Muscarinic induction of synchronous population activity in the entorhinal cortex

Diverse nature of interictal oscillations: EEG-based biomarkers in epilepsy

Interictal electroencephalogram (EEG) patterns, including high-frequency oscillations (HFOs), interictal spikes (ISs), and slow wave activities (SWAs), are defined as specific oscillations between seizure events. These interictal oscillations reflect specific dynamic changes in network excitability and play various roles in epilepsy. In this review, we briefly describe the electrographic characteristics of HFOs, ISs, and SWAs in the interictal state, and discuss the underlying cellular and network mechanisms. We also summarize representative evidence from experimental and clinical epilepsy to address their critical roles in ictogenesis and epileptogenesis, indicating their potential as electrophysiological biomarkers of epilepsy. Importantly, we put forwards some perspectives for further research in the field.

Evolving Mechanistic Concepts of Epileptiform Synchronization and their Relevance in Curing Focal Epileptic Disorders

The synchronized activity of neuronal networks under physiological conditions is mirrored by specific oscillatory patterns of the EEG that are associated with different behavioral states and cognitive functions. Excessive synchronization can, however, lead to focal epileptiform activity characterized by interictal and ictal discharges in epileptic patients and animal models. This review focusses on studies that have addressed epileptiform synchronization in temporal lobe regions by employing in vitro and in vivo recording techniques. First, we consider the role of ionotropic and metabotropic excitatory glutamatergic transmission in seizure generation as well as the paradoxical role of GABAA signaling in initiating and perhaps maintaining focal seizure activity. Second, we address non-synaptic mechanisms (which include voltage-gated ionic currents and gap junctions) in the generation of epileptiform synchronization. For each mechanism, we discuss the actions of antiepileptic drugs that are presumably modulating excitatory or inhibitory signaling and voltage-gated currents to prevent seizures in epileptic patients. These findings provide insights into the mechanisms of seizure initiation and maintenance, thus leading to the development of specific pharmacological treatments for focal epileptic disorders.

Carbachol-Induced theta-like oscillations in the rodent brain limbic system: Underlying mechanisms and significance

Theta oscillations (4-12 Hz) represent one of the most prominent physiological oscillatory activity in the mammalian EEG. They are observed in several areas of the hippocampus and in parahippocampal structures. Theta oscillations play important roles in modulating synaptic plasticity during memory and learning; moreover, they are dependent on septal cholinergic inputs. Theta oscillations can be reproduced in vitro in several regions of the temporal lobe in the absence of the septum by employing the cholinergic agonist carbachol (CCh). Here, we review the mechanisms underlying CCh-induced theta oscillations. We address: (i) the ability of temporal lobe neuronal networks to oscillate independently at theta frequency during CCh treatment; (ii) the contribution of intrinsic ionic currents; (iii) the participation of principal cells and interneurons; and (iv) their pharmacological profiles. We also discuss the similarities between CCh-induced theta oscillations and physiological type II theta activity, as well as their roles in synaptic plasticity. Finally, we consider experimental evidence pointing to the contribution of spontaneous and CCh-induced theta activity to epileptiform synchronization.

Single-unit Activity in the in vitro Entorhinal Cortex During Carbachol-induced Field Oscillations

The muscarinic receptor agonist carbachol (CCh) can induce activity in the theta range (4-15 Hz) in the entorhinal cortex (EC), but the underlying network mechanisms remain unclear. Here, we investigated the interplay between interneurons and principal cells in the EC during CCh-induced theta-like field oscillations in an in vitro brain slice preparation using tetrodes. Field oscillations at 10.1 Hz (IQR = 9.5-10.9 Hz) occurred during bath application of CCh (100 μM; n = 32 experiments) and were associated with single-unit (n = 189) firing. Interneuron activity increased before principal cell activity at the onset of the oscillations and both interneurons and principal cells fired at specific oscillation phases with interneurons preceding principal cells, suggesting that interneurons modulate principal cell activity during such oscillations. The regularity of occurrence of CCh-induced oscillations was abolished by applying the GABA_A receptor antagonist picrotoxin (100 μM; n = 13). These effects were accompanied by changes in firing with principal cells discharging action potentials before interneurons, along with a loss of preferred firing phase for principal cells in relation to the oscillation peaks. Blocking ionotropic glutamatergic transmission abolished CCh-induced field oscillations (n = 6), suggesting that ionotropic glutamatergic receptor signaling is necessary for their generation. Our results show that neuronal network interactions leading to CCh-induced theta-like field oscillations rest on the close interplay between interneurons and principal cells and that interneurons modulate principal cell activity during such oscillatory activity. Moreover, they underscore the role of ionotropic glutamatergic transmission in this type of oscillations.

Computational model of interictal discharges triggered by interneurons

Interictal discharges (IIDs) are abnormal waveforms registered in the periods before or between seizures. IIDs that are initiated by GABAergic interneurons have not been mathematically modeled yet. In the present study, a mathematical model that describes the mechanisms of these discharges is proposed. The model is based on the experimental recordings of IIDs in pyramidal neurons of the rat entorhinal cortex and estimations of synaptic conductances during IIDs. IIDs were induced in cortico-hippocampal slices by applying an extracellular solution with 4-aminopyridine, high potassium, and low magnesium concentrations. Two different types of IIDs initiated by interneurons were observed. The first type of IID (IID1) was pure GABAergic. The second type of IID (IID2) was induced by GABAergic excitation and maintained by recurrent interactions of both GABA- and glutamatergic neuronal populations. The model employed the conductance-based refractory density (CBRD) approach, which accurately approximates the firing rate of a population of similar Hodgkin-Huxley-like neurons. The model of coupled excitatory and inhibitory populations includes AMPA, NMDA, and GABA-receptor-mediated synapses and gap junctions. These neurons receive both arbitrary deterministic input and individual colored Gaussian noise. Both types of IIDs were successfully reproduced in the model by setting two different depolarized levels for GABA-mediated current reversal potential. It was revealed that short-term synaptic depression is a crucial factor in ceasing each of the discharges, and it also determines their durations and frequencies.

Cholinergic modulation of the hippocampal region and memory function

Acetylcholine (ACh) plays an important role in memory function and has been implicated in aging-related dementia, in which the impairment of hippocampus-dependent learning strongly manifests. Cholinergic neurons densely innervate the hippocampus, mediating the formation of episodic as well as semantic memory. Here, we will review recent findings on acetylcholine's modulation of memory function, with a particular focus on hippocampus-dependent learning, and the circuits involved. In addition, we will discuss the complexity of ACh actions in memory function to better understand the physiological role of ACh in memory. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

The Long and Winding Road to Gamma-Amino-Butyric Acid as Neurotransmitter

This review centers on the discoveries made during more than six decades of neuroscience research on the role of gamma-amino-butyric acid (GABA) as neurotransmitter. In doing so, special emphasis is directed to the significant involvement of Canadian scientists in these advances. Starting with the early studies that established GABA as an inhibitory neurotransmitter at central synapses, we summarize the results pointing at the GABA receptor as a drug target as well as more recent evidence showing that GABAA receptor signaling plays a surprisingly active role in neuronal network synchronization, both during development and in the adult brain. Finally, we briefly address the involvement of GABA in neurological conditions that encompass epileptic disorders and mental retardation.

Models of drug-induced epileptiform synchronization in vitro

Models of epileptiform activity in vitro have many advantages for recording and experimental manipulation. Neural tissues can be maintained in vitro for hours, and in neuronal or organotypic slice cultures for several weeks. A variety of drugs and other agents increase activity in these in vitro conditions, in many cases resulting in epileptiform activity, thus providing a direct model of symptomatic seizures. We review these preparations and the experimental manipulations used to induce epileptiform activity. The most common of drugs used are GABAA receptor antagonists and potassium channel blockers (notably 4-aminopyridine). Muscarinic agents also can induce epileptiform synchronization in vitro, and include potassium channel inhibition amongst their cellular actions. Manipulations of extracellular ions are reviewed in another paper in this special issue, as are ex vivo slices prepared from chronically epileptic animals and from people with epilepsy. More complex slices including extensive networks and/or several connected brain structures can provide insights into the dynamics of long range connections during epileptic activity. Visualization of slices also provides opportunities for identification of living neurons and for optical recording/stimulation and manipulation. Overall, the analysis of the epileptiform activity induced in brain tissue in vitro has played a major role in advancing our understanding of the cellular and network mechanisms of epileptiform synchronization, and it is expected to continue to do so in future.

Layer-specific modulation of entorhinal cortical excitability by presubiculum in a rat model of temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common form of epilepsy in adults and is often refractory to antiepileptic medications. The medial entorhinal area (MEA) is affected in TLE but mechanisms underlying hyperexcitability of MEA neurons require further elucidation. Previous studies suggest that inputs from the presubiculum (PrS) contribute to MEA pathophysiology. We assessed electrophysiologically how PrS influences MEA excitability using the rat pilocarpine model of TLE. PrS-MEA connectivity was confirmed by electrically stimulating PrS afferents while recording from neurons within superficial layers of MEA. Assessment of alterations in PrS-mediated synaptic drive to MEA neurons was made following focal application of either glutamate or NBQX to the PrS in control and epileptic animals. Here, we report that monosynaptic inputs to MEA from PrS neurons are conserved in epileptic rats, and that PrS modulation of MEA excitability is layer-specific. PrS contributes more to synaptic inhibition of LII stellate cells than excitation. Under epileptic conditions, stellate cell inhibition is significantly reduced while excitatory synaptic drive is maintained at levels similar to control. PrS contributes to both synaptic excitation and inhibition of LIII pyramidal cells in control animals. Under epileptic conditions, overall excitatory synaptic drive to these neurons is enhanced while inhibitory synaptic drive is maintained at control levels. Additionally, neither glutamate nor NBQX applied focally to PrS now affected EPSC and IPSC frequency of LIII pyramidal neurons. These layer-specific changes in PrS-MEA interactions are unexpected and of significance in unraveling pathophysiological mechanisms underlying TLE.

Limbic networks and epileptiform synchronization: the view from the experimental side

In this review, we summarize findings obtained in acute and chronic epilepsy models and in particular experiments that have revealed how neuronal networks in the limbic system-which is closely involved in the pathophysiogenesis of mesial temporal lobe epilepsy (MTLE)-produce hypersynchronous discharges. MTLE is often associated with a typical pattern of brain damage known as mesial temporal sclerosis, and it is one of the most refractory forms of partial epilepsy in adults. Specifically, we will address the cellular and pharmacological features of abnormal electrographic events that, as in MTLE patients, can occur in in vivo and in vitro animal models; these include interictal and ictal discharges along with high-frequency oscillations. In addition, we will consider how different limbic structures made hyperexcitable by acute pharmacological manipulations interact during epileptiform discharge generation. We will also review the electrographic characteristics of two types of seizure onsets that are most commonly seen in human and experimental MTLE as well as in in vitro models of epileptiform synchronization. Finally, we will address the role played by neurosteroids in reducing epileptiform synchronization and in modulating epileptogenesis.

Frequency preference in two-dimensional neural models: a linear analysis of the interaction between resonant and amplifying currents

Many neuron types exhibit preferred frequency responses in their voltage amplitude (resonance) or phase shift to subthreshold oscillatory currents, but the effect of biophysical parameters on these properties is not well understood. We propose a general framework to analyze the role of different ionic currents and their interactions in shaping the properties of impedance amplitude and phase in linearized biophysical models and demonstrate this approach in a two-dimensional linear model with two effective conductances g L and g1. We compute the key attributes of impedance and phase (resonance frequency and amplitude, zero-phase frequency, selectivity, etc.) in the g(L) - g1 parameter space. Using these attribute diagrams we identify two basic mechanisms for the generation of resonance: an increase in the resonance amplitude as g1 increases while the overall impedance is decreased, and an increase in the maximal impedance, without any change in the input resistance, as the ionic current time constant increases. We use the attribute diagrams to analyze resonance and phase of the linearization of two biophysical models that include resonant (I h or slow potassium) and amplifying currents (persistent sodium). In the absence of amplifying currents, the two models behave similarly as the conductances of the resonant currents is increased whereas, with the amplifying current present, the two models have qualitatively opposite responses. This work provides a general method for decoding the effect of biophysical parameters on linear membrane resonance and phase by tracking trajectories, parametrized by the relevant biophysical parameter, in pre-constructed attribute diagrams.

Diversity and excitability of deep-layer entorhinal cortical neurons in a model of temporal lobe epilepsy

The entorhinal cortex (ERC) is critically implicated in temporal lobe epileptogenesis--the most common type of adult epilepsy. Previous studies have suggested that epileptiform discharges likely initiate in seizure-sensitive deep layers (V-VI) of the medial entorhinal area (MEA) and propagate into seizure-resistant superficial layers (II-III) and hippocampus, establishing a lamina-specific distinction between activities of deep- versus superficial-layer neurons and their seizure susceptibilities. While layer II stellate cells in MEA have been shown to be hyperexcitable and hypersynchronous in patients and animal models of temporal lobe epilepsy (TLE), the fate of neurons in the deep layers under epileptic conditions and their overall contribution to epileptogenicity of this region have remained unclear. We used whole cell recordings from slices of the ERC in normal and pilocarpine-treated epileptic rats to characterize the electrophysiological properties of neurons in this region and directly assess changes in their excitatory and inhibitory synaptic drive under epileptic conditions. We found a surprising heterogeneity with at least three major types and two subtypes of functionally distinct excitatory neurons. However, contrary to expectation, none of the major neuron types characterized showed any significant changes in their excitability, barring loss of excitatory and inhibitory inputs in a subtype of neurons whose dendrite extended into layer III, where neurons are preferentially lost during TLE. We confirmed hyperexcitability of layer II neurons in the same slices, suggesting minimal influence of deep-layer input on superficial-layer neuron excitability under epileptic conditions. These data show that deep layers of ERC contain a more diverse population of excitatory neurons than previously envisaged that appear to belie their seizure-sensitive reputation.

Hippocampal microRNA-132 mediates stress-inducible cognitive deficits through its acetylcholinesterase target

Diverse stress stimuli induce long-lasting cognitive deficits, but the underlying molecular mechanisms are still incompletely understood. Here, we report three different stress models demonstrating that stress-inducible increases in microRNA-132 (miR-132) and consequent decreases in its acetylcholinesterase (AChE) target are causally involved. In a mild model of predator scent-induced anxiety, we demonstrate long-lasting hippocampal elevation of miR-132, accompanied by and associated with reduced AChE activity. Using lentiviral-mediated suppression of "synaptic" AChE-S mRNA, we quantified footshock stress-inducible changes in miR-132 and AChE and its corresponding cognitive damages. Stressed mice showed long-lasting impairments in the Morris water maze. In contrast, pre-stress injected AChE-suppressing lentivirus, but not a control virus, reduced hippocampal levels of both miR-132 and AChE and maintained similar cognitive performance to that of naïve, non-stressed mice. To dissociate between miR-132 and synaptic AChE-S as potential causes for stress-inducible cognitive deficits, we further used engineered TgR mice with enforced over-expression of the soluble "readthrough" AChE-R variant without the 3'-untranslated region binding site for miR-132. TgR mice displayed excess AChE-R in hippocampal neurons, enhanced c-fos labeling and correspondingly intensified reaction to the cholinergic agonist pilocarpine. They further showed excessive hippocampal expression of miR-132, accompanied by reduced host AChE-S mRNA and the GTPase activator p250GAP target of miR-132. At the behavioral level, TgR mice showed abnormal nocturnal locomotion patterns and serial maze mal-performance in spite of their reduced AChE-S levels. Our findings attribute stress-inducible cognitive impairments to cholinergic-mediated induction of miR-132 and consequently suppressed ACHE-S, opening venues for intercepting these miR-132-mediated damages.

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

GABA is the main inhibitory neurotransmitter in the adult forebrain, where it activates ionotropic type A and metabotropic type B receptors. Early studies have shown that GABA(A) receptor-mediated inhibition controls neuronal excitability and thus the occurrence of seizures. However, more complex, and at times unexpected, mechanisms of GABAergic signaling have been identified during epileptiform discharges over the last few years. Here, we will review experimental data that point at the paradoxical role played by GABA(A) receptor-mediated mechanisms in synchronizing neuronal networks, and in particular those of limbic structures such as the hippocampus, the entorhinal and perirhinal cortices, or the amygdala. After having summarized the fundamental characteristics of GABA(A) receptor-mediated mechanisms, we will analyze their role in the generation of network oscillations and their contribution to epileptiform synchronization. Whether and how GABA(A) receptors influence the interaction between limbic networks leading to ictogenesis will be also reviewed. Finally, we will consider the role of altered inhibition in the human epileptic brain along with the ability of GABA(A) receptor-mediated conductances to generate synchronous depolarizing events that may lead to ictogenesis in human epileptic disorders as well.

Involvement of inward rectifier and M-type currents in carbachol-induced epileptiform synchronization

Exposure to cholinergic agonists is a widely used paradigm to induce epileptogenesis in vivo and synchronous activity in brain slices maintained in vitro. However, the mechanisms underlying these effects remain unclear. Here, we used field potential recordings from the lateral entorhinal cortex in horizontal rat brain slices to explore whether two different K(+) currents regulated by muscarinic receptor activation, the inward rectifier (K(IR)) and the M-type (K(M)) currents, have a role in carbachol (CCh)-induced field activity, a prototypical model of cholinergic-dependent epileptiform synchronization. To establish whether K(IR) or K(M) blockade could replicate CCh effects, we exposed slices to blockers of these currents in the absence of CCh. K(IR) channel blockade with micromolar Ba(2+) concentrations induced interictal-like events with duration and frequency that were lower than those observed with CCh; by contrast, the K(M) blocker linopirdine was ineffective. Pre-treatment with Ba(2+) or linopirdine increased the duration of epileptiform discharges induced by subsequent application of CCh. Baclofen, a GABA(B) receptor agonist that activates K(IR), abolished CCh-induced field oscillations, an effect that was abrogated by the GABA(B) receptor antagonist CGP 55845, and prevented by Ba(2+). Finally, when applied after CCh, the K(M) activators flupirtine and retigabine shifted leftward the cumulative distribution of CCh-induced event duration; this effect was opposite to what seen during linopirdine application under similar experimental conditions. Overall, our findings suggest that K(IR) rather than K(M) plays a major regulatory role in controlling CCh-induced epileptiform synchronization.

Effects of benzodiazepine treatment on cortical GABA(A) and muscarinic receptors: studies in schizophrenia and rats

Changes in cortical γ-aminobutyric acid A (GABA(A)) receptors and muscarinic receptors have been reported in schizophrenia, a disorder treated with antipsychotic drugs and benzodiazepines. As there is a reported functional relationship between the GABAergic and cholinergic systems in the human central nervous system we have investigated whether there are changes in the GABA(A) and muscarinic receptors in the cortex of subjects from APD-treated subjects with schizophrenia and whether changes were different in subjects who had also received benzodiazepine treatment. We failed to show any strong correlations between changes in GABA(A) and muscarinic receptors in the CNS of subjects with schizophrenia. We showed that subjects with schizophrenia treated with benzodiazepines had lower levels of muscarinic receptors; which was not the case in rats treated with APDs, benzodiazepines or a combination of both drugs. Further, the benzodiazepine binding site, but not the muscimol binding site, was decreased in the parietal cortex of subjects with schizophrenia independent of benzodiazepine status at death. These data would therefore support our previously stated hypotheses that changes in the cortical cholinergic and GABAergic systems are involved in the pathophysiology of schizophrenia.

A parallel cholinergic brainstem pathway for enhancing locomotor drive

The brainstem locomotor system is believed to be organized serially from the mesencephalic locomotor region (MLR) to reticulospinal neurons, which in turn project to locomotor neurons in the spinal cord. We identified brainstem muscarinoceptive neurons in lampreys (Petromyzon marinus) that received parallel inputs from the MLR and projected back to reticulospinal cells to amplify and extend the duration of locomotor output. These cells responded to muscarine with extended periods of excitation, received direct muscarinic excitation from the MLR and projected glutamatergic excitation to reticulospinal neurons. Targeted blockade of muscarine receptors over these neurons profoundly reduced MLR-induced excitation of reticulospinal neurons and markedly slowed MLR-evoked locomotion. The presence of these neurons forces us to rethink the organization of supraspinal locomotor control, to include a sustained feedforward loop that boosts locomotor output.

Spatio-temporal dynamics of theta oscillations in hippocampal-entorhinal slices

Theta oscillations (4-12 Hz) are associated with learning and memory and are found in the hippocampus and the entorhinal cortex (EC). The spatio-temporal organization of rhythmic activity in the hippocampal-EC complex was investigated in vitro. The voltage sensitive absorption dye NK3630 was used to record the changes in aggregated membrane voltage simultaneously from the neuronal networks involved. Oscillatory activity at 7.0 Hz (range, 5.8-8.2) was induced in the slice with the muscarinic agonist carbachol (75-100 microM) in the presence of bicuculline (5 microM). Time relations between all recording sites were analyzed using cross-correlation functions which revealed systematic phase shifts in the theta oscillation recorded from the different entorhinal and hippocampal subregions. These phase shifts could be interpreted as propagation delays. The oscillation propagates over the slice in a characteristic spatio-temporal sequence, where the entorhinal cortex leads, followed by the subiculum and then the dentate gyrus (DG), to finally reach the CA3 and the CA1 area. The delay from dentate gyrus to the CA3 area was 12.4 +/- 1.1 ms (mean +/- s.e.m.) and from the CA3 to the CA1 region it was 10.9 +/- 1.9 ms. The propagation delays between the hippocampal subregions resemble the latencies of electrically evoked responses in the same subregions. Removing the entorhinal cortex from the slice changed the spatiotemporal pattern into a more clustered pattern with higher local synchrony. We conclude that in the slice, carbachol-induced theta oscillations are initiated in the entorhinal cortex. The EC could serve to control the information flow through the neuronal network in the subregions of the hippocampus by synchronizing and/or entraining their responses to external inputs.

In vitro effects of antiepileptic drugs on acetylcholinesterase and ectonucleotidase activities in zebrafish (Danio rerio) brain

Carbamazepine (CBZ), phenytoin (PHT), and gabapentine (GBP) are classical antiepileptic drugs (AEDs) that act through a variety of mechanisms. We have tested the in vitro effects of CBZ, PHT, and GBP at different concentrations on ectonucleotidase and acetylcholinesterase activities in zebrafish brain. CBZ inhibited ATP hydrolysis at 1000 microM (32%) whereas acetylcholine hydrolysis decreased at 500 microM (25.2%) and 1000 microM (38.7%). PHT increased AMP hydrolysis both at 500 microM (65%) and 1000 microM (64.8%). GBP did not promote any significant changes on ectonucleotidase and acetylcholinesterase activities. These results have shown that CBZ can reduce NTPDase (nucleoside triphosphate diphosphohydrolase) and PHT enhance ecto 5'-nucleotidase activities. Therefore, it is possible to suggest that the AEDs induced-effects on ectonucleotidases are related to enzyme anchorage form. Our findings have also shown that high CBZ concentrations inhibit acetylcholinesterase activity, which can induce an increase of acetylcholine levels. Taken together, these results showed a complex interaction among AEDs, purinergic, and cholinergic systems, providing a better understanding of the AEDs pharmacodynamics.

Functional interactions within the parahippocampal region revealed by voltage-sensitive dye imaging in the isolated guinea pig brain

The massive transfer of information from the neocortex to the entorhinal cortex (and vice versa) is hindered by a powerful inhibitory control generated in the perirhinal cortex. In vivo and in vitro experiments performed in rodents and cats support this conclusion, further extended in the present study to the analysis of the interaction between the entorhinal cortex and other parahippocampal areas, such as the postrhinal and the retrosplenial cortices. The experiments were performed in the in vitro isolated guinea pig brain by a combined approach based on electrophysiological recordings and fast imaging of optical signals generated by voltage-sensitive dyes applied to the entire brain by arterial perfusion. Local stimuli delivered in different portions of the perirhinal, postrhinal, and retrosplenial cortex evoked local responses that did not propagate to the entorhinal cortex. Neither high- and low-frequency-patterned stimulation nor paired associative stimuli facilitated the propagation of activity to the entorhinal region. Similar stimulations performed during cholinergic neuromodulation with carbachol were also ineffective in overcoming the inhibitory network that controls propagation to the entorhinal cortex. The pharmacological inactivation of GABAergic transmission by local application of bicuculline (1 mM) in area 36 of the perirhinal cortex facilitated the longitudinal (rostrocaudal) propagation of activity into the perirhinal/postrhinal cortices but did not cause propagation into the entorhinal cortex. Bicuculline injection in both area 35 and medial entorhinal cortex released the inhibitory control and allowed the propagation of the neural activity to the entorhinal cortex. These results demonstrate that, as for the perirhinal-entorhinal reciprocal interactions, also the connections between the postrhinal/retrosplenial cortices and the entorhinal region are subject to a powerful inhibitory control.

Modulation of GABAergic transmission by muscarinic receptors in the entorhinal cortex of juvenile rats

Whereas the entorhinal cortex (EC) receives profuse cholinergic innervations from the basal forebrain and activation of cholinergic receptors has been shown to modulate the activities of the principal neurons and promote the intrinsic oscillations in the EC, the effects of cholinergic receptor activation on GABAergic transmission in this brain region have not been determined. We examined the effects of muscarinic receptor activation on GABA(A) receptor-mediated synaptic transmission in the superficial layers of the EC. Application of muscarine dose-dependently increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) recorded from the principal neurons in layer II/III via activation of M(3) muscarinic receptors. Muscarine slightly reduced the frequency but had no effects on the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. Muscarine reduced the amplitude of IPSCs evoked by extracellular field stimulation and by depolarization of GABAergic interneurons in synaptically connected interneuron and pyramidal neuron pairs. Application of muscarine generated membrane depolarization and increased action potential firing frequency but reduced the amplitude of action potentials in GABAergic interneurons. Muscarine-induced depolarization of GABAergic interneurons was mediated by inhibition of background K(+) channels and independent of phospholipase C, intracellular Ca(2+) release, and protein kinase C. Our results demonstrate that activation of muscarinic receptors exerts diverse effects on GABAergic transmission in the EC.

Nonlinear dynamical analysis of carbachol induced hippocampal oscillations in mice

AIM

Hippocampal neuronal network and synaptic impairment underlie learning and memory deficit in Alzheimer's disease (AD) patients and animal models. In this paper, we analyzed the dynamics and complexity of hippocampal neuronal network synchronization induced by acute exposure to carbachol, a nicotinic and muscarinic receptor co-agonist, using the nonlinear dynamical model based on the Lempel-Ziv estimator. We compared the dynamics of hippocampal oscillations between wild-type (WT) and triple-transgenic (3xTg) mice, as an AD animal model. We also compared these dynamic alterations between different age groups (5 and 10 months). We hypothesize that there is an impairment of complexity of CCh-induced hippocampal oscillations in 3xTg AD mice compared to WT mice, and that this impairment is age-dependent.

METHODS

To test this hypothesis, we used electrophysiological recordings (field potential) in hippocampal slices.

RESULTS

Acute exposure to 100 micromol/L CCh induced field potential oscillations in hippocampal CA1 region, which exhibited three distinct patterns: (1) continuous neural firing, (2) repeated burst neural firing and (3) the mixed (continuous and burst) pattern in both WT and 3xTg AD mice. Based on Lempel-Ziv estimator, pattern (2) was significantly lower than patterns (1) and (3) in 3xTg AD mice compared to WT mice (P<0.001), and also in 10-month old WT mice compared to those in 5-month old WT mice (P<0.01).

CONCLUSION

These results suggest that the burst pattern (theta oscillation) of hippocampal network is selectively impaired in 3xTg AD mouse model, which may reflect a learning and memory deficit in the AD patients.

Differential induction of long-term potentiation in the horizontal versus columnar superficial connections to layer II cells of the entorhinal cortex

The entorhinal cortex (EC) is a nodal and independent mnemonic element of the medial temporal lobe memory circuit as it forms a bidirectional interface between the neocortex and hippocampus. Within the EC, intra- and inter-lamellar associational connections occur via horizontal and columnar projections, respectively. We undertook a comparative study of these two inputs as they converge upon EC layer II cells using whole-cell patch techniques in an adult rat EC horizontal slice preparation in which the deepest layers (V-VI) had been dissected out. Electrical stimulation of layers I and III during GABA blockade allowed us to study excitatory synaptic properties and plasticity in the horizontal and columnar fibre systems, respectively. Both pathways exhibited AMPA- and NMDA-receptor mediated transmission and both exhibited long-term potentiation (LTP) after high-frequency (tetanic) stimulation. LTP in the horizontal, but not in the columnar pathway, was blocked by NMDA receptor antagonism. Intriguingly, LTP in both appeared to be mediated by post synaptic increases in Ca²⁺ that may be coupled to differing second messenger pathways. Thus, the superficial excitatory horizontal and columnar associative pathways to layer II have divergent mechanisms for LTP which may endow the EC with even more complex and dynamic processing characteristics than previously thought.

Network hyperexcitability within the deep layers of the pilocarpine-treated rat entorhinal cortex

In this study we report that in the presence of normal buffer, epileptiform discharges occur spontaneously (duration = 2.60 +/- 0.49 s) or can be induced by electrical stimuli (duration = 2.50 +/- 0.62 s) in the entorhinal cortex (EC) of brain slices obtained from pilocarpine-treated rats but not in those from age-matched, nonepileptic control (NEC) animals. These network-driven epileptiform events consist of field oscillatory sequences at frequencies greater than 200 Hz that most often initiate in the lateral EC and propagate to the medial EC with 4-63 ms delays. The NMDA receptor antagonist CPP depresses the rate of occurrence (P < 0.01) of these spontaneous epileptiform discharges but fails in blocking them. Paradoxically, stimulus-induced epileptiform responses are enhanced in duration during CPP application. However, concomitant application of NMDA and non-NMDA glutamatergic antagonists abolishes spontaneous and stimulus-induced epileptiform events. Intracellular recordings from lateral EC layer V cells indicate a lower frequency of spontaneous hyperpolarizing postsynaptic potentials in pilocarpine-treated tissue than in NEC (P < 0.002) both under control conditions and with glutamatergic receptor blockade; the reversal potential of pharmacologically isolated GABA(A) receptor-mediated inhibitory postsynaptic potentials has similar values in the two types of tissue. Finally, immunohistochemical analysis shows that parvalbumin-positive interneurons are selectively reduced in number in EC deep layers. Collectively, these results indicate that reduced inhibition within the pilocarpine-treated EC layer V may promote network epileptic hyperexcitability.

Acute induction of epileptiform discharges by pilocarpine in the in vitro isolated guinea-pig brain requires enhancement of blood-brain barrier permeability

Systemic application of the muscarinic agonist, pilocarpine, is commonly utilized to induce an acute status epilepticus that evolves into a chronic epileptic condition characterized by spontaneous seizures. Recent findings suggest that the status epilepticus induced by pilocarpine may be triggered by changes in the blood-brain barrier (BBB) permeability. We tested the role of the BBB in an acute pilocarpine model by using the in vitro model brain preparation and compared our finding with in vivo data. Arterial perfusion of the in vitro isolated guinea-pig brain with <1 mM pilocarpine did not cause epileptiform activity, but rather reduced synaptic transmission and induced steady fast (20-25 Hz) oscillatory activity in limbic cortices. These effects were reversibly blocked by co-perfusion of the muscarinic antagonist atropine sulfate (5 microM). Brain pilocarpine measurements in vivo and in vitro suggested modest BBB penetration. Pilocarpine induced epileptiform discharges only when perfused with compounds that enhance BBB permeability, such as bradykinin (n=2) or histamine (n=10). This pro-epileptic effect was abolished when the BBB-impermeable muscarinic antagonist atropine methyl bromide (5 microM) was co-perfused with histamine and pilocarpine. In the absence of BBB permeability enhancing drugs, pilocarpine induced epileptiform activity only after arterial perfusion at concentrations >10 mM. Ictal discharges correlated with a high intracerebral pilocarpine concentration measured by high pressure liquid chromatography. We propose that acute epileptiform discharges induced by pilocarpine treatment in the in vitro isolated brain preparation are mediated by a dose-dependent, atropine-sensitive muscarinic effect promoted by an increase in BBB permeability. Pilocarpine accumulation secondary to BBB permeability changes may contribute to in vivo ictogenesis in the pilocarpine epilepsy model.

In vivo and in vitro effects of pilocarpine: relevance to ictogenesis

OBJECTIVES

A common experimental model of status epilepticus (SE) utilizes intraperitoneal administration of the cholinergic agonist pilocarpine preceded by methyl-scopolamine treatment. Currently, activation of cholinergic neurons is recognized as the only factor triggering pilocarpine SE. However, cholinergic receptors are also widely distributed systemically and pretreatment with methyl-scopolamine may not be sufficient to counteract the effects of systemically injected pilocarpine. The extent of such peripheral events and the contribution to SE are unknown and the possibility that pilocarpine also induces SE by peripheral actions is yet untested.

METHODS

We measured in vivo at onset of SE: brain and blood pilocarpine levels, blood-brain barrier (BBB) permeability, T-lymphocyte activation and serum levels of IL-1beta and TNF-alpha. The effects of pilocarpine on neuronal excitability was assessed in vitro on hippocampal slices or whole guinea pig brain preparations in presence of physiologic or elevated [K+](out).

RESULTS

Pilocarpine blood and brain levels at SE were 1400 +/- 200 microM and 200 +/- 80 microM, respectively. In vivo, after pilocarpine injection, increased serum IL-1beta, decreased CD4:CD8 T-lymphocyte ratios and focal BBB leakage were observed. In vitro, pilocarpine failed to exert significant synchronized epileptiform activity when applied at concentrations identical or higher to levels measured in vivo. Intense electrographic seizure-like events occurred only in the copresence of levels of K+ (6 mM) mimicking BBB leakage.

CONCLUSIONS

Early systemic events increasing BBB permeability may promote entry of cofactors (e. g. K+) into the brain leading to pilocarpine-induced SE. Disturbance of brain homeostasis represents an etiological factor contributing to pilocarpine seizures.

Cholinergic suppression of excitatory synaptic responses in layer II of the medial entorhinal cortex

Theta-frequency (4-12 Hz) electroencephalographic activity is thought to play a role in mechanisms mediating sensory and mnemonic processing in the entorhinal cortex and hippocampus, but the effects of acetylcholine on excitatory synaptic inputs to the entorhinal cortex are not well understood. Field excitatory postsynaptic potentials (fEPSPs) evoked by stimulation of the piriform (olfactory) cortex were recorded in the medial entorhinal cortex during behaviors associated with theta activity (active mobility) and were compared with those recorded during nontheta behaviors (awake immobility and slow wave sleep). Synaptic responses were smaller during behavioral activity than during awake immobility and sleep, and responses recorded during movement were largest during the negative phase of the theta rhythm. Systemic administration of cholinergic agonists reduced the amplitude of fEPSPs, and the muscarinic receptor blocker scopolamine strongly enhanced fEPSPs, suggesting that the theta-related suppression of fEPSPs is mediated in part by cholinergic inputs. The reduction in fEPSPs was investigated using in vitro intracellular recordings of EPSPs in Layer II neurons evoked by stimulation of Layer I afferents. Constant bath application of the muscarinic agonist carbachol depolarized membrane potential and suppressed EPSP amplitude in Layer II neurons. The suppression of EPSPs was not associated with a substantial change in input resistance, and could not be accounted for by a depolarization-induced reduction in driving force on the EPSP. The GABA(A) receptor-blocker bicuculline (50 microM) did not prevent the cholinergic suppression of EPSPs, suggesting that the suppression is not dependent on inhibitory mechanisms. Paired-pulse facilitation of field and intracellular EPSPs were enhanced by carbachol, indicating that the suppression is likely due to inhibition of presynaptic glutamate release. These results indicate that, in addition to well known effects on postsynaptic conductances that increase cellular excitability, cholinergic activation in the entorhinal cortex results in a strong reduction in strength of excitatory synaptic inputs from the piriform cortex.

Effects of serotonin on the intrinsic membrane properties of layer II medial entorhinal cortex neurons

Although serotonin (5-HT) is an important neuromodulator in the superficial layers of the medial entorhinal cortex (mEC), there is some disagreement concerning its influences upon the membrane properties of neurons within this region. We performed whole cell recordings of mEC Layer II projection neurons in rat brain slices in order to characterize the intrinsic influences of 5-HT. In current clamp, 5-HT evoked a biphasic response consisting of a moderately short latency and large amplitude hyperpolarization followed by a slowly developing, long lasting, and small amplitude depolarization. Correspondingly, in voltage clamp, 5-HT evoked a robust outward followed by a smaller inward shift of holding current. The outward current evoked by 5-HT showed a consistent current/voltage (I/V) relationship across cells with inward rectification, and demonstrating a reversal potential that was systematically dependent upon the extracellular concentration of K(+), suggesting that it was predominantly carried by potassium ions. However, the inward current showed a less consistent I/V relationship across different cells, suggesting multiple independent ionic mechanisms. The outward current was mediated through activation of 5-HT(1A) receptors via a G-protein dependent mechanism while inward currents were evoked in a 5-HT(1A)-independent fashion. A significant proportion of the inward current was blocked by the I(h) inhibitor ZD7288 and appeared to be due to 5-HT modulation of I(h) as 5-HT shifted the activation curve of I(h) in a depolarizing fashion. Serotonin is thus likely to influence, in a composite fashion, the information processing of Layer II neurons in the mEC and thus, the passage of neocortical information via the perforant pathway to the hippocampus.

Muscarinic receptor activation elicits sustained, recurring depolarizations in reticulospinal neurons

In lampreys, brain stem reticulospinal (RS) neurons constitute the main descending input to the spinal cord and activate the spinal locomotor central pattern generators. Cholinergic nicotinic inputs activate RS neurons, and consequently, induce locomotion. Cholinergic muscarinic agonists also induce locomotion when applied to the brain stem of birds. This study examined whether bath applications of muscarinic agonists could activate RS neurons and initiate motor output in lampreys. Bath applications of 25 microM muscarine elicited sustained, recurring depolarizations (mean duration of 5.0 +/- 0.5 s recurring with a mean period of 55.5 +/- 10.3 s) in intracellularly recorded rhombencephalic RS neurons. Calcium imaging experiments revealed that muscarine induced oscillations in calcium levels that occurred synchronously within the RS neuron population. Bath application of TTX abolished the muscarine effect, suggesting the sustained depolarizations in RS neurons are driven by other neurons. A series of lesion experiments suggested the caudal half of the rhombencephalon was necessary. Microinjections of muscarine (75 microM) or the muscarinic receptor (mAchR) antagonist atropine (1 mM) lateral to the rostral pole of the posterior rhombencephalic reticular nucleus induced or prevented, respectively, the muscarinic RS neuron response. Cells immunoreactive for muscarinic receptors were found in this region and could mediate this response. Bath application of glutamatergic antagonists (6-cyano-7-nitroquinoxaline-2,3-dione/D-2-amino-5-phosphonovaleric acid) abolished the muscarine effect, suggesting that glutamatergic transmission is needed for the effect. Ventral root recordings showed spinal motor output coincides with RS neuron sustained depolarizations. We propose that unilateral mAchR activation on specific cells in the caudal rhombencephalon activates a circuit that generates synchronous sustained, recurring depolarizations in bilateral populations of RS neurons.

Entorhinal cortex of the monkey: VII. intrinsic connections

The organization of intrinsic connections within the entorhinal cortex was investigated in Macaca fascicularis monkeys. Anterograde tracers ((3)H-amino acids, Phaseolus vulgaris-leucoagglutinin, biotinylated dextran amine, or Fluoro-Ruby) were injected into the deep or superficial layers of the entorhinal cortex in 24 animals. These injections labeled extensive intrinsic projections that terminated throughout all layers of the entorhinal cortex. Labeling was typically continuous i.e., there was no evidence of a patchy or columnar organization. Each injection produced a rostrocaudally oriented band of labeled fibers and terminals that extended for one-third to one-half of the length of the entorhinal cortex. The more extensive distributions of labeled fibers were more typical of caudally placed injection sites. Taken together, the projections identified at least two mediolaterally differentiated bands: a lateral band that encompasses fields Elr, Elc, and the most lateral aspect of fields Ec and Ecl and a wider, medially situated band that encompasses much of fields Er, Ei, Ec, and Ecl. We obtained some evidence that field Eo constitutes a third, very medially placed band. The rostrocaudal organization of labeled fibers and the extent of labeling within the deep and superficial layers were unrelated to the laminar position of the injection. These data suggest that intrinsic associatonal connections in the monkey entorhinal cortex are organized into separate associational networks. Our findings are discussed with reference to the role of interlaminar connections in mediating physiological interactions between the neocortex and the hippocampus.

Antiepileptic drugs and muscarinic receptor-dependent excitation in the rat subiculum

Field and intracellular recordings were made in an in vitro slice preparation to establish whether the antiepileptic drugs topiramate and lamotrigine modulate cholinergic excitation in the rat subiculum. Bath application of carbachol (CCh, 70-100microM) induced: (i) spontaneous and synchronous field oscillations (duration=up to 7s) that were mirrored by intracellular depolarizations with rhythmic action potential bursts; and (ii) depolarizing plateau potentials (DPPs, duration=up to 2.5s) associated with action potential discharge in response to brief (50-100ms) intracellular depolarizing current pulses. Ionotropic glutamatergic receptor antagonists abolished the field oscillations without influencing DPPs, while atropine (1microM) markedly reduced both types of activity. Topiramate (10-100microM, n=8-13 slices) or lamotrigine (50-400microM, n=3-12) decreased in a dose-dependent manner, and eventually abolished, CCh-induced field oscillations. During topiramate application, these effects were accompanied by marked DPP reduction. When these antiepileptic drugs were tested on DPPs recorded in the presence of CCh+ionotropic glutamatergic and GABA receptor antagonists, only topiramate reduced DPPs (n=5-19/dose; IC(50)=18microM, n=48). Similar effects were induced by topiramate during metabotropic glutamate receptor antagonism (n=5), which did not influence DPPs. Thus, topiramate and lamotrigine reduce CCh-induced epileptiform synchronization in the rat subiculum but only topiramate is effective in controlling DPPs. We propose that muscarinic receptor-mediated excitation represents a target for the action of some antiepileptic drugs such as topiramate.

Theta phase coding in a network model of the entorhinal cortex layer II with entorhinal-hippocampal loop connections

We investigated successive firing of the stellate cells within a theta cycle, which replicates the phase coding of place information, using a network model of the entorhinal cortex layer II with loop connections. Layer II of the entorhinal cortex (ECII) sends signals to the hippocampus, and the hippocampus sends signals back to layer V of the entorhinal cortex (ECV). In addition to this major pathway, projection from ECV to ECII also exists. It is, therefore, inferred that reverberation activity readily appears if projections from ECV to ECII are potentiated. The frequency of the reverberation would be in a gamma range because it takes signals 20-30 ms to go around the entorhinal-hippocampal loop circuits. On the other hand, it has been suggested that ECII is a theta rhythm generator. If the reverberation activity appears in the entorhinal-hippocampal loop circuits, gamma oscillation would be superimposed on a theta rhythm in ECII like a gamma-theta oscillation. This is a reminiscence of the theta phase coding of place information. In this paper, first, a network model of ECII will be developed in order to reproduce a theta rhythm. Secondly, we will show that loop connections from one stellate cell to the other one are selectively potentiated by afferent signals to ECII. Frequencies of those afferent signals are different, and transmission delay of the loop connections is 20 ms. As a result, stellate cells fire successively within one cycle of the theta rhythm. This resembles gamma-theta oscillation underlying the phase coding. Our model also replicates the phase precession of stellate cell firing within a cycle of subthreshold oscillation (theta rhythm).

Spatial representation and the architecture of the entorhinal cortex

It has recently been recognized that the entorhinal cortex has a crucial role in spatial representation and navigation. How the position of an animal is computed within the entorhinal circuitry remains to be determined, but the architectural organization of this brain area might provide some clues. Here, we review three organizational principles--recurrent connectivity, interlaminar connectivity and modular organization--and propose how each of them might contribute to the emergence and maintenance of positional representations in entorhinal neural networks.

Functional role of entorhinal cortex in working memory processing

Our learning and memory system has the challenge to work in a world where items to learn are dispersed in space and time. From the information extracted by the perceptual systems, the learning system must select and combine information. Both these operations may require a temporary storage where significance and correlations could be assessed. This work builds on the common hypothesis that hippocampus and subicular, entorhinal and parahippocampal/postrhinal areas are essential for the above-mentioned functions. We bring up two examples of models; the first one is modeling of in vivo and in vitro data from entorhinal cortex layer II of delayed match-to-sample working memory experiments, the second one studying mechanisms in theta rhythmicity in EC. In both cases, we discuss how cationic currents might be involved and relate their kinetics and pharmacology to behavioral and cellular experimental results.

Physiological changes in chronic epileptic rats are prominent in superficial layers of the medial entorhinal area

PURPOSE

We investigated whether the functional network properties of the medial entorhinal area (MEA) of the entorhinal cortex were altered in a rat model of chronic epilepsy that is characterized by extensive cell loss in MEA layer III.

METHODS

Responses were evoked in the entorhinal cortex by electrical stimulation of the subiculum in anesthetized chronic epileptic rats, 2-4 months after status epilepticus, induced by systemic kainate (KA) injections. Laminar field potentials were measured using a 16-channel silicon probe that covered all six layers of the MEA; an estimate of the local transmembrane currents was made using current source density analysis.

RESULTS

Double-pulse stimulation of the subiculum evoked responses in deep and superficial layers of the MEA in control and KA rats. A current sink in layer I and at the border of layer I and II that was induced by antidromic activation of MEA-II, was much more prominent in KA rats with extensive neuronal loss in MEA-III than in control rats or KA rats with minor MEA-III loss. Furthermore, KA rats that displayed MEA-III loss presented a series of oscillations induced by subicular stimulation in the beta/gamma-frequency range (20-100 Hz), which were confined to superficial layers of MEA. These oscillations were never observed in control rats or KA rats with minor MEA-III loss.

CONCLUSIONS

These results indicate that the observed alterations in the superficial MEA responses to subiculum stimulation and the occurrence of beta/gamma-oscillations are related phenomena, which are a consequence of altered and impaired inhibition within these MEA layers in chronic epileptic rats.

Sequential 123I-iododexetimide scans in temporal lobe epilepsy: comparison with neuroimaging scans (MR imaging and 18F-FDG PET imaging)

PURPOSE

Muscarinic acetylcholine receptors (mAChRs) play an important role in the generation of seizures. Single-photon emission computed tomography (SPECT) with 12I-iododexetimide (IDEX) depicts tracer uptake by mAChRs. Our aims were to: (a) determine the optimum time for interictal IDEX SPECT imaging; (b) determine the accuracy of IDEX scans in the localisation of seizure foci when compared with video EEG and MR imaging in patients with temporal lobe epilepsy (TLE); (c) characterise the distribution of IDEX binding in the temporal lobes and (d) compare IDEX SPECT and 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) in identifying seizure foci.

METHODS

We performed sequential scans using IDEX SPECT imaging at 0, 3, 6 and 24 h in 12 consecutive patients with refractory TLE undergoing assessment for epilepsy surgery. Visual and region of interest analyses of the mesial, lateral and polar regions of the temporal lobes were used to compare IDEX SPECT, FDG PET and MR imaging in seizure onset localisation.

RESULTS

The 6-h IDEX scan (92%; kappa=0.83, p=0.003) was superior to the 0-h (36%; kappa=0.01, p>0.05), 3-h (55%; kappa=0.13, p>0.05) and 24-h IDEX scans in identifying the temporal lobe of seizure origin. The 6-h IDEX scan correctly predicted the temporal lobe of seizure origin in two patients who required intracranial EEG recordings to define the seizure onset. Reduced ligand binding was most marked at the temporal pole and mesial temporal structures. IDEX SPECT was superior to interictal FDG PET (75%; kappa=0.66, p=0.023) in seizure onset localisation. MR imaging was non-localising in two patients in whom it was normal and in another patient in whom there was bilateral symmetrical hippocampal atrophy.

CONCLUSION

The 6-h IDEX SPECT scan is a viable alternative to FDG PET imaging in seizure onset localisation in TLE.

Ionic mechanisms in the generation of subthreshold oscillations and action potential clustering in entorhinal layer II stellate neurons

A multicompartmental biophysical model of entorhinal cortex layer II stellate cells was developed to analyze the ionic basis of physiological properties, such as subthreshold membrane potential oscillations, action potential clustering, and the medium afterhyperpolarization. In particular, the simulation illustrates the interaction of the persistent sodium current (I(Nap)) and the hyperpolarization activated inward current (Ih) in the generation of subthreshold membrane potential oscillations. The potential role of Ih in contributing to the medium hyperpolarization (mAHP) and rebound spiking was studied. The role of Ih and the slow calcium-activated potassium current Ikappa(AHP) in action potential clustering was also studied. Representations of Ih and I(Nap) were developed with parameters based on voltage-clamp data from whole-cell patch and single channel recordings of stellate cells (Dickson et al., J Neurophysiol 83:2562-2579, 2000; Magistretti and Alonso, J Gen Physiol 114:491-509, 1999; Magistretti et al., J Physiol 521:629-636, 1999a; J Neurosci 19:7334-7341, 1999b). These currents interacted to generate robust subthreshold membrane potentials with amplitude and frequency corresponding to data observed in the whole cell patch recordings. The model was also able to account for effects of pharmacological manipulations, including blockade of Ih with ZD7288, partial blockade with cesium, and the influence of barium on oscillations. In a model with a wider range of currents, the transition from oscillations to single spiking, to spike clustering, and finally tonic firing could be replicated. In agreement with experiment, blockade of calcium channels in the model strongly reduced clustering. In the voltage interval during which no data are available, the model predicts that the slow component of Ih does not follow the fast component down to very short time constants. The model also predicts that the fast component of Ih is responsible for the involvement in the generation of subthreshold oscillations, and the slow component dominates in the generation of spike clusters.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of principal neurons in the entorhinal cortex of macaque monkeys

Little is known about the neuroanatomical or electrophysiological properties of individual neurons in the primate entorhinal cortex. We have used intracellular recording and biocytin-labeling techniques in the entorhinal slice preparation from macaque monkeys to investigate the morphology and intrinsic electrophysiology of principal neurons. These neurons have previously been studied most extensively in rats. In monkeys, layer II neurons are usually stellate cells, as in rats, but they occasionally have a pyramidal shape. They tend to discharge trains, not bursts, of action potentials, and some display subthreshold membrane potential oscillations. Layer III neurons are pyramidal, and they do not appear to display membrane potential oscillations. The distribution of dendrites and of axon collaterals suggests that neurons in layers II and III are interconnected by a network of associational fibers. Layer V and VI neurons are pyramidal and tend to discharge trains of action potentials. The distribution of dendrites and axon collaterals suggests that there is an associative network of principal neurons in layers V and VI, and they also project axon collaterals toward superficial layers. Importantly, entorhinal cortical neurons in monkeys appear to exhibit significant differences from those in rats. Morphologically, neurons in monkey entorhinal layers II and III have more primary dendrites, more dendritic branches, and greater total dendritic length than in rats. Electrophysiologically, layer II neurons in monkeys exhibit less sag, and subthreshold oscillations are less robust and slower. Some monkey layer III neurons discharge bursts of action potentials that are not found in rats. The interspecies differences revealed by this study may influence information processing and pathophysiological processes in the primate entorhinal cortex. J. Comp. Neurol. 470:317-329, 2004.

Overwhelmingly asynchronous firing of rat subthalamic nucleus neurones in brain slices provides little evidence for intrinsic interconnectivity

In Parkinson's disease the neurones of the subthalamic nucleus show increased synchrony and oscillatory burst discharge, thought to reflect a breakdown of parallel processing in basal ganglia circuitry. To understand better the mechanisms underlying this transition, we sought to mimic this change in firing pattern within sagittal slices of rat midbrain. The firing patterns of up to four simultaneously extracellularly recorded subthalamic nucleus (STN) neurones were analysed using burst and oscillation detection programs, and correlated activity between pairs of neurones assessed. In control conditions all but 11 of 488 (2%) neurones fired in a predominantly tonic pattern (with mean oscillation frequency >3 Hz), with no significantly cross-correlated activity in any of 393 pairs of neurones. The glutamate antagonists DL-2-amino-phosphonopentanoic acid (APV), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-methyl-2-(phenylethynyl)pyridine (MPEP) did not change the firing rate or pattern of these cells, providing no evidence for a role of glutamatergic collaterals within the STN under these conditions. The GABA(A) receptor antagonist bicuculline and GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl]phenylmethyl phosphinic acid (CGP 55845) were also without effect on firing rate or pattern in these cells, suggesting that there was no active input from other GABAergic basal ganglia nuclei in this slice. The dopamine receptor antagonist haloperidol caused no significant change to firing rate or pattern of firing in these cells, suggesting that there was no active dopaminergic input in this slice. Excitations of STN neurones by muscarine, (+)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD), N-methyl-D-aspartic acid (NMDA) or dopamine were all unaccompanied by a change in firing pattern or any significant correlated activity between STN neurone pairs. Burst firing could be induced in STN neurones with either the potassium channel blocker tetraethylammonium (TEA; 10 mM; in 100/138 [72%] of cells) or with a combination of NMDA and the calcium-activated potassium channel blocker apamin (in 101/216 [47%] of cells). Burst firing in TEA was unchanged by CNOX and APV, MPEP, CGP55845, haloperidol, dopamine, and ACPD, although muscarine produced a significant increase in oscillation frequency. Burst firing in NMDA and apamin was unchanged by CNQX and APV, dopamine, muscarine and ACPD, although bicuculline caused a significant increase in oscillation frequency. Such burst firing was not accompanied by synchrony in any condition, either alone, or during application of excitatory agents or glutamate or GABA antagonists. As the bursting seen here was unaccompanied by the synchronous activity that has often been observed (pathologically) in vivo, it probably reflects solely intrinsic STN neuronal properties, rather than network activity. No functional role was found for glutamatergic collaterals within the STN, either when cells are firing tonically or burst firing. The circuitry needed to produce synchrony in the STN is most likely not intrinsic to the STN itself, but requires connections with other basal ganglia nuclei, and/or the cortex, which are not present in this preparation.

Ca2+-independent muscarinic excitation of rat medial entorhinal cortex layer V neurons

Cholinergic activation of entorhinal cortex (EC) layer V neurons plays a crucial role in the medial temporal lobe memory system and in the pathophysiology of temporal lobe epilepsy. Here, we demonstrate that muscarinic activation by focal application of carbachol depolarizes EC layer V neurons and induces epileptiform activity in rat brain slices. These seizure-like bursts are associated with a somatic [Ca2+]i increase of 293 +/- 82 nm and are blocked by the glutamate receptor antagonists CNQX and APV. Muscarinic activation did not directly evoke a [Ca2+]i increase, but subthreshold and suprathreshold depolarization did. Functional axon mapping revealed local axon branching as well as axon collaterals ascending to layers II and III. During blockade of ionotropic glutamatergic AMPA and NMDA receptors, carbachol depolarized layer V neurons by +7.5 +/- 3.4 mV. This direct muscarinic depolarization was associated with a conductance increase of 35 +/- 10.3% (+4.3 +/- 1.25 nS). Intracellular buffering of [Ca2+]i changes did not block this depolarization, but prolonged action potential duration and reduced adaptation of action potential firing. The muscarinic depolarization was neither blocked by combining intracellular Ca2+-buffering (EGTA or BAPTA) with non-specific Ca2+-channel inhibition by Ni+ (1 mm), nor by Ba2+ (1 mm) nor during inhibition of the h-current by 2 mm Cs+. In whole-cell patch-clamp recording, reversal of the muscarinic current occurred at about -45 mV and -5 mV with complete substitution of intrapipette K+ with Cs+. Thus, muscarinic depolarization of EC layer V neurons appears to be primarily mediated by Ca2+-independent activation of non-specific cation channels that conduct K+ about three times as well as Na+.

Electrophysiological characterization of interlaminar entorhinal connections: an essential link for re-entrance in the hippocampal-entorhinal system

The hippocampal formation communicates with the neocortex mainly through the adjacent entorhinal cortex. Neurons projecting to the hippocampal formation are found in the superficial layers of the entorhinal cortex and are largely segregated from the neurons receiving hippocampal output, which are located in deep entorhinal layers. We studied the communication between deep and superficial entorhinal layers in the anaesthetized rat using field potential recordings, current source density analysis and single unit measurements. We found that subiculum stimulation was able to excite entorhinal neurons in deep layers. This response was followed by current sinks in superficial layers. Both responses were subject to frequency dependent facilitation, but not depression. Selective blockade of deep layer responses also abolished subsequent superficial layer responses. This clearly demonstrates a functional deep-to-superficial layer communication in the entorhinal cortex, which can be triggered by hippocampal output. This pathway may provide a means by which processed hippocampal output is integrated or compared with new incoming information in superficial entorhinal layers, and it constitutes an important link in the process of re-entrance of activity in the hippocampal-entorhinal network, which may be important for consolidation of memories or retaining information for short periods.

Dopamine depresses cholinergic oscillatory network activity in rat hippocampus

The dopaminergic neuronal system is implicated in cognitive processes in a variety of brain regions including the mesolimbic system. We have investigated whether dopamine also affects synchronized network activity in the hippocampus, which has been ascribed to play a pivotal role in memory formation. Gamma frequency (20-80 Hz) oscillations were induced by the cholinergic agonist carbachol. Oscillatory activity was examined in area CA3 of Wistar rat hippocampal slices, employing field potential and intracellular recordings. Application of carbachol initiated synchronized population activity in the gamma band at 40 Hz. Induced gamma activity persisted over hours and required GABAA receptors. Dopamine reversibly decreased the integrated gamma band power of the carbachol rhythm by 62%, while its frequency was not changed. By contrast, individual pyramidal cells recorded during carbachol-induced field gamma activity exhibited theta frequency (5-15 Hz) membrane potential oscillations that were not altered by dopamine. The dopamine effect on the field gamma activity was mimicked by the D1 receptor agonist SKF-383393 and partially antagonized by the D1 antagonist SCH-23390. Conversely, the D2 receptor agonist quinpirole failed to depress the oscillations, and the D2 antagonist sulpiride did not prevent the suppressive dopamine effect. The data indicate that dopamine strongly depresses cholinergic gamma oscillations in area CA3 of rat hippocampus by activation of D1-like dopamine receptors and that this effect is most likely mediated via impairment of interneurons involved in generation and maintenance of the carbachol-induced network rhythm.

Gamma oscillations induced by kainate receptor activation in the entorhinal cortex in vitro

Gamma frequency (30-80 Hz) oscillations are recordable from human and rodent entorhinal cortex. A number of mechanisms used by neuronal networks to generate such oscillations in the hippocampus have been characterized. However, it is as yet unclear as to whether these mechanisms apply to other anatomically disparate brain regions. Here we show that the medial entorhinal cortex (mEC) in isolation in vitro generates gamma frequency oscillations in response to kainate receptor agonists. Oscillations had the same horizontal and laminar spatiotemporal distribution as seen in vivo and in the isolated whole-brain preparation. Oscillations occurred in the absence of input from the hippocampal formation and did not spread to lateral entorhinal regions. Pharmacological similarities existed between oscillations in the hippocampus and mEC in that the latter were also sensitive to GABAA receptor blockade, barbiturates, AMPA receptor blockade, and reduction in gap junctional conductance. Stellate and pyramidal neuron recordings revealed a large GABAergic input consisting of gamma frequency IPSP trains. Fast spiking interneurons in the superficial mEC generated action potentials at gamma frequencies phase locked to the local field. Stellate cells also demonstrated a subthreshold membrane potential oscillation at theta frequencies that was temporally correlated with a theta-frequency modulation in field gamma power. Disruption in this stellate theta frequency oscillation by the hyperpolarisation activated current (Ih) blocker ZD7288 also disrupted theta modulation of field gamma frequency oscillations. We propose that similar cellular and network mechanisms to those seen in the hippocampus generate and modulate persistent gamma oscillations in the entorhinal cortex.

Reduced inhibition and increased output of layer II neurons in the medial entorhinal cortex in a model of temporal lobe epilepsy

Temporal lobe epilepsy is the most common type of epilepsy in adults, and its underlying mechanisms are unclear. To investigate how the medial entorhinal cortex might contribute to temporal lobe epilepsy, we evaluated the histology and electrophysiology of slices from rats 3-7 d after an epileptogenic injury (pilocarpine-induced status epilepticus). Nissl staining, NeuN immunocytochemistry, and in situ hybridization for GAD65 mRNA were used to verify the preferential loss of glutamatergic neurons and the relative sparing of GABAergic interneurons in layer III. From slices adjacent to those that were used for anatomy, we obtained whole-cell patch recordings from layer II medial entorhinal cortical neurons. Recordings under current-clamp conditions revealed similar intrinsic electrophysiological properties (resting membrane potential, input resistance, single spike, and repetitive firing properties) to those of controls. Spontaneous IPSCs were less frequent (68% of controls), smaller in amplitude (57%), and transferred less charge (51%) than in controls. However, the frequency, amplitude, and rise time of miniature IPSCs were normal. These findings suggest that after epileptogenic injuries the layer II entorhinal cortical neurons receive less GABA(A) receptor-mediated synaptic input because presynaptic inhibitory interneurons become less active. To investigate the possible consequences of reduced spontaneous inhibitory input to layer II neurons, we recorded field potentials in the dentate gyrus, their major synaptic target. At 5 d after pilocarpine-induced status epilepticus the spontaneous field potentials recorded in vivo were over three times more frequent than in controls. These findings suggest that an epileptogenic injury reduces inhibition of layer II neurons and results in excessive synaptic input to the dentate gyrus.