Quantitative evidence for cochlear, non-neuronal norepinephrine

Simultaneous HPLC quantification of monoamines and metabolites in the blood-free rat cochlea

Monoamine quantification in peripheral sensory receptors, such as the cochlea, is of major interest since monoamines could play a role in neurotransmission. A three-step biochemical protocol was developed to analyze monoamine content within the cochlea. Removal of the blood by aortic perfusion was carried out with an anticoagulant solution prior to the dissection of the cochlea from the temporal bone. The cochlear monoamines and some of their metabolites were then quantified, from homogenated cochlear tissue, by a new application of high performance liquid chromatography coupled to electrochemical detection. This method demonstrated enough sensitivity to detect norepinephrine (NE), dopamine (DA), serotonin (5-HT) and some of their metabolites (3,4-dihydroxyphenylacetic acid, DOPAC; homovanillic acid, HVA; and 5-hydroxyindole-3-acetic acid, 5-HIAA). Furthermore, it enabled the demonstration of noise-induced changes in the cochlear concentrations of NE, DA, DOPAC and HVA. In addition, the aortic perfusion allowed removal of the blood-borne 5-HT from the cochlea without inducing systemic alterations or monoamine degradation, as shown by the absence of effects on NE, DA, DOPAC, HVA or 5-HIAA concentrations. The present methodology may constitute a useful strategy to analyze monoamine turnover in the cochlea and other peripheral sensory receptors.

K(+) cycling and its regulation in the cochlea and the vestibular labyrinth

Potassium (K(+)) plays a very important role in the cochlea. K(+) is the major cation in endolymph and the charge carrier for sensory transduction and the generation of the endocochlear potential. The importance of K(+) handling in the cochlea is marked by the discovery of several forms of hereditary deafness that are due to mutations of K(+) channels. Deafness results from mutations of KCNQ4, a K(+) channel in the sensory hair cells, as well as from mutations of the gap junction proteins GJB2, GJB3 and GJB6 that may facilitate cell-to-cell movements of K(+). Deafness results also from mutations of KCNQ1 or KCNE1, subunits of a K(+) channel that carries K(+) from strial marginal cells and vestibular dark cells into endolymph. Further, deafness results from mutations of KCNJ10, a K(+) channel that generates the endocochlear potential in conjunction with the high K(+) concentration in strial intermediate cells and the low K(+) concentration in the intrastrial fluid spaces. This review details recent advances in the understanding of K(+) transport and its regulation in the cochlea and the vestibular labyrinth.

Noise stimulation decreases the concentration of norepinephrine in the rat cochlea

The present study was designed to analyze, by using high performance liquid chromatography (HPLC), the effect of acoustic stimulation on the cochlear concentration of norepinephrine (NE). Independently of the rat strain (Long-Evans or Wistar strains), NE concentration decreased about 18% when animals were exposed to white noise (90 dB SPL for 1 h). The same decrease was observed in animals perfused by aortic pathway to remove the blood, indicating that this decrease corresponds exclusively to a neurophysiological process. In fact, these findings could indicate that noise stimulation is involved in the NE release from sympathetic fibers innervating the cochlea. This likely release of NE supports that sympathetic fibers play a functional role in cochleae exposed to noisy situations.

HPLC detection of serotonin within the rat cochlea

This study was performed to analyse the cochlear concentrations of serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA), their sources and modifications induced by noise exposure. Superior cervical ganglionectomy did not modify these concentrations. However, removal of the blood by aortic perfusion reduced significantly (about 76%) the cochlear concentration of 5-HT without affecting the 5-HIAA concentration. These results indicate that blood constitutes an important source of 5-HT to the cochlea, opposite to the superior cervical ganglion. Exposure to noise at 90 dB SPL did not modify the total cochlear concentrations of 5-HT and 5-HIAA, or the concentrations remaining after removal of the blood, suggesting that 5-HT could have a modulatory role in the cochlea distinct from that of olivocochlear neurotransmitters.

Effect of superior cervical ganglionectomy on catecholamine concentration in rat cochlea

Both noradrenergic and dopaminergic nerve terminals have been described in the cochlea. The present report focused on the effect of superior cervical ganglionectomy (SCGx) on monoamine concentration in adult rat cochlea. In homogenates of whole cochleas, we measured the concentrations of norepinephrine (NE), dopamine (DA) and its main metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), by HPLC coupled to electrochemical detection. Measurements were carried out 4 h, 24 h or 6 days after unilateral SCGx. Most of the NE (approximately 82%) was lost after sympathectomy on the ipsilateral side, indicating that the principal localization of cochlear NE is in peripheral sympathetic fibers. Since about 18% of NE remained detectable 6 days after SCGx, a second origin of cochlear noradrenergic fibers may exist. Cochlear concentrations of DA or its metabolites did not change after SCGx. Therefore, DA and NE are located in two different populations of fibers within the cochlea, and are presumably related to distinct functional roles.

Serotonergic innervation of the organ of Corti of the cat cochlea

The presence and distribution of serotonin-containing fibers within the adult cat cochlea is reported here for the first time. The 5-HT-like immunoreactive fibers were mainly found in the middle coils of the cat cochlea, following a peripheral distribution similar to that of the olivocochlear lateral efferent system (OLES). Thus, 5-HT-like immunoreactive varicose fibers were found within the intraganglionic spiral bundle, the inner and the tunnel bundles, although, some variation with the distribution of other neuroactive substances of the OLES were observed. In particular, some isolated fibers from the tunnel bundle branched towards the first row of the outer hair cells. Cochlear serotonergic fibers might be involved in the auditory processing within the cochlea as efferent modulating fibers.

Presence of catecholamines and serotonin in the rat vestibule

The concentrations of norepinephrine (NE), dopamine (DA) and its metabolites DOPAC and HVA, and serotonin (5-HT) and its metabolite 5-HIAA, were quantified in the rat vestibule. For this purpose, homogenates of vestibules, of albino and pigmented rats, were analyzed using HPLC with electrochemical detection. Vestibules of pigmented rats showed higher DOPAC and HVA concentrations than those of albino rats, and male pigmented rats also showed significantly more DA than male albino rats. These results could indicate that the rate of DA metabolism in vestibules was higher in pigmented than in albino rats. The vestibular concentrations of NE and 5-HT did not differ significantly between the two strains. In contrast, 5-HIAA concentration was higher in vestibules of pigmented rats than in those of albino rats, suggesting an increased 5-HT metabolism for the former strain. Differences in monoamine concentrations between the two sexes o the same strain were scarce. Only, a higher HVA concentration in vestibules of females could indicate a higher DA metabolism.

Piribedil affects dopamine turnover in cochleas stimulated by white noise

The presence of dopamine (DA) within the cochlea has been previously reported, indicating that its turnover increases under noise stimulation. In the present report, piribedil, a dopaminergic D2 agonist, was used in order to provide evidence of the activity of D2 receptors in the turnover of DA under noise stimulation. Long-Evans rats were intraperitoneally injected with distilled water or with a solution of piribedil one hour previously to either noise or silence exposure. Noise stimulation was performed in an anechoic chamber at 70, 90 or 110 dB SPL for one hour. The animals were then sacrificed and the cochlear contents of DA and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were quantified by HPLC with electrochemical detection. The administration of piribedil to animals kept in silence did not modify the cochlear DA, DOPAC and HVA content. Noise stimulation resulted in a decrease of the cochlear DA content and an increase of the cochlear DOPAC and HVA contents in vehicle treated animals. The administration of piribedil resulted in a blockade of this noise induced cochlear DA turnover. These results suggest that piribedil stimulates cochlear D2 receptors controlling the cochlear DA release. Piribedil action on D2 receptors could explain the improvement observed in some cochleo-vestibular diseases signs after piribedil treatment.

Distribution of Tyrosine Hydroxylase-Like Immunofluorescence in Guinea Pig Vestibular Ganglia and Sensory Areas

The distribution of tyrosine hydroxylase (TH)-like immunofluorescence was analyzed in the guinea pig vestibular ganglia and end organs using a monoclonal antibody to TH. TH was chosen as a marker for the sympathetic fibers because TH regulates the first step of catecholamine synthesis by converting tyrosine to dopa. In the vestibular ganglia, there were TH-positive nerve fibers having distinct varicosities surrounding the vestibular ganglion cells. In the sensory areas, there was a sympathetic plexus in the subepithelial tissue of the saccule, the utricle, and the crista ampullaris. We speculated that the sympathetic innervation has a direct influence on the vestibular ganglion cells and diffuse influence on the capillary permeability.

HPLC detection of dopamine and noradrenaline in the cochlea of adult and developing rats

The presence and postnatal development of dopamine (DA) and noradrenaline (NA) in the rat cochlea were detected by high-performance liquid chromatography coupled with electrochemical detection. Cochlear DA content rose gradually after birth. Conversely, NA concentrations rose rapidly between postnatal day 1 and 8; then, up to day 30, it increased more slowly. On day 30, both DA and NA levels were around 5 times higher than on day 1. In the adult rat cochlea, NA mean content was 234 +/- 22.2 pg/mg protein, while DA mean content was 23.6 +/- 1.1 pg/mg protein. Adrenaline was always undetectable. The present study is the first report describing directly the presence of DA in the rat cochlea. DA might serve as one of the lateral efferent neurotransmitters, whereas NA probably acts as a neurotransmitter of the sympathetic cochlear innervation. Nevertheless, their influences on the cochlear physiology, either in adulthood or during development are still a matter of discussion.

Vestibular sympathetic nervous system in guinea pig

The vestibular sympathetic fibers were examined in 20 guinea pigs by the immunohistochemical demonstration of tyrosine hydroxylase and dopamine B-hydroxylase. The vestibular sympathetics originated in the ipsilateral superior cervical ganglion and entered the internal auditory meatus along the labyrinthine artery. At the Schwann-glial border, some of the sympathetic fibers left the artery and went into the superior and inferior divisions of the vestibular nerve and formed a loose meshwork among the Scarpa's ganglion cells, while other fibers continued to follow the labyrinthine artery. Both groups of fibers entered the cristae ampullares and saccular and utricular maculas after several bifurcations in the cribrose areas and terminated either near the capillaries beneath the sensory epithelia or among the vestibular nerve fibers. These fibers traveled freely in the vestibular labyrinth without being restricted to following blood vessels or vestibular nerve fibers. Some sympathetic fibers made direct contact with the vestibular efferent fibers or the vestibular afferent fibers at the node of Ranvier. Sympathetic fibers were not observed in the sensory epithelia or semicircular canals, and were rarely found in the vicinity of the dark cells.

Use of albino animals for auditory research

Since Hearing Research commenced publication, 51% of reports involving the use of guinea pigs provide no information on whether the animals used were albino or pigmented. Recent evidence creates a strong suspicion that the albino mutation can influence both peripheral and central auditory function. It is concluded that information on pigmentation should always be provided in reports of auditory experiments on guinea pigs, and that the only justification for using albino animals in auditory research is to study effects of the albino mutation on hearing.

Distribution and significance of norepinephrine in the lateral cochlear wall of pigmented and albino rats

Picogram quantities of norepinephrine were found in cochlear regions of pigmented and non-pigmented rats. These regions of the cochlea were the modiolus, organ of Corti--osseous spiral lamina and the lateral cochlear wall. The content of norepinephrine in the modiolus and lateral cochlear wall of the pigmented rat was significantly greater than that in areas of the non-pigmented rat. In contrast, there was no statistical difference between the norepinephrine content of the organ of Corti--osseous spiral lamina region of the pigmented rat and that of the albino rat. Since a major difference between the pigmented and albino rats is the presence of melanin-containing melanocytes in the modiolus and lateral cochlear wall region of the pigmented animals, it is possible that norepinephrine is stored in cochlear melanocytes.