Current research in our laboratory focuses on processing of auditory signals in the cochlear nucleus. Although much is known about how auditory information is represented by firing patterns of the auditory nerve, there are many outstanding questions about how these signals are processed in the brain. Since the inception of the laboratory two years ago, we have begun addressing questions about the function of the dorsal division of the cochlear nucleus (DCN). Principal neurons of the DCN, or fusiform cells, exhibit complex and nonlinear responses to sounds. In particular, they have been shown to respond to specific features of sounds known as spectral notches, which arise from the position-dependent acoustic filtering properties of the external ear. This feature has led to the hypothesis that one function of the DCN is to encode the location of sounds along the vertical axis. We are currently investigating the cellular and synaptic mechanisms underlying fusiform cell responses to sounds. Our experimental approach combines electrophysiology and cellular imaging to study synaptic transmission and plasticity in the cochlear nucleus. At the present time there are two main projects in the lab, both focusing on the DCN. The first project addresses the role of endocannabinoids in modulating synaptic transmission at synapses formed by parallel fibers onto cartwheel cells, glycinergic interneurons that influence fusiform cell activity. Endocannabinoids act as retrograde messengers that are released from postsynaptic neurons and regulate synaptic strength over short and long time scales. Endocannabinoids regulate transmission at synapses in many brain region including auditory synapses but their role in auditory function is not known. Neuronal depolarization can evoke global endocannabinoid release throughout a neuron's dendritic arbor by a mechanism that relies on elevation of postsynaptic calcium. Cartwheel cells release endocannabinoids in response to brief depolarization, causing suppression of parallel fiber synapses that lasts tens of seconds. However, the high calcium levels required for this form of synaptic suppression may not be reached under physiological conditions. Therefore, we examined dendritic calcium signals in cartwheel cells and the resulting suppression of parallel fiber synapses that occur during more realistic firing patterns, using patch clamp techniques and two-photon calcium imaging. Cartwheel cells are spontaneously active neurons that fire both simple and complex spikes. During spike trains, dendritic calcium reached a concentration plateau in the low micromolar range. These spike trains strongly suppressed parallel fiber synapses and this suppression required the activation of CB1 receptors. Thus, prolonged but modest elevation of dendritic calcium in cartwheel cells evokes endocannabinoid release that regulates the strength of PF synapses. These findings suggest that endocannabinoid signaling occurs under physiological conditions and influences the output of the DCN. Further experiments are currently being conducted to examine the role of parallel fiber activity in evoking localized endocannabinoid release from cartwheel cells in an activity-dependent manner. The goal of the second project is to develop techniques for in vivo recordings from individual neurons in the mouse DCN in response to sounds. Since December 2008, we have assembled a rig for recording electrophysiological responses and sound presentation. We have also written a software package for single neuron recording and characterization as well as iontophoresis and juxtacellular labeling. We have developed the surgical techniques necessary for recording from the decerebrate mouse dorsal cochlear nucleus preparation and have collected preliminary data from putative DCN principal neurons. Our first goal is to combine electrophysiological recording and anatomical labeling techniques to determine basic sound-driven response properties of DCN principal- and inter-neurons in the mouse preparation. Data collection is currently underway and we project completion of the initial stages of this work by December 2009. Further experiments are planned to examine several aspects of cochlear nucleus function, including the hypothesis that endocannabinoids modulate synaptic transmission in the DCN and influence fusiform cell response properties. These studies contribute to our understanding of how sensory stimuli are represented by neuronal activity and will enable improvements in our understanding and treatment of hearing disorders. Recent evidence implicates the DCN in tinnitus, a prevalent hearing disorder that impairs the quality of life of many people. Animal models of tinnitus demonstrate elevated activity levels in fusiform cells. However the mechanisms that produce this elevated activity are not known. A better understanding of the synaptic mechanisms responsible for the response properties of DCN neurons will enable us to test specific hypotheses about the alterations in the DCN that can cause tinnitus.
