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In this study we investigate temporal changes in Fos expression in cuneate neurons after a high-threshold electrical stimulation of the transected median nerve in rats. Two hours after injury of the median nerve when given electrical stimulation, c-Fos-immunoreactive (c-Fos-IR) cells were barely detected in the ipsilateral cuneate nucleus (CN). A few c-Fos-IR cells, however, were observed in the ipsilateral CN at 5 days. A marked increase in c-Fos-IR cells was observed at 2, 3, and 4 weeks, but levels subsided thereafter. Labeled cells were totally diminished by 16 weeks. The statistical analysis showed that the mean density of c-Fos-IR cells throughout the CN at 4 weeks was significantly higher than at other post-surgical time points, except for 3 weeks. Furthermore, the mean density of c-Fos-IR cells in the middle region of the CN was markedly higher than in other areas of the nucleus. The mean density of c-Fos-IR cells in the middle region at 4 weeks (mean density = 35.9 +/- 3.0 cells/section) was considerably higher than at other time points. Combined retrograde Fluorogold (FG) labeling and c-Fos immunocytochemistry showed that throughout the CN about 60% (2270/3652) of the c-Fos-IR cells contained FG, confirming that they were cuneothalamic projection neurons (CTNs). Moreover, the percentage of double-labeled cells in the middle region at 2 weeks (78.9 +/- 0.6%) was significantly greater than at 3 (70.2 +/- 3.4%) and 4 weeks (66.0b +/- 1.4%) after injury. Although the mechanism leading to the vigorous c-Fos expression in the CTNs following the electrical stimulation of the transected median nerve remains unclear the hyperexcitable CTNs may transmit the neuropathic nociceptive sensation to the thalamus after the median nerve injury.

This study applied the vestibular evoked myogenic potential (VEMP) test to guinea pigs coupled with electronic microscopic examination to determine whether VEMPs are dependent on type I or II hair cell activity of the saccular macula. An amount of 0.05 ml of gentamicin (40 mg/ml) was injected directly overlaying, but not through, the round window membrane of the left ear in guinea pigs.One week after surgery, auditory brainstem response test revealed normal responses in 12 animals (80%), and elevated thresholds in 3 animals (20%). The VEMP test using click stimulation showed absent responses in all 15 animals (100%). Another 6 gentamicin-treated animals underwent the VEMP test using galvanic stimulation and all 6 also displayed absent responses. Ultrathin sections of the saccular macula in the gentamicin-treated ears displayed morphologic alterations in type I or II hair cells, including shrinkage and/or vacuolization in the cytoplasm, increased electron density of the cytoplasm and nuclear chromatin, and cellular lucency. However, extrusion degeneration was rare and only present in type II hair cells. Quantitative analysis demonstrated that the histological density of intact type I hair cells was 1.1 ± 1.2/4000 μm² in the gentamicin-treated ears, showing significantly less than that in control ears (4.5 ± 1.8/4000 μm²). However, no significant difference was observed in the densities of intact type II hair cells and supporting cells between treated and control ears. Furthermore, the calyx terminals surrounding the damaged type I hair cells were swollen and disrupted, while the button afferents contacting the damaged type II hair cells were not obviously deformed. Based on the above results, we therefore conclude that VEMPs are heavily dependent on type I hair cell activity of the saccular macula in guinea pigs.

To investigate whether the saccule exhibits temporary or permanent functional loss resembling threshold shifts in auditory brainstem response (ABR) of guinea pigs following noise exposure.Design: Randomly bred guinea pigs were divided into 3 groups: A (short-term noise exposure, 30 minutes, n=15), B (long-term noise exposure, 40 hours, n=9), and C (no noise exposure, n = 5).Setting: University hospital. Main Outcome Measures: All animals underwent vestibular-evoked myogenic potential (VEMP) and ABR tests. Chronological changes of VEMP and ABR responses following noise exposure were analyzed and compared. After audiovestibular function testing, animals were killed for morphological study with light and electron microscopy. Results: In group A, temporary VEMP loss and ABR METHODS ANIMAL PREPARATIONRandomly bred Hartley-strain guinea pigs weighing 250 to 300 g were housed at a mean