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

Lights on Endocannabinoid-Mediated Synaptic Potentiation

The endocannabinoid (eCB) system is a lipid-based neurotransmitter complex that plays crucial roles in the neural control of learning and memory. The current model of eCB-mediated retrograde signaling is that eCBs released from postsynaptic elements travel retrogradely to presynaptic axon terminals, where they activate cannabinoid type-1 receptors (CB₁Rs) and ultimately decrease neurotransmitter release on a short- or long-term scale. An increasing body of evidence has enlarged this view and shows that eCBs, besides depressing synaptic transmission, are also able to increase neurotransmitter release at multiple synapses of the brain. This indicates that eCBs act as bidirectional regulators of synaptic transmission and plasticity. Recently, studies unveiled links between the expression of eCB-mediated long-term potentiation (eCB-LTP) and learning, and between its dysregulation and several pathologies. In this review article, we first distinguish the various forms of eCB-LTP based on their mechanisms, resulting from homosynaptically or heterosynaptically-mediated processes. Next, we consider the neuromodulation of eCB-LTP, its behavioral impact on learning and memory, and finally, eCB-LTP disruptions in various pathologies and its potential as a therapeutic target in disorders such as stress coping, addiction, Alzheimer’s and Parkinson’s disease, and pain. Cannabis is gaining popularity as a recreational substance as well as a medicine, and multiple eCB-based drugs are under development. In this context, it is critical to understand eCB-mediated signaling in its multi-faceted complexity. Indeed, the bidirectional nature of eCB-based neuromodulation may offer an important key to interpret the functions of the eCB system and how it is impacted by cannabis and other drugs.

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.

Structure-efficiency relationship of photoinduced electron transfer-triggered nitric oxide releasers

Spatiotemporally controllable nitric oxide (NO) releasers are required for biological studies and as candidate therapeutic agents. Here, we investigate the structure-efficiency relationship of a series of photoinduced electron transfer-triggered NO releasers based on our reported yellowish-green light-controllable NO releaser, NO-Rosa. The distance between the NO-releasing N-nitrosoaminophenol moiety and the rosamine antenna moiety was critical for efficient NO release. Notably, substitution at the phenolic hydroxyl group blocked NO release. We synthesized NO-Rosa-Gal bearing D-galactose (Gal) at this location, and showed that hydrolysis by β-galactosidase restored the photoresponse. This represents proof-of-concept of a strategy for highly specific control of NO release by using a double-lock system involving both enzymatic reactivation and photo-control.

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.

Modulation of spinal motor networks by astrocyte-derived adenosine is dependent on D1-like dopamine receptor signaling

Astrocytes modulate many neuronal networks, including spinal networks responsible for the generation of locomotor behavior. Astrocytic modulation of spinal motor circuits involves release of ATP from astrocytes, hydrolysis of ATP to adenosine, and subsequent activation of neuronal A₁ adenosine receptors (A₁Rs). The net effect of this pathway is a reduction in the frequency of locomotor-related activity. Recently, it was proposed that A₁Rs modulate burst frequency by blocking the D₁-like dopamine receptor (D₁LR) signaling pathway; however, adenosine also modulates ventral horn circuits by dopamine-independent pathways. Here, we demonstrate that adenosine produced upon astrocytic stimulation modulates locomotor-related activity by counteracting the excitatory effects of D₁LR signaling and does not act by previously described dopamine-independent pathways. In spinal cord preparations from postnatal mice, a D₁LR agonist, SKF 38393, increased the frequency of locomotor-related bursting induced by 5-hydroxytryptamine and N-methyl-d-aspartate. Bath-applied adenosine reduced burst frequency only in the presence of SKF 38393, as did adenosine produced after activation of protease-activated receptor-1 to stimulate astrocytes. Furthermore, the A₁R antagonist 8-cyclopentyl-1,3-dipropylxanthine enhanced burst frequency only in the presence of SKF 38393, indicating that endogenous adenosine produced by astrocytes during network activity also acts by modulating D₁LR signaling. Finally, modulation of bursting by adenosine released upon stimulation of astrocytes was blocked by protein kinase inhibitor-(14-22) amide, a protein kinase A (PKA) inhibitor, consistent with A₁R-mediated antagonism of the D₁LR/adenylyl cyclase/PKA pathway. Together, these findings support a novel, astrocytic mechanism of metamodulation within the mammalian spinal cord, highlighting the complexity of the molecular interactions that specify motor output. NEW & NOTEWORTHY Astrocytes within the spinal cord produce adenosine during ongoing locomotor-related activity or when experimentally stimulated. Here, we show that adenosine derived from astrocytes acts at A₁ receptors to inhibit a pathway by which D₁-like receptors enhance the frequency of locomotor-related bursting. These data support a novel form of metamodulation within the mammalian spinal cord, enhancing our understanding of neuron-astrocyte interactions and their importance in shaping network activity.

Gliotransmission and adenosinergic modulation: insights from mammalian spinal motor networks

Astrocytes are proposed to converse with neurons at tripartite synapses, detecting neurotransmitter release and responding with release of gliotransmitters, which in turn modulate synaptic strength and neuronal excitability. However, a paucity of evidence from behavioral studies calls into question the importance of gliotransmission for the operation of the nervous system in healthy animals. Central pattern generator (CPG) networks in the spinal cord and brain stem coordinate the activation of muscles during stereotyped activities such as locomotion, inspiration, and mastication and may therefore provide tractable models in which to assess the contribution of gliotransmission to behaviorally relevant neural activity. We review evidence for gliotransmission within spinal locomotor networks, including studies indicating that adenosine derived from astrocytes regulates the speed of locomotor activity via metamodulation of dopamine signaling.

De novo transcriptome assembly for the lobster Homarus americanus and characterization of differential gene expression across nervous system tissues

BACKGROUND

The American lobster, Homarus americanus, is an important species as an economically valuable fishery, a key member in marine ecosystems, and a well-studied model for central pattern generation, the neural networks that control rhythmic motor patterns. Despite multi-faceted scientific interest in this species, currently our genetic resources for the lobster are limited. In this study, we de novo assemble a transcriptome for Homarus americanus using central nervous system (CNS), muscle, and hybrid neurosecretory tissues and compare gene expression across these tissue types. In particular, we focus our analysis on genes relevant to central pattern generation and the identity of the neurons in a neural network, which is defined by combinations of genes distinguishing the neuronal behavior and phenotype, including ion channels, neurotransmitters, neuromodulators, receptors, transcription factors, and other gene products.

RESULTS

Using samples from the central nervous system (brain, abdominal ganglia), abdominal muscle, and heart (cardiac ganglia, pericardial organs, muscle), we used RNA-Seq to characterize gene expression patterns across tissues types. We also compared control tissues with those challenged with the neuropeptide proctolin in vivo. Our transcriptome generated 34,813 transcripts with known protein annotations. Of these, 5,000-10,000 of annotated transcripts were significantly differentially expressed (DE) across tissue types. We found 421 transcripts for ion channels and identified receptors and/or proteins for over 20 different neurotransmitters and neuromodulators. Results indicated tissue-specific expression of select neuromodulator (allostatin, myomodulin, octopamine, nitric oxide) and neurotransmitter (glutamate, acetylcholine) pathways. We also identify differential expression of ion channel families, including kainite family glutamate receptors, inward-rectifying K(+) (IRK) channels, and transient receptor potential (TRP) A family channels, across central pattern generating tissues.

CONCLUSIONS

Our transcriptome-wide profiles of the rhythmic pattern generating abdominal and cardiac nervous systems in Homarus americanus reveal candidates for neuronal features that drive the production of motor output in these systems.

A behaviorally related developmental switch in nitrergic modulation of locomotor rhythmogenesis in larval Xenopus tadpoles

Locomotor control requires functional flexibility to support an animal's full behavioral repertoire. This flexibility is partly endowed by neuromodulators, allowing neural networks to generate a range of motor output configurations. In hatchling Xenopus tadpoles, before the onset of free-swimming behavior, the gaseous modulator nitric oxide (NO) inhibits locomotor output, shortening swim episodes and decreasing swim cycle frequency. While populations of nitrergic neurons are already present in the tadpole's brain stem at hatching, neurons positive for the NO-synthetic enzyme, NO synthase, subsequently appear in the spinal cord, suggesting additional as yet unidentified roles for NO during larval development. Here, we first describe the expression of locomotor behavior during the animal's change from an early sessile to a later free-swimming lifestyle and then compare the effects of NO throughout tadpole development. We identify a discrete switch in nitrergic modulation from net inhibition to overall excitation, coincident with the transition to free-swimming locomotion. Additionally, we show in isolated brain stem-spinal cord preparations of older larvae that NO's excitatory effects are manifested as an increase in the probability of spontaneous swim episode occurrence, as found previously for the neurotransmitter dopamine, but that these effects are mediated within the brain stem. Moreover, while the effects of NO and dopamine are similar, the two modulators act in parallel rather than NO operating serially by modulating dopaminergic signaling. Finally, NO's activation of neurons in the brain stem also leads to the release of NO in the spinal cord that subsequently contributes to NO's facilitation of swimming.

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.

Astrocytes in endocannabinoid signalling

Astrocytes are emerging as integral functional components of synapses, responding to synaptically released neurotransmitters and regulating synaptic transmission and plasticity. Thus, they functionally interact with neurons establishing tripartite synapses: a functional concept that refers to the existence of communication between astrocytes and neurons and its crucial role in synaptic function. Here, we discuss recent evidence showing that astrocytes are involved in the endocannabinoid (ECB) system, responding to exogenous cannabinoids as well as ECBs through activation of type 1 cannabinoid receptors, which increase intracellular calcium and stimulate the release of glutamate that modulates synaptic transmission and plasticity. We also discuss the consequences of ECB signalling in tripartite synapses on the astrocyte-mediated regulation of synaptic function, which reveal novel properties of synaptic regulation by ECBs, such as the spatially controlled dual effect on synaptic strength and the lateral potentiation of synaptic efficacy. Finally, we discuss the potential implications of ECB signalling for astrocytes in brain pathology and animal behaviour.

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.

Endocannabinoids Induce Lateral Long-Term Potentiation of Transmitter Release by Stimulation of Gliotransmission

Endocannabinoids (eCBs) play key roles in brain function, acting as modulatory signals in synaptic transmission and plasticity. They are recognized as retrograde messengers that mediate long-term synaptic depression (LTD), but their ability to induce long-term potentiation (LTP) is poorly known. We show that eCBs induce the long-term enhancement of transmitter release at single hippocampal synapses through stimulation of astrocytes when coincident with postsynaptic activity. This LTP requires the coordinated activity of the 3 elements of the tripartite synapse: 1) eCB-evoked astrocyte calcium signal that stimulates glutamate release; 2) postsynaptic nitric oxide production; and 3) activation of protein kinase C and presynaptic group I metabotropic glutamate receptors, whose location at presynaptic sites was confirmed by immunoelectron microscopy. Hence, while eCBs act as retrograde signals to depress homoneuronal synapses, they serve as lateral messengers to induce LTP in distant heteroneuronal synapses through stimulation of astrocytes. Therefore, eCBs can trigger LTP through stimulation of astrocyte-neuron signaling, revealing novel cellular mechanisms of eCB effects on synaptic plasticity.

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.

Physical activity and the endocannabinoid system: an overview

Recognized as a "disease modifier", physical activity (PA) is increasingly viewed as a more holistic, cost-saving method for prevention, treatment and management of human disease conditions. The traditional view that PA engages the monoaminergic and endorphinergic systems has been challenged by the discovery of the endocannabinoid system (ECS), composed of endogenous lipids, their target receptors, and metabolic enzymes. Indeed, direct and indirect evidence suggests that the ECS might mediate some of the PA-triggered effects throughout the body. Moreover, it is now emerging that PA itself is able to modulate ECS in different ways. Against this background, in the present review we shall discuss evidence of the cross-talk between PA and the ECS, ranging from brain to peripheral districts and highlighting how ECS must be tightly regulated during PA, in order to maintain its beneficial effects on cognition, mood, and nociception, while avoiding impaired energy metabolism, oxidative stress, and inflammatory processes.

Neuromodulation in developing motor microcircuits

Neuromodulation confers operational flexibility on motor network output and resulting behaviour. Furthermore, neuromodulators play crucial long-term roles in the assembly and maturational shaping of the same networks as they develop. Although previous studies have identified such modulator-dependent contributions to microcircuit ontogeny, some of the underlying mechanisms are only now being elucidated. Deciphering the role of neuromodulatory systems in motor network development has potentially important implications for post-lesional regenerative strategies in adults.

Neuromodulation as a mechanism for the induction of repetition priming

It is becoming apparent that the activity of many neural networks is shaped by effects of endogenous neuromodulators. Modulators exert second messenger-mediated actions that persist. We consider how this may impact network function and its potential role in the induction of repetition priming (increased performance when behavior is repeated). When effects of modulators persist and modulatory substances are repeatedly released, their effects will accumulate (summate) and become more pronounced. If this enhances the ability of a network to generate a particular output, performance will improve. We review data that support this model, and consider its implications for task switching. This model predicts that priming of one type of network activity will negatively impact the rapid transition to an incompatible type.

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.

The Nordic countries meeting on the zebrafish as a model for development and disease 2012

The first Nordic Countries Meeting on the Zebrafish as a Model for Development and Disease took place at Karolinska Institutet in Stockholm, November 21-23, 2012. The meeting gathered 130 scientists, students, and company representatives from Iceland, Finland, Norway, Denmark, and Sweden, as well as invited guests and keynote speakers from England, Scotland, Germany, Poland, The Netherlands, Singapore, Japan, and the United States. Presentations covered a wide range of topics, including developmental biology, genetics, evolutionary biology, toxicology, behavioral studies, and disease mechanisms. The need for formal guidance and training in zebrafish housing, husbandry, and health monitoring was recognized, and the meeting expressed its support for the joint working group of the FELASA/COST action BM0804 EuFishBioMed. The decision was made to turn the Nordic meeting into an annual event and create a Nordic network of zebrafish researchers.

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.