Lloyd B. Minor - US grants

The long-term goal of this research is to understand the functional organization of the central pathways mediating the vestibulo-ocular reflex (VOR). These central pathways receive information about head velocity that has been transduced by afferents innervating the semicircular canals. Recent morphophysiological studies have shown that semicircular canal afferents can be divided into three classes: (1) low-gain, irregular afferents; (2) high-gain, irregular afferents; (3) low-gain, regular afferents There are several subclasses of secondary neurons with somas located in the vestibular nuclei that are distinguishable in terms of their discharge properties in relation to oculomotor signals. A paradigm that we have developed for silencing irregular afferents will be used to study the profile of afferent inputs received by secondary neurons. The method is based on differences in the electrical sensitivity of vestibular afferents as a function of discharge regularity. Irregular afferents are, on average, 10x more sensitive to dc currents than are regular afferents. An anodal (inhibitory) current of 100 muA is presented to both ears. This current has been shown to silence most irregular afferents to the extent that they no longer respond to rotational stimuli. The rotational sensitivity of regular afferents is not significantly affected by the currents. Squirrel monkeys are prepared for chronic recording sessions by surgically implanting a head restraining bolt, scleral search coil, recording chamber, labyrinthine stimulating electrodes, and a stimulating electrode in the rostral medial longitudinal fasciculus (MLF). The animals are trained to fixate target lights, cancel their VOR, and pursue moving targets. Single-unit activity is extracellularly recorded in the superior and medial vestibular nuclei. Once a unit is isolated, its eye position and eye velocity sensitivity are determined. The unit is tested for monosynaptic activation from the ipsilateral vestibular nerve with cathodal (excitatory) short shock stimuli and its cathodal galvanic sensitivity is recorded. The polarization paradigm is then used to measure the rotational responses in the presence and absence of 100 muA anodal currents delivered to each labyrinth independently and then to both labyrinths simultaneously. Sinusoidal chair rotations of 0.5 Hz plus or minus 400/sec and 4.0 Hz plus or minus 200/sec are used. The animal cancels its VOR during the 0.5 Hz rotations by fixating a target located straight ahead. The rotational gain of those neurons receiving irregular fiber inputs will be reduced by the anodal polarizations and the response phase will be shifted. Low- and high-gain irregular afferent inputs will be distinguished by calculating the ratio of the secondary neurons's rotational gain to its galvanic sensitivity. This ratio will be significantly lower for those units receiving predominantly low-gain irregular afferent inputs. The responses of the neuron are recorded with the animal in three different positions. Simultaneous equations are solved to determine the extent to which the neuron is receiving convergent inputs from orthogonally related semicircular canals. The neuron is then tested for antidromic activation from the MLF.

Vestibulo-ocular reflexes (VORs) are responsible for maintaining stability of images on the retina during head movements. Many of the behavioral features and neurophysiological mechanisms underlying control of angular VORs have been elucidated. Comparatively little is known about the organization of otolith-ocular reflexes. These reflexes can be divided into three categories according to the type of compensatory eye movements arising from their activation. Ocular counterrolling is the otolith-ocular reflex that causes torsional movement of the eyes around a line of sight in response to dynamic and static lateral head tilt. Translational linear VORs are eye movements that occur in response to dynamic linear movements of the head. The nystagmus induced by constant velocity rotation about an off-vertical axis results from an angular velocity signal generated by central processing of otolith activity. Vestibular-nerve afferents innervating the otoliths cannot distinguish between movements leading to each of these three responses. Research described in this project will define mechanisms involved in specification and adaptive control of otolith-ocular reflexes. Three dimensional and vergence eye movements as well as single-unit activity in the vestibular nuclei will be recorded in alert squirrel monkeys before and after bilateral inactivation of all semicircular canals with a plugging procedure. Hypotheses related to processes that distinguish tilt from translational responses will be tested. The role of angular velocity signals, phasic-tonic responses of irregularly discharging otolith afferents, and frequency-specific filtering of otolith signals in these processes will be determined. The physiological properties of neurons in the vestibular nuclei involved in otolith-ocular control will be defined. Profiles of otolith and semicircular canal afferent inputs to these neurons will be identified in relation to translational and tilt stimuli. Studies of adaptation introduced by optokinetic stimuli in association with motions producing specific profiles of otolith and semicircular canal activation will provide further information about signals used to distinguish these responses. Transfer of adapted VOR gain to reflexes induced by differing combinations of semicircular canal and otolith activity will identify points of convergence in these pathways and define mechanisms involved in generation of angular velocity signals from otolith activity. This research will enhance our understanding of otolith-ocular responses and the processes involved in short-term VOR adaptation. The studies are directly relevant to the diagnosis and treatment of dysfunction in human otolith-ocular reflexes.

DESCRIPTION (provided by applicant): Disruption of vestibular signals from one labyrinth results in asymmetries in the angular vestibuloocular reflex (VOR) evoked by high-frequency, high-acceleration head movements. Studies during the previous funding period have elucidated linear and nonlinear components of the angular VOR in the squirrel monkey, demonstrated selective adaptation of these components with magnifying or minimizing spectacles, compared the horizontal angular VOR in squirrel monkeys and macaques in response to rapid rotations, defined the response properties of vestibular-nerve afferents to rapid head rotations, and analyzed changes in the angular VOR, afferents, and hair cells following unilateral ototoxic vestibular injury with gentamicin. The proposed research builds upon previous accomplishments through studies that are organized into three specific aims. The experiments are performed in chinchillas and in macaques. Studies in Aim I will define the responses of vestibular-nerve afferents to steps of acceleration and study the contributions of irregularly discharging afferents to the horizontal VOR evoked by these rapid head rotations. Bilateral, anodal galvanic currents delivered to each labyrinth will be used to substantially attenuate or silence irregular afferents during rapid head rotations. The experiments that are conducted in macaques will involve analysis of the normal VOR as well as following spectacle-induced adaptation. Aim II will investigate the contribution to VOR compensation of preserved resting rate in afferents on the side of a unilateral vestibular lesion and of afferents from the contralesional (intact) labyrinth. The unilateral lesion that preserves resting rate but abolishes or markedly attenuates responses to motion involves intratympanic injection of gentamicin. Aim III will examine neural correlates of compensatory changes in the horizontal VOR in macaques after unilateral labyrinthectomy. These experiments will determine if the discharge properties of vestibular-nerve afferents on the contralateral side change following unilateral labyrinthectomy. The dynamics of vestibular nuclei neurons that mediate the VOR will be studied following unilateral labyrinthectomy. The role of proprioceptive inputs and anticipatory mechanisms in modifying the responses of these central neurons will be determined by comparing neuronal responses during actively and passively generated head-on-body and whole-body rotations.

DESCRIPTION:(provided by applicant) The objective of the proposed research is to understand the pathophysiology of the vestibular disturbances in Meniere's disease and how to treat them. The specific effects (both qualitative and quantitative) on vestibular function of both Meniere's disease itself and of intratympanic gentamicin used to alleviate vertigo are unknown and will be determined in the proposed research. The research strategy is to analyze the vestibuloocular reflex (VOR) in three dimensions from responses to stimuli that activate the semicircular canals or the otoliths. Vestibular function will also be evaluated from measurements of the subjective visual vertical and from vestibular-evoked myogenic potentials. The angular VOR evoked by high-frequency, high-acceleration head thrusts will be studied in order to determine the effects of Meniere's disease and of intratympanic gentamicin on the function of individual semicircular canals. The translational VOR in these patients will be evaluated from the responses to rapid, lateral translations of the head. Through comparisons with findings in subjects with normal vestibular function and those with known surgical unilateral vestibular destruction (UVD), these studies will provide a new understanding of the effects of Meniere's disease itself, and the effects of treatment with gentamicin, on individual vestibular end organs. Recovery of the VOR after intratympanic gentamicin and after surgical UVD will be assessed through analyses of the trajectories of eye velocity. The corrective eye movements that reduce the gaze errors that occur as a consequence of diminished vestibular function in the responses to high acceleration angular and translational head movements will also be analyzed. The information derived from this research will have practical import on which vestibular tests are most useful in Meniere's disease, and on deciding when and with what to treat patients with Meniere's disease.

DESCRIPTION (provided by applicant): The goal of our program is to train and to develop outstanding physician-scientists in otolaryngology--head and neck surgery. Our specialty requires multidisciplinary approaches for the understanding and treatment of communication disorders and diseases of the head and neck. In order to meet this objective, our residents need to be educated in both clinical and research skills. Research training is a key element to our training program because it serves as a means to enhance critical thinking skills, to strengthen the understanding of scientific and medical literature, to provide training in investigative techniques, and to develop the ability to pose testable hypotheses. One outstanding feature of our current training program is that each year, two residents embark in our now-established 7-year track. This track provides 2 years of continuous research training where research opportunities include, but are not limited to, topics in molecular biology of head and neck cancers, basic mechanisms of dizziness and balance, studies of the auditory nervous system, and clinical outcomes. The strength of this training plan is measurable by the breadth and experience of the involved faculty. The success of the training program is evident in the accomplishments of those otolaryngologists who have completed their training on this sequence and who are now building successful academic careers.