Blast‐Induced tinnitus and spontaneous firing changes in the rat dorsal cochlear nucleus

Abstract

Exposure to high-pressure blast shock waves is known to cause tinnitus. Although the underlying mechanisms may involve damage to structures in the ear and/or direct brain impact, which triggers a cascade of neuroplastic changes in both auditory and nonauditory centers, it remains unclear how the induced neuroplasticity manifests neurophysiologically. This study investigates the influence of blast exposure on spontaneous firing rates (SFRs) in the dorsal cochlear nucleus (DCN) and its time course in rats with blast-induced tinnitus. Each rat was exposed to a single blast at 22 psi. Behavioral evidence of tinnitus was measured by using a gap-detection acoustic startle-reflex paradigm. SFRs were measured 1 day, 1 month, and 3 months after blast exposure. The results showed that nine rats with blast-induced tinnitus and hearing loss developed hyperactivity immediately and that the induced hyperactivity persisted in six rats with tinnitus at 1 month after blast exposure. At 3 months after blast exposure, however, the induced hyperactivity of four rats with tinnitus transitioned to hypoactivity. In addition, the 20-30-kHz, and >30-kHz regions in the DCN of rats with and without blast-induced tinnitus were more affected than other frequency regions at different recovery time points after blast exposure. These results demonstrate that the neural mechanisms underlying blast-induced tinnitus are substantially different from those underlying noise-induced tinnitus.

Blast-induced tinnitus and hyperactivity in the auditory cortex of rats

“…As shown in our recent study (see Figures 1-2 in (Luo et al, 2014b), there were nine tinnitus positive rats at one day following blast exposure, six tinnitus positive and three tinnitus negative rats at one month after blast, and four tinnitus positive and five tinnitus negative rats at three months after blast. Significant hearing threshold elevations were found in the exposed ears of rats at one day after blast, while rats exhibited threshold recovery at one and three months after blast exposure.…”

Blast Exposure Disrupts the Tonotopic Frequency Map in the Primary Auditory Cortex

“…Quantitative analysis indicates that AI neurons in the blast-exposed animals had higher thresholds than those of the sham-exposed animals (one-way ANOVA, F 1,8 = 5.97, p = 0.040; Figure 3). This is consistent with a high ABR threshold in the blast-compared to the sham-exposed animals (Luo et al, 2014b).…”

Gap-Prepulse Inhibition of the Acoustic Startle Reflex (GPIAS) for Tinnitus Assessment: Current Status and Future Directions

“…Therefore, different scientists used their own criteria to interpret gap detection data and often these criteria can vary dramatically from one lab to the other. Table 1 summarizes the main characteristics of studies that have used the GPIAS paradigm across species, namely, rats ( 12 , 16 – 34 ), mice ( 35 – 41 ), guinea pigs ( 42 – 45 ), gerbils ( 46 ), hamsters ( 47 ), and humans ( 48 – 52 ).…”

“…However, the extensive consequential damage resulting from blast exposure would make the development of such a tinnitus animal model very challenging. For these reasons, this model is only being developed in one laboratory ( 32 – 34 , 56 ). These studies have suggested that after a single or multiple unilateral blast exposures animals typically developed gap detection deficits, which are evident across a wide range of sound frequencies.…”