Differential effects of metabotropic glutamate receptor antagonists on bursting activity in the amygdala.

Differential effects of metabotropic glutamate receptor antagonists on bursting activity in the amygdala. Metabotropic glutamate receptors (mGluRs) are implicated in both the activation and inhibition of epileptiform bursting activity in seizure models. We examined the role of mGluR agonists and antagonists on bursting in vitro with whole cell recordings from neurons in the basolateral amygdala (BLA) of amygdala-kindled rats. The broad-spectrum mGluR agonist 1S,3R-1-aminocyclopentane dicarboxylate (1S,3R-ACPD, 100 microM) and the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 20 microM) evoked bursting in BLA neurons from amygdala-kindled rats but not in control neurons. Neither the group II agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG-I, 10 microM) nor the group III agonist L-2-amino-4-phosphonobutyrate (L-AP4, 100 microM) evoked bursting. The agonist-induced bursting was inhibited by the mGluR1 antagonists (+)-alpha-methyl-4-carboxyphenylglycine [(+)-MCPG, 500 microM] and (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG, 300 microM]. Kindling enhanced synaptic strength from the lateral amygdala (LA) to the BLA, resulting in synaptically driven bursts at low stimulus intensity. Bursting was abolished by (S)-4C3HPG. Further increasing stimulus intensity in the presence of (S)-4C3HPG (300 microM) evoked action potential firing similar to control neurons but did not induce epileptiform bursting. In kindled rats, the same threshold stimulation that evoked epileptiform bursting in the absence of drugs elicited excitatory postsynaptic potentials in (S)-4C3HPG. In contrast (+)-MCPG had no effect on afferent-evoked bursting in kindled neurons. Because (+)-MCPG is a mGluR2 antagonist, whereas (S)-4C3HPG is a mGluR2 agonist, the different effects of these compounds suggest that mGluR2 activation decreases excitability. Together these data suggest that group I mGluRs may facilitate and group II mGluRs may attenuate epileptiform bursting observed in kindled rats. The mixed agonist-antagonist (S)-4C3HPG restored synaptic transmission to control levels at the LA-BLA synapse in kindled animals. The different actions of (S)-4C3HPG and (+)-MCPG on LA-evoked bursting suggests that the mGluR1 antagonist-mGluR2 agonist properties may be the distinctive pharmacology necessary for future anticonvulsant compounds.     

Role of synaptic metabotropic glutamate receptors in epileptiform discharges in hippocampal slices

Application of group I metabotropic glutamate receptor (mGluR) agonists elicits seizure discharges in vivo and prolonged ictal-like activity in in vitro brain slices. In this study we examined 1) if group I mGluRs are activated by synaptically released glutamate during epileptiform discharges induced by convulsants in hippocampal slices and, if so, 2) whether the synaptically activated mGluRs contribute to the pattern of the epileptiform discharges. The GABA(A) receptor antagonist bicuculline (50 microM) was applied to induce short synchronized bursts of approximately 250 ms in mouse hippocampal slices. Addition of 4-aminopyridine (4-AP; 100 microM) prolonged these bursts to 0.7-2 s. The mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385; 25-100 microM) and the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10-50 microM), applied separately, significantly reduced the duration of the synchronized discharges. The effects of these antagonists were additive when applied together, suggesting that mGluR1 and mGluR5 exert independent actions on the epileptiform bursts. In phospholipase C beta1 (PLCbeta1) knockout mice, bicuculline and 4-AP elicited prolonged synchronized discharges of comparable duration as those observed in slices from wild-type littermates. Furthermore, mGluR1 and mGluR5 antagonists reduced the duration of the epileptiform discharges to the same extent as they did in the wild-type preparations. The results suggest that mGluR1 and mGluR5 are activated synaptically during prolonged epileptiform discharges induced by bicuculline and 4-AP. Synaptic activation of these receptors extended the duration of synchronized discharges. In addition, the data indicate that the synaptic effects of the group I mGluRs on the duration of epileptiform discharges were mediated by a PLCbeta1-independent mechanism.     

Status epilepticus induces time-dependent neuronal and astrocytic expression of interleukin-1 receptor type I in the rat limbic system

Interleukin-1beta is rapidly synthesized by glia after the induction of seizures. Recent evidence shows that endogenous IL-1beta has proconvulsant actions mediated by interleukin-1 receptor type I. This receptor also mediates interleukin-1beta effects on neuronal susceptibility to neurotoxic insults. In this study, we investigated the basal and seizure-induced expression of interleukin-1 receptor type I in rat forebrain to identify the cells targeted by interleukin-1beta during epileptic activity. Self-sustained limbic status epilepticus was induced in rats by electrical stimulation of the ventral hippocampus. Interleukin-1 receptor type I immunoreactivity was barely detectable in neurons in control brain tissue. During status epilepticus, interleukin-1 receptor type I was induced in the hippocampal neurons firstly, and several hours later in astrocytes localized in limbic and extralimbic areas. Neuronal interleukin-1 receptor type I expression in the hippocampus outlasted the duration of spontaneous electroencephalographic seizure and was not observed in degenerating neurons. Astrocytic expression occurred transiently, between six and 18 h after the induction of status epilepticus and was invariably found in regions of neuronal damage. These time-dependent, cell- and region-specific changes in interleukin-1 receptor type I expression during status epilepticus suggest that interleukin-1 receptor type I in neurons mediates interleukin-1beta-induced fast changes in hippocampal excitability while interleukin-1 receptor type I receptors in astrocytes may mediate interleukin-1beta effects on neuronal survival in hostile conditions.     

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.     

Involvement of non-NMDA receptors in picrotoxin-induced epileptiform activity in the hippocampus

The ability of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to suppress picrotoxin-induced epileptiform burst activity was examined. Intracellular recordings were obtained from hippocampal CA1 and CA3 pyramidal neurons maintained in vitro. Bath application of CNQX (5 microM) significantly reduced or abolished evoked paroxysmal depolarizing shifts (PDSs) in all CA1 and CA3 neurons tested. In cells where a CNQX-insensitive component in the PDS was manifest, this remaining activity was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovaleric acid (20 microM), suggesting the existence of a NMDA-mediated synaptic potential. Our results indicate that non-NMDA receptor antagonists are capable of markedly reducing picrotoxin-induced epileptiform activity and that these receptors play an important role in generation of PDSs.     

N-methyl-D-aspartate receptor activation regulates refractoriness of status epilepticus to diazepam.

Status epilepticus, prolonged intermittent or continuous seizure activity lasting 30 min or longer, is associated with high morbidity and mortality. The longer a seizure persists, the more refractory to treatment it becomes. The pilocarpine model of status epilepticus in rodents develops refractoriness to many first-line treatments as seizure duration increases, rendering it a good model to study refractory status epilepticus. This study was initiated to study the development of refractoriness of pilocarpine-induced status epilepticus to diazepam. Early pilocarpine-induced status epilepticus responded rapidly to diazepam treatment, whereas status epilepticus of longer duration became increasingly less responsive to treatment. Dizocilpine maleate-pretreated animals responded rapidly to diazepam treatment, even after 60 min of status epilepticus. Animals administered dizocilpine maleate at 15, 30 or 60 min after the onset of status epilepticus also demonstrated a rapid response to diazepam compared to pilocarpine-alone-treated animals. The longer the status epilepticus progressed prior to dizocilpine maleate injection, the longer the status epilepticus lasted after diazepam treatment. However, in all cases where dizocilpine maleate was administered, one injection of diazepam was able to terminate the status epilepticus, in contrast to the animals that did not receive dizocilpine maleate, in which the seizure was only attenuated. The results indicate that N-methyl-D-aspartate receptor activation plays a role in the seizure-induced refractoriness to benzodiazepines in status epilepticus, and blocking N-methyl-D-aspartate receptor activation converts refractory status epilepticus to a seizure responsive to benzodiazepine therapy. These findings offer insights into developing novel therapeutic interventions to improve the treatment of status epilepticus. Understanding the molecular mechanisms that mediate the effects of N-methyl-D-aspartate receptor activation on the development of resistance to treatment in status epilepticus will provide rational insights into more rapid methods to terminate seizure activity in this condition.     

Low-calcium epileptiform activity in the hippocampus in vivo

It has been clearly established that nonsynaptic interactions are sufficient for generating epileptiform activity in brain slices. However, it is not known whether this type of epilepsy model can be generated in vivo. In this paper we investigate low-calcium nonsynaptic epileptiform activity in an intact hippocampus. The calcium chelator EGTA was used to lower [Ca2+]o in the hippocampus of urethane anesthetized rats. Spontaneous and evoked field potentials in CA1 pyramidal stratum and in CA1 stratum radiatum were recorded using four-channel silicon recording probes. Three different types of epileptic activity were observed while synaptic transmission was gradually blocked by a decline in hippocampal [Ca2+]o. A short latency burst, named early-burst, occurred during the early period of EGTA application. Periodic slow-waves and a long latency high-frequency burst, named late-burst, were seen after synaptic transmission was mostly blocked. Therefore these activities appear to be associated with nonsynaptic mechanisms. Moreover, the slow-waves were similar in appearance to the depolarization potential shifts in vitro with low calcium. In addition, excitatory postsynaptic amino acid antagonists could not eliminate the development of slow-waves and late-bursts. The slow-waves and late-bursts were morphologically similar to electrographic seizure activity seen in patients with temporal lobe epilepsy. These results clearly show that epileptic activity can be generated in vivo in the absence of synaptic transmission. This type of low-calcium nonsynaptic epilepsy model in an intact hippocampus could play an important role in revealing additional mechanisms of epilepsy disorders and in developing novel anti-convulsant drugs.     

EEG alpha power changes reflect response inhibition deficits after traumatic brain injury (TBI) in humans

We studied the effect of cannabinoids on the activity of N-methyl-d-aspartate (NMDA) receptors in the locus coeruleus from rat brain slices by single-unit extracellular recordings. As expected, NMDA (100 microM) strongly excited (by nine fold) the cell firing activity of the locus coeruleus. Perfusion with the endocannabinoid anandamide (1 and 10 microM) or the anandamide transport inhibitor AM 404 (30 microM) enhanced the NMDA-induced excitation of locus coeruleus neurons. Similarly, the synthetic agonists R(+)-WIN 55212-2 (10 microM) and CP 55940 (30 microM) enhanced the effect of NMDA. In the presence of the CB(1) receptor antagonists SR 141716A (1 microM) or AM 251 (1 microM), the enhancement induced by anandamide (10 microM) was blocked. Our results suggest that cannabinoids modulate the activity of NMDA receptors in the locus coeruleus through CB(1) receptors.     

Block of spontaneous termination of paroxysmal depolarizations by forskolin (buccal ganglia, Helix pomatia)

Effects of cAMP-activated protein kinases (PKA) on epileptic activity are at present studied in a model nervous system. Identified neurons in the buccal ganglia of the snail Helix pomatia were recorded with intracellular microelectrodes in a continuously perfused experimental chamber. Epileptiform activity appeared regularly in neuron B3 when the saline contained pentylenetetrazol (20-40 mM). Epileptiform activity consisted of a series of paroxysmal depolarization shifts (PDS). Epileptiform activity was quantified by calculating the percentage of PDS-duration of PDS-periods. High percentage of PDS-duration was regularly found 15-30 min after the start of treatment with pentylenetetrazol. Subsequently, percentage of PDS decreased spontaneously. Adding forskolin (50 microM) to the pentylenetetrazol-containing solution increased percentage of PDS-duration. The increase during forskolin corresponded to the amount of decrease which had taken place spontaneously before. During application of forskolin for up to 4 h, spontaneous PDS decrease was absent, i.e., epileptiform activity corresponded to status epilepticus. Forskolin was not able to induce epileptiform activity when applied without pentylenetetrazol. 1,6-Dideoxy-forskolin (50 microM) did not accelerate epileptiform activity. When pentylenetetrazol was applied twice (1 h each) separated by 2.5 h of control conditions, PDS decrease obtained during the first application was found to be largely preserved during control conditions. When forskolin was applied for 30 min in between both applications of pentylenetetrazol, the second response to pentylenetetrazol did not show a preserved PDS decrease. Results suggest that forskolin blocks an endogenous antiepileptic process and that activation of PKA can maintain epileptic activity and induce status epilepticus.     

Epileptiform activity in rat hippocampus strengthens excitatory synapses

Although epileptic seizures are characterized by excessive excitation, the role of excitatory synaptic transmission in the induction and expression of epilepsy remains unclear. Here, we show that epileptiform activity strengthens excitatory hippocampal synapses by increasing the number of functional (RS)-α-amino-3hydroxy-5methyl-4-isoxadepropionate (AMPA)-type glutamate receptors in CA3–CA1 synapses. This form of synaptic strengthening occludes long-term potentiation (LTP) and enhances long-term depression (LTD), processes involved in learning and memory. These changes in synaptic transmission and plasticity, which are fully blocked with N -methyl-D-aspartate (NMDA) receptor antagonists, may underlie epilepsy induction and seizure-associated memory deficits.     

Epileptogenesis is associated with enhanced glutamatergic transmission in the perforant path

The perforant path provides the main excitatory input into the hippocampus and has been proposed to play a critical role in the generation of temporal lobe seizures. It has been hypothesized that changes in glutamatergic transmission in this pathway promote the epileptogenic process and seizure generation. We therefore asked whether epileptogenesis is associated with enhanced glutamatergic transmission from the perforant path to dentate granule cells. We used a rat model of temporal lobe epilepsy in which spontaneous seizures occur after an episode of pilocarpine-induced status epilepticus. Whole cell patch-clamp recordings were obtained from dentate granule cells in hippocampal slices from control and epileptic animals 3 wk after pilocarpine-induced status epilepticus. The paired pulse ratio of perforant path-evoked AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) was reduced in tissue obtained from epileptic rats. This is consistent with an increase in release probability. N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs were also prolonged. This prolongation could not be accounted for by decreased activity of glutamate transporters or by a change in NMDA receptor subunit composition in dentate granule cells, implying a change in NMDA receptor kinetics. This change in NMDA receptor kinetics was associated with the emergence of significant synaptic cross-talk, detected as a use-dependent block of receptors activated by medial perforant path synapses after lateral perforant path stimulation in MK-801. Enhanced glutamatergic transmission and the emergence of cross-talk among perforant path-dentate granule cell synapses may contribute to lowering seizure threshold.     

Muscarinic depression of synaptic transmission in the epileptogenic GABA withdrawal syndrome focus

The GABA withdrawal syndrome (GWS) is a model of local status epilepticus consecutive to the interruption of a prolonged GABA infusion into the rat somatomotor cortex. Bursting patterns in slices from GWS rats include intrinsic bursts of action potentials (APs) induced by intracellular depolarizing current injection and/or paroxysmal depolarization shifts (PDSs) induced by white matter stimulation. Possible changes in the effects of cholinergic drugs after in vivo induction of GWS were investigated on bursting cells (n = 30) intracellularly recorded in neocortical slices. In GWS slices, acetylcholine (Ach, 200-1000 microM) or carbachol (Cch, 50 microM) applications increased the number of bursts induced by depolarizing current injection while synaptically induced PDSs were significantly diminished (by 50-60%) or even blocked independently of the cholinergic-induced depolarization. The intrinsic burst facilitation and PDS depression provoked by Ach or Cch were mimicked by methyl-acetylcholine (mAch, 100-400 microM, n = 11), were reversed by atropine application (1-50 microM, n = 3), and were not mimicked by nicotine (50-100 microM, n = 4), indicating the involvement of muscarinic receptors. In contrast, in nonbursting cells from the same epileptic area (n = 42) or from equivalent area in control rats (n = 24), a nonsignificant muscarinic depression of EPSPs was induced by Cch and Ach. The mAch depression of excitatory postsynaptic potential (EPSPs) was significantly lower than that seen for PDSs in GWS rats. None of the cholinergic agonists caused bursting appearance in these cells. Therefore the present study demonstrates a unique implication of muscarinic receptors in exerting opposite effects on intrinsic membrane properties and on synaptic transmission in epileptiform GWS. Muscarinic receptor mechanisms may therefore have a protective role against the development and spread of epileptiform activity from the otherwise-activated epileptic focus.     

Endocannabinoid-dependent LTD in a nociceptive synapse requires activation of a presynaptic TRPV-like receptor

Recent studies have found that some forms of endocannabinoid-dependent synaptic plasticity in the hippocampus are mediated through activation of transient potential receptor vanilloid (TRPV) receptors instead of cannabinoid receptors CB1 or CB2. The potential role for synaptic localization of TRPV receptors during endocannabinoid modulation of nociceptive synapses was examined in the leech CNS where it is possible to record from the same pair of neurons from one preparation to the next. Long-term depression (LTD) in the monosynaptic connection between the nociceptive (N) sensory neuron and the longitudinal (L) motor neuron was found to be endocannabinoid-dependent given that this depression was blocked by RHC-80267, an inhibitor of DAG lipase that is required for 2-arachidonoyl glycerol (2AG) synthesis. Intracellular injection of a second DAG lipase inhibitor, tetrahyrdolipstatin (THL) was also able to block this endocannabinoid-dependent LTD (ecLTD) when injected postsynaptically but not presynaptically. N-to-L ecLTD was also inhibited by the TRPV1 antagonists capsazepine and SB 366791. Bath application of 2AG or the TRPV1 agonists capsaicin and resiniferatoxin mimicked LTD and both capsaicin- and 2AG-induced depression were blocked by capsazepine. In addition, pretreatment with 2AG or capsaicin occluded subsequent expression of LTD induced by repetitive activity. Presynaptic, but not postsynaptic, intracellular injection of capsazepine blocked both activity- and 2AG-induced ecLTD, suggesting that a presynaptic TRPV-like receptor in the leech mediated this form of synaptic plasticity. These findings potentially extend the role ecLTD to nociceptive synapses and suggest that invertebrate synapses, which are thought to lack CB1/CB2 receptor orthologues, utilize a TRPV-like protein as an endocannabinoid receptor.     

Dopamine enhances spatiotemporal spread of activity in rat prefrontal cortex

Dopaminergic modulation of prefrontal cortex (PFC) is important for neuronal integration in this brain region known to be involved in cognition and working memory. Because of the complexity and heterogeneity of the effect of dopamine on synaptic transmission across layers of the neocortex, dopamine's net effect on local circuits in PFC is difficult to predict. We have combined whole cell patch-clamp recording and voltage-sensitive dye imaging to examine the effect of dopamine on the excitability of local excitatory circuits in rat PFC in vitro. Whole cell voltage-clamp recording from visually identified layer II/III pyramidal neurons in rat brain slices revealed that, in the presence of bicuculline (10 microM), bath-applied dopamine (30-60 microM) increased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by weak intracortical stimulus. The effect was mimicked by the selective D1 receptor agonist SKF 81297 (1 microM). Increasing stimulation resulted in epileptiform discharges. SKF 81297 (1 microM) significantly lowered the threshold stimulus required for generating epileptiform discharges to 83% of control. In the imaging experiments, bath application of dopamine or SKF 81297 enhanced the spatiotemporal spread of activity in response to weak stimulation and previously subthreshold stimulation resulted in epileptiform activity that spread across the whole cortex. These effects could be blocked by the selective D1 receptor antagonist SCH 23390 (10 microM) but not by the D2 receptor antagonist eticlopride (5 microM). These results indicate that dopamine, by a D1 receptor-mediated mechanism, enhances spatiotemporal spread of synaptic activity and lowers the threshold for epileptiform activity in local excitatory circuits within PFC.     

Osmolality-induced changes in extracellular volume alter epileptiform bursts independent of chemical synapses in the rat: Importance of non-synaptic mechanisms in hippocampal epileptogenesis

The contribution of non-synaptic mechanisms to the seizure susceptibility of rat CA1 hippocampal pyramidal cells was examined in vitro by testing the effects of osmolality on synchronous neuronal activity, using solutions which blocked chemical synaptic transmission both pre- and post-synaptically. Decreases in osmolality, which shrink the extracellular volume, caused or enhanced epileptiform bursting. Increases in osmolality with membrane-impermeant solutes, which expand the extracellular volume, blocked or greatly reduced epileptiform discharges. Reductions in the extracellular volume, therefore, can enhance synchronization among CA1 hippocampal neurons through non-synaptic mechanisms. Since similar osmotic treatments are known to modify epileptiform discharges in several models of epilepsy, non-synaptic mechanisms are probably more important in hippocampal epileptogenesis than previously realized and may contribute to the high susceptibility of this brain region to epileptic seizures in animals and humans. These data also provide a possible explanation for the observation in humans that decreased plasma osmolality, which can be associated with a wide range of clinical syndromes, leads to seizures.     

Status epilepticus causes a long-lasting redistribution of hippocampal cannabinoid type 1 receptor expression and function in the rat pilocarpine model of acquired epilepsy

Activation of the cannabinoid type 1 (CB1) receptor, a major G-protein-coupled receptor in brain, acts to regulate neuronal excitability and has been shown to mediate the anticonvulsant effects of cannabinoids in several animal models of seizure, including the rat pilocarpine model of acquired epilepsy. However, the long-term effects of status epilepticus on the expression and function of the CB1 receptor have not been described. Therefore, this study was initiated to evaluate the effect of status epilepticus on CB1 receptor expression, binding, and G-protein activation in the rat pilocarpine model of acquired epilepsy. Using immunohistochemistry, we demonstrated that status epilepticus causes a unique "redistribution" of hippocampal CB1 receptors, consisting of specific decreases in CB1 immunoreactivity in the dense pyramidal cell layer neuropil and dentate gyrus inner molecular layer, and increases in staining in the CA1-3 strata oriens and radiatum. In addition, this study demonstrates that the redistribution of CB1 receptor expression results in corresponding functional changes in CB1 receptor binding and G-protein activation using [3H] R+-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl](1-napthalen-yl)methanone mesylate (WIN55,212-2) and agonist-stimulated [35S]GTPgammaS autoradiography, respectively. The redistribution of CB1 receptor-mediated [35S]GTPgammaS binding was 1) attributed to an altered maximal effect (Emax) of WIN55,212-2 to stimulate [35S]GTPgammaS binding, 2) reversed by the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), 3) confirmed by the use of other CB1 receptor agonists, and 4) not reproduced in other G-protein-coupled receptor systems examined. These results demonstrate that status epilepticus causes a unique and selective reorganization of the CB1 receptor system that persists as a permanent hippocampal neuronal plasticity change associated with the development of acquired epilepsy.     

Involvement of electrical coupling in the in vivo ictal epileptiform activity induced by 4-aminopyridine in the neocortex

In the present study we have investigated the possible role of gap junctions in the induction and manifestation of 4-aminopyridine-induced acute seizure activity both at the primary focus and at the mirror focus in anaesthetized rats by combining electrophysiological, pharmacological and molecular biological techniques. In the course of the intracellular recordings, unusual firing patterns that are assumed to be mediated by electrical coupling and appearing either randomly or in close time-locked manner with the ictal discharges were observed. In another series of experiments, a significant decrease in the intensity of seizure activity of the already active epileptic foci was detected when electrical synaptic transmission was blocked by carbenoxolone either at the primary focus or at the mirror focus. When electrical synaptic transmission was depressed relative to the initial baseline prior to the induction of epileptic focus, only a mild influence on the induction of seizure discharges occurred. The role of the gap junctional communication in the epileptiform activity was further investigated by following the expression pattern of two connexin genes. Both, connexin-32 and connexin-43 mRNA levels were significantly elevated at the primary focus as well as at the mirror focus, after 60 min of repeated ictal discharges.We conclude that gap junction communication probably became a part of the neuronal synchronization both in the primary and in the secondarily-induced acute epileptiform activity in the neocortex in vivo. These results, together with earlier observations, indicate a direction for the development of new drugs targeting gap junctions for therapeutic intervention.     

戊四氮致癫癎状态对大鼠长期行为和脑电的影响

目的：建立一种戊四氮（ＰＴＺ） 致癫癎状态（ＳＥ） 模型,并探讨ＰＴＺ 致惊厥与癫癎形成之间的关系。方法：观察ＰＴＺ 致抽搐发作（ 抽搐组） 和ＰＴＺ致抽搐延长发作即癫癎状态（ＳＥ 组） 对大鼠长期行为和脑电（ＥＥＧ） 的影响,同时观察二者是否产生海马、皮质神经元损伤。结果：ＰＴＺ 致抽搐延长发作能够产生具有某些癫癎特征的长期效应,如自发癎样放电、惊厥阈下剂量ＰＴＺ 可诱导癫癎发作以及皮质和海马神经元损伤,而单次抽搐发作不具有这些长期效应。结论：ＰＴＺ 致抽搐延长发作模型更符合ＳＥ 模型特点,惊厥持续时间与癫癎形成密切相关。     

Shortened-duration GABAA receptor-mediated synaptic potentials underlie enhanced CA1 excitability in a chronic model of temporal lobe epilepsy

Intracellular recording techniques were used to examine GABA_A receptor-mediated synaptic inhibition in pyramidal cells of the CA1 region of the rat hippocampus in the post-self sustaining limbic status epilepticus model of temporal lobe epilepsy. Orthodromically evoked, monosynaptic inhibitory postsynaptic potentials were recorded in vitro following pharmacological blockade of ionotropic glutamate and GABA_B receptors. Inhibitory postsynaptic potentials from epileptic tissue were kinetically altered relative to controls; both the 10–90% rise-time and width (measured at half-maximum amplitude) were reduced by approximately 50% resulting in significant shortening of duration. The degree of pyramidal cell hyperexcitability, assessed before pharmacological treatment as the number of action potentials evoked by maximum intensity afferent stimulation, correlated significantly with the magnitude of synaptic potential duration reduction determined following blockade of glutamatergic neurotransmission. Bath application of the benzodiazepine type 1 receptor agonist zolpidem reduced post-self sustaining limbic status epilepticus CA1 pyramidal cell hyperexcitability substantially (but not completely) via a marked increase in inhibitory postsynaptic potential area. Post-self sustaining limbic status epilepticus inhibitory postsynaptic potentials which exhibited the most pronounced shortening were augmented by zolpidem to a greater degree than longer duration synaptic potentials. In contrast, zolpidem-induced augmentation of control inhibitort postsynaptic potential area was much less robust.

It is suggested that a deficiency in post-self sustaining limbic status epilepticus GABA_A receptor-mediated synaptic inhibition contributes to a state of partial disinhibition which is a major factor in enhanced CA1 excitability in chronic limbic epilepsy. Possible mechanisms underlying post-self sustaining limbic status epilepticus kinetic abnormalities are discussed.     

Retrograde signaling changes the venue of postsynaptic inhibition in rat substantia nigra

Both endocannabinoids through cannabinoid receptor type I (CB1) receptors and dopamine through dopamine receptor type D1 receptors modulate postsynaptic inhibition in substantia nigra by changing GABA release from striatonigral terminals. By recording from visually identified pars compacta and pars reticulata neurons we searched for a possible co-release and interaction of endocannabinoids and dopamine. Depolarization of a neuron in pars reticulata or in pars compacta transiently suppressed evoked synaptic currents which were blocked by GABA(A) receptor antagonists (inhibitory postsynaptic currents [IPSCs]). This depolarization-induced suppression of inhibition (DSI) was abrogated by the cannabinoid CB1 receptor antagonist AM251 (1 microM). A correlation existed between the degree of DSI and the degree of reduction of evoked IPSCs by the CB1 receptor agonist WIN55,212-2 (1 microM). The cholinergic receptor agonist carbachol (0.5-5 microM) enhanced DSI, but suppression of spontaneous IPSCs was barely detectable pointing to the existence of GABA release sites without CB1 receptors. In dopamine, but not in GABAergic neurons DSI was enhanced by the dopamine D1 receptor antagonist SCH23390 (3-10 microM). Both the antagonist for CB1 receptors and the antagonist for dopamine D1 receptors enhanced or reduced, respectively, the amplitudes of evoked IPSCs. This tonic influence persisted if the receptor for the other ligand was blocked. We conclude that endocannabinoids and dopamine can be co-released. Retrograde signaling through endocannabinoids and dopamine changes inhibition independently from each other. Activation of dopamine D1 receptors emphasizes extrinsic inhibition and activation of CB1 receptors promotes intrinsic inhibition.