Bradley D. Winters, Ph.D. - US grants

? DESCRIPTION (provided by applicant): Hearing using 2 ears allows the extraction of sound timing information that animals, including humans, use to localize sounds in space and comprehend communication signals in complicated auditory environments. Understanding how the binaural hearing system develops is critical for implementing treatments for individuals with impairments. The medial superior olive (MSO) nucleus in the brainstem of mammals houses one of the first stages of processing combined information from both ears. MSO neurons enable detection of extremely minute timing differences between the arrivals of sounds at the 2 ears by responding maximally to binaural excitation at specific interaural time differences (ITDs). Glycinergic inhibitory inputs onto MSO neurons are critical for shaping their ITD responses. Prior to hearing onset, supernumerary inhibitory inputs are evenly distributed over the dendrites and soma of MSO neurons. After hearing onset, inhibition is dramatically refined. Most of the inhibitory inputs are pruned and those that remain are well timed with binaural excitation and concentrated onto the soma. Despite the relevance to the functioning of this important circuit, almost nothing is known about the cellular mechanisms that guide refinement of inhibition in the MSO. Synaptic plasticity is likely to be the key to maintaining a specific set of synapses in an experience-dependent process. The work proposed here seeks to understand inhibitory synaptic plasticity in the MSO and how it contributes to the development of inhibitory drive after hearing onset. This project utilizes electrophysiological and calcium imaging techniques in acute brain slices from Mongolian gerbils combined with in vivo manipulation of binaural auditory experience. Aim 1 will reveal mechanisms of synaptic plasticity that guide the strengthening and synchronization of inhibitory drive with binaural excitation in the MSO. Preliminary results suggest a model of N-methyl D-aspartate receptor (NMDAR)- dependent inhibitory long-term potentiation (iLTP) in the MSO in which glutamate, either co-released with glycine or through spillover from adjacent excitatory inputs, binds NMDARs and action potential (AP)-driven depolarization from binaural excitatory drive relieves the magnesium block. The locus of NMDAR activation and source of glutamate will be determined. Aim 2 seeks to understand what guides the developmental concentration of inhibitory inputs onto the soma of MSO neurons. Over the first 2 weeks of hearing, changes to intrinsic membrane properties of MSO neurons reduce the invasion of AP depolarization into the dendrites. My hypothesis is that inhibitory synapses in the dendrites progressively do not receive sufficient depolarization for iLTP and without this continued reinforcement are selectively pruned. To test this hypothesis, I will rear animals in omnidirectional noise, a manipulation known to disrupt binaural cues. Then I will determine whether iLTP, intrinsic changes, and somatic segregation of inhibition are retarded. Together, this work will give us a deeper understanding of the experience-dependent developmental refinement of inhibition in this important center for processing of binaural cues.

Project abstract Principal neurons (PNs) of the lateral superior olive (LSO) in the brainstem of mammals are a key component in the processing of binaural cues used for sound localization that underlie selective attention. They accomplish this by comparing excitatory synaptic inputs driven by the ipsilateral ear with inhibitory inputs driven by the contralateral ear. It is increasingly appreciated that along with their classical role of interaural intensity difference (IID) coding, LSO PNs also encode interaural time differences (ITDs). These two functional roles, along with the tonotopic organization of the LSO, place different demands on the cellular properties of LSO neurons. My major hypothesis is that there is functional segregation of LSO PNs for IID and ITD coding. This functional segregation may be defined by transmitter released, projection pattern, morphology, dendritic integration functions, or synaptic inputs. This proposal will develop core methodologies to access these cellular features of the LSO. Excitatory LSO PNs are biased to higher frequency regions and largely project contralaterally while inhibitory cells are biased to lower frequencies and project Ipsilaterally. Firing response characteristics associated with phase locking and ITD coding are biased toward lower frequency regions, potentially associating with inhibitory PNs. I will investigate the possibility that ipsilateral projecting inhibitory PNs are better adapted for ITD coding while contralateral projecting excitatory PNs are better adapted for IID coding. This potentially provides a means to segregate this information in upstream centers. Critical for understanding whether inhibitory and excitatory cells have distinct functional roles within the circuit is their relative intrinsic cellular properties. To efficiently investigate this I will develop a transgenic mouse line that will allow me to target excitatory and inhibitory cell types during brain slice physiology experiments. My expectation is that inhibitory/ITD coding cells would have lower input resistances, faster membrane time constants, larger diameter and less complicated dendrites, and phasic firing type, whereas, excitatory/IID coding will be associated with more integrative membrane properties. These experiments will yield foundational insights into the cellular organization of the LSO. Efficacy of propagation of action potentials and synaptic potentials in dendrites is a critical component of integrative functions and synaptic plasticity in neurons which cannot be measured from somatic recordings alone. Recent work has revealed dendritic properties that have adapted for ITD coding. In contrast, almost nothing is known of the electrical properties of LSO dendrites or what aspects of dendritic physiology best support IID coding. I will develop methodologies using multiphoton imaging to make unbiased dual dendritic/somatic patch recordings from LSO neurons which allow for the analysis not only of local responses in the dendrites but also signal transformations with propagation to the soma. Combining this information with our developing understanding of the different coding demands on cells along the tonotopic axis, and potentially between PN types, will yield new insights into the relationship between cellular properties and circuit function.