The Structural, Molecular And Functional Development Of The Mouse Dorsal Cochlear Nucleus

Abstract

The dorsal cochlear nucleus (DCN) is a major subdivision of the cochlear nucleus (CN), which is the first auditory nucleus in the central nervous system and the termination point of the cochlear nerve. The DCN is thought to be involved in sound localization in the vertical plane and identification of the sound source through feature extraction of sound stimuli and integration of auditory and non-auditory inputs. However, relatively little is known about its development and how these functional properties are established compared to the other CN subdivisions. This thesis investigated the structural and functional DCN development in the postnatal mouse (P0 to P21) as well as the developmental expression of key neurotransmitter receptor subunits, using histology, immunohistochemistry (IHistC) and electrophysiology. Morphologically, cell types were well defined and the layered structure established by P6, well before the increased sensitivity to sound (“onset of hearing”, P11/12) with some subsequent cell growth and further definition of the neural layers. Synapse distribution (synaptophysin IHistC) and dendritic morphology (MAP2 IHistC) also showed substantial development by P12 with further refinement of the distribution of presynaptic terminals, which became more localized subsequently. Expression of GluA2-4 glutamate AMPA receptor subunits was substantial and diffuse throughout the DCN at birth but became more localized to major cell types after P9. On the other hand, the GluA1 subunit was diffusely and weakly expressed in immature animals and expression increased by P21, especially in the molecular layer. In contrast, there was very weak expression of the inhibitory neurotransmitter receptors (GABAA α1 and glycine α1 receptor subunits) at birth but their expression rapidly approached the adult-like pattern by P9 with high levels of labelling localized to the deeper layers. Functional maturation of pyramidal cells followed the structural development. Neuronal excitability and action potential (AP) amplitude increased between P3 and P18 while their kinetics became faster. Putative excitatory postsynaptic currents (EPSCs) were recorded in pyramidal cells by P3 but inhibitory postsynaptic currents (IPSCs) could not be recorded until P6, consistent with the timeline of excitatory and inhibitory receptor expression. The frequency, amplitude and kinetics of EPSCs did not change substantially with age but both the amplitude and frequency of IPSCs increased significantly between P6 and P12 while faster kinetics were observed after P9. The proportion of IPSCs and their amplitudes exceeded those of EPSCs from P9 indicating a dominance of inhibition over excitation in DCN pyramidal cells with maturation. Thus, there are substantial structural, molecular and functional changes in the mouse DCN before the onset of hearing, with subsequent refinement of synaptic distribution and functional properties suggesting that establishment of the basic circuitry does not require significant acoustic input. However, the refinement of many features after the onset of hearing implies that auditory experience is important in shaping and refining the final circuitry in the DCN.