The role of individual subunits in the formation and function of presynaptic Kv3 potassium channels

Kv3 potassium channels, composed of four subunits, mediate fast action potential (AP) repolarisation. In the auditory brainstem they enable neurons to faithfully follow high-frequency sound stimuli with temporal precision, vital for binaural integration and sound localisation. Of four Kv3 subunits, only Kv3.1 and Kv3.3 are present in the auditory brainstem. A neurodegenerative disease (spinocerebellar ataxia type 13, SCA13) associated with impaired sound localisation, is caused by mutations of Kv3.3, highlighting the importance of these subunits in this system. The contributions of specific subunits to pre and post- synaptic Kv3 channels was examined in this thesis, with a particular focus given to presynaptic Kv3 channels (at the giant calyx of Held terminal) and their ability to modulate transmitter release from the synapse. Whole-cell patch clamp was conducted on in vitro brainstem slices from wildtype, Kv3.1 and Kv3.3 knockout mice, in combination with histology. Experiments were repeated in mice harbouring an SCA13 mutation (R420H) that has been linked with auditory deficits in humans. Lastly, whole-cell patch clamp was used in expression systems to determine whether different Kv3 subunits could form heteromers. These studies showed Kv3.1 and Kv3.3 had equal importance for AP repolarisation in neurons of the medial nucleus of the trapezoid body (MNTB) while Kv3.3 subunits dominated in neurons of the lateral superior olive (LSO) and at the presynaptic calyx of Held terminal. Kv3.3 was required at this synapse to limit AP duration, preventing excessive transmitter release, particularly during high-frequency stimulation. AP durations in MNTB and LSO neurons of the SCA13 mouse model were identical to those in Kv3.3 knockouts showing that the edited subunits do not form functional channels. Lastly, co-expression studies in CHO cells revealed that Kv3.3 subunits can form heteromeric channels with Kv3.1 but not with Kv3.4.