Serotonin blocks different patterns of low Mg2+-induced epileptiform activity in rat entorhinal cortex, but not hippocampus

Dissecting the role of subiculum in epilepsy: Research update and translational potential

The subiculum serves as the strategic core output of the hippocampus, through which neural activity exits the hippocampal proper and targets the entorhinal cortex and other more distant subcortical and cortical areas. The past decade has witnessed a growing interest in the subiculum, owing to discoveries revealing its critical role in regulating many physiological and pathophysiological processes. Notably, accumulating evidence from both clinical and experimental studies suggests that the subiculum plays a vital role in seizure initiation and propagation, in epilepsy. In this review, we briefly describe the structure and connectivity of the subiculum and then summarize the molecular and cellular mechanisms in the subiculum underlying the epileptic brain, in both epilepsy patients and animal models. Next, we review some translational approaches targeting the malfunctioned subiculum to treat epilepsy. Finally, we pose open questions for future research in the subiculum and their clinical translation challenges.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Intra- and Interhemispheric Propagation of Electrophysiological Synchronous Activity and Its Modulation by Serotonin in the Cingulate Cortex of Juvenile Mice

Disinhibition of the cortex (e.g., by GABA -receptor blockade) generates synchronous and oscillatory electrophysiological activity that propagates along the cortex. We have studied, in brain slices of the cingulate cortex of mice (postnatal age 14–20 days), the propagation along layer 2/3 as well as the interhemispheric propagation through the corpus callosum of synchronous discharges recorded extracellularly and evoked in the presence of 10 μM bicuculline by electrical stimulation of layer 1. The latency of the responses obtained at the same distance from the stimulus electrode was longer in anterior cingulate cortex (ACC: 39.53 ± 2.83 ms, n = 7) than in retrosplenial cortex slices (RSC: 21.99 ± 2.75 ms, n = 5; p<0.05), which is equivalent to a lower propagation velocity in the dorso-ventral direction in ACC than in RSC slices (43.0 mm/s vs 72.9 mm/s). We studied the modulation of this propagation by serotonin. Serotonin significantly increased the latency of the intracortical synchronous discharges (18.9% in the ipsilateral hemisphere and 40.2% in the contralateral hemisphere), and also increased the interhemispheric propagation time by 86.4%. These actions of serotonin were mimicked by the activation of either 5-HT_1B or 5-HT_2A receptors, but not by the activation of the 5-HT_1A subtype. These findings provide further knowledge about the propagation of synchronic electrical activity in the cerebral cortex, including its modulation by serotonin, and suggest the presence of deep differences between the ACC and RSC in the structure of the local cortical microcircuits underlying the propagation of synchronous discharges.

Abnormal cell-intrinsic and network excitability in the neocortex of serotonin-deficient Pet-1 knockout mice

Neurons originating from the raphe nuclei of the brain stem are the exclusive source of serotonin (5-HT) to the cortex. Their serotonergic phenotype is specified by the transcriptional regulator Pet-1, which is also necessary for maintaining their neurotransmitter identity across development. Transgenic mice in which Pet-1 is genetically ablated (Pet-1(-/-)) show a dramatic reduction (∼80%) in forebrain 5-HT levels, yet no investigations have been carried out to assess the impact of such severe 5-HT depletion on the function of target cortical neurons. Using whole cell patch-clamp methods, two-dimensional (2D) multielectrode arrays (MEAs), 3D morphological neuronal reconstructions, and animal behavior, we investigated the impact of 5-HT depletion on cortical cell-intrinsic and network excitability. We found significant changes in several parameters of cell-intrinsic excitability in cortical pyramidal cells (PCs) as well as an increase in spontaneous synaptic excitation through 5-HT3 receptors. These changes are associated with increased local network excitability and oscillatory activity in a 5-HT2 receptor-dependent manner, consistent with previously reported hypersensitivity of cortical 5-HT2 receptors. PC morphology was also altered, with a significant reduction in dendritic complexity that may possibly act as a compensatory mechanism for increased excitability. Consistent with this interpretation, when we carried out experiments with convulsant-induced seizures to asses cortical excitability in vivo, we observed no significant differences in seizure parameters between wild-type and Pet-1(-/-) mice. Moreover, MEA recordings of propagating field potentials showed diminished propagation of activity across the cortical sheath. Together these findings reveal novel functional changes in neuronal and cortical excitability in mice lacking Pet-1.

Epileptiform activity in the hippocampus of mice with different predisposition to pinch catalepsy

Experiments on hippocampal slices of CBA mice showed that the probability of spontaneous epileptiform discharges recorded in the field CA1 pyramidal layer is higher in animals predisposed to catalepsy compared to mice with low predisposition to catalepsy. Presumably, some factors determining predisposition to catalepsy modulate synchronization of neuronal activity in the hippocampus; this suggests using hippocampal slices as a model for studies of neurophysiological mechanisms of hereditary predisposition to catalepsy.

Serotonin increases GABA release in rat entorhinal cortex by inhibiting interneuron TASK-3 K+ channels

Whereas the entorhinal cortex (EC) receives profuse serotonergic innervations from the raphe nuclei in the brain stem and is critically involved in the generation of temporal lobe epilepsy, the function of serotonin (5-hydroxytryptamine, 5-HT) in the EC and particularly its roles in temporal lobe epilepsy are still elusive. Here we explored the cellular and molecular mechanisms underlying 5-HT-mediated facilitation of GABAergic transmission and depression of epileptic activity in the superficial layers of the EC. Application of 5-HT increased sIPSC frequency and amplitude recorded from the principal neurons in the EC with no effects on mIPSCs recorded in the presence of TTX. However, 5-HT reduced the amplitude of IPSCs evoked by extracellular field stimulation and in synaptically connected interneuron and pyramidal neuron pairs. Application of 5-HT generated membrane depolarization and increased action potential firing frequency but reduced the amplitude of action potentials in presynaptic interneurons suggesting that 5-HT still increases GABA release whereas the depressant effects of 5-HT on evoked IPSCs could be explained by 5-HT-induced reduction in action potential amplitude. The depolarizing effect of 5-HT was mediated by inhibition of TASK-3 K(+) channels in interneurons and required the functions of 5-HT(2A) receptors and Galpha(q/11) but was independent of phospholipase C activity. Application of 5-HT inhibited low-Mg(2+)-induced seizure activity in slices via 5-HT(1A) and 5-HT(2A) receptors suggesting that 5-HT-mediated depression of neuronal excitability and increase in GABA release contribute to its anti-epileptic effects in the EC.

Serotonin Inhibits Neuronal Excitability by Activating Two-Pore Domain K+ Channels in the Entorhinal Cortex

The entorhinal cortex (EC) is regarded as the gateway to the hippocampus; the superficial layers (layers I-III) of the EC convey the cortical input projections to the hippocampus, whereas deep layers of the EC relay hippocampal output projections back to the superficial layers of the EC or to other cortical regions. The superficial layers of the EC receive strong serotonergic projections from the raphe nuclei. However, the function of serotonin in the EC is still elusive. In the present study, we examined the molecular and cellular mechanisms underlying serotonin-mediated inhibition of the neuronal excitability in the superficial layers (layers II and III) of the EC. Application of serotonin inhibited the excitability of stellate and pyramidal neurons in the superficial layers of the EC by activating the TWIK-1 type of the two-pore domain K(+) channels. The effects of 5-HT were mediated via 5-HT(1A) receptors and required the function of Galpha(i3) subunit and protein kinase A. Serotonin-mediated inhibition of EC activity resulted in an inhibition of hippocampal function. Our study provides a cellular mechanism that might at least partially explain the roles of serotonin in many physiological functions and neurological diseases.

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.

The subiculum: a potential site of ictogenesis in human temporal lobe epilepsy

PURPOSE

This study determines synaptic and intrinsic alterations of subicular pyramidal cells that are associated with activity recorded in patients suffering from temporal lobe epilepsy.

METHODS

Electroencephalograms with sphenoidal electrodes were correlated with in vitro single cell recordings of subicular pyramidal cells from the corresponding resected epileptic tissue. We determined alterations of synaptic and intrinsic properties of subicular pyramidal cells that accompany spontaneous rhythmic activity in human sclerotic and nonsclerotic epileptic tissue.

RESULTS

We found that in sclerotic, but also in nonsclerotic hippocampal tissue, the subiculum showed cellular and synaptic changes that were associated with spontaneous rhythmic activity correlated to the occurrence and frequency of interictal discharges recorded in the electroencephalograms of the corresponding patients.

CONCLUSIONS

Even though Ammon's horn sclerosis (AHS) in resected hippocampi from patients suffering from temporal lobe epilepsy has important prognostic implications for freedom from seizures postoperatively, we report here that both synaptic and intrinsic alterations enhance seizure susceptibility of the subiculum also in the absence of classical AHS.

Adenosine acting via A1 receptors, controls the transition to status epilepticus-like behaviour in an in vitro model of epilepsy

Adenosine has powerful inhibitory effects in the central nervous system. In this study, we aim to understand how adenosine controls the progression of seizure-like events (SLEs) in a seizure-prone region of the brain, the entorhinal cortex. We chose to use a low Mg(2+) model of epilepsy in an in vitro slice preparation where, in the entorhinal cortex, SLEs progress into a type of epileptiform activity called late recurrent discharges (LRDs) that bear resemblance to status epilepticus. Adenosine, acting via its A1 receptor, exerted powerful inhibitory effects to prevent the spontaneous progression to LRDs while the potent A1 receptor antagonist, DPCPX, accelerated the progression in a concentration dependent manner. The spontaneous progression from SLEs to LRDs was associated with a decline in total cellular ATP levels and studies with metabolic inhibitors indicated a key role for the production of endogenous adenosine from ATP. We therefore hypothesise that when ATP becomes rate limiting, extracellular adenosine levels fall, the normal inhibitory brake is removed and the progression from SLEs to LRDs or status epilepticus-like activity can ensue. Moreover, under these conditions, inhibition of the adenine nucleotide salvage pathways reversed the status epilepticus-like activity. Our findings suggest a powerful role for adenosine for the control of the progression to status epilepticus-like activity in an epilepsy model that is refractory to most anti-epileptic drugs. On this basis, manipulation of adenine nucleotide metabolism may represent a potential therapeutic approach for the treatment of status epilepticus.

Does Serotonin Play a Role in Epilepsy?

Studies in experimental models have suggested a potential role for serotonergic transmission in epilepsy, and interest in this research has been increased by the development of positron emission tomography (PET) ligands that can be used to study 5-hydroxytryptamine (5-HT) receptors and transporters. The serotonergic system is very complex. At least 13 distinct G protein-coupled 5-HT receptors and one ligand-gated ion channel receptor (5-HT(3)) are divided into seven distinct classes (5-HT(1) to 5-HT(7)) ((1)). The receptors vary widely in their distribution and effects, innervating vascular structures and gut smooth muscle as well as neuronal tissue. Several receptor subtypes may be relevant to epilepsy.

Seizure, neurotransmitter release, and gene expression are closely related in the striatum of 4-aminopyridine-treated rats

The present experiments aimed to compare the length of seizure activity with the time-related increase of transmitter release and the induction of c-fos gene expression in the striatum of the rat. Anesthetized Wistar rats were intraperitoneally treated with 7 mg/kg 4-aminopyridine, and the transmitter levels in the striatum were measured by means of in vivo microdialysis, 30, 60, 90, 120, and 150 min following the treatment. Striatal and neocortical electric activity was monitored with depth and surface electrodes, respectively. The expression level of the c-fos gene was estimated by counting the striatal c-fos-immunostained cell nuclei at the time intervals of the microdialysis. 4-aminopyridine elicited high-frequency seizure discharges in the EEG and significantly increased glutamate, aspartate, GABA, serotonin, noradrenaline, and dopamine levels in the extracellular dialysates. The number of c-fos-stained cell nuclei in the striatum displayed a prolonged increase, showing significantly elevated numbers throughout the experiment. The increase of c-fos expression in time correlated best with the increase of glutamate release, which was also significantly elevated at every sampling time. The GABA release, culminating at 60 min after the seizure onset, correlated best with the cessation of the electrographic seizure. Aspartate, norepinephrine, serotonin, and dopamine displayed transient but significant elevations. We conclude that glutamate plays the essential role (most probably through ionotropic and metabotropic receptors) in the extracellular signaling, which eventually leads to intracellular cascades and c-fos gene expression in the striatum during convulsions.

Free radical-mediated cell damage after experimental status epilepticus in hippocampal slice cultures

Generation of free radicals may have a key role in the nerve cell damage induced by prolonged or frequently recurring convulsions (status epilepticus). Mitochondrial function may also be altered due to production of free radicals during seizures. We therefore studied changes in field potentials (fp) together with measurements of extracellular, intracellular, and intramitochondrial calcium concentration ([Ca(2+)]e, [Ca(2+)]i, and [Ca(2+)]m, respectively), mitochondrial membrane potential (deltapsi), NAD(P)H auto-fluorescence, and dihydroethidium (HEt) fluorescence in hippocampal slice cultures by means of simultaneous electrophysiological and microfluorimetric measurements. As reported previously, each seizure-like event (SLE) resulted in mitochondrial depolarization associated with a delayed rise in oxidation of HEt to ethidum, presumably indicating ROS production. We show here that repeated SLEs led to a decline in intracellular and intramitochondrial Ca(2+) signals despite unaltered Ca(2+) influx. Also, mitochondrial depolarization and the NAD(P)H signal became smaller during recurring SLEs. By contrast, the ethidium fluorescence rises remained constant or even increased from SLE to SLE. After about 15 SLEs, activity changed to continuous afterdischarges with steady depolarization of mitochondrial membranes. Staining with a cell death marker, propidium iodide, indicated widespread cell damage after 2 h of recurring SLEs. The free radical scavenger, alpha-tocopherol, protected the slice cultures against this damage and also reduced the ongoing impairment of NAD(P)H production. These findings suggest involvement of reactive oxygen species (ROS) of mitochondrial origin in the epileptic cell damage and that free radical scavenging may prevent status epilepticus-induced cell loss.

Far field effects of seizure like events induced by application of 4-AP in combined entorhinal cortex hippocampal slices

Epileptiform activity induced by 4-AP in hippocampal area CA1 is characterised by short recurrent discharges. These are occasionally superimposed by slow field potential (fp) shifts. Simultaneous recordings of fps and [K(+)](o) in area CA1 and temporal cortex showed a slow fp shift in both regions, but associated rises in [K(+)](o) occurred only in the cortex. Slow fps in area CA1 persisted after disruption of the perforant path, but were abolished after removal of the adjacent cortex from the hippocampus. These findings suggest that slow fps in CA1 can represent far field effects of seizure like events generated in neighbouring cortex.

Nitric oxide modulates low-Mg2+-induced epileptiform activity in rat hippocampal-entorhinal cortex slices

The production of nitric oxide (NO) during low-Mg2+-induced epileptiform activity in rat hippocampal-entorhinal cortex slices was investigated by real-time monitoring using 1,2-diaminoanthraquinone (DAQ). NO reacts with the aromatic amino groups of DAQ at neutral pH and in the presence of oxygen to form the fluorescence product 1H-anthra-[1,2d]-[1,2,3]triazole-6,11-dione (ATD). The DAQ-induced formation of ATD required NO and was insensitive to radical oxygen species. Removal of Mg2+ ions from the artificial cerebrospinal fluid (ACSF) induced a significant elevation in the ATD fluorescence signal. The application of L-arginine (2 mM), a substrate of nitric oxide synthase (NOS), caused a comparable increase in the ATD fluorescence signal. Furthermore, ATD signal increase induced either by low-Mg2+ ACSF or by L-arginine was sensitive to N-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor. The application of L-NAME (200 microM) caused a complete blockade of low-Mg2+-induced epileptiform activity. Under this condition, increasing NO concentration by addition of the NO donor S-nitroso-N-acetylpenicillamine (200 microM) reinduced the epileptiform activity. It has been concluded that onset and maintenance of low-Mg2+-induced spontaneous epileptiform activity are modulated by NO concentration. Further NO imaging studies may help to elucidate the role of NO in detail and may bring to light new means for epilepsy therapy.

Neuropeptide Y Y5 receptor protein in the cortical/limbic system and brainstem of the rat: expression on gamma-aminobutyric acid and corticotropin-releasing hormone neurons

Neuropeptide Y displays diverse modes of action in the CNS including the modulation of cortical/limbic function. Some of these physiological actions have been at least partially attributed to actions of neuropeptide Y on the Y5 receptor subtype. We utilized an antibody raised against the Y5 receptor to characterize the distribution of this receptor subtype in the rat cortical/limbic system and brainstem. Y5-like immunoreactivity was located primarily in neuronal cell bodies and proximal dendritic processes throughout the brain. In the cortex, Y5 immunoreactivity was limited to a subpopulation of small gamma-aminobutyric-acid interneurons (approximately 15 microm diameter) scattered throughout all cortical levels. Double label immunofluorescence was also used to demonstrate that all of the Y5 immunoreactive neurons in the cortex displayed intense corticotropin releasing hormone immunoreactivity. The most intense Y5 immunoreactive staining in the hippocampus was located in the pyramidal cell layer of the small CA2 subregion and the fasciola cinerea, with lower levels of staining in the hilar region of the dentate gyrus and CA3 subregion of the pyramidal cell layer. Nearly all of the Y5 immunoreactive neurons in the hilar region of the hippocampus displayed gamma-aminobutyric-acid immunoreactivity. In the brainstem, Y5 immunoreactivity was most intense in the Edinger-Westphal nucleus, locus coeruleus and the mesencephalic trigeminal nucleus. The present study provides neuroanatomical evidence for the possible sites of action of the neuropeptide Y/Y5 receptor system in the control of cortical/limbic function. The presence of Y5 immunoreactivity on cell bodies and proximal dendritic processes in specific regions of the hippocampus suggests that this receptor functions to modulate postsynaptic activity. These data also suggest that the neuropeptide Y/Y5 system may play a role in the modulation of a specific population of GABAergic neurons in the cortex, namely those that contain corticotropin-releasing hormone. The location of the Y5 receptor immunoreactivity fits with the known physiological actions of neuropeptide Y and this receptor.

Fenfluramine blocks low-Mg2+-induced epileptiform activity in rat entorhinal cortex

PURPOSE

The entorhinal cortex (EC) represents the main input structure to the hippocampus and seems to be critically involved in temporal lobe epilepsy. Considering that the EC receives a strong serotonergic projection from the raphe nuclei and expresses a high density of serotonin (5-HT) receptors, the effect of the 5-HT-releasing drug fenfluramine (FFA) on epileptiform activity generated in the EC was investigated in an in vitro model of epilepsy.

METHODS

The experiments were performed on 43 horizontal slices containing the EC, the subiculum, and the hippocampal formation obtained from 230-250 g adult Wistar rats. Using extracellular recording techniques, we investigated the effect of bath-applied FFA (200 micromol/L to 1 mmol/L) on epileptiform activity induced by omitting MgSO4 from the artificial cerebrospinal fluid.

RESULTS

We demonstrate that FFA reversibly blocks epileptiform activity in the EC. Surprisingly, in the presence of the 5-HT uptake blocker paroxetine, the FFA-induced effect was diminished. Coapplication of the 5-HT1A receptor antagonist WAY100635 prevented the FFA-induced anticonvulsive effect, suggesting that (a) the FFA-induced suppression of epileptiform activity is mediated by the release of 5-HT from synaptic terminals within the EC rather than by an unspecific effect of FFA and (b) released 5-HT most likely blocks the activity by activation of 5-HT1A receptors.

CONCLUSION

FFA, which is primarily used because of its anorectic activity, might get an additional therapeutic value in the treatment of temporal lobe epilepsy with parahippocampal involvement.

Kindling induces a transient suppression of afterhyperpolarization in rat subicular neurons

To determine whether chronic epilepsy induces persistent cellular changes in subicular neurons intracellular recordings were used to compare membrane properties of control and kindled rats. In both, control and kindled preparations the subiculum contained regular firing cells and an extensive sub-population of bursting cells expressing amplifying membrane characteristics. Subicular cells showed a transient depression of the fast and slow AHP in the course of kindling that may contribute to the induction but not permanence of the kindled state.

Properties of entorhinal cortex projection cells to the hippocampal formation

There are multiple connections from the entorhinal cortex (EC) to the hippocampus that carry the information from the EC to the hippocampus. Layer II cells of the medial EC innervating the dentate gyrus (DG)-molecular layer possess K(+)-outward currents and inward rectifier currents that are potentially modulated by changes in intracellular second messengers. Layer II cells responded to synaptic stimulation with a rather flat input-output curve, and much stronger stimuli are required to generate action potentials in these neurons than in EC layer III cells. During repetitive stimulation at frequencies of 10 Hz and more, EC layer II cells respond with increased likelihood to generate action potentials. Two different NMDA conductances can be demonstrated in these neurons. A slow, less Mg, less voltage-dependent component is responsible for the transient depolarization between the fast and slow IPSP. A second group of neurons also projects to the DG. These are either pyramidal or nonpyramidal cells in the deep layers of the EC. At least part of these neurons also possess rhythmogenic properties. In contrast to layer II cells, layer III neurons have a steep input-output curve and show during repetitive synaptic activation a tendency to repolarize and to display long-lasting inhibitions dependent on GABAB-, atropine-, and naloxone-sensitive components. As a consequence, they are readily activated during low frequency stimulation, but project only a few action potentials to area CA1 initially during higher (more than 10 Hz) frequency synaptic stimulation.

Potent depression of stimulus evoked field potential responses in the medial entorhinal cortex by serotonin

1. The entorhinal cortex (EC), main input structure to the hippocampus, gets innervated by serotonergic terminals from the raphe nuclei and expresses 5-HT-receptors at high density. Using extra- and intracellular recording techniques we here investigated the effects of serotonin on population and cellular responses within the EC. 2. Stimulation in the lateral entorhinal cortex resulted in complex field potential responses in the superficial EC. The potentials are composed of an early antidromic and a late orthodromic component reflecting the efferent and afferent circuitry. 3. Serotonin (5-HT) reduced synaptic potentials of the stimulus evoked extracellular field potential at all concentrations tested (0. 1 - 100 microM; 59%-depression by 10 microM serotonin), while the antidromic response was not significantly changed by up to 50 microM 5-HT. Depression of field potential responses by serotonin was associated with a significant increase in paired-pulse facilitation from 1.15 to 1.88. 4. The effects of serotonin on field potential responses were mimicked by 5-HT1A-receptor agonists (8-OH-DPAT, 5-CT) and partially prevented by the 5-HT1A-receptor antagonist (S-UH-301). Moreover, the 5-HT1A-receptor antagonist WAY100635 reduced the effect of 5-CT. 5. Fenfluramine, a serotonin releaser, mimics the effects of serotonin on stimulus-evoked field potential responses, indicating that synaptically released serotonin can produce the changes in reactivity to afferent stimulation. 6. Depression of isolated AMPA-receptor mediated EPSCs by serotonin as well as fenfluramine was associated with an increase in paired pulse facilitation, indicating a presynaptic locus of action. 7. We conclude that physiological concentrations of serotonin potently suppresses excitatory synaptic transmission in the superficial entorhinal cortex by a presynaptic mechanism.

A relative energy failure is associated with low-Mg2+ but not with 4-aminopyridine induced seizure-like events in entorhinal cortex

During seizure-like events (SLEs), intracellular Ca2+ concentration ([Ca2+]i) increases causing depolarization of the mitochondrial membrane and subsequent intramitochondrial accumulation of Ca2+. Mitochondrial depolarization results in an interruption of oxidative phosphorylation and increase in reactive oxygen species. Calcium activates enzymes of the citrate cycle. A characteristic feature of the low-Mg2+-induced SLEs is that they are transformed to a late activity refractory to anticonvulsant drugs, which may be regarded as a model system of difficult to treat status epilepticus. In contrast, 4-aminopyridine (4-AP)-induced activity rarely evolves to such late activity. The autofluorescence of NAD(P)H was used to monitor changes in cellular energy metabolism in the entorhinal cortex in two in vitro models of focal epilepsy. During repetitive 4-AP-induced SLEs there was a short decrease followed by a long-lasting overshoot of the NAD(P)H signal. This sequence remained unaltered during recurring SLEs. In contrast, during recurrent low-Mg2+-induced SLEs, the brief initial NADH signal reduction was unchanged but the following overshoot of NADH displayed a continuous decrease. This indicates a relative energy failure, which may contribute to the transformation to late activity in the low-Mg2+ model.

Phenytoin, phenobarbital, and midazolam fail to stop status epilepticus-like activity induced by low magnesium in rat entorhinal slices, but can prevent its development

OBJECTIVES

It was shown previously that low-Mg2+-induced epileptiform activity in rat entorhinal cortex slices changes with time from a pattern of serial seizure-like events (SLEs) to a state of continuously recurring epileptiform activity. Valproic acid blocked the early SLEs but not the late activity. It was proposed that the late activity is a model for pharmacoresistant status epilepticus since it was also refractory to phenytoin, carbamazepine, phenobarbital, and midazolam. In the present study, it is demonstrated that phenytoin (50 microM, n=6), phenobarbital (150 microM, n=7), and midazolam (50 microM, n=5) were able to block the early SLEs but not the late activity at the same concentrations. Carbamazepine (50 microM) reduced the duration of the SLEs from 21 +/-5 s to 4+/-3 s (P<0.01), the interictal interval from 123+/-27 s to 27+/-19 s (P<0.01), the SLE-associated rise of [K+]o from 7.7+/-0.5 mM to 5.7+/-0.8 mM (n=4, P<0.05), and the spread of the SLE between entorhinal cortex and neocortex from 4.0+/-0.6 s to 0.8+/-0.1 s (n=4, P<0.05). Lower concentrations of phenytoin (5 and 10 microM, n=5), carbamazepine (10 microM, n =6), and phenobarbital (50 microM, n = 4) had no effect. In conclusion, the hypothesis is supported that low-Mg2+-induced epileptiform activity in rat entorhinal cortex is an in vitro model for the transition from pharmacosensitive to pharmacoresistant status epilepticus.

Pattern and pharmacology of propagating epileptiform activity in mouse cerebral cortex

Multiple extracellular recording electrodes were used to study the intra- and interhemispheric spread of stimulus-evoked epileptiform responses in adult mouse neocortical slices. Bath application of 20 microM bicuculline methiodide induced epileptiform activity that propagated at approximately 0.08 m/s over several millimeters in rostro-caudal and medio-lateral direction within the ipsilateral hemisphere and across the corpus callosum to the contralateral hemisphere. A vertical incision from layer II to subcortical regions did not prevent the spread to remote cortical regions, indicating that layer I plays a major role in the lateral propagation of epileptiform activity. The intra- and interhemispheric spread was not influenced by application of an N-methyl-d-aspartate (NMDA) receptor antagonist, but blocked by an antagonist acting at the (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptor. The potential role of potassium channel activation in controlling the generation or spread of epileptiform activity was tested by applying the potassium channel opener cromakalim and the serotonin type 1A (5-HT1A) receptor agonist (+/-)-8-hydroxydipropylaminotetralin (8-OH-DPAT) to the disinhibited slices. Whereas cromakalim reduced the neuronal excitability and blocked all epileptiform responses, 8-OH-DAPT did not affect the activity pattern. Our results suggest that propagating epileptiform activity in disinhibited neocortical structures is predominantly mediated by activation of AMPA receptors and controllable by activation of a voltage-dependent potassium current.

Serotonin reduces polysynaptic inhibition via 5-HT1A receptors in the superficial entorhinal cortex

The superficial cells of the entorhinal cortex (EC), main input to the hippocampus, receive a serotonergic input from the raphe nuclei and express 5-hydroxytryptamine creatine sulfate complex (5-HT) receptors at high density. With the use of intracellular recordings, we investigated the effects of serotonin on synaptic inhibition of layer II and III neurons of the EC. Serotonin reduced both polysynaptic fast and slow inhibitory postsynaptic potentials (IPSPs) in projection neurons of the superficial EC. Polysynaptic fast and slow IPSPs were depressed by serotonin in a dose-dependent manner (0.1-100 microM). Serotonin in a concentration of 1 microM reduced the amplitudes of polysynaptic fast and slow IPSPs by approximately 40 and 50%, respectively. To identify the subtype of the 5-HT-receptor mediating the effects on polysynaptic IPSPs, we applied various 5-HT-receptor agonists and antagonists. Although the serotonin agonists for the 5-HT1B,2C,3 receptors were ineffective, the effects were mimicked by the 5-HT1A-receptor agonists (8-OH-DPAT, 5-CT) and prevented by the 5-HT1A-receptor antagonist NAN-190. To look at the direct effects of 5-HT on inhibitory interneurons, we elicited monosynaptic IPSPs in the absence of excitatory synaptic transmission. In contrast to the polysynaptic IPSPs, monosynaptic IPSPs were not significantly affected by serotonin. Recordings from putative inhibitory interneurons revealed that their excitatory postsynaptic potentials (EPSPs) were reversibly reduced by serotonin. We conclude that serotonin suppresses polysynaptic inhibition in projection neurons of layers II and III of the EC by depression of EPSPs on inhibitory interneurons via 5-HT1A receptors.

Mechanisms and experimental models of seizure generation

Over the past year evidence has accumulated against the idea that seizures require re-entrant activity between spatially separate structures. Seizures in vivo typically do involve interconnected, spatially separate brain regions, but they often show no net phase lag around the putative circuit. In many cases seizure-like events can arise from localized regions such as the entorhinal cortex or hippocampus proper, through mechanisms that are starting to be identified.

Comparison of the effects of serotonin in the hippocampus and the entorhinal cortex

Among the molecular, cellular, and systemic events that have been proposed to modulate the function of the hippocampus and the entorhinal cortex (EC), one of the most frequently cited possibilities is the activation of the serotonergic system. Neurons in the hippocampus and in the EC receive a strong serotonergic projection from the raphe nuclei and express serotonin (5-HT) receptors at high density. Here we review the various effects of 5-HT on intrinsic and synaptic properties of neurons in the hippocampus and the EC. Although similar membrane-potential changes following 5-HT application have been reported for neurons of the entorhinal cortex and the hippocampus, the effects of serotonin on synaptic transmission are contrary in both areas. Serotonin mainly depresses fast and slow inhibition of the principal output cells of the hippocampus, whereas it selectively suppresses the excitation in the entorhinal cortex. On the basis of these data, we discuss the possible role of serotonin under physiological and pathophysiological circumstances.