Cortical Amino Acid Release Figure 10. Hypothetical explanation of the experimental data showing differences in taurine and glutamate release in vivo. A reduction in medium osmolarity (Q[osm]e) in the rat cortex causes an increase in the extracellular levels of the excitatory amino acids glutamate and aspartate and the sulfonic acid taurine via a mechanism sensitive to the anion channel blocker DNDS. Despite these similarities, the excitatory amino acid and taurine release demonstrate different kinetics and are likely mediated by different transport pathways (1 and 2) and/or originate from different cellular sources. The taurine pathway (1) but not the excitatory amino acid pathway (2) is activated by isoosmotic lowering of [NaCl]e. Conversely, the swelling-activated excitatory amino acid release pathway (2) but not the taurine pathway (1) is potentiated by H2O2. Alternative transport pathways that were considered in this study include: [Na+]e-dependent taurine transporters (3), [Na+]e/[K+]i-dependent glutamate transporters in neurons (4), and in astrocytes (5), which are sensitive to TBOA; and vesicular glutamate release (6), which is sensitive to the voltage- gated Ca2+ channel blocker Cd2+. Based on the similarities of excitatory amino acid release in vivo and in cultured astrocytes, we speculate that glutamate release in vivo largely originates from glial cells. Similarities between taurine release in vivo and in synaptosomes suggest that taurine release may be of a neuronal origin. doi:10.1371/journal.pone.0003543.g010 the selective VRAC inhibitor DCPIB has been found to reduce intra-ischemic glutamate release and potently reduce ischemic infarction in a rat focal reversible ischemia model [15]. Consistent with in vivo findings, in a slice model of cerebral spreading depression, VRAC inhibitors delay the onset of glutamate- dependent depolarizations and reduce glutamate release, which is at least partially associated with cell swelling [84]. Interestingly, characteristics of intra-ischemic taurine release also show strong deviation from those of glutamate and aspartate. For instance, in a rat global ischemia model ischemic striatal taurine release was weekly sensitive to 1 and 10 mM DNDS, as compared to the strong inhibition of pathological excitatory amino acid release [11]. Although in the present experiments, both the hypoosmotic release of taurine and the excitatory amino acids were potently suppressed by DNDS, the global ischemia data are consistent with the idea that taurine and excitatory amino acids may be released from different cellular pools and/or via different transport mechanisms. Properties of swelling activated excitatory amino acid release in vivo strongly resembled the positive modulation of the excitatory amino acid release via a putative VRAC pathway, and in particular were strongly upregulated by H2O2, as seen in cultured astrocytes [69]. Since H2O2 levels are strongly upregu- References lated in ischemia and reperfusion [85,86], the additive effects of cell swelling and reactive oxygen species may contribute to excitotoxic tissue damage in stroke and perhaps other neuropa- thologies. In summary, we found that cellular swelling in the rat cortex in vivo triggers release of glutamate and aspartate with properties strongly resembling swelling-activated excitatory amino acid release in cultured astrocytes, which is thought to be mediated by VRAC [47]. In contrast, hypoosmotic medium-induced taurine release seemingly is derived from a different cellular source or mediated by different transporter mechanism(s). Since taurine release properties could be mimicked in synaptosomal prepara- tions, we speculate that such release may originate from a neuronal compartment. Author Contributions Conceived and designed the experiments: REHL AR AAM. Performed the experiments: REHL AR YJ. Analyzed the data: REHL AR PJF HK AAM. Contributed reagents/materials/analysis tools: PJF HK. Wrote the paper: REHL AR AAM. 1. Lang F, Busch GL, Ritter M, Volkl H, Waldegger S, et al. (1998) Functional significance of cell volume regulatory mechanisms. Physiol Rev 78: 247–306. 2. Kirk K, Strange K (1998) Functional properties and physiological roles of organic solute channels. Annu Rev Physiol 60: 719–739. PLoS ONE | www.plosone.org 10 October 2008 | Volume 3 | Issue 10 | e3543