Functional role of ambient GABA in refining neuronal circuits early in postnatal development

Abstract

Early in development, γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the mature brain, depolarizes and excites targeted neurons by an outwardly directed flux of chloride, resulting from the peculiar balance between the cation-chloride importer NKCC1 and the extruder KCC2. The low expression of KCC2 at birth leads to accumulation of chloride inside the cell and to the equilibrium potential for chloride positive respect to the resting membrane potential. GABA exerts its action via synaptic and extrasynaptic GABAA receptors mediating phasic and tonic inhibition, respectively. Here, recent data on the contribution of "ambient" GABA to the refinement of neuronal circuits in the immature brain have been reviewed. In particular, we focus on the hippocampus, where, prior to the formation of conventional synapses, GABA released from growth cones and astrocytes in a calcium- and SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor)-independent way, diffuses away to activate in a paracrine fashion extrasynaptic receptors localized on distal neurons. The transient increase in intracellular calcium following the depolarizing action of GABA leads to inhibition of DNA synthesis and cell proliferation. Tonic GABA exerts also a chemotropic action on cell migration. Later on, when synapses are formed, GABA spilled out from neighboring synapses, acting mainly on extrasynaptic α5, β2, β3, and γ containing GABAA receptor subunits, provides the membrane depolarization necessary for principal cells to reach the window where intrinsic bursts are generated. These are instrumental in triggering calcium transients associated with network-driven giant depolarizing potentials which act as coincident detector signals to enhance synaptic efficacy at emerging GABAergic and glutamatergic synapses.

Keywords: development; extrasynaptic GABAA receptor; hippocampus; network activity; tonic GABAA conductance.

FIGURE 1

In the immature hippocampus, ambient GABA depolarizes targeted cells and contributes to generate network-driven GDPs. (A) Late in embryonic, early in postnatal life, prior to synapses formation, GABA released from growth cones diffuses in the extracellular space (light blue), binds to GABA_A receptors located on the membrane of a neighboring cell (gray) and depolarizes the membrane through an outwardly directed flux of chloride. This results from the peculiar balance between the cation-chloride importer NKCC1 (large circle in green) and the poorly expressed cation-chloride extruder KCC2 (small circle in green), leading to accumulation of chloride inside the cell [Cl^-]_i. Tonic GABA current can be unveiled by applying picrotoxin (PTX, 100 μM; bar) as illustrated in the inset on the right. (B) After birth, during the first postnatal week, chemical GABAergic and glutamatergic synapses start appearing. GABA released by exocytosis from presynaptic vesicles (dark blue) acts on postsynaptic GABA_A receptors located in face to presynaptic release sites and generate synaptic currents (inset on left). Once released GABA spills out (light blue) to activate extrasynaptic GABA_A receptors. At this stage, glutamatergic synapses are also formed (yellow). The synergistic action of GABA and glutamate, both depolarizing and excitatory is crucial for GDPs generation. At this developmental stage, both phasic and tonic GABA are depolarizing since the cation-chloride extruder KCC2 is still poorly expressed on the membrane surface (small circle in green). (C) In adulthood, GABA acting on both synaptic and extrasynaptic GABA_A receptors hyperpolarizes the membrane. This occurs because, due to the developmental up-regulation of the cation-chloride extruder KCC2 expression (large circle in green), [Cl^-]_i is maintained at very low levels and when GABA opens GABA_A receptor channels, causes a net flux of chloride inside the cell leading to membrane hyperpolarization and inhibition of cell firing. In addition, the concomitant down-regulation of the cation-chloride importer NKCC1 with age (small circle in green), contributes to maintain a very low [Cl^-]_i. Note the opposite direction of both phasic (inset on left) and tonic (inset on the right) GABA and the reduced amplitude of the latter respect to neonates. (D) During the first postnatal week, depolarizing the membrane in the presence of AMPA/kainate/NMDA and GABA_A receptor antagonists (DNQX, APV, and PTX), induces the appearance of intrinsic voltage-dependent bursts which are instrumental in triggering GDPs (see text). On the right a single burst recorded on an expanded time scale. The shadow (light blue) represents the membrane depolarization induced by tonic GABA. (E) Individual whole cell (neonatal CA3 pyramidal neuron; upper trace) and concomitant extracellular field recordings of spontaneous network-driven GDPs (bottom trace). On the right a single GDP and a concomitant field potential are represented on an expanded time scale. (F) Intrinsic bursts induced in adulthood by depolarizing the membrane in the presence of DNQX, APV, and PTX. Note the difference in burst duration between adults and neonates. (G) Whole cell (upper trace) and concomitant extracellular recordings (bottom trace) obtained from an adult CA3 pyramidal cell. On the right a single spike is shown on an expanded time scale. Note the absence of network-driven correlated activity such as GDPs (D and F modified from Safiulina et al., 2008; E modified from Ben-Ari et al., 2007).