Slow Inhibitory Postsynaptic Currents Research Articles

Glutamate-Mediated Extrasynaptic Inhibition: Direct Coupling of NMDA Receptors to Ca 2+-Activated K + Channels

NMDA receptors (NMDARs) typically contribute to excitatory synaptic transmission in the CNS. While Ca 2+ influx through NMDARs plays a critical role in synaptic plasticity, direct actions of NMDAR-mediated Ca 2+ influx on neuronal excitability have not been well established. Here we show that Ca 2+ influx through NMDARs is directly coupled to activation of BK-type Ca 2+-activated K + channels in outside-out membrane patches from rat olfactory bulb granule cells. Repetitive stimulation of glutamatergic synapses in olfactory bulb slices evokes a slow inhibitory postsynaptic current (IPSC) in granule cells that requires both NMDARs and BK channels. The slow IPSC is enhanced by glutamate uptake blockers, suggesting that extrasynaptic NMDARs underlie the response. These findings reveal a novel inhibitory action of extrasynaptic NMDARs in the brain.

Interactions between Distinct GABA A Circuits in Hippocampus

Synchronous activity among synaptically connected interneurons is thought to organize temporal patterns such as gamma and theta rhythms in cortical circuits. Interactions between distinct interneuron circuits may underlie more complex patterns, such as nested rhythms. Here, we demonstrate such an interaction between two groups of CA1 interneurons, GABA A,slow and GABA A,fast cells, that may contribute to theta and gamma rhythms, respectively. Stratum lacunosum-moleculare (SL-M) stimuli that activate GABA A,slow inhibitory postsynaptic currents (IPSCs) in pyramidal cells simultaneously depress the rate and amplitude of spontaneous GABA A,fast IPSCs for several hundred milliseconds. This suppression has a similar pharmacological profile to GABA A,slow IPSCs, and SL-M stimuli elicit GABA A,slow IPSCs in interneurons. We conclude that GABA A,slow cells inhibit both pyramidal cells and GABA A,fast interneurons and postulate that this interaction contributes to nested theta/gamma rhythms in hippocampus.

Effects of corticosteroids on synaptic transmission in rat dorsolateral septal nucleus.

The effects of corticosteroids on synaptic transmission in the rat dorsolateral septal nucleus (DLSN) were examined, in vitro, by using intracellular and voltage-clamp recording methods. Prednisolone (100 microM) increased the amplitude of excitatory postsynaptic potential (EPSP) and depressed both fast and slow inhibitory postsynaptic potentials (IPSP). Under voltage-clamp conditions, prednisolone (100 microM) increased the amplitude of excitatory postsynaptic current (EPSC) and depressed the fast and slow inhibitory postsynaptic currents (IPSCs). Corticosterone (100 microM) mimicked the effects of prednisolone on the postsynaptic currents (PSCs). To examine the direct effects of prednisolone on the EPSC and slow IPSC, the fast IPSC was blocked by bicuculline (20 microM). Under these experimental conditions, prednisolone (100 microM) did not alter the isolated EPSC but depressed slow IPSC by 22 +/- 3% (n = 10). The fast IPSC was isolated by pretreatment with kynurenic acid and CGP55845A, where the EPSC and slow IPSC were blocked. Prednisolone (100 microM) depressed the isolated fast IPSC in DLSN neurons. Prednisolone (100 microM) did not change either the inward current produced by glutamate or the outward current produced by gamma-aminobutyric acid (GABA). The results suggest that corticosteroids facilitate excitatory synaptic transmission in the DLSN by reducing the release of GABA from the presynaptic nerve terminals of interneurons.

GABAB-Receptor-mediated currents in interneurons of the dentate-hilus border.

GABA(B)-receptor-mediated inhibition was investigated in anatomically identified inhibitory interneurons located at the border between the dentate gyrus granule cell layer and hilus. Biocytin staining was used to visualize the morphology of recorded cells. A molecular layer stimulus evoked a pharmacologically isolated slow inhibitory postsynaptic current (IPSC), recorded with whole cell patch-clamp techniques, in 55 of 63 interneurons. Application of the GABA(B) receptor antagonists, CGP 35348 (400 microM) or CGP 55845 (1 microM) to a subset of 25 interneurons suppressed the slow IPSC by an amount ranging from 10 to 100%. In 56% of these cells, the slow IPSC was entirely GABA(B)-receptor-mediated. However, in the remaining interneurons, a component of the slow IPSC was resistant to GABA(B) antagonists. Subtraction of this antagonist resistant current from the slow IPSC isolated the GABA(B) component (IPSC(B)). This IPSC(B) had a similar onset and peak latency to that recorded from granule cells but a significantly shorter duration. The GABA(B) agonist, baclofen (10 microM), produced a CGP 55845-sensitive outward current in 19 of 27 interneurons. In the eight cells that lacked a baclofen current, strong or repetitive ML stimulation also failed to evoke an IPSC(B), indicating that these cells lacked functional GABA(B) receptor-activated potassium currents. In cells that expressed a baclofen current, the amplitude of this current was approximately 50% smaller in interneurons with axons that projected into the granule cell dendritic layer (22.2 +/- 5.3 pA; mean +/- SE) than in interneurons with axons that projected into or near the granule cell body layer (46.1 +/- 10.0 pA). Similarly, the IPSC(B) amplitude was smaller in interneurons projecting to dendritic (9.4 +/- 2.7 pA) than perisomatic regions (34.3 +/- 5.1 pA). These findings suggest that GABA(B) inhibition more strongly regulates interneurons with axons that project into perisomatic than dendritic regions. To determine the functional role of GABA(B) inhibition, we examined the effect of IPSP(B) on action potential firing and synaptic excitation of these interneurons. IPSP(B) and IPSP(A) both suppressed depolarization-induced neuronal firing. However, unlike IPSP(A), suppression of firing by IPSP(B) could be easily overcome with strong depolarization. IPSP(B) markedly suppressed N-methyl-D-aspartate but not AMPA EPSPs, suggesting that GABA(B) inhibition may play a role in regulating slow synaptic excitation of these interneurons. Heterogeneous expression of GABA(B) currents in hilar border interneurons therefore may provide a mechanism for the differential regulation of excitation of these cells and thereby exert an important role in shaping neuronal activity in the dentate gyrus.

Full length article

Paired-pulse depression (PPD) and facilitation are found at many synapses in the central nervous system. In the present study, we aimed to characterise the paired-pulse behaviour of evoked postsynaptic potentials in the superficial layers of slices of the superior colliculus (SC) of adult guinea-pigs. We observed PPD for inter-stimulus intervals between 10 and 500 ms, for both high (90% of maximum) and low (30% of maximum) stimulus intensities. This depression could be converted into a facilitation when the probability of transmitter release was reduced in low Ca2+/high Mg2+ solution, but only when the low stimulus intensity was applied. Elimination of GABA(A) receptor mediated currents by bicuculline caused an enhanced general excitation and enhanced PPD. Application of the GABA(B) receptor antagonist CGP35348 reduced PPD, suggesting the contribution of slow inhibitory postsynaptic currents. The NMDA receptor antagonist D,L-amino-5-phosphonovaleric acid (APV) did not lead to major alterations of PPD. We conclude that presynaptic mechanisms affecting Ca2+-dependent glutamate release are crucial for PPD in the superficial SC. Nevertheless, postsynaptic inhibitory components and probably polysynaptic pathways also seem to contribute to PP behaviour in the SC. Moreover, PPD in the SC has a different profile compared with other brain areas. Here, PPD may be crucially involved in setting the threshold for novel vs. background stimulation, since the SC is known to trigger orienting responses towards novel sensory stimuli.

Voltage-clamp analysis of halothane effects on GABA A fast and GABA A slow inhibitory currents

Voltage-clamped GABA A fast and GABA A slow inhibitory postsynaptic currents (IPSCs) were selectively elicited in hippocampal area CA1 pyramidal neurons. Clinically relevant concentrations of halothane (1.2 vol.%) prolonged both GABA A fast and GABA A slow IPSC decay times approximately 2.5 fold, while having little to no effect on current amplitudes or rise times. Current–voltage analysis revealed that IPSC reversal potentials (−70 to −75 mV) remained constant in the presence of halothane. Under control conditions, GABA A slow IPSC decay times increased linearly with membrane depolarization, and this IPSC decay time voltage dependence was not significantly altered by halothane. These results confirm the existence of separable GABA A fast and GABA A slow IPSCs in hippocampus, and further elucidate the effects of halothane on these currents.

Endogenous GABA activates small-conductance K+ channels underlying slow IPSCs in rat hippocampal neurons.

The objective of this study was to determine the properties of K+ channels activated by endogenously released trasmitter under synaptic conditions. First, the levels of gamma-aminobutyric acid (GABA) were depleted in hippocampal nerve endings to establish the relative contribution of endogenously released GABA to the activation of GABA(B) receptors mediating slow inhibitory postsynaptic currents (IPSCs). Inhibition of glutamic acid decarboxylase and GABA reuptake effectively depleted >85% of the releasable GABA pool, producing parallel reductions of GABA(A) and GABA(B) receptor-mediated IPSCs, indicating that both classes of receptors are activated synaptically by endogenously released GABA. Whole cell patch-clamp recordings of stimulus-evoked slow IPSCs at potentials hyperpolarized from the potassium reversal potential were consistent with the activation of a nonrectifying (n = 3) or slightly outwardly rectifying (n = 4) K+ conductance by the endogenously released GABA. Spectral analysis of the decay phase of GABA(B) IPSCs revealed several time constants indicating complex underlying channel kinetics. Nonstationary variance analysis yielded a small unitary conductance in the range of 5-13 pS, consistent with a large number of channels activated during evoked currents. These results indicate that in granule cells of the dentate gyrus, GABA released synaptically from interneuron terminals activates an unusually small K+ conductance, with no or slight outward rectification. This conductance is therefore unlike those typically reported for neuronal G protein-coupled K+ channels or those activated by exogenously applied baclofen with larger, inwardly rectifying conductances.

Hippocampal CA1 lacunosum-moleculare interneurons: modulation of monosynaptic GABAergic IPSCs by presynaptic GABAB receptors

1. Whole cell patch-clamp recordings were employed to characterize monosynaptic inhibitory postsynaptic currents (IPSCs) in morphologically and electrophysiologically identified interneurons located in the stratum lacunosum moleculare, or near the border of the stratum radiatum (LM interneurons), in the CA1 region of hippocampal slices taken from 3- to 4-wk-old rats. Monosynaptic IPSCs, evoked in the presence of glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microM) and D-2-amino-5-phosphopentanoate (APV; 50 microM) were biphasic. The gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline (20 microM), blocked the fast IPSC, and the slow IPSC was blocked by the GABAB receptor antagonist CGP35348 (500 microM). 2. Monosynaptic IPSCs were evoked by electrical stimulation in several distant regions including the stratum radiatum, the stratum oriens, the stratum lacunosum-moleculare, and the molecular layer of dentate gyrus, suggesting an extensive network of inhibitory interneurons in the hippocampus. In paired recordings of CA1 interneurons and pyramidal cells, IPSCs were evoked by electrical stimulation of most of these distal regions with the exception of the molecular layer of dentate gyrus, which evoked an IPSC only in LM interneurons. 3. Frequent (> 0.1 Hz) stimulation depressed the evoked IPSCs. With a paired-pulse protocol, the second IPSC was depressed and the maximal depression (40-50%) was observed with an interstimulus interval of 100-200 ms. 4. The GABAB receptor agonist baclofen (1 microM) reduced the amplitude of evoked IPSCs and the paired-pulse depression of the second IPSC. The GABAB receptor antagonist CGP35348 (0.5-1 mM) had no significant effect on the amplitude of isolated IPSCs. However, CGP35348 reduced but did not fully block paired-pulse depression, suggesting that this depression is partly due to the activation of presynaptic GABAB receptors. 5. The paired-pulse depression depended on the level of transmitter release. Potentiation of synaptic release of GABA, by increasing the extracellular Ca2+ concentration to 4 mM and reducing the extracellular Mg2+ concentration to 0.1 mM, enhanced the depression. Reduction of transmitter release by increasing extracellular Mg2+ concentration to 7 mM diminished the paired-pulse depression of IPSCs. After potentiation of transmitter release, CGP35348 was less efficient in reducing the paired-pulse depression, suggesting that enhancement of depression by high-calcium/low-magnesium medium was preferentially due to the potentiation of a GABAB-independent component. 6. In summary, monosynaptic IPSCs recorded in LM interneurons show similar features to those recorded in pyramidal cells. The strong correlation between the level of transmitter release and the degree of paired-pulse depression may have important physiological consequences, because in synapses with a high level of activity and a high level of GABA release, inhibition is powerful, but depression can develop more readily.

Properties of isolated GABA B-mediated inhibitory postsynaptic currents in hippocampal pyramidal cells

Whole-cell recording techniques were used to record isolated slow inhibitory postsynaptic currents in CA1 pyramidal neurons from rat hippocampal slices. Application of 6-cyano-7-nitroquinoxaline-2,3-dione and 3-(2-car☐ypiperazin-4-yl)propyl-1-phosphonic acid eliminated excitatory synaptic transmission, resulting in a 38% reduction in slow inhibitory postsynaptic current magnitude. Subsequent addition of the GABA A antagonist picrotoxin caused a further decrease in slow inhibitory postsynaptic current amplitude. The remaining, isolated slow inhibitory postsynaptic current was blocked by the GABA B antagonist 2-hydroxysaclofen and when cesium was substituted for intracellular potassium. The kinetics of isolated slow inhibitory postsynaptic currents were characterized by single exponential, fourth power activation, and double exponential inactivation. These slow inhibitory postsynaptic currents had a reversal potential of−85.7 ± 1.6mV, and a slope conductance of935 ± 277pS. Single slow inhibitory postsynaptic currents carried a total charge flux of13.4 ± 7.6pC. Repetitive stimulation up to 1 Hz progressively reduced steady-state slow inhibitory postsynaptic current amplitude. This attenuation was characterized by a decrease in slope conductance, but slow inhibitory postsynaptic current reversal potential remained unchanged, as did slow inhibitory postsynaptic current kinetics. These results indicate that, under physiological conditions, both ionotropic glutamate- and GABA A-mediated transmission contribute to slow inhibitory postsynaptic current recruitment. Given this finding, activity-dependent decreases in GABA A transmission could contribute to slow inhibitory postsynaptic current depression, though not exclusively, since isolated slow inhibitory postsynaptic currents also demonstrated this property. The use-dependent depression of isolated slow inhibitory postsynaptic currents may be a consequence of a reduction in transmitter release.

Mechanisms underlying intracellular signal transduction of the slow IPSP in submucous neurones of the guinea-pig caecum.

1. Intracellular recordings were obtained from submucous plexus neurones of the guinea-pig caecum. 2. The resting membrane conductance displayed two types of inward rectification: one which developed at potentials more negative than -70 mV, and another that occurred at potentials more negative than the potassium equilibrium potential. The former inward rectification was blocked by extracellular caesium (Cs+; 1-2 mM) and the latter was blocked by Cs+ (1-2 mM) or barium (Ba2+; 30-100 microM). 3. The noradrenaline-induced current measured by subtraction of the current-voltage (I-V) relation before and after adding the agonist also showed an inward rectification around the resting potential. Ba2+ (30-100 microM) blocked both the outward and inward current induced by noradrenaline. The noradrenaline current was not affected by Cs+ (1-2 mM). Both the slow IPSP and the slow IPSC (inhibitory postsynaptic current) were reduced by Ba2+, but not by Cs+. 4. During the intracellular injection of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S), multiple repetitive stimulation or repeated applications of noradrenaline produced irreversible membrane hyperpolarizations with a decreased membrane input resistance, until the membrane had approached the potassium equilibrium potential. 5. Pertussis toxin (1-40 micrograms/ml) abolished both the slow IPSP and the noradrenaline hyperpolarization without affecting the nicotinic fast EPSP or the slow EPSP. 6. Superfusion with a Ca(2+)-free, high-Mg2+ (12 mM) solution caused a membrane depolarization associated with an increased input resistance. It eliminated the Ca2+ spikes, the slow after-hyperpolarizations following the spikes, and the synaptic potentials within 3 min. Prolonged exposure (longer than 20 min) to this solution resulted in a progressive decline of the noradrenaline hyperpolarization. 7. Intracellular injection of ethylene glycol-bis(beta-aminoethylether)N,N,N',N'-tetraacetic acid (EGTA) reduced the slow IPSP and the noradrenaline hyperpolarization. Superfusion with a membrane-permeable Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM; 10-200 microM) reduced the noradrenaline hyperpolarization. 8. Procaine reversibly reduced the slow IPSP and noradrenaline hyperpolarization without affecting the fast EPSP or slow EPSP at concentrations up to 300 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Block of GABAb-activated K+ conductance by kainate and quisqualate in rat CA3 hippocampal pyramidal neurones

Current and voltage-clamp techniques were used to study the effects of kainic (KA) and quisqualic (quis) acids on the slow synaptic inhibition evoked by mossy fibre stimulation in CA3 hippocampal pyramidal neurones in vitro. The K+ conductance underlying the slow synaptic inhibition is coupled to a gamma-aminobutyric acid b (GABAb) receptor by a guanosine-triphosphate (GTP)-binding protein. Both KA and quis reduce (after 7-10 min) the slow inhibitory post-synaptic current (IPSC) without changing the reversal potential. They also reduce the cellular response to exogenously applied (-)baclofen and 5-hydroxytryptamine, which are known to activate a similar K+ conductance. We conclude that KA and quis block the post-synaptic K+ conductance underlying the slow IPSC.

Electrogenesis of the slow inhibitory postsynaptic potential in bullfrog sympathetic ganglia.

The ionic mechanisms of the slow surface positive (P)-potential and the slow inhibitory postsynaptic potential (IPSP), an intracellularly recorded P-potential in sympathetic ganglia, were analysed by means of sucrose-gap, intracellular microelectrode techniques, and voltage clamp technique. Both the P-potential and the slow IPSP consist of two different potential components, namely the ouabain-sensitive and the ouabain-insensitive components. The ouabain-sensitive component was enhanced by a moderate conditioning hyperpolarization. This component was most reasonably explained as a potential change generated by an activation of the electrogenic Na+ pump. The ouabain-insensitive potential component of the P-potential and the slow IPSP decreased in the amplitude and finally reversed its polarity by conditioning hyperpolarization. The reversal potential of ouabain-insensitive component of slow IPSP and slow inhibitory postsynaptic current (IPSC) was close to the EK. The amplitude of ouabain-insensitive component of P-potential and slow IPSP was markedly decreased by an elevation of external K+ concentration. The reversal potential of ouabain-insensitive component shifted to a more positive potential level in high K+ Ringer's solution. On the other hand, it was augmented in K+-free Ringer's solution. A reduction of the membrane resistance was observed during the generation of the slow IPSP, when the membrane potential of ganglion cells was held at a membrane potential level more negative than -60 mV. The slow IPSC recorded by voltage-clamp method was associated with an increase in membrane conductance. It was concluded that the ouabain-insensitive component was generated by an activation of K+ conductance.