The ontogenetic mechanisms that are responsible for the adult patterns of innervation and function within the mammalian cochlea are poorly understood. The goal of this application is to investigate the mechanisms which underlie efferent synaptogenesis, specifically, the development of olivocochiear (OC) synapses. An additional goal is to test the generality of current models of synaptogenesis and competition in the maturation of the OC system, and ultimately, of auditory function. Since OC neurons demonstrate best frequency responses, are involved with the detection of signals in noise, and appear morphologically mature before the onset of hearing, understanding their development may help us understand hearing and disorders associated with hearing in preterm and newborn infants. The specific aims are to test the following hypotheses: 1) medial OC neurons exhibit a developmental waiting period, that is, they send projections to the cochlea that synapse on intermediate targets, 2) medial OC neurons are capable of releasing acetylcholine prior to synapsing on outer hair cells, 3) lateral OC neurons undergo a separate period of cochlear synaptogenesis from medial OC neurons, and 4) OC neurons are capable of electrical activity at the time of synapse formation. These investigations focus primarily on the first postnatal week in newborn hamsters and gerbils and include a variety of approaches such as: retrograde and anterograde labeling to visualize the temporal sequence of efferent projections; serial-section electron microscopy of labeled efferent terminals; histochemical, immunochemical, and in situ hybridization studies of the expression of neurotransmitters and developmental markers; characterization of enzyme activity to assess neurotransmitter mechanisms; and development of a brainstem slice preparation to explore the onset of activity and synaptic input to OC neurons. Importantly, this research should generate specific models that can be tested by experimental interventions. This ISA will significantly enhance the PI's development as an independent scientist by providing the much needed time to focus and direct his research, and by allowing the PI to develop and apply multidisciplinary approaches such as neurochemical, molecular, and slice physiological techniques. The institution's development plans include: 1) releasing the PI from nonessential teaching and administrative responsibilities, 2) encouraging additional mentoring between the PI and senior faculty within the department, and 3) allowing the PI to pursue research collaborations offcampus that will strengthen his research program.