Receptors for leptin in the otic labyrinth and the cochlear-vestibular nerve of guinea pig are modified in hormone-induced anorexia.

Metabolic syndromic inner ear pathology is a recognized condition in clinical practice but the possible causes remain controversial. We have previously reported that chronically-implanted estrogen implants in guinea pig results in hyperprolactinemia and hearing loss together with otic bone dysmorphology. The animals also present with anorexia. The hormone leptin has major roles in the regulation of satiety as well as bone metabolism and so we hypothesized that leptin might contribute to pathology of the otic labyrinth. We employed immunohistochemistry to investigate leptin receptor (ObR) expression. In control animals, ObR immunolabeling was not detected in the bone of the otic capsule but immunolabeling was observed in the cochlear-vestibular nerve. The labeling was associated with the astrocytic glial dome area, which marks the transition between central and peripheral parts of the nerve. In estrogen-treated animals, positive-ObR immunolabeling was observed in osteoblasts in new bone of the otic capsule and the ObR labeling was reduced in the cochlear-vestibular nerve compared to controls. The data provide evidence that leptin may target the labyrinth - affecting the bone and the nerve - and so could contribute to ongoing protection of the inner ear. Leptin disturbance might contribute to metabolic syndromes involving the audiovestibular system.

Effect of chronic estradiol administration on vimentin and GFAP immunohistochemistry within the inner ear.

Recent data show that hormone replacement therapy, involving estrogen together with progestin, can promote hearing loss (Guimaraes, P., Frisina, S.T., Mapes, F., Tadros, S.F., Frisina, D.R. and Frisina, R.D., 2006. Progestin negatively affects hearing in aged women. Proc. Natl. Acad. Sci. USA. 103, 14246-14249.). But long-term estradiol treatment, which induces hyperprolactinemia in guinea pigs, results in hearing loss and bone dysmorphology of the otic capsule-without much hair cell loss (Horner, K.C., Cazals, Y., Guieu, R., Lenoir, M. and Sauze, N., 2007. Experimental estrogen-induced hyperprolactinemia results in bone-related hearing loss in the guinea pig. Am. J. Physiol., Endocrinol. Metab. 293, E1224-1232.). Since estrogen receptor beta can protect the mouse cochlea against acoustic trauma (Meltser, I., Tahera, Y., Simpson, E., Hultcrantz, M., Charitidi, K., Gustafsson, J.A. and Canlon, B., 2008. Estrogen receptor beta protects against acoustic trauma in mice. J. Clin. Invest. 118, 1563-1570.), we hypothesized that estradiol might activate protective glial-like elements in the inner ear. Immunohistochemistry showed down-regulation of vimentin within the lateral wall and upregulation within the spiral limbus. Glial fibrillary acid protein was increased in the inner sulcus, Hensen cells and Claudius cells. Furthermore, there was increased expression of vimentin in type II cells of the spiral ganglion and type I vestibular hair cells. The observations suggested that estradiol treatment may affect the inner ear ionic homeostasis but protection may be afforded via activated intermediate filaments.

BDNF-TrkB signaling interacts with the GABAergic system to inhibit rhythmic swallowing in the rat.

Brain-derived neurotrophic factor (BDNF) acts as an anorexigenic factor in the dorsal vagal complex (DVC) of the adult rat brain stem. The DVC contains the premotoneurons controlling swallowing, a motor component of feeding behavior. Although rats with transected midbrain do not seek out food, they are able to swallow and to ingest food. Because BDNF and tropomyosin-related kinase B (TrkB) receptors are expressed in the DVC, this study hypothesized that BDNF could modify the activity of premotoneurons involved in swallowing. Repetitive electrical stimulation of the superior laryngeal nerve (SLN) induces rhythmic swallowing that can be recorded with electromyographic electrodes inserted in sublingual muscles. We show that a microinjection of BDNF in the swallowing network induced a rapid, transient, and dose-dependant inhibition of rhythmic swallowing. This BDNF effect appeared to be mediated via TrkB activation, since it no longer occurred when TrkB receptors were antagonized by K-252a. Interestingly, swallowing was inhibited when subthreshold doses of BDNF and GABA were coinjected, suggesting a synergistic interaction between these two signaling substances. Moreover, BDNF no longer had an inhibitory effect on swallowing when coinjected with bicuculline, a GABA(A) receptor antagonist. This blockade of BDNF inhibitory effect on swallowing was reversible, since it reappeared when BDNF was injected 15 min after bicuculline. Finally, we show that stimulation of SLN induced a decrease in BDNF protein within the DVC. Together, our results strongly suggest that BDNF inhibits swallowing via modulation of the GABAergic signaling within the central pattern generator of swallowing.

Glial fibrillary acidic protein (GFAP)-positive radial-like cells are present in the vicinity of proliferative progenitors in the nucleus tractus solitarius of adult rat.

The dorsal vagal complex (DVC), an integrative center of autonomic functions located dorsally in the caudal brainstem, comprises the nucleus tractus solitarius (NTS), the area postrema (AP), and the dorsal motor nucleus of the vagus nerve (DMNX). Recently, this area of the brainstem was shown to retain, during adulthood, the expression of developmental markers, which is consistent with several forms of morphological and functional plasticity. These data led us to attempt to determine the structural organization and phenotypical characteristics of the astroglial compartment in the adult DVC. We report a strikingly high density of glial fibrillary acidic protein (GFAP) immunoreactive cells in the NTS and the DMNX compared to other brainstem structures. Furthermore, we observed a subpopulation of atypical GFAP+ cells in the NTS. These cells expressed vimentin and nestin and displayed unbranched processes that radiate rostrocaudally from cuboid cell bodies located in the 4th ventricle wall. Interestingly, these radiating cells were found in close association with neural progenitors whose proliferation was stimulated by intracerebroventricular injection of epidermal growth factor/basic fibroblast growth factor or lesion of the vagus nerve. Newly born neurons in the NTS identified by doublecortin (DCX) immunolabeling were also preferentially found in the vicinity of the radiating cells. Altogether, these results indicate that the adult NTS retains, during adulthood, astroglial cells that display morphological and phenotypical features seen during development. The overlap in the distribution of proliferative neural progenitors, newborn neurons, and radiating GFAP-positive cells suggest a possible role of the glial compartment of the NTS in functional plasticity in this structure.

Effects of anesthetics on hypoglossal nerve discharge and c-Fos expression in brainstem hypoglossal premotor neurons.

This study examined the effects of anesthesia on the hypoglossal nerve and diaphragm activities and on c-Fos expression in brainstem hypoglossal premotor neurons (pmXII). Experiments were performed in 71 rats by using halothane inhalation, pentobarbital sodium, or mixtures of alpha-chloralose and urethane or ketamine and xylazine. First, various cardiorespiratory parameters were measured in the rats (n = 31) during both awake and anesthetized conditions. The volatile anesthetic halothane, but not the other anesthetics, was always associated with a strong phasic inspiratory activity in the hypoglossal nerve. Second, a double-immunohistochemical study was performed in awake and anesthetized rats (n = 40) to gauge the level of activity of pmXII neurons. Brainstem pmXII neurons were identified after microiontophoresis of the retrograde tracer Fluoro-Gold in the right hypoglossal motor nucleus. Patterns of c-Fos expression at different brainstem levels were compared in five groups of rats (i.e., awake or anesthetized with halothane, pentobarbital, chloralose-urethane, and ketamine-xylazine). Sections were processed for double detection of c-Fos protein and Fluoro-Gold by using the standard ABC method and a two-color peroxidase technique. Anesthesia with halothane induced the strongest c-Fos expression in a restricted pool of pmXII located in the pons at the level of the Kölliker-Fuse nucleus and the intertrigeminal region. The results demonstrated a major effect of halothane in inducing changes in hypoglossal activity and revealed a differential expression of c-Fos protein in pmXII neurons among groups of anesthetized rats. We suggest that halothane mediates changes in respiratory hypoglossal nerve discharge by altering activity of premotor neurons in the Kölliker-Fuse and intertrigeminal region.