Thanos Tzounopoulos - US grants

The dorsal cochlear nucleus (DCN) is an auditory brainstem center that integrates auditory signals with multimodal sensory signals thought to aid in orientation to sounds of interest. Our previous studies have identified a number of novel mechanisms of synaptic plasticity in the DCN and have uncovered their influence in the processing of auditory nerve inputs. Here, we propose to investigate the physiological role of an unconventional neurotransmitter that is coreleased with glutamate. DCN parallel fiber terminals contain uniquely high levels of Zn[2+] in their gluatamatergic synaptic vesicles. However, the physiological significance of synaptic Zn[2+] release is completely unknown for the auditory system and remains poorly understood for other brain areas. We hypothesize that synaptically released Zn[2+] is a neurotransmitter in the auditory system that regulates the intrinsic and synaptic properties of DCN neurons. To test this hypothesis we will employ a combination of physiological, anatomical and biochemical techniques that will be applied to brainstem slices prepared from wild type and genetically modified mice. Elucidating the mechanisms and the roles of this novel neurotransmitter system will contribute significantly towards the understanding of synaptic and intrinsic mechanisms that are involved during normal and pathological auditory processing.

DESCRIPTION (provided by applicant): Corticofugal - top-down - pathways from primary auditory cortex (A1) to the auditory brainstem powerfully modulate auditory processing. One important top-down pathway is mediated by corticocollicular neurons: a subset of layer (L) 5 pyramidal neurons in A1 with long-range axonal projections to the inferior colliculus (IC). The corticocollicular projection shapes the response properties of IC neurons, and mediates sound localization learning. However, previous physiological studies of A1 corticofugal mechanisms have not specifically targeted corticocollicular neurons. Therefore, basic physiological properties of corticocollicular neurons, such as their synaptic organization and their cellular properties, remain largely unknown. Our major goal in this proposal is to identify the cell-specific cellular and synaptic mechanisms that determine how L5 corticocollicular and L5 corticocallosal neurons - L5 principal neurons projecting to contralateral cortex - differentially process synaptic and acoustic input. The proposed experimental program will provide fundamental new information about the synaptic, cellular, and microcircuit properties of corticocollicular and corticocallosal L5 neurons. We expect that our findings will establish a new framework for understanding the roles of A1 projection neurons in top-down corticocollicular modulation of auditory processing in normal and disease states, such as in tinnitus and in pathological sound and speech perception.

In the auditory system, as sound properties of the auditory environment change, auditory cells in the brain adjust their dynamic range of their responses to match changing background noise. This type of adaptation to sound is extremely important for brain auditory processing, for it provides a mechanism for allowing us to focus on specific sounds in the presence of variable background sound levels. However, the mechanisms underlying this adaptation remain unknown. This project will establish a previously unknown link between the metal zinc in the brain and control of auditory processing in the cerebral cortex of the brain. Thus, findings from this project will advance understanding about mechanisms of adaptations of sound processing and create a new framework for approaching and interpreting the role of the auditory system in the processing of sound. This joint NSF/BSF project will establish a joint US-Israel student exchange program as well as target underserved student populations both in Israel and the US and train them in problem-solving at behavioral, neural and molecular levels of analysis. Broad dissemination of the work will be made possible by active engagement with the International Society for Zinc Biology through a variety of activities.

Zinc is packaged into glutamatergic vesicles by the ZnT3 zinc transporter, and released from synaptic terminals in an activity-dependent manner. Although the basic principles underlying zinc neurotransmission have begun to be deciphered, the sensory stimuli leading to the modulation of zinc signaling as well as the role of zinc in regulating sensory processing remain unknown. The central hypothesis of this project is that sound dependent changes in zinc signaling in the auditory cortex play a role in the sound level adaptation of neuronal input-output functions to match the prevailing stimulus intensity of the acoustic environment. This project aims to: i. Identify the consequences of experience-dependent plasticity of zinc-mediated modulation on input-output functions and receptive fields of layers 2/3 auditory cortical neurons in vivo, and ii. Establish the cellular and molecular mechanisms mediating the experience-dependent alterations in zinc-dependent auditory cortical processing. These studies will thus provide the first analysis of regulation of neuronal zinc and its effects on auditory processing, at the molecular, cellular and systems levels.

PROJECT SUMMARY In all sensory systems, peripheral sensory organ damage leads to compensatory cortical plasticity that supports a remarkable recovery of perceptual capabilities. In the auditory system, while auditory nerve input to the brainstem is significantly reduced after cochlear damage, sound-evoked cortical activity is maintained or even enhanced. This recovery is due to increased cortical sensitivity (gain) to the spared auditory input. Although this plasticity does not support features of sound processing encoded by the precise timing of neuronal firing, such as complex sound discrimination, it provides a remarkable recovery of sound detection. A major gap in knowledge is the lack of a precise mechanism that explains how this plasticity is implemented and distributed over the diverse excitatory and inhibitory cortical neurons, synapses and circuits. Here we propose a strategic, cooperative, time-dependent, cell type- and synapse-specific plasticity program that restores cortical sound processing. The results from our studies will advance the field to a new level of understanding regarding cortical plasticity after peripheral organ damage, and will inspire the development of well-timed, cell-specific treatments and rehabilitative paradigms and cures that may further enhance the recovery of perception after hearing loss, and mitigate the development of brain plasticity-related disorders, such as hyperacusis and tinnitus. Cortical principal neurons (PN) and interneurons (IN) are very diverse and thus capable of supporting a coordinated and collaborative plan for achieving cortical recovery. The major classes of cortical neurons include vasoactive intestinal-peptide (VIP), somatostatin (SOM) and parvalbumin (PV) expressing IN sub-classes, as well as intratelencephalic (IT), layer (L) 5 pyramidal tract (PT), and L6 corticothalamic (CT) PNs. Based on our preliminary results, we propose that: 1) PVs are the network ?stabilizers?; 2) VIPs are the ?enablers? that regulate SOM activity; and 3) SOMs are the ?modulators? that allow for high PN gain. At the cellular and synaptic level, we propose that soon after cochlear damage: 4) PVs and PTs exhibit a decrease in intrinsic excitability; 5) CTs exhibit an increase in intrinsic excitability; and 6) thalamic synaptic input to deep cortical layers is shifted from CT/PT equivalent to CT dominant. Overall, our proposed research and hypotheses provide an experimental platform to probe how multiple cortical neuronal sub-classes restore cortical processing after peripheral input loss (Aims 1 and 3). In combination with Aims 2 and 3, our proposed studies will determine the underlying intrinsic (Aim 2) and synaptic mechanisms (Aim 3) that mediate this plasticity.