The purpose of this grant is to investigate the efferent innervation of the peripheral vestibular apparatus. Knowledge of the chemical composition and projections of the efferent neurons in the brainstem to the peripheral labyrinth may help to understand the role of the vestibular efferent system in health and disease. Recent work has suggested that the neurons in the efferent nucleus may be heterogeneous in their transmitter neurochemistry and in their peripheral targets. This suggestion of heterogeneity, if confirmed, would change our long-held view that the vestibular efferents are a nonspecific system. The goal of this study is to define subpopulations of efferent neurons in the chinchilla based on their brain stem locations, their projections to one, the other or both ears, their neurochemical identities and their peripheral endings on hair cells, calyces and other afferent processes. Specific aims are: l) to characterize anatomical and neurochemical subpopulations of efferent neurons in the brainstem efferent neurons using retrograde tracers and transmitter immunohistochemistry, and 2) to relate the central subpopulations of vestibular efferent neurons to their terminations in the periphery. Efferent neurons in the brainstem will be identified by means of retrograde tracer injections into the peripheral vestibular apparatus and will be simultaneously characterized immunohistochemically by the use of antibodies to several putative neurotransmitters (acetylcholine, calcitonin gene-related peptide, nitric oxide synthase, met-enkephalin and others). The analysis will be extended to the periphery where electron microscopic immunohistochemical methods will be used to characterize neurochemically distinctive efferent boutons according to their terminations in the sensory epithelium, that is, by the class of afferents (calyx, dimorphic or bouton) or region (central or peripheral) that they innervate. Vestibular afferents will be identified by means of extracellular HRP injections, calretinin immunohistochemistry, or other means. The distribution of muscarinic receptor subtypes will also be examined in an attempt to understand the slow response of afferents to stimulation of the efferents. The significance of this project is that a precise knowledge of the various elements comprising the population of efferent neurons, and their targets in the periphery, will further our understanding of brain stem control of peripheral sensory processing. It is hoped that as a result of this work, it will no longer be sufficient merely to define subgroups in the brain stem without establishing their relationship to the periphery. Nor will it be sufficient to illustrate peripheral terminations without stating their specific location within the endorgan. The intent is to produce a body of knowledge about the structural basis of the efferents from which physiologically and pharmacologically testable hypotheses can be derived.