Neuromodulation in developing motor microcircuits

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ScienceDirect Neuromodulation in developing motor microcircuits Keith T Sillar1, Denis Combes2 and John Simmers2 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. Addresses 1 School of Psychology and Neuroscience, University of St Andrews, Westburn Lane, St Andrews, Fife KY16 9JP, Scotland, UK 2 Universite´ de Bordeaux, Institut de Neurosciences Cognitives et Inte´gratives d’Aquitaine, CNRS Unite´ Mixte de Recherche 5287, 146 rue Le´o Saignat, 33076 Bordeaux, France Corresponding author: Sillar, Keith T ([email protected])

Current Opinion in Neurobiology 2014, 29:73–81 This review comes from a themed issue on Neuromodulation 2014 Edited by David McCormick and Michael P Nusbaum

http://dx.doi.org/10.1016/j.conb.2014.05.009 0959-4388/# 2014 Published by Elsevier Ltd.

Introduction All neuronal networks underlying motor behaviour must adapt their output to prevailing organismal, developmental and environmental needs. Traditionally, however, such motor networks are portrayed as 2D electrochemical automata; anatomically hard-wired and physiologically stereotyped in their output. However, the dexterity of a dressage horse, the flexibility of an Olympic gymnast, and the ability to learn new motor skills, suggest this description is over simplistic. Which intrinsic neural processes impart such behavioural plasticity? Short-term adaptations of underlying network output can be conferred by differing discharge patterns of sensory or descending brain inputs. However, the longer-lasting, often 2nd messengermediated neuromodulation of neuronal electrical properties and synaptic strengths within motor networks is the essential source of adaptive motor circuit function [1,2]. In addition to the acute regulation of ongoing network operation, neuromodulation is intimately involved in the www.sciencedirect.com

actual assembly and fine-tuning of motor networks during ontogeny. Neuromodulators regulate the number, types and properties of neurons in developing motor networks, which in turn determines the detailed features of adult network output. Concomitantly, the neuromodulatory systems themselves may be changing as their own transmitters and actions evolve. Here we review how certain neuromodulators perform such instructive roles in the maturation of network architecture and function. Furthermore, recent evidence indicates that the same modulators, receptors and signalling pathways can be reactivated following spinal injury in adults suggesting that pathophysiology recapitulates ontogeny.

Neuromodulation of motor network assembly Many neuromodulators, but biogenic amines in particular, regulate the developmental configuration of the same networks they will modulate later in life. For example, during Xenopus tadpole development, serotonin deriving from raphespinal projections is causal to the maturation of a flexible larval locomotor pattern [3]. Similarly, descending serotonergic pathways contribute to the differentiation of motorneuron firing properties and the maturation of spinal motor networks in the perinatal rat [4]. Likewise, the respiratory network in the mammalian pre-Bo¨tzinger complex, which must function from birth, requires early acting noradrenergic influences for appropriate circuit assembly and maturation [5]. Mutant mice foetuses lacking the Phox2A gene (essential for the development of pre-Bo¨tzC noradrenergic inputs) die at birth and correspondingly, prenatal in vitro preparations express abnormal respiratory-related activity and hypoxic responsiveness. Similarly, alterations in the endogenous 5-HT environment lead to anomalous respiratory network maturation that can contribute to human neurodevelopmental disorders such as sudden infant death syndrome (SIDS) [6]. Dopamine has also been implicated in network maturation in zebrafish, with recent findings elegantly providing mechanistic details at the molecular and behavioural levels [7]. Experimentally interfering with dopamine signalling early in development has major consequences for motor behaviour later in the animal’s life. The dopamine effect is directly due to its release from midbrain neurons that constitute a highly conserved dopaminergic diencephalospinal tract (DDT) [9]. The DDT innervates the zebrafish spinal cord before free-swimming behaviour (Figure 1). The released amine acts upon spinal D4a receptors to control the number of motorneurons destined to innervate the axial swimming muscles. D4a receptors engage the hedgehog-signalling pathway to increase the generation of motorneurons from spinal Current Opinion in Neurobiology 2014, 29:73–81

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Figure 1

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Dopamine (DA) from diencephalic descending projections promotes motorneuron generation in the developing zebrafish spinal cord. Motorneurons (Mns; green circles) and V2 interneurons (yellow circles) are produced by specific progenitor cells along the ventral midline (green and yellow squares respectively) (a). The proportion of motorneurons to V2 interneurons is partly specified by DA’s action on the number of progenitor cells via activation of D4a receptors (see relative sizes of green and yellow squares in A; B and C). Addition of the DA agonist pergolide increases the number of motorneurons while preventing V2 interneuron formation (b). The opposite effects occur when embryos are treated with the D4 antagonist L-745870 (c). Figure adapted from [7,8] with permission.

progenitor cells and because the ventrally located progenitor pool homeostatically regulates the proportions of different cell types, there is a concomitant decrease in the number of V2a interneurons which results in an adverse effect on future swimming performance [7]. From day 3 to day 4 pf, D4a receptor activation triggers a switch from long swim episodes to the mature locomotor pattern of short, more frequent episodes [9]. Further evidence for dopamine’s role in spinal locomotor circuit formation derives from the abnormal development of dopaminergic pathways in transgenic zebrafish larvae lacking trpm7 [10], a protein involved in neuronal differentiation. The trpm7-depleted mutants fail to undergo a normal dopamine-dependent transition in locomotor output development, although the typical behavioural phenotype can be partially rescued by application of dopamine or its precursor levodopa. Dopamine-dependent signalling is also crucial for the correct development of the zebrafish GABAergic system [11]. By exerting a Current Opinion in Neurobiology 2014, 29:73–81

decreasing influence on the activity of protein kinase Akt, which is important for neurodevelopmental processes, early expressed dopamine acting via D2 receptors regulates the size of brain GABAergic neuronal populations and consequently larval motor behaviour.

Modulation and activity-dependence in network development Spontaneous activity is a ubiquitous feature that plays a fundamental role in network development and maturation [12,13]. Thus, by regulating network activity, precociously acting modulators can indirectly govern activity-dependent processes of motor network development ranging from neurotransmitter specification in embryonic spinal neurons [14], synaptic maturation [15], motorneuron neurite outgrowth [16] and axon pathfinding [17]. Interestingly, activity-dependent processes can also influence the developmental expression of the modulatory systems themselves, as for example in embryonic Xenopus, where hindbrain activity regulates www.sciencedirect.com

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the specification of serotonergic neurons and hence the latter’s modulatory control of swimming behaviour [18]. Recent studies have begun to establish a causal link between neuromodulatory and neurotransmitter signalling, electrical activity and motor circuit development. For example, the gaseous signalling molecule, nitric oxide (NO), is involved in the use-dependent tuning of synaptic efficacy and refinement of axonal projections in the developing CNS [19]. NO has recently been found to negatively regulate motor axon branching in the developing zebrafish spinal cord, with functional consequences for the ontogeny of locomotor behaviour [20] (also see below). Glutamatemediated electrical activity during a critical period shapes the dendritic topography of developing rat spinal motorneurons [16], while acetylcholine-driven spinal activity during mouse embryonic development ensures the correct configuration of locomotor pattern-generating circuitry [21]. Similarly, lipid-derived endocannabinoid modulators regulate glutamate release and thereby govern spontaneous activity of embryonic motor circuits in the chick spinal cord [22,23]. By tonically inhibiting AMPA miniature postsynaptic currents, endocannabinoid signalling reduces network discharge, with resultant implications for homeostatic processes involved in synaptic plasticity and consequently motor network maturation.

Repressive neuromodulatory control of circuit development Further to a permissive role in motor circuit development, neuromodulators can also restrain target network maturation. For example, serotonin from descending raphe´ projections to the embryonic mouse spinal cord slows the integration of inhibitory synaptic transmission [24] by delaying GABAergic interneuron maturation [25]. This repressive effect might ensure the coordinated maturation of spinal motor circuit and sensory input pathways, or the delay in GABAergic pathway function could relate to chloride homeostasis and the timing of GABA’s developmental switch from an excitatory to inhibitory action [26]. A modulation of chloride homeostasis by endogenous serotonin during maturation occurs in the spinal locomotor network of 3–4 day old zebrafish larvae [27,28]. Here, the amine acts to reduce inter-swim intervals to produce sustained, more adult-like motor activity. Interestingly, over the same developmental period, dopamine exerts suppressive effects on zebrafish locomotor circuitry [29]. Whereas endogenously released dopamine, acting via D2 receptors, prevents spontaneous swimming activity in 3day-old larvae, by day 5 the amine is no longer able to inhibit swim initiation, which now occurs more frequently. Such differential modulatory effects suggest multiple roles for dopamine, not only in locomotor circuit development [9], but also in inhibiting circuit output in early larval animals until an appropriate state of maturation [29]. An analogous repressive role for neuromodulation occurs in the crustacean stomatogastric ganglion where CPG www.sciencedirect.com

networks responsible for separate adult behaviours are already fully assembled and functional by mid-embryonic development [30], but their independent operation is withheld by modulatory influences until required in adulthood [31]. In the developing cricket, also, the CPG networks for stridulation are functional before hatching, but are held inactive by descending inhibition [32]. Likewise, in ranid frogs the respiratory circuitry for adult lung-breathing is already present in the gill-breathing pre-metamorphic tadpole, but is inhibited by a GABAB receptor-mediated mechanism until needed after metamorphosis, by which time the repressive signal has declined [33].

Changes in modulatory actions during network development In parallel with their target network development, neuromodulatory input systems themselves may change in terms of their transmitter substances, receptors and actions. Consequently, individual neuromodulators may exert very different or even opposing influences on the same motor circuit depending on the maturational stage of the animal. In lamprey spinal motor circuitry, for example, peptidergic modulation of synaptic transmission differs substantially in larvae and young adults [34]. Substance P initially potentiates larval inhibitory synapses, but as the circuits mature this switches to a peptidergic decrease in inhibition and an NMDA receptor-mediated enhancement of excitation in the adult. The serotonergic modulation of zebrafish spinal locomotor networks also changes during ontogeny, increasing spontaneous swim episode occurrences in larvae without affecting the temporal structure of the episodes themselves [27] (see above), but then decreasing the frequency of bursts within adult NMDA-elicited swim episodes [35]. The latter effect is mediated by a 5-HT-induced enhancement of mid-cycle inhibition that delays burst onset in the subsequent cycle. Similarly, the actions of neuromodulators on respiration change during amphibian development. Exogenous 5-HT application to the in vitro brainstem has little effect on burst activity related to lung ventilation in pre-metamorphic bullfrog tadpoles, but increases fictive lung ventilation in the post-metamorphic adult [36]. Noradrenaline, which is also without significant effects at pre-metamorphic stages, subsequently decreases fictive lung ventilation after metamorphosis via a1-adrenoceptor activation [37]. Conversely, NO is inhibitory to lung respiratory rhythm generation in pre-metamorphic tadpoles, but excitatory in the post-metamorphic young adult at the time of obligate aerial breathing [38], whereas substance P is excitatory throughout the entire transitional period [39]. The crustacean stomatogastric motor rhythms also display age-dependent differences in responsiveness to Current Opinion in Neurobiology 2014, 29:73–81

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modulators. Indeed, here the emergence of adult motor circuit operation appears to derive as much from developmentally regulated changes in the actions of modulatory inputs [31,40] that are acquired sequentially during embryonic and larval life [41–43], as from fundamental changes to the motor networks themselves.

Switching aminergic modulation in developing Xenopus spinal networks The monoaminergic modulation of spinal locomotor circuitry in hatchling Xenopus tadpoles during metamorphosis from larval tail-based to adult limb-based locomotion represents a striking example of switching modulatory actions during motor network development (Figure 2). Throughout the entire maturational process, 5-HT and NA exert overall opposing modulatory influences on the motor rhythms that drive undulatory and eventually appendicular swimming. In early postembryonic larvae, exogenous 5-HT increases the duration and intensity of axial motorneuron bursts with little effect on cycle periods [44], although at later pre-metamorphic stages, the amine increases both burst and cycle durations of spontaneous fictive swimming [45]. By contrast, NA slows down postembryonic swimming without altering motor burst durations [46], whereas at pre-metamorphosis, axial

burst durations are decreased but without significant changes in cycle periods [45]. During metamorphosis, both amines continue to exert opposing modulatory influences as limb CPG circuitry is being added, although intriguingly their roles now become reversed [45]. At metamorphic climax when the animal possesses coexisting tail-based and limbbased systems that can operate independently [47,48], 5-HT still slows fictive axial swimming in vitro, but accelerates the new appendicular rhythm to allow expression of a single coordinated rhythm by the two underlying spinal networks. Conversely, in preparations already displaying a conjoint rhythm, NA exerts the opposite effect of dissociating locomotor activity into separate faster axial and slower appendicular rhythms. Thus, through their opposing acute actions on internetwork connectivity of adjacent axial and limb spinal circuitry, 5-HT and NA provide a push-pull mechanism that presumably enables rapid adaptive alterations in the coupling of the two locomotor systems according to the animal’s immediate propulsive requirements. In postmetamorphic froglets where locomotion is now exclusively appendicular, the two amines continue to exert the same differential effects on the limb-kick rhythm as at hybrid mid-metamorphic stages.

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Modulation and metamodulation of Xenopus spinal locomotor networks during larval and metamorphic development. Nitric oxide (NO) and dopamine (DA) enhance (upward arrows) or decrease (downward arrows) fictive swim episode occurrences (upper panel). Serotonin (5-HT) and noradrenalin (NA) affect the temporal structure of ongoing episodes by modulating the duration (blue arrows) and/or cycle period (brown arrows) of spinal motor bursts (lower panel). The acute influences of each modulator also switch from increasing to decreasing or vice versa, and at different larval (NO, DA, NA) or metamorphic (5-HT, NA) stages of development. Opposing modulatory effects on swimming can be due to different modulators (e.g. 5-HT and NA) acting differentially on a locomotor network or a single modulator acting via the concentration-dependent recruitment of different receptor subtypes (e.g. DA). Moreover, at least some of these modulatory influences (e.g. NA, DA) are themselves subjected to metamodulation by brainstem NO (red arrows). Figure derived from [44–46,48,49,52,54]. Current Opinion in Neurobiology 2014, 29:73–81

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Dopamine is also a potent modulator of developing Xenopus spinal circuitry, but in contrast to 5-HT and NA, which regulate the temporal organization of burst activity within swim episodes, dopamine affects swim episode occurrence [49]. Dopamine also exerts opposing influences on network output, but accomplishes this by a concentration-dependent recruitment of D2-like and D1like receptors that increase and decrease swim episode occurrences, respectively (Figure 2). Also unlike 5-HT and NA, dopamine’s influences persist throughout metamorphosis, with D2 and D1 receptor activation exerting their respective inhibitory and excitatory actions on both the axial and appendicular networks. Interestingly, however, preliminary evidence suggests that dopamine’s influence on locomotor circuitry does indeed undergo a

developmental change, because at early larval stages the amine exerts uniquely an inhibitory effect on swimming regardless of concentration (L Picton and KT Sillar, unpublished observations).

Higher order metamodulation, NO and network development The ethereal nature of NO has hindered elucidation of its roles in neural circuit function such as in the hippocampus, cerebellum, brainstem and spinal cord. For the latter, NO exerts a net suppressive regulation of spinal locomotor CPG activity in Xenopus tadpoles via its facilitation of inhibitory synapses [50,51]. Subsequent experiments revealed NO’s role as a higher order ‘metamodulator’ (that is, a modulator of other modulatory

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Phylogenetic differences in metamodulation of locomotor networks by nitric oxide. In Xenopus tadpole and the lamprey, the balance between spinal synaptic excitation and inhibition determines the speed of swimming. In the tadpole (a), motorneurons (MN) are targeted by GABAergic and glycinergic synapses, with the latter themselves being modulated by noradrenergic inputs. NO governs swimming frequency by directly modulating GABA release and indirectly altering glycine release via NA input metamodulation. Thus, increasing NO from basal levels (a1 > a2) enhances inhibition and thereby slows swimming, while removing NO (a3 < a2) decreases inhibitory transmission and in turn promotes fast swimming. Figure adapted from [2]. In the lamprey (b), a metamodulatory interplay between endocannabinoids (EC) and NO regulates the balance between excitatory (glutamatergic) and inhibitory (glycinergic) drive to swim motorneurons (b1). EC alone selectively reduces glycine release to motorneurons without affecting excitatory transmission (b2). In the presence of NO, however, EC now evokes transmitter release from glutamatergic terminals (b3) and in synergy with the direct EC-mediated inhibition of glycine release, promote fast swimming. Figure adapted from [57]. www.sciencedirect.com

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systems) of spinal CPG function [52]. Thus, from discrete brainstem sources [53], NO directly facilitates GABA release from mid-hindbrain reticulospinal neurons to prematurely terminate and hence shorten locomotor episode duration, while simultaneously potentiating the effects of noradrenaline, which in turn facilitates reciprocal glycinergic inhibition to decrease swimming frequency [52] (Figure 3a). Like monoamines, the role of NO changes to an excitatory one during the course of Xenopus development [48,54] (see Figure 2), a switch coincident with both the transition to free-swimming behaviour and the appearance of NOS-expressing neurons in both spinal cord and brainstem [53]. Thus, in Xenopus tadpoles, NO is likely to contribute to the developmental emergence of motor behaviour, inhibiting swimming at stages when a predominantly sessile lifestyle prevails, but subsequently switching to an excitatory one when the now filter-feeding animal effectively swims continuously. A similar facilitatory role for NO occurs in the adult lamprey [55], but in this case complex interactions between NO and endocannabinoid signalling are involved [56]; NO enables the latter to simultaneously enhance excitatory neurotransmission and depress inhibitory glycinergic transmission. It therefore gates the polarity of another modulator’s effects on synaptic transmission [57] (Figure 3b), a further form of metamodulation in which NO plays a permissive role. A role for NO in controlling mammalian locomotor networks has also recently been reported [58]. From early neonatal stages of development, well before mouse pups can actually walk, a spinal locomotor CPG exists that can generate alternating flexor and extensor motor bursts. The CPG output is subject to modulation by numerous endogenous modulatory systems [2], including extrinsic aminergic pathways and intrinsic spinal modulators like adenosine [59] and acetylcholine [60,61], to which NO can now be added. NOS-positive propriospinal neurons are present from postnatal day 1 and NO donors slow the frequency and modulate the amplitude of locomotorrelated rhythmicity. NO scavengers or NOS inhibitors exert the opposite effect, indicating an endogenous NO tone that fine-tunes rhythmogenesis. Clearly, NO’s influence on motor control transcends phylogenetic boundaries and extends beyond acute effects on CPG circuitry itself. For example, nitrergic signalling affects behavioural performance during zebrafish development via control of the outgrowth of somatic motorneuron axons, their innervation of myotomal swimming muscle fibres [20] and the synaptic properties of neuromuscular junctions by affecting the kinetic properties of spontaneous miniature end plate currents [62]. Much still needs to be deciphered concerning NO’s control of motor networks but crucial species-specific and developmentally-regulated Current Opinion in Neurobiology 2014, 29:73–81

functions seem to place this gaseous modulator at the top of a hierarchy of subordinate modulatory control pathways. Moreover, NO’s variable and switching roles are likely to depend upon the location and responsiveness of its cellular and synaptic targets within particular network configurations at different stages of development.

Conclusions The development of functional motor circuitry occurs through a combination of genetically-programmed changes in a network’s intrinsic properties and extrinsic short-term and long-term influences from its equally changing neuromodulatory environment. Intriguingly, recent evidence suggests that certain roles played by neuromodulatory signalling during network development may also extend into the mature nervous system and promote regenerative plasticity in adult motor circuits after lesion. In a fascinating twist [7], it has been shown that the role of dopamine and D4a receptors in development is reinstated in adult zebrafish that have been subjected to spinal cord injury. Thus during successful spinal cord regeneration, endogenous release of dopamine in the spinal cord re-engages the developmental mechanisms that control motorneuron number to allow the injured fish to make new motorneurons by directing the fate of progenitor cells. The clinical implications of these data are far reaching because applications of D4 agonists augment the endogenous dopamine effect, heralding exciting avenues for development of new drugs that might assist in a range of spinal cord injury and disease states. More generally, gaining a better mechanistic understanding of the instructive relationship between neuromodulation and motor microcircuit development should provide insights relevant to the assembly of more complex brain networks, including those in humans [63].

Conflict of interest statement Nothing declared.

Acknowledgements We are grateful to the ‘Projet International de Coope´ration Scientifique’ (PICS) of the CNRS and LabEx BRAIN for recent support of collaborative interactions between the authors’ laboratories.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1. Marder E: Neuromodulation of neuronal circuits: back to the  future. Neuron 2012, 76:1-11. This review traces the history of our understanding of neural circuit neuromodulation and by focusing on the crustacean stomatogastric model system, the author describes how the neuromodulation of cellular and synaptic properties is able to specify variable motor network output and the ways in which the functional integrity of a circuit is maintained despite a multitude of modulator-induced parameter changes. 2.

Miles GB, Sillar KT: Neuromodulation of vertebrate locomotor control networks. Physiology 2011, 26:393-411. www.sciencedirect.com

Neuromodulation of motor control Sillar, Combes and Simmers 79

3.

Sillar KT, Woolston AM, Wedderburn JF: Involvement of brainstem serotonergic interneurons in the development of a vertebrate spinal locomotor circuit. Proc Biol Sci 1995, 259: 65-70.

4.

Vinay L, Brocard F, Clarac F, Norreel JC, Pearlstein E, Pflieger JF: Development of posture and locomotion: an interplay of endogenously generated activities and neurotrophic actions by descending pathways. Brain Res Brain Res Rev 2002, 40: 118-129.

5.

6.

Viemari JC, Be´vengut M, Burnet H, Coulon P, Pequignot JM, Tiveron MC, Hilaire G: Phox2a gene, A6 neurons, and noradrenaline are essential for development of normal respiratory rhythm in mice. J Neurosci 2004, 24:928-937. Hilaire G, Voituron N, Menuet C, Ichiyama RM, Subramanian HH, Dutschmann M: The role of serotonin in respiratory function and dysfunction. Respir Physiol Neurobiol 2010, 174:76-88.

7. 

Reimer MM, Norris A, Ohnmacht J, Patani R, Zhong Z, Dias TB, Kuscha V, Scott AL, Chen YC, Rozov S et al.: Dopamine from the brain promotes spinal motor neuron generation during development and adult regeneration. Dev Cell 2013, 25: 478-491. This multifaceted paper provides elegant and convincing evidence that dopamine released into the spinal cord of embryonic zebrafish from descending diencephalic projection neurons regulates axial motorneuron numbers via a D4a receptor-mediated suppression of PKA and resulting attenuation of the responsiveness of progenitor cells to sonic hedgehog signalling. Remarkably, the authors also show the same mechanism is reengaged in adults following spinal cord injury. 8.

Kong JH, Butler SJ, Novitch BG: My brain told me to do it. Dev Cell 2013, 25:437-448.

9. 

Lambert AM, Bonkowsky JL, Masino MA: The conserved dopaminergic diencephalospinal tract mediates vertebrate locomotor development in zebrafish. J Neurosci 2012, 32:13488-13500. The dopaminergic diencephalic tract neurons that influence zebrafish motoneuron numbers from day 1 post-fertilisation (see [7]) also play a role in the subsequent emergence of larval swimming behaviour, mediating the switch from immature to mature locomotor patterns between 3 and 4 days post-fertilisation. This influence also involves activation of D4a receptors.

10. Decker AR, McNeill MS, Lambert AM, Overton JD, Chen YC,  Lorca RA, Johnson NA, Brockerhoff SE, Mohapatra DP, MacArthur H et al.: Abnormal differentiation of dopaminergic neurons in zebrafish trpm7 mutant larvae impairs development of the motor pattern. Dev Biol 2014, 386: 428-439. Transient receptor potential melastatin-like 7 (Trpm7) is a cation channelkinase coupled protein involved in magnesium homeostasis and widely implicated in cell differentiation. In zebrafish, dopaminergic neurons in trpm-depleted mutants are deficient in tyrosine hydroxlase, the ratelimiting enzyme for dopamine synthesis. The mutant fish are hypomotile and do not undergo the normal developmental changes in swimming behaviour. However, this phenotype can be rescued by exposure to exogenous dopamine or its precursor levodopa.

15. Gonzalez-Islas C, Wenner P: Spontaneous network activity in the embryonic spinal cord regulates AMPAergic and GABAergic synaptic strength. Neuron 2006, 49:563-575. 16. Inglis FM1, Crockett R, Korada S, Abraham WC, Hollmann M, Kalb RGJ: The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons. J Neurosci 2002, 22:8042-8051. 17. Hanson MG, Landmesser LT: Normal patterns of spontaneous activity are required for correct motor axon guidance and the expression of specific guidance molecules. Neuron 2004, 43:687-701. 18. Demarque M, Spitzer NC: Activity-dependent expression of Lmx1b regulates specification of serotonergic neurons modulating swimming behavior. Neuron 2010, 67:321-334. 19. Gally JA, Montague R, Reeke GN, Edelmamn GM: The NO hypothesis: possible effects of a short-lived, rapidly diffusible signal in the development and function of the nervous system. Proc Natl Acad Sci USA 1990, 87:3547-3551. 20. Bradley S, Tossell K, Lockley R, McDearmid JR: Nitric oxide synthase regulates morphogenesis of zebrafish spinal cord motorneurons. J Neurosci 2010, 30:16818-16831. 21. Myers CP, Lewcock JW, Hanson MG, Gosgnach S, Aimone JB, Gage FH, Lee KF, Landmesser LT, Pfaff SL: Cholinergic input is required during embryonic development to mediate proper assembly of spinal locomotor circuits. Neuron 2005, 46: 37-49. 22. Gonzalez-Islas C, Garcia-Bereguiain MA, Wenner P: Tonic and  transient endocannabinoid regulation of AMPAergic miniature postsynaptic currents and homeostatic plasticity in embryonic motor networks. J Neurosci 2012, 32:13597-13607. In studying the developing chick spinal cord the authors reveal a fascinating interplay between spontaneous network activity driven by glutamate release and a resulting negative feedback role of tonic endocannabinoid signalling. The latter acts presynaptically to reduce the frequency of AMPA receptor-mediated miniature postsynaptic currents. The authors conclude that this endocannabinoid-regulated homeostatic synaptic plasticity plays an important role in spinal motor network maturation. 23. Wenner P: The effects of endocannabinoid signaling on network activity in developing and motor circuits. Ann N Y  Acad Sci 2013, 1279:135-142. The author reviews and compares the influence of endocannabinoids on motor network function in the embryonic and adult spinal cord and on developing hippocampal circuitry. Endocannabinoid signalling governs spontaneous activity of developing motor circuitry (with important implications for network maturation; see [21]) via the presynaptic regulation of glutamatergic and/or GABAergic neurotransmitter release. 24. Branchereau P, Chapron J, Meyrand P: Descending 5hydroxytryptamine raphe inputs repress the expression of serotonergic neurons and slow the maturation of inhibitory systems in mouse embryonic spinal cord. J Neurosci 2002, 22:2598-2606.

11. Souza BR, Romano-Silva MA, Tropepe V: Dopamine D2 receptor activity modulates Akt signaling and alters GABAergic neuron development and motor behavior in zebrafish larvae. J Neurosci 2011, 31:5512-5525.

25. Allain AE, Meyrand P, Branchereau P: Ontogenic changes of the spinal GABAergic cell population are controlled by the serotonin (5-HT) system: implication of 5-HT1 receptor family. J Neurosci 2005, 25:8714-8724.

12. Blankenship AG, Feller MB: Mechanisms underlying spontaneous patterned activity in developing neural circuits. Nat Rev Neurosci 2010, 11:18-29.

26. Allain AE, Se´gu L, Meyrand P, Branchereau P: Serotonin controls the maturation of the GABA phenotype in the ventral spinal cord via 5-HT1b receptors. Ann N Y Acad Sci 2010, 1198: 208-219.

13. Borodinsky LN, Belgacem YH, Swapna I: Electrical activity as a  developmental regulator in the formation of spinal cord circuits. Curr Opin Neurobiol 2012, 22:624-630. This short review underlines the crucial importance of bioelectrical activity for spinal network development. By drawing on studies on a variety of model organisms including Xenopus, zebrafish, chicken and rodents, the authors examine the role of activity-dependent regulation in the different steps of early spinal cord development, from cell generation and differentiation through to axon guidance and synapse/circuit formation. 14. Borodinsky LN, Root CM, Cronin JA, Sann SB, Gu X, Spitzer NC: Activity-dependent homeostatic specification of transmitter expression in embryonic neurons. Nature 2004, 429:523-530. www.sciencedirect.com

27. Brustein E, Chong M, Holmqvist B, Drapeau P: Serotonin patterns locomotor network activity in the developing zebrafish by modulating quiescent periods. J Neurobiol 2003, 57:303-322. 28. Brustein E, Drapeau PJ: Serotoninergic modulation of chloride homeostasis during maturation of the locomotor network in zebrafish. J Neurosci 2005, 25:10607-10616. 29. Thirumalai V, Cline HT: Endogenous dopamine suppresses initiation of swimming in prefeeding zebrafish larvae. J Neurophysiol 2008, 100:1635-1648. Current Opinion in Neurobiology 2014, 29:73–81

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30. Casasnovas B, Meyrand P: Functional differentiation of adult neural circuits from a single embryonic network. J Neurosci 1995, 15:5703-5718. 31. Le Feuvre Y, Fe´nelon VS, Meyrand P: Central inputs mask multiple adult neural networks within a single embryonic network. Nature 1999, 402:660-664. 32. Bentley DR, Hoy RR: Postembryonic development of adult motor patterns in crickets: a neural analysis. Science 1970, 170:1409-1411. 33. Straus C, Wilson RJ, Remmers JE: Developmental disinhibition: turning off inhibition turns on breathing in vertebrates. J Neurobiol 2000, 45:75-83. 34. Parker D, Gilbey T: Developmental differences in neuromodulation and synaptic properties in the lamprey spinal cord. Neuroscience 2007, 145:142-152. 35. Gabriel JP, Mahmood R, Kyriakatos A, So¨ll I, Hauptmann G, Calabrese RL, El Manira A: Serotonergic modulation of locomotion in zebrafish: endogenous release and synaptic mechanisms. J Neurosci 2009, 29:10387-10395. 36. Kinkead R, Belzile O, Gulemetova R: Serotonergic modulation of respiratory motor output during tadpole development. J Appl Physiol 2002, 93:936-946. 37. Fournier S, Kinkead R: Noradrenergic modulation of respiratory motor output during tadpole development: role of alphaadrenoceptors. J Exp Biol 2006, 209:3685-3694. 38. Hedrick MS, Chen AK, Jessop KL: Nitric oxide changes its role as a modulator of respiratory motor activity during development in the bullfrog (Rana catesbeiana). Comp Biochem Physiol 2005, 142:231-240. 39. Chen AK, Hedrick MS: Role of glutamate and substance P in the amphibian respiratory network during development. Respir Physiol Neurobiol 2008, 162:24-31. 40. Rehm KJ, Deeg KE, Marder E: Developmental regulation of neuromodulator function in the stomatogastric ganglion of the lobster, Homarus americanus. J Neurosci 2008, 28:9828-9839. 41. Fe´nelon VS, Kilman V, Meyrand P, Marder E: Sequential developmental acquisition of neuromodulatory inputs to a central pattern-generating network. J Comp Neurol 1999, 408:335-351. 42. Le Feuvre Y, Fenelon VS, Meyrand P: Ontogeny of modulatory inputs to motor networks: early established projection and progressive neurotransmitter acquisition. J Neurosci 2001, 21:1313-1326. 43. Richards KS, Simon DJ, Pulver SR, Beltz BS, Marder E: Serotonin in the developing stomatogastric system of the lobster, Homarus americanus. J Neurobiol 2003, 4:380-392. 44. Sillar KT, Wedderburn JF, Simmers AJ: Modulation of swimming rhythmicity by 5-hydroxytryptamine during post-embryonic development in Xenopus laevis. Proc Biol Sci 1992, 250: 107-114. 45. Rauscent A, Einum J, Le Ray D, Simmers J, Combes D: Opposing aminergic modulation of distinct spinal locomotor circuits and their functional coupling during amphibian metamorphosis. J Neurosci 2009, 29:1163-1174. 46. McDearmid JR, Scrymgeour-Wedderburn JF, Sillar KT: Aminergic modulation of glycine release in a spinal network controlling swimming in Xenopus laevis. J Physiol 1997, 503:111-117. 47. Combes D, Merrywest SD, Simmers J, Sillar KT: Developmental segregation of spinal networks driving axial and hindlimb based locomotion in metamorphosing Xenopus laevis. J Physiol 2004, 559:17-24. 48. Sillar KT, Combes D, Ramanathan S, Molinari M, Simmers J: Neuromodulation and developmental plasticity in the locomotor system of anuran amphibians during metamorphosis. Brain Res Rev 2008, 57:94-102. 49. Clemens S, Rauscent A, Simmers J, Combes D: Opposing  modulatory effects of D1- and D2-like receptor activation on a Current Opinion in Neurobiology 2014, 29:73–81

spinal central pattern generator. J Neurophysiol 2012, 107:2250-2259. This study provides evidence that a single neuromodulator (dopamine), depending on its concentration within in the spinal cord and therefore the differential activation of receptor targets, can exert opposite suppressive (via D2-like receptors) and permissive (via D1-like receptors) effects on locomotor episode occurrences in the pre-metamorphic Xenopus tadpole. 50. McLean DL, Sillar KT: The distribution of NADPH-diaphoraselabelled interneurons and the role of nitric oxide in the swimming system of Xenopus laevis larvae. J Exp Biol 2000, 203:705-713. 51. McLean DL, Sillar KT: Nitric oxide selectively tunes inhibitory synapses to modulate vertebrate locomotion. J Neurosci 2002, 22:4175-4184. 52. McLean DL, Sillar KT: Metamodulation of a spinal locomotor network by nitric oxide. J Neurosci 2004, 24:9561-9571. 53. Ramanathan S, Combes D, Molinari M, Simmers J, Sillar KT: Developmental and regional expression of NADPH-diaphorase/ nitric oxide synthase in spinal cord neurons correlates with the emergence of limb motor networks in metamorphosing Xenopus laevis. Eur J Neurosci 2006, 24:1907-1922. 54. Currie S: The development and neuromodulation of motor control systems in pro-metamorphic Xenopus laevis frog tadpoles. Scotland, UK: University of St Andrews; 2014, PhD thesis. 55. Kyriakatos A, Molinari M, Riyadh M, Grillner S, Sillar KT, El Manira A: Nitric oxide potentiation of locomotor activity in the spinal cord of the lamprey. J Neurosci 2009, 29:13283-13291. 56. El Manira A, Kyriakatos A: The role of endocannabinoid signaling in motor control. Physiology 2010, 25:230-238. 57. Song J, Kyriakatos A, El Manira A: Gating the polarity of  endocannabinoid-mediated synaptic plasticity by nitric oxide in the spinal locomotor network. J Neurosci 2012, 32:5097-5105. The authors demonstrate interactions between two intrinsic modulators in the spinal locomotor network of lamprey, the endocannabinoid 2-AG and NO. 2-AG is produced ‘on-demand’ by the network to speed up swimming by simultaneously reducing inhibition and increasing excitation. Effects on inhibition are mediated presynaptically via EC1 receptor activation on glycinergic interneurons while facilitation of excitatory transmission is achieved by NO metamodulation. The two mechanisms combine at the network level to increase locomotor frequency. 58. Foster JD, Dunford C, Sillar KT, Miles GB: Nitric oxide-mediated  modulation of the murine locomotor network. J Neurophysiol 2014, 111:659-674. This paper provides evidence that NO released from sources within the neonatal mice spinal cord modulates locomotor CPG output and thus may function as an intrinsic regulator that fine-tunes mammalian locomotor activity. 59. Witts EC, Panetta KM, Miles GB: Glial-derived adenosine  modulates spinal motor networks in mice. J Neurophysiol 2012, 107:1925-1934. This study explores the modulatory role of purinergic signalling in the mammalian spinal locomotor CPG and provides evidence for an intrinsic role of glial-derived adenosine acting via A1 receptors. Both ATP and its breakdown product adenosine reduce the frequency of the fictive locomotor rhythm in the isolated neonatal mouse spinal cord. 60. Zagoraiou L, Akay T, Martin JF, Brownstone RM, Jessell TM, Miles GB: A cluster of cholinergic interneurons modulates mouse locomotor activity. Neuron 2009, 64:645-662. 61. Witts EC, Zagoraiou L, Miles GB: Anatomy and function of  cholinergic C bouton inputs to motor neurons. J Anat 2014, 224:52-60. This timely review draws together recent anatomical, molecular genetic and physiological evidence on the powerful modulatory role of cholinergic C-bouton synapses onto spinal motorneurons. The source of C-boutons are intraspinal cholinergic V0c interneurons that are uniquely defined by their expression of the transcription factor pitx2. Evidence for a possible role of these interneurons in injury and disease states such as ALS are also reviewed. 62. Jay M, Bradley S, McDearmid JR: Effects of nitric oxide on  neuromuscular properties of developing zebrafish embryos. PLoS One 2014, 9:e86930doi: 10.1371/journal.pone.0086930. www.sciencedirect.com

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This study addresses the roles of nitric oxide during the formation and maturation of neuromuscular junctions in developing zebrafish. The authors show that exposure to NO during early development affects the maturation of the locomotor behaviour by changing the kinetics of spontaneous miniature end plate currents. 63. Posner MI, Rothbart MK, Sheese BE, Voelker P: Control  networks and neuromodulators of early development. Dev Psychol 2012, 48:827-835.

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This paper explores the neural processes involved in the specification of brain networks for attention control during human development. Two distinct regulative circuits, the so-called ‘orienting’ and ‘executive networks’, are modulated separately by acetylcholine and dopamine, respectively. The authors propose that during infant development until late childhood, the dominant attentional control system progressively switches from the orienting to the executive network, thus resulting in a change in the principle attentional regulator from cholinergic to dopaminergic modulation.

Current Opinion in Neurobiology 2014, 29:73–81