Abstract
Neurons in the superior olivary complex (SOC) integrate excitatory and inhibitory inputs to localize sounds in space. The majority of these inhibitory inputs have been thought to arise within the SOC from the medial nucleus of the trapezoid body (MNTB). However, recent work demonstrates that glycinergic innervation of the SOC persists in Egr2; En1CKO mice that lack MNTB neurons, suggesting that there are other sources of this innervation (Jalabi et al., 2013). To study the development of MNTB- and non-MNTB-derived glycinergic SOC innervation, we compared immunostaining patterns of glycine transporter 2 (GlyT2) at several postnatal ages in control and Egr2; En1CKO mice. GlyT2 immunostaining was present at birth (P0) in controls and reached adult levels by P7 in the superior paraolivary nucleus (SPN) and by P12 in the lateral superior olive (LSO). In Egr2; En1CKO mice, glycinergic innervation of the LSO developed at a similar rate but was delayed by one week in the SPN. Conversely, consistent reductions in the number of GlyT2+ boutons located on LSO somata were seen at all ages in Egr2; En1CKO mice, while these numbers reached control levels in the SPN by adulthood. Dendritic localization of GlyT2+ boutons was unaltered in both the LSO and SPN of adult Egr2; En1CKO mice. On the postsynaptic side, adult Egr2; En1CKO mice had reduced glycine receptor α1 (GlyRα1) expression in the LSO but normal levels in the SPN. GlyRα2 was not expressed by LSO or SPN neurons in either genotype. These findings contribute important information for understanding the development of MNTB- and non-MNTB-derived glycinergic pathways to the mouse SOC.
Highlights
The pontine superior olivary complex (SOC) is the first central auditory region that receives significant bilateral acoustic information, and it is the primary site for detecting interaural level and timing differences (ILDs and ITDs) critical for sound localization
We immunostained for glycine transporter 2 (GlyT2) to label glycinergic projections (Friauf et al, 1999) to the lateral superior olive (LSO) and superior paraolivary nucleus (SPN) throughout postnatal development starting on the day of birth (P0)
The numbers of GlyT2+ boutons/soma increased with age in Egr2; En1CKO mice (P < 0.001), but were consistently 20– 30% lower than control values at P7, P12, P14, and adulthood (6.73 ± 0.23 vs. 7.35 ± 0.20 vs. 7.86 ± 0.18 vs. 8.25 ± 0.16; N = 39–102 soma from 12 to 20 LSO sections/mouse, N = 2 mice/age; Figures 1E–M)
Summary
The pontine superior olivary complex (SOC) is the first central auditory region that receives significant bilateral acoustic information, and it is the primary site for detecting interaural level and timing differences (ILDs and ITDs) critical for sound localization. A critical player in both ILD and ITD detection is the medial nucleus of the trapezoid body (MNTB), which receives glutamatergic input from the contralateral cochlear nucleus (CN) and sends inhibitory projections to ipsilateral SOC nuclei. One major MNTB target is the lateral superior olive (LSO), which participates in ILD processing by integrating these inhibitory inputs with excitatory inputs from the ipsilateral CN (Boudreau and Tsuchitani, 1968; Moore and Caspary, 1983; Spangler et al, 1985; Sanes and Rubel, 1988; Helfert et al, 1989; Bledsoe et al, 1990; Wu and Kelly, 1991, 1992, 1995; Srinivasan et al, 2004). The SPN encodes temporal sound features and has been proposed to function in sound localization, rhythm coding, gap detection and/or as a discontinuity detector (Behrend et al, 2002; Dehmel et al, 2002; Kulesza et al, 2003, 2007; Kadner et al, 2006; Kadner and Berrebi, 2008; Felix et al, 2014)
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