Single Synapse Indicators of Glutamate Release and Uptake in Acute Brain Slices from Normal and Huntington Mice

Synapses are highly compartmentalized functional units that operate independently on each other. In Huntington&#39;s disease (HD) and other ...

Charité - University Medicine

Layer I of the developing cortex contains a dense GABAergic fiber plexus. These fibers provide excitatory inputs to Cajal-Retzius (CR) cells, the early born neurons in layer I. CR cells possess an extensive axonal projection and form synaptic contacts with excitatory, presumably pyramidal, neurons before birth. Interestingly, activity of CR cells declines during the first postnatal week, but mechanism(s) underlying this phenomenon is not yet known. Here we recorded inhibitory postsynaptic currents (IPSCs) in CR cells at postnatal day (P) 1-2 and P5-7. Blockade of adenosine A(1) receptors (A(1)Rs) increased the amplitude of evoked IPSCs (eIPSCs) and decreased paired-pulse ratio at P5-7 but not at P1-2. A(1)R activation decreased the mean eIPSC amplitude at P5-7, but failed to affect eIPSCs at P1-2. Ecto-adenosine triphosphatase (ATPase) inhibition completely abolished the A(1)R-mediated effects suggesting that extracellular ATP is the main source of adenosine. Because A(1)R blockade did not affect the median miniature IPSC amplitude, our results demonstrate that adenosine reduces gamma-aminiobutyric acid (GABA) release probability via presynaptic A(1)Rs at P5-7. As neuronal activity in layer I can depolarize pyramidal neurons influencing thereby glutamatergic synaptogenesis in the lower cortical layers, postnatal weakening of GABAergic transmission by adenosinergic system might reflect a developmental downregulation of this excitatory drive when glutamatergic synapses are formed.

Developmental downregulation of excitatory GABAergic transmission in neocortical layer I via presynaptic adenosine A(1) receptors.

Cajal-Retzius (CR) cells are principal cells of layer I in the developing neocortex. They are able to generate action potentials, make synaptic contacts in layer I and receive excitatory GABAergic inputs before birth. Although CR cells participate in neuronal network activity in layer I, the properties of their synaptic inputs are not yet characterized. We recorded miniature (mIPSCs) and evoked (eIPSCs) postsynaptic currents using the whole-cell patch-clamp technique. Most of CR cells displayed two types of mIPSCs, namely those with fast (mIPSC(F)) and slow (mIPSC(S)) rise kinetics. The mIPSC(F) mean amplitude was significantly larger than that of mIPSC(S), while their decay rates were not different. Peak-scaled non-stationary noise analysis revealed that mIPSC(S) and mIPSC(F) differed in their weighted single-channel conductance. In addition, zolpidem (100 nm), a modulator of alpha(1) subunit-containing GABA(A) receptors, selectively affected mIPSC(S) suggesting that different postsynaptic GABA(A) receptors mediate mIPSC(F) and mIPSC(S). eIPSCs also split into two populations with different rise kinetics. Fast eIPSCs (eIPSC(F)) displayed higher paired-pulse ratio (PPR) and lower GABA release probability than slowly rising eIPSCs (eIPSC(S)). As CGP55845, a GABA(B) receptor antagonist, eliminated the observed difference in PPR, the lower release probability at IPSC(F) connections probably reflects a stronger tonic GABA(B) receptor-mediated inhibition of IPSC(F) synapses. At low (0.1 Hz) stimulation frequency both inputs can effectively convert presynaptic action potentials into postsynaptic ones; however, only IPSC(F) connections reliably transfer the presynaptic activity patterns at higher stimulation rates. Thus, CR cells receive two GABAergic inputs, which differ in the quantal amplitude, the probability of GABA release and the frequency dependence of signal transfer.

Cajal Retzius cells in the mouse neocortex receive two types of pre- and postsynaptically distinct GABAergic inputs.

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.

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

GABAergic medium-sized striatal output neurons (SONs) provide the principal output for the neostriatum. In vitro and in vivo data indicate that spike discharge of SONs is tightly controlled by effective synaptic inhibition. Although phasic GABAergic transmission critically depends on ambient GABA levels, the role of GABA transporters (GATs) in neostriatal GABAergic synaptic transmission is largely unknown. In the present study we aimed at elucidating the role of GAT-1 in the developing mouse neostriatum (postnatal day (P) 7-34). We recorded GABAergic postsynaptic currents (PSCs) using the whole-cell patch-clamp technique. Based on the effects of NO-711, a specific GAT-1 blocker, we demonstrate that GAT-1 is operative at this age and influences GABAergic synaptic transmission by presynaptic and postsynaptic mechanisms. Presynaptic GABA(B)R-mediated suppression of GABA release was found to be functional at all ages tested; however, there was no evidence for persistent GABA(B)R activity under control conditions, unless GAT-1 was blocked (P12-34). In addition, whereas no tonic GABA(A)R-mediated conductances were detected in SONs until P14, application of a specific GABA(A)R antagonist caused distinct tonic outward currents later in development (P19-34). In the presence of NO-711, tonic GABA(A)R-mediated currents were also observed at P7-14 and were dramatically increased at more mature stages. Furthermore, GAT-1 block reduced the median amplitude of GABAergic miniature PSCs indicating a decrease in quantal size. We conclude that in the murine neostriatum GAT-1 operates in a net uptake mode. It prevents the persistent activation of presynaptic GABA(B)Rs (P12-34) and prevents (P7-14) or reduces (P19-34) tonic postsynaptic GABA(A)R activity.

GABA transporter 1 tunes GABAergic synaptic transmission at output neurons of the mouse neostriatum.

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.

Anton Dvorzhak

Synaptic Dysfunction Group,
Neuroscience Research Center,
Synaptic Dysfunction Group, Neuroscience Research Center

Charité - University Medicine

Single Synapse Indicators of Glutamate Release and Uptake in Acute Brain Slices from Normal and Huntington Mice

Anton Dvorzhak

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Charité - University Medicine

Single Synapse Indicators of Glutamate Release and Uptake in Acute Brain Slices from Normal and Huntington Mice

Synaptic Dysfunction Group,
Neuroscience Research Center,
Synaptic Dysfunction Group, Neuroscience Research Center