Neural events underlying escape swimming behaviour in the squat lobster 'Galathea strigosa' (Crustacea, Anomura)

1. The anatomy and physiology of escape swimming behaviour in the squat lobster, Galathea strigosa, have been investigated and the results discussed in the context of comparative mechanisms of escape in rela~ed species. 2. In contrast to many other decapods, G. strigosa, does not possess a giant-fibre system which underlies escape. 3. In terms of the number, size and position of neuronal somata, the fast flexor motorneuron pools in Galathea and crayfish are homologous. 4. A neuron, homologous with the crayfish MeG, has been studied. Unlike the crayfish neuron, MoGH is a typical, unspecialized fast flexor motorneuron. 5. The anatomy of afferent and efferent neurons involved in abdominal extension has been investigated. The extensor motorneuron and accessory neuron pools in crayfish and Galathea are largely homologous. 6. A small degree of intersegmental and interspecific variation in abdominal flexor and extensor motorneuron pools is reported. 7. The anatomy and physiology of the abdominal MRO's has been examined. These are found to be homologous in structure and function with other decapod MRO's. 8. The considerable differences between the phasic and tonic MRO sensory dendrites may account for their different response characteristics. 9. The MRO's excite both extensor motorneurons and the flexor inhibitor motorneuron via an apparently monosynaptic pathway. Similar input properties have been described for the crayfish MHO's . 10. The MRO's, which are shown to fire in response to abdominal flexion, produce EPSP's in extensor motorneurons which both summate and facilitate. This feature has not been decribed previously and may be important in the reflex function of the MHO's during escape swimming behaviour. 11. The relative roles of proprioceptive and exteroceptive feedback on the generation of the swimming rhythm have been studied using a variety of preparations involving restraint and deafferenta- tion. 12. Sensory feedback both excites and inhibits swimming. It is deduced that proprioceptive feedback has excitatory effects and extero- ceptive feedback inhibits swimming behaviour. 13. It is suggested that the MHO's may playa role in exciting the neural circuits underlying swimming bewvbur via both direct connections with the thoracic nervous system and a restricted portion of the abdominal motorneuron pool. 14. A deafferented preparation has been used to analyse the motor programme underlying swimming behaviour. The ability to record swimming activity, identical with that recorded in the intact animal, in the absence of sensory feedback from the abdomen, suggests that swimming behaviour is controlled by a central pattern generator (CPG). 15. A method of inducing swimming activity by high frequency electrical- stimulation of abdominal sensori-motor roots is described. 16. The CPG for swimming behaviour is shown to be most likely to reside in the suboesophageal or thoracic ganglion. 17. The activity of flexor and extensor motorneurons in abdominal ganglia has been analysed at the cellular level using both extra- cellular and intracellular recording techniques. 18. Fast flexor motorneurons are driven by a combination of brief unitary synaptic potentials and a large underlying oscillatory slow wave depolarization. 19. Current injection into the somata of fast flexor motorneurons during swimming has dramatic effects on slow wave amplitude and suggests that motorneuron drive results from powerful periodic excitation via chemical synapses. 20. In contrast to the fast flexor motorneurons, fast extensor motorneurons are driven by only brief unitary synaptic potentials and not by an underlying slow wave depolarization. The contrasting mechanisms for excitation in antagonistic sets of motorneurons is documented and a possible explanation presented. 21. Among the fast extensor motorneurons there is an apparent gradation in spike thresholds which can be correlated with a gradation in soma diameter. The largest of the available pool of extensor motorneurons has the highest spike threshold. 22. The activity of the phasic inhibitors of the extensor and flexor muscles has been analysed. The extensor inhibitor, which fires in antiphase with other extensor motorneurons during the flexion phase of the swim cycle, appears to receive the same slow wave depolarization as fast flexor motorneurons. The extensor inibitor motorneuron burst is terminated by a high frequency barrage of IPSP's superimposed upon the membrane slow wave. The flexor inhibitor motorneuron receives complex excitation and inhibition during swimming, involving both unitary events and membrane waves. 23. The coordination of segmented limb structures during swimming has been investigated. The walking legs are physically protracted during flexion while the unmodified male swimmerets are flicked posteriorly. 24. Swimmeret retraction during swimming is controlled by the activity of a single swimmeret motorneuron which appears to be part of the swimming circuit and which may also be a primitive homologue of the Segmental Giant neuron in crayfish. 25. It is concluded that escape swimming behaviour is homologous with non-giant backwards swimming in crayfish and may also be homologous with swimming in certain sand crab species. The evolutionary relationships of a number of decapods is discussed on the basis of escape circuitry and it is suggested that Galathea may represent an ancestral type of swimming decapod

Evolutionary Divergence in Developmental Strategies and Neuromodulatory Control Systems of Two Amphibian Locomotor Networks

Attempts to understand the neural mechanisms which produce behaviour must consider both prevailing sensory cues and the central cellular and synaptic changes they direct. At each level, neuromodulation can additionally shape the final output. We have investigated neuromodulation in the developing spinal motor networks in hatchling tadpoles of two closely related amphibians, Xenopus laevis and Rana temporaria to examine the subtle differences in their behaviours that could be attributed to their evolutionary divergence. At the point of hatching, both species can swim in response to a mechanosensory stimulus, however Rana embryos often display a more forceful, non-locomotory coiling behaviour. Whilst the synaptic drive that underlies these behaviours appears similar, subtle inter-specific differences in neuronal properties shape motor outputs in different ways. For example, Rana neurons express N-methyl-D-aspartate (NMDA)/serotonin (5-HT)-dependent oscillations, not present in hatchling Xenopus and many also exhibit a prominent slow spike after-hyperpolarisation. Such properties may endow the spinal circuitry of Rana with the ability to produce a more flexible range of outputs. Finally, we compare the roles of the neuromodulators 5-HT, noradrenaline (NA) and nitric oxide (NO) in shaping motor outputs. 5-HT increases burst durations during swimming in both Xenopus and Rana, but 5-HT dramatically slows the cycle period in Rana with little effect in Xenopus. Three distinct, but presumably homologous NO-containing brainstem clusters of neurons have been described, yet the effects of NO differ between species. In Xenopus, NO slows and shortens swimming in a manner similar to NA, yet in Rana NO and NA elicit the non-rhythmic coiling pattern

From tadpole to adult frog locomotion

Funding: Work in the Sillar lab is currently funded by BBSRC grant ref: BB/T015705/ 1.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.Publisher PDFPeer reviewe

Control of <i>Xenopus</i> tadpole locomotion via selective expression of Ih in excitatory interneurons

The authors are grateful for the support of the Biotechnology and Biological Science Research Council (BBSRC) [grant number BB/J01446X/1 and BB/M024946/1] and the Wellcome Trust-University of Edinburgh Institutional Strategic Support Fund (ISSF).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 depolarising 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.Publisher PDFPeer reviewe

Sodium pump regulation of locomotor control circuits

The authors are grateful for the financial support of the BBSRC (grant numbers: BB/M024946/1 and BB/JO1446X/1), the Carnegie Trust and the University of St Andrews.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. Here 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 afterhyperpolarisation (usAHP) of the membrane potential that lasts around 1 minute, 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.PostprintPeer reviewe

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

Authors are grateful for the financial support of this research by the University of St Andrews, the E&RS Neuroscience research fund and BBSRC grant BB/T015705/1.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.Publisher PDFPeer reviewe

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 brainstem and spinal cord from pro-metamorphic 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 CPG circuit. Here we have addressed how development sculpts complexity from this more basic circuit. The preparation generates bouts of fictive31 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 to younger larval stages and investigate the cellular level effects of nitrergic neuromodulation as well as the development of a sodium pump-mediated ultra-slow afterhyperpolarisation (usAHP) in these free-swimming larval animals.PostprintPeer reviewe

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

SPC was supported by a BBSRC studentship.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.PostprintPeer reviewe

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

Supported by PICS (Projet International de Coopération Scientifique) of the French CNRS and a LabEx BRAIN Visiting Professorship to KTS. SPC was a BBSRC research student. NWS was an MPhil student supported in part by the E & RS Research Fund of the University of St Andrews.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.Publisher PDFPeer reviewe

Short-Term Memory of Motor Network Performance via Activity-Dependent Potentiation of Na<sup style="border: 0px; font-size: 0.75em; font-weight: 100; margin: 0px; padding: 0px; vertical-align: super; line-height: 0;">+/K<sup style="border: 0px; font-size: 0.75em; font-weight: 100; margin: 0px; padding: 0px; vertical-align: super; line-height: 0;">+</sup> Pump Function</sup>

SummaryBrain networks memorize previous performance to adjust their output in light of past experience. These activity-dependent modifications generally result from changes in synaptic strengths or ionic conductances, and ion pumps have only rarely been demonstrated to play a dynamic role [1–4]. Locomotor behavior is produced by central pattern generator (CPG) networks and modified by sensory and descending signals to allow for changes in movement frequency, intensity, and duration [5–7], but whether or how the CPG networks recall recent activity is largely unknown. In Xenopus frog tadpoles, swim bout duration correlates linearly with interswim interval, suggesting that the locomotor network retains a short-term memory of previous output. We discovered an ultraslow, minute-long afterhyperpolarization (usAHP) in network neurons following locomotor episodes. The usAHP is mediated by an activity- and sodium spike-dependent enhancement of electrogenic Na+/K+ pump function. By integrating spike frequency over time and linking the membrane potential of spinal neurons to network performance, the usAHP plays a dynamic role in short-term motor memory. Because Na+/K+ pumps are ubiquitously expressed in neurons of all animals and because sodium spikes inevitably accompany network activity, the usAHP may represent a phylogenetically conserved but largely overlooked mechanism for short-term memory of neural network function