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Roles of taurine-mediated tonic GABAA receptor activation in the radial migration of neurons in the fetal mouse cerebral cortex.

Furukawa T, Yamada J, Akita T, Matsushima Y, Yanagawa Y, Fukuda A - Front Cell Neurosci (2014)

Bottom Line: This effect of GABAAR blockade in GAD67(GFP/GFP) mice suggested a role for alternative endogenous GABAAR agonists.GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531.In a taurine-deficient mouse model, the GABAAR-mediated tonic currents were greatly reduced, and radial migration was accelerated.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Shizuoka, Japan.

ABSTRACT
γ-Aminobutyric acid (GABA) depolarizes embryonic cerebrocortical neurons and continuous activation of the GABAA receptor (GABAAR) contributes to their tonic depolarization. Although multiple reports have demonstrated a role of GABAAR activation in neocortical development, including in migration, most of these studies have used pharmacological blockers. Herein, we performed in utero electroporation in GABA synthesis-lacking homozygous GAD67-GFP knock-in mice (GAD67(GFP/GFP)) to label neurons born in the ventricular zone. Three days after electroporation, there were no differences in the distribution of labeled cells between the genotypes. The dose-response properties of labeled cells to GABA were equivalent among genotypes. However, continuous blockade of GABAAR with the GABAAR antagonist SR95531 accelerated radial migration. This effect of GABAAR blockade in GAD67(GFP/GFP) mice suggested a role for alternative endogenous GABAAR agonists. Thus, we tested the role of taurine, which is derived from maternal blood but is abundant in the fetal brain. The taurine-evoked currents in labeled cells were mediated by GABAAR. Taurine uptake was blocked by a taurine transporter inhibitor, 2-(guanidino)ethanesulfonic acid (GES), and taurine release was blocked by a volume-sensitive anion channel blocker, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid, as examined through high-performance liquid chromatography. GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531. In a taurine-deficient mouse model, the GABAAR-mediated tonic currents were greatly reduced, and radial migration was accelerated. As the tonic currents were equivalent among the genotypes of GAD67-GFP knock-in mice, taurine, rather than GABA, might play a major role as an endogenous agonist of embryonic tonic GABAAR conductance, regulating the radial migration of neurons in the developing neocortex.

No MeSH data available.


Related in: MedlinePlus

GABA-, taurine-, and glycine-evoked currents in RFP-positive cells in rat fetuses at E18.5. Typical traces of the currents evoked by the application of 10 μM GABA (A), 10 mM taurine (B), and 3 mM glycine (C) in RFP-positive cells are shown. Black traces indicate the currents in the absence of blockers; red traces are the currents obtained after the application of 10 μM strychnine through bath perfusion in the same cells; and gray traces are the currents recorded after the further addition of 10 μM SR95531 to the bath solution. VH was -60 mV. (D) The means ± SEMs of the peak current densities in the absence and presence of strychnine (STR) and after the addition of SR95531 are plotted. (E) Effects of blockers on agonist-evoked currents. The peak current densities in the absence of blockers were normalized to 100%.
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Figure 6: GABA-, taurine-, and glycine-evoked currents in RFP-positive cells in rat fetuses at E18.5. Typical traces of the currents evoked by the application of 10 μM GABA (A), 10 mM taurine (B), and 3 mM glycine (C) in RFP-positive cells are shown. Black traces indicate the currents in the absence of blockers; red traces are the currents obtained after the application of 10 μM strychnine through bath perfusion in the same cells; and gray traces are the currents recorded after the further addition of 10 μM SR95531 to the bath solution. VH was -60 mV. (D) The means ± SEMs of the peak current densities in the absence and presence of strychnine (STR) and after the addition of SR95531 are plotted. (E) Effects of blockers on agonist-evoked currents. The peak current densities in the absence of blockers were normalized to 100%.

Mentions: The insensitivity of RFP-positive cells to glycine (Figures 5C,D) appears to be inconsistent with findings in rat fetuses (Flint et al., 1998; Nimmervoll et al., 2011). Therefore, we also examined the currents in RFP-positive cells in cortical slices obtained from rat fetuses at E18.5. The peak amplitudes of GABA (10 μM)-evoked inward currents recorded in RFP-positive cells located in the CP of rat cortices were 19.66 ± 3.22 pA/pF (n = 6; Figures 6A,D), and these currents were not blocked by strychnine (10 μM: 74.92 ± 7.84%, P = 0.062 by paired t-test, n = 6; Figures 6A,D,E) but were blocked by SR95531 (10 μM: 14.25 ± 3.07%, P < 0.001, n = 6; Figures 6A,D,E). Taurine (10 mM) also evoked inward currents (22.94 ± 2.87 pA/pF, n = 6; Figures 6B,D), but these currents were found to be significantly blocked by strychnine (29.09 ± 2.63%, P < 0.001, n = 6; Figures 6B,D,E). Additionally, the application of glycine (3 mM) indeed produced strychnine-sensitive inward currents in RFP-positive cells in rat cortices (30.32 ± 4.14 pA/pF, n = 6; blocked by strychnine to 14.13 ± 2.99%, P < 0.001, n = 6; Figures 6C,D,E). Thus, in the rat fetal neocortex, radially migrating neurons express GlyRs (Nimmervoll et al., 2011), and the taurine-evoked inward currents are mainly mediated by these GlyRs.


Roles of taurine-mediated tonic GABAA receptor activation in the radial migration of neurons in the fetal mouse cerebral cortex.

Furukawa T, Yamada J, Akita T, Matsushima Y, Yanagawa Y, Fukuda A - Front Cell Neurosci (2014)

GABA-, taurine-, and glycine-evoked currents in RFP-positive cells in rat fetuses at E18.5. Typical traces of the currents evoked by the application of 10 μM GABA (A), 10 mM taurine (B), and 3 mM glycine (C) in RFP-positive cells are shown. Black traces indicate the currents in the absence of blockers; red traces are the currents obtained after the application of 10 μM strychnine through bath perfusion in the same cells; and gray traces are the currents recorded after the further addition of 10 μM SR95531 to the bath solution. VH was -60 mV. (D) The means ± SEMs of the peak current densities in the absence and presence of strychnine (STR) and after the addition of SR95531 are plotted. (E) Effects of blockers on agonist-evoked currents. The peak current densities in the absence of blockers were normalized to 100%.
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Figure 6: GABA-, taurine-, and glycine-evoked currents in RFP-positive cells in rat fetuses at E18.5. Typical traces of the currents evoked by the application of 10 μM GABA (A), 10 mM taurine (B), and 3 mM glycine (C) in RFP-positive cells are shown. Black traces indicate the currents in the absence of blockers; red traces are the currents obtained after the application of 10 μM strychnine through bath perfusion in the same cells; and gray traces are the currents recorded after the further addition of 10 μM SR95531 to the bath solution. VH was -60 mV. (D) The means ± SEMs of the peak current densities in the absence and presence of strychnine (STR) and after the addition of SR95531 are plotted. (E) Effects of blockers on agonist-evoked currents. The peak current densities in the absence of blockers were normalized to 100%.
Mentions: The insensitivity of RFP-positive cells to glycine (Figures 5C,D) appears to be inconsistent with findings in rat fetuses (Flint et al., 1998; Nimmervoll et al., 2011). Therefore, we also examined the currents in RFP-positive cells in cortical slices obtained from rat fetuses at E18.5. The peak amplitudes of GABA (10 μM)-evoked inward currents recorded in RFP-positive cells located in the CP of rat cortices were 19.66 ± 3.22 pA/pF (n = 6; Figures 6A,D), and these currents were not blocked by strychnine (10 μM: 74.92 ± 7.84%, P = 0.062 by paired t-test, n = 6; Figures 6A,D,E) but were blocked by SR95531 (10 μM: 14.25 ± 3.07%, P < 0.001, n = 6; Figures 6A,D,E). Taurine (10 mM) also evoked inward currents (22.94 ± 2.87 pA/pF, n = 6; Figures 6B,D), but these currents were found to be significantly blocked by strychnine (29.09 ± 2.63%, P < 0.001, n = 6; Figures 6B,D,E). Additionally, the application of glycine (3 mM) indeed produced strychnine-sensitive inward currents in RFP-positive cells in rat cortices (30.32 ± 4.14 pA/pF, n = 6; blocked by strychnine to 14.13 ± 2.99%, P < 0.001, n = 6; Figures 6C,D,E). Thus, in the rat fetal neocortex, radially migrating neurons express GlyRs (Nimmervoll et al., 2011), and the taurine-evoked inward currents are mainly mediated by these GlyRs.

Bottom Line: This effect of GABAAR blockade in GAD67(GFP/GFP) mice suggested a role for alternative endogenous GABAAR agonists.GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531.In a taurine-deficient mouse model, the GABAAR-mediated tonic currents were greatly reduced, and radial migration was accelerated.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurophysiology, Hamamatsu University School of Medicine Hamamatsu, Shizuoka, Japan.

ABSTRACT
γ-Aminobutyric acid (GABA) depolarizes embryonic cerebrocortical neurons and continuous activation of the GABAA receptor (GABAAR) contributes to their tonic depolarization. Although multiple reports have demonstrated a role of GABAAR activation in neocortical development, including in migration, most of these studies have used pharmacological blockers. Herein, we performed in utero electroporation in GABA synthesis-lacking homozygous GAD67-GFP knock-in mice (GAD67(GFP/GFP)) to label neurons born in the ventricular zone. Three days after electroporation, there were no differences in the distribution of labeled cells between the genotypes. The dose-response properties of labeled cells to GABA were equivalent among genotypes. However, continuous blockade of GABAAR with the GABAAR antagonist SR95531 accelerated radial migration. This effect of GABAAR blockade in GAD67(GFP/GFP) mice suggested a role for alternative endogenous GABAAR agonists. Thus, we tested the role of taurine, which is derived from maternal blood but is abundant in the fetal brain. The taurine-evoked currents in labeled cells were mediated by GABAAR. Taurine uptake was blocked by a taurine transporter inhibitor, 2-(guanidino)ethanesulfonic acid (GES), and taurine release was blocked by a volume-sensitive anion channel blocker, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid, as examined through high-performance liquid chromatography. GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531. In a taurine-deficient mouse model, the GABAAR-mediated tonic currents were greatly reduced, and radial migration was accelerated. As the tonic currents were equivalent among the genotypes of GAD67-GFP knock-in mice, taurine, rather than GABA, might play a major role as an endogenous agonist of embryonic tonic GABAAR conductance, regulating the radial migration of neurons in the developing neocortex.

No MeSH data available.


Related in: MedlinePlus