Aspects of inhibitory synaptic transmission to the medial superior olive
thesisposted on 15.12.2014, 10:34 by Steven Owens
The medial superior olive (MSO) is a nucleus located in the auditory brainstem receiving binaural input to detect inter-aural time differences of sounds arriving at the two ears. It receives binaural excitatory inputs from the anteroventral cochlear nucleus and a monaural inhibitory input from the ipsilateral medial nucleus of the trapezoid body (MNTB). Investigations in this thesis concentrate on the inhibitory synaptic input and introduce aspects of the excitatory input. Lister Hooded rats aged 3-14 day old were killed by decapitation and transverse brainstem slices (150-200 urn) were prepared. Whole-cell voltage clamp recordings were made from visually identified MSO neurones and synaptic currents evoked by a bipolar stimulating electrode placed across the ipsilateral MNTB. The inhibitory postsynaptic current (IPSC) had a GABAergic component in animals under 6 days old and negligible there after. After 6 days the IPSC was predominantly glycine mediated. The decay time course of the glycinergic IPSC had two components in animals under 11 days old. After 11 days the glycinergic IPSC decay time course accelerated accompanied by a loss of the second slower component. IPSCs were modulated by metabotropic glutamate and GABAb receptors with GABAb receptors acting presynaptically to inhibit neurotransmitter release. Glycine release from the MNTB synapse was mediated predominantly by P/Q-type Ca++ channels, but with a significant contribution from N-type Ca++ channels. Spontaneous miniature IPSCs were variable in amplitude and were of large conductance. Excitatory inputs were mediated by AMPA and NMDA receptors. Excitatory postsynaptic currents (EPSCs) displayed fast decay kinetics compared to EPSCs from other regions of the brain. These studies describe evidence detailing the development and modulation of inhibitory synaptic input to the MSO.