Stress and the Central Nervous System
Mazurek et al. (2015) emphasized that stress causes changes in neuroplasticity. Auditory neural plasticity may be defined as the dynamic changes that occur in the structural and functional characteristics of auditory neurons in response to changes in, or in the significance of, the sound they receive (Irvine 2010). Synaptic plasticity affecting the glutamate postsynaptic system and especially the AMPA and NMDA receptors appears to be regulated by stress (Hubert et al. 2014; Timmermans et al.
- 2013). A stressful acoustic stimulus, such as noise, causes amygdala-mediated release of stress hormones via the HPA-axis, which may have negative effects on the central nervous system (Fig. 5.2). The hippocampus can affect auditory processing by being able to mediate novelty detection. Noise exposure affects hippocampal neurogenesis and LTP in a manner that affects structural plasticity, learning, and memory (Kraus and Canlon 2012). High stress levels at the time of a moderate auditory trauma led to a “tinnitus-specific” central responsiveness, including more severe IHC ribbon loss, reduction of ABR amplitudes, and the decline of Arc/Arg3.1 expression levels in the hippocampal CA1 or auditory cortex (Singer et al. 2013). In contrast, moderate stress levels at the time of trauma could prevent such a tinnitus- specific central response and restore adaptive central responses (Knipper et al.
Nava et al. (2017) studied the time course of acute stress by foot shock on dendritic remodeling within the prelimbic (PL) region of the rodent prefrontal cortex (PFC). They analyzed dendritic length and spine density at 1 day, 7 days, and 14 days after inducing stress. At day 1, they found increased small-spine density and dendritic retraction, together with significant atrophy of apical dendrites. After 7 and 14 days recovery, complete normalization of spine density was observed. Nava et al. (2017) concluded that acute stressors may induce rapid and sustained changes of PL neurons. These changes in the PFC could affect the protective gating effect that was hypothesized to prevent tinnitus (Leaver et al. 2011; next section).