Background: The extracellular baskets of cochlear inner hair cell (IhC) ribbon synapses have been suggested to regulate synaptic coupling. This study aimed to investigate the expression of components of the extracellular baskets of the IhCs, chondroitin sulfate proteoglycans (CSPGs) and a hyaluronan and proteoglycan link protein 1 (HAPLN1) in the cochlea and auditory nerve. In addition, changes in CSPGs and HAPLN1 in noise-injured cochleae were examined.
Methods: The expression of CSPGs, including aggrecan (ACAN), brevican (BCAN), neurocan (NCAN), and HAPLN1, was evaluated in the cochleae of 2-month-old Sprague-Dawley (SD) rats. The expression of CSPGs and HAPLN1 in cochleae and auditory nerves was compared to that in 3-month-old noise-exposed SD rats during the developmental period. The cochlear immunohistochemistry (IHC) and cochlear whole mount immunofluorescence studies were conducted for ACAN, BCAN, NCAN, and HAPLN1. To examine the large ganglial cells in auditory nerves, IHC was conducted for parvalbumin (PV), glutamate decarboxylase 67 (GAD67), postsynaptic density protein 95 (PSD95), and glial fibrillary acidic protein (GFAP). In situ hybridization was performed for BCAN.
Results: ACAN, BCAN, and HAPLN1 expression was detected in the IhCs and was decreased tendency in noise-injured cochleae. In the spiral ganglial cell (SGC) region, ACAN and NCAN were expressed without the expression of BCAN. In auditory nerves, large ganglionic cells (LGCs) are encased with perineuronal nets (PNNs), which express PV, GAD67, and PSD95. The mRNA expression of BCAN was noted in SGCs and glial cells of auditory nerves.
Conclusions: The extracellular baskets of IhCs revealed the expression of CSPGs and HAPLN1, which was attenuated in noise-exposed cochleae. In auditory nerves, PV-positive LGCs with inhibitory synapses presented PNNs. The protein expression of BCAN was restricted to the extracellular baskets of IhC but not to the SGC region. However, the mRNA expression of BCAN in SGCs was not affected.
Keywords: Brevican; Cochlea; Hearing loss; Noise; Perineuronal nets.