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In recent years, exposures to organophosphate pesticide have been highlighted as a possible cause or aggravating factor of autism spectrum disorder (ASD). The present study examined if Wistar rats prenatally exposed to chlorpyrifos (CPF) at a dose of 1 mg/kg in GD 12.5-15.5 could express similar behaviors to those exposed to valproic acid (VPA, 400 mg/kg) during the same administration window, which is an accepted animal model of autism. The 3-chambered test was employed to evaluate sociability and reaction to social novelty in two experiments, the first in adolescence and the second in adulthood. The results obtained in this study show that animals prenatally treated with CPF or VPA show a similar behavioral phenotype compared to the control group (CNT). In adolescence, the CPF animals showed a negative index in the reaction to social novelty, followed closely by the VPA, while both experimental groups showed a recovery in this aspect during adulthood. This study therefore provides evidence to suggest that prenatal exposure to CPF in rats could have similar effects on certain components of sociability to those seen in autistic models., (© 2024. The Author(s).)

Behavioral phenotyping devices have been successfully used to build ethograms, but many aspects of behavior remain out of reach of available phenotyping systems. We now report on a novel device, which consists in an open-field platform resting on highly sensitive piezoelectric (electromechanical) pressure-sensors, with which we could detect the slightest movements (up to individual heart beats during rest) from freely moving rats and mice. The combination with video recordings and signal analysis based on time-frequency decomposition, clustering, and machine learning algorithms provided non-invasive access to previously overlooked behavioral components. The detection of shaking/shivering provided an original readout of fear, distinct from but complementary to behavioral freezing. Analyzing the dynamics of momentum in locomotion and grooming allowed to identify the signature of gait and neurodevelopmental pathological phenotypes. We believe that this device represents a significant progress and offers new opportunities for the awaited advance of behavioral phenotyping., (© 2021. The Author(s), under exclusive licence to American College of Neuropsychopharmacology.)

The hypothesis that reversed, excitatory GABA may be involved in various brain pathologies, including epileptogenesis, is appealing but controversial because of the technical difficulty of probing endogenous GABAergic synaptic function in vivo. We overcome this challenge by non-invasive extracellular recording of neuronal firing responses to optogenetically evoked and spontaneously occurring inhibitory perisomatic GABAergic field potentials, generated by individual parvalbumin interneurons on their target pyramidal cells. Our direct probing of GABAergic transmission suggests a rather anecdotal participation of excitatory GABA in two specific models of epileptogenesis in the mouse CA3 circuit in vivo, even though this does not preclude its expression in other brain areas or pathological conditions. Our approach allows the detection of distinct alterations of inhibition during spontaneous activity in vivo, with high sensitivity. It represents a promising tool for the investigation of excitatory GABA in different pathological conditions that may affect the hippocampal circuit., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

The formation of functional cortical maps in the cerebral cortex results from a timely regulated interaction between intrinsic genetic mechanisms and electrical activity. To understand how transcriptional regulation influences network activity and neuronal excitability within the neocortex, we used mice deficient for Nr2f1 (also known as COUP-TFI), a key determinant of primary somatosensory (S1) area specification during development. We found that the cortical loss of Nr2f1 impacts on spontaneous network activity and synchronization of S1 cortex at perinatal stages. In addition, we observed alterations in the intrinsic excitability and morphological features of layer V pyramidal neurons. Accordingly, we identified distinct voltage-gated ion channels regulated by Nr2f1 that might directly influence intrinsic bioelectrical properties during critical time windows of S1 cortex specification. Altogether, our data suggest a tight link between Nr2f1 and neuronal excitability in the developmental sequence that ultimately sculpts the emergence of cortical network activity within the immature neocortex., (© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: In recent years, ultrasonic vocalizations (USV) in pups has become established as a good tool for evaluating behaviors related to communication deficits and emotional states observed in autism spectrum disorder (ASD). Prenatal valproic acid (VPA) exposure leads to impairments and social behavior deficits associated with autism, with the effects of VPA being considered as a reliable animal model of ASD. Some studies also suggest that prenatal exposure to chlorpyrifos (CPF) could enhance autistic-like behaviors., Methods: In order to explore these similarities, in the present study we tested whether prenatal exposure to CPF at GD12.5-14.5 produces effects that are comparable to those produced by prenatal VPA exposure at GD12.5 in infant Wistar rats. Using Deep Squeek software, we evaluated total number of USVs, latency to the first call, mean call duration, principal frequency peak, high frequency peak, and type of calls., Results: Consistent with our hypothesis, we found that exposure to both CPF and VPA leads to a significantly smaller number of calls along with a longer latency to produce the first call. No significant effects were found for the remaining dependent variables., Conclusions: These results suggest that prenatal exposure to CPF could produce certain behaviors that are reminiscent of those observed in ASD patients.

In fragile X syndrome (FXS), sensory hypersensitivity and impaired habituation is thought to result in attention overload and various behavioral abnormalities in reaction to the excessive and remanent salience of environment features that would normally be ignored. This phenomenon, termed sensory defensiveness, has been proposed as the potential cause of hyperactivity, hyperarousal, and negative reactions to changes in routine that are often deleterious for FXS patients. However, the lack of tools for manipulating sensory hypersensitivity has not allowed the experimental testing required to evaluate the relevance of this hypothesis. Recent work has shown that BMS-204352, a BK Ca channel agonist, was efficient to reverse cortical hyperexcitability and related sensory hypersensitivity in the Fmr1-KO mouse model of FXS. In the present study, we report that exposing Fmr1-KO mice to novel or unfamiliar environments resulted in multiple behavioral perturbations, such as hyperactivity, impaired nest building and excessive grooming of the back. Reversing sensory hypersensitivity with the BK Ca channel agonist BMS-204352 prevented these behavioral abnormalities in Fmr1-KO mice. These results are in support of the sensory defensiveness hypothesis, and confirm BK Ca as a potentially relevant molecular target for the development of drug medication against FXS/ASD.

The possible effects on cognitive processes of external electric fields, such as those generated by power line pillars and household appliances are of increasing public concern. They are difficult to study experimentally, and the relatively scarce and contradictory evidence make it difficult to clearly assess these effects. In this study, we investigate how, why and to what extent external perturbations of the intrinsic neuronal activity, such as those that can be caused by generation, transmission and use of electrical energy can affect neuronal activity during cognitive processes. For this purpose, we used a morphologically and biophysically realistic three-dimensional model of CA1 pyramidal neurons. The simulation findings suggest that an electric field oscillating at power lines frequency, and environmentally measured strength, can significantly alter both the average firing rate and temporal spike distribution properties of a hippocampal CA1 pyramidal neuron. This effect strongly depends on the specific and instantaneous relative spatial location of the neuron with respect to the field, and on the synaptic input properties. The model makes experimentally testable predictions on the possible functional consequences for normal hippocampal functions such as object recognition and spatial navigation. The results suggest that, although EF effects on cognitive processes may be difficult to occur in everyday life, their functional consequences deserve some consideration, especially when they constitute a systematic presence in living environments., (© 2016 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Introduction: The inhibition of the Histone Deacetylase 6 (HDAC6) increases tubulin acetylation, thus stimulating intracellular vesicle trafficking and brain-derived neurotrophic factor (BDNF) release, that is, cellular processes markedly reduced in Huntington's disease (HD)., Methods: We therefore tested that reducing HDAC6 levels by genetic manipulation would attenuate early cognitive and behavioral deficits in R6/1 mice, a mouse model which develops progressive HD-related phenotypes., Results: In contrast to our initial hypothesis, the genetic deletion of HDAC6 did not reduce the weight loss or the deficits in cognitive abilities and nest-building behavior shown by R6/1 mice, and even worsened their social impairments, hypolocomotion in the Y-maze, and reduced ultrasonic vocalizations., Conclusions: These results weaken the validity of HDAC6 reduction as a possible therapeutic strategy for HD. The data are discussed in terms of additional cellular consequences and anatomical specificity of HDAC6 that could explain these unexpected effects.

There is growing evidence that Parkinson's disease, generally characterized by motor symptoms, also causes cognitive impairment such as spatial disorientation. The hippocampus is a critical structure for spatial navigation and receives sparse but comprehensive dopamine (DA) innervation. DA loss is known to be the cause of Parkinson's disease and therefore it has been hypothesized that the associated spatial disorientation could result from hippocampal dysfunction. Because DA is involved in the prediction of reward expectation, it is possible to infer that spatial disorientation in DA depleted subjects results from the loss of the ability to detect the rewarding features within the environment. Amongst hippocampal formation subdivisions, CA3 properties such as the high liability of its place fields make it a serious candidate for interfacing DA reward system and spatial information encoding. We addressed this issue using multiple electrode recordings of CA3 in normal and dopamine depleted rats performing a spatial learning in a Y-maze. Our data confirm that DA is essential to spatial learning as its depletion results in spatial impairments. The present work also shows that CA3 involvement in the detection of spatial feature contextual significance is under DA control. Finally, it also shows that CA3 contributes to the decision making processes of navigation tasks. The data also reveal a lateralization effect of DA depletion underlined by neural correlates., (© 2013.)

It was recently shown that perisomatic GABAergic inhibitory postsynaptic potentials (IPSPs) originating from basket and chandelier cells can be recorded as population IPSPs from the hippocampal pyramidal layer using extracellular electrodes (eIPSPs). Taking advantage of this approach, we have investigated the recruitment of perisomatic inhibition during spontaneous hippocampal activity in vitro. Combining intracellular and extracellular recordings from pyramidal cells and interneurons, we confirm that inhibitory signals generated by basket cells can be recorded extracellularly, but our results suggest that, during spontaneous activity, eIPSPs are mostly confined to the CA3 rather than CA1 region. CA3 eIPSPs produced the powerful time-locked inhibition of multi-unit activity expected from perisomatic inhibition. Analysis of the temporal dynamics of spike discharges relative to eIPSPs suggests significant but moderate recruitment of excitatory and inhibitory neurons within the CA3 network on a 10 ms time scale, within which neurons recruit each other through recurrent collaterals and trigger powerful feedback inhibition. Such quantified parameters of neuronal interactions in the hippocampal network may serve as a basis for future characterisation of pathological conditions potentially affecting the interactions between excitation and inhibition in this circuit.

The role of amyloid beta (Aβ) in brain function and in the pathogenesis of Alzheimer's disease (AD) remains elusive. Recent publications reported that an increase in Aβ concentration perturbs pre-synaptic release in hippocampal neurons. In particular, it was shown in vitro that Aβ is an endogenous regulator of synaptic transmission at the CA3-CA1 synapse, enhancing its release probability. How this synaptic modulator influences neuronal output during physiological stimulation patterns, such as those elicited in vivo, is still unknown. Using a realistic model of hippocampal CA1 pyramidal neurons, we first implemented this Aβ-induced enhancement of release probability and validated the model by reproducing the experimental findings. We then demonstrated that this synaptic modification can significantly alter synaptic integration properties in a wide range of physiologically relevant input frequencies (from 5 to 200 Hz). Finally, we used natural input patterns, obtained from CA3 pyramidal neurons in vivo during free exploration of rats in an open field, to investigate the effects of enhanced Aβ on synaptic release under physiological conditions. The model shows that the CA1 neuronal response to these natural patterns is altered in the increased-Aβ condition, especially for frequencies in the theta and gamma ranges. These results suggest that the perturbation of release probability induced by increased Aβ can significantly alter the spike probability of CA1 pyramidal neurons and thus contribute to abnormal hippocampal function during AD.

The pathophysiology of Huntington's disease (HD) is primarily associated with striatal degeneration and a number of behavioral symptoms such as involuntary movements, cognitive decline, psychiatric disorders, and in the most juvenile-onset cases with epilepsy. In addition to several changes in cellular and synaptic properties previously reported in HD, attention was recently driven towards the potential relationships between cognitive deficits and sleep disturbances in patients and animal models of Huntington's disease. In the present study, we have investigated whether the population-activity patterns normally expressed by the hippocampal and neocortical circuits during active and slow-wave states are affected in R6/1 mice, a model of Huntington's disease. By performing electrophysiological recordings from the hippocampus and neocortex of R6/1 mice that were either freely moving, head restrained or anesthetized, we observed an altered segregation of active and slow wave brain states, in relation with an epileptic phenotype. Slow-wave state (SWS) in R6/1 was characterized by the intrusion of active-state features (increased 6-10 Hz theta power and depressed 2-3 Hz delta power) and transient, temporally misplaced ("ectopic") theta oscillations. The epileptic phenotype, in addition to previously reported occasional ictal seizures, was characterized by the systematic presence of interictal activity, confined to SWS. Ectopic theta episodes, which could be reversed by the cholinergic antagonist atropine, concentrated interictal spikes and phase-locked hippocampal sharp-wave-ripples. These results point to major alterations of neuronal activity during rest in R6/1 mice, potentially involving anomalous activation of the cholinergic system, which may contribute to the cognitive deficits observed in Huntington's disease., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Oscillatory patterns of activity in various frequency ranges are ubiquitously expressed in cortical circuits. While recent studies in humans emphasized rhythmic modulations of neuronal oscillations ("second-order" rhythms), their potential involvement in information coding remains an open question. Here, we show that a rhythmic (~0.7 Hz) modulation of hippocampal theta power, unraveled by second-order spectral analysis, supports encoding of spatial and behavioral information. The phase preference of neuronal discharge within this slow rhythm significantly increases the amount of information carried by action potentials in various motor/cognitive behaviors by (1) distinguishing between the spikes fired within versus outside the place field of hippocampal place cells, (2) disambiguating place firing of neurons having multiple place fields, and (3) predicting between alternative future spatial trajectories. This finding demonstrates the relevance of second-order spectral components of brain rhythms for decoding neuronal information., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Theta oscillations represent the neural network configuration underlying active awake behavior and paradoxical sleep. This major EEG pattern has been extensively studied, from physiological to anatomical levels, for more than half a century. Nevertheless the cellular and network mechanisms accountable for the theta generation are still not fully understood. This review synthesizes the current knowledge on the circuitry involved in the generation of theta oscillations, from the hippocampus to extra hippocampal structures such as septal complex, entorhinal cortex and pedunculopontine tegmentum, a main trigger of theta state through direct and indirect projections to the septal complex. We conclude with a short overview of the perspectives offered by technical advances for deciphering more precisely the different neural components underlying the emergence of theta oscillations., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Sensorimotor coordination emerges early in development. The maturation period is characterized by the establishment of somatotopic cortical maps, the emergence of long-range cortical connections, heightened experience-dependent plasticity and spontaneous uncoordinated skeletal movement. How these various processes cooperate to allow the somatosensory system to form a three-dimensional representation of the body is not known. In the visual system, interactions between spontaneous network patterns and afferent activity have been suggested to be vital for normal development. Although several intrinsic cortical patterns of correlated neuronal activity have been described in developing somatosensory cortex in vitro, the in vivo patterns in the critical developmental period and the influence of physiological sensory inputs on these patterns remain unknown. We report here that in the intact somatosensory cortex of the newborn rat in vivo, spatially confined spindle bursts represent the first and only organized network pattern. The localized spindles are selectively triggered in a somatotopic manner by spontaneous muscle twitches, motor patterns analogous to human fetal movements. We suggest that the interaction between movement-triggered sensory feedback signals and self-organized spindle oscillations shapes the formation of cortical connections required for sensorimotor coordination.

Because developmental activity-dependent synaptic plasticity has been hypothesized to participate in network refinement, leading to the precise mapping of synaptic contacts constituting a functional brain, it is important to investigate the spatio-temporal structure of immature network activities. This article is briefly reviewing 15 years of studies on the immature rat hippocampus which, together with recent results obtained from awake rat pups, represent an important step toward the understanding of spontaneous patterns of activity and their potential implication in network maturation. Due to synergistic excitatory actions of GABA and glutamate receptor mediated signals during early postnatal life, spontaneous patterns of hippocampal activity that have been characterized both in vitro and in vivo are likely to provide hebbian modulation of developing glutamatergic and GABAergic synapses. Together with studies on trophic actions of these transmitters, study of the immature hippocampal network patterns and plasticities allows for multiple technical and conceptual approaches and represents an interesting experimental model for development studies.

The behavior of immature cortical networks in vivo remains largely unknown. Using multisite extracellular and patch-clamp recordings, we observed recurrent bursts of synchronized neuronal activity lasting 0.5 to 3 seconds that occurred spontaneously in the hippocampus of freely moving and anesthetized rat pups. The influence of slow rhythms (0.33 and 0.1 hertz) and the contribution of both gamma-aminobutyric acid A-mediated and glutamate receptor-mediated synaptic signals in the generation of hippocampal bursts was reminiscent of giant depolarizing potentials observed in vitro. This earliest pattern, which diversifies during the second postnatal week, could provide correlated activity for immature neurons and may underlie activity-dependent maturation of the hippocampal network.

According to the temporal coding hypothesis, neurons encode information by the exact timing of spikes. An example of temporal coding is the hippocampal phase precession phenomenon, in which the timing of pyramidal cell spikes relative to the theta rhythm shows a unidirectional forward precession during spatial behaviour. Here we show that phase precession occurs in both spatial and non-spatial behaviours. We found that spike phase correlated with instantaneous discharge rate, and processed unidirectionally at high rates, regardless of behaviour. The spatial phase precession phenomenon is therefore a manifestation of a more fundamental principle governing the timing of pyramidal cell discharge. We suggest that intrinsic properties of pyramidal cells have a key role in determining spike times, and that the interplay between the magnitude of dendritic excitation and rhythmic inhibition of the somatic region is responsible for the phase assignment of spikes.

The interplay between principal cells and interneurons plays an important role in timing the activity of individual cells. We investigated the influence of single hippocampal CA1 pyramidal cells on putative interneurons. The activity of CA1 pyramidal cells was controlled intracellularly by current injection, and the activity of neighboring interneurons was recorded extracellularly in the urethane-anesthetized rat. Spike transmission probability between monosynaptically connected pyramidal cell-interneuron pairs was frequency dependent and highest between 5 and 25 Hz. In the awake animal, interneurons were found that had place-modulated firing rates, with place maps similar to their presynaptic pyramidal neuron. Thus, single pyramidal neurons can effectively determine the firing patterns of their interneuron targets.

Cortical pyramidal cells fire single spikes and complex spike bursts. However, neither the conditions necessary for triggering complex spikes, nor their computational function are well understood. CA1 pyramidal cell burst activity was examined in behaving rats. The fraction of bursts was not reliably higher in place field centers, but rather in places where discharge frequency was 6-7 Hz. Burst probability was lower and bursts were shorter after recent spiking activity than after prolonged periods of silence (100 ms-1 s). Burst initiation probability and burst length were correlated with extracellular spike amplitude and with intracellular action potential rising slope. We suggest that bursts may function as "conditional synchrony detectors," signaling strong afferent synchrony after neuronal silence, and that single spikes triggered by a weak input may suppress bursts evoked by a subsequent strong input.

What determines the firing rate of cortical neurons in the absence of external sensory input or motor behavior, such as during sleep? Here we report that, in a familiar environment, the discharge frequency of simultaneously recorded individual CA1 pyramidal neurons and the coactivation of cell pairs remain highly correlated across sleep-wake-sleep sequences. However, both measures were affected when new sets of neurons were activated in a novel environment. Nevertheless, the grand mean firing rate of the whole pyramidal cell population remained constant across behavioral states and testing conditions. The findings suggest that long-term firing patterns of single cells can be modified by experience. We hypothesize that increased firing rates of recently used neurons are associated with a concomitant decrease in the discharge activity of the remaining population, leaving the mean excitability of the hippocampal network unaltered.

Local versus distant coherence of hippocampal CA1 pyramidal cells was investigated in the behaving rat. Temporal cross-correlation of pyramidal cells revealed a significantly stronger relationship among local (<140 microm) pyramidal neurons compared with distant (>300 microm) neurons during non-theta-associated immobility and sleep but not during theta-associated running and walking. In contrast, cross-correlation between local pyramidal cell-interneuron pairs was significantly stronger than between distant pairs during theta oscillations but were similar during non-theta-associated behaviors. We suggest that network state-dependent functional clustering of neuronal activity emerges because of the differential contribution of the main excitatory inputs, the perforant path, and Schaffer collaterals during theta and non-theta behaviors.

1. The study of the physiological role of long-term potentiation (LTP) is often hampered by the challenge of finding a physiological event that can be used to assess synaptic strength. We explored the possibility of utilising a naturally occurring event, the hippocampal sharp wave (SPW), for the assessment of synaptic strength and the induction of LTP in vivo. 2. We used two methods in which hippocampal cells were either recorded intracellularly or extracellularly in vivo. In both cases, a linear association between the magnitude of the SPW and cellular responsiveness was observed. 3. LTP was induced by depolarising cells during SPWs by either direct intracellular current injection or extracellular microstimulation adjacent to the cell body. Both of these approaches led to an increase in the slope of the linear association between SPWs and cellular responsiveness. 4. This change was achieved without a rise in overall cell excitability, implying that the synapses providing input to CA1 cells during sharp waves had undergone potentiation. 5. Our findings show that the Hebbian pairing of cellular activation with spontaneous, naturally occurring synaptic events is capable of inducing LTP.

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.

The effects of increased dendritic calcium concentration ([Ca2+]i) induced by single action potentials on monosynaptic glutamatergic excitatory postsynaptic currents (EPSCs) were studied in cultured rat hippocampal neurons. To investigate the respective roles of pre- and postsynaptic elements in the depolarization-induced NMDAR inactivation, we have performed simultaneous paired whole-cell recordings from monosynaptically connected pre- and postsynaptic hippocampal neurons. We report that the single firing of the postsynaptic neuron did not result in inactivation of the NMDAR-EPSC, whereas a burst of depolarizing steps transiently depressed the NMDAR-EPSCs in both pyramidal cells and interneurons. This effect was mediated by postsynaptic voltage-gated Ca2+ influx, as it was prevented by: (i) buffering postsynaptic [Ca2+]i with 30 mM BAPTA; (ii) removing extracellular Ca2+; or (iii) applying Cd2+o (100 microM), a voltage-gated calcium channel blocker. It does not involve presynaptic mechanisms as it selectively affected NMDA but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor-mediated EPSCs. These results suggest that inactivation of NMDAR-channels by voltage-gated Ca influx is a general property of hippocampal neurons, which may play an important role in reducing postsynaptic NMDAR Ca2+ influx that leads to plasticity or excitotoxicity during sustained neuronal activity.

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.

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.

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.

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.

We have shown previously that a selective metabotropic glutamate receptor (mGluR) agonist, 1S,3R-1-aminocyclo-pentane-1, 3-dicarboxylate (1S,3R-ACPD), evokes an inward current in CA1 pyramidal neurons of rat hippocampal slices in the presence of K+ channel blockers (). This current has been characterized as a Ca2+-activated nonselective cationic (CAN) current. Using whole-cell patch-clamp recordings and intracellular dialysis, we now have identified the mGluR subtype and the mechanisms underlying the CAN current (ICAN) and report for the first time the presence of a synaptic ICAN in the mammalian CNS. First, we have shown pharmacologically that activation of ICAN by 1S,3R-ACPD involves the group I mGluRs (and not the groups II and III) and a G-protein-dependent process. We also report that ICAN is modulated by the divalent cations (Mg2+, Cd2+, and Zn2+). Second, we have isolated a slow synaptic inward current evoked by a high-frequency stimulation in the presence of K+ channel blockers, ionotropic glutamate, and GABAA receptor antagonists. This current shows similar properties to the exogenously evoked ICAN: its reversal potential is close to the reversal potential of the 1S, 3R-ACPD-evoked ICAN, and it is G-protein- and Ca2+-dependent. Because the amplitude and duration of ICAN increased in the presence of a glutamate uptake blocker, we suggest that this synaptic current is generated via the activation of mGluRs. We propose that the synaptic ICAN, activated by a brief tetanic stimulation and leading to a long-lasting inward current, may be involved in neuronal plasticity and synchronized network-driven oscillations.

In the adult central nervous system, GABAergic synaptic inhibition is known to play a crucial role in preventing the spread of excitatory glutamatergic activity. This inhibition is achieved by a membrane hyperpolarization through the activation of postsynaptic gamma-aminobutyric acidA (GABAA) and GABAB receptors. In addition, GABA also depress transmitter release acting through presynaptic GABAB receptors. Despite the wealth of data regarding the role of GABA in regulating the degree of synchronous activity in the adult, little is known about GABA transmission during early stages of development. In the following we report that GABA mediates most of the excitatory drive at early stages of development in the hippocampal CA3 region. Activation of GABAA receptors induces a depolarization and excitation of immature CA3 pyramidal neurons and increases intracellular Ca2+ ([Ca2+]i)] during the first postnatal week of life. During the same developmental period, the postsynaptic GABAB-mediated inhibition is poorly developed. In contrast, the presynaptic GABAB-mediated inhibition is well developed at birth and plays a crucial role in modulating the postsynaptic activity by depressing transmitter release at early postnatal stages. We have also shown that GABA plays a trophic role in the neuritic outgrowth of cultured hippocampal neurons.

We have developed a method allowing suitable morphological conservation combined with in situ hybridization, on hippocampal slices used in conventional electrophysiological studies. After a bath application of kainate (KA, 750 nM, 2 min 15 s), electrical stimulation of the mossy fibre zone evoked epileptiform activity for up to 2 h. In situ hybridization performed on these slices showed a marked increased in expression of the transcription factor Zif/268 over the pyramidal and the granule cells and the surrounding neuropils. Bath application of tetraethylammonium (TEA, 25 mM, 10 min) elicited long-term potentiation in CA1 lasting up to 4 h. This was associated with enhanced expression of Zif/268 which returned to control values after 2 h 30 min. These observations suggest that slice preparations are suitable for the study of the role of neuronal activity in the regulation of gene expression.