IVESTIGATION OF NEUROMODULATION OF THE ENDBULB OF HELD SYNAPSE IN THE COCHLEAR NUCLEUS BY DOPAMINE, SEROTONIN AND NOREPINEPHRINE

In this study, we investigated whether the endbulb of Held synapse formed by auditory nerve fibers onto bushy cells (BCs) in the anteroventral cochlear nucleus (AVCN) of mice is modulated by norepinephrine (NE), serotonin (5-HT) and dopamine (DA).
We used electron microscopy (EM) of the cochlear nucleus (CN) to investigate the presence of monoaminergic projections. Furthermore, we performed immunohistochemistry to study the localization of monoamine transporters in the AVCN, as well as monoamine receptors in the AVCN and in spiral ganglion neurons (SGNs). We performed patch-clamp recordings from BCs to study spontaneous and evoked synaptic transmission as well as short-term plasticity of the endbulb of Held synapse and to investigate the excitability of the BCs.
We found EM evidence for putative monoaminergic varicosities in both ventral and dorsal divisions of the CN. 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 several monoamine receptors both in the AVCN and in SGNs. Patch-clamp recordings from BCs revealed an increase in frequency of miniature excitatory postsynaptic currents (mEPSCs) upon application of NE but not 5-HT or DA. Evoked synaptic transmission was unaffected by the application of either NE or 5-HT, while DA increased the amplitude of the first EPSC at 100 Hz train stimulation.
In summary, we report evidence for the presence of monoaminergic innervation in the cochlear nucleus and for subtle functional NE and DA-neuromodulation at the endbulb of Held synapse.