Fictive Swimming Research Articles

Fictive Swimming Motor Patterns in Wild Type and Mutant Larval Zebrafish

Larval zebrafish provide a unique model for investigating the mechanisms involved in generating rhythmic patterns of behavior, such as swimming, due to the array of techniques available including genetics, optical imaging, and conventional electrophysiology. Because electrophysiological and imaging studies of rhythmic motor behaviors in paralyzed preparations depend on the ability to monitor the central motor pattern, we developed a fictive preparation in which the activity of axial motor neurons was monitored using extracellular recordings from peripheral nerves. We examined spontaneous and light induced fictive motor patterns in wild type and mutant larval zebrafish (4-6 days post-fertilization) paralyzed with curare. All spontaneous and light-induced preparations produced alternation of motor activity from side-to-side (mean contralateral phase = 50.7 +/- 7.0%; mean burst frequency = 35.6 +/- 4.7 Hz) and a progression of activity from head-to-tail (mean ipsilateral rostrocaudal delay = 0.8 +/- 0.5 ms per segment), consistent with lateral undulation and forward propulsion during swimming, respectively. The basic properties of the motor pattern were similar in spontaneous and light-induced swimming. This fictive preparation can be used in combination with conventional electrophysiological and imaging methods to investigate normal circuit function as well as to elucidate functional deficits in mutant lines. Toward this end, we show that two accordion class mutants, accordion and bandoneon, have alternating activity on opposite sides of the body, contradicting the hypothesis that their deficit results from the absence of the reciprocal glycinergic inhibition that is typically found in the spinal cord of swimming vertebrates.

A cephalic projection neuron involved in locomotion is dye coupled to the dopaminergic neural network in the medicinal leech

It is widely appreciated that the selection and modulation of locomotor circuits are dependent on the actions of higher-order projection neurons. In the leech, Hirudo medicinalis, locomotion is modulated by a number of cephalic projection neurons that descend from the subesophageal ganglion in the head. Specifically, descending brain interneuron Tr2 functions as a command-like neuron that can terminate or sometimes trigger fictive swimming. In this study, we demonstrate that Tr2 is dye coupled to the dopaminergic neural network distributed in the head brain. These findings represent the first anatomical evidence in support of dopamine (DA) playing a role in the modulation of locomotion in the leech. In addition, we have determined that bath application of DA to the brain and entire nerve cord reliably and rapidly terminates swimming in all preparations exhibiting fictive swimming. By contrast, DA application to nerve cords expressing ongoing fictive crawling does not inhibit this motor rhythm. Furthermore, we show that Tr2 receives rhythmic feedback from the crawl central pattern generator. For example, Tr2 receives inhibitory post-synaptic potentials during the elongation phase of each crawl cycle. When crawling is not expressed, spontaneous inhibitory post-synaptic potentials in Tr2 correlate in time with spontaneous excitatory post-synaptic potentials in the CV motor neuron, a circular muscle excitor that bursts during the elongation phase of crawling. Our data are consistent with the idea that DA biases the nervous system to produce locomotion in the form of crawling.

Metamodulation of a Spinal Locomotor Network by Nitric Oxide

Flexibility in the output of spinal networks can be accomplished by the actions of neuromodulators; however, little is known about how the process of neuromodulation itself may be modulated. Here we investigate the potential "meta"-modulatory hierarchy between nitric oxide (NO) and noradrenaline (NA) in Xenopus laevis tadpoles. NO and NA have similar effects on fictive swimming; both potentiate glycinergic inhibition to slow swimming frequency and GABAergic inhibition to reduce episode durations. In addition, both modulators have direct effects on the membrane properties of motor neurons. Here we report that antagonism of noradrenergic pathways with phentolamine dramatically influences the effect of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) on swimming frequency, but not its effect on episode durations. In contrast, scavenging extracellular NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) does not influence any of the effects of NA on fictive swimming. These data place NO above NA in the metamodulatory hierarchy, strongly suggesting that NO works via a noradrenergic pathway to control glycine release but directly promotes GABA release. We confirmed this possibility using intracellular recordings from motor neurons. In support of a natural role for NO in the Xenopus locomotor network, PTIO not only antagonized all of the effects of SNAP on swimming but also, when applied on its own, modulated both swimming frequency and episode durations in addition to the underlying glycinergic and GABAergic pathways. Collectively, our results illustrate that NO and NA have parallel effects on motor neuron membrane properties and GABAergic inhibition, but that NO serially metamodulates glycinergic inhibition via NA.

Divergent actions of serotonin receptor activation during fictive swimming in frog embryos.

We have investigated the pharmacology underlying locomotor system responses to serotonin (5-HT) in embryos of the frog, Rana temporaria, to provide a comparison to studies in embryos of its close relative, Xenopus laevis. Our findings suggest that two divergent mechanisms underlie the modulation of locomotion by 5-HT in Rana. Bath-applied 5-HT or 5-carboxamidotyptamine, a 5-HT(1,5A,7) receptor agonist, can modulate fictive swimming in a dose-dependent manner, increasing burst durations and cycle periods. However, activation of 5-HT(1,7) receptors with R8-OHDPAT or 8-OHDPAT fails to mimic 5-HT, and in some cases exerts exactly the opposite response; decreasing burst durations and cycle periods. Elevating endogenous 5-HT levels by blocking re-uptake with clomipramine transiently increases burst durations. The receptors involved in this endogenous response include 5-HT(1A) receptors, as in Xenopus, but also 5-HT(7) receptors. However, like the 8-OHDPAT enantiomers, prolonged re-uptake inhibition can result in a motor response in the opposite direction to exogenous 5-HT. This effect is not reversed by 5-HT(1A) and/or 5-HT(7) receptor antagonism, implicating 5-HT(1B/1D) receptors. Remarkably, antagonism of these receptors using methiothepin unmasks a dose-dependent response to clomipramine, reminiscent of exogenous 5-HT. Our data suggest that 5-HT(1A,7) and 5-HT(1B/1D) receptors act as gain-setters of burst durations, whilst 5-HT(5A) receptors are involved in the effects of bath-applied 5-HT on locomotion.

Serotonin patterns locomotor network activity in the developing zebrafish by modulating quiescent periods.

Developing neural networks follow common trends such as expression of spontaneous, recurring activity patterns, and appearance of neuromodulation. How these processes integrate to yield mature, behaviorally relevant activity patterns is largely unknown. We examined the integration of serotonergic neuromodulation and its role in the functional organization of the accessible locomotor network in developing zebrafish at behavioral and cellular levels. Locally restricted populations of serotonergic neurons and their projections appeared in the hindbrain and spinal cord of larvae after hatching (approximately day 2). However, 5-HT affected the swimming pattern only from day 4 on, when sustained spontaneous swimming appeared. 5-HT and its agonist quipazine increased motor output by reducing intervals of inactivity, observed behaviorally (by high-speed video) and in recordings from spinal neurons during fictive swimming (by whole-cell current clamp). 5-HT and quipazine had little effect on the properties of the activity periods, such as the duration of swim episodes and swim frequency. Further, neuronal input resistance, rheobasic current, and resting potential were not affected significantly. The 5-HT antagonists methysergide and ketanserin decreased motor output by prolonging the periods of inactivity with little effect on the active swim episode or neuronal properties. Our results suggest that 5-HT neuromodulation is integrated early in development of the locomotor network to increase its output by reducing periods of inactivity with little effect on the activity periods, which in contrast are the main targets of 5-HT neuromodulation in neonatal and adult preparations.

Spinal and supraspinal functions of noradrenaline in the frog embryo: consequences for motor behaviour.

The monoamine noradrenaline (NA) can initiate and/or modulate locomotion in a variety of vertebrates. Here we report that exogenous NA application can facilitate two completely different fictive behaviours in embryos of the common frog Rana temporaria, depending on whether spinal networks are connected to supraspinal centres. When the nervous system is intact, NA elicits a non-rhythmic coiling motor response, reminiscent of a spontaneous behaviour appropriate to drive hatching movements, but has only minor effects on evoked swimming activity. After the spinal cord has been severed from the brain, spontaneous coiling is no longer observed, nor can NA elicit it, but the amine can 'release' swimming rhythm generation in response to electrical skin stimulation. The rhythm is similar, but relatively inflexible when compared to fictive swimming recorded from intact animals. Our pharmacological tests indicate that alpha 1-adrenoreceptors are involved in the permissive role of NA during spinalised rhythmic swimming and that the fictive coiling response to NA in intact animals involves descending inputs and the activation of beta 1-adrenoreceptors. Furthermore, the subtle effects of NA on evoked swimming in intact animals were mimicked by either alpha 1- or alpha 2-adrenoreceptor activation, reversibly decreasing motor burst durations and increasing their frequency. We discuss our results with reference to the known synergistic actions of NA with another aminergic neuromodulator, serotonin, and raise the possibility that these amines may actively regulate the release of one another during locomotion, in addition to their respective post-synaptic targets in the spinal cord.

Activities of mesencephalic nucleus neurons during fictive swimming of the carp Cyprinus carpio

: Fish swim by undulatory contractions of the axial trunk musculature. It has been presumed that a descending signal from the brainstem activates central pattern generators in the spinal cord to make the swimming rhythm. In the carp the electrical or chemical stimulation of a mesencephalic nucleus, the nucleus fasciculi longitudinalis medialis (Nflm), could initiate swimming. However, it has not been established what signals originate from the Nflm in order to make the fish swim. The activity of the Nflm neurons was therefore examined electrophysiologically during fictive swimming in paralyzed carp. Three types of neuronal activities closely related to the swimming rhythms were obtained. The first type was a continuous tonic firing throughout an episode of fictive swimming. Neurons involved in this tonic activity may project to the spinal cord and contribute to the activation of spinal neurons to initiate fish swimming. The second type was accompanied by continual phasic firings occurring in rhythm with the activity of spinal motoneurons. Supposing that the swimming rhythm originates in the spinal cord, the synchronous activity in the brain neurons may suggest that in the nucleus there is a relay neuron conveying the rhythm information from the cord to other neurons there or in the brain. The third type exhibited reduced firing rates during fictive swimming. It is possible that the neurons engaged in this activity may be inhibitory and suppress the activities of other neurons in the nucleus and spinal cord during rest or during decelerating swimming.

The in vitro and in vivo enantioselectivity of etomidate implicates the GABA A receptor in general anaesthesia

General anaesthetics exhibiting enantioselectivity afford valuable tools to assess the fundamental mechanisms underlying anaesthesia. Here, we characterised the actions of the R-(+)- and S-(−)-enantiomers of etomidate. In mice and tadpoles, R-(+)-etomidate was more potent (~10-fold) than S-(−)-etomidate in producing loss of the righting reflex. In electrophysiological and radioligand binding assays, the enantiomers of etomidate positively regulated GABA A receptor function at anaesthetic concentrations and with an enantioselectivity paralleling their in vivo activity. GABA-evoked currents mediated by human recombinant GABA A receptors were potentiated by either R-(+)- or S-(−)-etomidate in a manner dependent upon receptor subunit composition. A direct, GABA-mimetic, effect was similarly subunit dependent. Modulation of GABA receptor activity was selective; R-(+)-etomidate inhibited nicotinic acetylcholine, or 5-hydroxytryptamine 3 receptor subtypes only at supra-clinical concentrations and ionotropic glutamate receptor isoforms were essentially unaffected. Acting upon reticulothalamic neurones in rat brain slices, R-(+)-etomidate prolonged the duration of miniature IPSCs and modestly enhanced their peak amplitude. S-(−)-etomidate exerted qualitatively similar, but weaker, actions. In a model of locomotor activity, fictive swimming in Xenopus laevis tadpoles, R-(+)- but not S-(−)-etomidate exerted a depressant influence via enhancement of GABAergic neurotransmission. Collectively, these observations strongly implicate the GABA A receptor as a molecular target relevant to the anaesthetic action of etomidate.

Fast and slow locomotor burst generation in the hemispinal cord of the lamprey.

A fundamental question in vertebrate locomotion is whether distinct spinal networks exist that are capable of generating rhythmic output for each group of muscle synergists. In many vertebrates including the lamprey, it has been claimed that burst activity depends on reciprocal inhibition between antagonists. This question was addressed in the isolated lamprey spinal cord in which the left and right sides of each myotome display rhythmic alternating activity. We sectioned the spinal cord along the midline and tested whether rhythmic motor activity could be induced in the hemicord with bath-applied D-glutamate or N-methyl-D-aspartate (NMDA) as in the intact spinal cord or by brief trains of electrical stimuli. Fast rhythmic bursting (2-12 Hz), coordinated across ventral roots, was observed with all three methods. Furthermore, to diminish gradually the crossed glycinergic inhibition, a progressive surgical lesioning of axons crossing the midline was implemented. This resulted in a gradual increase in burst frequency, linking firmly the fast hemicord rhythm [6.6 +/- 1.7 (SD) Hz] to fictive swimming in the intact cord (2.4 +/- 0.7 Hz). Ipsilateral glycinergic inhibition was not required for the hemicord burst pattern generation, suggesting that an interaction between excitatory glutamatergic neurons suffices to produce the unilateral burst pattern. In NMDA, burst activity at a much lower rate (0.1-0.4 Hz) was also encountered, which required the voltage-dependent properties of NMDA receptors in contrast to the fast rhythm. Swimming is thus produced by pairs of unilateral burst generating networks with reciprocal inhibitory connections that not only ensure left/right alternation but also downregulate frequency.

Changes in Electrophysiological Properties of Lamprey Spinal Motoneurons During Fictive Swimming

Electrophysiological properties of lamprey spinal motoneurons were measured to determine whether their cellular properties change as the spinal cord goes from a quiescent state to the active state of fictive swimming. Intracellular microelectrode recordings of membrane potential were made from motoneurons in the isolated spinal cord preparation. Electrophysiological properties were first characterized in the quiescent spinal cord, and then fictive swimming was induced by perfusion with D-glutamate and the measurements were repeated. During the depolarizing excitatory phase of fictive swimming, the motoneurons had significantly reduced rheobase and significantly increased input resistance compared with the quiescent state, with no significant changes in these parameters during the repolarizing inhibitory phase of swimming. Spike threshold did not change significantly during fictive swimming compared with the quiescent state. During fictive swimming, the slope of the spike frequency versus injected current (F-I) relationship decreased significantly as did spike-frequency adaptation and the amplitude of the slow after-spike hyperpolarization (sAHP). Serotonin is known to be released endogenously from the spinal cord during fictive swimming and is known to reduce the amplitude of the sAHP. Therefore the effects of serotonin on cellular properties were tested in the quiescent spinal cord. It was found that, in addition to reducing the sAHP amplitude, serotonin also reduced the slope of the F-I relationship and reduced spike-frequency adaptation, reproducing the changes observed in these parameters during fictive swimming. Application of spiperone, a serotonin antagonist, significantly increased the sAHP amplitude during fictive swimming but had no significant effect on F-I slope or adaptation. Because serotonin may act in part through reduction of calcium currents, the effect of calcium-free solution (cobalt substituted for calcium) was tested in the quiescent spinal cord. Similar to fictive swimming and serotonin application, the calcium-free solution significantly reduced the sAHP amplitude, the slope of the F-I relationship, and spike-frequency adaptation. These results suggest that there are significant changes in the firing properties of motoneurons during fictive swimming compared with the quiescent state, and it is possible that these changes may be attributed in part to the endogenous release of serotonin acting via reduction of calcium currents.

Neural correlates of swimming behavior in Melibe leonina.

The nudibranch Melibe leonina swims by rhythmically bending from side to side at a frequency of 1 cycle every 2-4 s. The objective of this study was to locate putative swim motoneurons (pSMNs) that drive these lateral flexions and determine if swimming in this species is produced by a swim central pattern generator (sCPG). In the first set of experiments, intracellular recordings were obtained from pSMNs in semi-intact, swimming animals. About 10-14 pSMNs were identified on the dorsal surface of each pedal ganglion and 4-7 on the ventral side. In general, the pSMNs in a given pedal ganglion fired synchronously and caused the animal to flex in that direction, whereas the pSMNs in the opposite pedal ganglion fired in anti-phase. When swimming stopped, so did rhythmic pSMN bursting; when swimming commenced, pSMNs resumed bursting. In the second series of experiments, intracellular recordings were obtained from pSMNs in isolated brains that spontaneously expressed the swim motor program. The pattern of activity recorded from pSMNs in isolated brains was very similar to the bursting pattern obtained from the same pSMNs in semi-intact animals, indicating that the sCPG can produce the swim rhythm in the absence of sensory feedback. Exposing the brain to light or cutting the pedal-pedal connectives inhibited fictive swimming in the isolated brain. The pSMNs do not appear to participate in the sCPG. Rather, they received rhythmic excitatory and inhibitory synaptic input from interneurons that probably comprise the sCPG circuit.

A three-enzyme microelectrode sensor for detecting purine release from central nervous system

As the purines, in particular adenosine, are important signaling agents in the nervous system we have devised a new biosensor for directly measuring their production in real time during physiological activity. Our amperometric adenosine biosensor is made by entrapping 3 enzymes (xanthine oxidase, purine nucleoside phosphorylase and adenosine deaminase) in a composite lactobionamide and amphiphillic polypyrrole matrix around a Pt microelectrode. The resulting sensors are small (25–100 μm diameter), fast responding (10–90% rise time, 2±0.23 s), sensitive (100–222 mA M −1 cm −2) and stable (100% activity after 5 days). The sensor was used in vivo to demonstrate the spatial localization of release of adenosine from Xenopus embryo spinal cord during fictive swimming.

Sensory Activation and Role of Inhibitory Reticulospinal Neurons that Stop Swimming in Hatchling Frog Tadpoles

Activity in neuronal networks underlying locomotion and other rhythmic actions can start and stop in response to specific sensory stimuli. In vertebrate locomotion, some reticulospinal neurons such as Mauthner neurons can initiate activity, but the neurons controlling stopping are not defined. We have studied swimming in tadpoles of the frog,Xenopus, which is started by touching the skin and stops when the head contacts a solid surface. Using an immobilized tadpole preparation, the same stimuli control fictive swimming. When head contact is imitated by pressure to the head skin sensory neurons in the trigeminal ganglion are active, spinal neurons receive GABAergic inhibition, and swimming stops. Here we record intracellularly from neurons in the hindbrain that are excited by pressure or electrical stimulation to the head skin. By intracellular filling with neurobiotin, we identify these anatomically as mid-hindbrain reticulospinal neurons (MHRs). These have contralateral descending projections to the spinal cord and GABA-like immunoreactivity. They are rhythmically inhibited during fictive swimming. Individual MHRs reliably stopped ongoing swimming when brief firing was induced by intracellular current injection. The ability of individual MHRs to stop swimming was blocked by the GABAAantagonist bicuculline. Our evidence indicates that MHRs receive direct excitation from trigeminal sensory neurons and in turn release GABA to directly inhibit spinal neurons and turn off the swimming central pattern generator.

Sensory activation and role of inhibitory reticulospinal neurons that stop swimming in hatchling frog tadpoles.

Activity in neuronal networks underlying locomotion and other rhythmic actions can start and stop in response to specific sensory stimuli. In vertebrate locomotion, some reticulospinal neurons such as Mauthner neurons can initiate activity, but the neurons controlling stopping are not defined. We have studied swimming in tadpoles of the frog, Xenopus, which is started by touching the skin and stops when the head contacts a solid surface. Using an immobilized tadpole preparation, the same stimuli control fictive swimming. When head contact is imitated by pressure to the head skin sensory neurons in the trigeminal ganglion are active, spinal neurons receive GABAergic inhibition, and swimming stops. Here we record intracellularly from neurons in the hindbrain that are excited by pressure or electrical stimulation to the head skin. By intracellular filling with neurobiotin, we identify these anatomically as mid-hindbrain reticulospinal neurons (MHRs). These have contralateral descending projections to the spinal cord and GABA-like immunoreactivity. They are rhythmically inhibited during fictive swimming. Individual MHRs reliably stopped ongoing swimming when brief firing was induced by intracellular current injection. The ability of individual MHRs to stop swimming was blocked by the GABA(A) antagonist bicuculline. Our evidence indicates that MHRs receive direct excitation from trigeminal sensory neurons and in turn release GABA to directly inhibit spinal neurons and turn off the swimming central pattern generator.

Nitric oxide selectively tunes inhibitory synapses to modulate vertebrate locomotion.

We have explored the possible modulation by nitric oxide (NO) of inhibitory synaptic transmission mediated by either glycine or GABA during episodes of rhythmic fictive swimming in postembryonic Xenopus laevis tadpoles. Extracellular ventral-root recordings suggest a stage-dependent increase in the reliability and extent of the NO donor S-nitroso-n-acetylpenicillamine (SNAP; 0.1-1 mm) to inhibit swimming by reducing the frequency and shortening the duration of swim episodes. These effects of SNAP on the swimming rhythm at both developmental stages are corroborated by intracellular recordings from presumed motor neurons with sharp microelectrodes, which also suggest that NO inhibits swimming by facilitating both glycinergic and GABAergic inhibition. However, we found no evidence for NO modulation of the excitatory drive for swimming. In addition to presynaptic effects on inhibitory transmitter release, a pronounced postsynaptic membrane depolarization ( approximately 5-10 mV) and conductance decrease ( approximately 10-20%) are associated with bath application of SNAP. Hence, NO exerts inhibitory effects on swimming through multiple but selective actions on both the electrical properties of spinal neurons and on particular synaptic interconnections. The presynaptic and postsynaptic effects of NO act in concert to tune inhibitory synapses.

Nitric Oxide Selectively Tunes Inhibitory Synapses to Modulate Vertebrate Locomotion

We have explored the possible modulation by nitric oxide (NO) of inhibitory synaptic transmission mediated by either glycine or GABA during episodes of rhythmic fictive swimming in postembryonicXenopus laevistadpoles. Extracellular ventral-root recordings suggest a stage-dependent increase in the reliability and extent of the NO donorS-nitroso-n-acetylpenicillamine (SNAP; 0.1–1 mm) to inhibit swimming by reducing the frequency and shortening the duration of swim episodes. These effects of SNAP on the swimming rhythm at both developmental stages are corroborated by intracellular recordings from presumed motor neurons with sharp microelectrodes, which also suggest that NO inhibits swimming by facilitating both glycinergic and GABAergic inhibition. However, we found no evidence for NO modulation of the excitatory drive for swimming. In addition to presynaptic effects on inhibitory transmitter release, a pronounced postsynaptic membrane depolarization (∼5–10 mV) and conductance decrease (∼10–20%) are associated with bath application of SNAP. Hence, NO exerts inhibitory effects on swimming through multiple but selective actions on both the electrical properties of spinal neurons and on particular synaptic interconnections. The presynaptic and postsynaptic effects of NO act in concert to tune inhibitory synapses.

A new method for recording activities of spinal motoneurons during fictive swimming in paralyzed teleost fish

: A new method for monitoring spinal motoneuron activities during fictive swimming in teleost fish was developed. Enamel-insulated copper wire electrodes were implanted in the trunk muscle of goldfish or carp. For each freely moving fish, an electromyogram (EMG) was recorded using the electrodes. In those fish paralyzed with curare, using the same electrode set, bursts of electrical activities consisted of spikes of smaller amplitude and of shorter duration compared with those recorded by EMG. Simultaneous recording of the extracellular activity and intracellular recording from the muscle revealed that the bursts of spikes recorded in the paralyzed fish were motor nerve impulses innervating the muscle and are considered to be fictive swimming activity. The method developed in the present study provides a useful tool with which to investigate neuronal mechanisms underlying swimming activity in teleost fish.

Activation of embryonic red and white muscle fibers during fictive swimming in the developing zebrafish.

Sub-threshold, motoneuron-evoked synaptic activity was observed in zebrafish embryonic red (ER) and white (EW) muscle fibers paralyzed with a dose of D-tubocurarine insufficient to abolish synaptic activity to determine whether muscle activation was coordinated to produce the undulating body movements required for locomotion. Paired whole-cell recordings revealed a synaptic drive that alternated between ipsilateral and contralateral myotomes and exhibited a rostral-caudal delay in timing appropriate for swimming. Both ER and EW muscle were activated during fictive swimming. However, at the fastest fictive swimming rates, ER fibers were de-recruited, whereas they could be active in isolation of EW fibers at the slowest fictive swimming rates. Prior to hatching, fictive swimming was preceded by a lower frequency, more robust and rhythmic synaptic drive resembling the "coiling" behavior of fish embryos. The motor activity observed in paralyzed zebrafish closely resembled the swimming and coiling behaviors observed in these developing fishes. At the early developmental stages examined in this study, myotomal muscle recruitment and coordination were similar to that observed in adult fishes during swimming. Our results indicate that the patterned activation of myotomal muscle is set from the onset of development.

Alpha-adrenoreceptor activation modulates swimming via glycinergic and GABAergic inhibitory pathways in Xenopus laevis tadpoles.

This study focuses upon the network pathways underlying the adrenoreceptor-mediated modulation of fictive swimming in the immobilized Xenopus laevis tadpole. As shown recently, noradrenaline (NA) increases cycle periods while simultaneously reducing the rostrocaudal delay in head-to-tail firing and the duration of swimming episodes. Furthermore, both swimming frequency and duration are reduced by selective pharmacological activation of alpha1- and/or alpha2-adrenoreceptors, while alpha1-receptor activation also reduces rostrocaudal delays. We show that NA could still modulate aspects of swimming after blocking either glycine or GABA(A) receptors with strychnine and bicuculline, respectively. Furthermore, after prior application of NA, strychnine could counteract noradrenergic effects on cycle periods and rostrocaudal delays, while bicuculline could counteract effects on cycle periods, suggesting that these two fast inhibitory pathways are both involved in the NA modulation of swimming. In addition, blocking glycine receptors reduced the effects of alpha1-receptors on cycle periods and delays, while blocking GABA(A) receptors had no effect. Blocking either glycine or GABA(A) receptors, however, lessened the reduction in swimming frequency by alpha2-receptors. In addition, pre-application of bicuculline prevented a reduction in episode durations by NA, alpha1- and alpha2-receptors. Our findings suggest that the noradrenergic modulation of Xenopus swimming is mediated via alpha-adrenoreceptors interacting with both glycinergic and GABAergic inhibitory pathways. Both alpha1- and alpha2-receptor activation influences the GABAergic pathway controlling the duration of swimming episodes and is involved in the glycinergic modulation of the swimming rhythm and its longitudinal co-ordination, with alpha2-receptors additionally affecting swimming frequency through GABAergic pathways.

Sensory modification of leech swimming: interactions between ventral stretch receptors and swim-related neurons.

The neuronal circuits that generate the leech swimming rhythm comprise oscillatory interneurons that provide appropriately phased output to drive swim-related motoneurons. Within ganglia, these interneurons express three phases; between ganglia there exists a phase delay between homologs. Our earlier experiments revealed that stretch receptors embedded in the body wall participate in intersegmental coordination and setting intersegmental phases. To identify the basis for these sensory effects, we mapped interactions between a ventral stretch receptor and swim-related neurons. Connections between this receptor and motoneurons are weak and variable in quiescent preparations, but during fictive swimming stretch receptor activation modulates motoneuron oscillations, hence, these effects are polysynaptic, mediated by interneurons. We identified a strong, nonrectifying, and apparently direct electrical connection between the stretch receptor and oscillator neuron 33. The ventral stretch receptor also interacts with most of the other oscillatory interneurons, including inhibitory inputs to cells 28 and 208, excitatory input to the contralateral cell 115, and mixed input to the ipsilateral cell 115. These direct and indirect interactions can account for previously described effects of body-wall stretch on motoneuron activity. They also could mediate the previously described modification of intersegmental phase relationships by appropriately phased stretch receptor activation.