Evidence for decreased calcium dependent potassium conductance in hippocampal CA3 neurons of genetically epilepsy-prone rats

Impact of optogenetic pulse design on CA3 learning and replay: A neural model

Optogenetic manipulation of hippocampal circuitry is an important tool for investigating learning in vivo. Numerous approaches to pulse design have been employed to elicit desirable circuit and behavioral outcomes. Here, we systematically test the outcome of different single-pulse waveforms in a rate-based model of hippocampal memory function at the level of mnemonic replay extension and de novo synaptic weight formation in CA3 and CA1. Lower-power waveforms with long forward or forward and backward ramps yield more natural sequence replay dynamics and induce synaptic plasticity that allows for more natural memory replay timing, in contrast to square or backward ramps. These differences between waveform shape and amplitude are preserved with the addition of noise in membrane potential, light scattering, and protein expression, improving the potential validity of predictions for in vivo work. These results inform future optogenetic experimental design choices in the field of learning and memory.

Activation of Calcium-Activated Chloride Channels Suppresses Inherited Seizure Susceptibility in Genetically Epilepsy-Prone Rats

Inherited seizure susceptibility in genetically epilepsy-prone rats (GEPR-3s) is associated with increased voltage-gated calcium channel currents suggesting a massive calcium influx resulting in increased levels of intraneuronal calcium. Cytosolic calcium, in turn, activates many processes, including chloride channels, to restore normal membrane excitability and limit repetitive firing of the neurons. Here we used EACT and T16Ainh-A01, potent activator and inhibitor of calcium-activated channels transmembrane protein 16A (TMEM16A), respectively, to probe the role of these channels in the pathophysiology of acoustically evoked seizures in the GEPR-3s. We used adult male and female GEPR-3s. Acoustically evoked seizures consisted of wild running seizures (WRSs) that evolved into generalized tonic-clonic seizures (GTCSs) and eventually culminated into forelimb extension (partial tonic seizures). We found that acute EACT treatment at relatively higher tested doses significantly reduced the incidences of WRSs and GTCSs, and the seizure severity in male GEPR-3s. Furthermore, these antiseizure effects were associated with delayed seizure onset and reduced seizure duration. Interestingly, the inhibition of TMEM16A channels reversed EACT’s antiseizure effects on seizure latency and seizure duration. No notable antiseizure effects were observed in female GEPR-3s. Together, these findings suggest that activation of TMEM16A channels may represent a putative novel cellular mechanism for suppressing GTCSs.

Activation of small conductance calcium-activated potassium channels suppresses seizure susceptibility in the genetically epilepsy-prone rats

Small conductance calcium-activated potassium (SK) channels dampen neuronal excitability by contributing to slow afterhyperpolarization (AHP) that follows a series of action potentials, and therefore may represent an intrinsic inhibitory mechanism to prevent seizures. We have previously reported that susceptibility to acoustically evoked seizures was associated with downregulation of SK1 and SK3 subtypes of SK channels in the inferior colliculus of the moderated seizure severity strain of the genetically epilepsy-prone rats (GEPR-3s). Here, we evaluated the effects of 1-ethyl-2-benzimidazolinone (1-EBIO), a potent activator of SK channels, on acoustically evoked seizures in both male and female adult GEPR-3s at various time points post-treatment. Systemic administration of 1-EBIO at various tested doses suppressed seizure susceptibility in both male and female GEPR-3s; however, the complete seizure suppression was only observed following administration of relatively higher doses of 1-EBIO in females. These findings indicate that activation of SK channels results in anticonvulsive action against generalized tonic-clonic seizures in both male and female GEPR-3s, with males exhibiting higher sensitivity than females.

The Cannabinoid-Like Compound, VSN16R, Acts on Large Conductance, Ca2+-Activated K+ Channels to Modulate Hippocampal CA1 Pyramidal Neuron Firing

Large conductance, Ca2+-activated K+ (BKCa) channels are widely expressed in the central nervous system, where they regulate action potential duration, firing frequency and consequential neurotransmitter release. Moreover, drug action on, mutations to, or changes in expression levels of BKCa can modulate neuronal hyperexcitability. Amongst other potential mechanisms of action, cannabinoid compounds have recently been reported to activate BKCa channels. Here, we examined the effects of the cannabinoid-like compound (R,Z)-3-(6-(dimethylamino)-6-oxohex-1-en-1-yl)-N-(1-hydroxypropan-2-yl) benzamide (VSN16R) at CA1 pyramidal neurons in hippocampal ex vivo brain slices using current clamp electrophysiology. We also investigated effects of the BKCa channel blockers iberiotoxin (IBTX) and the novel 7-pra-martentoxin (7-Pra-MarTx) on VSN16R action. VSN16R (100 μM) increased first and second fast after-hyperpolarization (fAHP) amplitude, decreased first and second inter spike interval (ISI) and shortened first action potential (AP) width under high frequency stimulation protocols in mouse hippocampal pyramidal neurons. IBTX (100 nM) decreased first fAHP amplitude, increased second ISI and broadened first and second AP width under high frequency stimulation protocols; IBTX also broadened first and second AP width under low frequency stimulation protocols. IBTX blocked effects of VSN16R on fAHP amplitude and ISI. 7-Pra-MarTx (100 nM) had no significant effects on fAHP amplitude and ISI but, unlike IBTX, shortened first and second AP width under high frequency stimulation protocols; 7-Pra-MarTx also shortened second AP width under low frequency stimulation protocols. However, in the presence of 7-Pra-MarTx, VSN16R retained some effects on AP waveform under high frequency stimulation protocols; moreover, VSN16R effects were revealed under low frequency stimulation protocols. These findings demonstrate that VSN16R has effects in native hippocampal neurons consistent with its causing an increase in initial firing frequency via activation of IBTX-sensitive BKCa channels. The differential pharmacological effects described suggest that VSN16R may differentially target BKCa channel subtypes.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Pharmacokinetic/pharmacodynamic considerations for epilepsy - depression comorbidities

INTRODUCTION

Epilepsy may be frequently associated with psychiatric disorders and its co-existence with depression usually results in the reduced quality of life of patients with epilepsy. Also, the efficacy of antiepileptic treatment in depressed patients with epilepsy may be significantly reduced.

AREAS COVERED

Results of experimental studies indicate that antidepressants co-administered with antiepileptic drugs may either increase their anticonvulsant activity, remain neutral or decrease the protective action of antiepileptic drugs in models of seizures. Apart from purely pharmacodynamic interactions, pharmacokinetic mechanisms have been proven to contribute to the final outcome. We report on clinical data regarding the pharmacokinetic interactions of enzyme-inducing antiepileptic drugs with various antidepressants, whose plasma concentration may be significantly reduced. On the other hand, antidepressants (especially selective serotonin reuptake inhibitors) may influence the metabolism of antiepileptics, in many cases resulting in the elevation of plasma concentration of antiepileptic drugs.

EXPERT OPINION

The preclinical data may provide valuable clues on how to combine these two groups of drugs - antidepressant drugs neutral or potentiating the anticonvulsant action of antiepileptics are recommended in this regard. Avoidance of antidepressants clearly decreasing the convulsive threshold or decreasing the anticonvulsant efficacy of antiepileptic drugs (f.e. bupropion or mianserin) in patients with epilepsy is recommended.

Presynaptic BK channels control transmitter release: physiological relevance and potential therapeutic implications

BK channels are large conductance potassium channels characterized by four pore-forming α subunits, often co-assembled with auxiliary β and γ subunits to regulate Ca(2+) sensitivity, voltage dependence and gating properties. Abundantly expressed in the CNS, they have the peculiar characteristic of being activated by both voltage and intracellular calcium rise. The increase in intracellular calcium via voltage-dependent calcium channels (Cav ) during spiking triggers conformational changes and BK channel opening. This narrows the action potential and induces a fast after-hyperpolarization that shuts calcium channels. The tight coupling between BK and Cav channels at presynaptic active zones makes them particularly suitable for regulating calcium entry and neurotransmitter release. While in most synapses, BK channels exert a negative control on transmitter release under basal conditions, in others they do so only under pathological conditions, serving as an emergency brake to protect against hyperactivity. In particular cases, by interacting with other channels (i.e. limiting the activation of the delayed rectifier and the inactivation of Na(+) channels), BK channels induce spike shortening, increase in firing rate and transmitter release. Changes in transmitter release following BK channel dysfunction have been implicated in several neurological disorders including epilepsy, schizophrenia, fragile X syndrome, mental retardation and autism. In particular, two mutations, one in the α and one in the β3 subunit, resulting in a gain of function have been associated with epilepsy. Hence, these discoveries have allowed identification of BK channels as new drug targets for therapeutic intervention.

Potassium Channels in Epilepsy

This review attempts to give a concise and up-to-date overview on the role of potassium channels in epilepsies. Their role can be defined from a genetic perspective, focusing on variants and de novo mutations identified in genetic studies or animal models with targeted, specific mutations in genes coding for a member of the large potassium channel family. In these genetic studies, a demonstrated functional link to hyperexcitability often remains elusive. However, their role can also be defined from a functional perspective, based on dynamic, aggravating, or adaptive transcriptional and posttranslational alterations. In these cases, it often remains elusive whether the alteration is causal or merely incidental. With ∼80 potassium channel types, of which ∼10% are known to be associated with epilepsies (in humans) or a seizure phenotype (in animals), if genetically mutated, a comprehensive review is a challenging endeavor. This goal may seem all the more ambitious once the data on posttranslational alterations, found both in human tissue from epilepsy patients and in chronic or acute animal models, are included. We therefore summarize the literature, and expand only on key findings, particularly regarding functional alterations found in patient brain tissue and chronic animal models.

The riluzole derivative 2-amino-6-trifluoromethylthio-benzothiazole (SKA-19), a mixed KCa2 activator and NaV blocker, is a potent novel anticonvulsant

Inhibitors of voltage-gated sodium channels (Na(v)) have been used as anticonvulsants since the 1940s, while potassium channel activators have only been investigated more recently. We here describe the discovery of 2-amino-6-trifluoromethylthio-benzothiazole (SKA-19), a thioanalog of riluzole, as a potent, novel anticonvulsant, which combines the two mechanisms. SKA-19 is a use-dependent NaV channel blocker and an activator of small-conductance Ca(2+)-activated K(+) channels. SKA-19 reduces action potential firing and increases medium afterhyperpolarization in CA1 pyramidal neurons in hippocampal slices. SKA-19 is orally bioavailable and shows activity in a broad range of rodent seizure models. SKA-19 protects against maximal electroshock-induced seizures in both rats (ED50 1.6 mg/kg i.p.; 2.3 mg/kg p.o.) and mice (ED50 4.3 mg/kg p.o.), and is also effective in the 6-Hz model in mice (ED50 12.2 mg/kg), Frings audiogenic seizure-susceptible mice (ED50 2.2 mg/kg), and the hippocampal kindled rat model of complex partial seizures (ED50 5.5 mg/kg). Toxicity tests for abnormal neurological status revealed a therapeutic index (TD50/ED50) of 6-9 following intraperitoneal and of 33 following oral administration. SKA-19 further reduced acute pain in the formalin pain model and raised allodynic threshold in a sciatic nerve ligation model. The anticonvulsant profile of SKA-19 is comparable to riluzole, which similarly affects Na(V) and KCa2 channels, except that SKA-19 has a ~4-fold greater duration of action owing to more prolonged brain levels. Based on these findings we propose that compounds combining KCa2 channel-activating and Na(v) channel-blocking activity exert broad-spectrum anticonvulsant and analgesic effects.

BK channel activators and their therapeutic perspectives

The large conductance calcium- and voltage-activated K⁺ channel (KCa1.1, BK, MaxiK) is ubiquitously expressed in the body, and holds the ability to integrate changes in intracellular calcium and membrane potential. This makes the BK channel an important negative feedback system linking increases in intracellular calcium to outward hyperpolarizing potassium currents. Consequently, the channel has many important physiological roles including regulation of smooth muscle tone, neurotransmitter release and neuronal excitability. Additionally, cardioprotective roles have been revealed in recent years. After a short introduction to the structure, function and regulation of BK channels, we review the small organic molecules activating BK channels and how these tool compounds have helped delineate the roles of BK channels in health and disease.

A new rapid kindling variant for induction of cortical epileptogenesis in freely moving rats

Kindling, one of the most used models of experimental epilepsy is based on daily electrical stimulation in several brain structures. Unlike the classic or slow kindling protocols (SK), the rapid kindling types (RK) described until now require continuous stimulation at suprathreshold intensities applied directly to the same brain structure used for subsequent electrophysiological and immunohistochemical studies, usually the hippocampus. However, the cellular changes observed in these rapid protocols, such as astrogliosis and neuronal loss, could be due to experimental manipulation more than to epileptogenesis-related alterations. Here, we developed a new RK protocol in order to generate an improved model of temporal lobe epilepsy (TLE) which allows gradual progression of the epilepsy as well as obtaining an epileptic hippocampus, thus avoiding direct surgical manipulation and electric stimulation over this structure. This new protocol consists of basolateral amygdala (BLA) stimulation with 10 trains of biphasic pulses (10 s; 50 Hz) per day with 20 min-intervals, during 3 consecutive days, using a subconvulsive and subthreshold intensity, which guarantees tissue integrity. The progression of epileptic activity was evaluated in freely moving rats through electroencephalographic (EEG) recordings from cortex and amygdala, accompanied with synchronized video recordings. Moreover, we assessed the effectiveness of RK protocol and the establishment of epilepsy by evaluating cellular alterations of hippocampal slices from kindled rats. RK protocol induced convulsive states similar to SK protocols but in 3 days, with persistently lowered threshold to seizure induction and epileptogenic-dependent cellular changes in amygdala projection areas. We concluded that this novel RK protocol introduces a new variant of the chronic epileptogenesis models in freely moving rats, which is faster, highly reproducible and causes minimum cell damage with respect to that observed in other experimental models of epilepsy.

Dual mechanisms diminishing tonic GABAA inhibition of dentate gyrus granule cells in Noda epileptic rats

The Noda epileptic rat (NER), a Wistar colony mutant, spontaneously has tonic-clonic convulsions with paroxysmal discharges. In the present study, we measured phasic and tonic γ-aminobutyric acid A (GABAA) current (I tonic) in NER hippocampal dentate gyrus granule cells and compared the results with those of normal parent strain Wistar rats (WIS). I tonic, revealed by a bicuculline-induced outward shift in holding current, was significantly smaller in NER than in WIS (P < 0.01). The frequency of inhibitory postsynaptic currents (IPSCs) was also significantly lower in NER than in WIS (P < 0.05), without significant differences in the IPSC amplitude or decay time between WIS and NER. I tonic attenuation in NER was further confirmed in the presence of GABA transporter blockers, NO-711 and nipecotic acid, with no difference in neuronal GABA transporter expression between WIS and NER. I tonic responses to extrasynaptic GABAA receptor agonists (THIP and DS-2) were significantly reduced in NER compared with WIS (P < 0.05). Allopregnanolone caused less I tonic increase in NER than in WIS, while it prolonged the IPSC decay time to a similar rate in the two groups. Expression of the GABAA receptor δ-subunit was decreased in the dentate gyrus of NER relative to that of WIS. Taken together, our results showed that a combination of attenuated presynaptic GABA release and extrasynaptic GABAA receptor expression reduced I tonic amplitude and its sensitivity to neurosteroids, which likely diminishes the gating function of dentate gyrus granule cells and renders NER more susceptible to seizure propagation.

Rhythm and blues: animal models of epilepsy and depression comorbidity

Clinical evidence shows a strong, bidirectional comorbidity between depression and epilepsy that is associated with decreased quality of life and responsivity to pharmacotherapies. At present, the neurobiological underpinnings of this comorbidity remain hazy. To complicate matters, anticonvulsant drugs can cause mood disturbances, while antidepressant drugs can lower seizure threshold, making it difficult to treat patients suffering from both depression and epilepsy. Animal models have been created to untangle the mechanisms behind the relationship between these disorders and to serve as screening tools for new therapies targeted to treat both simultaneously. These animal models are based on chemical interventions (e.g. pentylenetetrazol, kainic acid, pilocarpine), electrical stimulations (e.g. kindling, electroshock), and genetic/selective breeding paradigms (e.g. genetically epilepsy-prone rats (GEPRs), genetic absence epilepsy rat from Strasbourg (GAERS), WAG/Rij rats, swim lo-active rats (SwLo)). Studies on these animal models point to some potential mechanisms that could explain epilepsy and depression comorbidity, such as various components of the dopaminergic, noradrenergic, serotonergic, and GABAergic systems, as well as key brain regions, like the amygdala and hippocampus. These models have also been used to screen possible therapies. The purpose of the present review is to highlight the importance of animal models in research on comorbid epilepsy and depression and to explore the contributions of these models to our understanding of the mechanisms and potential treatments for these disorders.

Selective changes in inhibition as determinants for limited hyperexcitability in the insular cortex of epileptic rats

The insular cortex (IC) is involved in the generalization of epileptic discharges in temporal lobe epilepsy (TLE), whereas seizures originating in the IC can mimic the epileptic phenotype seen in some patients with TLE. However, few studies have addressed the changes occurring in the IC in TLE animal models. Here, we analyzed the immunohistochemical and electrophysiological properties of IC networks in non-epileptic control and pilocarpine-treated epileptic rats. Neurons identified with a neuron-specific nuclear protein antibody showed similar counts in the two types of tissue but parvalbumin- and neuropeptide Y-positive interneurons were significantly decreased (parvalbumin, approximately -35%; neuropeptide Y, approximately -38%; P < 0.01) in the epileptic IC. Non-adapting neurons were seen more frequently in the epileptic IC during intracellular injection of depolarizing current pulses. In addition, single-shock electrical stimuli elicited network-driven epileptiform responses in 87% of epileptic and 22% of non-epileptic control neurons (P < 0.01) but spontaneous postsynaptic potentials had similar amplitude, duration and intervals of occurrence in the two groups. Finally, pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials had more negative reversal potential (P < 0.01) and higher peak conductance (P < 0.05) in epileptic tissue. These data reveal moderate increased network excitability in the IC of pilocarpine-treated epileptic rats. We propose that this limited degree of hyperexcitability originates from the loss of parvalbumin- and neuropeptide Y-positive interneurons that is compensated by an increased drive for GABA(A) receptor-mediated inhibition.

Protein expression of small conductance calcium-activated potassium channels is altered in inferior colliculus neurons of the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) exhibits inherited predisposition to sound stimuli-induced generalized tonic-clonic seizures (audiogenic reflex seizures) and is a valid model to study the physiopathology of epilepsy. In this model, the inferior colliculus (IC) exhibits enhanced neuronal firing that is critical in the initiation of reflex audiogenic seizures. The mechanisms underlying IC neuronal hyperexcitability that leads to seizure susceptibility are not as yet fully understood. The present report shows that the levels of protein expression of SK1 and SK3 subtypes of the small conductance Ca2+-activated K+ channels were significantly decreased, while SK2 channel proteins were increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3), as compared to control Sprague-Dawley rats. No significant change was found in the expression of BK channel proteins in IC neurons of SN-GEPR-3s. Single episode of reflex audiogenic seizures in the GEPR-3s did not significantly alter the protein expression of SK1-3 and BK channels in IC neurons compared to SN-GEPR-3s. Thus, downregulation of SK1 and SK3 channels and upregulation of SK2 channels provide direct evidence that these Ca2+-activated K+ channels play important roles in IC neuronal hyperexcitability that leads to inherited seizure susceptibility in the GEPR.

Calcium channel dysfunction in inferior colliculus neurons of the genetically epilepsy-prone rat

Voltage-gated calcium (Ca(2+)) channels are thought to play an important role in epileptogenesis and seizure generation. Here, using the whole cell configuration of patch-clamp techniques, we report on the modifications of biophysical and pharmacological properties of high threshold voltage-activated Ca(2+) channel currents in inferior colliculus (IC) neurons of the genetically epilepsy-prone rats (GEPR-3s). Ca(2+) channel currents were measured by depolarizing pulses from a holding potential of - 80 mV using barium (Ba(2+)) as the charge carrier. We found that the current density of high threshold voltage-activated Ca(2+) channels was significantly larger in IC neurons of seizure-naive GEPR-3s compared to control Sprague-Dawley rats, and that seizure episodes further enhanced the current density in the GEPR-3s. The increased current density was reflected by both a - 20 mV shifts in channel activation and a 25% increase in the non-inactivating fraction of channels in seizure-naive GEPR-3s. Such changes were reduced by seizure episodes in the GEPR-3s. Pharmacological analysis of the current density suggests that upregulation of L-, N- and R-type of Ca(2+) channels may contribute to IC neuronal hyperexcitability that leads to seizure susceptibility in the GEPR-3s.

The slow afterhyperpolarization modulates high pH-induced changes in the excitability of rat CA1 pyramidal neurons

Extra- and intracellular recordings from the CA1 region of rat hippocampal slices were employed to examine the role of the slow afterhyperpolarization (sAHP) in modulating the increases in neuronal excitability observed on increasing extracellular pH (pHo) from 7.4 to 7.7. In the majority of experiments, an antidromic conditioning stimulus applied in the presence of D(-)-2-amino-5-phosphonopentanoic acid (D-APV), 6-cyano-7-nitroquinoxaline-2,3-dione disodium salt (CNQX) and bicuculline was employed to elicit a sAHP, and an antidromic test stimulus was applied during the sAHP. At pHo 7.4, a single conditioning stimulus elicited an action potential followed by a sAHP, which in turn inhibited the response to the test stimulus compared with the conditioning stimulus. Increasing the number of action potentials in the conditioning stimulus augmented the sAHP and further inhibited the test response, whereas isoproterenol inhibited the sAHP and prevented the relative inhibition of the test response. At pHo 7.7, a single conditioning stimulus elicited a burst of action potentials followed by a large sAHP, which in turn prevented the test stimulus from eliciting a burst of action potentials and, in extracellular recordings, further increased the inhibition of the test response. The latter effect did not solely reflect a high pHo-induced increase in the conditioning response (and, thus, the subsequent sAHP), but rather involved a more direct effect of high pHo to augment the sAHP. The results indicate that increasing pHo increases the excitability of CA1 neurons to an initial stimulus; however, a high pHo-dependent increase in the sAHP evoked by the initial stimulus limits the response to subsequent stimuli.

Normal electrical properties of hippocampal neurons modelling early Huntington disease pathogenesis

Huntington disease (HD) is a neurodegenerative disorder caused by an unstable and progressive expansion of a CAG trinucleotide repeat tract in the HD gene. Previous studies using truncated forms of the HD gene have shown pronounced deficits in synaptic transmission and plasticity but rather modest changes in intrinsic cellular properties, despite overt pathology. The knock-in mice carrying a 72-80 CAG repeat mutation is an accurate genetic model of early stage HD, displaying a more subtle disease phenotype. To relate full-length HD gene expression and differential polyglutamine expansion with possible pathophysiological changes in salient electrophysiological properties of neurons that may underlie early symptoms of HD, including mood and cognitive impairments, we have conducted whole-cell recordings from hippocampal area CA1 pyramidal cells in Hdh6/Q72 and Hdh4/Q80 knock-in mice. Electrophysiological characterisation of cells obtained from young adult (<4 months) HD mice harbouring an expanded CAG repeat stretch and age-matched wild type (WT) mice revealed no significant differences in any of the active or passive membrane properties investigated. Similar findings, showing a lack of significant differences in cellular electrical properties, were obtained from cells of aged (>18 months) HD mice and WT controls, despite modest levels of repeat length variability demonstrated by single cell PCR. Thus, the current study indicates a lack of overt changes in the electrical membrane properties of pyramidal cells in HD mice accurately modelling early stage HD pathology. Furthermore, together with our previous work, these findings point to a synaptic rather than cellular locus of HD-related pathology.

In vivo characterisation of the small-conductance KCa (SK) channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) as a potential anticonvulsant

Owing to their activation by increased intracellular Ca(2+) levels following burst firing, and the resultant hyperpolarisation and dampening of neuronal excitability, the small-conductance Ca(2+)-activated K(+) (SK(Ca)) channels have been proposed as a potential target for novel antiepileptic drugs. Indeed, the channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) has been shown to reduce epileptiform activity in vitro. Accordingly, this study has investigated the therapeutic potential of 1-EBIO using a range of in vivo seizure models, and assessed the adverse effect liability with the rotarod and locomotor activity paradigms. To aid benchmarking of 1-EBIO's therapeutic and adverse effect potential, it was tested alongside two currently marketed antiepileptic drugs, phenytoin and levetiracetam. 1-EBIO was found to be effective at reducing seizure incidence in mice following maximal electroshock (ED(50) 36.0 mg/kg) as well as increasing the threshold to electrically- and pentylenetetrazole-induced seizures (TID(10)s 7.3 and 21.5 mg/kg, respectively). However, results from the mouse rotarod test revealed a strong adverse effect potential within the therapeutic dose range (ID(50) 35.6 mg/kg), implying a significantly inferior therapeutic index with respect to the comparator compounds. These results, therefore, support the in vitro data detailing 1-EBIO's reduction of epileptiform activity. However, the use of in vivo models has revealed a significant adverse effect potential within the therapeutic dose range. Nevertheless, given the multiplicity of SK(Ca) channel subunits and that 1-EBIO has been shown to enhance additional, non-SK(Ca) carried currents, these findings do not preclude the possibility that more selective enhancers of SK(Ca) function could prove to be effective as antiepileptic medications.

Inhibition modifies the effects of slow calcium-activated potassium channels on epileptiform activity in a neuronal network model

Generation of epileptiform activity typically results from a change in the balance between network excitation and inhibition. Experimental evidence indicates that alterations of either synaptic activity or intrinsic membrane properties can produce increased network excitation. The slow Ca(2+)-activated K(+) currents (sI(AHP)) are important modulators of neuronal firing rate and excitability and have important established and potential roles in epileptogenesis. While the effects of changes in sI(AHP) on individual neuronal excitability are readily studied and well established, the effects of such changes on network behavior are less well known. The experiments here utilize a defined small network model of multicompartment pyramidal cells and an inhibitory interneuron to study the effects of changes in sI(AHP) on network behavior. The benefits of this model system include the ability to observe activity in all cells in a network and the effects of interactions of multiple simultaneous influences. In the model with no inhibitory interneuron, increasing sI(AHP) results in progressively decreasing burst activity. Adding an inhibitory interneuron changes the observed effects; at modest inhibitory strengths, increasing sI(AHP) in all network neurons actually results in increased network bursting (except at very high values). The duration of the burst activity is influenced by the length of delay in a feedback loop, with longer loops resulting in more prolonged bursting. These observations illustrate that the study of potential antiepileptogenic membrane effects must be extended to realistic networks. Network inhibition can dramatically alter the observations seen in pure excitatory networks.

Reduction of intracellular calcium removal rate can explain changes in seizure dynamics: studies in neuronal network models

Complex partial seizures originating from mesial temporal structures are characterized by relatively short durations of organized rhythmic activity (ORA) of 5-8 Hz, typically lasting less than 60s. Previous investigations into seizure dynamics have revealed that this ORA undergoes a monotonic decline prior to seizure evolution into intermittent bursting and subsequent seizure termination. Large neural network models of simplified single-compartment neurons were employed to address the hypothesis that changes in the free intracellular calcium ([Ca2+]i) removal rate during network bursting can result in the alterations of rhythmic seizure activity similar to that observed in recordings from humans. Both exponential and linear models of decreasing calcium removal rates resulted in changes in the predominant frequency of network bursting very similar in frequency and time course to those seen in human intracranial recordings. This supports the concept that changes in [Ca2+]i removal can explain this important network behavior, while not excluding alternative hypotheses. Identifying potential mechanisms underlying the dynamic changes seen in epileptogenic activity in large neural networks can provide important insights into seizure evolution and termination. Model neural network ensembles are attractive systems to address these questions that are difficult to investigate in biological preparations.

Synaptic regulation of the slow Ca2+-activated K+ current in hippocampal CA1 pyramidal neurons: implication in epileptogenesis

The slow Ca2+-activated K+ current (sI(AHP)) plays a critical role in regulating neuronal excitability, but its modulation during abnormal bursting activity, as in epilepsy, is unknown. Because synaptic transmission is enhanced during epilepsy, we investigated the synaptically mediated regulation of the sI(AHP) and its control of neuronal excitability during epileptiform activity induced by 4-aminopyridine (4AP) or 4AP+Mg2+-free treatment in rat hippocampal slices. We used electrophysiological and photometric Ca2+ techniques to analyze the sI(AHP) modifications that parallel epileptiform activity. Epileptiform activity was characterized by slow, repetitive, spontaneous depolarizations and action potential bursts and was associated with increased frequency and amplitude of spontaneous excitatory postsynaptic currents and a reduced sI(AHP.) The metabotropic glutamate receptor (mGluR) antagonist (S)-alpha-methyl-4-carboxyphenylglycine did not modify synaptic activity enhancement but did prevent sI(AHP) inhibition and epileptiform discharges. The mGluR-dependent regulation of the sI(AHP) was not caused by modulated intracellular Ca2+ signaling. Histamine, isoproterenol, and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid reduced the sI(AHP) but did not increase synaptic activity and failed to evoke epileptiform activity. We conclude that 4AP or 4AP+Mg-free-induced enhancement of synaptic activity reduced the sI(AHP) via activation of postsynaptic group I/II mGluRs. The increased excitability caused by the lack of negative feedback provided by the sI(AHP) contributes to epileptiform activity, which requires the cooperative action of increased synaptic activity.

Changes in neuronal excitability and synaptic function in a chronic model of temporal lobe epilepsy

Long-term potentiation and depression of glutamatergic synaptic responses are accompanied by an increased firing probability of neurons in response to a given excitatory input. This property, named excitatory postsynaptic potential/spike potentiation, has also been described in epileptic tissue and has pro-epileptic consequences. In this study, we show that excitatory postsynaptic potential/spike potentiation can be reversed in the kainic acid lesioned rat hippocampus, a chronic model of temporal lobe epilepsy. Simultaneous in vitro extracellular recordings in stratum radiatum and stratum pyramidale were performed in the CA1 area of the kainic acid lesioned rat hippocampal slices. Fifteen minutes, application of the K(+) channel blocker tetraethylammonium resulted in excitatory postsynaptic potential/spike potentiation (measured 90min after the start of the washout period) which could be reversed by subsequent low-frequency or tetanic stimuli. Excitatory postsynaptic potential/spike potentiation and its subsequent reversal by an electrical conditioning stimulus were found to have a N-methyl-D-aspartate receptor-independent component. Tetraethylammonium treatment also resulted in excitatory postsynaptic potential/spike potentiation of pharmacologically isolated N-methyl-D-aspartate receptor-mediated responses which could be reversed by subsequent low-frequency or tetanic stimuli. We conclude that excitatory postsynaptic potential/spike potentiation can be reversed in epileptic tissue, even in the absence of synaptic plasticity. These results suggest the presence of endogenous regulatory mechanisms which are able to decrease cell excitability.

Block of rat brain recombinant SK channels by tricyclic antidepressants and related compounds

SK channels are small conductance, Ca(2+)-activated K(+) channels that underlie neuronal slow afterhyperpolarization and mediate spike frequency adaptation. Using the patch clamp technique, we tested the effects of eight clinically relevant psychoactive compounds structurally related to the tricyclic antidepressants, on SK2 subtype channels cloned from rat brain and functionally expressed in the human embryonic kidney cell line, HEK293. Amitriptyline, carbamazepine, chlorpromazine, cyproheptadine, imipramine, tacrine and trifluperazine blocked SK2 channel currents with micromolar affinity. The block was reversible and concentration-dependent. The potency differed according to chemical structure. In contrast, the cognitive enhancer linopirdine was ineffective at blocking these channels. Our results point to a distinct pharmacological profile for SK channels.

Patch-clamp analysis of anesthetic interactions with recombinant SK2 subtype neuronal calcium-activated potassium channels

Small conductance calcium-activated potassium channels (SK) mediate spike frequency adaptation and underlie the slow afterhyperpolarization in central neurons. We tested the actions of several anesthetics on the SK2 subtype of recombinant SK channels, cloned from rat brain and functionally expressed in a mammalian cell line. Butanol, ethanol, ketamine, lidocaine, and methohexital blocked recombinant SK2 channel currents, measured in the whole-cell patch clamp recording mode. The block was reversible, dose-dependent, and of variable efficacy. The inhaled anesthetics chloroform, desflurane, enflurane, halothane, isoflurane, and sevoflurane produced little or no block when applied at 1 minimum alveolar anesthetic concentration; varying degrees of modulation were observed at very large concentrations (10 minimum alveolar concentration). The extent of block by inhaled anesthetics did not appear to depend on concentration or membrane voltage.

IMPLICATIONS

We describe differential effects of anesthetics on cloned small conductance calcium-activated potassium channels from brain that may play a role in generating the effects or side effects of anesthetics.

Two electrophysiologically distinct types of granule cells in epileptic human hippocampus

We investigated the electrophysiology of morphologically identified human granule cells with conventional current-clamp recordings. Slices were prepared from 14 human epileptic sclerotic hippocampi. Granule cells appeared to have a diverse electrophysiology. Each cell was distinguished by the shape of the afterhyperpolarization following single action potentials. Two types could be discerned: type I afterhyperpolarizations were monophasic and brief (typically 10-40 ms), whilst type II afterhyperpolarizations were biphasic and long (typically 50-100 ms). The two types also differed in their repetitive firing behaviour and action potential morphology: type I cells had significantly weaker spike frequency adaptation, lower action potential amplitude and smaller action potential upstroke/downstroke ratio. Thus, the firing pattern of type I cells resembled that of rodent dentate interneurons. In contrast, the corresponding parameters of type II cells were comparable to rodent dentate granule cells. Despite the distinct firing patterns, membrane properties were not different. The two types of cells also differed in their synaptic responses to stimulation of the perforant path. At strong suprathreshold stimulation intensity, type I cells always generated multiple action potentials, whereas type II cells usually spiked once only. Slow inhibitory postsynaptic potentials were not detected in type I neurons, but were easily identified in type II neurons. Extracellular recordings of perforant path-evoked field potentials in the cell layer confirmed that the majority of granule cells showed multiple discharges even when we recorded simultaneously from a type II cell that generated one action potential only. The morphology of both types of cells was characteristic of what has been described for primate dentate granule cells. Based on comparisons with previous studies on rodent and human granule cells, we tentatively hypothesize that: (i) the majority of granule cells from sclerotic hippocampus display an hyperexcitable epileptogenic electrophysiology; (ii) there is a subset of granule cells whose electrophysiology is preserved and is more comparable to granule cells from non-epileptic hippocampus.

Differential roles in the neuronal network for audiogenic seizures are observed among the inferior colliculus subnuclei and the amygdala

The inferior colliculus (IC) is established as the initiation site within the neuronal network for audiogenic seizures (AGS), but the relative importance of the IC subnuclei in AGS is controversial. The lateral and basolateral subdivisions of the amygdala are implicated in the expansion of the AGS network that occurs during AGS kindling. However, the role of the amygdala in the AGS network in nonkindled AGS is unknown. NMDA receptors are implicated in modulation of AGS and in neurotransmission in both the IC and amygdala. Therefore, changes in AGS severity in genetically epilepsy-prone rats (GEPR-9s) were examined after bilateral focal microinjection into IC subnuclei or lateral/basolateral subdivisions of the amygdala of a competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP). Blockade of AGS in IC central nucleus (ICc) and external cortex (ICx) was observed at identical doses of CPP, but these doses were ineffective in IC dorsal cortex (ICd). Microinjection of CPP into the amygdala did not produce significant changes in AGS severity except at doses 20 times those effective in IC. The latter data contrast with the anticonvulsant effects of amygdala microinjections on seizure severity in kindled AGS reported previously. The present data in concord with neuronal recording studies of these nuclei suggest that the ICc is the most critical site in AGS initiation, the ICx in propagation, and that the ICd plays a lesser role in the AGS network. The amygdala does not appear to play a requisite role in the neuronal network for AGS in animals that have not been subjected to AGS kindling.

Evidence of increased excitability in GEPR hippocampus preceding development of seizure susceptibility

The genetically epilepsy-prone rat (GEPR) provides a valuable model to study the mechanism of neonatal seizure susceptibility because seizure predisposition in GEPRs is determined by factors present from birth. We have previously shown that reduced afterhyperpolarization (AHP), reduced spike frequency adaptation and increased excitation with repetitive stimulation are present in the adult GEPRs. To investigate whether these abnormalities are present at birth or appear at the time when GEPRs show seizure susceptibility and to elucidate whether these abnormalities were a consequence of seizure experience (the adult rats previously tested were induced to seize in three tests), we studied the membrane and synaptic properties of CA3 hippocampal neurons in preseizing offspring of GEPR-9s (seizure naive GEPRs). Electrophysiological recordings were done in the in vitro brain slice preparation during three different stages of early postnatal development (postnatal day (P) 7-10, P12-15 and P18-28) in GEPRs and compared to age matched control Sprague-Dawley (SD) rats. Reduction in AHP amplitude and duration and reduced inhibitory post synaptic potentials (IPSPs) were observed in the CA3 region in all the three stages tested. Reduction in spike frequency adaptation in 40% of CA3 neurons and reduction in fast AHP occurred in the 3rd and 4th weeks of postnatal development in GEPRs. Therefore, our results suggest that reduced synaptic inhibition and increased membrane excitability in the CA3 circuitry are present from early postnatal development and may represent few of the general cortical features that might eventually contribute to development of enhanced seizure susceptibility in developing GEPRs.

Effects of prenatal cocaine exposure on the developing hippocampus: intrinsic and synaptic physiology

A variety of neurological complications has been reported in infants exposed to cocaine during gestation. In the present study, intrinsic cell properties of hippocampal neurons from CA1, CA3, and dentate gyrus regions were measured and compared in tissue from neonatal rats exposed to saline or cocaine in utero. Synaptic properties of the CA1 pyramidal cell region were analyzed at postnatal day (P) 20 with the use of extracellular and intracellular recording techniques. In vitro intracellular recordings (n = 223) obtained at P10, P15 and P20 in tissue from cocaine- and saline-exposed animals revealed no differences in standard cell properties such as resting membrane potential, input resistance, time constant, and action potential amplitude or duration. Hippocampal slices from cocaine-exposed animals exhibited a marked reduction of spike frequency adaptation for all three types of principal hippocampal neurons (e.g., CA1, CA3, and granule cells). The amplitudes of afterhyperpolarizations following a spike train were also decreased in CA1 and CA3 cells in tissue from cocaine-exposed animals. Extracellular and intracellular recordings in the CA1 pyramidal cell region at P20 were obtained to assess and compare synaptic function in tissue from cocaine- and saline-exposed animals. In hippocampal slices from cocaine-exposed animals, synaptic responses in the CA1 region were characterized by multiple population spike activity and reduced inhibitory postsynaptic potentials. The reduction in fast inhibitory postsynaptic potential conductance was not associated with a change in reversal potential. These results suggest that gestational cocaine exposure induces significant changes in intrinsic and synaptic electrophysiological properties of hippocampal neurons in the developing animal. The cell and synaptic features are consistent with an increase in hippocampal excitability, which may contribute to the neurobehavioral deficits and epileptogenic predisposition reported in this infant population. As such, this in utero drug exposure model may provide a useful system in which to elucidate and study the basic cellular mechanisms underlying neurological complications associated with maternal cocaine abuse.