Francisco Carricondo

Serotonergic innervation of the organ of Corti.

The olivocochlear efferent system of the mammalian cochlea, which is divided into two lateral and medial bundles, contains numerous neuroactive substances (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). These have been located at the brainstem in neurons belonging to the lateral superior olive (lateral efferent system) or in neurons of the periolivary region around the medial superior olive and the trapezoid body (medial efferent system). All of these substances were found in well-characterized projections corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respectively. All could be involved in the modulation of the auditory process, as is suggested by the cochlear turnover increases observed in some of them (i.e. enkephalins or dopamine) induced by sound stimulation. Recently, the presence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly particular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a previously unknown cochlear innervation.The olivocochlear efferent system of the mammalian cochlea, which is divided into two lateral and medial bundles, contains numerous neuroactive substances (acetylcholine, GABA, dopamine, enkephalins, dynorphins and CGRP). These have been located at the brainstem in neurons belonging to the lateral superior olive (lateral efferent system) or in neurons of the periolivary region around the medial superior olive and the trapezoid body (medial efferent system). All of these substances were found in well-characterized projections corresponding to lateral and medial nerve fibres and terminals which connect to the type I afferent dendrites and the outer hair cells, respectively. All could be involved in the modulation of the auditory process, as is suggested by the cochlear turnover increases observed in some of them (i.e. enkephalins or dopamine) induced by sound stimulation. Recently, the presence and distribution of serotonin-containing fibres has been included in the long list of cochlear neuroactive substances. However, its highly particular peripheral pattern of distribution together with the lack of response to sound stimulation could suggest that serotonergic fibres constitute a previously unknown cochlear innervation.

Biochemical evidence for the presence of serotonin transporters in the rat cochlea.

Cochlear serotonergic innervation is constituted by efferent fibers projecting both to the area below the inner and the outer hair cells. Previous detection of serotonin (5-HT) metabolites and 5-HT receptor mRNAs suggests the existence of serotonergic synaptic activity in the cochlea. The present study explores this possibility through the effect of 6-nitroquipazine (6-NQ), a 5-HT selective reuptake inhibitor, on the basal turnover of 5-HT. The concentrations of 5-HT and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA) were quantified by high performance liquid chromatography with electrochemical detection in blood-free cochleae of rats treated with 6-NQ or saline and kept under silent conditions. Treatment with 6-NQ induced a significant increase of the cochlear concentration of 5-HT and a significant reduction of 5-HIAA concentration with respect to saline treatment. These findings could indicate that 6-NQ induced the blockade of the 5-HT selective reuptake to the cochlear serotonergic fibers. This suggests that plasma membrane 5-HT transporters are present in cochlear serotonergic fibers. Even though the role of serotonergic innervation on cochlear physiology remains unknown, the existence of cochlear serotonergic synaptic activity is strongly supported by present contributions.

Trimetazidine prevents cochlear lesions induced by intraperitoneal and perilymphatic administration of kainic acid.

The protective activity of trimetazidine (TMZ) against cochlear neurotoxicity induced by intraperitoneal and intracochlear administration of kainic acid (KA) has been analyzed. The amplitude of the CAP N1 wave was significantly higher in KA rats pretreated with TMZ, independently of the administration route, than in those only treated with KA. However, CAP N1 amplitude of both TMZ pretreated and non-pretreated animals was always lesser than the N1 wave amplitude observed in the control group. The CAP N1 latency did not show any significant difference between KA and TMZ+KA groups except at high intensities of 8 and 12 kHz. As a complementary control, we have demonstrated that the intraperitoneal administration of TMZ (5 mg/kg) alone did not affect either the electrophysiological activity or the morphology of the auditory nerve. Morphological results fit well with electrophysiology. Some isolated swollen afferent fibers were observed in TMZ+KA cochleae, the swollen dendrites being sparser than in the KA only treated animals. In TMZ+KA animals, the cochlear apical coils were less affected than the basal coils. Our results are in agreement with recent clinical studies and suggest that TMZ could be an active drug on cochlear impairment linked to hypoxic-ischaemic syndromes.

Cochlear microphonic potentials: a new recording technique.

A new instrumentation and a particular method for detecting and recording cochlear microphonic potentials (CMPs) are described here. The CMPs were recorded in rats by means of pure tones (4,000, 2,000, 1,000, 500, and 250 Hz) and intraepidermic electrodes; the electrocochleography technique was avoided. An experimental design that included the use of a glutamatergic agonist (kainic acid [KA]) and an aminoglycoside antibiotic (kanamycin [KANA]) was carried out to demonstrate the origin of the recorded potential. Morphological studies showed that KA selectively eliminated the afferent type I dendrites of the spiral ganglion, while the administration of KANA resulted in the absence of outer hair cells. When CMPs were recorded after KA administration, no alterations were detected. In contrast, KANA administration resulted in the absence of any selective electrophysiological activity corresponding to CMPs. All these results were compared with the recording of the compound action potential of the eighth nerve obtained by electrocochleography. These findings and the great specificity of the reproduction of the sound stimulus confirm that the CMPs can be recorded by the new equipment.

Cochlear microphonic audiometry: a new hearing test for objective diagnosis of deafness

Conclusions. Objective audiometric tests could constitute a valuable tool for detection of deafness. This could be especially useful in children (universal newborn hearing screening) and non-collaborative patients, who are especially difficult candidates for classic audiometry. The cochlear microphonic audiometry (CMA) technique offers the possibility of obtaining objective audiometric profiles, highly correlated with those obtained by pure tone audiometry (PTA). Therefore, CMA could be used as an alternative test to obtain the audiometric profile of these patients. Objective. The main purpose of the present study was to demonstrate that CMA provides objective audiometric profiles by avoiding active participation by the patient. Subjects and methods. CMA specific equipment, improved for non-invasive recording of cochlear microphonic potentials, was used. This tool plots the recordings obtained as the classic audiogram. Verification of the method was carried out in adult patients by comparing the PTA with the CMA audiometric profiles obtained for each patient. Results. Our findings showed that audiometric profiles obtained from CMA are highly correlated, without statistical differences, to those obtained with PTA. More than 81% of patients explored (91.67% at 250 Hz) exhibited differences below 10 dB(HL) between tests at all exploration frequencies, while a low number of cases showed differences over 20 dB(HL).

Cellular and molecular bases of neuroplasticity: brainstem effects after cochlear damage

After a cochlear lesion or auditory nerve damage, afferent connections from auditory ganglia can be highly altered. This results in a clear reduction of auditory input and an alteration of connectivity of terminals on cochlear nuclei neurons. Such a process could stimulate the reorganization of the neural circuits and neuroplasticity. Cochlea removal has been demonstrated to be a good model in which to analyse brainstem neuroplasticity, particularly with regard to the cochlear nuclei. After cochlea removal three main periods of degeneration and regeneration were observed. Early effects, during the first week post lesion, involved acute degeneration with nerve ending oedema and degeneration. During the second and, probably, the third post lesion weeks, degeneration was still present, even though a limited and diffuse expression of GAP-43 started. Around 1 month post lesion, degeneration at the cochlear nuclei progressively disappeared and a relevant GAP-43 expression was found. We conclude that neuroplasticity leads neurons to modify their activity and/or their synaptic tree as a consequence of animal adaptation to learning and memory. For the human being neuroplasticity is involved in language learning and comprehension, particularly the acquisition of a second language. Neuroplasticity is important for therapeutic strategies, such as hearing aids and cochlear implants.

Effects of aging on C57BL/6J mice: an electrophysiological and morphological study.

Presbycusis is a progressive hearing loss associated with aging that manifests as deafness linked to cochlear morphological degeneration. The effects of aging on the auditory system were studied in C57BL/6J mice using electrophysiological (brainstem auditory evoked potentials; BAEP) and morphological techniques. Cochleae of animals aged 1, 6, 9, 12, 15, 18, 21, or 24 months old were used for that purpose. The BAEP showed a progressive increase in latency and a reduction in amplitude. Morphological studies demonstrated total degeneration of the organ of Corti, which was replaced by a single epithelial layer. An affinity histochemistry study demonstrated minor modifications of glycoconjugates in the organ of Corti during the aging process.

The impact of comorbidity on the survival of patients with laryngeal squamous cell carcinoma

Abstract Conclusions: In survival analysis, the combined Charlson comorbidity index (CCI) can be considered as a prognostic factor independent of the tumor node metastasis (TNM) classification, tumor stage, and tumor location. Severe comorbidity was the factor that had the greatest impact on prognosis in cases of initial tumor. Objective: To study the influence of comorbidity on the survival of patients undergoing surgery for larynx cancer. Methods: This was a retrospective study of the survival of 231 patients with laryngeal cancer who underwent surgery between 1995 and 2002. The CCI was used to assess comorbidity, the Kaplan–Meier method was used for survival analysis, and the Cox proportional risk regression model was used to identify independent prognostic factors. Results: The multivariate analysis of specific mortality showed that patients classified as having severe comorbidity (CCI) were more likely to die (adjusted hazard ratio (adjHR) 1.85, 95% confidence interval (CI) 1.07–3.17). This difference was more important in patients with early tumor stages than in those with advanced stages.

Immunocytochemical detection of synaptophysin in C57BL/6 mice cochlea during aging process

Aged mammals frequently exhibit a bilateral, progressive, and symmetric deafness related to the degeneration of auditory receptor. However, little is still known about aging effects on synapses in this receptor. Synaptophysin (Syp) is a 38 kDa Ca2+ binding glycoprotein widely found in presynaptic membrane and vesicles. The Syp has been found in presynaptic buttons of efferent auditory fibers, within the developing and adult auditory receptor. The detection of Syp in aged cochleae could provide relevant information about synaptic changes and receptor degeneration process observed in old animals. This paper focuses on aging linked changes related to the presence of Syp in cochleae of C57BL/6J mice (from 1 to 24 months old). Results showed that during the first months of age, no significant changes were observed in the Syp distribution under the basal pole of inner (IHCs) neither the outer (OHCs) hair cells. At six months of age, a significant decrease of Syp immunocytochemical detection appeared in fibers under the most external row of OHCs, but restricted to the cochlear basal coil. Only a very scarce reduction of Syp was noted under the IHC and the other OHC rows, also at the basal coil. From mice 9 months old on, a progressive decrease of the presence of Syp was found under IHC and all OHC rows starting at the basal coil and reaching the apical coil in the oldest mice. All these data could indicate that the cochlea aging process early affects to presynaptic membrane proteins of efferent endings fibers. This early alteration of cochleae efferent synapses could be involved in the whole degeneration of the Cortis organ.

In vitro long-term development of cultured inner ear stem cells of newborn rat

The adult mammalian auditory receptor lacks any ability to repair and/or regenerate after injury. However, the late developing cochlea still contains some stem-cell-like elements that might be used to regenerate damaged neurons and/or cells of the organ of Corti. Before their use in any application, stem cell numbers need to be amplified because they are usually rare in late developing and adult tissues. The numerous re-explant cultures required for the progressive amplification process can result in a spontaneous differentiation process. This aspect has been implicated in the tumorigenicity of stem cells when transplanted into a tissue. The aim of this study has been to determine whether cochlear stem cells can proliferate and differentiate spontaneously in long-term cultures without the addition of any factor that might influence these processes. Cochlear stem cells, which express nestin protein, were cultured in monolayers and fed with DMEM containing 5% FBS. They quickly organized themselves into typical spheres exhibiting a high proliferation rate, self-renewal property, and differentiation ability. Secondary cultures of these stem cell spheres spontaneously differentiated into neuroectodermal-like cells. The expression of nestin, glial-fibrillary-acidic protein, vimentin, and neurofilaments was evaluated to identify early differentiation. Nestin expression appeared in primary and secondary cultures. Other markers were also identified in differentiating cells. Further research might demonstrate the spontaneous differentiation of cochlear stem cells and their teratogenic probability when they are used for transplantation.