Glutamate release

The presynaptic specialization of the hair cells is characterized by an electron dense body (Friedmann and Ballantyne, 1984). This is localized near the centre of the synapse and is surrounded by synaptic vesicles. As discussed below, it is believed that synaptic release occurs mainly or exclusively at the site of the presynaptic dense body (Furukawa et al., 1982 Parsons et al., 1994). 

A large variety of glutamate receptors of every major family has been demonstrated at the protein or mRNA level in spiral and vestibular ganglion cells (Kuriyama et al., 1993, 1994 Fujita et al., 1994 Dememes et al., 1995 Niedzielski and Wenthold, 1995 also see Fig. 6A). The challenge has been to identify the receptor types that are expressed at the afferent synapse itself and which take part in hair cell transmission. 

with Nakagawa et al., 1991a). In agreement, the outer hair cells have been found to exhibit a low rate of vesicle recycling compared to the inner hair cells (Siegel and Brownell, 1986). 

We can thus conclude from the immunoelectron microscopical data that AMPA receptors composed of GluR2/3 and GluR4 subunits are likely to be involved in afferent hair cell transmission. This would be in line with physiological studies. Patch-clamp analyses have identified functional AMPA receptors in isolated spiral ganglion cells (Nakagawa et al.. 

1991b Ruel et al., 1999), and antisense knockdown of GluR2 was shown to reduce the compound action potential and diminish spontaneous activity of single auditory nerve fibres (D Aldin et al., 1998). The effect of the antisense probe was confirmed by demonstrating a reduction in GluR2/3 immunoreactivity in ganglion cells. 

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Hundt W, Holter SM, Spanagel R Discriminative stimulus effects of glutamate release inhibitors in rats trained to discriminate ethanol. Pharmacol Biochem Behav 59 691-695, 1998... 

The GMT in human serum reacts most rapidly with Y-glutamyl-p-nitroanilide at pH 8.2. The same activity is found in 2-amino-2-methylpropane-l 3 diol, diethanolamine, triethanolamine and tris buffers. Magnesium ions have no effect on the activity but favor the solubilization of the substrate. Bondar and Moss (54) found that free glutamate, due to elevated serum glutamate concentrations or glutamate released by substrate breakdown, increases the apparent GMT activity. They concluded that the assay should be performed in the presence of 1.0 vM/1 glutamate in order to reduce the possibility of falsely elevated results. This was not observed by others. Rowe and co-workers have indicated that certain batches of p-nitroanilide substrate contain impurities which may reduce GMT activity and increase the values ( ). Huesby and Stromme (56) confirmed the presence of such impurities and recommended pyridine extraction for substrate purification. 

Other Links Between Chemokines and Excitotoxic Injury Glutamate Release... 

Chemokines as Neuromodulators Regulation of Glutamatergic Transmission by CXCR4-Mediated Glutamate Release From Astrocytes... 

Bezzi P, Carmignoto G, Pasti L, Vesce S, Rossi D, Rizzini BL, Pozzan T, Volterra A (1998) Prostaglandins stimulate calcium-dependent glutamate release in astrocytes. Nature 391 281-285... 

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