The presence of opioid receptors in rat inner ear

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.

Hearing status in patients with overdose of illicit drugs

Background: The overdose of illicit drugs is not always fatal but can lead to various complications. One of the unusual medical complications is a sensorineural hearing loss (SNHL). There are multiple case reports about this subject. Considering the importance of hearing loss on quality of life, we investigated hearing status in patients with overdose of illicit drugs.

Methods: This cross-sectional study was performed in Loghman Hakim hospital in Shahid Beheshti University of Medical Sciences, Tehran, Iran, in 2016-2017. The hearing status of 95 patients with illicit drugs overdose and 44 healthy individuals were assessed using standard pure tone audiometry and distortion product otoacoustic emissions. The patient group was categorized based on hearing status and compared based on some variables. We applied 2 independent t tests, Mann-Whitney, Chi-square, and binary logistic regression tests. All analyses were conducted in Stata 12 (STATA Corp, USA) and significance level was set at less than 0.05.

Results: We found higher percentage frequency of SNHL in the patient group than the control group (15.8% vs 2.3%; p=0.021). The frequency of hearing loss was 21.7% in opioid users, 5.3% in stimulant users, and 6.3% in concomitant use of both. There was a significant relationship between SNHL and overdose of illicit drugs (aOR = 14.48, 95% CI = 1.53-136.44; p=0.019) with adjusting age, sex, and smoking.

Conclusion: Illicit drugs overdose can potentially affect the hearing system. Opioid drugs, especially methadone and tramadol, have been found to affect the hearing system. Therefore, it is important to conduct longitudinal studies to demonstrate the role of opioid drugs on the hearing system.

Drug-Induced Ototoxicity: A Comprehensive Review and Reference Guide

OBJECTIVE

In an era of increasing polypharmacy, adverse drug effects such as ototoxicity have significant public health implications. Despite the availability of evidence, many health care professionals may not know the risk of ototoxicity in common medications. Therefore, the aim of this review is to provide a comprehensive, easy to use, ototoxic profile of medications with an assessment of supporting evidence.

METHODS

Medications of interest were identified through adverse drug reaction reports derived from Micromedex (IBM), Lexicomp (Wolters Kluwer), and the textbook, Drug Induced Diseases: Prevention, Detection, and Management. Additional evidence was identified though a query of PubMed and the Cochrane database. Evidence of causality was graded according to the following: A (randomized, controlled clinical trials), B (nonrandomized clinical trials, prospective observational studies, cohort studies, retrospective studies, case-controlled studies, and/or postmarketing surveillance studies), and C (case reports/case series).

RESULTS

A total of 194 systemically administered medications associated with ototoxicity were identified, most commonly antimicrobials (53), psychotropics (21), antihypertensive/antiarrhythmics (19), nonsteroidal antiinflammatory drugs (18), and antineoplastics (16). There was evidence of cochleotoxicity in 165 medications (evidence grading A [22], B [77], C [69]), vestibulotoxicity in 100 medications (evidence grading A [23], B [47], and C [30]), and dizziness in 142 medications (evidence grading A [50], B [76], and C [16]). In addition, a review of the evidence of ototoxicity in ototopical medications is also reviewed.

CONCLUSION

The effect and severity of ototoxicity can vary immensely depending on pharmacological and individual patient risk factors. The intent of this comprehensive review was to help health care providers of all sectors obtain a deeper knowledge of drug-induced ototoxicity to make more informed management decisions for their patients.

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.

Methadone associated long term hearing loss and nephrotoxicity; a case report and literature review

Background

Methadone is a long-acting opioid receptor agonist. Reported adverse effects of methadone include constipation, respiratory depression, dizziness, nausea, vomiting, itching, sweating, rhabdomyolysis, QT prolongation, and orthostatic hypotension. Hearing loss has been rarely reported following methadone use, and when reported, long term follow-up is rare.

Herein we report a case of methadone poisoning with rhabdomyolysis, acute kidney injury, and persistent hearing loss documented by a 2 year follow up.

Case presentation

The patient was a 34 years old male who presented with a reduced level of consciousness and acute hearing loss after suicidal ingestion of 40 mg of methadone while experiencing family-related stresses. He had no prior history of methadone use, abuse, or addiction.

Initial laboratory testing was significant for a serum creatinine concentration of 4.1 mg/dl, a mixed metabolic and respiratory acidosis, thrombocytopenia, abnormal hepatic transaminases, and coagulation tests. The patient then developed severe rhabdomyolysis. Also, audiometry showed a bilateral sensorineural hearing loss. The patient required hemodialysis for 11 days while his metabolic abnormalities gradually resolved. However, his hearing loss was persistent, as demonstrated by 2 years of follow up.

Conclusion

Our patient simultaneously had kidney failure, rhabdomyolysis, and permanent hearing loss following methadone poisoning. Although rare, ototoxicity and permanent hearing loss may happen in cases of methadone poisoning. While opioid-induced hearing loss is uncommon, methadone toxicity should be taken into account for any previously healthy patient presenting with acute hearing loss with or without rhabdomyolysis.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

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.

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.

Opioids and GI Motility-Friend or Foe?

OPINION STATEMENT

The use of opioids for the treatment of chronic non-cancer pain is growing at an alarming rate. Opioid-induced bowel dysfunction (OBD) is a common adverse effect of long-term opioid treatment manifesting as constipation, nausea, and vomiting. These effects are primarily mediated by peripheral μ-opioid receptors with resultant altered GI motility and function. As a result, patients may present with opioid-induced constipation (OIC), opioid-induced nausea and vomiting (OINV), and/or narcotic bowel syndrome (NBS). This often leads to decreased quality of life and in many cases, discontinuation of opioid therapy. There is limited evidence to support the use of traditional anti-emetics and laxatives in the treatment of OBD. Tapering the dose of opioids, switching to transdermal application, opioid rotation, or dual-action opioids, such as tapentadol, may be helpful in the treatment of OBD. Novel agents, such as peripherally acting μ-opioid receptor antagonists which target the cause of OIC, show promise in the treatment of OBD and should be considered when conventional laxatives fail. This chapter will review the pathophysiology of OBD, including OINV and OIC, and treatment options available.

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.

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

Opioid peptides and their receptors have been localized to the inner ear of the rat and guinea pig mammalian models. The expression of mu opioid receptor (MOR) in the human and mouse cochlea is not yet known. We present MOR protein localization by immunohistochemistry and mRNA expression by in situ hybridization in the human and mouse spiral ganglia (SG) and organ of Corti. In the human most of the (SG) neurons were immunoreactive; a subset was non-immunoreactive. In situ hybridization revealed a similar labeling pattern across the neurons of the SG. A similar distribution MOR pattern was demonstrated in the mouse SG. In the mouse organ of Corti MOR was expressed in inner and outer hair cells. Fibers underneath the inner hair cells were also MOR immunoreactive. These results are consistent with a role of MOR in neuromodulation of the auditory periphery. The present results show that the expression of MORs is well-conserved across multiple mammalian species, indicative of an important role in auditory processing.

Dynorphin release by the lateral olivocochlear efferents may inhibit auditory nerve activity: a cochlear drug delivery study

Dynorphin (dyn) is suggested to excite the auditory nerve (AN) when released by the lateral olivocochlear (LOC) efferents. However, previous studies evaluated either intravenously delivered dyn-like agents, raising the potential for systemic (central) effects, or agent concentrations unlikely to be achieved via endogenous cochlear release. This study tested the hypothesis that biologically relevant increases in dyn levels in the cochlea achieved via diffusion of the drug of (-)pentazocine across the round window membrane enhances AN firing. In general, amplitude of the cochlear whole-nerve action potential (CAP) was depressed following drug application. These results suggest that dyn released by the LOC neurons would likely act as an inhibitory transmitter substance in the LOC system; neurotransmission is one of the LOC system's vast unknowns.

A case of heroin induced sensorineural hearing loss

A case of a 31-year-old male who developed profound sensorineural hearing loss following a heroin overdose is presented. The patient subsequently had a full recovery of his hearing. Other cases of this rare phenomenon are reviewed and management options are discussed.

Opioid induced nausea and vomiting

Opioids are broad spectrum analgesics that are an integral part of the therapeutic armamentarium to combat pain in the palliative care population. Unfortunately, among the adverse effects of opioids that may be experienced along with analgesia is nausea, vomiting, and/or retching. Although it is conceivable that in the future, using combination agents (opioids combined with agents which may nullify emetic effects), currently nausea/vomiting remains a significant issue for certain patients. However, there exists potential current strategies that may be useful in efforts to diminish the frequency and/or intensity of opioid-induced nausea/vomiting (OINV).

Nebulized lidocaine alone or combined with fentanyl as a premedication to general anesthesia in spontaneously breathing pediatric patients undergoing rigid bronchoscopy

INTRODUCTION

Pediatric bronchoscopy is an intensely stimulating procedure that requires a deep level of anesthesia to prevent hemodynamic overstimulation and straining. Topical anesthesia of the airway may be a beneficial component of the anesthetic technique to achieve adequate depth without residual sedation. Experimental evidence suggests that in addition to its central effects, locally applied opioids elicit potent analgesic effects.

METHODS

Forty-five patients aged 1-6 years scheduled for rigid bronchoscopy for foreign body removal were selected and subjected preoperatively to a nebulizer setting according to its components patients were divided into three groups. Group A: Nebulized solution contains 4 mg·kg(-1) lidocaine 1%. Group B: Nebulized solution contains 4 mg·kg(-1) lidocaine 1% plus 2 μg·kg(-1) fentanyl. Group C: Nebulized solution contains 0.9% normal saline. Anesthesia was induced with 8% sevoflurane in 100% oxygen and maintained with continuous infusion of propofol 200 μg·kg(-1). Increments of 500 μg·kg(-1) propofol were given to the patient in case of straining or coughing. Patients were followed for the hemodynamics, the intraoperative difficulties, postoperative sedation score, time to full wakefulness and the postoperative complications.

RESULTS

The hemodynamic parameters were much more stable in the fentanyl group relative to the other two groups. Also, the incidence of intraoperative difficulties was less significantly evident among patients in the fentanyl group (As regards cough P(1) = 0.003, P(2) = 0.0001, As regards the need to manual ventilation P(1) = 0.037, P(2) = 0.001, As regards Propofol increments P(1) = 0.001, P(2) = 0.001 where P(1) refers to the fentanyl group relative to the lidocaine group, and P(2) refers to fentanyl group relative to the placebo group). The postoperative sedation score was significantly higher, and the time to full wakefulness was significantly prolonged among patients in the fentanyl group relative to the other groups (P = 0.0001).

CONCLUSION

It is concluded that preoperative nebulized fentanyl reduces the hemodynamic response to bronchoscopy and decreases the intraoperative coughing in response to surgical manipulation without significant side effects except prolonged time to full wakefulness of patients.

Is musical hallucination an otological phenomenon? a review of the literature

BACKGROUND

Musical hallucination is the subjective experience of hearing music, or aspects of music, when none is being played, and as such is a disorder of the processing of complex sounds.

OBJECTIVE OF REVIEW

To determine the extent to which the otological system is responsible for musical hallucination, and to evaluate approaches to clinical management.

TYPE OF REVIEW

A review of all studies and literature reviews pertaining to musical hallucination, supplemented by inclusion of informative case reports.

SEARCH STRATEGY

A systematic search of multiple databases carried out on 22nd March 2009 was complemented by referral to the reference lists of included manuscripts.

RESULTS

Although not always troublesome in itself, musical hallucination can be a marker of underlying pathology in the ear or brain, or indicate obsessive-compulsive traits or social isolation, and is likely to be clinically underreported. Associations have been reported with hearing loss, female gender, social isolation and being over 60 years of age, although it is rare even in this group, and these may well not be independent risk factors. Robust comparative analysis of these factors with controls has yet to be undertaken. Underlying causes include neurovascular pathology, psychiatric disorders and opioid medications, however these are absent in the majority of cases.

CONCLUSIONS

This review supports the proposal that the otological system plays a role in the pathogenesis of musical hallucination. Hearing impairment may act as an initiating factor, and the primary dysfunction is overactivity of auditory association cortex, although an impairment of higher-level inhibition does also seem necessary. Once underlying sinister causes have been excluded, first line treatment should be explanation of the condition and optimisation of hearing. Medications have a role only in selected patient groups.

Topical fentanyl in a randomized, double-blind study in patients with corneal damage

OBJECTIVES

Corneal damage causes severe pain. This study investigated whether peripheral opioid receptors are present in the human cornea and assessed the efficacy of topical fentanyl in patients with corneal erosion.

METHODS

Immunohistochemical staining experiments were performed to examine the presence of both mu and delta-receptors on peripheral nerve fibers within human corneal tissue. In a randomized, double-blind clinical trial dexpanthenol (n=20) or dexpanthenol plus 10 microg fentanyl (n=20) were topically applied every 4 hours to the eye of patients with a surgical intervention of corneal damage and subjective pain intensity was determined by a numerical rating scale.

RESULTS

Immunohistochemical staining identified peripheral nerve fibers in human cornea expressing both mu and delta-opioid receptors. In patients with corneal damage the ophthalmic intervention in local anesthesia decreased the subjective pain intensity significantly. At 4-hour intervals after the ophthalmic intervention, moderate pain intensity levels were not altered by the application of dexpanthenol with or without fentanyl. At 24 hours pain intensity dropped significantly, most likely owing to a natural decrease in pain, because the erosion was almost healed.

DISCUSSION

Both mu and delta-receptors are localized on nerve fibers within the cornea, which are accessible for topical opioid treatment. However, our formulation and dose of topical fentanyl in combination with dexpanthenol did not show any benefit in relieving pain from corneal erosion. Future studies are planned to determine the optimal protocol and dose of topical opioid treatment.

Sudden sensorineural hearing loss after heroin injection

Sudden sensorineural hearing loss is a symptom of cochlear injury. Potential aetiologies are vascular diseases, viral infections, allergic reactions, autoimmune disorders, and traumatic rupture of the intralabyrinthe membrane. Unlike in unilateral cases bilateral sensorineural hearing loss is often associated with specific disease entities. We report a case of sudden bilateral deafness after intravenous heroin abuse. The putative pathophysiological mechanisms are discussed.

Bi-phasic intensity-dependent opioid-mediated neural amplitude changes in the chinchilla cochlea: partial blockade by an N-Methyl-D-Aspartate (NMDA)-receptor antagonist

Dynorphins, glutamate, and glutamate-sensitive N-Methyl-D-Aspartate (NMDA) receptors exist in the mammalian cochlea. Dynorphins produce neural excitation and excitotoxic effects in the spinal cord through a kappa-opioid facilitation of NMDA receptor-sensitivity to glutamate. The kappa-opioid receptor drug agonists N-dimethylallyl-normetazocine [(-)-pentazocine (50 mmol)] and trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide [U-50488H (100 mmol)] were administered across the cochlear round window membrane in the chinchilla. Each drug produced significant post-baseline amplitude changes in the click-evoked auditory nerve compound action potential. Amplitude changes at threshold amounted to increases in sensitivity that ranged from 4-8 decibels, measured in sound pressure level (dB SPL). The large neural amplitude increases at threshold were accompanied by progressively smaller amplitude changes at 5 and 10 dB above threshold (dB SL). However, at stimulus intensities > or =20 dB SL, post-baseline neural amplitudes were suppressed to levels below baseline and control values. These bi-phasic intensity-dependent neural amplitude changes have never before been observed following i.v. administered (-)-pentazocine in this species. Finally, the bi-phasic neural amplitude changes in U-50488H-treated (100 mmol) animals were partially blocked (except at 20 dB SL), following a round window pre-treatment with the NMDA receptor drug antagonist, dizocilpine hydrogen maleate [(+)-MK-801 (8 mmol)]. Our data suggests that endogenous dynorphins within lateral efferent olivocochlear neurons differentially modulate auditory neural excitation, possibly through cochlear NMDA receptors and glutamate. The role played by lateral efferent opioid neuromodulation at cochlear NMDA receptors, is discussed.

Immunolocalization of orphanin FQ in rat cochlea

Orphanin FQ/nociceptin (OFQ/N) and its receptor (ORL-1) have been proposed to play a role in the regulation of hearing. In this study, we investigate the localization of OFQ/N-like immunoreactivity in the mammalian cochlea. Sprague-Dawley rat temporal bones were harvested and decalcified. The organ of Corti was microdissected, and indirect immunohistochemistry was performed using a rabbit polyclonal antibody raised against OFQ/N. Immunoreactivity was seen in the tunnel crossing fibers and the large boutons terminating onto outer hair cells, and in the fibers terminating onto the afferents to the inner hair cells. The findings are consistent with OFQ/N expression in lateral and medial olivocochlear efferents.

Relationship of opioid receptors with GABAergic neurons in the rat inferior colliculus

The inferior colliculus is a critical structure for processing auditory information and receives ascending and descending synaptic auditory projections. In addition to GABAergic and glutamatergic innervations, other neurotransmitter systems are also reported in the inferior colliculus, including opioid peptides. In the present study, the relative distribution of each type of opioid receptor, mu (MOR), delta (DOR) and kappa (KOR) within GABAergic neurons in the inferior colliculus was examined. GABA immunoreactivity was expressed by small, medium and large neurons and distributed in the central nucleus and the pericentral nucleus of the inferior colliculus. Immunostaining for MOR, DOR and KOR receptors was found in both disc-shaped cells and stellate cells. Punctiform beta-endorphin immunolabelling was observed in the proximity of GABA-positive neurons. Co-localization of GABA and MOR receptors was observed in neurons and nerve terminals in the central nucleus, dorsal cortex and external cortex of the inferior colliculus. Quantification of the co-localization patterns determined that a higher proportion of GABA neurons was associated with MOR receptors compared with KOR or DOR receptors.

The existence of opioid receptors in the cochlea of guinea pigs

Several independent investigations have demonstrated the presence of opioid peptides in the inner ear organ of Corti and in particular in the efferent nerve fibers innervating the cochlear hair cells. However, the precise innervation pattern of opioid fibers remains to be investigated. In the present study the expression of opioid receptors and their peptides is demonstrated in young adult guinea pig cochlea. Opioid receptors are mainly expressed in hair cells of the organ of Corti and in inner and outer spiral bundles with different characteristics for each type of receptor. Co-localization studies were employed to compare the distribution of mu-, delta- and kappa-opioid receptors and their respective peptides, beta-endorphin, leu-enkephalin and dynorphin. Additionally, immunostaining of synaptophysin was used in this study to identify the presynaptic site. Immunoreactivity for enkephalin and dynorphin was found in the organ of Corti. Leu-enkephalin was co-localized with synaptophysin prominently in the inner spiral bundle (ISB). Dynorphin was co-localized with synaptophysin in both inner and outer spiral bundles. Delta-opioid receptor was most prominently co-localized with its peptide in the ISB bundle. Kappa-opioid receptor was seemingly present with dynorphin in both inner and outer spiral bundles. The co-staining of both peptides and receptors with synaptophysin in the same areas suggests that some of the opioid receptors may act as auto-receptors. The results provide further evidence that opioids may function as neurotransmitters or neuromodulators in the cochlea establishing the basis for further electrophysiological and pharmacological investigations to understand better the roles of the opioid system in auditory function.

Opioid modulation of GABA release in the rat inferior colliculus

BACKGROUND

The inferior colliculus, which receives almost all ascending and descending auditory signals, plays a crucial role in the processing of auditory information. While the majority of the recorded activities in the inferior colliculus are attributed to GABAergic and glutamatergic signalling, other neurotransmitter systems are expressed in this brain area including opiate peptides and their receptors which may play a modulatory role in neuronal communication.

RESULTS

Using a perfusion protocol we demonstrate that morphine can inhibit KCl-induced release of [3H]GABA from rat inferior colliculus slices. DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin) but not DADLE ([D-Ala2, D-Leu5]-enkephalin or U69593 has the same effect as morphine indicating that micro rather than delta or kappa opioid receptors mediate this action. [3H]GABA release was diminished by 16%, and this was not altered by the protein kinase C inhibitor bisindolylmaleimide I. Immunostaining of inferior colliculus cryosections shows extensive staining for glutamic acid decarboxylase, more limited staining for micro opiate receptors and relatively few neurons co-stained for both proteins.

CONCLUSION

The results suggest that micro-opioid receptor ligands can modify neurotransmitter release in a sub population of GABAergic neurons of the inferior colliculus. This could have important physiological implications in the processing of hearing information and/or other functions attributed to the inferior colliculus such as audiogenic seizures and aversive behaviour.

The opioid receptors in inner ear of different stages of postnatal rats

There is increasing evidence that the opioid system has a role in hearing. To provide further evidence for such a role, the expression of opioid receptor mRNAs and proteins in the inner ear of rats was studied during development from birth (P0) to postnatal day 16 (P16). A semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) was employed to detect changes in the expression of delta- (DOR) kappa- (KOR) and mu- (MOR) opioid receptor mRNAs in rat cochleae at P0, P4, P8 and P16. Expression of DOR mRNA levels steadily increased from P0 to P8 with no further increases by P16. KOR mRNA was expressed at a relatively high level at P0 and P4 followed by a decrease while MOR mRNA was expressed at a low level at P0 and P4 followed by an increase by P8 and P16. Immunocytochemical labelling of inner ear sections revealed unique developmental and distribution patterns of opioid receptors. In the organ of Corti DOR immunoreactivity (DOR-IR) was detected in hair cells from P4. In contrast MOR-IR was present only in supporting cells at P0-P16. In the spiral ganglion all three receptor subtypes were expressed from P0 on nerve cell soma and qualitatively appeared to increase with age. Also DOR-IR and MOR-IR were detected at P8 and P16 in nerve fibers within the spiral ganglion. In the limbus DOR-IR was detected at P8 and P16 on cells proximal to the tectorial membrane while MOR-IR was detected more distally. In general these findings demonstrate that within the inner ear each receptor subtype follows specific temporal and spatial developmental patterns, some of which may be associated to the onset of hearing. The data provide further evidence that the opioid system may play a role in the development and functioning of the inner ear.