Expression of leptin receptor by glial cells of the nucleus tractus solitarius: possible involvement in energy homeostasis.

Leptin, an adipocyte-derived hormone, regulates food intake and body weight by acting principally on the hypothalamus, which displays the highest expression of leptin receptor (Ob-R). Nevertheless, other regions of the brain express Ob-R and constitute leptin's target sites. The dorsal vagal complex (DVC), an integrative centre of autonomic functions located in the caudal brainstem, is one of these structures. Leptin, by acting through the DVC, affects autonomic and neuroendocrine functions, such as control of food intake and gastric motility. In the present study, we observed Ob-R labelling within the DVC in cells that correspond to neuronal cell bodies. We showed for the first time Ob-R expression in a subpopulation of glial fibrillary acid protein positive cells located at the border between the area postrema and the nucleus tractus solitarius (NTS). These glial cells exhibit an atypical morphology consisting of unbranched processes that radiate rostro-caudally from the fourth ventricle wall. In vitro, the glial cells exhibited both long and short Ob-R expression with a preferential expression of the Ob-Ra and-f isoforms. Interestingly, using i.v and i.c.v. injection of the fluorescent tracer hydroxystilbamidine, we provided evidence that these cells may constitute a diffusion barrier which might regulate entry of molecules into the NTS. Finally, modulation of energy status, by acute or chronic reduction of food intake, modulated especially the short Ob-R isoforms in the DVC. In the light of these results, we hypothesise that Ob-R positive glial cells of the DVC participate in the transport of leptin into the brainstem and thus contribute to regulation of energy homeostasis.

Stress in hearing and balance in Meniere's disease.

Stress is an unavoidable every-day phenomenon. Physiological coping with stress depends on the appropriate release of stress hormones as well as their alleviation at the termination of the stress. Despite quite a body of research indicating that stress affects inner ear function, this concept has found little application in otolaryngology. Today's evidence clearly indicates that the inner ear is equipped to detect stress hormones and some of these hormones have been shown to affect the inner ear function. Major stress control pathways shown to affect the inner ear include several third order axes, the hypothalamus-pituitary-adrenal axis, the hypothalamus-pituitary-thyroid axis and the hypothalamus-pituitary-gonadal axis whose functioning are interactive and inter-dependent. Less well-studied are the second order hypothalamus-pituitary control axis and its interaction with other hormones. To explore these we carried out a retrospective study on a series of Ménière's patients who had undergone a neurotomy of the vestibular nerve in the dept of ORL at the Hopital Nord, Marseille. Ménière's patients were particularly appropriate for this study since stress has long been recognised as a factor associated with the triggering of the symptoms of this pathology. Patients with acoustic neuroma and facial spasm were taken as a control population. We investigated the level of a battery of stress hormones including prolactin beta-endorphin and growth hormone. The blood sample was taken on the morning before surgery. The most striking observation was the presence of hyperprolactinemia in 30% of the Ménière patients (more than 20 microg/l) with confirmation of prolactinoma in 6 patients. The level of beta-endorphin could also be elevated. Horner, K.C., Guieu, R., Magnan, J., Chays, A. and Cazal, Y. Neuropysychopharmacology, (2001) 26:135-138. These observations suggest that neuroendocrinological feedback pathways controlling stress can be disturbed in Ménière's patients and depression of hypothalamic dopaminergic inhibition of prolactin secretion might be implicated. A further study on non-operated Ménière's patients presenting hyperprolactinemia and on dopamine agonist treatment, is needed in order to assess the role of stress in Ménière's patients. Progress in this domain could open the door towards integration of the stress concept into clinical management of various inner ear disorders.

The emotional ear in stress.

Stress of some kind is encountered everyday and release of stress hormones is essential for adaptation to change. Stress can be physical (pain, noise exposure, etc.), psychological (apprehension to impending events, acoustic conditioning, etc.) or due to homeostatic disturbance (hunger, blood pressure, inner ear pressure, etc.). Persistent elevated levels of stress hormones can lead to disease states. The aim of the present review is to bring together data describing morphological or functional evidence for hormones of stress within the inner ear. The present review describes possible multiple interactions between the sympathetic and the complex feed-back neuroendocrine systems which interact with the immune system and so could contribute to various inner ear dysfunctions such as tinnitus, vertigo, hearing losses. Since there is a rapidly expanding list of genes specifically expressed within the inner ear this clearly allows for possible genomic and non-genomic local action of steroid hormones. Since stress can be encountered at any time throughout the life-time, the effects might be manifested starting from in-utero. These are avenues of research which remain relatively unexplored which merit further consideration. Progress in this domain could lead towards integration of stress concept into the overall clinical management of various inner ear pathologies.

Sympathectomy improves the ear's resistance to acoustic trauma--could stress render the ear more sensitive?

Emotional stress is a phenomenon experienced by many people at some time in their lives. Some of its early manifestations, such as unbearable loudness of ambient sounds and sensations of dizziness, might be linked to inner ear dysfunction. Although the inner ear is supplied with a substantial sympathetic innervation, previous studies have failed to demonstrate any significant functional impact. We show here that in the awake guinea pig and following unilateral ablation of the superior cervical ganglion, the temporary threshold shift induced by a 1-min exposure to 8 kHz pure tone at 96 dB sound pressure level was reduced by as much as 40 dB. Of interest, the protective effect was bilateral suggesting an intimate relationship between the sympathetic and the olivocochlear efferent systems. The data presented here provide new evidence for a key role for the sympathetic system in modulating temporary threshold shifts following exposure to moderate sound stimulation. This opens new perspectives for investigation of sympathetic control in noise-induced permanent hearing losses.

Alterations in average spectrum of cochleoneural activity by long-term salicylate treatment in the guinea pig: a plausible index of tinnitus.

Salicylate, one of the most widely used drugs, produces at repetitive high doses reversible tinnitus and hearing loss. Neural correlates of hearing loss have long been established, whereas they remain elusive for tinnitus. The average spectrum of electrophysiological cochleoneural activity (ASECA), a measure of spontaneous auditory nerve activity, was monitored in guinea pigs over weeks of salicylate administration. Auditory nerve compound action potential (CAP) was also recorded to monitor acoustic sensitivity. In the first days of treatment, ASECA decreased acutely during hours after salicylate administration; after several days this decrease could be reduced. Over weeks of treatment the level of ASECA increased progressively. No change in CAP threshold was observed. The ASECA decrease induced by a contralateral broadband noise remained unchanged. At the end of treatment, acoustic tuning of ASECA showed a partially decreased sensitivity. After cessation of treatment the ASECA level returned progressively to initial values. In control animals delivery of an ipsilateral acoustic noise could reproduce the ASECA increase observed in long-term salicylate-treated animals. This white noise was of moderate sound pressure level and it elevated slightly CAP thresholds at high frequencies. These data provide evidence for salicylate-induced ASECA alterations without changes in CAP thresholds, in accord with clinical reports of tinnitus being the first subjective sign of salicylate ototoxicity. The similarities in occurrence, development, reversibility, frequency content, and acoustic level support the idea that ASECA changes, which indicates alterations of spontaneous eighth nerve activity and reflects the presence of salicylate-induced high-pitch tinnitus.

Efferent-mediated protection of the cochlear base from acoustic overexposure by low doses of lithium.

Many studies on anaesthetized animals and a few on awake animals have suggested that the cholinergic olivocochlear efferent feedback to outer hair cells can participate in the protection of the cochlea from acoustic overexposure. Lithium is known to stimulate acetylcholine synthesis and release in the brain and it is likely to act similarly at the level of the cochlear efferent synapses. We demonstrate here that, in the awake guinea-pig with a chronically implanted electrode on the round window of the cochlea, the temporary threshold shift induced by 1 minute exposure to different pure tones at around 90 dB sound pressure level (SPL) was reduced by as much as 40 dB, when exposure occurred after lithium treatment. The protection effect was not observed in anaesthetized animals. The effect was seen across the test frequency range of 6.4-12.5 kHz, suggesting that both 'fast' and 'slow' efferent effects are likely to be mediated by acetylcholine. Together our results provide new evidence that the olivocochlear efferents can provide a more efficient protection from acoustic overexposure when animals are awake.

Immunoreactivity for taurine in the cochlea: its abundance in supporting cells.

Taurine is the second most abundant free amino acid in the brain where its osmoregulatory function is well established. Taurine-deprived kittens show retinal pathology leading to blindness. In the inner ear, taurine has been reported to be the most abundant free amino acid although its role in inner ear function is not known. Immunohistochemistry was employed here to investigate the localisation of taurine in normal cochleae of the guinea pig compared with two different conditions: experimentally induced endolymphatic hydrops and after oral administration of glycerol. In normal cochleae, by light microscopy, taurine-like immunoreaction was never observed in the sensory outer hair cells and appeared absent from the inner hair cells. In contrast taurine-like immunolabeling was found to be present in all supporting tissue with the striking exception of the tectorial membrane and the outer pillar cell which had no or little taurine immunoreactivity respectively. In early experimental endolymphatic hydrops, the distribution of taurine-like immunoreactivity appeared similar to that observed for normal cochleae. In long-term hydrops, degenerated outer hair cells were replaced by the swelling of the phalangeal process of the Deiters' cells which became highly immunoreactive to taurine. After glycerol administration, the tectorial membrane became more tightly bound to the apical surface of the sensory hair cells and distinctly immunoreactive to taurine. The localisation of taurine in the organ of Corti shown here is consistent with taurine being involved in the maintenance of osmotic equilibrium in the normal and perhaps also in the restructuration of the pathological organ of Corti.

Reversible hearing impairment induced by lithium in the guinea pig.

Lithium salts remain one of the most widely used treatments for depressive illness. The mechanisms involved probably include reduction in free inositol. Visual perceptive disturbances can be a side effect of the treatment. We report here for the first time that chronic lithium treatment in the guinea pig induces a predominantly low frequency hearing loss and, in the longer term, loss of sensitivity is observed across the whole audiogram. The changes are reversed when treatment is arrested. The observations could be accounted for, at least partially, by a lithium-induced perturbation of the phosphoinositide cascade within the inner ear.

Ultrastructural changes in the hydropic cochlea of the guinea-pig.

Transmission electron microscopy of the cochlear organ of Corti in experimental endolymphatic hydrops revealed two principal features. Starting 1 month after induction of hydrops, osmiophilic inclusions thought to represent lipofuscin accumulation were frequently observed in the subcuticular cytoplasm of the outer hair cells along the length of the cochlea. Starting 3 months after induction of hydrops the efferent terminals on the outer hair cells appeared to be vacuolated. These data suggest that oxidative insult is likely to contribute to the pathology associated with endolymphatic hydrops and thus that free radical scavengers might be useful in the treatment of Menière's disease patients. In addition the early modification of the efferent innervation of the hydropic cochlea might underlie the known hypersensitivity to various insults, including noise stimulation, glycerol administration and hypoxia.

Auditory and vestibular function in experimental hydrops.

Endolymphatic hydrops is the characteristic morphopathology observed at postmortem examination of temporal bones of Meniere's disease. Surgical induction of endolymphatic hydrops in the guinea pig provides a reproducible animal model for the investigation of some effects of endolymphatic hydrops on inner ear function. Episodic and incapacitating vertigo are classic features of Meniere's disease, although very limited vestibular dysfunction has ever been described for the animal model. On the other hand, the auditory deficits peculiar to Meniere's disease, including fluctuant sensitivity losses and a peak audiogram, are also observed at different stages in the development of experimental hydrops. In future studies the model is likely to provide a means of investigating osmoregulatory function of the inner ear.

Immediate and long-term effects of daily glycerol administration on guinea pig hydropic ears.

Oral administration of glycerol at 0.5 g/kg and 1.0 g/kg body weight in the guinea pig with experimentally induced hydrops provoked no obvious modifications of the CAP audiogram within a 3-h recording session. Daily administration of glycerol at the same dose level during a 4-month period did not modify the deterioration of CAP thresholds although some limited beneficial effect was detected for some animals after 8 weeks of treatment.

The effect of a taurine modified diet on normal and hydropic ears of the guinea pig.

Taurine is recognised as the major osmoregulating amino acid in the brain as well as other organs. Its possible influence on normal cochlear function and on hydropic cochleas of the guinea pig was investigated here. Normally hearing adult guinea pigs were placed on either a taurine-depleting diet (B-alanine) or a taurine-supplemented diet. At the end of 6 weeks the compound action potential audiograms (CAP) in the two groups were similar and appeared not to be different from a third control group receiving a non modified diet which suggested that a taurine-modified diet had not affected normal cochlear function. In hydropic ears the taurine-depleting diet was observed, in some cases, to moderately slow down the early phase of the characteristic progression of the CAP sensitivity loss. These data are in favor of a homeostatic role for taurine in the inner ear of the guinea pig under particular conditions such as osmoregulatory stress.

Functional changes associated with experimentally induced endolymphatic hydrops.

A guinea pig model with surgically induced endolymphatic hydrops of the inner ear has been developed and studied over the past thirty years. The aim of such studies is to obtain insight into physiological processes associated with endolymphatic hydrops in man and in particular in Menière's disease where endolymphatic hydrops is systematically encountered at post-mortem examination of the temporal bones. The present review attempts to draw together the data pertaining to functional modifications of inner ear function in the animal model. For simplicity the data are categorised under five main titles: electrochemical modifications, electrophysiological modifications, pressure and hydrops, sensitivity to other insults and vestibular dysfunction. One of the most striking observations that can be made is that the data originating from different authors are very variable. There is, however, some evidence suggesting that the evolution of the auditory dysfunction could be considered as consisting of a series of different phases. This kind of information could serve as a basic framework for future research on the animal model.

Review: morphological changes associated with endolymphatic hydrops.

Endolymphatic hydrops of the inner ear is identified as a swelling of the endolymphatic spaces. This morphopathology in man can only be confirmed at post-mortem examination although it is believed to underlie the auditory dysfunction and vestibular disturbances associated with Menière's disease. This is an illusive inner ear disorder characterized typically by the fluctuant hearing loss, tinnitus and episodes of vertigo. Menière's disease remains a major problem in otorhinolaryngology since the cause of the disease is not known and various treatments are recommended, often with unsatisfactory results. Experimentally induced endolymphatic hydrops in the animal model has been developed in order to understand better the consequences of this morphopathology on inner ear structure and function. Further investigations on the model might, in the future, lead to a more efficient management of the disorder.

Cochlear and vestibular epithelia from a patient with Menière's disease: a case study.

Scanning electron microscopy observations were carried out on the cochlear and vestibular epithelia of the left temporal bone of a Menière's patient. There was almost complete absence of hair cells in the basal turn of the cochlea. The outer hair cells of the second turn presented an abnormal shortening of the shorter stereocilia within a tuft, reminiscent of the specific atrophy of the short and middle stereocilia in the ciliary tufts of outer hair cells in the guinea pig with experimental hydrops. The cilia of the inner hair cells showed fusion and giant cilia formation. Hair cells were observed in the apical turn which showed no pathological features in particular. In the saccular epithelium there were a number of striking features including, loss of the kinocilium, loss of ciliary tufts, swelling of the sensory cells, holes in the epithelium, and sensory cells pushed out and lying on the surface. The utricular epithelium was less perturbed and showed only relatively small protrusions from the epithelial surface. Similar observations have earlier been made on the vestibular epithelium in experimental hydrops. After taking into consideration the relatively long delay to fixation (12 hours) it appeared that the sacculus was more fragile and prone to autolysis than the other organs suggesting that the in-vivo pathology was manifested in particular in that organ as would be predicted from Menière's symptoms.

Time-related alteration of endolymph composition in an experimental model of endolymphatic hydrops.

The electrochemical changes of the inner ear fluids were studied in the guinea pig during the development of endolymphatic hydrops in an experimental model of Meniere's disease obtained by the blockage of the vestibular aqueduct. The endocochlear potential (first and third turns) was recorded, and the sodium, potassium, and chloride concentrations, and osmolality of the endolymph (first and third turns) and perilymph were determined at different intervals from 2 to 24 weeks after the induction of the hydrops. The development of hydrops was monitored by the compound action potential once a week during the observation period. In normal, nonoperated guinea pigs, longitudinal endolymphatic gradients of endocochlear potential, potassium and chloride concentrations, and osmolality, increasing from the apex to the base of the cochlea, were observed. After 2 weeks of hydrops, no alteration of this pattern was detected. After 6 and 9 weeks of hydrops, a progressive decrease of endocochlear potential, potassium and chloride concentrations, and osmolality was noticed at the first turn (6 and 9 weeks) and then at the third turn (9 weeks) which resulted in the disappearance of longitudinal gradients. At 24 weeks, the endocochlear potential was still diminished by 60%, whereas potassium and chloride concentrations and osmolality increased as compared to 9-week values but remained lower than in controls. The changes in composition of endolymph induced by the development of the hydrops could be related to the progressive alteration of the ionic permeability of the cochlear epithelium, which should be localized at the distended Reissner's membrane.

The vestibular epithelia in experimental hydrops.

Morpho-pathological features, observed by scanning electron microscopy, in guinea pigs with endolymphatic hydrops of 4-14 months included shortening of the hair cell tufts, loss of tufts, retraction of sensory hair cells away from the surrounding tissue and hair cell loss. After 22 months of hydrops, there was complete loss of hair cells with indifferentiation of the epithelium. The loss of ciliary tufts involved loss of both stereocilia and kinocilia identified as short stubs and holes respectively. Control macular epithelia showed no hair tuft loss although ampullae in control ears could show some loss.

Contribution of increased endolymphatic pressure to hearing loss in experimental hydrops.

After the induction of experimental hydrops there follows a strict sequence of compound action potential (CAP) audiogram changes. Within days a low-frequency loss (below 8 kHz) is detected; within weeks a very high-frequency loss (above 8 kHz) develops; and finally within months the 8-kHz region is also affected. Following the application of excess pressure, presumably to the endolymphatic spaces, via a cannula placed in the endolymphatic duct, a sequence of CAP audiogram changes occurs that is not similar to that observed with hydrops. There is first a very high-frequency loss (above 4 kHz), then a very low-frequency loss (below 4 kHz), and finally the 4 kHz region is also affected and thresholds for all frequencies become even more raised. The data thus suggest that a slow accumulation of endolymph as induced by blockage of the endolymphatic duct does not produce the same disorder as a rapid onset of a high pressure applied at the base of the cochlea. In addition the data suggest that raised endolymphatic pressure is not likely to be significant in early stages of hydrops, but could contribute to the later high-frequency sensitivity losses.

Atrophy of outer hair cell stereocilia and hearing loss in hydropic cochleae.

We have earlier described selective atrophy of short and middle stereocilia on outer hair cells of the three upper cochlear turns in hydropic cochleae of guinea pigs. The present study describes sequential early stages of stereocilia degeneration leading to this specific atrophy. Comparison of the morpho-pathology with the ultimate CAP audiograms taken before sacrifice indicated a close association between the low frequency hearing loss and this atrophy of stereocilia. The atrophy appeared to be associated first with the short and then the middle stereocilia of the 2nd and 3rd rows of outer hair cells between 0.5 kHz and 2 kHz and with time included the 1st row of all outer hair cells of the upper cochlear turns down to the 8 kHz region.

Old theme and new reflections: hearing impairment associated with endolymphatic hydrops.

The cause(s) of the hearing impairment associated with Menière's disease are not understood but are undoubtedly associated with the inner ear endolymphatic hydrops. Two major hypotheses have been proposed and widely received: endolymphatic overpressure followed by leaky membranes and subsequently the mixing of high K+ endolymph with perilymph. Our recent data on an experimental model of endolymphatic hydrops have provided grounds for renewed reflections on the pathology. Indeed our data might be interpreted without involving either of the above hypotheses and suggests that the symptoms of Menière's disease might be accounted for by a flow of perilymph from scala vestibuli towards scala tympani with the mixing of the two perilymphs which are similar but not identical in composition. The higher K+ concentration arriving from the scala vestibuli into the scala tympani at the apex of the cochlea via the helicotrema is likely to be toxic to hair cell and auditory nerve fiber function. The mixing of the two perilymphs could result in deterioration of low frequency sensitivity, provoke low frequency tinnitus and in the long term cause spiral ganglion cell degeneration at the apex of the cochlea. The feeling of fullness in the ear might be the result of the decreasing perilymph volume in the scala vestibuli which could give rise to inner ear conductive losses. The patency of the cochlear aqueduct might play a role in determining the high risk group of individuals likely to manifest the symptoms of endolymphatic hydrops.