Martha W. Bagnall - US grants

DESCRIPTION (provided by applicant): Good control of posture and orientation is vital for animals as they make movements or navigate the environment. Vertebrates rely on the vestibulospinal system to translate gravity sensations from the inner ear into appropriate compensatory trunk (axial) and limb movements to stabilize and orient themselves. Although this system exists in all vertebrates and is crucial for survival, research on it has languished du to the technical difficulties in recording from vestibular and spinal neurons, especially during animal motion. My long-term goal is to define the means by which vestibular and cerebellar pathways influence spinal circuit activity patterns to fine-tune behavioral outputs. The objective of this proposal is to determine how vestibular signals are translated into appropriate compensatory postural adjustments by defining the synaptic circuit by which vestibular neurons govern the activity of spinal motor neurons and interneurons. To surmount the technical difficulties that have limited prior efforts, I propose to use the larval zebrafish. Zebrafish are n excellent system for this line of research because of the accessibility of their brainstem and spinal column, and the strong homologies between zebrafish and mammalian spinal circuits. Thus, circuit mapping between the brainstem and spinal cord can be performed with much greater ease than in mammalian systems, and the results are likely to be applicable across vertebrates. Microcircuit activity can then be translated into behavioral output due to the relativ simplicity of the zebrafish body plan, yielding a complete picture of this vital sensorimotor transformation. In Aim 1, a combination of calcium signaling and electrophysiology in vivo will be used to examine differential recruitment of dorsal and ventral musculature while the animal attempts to right itself from side-lying to upright. The requirement for vestibular signals will be tested in mutant animals missing their otoliths (gravity sensors). These experiments will identify how motor pools are activated by vestibular signals to drive self-righting. In Aim 2, vestibular neurons will be stimulated during in vivo recordings from identified spinal motor neurons to test how vestibulospinal drive is distributed to the appropriate pools of motor neurons for self-righting. Finally, Aim 3 will extend this research to spinal interneurons, to identify how descending inputs regulate interneuronal circuits for highly specific modulation of movement. Impairments in vestibulospinal signaling can cause vertigo and falls, a major health hazard in the elderly. Thus, a complete sensory-to-motor analysis of vestibulospinal signaling will advance our understanding of descending control of behavior and potentially identify strategies for improving human postural control. PUBLIC HEALTH RELEVANCE: Vertigo and falls represent a major health hazard in the elderly and those with common neuropathic ailments like diabetes. Although falls are frequently due to impairments in the balance system, it is not known how balance signals from the inner ear normally interact with the spinal cord to drive movements that help maintain posture. This proposal seeks to identify the precise connections between the inner ear and the spinal cord that underlie compensatory postural adjustments, providing the basis for future therapeutic interventions in humans with balance deficits.

Project Summary Animals must cope with the pervasive force of gravity as they navigate the environment. To sense and respond to this force, vertebrates rely on signals from the inner ear, where gravito-inertial sensors called otoliths drive activity in peripheral vestibular circuits. This information is then processed by central vestibular neurons in the brainstem and transformed into postural outputs via projections to the spinal cord. Because this vestibulospinal circuit is formed early in life, it has been technically challenging to examine how it develops. The objective of this proposal is to determine how sensory computations arise in vestibulospinal neurons, and whether normal sensory activity is required for this development. To surmount the technical challenges of examining these circuits at early stages, I propose to use the larval zebrafish. Zebrafish are an excellent system for this line of research because of their accessibility, transparency, and homology to other vertebrates. Furthermore, we can carry out many experiments that are not feasible in mammalian models, including in vivo whole cell patch-clamp analysis of synaptic responses to sensory stimuli. This technical advance permits us to record sensory-evoked activity in the intact brain, over the time period when postural behaviors develop. In addition, we can exploit a mutant fish line in which otolith development is delayed by two weeks, providing in effect a reversible sensory deprivation to vestibular circuits. The proposed experiments will therefore reveal how sensory information is encoded during development, both under normal conditions and those of sensory delay. In Aim 1, we will use a combination of behavior, imaging, and physiology to define the anatomy, sensory responses, and functional role of vestibulospinal neurons in vivo. These experiments will identify anatomically distinctive vestibulospinal neurons for characterization across animals. In Aim 2, we will examine the development of vestibular encoding during the time period in which animals begin to self-right with respect to gravity. Here we will use both anatomical imaging of vestibular afferents to the central vestibulospinal neurons as well as physiological analyses of the development of sensory encoding. Finally, in Aim 3 we will use an animal model of sensory deprivation to ask whether the first two weeks represent a critical period in vestibular development: does the vestibular circuit develop normally in the absence of normally patterned sensory input? Because this model develops an otolith on a grossly delayed time scale, it serves as a model of reversible sensory deprivation so that we can also examine whether and how the circuit recovers. Together, the proposed experiments will provide a view of vestibular development under both normal and abnormal sensory conditions.

Project Summary Animals must cope with the pervasive force of gravity as they navigate the environment. To sense and respond to this force, vertebrates rely on signals from the inner ear, where gravito-inertial sensors called otoliths drive activity in peripheral vestibular circuits. This information is then processed by central vestibular neurons in the brainstem and transformed into postural outputs via projections to the spinal cord. Although studies in slice preparation have indicated that vestibular neurons make linear computations of their inputs, this concept has not been tested in vivo. The objective of this proposal is to determine how vestibulospinal neurons carry out computations of sensory inputs. To surmount the technical challenges of examining synaptic and cellular properties of this circuit, I propose to use the larval zebrafish. Zebrafish are an excellent system for this line of research because of their accessibility, transparency, and homology to other vertebrates. Furthermore, we can carry out many experiments that are not feasible in mammalian models, including in vivo whole cell patch- clamp analysis of synaptic responses to sensory stimuli. This technical advance permits us to record sensory- evoked activity in the intact brain, over the time period in which postural behaviors develop. In addition, we can exploit a mutant fish line in which otolith development is delayed by two weeks, providing in effect a high selective sensory deprivation to vestibular circuits. The proposed experiments will therefore reveal how sensory information is encoded during development, both under normal conditions and those of sensory delay. In Aim 1, we will use a combination of behavior, imaging, and physiology to define the anatomy, sensory responses, and functional role of vestibulospinal neurons in vivo. These experiments will define the homology between zebrafish and mammalian vestibulospinal nuclei. In Aim 2, we will quantify how sensory afferents converge to produce central tuning. We will further ask how this convergence develops over the time period in which animals begin to self-right with respect to gravity. Here we will use both ultrastructural reconstructions of vestibular afferents to the central vestibulospinal neurons as well as physiological analyses of the development of sensory encoding. Finally, in Aim 3 we will examine the functional contributions of inhibition to sensory tuning and develop a highly constrained model of vestibular computations. Together, the proposed experiments will provide a rigorous and quantitative analysis of how sensory tuning is constructed in central vestibular neurons.