Central Nervous Activity upon Systemic Salicylate Application in Animals with Kanamycin-Induced Hearing Loss--A Manganese-Enhanced MRI (MEMRI) Study

Microglial activation in the lateral amygdala promotes anxiety-like behaviors in mice with chronic moderate noise exposure

BACKGROUND

Long-term non-traumatic noise exposure, such as heavy traffic noise, can elicit emotional disorders in humans. However, the underlying neural substrate is still poorly understood.

METHODS

We exposed mice to moderate white noise for 28 days to induce anxiety-like behaviors, measured by open-field, elevated plus maze, and light-dark box tests. In vivo multi-electrode recordings in awake mice were used to examine neuronal activity. Chemogenetics were used to silence specific brain regions. Viral tracing, immunofluorescence, and confocal imaging were applied to define the neural circuit and characterize the morphology of microglia.

RESULTS

Exposure to moderate noise for 28 days at an 85-dB sound pressure level resulted in anxiety-like behaviors in open-field, elevated plus maze, and light-dark box tests. Viral tracing revealed that fibers projecting from the auditory cortex and auditory thalamus terminate in the lateral amygdala (LA). A noise-induced increase in spontaneous firing rates of the LA and blockade of noise-evoked anxiety-like behaviors by chemogenetic inhibition of LA glutamatergic neurons together confirmed that the LA plays a critical role in noise-induced anxiety. Noise-exposed animals were more vulnerable to anxiety induced by acute noise stressors than control mice. In addition to these behavioral abnormalities, ionized calcium-binding adaptor molecule 1 (Iba-1)-positive microglia in the LA underwent corresponding morphological modifications, including reduced process length and branching and increased soma size following noise exposure. Treatment with minocycline to suppress microglia inhibited noise-associated changes in microglial morphology, neuronal electrophysiological activity, and behavioral changes. Furthermore, microglia-mediated synaptic phagocytosis favored inhibitory synapses, which can cause an imbalance between excitation and inhibition, leading to anxiety-like behaviors.

CONCLUSIONS

Our study identifies LA microglial activation as a critical mediator of noise-induced anxiety-like behaviors, leading to neuronal and behavioral changes through selective synapse phagocytosis. Our results highlight the pivotal but previously unrecognized roles of LA microglia in chronic moderate noise-induced behavioral changes.

The Role of the Brainstem in Generating and Modulating Tinnitus

Purpose The purpose of this work is to present a perspective article summarizing ideas pertaining to the brainstem's role in generating and modulating tinnitus. It is organized in 4 sections: Part 1, the role of the brainstem as a tinnitus generator; Part 2, the role of the brainstem in modulating tinnitus; Part 3, the role of the brainstem in nonauditory comorbid conditions associated with tinnitus; and Part 4, clinical implications. In Part 1, well-established neurophysiological models are discussed providing the framework of evidence that auditory brainstem nuclei play a role in generating tinnitus. In Part 2, ideas are presented explaining modulatory effects on tinnitus related to underlying pathways originating from or projecting to brainstem auditory and nonauditory nuclei. This section addresses multiple phenomena including somatic-related, attention-mediated, and emotion-mediated changes in the tinnitus percept. In Part 3, the role of the brainstem in common nonauditory comorbidities that occur in patients with tinnitus is discussed. Part 4 presents clinical implications of these new ideas related to the brainstem's involvement in generating and modulating tinnitus. Impact Knowledge of the brainstem's involvement in generating and modulating tinnitus provides a context for health care professionals to understand the temporal relationship between tinnitus and common nonauditory comorbid conditions.

In vivo transcranial flavoprotein autofluorescence imaging of tonotopic map reorganization in the mouse auditory cortex with impaired auditory periphery

Ototoxic-drug-induced hearing disturbances in the auditory periphery are associated with tonotopic map reorganization and neural activity modulation, as well as changes in neural correlates in the central auditory pathway, including the auditory cortex (AC). Previous studies have reported that peripheral auditory impairment induces AC plasticity that involves changes in the balance of excitatory vs. inhibitory synapses, within existing and newly forming patterns of connectivity. Although we know that such plastic changes modulate sound-evoked neural responses and the organization of tonotopic maps in the primary AC (A1), little is known about the effects of peripheral impairment on other frequency-organized AC subfields, such as the anterior auditory field (AAF) and the secondary auditory cortex (A2). Therefore, to examine ototoxic-drug-induced spatiotemporal effects on AC subfields, we measured sound-evoked neural activity in mice before and after the administration of kanamycin sulfate (1 mg/g body weight) and bumetanide (0.05 mg/g body weight), using in vivo transcranial flavoprotein autofluorescence imaging over a 4-week period. At first, ototoxic treatment gradually reduced responses driven by tone bursts with lower- (≤8 kHz) and middle- (e.g., 16 kHz) range frequencies in all AC subfields. Subsequently, response intensities in the A1 recovered to more than 78% of the pre-drug condition; however, in the AAF and A2, they remained significantly lower and were unchanged over 3 weeks. Furthermore, after drug administration, the best frequency (BF) areas of the lower (4 and 8 kHz) and higher (25 and 32 kHz) ranges in all subfields were reduced and shifted to those of a middle range (centered around 16 kHz) during the 3 weeks following drug administration. Our results also indicated that, compared with A1, BF distributions in the AAF and A2 were sharper around 16 kHz 3 weeks after drug administration. These results indicate that the ototoxic-damage-induced tonotopic map reorganizations that occurred in each of the three AC subfields were similar, but that there were subfield-dependent differences in the extent of response intensities and in the activated areas that were responsive to tone bursts with specific frequencies. Thus, by examining cortical reorganization induced by ototoxic drugs, we may contribute to the understanding of how this reorganization can be caused by peripheral damage.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.