Mu-opioid receptor (MOR) expression in the human spiral ganglia

Opioid receptor activation modulates the calcium current in the cochlear outer hair cells of the rat

Previous works showed that opioid peptides are produced by olivocochlear efferent neurons, while cochlear hair cells express opioid receptors. It has been proposed that opioids protect the auditory system from damage by intense stimulation, although their use for therapeutic or illicit purposes links to hearing impairment. Therefore, it is relevant to study the effect of opioids in the auditory system to define their functional expression and mechanism of action. This study investigated the modulation of the Ca²⁺ currents by opioid peptides in the rat outer hair cells (OHC) using the whole-cell patch-clamp technique. The influence of agonists of the three opioid receptor subtypes (μ, δ, and κ) was studied. The κ opioid receptor agonist U-50488 inhibits the Ca²⁺ currents in a partially reversible form. Coincidently, norbinaltorphimine (a κ receptor antagonist) blocked the U-50488 inhibitory effect on the Ca²⁺ current. The δ- and the μ opioid receptor agonists did not significantly affect the Ca²⁺ currents. These results indicate that the κ opioid receptor activation inhibits the Ca²⁺ current in OHC, modulating the intracellular Ca²⁺ concentration when OHCs depolarize. The modulation of the auditory function by opioids constitutes a relevant mechanism with a potential role in the physiopathology of auditory disturbances.

Drug-Induced Deafness: A Rare Case of Bilateral Sensorineural Hearing Loss Following Speedballing

Many users of recreational drugs use cocaine and opioids together, often called “speedballing.” Hearing loss is a rarely reported adverse effect following recreational drug abuse. Only one case has been reported in history with hearing loss caused by speedballing. Here, we present the case of a 38-year-old female who presented with speedball abuse and new-onset bilateral hearing loss to the emergency department. A computed tomography scan of the head was unremarkable. She was treated with thiamine, folate, multivitamins, and intravenous fluids. The hearing loss improved without any acute intervention. The significance of sudden hearing loss due to recreational drug use is highlighted by this case. Apart from a few animal studies, there is no detailed research explaining the pathophysiology of speedball-induced hearing loss. Further studies and trials are needed to better understand the effects of combined and separate cocaine and opioid use on audiologic physiology.

Diverse identities and sites of action of cochlear neurotransmitters

Accurate encoding of acoustic stimuli requires temporally precise responses to sound integrated with cellular mechanisms that encode the complexity of stimuli over varying timescales and orders of magnitude of intensity. Sound in mammals is initially encoded in the cochlea, the peripheral hearing organ, which contains functionally specialized cells (including hair cells, afferent and efferent neurons, and a multitude of supporting cells) to allow faithful acoustic perception. To accomplish the demanding physiological requirements of hearing, the cochlea has developed synaptic arrangements that operate over different timescales, with varied strengths, and with the ability to adjust function in dynamic hearing conditions. Multiple neurotransmitters interact to support the precision and complexity of hearing. Here, we review the location of release, action, and function of neurotransmitters in the mammalian cochlea with an emphasis on recent work describing the complexity of signaling.

Opioid-Associated Hearing Loss: A 20-Year Review from the New Jersey Poison Center

BACKGROUND

Opioid-associated ototoxicity is a known complication of opioid exposure, although the mechanism remains unclear. While historically most closely linked to heroin and oxycodone, evolving reports suggest that it may be a class effect of opioids. However, the evidence is limited to case reports.

METHODS

A retrospective review of the New Jersey Poison Center records (ToxiCALL®) identified cases that included both hearing loss and recent opioid exposure between January 1, 1999, and September 21, 2018.

RESULTS

Forty-one cases were identified, mean age 29.4 years, 51% (n = 21) were male. Reported heroin exposures comprised 51% (n = 22), 18 of which were heroin alone. The next most commonly cited opioids were oxycodone (n = 7), methadone, (n = 4), and tramadol (n = 3). Hearing loss was described as tinnitus in 24% of cases, hypoacusis in 37% of cases, deafness in 29% of cases, and mixed tinnitus/hypoacusis in 10% of cases. Only 34% (n = 14) of cases were associated with a potential hypoxic event. Of the cases that documented resolution data, 21% (n = 4 of 19) reported no improvement at time of hospital discharge.

DISCUSSION

Opioid-associated ototoxicity appears to be a hypoxia-independent adverse effect since most of the reported cases did not involve a known contributory hypoxic event. It occurs with a wide array of opioids, which supports an opioid receptor-mediated mechanism. The ototoxic effect may be self-limited in many patients.

CONCLUSION

Opioid-associated ototoxicity was most commonly associated with heroin exposure and appeared independent of hypoxic events. Further investigation that clarifies the risk factors and long-term outcomes is needed.

Early Diagnosis of Hearing Loss in Patients Under Methadone Maintenance Treatment

Background and Objective: Methadone maintenance treatment (MMT) as the most effective treatment for opioid addictions could induce both reversible and permanent hearing loss. Therefore, early detection of methadone-induced hearing loss is necessary to prevent irreversible cochlear damage. The present study aims to identify the early onset of hearing loss in patients who underwent MMT and to compare them with the age and gender matched normal hearing peers. Methods: This was an analytic cross-sectional study conducted on patients (n = 27 males; age range: 18-53 years old) who received 3 months MMT course (MMT group) and a control group consisting of age and gender matched healthy individuals (n = 27 males). Before MMT, all patients underwent conventional audiometry (250-8,000 Hz) and those with normal hearing threshold participated into the study. One month after MMT termination, the patients were assessed for possible hearing loss using conventional pure tone audiometry (PTA), extended high frequency (EHF) audiometry, and distortion product otoacoustic emissions (DPOAEs). Results: Our results demonstrated that the mean EHF thresholds in the MMT patients were significantly greater than the age- and gender-matched healthy controls across all frequencies (p < 0.001). However, there was no statistically significant difference in conventional PTA thresholds between both groups (p > 0.05). DPOAE amplitudes significantly reduced at higher frequencies (3,000-8,000 Hz) in the MMT group, compared to the healthy control group. In contrast to the conventional PTA audiometry, the EHF and DPOAE assessments identified hearing impairments in 11 (40.74%), and 14 (51.85%) of the MMT patients, respectively. The main mechanisms proposed for methadone induced hearing loss are cochlear ischemia following vasospasm or vasculitis, direct effect of opioids on opioid receptors present in cochlear stria vascularis of inner ear, blood-labyrinth selective transport of opioidproteins and receptors, and genetic polymorphism and mutations. Conclusion: The EHF and DPOAE tests have the potential to detect earlier changes in auditory function than conventional frequency audiometry in the MMT patients.

Immunohistochemical localization of Nrf2 in the human cochlea

Oxidative stress plays an important role in several inner ear diseases and normal aging. Nuclear (erythroid-derived 2)-like 2, also known as Nrf2, is a transcription factor encoded by the NFE2L2 gene that controls the expression of genes necessary to reduce oxidative stress. There are no studies to the date on the localization of Nrf2 in the human inner ear in normal or pathological conditions. Therefore, we investigated the immunohistochemical localization of Nrf2 in the human cochlea and vestibule using formalin-fixed celloidin-embedded human temporal bone sections. Nrf2 immunoreactivity (IR) was found in the inner and outer hair cells and supporting cells of the organ of Corti throughout the cochlea. Nfr2-IR was also found in hair cells and supporting cells of the maculae and cristae vestibular sensory epithelia. Nrf2-IR was decreased in the organ of Corti of older age individuals. The immunolocalization of Nrf2 in both auditory and vestibular sensory epithelia suggest that this transcription factor may play a relevant role in protecting sensory hair cells from oxidative stress.

Connexin 26 Immunohistochemistry in Temporal Bones With Cochlear Otosclerosis

HYPOTHESIS

Connexin-26 (Cx26) expression is diminished in the spiral ligament of subjects with hearing loss and cochlear otosclerosis (CO).

BACKGROUND

Human temporal bone (HTB) studies have demonstrated that CO is associated with hyalinization of the spiral ligament. We hypothesize that hyalinization is associated with a loss of fibrocytes with a consequent decline in Cx26 expression. Cx26 and Connexin-30 (Cx30) encode gap junction proteins expressed in supporting cells of the organ of Corti, the spiral limbus, stria vascularis, and in fibrocytes of the spiral ligament. These gap junctions are critical for potassium recycling and maintenance of the endocochlear potential. Diminished expression of these proteins would likely be associated with hearing dysfunction.

METHODS

Histopathology and clinical characteristics of 45 HTB specimens with CO and spiral ligament hyalinization were reviewed. Those with sensorineural or mixed hearing loss but normal or near-normal hair cell counts were analyzed with light microscopy, and Cx26-immunoreactive (IR) signal was qualitatively assessed.

RESULTS

H&E staining demonstrated hyalinization in the spiral ligament and loss of type II and type III fibrocytes. Cx26-IR was diminished throughout the cochlea affected with CO compared with normal controls.

CONCLUSIONS

Cx26-IR reduction in the spiral ligament of subjects with CO likely plays a role in hearing loss.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

Immunohistochemical techniques for the human inner ear

In this review, we provide a description of the recent methods used for immunohistochemical staining of the human inner ear using formalin-fixed frozen, paraffin and celloidin-embedded sections. We also show the application of these immunohistochemical methods in auditory and vestibular endorgans microdissected from the human temporal bone. We compare the advantages and disadvantages of immunohistochemistry (IHC) in the different types of embedding media. IHC in frozen and paraffin-embedded sections yields a robust immunoreactive signal. Both frozen and paraffin sections would be the best alternative in the case where celloidin-embedding technique is not available. IHC in whole endorgans yields excellent results and can be used when desiring to detect regional variations of protein expression in the sensory epithelia. One advantage of microdissection is that the tissue is processed immediately and IHC can be made within 1 week of temporal bone collection. A second advantage of microdissection is the excellent preservation of both morphology and antigenicity. Using celloidin-embedded inner ear sections, we were able to detect several antigens by IHC and immunofluorescence using antigen retrieval methods. These techniques, previously applied only in animal models, allow for the study of numerous important proteins expressed in the human temporal bone potentially opening up a new field for future human inner ear research.

Noise-induced hearing loss: Neuropathic pain via Ntrk1 signaling

Severe noise-induced damage to the inner ear leads to auditory nerve fiber degeneration thereby reducing the neural input to the cochlear nucleus (CN). Paradoxically, this leads to a significant increase in spontaneous activity in the CN which has been linked to tinnitus, hyperacusis and ear pain. The biological mechanisms that lead to an increased spontaneous activity are largely unknown, but could arise from changes in glutamatergic or GABAergic neurotransmission or neuroinflammation. To test this hypothesis, we unilaterally exposed rats for 2h to a 126dB SPL narrow band noise centered at 12kHz. Hearing loss measured by auditory brainstem responses exceeded 55dB from 6 to 32kHz. The mRNA from the exposed CN was harvested at 14 or 28days post-exposure and qRT-PCR analysis was performed on 168 genes involved in neural inflammation, neuropathic pain and glutamatergic or GABAergic neurotransmission. Expression levels of mRNA of Slc17a6 and Gabrg3, involved in excitation and inhibition respectively, were significantly increased at 28days post-exposure, suggesting a possible role in the CN spontaneous hyperactivity associated with tinnitus and hyperacusis. In the pain and inflammatory array, noise exposure upregulated mRNA expression levels of four pain/inflammatory genes, Tlr2, Oprd1, Kcnq3 and Ntrk1 and decreased mRNA expression levels of two more genes, Ccl12 and Il1β. Pain/inflammatory gene expression changes via Ntrk1 signaling may induce sterile inflammation, neuropathic pain, microglial activation and migration of nerve fibers from the trigeminal, cuneate and vestibular nuclei into the CN. These changes could contribute to somatic tinnitus, hyperacusis and otalgia.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).