Investigation of neuromodulation of the endbulb of Held synapse in the cochlear nucleus by serotonin and norepinephrine

Maria Boykova Groshkova^1,2,3,4,5,6Theocharis Alvanos^1,3,4Yumeng Qi⁷Fangfang Wang⁷Carolin Wichmann^4,8,9Yunfeng Hua⁷Tobias Moser^10,2,3,4,9*

• ¹Auditory Neuroscience and Nanophysiology Group, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Lower Saxony, Germany
• ²University Medical Center Göttingen, Göttingen, Germany
• ³Institute for Auditory Neuroscience, University Medical Center Göttingen, Göttingen, Lower Saxony, Germany
• ⁴Collaborative Research Center SFB 1286 “Quantitative Synaptology”, University of Göttingen, Göttingen, Germany
• ⁵Göttingen Graduate School for Neurosciences and Molecular Biosciences, University of Göttingen, Göttingen, Germany
• ⁶International Max Planck Research School for Molecular biology (IMPRS), Göttingen, Germany
• ⁷Shanghai Institute Precision Medicine, Shanghai Ninth People’s Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, China
• ⁸Molecular architecture of synapses group and Center for Biostructural Imaging of Neurodegeneration, Institute for Auditory Neuroscience, University Medical Center Göttingen, Göttingen, Lower Saxony, Germany
• ⁹Cluster of Excellence Multiscale Bioimaging: from molecular Machines to Networks of Excitable Cells, University Medical Center Göttingen, Göttingen, Lower Saxony, Germany
• ¹⁰Max Planck Institute for Multidisciplinary Sciences, Göttingen, Lower Saxony, Germany

Synapses vary greatly in synaptic strength and plasticity, even within the same circuitry or set of preand postsynaptic neurons. Neuromodulation is a candidate mechanism to explain some of this variability. Neuromodulators such as monoamines can differentially regulate presynaptic function as well as neuronal excitability. Variability is found also for the large calyceal synapses of the auditory pathway that display high synaptic vesicle (SV) release probability (Pvr) and large postsynaptic currents in vitro enabling reliable and temporally precise transmission of auditory information. Here we investigated whether the endbulb of Held synapse formed by auditory nerve fibers onto bushy cells (BCs) in the anteroventral cochlear nucleus (AVCN) is modulated by norepinephrine (NE) and serotonin (5-HT).Using electron microscopy (EM) of the cochlear nucleus we found evidence for putative monoaminergic varicosities in both ventral and dorsal divisions. Immunostaining for vesicular 5-HT and NE transporters revealed NE-containing and 5-HT-containing varicosities in the AVCN, juxtaposed to both endbulbs and BCs. Furthermore, we detected immunofluorescence for 5-HT1B, 5-HT4, 5-HT7 receptors (R) and α2C-adrenergic receptors (AR) in BCs. We used voltage-clamp recordings from mouse BCs in order to uncover potential presynaptic effects of neuromodulation, which revealed an increase in frequency of miniature excitatory postsynaptic currents (mEPSCs) upon application of NE but not 5-HT. Evoked synaptic transmission was unaffected by the application of either NE or 5-HT. Likewise, while studying the biophysical properties of the BCs, we did not observe effects of NE or 5-HT on low-voltage-activated K + (K + LVA) and hyperpolarization-activated mixed cation (HCN) channels during application. In summary, we report evidence for the presence of monoaminergic innervation in the cochlear nucleus and for subtle functional NE-neuromodulation at the endbulb of Held synapse.