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GABA Signaling and Neuroactive Steroids in Adrenal Medullary Chromaffin Cells.

Harada K, Matsuoka H, Fujihara H, Ueta Y, Yanagawa Y, Inoue M - Front Cell Neurosci (2016)

Bottom Line: GABA has two actions mediated by GABAA receptors in chromaffin cells: it induces catecholamine secretion by itself and produces an inhibition of synaptically evoked secretion by a shunt effect.This function of GABA may be facilitated by expression of the immature isoforms of GAD and GABAA receptors and the lack of expression of plasma membrane GABA transporters (GATs).In this review, we will consider how the para/autocrine function of GABA is achieved, focusing on the structural and molecular mechanisms for GABA signaling.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell and Systems Physiology, University of Occupational and Environmental Health School of Medicine Kitakyushu, Japan.

ABSTRACT
Gamma-aminobutyric acid (GABA) is produced not only in the brain, but also in endocrine cells by the two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67. In rat adrenal medullary chromaffin cells only GAD67 is expressed, and GABA is stored in large dense core vesicles (LDCVs), but not synaptic-like microvesicles (SLMVs). The α3β2/3γ2 complex represents the majority of GABAA receptors expressed in rat and guinea pig chromaffin cells, whereas PC12 cells, an immortalized rat chromaffin cell line, express the α1 subunit as well as the α3. The expression of α3, but not α1, in PC12 cells is enhanced by glucocorticoid activity, which may be mediated by both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). GABA has two actions mediated by GABAA receptors in chromaffin cells: it induces catecholamine secretion by itself and produces an inhibition of synaptically evoked secretion by a shunt effect. Allopregnanolone, a neuroactive steroid which is secreted from the adrenal cortex, produces a marked facilitation of GABAA receptor channel activity. Since there are no GABAergic nerve fibers in the adrenal medulla, GABA may function as a para/autocrine factor in the chromaffin cells. This function of GABA may be facilitated by expression of the immature isoforms of GAD and GABAA receptors and the lack of expression of plasma membrane GABA transporters (GATs). In this review, we will consider how the para/autocrine function of GABA is achieved, focusing on the structural and molecular mechanisms for GABA signaling.

No MeSH data available.


Related in: MedlinePlus

Suppression of the synaptically evoked Ca2+ signal in rat adrenal chromaffin cells by GABA. (A) Confocal images of Fluo-4 fluorescence in rat adrenal medulla. The adrenal gland was retrogradely loaded with Fluo-4 AM, a Ca2+ indicator via the adrenal vein, and then part of the adrenal cortex covering the medulla was removed by microscissors under a stereoscope (Warashina and Inoue, 2012). The adrenal gland was placed between a pair of silver circles for electrical stimulation. Confocal images of FITC-like fluorescence in adrenal chromaffin cells were acquired every 5 s. (B) Relative values of the change in fluorescence intensity are plotted against time. The adrenal gland was retrogradely perfused with saline via the adrenal vein, and 30 μM GABA was added to the perfusion solution during the indicated period (dotted line). Nerve fibers were electrically stimulated with 60 V pulses of 1.5 ms duration at 10 Hz for 30 s (bars). Fluorescence intensities in the areas (x and y) indicated in (A) part c were measured and presented as filled (x) and open (y) symbols, respectively. Note that the cells in the region indicated by y showed no response to the application of GABA, whereas those in the region marked x did. After correction for the decline due to photobleaching, the increase in fluorescence intensity was expressed as a fraction of the resting level. The images marked a, b, and c in (A) correspond to the points labeled in (B). (A,B) Are reproduced from Matsuoka et al. (2008).
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Figure 3: Suppression of the synaptically evoked Ca2+ signal in rat adrenal chromaffin cells by GABA. (A) Confocal images of Fluo-4 fluorescence in rat adrenal medulla. The adrenal gland was retrogradely loaded with Fluo-4 AM, a Ca2+ indicator via the adrenal vein, and then part of the adrenal cortex covering the medulla was removed by microscissors under a stereoscope (Warashina and Inoue, 2012). The adrenal gland was placed between a pair of silver circles for electrical stimulation. Confocal images of FITC-like fluorescence in adrenal chromaffin cells were acquired every 5 s. (B) Relative values of the change in fluorescence intensity are plotted against time. The adrenal gland was retrogradely perfused with saline via the adrenal vein, and 30 μM GABA was added to the perfusion solution during the indicated period (dotted line). Nerve fibers were electrically stimulated with 60 V pulses of 1.5 ms duration at 10 Hz for 30 s (bars). Fluorescence intensities in the areas (x and y) indicated in (A) part c were measured and presented as filled (x) and open (y) symbols, respectively. Note that the cells in the region indicated by y showed no response to the application of GABA, whereas those in the region marked x did. After correction for the decline due to photobleaching, the increase in fluorescence intensity was expressed as a fraction of the resting level. The images marked a, b, and c in (A) correspond to the points labeled in (B). (A,B) Are reproduced from Matsuoka et al. (2008).

Mentions: The intracellular concentration of Cl− in rat chromaffin cells was electrophysiologically estimated to be 31 mM, as the equilibrium potential for Cl− ([ECl]) is -38 mV (Matsuoka et al., 2008; Inoue et al., 2010). It is likely that the chloride gradient and ECl in rat chromaffin cells is determined by finely tuned expression and/or function of NKCC1 and KCC2 (Kaila et al., 2014). This value of ECl may allow GABA to have a dual action; GABA by itself induces excitation, but inhibits the much larger excitation resulting from a volley of neuronal inputs. As shown in Figure 3, GABA alone induced a depolarization with the consequent activation of voltage-dependent Ca2+ channels by stimulating GABAA receptors, resulting in catecholamine secretion. When the adrenal medulla was electrically stimulated at a high frequency (5–10 Hz) during GABAA receptor stimulation, the total amplitude of Ca2+ signals, which consisted of GABA-induced and synaptically evoked Ca2+ responses, was smaller than that evoked synaptically alone (Figure 3). This decrease in the overall Ca2+ signal is ascribed to the fact that the membrane electrical shunt induced by GABA (reversal potential at about −38 mV) reduces the depolarization resulting from nerve stimulation and so reduces Ca2+ channel activation.


GABA Signaling and Neuroactive Steroids in Adrenal Medullary Chromaffin Cells.

Harada K, Matsuoka H, Fujihara H, Ueta Y, Yanagawa Y, Inoue M - Front Cell Neurosci (2016)

Suppression of the synaptically evoked Ca2+ signal in rat adrenal chromaffin cells by GABA. (A) Confocal images of Fluo-4 fluorescence in rat adrenal medulla. The adrenal gland was retrogradely loaded with Fluo-4 AM, a Ca2+ indicator via the adrenal vein, and then part of the adrenal cortex covering the medulla was removed by microscissors under a stereoscope (Warashina and Inoue, 2012). The adrenal gland was placed between a pair of silver circles for electrical stimulation. Confocal images of FITC-like fluorescence in adrenal chromaffin cells were acquired every 5 s. (B) Relative values of the change in fluorescence intensity are plotted against time. The adrenal gland was retrogradely perfused with saline via the adrenal vein, and 30 μM GABA was added to the perfusion solution during the indicated period (dotted line). Nerve fibers were electrically stimulated with 60 V pulses of 1.5 ms duration at 10 Hz for 30 s (bars). Fluorescence intensities in the areas (x and y) indicated in (A) part c were measured and presented as filled (x) and open (y) symbols, respectively. Note that the cells in the region indicated by y showed no response to the application of GABA, whereas those in the region marked x did. After correction for the decline due to photobleaching, the increase in fluorescence intensity was expressed as a fraction of the resting level. The images marked a, b, and c in (A) correspond to the points labeled in (B). (A,B) Are reproduced from Matsuoka et al. (2008).
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Figure 3: Suppression of the synaptically evoked Ca2+ signal in rat adrenal chromaffin cells by GABA. (A) Confocal images of Fluo-4 fluorescence in rat adrenal medulla. The adrenal gland was retrogradely loaded with Fluo-4 AM, a Ca2+ indicator via the adrenal vein, and then part of the adrenal cortex covering the medulla was removed by microscissors under a stereoscope (Warashina and Inoue, 2012). The adrenal gland was placed between a pair of silver circles for electrical stimulation. Confocal images of FITC-like fluorescence in adrenal chromaffin cells were acquired every 5 s. (B) Relative values of the change in fluorescence intensity are plotted against time. The adrenal gland was retrogradely perfused with saline via the adrenal vein, and 30 μM GABA was added to the perfusion solution during the indicated period (dotted line). Nerve fibers were electrically stimulated with 60 V pulses of 1.5 ms duration at 10 Hz for 30 s (bars). Fluorescence intensities in the areas (x and y) indicated in (A) part c were measured and presented as filled (x) and open (y) symbols, respectively. Note that the cells in the region indicated by y showed no response to the application of GABA, whereas those in the region marked x did. After correction for the decline due to photobleaching, the increase in fluorescence intensity was expressed as a fraction of the resting level. The images marked a, b, and c in (A) correspond to the points labeled in (B). (A,B) Are reproduced from Matsuoka et al. (2008).
Mentions: The intracellular concentration of Cl− in rat chromaffin cells was electrophysiologically estimated to be 31 mM, as the equilibrium potential for Cl− ([ECl]) is -38 mV (Matsuoka et al., 2008; Inoue et al., 2010). It is likely that the chloride gradient and ECl in rat chromaffin cells is determined by finely tuned expression and/or function of NKCC1 and KCC2 (Kaila et al., 2014). This value of ECl may allow GABA to have a dual action; GABA by itself induces excitation, but inhibits the much larger excitation resulting from a volley of neuronal inputs. As shown in Figure 3, GABA alone induced a depolarization with the consequent activation of voltage-dependent Ca2+ channels by stimulating GABAA receptors, resulting in catecholamine secretion. When the adrenal medulla was electrically stimulated at a high frequency (5–10 Hz) during GABAA receptor stimulation, the total amplitude of Ca2+ signals, which consisted of GABA-induced and synaptically evoked Ca2+ responses, was smaller than that evoked synaptically alone (Figure 3). This decrease in the overall Ca2+ signal is ascribed to the fact that the membrane electrical shunt induced by GABA (reversal potential at about −38 mV) reduces the depolarization resulting from nerve stimulation and so reduces Ca2+ channel activation.

Bottom Line: GABA has two actions mediated by GABAA receptors in chromaffin cells: it induces catecholamine secretion by itself and produces an inhibition of synaptically evoked secretion by a shunt effect.This function of GABA may be facilitated by expression of the immature isoforms of GAD and GABAA receptors and the lack of expression of plasma membrane GABA transporters (GATs).In this review, we will consider how the para/autocrine function of GABA is achieved, focusing on the structural and molecular mechanisms for GABA signaling.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell and Systems Physiology, University of Occupational and Environmental Health School of Medicine Kitakyushu, Japan.

ABSTRACT
Gamma-aminobutyric acid (GABA) is produced not only in the brain, but also in endocrine cells by the two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67. In rat adrenal medullary chromaffin cells only GAD67 is expressed, and GABA is stored in large dense core vesicles (LDCVs), but not synaptic-like microvesicles (SLMVs). The α3β2/3γ2 complex represents the majority of GABAA receptors expressed in rat and guinea pig chromaffin cells, whereas PC12 cells, an immortalized rat chromaffin cell line, express the α1 subunit as well as the α3. The expression of α3, but not α1, in PC12 cells is enhanced by glucocorticoid activity, which may be mediated by both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). GABA has two actions mediated by GABAA receptors in chromaffin cells: it induces catecholamine secretion by itself and produces an inhibition of synaptically evoked secretion by a shunt effect. Allopregnanolone, a neuroactive steroid which is secreted from the adrenal cortex, produces a marked facilitation of GABAA receptor channel activity. Since there are no GABAergic nerve fibers in the adrenal medulla, GABA may function as a para/autocrine factor in the chromaffin cells. This function of GABA may be facilitated by expression of the immature isoforms of GAD and GABAA receptors and the lack of expression of plasma membrane GABA transporters (GATs). In this review, we will consider how the para/autocrine function of GABA is achieved, focusing on the structural and molecular mechanisms for GABA signaling.

No MeSH data available.


Related in: MedlinePlus