Effects of pentylenetetrazol kindling on glutamate receptor genes expression in the rat hippocampus

Pentylenetetrazole: A review

Pentylenetetrazole (PTZ), a tetrazole derivative, is commonly used as a chemical agent to induce neurological disorders and replicate the characteristics of human epileptic seizures in animal models. This review offers a comprehensive analysis of the behavioral, neurophysiological, and neurochemical changes induced by PTZ. The epileptogenic and neurotoxic mechanisms of PTZ are associated with an imbalance between the GABAergic and glutamatergic systems. At doses exceeding 60 mg/kg, PTZ exerts its epileptic effects by non-competitively antagonizing GABAA receptors and activating NMDA receptors, resulting in an increased influx of cations such as Na+ and Ca2+. Additionally, PTZ promotes oxidative stress, microglial activation, and the synthesis of pro-inflammatory mediators, all of which are features characteristic of glutamatergic excitotoxicity. These mechanisms ultimately lead to epileptic seizures and neuronal cell death, which depend on the dosage and method of administration. The behavioral, electroencephalographic, and histological changes associated with PTZ further establish it as a valuable preclinical model for the study of epileptic seizures, owing to its simplicity, cost-effectiveness, and reproducibility.

Hippocampus, hippocampal sclerosis and epilepsy

Hippocampal sclerosis (HS) is considered one of the major pathogenic factors of drug-resistant temporal lobe epilepsy. HS is characterized by selective loss of pyramidal neurons - especially of sectors CA1 and CA3 of the hippocampus - pathological proliferation of interneuron networks, and severe glia reaction. These changes occur in the course of long-term and complex epileptogenesis. The authors, on the basis of a review of the literature and own experience, present the pathomechanisms leading to hippocampal sclerosis and epileptogenesis, including various morphological and functional elements of this structure of the brain and pharmacological possibilities of preventing these processes.

Effect of minocycline on pentylenetetrazol-induced chemical kindled seizures in mice

Inflammation is one of the mechanisms involved in seizure induction. In this study, the effect of minocycline, an anti-inflammatory drug, was investigated on kindling acquisition. Chemical kindling was induced by injection of a subthreshold dose of pentylenetetrazol (PTZ; 37.5 mg/kg) in mice on every other day. Two groups of animals received minocycline (25 mg/kg) at 1 h before or 1 h after PTZ injection. Following the last PTZ injection, the changes in gene expression of TNF-α receptor, γ2 subunit of GABAA receptor and NR2A subunit of NMDA receptor were assessed in the hippocampus and piriform cortex. Injection of minocycline before PTZ increased the latency to stage 4 seizure, and decreased the duration of stages 4 and 5 seizure. It also prevented the increase in the mRNA of NR2A subunit of NMDA receptor in the hippocampus and removed the PTZ-induced increase in mRNA of γ2 subunit of GABAA receptor in piriform cortex of PTZ kindled mice. Minocycline also prevented the increase in TNF-α receptor gene expression in both hippocampus and piriform cortex. Injection of minocycline after PTZ had no significant effect on measured parameters. Therefore, it can be concluded that minocycline may exert an anticonvulsant effect through preventing the increase in GABAA and NMDA receptor subunits. These effects are accompanied by a reduction in an important inflammation index, TNF-α receptor.

Comparison between standard protocol and a novel window protocol for induction of pentylenetetrazol kindled seizures in the rat

Experimental models of epilepsy, including pentylenetetrazol (PTZ) chemical kindling, are very important in studying the pathophysiology of epilepsy. The aim of the present study was to provide behavioral, electrophysiological and molecular evidences to confirm the similarities between standard and a modified protocol named window- (win-) PTZ kindling method. Standard PTZ kindling model was induced by injection of PTZ (37.5mg/kg) every other days. In win-PTZ kindling method, animals received 4 initial PTZ injections and the time of 3 last PTZ injections were determined according to the number of PTZ injections in standard PTZ kindling model. The behavioral signs of kindled seizures were observed for 20 min after each PTZ injection. Field potential recordings were done from the dentate gyrus granular cells following perforant path stimulation. In addition, the expression of γ2 subunit of GABAA receptor, NR2A subunit of NMDA receptor, adenosine A1 receptor, α-CaMKII and GAP-43 were evaluated in the hippocampus and dentate gyrus using RT-PCR technique. All the animals in win-PTZ kindling method group achieved fully kindled state after 3 last PTZ injections. There was no significant difference in population spike amplitude and expression of the mentioned genes during kindling acquisition between standard PTZ kindling model and win-PTZ kindling method. The similarities in electrophysiological and molecular parameters remained after 8 days post fully kindled state. Obtained data showed the similarities between this win-PTZ kindling method and standard PTZ kindling model. Thus, it may be suggested that not all PTZ injections are need for induction of PTZ induced fully kindled state.

L-type calcium channel mediates anticonvulsant effect of cannabinoids in acute and chronic murine models of seizure

The anticonvulsant activities of cannabinoid compounds have been shown in various models of seizure and epilepsy. At least, part of antiseizure effects of cannabinoid compounds is mediated through calcium (Ca(2+)) channels. The L-type Ca(2+) channels have been shown to be important in various epilepsy models. However, there is no data regarding the role of L-type Ca(2+) channels in protective action of cannabinoids on acute and chronic models of seizure. In this study, the effects of cannabinoid compounds and L-type Ca(2+) channels blockers, either alone or in combination were investigated using acute model of pentylenetetrazole (PTZ)-induced seizure in mice and chronic model electrical kindling of amygdala in rats. Pretreatment of mice with both cannabinoid CB1 receptor agonist arachidonyl-2'-chloroethylamide (ACEA) and endocannabinoid degradating enzyme inhibitor cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB597) produced a protective effect against PTZ-induced seizure. Administration of various doses of the two L-type Ca(2+) channel blockers verapamil and diltiazem did not alter PTZ-induced seizure threshold. However, co-administration of verapamil and either ACEA or URB597 attenuated the protective effect of cannabinoid compounds against PTZ-induced seizure. Also, pretreatment of mice with diltiazem blocked the anticonvulsant activity of both ACEA and URB597. Moreover, (R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate (WIN55,212-2), the non-selective cannabinoid CB1 and CB2 receptor agonist showed anticonvulsant effect in amygdala-kindled rats. However, co-administration of WIN55,212-2 and verapamil attenuated the protective properties of WIN55,212-2. Our results showed that the anticonvulsant activity of cannabinoid compounds is mediated, at least in part, by L-type Ca(2+) channels in these two models of convulsion and epilepsy.

RNA editing (R/G site) and flip-flop splicing of the AMPA receptor subunit GluR2 in nervous tissue of epilepsy patients

Editing and alternative splicing of mRNA are posttranscriptional steps probably involved in pathophysiological aspects of epilepsy. The present study analyses the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR2 with respect to the expression of (i) editing at the R/G site and (ii) flip-flop cassettes. Nervous tissue from patients with temporal lobe epilepsy was analysed by RT-PCR followed by restriction enzyme assays. Human autoptic tissue served as control. R/G editing status: the relative amount of edited RNA was significantly increased in the hippocampal tissue, whereas no changes were found in neocortical tissues. Flip-flop expression: no significant alterations were found in relative abundance of spliced variants containing the flip exon. The increased editing at the R/G site in the hippocampal tissue of epilepsy patients may enhance responses to glutamate, resulting in a synapse operating at an increased gain.

Anticonvulsant and antiepileptogenic effects of GABAA receptor ligands in pentylenetetrazole-kindled mice

Although animal models based on pentylenetetrazole (PTZ) are widely used, the mechanism by which PTZ elicits its action is not very well understood. At the molecular level, a generally accepted mechanism of PTZ is noncompetitive antagonism of the gamma-aminobutyric acid (GABA)(A) receptor complex. By a systematic pharmacological investigation of various GABA(A) receptor ligands, our aim was to gain a better understanding of the GABAergic mechanisms involved in different PTZ-induced seizures. We investigated anticonvulsant effects of various specific GABA(A) receptor ligands, which are believed to bind to different binding sites on the GABA(A) receptor complex, on PTZ-induced clonic seizures in drug naive and PTZ-kindled mice as well as their effects on the development of PTZ kindling. Diazepam and alphaxalone produced potent anticonvulsant effects and completely suppressed the development of kindling. In contrast, the antagonists bicuculline and dehydroepiandrosterone sulfate (DHEAS) displayed neither anticonvulsant nor antiepileptogenic effects. Flumazenil, often used as a reference antagonist at the GABA(A) receptor benzodiazepine (BZ) binding site, lacked anticonvulsant effects but surprisingly inhibited the development of PTZ-kindled seizures. The agonist 4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin-3-ol (THIP) was devoid of both anticonvulsant and antiepileptogenic effects. Marked differences in drug sensitivity were observed between models based on single and chronic administration of PTZ showing that the two sets of models are fundamentally different. These results describe the pharmacology of a set of ligands believed to bind to different sites at the GABA(A) receptor complex in animal models based on PTZ and demonstrate that a drug's action in these models cannot be readily explained by agonistic or antagonistic properties at the receptor level.

The effect of drugs of abuse on NMDAR1 receptor expression in the rat limbic system

An increasing body of evidence points to the role of N-methyl-D-aspartate (NMDA) receptors in the limbic system in the mechanism of drug dependence. We studied the influence of acute and repeated morphine (20 mg/kg i.p. or increasing dose for 10 days) and cocaine (3x20 mg/kg i.p. per day at hourly intervals, for 1 or 5 days) administration on the expression of glutamate NMDA receptor subunit 1 (NMDAR1) in the central and basolateral nuclei of the rat amygdala and hippocampal formation. Acute or chronic morphine and cocaine administration increased NMDAR1 mRNA level in the central and basolateral nuclei of the amygdala; morphine did so 3 h after the last dose and 48 h after withdrawal, cocaine 3 h after acute and last chronic dose. Morphine did not change the NMDAR1 mRNA level in the hippocampal formation, but chronic cocaine did decrease it in the dentate gyrus only. Our study suggests a possible link between the expression of NMDAR1 and changes in limbic system neuronal activity and behaviour after administration of morphine and cocaine. In summary, the present study demonstrated that morphine and cocaine influenced the expression of NMDAR1 in the structure of the limbic system which could be involved in dependence phenomena.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Epilepsy as an example of neural plasticity

Epilepsy is a devastating disease affecting more than 1% of the population. Yet, if one considers the neurobiological substrates of this disease, what is revealed is an array of phenomenon that exemplify the remarkable capacity for the brain to change its basic structure and function, that is, neural plasticity. Some of these alterations are transient and merely impressive for their extent, or for their robust nature across animal models and human epilepsy. Others are notable for their persistence, often enduring for months or years. As an example, the dentate gyrus, and specifically the principal cell of the dentate gyrus, the granule cell, is highlighted. This area of the brain and this particular cell type, for reasons that are currently unclear, hold an uncanny capacity to change after seizures. For those interested in plasticity, it is suggested that perhaps the best examples for studying plasticity lie in the field of epilepsy.

Changes in AMPA receptor binding and subunit messenger RNA expression in hippocampus and cortex in the pentylenetetrazole-induced 'kindling' model of epilepsy

'Kindling' is a phenomenon of epileptogenesis, which has been widely used as an experimental model of temporal lobe epilepsy. In the present study, we have examined the contribution of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptors and their subunits (GluR-A, -B, -C and -D) to the acquisition and maintenance of the kindled state in the pentylenetetrazole (PTZ)-induced 'kindling' mouse model, by using quantitative autoradiography and in situ hybridization. Region-specific increases in [3H]AMPA binding were seen in kindled animals in the CA3 region of hippocampus and in the temporal cortex 1 week after the last PTZ injection. At the same time, a significant decrease in the level of transcripts encoding the GluR-B and -C subunits was detected in the hippocampal CA1 region and dentate gyrus, suggestive of a higher proportion of Ca(2+)-permeable AMPA receptors in these neurons. These changes did not persist 1 month after establishment of kindling, indicating a transient role of AMPA receptors in the acquisition of the kindled state. At 1 month after the last PTZ injection, an upregulation in [3H]AMPA binding appeared in the motor cortex and the basal ganglia of kindled animals, which is consistent with electrophysiological data showing hyperexcitability in the cortex of the PTZ-kindled animals at that time. Interestingly, an increase in mRNA for the GluR-B subunit appeared in the outer layers of motor and somatosensory cortices of the kindled animals 1 month after acquisition of the kindled state, possibly as part of a gene-regulated, compensatory mechanism against seizure susceptibility, since this change should give rise to a higher proportion of Ca(2+)-impermeable AMPA receptors. These results support the evidence of a transient role of hippocampal AMPA receptors in the acquisition of the 'kindling' phenomenon and they also suggest an involvement of AMPA receptors in the maintenance of kindled state at least in two brain areas, cortex and basal ganglia.

Long-term increase of GluR2 alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor subunit in the dispersed dentate gyrus after intrahippocampal kainate injection in the mouse

Intrahippocampal injection of a subtoxic dose of kainate in mice has been shown to induce a dispersion of granule cells of the dentate gyrus, which is a characteristic morphological change often seen in human hippocampal sclerosis. In addition, it has been shown recently that such injections lead to recurrent hippocampal seizures and changes in glucose metabolism, which are reminiscent of temporal lobe epilepsy. Previous reports on human hippocampal sclerosis have shown an increase of the expression of the GluR2 alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate subunits in the dispersed granule cell somata. However, no such changes have been observed so far in animal models of epilepsy with hippocampal sclerosis. In this study, the expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor subunits was examined by immunohistochemistry following intrahippocampal injection of kainate in mice and rats. In mice, such injection induced a persistent increase of GluR2 immunoreactivity in the granule cells for up to 180 days. By contrast, GluR1 immunoreactivity was transiently increased during the first four days after the injection and progressively decreased thereafter. By contrast, intrahippocampal injection of kainate in rats did not result in granule cell dispersion and no changes in GluR1 immunoreactivity or GluR2 immunoreactivity were observed. These results show that, in addition to morphological, clinical and metabolical similarities, intrahippocampal injection of kainate results in a persistent increase of GluR2 associated with granule cell dispersion, as in human hippocampal sclerosis. These data suggest the existence of common mechanisms between granule cell dispersion and regulation of GluR2 subunits associated with hippocampal sclerosis.

Primed-burst potentiation occludes the potentiation phenomenon and enhances the epileptiform activity induced by transient pentylenetetrazol in the CA1 region of rat hippocampal slices

The effects of pentylenetetrazol (PTZ) following induction of long-term potentiation (LTP) on population spikes in CA1 of hippocampal slices were investigated. Population spikes were evoked by activation of Schaffer collaterals with a range of stimulation intensities. LTP was induced using θ-pattern primed burst tetanic stimulation. Changes in the population spike amplitude and number of population spikes were used as indices to quantify the effects of PTZ exposure in the control (non-tetanized) and LTP (tetanized) conditions. The amplitude of population spike was measured 20 min before, during 20 min chemical application (3 mM), and also after 30 or 60 min washout period. In non-tetanized slices, the population spike input-output curve was significantly increased 20 min after PTZ application and persisted at least for 60 min. Multiple population spikes or after potentials also appeared, but did not persist. When PTZ was applied on tetanized slices, 60 min after LTP induction, the amplitude increase produced by PTZ was smaller than the increase seen in the control condition. Also LTP induction preceding PTZ exposure increased the number of population spikes evoked by stimulation of Schaffer collaterals. It is concluded that a transient PTZ application produces a long-lasting increase in population spike amplitude. Primed burst LTP occludes PTZ-induced potentiation while also increasing the epileptogenic effect of PTZ.

Gene expression in the forebrain of dexamethasone-treated pigs: effects on stress neuropeptides in the hypothalamus and hippocampus and glutamate receptor subunits in the hippocampus

Gene expression studies advance our understanding of the effects of stress and glucocorticoids on brain function and give a new direction to animal welfare research. In this context, the presence of messenger RNA s (m RNA s) for corticotrophin releasing hormone (CRH) and vasopressin (VP) in the porcine hypothalamus has recently been documented. This study investigated the expression of CRH, VP and ionotropic glutamate receptor (iGluR) subunit m RNA s in the brains of pigs treated with the synthetic glucocorticoid dexamethasone (Dex; 5 mg kg(-1)i.v.). In the hypothalamus, VP, but not CRH, m RNA was reduced 3 hours after Dex. In the hippocampus, expression of m RNA s for some iGluR subunits appeared to be differentially regulated 6 hours after Dex. In addition, CRH message was detected in the hippocampus and significantly upregulated in the CA1 region 3 hours after Dex. The relevance of these findings to stress neurobiology of the growing pig is discussed.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Behavioral analysis of PTZ-kindled rats after acute and chronic ethanol treatments

The present study was designed to examine the response of PTZ-kindled and saline-injected animals to both acute and chronic ethanol treatment. Acute injection of ethanol (3.0 g/kg; IP) resulted in a rapid onset of loss of righting reflex (LORR) in both PTZ-kindled and saline-injected animals. However, the PTZ-kindled animals recovered from LORR significantly more quickly than control animals. Using a tilt-plane test as a measure of motor incoordination, the PTZ-kindled animals had significantly less motor incoordination compared to controls. Blood alcohol levels (BAL) were not significantly different between the groups. We also compared the degree of tolerance and dependence in chronic ethanol-treated, PTZ-kindled, and control animals. PTZ-kindled, saline-injected and naive control animals were chronically treated with ethanol vapor. The PTZ-kindled group tolerated high vapor concentrations (in terms of food consumed/rat) and, at the end of the treatment, displayed intoxication characteristics different from those of the control groups despite having similar blood alcohol levels. The PTZ-kindled group also displayed withdrawal behavior that was similar to a group of ethanol-treated animals that had experienced a prior cycle of dependency and withdrawal. These data show many intriguing similarities between animals that are PTZ-kindled and chronically treated with ethanol and suggest the use of PTZ-kindled animals as a model for alcohol withdrawal kindling.