GABA Not Only a Neurotransmitter: Osmotic Regulation by GABA(A)R Signaling

Abstract

Mature macroglia and almost all neural progenitor types express γ-aminobutyric (GABA) A receptors (GABA(A)Rs), whose activation by ambient or synaptic GABA, leads to influx or efflux of chloride (Cl(-)) depending on its electro-chemical gradient (E(Cl)). Since the flux of Cl(-) is indissolubly associated to that of osmotically obliged water, GABA(A)Rs regulate water movements by modulating ion gradients. In addition, since water movements also occur through specialized water channels and transporters, GABA(A)R signaling could affect the movement of water by regulating the function of the channels and transporters involved, thereby affecting not only the direction of the water fluxes but also their dynamics. We will here review recent observations indicating that in neural cells GABA(A)R-mediated osmotic regulation affects the cellular volume thereby activating multiple intracellular signaling mechanisms important for cell proliferation, maturation, and survival. In addition, we will discuss evidence that the osmotic regulation exerted by GABA may contribute to brain water homeostasis in physiological and in pathological conditions causing brain edema, in which the GABAergic transmission is often altered.

Keywords: GABA; GABAAR; cell volume; chloride; osmotic swelling.

Figure 1

Basic properties of water and anions fluxes. (A) Water can diffuse according to the osmotic pressure through the membrane lipid bilayer or via the dedicated channels AQP. Additionally it can be transported against its gradient by cotransporters, such as those for GABA and glutamate that use the driving force of Na⁺ to move the neurotransmitters and eventually water against their gradient. (B,C) The activity of different transporters and exchangers determines the steady-state gradient for Cl⁻ and consequently regulates the ${\text{E}}_{\text{Cl}}$. Cl⁻ transport via cation–chloride cotransporters is fueled by the Na⁺ and K⁺ gradients generated by the Na–K ATPase. Two of the major players in neural cells are NKCC1 and KCC2, by which Cl⁻ is transported inside and outside the cells respectively. When ${\text{E}}_{\text{Cl}}$ (see Box 1), is more positive that the E_M, opening of GABA_AR would mediate an efflux of Cl⁻ (for convention called inward current) that depolarizes the cells (B). Conversely when ${\text{E}}_{\text{Cl}}$ is more negative that the E_M, GABAergic currents (outward) bring the membrane potential close to these values and are hyperpolarizing (C). In this case to determine the E_GABA is important to consider also the flux of ${\text{HCO}}_3^{-}$ since GABA_ARs are permeable to this anion. Due to its higher level inside the cells, opening of GABA_AR would drive a depolarizing efflux of ${\text{HCO}}_3^{-}$ that, counteracting the influx of Cl⁻, contributes to deviate E_GABA to value more positive than that of ${\text{HCO}}_3^{-}$ (For Rev, see Andrew et al., ; Blaesse et al., ; Risher et al., 2009).

Figure 2

GABA-mediated osmotic regulation in non-neuronal cells. Schematic representation summarizing the current knowledge concerning the expression of the neurotransmitter GABA, the synthesizing enzyme GAD, the membrane transporter GAT, which can work in both directions, and the ionic GABA_AR in the indicated non-neuronal cell types of the mammalian CNS. Depicted is also the direction of water flux and of the Cl⁻ gradient, as estimated in resting physiological conditions. The scheme also illustrates the expression of the aquaporin (AQP) water channels and of the Na⁺/K⁺/Cl⁻ (NKCC) and K⁺/Cl⁻ (KCC) transporters in those cell types where their expression has been directly investigated. As in neurons, GABA promotes swelling in neural stem cells (NSC) but not in NG2⁺ oligodendrocyte precursors (OPCs) and in mature macroglia, where GABA_AR activation induces Cl⁻ exit and water efflux. Note that here the information concerning neural stem cells (NSCs) is gathered from the analysis of neural precursors isolated from the postnatal SVZ on the basis of Prominin expression. We did not include in this diagram the information concerning GFAP expressing cells in neurogenic regions of the adult brain because they mainly consist of niche astrocytes. Moreover, the properties illustrated in the diagram were not directly investigated in these populations. However, as discussed in the text, the available information indicates that these GFAP populations resemble mature astrocytes with respect to the direction of the GABA evoked Cl⁻ currents.

Figure 3

GABA_AR signaling promotes regular Ca²⁺ transients on proliferating neural precursors. Analysis of spontaneous Ca²⁺ transients in a population enriched in neural stem cells, purified by fluorescence activated cell sorting from the postnatal SVZ on the basis of the expression of high levels of EGFR (EGFR^high cells; Ciccolini et al., 2005). After isolation the cells were plated on coverslips and maintained in medium containing EGF and FGF-2 for 2 days (Cesetti et al., 2009). Upon loading with the Ca²⁺ indicator fluo-3, in control conditions the EGFR^high cells displayed a very low incidence of regular Ca²⁺ oscillation (regular oscillation defined as at least three spikes with similar amplitude and interspike interval within 15 min recording) (A,B). However the addition of GABA (100 μM) to the medium before (15 min) and during the imaging led to a dramatic increase in the incidence of the cells displaying regular Ca²⁺ oscillation (A,B), an effect that was blocked by the GABA_AR bicuculline (50 μM) (A). In (B) three representative traces of Ca²⁺ changes are shown, respectively in control condition and upon GABA treatment, after background subtraction, and normalization to baseline values.