Effects of anticonvulsive drugs on pentylenetetrazol kindling and long-term potentiation in freely moving rats

Methanol extract of Ficus platyphylla ameliorates seizure severity, cognitive deficit and neuronal cell loss in pentylenetetrazole-kindled mice

Decoctions of Ficus plathyphylla are used in Nigeria's folk medicine to manage epilepsy for many years and their efficacies are widely acclaimed among the rural communities of Northern Nigeria. In this study, we examined the ameliorative effects of the standardized methanol extract of Ficus platyphylla (FP) stem bark on seizure severity, cognitive deficit and neuronal cell loss in pentylenetetrazole-kindled mice. The (35)S-GTPγS, glutamate and γ-aminobutyric acid receptors binding properties of the extract were also evaluated. Male CD-1 mice were kindled with an initial subeffective dose of pentylenetetrazole (PTZ, 37.5mg/kg, i.p.) for a total of 13 convulsant injections and the treatment groups concurrently received FP (100 and 200mg/kg). Control animals received the same number of saline injections. Twenty-four h after kindling completion the animals' learning performance was tested in a two-way shuttle-box. The animals were challenged with another subeffective dose of PTZ (32.5mg/kg, i.p.) on day 7 after kindling completion. Animals were sacrificed a day after the challenged experiment and the brains were processed for histological investigation. FP ameliorates seizure severity, cognitive deficits and neuronal cell loss in PTZ kindled mice. Components of the extract showed affinity for GABAergic and glutamatergic receptors. Glutamate release was diminished and the (35)S-GTPγS binding assay revealed no intrinsic activity at glutamatergic receptors. Our results revealed that FP contains psychoactive secondary metabolites with anticonvulsant properties, thus supporting the isolation and development of the biologically active components of this medicinal plant as antiepileptic agents.

Short-term synaptic plasticity in the dentate gyrus of monkeys

The hippocampus plays an important role in learning and memory. Synaptic plasticity in the hippocampus, short-term and long-term, is postulated to be a neural substrate of memory trace. Paired-pulse stimulation is a standard technique for evaluating a form of short-term synaptic plasticity in rodents. However, evidence is lacking for paired-pulse responses in the primate hippocampus. In the present study, we recorded paired-pulse responses in the dentate gyrus of monkeys while stimulating to the medial part of the perforant path at several inter-pulse intervals (IPIs) using low and high stimulus intensities. When the stimulus intensity was low, the first pulse produced early strong depression (at IPIs of 10–30 ms) and late slight depression (at IPIs of 100–1000 ms) of field excitatory postsynaptic potentials (fEPSPs) generated by the second pulse, interposing no depression IPIs (50–70 ms). When the stimulus intensity was high, fEPSPs generated by the second pulse were depressed by the first pulse at all IPIs except for the longest one (2000 ms). Population spikes (PSs) generated by the second pulse were completely blocked or strongly depressed at shorter IPIs (10–100 or 200 ms, respectively), while no depression or slight facilitation occurred at longer IPIs (500–2000 ms). Administration of diazepam slightly increased fEPSPs, while it decreased PSs produced by the first pulse. It also enhanced the facilitation of PSs produced by the second stimulation at longer IPIs. The present results, in comparison with previous studies using rodents, indicate that paired-pulse responses of fEPSPs in the monkey are basically similar to those of rodents, although paired-pulse responses of PSs in the monkey are more delayed than those in rodents and have a different sensitivity to diazepam.

Fast Fourier transformation analysis of kindling-induced afterdischarge in the rabbit hippocampus

Kindling is a widely used animal model of intractable temporal lobe epilepsy. In the present study, we performed fast Fourier transformation (FFT) analysis of kindling-induced afterdischarge (AD) in the rabbit hippocampus. Ten adult rabbits were used. Kindling stimulation to the right hippocampus was delivered as a train of biphasic pulses (1 ms duration each) of 50 Hz for 1s, with suprathreshold intensity for AD. Motor responses were classified into five stages according to the conventional criteria. Of 10 animals, five developed stage 5 convulsions with a mean of 21 stimulations (kindled (K) group), while the remaining five animals did not (incomplete kindling (IK) group). We standardized each ratio of power spectral density of lower frequency band component (LFB: 0-9 Hz) and the higher frequency band (HFB: 12-30 Hz) in the initial stage as 1.0. The IK group exhibited small decrements (0.99 and 0.94 times) in LFB and HFB components at the final stage. In contrast, the K group exhibited a significantly (p<0.05) large decrement (0.49 times) in the LFB component and a very large increment (4.45 times) of HFB component at the final stage. Correlation analyses were performed between alteration of power spectral density ratio of the HFB component and AD duration, interictal discharge frequency, and behavioral stage during kindling progression. Fairly strong positive correlations were found in all cases in the K group. FFT analysis of kindling-induced AD demonstrated an important role of the HFB component: enhancement of the HFB component is associated with kindled stage, while decrement of it is associated with incomplete kindling stage. These findings suggest that FFT analysis of stimulus-induced and spontaneous seizure discharges is useful for examination of the progression of epileptic disorders.

Different phases of afterdischarge during rapid kindling procedure in mice

The basic mechanisms of hippocampal networks in epileptogenesis are not entirely understood. To help achieve a better understanding of these mechanisms, we studied the extra-cellular electrically evoked responses in the hippocampi of mice during rapid kindling. Kindling protocol was achieved by stimulating the dorsal right hippocampus six times daily for four days using bipolar electrodes to produce sub-convulsive electrical discharges. Motor responses and analyzed electroencephalographic recordings showed progression from partial complex seizures to generalized seizures associated with different consecutive patterns within the afterdischarges. A spike-wave pattern appeared immediately after stimulation in combination with a poly-spike complex superimposed over the wave (AD1). AD1 was followed by a poly-spike complex (AD2), which was followed by a progressive modification of repetitive spikes (AD3). An ictal depression event was observed at the end of each AD3. Theta oscillations were observed at stage 1-2 of kindling, while beta/gamma oscillations appeared within AD2, associated with stage 4-5 from Racine's score. Benzodiazepine, a GABA (A) agonist (Diazepam) administered at non-sedative doses and only on days 3 and 4 of kindling, limited beta and gamma frequency bands and the progression of seizure severity, suggesting that the failure of GABA (A) agonism mediates the propagation or generalization of seizures. We conclude that different phases of afterdischarge occur during kindling and that high frequencies mediate generalization of seizures.

Antiepileptic ceramides from the Red Sea sponge Negombata corticata

A new antiepileptic ceramide mixture 1 was isolated from the Red Sea sponge Negombata corticata. The structures of the metabolites were determined by extensive spectroscopic analysis. The anticonvulsant activity of 1 was measured in vivo using the pentylenetetrazole-induced seizure model. This finding has important implications for biological studies with this class of compounds.

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.

Phenytoin reverses the chronic stress-induced impairment of memory consolidation for water maze training and depression of LTP in rat hippocampal CA1 region, but does not affect motor activity

Previous studies have shown that phenytoin can protect hippocampal structure from damage by chronic stress, while whether it can reverse the hippocampal malfunction induced by chronic stress is unknown. We investigated the effects of phenytoin on motor activity of stressed rats and on the long-term memory of water maze spatial training, which is known to depend on hippocampal function. We also explored whether phenytoin could protect long-term potentiation (LTP) in hippocampal CA1 region from depression of chronic stressed rats. Isolated hippocampal slices of rats were used to observe the changes of LTP in hippocampal CA1 field with electrophysiological technique. The results showed that the motor activity of chronic forced-swimming rats was markedly higher than that of control rats, and phenytoin could not affect this change. The performance of water maze spatial training indicated that chronic stress damages long-term memory but not short-term memory, and phenytoin could reverse this long-term memory deficit. The increases of LTP after HFS in control and stress-phenytoin groups were significantly greater than those in stress-saline group (P < 0.05). There were no significant differences between control group and stress-phenytoin group (P > 0.05) and between control and control-phenytoin groups (P > 0.05). These findings provided the first evidence with behavioral and electrophysiological technique that phenytoin could reverse the hippocampal-dependent memory deficit and depression of LTP induced by chronic stress, which may be helpful for exploring the pathogenesis and improving the therapy of depression.

Group I metabotropic glutamate receptors interfere in different ways with pentylenetetrazole seizures, kindling, and kindling-related learning deficits

LY 367385 (mGluR1) and MPEP (mGluR5), which are group I metabotropic glutamate receptor (mGluR) antagonists, were used to investigate their effects on pentylenetetrazole (PTZ) seizures, kindling, and kindling-related learning deficits. Both substances showed anticonvulsant efficacy against seizures induced by lower doses of PTZ (40 mg/kg), but they were ineffective in counteracting seizures evoked by higher PTZ doses. When these substances were given in the course of kindling induction, LY significantly depressed the progression of kindled seizure severity. In contrast, MPEP was ineffective in this experiment. Treatment with either LY or MPEP did not modify the reaction to challenge dose of PTZ. Kindling results in a worsening of shuttle-box learning. LY improved shuttle-box learning when administered in the course of kindling development or when given prior to the learning experiment. This suggests protective and restorative effectiveness. In contrast, MPEP was only effective on the learning performance of kindled rats when given prior to the shuttle-box experiment, which demonstrates restorative effectiveness. Kindling is associated with an increase in glutamate binding. LY counteracted this increase whereas MPEP was ineffective. It was concluded that mGluR1 and mGluR5 play a specific role in the convulsive component of kindling. The beneficial action of the antagonists on kindling-induced impairments in shuttle-box learning may be associated with their effect on glutamatergic synaptic activity.

Aging impairs the late phase of long-term potentiation at the medial perforant path-CA3 synapse in awake rats

The effects of aging on long-term potentiation (LTP) in the dentate gyrus (DG) and CA1 are well documented, but LTP at the medial perforant path (MPP)-CA3 synapse of aged animals has remained unexplored. Because the MPP-DG and Schaffer-collateral-CA1 synapses account for only about 20% of total hippocampal synapses, global understanding of how aging affects hippocampal plasticity has remained limited. Much is known about LTP induction in the hippocampal formation, whereas the mechanisms that regulate LTP maintenance are less understood, especially during aging. We investigated the effects of aging on MPP-CA3 LTP induction and maintenance in awake rats. As is the case in the DG and CA1, high-frequency stimulation-induced LTP at the MPP-CA3 synapse is normal in aged rats. These data indicate that N-methyl-D-aspartate (NMDA) receptor-mediated processes are intact at the MPP-CA3 synapse in aged rats. In contrast, aging impaired the magnitude and duration of MPP-CA3 LTP over a period of days. Also, these data are consistent with reports that area CA3 is especially susceptible to age-related changes. Our data suggest that aging impairs mechanisms that regulate the late phase of MPP-CA3 LTP and contribute to a more global understanding of how aging affects hippocampal plasticity.

Simultaneous activation and opioid modulation of long-term potentiation in the dentate gyrus and the hippocampal CA3 region after stimulation of the perforant pathway in freely moving rats

Recent investigations indicate monosynaptic activation by the perforant pathway (pp) of the dentate gyrus and the CA3 region. While short-term potentiation and long-term potentiation (LTP) and its opioid modulation are frequently described for the dentate gyrus, data for the CA3 region are rare. Therefore, evoked potentials and opioid modulation of LTP were directly compared in both target regions of the pp. Male Wistar rats were chronically implanted with a bipolar stimulation electrode in the pp (angular bundle) and two recording electrodes in the dorsal dentate gyrus and the CA3 region. Stimulation of the pp in the freely behaving animals induced short-latency evoked potentials in both target structures which were compared with respect to waveform, latency, amplitude and signs of short- and long-term neuronal plasticity. The short-latency potential in the CA3 region seemed to be a monosynaptic potential which displayed LTP sensitive to the N-methyl-D-aspartate receptor antagonist, MK 801, and depotentiating stimulation. After application of specific opioid antagonists at the mu-, delta- and kappa-opioid receptor subtypes, naloxone, funaltrexamine, naltrindole and binaltorphimine, different effects on induction and maintenance of LTP of the population spike were found both within the dentate gyrus and between the dentate gyrus and the CA3 region. The results show marked diminution of LTP in the dentate gyrus only for naloxone and naltrindole and only small, if any, effects of naloxone on LTP in the CA3 region. Thus, neuronal plasticity in the direct perforant pathway input to the CA3 region seems not to be under such substantial opioidergic control. LTP would be inducible in that region even when LTP in the input formation, the dentate gyrus, and transsynaptic LTP via the mossy fibres are blocked.

Activation of the dentate gyrus by stimulation of the contralateral perforant pathway: evoked potentials and long-term potentiation after ipsi- and contralateral induction

Rats were chronically implanted with stimulation electrodes in the perforant pathway (pp) bilaterally and a recording electrode in the dentate gyrus (DG) unilaterally. Evoked field potentials (EPs) were recorded upon alternating stimulation of the pp on both sides, and long-term potentiation (LTP) was induced. Besides the EP after ipsilateral stimulation, an EP with a latency of approximately 5.5-6.5 ms was also seen upon stimulation of the contralateral pp. This potential was reversibly abolished during pentobarbital anesthesia and irreversibly after lesioning of the ipsilateral angular bundle. Paired-pulse facilitation and paired-pulse depression, depending on interstimulus interval and intensity, were also observed. Therefore, this long-latency potential could be characterized as polysynaptic and induced perhaps by transsynaptic activation via the ipsilateral entorhinal cortex. Ipsilateral tetanization induced strong E/S potentiation of both the ipsilaterally and contralaterally evoked EP, but with different time courses. Tetanization of the contralateral pp did not induce LTP of the ipsilaterally induced EP in the first 4 h. But afterwards a late and slowly developing potentiation occurred. The contralaterally induced EP also showed potentiation of the population spike, which was not immediately detectable but developed slowly over time. The results can be interpreted such that, after stimulation of the pp, the DG on the opposite side cannot only be activated via the weak crossed entorhinal projection but also transsynaptically via an entorhino/entorhinal connection.

Repeated long-term potentiation induces mossy fibre sprouting and changes the sensibility of hippocampal granule cells to subconvulsive doses of pentylenetetrazol

Electrical and chemical kindling induces sprouting of the mossy fibre system and potentiation of evoked field potentials in the dentate gyrus. It has been postulated that such changes may also be induced by repeated induction of long-term potentiation (LTP) with tetanic stimulation of the perforant pathway. LTP was induced in rats chronically implanted with stimulation electrodes in the ipsilateral and contralateral angular bundles and with a recording electrode in the ipsilateral dorsal dentate gyrus. The animals were stimulated 10 times on 10 consecutive days but with different tetanization strengths. Sprouting of the mossy fibres terminating in the CA3 region was significantly induced only in the group of 'strongly' tetanized animals, but not in that of 'weakly' tetanized animals, or in low-frequency stimulated animals. Additionally, a novel form of potentiation which was previously found in pentylenetetrazol (PTZ)-kindled animals was also observed in the group of 'strongly' and 'weakly' tetanized rats. Differences in duration of this potentiation were found between the two groups of animals tetanized with different strengths. The results further demonstrate that morphological and functional changes in the hippocampus, similar to those seen after kindling, can also occur in an activation paradigm leading to long-lasting synaptic plasticity but not accompanied by seizure activity.