Development of Intrinsic Connectivity in the Central Nucleus of the Mouse Inferior Colliculus

Abstract

The inferior colliculus (IC) in the mammalian midbrain is the major subcortical auditory integration center receiving ascending inputs from almost all auditory brainstem nuclei as well as descending inputs from the thalamus and cortex. In addition to these extrinsic inputs, the IC also contains a dense network of local, intracollicular connections, which are thought to provide gain control and contribute to the selectivity for complex acoustic features. However, in contrast to the organization of extrinsic IC afferents, the development and functional organization of intrinsic connections in the IC has remained poorly understood. Here we used laser-scanning photostimulation with caged glutamate to characterize the spatial distribution and strength of local synaptic connections in the central nucleus of the inferior colliculus of newborn mice until after hearing onset (P2-P22). We demonstrate the presence of an extensive excitatory and inhibitory intracollicular network already at P2. Excitatory and inhibitory synaptic maps to individual IC neurons formed continuous maps that largely overlapped with each other and that were aligned with the presumed isofrequency axis of the central nucleus of the IC. Although this characteristic organization was present throughout the first three postnatal weeks, the size of input maps was developmentally regulated as input maps underwent an expansion during the first week that was followed by a dramatic refinement after hearing onset. These changes occurred in parallel for excitatory and inhibitory input maps. However, the functional elimination of intrinsic connections was greater for excitatory than for inhibitory connections, resulting in a predominance of intrinsic inhibition after hearing onset.

Commissural functional topography of the inferior colliculus assessed in vitro

“…By maintaining intact connections in the IC commissural pathway, our preparation now enables extended investigations of the functional topography of these excitatory and inhibitory inputs with laser-scanning photostimluation via glutamate uncaging. In this regard, our initial assessment of the functional topography of intrinsic collicular projections are in agreement with recent comprehensive studies of the development of the intrinsic IC connections in the mouse, which have shown overlapping excitatory and inhibitory domains (Sturm et al, 2014). The commissural collicular projections also appear to receive excitatory and inhibitory inputs, with at least one source of excitatory inputs originating from a homotopic contralateral collicular locus, in agreement with neuroanatomical studies (Schweizer, 1981, Ross et al, 1988, Malmierca et al, 1995, Cant, 2013).…”

“…Such limitations naturally gives rise to concerns of whether the functional organization assessed in juvenile mice are applicable to adults and whether similar conditions also preserve the IC commissural connections in the adult due to maturation of structural geometries. Indeed, analysis of intrinsic IC connections during development in the mouse demonstrate profound changes for excitatory and inhibitory circuits (Sturm et al, 2014), possibly due to developmental refinements of axonal arbors or alterations to synaptic properties. Consequently, potential modifications for preserving these connections in older animals may likely be required and suitable caveats for the interpretation of data obtained in juvenile animals must be adhered.…”

“…The functional organization of the commissural IC connections, in particular, would benefit greatly from such an in vitro slice preparation, in the same manner as recent studies of the development of functional IC intrinsic connections in the mouse have demonstrated (Sturm et al, 2014). Prior neuroanatomical studies of the commissural IC projections indicate highly topographic connections between IC nuclei in the mouse (González Hernández et al, 1986, Frisina et al, 1997, Frisina et al, 1998); however, due to the limitations of bulk IC tracer injections, precise cell-type specific determinations of commissural convergence and divergence remain unresolved (Saldaña and Marchán, 2005).…”

“…Prior studies of the synaptic properties in the IC in vitro have generally maintained only the afferent fibers without a functional assessment of whether the afferent nuclei are also intact in the slice preparation (Smith, 1992, Moore et al, 1998, Li et al, 1999, Reetz and Ehret, 1999, Sivaramakrishnan and Oliver, 2001). Maintaining such intact connections between nuclei in a slice preparation enables the probing of the functional topography of pathways using laser-scanning photostimulation via uncaging of glutamate (Papageorgiou et al, 1999, Shepherd, 2012), yielding semi-quantitative evaluations of the functional convergence from multiple input sources (Llano and Sherman, 2009, Lee and Imaizumi, 2013, Sturm et al, 2014). In addition, the auditory slice preparations that maintain these intact connections enable assessment of the divergence of connections, using optical imaging methods that elucidate the functional divergence from their respective input sources (Llano et al, 2009, Broicher et al, 2010, Theyel et al, 2010, Hackett et al, 2011).…”

“…Furthermore, the neurochemical identity and specificity of the commissural IC projections has only roughly been assigned by prior anatomical (Saldaña and Merchán, 1992, González-Hernández et al, 1996, Saint Marie, 1996) and electrophysiological (Smith, 1992, Chakravarty and Faingold, 1997, Moore et al, 1998, Li et al, 1999, Reetz and Ehret, 1999, Malmierca et al, 2003, Malmierca et al, 2005) studies, which indicate both excitatory and inhibitory components to the commissural pathway. Remaining to be determined are the cell-type specific origins and terminations of such commissural connections and their refinement with development (Sturm et al, 2014). …”