Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles.

Nonvisual photoreceptors are widely distributed in the retina and brain, but their roles in animal behavior remain poorly understood. Here we document a previously unidentified form of deep-brain photoreception in Xenopus laevis frog tadpoles. The isolated nervous system retains sensitivity to light even when devoid of input from classical eye and pineal photoreceptors. These preparations produce regular bouts of rhythmic swimming activity in ambient light but fall silent in the dark. This sensitivity is tuned to short-wavelength UV light; illumination at 400 nm initiates motor activity over a broad range of intensities, whereas longer wavelengths do not cause a response. The photosensitive tissue is located in a small region of caudal diencephalon-this region is necessary to retain responses to illumination, whereas its focal illumination is sufficient to drive them. We present evidence for photoreception via the light-sensitive proteins opsin (OPN)5 and/or cryptochrome 1, because populations of OPN5-positive and cryptochrome-positive cells reside within the caudal diencephalon. This discovery represents a hitherto undescribed vertebrate pathway that links luminance detection to motor output. The pathway provides a simple mechanism for light avoidance and/or may reinforce classical circadian systems.

Sodium Pumps Mediate Activity-Dependent Changes in Mammalian Motor Networks.

Ubiquitously expressed sodium pumps are best known for maintaining the ionic gradients and resting membrane potential required for generating action potentials. However, activity- and state-dependent changes in pump activity can also influence neuronal firing and regulate rhythmic network output. Here we demonstrate that changes in sodium pump activity regulate locomotor networks in the spinal cord of neonatal mice. The sodium pump inhibitor, ouabain, increased the frequency and decreased the amplitude of drug-induced locomotor bursting, effects that were dependent on the presence of the neuromodulator dopamine. Conversely, activating the pump with the sodium ionophore monensin decreased burst frequency. When more "natural" locomotor output was evoked using dorsal-root stimulation, ouabain increased burst frequency and extended locomotor episode duration, whereas monensin slowed and shortened episodes. Decreasing the time between dorsal-root stimulation, and therefore interepisode interval, also shortened and slowed activity, suggesting that pump activity encodes information about past network output and contributes to feedforward control of subsequent locomotor bouts. Using whole-cell patch-clamp recordings from spinal motoneurons and interneurons, we describe a long-duration (∼60 s), activity-dependent, TTX- and ouabain-sensitive, hyperpolarization (∼5 mV), which is mediated by spike-dependent increases in pump activity. The duration of this dynamic pump potential is enhanced by dopamine. Our results therefore reveal sodium pumps as dynamic regulators of mammalian spinal motor networks that can also be affected by neuromodulatory systems. Given the involvement of sodium pumps in movement disorders, such as amyotrophic lateral sclerosis and rapid-onset dystonia parkinsonism, knowledge of their contribution to motor network regulation also has considerable clinical importance. SIGNIFICANCE STATEMENT: The sodium pump is ubiquitously expressed and responsible for at least half of total brain energy consumption. The pumps maintain ionic gradients and the resting membrane potential of neurons, but increasing evidence suggests that activity- and state-dependent changes in pump activity also influence neuronal firing. Here we demonstrate that changes in sodium pump activity regulate locomotor output in the spinal cord of neonatal mice. We describe a sodium pump-mediated afterhyperpolarization in spinal neurons, mediated by spike-dependent increases in pump activity, which is affected by dopamine. Understanding how sodium pumps contribute to network regulation and are targeted by neuromodulators, including dopamine, has clinical relevance due to the role of the sodium pump in diseases, including amyotrophic lateral sclerosis, parkinsonism, epilepsy, and hemiplegic migraine.

Developmental changes in spinal neuronal properties, motor network configuration, and neuromodulation at free-swimming stages of Xenopus tadpoles.

We describe a novel preparation of the isolated brain stem and spinal cord from prometamorphic tadpole stages of the South African clawed frog ( Xenopus laevis) that permits whole cell patch-clamp recordings from neurons in the ventral spinal cord. Previous research on earlier stages of the same species has provided one of the most detailed understandings of the design and operation of a central pattern generator circuit. Here we have addressed how development sculpts complexity from this more basic circuit. The preparation generates bouts of fictive swimming activity either spontaneously or in response to electrical stimulation of the optic tectum, allowing an investigation into how the neuronal properties, activity patterns, and neuromodulation of locomotor rhythm generation change during development. We describe an increased repertoire of cellular responses compared with younger larval stages and investigate the cellular-level effects of nitrergic neuromodulation as well as the development of a sodium pump-mediated ultraslow afterhyperpolarization (usAHP) in these free-swimming larval animals. NEW & NOTEWORTHY A novel in vitro brain stem-spinal cord preparation is described that enables whole cell patch-clamp recordings from spinal neurons in prometamorphic Xenopus tadpoles. Compared with the well-characterized earlier stages of development, spinal neurons display a wider range of firing properties during swimming and have developed novel cellular properties. This preparation now makes it feasible to investigate in detail spinal central pattern generator maturation during the dramatic switch between undulatory and limb-based locomotion strategies during amphibian metamorphosis.

Sodium pump regulation of locomotor control circuits.

Sodium pumps are ubiquitously expressed membrane proteins that extrude three Na+ ions in exchange for two K+ ions, using ATP as an energy source. Recent studies have illuminated additional, dynamic roles for sodium pumps in regulating the excitability of neuronal networks in an activity-dependent fashion. We review their role in a novel form of short-term memory within rhythmic locomotor networks. The data we review derives mainly from recent studies on Xenopus tadpoles and neonatal mice. The role and underlying mechanisms of pump action broadly match previously published data from an invertebrate, the Drosophila larva. We therefore propose a highly conserved mechanism by which sodium pump activity increases following a bout of locomotion. This results in an ultraslow afterhyperpolarization (usAHP) of the membrane potential that lasts around 1 min, but which only occurs in around half the network neurons. This usAHP in turn alters network excitability so that network output is reduced in a locomotor interval-dependent manner. The pumps therefore confer on spinal locomotor networks a temporary memory trace of recent network performance.

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.

Nitric oxide-mediated modulation of the murine locomotor network.

Spinal motor control networks are regulated by neuromodulatory systems to allow adaptability of movements. The present study aimed to elucidate the role of nitric oxide (NO) in the modulation of mammalian spinal locomotor networks. This was investigated with isolated spinal cord preparations from neonatal mice in which rhythmic locomotor-related activity was induced pharmacologically. Bath application of the NO donor diethylamine NONOate (DEA/NO) decreased the frequency and modulated the amplitude of locomotor-related activity recorded from ventral roots. Removal of endogenous NO with coapplication of a NO scavenger (PTIO) and a nitric oxide synthase (NOS) blocker [nitro-l-arginine methyl ester (l-NAME)] increased the frequency and decreased the amplitude of locomotor-related activity. This demonstrates that endogenously derived NO can modulate both the timing and intensity of locomotor-related activity. The effects of DEA/NO were mimicked by the cGMP analog 8-bromo-cGMP. In addition, the soluble guanylyl cyclase (sGC) inhibitor ODQ blocked the effects of DEA/NO on burst amplitude and frequency, although the frequency effect was only blocked at low concentrations of DEA/NO. This suggests that NO-mediated modulation involves cGMP-dependent pathways. Sources of NO were studied within the lumbar spinal cord during postnatal development (postnatal days 1-12) with NADPH-diaphorase staining. NOS-positive cells in the ventral horn exhibited a rostrocaudal gradient, with more cells in rostral segments. The number of NOS-positive cells was also found to increase during postnatal development. In summary, we have shown that NO, derived from sources within the mammalian spinal cord, modulates the output of spinal motor networks and is therefore likely to contribute to the fine-tuning of locomotor behavior.

Development of a spinal locomotor rheostat.

Locomotion in immature animals is often inflexible, but gradually acquires versatility to enable animals to maneuver efficiently through their environment. Locomotor activity in adults is produced by complex spinal cord networks that develop from simpler precursors. How does complexity and plasticity emerge during development to bestow flexibility upon motor behavior? And how does this complexity map onto the peripheral innervation fields of motorneurons during development? We show in postembryonic Xenopus laevis frog tadpoles that swim motorneurons initially form a homogenous pool discharging single action potential per swim cycle and innervating most of the dorsoventral extent of the swimming muscles. However, during early larval life, in the prelude to a free-swimming existence, the innervation fields of motorneurons become restricted to a more limited sector of each muscle block, with individual motorneurons reaching predominantly ventral, medial, or dorsal regions. Larval motorneurons then can also discharge multiple action potentials in each cycle of swimming and differentiate in terms of their firing reliability during swimming into relatively high-, medium-, or low-probability members. Many motorneurons fall silent during swimming but can be recruited with increasing locomotor frequency and intensity. Each region of the myotome is served by motorneurons spanning the full range of firing probabilities. This unfolding developmental plan, which occurs in the absence of movement, probably equips the organism with the neuronal substrate to bend, pitch, roll, and accelerate during swimming in ways that will be important for survival during the period of free-swimming larval life that ensues.

Archerfish foraging success varies with immediate competition level but not group size.

Group living can lead to kleptoparasitism, the theft of resources by competitors. Under such conditions, foragers may alter their behavior to minimize competition. However, it is unclear how such behavioral changes impact foraging performance. Archerfish (Toxotes spp.) are a good model for investigating the behavioral responses to kleptoparasitism, as their hunting method (shooting waterjets at insects perched above the water) leaves them vulnerable to theft. They must hit the target prey with sufficient force to dislodge it; thus, the prey may land some distance away from the shooter. Kleptoparasitism rates increase with group size in archerfish, and individuals alter their behavior around conspecifics. We investigated whether group size affected shooting success, using 7-spot archerfish T. chatareus. We considered a fish's shot to be successful if it knocked a fly, placed on a transparent platform above the tank, into the water. The probability of shooting success was modeled as a function of group size, aiming duration, nearest neighbor distance and position, and trial number. We found no effect of group size, aiming duration, or nearest neighbor distance or position on shooting success. Shooting success increased as trials progressed, likely due to the fish becoming more familiar with the task. We also found no change in the kleptoparasitism rate between group sizes. Instead, the likelihood of the shooter consuming the prey depended on the types of competition present at the time of shooting. We suggest that archerfish shooting behavior can be influenced by the presence of conspecifics in ways not previously considered.

From tadpole to adult frog locomotion.

The transition from larval to adult locomotion in the anuran, Xenopus laevis, involves a dramatic switch from axial to appendicular swimming including intermediate stages when the tail and hindlimbs co-exist and contribute to propulsion. Hatchling tadpole swimming is generated by an axial central pattern generator (CPG) which matures rapidly during early larval life. During metamorphosis, the developing limbs are controlled by a de novo appendicular CPG driven initially by the axial system before segregating to allow both systems to operate together or independently. Neuromodulation plays important roles throughout, but key modulators switch their effects from early inhibitory influences to facilitating locomotion. Temperature affects the construction and operation of locomotor networks and global changes in environmental temperature place aquatic poikilotherms, like amphibians, at risk. The locomotor control strategy of anurans differs from other amphibian groups such as salamanders, where evolution has acted upon the thyroid hormone pathway to sculpt different developmental outcomes.

Neuromodulation of vertebrate locomotor control networks.

Vertebrate locomotion must be adaptable in light of changing environmental, organismal, and developmental demands. Much of the underlying flexibility in the output of central pattern generating (CPG) networks of the spinal cord and brain stem is endowed by neuromodulation. This review provides a synthesis of current knowledge on the way that various neuromodulators modify the properties of and connections between CPG neurons to sculpt CPG network output during locomotion.

Bimodal modulation of short-term motor memory via dynamic sodium pumps in a vertebrate spinal cord.

Dynamic neuronal Na+/K+ pumps normally only respond to intense action potential firing owing to their low affinity for intracellular Na+. Recruitment of these Na+ pumps produces a post-activity ultraslow afterhyperpolarization (usAHP) up to ∼10 mV in amplitude and ∼60 s in duration, which influences neuronal properties and future network output. In spinal motor networks, the usAHP underlies short-term motor memory (STMM), reducing the intensity and duration of locomotor network output in a manner dependent on the interval between locomotor bouts. In contrast to tonically active Na+ pumps that help set and maintain the resting membrane potential, dynamic Na+ pumps are selectively antagonized by low concentrations of ouabain, which, we show, blocks both the usAHP and STMM. We examined whether dynamic Na+ pumps and STMM can be influenced by neuromodulators, focusing on 5-HT and nitric oxide. Bath-applied 5-HT alone had no significant effect on the usAHP or STMM. However, this is due to the simultaneous activation of two distinct 5-HT receptor subtypes (5-HT7 and 5-HT2a) that have opposing facilitatory and suppressive influences, respectively, on these two features of the locomotor system. Nitric oxide modulation exerts a potent inhibitory effect that can completely block the usAHP and erase STMM. Using selective blockers of 5-HT7 and 5-HT2a receptors and a nitric oxide scavenger, PTIO, we further provide evidence that the two modulators constitute an endogenous control system that determines how the spinal network self-regulates the intensity of locomotor output in light of recent past experience.

The nitric oxide/cGMP pathway tunes the thermosensitivity of swimming motor patterns in Xenopus laevis tadpoles.

We investigated the role of the nitric oxide (NO)/cGMP pathway in setting thresholds for failure and recovery during hyperthermic stress of the swimming central pattern generator of immobilized Xenopus tadpoles (stage 42). We recorded swimming motor patterns induced by tail skin stimulation (TS) (1 ms current pulse) or by bath application of 50 microm NMDA. Swimming rhythm frequency increased in a linear manner with increasing temperature. In the presence of the NO donor S-nitroso-N-acetylpenicillamine (SNAP), recovery from hyperthermic failure was greatly slowed, often taking longer than the duration of the experiment. Pharmacological activation of the NO/cGMP pathway using SNAP or 8-bromo-cGMP (1) decreased the duration of TS-evoked swim episodes; (2) decreased the temperature threshold for hyperthermic circuit failure; (3) decreased the temperature at which the circuit recovered; and (4) increased the time taken to recover. Pharmacological inhibition of the NO/cGMP pathway using the NO scavenger CPTIO, the nitric oxide synthase (NOS) inhibitor L-NAME or the guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) had the opposite effects. NMDA rhythms were more resistant to hyperthermic failure than TS-evoked swim episodes, but the effects of SNAP on the temperature sensitivity of swimming evoked by NMDA were similar to those on TS-evoked swimming, suggesting that drug effects occur on central pattern-generating networks rather than sensory pathways. We conclude that the NO/cGMP pathway is involved in setting the threshold temperatures for hyperthermic failure and subsequent recovery of fictive swimming in tadpoles, and we suggest that this is part of a variable response to prevent overexcitation during abiotic stress under different environmental conditions.

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.

Developmental stage-dependent switching in the neuromodulation of vertebrate locomotor central pattern generator networks.

Neuromodulation plays important and stage-dependent roles in regulating locomotor central pattern (CPG) outputs during vertebrate motor system development. Dopamine, serotonin and nitric oxide are three neuromodulators that potently influence CPG outputs in the development of Xenopus frog tadpole locomotion. However, their roles switch from predominantly inhibitory early in development to mainly excitatory at later stages. In this review, we compare the stage-dependent switching in neuromodulation in Xenopus with other vertebrate systems, notably the mouse and the zebrafish, and highlight features that appear to be phylogenetically conserved.

Electrical coupling synchronises spinal motoneuron activity during swimming in hatchling Xenopus tadpoles.

The role of electrical coupling between neurons in the swimming rhythm generator of Xenopus embryos has been studied using pharmacological blockade of gap junctions. A conspicuous effect of 18beta-glycyrrhetinic acid (18beta-GA) and carbenoxolone, which have been shown to block electrical coupling in this preparation, was to increase the duration of ventral root bursts throughout the spinal cord during swimming. The left-right coordination, the swimming frequency and the duration of swimming episodes were not affected by concentrations of 18beta-GA which significantly increased burst durations. However, the longitudinal coupling was affected such that 18beta-GA led to a significant correlation between rostrocaudal delays and cycle periods, which is usually only present in older larval animals. Patch clamp recordings from spinal motoneurons tested whether gap junction blockers affect the spike timing and/or firing pattern of motoneurons during fictive swimming. In the presence of 18beta-GA motoneurons continued to fire a single, but broader action potential in each cycle of swimming, and the timing of their spikes relative to the ventral root burst became more variable. 18beta-GA had no detectable effect on the resting membrane potential of motoneurons, but led to a significant increase in input resistance, consistent with the block of gap junctions. This effect did not result in increased firing during swimming, despite the fact that multiple spikes can occur in response to current injection. Applications of 18beta-GA at larval stage 42 had no discernible effect on locomotion. The results, which suggest that electrical coupling primarily functions to synchronize activity in synergistic motoneurons during embryo swimming, are discussed in the context of motor system development.

Neuromodulation and developmental plasticity in the locomotor system of anuran amphibians during metamorphosis.

Metamorphosis in frogs has long fascinated laymen and scientists alike. This remarkable developmental transformation involves the simultaneous remodelling of almost every organ in the body, including the gut, associated with a switch in diet from filter feeder to predator, and the visual system, from laterally-directed monocular to forward-directed binocular vision. In the context of locomotion there is the complete loss of the tail, the main structure involved in generating thrust during swimming in larvae, and the gain of the limbs which produce rhythmic extension-flexion kicks during swimming and jumping. Here we review recent evidence from experiments utilizing novel in vitro isolated preparations of the Xenopus laevis spinal cord and brainstem which remain viable for several days and can generate motor rhythms similar to those that would normally drive locomotion in vivo. The results indicate that the developing limb circuitry is born from within the existing axial-based network, which acts like a functional scaffold. Initially the limb activity shares the same left-right alternation coordination and relatively high frequency as the tail swimming network. Only later, once the limbs are fully functional, does the limb network break free to produce left-right synchrony of limb motoneuron bursting and with a different, slower cadence than the tail-based system. During the initial formation of the limb networks nitric oxide-producing neurons appear in the spinal cord, but occupy regions other than those in which the new limb circuitry is developing. Now exogenous nitric oxide facilitates locomotor activity, in contrast to its inhibitory effects on swimming at earlier larval stages of development.

The dyslexia susceptibility KIAA0319 gene shows a specific expression pattern during zebrafish development supporting a role beyond neuronal migration.

Dyslexia is a common neurodevelopmental disorder caused by a significant genetic component. The KIAA0319 gene is one of the most robust dyslexia susceptibility factors but its function remains poorly understood. Initial RNA-interference studies in rats suggested a role in neuronal migration whereas subsequent work with double knock-out mouse models for both Kiaa0319 and its paralogue Kiaa0319-like reported effects in the auditory system but not in neuronal migration. To further understand the role of KIAA0319 during neurodevelopment, we carried out an expression study of its zebrafish orthologue at different embryonic stages. We used different approaches including RNAscope in situ hybridization combined with light-sheet microscopy. The results show particularly high expression during the first few hours of development. Later, expression becomes localized in well-defined structures. In addition to high expression in the brain, we report for the first time expression in the eyes and the notochord. Surprisingly, kiaa0319-like, which generally shows a similar expression pattern to kiaa0319, was not expressed in the notochord suggesting a distinct role for kiaa0319 in this structure. This observation was supported by the identification of notochord enhancers enriched upstream of the KIAA0319 transcription start site, in both zebrafish and humans. This study supports a developmental role for KIAA0319 in the brain as well as in other developing structures, particularly in the notochord which, is key for establishing body patterning in vertebrates.

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].

Control of Xenopus Tadpole Locomotion via Selective Expression of Ih in Excitatory Interneurons.

Locomotion relies on the coordinated activity of rhythmic neurons in the hindbrain and spinal cord and depends critically on the intrinsic properties of excitatory interneurons. Therefore, understanding how ion channels sculpt the properties of these interneurons, and the consequences for circuit function and behavior, is an important task. The hyperpolarization-activated cation current, Ih, is known to play important roles in shaping neuronal properties and for rhythm generation in many neuronal networks. We show in stage 42 Xenopus laevis frog tadpoles that Ih is strongly expressed only in excitatory descending interneurons (dINs), an important ipsilaterally projecting population that drives swimming activity. The voltage-dependent HCN channel blocker ZD7288 completely abolished a prominent depolarizing sag potential in response to hyperpolarization, the hallmark of Ih, and hyperpolarized dINs. ZD7288 also affected dIN post-inhibitory rebound firing, upon which locomotor rhythm generation relies, and disrupted locomotor output. Block of Ih also unmasked an activity-dependent ultraslow afterhyperpolarization (usAHP) in dINs following swimming, mediated by a dynamic Na/K pump current. This usAHP, unmasked in dINs by ZD7288, resulted in suprathreshold stimuli failing to evoke swimming at short inter-swim intervals, indicating an important role for Ih in maintaining swim generation capacity and in setting the post-swim refractory period of the network. Collectively, our data suggest that the selective expression of Ih in dINs determines specific dIN properties that are important for rhythm generation and counteracts an activity-dependent usAHP to ensure that dINs can maintain coordinated swimming over a wide range of inter-swim intervals.

Synthesis, conformation and biological evaluation of the enantiomers of 3-fluoro-gamma-aminobutyric acid ((R)- and (S)-3F-GABA): an analogue of the neurotransmitter GABA.

Gamma-aminobutyric acid or GABA (1) is one of the major inhibitory amino acid neurotransmitters of the central nervous system. This article describes the first synthesis of both the (R)- and (S)- enantiomers of 3-fluoro-GABA (2, 3F-GABA). DFT calculations were carried out in a continuum solvent model (PCM-B3LYP) to estimate the preferred conformations of 3F-GABA in aqueous solution. NMR coupling constants were calculated for each conformer and were then used to simulate the NMR spectra to evaluate the solution conformation of 3F-GABA. A preliminary evaluation of the 3F-GABA enantiomers shows that they act similarly as agonists of cloned GABA(A) receptors; however, they behave quite differently in a whole animal (Xenopus laevis tadpole model).