Opioid modulation of cochlear auditory responses in the rat inner ear

Bilateral sensorineural hearing loss after opioid overdose in an adolescent male

Opioid-induced hearing loss has been described as a form of sudden sensorineural hearing loss that can occur with chronic or acute opioid use. Here, we report a case of a 16-year-old patient with sudden onset hearing loss after opioid overdose requiring prolonged intubation with fentanyl sedation.

Auditory brainstem responses are resistant to pharmacological modulation in Sprague Dawley wild-type and Neurexin1α knockout rats

Sensory processing in the auditory brainstem can be studied with auditory brainstem responses (ABRs) across species. There is, however, a limited understanding of ABRs as tools to assess the effect of pharmacological interventions. Therefore, we set out to understand how pharmacological agents that target key transmitter systems of the auditory brainstem circuitry affect ABRs in rats. Given previous studies, demonstrating that Nrxn1α KO Sprague Dawley rats show substantial auditory processing deficits and altered sensitivity to GABAergic modulators, we used both Nrxn1α KO and wild-type littermates in our study. First, we probed how different commonly used anesthetics (isoflurane, ketamine/xylazine, medetomidine) affect ABRs. In the next step, we assessed the effects of different pharmacological compounds (diazepam, gaboxadol, retigabine, nicotine, baclofen, and bitopertin) either under isoflurane or medetomidine anesthesia. We found that under our experimental conditions, ABRs are largely unaffected by diverse pharmacological modulation. Significant modulation was observed with (i) nicotine, affecting the late ABRs components at 90 dB stimulus intensity under isoflurane anesthesia in both genotypes and (ii) retigabine, showing a slight decrease in late ABRs deflections at 80 dB stimulus intensity, mainly in isoflurane anesthetized Nrxn1α KO rats. Our study suggests that ABRs in anesthetized rats are resistant to a wide range of pharmacological modulators, which has important implications for the applicability of ABRs to study auditory brainstem physiology.

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.

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.