Intracellular Na+ concentration influences short-term plasticity of glutamate transporter-mediated currents in neocortical astrocytes.

Fast synaptic transmission requires a rapid clearance of the released neurotransmitter from the extracellular space. Glial glutamate transporters (excitatory amino acid transporters, EAATs) strongly contribute to glutamate removal. In this work, we investigated the paired-pulse plasticity of synaptically activated, glutamate transporter-mediated currents (STCs) in cortical layer 2/3 astrocytes. STCs were elicited by local electrical stimulation in layer 4 in the presence of ionotropic glutamate (AMPA and NMDA), GABAA, and GABAB receptor antagonists. In experiments with low [Na(+)]i (5 mM) intrapipette solution, STCs elicited by paired-pulse stimulation demonstrated paired-pulse facilitation (PPF) at short (<250 ms) interstimulus intervals (ISIs) and paired-pulse depression at longer ISIs. In experiments with close to physiological, high [Na(+)]i (20 mM) intrapipette solution, PPF of STCs at short ISIs was significantly reduced. In addition, the STC kinetics was slowed in the presence of high [Na(+)]i. Exogenous GABA increased astrocytic [Na(+)]i, reduced the mean STC amplitude, decreased PPF at short ISIs, and slowed STC kinetics. All GABA-induced changes were blocked by NO-711 and SNAP-5114, GABA transporter (GATs) antagonists. In experiments with the low intrapipette solution, GAT blockade under control conditions decreased PPF at short ISIs both at room and at near physiological temperatures. Dialysis of single astrocyte with low [Na(+)]i solution increased the amplitude and reduced PPR of evoked field potentials recorded in the vicinity of the astrocyte. We conclude that (1) endogenous GABA via GATs may influence EAAT functioning and (2) astrocytic [Na(+)]i modulates the short-term plasticity of STCs and in turn the efficacy of glutamate removal.

GABAergic projections from the subplate to Cajal-Retzius cells in the neocortex.

Subplate neurons and Cajal-Retzius cells play an important role in the corticogenesis. Despite morphological evidence, the question whether subplate neurons innervate Cajal-Retzius cells has not been studied yet. We report that electrical stimulation in the subplate resulted in evoked GABAergic inhibitory postsynaptic currents (eIPSCs) in Cajal-Retzius cells. The eIPSC latency showed minor variability and amounted to approximately 4 ms, suggesting the monosynaptic connection. During the first postnatal week: (i) eIPSC amplitude increased, (ii) eIPSC kinetics sped up, (iii) the size of readily releasable pool increased, and (iv) γ-aminobutyric acid release probability decreased. We conclude that GABAergic subplate neurons innervate Cajal-Retzius cells. Surprisingly, despite the transient nature of both cell populations, these projections show developmental adjustments typical for many nontransient synaptic connections.

Estimation of ambient GABA levels in layer I of the mouse neonatal cortex in brain slices.

GABAergic synapses on Cajal-Retzius neurons in layer I of the murine neocortex experience GABA(B) receptor (GABA(B)R)-mediated tonic inhibition. Extracellular GABA concentration ([GABA](o)) that determines the strength of GABA(B)R-mediated inhibition is controlled by GABA transporters (GATs). In this study, we hypothesized that the strength of presynaptic GABA(B)R activation reflects [GABA](o) in the vicinity of synaptic contacts. Slices obtained from two age groups were used, namely postnatal days (P)2-3 and P5-7. GABAergic postsynaptic currents (IPSCs) were recorded using the whole-cell patch-clamp technique. Minimal electrical stimulation in layer I was applied to elicit evoked IPSCs (eIPSCs) using a paired-pulse protocol. Three parameters were selected for comparison: the mean eIPSC amplitude, paired-pulse ratio, and failure rate. When GAT-1 and GAT-2/3 were blocked by NO-711 (10 microM) and SNAP-5114 (40 microM), respectively, no tonic GABA(B)R-mediated inhibition was observed. In order to restore the control levels of GABA(B)R-mediated inhibition, 250 and 125 nm exogenous GABA was required at P2-3 and P5-7, respectively. Addition of 3-mercaptopropionic acid, a glutamate decarboxylase inhibitor, did not significantly change the obtained values arguing against the suggestion that a mechanism different from GATs contributes to [GABA](o) control. We conclude that juxtasynaptic [GABA](o) is higher (about 250 nM) at P2-3 than at P5-7 (about 125 nM). As both radial cell migration and corticogenesis in general are strongly dependent on [GABA](o) and the formation of the last layer 2/3 is finished by P4 in rodents, the observed [GABA](o) reduction in layer I might reflect this crucial event in the cortical development.

Postsynaptically different inhibitory postsynaptic currents in Cajal-Retzius cells in the developing neocortex.

Fast and slowly rising inhibitory postsynaptic currents (IPSCs, IPSCF and IPSCS) in neocortical Cajal-Retzius cells are observed. In this study, zolpidem, a benzodiazepine agonist that specifically modulates gamma-aminobutyric acid type A receptors (GABAARs) containing gamma2 subunit, was used to characterize GABAARs mediating IPSCF and IPSCS. One-hundred-nanomolar zolpidem prolonged IPSCS, increased evoked IPSCS (eIPSCS) amplitude, and decreased paired-pulse ratio (PPR) of eIPSCS. Two micromolar zolpidem prolonged both IPSCF and IPSCS, increased miniature IPSCF and eIPSCF amplitudes, increased eIPSCS amplitude but not miniature IPSCS amplitude, decreased PPR of eIPSCS, but failed to affect PPR of eIPSCF. We conclude that IPSCF are mediated by alpha2/3-containing GABAARs, which are not saturated by synaptic GABA release, whereas IPSCS are mediated by alpha1-containing and alpha2/3-containing GABAARs, which are saturated by quantal GABA release.