Noise trauma impairs neurogenesis in the rat hippocampus

Abstract

The hippocampus, a major site of neurogenesis in the adult brain, plays an important role in memory. Based on earlier observations where exposure to high-intensity noise not only caused hearing loss but also impaired memory function, it is conceivably that noise exposure may suppress hippocampal neurogenesis. To evaluate this possibility, nine rats were unilaterally exposed for 2 h to a high-intensity, narrow band of noise centered at 12 kHz at 126 dB SPL. The rats were also screened for noise-induced tinnitus, a potential stressor which may suppress neurogenesis. Five rats developed persistent tinnitus-like behavior while the other four rats showed no signs of tinnitus. Age-matched sham controls showed no signs of hearing loss or tinnitus. The inner ear and hippocampus were evaluated for sensory hair cell loss and neurogenesis 10 weeks post-exposure. All noise exposed rats showed severe loss of sensory hair cells in the noise-exposed ear, but essentially no damage in the unexposed ear. Frontal sections from the hippocampus were immunolabeled for doublecortin to identify neuronal precursor cells, or Ki67 to label proliferating cells. Noise-exposed rats showed a significant reduction of neuronal precursors and fewer dividing cells as compared to sham controls. However, we could not detect any difference between rats with behavioral evidence of tinnitus versus rats without tinnitus. These results show for the first time that high intensity noise exposure not only damages the cochlea but also causes a significant and persistent decrease in hippocampal neurogenesis that may contribute to functional deficits in memory.

Figure 1

Gap prepulse-induced inhibition of the acoustic startle (GPIAS) paradigm used for tinnitus screening in rats after noise trauma: A noise burst at 100 dB SPL, embedded in a continuous noise floor of moderate SPL (60 dB), results in an acoustic startle response (A). When the background noise floor contains a short gap immediately before the startle-eliciting stimulus, the amplitude of the startle is reduced (B) showing that the rat is able to detect the silent gap by inhibiting its response to the subsequent startle stimulus. A rat with tinnitus however, shows no difference in startle amplitudes in the “no-gap” (C) and “gap” (D) condition, since the perceived phantom sound fills the gap and thus prevents the rat from detecting the silent gap thereby eliminating pre-pulse inhibition of the startle response.

Figure 2

Cochleograms showing degree of outer hair cell (OHC, dashed line) and inner hair cell (IHC, solid line) loss as function of percent distance from the apex of the noise exposed cochlea (126 dB, 100 Hz narrowband noise centered at 12 kHz, 2 hours) in nine rats allowed to survive for 10 weeks. All nine Noise Trauma rats showed severe loss of both IHC and OHC throughout most of the cochlea except for the most apical, low-frequency region. OHC loss was greater than IHC loss.

Figure 3

Gap pre-pulse induced inhibition of the acoustic startle (GPIAS) in Sham Controls (open symbols) and Noise Trauma rats (filled symbols): For each rat, frequency, and testing period, the diagrams show the average amplitude at the “gap” condition, relative to the average amplitude at the “no-gap” condition, which was normalized and set to 1 (horizontal dashed line). In case of gap detection, the average amplitude at the “gap” condition was significantly smaller than the amplitude at the “no gap” condition (inhibition). Circles: Before any treatment, all rats showed significant inhibition at the “gap” condition at all frequencies tested (“Baseline”). Average inhibition ranged from approximately 30% to 60% and varied among individual rats, but was significant at all frequencies tested for all rats. Triangles: At days 1 – 10 after noise exposure, five Noise Trauma rats (Rat C1, D1, C3, A4 and B4) showed changes in gap detection. At one or more frequencies tested, these rats now showed no significant difference in amplitudes at “no-gap” conditions and “gap” conditions (“T”), demonstrating no or only very poor gap-detection and thus signs of severe tinnitus. Some of these rats showed a significant increase of amplitudes (facilitation) which may be related to increased stimulus aversiveness. The other four Noise Trauma rats (B1, D3, E and F), along with age-matched Sham Controls (A, BB and CC), continued showing significant inhibition at all frequencies. Diamonds: When rats were re-tested at week 8-10 after the noise exposure, the five Noise Trauma rats with tinnitus-like behavior during days 1-10 again showed evidence of tinnitus (bold “T”) and facilitation. The other four Noise Trauma rats and Sham Controls continued showing significant inhibition at all frequencies.

Figure 4

Neuronal precursor cells immunostained for DCX in the adult rat hippocampus in a normal hearing rat (A) and a rat with severe noise trauma 10 weeks after the noise exposure (B). A: In the normal rat, cell bodies of neuronal precursor cells (arrowheads) form a thin line along the subgranular cell zone (SGZ) in the dentate gyrus (DG). B: The noise exposed rat showed a strongly reduced number of neuronal precursor cells. Arrowheads point to some of the few remaining cells. C, D: Single DCX neuronal precursor cells shown at high magnification in the normal hearing rat (C) and in the rat with noise trauma (D). Scale bars: 200 μm in Fig. 4B for Fig. 4A, B; 50 μm in Fig. 4D for Fig. 4C, D.

Figure 5

Dividing cells immunostained for Ki67 in the adult rat hippocampus in a normal hearing rat (A, C, D) and a rat with severe noise trauma 10 weeks after the noise exposure (B, E, F). A, B: In the normal rat as well as the noise-exposed rat, Ki67-immunopositive nuclei (arrowheads) are present in the subgranular cell zone (SGZ) in the dentate gyrus (DG). Ki67 immunopositive cells were typically clustered in pairs or small groups in normal hearing rats (5C, D) as well as in noise exposed rats (5E, F). Scale bars: 200 μm in Fig. 5B for Fig. 5A, B; 50 μm in Fig. 5C for Fig. 5C-F.

Figure 6

Reduction of density of DCX labeled cells (A) and Ki67 labeled cells (B) in the hippocampus after noise trauma. Numbers were normalized to average number of cells in Sham Controls (white bars), which was set to 1. Noise Exposed rats (black bars) showed a strong and significant reduction in numbers of DCX positive cells as well as Ki67 positive cells.

Figure 7

DCX (A) and Ki67 (B) cell density in the SGZ after Noise Trauma in rats without signs of tinnitus and in rats with signs of tinnitus: We were not able to detect any difference in cell numbers between the two groups.