Frequency Of Spontaneous GABAergic Currents Research Articles

MGlu1α-dependent recruitment of excitatory GABAergic input to neocortical Cajal-Retzius cells

Cajal-Retzius cells are thought to play an important role for cortical development, and receive primarily spontaneous GABAergic input mediated by GABAA receptors. However, neither the effects of synaptically-released GABA on their excitability nor the cellular source(s) of spontaneous GABAergic currents have been yet determined. By directly recording electrophysiological responses from identified Cajal-Retzius cells of the CXCR4-EGFP mouse, we show that GABAergic input can trigger supra-threshold responses, and that the pharmacological activation of mGlu1α receptors with the group I agonist DHPG powerfully increases the frequency of spontaneous GABAergic currents. These effects appeared mediated by a network mechanism, because responses to DHPG were completely prevented both by surgical disconnection of layer I from lower layers and by exposure of slices to TTX.We propose that the cellular source underlying the observed effect of DHPG are layer I-targeting Martinotti-like interneurons, which we show express functional group I mGluRs and respond to DHPG with supra-threshold depolarization already at early developmental stages.In conclusion, our work suggests that conditions of enhanced glutamate release may be critical at early developmental stages for the recruitment of an mGlu1α-dependent micro-circuit, which then leads to the activation of Cajal-Retzius cells.

The cation‐chloride cotransporter NKCC1 promotes sharp waves in the neonatal rat hippocampus

Earlier studies indicate a crucial role for the interconnected network of intrinsically bursting CA3 pyramidal neurons in the generation of in vivo hippocampal sharp waves (SPWs) and their proposed neonatal in vitro counterparts, the giant depolarizing potentials (GDPs). While mechanisms involving ligand- and voltage-gated channels have received lots of attention in the generation of CA3 network events in the immature hippocampus, the contribution of ion-transport mechanisms has not been extensively studied. Here, we show that bumetanide, a selective inhibitor of neuronal Cl- uptake mediated by the Na+-K+-2Cl- cotransporter isoform 1 (NKCC1), completely and reversibly blocks SPWs in the neonate (postnatal days 7-9) rat hippocampus in vivo, an action also seen on GDPs in slices (postnatal days 1-8). These findings strengthen the view that GDPs and early SPWs are homologous events. Gramicidin-perforated patch recordings indicated that NKCC1 accounts for a large ( approximately 10 mV) depolarizing driving force for the GABAA current in the immature CA3 pyramids. Consistent with a reduction in the depolarization mediated by endogenous GABAA-receptor activation, bumetanide inhibited the spontaneous bursts of individual neonatal CA3 pyramids, but it slightly increased the interneuronal activity as seen in the frequency of spontaneous GABAergic currents. An inhibitory effect of bumetanide was seen on the in vitro population events in the absence of synaptic GABAA receptor-mediated transmission, provided that a tonic GABAA receptor-mediated current was present. Our work indicates that NKCC1 expressed in CA3 pyramidal neurons promotes network activity in the developing hippocampus.

Activation of Nicotinic Acetylcholine Receptors Increases the Frequency of Spontaneous GABAergic IPSCs in Rat Basolateral Amygdala Neurons

The basolateral amygdala (BLA) is a critical component of the amygdaloid circuit, which is thought to be involved in fear conditioned responses. Using whole cell patch-clamp recording, we found that activation of nicotinic acetylcholine receptors (nAChRs) leads to an action potential-dependent increase in the frequency of spontaneous GABAergic currents in principal neurons in the BLA. These spontaneous GABAergic currents were abolished by a low-Ca2+/high-Mg2+ bathing solution, suggesting that they are spontaneous inhibitory postsynaptic currents (sIPSCs). Blockade of ionotropic glutamate receptors did not prevent this increased frequency of sIPSCs nor did blockade of alpha7 nAChRs. Among the nAChR agonists tested, cystisine was more effective at increasing the frequency of the sIPSCs than nicotine or 1,1-dimethyl-4-phenyl piperazinium iodide, consistent with a major contribution of beta4 nAChR subunits. The nicotinic antagonist, dihydro-beta-erythroidine, was less effective than d-tubocurarine in blocking the increased sIPSC frequency induced by ACh, suggesting that alpha4-containing nAChR subunits do not play a major role in the ACh-induced increased sIPSC frequency. Although alpha2/3/4/7 and beta2/4 nAChR subunits were found in the BLA by RT-PCR, the agonist and antagonist profiles suggest that the ACh-induced increase in sIPSC frequency involves predominantly alpha3beta4-containing nAChR subunits. Consistent with this, alpha-conotoxin-AuIB, a nAChR antagonist selective for the alpha3beta4 subunit combination, inhibited the ACh-induced increase in the frequency of sIPSCs. The observations suggest that nicotinic activation increases the frequency of sIPSCs in the BLA by acting mainly on alpha3beta4-containing nicotinic receptors on GABAergic neurons and may play an important role in the modulation of synaptic transmission in the amygdala.

Nicotinic receptors mediate increased GABA release in brain through a tetrodotoxin-insensitive mechanism during prolonged exposure to nicotine

The effects of nicotine on the spontaneous release of GABA from nerve terminals in the chick lateral spiriform nucleus were examined using whole cell patch-clamp recording in brain slices. Exposure to 1 μM nicotine produced an early immediate increase in the frequency of spontaneous postsynaptic GABAergic currents. This effect was blocked in the presence of 0.5 μM tetrodotoxin. However, a prolonged application of 0.1–1 μM nicotine (>3 min) caused a tetrodotoxin-insensitive increase in the frequency of spontaneous GABAergic currents. This late tetrodotoxin-insensitive effect was blocked by the nicotinic antagonists dihydro-β-erythroidine (30 μM) and mecamylamine (10 μM), but not by methyllycaconitine (50–100 nM), indicating that activation of high affinity nicotine receptors was mainly responsible for this effect. This enhancement was inhibited by the high threshold Ca 2+ channel blocker Cd 2+ (100 μM), but not by dantrolene or ryanodine. The tetrodotoxin-insensitive enhancement of the frequency of GABA currents by nicotine was reduced by inhibition of cAMP-dependent protein kinase with HA1004 (30 μM), but not by inhibition of protein kinase C with staurosporine (1 μM), and was facilitated by forskolin (10 μM) or bromo-cAMP (50 μM). The results indicate that nicotine-enhanced GABA release can operate through both tetrodotoxin-sensitive and -insensitive mechanisms in a single brain region and that a second messenger cascade may be involved in the tetrodotoxin-insensitive enhancement by nicotine.

Distribution of spontaneous currents along the somato-dendritic axis of rat hippocampal CA1 pyramidal neurons

Excitatory and inhibitory pathways have specific patterns of innervation along the somato-dendritic axis of neurons. We have investigated whether this morphological diversity was associated with variations in the frequencies of spontaneous and miniature GABAergic and glutamatergic synaptic currents along the somato-dendritic axis of rat hippocampal CA1 pyramidal neurons. Using in vitro whole cell recordings from somata, apical dendrites and basal dendrites (for which we provide the first recordings) of CA1 pyramidal neurons, we report that over 90% of the spontaneous currents were GABAergic, <10% being glutamatergic. The frequency of spontaneous GABAergic currents was comparable in the soma and in the dendrites. In both somata and dendrites, the Na + channel blocker tetrodotoxin abolished more than 80% of the spontaneous glutamatergic currents. In contrast, tetrodotoxin abolished most dendritic (>90%) but not somatic (<40%) spontaneous GABAergic currents. Computer simulations suggest that in our experimental conditions, events below 40 pA are electrotonically filtered to such a degree that they are lost in the recording noise. We conclude that, in vitro, inhibition is massively predominant over excitation and quantitatively evenly distributed throughout the cell. However, inhibition appears to be mainly activity-dependent in the dendrites whereas it can occur in the absence of interneuron firing in the soma. These results can be used as a benchmark to compare values obtained in pathological tissue, such as epilepsies, where changes in the balance between excitation and inhibition would dramatically alter cell behaviour.

Nicotine modulates evoked GABAergic transmission in the brain.

The effects of nicotine on evoked GABAergic synaptic transmission were examined using whole cell recordings from neurons of the lateral spiriform nucleus in embryonic chick brain slices. All synaptic activities were abolished by the GABA(A) receptor antagonist, bicuculline (20 microM). Under voltage-clamp with KCl-filled pipettes (holding potential -70 mV), nicotine (0.1-1.0 microM) increased the frequency of spontaneous GABAergic currents in a dose-dependent manner. Nicotine enhanced electrically evoked GABAergic transmission only at relatively low concentrations of 50-100 nM (but not 25 nM), which approximate the concentrations of nicotine in the blood produced by cigarette smoking. At higher concentrations nicotine had either no effect (0.25 microM) or diminished (0.5-1.0 microM) evoked GABAergic neurotransmission. Nicotine had no significant effect on the postsynaptic current induced by exogenous GABA (30-50 microM). These data imply that nicotine levels attained in smokers are sufficient to enhance evoked GABAergic transmission in the brain, and that this effect is most likely mediated through activation of presynaptic nicotinic receptors.

Operative GABAergic inhibition in hippocampal CA1 pyramidal neurons in experimental epilepsy.

Patch-clamp recordings of CA1 interneurons and pyramidal cells were performed in hippocampal slices from kainate- or pilocarpine-treated rat models of temporal lobe epilepsy. We report that gamma-aminobutyric acid (GABA)ergic inhibition in pyramidal neurons is still functional in temporal lobe epilepsy because: (i) the frequency of spontaneous GABAergic currents is similar to that of control and (ii) focal electrical stimulation of interneurons evokes a hyperpolarization that prevents the generation of action potentials. In paired recordings of interneurons and pyramidal cells, synchronous interictal activities were recorded. Furthermore, large network-driven GABAergic inhibitory postsynaptic currents were present in pyramidal cells during interictal discharges. The duration of these interictal discharges was increased by the GABA type A antagonist bicuculline. We conclude that GABAergic inhibition is still present and functional in these experimental models and that the principal defect of inhibition does not lie in a complete disconnection of GABAergic interneurons from their glutamatergic inputs.

Activation of metabotropic glutamate receptors increase the frequency of spontaneous GABAergic currents through protein kinase A in neonatal rat hippocampal neurons.

1. The tight-seal whole cell recording technique was used to study the effects of the metabotropic glutamate receptor (mGluR) agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) on spontaneous gamma-aminobutyric acid (GABA)-mediated synaptic currents in neonatal rat CA1 hippocampal neurons in slices obtained from postnatal (P) days P6-P12. 2. Bath application of t-ACPD (3-30 microM), in the presence of kynurenic acid, induced a concentration-dependent increase in frequency but not in amplitude of spontaneous GABAergic currents. The mean frequency ratio (t-ACPD 10 microM over control) was 2.6 +/- 1 (mean +/- SD), whereas the mean amplitude ratio was 1.1 +/- 0.3. 3. The effect of t-ACPD was partially antagonized by the mGluR antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mM). 4. t-ACPD (10-30 microM) did not modify the frequency of miniature GABAergic synaptic currents recorded in tetrodotoxin (the mean frequency ratio of t-ACPD over control was 0.7 +/- 0.3). 5. Forskolin (30 microM), but not its analogue 1,9 dideoxyforskolin (30 microM), mimicked the effect of t-ACPD. Similar effects were obtained with 3-isobutyl-1-methylxanthine (IBMX, 200 microM). 6. The potentiating effect of t-ACPD on spontaneous GABAergic currents was prevented by Rp-cAMPS (30 microM), a specific antagonist of protein kinase A. This suggests that mGluRs localized at the soma-dendritic level of GABAergic interneurons and positively coupled to cyclic AMP may modulate GABA release during a critical period of postnatal development.