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.