Local synaptic circuits and epileptiform activity in slices of neocortex from children with intractable epilepsy

1992 ◽  
Vol 67 (3) ◽  
pp. 496-507 ◽  
Author(s):  
J. G. Tasker ◽  
W. J. Peacock ◽  
F. E. Dudek
Keyword(s):  
Electrical Stimulation ◽  
Gabaa Receptor ◽  
Epileptiform Activity ◽  
Intractable Epilepsy ◽  
Pyramidal Cells ◽  
Synaptic Inhibition ◽  
Gamma Aminobutyric Acid ◽  
Total N ◽  
Postsynaptic Potentials ◽  
Synaptic Circuits

1. Single and dual intracellular recordings were performed in neocortical slices obtained from tissue samples surgically removed from children (8 mo to 15 yr) for the treatment of intractable epilepsy. Electrical stimulation and glutamate microapplication were used to study local synaptic inputs to pyramidal cells. 2. In recordings with potassium-acetate electrodes, activation of presynaptic neocortical neurons with glutamate microdrops did not elicit a clear increase in postsynaptic potentials (PSPs) but did suppress current-evoked repetitive spike firing in recorded neurons. Bicuculline (10 microM) blocked this effect, suggesting it was caused by the activation of presynaptic gamma-aminobutyric acid (GABA) cells. In recordings with KCl electrodes, glutamate microdrops elicited an increase in the frequency and amplitude of depolarizing PSPs. Bicuculline (5-10 microM) blocked the glutamate-evoked PSPs, suggesting they were reversed GABAA-receptor-mediated inhibitory postsynaptic potentials (IPSPs). In one cell recorded with a KCl electrode (total n = 8), current-evoked spike trains elicited afterdischarges of reversed IPSPs, thus revealing a recurrent inhibitory circuit. Therefore local inhibitory synaptic circuits were robust and could be observed in tissue from patients as young as 11 mo. 3. In addition to short-latency (10-25 ms), monosynaptic excitatory postsynaptic potentials (EPSPs), electrical stimulation at low intensities sometimes elicited delayed EPSPs (20-60 ms). When GABAA-receptor-mediated synaptic inhibition was partially reduced in bicuculline (5-10 microM), electrical stimulation evoked large EPSPs at long and variable latencies (100-300 ms). Glutamate microapplication caused an increase in the frequency and amplitude of EPSPs; preliminary results suggest that glutamate microdrops were less likely to evoke EPSPs in tissue from younger patients (8-12 mo) than in slices from patients greater than 4 yr. Evidence for local excitatory synaptic circuits was thus found when synaptic inhibition was partially reduced. 4. After further reduction of inhibition in bicuculline (5-50 microM), electrical stimulation elicited epileptiform bursts. In pairs of simultaneously recorded neurons, bursts were generated synchronously from long-latency EPSPs (100-300 ms) in slices from patients as young as 8 mo. Reflected EPSPs at very long and variable latencies (500-1,100 ms) and repetitive epileptiform bursts could be evoked synchronously in pairs of cells. Glutamate activation of local presynaptic neurons elicited robust epileptiform events in recorded cells. This was seen in slices from patients as young as 16 mo. 5. These data provide physiological evidence for the presence of local inhibitory and excitatory synaptic circuits in human neocortex at least as early as 11 and 8 mo, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of HCO3- ions in depolarizing GABAA receptor-mediated responses in pyramidal cells of rat hippocampus

1993 ◽  
Vol 69 (5) ◽  
pp. 1541-1555 ◽  
Author(s):  
L. M. Grover ◽  
N. A. Lambert ◽  
P. A. Schwartzkroin ◽  
T. J. Teyler
Keyword(s):  
Gabaa Receptor ◽  
Gabab Receptor ◽  
Reversal Potential ◽  
Pyramidal Cells ◽  
Input Resistance ◽  
Gamma Aminobutyric Acid ◽  
Free Medium ◽  
Postsynaptic Potentials ◽  
Ca1 Pyramidal Cells ◽  
Ionic Basis

1. Activation of GABAA receptors can produce both hyperpolarizing and depolarizing responses in CA1 pyramidal cells. The hyperpolarizing response is mediated by a Cl- conductance, but the ionic basis of the depolarizing response is not clear. We compared the GABAA receptor-mediated depolarizations induced by synaptically released gamma-aminobutyric acid [GABA; depolarizing inhibitory postsynaptic potentials (dIPSPs)] with those produced by exogenous GABA (depolarizing GABA responses). Short trains of high-frequency (200 Hz) stimuli were used to generate dIPSPs. We found that dIPSPs generated by trains of stimuli and depolarizing responses to exogenous GABA were accompanied by a conductance increase and had a similar reversal potential, indicating a similar ionic basis for both responses. 2. We wished to determine whether an HCO3- current contributed to the GABAA-mediated depolarizations. We found that dIPSPs and depolarizing GABA responses were sensitive to perfusion with HCO3(-)-free medium. Interpretation of these data was complicated by the mixed nature of the responses: dIPSPs were invariably accompanied by conventional, Cl(-)-mediated fast hyperpolarizing IPSPs (fIPSPs), and response to exogenous GABA usually consisted of biphasic hyperpolarizing and depolarizing responses. However, it was sometimes possible to elicit responses to GABA that appeared purely depolarizing (monophasic depolarizing GABA responses). 3. We analyzed monophasic depolarizing GABA responses and found no change in reversal potential when slices were perfused with HCO(3-)-free medium. We also made whole-cell recordings from CA1 pyramidal cells, attempting to reduce [HCO3-]i, and compared the reversal potential for monophasic depolarizing GABA responses with similar responses recorded with fine intracellular microelectrodes. We found no difference in reversal potential. We also examined effects of the carbonic anhydrase inhibitor acetazolamide (ACTZ) on depolarizing GABA responses. ACTZ reduced these responses but did not change their reversal potential. 4. Effects of HCO(3-)-free medium were not specific to GABAA receptor-mediated responses. GABAB receptor-mediated slow IPSPs (sIPSPs) were also reduced, as were excitatory postsynaptic potentials (EPSPs). Analyses of field potentials and spontaneous fIPSPs suggested a decrease in presynaptic excitability during perfusion with HCO(3-)-free medium. In addition, pyramidal cells showed decreased input resistance when perfused with HCO(3-)-free medium. 5. The sensitivity of GABAA receptor-mediated depolarizations to HCO(3-)-free medium can be explained by a decrease in presynaptic excitability and an increased resting conductance in postsynaptic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Agent-selective Effects of Volatile Anesthetics on GABAAReceptor–mediated Synaptic Inhibition in Hippocampal Interneurons

2001 ◽  
Vol 94 (2) ◽  
pp. 340-347 ◽  
Author(s):  
Koichi Nishikawa ◽  
M. Bruce MacIver
Keyword(s):  
Gabaa Receptor ◽  
Hippocampal Slices ◽  
Pyramidal Cells ◽  
Volatile Anesthetics ◽  
Stratum Radiatum ◽  
Synaptic Inhibition ◽  
Gamma Aminobutyric Acid ◽  
Ca1 Region ◽  
Whole Cell Patch Clamp ◽  
Cortical Regions

Background A relatively small number of inhibitory interneurons can control the excitability and synchronization of large numbers of pyramidal cells in hippocampus and other cortical regions. Thus, anesthetic modulation of interneurons could play an important role for the maintenance of anesthesia. The aim of this study was to compare effects produced by volatile anesthetics on inhibitory postsynaptic currents (IPSCs) of rat hippocampal interneurons. Methods Pharmacologically isolated gamma-aminobutyric acid type A (GABAA) receptor-mediated IPSCs were recorded with whole cell patch-clamp techniques in visually identified interneurons of rat hippocampal slices. Neurons located in the stratum radiatum-lacunosum moleculare of the CA1 region were studied. The effects of clinically relevant concentrations (1.0 rat minimum alveolar concentration) of halothane, enflurane, isoflurane, and sevoflurane were compared on kinetics of both stimulus-evoked and spontaneous GABAA receptor-mediated IPSCs in interneurons. Results Halothane (1.2 vol% approximately 0.35 mm), enflurane (2.2 vol% approximately 0.60 mm), isoflurane (1.4 vol% approximately 0.50 mm), and sevoflurane (2.7 vol% approximately 0.40 mm) preferentially depressed evoked IPSC amplitudes to 79.8 +/- 9.3% of control (n = 5), 38.2 +/- 8.6% (n = 6), 52.4 +/- 8.4% (n = 5), and 46.1 +/- 16.0% (n = 8), respectively. In addition, all anesthetics differentially prolonged the decay time constant of evoked IPSCs to 290.1 +/- 33.2% of control, 423.6 +/- 47.1, 277.0 +/- 32.2, and 529 +/- 48.5%, respectively. The frequencies of spontaneous IPSCs were increased by all anesthetics (twofold to threefold). Thus, the total negative charge transfer mediated by GABAA receptors between synaptically connected interneurons was enhanced by all anesthetics. Conclusions Volatile anesthetics differentially enhanced GABAA receptor-mediated synaptic inhibition in rat hippocampal interneurons, suggesting that hippocampal interneuron circuits are depressed by these anesthetics in an agent-specific manner.

Enhanced NMDAR-dependent epileptiform activity is controlled by oxidizing agents in a chronic model of temporal lobe epilepsy

1996 ◽  
Vol 76 (6) ◽  
pp. 4185-4189 ◽  
Author(s):  
J. C. Hirsch ◽  
O. Quesada ◽  
M. Esclapez ◽  
H. Gozlan ◽  
Y. Ben-Ari ◽  
...  
Keyword(s):  
Synaptic Transmission ◽  
Ex Vivo ◽  
Epileptiform Activity ◽  
Hippocampal Slices ◽  
Nmda Antagonist ◽  
Pyramidal Cells ◽  
Resting Membrane Potential ◽  
Postsynaptic Potentials ◽  
Epileptiform Discharges ◽  
Late Part

1. Graded N-methyl-D-aspartate receptor (NMDAR)-dependent epileptiform discharges were recorded from ex vivo hippocampal slices obtained from rats injected a week earlier with an intracerebroventricular dose of kainic acid. Intracellular recordings from pyramidal cells of the CA1 area showed that glutamate NMDAR actively participated in synaptic transmission, even at resting membrane potential. When NMDAR were pharmacologically isolated, graded burst discharges could still be evoked. 2. The oxidizing reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM, 15 min) suppressed the late part of the epileptiform burst that did not recover after wash but could be reinstated by the reducing agent tris (2-carboxyethyl) phosphine (TCEP, 200 microM, 15 min) and again abolished with the NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-APV). 3. Pharmacologically isolated NMDAR-mediated responses were decreased by DTNB (56 +/- 10%, mean +/- SD, n = 6), an effect reversed by TCEP. 4. When only the fast glutamateric synaptic component was blocked, NMDA-dependent excitatory postsynaptic potentials (EPSPs) could be evoked despite the presence of underlying fast and slow inhibitory postsynaptic potentials (IPSPs). DTNB decreased EPSPs to 48 +/- 12% (n = 5) of control. 5. Since a decrease of the NMDAR-mediated response by +/- 50% is sufficient to suppress the late part of the burst, we suggest that epileptiform activity can be controlled by manipulation of the redox sites of NMDAR. Our observations raise the possibility of developing new anticonvulsant drugs that would spare alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-R (AMPAR)-mediated synaptic responses and decrease NMDAR-mediated synaptic transmission without blocking it completely.

On the synchronous activity induced by 4-aminopyridine in the CA3 subfield of juvenile rat hippocampus

1993 ◽  
Vol 70 (3) ◽  
pp. 1018-1029 ◽  
Author(s):  
M. Avoli ◽  
C. Psarropoulou ◽  
V. Tancredi ◽  
Y. Fueta
Keyword(s):  
Nmda Receptor ◽  
Gabaa Receptor ◽  
Action Potentials ◽  
Hippocampal Slices ◽  
Field Potential ◽  
Pyramidal Cells ◽  
Occurrence Rate ◽  
Gamma Aminobutyric Acid ◽  
Synchronous Activity ◽  
Ca3 Pyramidal Cells

1. Extracellular field potential and intracellular recordings were made in the CA3 subfield of hippocampal slices obtained from 10- to 24-day-old rats during perfusion with artificial cerebrospinal fluid (ACSF) containing the convulsant 4-aminopyridine (4-AP, 50 microM). 2. Three types of spontaneous, synchronous activity were recorded in the presence of 4-AP by employing extracellular microelectrodes positioned in the CA3 stratum (s.) radiatum: first, inter-ictal-like discharges that lasted 0.2-1.2 s and had an occurrence rate of 0.3-1.3 Hz; second, ictal-like events (duration: 3-40 s) that occurred at 4-38 x 10(-3) Hz; and third, large-amplitude (up to 8 mV) negative-going potentials that preceded the onset of the ictal-like events and thus appeared to initiate them. 3. None of these synchronous activities was consistently modified by addition of antagonists of the N-methyl-D-aspartate (NMDA) receptor to the ACSF. In contrast, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 2-10 microM) reversibly blocked interictal- and ictallike discharges. The only synchronous, spontaneous activity recorded in this type of medium consisted of the negative-going potentials that were abolished by the GABAA receptor antagonists bicuculline methiodide (5-20 microM) or picrotoxin (50 microM). Hence they were mediated through the activation of the GABAA receptor. 4. Profile analysis of the 4-AP-induced synchronous activity revealed that the gamma-aminobutyric acid (GABA)-mediated field potential had maximal negative amplitude in s. lacunosum-moleculare, attained equipotentiality at the border between s. radiatum and s. pyramidale, and became positive-going in s. oriens. These findings indicated that the GABA-mediated field potential presumably represented a depolarization occurring in the dendrites of CA3 pyramidal cells. 5. This conclusion was supported by intracellular analysis of the 4-AP-induced activity. The GABA-mediated potential was reflected by a depolarization of the membrane of CA3 pyramidal cells that triggered a few variable-amplitude, fractionated spikes or fast action potentials. By contrast, the ictal-like discharge was associated with a prolonged depolarization during which repetitive bursts of action potentials occurred. Short-lasting depolarizations with bursts of action potentials occurred during each interictal-like discharge. 6. The GABA-mediated potential recorded intracellularly in the presence of CNQX consisted of a prolonged depolarization (up to 12 s) that was still capable of triggering a few fast action potentials and/or fractionated spikes.(ABSTRACT TRUNCATED AT 400 WORDS)

Ammonia does not selectively block IPSPs in rat hippocampal pyramidal cells

1983 ◽  
Vol 49 (6) ◽  
pp. 1381-1391 ◽  
Author(s):  
B. E. Alger ◽  
R. A. Nicoll
Keyword(s):  
Reversal Potential ◽  
Pyramidal Cells ◽  
Gamma Aminobutyric Acid ◽  
Postsynaptic Potentials ◽  
Calcium Dependent ◽  
Nonspecific Effects ◽  
Chloride Pump ◽  
High Concentrations ◽  
Conductance Increase ◽  
Potassium Response

Intracellular recordings from CA1 pyramidal cells in the rat hippocampal slice preparation have been used to study the action of ammonia on inhibitory postsynaptic potentials (IPSPs). Concentrations of ammonia less than 2 mM had little effect on IPSPs or the action of iontophoretically applied gamma-aminobutyric acid (GABA). This concentration has been reported to be fully effective in blocking hyperpolarizing IPSPs in spinal cord and neocortex. Concentrations above 2 mM did cause a depolarizing shift in the IPSP and GABA reversal potentials, but this effect was accompanied by several generalized effects. The conductance increase during the IPSP but not during the GABA response was depressed, indicating that ammonia has a presynaptic depressant effect on the IPSP. Ammonia also depressed excitatory postsynaptic potentials (EPSPs), presynaptic fiber potentials, and pyramidal cell population spikes. In addition, the calcium-dependent potassium response elicited by depolarizing current pulses was depressed. This depression was due, in part, to a depolarizing shift in the reversal potential for this response. Responses recorded with potassium-sensitive microelectrodes indicate that ammonia releases potassium into the extracellular space. The possibility is discussed that the shifts in IPSP reversal potential seen with high concentrations of ammonia are a consequence of generalized nonspecific effects. We conclude that the relative insensitivity of hippocampal IPSPs to blockade by ammonia suggests that a mechanism fundamentally unlike an ammonia-sensitive chloride pump must maintain the hippocampal IPSP gradient.

GABA as an inhibitory neurotransmitter in human cerebral cortex

1989 ◽  
Vol 62 (5) ◽  
pp. 1018-1027 ◽  
Author(s):  
D. A. McCormick
Keyword(s):  
Cerebral Cortex ◽  
Cortical Neurons ◽  
Epileptiform Activity ◽  
Reversal Potential ◽  
Membrane Conductance ◽  
Pyramidal Cells ◽  
Excitatory Postsynaptic Potential ◽  
Gamma Aminobutyric Acid ◽  
Human Cerebral Cortex ◽  
Inhibitory Neurotransmitter

1. The possible role of gamma-aminobutyric acid (GABA) as an inhibitory neurotransmitter in the human cerebral cortex was investigated with the use of intracellular recordings from neocortical slices maintained in vitro. 2. Electrical stimulation of afferents to presumed pyramidal cells resulted in an initial excitatory postsynaptic potential (EPSP) followed by fast and slow inhibitory postsynaptic potentials (IPSPs). The early IPSP had an average reversal potential of -68 mV, was associated with a mean 67-nS increase in membrane conductance, was reduced by the GABAA antagonist bicuculline, was sensitive to the intracellular injection of Cl-, and was mimicked by the GABAA agonist muscimol. 3. The late IPSP, in contrast, had an average reversal potential of -95 mV, was associated with a mean 12-nS increase in membrane conductance, was reduced by the GABAB antagonist phaclofen, and was mimicked by the GABAB agonist baclofen. 4. Block of the early IPSP by bicuculline or picrotoxin led to the generation of paroxysmal epileptiform activity, which could be further enhanced by reduction of the late IPSP. 5. These data strongly support the hypothesis that GABA is a major inhibitory neurotransmitter in the human cerebral cortex and that GABAergic IPSPs play an important role in controlling the excitability and responsiveness of cortical neurons.

Diverse neuronal populations mediate local circuit excitation in area CA3 of developing hippocampus

1995 ◽  
Vol 74 (2) ◽  
pp. 650-672 ◽  
Author(s):  
K. L. Smith ◽  
D. H. Szarowski ◽  
J. N. Turner ◽  
J. W. Swann
Keyword(s):  
Confocal Microscopy ◽  
Gabaa Receptor ◽  
Action Potentials ◽  
Lucifer Yellow ◽  
Single Cells ◽  
Pyramidal Cells ◽  
Gamma Aminobutyric Acid ◽  
Gabaa Receptor Antagonist ◽  
Fast Spiking ◽  
Area Ca3

1. Studies were undertaken to better understand why the developing hippocampus has a marked capacity to generate prolonged synchronized discharges when exposed to gamma-aminobutyric acid-A (GABAA) receptor antagonists. 2. Excitatory synaptic interactions were studied in small microdissected segments of hippocampal area CA3. Slices were obtained from 10- to 16-day-old rats. Application of the GABAA receptor antagonist penicillin produced prolonged synchronized discharges in minislices that were very similar, if not identical, to those recorded in intact slices. The sizes of minislices were systematically varied. Greater than 90% of those that measured 600 microns along the cell body layer produced prolonged synchronized discharges, whereas most minislices measuring 300 microns produced only brief interictal spikes. 3. Action potentials in the majority (75%, 158 of 254) of cells impaled with microelectrodes were able to entrain the entire CA3 population. They were also able to increase (on average 26%) the frequency of spontaneous population discharges. The population discharges were followed by a refractory period that lasted 5–60 s, during which single cells were unable to initiate a population discharge. 4. The majority (87%) of neurons with intrinsic burst properties were found to entrain the CA3 population. The electrophysiological characteristics of these cells were reminiscent of recordings obtained from more mature rats. Action potentials were quite prolonged and demonstrated a secondary shoulder or hump on the down-slope of the spike. 5. When bursting cells were filled with Lucifer yellow and imaged during recording sessions by videomicroscopy and later using confocal microscopy, they showed the anatomic features of CA3 hippocampal pyramidal cells. Confocal microscopy permitted detailed characterization of individual neurons and showed substantial variation in cellular microanatomy. 6. Another class of cells that were found to entrain the CA3 population but did not demonstrate intrinsic bursts were termed regular-firing cells. These cells possessed many of the anatomic and physiological features of bursting cells with the exception of burst firing. They were rarely encountered in intracellular recordings. 7. The third physiological class of cells was termed fast-spiking cells. These had action potentials that were shorter in duration than the other two cell types. They were distinct in the rapid rate of spike repolarization. They demonstrated modest degrees of spike frequency adaptation and fired repeatedly and at relatively high frequencies. Compared with reports on fast-spiking cells in mature hippocampus and neocortex, action potentials appear to be slower and repetitive discharging appeared to be of a lower frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

The quantitative structure-activity relationships between GABAA receptor and ligands based on binding interface characteristic

2020 ◽  
Vol 16 ◽  
Author(s):  
Shu Cheng ◽  
Yanrui Ding
Keyword(s):  
Gabaa Receptor ◽  
Molecular Descriptors ◽  
Qsar Model ◽  
Coefficient Of Determination ◽  
Gradient Boosting ◽  
Receptor Interaction ◽  
Gamma Aminobutyric Acid ◽  
Quantitative Structure ◽  
Binding Interface ◽  
Structure Activity

Background: Quantitative Structure Activity Relationship (QSAR) methods based on machine learning play a vital role in predicting biological effect. Objective: Considering the characteristics of the binding interface between ligands and the inhibitory neurotransmitter Gamma Aminobutyric Acid A(GABAA) receptor, we built a QSAR model of ligands that bind to the human GABAA receptor. Method: After feature selection with Mean Decrease Impurity, we selected 53 from 1,286 docked ligand molecular descriptors. Three QSAR models are built using gradient boosting regression tree algorithm based on the different combinations of docked ligand molecular descriptors and ligand-receptor interaction characteristics. Results: The features of the optimal QSAR model contain both the docked ligand molecular descriptors and ligand-receptor interaction characteristics. The Leave-One-Out-Cross-Validation (Q2 LOO) of the optimal QSAR model is 0.8974, the Coefficient of Determination (R2) for the testing set is 0.9261, the Mean Square Error (MSE) is 0.1862. We also used this model to predict the pIC50 of two new ligands, the differences between the predicted and experimental pIC50 are -0.02 and 0.03 respectively. Conclusion : We found the BELm2, BELe2, MATS1m, X5v, Mor08v, and Mor29m are crucial features, which can help to build the QSAR model more accurately.

Developmental study of benzodiazepine effects on monosynaptic GABAA-mediated IPSPs of rat hippocampal neurons

1993 ◽  
Vol 70 (3) ◽  
pp. 1076-1085 ◽  
Author(s):  
C. Rovira ◽  
Y. Ben-Ari
Keyword(s):  
Hippocampal Neurons ◽  
Receptor Agonist ◽  
Hippocampal Slices ◽  
Pyramidal Cells ◽  
Developmental Study ◽  
Type I ◽  
Gamma Aminobutyric Acid ◽  
Adult Rats ◽  
Young Rats ◽  
In Cells

1. The effects of type I (BZ1) and type II (BZ2) benzodiazepine receptor ligands on monosynaptic gamma-aminobutyric acid (GABA)A-mediated inhibitory postsynaptic potentials (IPSPs) and on responses to exogenously applied GABA were studied using intracellular recordings from CA3 pyramidal cells of rat hippocampal slices taken at different postnatal stages [postnatal day 4 (P4)-P35)]. 2. The effects of midazolam, a BZ1 and BZ2 receptor agonist, were tested on the monosynaptic IPSPs at different stages. Monosynaptic, bicuculline-sensitive IPSPs were evoked by hilar stimulation in presence of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) antagonists [6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) and D(-)2-amino-5-phosphonopentanoic acid (50 microM)]. Midazolam at 300 nM maximally increased the duration and amplitude of monosynaptic GABAA-mediated IPSPs in neurons from pups (P4-P6, n = 6) and young (P7-P12, n = 8) and adult (P25-P35, n = 9) rats. All the effects of midazolam on IPSPs were reversed by the antagonist Ro 15-1788 (10 microM). 3. The effect of midazolam was also tested on the response to exogenously applied GABA (5 mM) in the presence of tetrodotoxine [TTX (1 microM)]. In neurons from young rats (n = 9), midazolam (1 nM-1 microM) did not change the responses to exogenously applied GABA, whereas in adult rats (n = 8) midazolam maximally increased GABA currents at 30 nM. 4. The effect of zolpidem, a BZ1 receptor agonist, was tested on monosynaptic IPSPs and GABA currents at different stages. Zolpidem (10 nM-1 microM) was inactive in cells from young rats (n = 12). In neurons from adult rats, zolpidem maximally increased the duration and amplitude of the monosynaptic IPSPs at 300 nM (n = 5) and the amplitude of GABA current at 30-100 nM (n = 5). 5. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) (300 nM), an inverse agonist of BZ1 and BZ2 receptors, decreased the amplitude and duration of monosynaptic IPSPs in neurons from pups (n = 3) and young (n = 4) and adult (n = 5) rats. In all cases, full recovery was obtained after exposure to R0 15-1788 (10 microM). DMCM (300 nM-10 microM) failed to reduce GABA responses in cells from young (n = 3) or adult (n = 7) rats. 6. Results indicate that the regulation by benzodiazepine of GABAA-mediated IPSPs varies with the developmental stage.(ABSTRACT TRUNCATED AT 400 WORDS)

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