Carbenoxolone depresses spontaneous epileptiform activity in the CA1 region of rat hippocampal slices

Seizure suppression by shakB2, a gap junction mutation in Drosophila

Gap junction proteins mediate electrical synaptic transmission. In Drosophila, flies carrying null mutations in the shakB locus, such as shakB2, have behavioral and electrophysiological defects in the giant fiber (GF) system neurocircuit consistent with a loss of transmission at electrical synapses. The shakB2 mutation also affects seizure susceptibility. Mutant flies are especially seizure-resistant and have a high threshold to evoked seizures. In addition, in some double mutant combinations with "epilepsy" mutations, shakB2 appears to act as a seizure-suppressor mutation: shakB2 restores seizure susceptibility to the wild-type range in the double mutant. In double mutant combinations, shakB2 completely suppresses seizures caused by slamdance (sda), knockdown (kdn), and jitterbug (jbug) mutations. Seizures caused by easily shocked (eas) and technical knockout (tko) mutations are partially suppressed by shakB2. Seizures caused by bang-sensitive (bas2) and bang-senseless (bss1, bss2 alleles) mutations are not suppressed by shakB2. These results show the use of Drosophila as a model system for studying the kinds of genetic interactions responsible for seizure susceptibility, bringing us closer to unraveling the complexity of seizure disorders in humans.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Anticonvulsant effects of carbenoxolone in genetically epilepsy prone rats (GEPRs)

Carbenoxolone (CBX), the succinyl ester of glycyrrhetinic acid, is an inhibitor of gap junctional intercellular communication. Systemic administration of CBX was able to decrease the seizure severity score and to increase the latency time of seizure onset in genetically epilepsy prone rats (GEPRs). In particular, intravenous or intraperitoneal administration of carbenoxolone (5-30 mg/kg) produced a dose-dependent and significant reduction in the clonic and tonic phases of the audiogenic seizures in GEPRs. The anticonvulsant doses were not associated with an impairment of motor coordination. The bilateral microinjection of CBX (0.001-0.50 microg/0.5 microl) into the inferior colliculi, the substantia nigra (pars reticulata or compacta) and the inferior olivary complex was able to reduce the seizure severity score in a dose-dependent manner. The anticonvulsant effects were longer lasting after focal microinjection than after systemic administration. No anticonvulsant effects were observed following focal bilateral microinjections of glycyrrhizin into the same brain areas where CBX was shown to be effective.

Single-column thalamocortical network model exhibiting gamma oscillations, sleep spindles, and epileptogenic bursts

To better understand population phenomena in thalamocortical neuronal ensembles, we have constructed a preliminary network model with 3,560 multicompartment neurons (containing soma, branching dendrites, and a portion of axon). Types of neurons included superficial pyramids (with regular spiking [RS] and fast rhythmic bursting [FRB] firing behaviors); RS spiny stellates; fast spiking (FS) interneurons, with basket-type and axoaxonic types of connectivity, and located in superficial and deep cortical layers; low threshold spiking (LTS) interneurons, which contacted principal cell dendrites; deep pyramids, which could have RS or intrinsic bursting (IB) firing behaviors, and endowed either with nontufted apical dendrites or with long tufted apical dendrites; thalamocortical relay (TCR) cells; and nucleus reticularis (nRT) cells. To the extent possible, both electrophysiology and synaptic connectivity were based on published data, although many arbitrary choices were necessary. In addition to synaptic connectivity (by AMPA/kainate, NMDA, and GABA(A) receptors), we also included electrical coupling between dendrites of interneurons, nRT cells, and TCR cells, and--in various combinations--electrical coupling between the proximal axons of certain cortical principal neurons. Our network model replicates several observed population phenomena, including 1) persistent gamma oscillations; 2) thalamocortical sleep spindles; 3) series of synchronized population bursts, resembling electrographic seizures; 4) isolated double population bursts with superimposed very fast oscillations (>100 Hz, "VFO"); 5) spike-wave, polyspike-wave, and fast runs (about 10 Hz). We show that epileptiform bursts, including double and multiple bursts, containing VFO occur in rat auditory cortex in vitro, in the presence of kainate, when both GABA(A) and GABA(B) receptors are blocked. Electrical coupling between axons appears necessary (as reported previously) for persistent gamma and additionally plays a role in the detailed shaping of epileptogenic events. The degree of recurrent synaptic excitation between spiny stellate cells, and their tendency to fire throughout multiple bursts, also appears critical in shaping epileptogenic events.

Electrical coupling underlies theta oscillations recorded in hippocampal formation slices

The role of gap junction coupling in generation of carbachol-induced theta-like activity (TLA) in hippocampal formation (HPC) slices was investigated in this study. Two gap junction (GJ) blockers, carbenoxolone (100 microM) and quinine (100 microM), were tested. Both GJ blockers abolished cholinergically induced theta-like activity and related cell discharges. Abolishing effects were observed after 40-45 min of drug perfusion. These effects were found to be slowly and partially reversible. Our results provide evidence for the contribution of gap junction communication in mechanisms of neural synchrony, underlying the production of theta oscillations in limbic cortex maintained in vitro.

Electrotonic coupling between stratum oriens interneurones in the intact in vitro mouse juvenile hippocampus

Using the isolated juvenile (7-14 days) mouse whole hippocampus preparation, which contains intact complex local circuitry, 145 dual whole cell recordings were made from stratum oriens (s.o.) interneurones under infrared microscopy. In 11.7% of paired recordings, evidence for direct electrotonic coupling between the s.o. interneurones was obtained from the response of one interneurone to a long (400-600 ms) constant current pulse passed into the coupled interneurone. When specifically orienting the dual recordings in the transectional plane of the hippocampus, 18.5% of paired recordings showed electrotonic coupling. The coupling coefficient, estimated from averaged data, was 6.9 +/- 4.7%, ranging from 1.3 to 17.6%. The time constant of the electrotonically transmitted hyperpolarization was inversely related to the coupling coefficient between the two neurones. The electrotonic responses of one neurone to constant current pulses injected into the other coupled neurone were intermittent. Spikes in one of the coupled neurones were associated with small electrotonic EPSPs (spikelets) in the other coupled neurone, in those neuronal pairs with coupling coefficients greater than 10%. Failure of spikelet production following a spike in the coupled cell occurred 5-10% of the time. Electrotonic coupling and spikelets persisted in the presence of chemical synaptic transmission blockade by CNQX, APV and bicuculline, or in zero Ca(2+) perfusate, but were abolished by carbenoxolone (100 microm), a gap junctional blocker. These data confirm the existence of electrotonic coupling between s.o. interneurones, presumably via gap junctions located in dendrites.

The sleep lipid oleamide may represent an endogenous anticonvulsant: an in vitro comparative study in the 4-aminopyridine rat brain-slice model

cis-Oleamide (cOA) is a putative endocannabinoid, which modulates GABA(A) receptors, Na+ channels and gap-junctions (important targets for clinical and experimental anticonvulsants). Here we address the hypothesis that cOA possesses seizure limiting properties and might represent an endogenous anticonvulsant. Field potentials were recorded from the rat hippocampus and visual cortex. The effects of cOA, were compared to carbamazepine (CBZ), pentobarbital (PB) and carbenoxolone (CRX) on 4-Aminopyridine(4AP)-induced epileptiform discharges. CBZ (100 microM), PB (50 microM) and CRX (100 microM), but not cOA (64 microM), significantly attenuated the duration of the evoked epileptiform discharges in CA1. Interictal activity in CA3 was significantly depressed by CRX and cOA (irreversible by AM251), increased by CBZ and remained unaffected by PB. CBZ, PB and CRX abolished spontaneous ictal events and attenuated evoked ictal discharges in the visual cortex. cOA did not abolish spontaneous ictal events, but significantly (albeit weakly) reduced the duration of evoked ictal events. cOA and CRX, in contrast to CBZ or PB, caused a significant delay in the development of the evoked (tonic phase) epileptiform discharges. The weak effects of cOA seem independent of cannabinoid (CB1) receptors. Enzymatic cleavage and lack of specific antagonists for cOA confound simple interpretations of its actions in slices. Its high lipophilicity, imposing a permeability barrier, may also explain the lack of anticonvulsant activity. The effects of cOA may well be masked by release of the endogenous ligand upon ictal depolarisation as we demonstrate here for established endocannabinoids. cOA does not possess profound antiepileptic actions in our hands compared to CBZ, PB or CRX.

Modulation of muscarinic facilitation of epileptiform discharges in immature rat neocortex

We examined the cholinergic effects on epileptiform discharge generation in immature (postnatal days 10-20) rat neocortex. Evoked and spontaneous field potentials were recorded from the deep layers of neocortical slices during GABA(A) receptor blockade by bicuculline methiodide (BMI, 50 microM). The anticholinesterase eserine (10 microM) as well as the ACh-analog carbamylcholine chloride (CCh, 25 microM) decreased the amplitude and duration of evoked field potentials and in parallel, increased significantly the rate of occurrence of spontaneous discharges. These effects were reversed by the muscarinic antagonist atropine (2.5 microM, n = 20), but not by the nicotinic receptor antagonist hexamethonium (50 microM, n = 3). The M1 subtype-selective muscarinic antagonist pirenzepine (1 microM, n = 12) blocked spontaneous discharges in 8/12 slices, while muscarinic antagonists of the M2 (AFDX 116 n = 4), M3 (4-DAMP n = 4) and M4 (gallamine n = 5, tropicamide n = 6) type, all at 1 microM, only reduced their frequency. CCh-induced spontaneous discharges were blocked by the combination of the glutamate receptor antagonists AP5 and CNQX (both at 10 microM; n = 11). Gap junction blockers abolished them (halothane, n = 7) or reduced their frequency by 65% (carbenoxolone, n = 8). Inhibiting Ca2+ release from intracellular stores by dantrolene (100 microM, n = 5) or thapsigargin (1 microM, n = 5) also depressed their frequencies by 55-65%. By contrast, their rates were not altered by perfusion with high Ca2+ (7 mM; n = 6) medium, a manipulation suppressing polysynaptic connections. These findings demonstrate that activation of muscarinic receptors, notably of the M1 type, in immature rat neocortex facilitates the generation of glutamatergic epileptiform discharges. These discharges are strongly inhibited by gap junction blockers, and are also partly mediated by the, presumably muscarinic receptor-dependent, mobilization of intracellular calcium.

Low micromolar concentrations of 4-aminopyridine facilitate fictive locomotion expressed by the rat spinal cord in vitro

Upregulating the operation of spinal locomotor networks is one mechanism to restore, at least partially, lesion-impaired locomotion. We investigated if the K+ channel blocker 4-aminopyridine (4-AP) could facilitate spinal locomotor networks in addition to its well-known effect on motor nerve conduction. Fictive locomotor patterns were recorded from ventral roots (VRs) of the isolated spinal cord of the neonatal rat. 4-AP (0.1-50 microM) produced synchronous VR oscillations which did not develop into fictive locomotion. These oscillations had network origin, required intact glutamatergic transmission and were probably amplified via electrotonic coupling because of their depression by the selective gap junction blocker carbenoxolone. 4-AP (5 microM) slightly increased input resistance of lumbar motoneurons without affecting their action or resting potentials. Dorsal root (DR) evoked synaptic responses were enhanced (217 +/- 65%) by 5 microM 4-AP without changes in axon conduction. 4-AP (5 microM) accelerated fictive locomotion induced by N-methyl-d-aspartate (NMDA) and serotonin (5-HT) without altering cycle amplitude and facilitated the onset of fictive locomotion in the presence of sub-threshold concentrations of NMDA and 5-HT. Furthermore, in the presence of 4-AP, weak DR stimuli, previously insufficient to activate locomotor patterns, generated alternating VR discharges. Thus, although 4-AP per se could not directly activate the locomotor network of the spinal cord, it could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli. These data suggest that the reported improvement by 4-AP in locomotor activity of spinal-injury patients may include activation of locomotor networks when low concentrations of this drug are administered in coincidence with appropriate stimuli.

Influence of carbenoxolone on the anticonvulsant efficacy of conventional antiepileptic drugs against audiogenic seizures in DBA/2 mice

Carbenoxolone, the succinyl ester of glycyrrhetinic acid, is an inhibitor of 11beta-hydroxy steroid dehydrogenase and gap junctional intercellular communication. It is currently used in clinical treatment of ulcer diseases. Systemic administration of carbenoxolone (1-40 mg/kg, intraperitoneally (i.p.)) was able to produce a dose-dependent decrease in DBA/2 audiogenic seizure severity score. Glycyrrhizin, an analogue of carbenoxolone inactive at the gap-junction level, was unable to affect audiogenic seizures at doses up to 30 mg/kg. In combination with conventional antiepileptic drugs, carbenoxolone, 0.5 mg/kg, i.p., which per se did not significantly affect the occurrence of audiogenic seizures in DBA/2 mice, potentiated the anticonvulsant activity of carbamazepine, diazepam, felbamate, gabapentin, lamotrigine, phenytoin, phenobarbital and valproate against sound-induced seizures in DBA/2 mice. This effect was not observed after the combination of glycyrrhizin (10 mg/kg, i.p.) with some conventional antiepileptic drugs. The degree of potentiation induced by carbenoxolone was greater for diazepam, felbamate, gabapentin, phenobarbital and valproate, less for lamotrigine, phenytoin and carbamazepine. This increase was associated with a comparable impairment in motor activity; however, the therapeutic index of combined treatment of antiepileptic drugs with carbenoxolone was more favourable than the combination with glycyrrhizin or saline. Since carbenoxolone did not significantly influence the total and free plasma levels of diazepam, felbamate, gabapentin, lamotrigine, phenytoin, phenobarbital, valproate and carbamazepine, pharmacokinetic interactions are not likely. However, the possibility that carbenoxolone can modify the brain clearance of the anticonvulsant drugs studied may not be excluded. In addition, carbenoxolone did not significantly affect the hypothermic effects of the anticonvulsants tested. In conclusion, carbenoxolone showed an additive anticonvulsant effect when administered in combination with some classical anticonvulsants, most notably diazepam, felbamate, gabapentin, phenobarbital, and valproate, implicating a possible therapeutic relevance of such drug combinations.

Involvement of gap junctions in the manifestation and control of the duration of seizures in rats in vivo

PURPOSE

The possible role of gap junctions in the manifestation and control of the duration of seizures was tested on the 4-aminopyridine-induced epilepsy model in rats in vivo, by using electrophysiologic, pharmacologic, and molecular biologic techniques. METHODS; In electrophysiologic experiments, the functional states of the gap junctions were manipulated with a specific blocker (carbenoxolone) or opener (trimethylamine) at the already active focus of adult, anesthetized rats, 60 min after the induction of the first seizure, which was repeated spontaneously thereafter. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) amplification was used to measure the levels of connexin (Cx) 32, 43, and 36 messenger RNAs (mRNAs) prepared from the areas of the already active primary and mirror foci.

RESULTS

After repeated seizures, the expression levels of Cx32, Cx43, and Cx36 mRNAs at the epileptic foci were increased significantly. Blockade of the gap junctions with carbenoxolone shortened the duration of seizures and decreased the amplitude of the seizure discharges, whereas their opening with trimethylamine lengthened the duration and increased the amplitude. Secondary epileptogenesis was facilitated when the gap junctions were opened. CONCLUSIONS; Our findings support the idea that, in epileptic foci, the gap junctions are involved in the expression of rhythmic ictal discharges and in the control of the duration and propagation of the individual seizures in vivo.

Desynchronisation of spontaneously recurrent experimental seizures proceeds with a single rhythm

Here we investigate the temporal properties of recurrent seizure-like events (SLEs) in a low-[Mg(2+)] model of experimental epilepsy. Simultaneous intra- and extracellular electric signals were recorded in the CA3 region of rat hippocampal slices whereby cytosolic [Ca(2+)] transients were imaged by fluorescence detection. Recurrence pattern analysis was applied to give a measure of synchrony of simultaneously recorded intra- and extracellular electric signals and the SLE frequencies were extracted by complex wavelet analysis. Slices from the juvenile, but not the young adult rats, displayed several high-amplitude triplets of electric and [Ca(2+)] transients, termed paroxysmal spikes, followed by an SLE. Occurrence of the full-blown SLE was associated with decreased synaptic activity between the paroxysmal spikes that were seen as incomplete SLE starting sequences. The time series of recurrent SLEs provide evidence for a single SLE rhythm as continuously declining from about 200 Hz to below 1 Hz at the onset and termination of SLE, respectively, with an intermediate spectral discontinuity, tentatively identified with the tonic/clonic transition. All other frequency components were the harmonics of the fundamental rhythm, whereby the gamma and the theta band oscillations were not detected as separate activities. Recurrence showed decreasing temporal synchrony of intra- and extracellular signals during the SLE, suggesting that coincidence is destroyed by the SLE. Blockade of gap junctions with 200 microM carbenoxolone ceased recurrent SLEs. Release of gap junction blockade shortened both SLEs and their tonic phase indicating that persistent changes occurred via an altered gap junction coupling. We conclude that the initially precise temporal synchrony is gradually destroyed during ictal events with a single rhythm of continuously decreasing frequency. Blockade of gap junction coupling might prevent epileptic synchronisation.

Carbenoxolone blockade of neuronal network activity in culture is not mediated by an action on gap junctions

Spontaneous activity in the central nervous system is strongly suppressed by blockers of gap junctions (GJs), suggesting that GJs contribute to network activity. However, the lack of selective GJ blockers prohibits the determination of their site of action, i.e. neuronal versus glial. Astrocytes are strongly coupled through GJs and have recently been shown to modulate synaptic transmission, yet their role in neuronal network activity was not analysed. The present study investigated the effects and site of action of the GJ blocker, carbenoxolone (CBX), on neuronal network activity. To this end, we used cultures of hippocampal or cortical neurons, plated on astrocytes. In these cultures neurons display spontaneous synchronous activity and GJs are found only in astrocytes. CBX induced in these neurons a reversible suppression of spontaneous action potential discharges, synaptic currents and synchronised calcium oscillations. Moreover, CBX inhibited oscillatory activity induced by the GABAA antagonist, bicuculline. These effects were not due to blockade of astrocytic GJs, since they were not mimicked nor occluded by endothelin-1 (ET-1), a peptide known to block astrocytic GJs. Also, these effects were still present in co-cultures of wild-type neurons plated on astrocytes originating from connexin-43 (Cx43) knockout mice, and in neuronal cultures which contain few isolated astrocytes. CBX was not likely to exert its effect through neuronal GJs either, as immunostaining for major neuronal connexins (Cx) as well as dye or electrical coupling, were not detected in the different models of cultured neurons examined. Finally while CBX (at 100 microM) did not modify presynaptic transmitter release and postsynaptic responses to glutamate, it did cause an increase in the action potential threshold and strongly decreased the firing rate in response to a sustained depolarising current. These data demonstrate that CBX does not exert its action on network activity of cultured neurons through astrocytic GJs and suggest that it has direct effects on neurons, not involving GJs.

Epileptiform activity in hippocampal slice cultures exposed chronically to bicuculline: increased gap junctional function and expression

Chronic (18 h) exposure of cultured hippocampal slices to the type-A GABA receptor blocker, bicuculline methiodide (BMI) 10 micro m increased the levels of connexin 43 (Cx43) and connexin 32 (Cx32) mRNAs, but not connexin 26 and connexin 36, as demonstrated by RNase protection assays. The levels of Cx43 and Cx32 proteins in membrane fractions detected by western blotting were also significantly increased. Immunoblotting indicated that BMI also promoted a significant expression of the transcription protein c-fos. The rate of fluorescence recovery after photobleaching, an index of gap junctional coupling, was also significantly increased, whereas it was blocked by the gap junctional blocker, carbenoxolone (100 micro m). Extracellular recordings in CA1 stratum pyramidale, performed in BMI-free solution, demonstrated that BMI-exposed cultures possessed synaptic responses characteristic of epileptiform discharges: (i) significantly greater frequency of spontaneous epileptiform discharges, (ii) post-synaptic potentials with multiple population spikes, and (iii) significantly longer duration of primary afterdischarges. Carbenoxolone (100 micro m), but not its inactive analog, oleanolic acid (100 micro m), reversibly inhibited spontaneous and evoked epileptiform discharges. The findings of BMI-induced parallel increases in levels of gap junction expression and function, and the increase in epileptiform discharges, which were sensitive to gap junctional blockers, are consistent with the hypothesis that increased gap junctional communication plays an intrinsic role in the epileptogenic process.

Anticonvulsant, sedative and muscle relaxant effects of carbenoxolone in mice

BACKGROUND

Carbenoxolone, as an antiulcer medicine, has some pharmacological properties such as: the inhibition of gap junctional (GJ) intercellular communication. In vitro studies have shown, carbenoxolone to abolish the generation of full or partial ectopic spike generation, by 4-aminopyridine, as well as spontaneous epileptiform activity in CA3 or CA1 regions of the rat hippocampal slices via closing GJ channels. Thus, we considered the possible anticonvulsant effects of carbenoxolone in animal seizure models.

RESULTS

ED50 values of diazepam and carbenoxolone in the pentylenetetrazole model were 1.13 mg/kg and 283.3 mg/kg, respectively. In this model, carbenoxolone in doses of 200 and 300 mg/kg prolonged the onset time of seizure and decreased the duration of seizures. In the maximal electroshock model, carbenoxolone in a dose of 400 mg/kg decreased the duration of seizure producing protection against seizure but failing to protect against mortality in comparison with diazepam. In the potentiation of pentobarbitone sleep test, carbenoxolone significantly increased sleeping time and decreased latency in doses of 100, 200 and 300 mg/kg in mice dose dependently. In the traction test, carbenoxolone (400 mg/kg) showed muscle relaxant activity and in the accelerated rotarod test, carbenoxolone in doses of 200 and 300 mg/kg showed a decline in motor coordination.

CONCLUSION

It can be concluded that carbenoxolone possesses anticonvulsant, muscle relaxant and hypnotic effects, which could contribute to the control of petit mal and grand mal seizures.

Sharp wave-like activity in the hippocampus in vitro in mice lacking the gap junction protein connexin 36

Bath application of kainate (100-300 nM) induced a persistent gamma-frequency (30-80 Hz) oscillation that could be recorded in stratum radiatum of the CA3 region in vitro. We have previously described that in knockout mice lacking the gap junction protein connexin 36 (Cx36KO), gamma-frequency oscillations are reduced but still present. We now demonstrate that in the Cx36KO mice, but not in wild-type (WT), large population field excitatory postsynaptic potentials, or sharp wave-burst discharges, also occurred during the on-going gamma-frequency oscillation. These spontaneous burst discharges were not seen in WT mice. Burst discharges in the Cx36KO mice occurred with a mean frequency of 0.23 +/- 0.11 Hz and were accompanied by a series of fast (approximately 60-115 Hz) population spikes or "ripple" oscillations in many recordings. Intracellular recordings from CA3 pyramidal cells showed that the burst discharges consisted of a depolarizing response and presumed coupling potentials (spikelets) could occasionally be seen either before or during the burst discharge. The burst discharges occurring in Cx36KO mice were sensitive to gap junctions blockers as they were fully abolished by carbenoxolone (200 microM). In control mice we made several attempts to replicate this pattern of sharp wave activity/ripples occurring with the on-going kainate-evoked gamma-frequency oscillation by manipulating synaptic and electrical signaling. Partial disruption of inhibition, in control slices, by bath application of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline (1-4 microM) completely abolished all gamma-frequency activity before any burst discharges occurred. Increasing the number of open gap junctions in control slices by using trimethylamine (TMA; 2-10 mM), in conjunction with kainate, failed to elicit any sharp wave bursts or fast ripples. However, bath application of the potassium channel blocker 4-aminopyridine (4-AP; 20-80 microM) produced a pattern of activity in control mice (13/16 slices), consisting of burst discharges occurring in conjunction with kainate-evoked gamma-frequency oscillations, that was similar to that seen in Cx36KO mice. In a few cases (n = 9) the burst discharges were accompanied by fast ripple oscillations. Carbenoxolone also fully blocked the 4-AP-evoked burst discharges (n = 5). Our results show that disruption of electrical signaling in the interneuronal network can, in the presence of kainate, lead to the spontaneous generation of sharp wave/ripple activity similar to that observed in vivo. This suggests a complex role for electrically coupled interneurons in the generation of hippocampal network activity.

Pharmacological enhancement of hemi-gap-junctional currents in Xenopus oocytes

Hemichannels formed by expressing connexin subunits in Xenopus oocytes provide a valuable tool for revealing the gating properties of intercellular gap junctions in electrically coupled cells. We used the two electrode voltage-clamp technique to demonstrate that activation of the time-dependent outward hemichannel currents brings into play a sodium current of similar time course and opposite polarity; the interaction between these opposing currents had not been explored previously. Using the endogenous connexin (Cx38) of Xenopus oocytes as a model system, we have shown that substituting choline for sodium in the bath solution eliminates the sodium current, thereby unmasking large hemichannel currents, and enabling pharmacological studies of agents that are known to modulate gap-junctional conductances. The cinchona alkaloid quinine also effectively blocked the inward current, and in addition, enhanced significantly the Cx38 hemichannel currents in a dose-dependent fashion; the Hill coefficient of 1.9 suggests that the binding of at least two molecules of quinine is required to produce the effect. Intracellular quinine had no effect on hemichannel currents, and experiments on the displacement of quinine suggest that binding is at an external site near or within the mouth of the hemichannel. Intracellular acidification suppressed the quinine-enhanced hemichannel currents, indicating that quinine does not block the proton binding site. We found that retinoic acid (RA) and carbenoxolone, agents that block gap-junctional channels in coupled neurons and other cell types, also suppressed Cx38 hemichannel currents with an IC(50) of approximately 2 and 34 microM for RA and carbenoxolone, respectively. Raising extracellular calcium to 3 mM suppressed both the hemichannel current and the inward sodium current. These results provide a foundation upon which to further characterize the gating of hemichannel currents mediated by connexins expressed in Xenopus oocytes.

Anticonvulsant actions of gap junctional blockers in an in vitro seizure model

Gap junctions (gjs) are increasingly recognized as playing a significant role in seizures. We demonstrate that different types of gap junctional blocking agents reduce the duration of evoked seizure-like primary afterdischarges (PADs) in the rat in vitro CA1 hippocampal pyramidal region, following repetitive tetanization of the Schaffer collaterals. Intracellular acidosis, which is known to block gap junctional communication, decreased the PADs, whereas alkalinization increased the PADs. Cellular excitability was not significantly depressed as determined by input/output relations recorded before and during perfusion of the gj blockers blockers carbenoxolone and sodium propionate. There was a small decrease following 1-octanol perfusion and a large decrease following NH(4)Cl application. Carbenoxolone diminished PAD duration, but increased neuronal excitability in whole-cell recordings. After robust PADs were established, the expression of several gj proteins including connexins (Cxs) 26, 32, 36, and 43, as measured by Western blotting, was unchanged, although the level of nonphosphorylated Cx43 was decreased. Our data support the concept that blocking gap junctional communication is an anticonvulsant mechanism.

Fast network oscillations induced by potassium transients in the rat hippocampus in vitro

Brief pressure ejection of solutions containing potassium, caesium or rubidium ions into stratum radiatum of the CA1 or CA3 regions of the hippocampal slice evoked a fast network oscillation. The activity evoked lasted approximately 3-25 s with the predominant frequency component being in the gamma frequency range (30-80 Hz), although beta frequency (15-30 Hz) and ultrafast (> 80 Hz) components could also be seen. The gamma frequency component of the oscillation remained constant, even when large changes in power occurred, and was synchronous across the CA1 region. Measurements with potassium ion-sensitive electrodes revealed that the network oscillation was accompanied by increases (0.5 to 2.0 mM) in the extracellular potassium concentration [K+]o. Bath application of the N-methyl-D-aspartate (NMDA) receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 50 microM) had no significant effect but the alpha-amino-3-hydroxy-5-methyl-4-isooxazolepropionic acid (AMPA)/kainate receptor antagonist 2,3,-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide disodium (NBQX; 20 microM) caused a significant reduction (86.7 +/- 4.5 %) in the power in the gamma frequency range. Residual rhythmic activity, presumably arising in the interneuronal network, was blocked by the GABA(A) receptor antagonist bicuculline. The putative gap junction blocker octanol caused a decrease in the power of the gamma frequency component of 75.5 +/- 5.6 %, while carbenoxolone produced a reduction of only 14 +/- 42 %. These experiments demonstrate that a modest increase in exogenous [K+]o in the hippocampus in vitro is sufficient to evoke a fast network oscillation, which is an emergent property of the synaptically and electrically interconnected neuronal network.

Reduction of high-frequency network oscillations (ripples) and pathological network discharges in hippocampal slices from connexin 36-deficient mice

Recent evidence suggests that electrotonic coupling is an important mechanism for neuronal synchronisation in the mammalian cortex and hippocampus. Various types of network oscillations have been shown to depend on, or be sharpened by, gap junctions between inhibitory interneurones or excitatory projection cells. Here we made use of a targeted disruption of the gene coding for Cx36, a recently discovered neuronal gap junction subunit, to analyse its role in hippocampal network behaviour. Mice lacking Cx36 are viable and lack obvious morphological or behavioural abnormalities. Stimulation of afferent and efferent fibre pathways in hippocampal slices revealed a largely normal function of the synaptic circuitry, including tetanically evoked network oscillations. Spontaneous sharp waves and ripple (approximately 200 Hz) oscillations, however, occurred less frequently in slices from Cx36 -/- mice, and ripples were slightly slower than in littermate controls. Moreover, epileptiform discharges elicited by 4-aminopyridine were attenuated in slices from Cx36 -/- mice. Our findings indicate that Cx36 plays a role in the generation of certain forms of network synchronisation in the hippocampus, namely sharp wave-ripple complexes and hypersynchronous epileptiform discharges.

Ischemia-induced brain damage depends on specific gap-junctional coupling

Ischemic brain injury results in neuronal loss and associated neurologic deficits. Although there is some evidence that intercellular communication via gap junctions can spread oxidative cell injury, the possible role of gap-junctional communication in ischemia-induced cell death is the object of debate. Because gap junctions directly connect the cytoplasms of coupled cells, they offer a way to propagate stress signals from cell to cell. The authors investigated the contribution of gap-junctional communication to cell death using an in vitro ischemia model, which was reproduced by submersion of organotypic hippocampal slices into glucose-free deoxygenated medium. The gap-junctional blocker carbenoxolone significantly decreased the spread of cell death, as measured by propidium iodide staining, over a 48-hour period after the ischemic episode. Carbenoxolone ameliorated the hypoxia-induced impairment of the intrinsic neuronal electrophysiologic characteristics, as measured by whole-cell patch clamp recordings. To determine whether specific connexins were involved in the spread of postischemic cell death, the authors partially reduced the synthesis of specific connexins using antisense oligodeoxynucleotides. Simultaneous knockdown of two connexins localized mostly in neurons, connexins 32 and 26, resulted in significant neuroprotection 48 hours after the hypoxic-hypoglycemic episode. Similarly, partial reduction of the predominant glial connexin 43 significantly decreased cell death. These results indicate that gap-junctional communication contributes to the propagation of hypoxic injury and that specific gap junctions could be a novel target to reduce brain damage.

Specific gap junctions enhance the neuronal vulnerability to brain traumatic injury

Traumatic brain injury results in neuronal loss and associated neurological deficits. Although most research on the factors leading to trauma-induced damage focuses on synaptic or ionic mechanisms, the possible role of direct intercellular communication via gap junctions has remained unexplored. Gap junctions connect directly the cytoplasms of coupled cells; hence, they offer a way to propagate stress signals from cell to cell. We investigated the contribution of gap junctional communication (GJC) to cell death using an in vitro trauma model. The impact injury, induced by a weight dropped on the distal CA1 area of organotypic hippocampal slices, results in glutamate-dependent cell loss. The gap junctional blockers carbenoxolone and octanol decreased significantly post-traumatic cell death, measured by propidium iodide staining over a 72 hr period after the impact. Dye coupling in the pyramidal layers was enhanced immediately after the injury and decreased over the following 24 hr. To determine whether specific connexins were involved in the spread of trauma-induced cell death, we used organotypic slices from connexin43 (Cx43) knock-out mice, as well as acute knock-outs by incubation with antisense oligodeoxynucleotides. Simultaneous knockdown of two neuronal connexins resulted in significant neuroprotection. Slices from the null-mutant Cx43 mice, as well as the acute Cx43 knockdown, also showed decreased cell death after the impact. The gap junctional blockers alleviated the trauma-induced impairment of synaptic function as measured by electrophysiological field potential recordings. These results indicate that GJC enhances the cellular vulnerability to traumatic injury. Hence, specific gap junctions could be a novel target to reduce injury and secondary damage to the brain and maximize recovery from trauma.

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.

Prolonged epileptiform bursting induced by 0-Mg(2+) in rat hippocampal slices depends on gap junctional coupling

The transition from brief interictal to prolonged seizure, or 'ictal', activity is a crucial event in epilepsy. In vitro slice models can mimic many phenomena observed in the electroencephalogram of patients, including transition from interictal to ictaform or seizure-like activity. In field potential recordings, three discharge types can be distinguished: (1) primary discharges making up the typical interictal burst, (2) secondary bursts, lasting several hundred milliseconds, and (3) tertiary discharges lasting for seconds, constituting the ictal series of bursts. The roles of chemical synapses in these classes of burst have been explored in detail. Here we test the hypothesis that gap junctions are necessary for the generation of secondary bursts. In rat hippocampal slices, epileptiform activity was induced by exposure to 0-Mg(2+). Epileptiform discharges started in the CA3 subfield, and generally consisted of primary discharges followed by 4-13 secondary bursts. Three drugs that block gap junctions, halothane (5-10 mM), carbenoxolone (100 microM) and octanol (0.2-1.0 mM), abolished the secondary discharges, but left the primary bursts intact. The gap junction opener trimethylamine (10 mM) reversibly induced secondary and tertiary discharges. None of these agents altered intrinsic or synaptic properties of CA3 pyramidal cells at the doses used. Surgically isolating the CA3 subfield made secondary discharges disappear, and trimethylamine under these conditions was able to restore them.We conclude that gap junctions can contribute to the prolongation of epileptiform discharges.

Can gap-junction blockade preferentially inhibit neuronal hypersynchrony vs. excitability?

Epileptic activity involves a hypersynchronous firing of large populations of brain neurons, some of which are hyperexcitable. This study explored to what extent gap-junction blockade inhibits neuronal synchronization vs. neuronal excitability. We investigated the effects of the gap-junction blockers (GJBs) 1-heptanol, 1-octanol and carbenoxolone vs. the loop diuretic furosemide on spontaneous and evoked epileptiform field potentials, induced in CA3 area of rat hippocampal slices by a 'high K(+)-low Ca(2+)' perfusion fluid. This milieu induced frequent (>30 min(-1)) spontaneous bursts, led single fimbrial stimuli to evoke repetitive population spikes (PSs), and increased PS amplitudes. Both furosemide and the three GJBs gradually reduced spontaneous field bursting, or even stopped it within one hour. The anti-bursting activity of carbenoxolone showed dose-response dependence in the concentration range 50-400 microM. 1-heptanol and 1-octanol markedly and similarly depressed all the epileptiform markers of the evoked responses, whereas carbenoxolone did not reduce the number of repetitive PSs evoked by single stimuli. By its minor effect on evoked responses, carbenoxolone resembled furosemide, rather than its congeners GJBs. These results favor the possibility that selective gap-junction blockade might antagonize epileptic synchronization, without depressing neuronal excitability.

Gap junctions synchronize the firing of inhibitory interneurons in guinea pig hippocampus

The convulsant 4-aminopyridine (4AP) facilitates the synchronous firing of interneurons in the hippocampus, eliciting giant inhibitory postsynaptic potentials (IPSPs) in CA3 pyramidal cells. We used the gap junction blocker carbenoxolone to investigate the role of electrotonic coupling in both the initiation and the maintenance of 4AP-facilitated inhibitory circuit oscillations. Carbenoxolone abolished all synchronized IPSPs in CA3 cells elicited by 4AP in the presence of ionotropic glutamate receptor blockers. Carbenoxolone also blocked the isolated synchronized GABA(B) IPSPs generated in CA3 cells by a subpopulation of interneurons. These data confirm that: (1) the interneurons producing GABA(B) responses in CA3 cells are electrotonically coupled, and (2) gap junctions among interneurons are essential for initiating synchronized interneuron oscillatory firing in 4AP.