Glutamate induced modulation of free Ca(2+) in isolated inner hair cells of the guinea pig cochlea

Pre- and Postsynaptic Effects of Glutamate in the Frog Labyrinth

The role of glutamate in quantal release at the cytoneural junction was examined by measuring mEPSPs and afferent spikes at the posterior canal in the intact frog labyrinth. Release was enhanced by exogenous glutamate, or dl-TBOA, a blocker of glutamate reuptake. Conversely, drugs acting on ionotropic glutamate receptors did not affect release; the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) blocker CNQX decreased mEPSP size in a dose-dependent manner; the NMDA-R blocker d-AP5 at concentrations <200 µM did not affect mEPSP size, either in the presence or absence of Mg and glycine. In isolated hair cells, glutamate did not modify Ca currents. Instead, it systematically reduced the compound delayed potassium current, IKD, whereas the metabotropic glutamate receptor (mGluR)-II inverse agonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY341495), increased it. Given mGluR-II decrease cAMP production, these finding are consistent with the reported sensitivity of IKD to protein kinase A (PKA)-mediated phosphorylation. LY341495 also enhanced transmitter release, presumably through phosphorylation-mediated facilitation of the release machinery. The observed enhancement of release by glutamate confirms previous literature data, and can be attributed to activation of mGluR-I that promotes Ca release from intracellular stores. Glutamate-induced reduction in the repolarizing IKD may contribute to facilitation of release. Overall, glutamate exerts both a positive feedback action on mGluR-I, through activation of the phospholipase C (PLC)/IP₃ path, and the negative feedback, by interfering with substrate phosphorylation through G_i/0-coupled mGluRs-II/III. The positive feedback prevails, which may explain the increase in overall rates of release observed during mechanical stimulation (symmetrical in the excitatory and inhibitory directions). The negative feedback may protect the junction from over-activation.

Excitotoxic effects of glutamate on cochlear organotypic cultures

Glutamate (Glu) is the major afferent excitatory neurotransmitter in the auditory system, and excessive Glu may play an important role in cochlear dysfunction. It is unclear how excessive Glu plays roles in cochlear dysfunction in cochlear organotypic cultures. In this study neonatal rat cochlear organotypic cultures were prepared, and then the cochlear tissues were incubated with a new medium containing specific concentrations of Glu (0.1, 0.5, 1, 10 or 20 mmol/L) for 24 h, or incubated with the medium containing a concentration of 20 mmol/L Glu for 6, 12, 24 or 72 h, respectively. It was found that when the cochlear tissues were cultured for 24 h, the inner hair cells (IHCs) were damaged at the concentration of 0.5 mmol/L Glu, and with the increases of the concentrations, the injury was gradually aggravated, and 20 mmol/L Glu resulted in the significant loss of IHCs. In the 20 mmol/L Glu groups, the stereocilia bundles were missing or disarrayed on a few IHCs after culture for 6 h and the damage effect was time-dependent. The missing of IHCs was more significant in the basal turn of the cochlea than in the middle turn of the cochlea under the same concentration of Glu exposure. These results suggest that excessive exogenous Glu affects the morphology of IHCs, but not affects the outer hair cells (OHCs) in cochlear organotypic cultures, and the excitotoxic effects are different on IHCs of different parts of the cochlea under the same concentration of Glu exposure.

Reference values of elements in human hair: a systematic review

BACKGROUND

The lack of systematic review on reference values of elements in human hair with the consideration of methodological approach. The absence of worldwide accepted and implemented universal reference ranges causes that hair mineral analysis has not become yet a reliable and useful method of assessment of nutritional status and exposure of individuals.

OBJECTIVES

Systematic review of reference values of elements in human hair.

DATA SOURCES

PubMed, ISI Web of Knowledge, Scopus.

STUDY ELIGIBILITY CRITERIA

Humans, hair mineral analysis, elements or minerals, reference values, original studies.

RESULTS

The number of studies screened and assessed for eligibility was 52. Eventually, included in the review were 5 papers. The studies report reference ranges for the content of elements in hair: macroelements, microelements, toxic elements and other elements. Reference ranges were elaborated for different populations in the years 2000-2012. The analytical methodology differed, in particular sample preparation, digestion and analysis (ICP-AES, ICP-MS). Consequently, the levels of hair minerals reported as reference values varied.

CONCLUSIONS

It is necessary to elaborate the standard procedures and furtherly validate hair mineral analysis and deliver detailed methodology. Only then it would be possible to provide meaningful reference ranges and take advantage of the potential that lies in Hair Mineral Analysis as a medical diagnostic technique.

Glutamate agonist causes irreversible degeneration of inner hair cells

Glutamate neurotoxicity in cochlear hair cells was investigated by administering the glutamate agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) into the scala tympani of Mongolian gerbils. AMPA administration caused the formation of large number of vacuoles in the inner hair cells (IHCs) and dendritic terminals. The number of degenerated hair cells was counted using rhodamine-phalloidin and Hoechst 33342 staining. The administration of 50 microM AMPA caused reversible elevation of the auditory brainstem response threshold without loss of IHCs. In contrast, 200 microM AMPA induced a substantial elevation of the auditory brainstem response threshold with the characteristic disappearance of IHCs. As cochlear ischemia involves excessive glutamate release, these results suggest that an elevated glutamate level in the cochlea is responsible for the progressive IHC death related to ischemic injury.

Gentamicin induced nitric oxide-related oxidative damages on vestibular afferents in the guinea pig

Gentamicin is a well-known ototoxic aminoglycoside. However, the mechanism underlying this ototoxicity remains unclear. One of the mechanisms which may be responsible for this ototoxicity is excitotoxic damage to hair cells. The overstimulation of the N-methyl-d-aspartate (NMDA) receptors increases the production of nitric oxide (NO), which induces oxidative stress on hair cells. In order to determine the mechanism underlying this excitotoxicity, we treated guinea pigs with gentamicin by placing gentamicin (0.5 mg) pellets into a round window niche. After the sacrifice of the animals, which occurred at 3, 7 and 14 days after the treatment, the numbers of hair cells in the animals were counted with a scanning electron microscope. We then performed immunostaining using neuronal nitric oxide synthase (nNOS), inducible NOS (iNOS) and nitrotyrosine antibodies. The number of hair cells in the animals was found to decrease significantly after 7 days. nNOS and iNOS expression levels were observed to have increased 3 days after treatment. Nitrotyrosine was expressed primarily at the calyceal afferents of the type I hair cells 3 days after treatment. Terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) staining revealed positive hair cells 3 days after treatment. Our results suggest that inner ear treatment with gentamicin may upregulate nNOS and iNOS to induce oxidative stress in the calyceal afferents of type I hair cells, via nitric oxide overproduction.

AMPA/kainate-type glutamate receptor antagonist reduces progressive inner hair cell loss after transient cochlear ischemia

We investigated the effect of glutamate receptor antagonists on progressive inner hair cell (IHC) loss following transient cochlear ischemia in gerbils. Transient cochlear ischemia was induced by 15-min bilateral vertebral artery occlusion. An alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate-type glutamate receptor antagonist, 6-7-dinitroquinoxaline-2,3-dione (DNQX), or an N-methyl-D-aspartate (NMDA)-type receptor antagonist, MK-801, was administered 10 min before the ischemic insult. Hearing was assessed by sequentially recording compound action potentials (CAPs) before, during, and after the ischemia. The degree of hair cell loss in the organ of Corti was evaluated in specimens stained with rhodamine-phalloidin and Hoechst 33342. On the seventh day after ischemia, the increases in the CAP threshold and the progressive IHC loss were significantly reduced in cochleae treated with DNQX, while MK-801 was ineffective. These results suggest that the AMPA receptor plays a critical role in the development of the progressive IHC loss induced by ischemia/reperfusion injury in the cochlea.

Glutamate induced currents in isolated inner hair cells from guinea-pig cochlea

We investigated the direct action of glutamate (Glu) on the membrane current of isolated inner hair cells of guinea-pig cochlea. Glu elicited inward currents at a holding potential of -70 mV. Eight of 13 cells showed a steady inward current, while five of 13 cells showed a fast and rapidly desensitized current. I-V relationships demonstrated that the reversal potential of Glu-induced current was near 0 mV. Glu-induced currents were dose-dependent, where the half maximum concentration (K(d)) was 41 microM and Hill coefficient (n) was 1.75.