The existence of opioid receptors in the cochlea of guinea pigs

Abstract

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.

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

“…Therefore, it is unlikely that opioid receptors exist at or near the large medial efferent olivocochlear terminals located near the bases of the outer hair cells. It is more plausible that opioid κ-receptors in the chinchilla are anatomically positioned within Type I auditory dendrites found near the bases of the inner hair cells, similar to that reported in the rat (Jongkamonwiwat et al, 2003) and guinea pig cochlea (Jongkamonwiwat et al, 2006). Consequently, the available evidence suggests a κ-opioidinduced neural modulatory role for endogenous LEOC dynorphins in the mammalian cochlea.…”

“…There is direct evidence that κ-opioid receptors exist in the cochlea of the rat (Jongkamonwiwat et al, 2003) and guinea pig (Jongkamonwiwat et al, 2006). In both species, κ-opioid receptor immunoreactivity was detected on the sensory Type I auditory dendrites that are anatomically positioned at the bases of the cochlear inner hair cells.…”

Molecular aspects of tinnitus

“…The hypothesis is based on dynorphin and enkephalin-like opioids that are co-distributed within lateral efferent brainstem nuclei. Also enkephalin-and dynorphin-positive lateral efferent fibers have been described in inner and outer spiral bundle in the cochlea of species such as the guinea pig and rat (Abou-Madi et al, 1987;Altschuler et al, 1988;Jongkamonwiwat et al, 2003Jongkamonwiwat et al, , 2006. Furthermore, elevated levels of Met5-enkephalin-like opioid peptides after exposure to intense, stressful noise were described (Drescher et al, 1983;Drescher and Drescher, 1985).…”

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

“…Dyn opioid peptides primarily act as agonists at kappa-opioid receptors (KOR) but also have some affinity for μ-opioid receptors (MOR), which they antagonize. KOR and MOR immunoreactivity have been confirmed in rat [17] and guinea pig [16] cochleae. Unlike the MOC system, very little is known about the LOC system, including LOC neurotransmission.…”