PROJECT SUMMARY Vestibular receptors encode information about head movements and send these signals through the vestibular nerve to the brain stem. In turn, receptors and afferents receive bilateral inputs from cholinergic efferent fibers that originate in the brain stem. To date, the function of the efferent pathway has remained elusive. It has been suggested that efferents might play a role in normal development of vestibular reflexes and compensation after lesions. In particular, efferents mainly affect activity of afferents with calyx terminals that completely cover the basolateral walls of vestibular hair cells, providing a unique form of synapse. Furthermore, in addition to acetylcholine, sources for gamma amino butyric acid (GABA) have also been found in the vestibular periphery, in the supporting cells and probably also in some of the efferent fibers. However, there is little known about the synaptic properties, receptor types, and interaction between cholinergic and GABAergic inputs and their effect on responses of vestibular afferents. The goal of the present proposal is to investigate the underlying mechanisms that mediate changes in afferent response properties by cholinergic and GABAergic inputs. We will use an in vitro whole preparation of the vestibular sensory epithelium to study the effect of specific agonists of different subtypes of receptors on hair cells and afferent terminals in mouse. We will also use an in vivo mouse preparation to record response properties of vestibular nerve afferents along with optical stimulation of cholinergic fibers/ GABAergic cells and application of agonists and antagonists of specific receptors in the inner ear. Finally, we investigate the effect of these drugs on vestibular function in alert mice by measuring the vestibulo-ocular reflex. Results of these studies provide the information to correlate changes at the level of single synapses in the end organs, to vestibular nerve activity, and behavioral responses. The ultimate goal of these studies is to pave the way for finding new treatment options through local application of drugs into the ear to improve vestibular compensation in patients.