Contribution of ionotropic glutamate receptors and voltage-dependent calcium channels to the potentiation phenomenon induced by transient pentylenetetrazol in the CA1 region of rat hippocampal slices

Antiepileptic effects of antioxidant potent extract from Urtica dioica Linn. root on pentylenetetrazole and maximal electroshock induced seizure models

Urtica dioica Linn. (Urticaceae) is a medicinal plant that has shown various therapeutic utilities in folklore medicine along with its use in the treatment of epilepsy. The entire plant has a sensible reservoir of nutritional elements and micronutrients. The purpose of the present study was to investigate the antiepileptic effect of antioxidant potent extract of Urtica dioica root on animal models. Antioxidant activity of various solvent extracts i.e. Petroleum ether extract (PEE), Ethyl acetate extract (EAE), Chloroform extract (CE) and Ethanolic extract (EE) were screened by DPPH radical scavenging assay using Ascorbic acid as the standard. Further the most potent antioxidant extract was subjected to antiepileptic activity against MES and PTZ model. The IC₅₀ values of different Urtica dioica extracts (PEE, CE, EAE, and EE) in antioxidant assay were found to be 167.54 ± 1.97, 134.41 ± 0.82, 88.15 ± 1.39 and 186.38 ± 1.91 μg/ml in DPPH radical scavenging assay, respectively. The EAE has showed the potent antioxidant activity. In experimental study the EAE (100 and 200 mg/kg, p.o) has found to be effective and significant against MES and PTZ induced seizures. The present study also suggested that antioxidant potent extract (EAE) of Urtica dioica root has antiepileptic effect against MES and PTZ induced seizures. However, further research studies will investigate the active component(s) of Urtica dioica responsible for the observed anticonvulsant effects.

Anticonvulsant effect of the hydroethanolic leaf extract of Psydrax subcordata (DC.) Bridson in murine models

ETHNOPHARMACOLOGICAL RELEVANCE

Psydrax subcordata (DC.) Bridson is a tropical medicinal plant used traditionally for the management of epilepsy. However, there is little scientific evidence to support its use.

AIM OF STUDY

The current study investigated the anticonvulsant properties of the hydroethanolic leaf extract of Psydrax subcordata (PSE) in animal models.

MATERIALS AND METHODS

The anticonvulsant effects were evaluated in mouse models of acute seizures (pentylenetetrazole-, picrotoxin-, 4-aminopyridine-, strychnine- and maximal electroshock-induced seizure tests) and status epilepticus (Lithium/pilocarpine-induced SE). The role of GABAergic mechanisms in the actions of the extract was also examined by pre-treatment of animals with flumazenil in the pentylenetetrazole test.

RESULTS

The extract (30, 100 and 300mg/kg, p.o.) significantly delayed the onset and decreased the duration and frequency of pentylenetetrazole- and picrotoxin-convulsions. PSE also reduced the duration of tonic hind limb extensions in the maximal electroshock-induced seizure test. Furthermore, PSE pre-treatment significantly delayed the onset of seizures and improved survival in the 4-aminopyridine-induced seizure test. In the strychnine-induced seizure test, PSE treatment did not significantly affect the latency to convulsions and time until death when compared to controls. PSE exhibited anticonvulsant effects in the lithium/pilocarpine test by delaying the onset of seizures and status epilepticus as well as reducing the severity of seizures and mortality of mice. Again, the anticonvulsant effect of PSE (100mg/kg, p.o.) was blocked by pre-treatment with flumazenil in the PTZ test.

CONCLUSION

PSE has anticonvulsant activity in animal models, and this effect may be mediated, at least partly, through GABAergic mechanisms.

Increased sensitivity to kindling in mice lacking TSP1

The development of a hyperexcitable neuronal network is thought to be a critical event in epilepsy. Thrombospondins (TSPs) regulate synaptogenesis by binding the neuronal α2δ subunit of the voltage-gated calcium channel. TSPs regulate synapse formation during development and in the mature brain following injury. It is unclear if TSPs are involved in hyperexcitability that contributes to the development of epilepsy. Here we explore the development of epilepsy using a pentylenetetrazole (PTZ) kindling model in mice lacking TSP1 and TSP2. Unexpectedly, we found increased sensitivity to PTZ kindling in mice lacking TSP1, while mice lacking TSP2 kindled similar to wild-type. We found that the increased seizure susceptibility in the TSP1 knockout (KO) mice was not due to a compensatory increase in TSP2 mRNA as TSP1/2 KO mice were sensitive to PTZ, similar to the TSP1 KO mice. Furthermore, there were similar levels of TGF-B signal activation during kindling in the TSP1 KO mice compared to wild-type. We observed decreased expression of voltage-dependent calcium channel subunit CACNA2D1 mRNA in TSP1, TSP2, and TSP1/2 KO mice. Decreased CACNA2D2 mRNA was only detected in mice that lacked TSP1 and α2δ-1/2 protein levels in the cortex were lower in the TSP 1/2 KO mice. CACNA2D2 knockout mice have spontaneous seizures and increased PTZ seizure susceptibility. Here we report similar findings, TSP1, and TSP1/2 KO mice have low levels of CACNA2D2 mRNA expression and α2δ-1/2 receptor level in the cortex, and are more susceptible to seizures. CACNA2D2 mutations in mice and humans can cause epilepsy. Our data suggest TSP1 in particular may control CACNA2D2 levels and could be a modifier of seizure susceptibility.

Threshold for epileptiform activity is elevated in prion knockout mice

Prion protein (PrP) is abundant in the nervous system, but its role remains uncertain. Prion diseases depend on an aggregation of the protein that is likely to interfere with its normal function. Loss of function does not in itself cause neurodegeneration, but whether it contributes to the clinical features of the disease remains an open question. Patients with classical Creutzfeldt-Jakob disease (CJD) have a higher than expected incidence of epilepsy. To study the mechanisms by which loss of PrP function may underlie changes in vulnerability to epilepsy in disease, we used several acute epilepsy models: we applied a variety of convulsant treatments (zero-magnesium, bicuculline, and pentylenetetrazol) to slices in vitro from PrP knockout (Prnp0/0) and control mice. In all three epilepsy models, we found that longer delays and/or higher concentrations of convulsants were necessary to generate spontaneous epileptiform activity in Prnp0/0 mice. These results together indicate an increased seizure threshold in Prnp0/0 mice, suggesting that loss of PrP function cannot explain a predisposition to seizures initiation in CJD.

Mapping convulsants' binding to the GABA-A receptor chloride ionophore: a proposed model for channel binding sites

Gamma-aminobutyric acid (GABA) type A receptors play a key role in brain inhibitory neurotransmission, and are ligand-activated chloride channels blocked by numerous convulsant ligands. Here we summarize data on binding of picrotoxin, tetrazoles, beta-lactams, bicyclophosphates, butyrolactones and neurotoxic pesticides to GABA-A ionophore, and discuss functional and structural overlapping of their binding sites. The paper reviews data on convulsants' binding sensitivity to different point mutations in ionophore-lining second trans-membrane domains of GABA-A subunits, and maps possible location of convulsants' sites within the chloride ionophore. We also discuss data on inhibition of glycine, glutamate, serotonin (5-HT3) and N-acetylcholine receptors by GABA-A channel blockers, and examine the applicability of this model to other homologous ionotropic receptors. Positioning various convulsant-binding sites within ionophore of GABA-A receptors, this model enables a better understanding of complex architectonics of ionotropic receptors, and may be used for developing new channel-modulating drugs.

Cyclothiazide induces robust epileptiform activity in rat hippocampal neurons both in vitro and in vivo

Cyclothiazide (CTZ) is a potent blocker of AMPA receptor desensitization. We have recently demonstrated that CTZ also inhibits GABA(A) receptors. Here we report that CTZ induces robust epileptiform activity in hippocampal neurons both in vitro and in vivo. We first found that chronic treatment of hippocampal cultures with CTZ (5 microM, 48 h) results in epileptiform activity in the majority of neurons (80%). The epileptiform activity lasts more than 48 h after washing off CTZ, suggesting a permanent change of the neural network properties after CTZ treatment. We then demonstrated in in vivo recordings that injection of CTZ (5 micromol in 5 microl) into the lateral ventricles of anaesthetized rats also induces spontaneous epileptiform activity in the hippocampal CA1 region. The epileptogenic effect of CTZ is probably due to its enhancing glutamatergic neurotransmission as shown by increasing the frequency and decay time of mEPSCs, and simultaneously inhibiting GABAergic neurotransmission by reducing the frequency of mIPSCs. Comparing to a well-known epileptogenic agent kainic acid (KA), CTZ affects neuronal activity mainly through modulating synaptic transmission without significant change of the intrinsic membrane excitability. Unlike KA, which induces significant cell death in hippocampal cultures, CTZ treatment does not result in any apparent neuronal death. Therefore, the CTZ-induced epilepsy model may provide a novel research tool to elucidate the molecular and cellular mechanisms of epileptogenesis without any complication from drug-induced cell death. The long-lasting epileptiform activity after CTZ washout may also make it a very useful model in screening antiepileptic drugs.

Pentylenetetrazol-induced seizures affect the levels of prolyl oligopeptidase, thimet oligopeptidase and glial proteins in rat brain regions, and attenuation by MK-801 pretreatment

The regulatory mechanisms of neuropeptide-metabolizing enzymes often play a critical role in the pathogenesis of neuronal damage. A systemic administration of pentylenetetrazol (PTZ), an antagonist of GABA(A) receptor ion channel binding site, causes generalized epilepsy in an animal model. In the present study, we examined the involvement of prolyl oligopeptidase (POP), thimet oligopeptidase/neurolysin (EP 24.15/16) and glial proteins in PTZ-treated rat brain regions, and the suppressive effect of MK-801, a non-competitive NMDA receptor antagonist, pretreatment for their proteins. The activity of POP significantly decreased in the hippocampus at 30min and 3h, and in the frontal cortex at 3h after PTZ treatment, and pretreatment with MK-801 recovered the activity in the cortex at 3h. The activity of EP 24.15/16 significantly decreased in the hippocampus at 3h and 1 day, and in the cortex at 3h after the PTZ administration, whereas pretreatment with MK-801 recovered the change of the activity. The Western blot analysis of EP 24.15 showed significant decrease of the protein level in the hippocampus 3h after the PTZ treatment, whereas pretreatment with MK-801 recovered. The expression of GFAP and CD11b immunohistochemically increased in the hippocampus of the PTZ-treated rat as compared with controls. Pretreatment with MK-801 also recovered the GFAP and CD11b expression. These data suggest that PTZ-induced seizures of the rats cause indirect activation of glutamate NMDA receptors, then decrease POP and EP 24.15/16 enzyme activities and EP 24.15 immunoreactivity in the neuronal cells of the hippocampal formation. We speculate that changes of those peptidases in the brain may be related to the levels of the neuropeptides regulating PTZ-induced seizures.

Reversal of pentylenetetrazol-induced potentiation phenomenon by theta pulse stimulation in the CA1 region of rat hippocampal slices

The effect of theta pulse stimulation (TPS) on pentylenetetrazol (PTZ)-induced long-term potentiation of population spikes was studied in the CA1 region of rat hippocampal slices. The field excitatory postsynaptic potential (fEPSP) and population spikes (PS) were recorded from strata radiatum and pyramidale, respectively, following stimulation of Schaffer collaterals. A transient PTZ application produced a long-lasting enhancement of PS amplitude. A 3-min episode of TPS delivered at test-pulse intensity failed to reverse the PTZ potentiation. However, the same stimulation at a higher intensity produced complete reversal of the PTZ potentiation when delivered during the last minutes of PTZ application. Prior application of high-intensity TPS also decreased the amount of PTZ potentiation, whereas it had no long-lasting effect on baseline synaptic responses. High-intensity TPS induced reversal was blocked by adenosine A1 receptor antagonist and, furthermore, was reduced by protein phosphatase 1 inhibitor. The results suggest that mechanism of PTZ-induced LTP reversal involves activation of adenosine receptors and protein phosphatases.