Glycol Methacrylate Embedding and Microwave Staining for Light Microscopy of the Mouse Cochlea

This study examined the utility of a methacrylate-based embedding medium and microwave staining for light microscopic quantification of hair cells and spiral ganglion cells in the mouse cochlea. The most important phase of the preparation process involved slowing down the polymerization process. The tissue molecules so locked within the plastic matrix produced excellent preservation of the organ of Corti and adjacent structures including the spiral ganglion, as well as tissue ionic charges. Excitable by microwaves, these ionic charges accelerated the movement of the basic dye (hematoxylin) into the tissue, reducing the time for this segment of the staining process from approximately 45 minutes to 1-2 minutes. When embedded in glycol methacrylate (GMA), acidic dyes show less stain-cell affinity so that staining intensity and time cannot be improved significantly. However, addition of color extenders to the counterstain eosin produced distinguished staining of all tissue constituents. Thus, a combination of GMA embedding medium, use of the microwave for staining and addition of color extenders to the counterstain generated excellent structural resolution and contrast. This made both hair cell and spiral ganglion cell counts possible from within the same specimen and provided an opportunity for qualitative evaluation as well

AP threshold elevation in the guinea pig following exposure to a broadband noise

Sixty guinea pigs were exposed to a steady-state broadband noise with a falling frequency spectrum. The sound-pressure level was varied between 96 and 117 dB SPL, and the duration of the exposure was varied from 3 to 12 h. After 4-5 weeks, the auditory thresholds were determined by electrocochleography at 14 frequencies, and the results were compared with a control group. With increasing sound-pressure level, the thresholds became elevated at all frequencies. The maximum threshold elevation also exhibited a slight shift toward higher frequencies. With increasing exposure time, the threshold elevations increased and shifted into the high-frequency region, whereas the low-frequency region was less affected. Linear regression analysis showed that the average threshold elevation between 1 and 20 kHz did not deviate from that predicted by the equal-energy hypothesis. However, the high-frequency loss at 5-20 kHz was very dependent on the exposure time, whereas the 1- to 4-kHz loss was not. There was no sign of any critical intensity with sudden increments in threshold elevation

Right-left correlation in guinea pig ears after noise exposure

In order to investigate whether experimental material consisting of noise-exposed animals should be regarded as 'ears' or 'animals', a comparison was made between the permanent threshold shifts in the right and left ears in 53 groups of noise-exposed guinea-pigs, with 5 animals in each group. The action potential (N1) thresholds were measured at fourteen frequencies in both ears. In the noise-exposed animals, the threshold elevations were up to 40 dB. The average right-left correlation coefficient within each group was about 0.79, regardless of noise energy. The degree of correlation was significantly greater after impact noise than after continuous noise. As a consequence of the high right-left correlation, the informative value of measuring a second ear in the same animal was, in our experiments, only 11% of that of the first one. No correlation could be found between the degree of skin pigmentation and the threshold elevation

Noise-induced threshold elevation as a function of peak sound pressure level

Thirty-three groups of guinea pigs, consisting of five animals in each group, were exposed to a simulated impact noise with peak levels ranging between 119.5 and 134.5 dB SPL. By varying the repetition rate, different equivalent levels could be set at each peak level. The equivalent levels ranged from 96 to 117 dB SPL, and the exposure duration was 1.5 to 24 hours. The compound action potential thresholds were measured in 1/3-octave steps between 1 and 20 kHz, one month after the exposure. Higher peak levels resulted in a peak-shaped threshold elevation with a maximum around 8 kHz. For constant peak levels, the equal energy theory was supported. For exposures of equal energy but different peak levels, significantly higher threshold elevations resulted after exposure to higher peak levels