Enkephalin-like immunoreactivity of olivocochlear nerve fibers in cochlea of guinea pig and cat

Abstract

The distribution ofenkephalin-like immunoreactivity in the cochlea ofthe guinea pig and cat was studied. Indirect immunofluorescence immunohistochemistry using antisera generated against a methionine enkephalin-bovine thyroglobulin conjugate was applied to surface preparations of the organ of Corti and cryostat sections of the whole of the cochlea. In the cochlear osseous spiral lamina, immunofluorescence was localized to unmyelinated fibers ofthe intraganglionic spiral bundle. In the organ of Corti, immunofluorescence was localized to a small number of fibers at inner hair cells, the inner spiral bundle, and tunnel spiral bundle, to tunnel crossing fibers at the level of the tunnel floor, to an occasional spiral outer fiber, and to the synaptic region ofouter hair cells in the three rows of the basal turn of the cochlea. Less immunofluorescence was found in this region as one progressed towards the apex, with none seen at the apex. At the most apical region the inner spiral bundle became patchy and the tunnel spiral bundle developed arcades. There was no immunofluorescence found in spiral ganglion cells, in auditory nerve fibers, or in the hair cells of the organ of Corti. The findings were the same in cat as in guinea pig, the latter being studied in more detail. It was concluded that efferent, olivocochlear neurons ofthe cochlea, synapsing predominantly with primary auditory nerve fibers from the inner sensory cells or with the sensory cells, contain enkephalin-like immunoreactivity. Also, the findings indicate that endings of olivocochlear neurons that synapse predominantly with outer hair cells contain enkephalin-like immunoreactivity. It has previously been shown that ofivocochlear neurons are likely to be cholinergic.Methionine enkephalin and leucine enkephalin are opioid pentapeptides originally isolated from the whole brain (1, 2). Immunohistochemical mapping of enkephalin-like immunoreactivity in the central nervous system was done (3-5), and it was noted that in certain regions enkephalin-like immunoreactivity and opiate receptors had a similar distribution (3, 5). A much entertained working hypothesis is that enkephalins may act as neurotransmitters or modulate the action of neurotransmitters (6).In the organ ofCorti, each inner hair cell synapses with about 20 auditory nerve boutons from about 90% of the total spiral ganglion cell population (7-9). The outer hair cells make relatively few synapses with auditory nerve boutons from about 10% of the spiral ganglion cells (7-9). The auditory receptor organ also receives efferent, centrifugal innervation from the brainstem, through olivocochlear neurons. In the cochlea, myelinated and unmyelinated olivocochlear fibers spiral in the fascicles ofthe intraganglionic bundle (10, 11) and then fan out to the organ of Corti. In the organ of Corti, such fibers form the inner spiral bundle and the tunnel spiral bundle (12)(13)(14). Inner hair cells and auditory nerve dendrites under inner hair cells receive efferent innervation from the inner spiral bundl...

Immunocytochemical distribution of Met-enkephalin-Arg6-Gly7-Leu8 (Met-8) in the auditory system of the rat

“…The ¢rst indirect evidence that suggested an association of enkephalin with the olivocochlear system was made using guinea pig as an experimental animal (Fex and Altschuler, 1981). These authors already suggested that enkephalin could be involved in both the LOC and the medial olivocochlear system (MOC).…”

“…Neurons in the cochlear nucleus give rise to a number of parallel ascending pathways that are involved in the processing of di¡erent properties (monaural, binaural, temporal and spectral) of the auditory stimulus (for reviews see Malmierca, 2003;Malmierca and Mercha ¤n, 2004). The ¢rst report on enkephalin immunostaining in the auditory system by Fex and Altschuler (1981) was made in guinea pig. This study ¢rst showed the potential involvement of enkephalin in the modulation of the cochlea through the descending olivocochlear system.…”

Lack of Evidence of an Interaction between Leu-Enkephalin and Muscarinic-Like Responses in the Frog Semicircular Canal

“…The opposite effects of ACh suggest the existence of two types of receptors for ACh, muscarinic-like and nicotinic-like receptors. Considering the fact that ACh frequently co-localizes with neuropeptides in the cochlear efferent neurons, the latter are viewed as neuromodulators [9][10][11][12]. Recently, such a co-localization has been proposed for efferent vestibular system as well [1].…”

Desde la corteza auditiva a la cóclea: Progresos en el sistema eferente auditivo

“…Las encefalinas fueron descritas en el cerebro desde mediados de los años 70 18 , pero caracterizadas en el sistema eferente auditivo una década después 29 . Se postula que las encefalinas tienen funciones inhibitorias dentro del sistema auditivo de mamíferos, rol funcional que ha sido constatado de manera indirecta a través de la verificación del aumento de su concentración en ambientes ruidosos y disminución en ambientes silenciosos 16 .…”