Martha W. Bagnall

Transgenic Mouse Lines Subdivide Medial Vestibular Nucleus Neurons into Discrete, Neurochemically Distinct Populations

The identification of neuron types within circuits is fundamental to understanding their relevance to behavior. In the vestibular nuclei, several classes of neurons have been defined in vivo on the basis of their activity during behavior, but it is unclear how those types correspond to neurons identified in slice preparations. By targeting recordings to neurons labeled in transgenic mouse lines, this study reveals that the continuous distribution of intrinsic parameters observed in medial vestibular nucleus (MVN) neurons can be neatly subdivided into two populations of neurons, one of which is GABAergic and the other of which is exclusively glycinergic or glutamatergic. In slice recordings, GABAergic neurons labeled in the EGFP (enhanced green fluorescent protein)-expressing inhibitory neuron (GIN) line displayed lower maximum firing rates (<250 Hz) than glycinergic and glutamatergic neurons labeled in the yellow fluorescent protein-16 (YFP-16) line (up to 500 Hz). In contrast to cortical and hippocampal interneurons, GABAergic MVN neurons exhibited wider action potentials than glutamatergic (and glycinergic) neurons. Responses to current injection differed between the neurons labeled in the two lines, with GIN neurons modulating their firing rates over a smaller input range, adapting less during steady depolarization, and exhibiting less rebound firing than YFP-16 neurons. These results provide a scheme for robust classification of unidentified MVN neurons by their physiological properties. Finally, dye labeling in slices shows that both GABAergic and glycinergic neurons project to the contralateral vestibular nuclei, indicating that commissural inhibition is accomplished through at least two processing streams with differential input and output properties.

Glycinergic Projection Neurons of the Cerebellum

The cerebellum funnels its entire output through a small number of presumed glutamatergic premotor projection neurons in the deep cerebellar nuclei and GABAergic neurons that feed back to the inferior olive. Here we use transgenic mice selectively expressing green fluorescent protein in glycinergic neurons to demonstrate that many premotor output neurons in the medial cerebellar (fastigial) nuclei are in fact glycinergic, not glutamatergic as previously thought. These neurons exhibit similar firing properties as neighboring glutamatergic neurons and receive direct input from both Purkinje cells and excitatory fibers. Glycinergic fastigial neurons make functional projections to vestibular and reticular neurons in the ipsilateral brainstem, whereas their glutamatergic counterparts project contralaterally. Together, these data suggest that the cerebellum can influence motor outputs via two distinct and complementary pathways.

Bidirectional plasticity gated by hyperpolarization controls the gain of postsynaptic firing responses at central vestibular nerve synapses

Linking synaptic plasticity with behavioral learning requires understanding how synaptic efficacy influences postsynaptic firing in neurons whose role in behavior is understood. Here, we examine plasticity at a candidate site of motor learning: vestibular nerve synapses onto neurons that mediate reflexive movements. Pairing nerve activity with changes in postsynaptic voltage induced bidirectional synaptic plasticity in vestibular nucleus projection neurons: long-term potentiation relied on calcium-permeable AMPA receptors and postsynaptic hyperpolarization, whereas long-term depression relied on NMDA receptors and postsynaptic depolarization. Remarkably, both forms of plasticity uniformly scaled synaptic currents evoked by pulse trains, and these changes in synaptic efficacy were translated into linear increases or decreases in postsynaptic firing responses. Synapses onto local inhibitory neurons were also plastic but expressed only long-term depression. Bidirectional, linear gain control of vestibular nerve synapses onto projection neurons provides a plausible mechanism for motor learning underlying adaptation of vestibular reflexes.

Frequency-Independent Synaptic Transmission Supports a Linear Vestibular Behavior

The vestibular system is responsible for transforming head motion into precise eye, head, and body movements that rapidly stabilize gaze and posture. How do central excitatory synapses mediate behavioral outputs accurately matched to sensory inputs over a wide dynamic range? Here we demonstrate that vestibular afferent synapses in vitro express frequency-independent transmission that spans their in vivo dynamic range (5-150 spikes/s). As a result, the synaptic charge transfer per unit time is linearly related to vestibular afferent activity in both projection and intrinsic neurons of the vestibular nuclei. Neither postsynaptic glutamate receptor desensitization nor saturation affect the relative amplitude or frequency-independence of steady-state transmission. Finally, we show that vestibular nucleus neurons can transduce synaptic inputs into linear changes in firing rate output without relying on one-to-one calyceal transmission. These data provide a physiological basis for the remarkable linearity of vestibular reflexes.

Multiple clusters of release sites formed by individual thalamic afferents onto cortical interneurons ensure reliable transmission

Thalamic afferents supply the cortex with sensory information by contacting both excitatory neurons and inhibitory interneurons. Interestingly, thalamic contacts with interneurons constitute such a powerful synapse that even one afferent can fire interneurons, thereby driving feedforward inhibition. However, the spatial representation of this potent synapse on interneuron dendrites is poorly understood. Using Ca imaging and electron microscopy we show that an individual thalamic afferent forms multiple contacts with the interneuronal proximal dendritic arbor, preferentially near branch points. More contacts are correlated with larger amplitude synaptic responses. Each contact, consisting of a single bouton, can release up to seven vesicles simultaneously, resulting in graded and reliable Ca transients. Computational modeling indicates that the release of multiple vesicles at each contact minimally reduces the efficiency of the thalamic afferent in exciting the interneuron. This strategy preserves the spatial representation of thalamocortical inputs across the dendritic arbor over a wide range of release conditions.

Modular Organization of Axial Microcircuits in Zebrafish

Spinal Circuit Complexity in Fish Rapid coordination of opposing muscle groups helps zebrafish zip through water. Bagnall and McLean (p. 197) now describe the neuronal circuits that stabilize swimming fish in their three-dimensional environment. By studying the self-righting behavior of larval zebrafish immobilized in agar, the authors identified parallel excitatory and inhibitory circuits driving dorsal and ventral hemisegments that could be activated independently. Larval zebrafish show more refined musculature control than expected. Locomotion requires precise control of spinal networks. In tetrapods and bipeds, dynamic regulation of locomotion is simplified by the modular organization of spinal limb circuits, but it is not known whether their predecessors, fish axial circuits, are similarly organized. Here, we demonstrate that the larval zebrafish spinal cord contains distinct, parallel microcircuits for independent control of dorsal and ventral musculature on each side of the body. During normal swimming, dorsal and ventral microcircuits are equally active, but, during postural correction, fish differentially engage these microcircuits to generate torque for self-righting. These findings reveal greater complexity in the axial spinal networks responsible for swimming than previously recognized and suggest an early template of modular organization for more-complex locomotor circuits in later vertebrates.

Multiple Types of Cerebellar Target Neurons and Their Circuitry in the Vestibulo-ocular Reflex

The cerebellum influences behavior and cognition exclusively via Purkinje cell synapses onto neurons in the deep cerebellar and vestibular nuclei. In contrast with the rich information available about the organization of the cerebellar cortex and its synaptic inputs, relatively little is known about microcircuitry postsynaptic to Purkinje cells. Here we examined the cell types and microcircuits through which Purkinje cells influence an oculomotor behavior controlled by the cerebellum, the horizontal vestibulo-ocular reflex, which involves only two eye muscles. Using a combination of anatomical tracing and electrophysiological recordings in transgenic mouse lines, we identified several classes of neurons in the medial vestibular nucleus that receive Purkinje cell synapses from the cerebellar flocculus. Glycinergic and glutamatergic flocculus target neurons (FTNs) with somata densely surrounded by Purkinje cell terminals projected axons to the ipsilateral abducens and oculomotor nuclei, respectively. Of three additional types of FTNs that were sparsely innervated by Purkinje cells, glutamatergic and glycinergic neurons projected to the contralateral and ipsilateral abducens, respectively, and GABAergic neurons projected to contralateral vestibular nuclei. Densely innervated FTNs had high spontaneous firing rates and pronounced postinhibitory rebound firing, and were physiologically homogeneous, whereas the intrinsic excitability of sparsely innervated FTNs varied widely. Heterogeneity in the molecular expression, physiological properties, and postsynaptic targets of FTNs implies that Purkinje cell activity influences the neural control of eye movements in several distinct ways. These results indicate that the cerebellum regulates a simple reflex behavior via at least five different cell types that are postsynaptic to Purkinje cells.

Systematic Shifts in the Balance of Excitation and Inhibition Coordinate the Activity of Axial Motor Pools at Different Speeds of Locomotion

An emerging consensus from studies of axial and limb networks is that different premotor populations are required for different speeds of locomotion. An important but unresolved issue is why this occurs. Here, we perform voltage-clamp recordings from axial motoneurons in larval zebrafish during “fictive” swimming to test the idea that systematic differences in the biophysical properties of axial motoneurons are associated with differential tuning in the weight and timing of synaptic drive, which would help explain premotor population shifts. We find that increases in swimming speed are accompanied by increases in excitation preferentially to lower input resistance (Rin) motoneurons, whereas inhibition uniformly increases with speed to all motoneurons regardless of Rin. Additionally, while the timing of rhythmic excitatory drive sharpens within the pool as speed increases, there are shifts in the dominant source of inhibition related to Rin. At slow speeds, anti-phase inhibition is larger throughout the pool. However, as swimming speeds up, inhibition arriving in-phase with local motor activity increases, particularly in higher Rin motoneurons. Thus, in addition to systematic differences in the weight and timing of excitation related to Rin and speed, there are also speed-dependent shifts in the balance of different sources of inhibition, which is most obvious in more excitable motor pools. We conclude that synaptic drive is differentially tuned to the biophysical properties of motoneurons and argue that differences in premotor circuits exist to simplify the coordination of activity within spinal motor pools during changes in locomotor speed.

A New Locus for Synaptic Plasticity in Cerebellar Circuits

Experimental and computational analyses of cerebellar function indicate that excitatory synapses onto deep nucleus neurons are likely to be a critical site of plasticity during motor learning. In this issue of Neuron, Pugh and Raman report that unconventional stimulus protocols can drive synaptic plasticity in the deep cerebellar nuclei.

Development of vestibular behaviors in zebrafish

Most animals orient their bodies with respect to gravity to facilitate locomotion and perception. The neural circuits responsible for these orienting movements have long served as a model to address fundamental questions in systems neuroscience. Though postural control is vital, we know little about development of either balance reflexes or the neural circuitry that produces them. Recent work in a genetically and optically accessible vertebrate, the larval zebrafish, has begun to reveal the mechanisms by which such vestibular behaviors and circuits come to function. Here we highlight recent work that leverages the particular advantages of the larval zebrafish to illuminate mechanisms of postural development, the role of sensation for balance circuit development, and the organization of developing vestibular circuits. Further, we frame open questions regarding the developmental mechanisms for functional circuit assembly and maturation where studying the zebrafish vestibular system is likely to open new frontiers.