5-HT(1A) receptor subtype mRNA expression in cochlear nucleus

Antidepressants and risk of sudden sensorineural hearing loss: a population-based cohort study

BACKGROUND

Increasing numbers of animal studies have found that sudden sensorineural hearing loss (SSNHL) is related to the mechanism of serotonergic modulation. However, the relationship between antidepressants and SSNHL is unclear in humans. Therefore, this study aimed to evaluate the association between antidepressant use and risk of SSNHL.

METHODS

Data from 218 466 antidepressant users and 1 116 518 nonusers were obtained from the Taiwan Longitudinal Health Insurance Database. We used propensity-score matching (PSM) and inverse-probability treatment weighting (IPTW) to eliminate any bias. Each patient was tracked for 5 years to ascertain whether or not they were diagnosed with SSNHL. Cox proportional-hazard regression analyses were performed to calculate the SSNHL risk.

RESULTS

The adjusted hazard ratio (aHR) of SSNHL for antidepressant users was 1.36 compared with nonusers in the full cohort study. The aHR for antidepressant users was 1.44 and 1.49 compared with the nonusers in the IPTW and PSM cohorts, respectively. All classes of antidepressants consistently increased the SSNHL risk. Additionally, patients receiving four classes of antidepressants were associated with a much higher SSNHL risk (aHR, 2.05) and those receiving one or two classes of antidepressants had a relatively lower SSNHL risk.

CONCLUSION

Antidepressants increased SSNHL risk, regardless of their class. Furthermore, patients who took a higher number of antidepressant classes showed an increased risk of developing SSNHL than those who took a lower number of antidepressant classes. Therefore, physicians should estimate the risks and benefits of antidepressant use and avoid prescribing antidepressants concurrently.

Serotonergic Modulation of Sensory Representation in a Central Multisensory Circuit Is Pathway Specific

Many studies have explored how neuromodulators affect synaptic function, yet little is known about how they modify computations at the microcircuit level. In the dorsal cochlear nucleus (DCN), a region that integrates auditory and multisensory inputs from two distinct pathways, serotonin (5-HT) enhances excitability of principal cells, predicting a generalized reduction in sensory thresholds. Surprisingly, we found that when looked at from the circuit level, 5-HT enhances signaling only from the multisensory input, while decreasing input from auditory fibers. This effect is only partially explained by an action on auditory nerve terminals. Rather, 5-HT biases processing for one input pathway by simultaneously enhancing excitability in the principal cell and in a pathway-specific feed-forward inhibitory interneuron. Thus, by acting on multiple targets, 5-HT orchestrates a fundamental shift in representation of convergent auditory and multisensory pathways, enhancing the potency of non-auditory signals in a classical auditory pathway.

Cervical spine disorders and its association with tinnitus: The "triple" hypothesis

Subjective tinnitus and cervical spine disorders (CSD) are among the most common complaints encountered by physicians. Although the relationship between tinnitus and CSD has attracted great interest during the past several years, the pathogenesis of tinnitus induced by CSD remains unclear. Conceivably, CSD could trigger a somatosensory pathway-induced disinhibition of dorsal cochlear nucleus (DCN) activity in the auditory pathway; furthermore, CSD can cause inner ear blood impairment induced by vertebral arteries hemodynamic alterations and trigeminal irritation. In genetically -predisposed CSD patients with reduced serotoninergic tone, signals from chronically stimulated DCNs could activate specific cortical neuronal networks and plastic neural changes resulting in tinnitus. Therefore, an early specific tailored CSD treatments and/or boosting serotoninergic activity may be required to prevent the creation of 'tinnitus memory circuits' in CSD patients.

Serotonergic regulation of excitability of principal cells of the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is one of the first stations within the central auditory pathway where the basic computations underlying sound localization are initiated and heightened activity in the DCN may underlie central tinnitus. The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT), is associated with many distinct behavioral or cognitive states, and serotonergic fibers are concentrated in the DCN. However, it remains unclear what is the function of this dense input. Using a combination of in vitro electrophysiology and optogenetics in mouse brain slices, we found that 5-HT directly enhances the excitability of fusiform principal cells via activation of two distinct 5-HT receptor subfamilies, 5-HT2A/2CR (5-HT2A/2C receptor) and 5-HT7R (5-HT7 receptor). This excitatory effect results from an augmentation of hyperpolarization-activated cyclic nucleotide-gated channels (Ih or HCN channels). The serotonergic regulation of excitability is G-protein-dependent and involves cAMP and Src kinase signaling pathways. Moreover, optogenetic activation of serotonergic axon terminals increased excitability of fusiform cells. Our findings reveal that 5-HT exerts a potent influence on fusiform cells by altering their intrinsic properties, which may enhance the sensitivity of the DCN to sensory input.

Context-dependent modulation of auditory processing by serotonin

Context-dependent plasticity in auditory processing is achieved in part by physiological mechanisms that link behavioral state to neural responses to sound. The neuromodulator serotonin has many characteristics suitable for such a role. Serotonergic neurons are extrinsic to the auditory system but send projections to most auditory regions. These projections release serotonin during particular behavioral contexts. Heightened levels of behavioral arousal and specific extrinsic events, including stressful or social events, increase serotonin availability in the auditory system. Although the release of serotonin is likely to be relatively diffuse, highly specific effects of serotonin on auditory neural circuitry are achieved through the localization of serotonergic projections, and through a large array of receptor types that are expressed by specific subsets of auditory neurons. Through this array, serotonin enacts plasticity in auditory processing in multiple ways. Serotonin changes the responses of auditory neurons to input through the alteration of intrinsic and synaptic properties, and alters both short- and long-term forms of plasticity. The infrastructure of the serotonergic system itself is also plastic, responding to age and cochlear trauma. These diverse findings support a view of serotonin as a widespread mechanism for behaviorally relevant plasticity in the regulation of auditory processing. This view also accommodates models of how the same regulatory mechanism can have pathological consequences for auditory processing.

Subjective tinnitus, temporomandibular joint dysfunction, and serotonin modulation of neural plasticity: causal or casual triad?

Tinnitus and temporomandibular joint dysfunction (TMJD) are among the most common complaints encountered by physicians. Though the relationship between tinnitus and TMJD has attracted great interest during the past several years, theories attempting to explain this association are still few and inconsistent. Conceivably, TMJD could irritate auricolo-temporal nerve (ATN), triggering a somatosensory pathway-induced disinhibition of dorsal cochlear nucleus (DCN) activity in the auditory pathway. In genetically-predisposed TMJD patients, signals from cronically stimulated DCNs activating specific cortical neuronal networks, could yield plastic neural changes resulting in tinnitus. Based on current evidence of serotoninergic modulation of neural activity and plasticity in sensory pathways, reduced serotoninergic tone could promote plastic changes underlying tinnitus through diminished filtering of incoming signals. Therefore, the early establishment of specific treatments aimed at improving TMJD and/or boosting serotoninergic activity may be required to prevent the creation of 'tinnitus memory circuits'.