Functional Specializations of Neuroglia as Critical Determinants for precise neuronal signaling in the auditory brainstem

• Applicant: Dr. Jonathan Stephan
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 279540474

The present proposal addresses the issue of astrocytic specializations regarding synaptic neurotransmission in the mammalian auditory system, in which temporal precision is fundamental to feature extraction of acoustic signals. To this end, astrocyte function will be analyzed in two conspicuous brainstem nuclei, the lateral superior olive (LSO) and the inferior colliculus (IC). The neuronal pathways to the LSO, which is involved in sound localization, feature unusual specializations, allowing very precise temporal processing down to the µs-range. In contrast, no such specializations have been identified in the input to the IC, in which temporal processing occurs at a much longer time scale (ms to hundreds of ms). We hypothesize that the heterogeneity between LSO and IC seen at the neuronal level finds its counterpart at the astrocytic level. To test this hypothesis, we will characterize astrocyte function regarding two major aspects, namely neurotransmitter uptake and spatial buffering. Neurotransmitter uptake will be assessed at inhibitory inputs to both nuclei, and the role of the astrocytic glycine transporter GlyT1 and the GABA transporter GAT 3 will be analyzed. This will involve electrophysiological recordings from astrocytes and neurons in brainstem slices and acute pharmacological blockade as well as a genetic approach. For the latter, we will knock out GlyT1 and GAT 3 in an astrocyte-specific and tamoxifen-inducible manner, making use of GLASTCreERT2 transgenic mice. Prolonged activation of the inhibitory inputs (> 21,000 stimuli) will form an essential part of our stimulus paradigm, and response kinetics as well as response robustness will be analyzed. Differences in spatial buffering will be assessed by investigating synaptic transmission in Kir4.1 knock-out and Cx43/30 double knock-out mice, which are devoid of inwardly rectifying potassium channels of the 4.1 subtype and gap junction coupling, respectively. The spatial buffering experiments will be complemented by Na+ imaging experiments. To further assess nucleus-specific features of the astrocytic transporters at the subcellular level, we will perform electron microscopic immunogold labeling experiments aimed at determining the location of GlyT1 and GAT 3 in the LSO and IC qualitatively and quantitatively. Our research program promises to yield fundamental insights into the determinant role of astrocytes in synaptic neurotransmission with very high temporal acuity.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1608: Ultrafast and Temporally Precise Information Processing: Normal and Dysfunctional Hearing
• Co-Investigator: Professor Dr. Eckhard Friauf