Cochlear protection against noise exposure requires serotonin type 3A receptor via the medial olivocochlear system

The cochlear efferent feedback system plays important roles in auditory processing, including regulation of the dynamic range of hearing, and provides protection against acoustic trauma. These functions are performed through medial olivocochlear (MOC) neurons. However, the underlying cellular and molecular mechanisms are not fully understood. The serotonin type 3A (5-HT3A) receptor is widely expressed throughout the nervous system, which suggests important roles in various neural functions. However, involvement of the 5-HT3A receptor in the MOC system remains unclear. We used mice in this study and found that the 5-HT3A receptor was expressed in MOC neurons that innervated outer hair cells in the cochlea and was involved in the activation of MOC neurons by noise exposure. 5-HT3A receptor knockout impaired MOC functions, potentiated noise-induced hearing loss, and increased loss of ribbon synapses following noise exposure. Furthermore, 5-HT3 receptor agonist treatment alleviated the noise-induced hearing loss and loss of ribbon synapses, which enhanced cochlear protection provided by the MOC system. Our findings demonstrate that the 5-HT3A receptor plays fundamental roles in the MOC system and critically contributes to protection from noise-induced hearing impairment.

Bilirubin Induces Pain Desensitization in Cholestasis by Activating 5-Hydroxytryptamine 3A Receptor in Spinal Cord

Background

Cholestasis patients often suffer from pain desensitization, resulting in serious complications in perioperative period. This study was aim to investigate the mechanism of bilirubin in cholestasis mediating pain desensitization through 5-hydroxytryptamine 3A (5-HT_3A ) receptor activation in spinal dorsal horn (SDH).

Methods

A cholestasis model was established by bile duct ligation (BDL) in rats. Pain thresholds of rats were measured after BDL or intrathecally injecting bilirubin in the presence or absence of agonist (mCPBG) and antagonists (ondansetron, bicuculline, or CGP55845). Expression of 5-HT₃ receptors, and the affinity and binding mode of bilirubin to 5-HT_3A receptor were determined. Effects of bilirubin on γ-aminobutyric acid (GABA) pathway and the interactions with 5-HT_3A receptor were tested.

Results

Bilirubin was elevated significantly in both serum and CSF in BDL rats, accompanied with the up-regulation of pain thresholds. Both of 5-HT_3A receptor and GABA _A receptor antagonists could reverse the increased pain threshold in BDL rats. Further, 5-HT_3A and GABA _A receptor expressions were increased in BDL rats or intervention with bilirubin. Molecular docking suggested that bilirubin entered the hydrophobic pocket pre-formed in 5-HT_3A receptor with potential hydrogen bonding. Bilirubin also increased GABA concentrations in CSF and GABAergic spontaneous inhibitory postsynaptic current in spinal cord, and directly induced inward currents in HEK293 cells which were overexpressed 5-HT_3A receptor by lentivirus.

Conclusion

In conclusion, bilirubin induced pain desensitization in cholestasis by activating 5-HT_3A receptor in spinal cord. The activation of 5-HT_3A receptor might regulate pain threshold by acting on the GABA pathway.

Gene expression of serotonergic markers in peripheral blood mononuclear cells of patients with late-onset Alzheimer's disease

Serotonin or 5-hydroxytryptamine (5-HT) is primarily involved in the regulation of learning and memory. Pathological changes in metabolism or functional imbalance of 5-HT has been associated with Alzheimer's disease (AD). The hypothesis tested is that in peripheral blood, markers of the serotonergic pathway can be used as a diagnostic tool for AD. The current study measured the relative expression of 5-HT receptors (5-HTR2A and 5-HTR3A) as well as the 5-HT catalytic enzyme, Monoamine oxidase A (MAO-A) mRNA in Peripheral Blood Mononuclear Cells (PBMCs) of patients with late-onset Alzheimer's disease (LOAD) and age-matched controls. 5-HTR2A, 5-HTR3A, and MAO-A mRNA expressions were examined in PBMCs of 30 patients with LOAD and 30 control individuals. Real-time quantitative PCR was used to measure mRNA expression. The dementia status of patients in this study was assessed using a Mini-Mental State Examination (MMSE). Mean data of relative mRNA expression of 5-HTR2A, 5-HTR3A and MAO-A were significantly lower in PBMCs of patients with LOAD compared with controls. Based on the down-regulation of serotonergic markers in PBMCs, our findings may be another claim to the systemic nature of LOAD. The role of peripheral serotonergic downregulation, in the pathogenesis of AD, needs to be further studied. Given the extremely convenient access to PBMCs, these molecular events may represent more complete dimensions of AD neuropathophysiology or possibly lead to a new direction in studies focused on blood-based markers.

Function of P2X4 Receptors Is Directly Modulated by a 1:1 Stoichiometric Interaction With 5-HT3A Receptors

Interacting receptors at the neuronal plasma membrane represent an additional regulatory mode for intracellular transduction pathways. P2X4 receptor triggers fast neurotransmission responses via a transient increase in intracellular Ca²⁺ levels. It has been proposed that the P2X4 receptor interacts with the 5-HT₃A receptor in hippocampal neurons, but their binding stoichiometry and the role of P2X4 receptor activation by ATP on this crosstalking system remains unknown. Via pull-down assays, total internal reflection fluorescence (TIRF) microscopy measurements of the receptors colocalization and expression at the plasma membrane, and atomic force microscopy (AFM) imaging, we have demonstrated that P2X4/5-HT₃A receptor complexes can interact with each other in a 1:1 stoichiometric manner that is preserved after ATP binding. Also, macromolecular docking followed by 100 ns molecular dynamics (MD) simulations suggested that the interaction energy of the P2X4 receptor with 5-HT₃A receptor is similar at the holo and the apo state of the P2X4 receptor, and the interacting 5-HT₃A receptor decreased the ATP binding energy of P2X4 receptor. Finally, the P2X4 receptor-dependent Ca²⁺ mobilization is inhibited by the 5-HT₃A interacting receptor. Altogether, these findings provide novel molecular insights into the allosteric regulation of P2X4/5-HT₃A receptor complex in lipid bilayers of living cells via stoichiometric association, rather than accumulation or unspecific clustering of complexes.

Increased levels of 5HT2A receptor mRNA expression in peripheral blood mononuclear cells of patients with major depression: correlations with severity and duration of illness

BACKGROUND

Neuroimaging, immunologic, and pharmacologic studies have emphasized the role of 5-HT2A and 5-HT3A serotonin receptors in the pathophysiology of major depression.

AIM

The aim of this study was to measure the relative expression of 5-HT2A and 5-HT3A receptor mRNA in peripheral blood mononuclear cells (PBMCs) of patients with major depressive disorder (MDD).

METHOD

5-HT2A and 5-HT3A receptor mRNA expressions were examined in PBMCs of 25 medication-naïve-patients with MDD, 25 medication-free MDD patients, and 25 healthy controls. 5-HT2A and 5-HT3A receptor mRNA expressions were measured using real-time quantitative PCR. This study evaluated patients' clinical symptoms using the Hamilton Depression Rating Scale-17 items (HDRS) and the Beck Depression Inventory (BDI).

RESULTS

Relative 5-HTR2A mRNA expression was significantly higher in PBMCs of all MDD patients when compared with healthy controls (Z = -3.875, p < 0.05). However, there was no significant difference in the relative levels of 5-HTR3A mRNA expression in PBMCs of all MDD patients when compared with healthy controls (Z = -1.328, p > 0.05). MDD patients showed significant correlations between 5-HTR2A mRNA expression and HDRS scores (rs = 0.902, p < 0.001) and BDI scores (rs = 0.878, p < 0.001).

CONCLUSION

This study showed that depressed patients, irrespective of treatment, have higher 5-HTR2A mRNA levels in PBMCs than healthy subjects. It also provided evidence that 5-HTR2A mRNA levels in PBMCs of MDD patients could be associated with the severity of depression and the duration of the illness.

Kampo Medicine: Evaluation of the Pharmacological Activity of 121 Herbal Drugs on GABAA and 5-HT3A Receptors

Kampo medicine is a form of Japanese phytotherapy originating from traditional Chinese medicine (TCM). During the last several decades, much attention has been paid to the pharmacological effects of these medical plants and their constituents. However, in many cases, a systematic screening of Kampo remedies to determine pharmacologically relevant targets is still lacking. In this study, a broad screening of Kampo remedies was performed to look for pharmacologically relevant 5-HT_3A and GABA_A receptor ligands. Several of the Kampo remedies are currently used for symptoms such as nausea, emesis, gastrointestinal motility disorders, anxiety, restlessness, or insomnia. Therefore, the pharmacological effects of 121 herbal drugs from Kampo medicine were analyzed as ethanol tinctures on heterologously expressed 5-HT_3A and GABA_A receptors, due to the involvement of these receptors in such pathophysiological processes. The tinctures of Lindera aggregata (radix) and Leonurus japonicus (herba) were the most effective inhibitory compounds on the 5-HT_3A receptor. Further investigation of known ingredients in these compounds led to the identification of leonurine from Leonurus as a new natural 5-HT_3A receptor antagonist. Several potentiating herbs (e.g., Magnolia officinalis (cortex), Syzygium aromaticum (flos), and Panax ginseng (radix)) were also identified for the GABA_A receptor, which are all traditionally used for their sedative or anxiolytic effects. A variety of tinctures with antagonistic effects Salvia miltiorrhiza (radix) were also detected. Therefore, this study reveals new insights into the pharmacological action of a broad spectrum of herbal drugs from Kampo, allowing for a better understanding of their physiological effects and clinical applications.

Identification of Glycyrrhiza as the rikkunshito constituent with the highest antagonistic potential on heterologously expressed 5-HT3A receptors due to the action of flavonoids

The traditional Japanese phytomedicine rikkunshito is traditionally used for the treatment of gastrointestinal motility disorders, cachexia and nausea. These effects indicate 5-HT₃ receptor antagonism, due to the involvement of these receptors in such pathophysiological processes. E.g., setrons, specific 5-HT₃ receptor antagonists are the strongest antiemetics, developed so far. Therefore, the antagonistic effects of the eight rikkunshito constituents at heterologously expressed 5-HT_3Areceptors were analyzed using the two-electrode voltage-clamp technique. The results indicate that tinctures from Aurantii, Ginseng, Zingiberis, Atractylodis and Glycyrrhiza inhibited the 5-HT_3A receptor response, whereas the tinctures of Poria cocos, Jujubae and Pinellia exhibited no effect. Surprisingly, the strongest antagonism was found for Glycyrrhiza, whereas the Zingiberis tincture, which is considered to be primarily responsible for the effect of rikkunshito, exhibited the weakest antagonism of 5-HT_3A receptors. Rikkunshito contains various vanilloids, ginsenosides and flavonoids, a portion of which show an antagonistic effect on 5-HT₃ receptors. A screening of the established ingredients of the active rikkunshito constituents and related substances lead to the identification of new antagonists within the class of flavonoids. The flavonoids (-)-liquiritigenin, glabridin and licochalcone A from Glycyrrhiza species were found to be the most effective inhibitors of the 5-HT-induced currents in the screening. The flavonoids (-)-liquiritigenin and hesperetin from Aurantii inhibited the receptor response in a non-competitive manner, whereas glabridin and licochalcone A exhibited a potential competitive antagonism. Furthermore, licochalcone A acts as a partial antagonist of 5-HT_3A receptors. Thus, this study reveals new 5-HT_3A receptor antagonists with the aid of increasing the comprehension of the complex effects of rikkunshito.