Biomedical subjects

R Khazipov

Publications and source records attributed to R Khazipov.

At least 19 recordsLinked to original sources

PubMed2001

Early development of neuronal activity in the primate hippocampus in utero.

Morphological studies suggest that the primate hippocampus develops extensively before birth, but little is known about its functional development. Patch-clamp recordings of hippocampal neurons and reconstruction of biocytin-filled pyramidal cells were performed in slices of macaque cynomolgus fetuses delivered by cesarean section. We found that during the second half of gestation, axons and dendrites of pyramidal cells grow intensively by hundreds of micrometers per day to attain a high level of maturity near term. Synaptic currents appear around midgestation and are correlated with the level of morphological differentiation of pyramidal cells: the first synapses are GABAergic, and their emergence correlates with the growth of apical dendrite into stratum radiatum. A later occurrence of glutamatergic synaptic currents correlates with a further differentiation of the axodendritic tree and the appearance of spines. Relying on the number of dendritic spines, we estimated that hundreds of new glutamatergic synapses are established every day on a pyramidal neuron during the last third of gestation. Most of the synaptic activity is synchronized in spontaneous slow ( approximately 0.1 Hz) network oscillations reminiscent of the giant depolarizing potentials in neonatal rodents. Epileptiform discharges can be evoked by the GABA(A) receptor antagonist bicuculline by the last third of gestation, and postsynaptic GABA(B) receptors contribute to the termination of epileptiform discharges. Comparing the results obtained in primates and rodents, we conclude that the template of early hippocampal network development is conserved across the mammalian evolution but that it is shifted toward fetal life in primate.

Animals↗

PubMed2001

Neuronal mechanisms of the anoxia-induced network oscillations in the rat hippocampus in vitro.

1. A spindle of fast network oscillations precedes the ischaemia-induced rapid depolarisation in the rat hippocampus in vivo. However, this oscillatory pattern could not be reproduced in slices and the underlying mechanisms remain poorly understood. We have found that anoxia-induced network oscillations (ANOs, 20-40 Hz, lasting for 1-2 min) can be reproduced in the intact hippocampi of postnatal day P7-10 rats in vitro, and we have examined the underlying mechanisms using whole-cell and extracellular field potential recordings in a CA3 pyramidal layer. 2. ANOs were generated at the beginning of the anoxic depolarisation, when pyramidal cells depolarised to subthreshold values. Maximal power of the ANOs was attained when pyramidal cells depolarised to -56 mV; depolarisation above -47 mV resulted in a depolarisation block of pyramidal cells and a waning of ANOs. 3. A multiple unit activity in extracellular field recordings was phase locked to the negative and ascending phases of ANOs. Pyramidal cells recorded in current-clamp mode generated action potentials with an average probability of about 0.05 per cycle. The AMPA receptor-mediated EPSCs and the GABA receptor-mediated IPSCs in CA3 pyramidal cells were also phase locked with ANOs. 4. ANOs were prevented by tetrodotoxin and glutamate receptor antagonists CNQX and APV, and were slowed down by the allosteric GABA(A) receptor modulator diazepam. In the presence of the GABA(A) receptor antagonist bicuculline, ANOs were transformed to epileptiform discharges. 5. In the presence of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the anoxia induced an epileptiform activity and no ANOs were observed. 6. In normoxic conditions, a rise of extracellular potassium to 10 mM induced an epileptiform activity. Increasing extracellular potassium in conjunction with a bath application of the adenosine A1 receptor agonist cyclopentyladenosine induced oscillations similar to ANOs. 7. Multisite recordings along the septo-temporal hippocampal axis revealed that ANOs and anoxic depolarisation originate in the temporal part, and propagate towards the septal pole at a speed of 1.9 mm x min(-1). 8. ANOs were observed starting from P7, i.e. at a developmental stage when the effects of GABA change from depolarisation to hyperpolarisation. 9. These results suggest that the synchronisation of anoxia-induced oscillations relies on synaptic mechanisms; that the inhibition by GABA and adenosine sets the tune for a generation of oscillations and prevents an epileptiform activity; and that a synchronous GABAergic inhibition is instrumental in a phase locking neuronal activity similarly to other types of oscillatory activities in the gamma frequency range.

2-Amino-5-phosphonovalerate↗

PubMed2000

Seizures accelerate anoxia-induced neuronal death in the neonatal rat hippocampus.

Seizures occurring in infants with hypoxia are frequently associated with an ominous prognosis. There is, however, no direct evidence that seizures are involved in the pathogenesis of hypoxia-induced neuronal damage. Here, we report that seizures significantly aggravate the hypoxic state by accelerating rapid anoxic depolarization (AD) and associated neuronal death in preparations of the intact hippocampus of neonatal rats in vitro. Under control conditions, prolonged episodes of anoxia/aglycemia induced rapid suppression of synaptic activity followed sequentially by brief bursts of epileptiform activity and then by rapid AD. AD was associated with irreversible neuronal damage manifested by irreversible loss of the membrane potential, synaptic responses, and neuronal degeneration. Aggravation of electrographic seizure activity during anoxic episodes by the adenosine A1 receptor antagonists DPCPX and caffeine or the gamma-aminobutyric acid-A receptor antagonist bicuculline or pretreatment with 4-aminopyridine accelerated AD and associated neuronal death by up to twofold, whereas blockade of seizure activity by the glutamate receptor antagonists or tetrodotoxin significantly delayed the onset of AD. This report provides direct evidence for the need to prevent seizures during neonatal brain hypoxia.

Action Potentials↗

PubMed1999

Maturation of kainate-induced epileptiform activities in interconnected intact neonatal limbic structures in vitro.

In vivo studies suggest that ontogenesis of limbic seizures is determined by the development of the limbic circuit. We have now used the newly-developed in vitro intact interconnected neonatal rat limbic structures preparation to determine the developmental profile of kainate-induced epileptiform activity in the hippocampus and its propagation to other limbic structures. We report gradual alterations in the effects of kainate during the first postnatal week on an almost daily basis; from no epileptiform activity at birth, through interictal seizures around postnatal day (P) 2 and ictal seizures by the end of the first week. The developmental profile of kainate-induced hippocampal seizures is paralleled by the expression of postsynaptic kainate receptor-mediated currents in CA3 pyramidal cells. Intralimbic propagation of the hippocampal seizures is also age-dependent: whereas seizures readily propagate to the septum and to the contralateral hippocampus via the commissures on P2, propagation to the entorhinal cortex only takes place from P4 onwards. Finally, repeated brief applications of kainate to the hippocampus induce recurrent spontaneous glutamatergic ictal and interictal discharges which persist for several hours after the kainate is washed away and which replace the physiological pattern of network activity. Paroxysmal activities are thus generated by kainate in the hippocampus at an early developmental stage and are initially restricted to this structure. Before the end of the first week of postnatal life, kainate generates the epileptiform activities that may perturb activity-dependent mechanisms that modulate neuronal development. Although at this stage neurons are relatively resistant to the pathological effects of kainate, the epileptiform activities that it generates will perturb activity-dependent mechanisms that modulate neuronal development.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1999

Three-independent-compartment chamber to study in vitro commissural synapses.

We describe a novel chamber in which the two intact neonatal rat hippocampi and the commissural fibers are placed in three independent compartments separated by latex membranes and perfused selectively with different solutions. A set of control tests showed that the compartments are well isolated: 1) methylene blue or eosin applied to one compartment did not diffuse to other compartments when verified via the microscope, and spectrophotometry revealed that <1/10.000th of the dye diffuses to other compartments; 2) tetrodotoxin (1 microM) applied to the commissural compartment blocked the synaptic responses evoked contralaterally without affecting those evoked on the ipsilateral side. This chamber enables a wide range of experiments that cannot be performed in conventional chambers, e.g., to study the maturation and plasticity of the commissural connections, bilateral synchronization of the rhythmic activities in the limbic system, commissural propagation of the epileptiform activities, etc.

Animals↗

PubMed1999

Dual role of GABA in the neonatal rat hippocampus.

The effects of modulators of GABA-A receptors on neuronal network activity were studied in the neonatal (postnatal days 0-5) rat hippocampus in vitro. Under control conditions, the physiological pattern of activity of the neonatal hippocampal network was characterized by spontaneous network-driven giant depolarizing potentials (GDPs). The GABA-A receptor agonist isoguvacine (1-2 microM) and the allosteric modulator diazepam (2 microM) induced biphasic responses: initially the frequency of GDPs increased 3 to 4 fold followed by blockade of GDPs and desynchronization of the network activity. The GABA-A receptor antagonists bicuculline (10 microM) and picrotoxin (100 microM) blocked GDPs and induced glutamate (AMPA and NMDA)-receptor-mediated interictal- and ictal-like activities in the hippocampal slices and the intact hippocampus. These data suggest that at early postnatal ages GABA can exert a dual - both excitatory and inhibitory - action on the network activity.

Animals↗

PubMed1999

Epileptogenic action of caffeine during anoxia in the neonatal rat hippocampus.

Excessive maternal caffeine consumption can lead to fetal and neonatal pathology, but the underlying mechanisms have not been determined. Here, we report that low doses of caffeine generate seizures when applied in conjunction with brief anoxic episodes in the hippocampus of neonatal rats in vitro. In control conditions, brief (4-6 minutes) anoxic episodes reversibly depressed evoked synaptic responses and blocked the physiological pattern of network activity. In the presence of caffeine (50 microM), similar anoxic episodes generated ictal (29%) or interictal (33%) epileptiform activities often followed during reoxygenation by recurrent spontaneous seizure activity that persisted for several hours. These effects are likely mediated by a blockade of adenosine receptors by caffeine because (1) in control conditions, caffeine antagonized the inhibitory effect of selective A1 receptor agonist N6-cyclopentyladenosine on excitatory synaptic responses, and (2) epileptogenic effects of caffeine were reproduced by selective A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and theophylline. Our findings suggest that endogenous adenosine released during anoxia acting via A1 receptors prevents seizures in the neonatal hippocampus and that the antagonism of these receptors by caffeine leads to epileptogenesis. This study suggests concerns about the safety of caffeine in the fetus and newborn.

Action Potentials↗

PubMed1999

GABA is the principal fast-acting excitatory transmitter in the neonatal brain.

gamma-aminobutyric acid (GABA) is the principal neurotransmitter of inhibition in the adult mammalian brain. However, at early stages of development, including the embryonic period and first week of postnatal life, GABA plays the role of main neurotransmitter of excitation. The paradoxical excitatory effect of GABA is caused by an inverted chloride gradient and, therefore, a depolarizing direction of GABA type A (GABAA) receptor mediated responses. In addition, another type of GABAergic inhibition mediated by postsynaptic GABA type B (GABAB) receptors is not functional at early stage of life. In the neonatal rat hippocampus, GABA, acting via GABAA receptors, activates voltage-gated sodium and calcium channels and potentiates the activity of N-methyl-D-aspartate (NMDA) receptors by reducing their voltage-dependent Mg2+ block. The temporal window when GABA exerts excitatory actions coincides with a particular pattern of activity of hippocampal neuronal network that is characterized by periodical giant depolarizing potentials (GDPs) reminiscent of interictal-like epileptiform discharges. Recent studies have shown that GDPs result from the synchronous discharge of GABAergic interneurons and principal glutamatergic pyramidal cells, and they are mediated by the synergistic excitatory actions of GABAA and glutamate receptors. GDPs provide synchronous intracellular Ca2+ oscillations and may, therefore, be implicated in hebbian modulation of developing synapses and activity-dependent formation of the hippocampal network.

Animals↗

PubMed1998

Giant depolarizing potentials: the septal pole of the hippocampus paces the activity of the developing intact septohippocampal complex in vitro.

In neonatal hippocampal slices, recurrent spontaneous giant depolarizing potentials (GDPs) provide neuronal synchronized firing and Ca2+ oscillations. To investigate the possible role of GDPs in the synchronization of neuronal activity in intact neonatal limbic structures, we used multiple simultaneous electrophysiological recordings in the recently described preparation of intact neonatal septohippocampal complex in vitro. Combined whole-cell (in single or pairs of cells) and extracellular field recordings (one to five simultaneous recording sites) from the CA3 hippocampal region and various parts of the septum indicated that spontaneous GDPs, which can be initiated anywhere along the longitudinal hippocampal axis, are most often initiated in the septal poles of hippocampus and propagate to medial septum and temporal poles of both hippocampi simultaneously. GDPs were abolished in the medial septum but not in the hippocampus after surgical separation of both structures, suggesting hippocampal origin of GDPs. The preferential septotemporal orientation of GDP propagation observed in the intact hippocampus was associated with a corresponding gradient of GDP frequency in isolated portions of hippocampus. Accordingly, most GDPs propagated in the septotemporal direction in both septal and temporal hippocampal isolated halves, and whereas GDP frequency remained similar in the septal part of hippocampus after its surgical isolation, it progressively decreased in more temporally isolated portions of the hippocampus. Because GDPs provide most of the synaptic drive of neonatal neurons, they may modulate the development of neuronal connections in the immature limbic system.

Animals↗

PubMed1997

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.

Action Potentials↗

PubMed1997

Synchronization of GABAergic interneuronal network in CA3 subfield of neonatal rat hippocampal slices.

1. Cell-attached and whole-cell recordings from interneurons localized in the stratum radiatum of the CA3 subfield (SR-CA3) of neonatal (postnatal days 2-5) rat hippocampal slices were performed to study their activity during the generation of GABAergic giant depolarizing potentials (GDPs) in CA3 pyramidal cells. 2. Dual recordings revealed that during the generation of GDPs in CA3 pyramidal cells, the interneurons fire bursts of spikes, on average 4.5 +/- 1.4 spikes per burst (cell-attached mode). There bursts were induced by periodical large inward currents (interneuronal GDPs) recorded in whole-cell mode. 3. Interneuronal GDPs revealed typical features of polysynaptic neuronal network-driven events: they were blocked by TTX and by high divalent cation medium and they could be evoked in an all-or-none manner by electrical stimulation in different regions of the hippocampus. The network elements required for the generation of GDPs are present in local CA3 circuits since spontaneous GDPs were present in the isolated CA3 subfield of the hippocampal slice. 4. Interneuronal GDPs were mediated by GABAA and glutamate receptors, since: (i) their reversal potential strongly depended on [Cl-]i; (ii) at the reversal potential of GABAA postsynaptic currents an inward component of GDPs was composed of events with the same kinetics as alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-mediated EPSCs; and (iii) once GABAA receptors were blocked intracellularly by dialysis with F(-)-MgATP-free solution, the remaining component of interneuronal GDPs reversed near 0 mV and rectified at membrane potentials more negative than -20 mV, suggesting an important contribution of NMDA receptors in addition to AMPA receptors. 5. In cell-attached recordings from interneurons, electrical stimulation in the stratum radiatum evoked a burst of spikes that corresponded to evoked GDPs. Pharmacological study of this response revealed that excitation of SR-CA3 interneurons during GDPs is determined by the co-operative depolarizing actions mediated by GABAA and glutamate (AMPA and NMDA) receptors. Interestingly, after blockade of AMPA receptors, GABAA receptor-mediated depolarization enabled the activation of NMDA receptors presumably via attenuation of their voltage-dependent magnesium block. 6. It is concluded that synchronous activation of SR-CA3 interneurons during generation of GDPs is mediated synaptically and is determined by the co-operation of (i) excitatory GABAergic connections between interneurons and (ii) glutamatergic connections to interneurons originating presumably from the pyramidal cells.

Animals↗

PubMed1997

Bicuculline induces ictal seizures in the intact hippocampus recorded in vitro.

The effects of the gamma-aminobutyric acid (GABA) receptor antagonist bicuculline on rat hippocampal neurons recorded in slices and in the intact hippocampi kept in vitro were studied using whole-cell patch-clamp recordings. Bicuculline (10 microM) evoked ictal discharges in the intact hippocampus but only interictal discharges in conventional slices. Recording from the intact hippocampus in vitro is an alternative preparation to study the organization of the hippocampal neuronal network.

Animals↗

PubMed1997

Redox modulation of synaptic responses and plasticity in rat CA1 hippocampal neurons.

Effects of redox reagents on excitatory and inhibitory synaptic responses as well as on the bidrectional plasticity of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses were studied in CA1 pyramidal neurons in rat hippocampal slices. The oxidizing agent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM) did not affect AMPA, GABAA or GABAB receptor-mediated synaptic responses or the activation of presynaptic metabotropic receptors. However, DTNB irreversibly decreased (by approximately 50%) currents evoked by focal application of NMDA. DTNB also decreased the NMDA component of the EPSC. The reversal potential of NMDA currents and the Mg2+ block were not modified. In the presence of physiological concentrations of Mg2+ (1.3 mM), DTNB did not affect the NMDA receptor-dependent induction of long-term potentiation (LTP) or long-term depression (LTD) expressed by AMPA receptors. In contrast, DTNB fully prevented LTP and LTD induced and expressed by NMDA receptors. Plasticity of NMDA receptor-mediated synaptic responses could be reinstated by the reducing agent tris-(2-carboxyethyl) phosphine (TCEP, 200 microM). These results suggest that persistent, bidirectional changes in synaptic currents mediated by NMDA receptors cannot be evoked when these receptors are in an oxidized state, whereas NMDA-dependent LTP and LTD are still expressed by AMPA receptors. Our observations raise the possibility of developing therapeutic agents that would prevent persistent excitotoxic enhancement of NMDA receptor-mediated events without blocking longterm modifications of AMPA receptor-mediated synaptic responses, thought to underlie memory processes.

Animals↗

PubMed1997

GABAA, NMDA and AMPA receptors: a developmentally regulated 'ménage à trois'.

The main ionotropic receptors (GABAA, NMDA and AMPA) display a sequential participation in neuronal excitation in the neonatal hippocampus. GABA, the principal inhibitory transmitter in the adult CNS, acts as an excitatory transmitter in early postnatal stage. Glutamatergic synaptic transmission is first purely NMDA-receptor based and lacks functional AMPA receptors. Therefore, initially glutamatergic synapses are 'silent' at resting membrane potential, NMDA channels being blocked by Mg2+. However, when GABA and glutamatergic synapses are coactivated during the physiological patterns of activity, GABAA receptors can facilitate the activation of NMDA receptors, playing the role conferred to AMPA receptors later on in development. Determining the mechanisms underlying the development of this 'ménage à trois' will shed light not only on the wide range of trophic roles of glutamate and GABA in the developing brain, but also on the significance of the transition from neonatal to adult forms of plasticity.

Animals↗

PubMed1997

Ca2+ oscillations mediated by the synergistic excitatory actions of GABA(A) and NMDA receptors in the neonatal hippocampus.

We asked whether GABA(A) and NMDA receptors may act in synergy in neonatal hippocampal slices, at a time when GABA exerts a depolarizing action. The GABA(A) receptor agonist isoguvacine reduced the voltage-dependent Mg2+ block of single NMDA channels recorded in cell-attached configuration from P(2-5) CA3 pyramidal neurons and potentiated the Ca2+ influx through NMDA channels. The synaptic response evoked by electrical stimulation of stratum radiatum was mediated by a synergistic interaction between GABA(A) and NMDA receptors. Network-driven Giant Depolarizing Potentials, which are a typical feature of the neonatal hippocampal network, provided coactivation of GABA(A) and NMDA receptors and were associated with spontaneous and synchronous Ca2+ increases in CA3 pyramidal neurons. Thus, at the early stages of development, GABA is a major excitatory transmitter that acts in synergy with NMDA receptors. This provides in neonatal neurons a hebbian stimulation that may be involved in neuronal plasticity and network formation in the developing hippocampus.

Action Potentials↗

PubMed1997

A novel in vitro preparation: the intact hippocampal formation.

The intact hippocampal formation (IHF) of neonatal or young rats can be kept alive for an extended period in a fully submerged chamber with excellent morphological preservation. Field or patch-clamp recordings, intracellular Ca2+ measurements, and 3-D reconstruction of biocytin-filled neurons can be performed routinely. The generation and propagation of network-driven activities can be studied within the IHF or between connected intact structures such as the septum and the hippocampus or two hippocampi, and the use of a dual chamber enables the application of drugs separately to each structure. This preparation will be useful to study intact neuronal networks in the developing hippocampus in vitro.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1997

Blocking GABA(A) inhibition reveals AMPA- and NMDA-receptor-mediated polysynaptic responses in the CA1 region of the rat hippocampus.

We have investigated the conditions required to evoke polysynaptic responses in the isolated CA1 region of hippocampal slices from Wistar adult rats. Experiments were performed with extracellular and whole cell recording techniques. In the presence of bicuculline (10 microM), 6-cyano-7-nitroquinoxaline-2-3-dione (10 microM), glycine (10 microM), and a low external concentration of Mg2+ (0.3 mM), electrical stimulation of the Schaffer collaterals/commissural pathway evoked graded N-methyl-D-aspartate (NMDA)-receptor-mediated late field potentials in the stratum radiatum of the CA1 region. These responses were generated via polysynaptic connections because their latency varied strongly and inversely with the stimulation intensity and they were abolished by a high concentration of divalent cations (7 mM Ca2+). These responses likely were driven by local collateral branches of CA1 pyramidal cell axons because focal application of tetrodotoxin (30 microM) in the stratum oriens strongly reduced the late synaptic component and antidromic stimulation of CA1 pyramidal cells could evoke the polysynaptic response. Current-source density analysis suggested that the polysynaptic response was generated along the proximal part of the apical dendrites of CA1 pyramidal cells (50-150 microm below the pyramidal cell layer in the stratum radiatum). In physiological concentration of Mg2+ (1.3 mM), the pharmacologically isolated NMDA-receptor-mediated polysynaptic response was abolished. In control artificial cerebrospinal fluid (with physiological concentration of Mg2+), bicuculline ( 10 microM) generated a graded polysynaptic response. Under these conditions, this response was mediated both by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/NMDA receptors. In the presence of D-2-amino-5-phosphonovalerate (50 microM), the polysynaptic response could be mediated by AMPA receptors, although less efficiently. In conclusion, suppression of gamma-aminobutyric acid-A inhibition reveals glutamate receptor-mediated network-driven events in the isolated CA1 region. These polysynaptic responses are mediated by AMPA and/or NMDA receptors depending on the pharmacological conditions and the external concentration of Mg2+ used. We suggest that these responses are driven by local recurrent collaterals of CA1 pyramidal cells.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1996

Spontaneous release of GABA activates GABAB receptors and controls network activity in the neonatal rat hippocampus.

1. We investigated the effects of the selective gamma-aminobutyric acid-B (GABAB) receptor antagonist, P-3 aminopropyl-P-diethoxymethyl phosphoric acid (CGP 35348), on spontaneous and evoked postsynaptic potentials (PSPs) and currents (PSCs) in CA3 pyramidal cells and interneurons of hippocampal slices obtained between postnatal day 3 and 7 with the use of intracellular and whole cell recording techniques. The intracellular pipette solution contained either 2 M CsCl or 50 mM 2(triethylamino)-N-(2,6-dimethylphenyl) acetamine (QX314) dissolved in 2 M KMeSO4. Cesium and QX314 block postsynaptic responses mediated by GABAB receptors. 2. Under control conditions, bath application of CGP 35348 (0.5-1 mM) progressively increased the duration of spontaneous and evoked polysynaptic giant GABAergic PSPs leading to the appearance of ictal-like discharges. The effects of CGP 35348 were dose dependent and voltage independent. 3. In CA3 pyramidal neurons, CGP 35348 (0.5 mM) had no effect on monosynaptic GABAergic inhibitory PSPs (IPSPs) that were isolated in the presence of ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D(-)2-amino-5-phosphovaleric acid (D-APV, 50 microM). Similarly, CGP 35348 (0.5 mM) had no effect on monosynaptic glutamatergic excitatory PSPs (EPSPs) that were isolated in the presence of bicuculline (10 microM) and high divalent cation artificial cerebrospinal fluid (ACSF; 6 mM Mg2+/4 mM Ca2+). 4. In CA3 pyramidal neurons exposed to CNQX (20 microM) and D-APV (50 microM), application of the potassium channel blocker 4-aminopyridine (4-AP, 50 microM) generated synchronous giant GABAergic PSPS that were blocked in the presence of high divalent cation ACSF (6 mM Mg2+/4 mM Ca2+) or bicuculline (10 microM). The duration of these synchronous GABAergic PSPs was prolonged in the presence of CGP 35348 (0.5 mM) but did not lead to the appearance of ictal-like discharges. 5. In the presence of bicuculline, interictal giant glutamatergic potentials were observed in simultaneously recorded CA3 pyramidal cells and interneurons. CGP 35348 (0.5 mM) progressively increased the duration of these bicuculline-induced glutamatergic bursts leading to the simultaneous appearance of ictal discharges in both pyramidal cells and interneurons. 6. These results suggest that in the neonatal CA3 hippocampal region, when synchronous giant polysynaptic GABAergic PSPs are present (i.e., under basal, control conditions), spontaneously released GABA reaches a critical level and activates GABAB receptors on both pyramidal cells and interneurons thus regulating the level of glutamatergic and GABAergic activity in the CA3 neuronal network.

Animals↗