The effects of locally applied kainic acid on cells and fibers in the rat cochlea were examined in a quantitative and ultrastructural study. Doses of 5 nM per microliter of artificial perilymph destroyed part of the spiral ganglion type I cell population, with no ototoxic effects on cochlear hair cells or supporting cells. Type II cells also appeared unaffected. A quantitative evaluation of the cell loss with the 5 nM dosage showed that 34% of spiral ganglion neurons were lost 10 days after treatment. Doses of 20 nM per microliters and 40 nM per microliters did not result in increasing neuronal loss. This differential toxicity could reflect the presence of a sub-population of spiral ganglion cells with an increased number of KA receptors.

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We examined the role of chromium reduction in the Golgi-Colonnier method, correlating the quality of neuronal impregnation with the levels of hexavalent (CrVI) and trivalent (CrIII) chromium in the tissue and in the chromation fluid (CF). The concentrations of both chromium species were assessed by measuring spectrophotometrically the CrVI before and after oxidizing the sample and by calculating the ratio of CrVI to total chromium (chromium ratio, CrR). The CrR was almost identical in the tissue and the CF, decreasing exponentially during chromation due to a progressive consumption of CrVI to form CrIII. Satisfactory cell impregnation was obtained only when the CrR was 0.45-0.7, regardless of other factors. The CrR values could be accurately predicted by the pH increase of the CF; this increase has proven to be a most reliable criterion to decide the endpoint of the chromation process. The dependence of cell staining on the [CrIII], together with the well-known ability of this species to bridge proteins, suggests that the key event for cell impregnation is the cross-linking of neuronal proteins by CrIII polymers.