In the mammalian brain, the medial superior olive (MSO) forms a major ascending auditory pathway that conveys information about the temporal structure and location of low-frequency sounds. Sounds arising from off-center locations along the horizontal plane reach the two ears at slightly different times. The projecting, or principal neurons of the MSO compute these temporal differences with a resolution on the order of tens of microseconds. MSO neurons then transmit these cues with precision to higher auditory centers where they are combined with other cues used in the localization of sounds. Within MSO principal neurons, the integration of bilateral synaptic inputs occurs in the dendrites, the narrow branched structures that receive the majority of excitatory synaptic contacts. As in other neurons, synaptic activity is propagated along the dendrites to the soma and axon. In the axon, the processed synaptic activity is translated into electrical impulses that are then conveyed along the axon to the sites of neurotransmitter release. In studies of non-auditory brain areas, the recent application of patch-clamp recording techniques to dendrites has revealed that synaptic activity interacts with a diverse array of dendritic ion channels, endowing neurons with complex computational properties. While the biophysical properties of MSO neurons appear specialized for rapid and precise neural signaling, the role of dendrites in processing binaural auditory information remains unexplored experimentally. The present study will, for the first time, employ dendritic patch-clamp recording techniques to study the integration of synaptic input in mammalian central auditory neurons. MSO principal neurons will be visualized in gerbil brain slices using differential interference contrast video microscopy. Electrophysiological recordings will be made from MSO neuron dendrites to examine a) how synaptic activity sum in the dendrites, b) to assess interactions between synaptic input and voltage-gated ion channels, and c) to quantify the electrical influence of dendritic structure on synaptic activity. Electrophysiological approaches will also examine the subtype expression, spatial distribution, and functional role of low-threshold potassium channels along the soma and dendrites. These channels have the strongest impact on the threshold and precision of action potential signaling, critical factors underlying the processing of sound localization cues.
