Neuromodulation via conditional release of endocannabinoids in the spinal locomotor network

Cannabinoids, Endocannabinoids, and Synthetic Cannabimimetic Molecules in Neuromuscular Disorders

Neuromuscular disorders (NMDs) encompass a large heterogeneous group of hereditary and acquired diseases primarily affecting motor neurons, peripheral nerves, and the skeletal muscle system. The symptoms of NMDs may vary depending on the specific condition, but some of the most common ones include muscle weakness, pain, paresthesias, and hyporeflexia, as well as difficulties with swallowing and breathing. NMDs are currently untreatable. Therapeutic options include symptomatic and experimental medications aimed at delaying and alleviating symptoms, in some cases supplemented by surgical and physical interventions. To address this unmet medical need, ongoing research is being conducted on new treatments, including studies on medical cannabis, endocannabinoids, and related molecules with cannabimimetic properties. In this context, a significant amount of knowledge about the safety and effectiveness of cannabinoids in NMDs has been obtained from studies involving patients with multiple sclerosis experiencing pain and spasticity. In recent decades, numerous other preclinical and clinical studies have been conducted to determine the potential benefits of cannabinoids in NMDs. This review article aims to summarize and provide an unbiased point of view on the current knowledge about the use of cannabinoids, endocannabinoids, and synthetic analogs in NMDs, drawing from an array of compelling studies.

Endocannabinoid signaling in microglia

Microglia, the innate immune cells of the central nervous system (CNS), execute their sentinel, housekeeping and defense functions through a panoply of genes, receptors and released cytokines, chemokines and neurotrophic factors. Moreover, microglia functions are closely linked to the constant communication with other cell types, among them neurons. Depending on the signaling pathway and type of stimuli involved, the outcome of microglia operation can be neuroprotective or neurodegenerative. Accordingly, microglia are increasingly becoming considered cellular targets for therapeutic intervention. Among signals controlling microglia activity, the endocannabinoid (EC) system has been shown to exert a neuroprotective role in many neurological diseases. Like neurons, microglia express functional EC receptors and can produce and degrade ECs. Interestingly, boosting EC signaling leads to an anti-inflammatory and neuroprotective microglia phenotype. Nonetheless, little evidence is available on the microglia-mediated therapeutic effects of EC compounds. This review focuses on the EC signals acting on the CNS microglia in physiological and pathological conditions, namely on the CB1R, CB2R and TRPV1-mediated regulation of microglia properties. It also provides new evidence, which strengthens the understanding of mechanisms underlying the control of microglia functions by ECs. Given the broad expression of the EC system in glial and neuronal cells, the resulting picture is the need for in vivo studies in transgenic mouse models to dissect the contribution of EC microglia signaling in the neuroprotective effects of EC-derived compounds.

Astrocytic modulation of central pattern generating motor circuits

Central pattern generators (CPGs) generate the rhythmic and coordinated neural features necessary for the proper conduction of complex behaviors. In particular, CPGs are crucial for complex motor behaviors such as locomotion, mastication, respiration, and vocal production. While the importance of these networks in modulating behavior is evident, the mechanisms driving these CPGs are still not fully understood. On the other hand, accumulating evidence suggests that astrocytes have a significant role in regulating the function of some of these CPGs. Here, we review the location, function, and role of astrocytes in locomotion, respiration, and mastication CPGs and propose that, similarly, astrocytes may also play a significant role in the vocalization CPG.

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.

Orexinergic Modulation of Spinal Motor Activity in the Neonatal Mouse Spinal Cord

The role of orexin during development, and especially in terms of spinal cord function, is not well understood. It is for this reason that we focused on the network actions of orexin during the first week of development. We found that orexinergic fibers were present in the lumbar spinal cord of postnatal day 0 (P0) to P3 mice. The fibers were expressed mainly in the dorsal horn, but occasional fibers were observed in the ventral horn. Both orexin (OX) A and OXB increased the motoneurons (MNs) tonic neurogram discharge. However, only OXA was found to significantly increase spontaneous bursting activity and the frequency of fictive locomotor bursts. We show that OXA is able to act directly on MNs. To test the contribution of the recurrent MN collaterals, we blocked the nicotinic cholinergic drive and observed that OXA retained its ability to increase fictive locomotor activity. Additionally, we recorded neurograms from ventral lateral funiculi, where OXA had no effect on population discharge. These effects were also confirmed by recording from descending commissural interneurons via patch recordings. The loci of the effects of OXA were further investigated in a dorsal horn-removed preparation where OXA also shows an increase in the discharge from ventral root neurograms but no increase in the frequency of spontaneous or fictive locomotion burst activity. In summary, multiple lines of evidence from our work demonstrate the robust effects of orexins on spinal cord networks and MNs at the time of birth.

Principles Governing Locomotion in Vertebrates: Lessons From Zebrafish

Locomotor behaviors are critical for survival and enable animals to navigate their environment, find food and evade predators. The circuits in the brain and spinal cord that initiate and maintain such different modes of locomotion in vertebrates have been studied in numerous species for over a century. In recent decades, the zebrafish has emerged as one of the main model systems for the study of locomotion, owing to its experimental amenability, and work in zebrafish has revealed numerous new insights into locomotor circuit function. Here, we review the literature that has led to our current understanding of the neural circuits controlling swimming and escape in zebrafish. We highlight recent studies that have enriched our comprehension of key topics, such as the interactions between premotor excitatory interneurons (INs) and motoneurons (MNs), supraspinal and spinal circuits that coordinate escape maneuvers, and developmental changes in overall circuit composition. We also discuss roles for neuromodulators and sensory inputs in modifying the relative strengths of constituent circuit components to provide flexibility in zebrafish behavior, allowing the animal to accommodate changes in the environment. We aim to provide a coherent framework for understanding the circuitry in the brain and spinal cord of zebrafish that allows the animal to flexibly transition between different speeds, and modes, of locomotion.

Selective and Context-Dependent Social and Behavioral Effects of Δ9-Tetrahydrocannabinol in Weakly Electric Fish

Cannabinoid (CB) receptors are widespread in the nervous system and influence a variety of behaviors. Weakly electric fish have been a useful model system in the study of the neural basis of behavior, but we know nothing of the role played by the CB system. Here, we determine the overall behavioral effect of a nonselective CB receptor agonist, namely Δ9-tetrahydrocannabinol (THC), in the weakly electric fish Apte-ronotus leptorhynchus. Using various behavioral paradigms involving social stimuli, we show that THC decreases locomotor behavior, as in many species, and influences communication and social behavior. Across the different experiments, we found that the propensity to emit communication signals (chirps) and seek social interactions was affected in a context-dependent manner. We explicitly tested this hypothesis by comparing the behavioral effects of THC injection in fish placed in a novel versus a familiar social and physical environment. THC-injected fish were less likely to chirp than control fish in familiar situations but not in novel ones. The tendency to be in close proximity to other fish was affected only in novel environments, with control fish clustering more than THC-injected ones. By identifying behaviors affected by CB agonists, our study can guide further comparative and neurophysiological studies of the role of the CB system using a weakly electric fish as a model.

A study of cannabinoid-1 receptors during the early phase of excitotoxic damage to rat spinal locomotor networks in vitro

Endocannabinoids acting on cannabinoid-1 receptors (CB1Rs) are proposed to protect brain and spinal neurons from excitotoxic damage. The ability to recover from spinal cord injury (SCI), in which excitotoxicity is a major player, is usually investigated at late times after modulation of CB1Rs whose role in the early phases of SCI remains unclear. Using the rat spinal cord in vitro as a model for studying SCI initial pathophysiology, we investigated if agonists or antagonists of CB1Rs might affect SCI induced by the excitotoxic agent kainate (KA) within 24h from a transient (1h) application of this glutamate agonist. The CB1 agonist anandamide (AEA or pharmacological block of its degradation) did not limit excitotoxic depolarization of spinal networks: cyclic adenosine monophosphate (cAMP) assay demonstrated that CB1Rs remained functional 24h later and similarly expressed among dead or survived cells. Locomotor-like network activity recorded from ventral roots could not recover with such treatments and was associated with persistent depression of synaptic transmission. Motoneurons, that are particularly vulnerable to KA, were not protected by AEA. Application of 2-arachidonoylglycerol also did not attenuate the electrophysiological and histological damage. The intensification of damage by the CB1 antagonist AM251 suggested that endocannabinoids were operative after excitotoxic stimulation, yet insufficient to contrast it efficiently. The present data indicate that the early phases of excitotoxic SCI could not be arrested by pharmacologically exploiting the endocannabinoid system, consistent with the notion that AEA and its derivatives are more useful to treat late SCI phases.

Locomotion: Electrical Coupling of Motor and Premotor Neurons

A new study has found that zebrafish motor neurons can contribute to the generation of locomotor rhythms by feedback modulation of their premotor interneurons via gap junctions.

Cannabinoids to treat spinal cord injury

Spinal cord injury (SCI) is a devastating condition for which there is no standard treatment beyond rehabilitation strategies. In this review, we discuss the current knowledge on the use of cannabinoids to treat this condition. The endocannabinoid system is expressed in the intact spinal cord, and it is dramatically upregulated after lesion. Endogenous activation of this system counteracts secondary damage following SCI, and treatments with endocannabinoids or synthetic cannabinoid receptor agonists promote a better functional outcome in experimental models. The use of cannabinoids in SCI is a new research field and many questions remain open. Here, we discuss caveats and suggest some future directions that may help to understand the role of cannabinoids in SCI and how to take advantage of this system to regain functions after spinal cord damage.

Cannabinoids: Glutamatergic Transmission and Kynurenines

The endocannabinoid system (ECS) comprises a complex of receptors, enzymes, and endogenous agonists that are widely distributed in the central nervous system of mammals and participates in a considerable number of neuromodulatory functions, including neurotransmission, immunological control, and cell signaling. In turn, the kynurenine pathway (KP) is the most relevant metabolic route for tryptophan degradation to form the metabolic precursor NAD(+). Recent studies demonstrate that the control exerted by the pharmacological manipulation of the ECS on the glutamatergic system in the brain may offer key information not only on the development of psychiatric disorders like psychosis and schizophrenia-like symptoms, but it also may constitute a solid basis for the development of therapeutic strategies to combat excitotoxic events occurring in neurological disorders like Huntington's disease (HD). Part of the evidence pointing to the last approach is based on experimental protocols demonstrating the efficacy of cannabinoids to prevent the deleterious actions of the endogenous neurotoxin and KP metabolite quinolinic acid (QUIN). These findings intuitively raise the question about what is the precise role of the ECS in tryptophan metabolism through KP and vice versa. In this chapter, we will review basic concepts on the physiology of both the ECS and the KP to finally describe those recent findings combining the components of these two systems and hypothesize the future course that the research in this emerging field will take in the next years.

A Hardwired Circuit Supplemented with Endocannabinoids Encodes Behavioral Choice in Zebrafish

Animals constantly make behavioral choices to facilitate moving efficiently through their environment. When faced with a threat, animals make decisions in the midst of other ongoing behaviors through a context-dependent integration of sensory stimuli. In vertebrates, the mechanisms underlying behavioral selection are poorly understood. Here, we show that ongoing swimming in zebrafish is suppressed by escape. The selection of escape over swimming is mediated by switching between two distinct motoneuron pools. A hardwired circuit mediates this switch by acting as a clutch-like mechanism to disengage the swimming motoneuron pool and engage the escape motoneuron pool. Threshold for escape initiation is lowered and swimming suppression is prolonged by endocannabinoid neuromodulation. Thus, our results reveal a novel cellular mechanism involving a hardwired circuit supplemented with endocannabinoids acting as a clutch-like mechanism to engage/disengage distinct motor pools to ensure behavioral selection and a smooth execution of motor action sequences in a vertebrate system.

Dynamics and plasticity of spinal locomotor circuits

Spinal circuits generate coordinated locomotor movements. These hardwired circuits are supplemented with neuromodulation that provide the necessary flexibility for animals to move smoothly through their environment. This review will highlight some recent insights gained in understanding the functional dynamics and plasticity of the locomotor circuits. First the mechanisms governing the modulation of the speed of locomotion will be discussed. Second, advantages of the modular organization of the locomotor networks with multiple circuits engaged in a task-dependent manner will be examined. Finally, the neuromodulation and the resulting plasticity of the locomotor circuits will be summarized with an emphasis on endocannabinoids and nitric oxide. The intention is to extract general principles of organization and discuss some onto-genetic and phylogenetic divergences.

Expression of the cannabinoid CB1 receptor in the gymnotiform fish brain and its implications for the organization of the teleost pallium

Cannabinoid CB1 receptors (CB1R) are widely distributed in the brains of many vertebrates, but whether their functions are conserved is unknown. The weakly electric fish, Apteronotus leptorhynchus (Apt), has been well studied for its brain structure, behavior, sensory processing, and learning and memory. It therefore offers an attractive model for comparative studies of CB1R functions. We sequenced partial AptCB1R mRNAs and performed in situ hybridization to localize its expression. Partial AptCB1R protein sequence was highly conserved to zebrafish (90.7%) and mouse (81.9%) orthologs. AptCB1R mRNA was highly expressed in the telencephalon. Subpallial neurons (dorsal, central, intermediate regions and part of the ventral region, Vd/Vc/Vi, and Vv) expressed high levels of AptCB1R transcript. The central region of dorsocentral telencephalon (DC(core) ) strongly expressed CB1R mRNA; cells in DC(core) project to midbrain regions involved in electrosensory/visual function. The lateral and rostral regions of DC surrounding DC(core) (DC(shell) ) lack AptCB1R mRNA. The rostral division of the dorsomedial telencephalon (DM1) highly expresses AptCB1R mRNA. In dorsolateral division (DL) AptCB1R mRNA was expressed in a gradient that declined in a rostrocaudal manner. In diencephalon, AptCB1R RNA probe weakly stained the central-posterior (CP) and prepacemaker (PPn) nuclei. In mesencephalon, AptCB1R mRNA is expressed in deep layers of the dorsal (electrosensory) torus semicircularis (TSd). In hindbrain, AptCB1R RNA probe weakly labeled inhibitory interneurons in the electrosensory lateral line lobe (ELL). Unlike mammals, only few cerebellar granule cells expressed AptCB1R transcripts and these were located in the center of eminentia granularis pars posterior (EGp), a cerebellar region involved in feedback to ELL.

mGluR5 ablation in cortical glutamatergic neurons increases novelty-induced locomotion

The group I metabotropic glutamate receptor 5 (mGluR5) has been implicated in the pathology of various neurological disorders including schizophrenia, ADHD, and autism. mGluR5-dependent synaptic plasticity has been described at a variety of neural connections and its signaling has been implicated in several behaviors. These behaviors include locomotor reactivity to novel environment, sensorimotor gating, anxiety, and cognition. mGluR5 is expressed in glutamatergic neurons, inhibitory neurons, and glia in various brain regions. In this study, we show that deleting mGluR5 expression only in principal cortical neurons leads to defective cannabinoid receptor 1 (CB1R) dependent synaptic plasticity in the prefrontal cortex. These cortical glutamatergic mGluR5 knockout mice exhibit increased novelty-induced locomotion, and their locomotion can be further enhanced by treatment with the psychostimulant methylphenidate. Despite a modest reduction in repetitive behaviors, cortical glutamatergic mGluR5 knockout mice are normal in sensorimotor gating, anxiety, motor balance/learning and fear conditioning behaviors. These results show that mGluR5 signaling in cortical glutamatergic neurons is required for precisely modulating locomotor reactivity to a novel environment but not for sensorimotor gating, anxiety, motor coordination, several forms of learning or social interactions.

Modulation of calcium currents and membrane properties by substance P in the lamprey spinal cord

Substance P is endogenously released within the locomotor network of the adult lamprey, accelerates the burst frequency of fictive locomotion, and reduces the reciprocal inhibition. Previous studies have shown that dopamine, serotonin, and GABA regulate calcium channels, which control neurotransmitter release, action potential duration, and slow afterhyperpolarization (sAHP). Here we examine the effect of substance P on calcium channels in motoneurons and commissural interneurons using whole cell patch clamp in the lamprey spinal cord. This study analyzed the effects of substance P on calcium currents activated in voltage clamp. We examined the calcium-dependent sAHP in current clamp, to determine the involvement of three calcium channel subtypes modulated by substance P. The effects of substance P on membrane potential and during N-methyl-d-aspartic acid (NMDA) induced oscillations were also analyzed. Depolarizing voltage steps induced inward calcium currents. Substance P reduced the currents carried by calcium by 61% in commissural interneurons and by 31% in motoneurons. Using specific calcium channel antagonists, we show that substance P reduces the sAHP primarily by inhibiting N-type (CaV2.2) channels. Substance P depolarized both motoneurons and commissural interneurons, and we present evidence that this occurs due to an increased input resistance. We also explored the effects of substance P on NMDA-induced oscillations in tetrodotoxin and found it caused a frequency increase. Thus the reduction of calcium entry by substance P and the accompanying decrease of the sAHP amplitude, combined with substance P potentiation of currents activated by NMDA, may both contribute to the increase in fictive locomotion frequency.

The effects of endocannabinoid signaling on network activity in developing and motor circuits

Endocannabinoid signaling typically mediates a form of synaptic plasticity in which a postsynaptic cell acts retrogradely to reduce vesicle release from presynaptic terminals impinging on that cell. In the embryonic spinal cord, endocannabinoids inhibit spontaneously released glutamatergic vesicles in both a brief and ongoing tonic manner. Together these endocannabinoid-mediated forms of synaptic regulation appear to play an important role in regulating the frequency of a form of spontaneous network activity (SNA) that is expressed in the embryonic spinal cord. Because of the importance of SNA to the maturation of the developing network, fetal exposure to drugs that influence endocannabinoid signaling may have profound effects on spinal maturation. In this review, endocannabinoid signaling in the embryonic spinal cord is described and compared to signaling in the mature lamprey spinal cord as well as in the developing hippocampal network, which expresses a form of SNA.

Tonic and transient endocannabinoid regulation of AMPAergic miniature postsynaptic currents and homeostatic plasticity in embryonic motor networks

Endocannabinoid signaling has been shown to mediate synaptic plasticity by retrogradely inhibiting presynaptic transmitter release in several systems. We found that endocannabinoids act tonically to regulate AMPA miniature postsynaptic current (mPSC) frequency in embryonic motor circuits of the chick spinal cord. Further, strong postsynaptic depolarizations also induced a short-lived endocannabinoid-mediated suppression of mEPSC frequency. Unlike many previous studies, endocannabinoid signaling was not found to influence evoked transmitter release. The results suggest a special role for spontaneous glutamatergic mPSCs and their control by endocannabinoids in the developing spinal cord. We determined that blocking endocannabinoid signaling, which increases spontaneous glutamatergic release, increased spontaneous network activity in vitro and in vivo. Previous work in spinal motoneurons had shown that reducing spontaneous network activity (SNA) chronically in vivo led to homeostatic increases in AMPA and GABA mPSC amplitude (homeostatic synaptic plasticity). Blocking endocannabinoid signaling in vivo, and thus increasing SNA, triggered compensatory decreases of both AMPA and GABA mPSC amplitudes. These findings, combined with previous results, are consistent with the idea that this form of homeostatic synaptic plasticity is a bidirectional process in the living embryo. Together, our results suggest a role for tonic signaling of endocannabinoids as a potential mechanism to regulate the level of SNA, which is known to be critical for synaptic maturation in the embryonic spinal cord.

The evolution and comparative neurobiology of endocannabinoid signalling

CB(1)- and CB(2)-type cannabinoid receptors mediate effects of the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide in mammals. In canonical endocannabinoid-mediated synaptic plasticity, 2-AG is generated postsynaptically by diacylglycerol lipase alpha and acts via presynaptic CB(1)-type cannabinoid receptors to inhibit neurotransmitter release. Electrophysiological studies on lampreys indicate that this retrograde signalling mechanism occurs throughout the vertebrates, whereas system-level studies point to conserved roles for endocannabinoid signalling in neural mechanisms of learning and control of locomotor activity and feeding. CB(1)/CB(2)-type receptors originated in a common ancestor of extant chordates, and in the sea squirt Ciona intestinalis a CB(1)/CB(2)-type receptor is targeted to axons, indicative of an ancient role for cannabinoid receptors as axonal regulators of neuronal signalling. Although CB(1)/CB(2)-type receptors are unique to chordates, enzymes involved in biosynthesis/inactivation of endocannabinoids occur throughout the animal kingdom. Accordingly, non-CB(1)/CB(2)-mediated mechanisms of endocannabinoid signalling have been postulated. For example, there is evidence that 2-AG mediates retrograde signalling at synapses in the nervous system of the leech Hirudo medicinalis by activating presynaptic transient receptor potential vanilloid-type ion channels. Thus, postsynaptic synthesis of 2-AG or anandamide may be a phylogenetically widespread phenomenon, and a variety of proteins may have evolved as presynaptic (or postsynaptic) receptors for endocannabinoids.

Gating the polarity of endocannabinoid-mediated synaptic plasticity by nitric oxide in the spinal locomotor network

The final motor output underlying behavior arises from an appropriate balance between excitation and inhibition within neural networks. Retrograde signaling by endocannabinoids adapts synaptic strengths and the global activity of neural networks. In the spinal cord, endocannabinoids are mobilized postsynaptically from network neurons and act retrogradely on presynaptic cannabinoid receptors to potentiate the locomotor frequency. However, it is still unclear whether mechanisms exist within the locomotor networks that determine the sign of the modulation by cannabinoid receptors to differentially regulate excitation and inhibition. In this study, using the lamprey spinal cord in vitro, we first report that 2-AG (2-arachidonyl glycerol) is mobilized by network neurons and underlies a form of modulation that is embedded within the locomotor networks. We then show that the polarity of the endocannabinoid modulation is gated by nitric oxide to enable simultaneously potentiation of excitation and depression of inhibition within the spinal locomotor networks. Our results suggest that endocannabinoid and nitric oxide systems interact to mediate inversion of the polarity of synaptic plasticity within the locomotor networks. Thus, endocannabinoid and nitric oxide shift in the excitation-inhibition balance to set the excitability of the spinal locomotor network.

A Role for the Cannabinoid 1 Receptor in Neuronal Differentiation of Adult Spinal Cord Progenitors in vitro is Revealed through Pharmacological Inhibition and Genetic Deletion

In contrast to the adult brain, the adult spinal cord is a non-neurogenic environment. Understanding how to manipulate the spinal cord environment to promote the formation of new neurons is an attractive therapeutic strategy for spinal cord injury and disease. The cannabinoid 1 receptor (CB1R) has been implicated as a modulator of neural progenitor cell proliferation and fate specification in the brain; however, no evidence exists for modulation of adult spinal cord progenitor cells. Using adult rat spinal cord primary cultures, we demonstrated that CB1R antagonism with AM251 significantly decreased the number of Nestin(+) cells, and increased the number of βIII tubulin(+) and DCX(+) cells, indicative of neuronal differentiation. AM251’s effect was blocked by co-application of the CB1R agonists, WIN 55, 212-2, or ACEA. Consistent with our hypothesis, cultures, and spinal cord slices derived from CB1R knock-out (CB1−/−) mice had significantly higher levels of DCX(+) cells compared to those derived from wild type (CB1+/+) mice, indicative of enhanced neuronal differentiation in CB1−/− spinal cords. Moreover, AM251 promoted neuronal differentiation in CB1+/+, but not in CB1−/− cultures. Since CB1R modulates synaptic transmission, and synaptic transmission has been shown to influence progenitor cell fate, we evaluated whether AM251-induced neuronal differentiation was affected by chronic inactivity. Either the presence of the voltage-dependent sodium channel blocker tetrodotoxin (TTX), or the removal of mature neurons, inhibited the AM251-induced increase in DCX(+) cells. In summary, antagonism or absence of CB1R promotes neuronal differentiation in adult spinal cords, and this action appears to require TTX-sensitive neuronal activity. Our data suggest that the previously detected elevated levels of endocannabinoids in the injured adult spinal cord could contribute to the non-neurogenic environment and CB1R antagonists could potentially be used to enhance replacement of damaged neurons.

Initiation of locomotion in adult zebrafish

Motor behavior is generated by specific neural circuits. Those producing locomotion are located in the spinal cord, and their activation depends on descending inputs from the brain or on sensory inputs. In this study, we have used an in vitro brainstem-spinal cord preparation from adult zebrafish to localize a region where stimulation of descending inputs can induce sustained locomotor activity. We show that a brief stimulation of descending inputs at the junction between the brainstem and spinal cord induces long-lasting swimming activity. The swimming frequencies induced are remarkably similar to those observed in freely moving adult fish, arguing that the induced locomotor episode is highly physiological. The motor pattern is mediated by activation of ionotropic glutamate and glycine receptors in the spinal cord and is not the result of synaptic interactions between neurons at the site of the stimulation in the brainstem. We also compared the activity of motoneurons during locomotor activity induced by electrical stimulation of descending inputs and by exogenously applied NMDA. Prolonged NMDA application changes the shape of the synaptic drive and action potentials in motoneurons. When escape activity occurs, the swimming activity in the intact zebrafish was interrupted and some of the motoneurons involved became inhibited in vitro. Thus, the descending inputs seem to act as a switch to turn on the activity of the spinal locomotor network in the caudal spinal cord. We propose that recurrent synaptic activity within the spinal locomotor circuits can transform a brief input into a well coordinated and long-lasting swimming pattern.

Bilateral oscillatory hip movements induce windup of multijoint lower extremity spastic reflexes in chronic spinal cord injury

After spinal cord injury (SCI), alterations in intrinsic motoneuron properties have been shown to be partly responsible for spastic reflex behaviors in human SCI. In particular, a dysregulation of voltage-dependent depolarizing persistent inward currents (PICs) may permit sustained muscle contraction after the removal of a brief excitatory stimulus. Windup, in which the motor response increases with repeated activation, is an indicator of PICs. Although windup of homonymous stretch reflexes has been shown, multijoint muscle activity is often observed following imposed limb movements and may exhibit a similar windup phenomenon. The purpose of this study was to identify and quantify windup of multijoint reflex responses to repeated imposed hip oscillations. Ten chronic SCI subjects participated in this study. A custom-built servomotor apparatus was used to oscillate the legs about the hip joint bilaterally and unilaterally from 10° of extension to 40° flexion for 10 consecutive cycles. Surface electromyograms (EMGs) and joint torques were recorded from both legs. Consistent with a windup response, hip and knee flexion/extension and ankle plantarflexion torque and EMG responses varied according to movement cycle number. The temporal patterns of windup depended on the muscle groups that were activated, which may suggest a difference in the response of neurons in different spinal pathways. Furthermore, because windup was seen in muscles that were not being stretched, these results imply that changes in interneuronal properties are also likely to be associated with windup of spastic reflexes in human SCI.

Beyond connectivity of locomotor circuitry-ionic and modulatory mechanisms

Discrete neural networks in the central nervous system generate the repertoire of motor behavior necessary for animal survival. The final motor output of these networks is the result of the anatomical connectivity between the individual neurons and also their biophysical properties as well as the dynamics of their synaptic transmission. To illustrate how this processing takes place to produce coordinated motor activity, we have summarized some of the results available from the lamprey spinal locomotor network. The detailed knowledge available in this model system on the organization of the network together with the properties of the constituent neurons and the modulatory systems allows us to determine how the impact of specific ion channels and receptors is translated to the global activity of the locomotor circuitry. Understanding the logic of the neuronal and synaptic processing within the locomotor network will provide information about not only their normal operation but also how they react to disruption such as injuries or trauma.

Activation of group I metabotropic glutamate receptors modulates locomotor-related motoneuron output in mice

Fast glutamatergic transmission via ionotropic receptors is critical for the generation of locomotion by spinal motor networks. In addition, glutamate can act via metabotropic glutamate receptors (mGluRs) to modulate the timing of ongoing locomotor activity. In the present study, we investigated whether mGluRs also modulate the intensity of motor output generated by spinal motor networks. Application of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) reduced the amplitude and increased the frequency of locomotor-related motoneuron output recorded from the lumbar ventral roots of isolated mouse spinal cord preparations. Whole cell patch-clamp recordings of spinal motoneurons revealed multiple mechanisms by which group I mGluRs modulate motoneuron output. Although DHPG depolarized the resting membrane potential and reduced the voltage threshold for action potential generation, the activation of group I mGluRs had a net inhibitory effect on motoneuron output that appeared to reflect the modulation of fast, inactivating Na(+) currents and action potential parameters. In addition, group I mGluR activation decreased the amplitude of locomotor-related excitatory input to motoneurons. Analyses of miniature excitatory postsynaptic currents indicated that mGluRs modulate synaptic drive to motoneurons via both pre- and postsynaptic mechanisms. These data highlight group I mGluRs as a potentially important source of neuromodulation within the spinal cord that, in addition to modulating components of the central pattern generator for locomotion, can modulate the intensity of motoneuron output during motor behavior. Given that group I mGluR activation reduces motoneuron excitability, mGluRs may provide negative feedback control of motoneuron output, particularly during high levels of glutamatergic stimulation.

mGluR5 in cortical excitatory neurons exerts both cell-autonomous and -nonautonomous influences on cortical somatosensory circuit formation

Glutamatergic neurotransmission plays important roles in sensory map formation. The absence of the group I metabotropic glutamate receptor 5 (mGluR5) leads to abnormal sensory map formation throughout the mouse somatosensory pathway. To examine the role of cortical mGluR5 expression on barrel map formation, we generated cortex-specific mGluR5 knock-out (KO) mice. Eliminating mGluR5 function solely in cortical excitatory neurons affects, not only the whisker-related organization of cortical neurons (barrels), but also the patterning of their presynaptic partners, the thalamocortical axons (TCAs). In contrast, subcortical whisker maps develop normally in cortical-mGluR5 KO mice. In the S1 cortex of cortical-mGluR5 KO, layer IV neurons are homogenously distributed and have no clear relationship to the location of TCA clusters. The altered dendritic morphology of cortical layer IV spiny stellate neurons in cortical-mGluR5 KO mice argues for a cell-autonomous role of mGluR5 in dendritic patterning. Furthermore, morphometric analysis of single TCAs in both cortical- and global-mGluR5 KO mice demonstrated that in these mice, the complexity of axonal arbors is reduced, while the area covered by TCA arbors is enlarged. Using voltage-clamp whole-cell recordings in acute thalamocortical brain slices, we found that KO of mGluR5 from cortical excitatory neurons reduced inhibitory but not excitatory inputs onto layer IV neurons. This suggests that mGluR5 signaling in cortical excitatory neurons nonautonomously modulates the functional development of GABAergic circuits. Together, our data provide strong evidence that mGluR5 signaling in cortical principal neurons exerts both cell-autonomous and -nonautonomous influences to modulate the formation of cortical sensory circuits.

The role of endocannabinoid signaling in motor control

Cannabinoid receptors and endocannabinoid signaling are distributed throughout the rostrocaudal neuraxis. Retrograde signaling via endocannabinoid mediates synaptic plasticity in many regions in the central nervous system. Here, we review the role of endocannabinoid signaling in different parts of the vertebrate motor system from networks responsible for the execution of movement to planning centers in the basal ganglia and cortex. The ubiquity of endocannabinoid-mediated plasticity suggests that it plays an important role in producing motion from defined circuitries and also for reconfiguring networks to learn new motor skills. The long-term plasticity induced by endocannabinoids may provide a long-term buffer that stabilizes the organization of motor circuits and their activity.

Synaptically activated burst-generating conductances may underlie a group-pacemaker mechanism for respiratory rhythm generation in mammals

Breathing, chewing, and walking are critical life-sustaining behaviors in mammals that consist essentially of simple rhythmic movements. Breathing movements in particular involve the diaphragm, thorax, and airways but emanate from a network in the lower brain stem. This network can be studied in reduced preparations in vitro and using simplified mathematical models that make testable predictions. An iterative approach that employs both in vitro and in silico models argues against canonical mechanisms for respiratory rhythm in neonatal rodents that involve reciprocal inhibition and pacemaker properties. We present an alternative model in which emergent network properties play a rhythmogenic role. Specifically, we show evidence that synaptically activated burst-generating conductances-which are only available in the context of network activity-engender robust periodic bursts in respiratory neurons. Because the cellular burst-generating mechanism is linked to network synaptic drive we dub this type of system a group pacemaker.

Measured motion: searching for simplicity in spinal locomotor networks

Spinal interneurons are organized into networks that control the activity and output of the motor system. This review outlines recent progress in defining the rules that govern the assembly and function of spinal motor networks, focusing on three main areas. We first examine how subtle variations in the wiring diagrams and organization of locomotor networks in different vertebrates permits animals to adapt their motor programs to the demands of their physical environment. We discuss how the membrane properties of spinal interneurons, and their synaptic interactions, underlie the modulation of motor circuits and encoded motor behaviors. We also describe recent molecular genetic approaches to map and manipulate the connectivity and interactions of spinal interneurons and to assess the impact of such perturbations on network function and motor behavior.

Nitric oxide potentiation of locomotor activity in the spinal cord of the lamprey

To understand the intrinsic operation of spinal networks generating locomotion, we need to not only characterize the constituent neurons and their connectivity, but also determine the role of intrinsic modulation in shaping the final motor output. We have focused on the effects of nitric oxide (NO) on the locomotor frequency and the underlying synaptic mechanisms in the lamprey spinal cord. To identify the source of NO, we used NADPH-diaphorase histochemistry and nNOS immunocytochemistry. Gray matter and sensory neurons were positively labeled using both methods. Preparations preincubated with NO synthase inhibitors displayed slower locomotor frequency that increased upon washout of the inhibitors, suggesting that NO is an endogenous neuromodulator in the spinal cord. Application of NO donors increased the locomotor frequency that was blocked by an NO scavenger and partially reduced by an inhibitor of sGC. To analyze the synaptic modulation underlying the NO-induced increase of the locomotor frequency we performed intracellular recordings from motoneurons and interneurons. The NO-induced increase in locomotor frequency was associated with a decrease in the midcycle inhibition and an increase in on-cycle excitation. To determine the site of action of NO, we examined the effect of NO donors on miniature PSCs. NO increased both the frequency and amplitude of mEPSCs while it only decreased the frequency of mIPSCs, suggesting the increased excitation is mediated by both presynaptic and postsynaptic mechanisms, while the decrease in inhibition involves only presynaptic mechanisms. Our results demonstrate a significant role of NO in adult vertebrate motor control which, via modulation of both excitatory and inhibitory transmission, increases the locomotor burst frequency.

Transmitter phenotypes of commissural interneurons in the lamprey spinal cord

The fundamental network for locomotion in all vertebrates contains a central pattern generator or CPG that produces the required motor output in the spinal cord. In the lamprey spinal cord different classes of interneuron's forming the core CPG circuitry have been characterized based on their morphological and electrophysiological features. The commissural interneuron's (C-INs) represent one essential component of CPG that have been implicated in controlling left-right alternation of the motor activity during swimming. However, it is still unclear if the C-INs displays a homogenous neurotransmitter phenotype and how they are distributed. In this paper we investigated the segmental distribution of glycine, glutamate and GABA-immunoreactive (ir) C-INs by combining retrograde Neurobiotin tracing with specific antibodies for these transmitters. The C-INs were more abundant in caudal and rostral segments adjacent to the injection site and their number gradually decreased in more distal segments, suggesting that these interneurons project over a short distance. The glycine-ir neurons represented around 50% of the total C-INs, while glutamate-ir neurons represented only 29%. Both types of C-INs were homogenously distributed over different segments along the spinal cord. Finally, no Neurobiotin labeled C-INs displayed GABA-ir, although many interneurons were ir to GABA, suggesting that GABAergic interneurons are not directly responsible for controlling left-right alternation of activity during locomotion in lamprey. Overall, these results show that the C-INs display a gradual rostrocaudal distribution and consist of both glycine- and glutamate-ir neurons. The difference in the proportion of inhibitory and excitatory C-INs represents an anatomical substrate that can ensure the predominance of alternating activity during locomotion.

Endocannabinoids Mediate Tachykinin-Induced Effects in the Lamprey Locomotor Network

The spinal network underlying locomotion in lamprey is composed of excitatory and inhibitory interneurons mediating fast ionotropic action. In addition, several modulator systems are activated as locomotion is initiated, including the tachykinin system and the metabotropic glutamate receptor 1 (mGluR1), the latter operating partially via the endocannabinoid system. The effects of mGluR1 agonists and tachykinins resemble each other. Like mGluR1 agonists, the tachykinin substance P accelerates the burst rate and reduces the crossed inhibition in an activity-dependent fashion. The present study therefore explores whether tachykinins also use the endocannabinoid system to modulate the locomotor frequency. By monitoring fictive locomotion, we were able to compare the facilitatory effects exerted by applying substance P (1 μM, 20 min), on the burst frequency before and during application of the endocannabinoid CB1 receptor antagonist AM251 (2–5 μM). By using two different lamprey species, we showed that the response to substance P on the burst frequency is significantly reduced during the application of AM251. To examine whether endocannabinoids are involved in the substance P–mediated modulation of reciprocal inhibition, the commissural axons were stimulated, while recording intracellularly from motoneurons. We compare the effect of substance P on the amplitude of the contralateral compound glycinergic inhibitory postsynaptic potential (IPSP) in control and in the presence of AM251. The blockade of CB1 receptors reduced the substance P–mediated decrease in the amplitude by 29%. The present findings suggest that the effects of substance P on the increase in the locomotor burst frequency and depression of IPSPs are mediated partially via release of endocannabinoids acting through CB1 receptors.

Separate signalling mechanisms underlie mGluR1 modulation of leak channels and NMDA receptors in the network underlying locomotion

Metabotropic glutamate receptor subtype 1 (mGluR1) contributes importantly to the activity of the spinal locomotor network. For example, it potentiates NMDA current and inhibits leak conductance in lamprey spinal cord neurons. In this study we examined the signalling pathways underlying the mGluR1 modulation of NMDA receptors and leak channels, respectively. Our results show that mGluR1-induced potentiation of NMDA current required activation of phospholipase C (PLC) and was independent of the increase in the intracellular Ca2+ concentration because it was unaffected by the Ca2+ chelator BAPTA and by depletion of the internal Ca2+ stores with thapsigargin. We also show that the mGluR1-mediated inhibition of leak channels is mediated by activation of G-proteins. Finally, we show that blockade of protein kinase C (PKC) abolished the mGluR1-induced inhibition of leak current without affecting the potentiation of NMDA receptors. The contribution of mGluR1-mediated modulation of leak channels to the potentiation of the locomotor cycle frequency was assessed during fictive locomotion. Blockade of PKC significantly decreased the short-term potentiation of locomotor cycle frequency by mGluR1. These results show that the effects of mGluR1 activation on the two cellular targets, the NMDA receptor and leak channels, are mediated through separate signalling pathways.

AMPA and metabotropic glutamate receptors cooperatively generate inspiratory-like depolarization in mouse respiratory neurons in vitro

Excitatory transmission mediated by AMPA receptors is critical for respiratory rhythm generation. However, the role of AMPA receptors has not been fully explored. Here we tested the functional role of AMPA receptors in inspiratory neurons of the neonatal mouse preBötzinger complex (preBötC) using an in vitro slice model that retains active respiratory function. Immediately before and during inspiration, preBötC neurons displayed envelopes of depolarization, dubbed inspiratory drive potentials, that required AMPA receptors but largely depended on the Ca(2+)-activated non-specific cation current (I(CAN)). We showed that AMPA receptor-mediated depolarization opened voltage-gated Ca(2+) channels to directly evoke I(CAN). Inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca(2+) release also evoked I(CAN). Inositol 1,4,5-trisphosphate receptors acted downstream of group I metabotropic glutamate receptor activity but, here too, AMPA receptor-mediated Ca(2+) influx was essential to trigger the metabotropic glutamate receptor contribution to inspiratory drive potential generation. This study helps to elucidate the role of excitatory transmission in respiratory rhythm generation in vitro. AMPA receptors in preBötC neurons initiate convergent signaling pathways that evoke post-synaptic I(CAN), which underlies inspiratory drive potentials. The coupling of AMPA receptors with I(CAN) suggests that latent burst-generating intrinsic conductances are recruited by excitatory synaptic interactions among preBötC neurons in the context of respiratory network activity in vitro, exemplifying a rhythmogenic mechanism based on emergent properties of the network.

Gene expression profiling of two distinct neuronal populations in the rodent spinal cord

Background

In the field of neuroscience microarray gene expression profiles on anatomically defined brain structures are being used increasingly to study both normal brain functions as well as pathological states. Fluorescent tracing techniques in brain tissue that identifies distinct neuronal populations can in combination with global gene expression profiling potentially increase the resolution and specificity of such studies to shed new light on neuronal functions at the cellular level.

Methodology/Principal Findings

We examine the microarray gene expression profiles of two distinct neuronal populations in the spinal cord of the neonatal rat, the principal motor neurons and specific interneurons involved in motor control. The gene expression profiles of the respective cell populations were obtained from amplified mRNA originating from 50–250 fluorescently identified and laser microdissected cells. In the data analysis we combine a new microarray normalization procedure with a conglomerate measure of significant differential gene expression. Using our methodology we find 32 genes to be more expressed in the interneurons compared to the motor neurons that all except one have not previously been associated with this neuronal population. As a validation of our method we find 17 genes to be more expressed in the motor neurons than in the interneurons and of these only one had not previously been described in this population.

Conclusions/Significance

We provide an optimized experimental protocol that allows isolation of gene transcripts from fluorescent retrogradely labeled cell populations in fresh tissue, which can be used to generate amplified aRNA for microarray hybridization from as few as 50 laser microdissected cells. Using this optimized experimental protocol in combination with our microarray analysis methodology we find 49 differentially expressed genes between the motor neurons and the interneurons that reflect the functional differences between these two cell populations in generating and transmitting the motor output in the rodent spinal cord.

Group I mGluRs increase locomotor network excitability in Xenopus tadpoles via presynaptic inhibition of glycinergic neurotransmission

The group I metabotropic glutamate receptor agonist (S)-3,5-dihyroxyphenylglycine (DHPG) increases the frequency of rhythmic swimming activity in Xenopus tadpoles. This study explores the possibility that group I receptor modulation occurs in part via depression of inhibitory synaptic transmission. Applications of the glycine receptor antagonist strychnine occluded the effects of DHPG, providing preliminary evidence that group I receptors affect motor network output by reducing glycinergic transmission. This evidence was supported further by intracellular and whole-cell patch-clamp recordings from presumed motorneurons. DHPG applications produced two prominent effects: (i) during swimming activity, glycinergic mid-cycle IPSPs were reduced in amplitude; and (ii) during quiescent periods, the frequency of spontaneous miniature IPSPs was also reduced. No change in membrane potential or input resistance following group I receptor activation was detected. The reduction in fast synaptic inhibition provides a plausible explanation for the increased excitability of the locomotor network, although other contributory mechanisms activated in parallel by group I receptors cannot be discounted. Aspects of this work have been published previously in abstract form [R. J. Chapman & K. T. Sillar (2003) SFN Abstracts 277.8].

Long-term plasticity of the spinal locomotor circuitry mediated by endocannabinoid and nitric oxide signaling

Retrograde signaling by endocannabinoids is known to induce short- and long-term synaptic plasticity, but the significance of this modulation for the activity of neural networks underlying motor behavior is largely unclear. Here, we used the isolated lamprey spinal cord to show that endocannabinoids released by activation of metabotropic glutamate receptor 1 (mGluR1) induce long-term synaptic plasticity during an ongoing locomotor rhythm and how this is translated onto the integrated activity of the spinal circuitry. A brief activation of mGluR1 induces a long-term increase in the locomotor frequency that is mediated by a concomitant long-term depression of midcycle reciprocal inhibition and long-term potentiation of ipsilateral synaptic excitation arising from locomotor circuit interneurons. Blockade of cannabinoid receptors with AM251 prevented the mGluR1-mediated long-term plasticity of both inhibitory and excitatory synaptic transmission, as well as that of the locomotor activity. Similarly, inhibition of nitric oxide signaling blocked the mGluR1-mediated long-term plasticity. These results show that the locomotor circuitry is endowed with a "memory" capacity mediated by a long-term shift in the balance between synaptic inhibition and excitation. This is triggered by activation of mGluR1 and requires subsequent endocannabinoid and nitric oxide signaling.

Modulation of a spinal locomotor network by metabotropic glutamate receptors

We have explored the potential involvement of the three main classes of metabotropic glutamate receptor in the modulation of a spinal locomotor network using tadpoles of the anuran amphibian Xenopus laevis. Selective activation of group I receptors in Xenopus embryos and young larvae using the general group I agonist DHPG [(S)-3,5-dihyroxyphenylglycine] significantly increased the frequency of swimming and the number of spontaneously occurring swimming episodes, as monitored by extracellular recordings from ventral roots. Group I receptor activation was without significant effect on the duration or amplitude of motor bursts, the duration of swimming episodes, or the head-to-tail delay in the propagation of swimming activity. Activation of either group II or group III receptors, however, following bath applications of the specific agonists APDC [(2R,4R)-aminopyrrolidine-2,4-dicarboxylic acid] and L-AP4 (L-2-amino-4-phosphonobutanoate), respectively, produced a net inhibitory effect on many of the parameters of fictive swimming at both developmental stages, including a reduction in swimming frequency and episode duration, along with a significant reduction in motor burst amplitude and duration in larval animals only. Applications of selective antagonists provide evidence for activation of all three groups during swimming. The group II and III antagonists EGLU (1-ethyl-2-benzimidazolinone) and MAP4 [(S)-2-amino-2-methyl-4-phosphonobutanoate], respectively, increased, while group I antagonists, CPCCOEt and MPEP, decreased swim frequency. Our findings thus provide evidence for the presence and endogenous activation of three classes of metabotropic glutamate receptor which may function as an intrinsic modulatory control system during fictive swimming in Xenopus tadpoles.

Neural bases of goal-directed locomotion in vertebrates--an overview

The different neural control systems involved in goal-directed vertebrate locomotion are reviewed. They include not only the central pattern generator networks in the spinal cord that generate the basic locomotor synergy and the brainstem command systems for locomotion but also the control systems for steering and control of body orientation (posture) and finally the neural structures responsible for determining which motor programs should be turned on in a given instant. The role of the basal ganglia is considered in this context. The review summarizes the available information from a general vertebrate perspective, but specific examples are often derived from the lamprey, which provides the most detailed information when considering cellular and network perspectives.

Endocannabinoid signaling in the spinal locomotor circuitry

To understand how the spinal central pattern generators produce locomotor movements, it is necessary to characterize the network's connectivity, the intrinsic properties of the constituent neurons and the modulatory mechanisms. Modulation operating within spinal locomotor networks is required for the generation of the final motor output. In this review, we have summarized how endocannabinoids released by locomotor network neurons contribute to setting the baseline locomotor frequency. They are synthesized on demand as a result of activation of mGluR1 and act as retrograde messengers to depress inhibitory synaptic transmission. We also discuss how endogenous activation of mGluR1 contributes to the normal operation of the spinal locomotor network and the underlying cellular and synaptic mechanisms.

Endogenous Tachykinin Release Contributes to the Locomotor Activity in Lamprey

Tachykinins are present in lamprey spinal cord. The goal of this study was to investigate whether an endogenous release of tachykinins contributes to the activity of the spinal network generating locomotor activity. The locomotor network of the isolated lamprey spinal cord was activated by bath-applied N-methyl-d-aspartate (NMDA) and the efferent activity recorded from the ventral roots. When spantide II, a tachykinin receptor antagonist, was bath-applied after reaching a steady-state burst frequency (>2 h), it significantly lowered the burst rate compared with control pieces from the same animal. In addition, the time to reach the steady-state burst frequency (>2 h) was lengthened in spantide II. These data indicate that an endogenous tachykinin release contributes to the ongoing activity of the locomotor network by modulating the glutamate–glycine neuronal network responsible for the locomotor pattern. We also explored the effects of a 10-min exogenous application of substance P (1 μM), a tachykinin, and showed that its effect on the burst rate depended on the initial NMDA induced burst frequency. At low initial burst rates (∼0.5 Hz), tachykinins caused a marked further slowing to 0.1 Hz, whereas at higher initial burst rates, it instead caused an enhanced burst rate as previously reported, and in addition, a slower modulation (0.1 Hz) of the amplitude of the motor activity. These effects occurred during an initial period of ∼1 h, whereas a modest long-lasting increase of the burst rate remained after >2 h.

Biological pattern generation: the cellular and computational logic of networks in motion

In 1900, Ramón y Cajal advanced the neuron doctrine, defining the neuron as the fundamental signaling unit of the nervous system. Over a century later, neurobiologists address the circuit doctrine: the logic of the core units of neuronal circuitry that control animal behavior. These are circuits that can be called into action for perceptual, conceptual, and motor tasks, and we now need to understand whether there are coherent and overriding principles that govern the design and function of these modules. The discovery of central motor programs has provided crucial insight into the logic of one prototypic set of neural circuits: those that generate motor patterns. In this review, I discuss the mode of operation of these pattern generator networks and consider the neural mechanisms through which they are selected and activated. In addition, I will outline the utility of computational models in analysis of the dynamic actions of these motor networks.

Tuning the network: modulation of neuronal microcircuits in the spinal cord and hippocampus

Adaptation of an organism to its changing environment ultimately depends on the modification of neuronal activity. The dynamic interaction between cellular components within neuronal networks relies on fast synaptic interaction via ionotropic receptors. However, neuronal networks are also subject to modulation mediated by various metabotropic G-protein-coupled receptors that modify synaptic and neuronal function. Modulation increases the functional complexity of a network, because the same cellular components can produce different outputs depending on the behavioural state of the animal. This review, which is part of the TINS Microcircuits Special Feature, provides an overview of neuromodulation in two neuronal circuits that both produce oscillatory activity but differ fundamentally in function. Hippocampal circuits are compared with the spinal networks generating locomotion, with a view to exploring common principles of neuromodulatory activity.