Glutamate-like immunoreactivity during hair cell recovery after gentamicin exposure in the chinchilla vestibular sensory periphery

Synaptophysin immunohistochemistry during vestibular hair cell recovery after gentamicin treatment

In the present study, morphometric and immunohistochemical techniques were used to evaluate the degree of synaptic recovery in the chinchilla crista sensory epithelia during various post-gentamicin-treatment periods of hair cell loss and recovery. For this purpose, two groups of animals were treated with Gelfoam pellets impregnated with 50 micro g of gentamicin implanted in the perilymphatic space within the otic capsule of the superior semicircular canal. Animals were sacrificed 1, 2 and 4 weeks after treatment. The degree of synaptic reinnervation was evaluated in the horizontal crista of the first group of animals using immunohistochemical techniques and antibodies against synaptophysin, a marker for synaptic reinnervation and synaptogenesis. Quantification of immunoreactivity in this group was made in the mid-region of the crista using the NIH 'Image' program. The second group of animals was used for quantification of the number of hair cells and supporting cells in the horizontal crista. In the normal sensory epithelium, synaptophysin immunoreactivity was found in the areas corresponding to the known distribution of afferent and efferent nerve terminals. Immunoreactivity was predominantly located within the afferent calyces of type I hair cells. No immunoreactivity was found in the supporting cells. Seven days after treatment there was a significant loss of hair cells and synaptophysin-stained area (SSA). In the mid-region of the crista the loss of synaptophysin immunoreactivity was quantitatively the greatest within the central zone of this region (93%) while the loss of hair cells was the smallest. These results suggest that afferent and efferent nerve terminals were also severely affected by the ototoxic treatment. Four weeks after treatment corresponding to the end of the recovery phase of gentamicin ototoxicity, there was a proportional increase in the number of hair cells and of the degree of SSA in the mid-region of the crista. The number of hair cells recovered to 58% with a recovery of SSA to 54% of normal. These results suggest that a greater fraction of synaptophysin expression within the sensory epithelium depends on the presence of afferent calyceal endings, which are greatly affected by gentamicin. Also, these results demonstrate a significant level of reinnervation of the newly regenerated hair cells, forecasting the potential for functionality of the regenerated hair cells.

Subcellular immunolocalization of NMDA receptor subunit NR-1 in the chinchilla vestibular periphery

The immunohistochemical localization of N-methyl-D-aspartate (NMDA) glutamate receptor subunit, NR-1 was investigated in the chinchilla cristae ampullaris and utricular maculae at the light and electron microscopy level with the use of specific antipeptide antibodies. The afferent calyces that innervate type I hair cell, and the basolateral type I vestibular hair cell is NR-1 immunoreactive. The afferent boutons innervating type II hair cells and the basal portion of type II hair cell are NR-1 non-immunoreactive. These findings are consistent with NMDA receptor mediation of afferent excitatory neurotransmission from type I, but not type II hair cells to the primary afferent vestibular nerve. The NMDA receptors on the type I hair cell are located in areas of synaptic specialization, and may play a role in autoregulation. The localization of the NMDA receptor subunit in type I but not type II hair cells is intriguing.