial joeys, rodent pups and ruminant offspring. Mothers in these three groups exhibit distinctive patterns of birthing and postnatal care behaviours. To secure survival of the young, maternal care must compensate for behavioural inadequacies imposed by the limited sensory capacities the young possess at each stage. These sensory capacities develop in a predictable sequence in most mammals such that before birth the sequence progresses to an extent that parallels the degree of neurological maturity reached at birth. The extent of neurological maturity is likewise reflected in how long it takes after birth for the necessary brain circuit connectivity to develop sufficiently to support cortically based cognitive modulation of behaviour. This takes several months, days-to-weeks or minutes-to-hours in young that are, respectively, neurologically exceptionally immature, moderately immature, or mature at birth. Once achieved, cognitive awareness confers a high degree of behavioural flexibility that allows the young to respond more effectively to the unpredictability of their postnatal environments. It is shown that the onset of this cognitively based flexibility in the young of each group coincides with their first exposure to a variable environment that requires such behavioural flexibility.

Characteristics of the developing human locomotor system: Similarities to other mammals

Similarities in the development of locomotion between young children and other mammals are explored by reanalysis of data accrued over ~18 years. Supported stepping in children was tested on a treadmill. Although the time course of development is more protracted in humans compared to other mammals, the same trends are seen. For example, the duration of the stepping cycle shortens rapidly in the first 5 months of life. Hypermetric flexion of the hip and knee during stepping is seen in children <3 mo old. Stability of the locomotor rhythm both with respect to cycle duration within a limb and coupling between limbs improves slowly. Finally, coordination between the left and right legs can be manipulated with training, indicating experience-dependent learning at a young age. The possible reasons for these remarkably similar trends in development are explored as a function of maturational time tables for neural structures.

Towards effective non-invasive brain-computer interfaces dedicated to gait rehabilitation systems

In the last few years, significant progress has been made in the field of walk rehabilitation. Motor cortex signals in bipedal monkeys have been interpreted to predict walk kinematics. Epidural electrical stimulation in rats and in one young paraplegic has been realized to partially restore motor control after spinal cord injury. However, these experimental trials are far from being applicable to all patients suffering from motor impairments. Therefore, it is thought that more simple rehabilitation systems are desirable in the meanwhile. The goal of this review is to describe and summarize the progress made in the development of non-invasive brain-computer interfaces dedicated to motor rehabilitation systems. In the first part, the main principles of human locomotion control are presented. The paper then focuses on the mechanisms of supra-spinal centers active during gait, including results from electroencephalography, functional brain imaging technologies [near-infrared spectroscopy (NIRS), functional magnetic resonance imaging (fMRI), positron-emission tomography (PET), single-photon emission-computed tomography (SPECT)] and invasive studies. The first brain-computer interface (BCI) applications to gait rehabilitation are then presented, with a discussion about the different strategies developed in the field. The challenges to raise for future systems are identified and discussed. Finally, we present some proposals to address these challenges, in order to contribute to the improvement of BCI for gait rehabilitation.

Assessing forelimb function after unilateral cervical SCI using novel tasks: limb step-alternation, postural instability and pasta handling

Cervical spinal cord injury (cSCI) can cause devastating neurological deficits, including impairment or loss of upper limb and hand function. A majority of the spinal cord injuries in humans occur at the cervical levels. Therefore, developing cervical injury models and developing relevant and sensitive behavioral tests is of great importance. Here we describe the use of a newly developed forelimb step-alternation test after cervical spinal cord injury in rats. In addition, we describe two behavioral tests that have not been used after spinal cord injury: a postural instability test (PIT), and a pasta-handling test. All three behavioral tests are highly sensitive to injury and are easy to use. Therefore, we feel that these behavioral tests can be instrumental in investigating therapeutic strategies after cSCI.

Developmental constraints of quadrupedal coordination across crawling styles in human infants

Human infants can crawl using several very different styles; this diversity appears at first glance to contradict our previous findings from hands-and-knees crawling, which suggested that there were strict limitations on coordination, imposed either mechanically or by the developing nervous system. To determine whether coordination was similarly restricted across crawling styles, we studied free crawling overground in 22 infants who used a number of different locomotor strategies. Despite the wide variety in the use of individual limbs and even the number of limbs used, the duration of the stance phase increased with duration of cycle, whereas the duration of the swing phase remained more constant. Additionally, all infants showed organized, rhythmic interlimb coordination. Alternating patterns (e.g., trotlike) predominated (86% of infants). Alternatively, yet much less frequently, all limbs used could work in synchrony (14% of infants). Pacelike patterns were never observed, even in infants that crawled with the belly remaining in contact with the ground so that stability was not a factor. To explore the robustness of the interlimb coordination, a perturbation that prolonged swing of the leg was imposed on 14 additional infants crawling on hands and knees overground or on the treadmill. The perturbation led to a resetting of the crawling pattern, but never to a change in the coordination of the limbs. The findings concur with those regarding other infant animals, together suggesting that the nervous system itself limits the coordination patterns available at a young age.

Development of the spinal cord and peripheral nervous system in platypus (Ornithorhynchus anatinus) and short-beaked echidna (Tachyglossus aculeatus)

The modern monotremes (platypus and echidnas) are characterized by development of their young in a leathery egg that is laid into a nest or abdominal pouch. At hatching, the young are externally immature, with forelimbs capable of digitopalmar prehension, but hindlimbs little advanced beyond limb buds. The embryological collections at the Museum für Naturkunde in Berlin were used to examine the development of the spinal cord and early peripheral nervous system in developing monotremes and to correlate this with known behavioural development. Ventral root outgrowth to the bases of both the fore- and hindlimbs occurs at 6.0 mm crown-rump length (CRL), but invasion of both limbs does not happen until about 8.0-8.5 mm CRL. Differentiation of the ventral horn precedes the dorsal horn during incubation and separate medial and lateral motor columns can be distinguished before hatching. Rexed's laminae begin to appear in the dorsal horn in the first week after hatching, and gracile and cuneate fasciculi emerge during the first two post-hatching months. Qualitative and quantitative comparisons of the structure of the cervicothoracic junction spinal cord in the two monotremes with that in a diprotodont marsupial (the brush-tailed possum, Trichosurus vulpecula) of similar size at birth, did not reveal any significant structural differences between the monotremes and the marsupial. The precocious development of motor systems in the monotreme spinal cord is consistent with the behavioural requirements of the peri-hatching period, that is, rupture of embryonic membranes and egg, and digitopalmar prehension to grasp maternal hair or nest material.

Development of the vestibular apparatus and central vestibular connections in a wallaby (Macropus eugenii)

We have studied the early development of the vestibular apparatus and its central connections in the tammar wallaby (Macropus eugenii) in order to determine whether the vestibular system anatomy is sufficiently mature at birth to assist in climbing to the pouch. Structural development was studied with the aid of hematoxylin and eosin stained sections and immunoreactivity for GAP-43, whereas the development of vestibular system connections was examined by carbocyanine dye tracing. At the time of birth, the otocyst has distinct utricle, saccule and semicircular canals with immature sensory regions receiving innervation by GAP-43 immunoreactive fibers. Vestibular nerve fibers can be traced into the brainstem to the developing vestibular nuclei, which are not yet cytoarchitectonically distinct. The vestibular nuclei do not contribute direct projections to the lower cervical spinal cord at birth; most bulbospinal projections in the newborn appear to be derived bilaterally from the gigantocellular, lateral paragigantocellular reticular and ventral medullary nuclei. A substantial bilateral projection to the vestibular ganglion and apparatus from the region of the gigantocellular and lateral paragigantocellular nuclei was seen at birth, but not in subsequent ages. This is similar to a projection seen in newborn Ameridelphians. By postnatal day (P) 5, the vestibular apparatus had extensive projections to all vestibular nuclei and neurons projecting in the lateral vestibulospinal tract could be identified in the lateral vestibular nucleus. Cytoarchitectonic differentiation of the vestibular nuclei proceeded over the next 3 to 4 weeks with the emergence of discrete parvicellular and magnocellular components of the medial vestibular nucleus by P19. GAP-43 immunoreactivity stayed high in the lateral vestibulospinal tract for several months after birth, suggesting that the development of this tract followed a prolonged timecourse. Our findings indicate that central and peripheral connections of the vestibular ganglion are present at birth, but that there is no direct projection from the vestibular nuclei to the cervical spinal cord until P5. Nevertheless, the possibility remains that an indirect projection between the vestibular nuclei and the medial reticular formation is present at birth and mediates control of the climb.

Development of the olfactory system in a wallaby (Macropus eugenii)

We used carbocyanine dye tracing techniques in conjunction with hematoxylin and eosin staining, immunohistochemistry for GAP-43, and tritiated thymidine autoradiography to examine the development of the olfactory pathways in early pouch young tammar wallabies (Macropus eugenii). The overarching aim was to test the hypothesis that the olfactory system of newborn tammars is sufficiently mature at birth to contribute to the guidance of the pouch young to the nipple. Although GAP-43 immunoreactive fibers emerge from the olfactory epithelium and enter the olfactory bulb at birth, all other components of the olfactory pathway in newborn tammars are very immature at birth, postnatal day (P0). In particular, maturation of the vomeronasal organ and its projections to the accessory olfactory bulb appears to be delayed until P5 and the olfactory bulb is poorly differentiated until P12, with glomerular formation delayed until P25. The lateral olfactory tract is also very immature at birth with pioneer axons having penetrated only the most rostral portion of the piriform lobe. Interestingly, there were some early (P0) projections from the olfactory epithelium to the medial septal region and lamina terminalis (by the terminal nerve) and to olfactory tubercle and basal forebrain. The former of these is presumably serving the transfer of LHRH(+) neurons to the forebrain, as seen in eutherians, but neither of these very early pathways is sufficiently robust or connected to the more caudal neuraxis to play a role in nipple finding. Tritiated thymidine autoradiography confirmed that most piriform cortex pyramidal neurons are generated in the first week of life and are unlikely to be able to contribute to circuitry guiding the climb to the pouch. Our findings lead us to reject the hypothesis that olfactory projections contribute to guidance of the newborn tammar to the pouch and nipple. It appears far more likely that the trigeminal pathways play a significant role in this behavior because the central projections of the trigeminal nerve are more mature at birth in this marsupial.

Spinal and brain control of human walking: implications for retraining of walking

In this update, the authors will discuss evidence for both spinal and brain regulation of walking in humans. They will consider the sensory control of walking in young babies and spinal cord-injured adults, two models with weak descending input from the brain, to suggest that subcortical structures are important in shaping walking behavior. Based on evidence from development, the authors suggest that the primitive pattern of walking seen in babies forms the base upon which additional features are added by supraspinal input as independent walking develops. Increasing evidence suggests the motor cortex is important in the control of level-ground walking in adults, in contrast to quadrupeds. This brain input seems particularly important for distal flexors in the leg. Finally, the authors will consider evidence that the recovery of walking after incomplete spinal cord injuries is dependent on the presence of descending input from the motor cortex and our ability to strengthen that input. These findings imply that training methods for improving walking after injury to the nervous system must promote the involvement of both spinal and brain circuits.

Propriospinal circuitry underlying interlimb coordination in mammalian quadrupedal locomotion

Soon after birth, freely moving quadrupeds can express locomotor activity with coordinated forelimb and hindlimb movements. To investigate the neural mechanisms underlying this coordination, we used an isolated spinal cord preparation from neonatal rats. Under bath-applied 5-HT, N-methyl-d,l-aspartate (NMA), and dopamine (DA), the isolated cord generates fictive locomotion in which homolateral cervicolumbar extensor motor bursts occur in phase opposition, as does bursting in homologous (left-right) extensor motoneurons. This coordination corresponded to a walking gait monitored with EMG recordings in the freely behaving animal. Functional decoupling of the cervical and lumbar generators in vitro by sucrose blockade at the thoracic cord level revealed independent rhythmogenic capabilities with similar cycle frequencies in the two locomotor regions. When the cord was partitioned at different thoracic levels and 5-HT/NMA/DA was applied to the more caudal compartment, the ability of the lumbar generators to drive their cervical counterparts increased with the proportion of chemically exposed thoracic segments. Blockade of synaptic inhibition at the lumbar level caused synchronous bilateral lumbar rhythmicity that, surprisingly, also was able to impose bilaterally synchronous bursting at the unblocked cervical level. Furthermore, after a midsagittal section from spinal segments C1 to T7, and during additional blockade of cervical synaptic inhibition, the cord exposed to 5-HT/NMA/DA continued to produce a coordinated fictive walking pattern similar to that observed in control. Thus, in the newborn rat, a caudorostral propriospinal excitability gradient appears to mediate interlimb coordination, which relies more on asymmetric axial connectivity (both excitatory and inhibitory) between the lumbar and cervical generators than on differences in their inherent rhythmogenic capacities.

Anatomical organization of motoneurons and interneurons in the mudpuppy (Necturus maculosus) brachial spinal cord: the neural substrate for central pattern generation

The isolated brachial spinal cord of the mudpuppy is useful for studies of neural networks underlying forelimb locomotion, but information about its anatomy is scarce. We addressed this issue by combining retrograde labeling with fluorescent tracers and confocal microscopy. Remarkably, the central region of gray matter was aneural and contained only a tenuous meshwork of glial fibers and large extracellular spaces. Somata of motoneurons (MNs) and interneurons (INs), labeled retrogradely from ventral roots or axons in the ventro-lateral funiculus, respectively, were confined within a gray neuropil layer abutting the white matter borders, while their dendrites projected widely throughout the white matter. A considerable fraction of labeled INs was found contralaterally with axons crossing beneath a thick layer of ependyma surrounding the central canal. Dorsal roots (DRs) produced dense presynaptic arbors within a restricted dorsal region containing afferent terminations, within which dorsally directed MN and IN dendrites mingled with dense collections of synaptic boutons. Our data suggest that a major fraction of synaptic interactions takes place within the white matter. This study provides a detailed foundation for electrophysiological experiments aimed at elucidating the neural circuits involved in locomotor pattern generation.

Development of Ca2+-channel and BK-channel expression in embryos and larvae of Xenopus laevis

The patterns and density of channels expressed in neurons critically determine their electrical properties. We have examined developmental regulation of Ca2+-channel expression during the maturation of the spinal motor circuits in Xenopus as it develops from an embryo to a larva. In embryonic neurons approximately 60% of the current is carried by N-type channels, 8% by l-type channels and the remainder by an unidentified channel. As the embryo matures, omega-agatoxin-sensitive P/Q channels are gradually expressed and replace the unidentified HVA channel such that at stage 42 approximately 25% of the current is carried by P/Q channels. We have used fluorescent labelling of selective channel toxins to directly observe the distribution of P/Q, N and BK channels. The P/Q channel distribution was most prevalent on the cell surface proximal to the areas of the soma where processes emerge. Both N and BK channels were distributed throughout the soma but still exhibited concentration around the areas adjacent to the emergence of processes from the soma. The patterns of fluorescence labelling during development mirrored the development of the respective ionic currents. Both N and P/Q channels contribute roughly equally to activation of the BK current, suggesting that overlap in the distribution of the N, P/Q and BK channels is important in their functional interdependence. The newly expressed P/Q channels play a role in spike initiation and repetitive firing in larval spinal neurons and contribute to burst generation during swimming in the larva.

Forelimb locomotor generators and quadrupedal locomotion in the neonatal rat

The spinal localization of the forelimb locomotor generators and their interactions with other spinal segments were investigated on in vitro brainstem-spinal cord preparations of new-born rats. Superfusion of the cervicothoracic cord (C1-T4) with high K+/low Mg2+ artificial cerebrospinal fluid (aCSF) evoked rhythmic motor root activity that was limited to low cervical (C7, C8) and high thoracic (T1) spinal levels. This activity consisted of synchronous, homolateral bursts and a typical alternating bilateral pattern. Rhythmic activity with similar locomotor-like characteristics could be induced with either serotonin (5-HT, 5 microm), N-methyl-d-aspartate (NMDA, 5 microm), kainate (10 microm) or a "cocktail" of 5-HT (5 microm) and NMDA (5 microm). During 5-HT/NMDA perfusion of the cervicothoracic cord, induced bursting was no longer restricted to C7-T1 levels, but also occurred at cervical C3-C5 levels and with C5-C8 homolateral alternation. Spinal transections between C6 and C7 cervical segments did not abolish rhythmic activity in C7-T1, but suppressed locomotor-like rhythmicity at C3-C5 levels. Reduced regions comprising the C7-C8 or C8-T1 segments maintained rhythmicity. Superfusion of the whole cord with 5-HT/NMDA induced ventral root bursting with similar frequencies at all recorded segments (cervical, thoracic and lumbar). After isolation, the T3-T10 cord was unable to sustain any rhythmic activity while cervical and lumbar segmental levels continued to burst, albeit at different frequencies. We also found that the faster caudal and the slower rostral locomotor generators interact to produce coordinated locomotor-like activity in all segments of the intact spinal cord. In conclusion, C7-T1 spinal levels display a strong motor rhythmogenic ability; with the lumbar generators, they contribute to coordinated rhythmic activity along the entire spinal cord of a quadrupedal locomoting mammal.

Development of lumbar rhythmic networks: from embryonic to neonate locomotor-like patterns in the mouse

Different aspects of spinal locomotor organization have been studied in the mouse during embryonic and neonatal development using in vitro preparations of isolated lumbosacral cords. The first consideration was the embryonic development of an alternating bilateral pattern. From embryonic day (E) 12, perfusion of serotonin could induce relatively synchronous lumbar bursts across the cord. Bilateral activity became progressively alternate at E15 due to the appearance of glycinergic inhibitory interactions (revealed by strychnine application). Strictly alternating patterns were expressed at E18 and were maintained after birth. In a second step, we investigated cellular properties involved in lumbar rhythmogenesis in postnatal day 0-2 preparations which displayed spontaneous locomotor-like activity. Perfusion of receptor antagonists showed the co-operative involvement of N-methyl-D-aspartate (NMDA)- and non-NMDA-receptors for excitatory amino acids-mediated operation of locomotor networks. In a final step we investigated the localization of locomotor networks within the lumbar cord. Data obtained from preparations exhibiting spontaneous or Mg2+-free induced bursts revealed that the networks are present throughout the lumbar cord and that rhythmogenesis is distributed throughout all segmental levels.

alpha1-adrenergic receptor-induced slow rhythmicity in nonrespiratory cervical motoneurons of neonatal rat spinal cord

Previous studies have reported that the alpha1-adrenergic system can activate spinal rhythm generators belonging to the central respiratory network. In order to analyse alpha1-adrenergic effects on both cranial and spinal motoneuronal activity, phenylephrine (1-800 microM) was applied to in vitro preparations of neonatal rat brainstem-spinal cord. High concentration of phenylephrine superfusion exerted multiple effects on spinal cervical outputs (C2-C6), consisting of a lengthening of respiratory period and an increase in inspiratory burst duration. Furthermore, in 55% of cases a slow motor rhythm recorded from the same spinal outputs was superimposed on the inspiratory activity. However, this phenylephrine-induced slow motor rhythm generated at the spinal level was observed neither in inspiratory cranial nerves (glossopharyngeal, vagal and hypoglossal outputs) nor in phrenic nerves. Whole-cell patch-clamp recordings were carried out on cervical motoneurons (C4-C5), to determine first which motoneurons were involved in this slow rhythm, and secondly the cellular events underlying direct phenylephrine effects on motoneurons. In all types of motoneurons (inspiratory and nonrespiratory) phenylephrine induced a prolonged depolarization with an increase in neuronal excitability. However, only nonrespiratory motoneurons showed additional rhythmic membrane depolarizations (with spiking) occurring in phase with the slow motor rhythm recorded from the ventral root. Furthermore the tonic depolarization produced in all motoneurons results from an inward current [which persists in the presence of tetrodotoxin (TTX)] associated with a decrease in neuron input conductance, with a reversal potential varying as a Nernstian function of extracellular K+ concentration. Our results indicate that the alpha1-adrenoceptor activation: (i) affects both the central respiratory command (i.e. respiratory period and inspiratory burst duration) and spinal inspiratory outputs; (ii) induces slow spinal motor rhythmicity, which is unlikely to be related to the respiratory system; and (iii), increases motoneuronal excitability, probably through a decrease in postsynaptic leak K+ conductance.

Intercostal expiratory activity in an in vitro brainstem-spinal cord-rib preparation from the neonatal rat

1. We examined whether expiratory activity can be observed when central chemoreceptors are activated by a decrement in the extracellular pH in an isolated brainstem-spinal cord-rib preparation from 0- to 3-day-old rats. Expiratory activity was defined as the burst activity that occurs in an internal intercostal muscle (IIM) during the silent period between the periodic inspiratory bursts in the C4 ventral root (which contains phrenic motor axons). 2. During perfusion with modified Krebs solution (26 mM HCO3-, 5 % CO2, pH 7.4), there was no consistent activity in IIM, though rhythmic inspiratory motor activity always appeared in the C4 ventral root. 3. When the pH of the perfusate was lowered from about 7.4 to 7.1 by reducing [HCO3-] from 26 to 10 mM, the frequency of the C4 inspiratory rhythm increased, and rhythmic activity appeared in IIM. In most cases, the rhythmic burst in IIM started just after the cessation of the C4 inspiratory burst and coincided with movement of the ribs in a caudal direction. This intercostal expiratory burst was limited to the first half of the expiratory phase. 4. The coordinated reciprocal motor activity between the C4 ventral root and IIM changed to a largely overlapping pattern when strychnine (5-10 microM), a glycine receptor antagonist, was added to the perfusate. 5. These results suggest (i) that the neuronal mechanisms responsible for expiratory motor activity are preserved in this in vitro preparation and (ii) that the glycinergic inhibitory system plays an important role in the coordination between inspiratory and expiratory motor activity during respiration.

Reciprocal interactions in the turtle hindlimb enlargement contribute to scratch rhythmogenesis

We examined interactions between the spinal networks that generate right and left rostral scratch motor patterns in turtle hindlimb motoneurons before and after transecting the spinal cord within the anterior hindlimb enlargement. Our results provide evidence that reciprocal inhibition between hip circuit modules can generate hip rhythmicity during the rostral scratch reflex. "Module" refers here to the group of coactive motoneurons and interneurons that controls either flexion or extension of the hip on one side and coordinates that activity with synergist and antagonist motor pools in the same limb and in the contralateral limb. The "bilateral shared core" hypothesis states that hip flexor and extensor (HF and HE) circuit modules interact via crossed and uncrossed spinal pathways: HF modules make reciprocal inhibitory connections with contralateral HF and ipsilateral HE modules and mutual excitatory connections with contralateral HE modules. It is currently unclear how much reciprocal inhibition between modules contributes to scratch rhythmogenesis. To address this issue, fictive scratch motor patterns were recorded bilaterally as electroneurograms from HF, HE, knee extensor (KE), and respiratory (d.D8) muscle nerves in immobilized animals. D3-end (low-spinal) preparations had intact spinal cords posterior to a complete D2-D3 transection. Unilateral stimulation of rostral scratch in D3-end turtles elicited rhythmic alternation between ipsilateral HF and HE bursts in most cycles; consecutive HF bursts were separated by complete silent (HF-OFF ) periods. D3-D9 and D3-D8 preparations received a second spinal transection at the caudal end of segment D9 or D8, respectively, within the anterior hindlimb enlargement. This second transection disconnected most HE circuitry (located mainly in segments D10-S2 of the posterior enlargement) from the rostral scratch network and thereby reduced the HE-associated inhibition of HF circuitry. Unilateral stimulation of rostral scratch in most D3-D9 and D3-D8 preparations evoked rhythmic or weakly modulated ipsilateral HF discharge without HF-OFF periods between bursts and without ipsilateral HE activity in the majority of cycles. In contrast, bilateral stimulation in D3-D9 and D3-D8 preparations reconstructed the HF-OFF periods, increased HF rhythmicity (assessed by fast Fourier transform power spectra and autocorrelation analyses), and reestablished weak HE-phase motoneuron activity. We suggest that bilateral stimulation produced these effects by simultaneously activating reciprocally inhibitory hip modules on opposite sides (right and left HF) and the same side (HF and residual ipsilateral HE circuitry). Our data support the hypothesis that reciprocal inhibition can contribute to spinal rhythmogenesis during the scratch reflex.

The origin of spontaneous activity in developing networks of the vertebrate nervous system

Spontaneous neuronal activity has been detected in many parts of the developing vertebrate nervous system. Recent studies suggest that this activity depends on properties that are probably shared by all developing networks. Of particular importance is the high excitability of recurrently connected, developing networks and the presence of activity-induced transient depression of network excitability. In the spinal cord, it has been proposed that the interaction of these properties gives rise to spontaneous, periodic activity.

Reorganization of locomotor activity during development in the prenatal rat

Development of neuronal circuits generating locomotor activity was studied using an isolated lumbar spinal cord preparation from fetal and neonatal rats. Bath application of N-methyl-D-aspartate (NMDA) or 5-HT evoked patterned motor activity resembling that seen during normal fictive locomotion on embryonic day (E) 20.5. Glycine-mediated inhibition was essential to the formation of this coordinated motor activity. In preparations from fetuses at the earlier stages (E14.5-E16.5), we observed spontaneous motoneuronal activity and chemically induced rhythmic bursts, which were synchronized on the two sides in the corresponding ventral roots. The spontaneous activity was not blocked by kynurenate, the glutamate receptor blocker, although it was completely abolished by strychnine, the glycine receptor antagonist. A brief application of glycine evoked excitatory responses resembling the spontaneous bursts in both time course and amplitude. It is concluded that glycine functions transiently as excitatory transmitters at these stages. These results suggest that functional change in glycine-induced responses during development plays an important role in differentiation of the neuronal circuits generating locomotion.

Development of whisker-related patterns in marsupials: factors controlling timing

In mature rodents, whisker-related patterns are known to be present in three levels of the brain: the brainstem trigeminal nuclei, the ventrobasal thalamus and the somatosensory cortex. These patterns have been demonstrated using neuroanatomical tracing techniques, histological and histochemical staining methods and electrophysiological recordings. The development and topography of these patterns are dependent on an intact periphery. But what governs when patterns form at the three levels? Possibilities include a controlling signal from the periphery or local mechanisms at each site, such as the arrival of afferent inputs or the maturation of target tissue. In this review, we report on the maturation of the whisker pathway in a marsupial, the wallaby, where the slow tempo of development is a feature. At each level, afferent fibres grow into the region of termination many weeks before the whisker-related pattern emerges. The results suggest that the maturity of the target tissue as well as signals from the periphery combine to trigger pattern formation at each level of the pathway.

Development of the spatial pattern of 5-HT-induced locomotor rhythm in the lumbar spinal cord of rat fetuses in vitro

Developmental changes in the 5-hydroxytryptamine (5-HT)-induced locomotor rhythm were examined in isolated spinal cord preparations taken from fetal rats at embryonic day (E) 16.5, E18.5 and E20.5. Motor activity was recorded from L2/L3 and L5 ventral roots. Bath application of 5-HT evoked rhythmic bursts that were synchronized in all ventral roots studied at E16.5. At E18.5, 5-HT evoked rhythmic bursts that alternated between the two sides, while the bursts in the L2/L3 ventral root were synchronous with those in the ipsilateral L5 ventral root. At E20.5, 5-HT evoked alternate rhythmic bursts in L2/L3 and L5 ventral roots, representing activity in flexors and extensors, respectively. In the presence of strychnine, 5-HT induced rhythmic bursts that were synchronized in all ventral roots studied at E18.5 and E20.5, suggesting that the change in the pattern of rhythmic motor activity that occurs with age is due to the development of glycine-mediated inhibition.

Roles of ascending inhibition during two rhythmic motor patterns in Xenopus tadpoles

We have investigated the effects of ascending inhibitory pathways on two centrally generated rhythmic motor patterns in a simple vertebrate model, the young Xenopus tadpole. Tadpoles swim when touched, but when grasped respond with slower, stronger struggling movements during which the longitudinal pattern of motor activity is reversed. Surgical spinal cord transection to remove all ascending connections originating caudal to the transection (in tadpoles immobilized in alpha-bungarotoxin) did not affect "fictive" swimming generated more rostrally. In contrast, cycle period and burst duration both significantly increased during fictive struggling. Increases were progressively larger with more rostral transection. Blocking caudal activity with the anesthetic MS222 (pharmacological transection) produced equivalent but reversible effects. Reducing crossed-ascending inhibition selectively, either by midsagittal spinal cord division or rostral cord hemisection (1-sided transection) mimicked the effects of transection. Like transection, both operations increased cycle period and burst duration during struggling but did not affect swimming. The changes during struggling were larger with more rostral hemisection. Reducing crossed-ascending inhibition by spinal hemisection also increased the rostrocaudal longitudinal delay during swimming, and the caudorostral delay during struggling. Weakening inhibition globally with low concentrations of the glycine antagonist strychnine (10-100 nM) did not alter swimming cycle period, burst duration, or longitudinal delay. However, strychnine at 10-60 nM decreased cycle period during struggling. It also increased burst duration in some cases, although burst duration increased as a proportion of cycle period in all cases. Strychnine reduced longitudinal delay during struggling, making rostral and caudal activity more synchronous. At 100 nM, struggling was totally disrupted. By combining our results with a detailed knowledge of tadpole spinal cord anatomy, we conclude that inhibition mediated by the crossed-ascending axons of characterized, glycinergic, commissural interneurons has a major influence on the struggling motor pattern compared with swimming. We suggest that this difference is a consequence of the larger, reversed longitudinal delay and the extended burst duration during struggling compared with swimming.

Development of muscle afferents in the spinal cord of the tammar wallaby

The development of muscle afferents in the tammar wallaby was examined to address whether proprioceptive input contributes to the marked asymmetry of the fore and hindlimb movement. Anatomical tracing with biocytin showed that the muscle afferents had reached the brachial motor horn by postnatal day (P1), but were less advanced in the lumbar region. Labelled cells lying outside the motor horn, presumably filled via gap junctions, were evident in the neonatal lumbar cord. By the 4th postnatal week, the afferent innervation of both brachial and lumbar cords became similar. Afferent discharges from stretching the biceps muscle could be recorded at birth, but not until P4 from the hindlimb gastrocnemius muscle. The discharges were predominantly phasic until P35 when tonic activity could also be recorded. Short latency spinal reflex responses superimposed upon a longer lasting potential were present in the brachial cord at birth, appearing in the lumbar cord at P4. By the 3rd postnatal week, spinal reflex became comparable in both segmental levels. The time course of muscle afferent development was compared to the progression of natural cell death in the lumbar cord. Sensorimotor connections were established towards the end of the rapid phase cell death as observed in other vertebrates.

Developmental changes in expression of ion currents accompany maturation of locomotor pattern in frog tadpoles

1. The K+ currents of spinal neurons acutely dissociated from Xenopus larvae were studied and compared with those of neurons dissociated from Xenopus embryos. 2. The density of total outward current in the larval and embryonic neurons remained the same from stage 37/38 to stage 42. 3. Almost all neurons at stage 42 expressed a fast activating Ca2+-dependent K+ current (IKCa) that was largely absent from embryonic neurons. Whereas IKCa became larger and more prevalent during development, the delayed rectifier K+ currents were down-regulated. 4. About 53 % of IKCa was selectively blocked by iberiotoxin which had no effect on the delayed rectifier K+ currents or the K+ currents of embryonic neurons. 5. The firing properties of neurons isolated from embryos were unchanged by iberiotoxin. However, the toxin greatly increased the frequency of firing in larval neurons. 6. Iberiotoxin extended the duration of ventral root bursts during fictive swimming in larvae at stages 41 and 42 but had no effect at stage 40. The progressive expression of IKCa thus contributed to burst termination. 7. We have found that changes in expression of outward current closely correlate with the maturation of the motor pattern during development. At a time when the motor pattern has a need for a burst-terminating mechanism, the larval neurons express a channel with properties appropriate for such a role.