Long-term moderate noise exposure enhances the medial olivocochlear reflex

Characterization of Anxiety-Like Behaviors and Neural Circuitry following Chronic Moderate Noise Exposure in Mice

BACKGROUND

Commonly encountered nontraumatic, moderate noise is increasingly implicated in anxiety; however, the neural substrates underlying this process remain unclear.

OBJECTIVES

We investigated the neural circuit mechanism through which chronic exposure to moderate-level noise causes anxiety-like behaviors.

METHODS

Mice were exposed to chronic, moderate white noise [85 decibel (dB) sound pressure level (SPL)], 4 h/d for 4 wk to induce anxiety-like behaviors, which were assessed by open field, elevated plus maze, light-dark box, and social interaction tests. Viral tracing, immunofluorescence confocal imaging, and brain slice patch-clamp recordings were used to characterize projections from auditory brain regions to the lateral amygdala. Neuronal activities were characterized by in vivo multielectrode and fiber photometry recordings in awake mice. Optogenetics and chemogenetics were used to manipulate specific neural circuitry.

RESULTS

Mice chronically (4 wk) exposed to moderate noise (85 dB SPL, 4 h/d) demonstrated greater neuronal activity in the lateral amygdala (LA), and the LA played a critical role in noise-induced anxiety-like behavior in these model mice. Viral tracing showed that the LA received monosynaptic projections from the medial geniculate body (MG) and auditory cortex (ACx). Optogenetic excitation of the MG → LA or ACx → LA circuits acutely evoked anxiety-like behaviors, whereas their chemogenetic inactivation abolished noise-induced anxiety-like behavior. Moreover, mice chronically exposed to moderate noise were more susceptible to acute stress, with more neuronal firing in the LA, even after noise withdrawal.

DISCUSSION

Mice exposed to 4 wk of moderate noise (85 dB SPL, 4 h/d) demonstrated behavioral and physiological differences compared to controls. The neural circuit mechanisms involved greater excitation from glutamatergic neurons of the MG and ACx to LA neurons under chronic, moderate noise exposure, which ultimately promoted anxiety-like behaviors. Our findings support the hypothesis that nontraumatic noise pollution is a potentially serious but unrecognized public health concern. https://doi.org/10.1289/EHP12532.

The role of medial olivocochlear activity in contralateral suppression of auditory steady-state responses

OBJECTIVE

The auditory steady-state response (ASSR) amplitudes fall in the presence of contralateral noise. However, whether and to what extent medial olivocochlear (MOC) activity involves in contralateral suppression of ASSR remain unclear. Therefore, we assess the role of MOC activity in contralateral suppression of ASSR.

METHODS

Mice were treated with strychnine to completely eliminate MOC activity and then measured ASSR amplitudes in the presence of contralateral noise.

RESULTS

The contralateral noise reduces ASSR amplitudes at some stimulus intensity. After treating with the strychnine to eliminate MOC activity, ASSR amplitudes recovered again.

CONCLUSIONS

MOC activity participated in contralateral suppression of ASSR.

The purinergic receptors 2X3 on spiral ganglion neurons enhance the medial olivocochlear reflex in mice after long-term moderate noise exposure

Our purpose was to study the expression of purinergic receptors 2X2 (P2X2) and purinergic receptors 2X3 (P2X3) in spiral ganglion neurons (SGNs), the afferent nerves of medial olivocochlear (MOC) reflex, after long-term moderate noise exposure, and its relationship with the enhancement of MOC reflex. Mice were exposed a moderate broadband noise for 4 weeks consecutively. Then mouse hearing functions, including threshold auditory brainstem responses, distortion-product otoacoustic emissions, and MOC reflex, were evaluated and the expression of P2X2 and P2X3 on SGNs were assessed by cochlear immunofluorescence. AF-353 was injected before each noise exposure. Four weeks later, mice were also tested for hearing functions and expression of P2X2 and P2X3 on SGNs. The long-term moderate noise strengthened MOC reflex, and AF-353 reduced it in mice and P2X3 expression on SGNs increased after long-term moderate noise exposure, and AF-353 can downregulate it. The P2X3 on SGNs of mice increased after long-term moderate noise exposure, and the upregulation of it mediated the enhancement of MOC reflex.

Contralateral inhibition of distortion product otoacoustic emissions in young noise-exposed Veterans

Although animal models show a clear link between noise exposure and damage to afferent cochlear synapses, the relationship between noise exposure and efferent function appears to be more complex. Animal studies indicate that high intensity noise exposure reduces efferent medial olivocochlear (MOC) reflex strength, whereas chronic moderate noise exposure is associated with a conditioning effect that enhances the MOC reflex. The MOC reflex is predicted to improve speech-in-noise perception and protects against noise-induced auditory damage by reducing cochlear gain. In humans, MOC reflex strength can be estimated by measuring contralateral inhibition of distortion product otoacoustic emissions (DPOAEs). The objective of this study was to determine the impact of military noise exposure on efferent auditory function by measuring DPOAE contralateral inhibition in young Veterans and non-Veterans with normal audiograms. Compared with non-Veteran controls, Veterans with high levels of reported noise exposure demonstrated a trend of reduced contralateral inhibition across a broad frequency range, suggesting efferent damage. Veterans with moderate noise exposure showed trends of reduced inhibition from 3 to 4 kHz but greater inhibition from 1 to 1.5 kHz, consistent with conditioning. These findings suggest that, in humans, the impact of noise exposure on the MOC reflex differs depending on the noise intensity and duration.