The prodynorphin system in the rat hippocampus is differentially influenced by kainic acid and pentetrazole

Effect of U50488, a selective kappa opioid receptor agonist and levetiracetam against lithium-pilocarpine-induced status epilepticus, spontaneous convulsive seizures and related cognitive impairment

PURPOSE

Kappa opioid receptor (KOR) agonists have anticonvulsant effect but their antiepileptogenic effect is unknown. U50488, a selective KOR agonist is used to determine its effect on status epilepticus (SE), spontaneous convulsive seizures (SS) and cognitive impairment in rat lithium-pilocarpine SE model. Effect of an antiepileptic drug levetiracetam is also studied.

METHOD

Male Wistar rats with SE were divided into three groups namely, LiP, LiP + U50488 (10 mg/kg, i.p.) and LiP + levetiracetam (400 mg/kg, i.p.) group. SE was terminated after 90 min of its onset with diazepam (15 mg/kg, i.p.) and phenobarbitone (25 mg/kg, i.p.). Drug treatment was started after 15 min of onset of SE and repeated once after 4 h. Rats were video monitored 12 h daily (9 AM to 9 PM) to determine severity of SE using modified Racine scale and onset and frequency of SS from day 0 to day 21. Morris water maze (MWM) test was done at baseline i.e. day -1 (before lithium administration) and day 22, to assess cognitive impairment.

RESULTS

As compared to LiP, U50488 decreased the severity of SE (1.98 ± 0.13 vs 2.95 ± 0.12; p-value < 0.0001) but not levetiracetam (2.62 ± 0.09; p-value = 0.3112). Survival increased with both U50488 (90%, n = 10) and levetiracetam (81.8%, n = 11) as compared to NS (56.2%, n = 16). No effect on onset and frequency of SS was found in U50488/levetiracetam group. U50488 improved seizures-induced cognitive impairment. Levetiracetam group showed thigmotactic (wall hugging) behaviour in MWM in 8 out of 9 rats.

CONCLUSION

Acute treatment with U50488, a kappa opioid receptor agonist has a beneficial effect on SE, SE-related mortality and memory impairment. The dual protective effect of U50488 on seizures and related cognitive impairment is advantageous over currently used antiseizure drugs which are known to cause cognitive impairment.

The Kappa Opioid Receptor System in Temporal Lobe Epilepsy

Temporal lobe epilepsy is considered to be one of the most common and severe forms of focal epilepsies. Patients frequently develop cognitive deficits and emotional blunting along progression of the disease. The high incidence of refractoriness to antiepileptic drugs and a frequent lack of admissibility to surgery pose an unmet medical challenge. In the urgent quest for novel treatment strategies, neuropeptides and their receptors are interesting candidates. However, their therapeutic potential has not yet been fully exploited. This chapter focuses on the functional role of the dynorphins (Dyns) and the kappa opioid receptor (KOR) system in temporal lobe epilepsy and the hippocampus.Genetic polymorphisms in the prepro-dynorphin (pDyn) gene causing lower levels of Dyns in humans and pDyn gene knockout in mice increase the risk to develop epilepsy. This suggests a role of Dyns and KOR as modulators of neuronal excitability. Indeed, KOR agonists induce inhibition of presynaptic neurotransmitter release, as well as postsynaptic hyperpolarization in glutamatergic neurons, both producing anticonvulsant effects.The development of new approaches to modulate the complex KOR signalling cascade (e.g. biased agonism and gene therapy) opens up new exciting therapeutic opportunities with regard to seizure control and epilepsy. Potential adverse side effects of KOR agonists may be minimized through functional selectivity or locally restricted treatment. Preclinical data suggest a high potential of such approaches to control seizures.

The Opioid System in Temporal Lobe Epilepsy: Functional Role and Therapeutic Potential

Temporal lobe epilepsy is considered to be one of the most common and severe forms of focal epilepsies. Patients often develop cognitive deficits and emotional blunting along the progression of the disease. The high incidence of resistance to antiepileptic drugs and a frequent lack of admissibility to surgery poses an unmet medical challenge. In the urgent quest of novel treatment strategies, neuropeptides are interesting candidates, however, their therapeutic potential has not yet been exploited. This review focuses on the functional role of the endogenous opioid system with respect to temporal lobe epilepsy, specifically in the hippocampus. The role of dynorphins and kappa opioid receptors (KOPr) as modulators of neuronal excitability is well understood: both the reduced release of glutamate as well of postsynaptic hyperpolarization were shown in glutamatergic neurons. In line with this, low levels of dynorphin in humans and mice increase the risk of epilepsy development. The role of enkephalins is not understood so well. On one hand, some agonists of the delta opioid receptors (DOPr) display pro-convulsant properties probably through inhibition of GABAergic interneurons. On the other hand, enkephalins play a neuro-protective role under hypoxic or anoxic conditions, most probably through positive effects on mitochondrial function. Despite the supposed absence of endorphins in the hippocampus, exogenous activation of the mu opioid receptors (MOPr) induces pro-convulsant effects. Recently-expanded knowledge of the complex ways opioid receptors ligands elicit their effects (including biased agonism, mixed binding, and opioid receptor heteromers), opens up exciting new therapeutic potentials with regards to seizures and epilepsy. Potential adverse side effects of KOPr agonists may be minimized through functional selectivity. Preclinical data suggest a high potential of such compounds to control seizures, with a strong predictive validity toward human patients. The discovery of DOPr-agonists without proconvulsant potential stimulates the research on the therapeutic use of neuroprotective potential of the enkephalin/DOPr system.

Activation of κ opioid receptors increases intrinsic excitability of dentate gyrus granule cells

The dentate gyrus of the hippocampus is thought to control information flow into the rest of the hippocampus. Under pathological conditions, such as epilepsy, this protective feature is circumvented and uninhibited activity flows throughout the hippocampus. Many factors can modulate excitability of the dentate gyrus and ultimately, the hippocampus. It is therefore of critical importance to understand the mechanisms involved in regulating excitability in the dentate gyrus. Dynorphin, the endogenous ligand for the kappa (κ) opioid receptor (KOR), is thought to be involved in neuromodulation in the dentate gyrus. Both dynorphin and its receptor are widely expressed in the dentate gyrus and have been implicated in epilepsy and other complex behaviours such as stress-induced deficits in learning and stress-induced depression-like behaviours. Administration of KOR agonists can prevent both the behavioural and electroencephalographic measures of seizures in several different models of epilepsy. Antagonism of the KORs also prevents stress-induced behaviours. This evidence suggests the KORs as possible therapeutic targets for various pathological conditions. In addition, KOR agonists prevent the induction of LTP. Although there are several mechanisms through which dynorphin could mediate these effects, no studies to date investigated the effects of KOR activation on intrinsic membrane properties and cell excitability. We used whole-cell, patch-clamp recordings from acute mouse hippocampus slices to investigate the effect of KOR activation on dentate gyrus granule cell excitability. The agonist U69,593 (U6, 1 μM) resulted in a lower spike threshold, a decreased latency to first spike, an increased spike half-width, and an overall increase in spike number with current injections ranging from 15 to 45 pA. There was also a reduction in the interspike interval (ISI) both early and late in the spike train, with no change in membrane potential or input resistance. Preincubation of the slice with the selective KOR antagonist, nor-binalthorphimine (BNI, 1 μM) inhibited the effect of U6 on the latency to first spike and spike half-width suggesting that these effects are mediated through KORs. The inclusion of GDP-βS (1 mM) in the recording pipette prevented all of the U6 effects, suggesting that all effects are mediated via a G-protein-dependent mechanism. Inclusion of the A-type K+ current blocker, 4-aminopyridine (4-AP, 5 mM) in the pipette also antagonised the effects of U6. Kv4.2 is one of the channel α subunits thought to be responsible for carrying the A-type K+ current. Incubation of hippocampus slices with U6 resulted in a decrease in the Kv4.2 subunit protein at the cell surface. These results are consistent with an increase in cell excitability in response to KOR activation and may reflect new possibilities for additional opioid functions.

30 years of dynorphins--new insights on their functions in neuropsychiatric diseases

Since the first description of their opioid properties three decades ago, dynorphins have increasingly been thought to play a regulatory role in numerous functional pathways of the brain. Dynorphins are members of the opioid peptide family and preferentially bind to kappa opioid receptors. In line with their localization in the hippocampus, amygdala, hypothalamus, striatum and spinal cord, their functions are related to learning and memory, emotional control, stress response and pain. Pathophysiological mechanisms that may involve dynorphins/kappa opioid receptors include epilepsy, addiction, depression and schizophrenia. Most of these functions were proposed in the 1980s and 1990s following histochemical, pharmacological and electrophysiological experiments using kappa receptor-specific or general opioid receptor agonists and antagonists in animal models. However, at that time, we had little information on the functional relevance of endogenous dynorphins. This was mainly due to the complexity of the opioid system. Besides actions of peptides from all three classical opioid precursors (proenkephalin, prodynorphin, proopiomelanocortin) on the three classical opioid receptors (delta, mu and kappa), dynorphins were also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Moreover, discrepancies between the distribution of opioid receptor binding sites and dynorphin immunoreactivity contributed to the difficulties in interpretation. In recent years, the generation of prodynorphin- and opioid receptor-deficient mice has provided the tools to investigate open questions on network effects of endogenous dynorphins. This article examines the physiological, pathophysiological and pharmacological implications of dynorphins in the light of new insights in part obtained from genetically modified animals.

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.

Morphometry of a peptidergic transmitter system: dynorphin B-like immunoreactivity in the rat hippocampal mossy fiber pathway before and after seizures

While the morphometry of classical transmitter systems has been extensively studied, relatively little quantitative information is available on the subcellular distribution of peptidergic dense core vesicles (DCVs) within axonal arbors and terminals, and how distribution patterns change in response to neural activity. This study used correlated quantitative light and electron microscopic immunohistochemistry to examine dynorphin B-like immunoreactivity (dyn B-LI) in the rat hippocampal mossy fiber pathway before and after seizures. Forty-eight hours after seizures induced by two pentylenetetrazol injections, light microscopic dyn B-LI was decreased dorsally and increased ventrally. Ultrastructural examination indicated that, in the hilus of the dentate gyrus, these alterations resulted from changes that were almost entirely restricted to the profiles of the large mossy-like terminals formed by mossy fiber collaterals (which primarily contact spines), compared to the profiles of the smaller, less-convoluted terminals found on the same collaterals (which primarily contact aspiny dendritic shafts). Dorsally, mossy terminal profile labeled DCV (/DCV) density dropped substantially, while ventrally, both mossy terminal profile perimeter and /DCV density increased. In all terminal profile examined, /DCVs also were closely associated with the plasma membrane. Following seizures, there was a reorientation of /DCVs along the inner surface of mossy terminal profile membranes, in relation to the types of profiles adjacent to the membrane: in both the dorsal and ventral hilus, significantly fewer /DCVs were observed at sites apposed to dendrites, and significantly more were observed at sites apposed to spines. Thus, after seizures, changes specific to: (1) the dorsoventral level of the hippocampal formation, (2) the type of terminal, and (3) the type of profile in apposition to the portion of the terminal membrane examined were all observed. An explanation of these complex, interdependent alterations will probably require evoking multiple interrelated mechanisms, including selective prodynorphin synthesis, transport, and release.

Ethanol withdrawal enhances the prodynorphin system activity in the rat nucleus accumbens

The present study investigated the effects of ethanol withdrawal after its chronic administration on endogenous opioid systems in the nucleus accumbens of rats. An in situ hybridization study showed an increase in the prodynorphin mRNA level at 24 and 48 h (by 189 and 146%, respectively) after ethanol withdrawal, whereas the proenkephalin mRNA level remained unchanged. Furthermore, after a 48 h withdrawal period, the level of alpha-neoendorphin (alphaNEO), a prodynorphin-derived peptide, was significantly decreased (by 48%), that effect being associated with the enhancement of the K+-stimulated release of that peptide from nucleus accumbens slices. At 96 h after ethanol withdrawal, only the basal release of alphaNEO was elevated, while other parameters returned to the control level. Our data indicate that after 48 h of ethanol withdrawal, prodynorphin neurons are highly activated. The increased supply of endogenous kappa opioid receptor agonists in the nucleus accumbens at that time may promote aversive states during ethanol withdrawal.

Effects of pilocarpine and kainate-induced seizures on N-methyl-D-aspartate receptor gene expression in the rat hippocampus

The effects of pilocarpine- and kainate-induced seizures on N-methyl-D-aspartate receptor subunit-1 messenger RNA and [3H]dizocilpine maleate binding were studied in the rat hippocampal formation. Pilocarpine- but not kainate-induced seizures decreased N-methyl-D-aspartate receptor subunit-1 messenger RNA level in dentate gyrus at 24 and 72 h after drug injection. Both convulsants decreased the messenger RNA level in CA1 pyramidal cells at 24 and 72 h, the effects of kainate being more profound. Kainate also decreased the N-methyl-D-aspartate receptor subunit-1 messenger RNA level in CA3 region after 24 and 72 h, whereas pilocarpine decreased the messenger RNA level at 72 h only. At 3 h after kainate, but not pilocarpine, an increased binding of [3H]dizocilpine maleate in several apical dendritic fields of pyramidal cells was found. Pilocarpine reduced the [3H]dizocilpine maleate binding in stratum lucidum only at 3 and 24 h after the drug injection. Pilocarpine but not kainate induced prolonged decrease in N-methyl-D-aspartate receptor subunit-1 gene expression in dentate gyrus. However, at the latest time measured, kainate had the stronger effect in decreasing both messenger RNA N-methyl-D-aspartate receptor subunit-1 and [3H]dizocilpine maleate binding in CA1 and CA3 hippocampal pyramidal cells. The latter changes corresponded, however, to neuronal loss and may reflect higher neurotoxic potency of kainate. These data point to some differences in hippocampal N-methyl-D-aspartate receptor regulation in pilocarpine and kainate models of limbic seizures. Moreover, our results suggest that the N-methyl-D-aspartate receptor subunit-1 messenger RNA level is more susceptible to limbic seizures than is [3H]dizocilpine maleate binding in the rat hippocampal formation.

Rapid and transient increases in cellular immediate early gene and neuropeptide mRNAs in cortical and limbic areas after amygdaloid kindling seizures in the rat

Changes in transcription factor and neuropeptide gene expression are likely to be involved in the cascade of genetic and molecular events leading to permanent changes in neuronal activity associated with kindling and epilepsy. Both acute-transient and delayed-sustained changes in transcription factor or immediate early gene (IEG) activity have previously been reported in response to different stimuli. In the present study in situ hybridization was used to investigate the possible time course (30 min-8 week) of IEG and neuropeptide mRNA induction in forebrain in a kindling model of epilepsy. Kindling was produced by daily unilateral stimulation of the amygdala. IEG mRNAs were detected using [35S]-labelled oligonucleotide probes specific for c-fos, c-jun, NGFI-A (PC1) and PC3 transcripts. Possible changes in the level of mRNAs encoding the neuropeptides somatostatin (SOM) and neuropeptide Y (NPY) were also studied. Stimulation-induced seizures produced dramatic bilateral increases in all IEG mRNAs in the dentate gyrus after 30 min to 1 h. Ipsilateral or bilateral increases in c-fos and PC3 mRNA were observed in the piriform cortex of individual animals at 30 min post-stimulation. While the distribution and apparent basal expression of the different IEGs varied (NGFI-A and c-jun > c-fos and PC3), the degree of induction in the dentate gyrus was similar for all IEGs studied (i.e. 200-300%). No long-term changes in IEG mRNA expression were detected beyond 2 h and up to 8 week after the last seizure. Increased levels of preproSOM and preproNPY mRNAs were consistently observed in hilar interneurons, but not in pyramidal or granule cells of the hippocampus, after 1-2 h. These increases were not maintained at later times. The short-term effects on IEG and neuropeptide mRNAs observed suggest that these changes are consequence of seizure activity with the development of kindling. In contrast, no evidence was found of any substantial, long-lasting effects on these parameters associated with the established kindled state.

Dynorphin and epilepsy

Studies on dynorphin involvement in epilepsy are summarised in this review. Electrophysiological, biochemical and pharmacological data support the hypothesis that dynorphin is implicated in specific types of seizures. There is clear evidence that this is true for complex partial (limbic) seizures, i.e. those characteristic of temporal lobe epilepsy, because; (1) dynorphin is highly expressed in various parts of the limbic system, and particularly in the granule cells of the hippocampus; (2) dynorphin appears to be released in the hippocampus (and in other brain areas) during complex partial seizures; (3) released dynorphin inhibits excitatory neurotransmission at multiple synapses in the hippocampus via activation of kappa opioid receptors; (4) kappa opioid receptor agonists are highly effective against limbic seizures. Data on generalised tonic-clonic seizures are less straightforward. Dynorphin release appears to occur after ECS seizures and kappa agonists exert a clear anticonvulsant effect in this model. However, more uncertain biochemical data and lack of efficacy of kappa agonists in other generalised tonic-clonic seizure models argue that the involvement of dynorphin in this seizure type may not be paramount. Finally, an involvement of dynorphin in generalised absence seizures appears unlikely on the basis of available data. This may not be surprising, given the presumed origin of absence seizures in alterations of the thalamo-cortical circuit and the low representation of dynorphin in the thalamus. In conclusion, it may be suggested that dynorphin plays a role as an endogenous anticonvulsant in complex partial seizures and in some cases of tonic-clonic seizures, but most likely not in generalised absence. This pattern of effects may coincide with the antiseizure spectrum of selective kappa agonists.

Endogenous opioid regulation of hippocampal function

Endogenous opioid peptides modulate neural transmission in the hippocampus. Procnkephalin-derived peptides have been demonstrated to act at mu and delta opioid receptors to inhibit GABA release from inhibitory interneurons, resulting in increased excitability of hippocampal pyramidal cells and dentate gyrus granule cells. Prodynorphin-derived peptides primarily act at presynaptic kappa opioid receptors to inhibit excitatory amino acid release from perforant path and mossy fiber terminals. Opioid receptors reduce membrane excitability by modulating ion conductances, and in this way they may decrease voltage-dependent calcium influx and transmitter release. Synaptic plasticity in the hippocampus also is modulated by endogenous opioids. Enkephalins facilitate long-term potentiation, whereas dynorphins inhibit the induction of this type of neuroplasticity. Further, opioids may play important roles in hippocampal epilepsy. Recurrent seizures induce changes in the expression of opioid peptides and receptors. Also, enkephalins have proconvulsant effects in the epileptic hippocampus, whereas dynorphins may function as endogenous anticonvulsants.

Seizure related changes in the regulation of opioid genes and transcription factors in the dentate gyrus of rat hippocampus

An in situ hybridization study showed that limbic seizures induced by kainate strongly augmented the prodynorphin and proenkephalin messenger RNA levels in granular cells of the rat hippocampal dentate gyrus. Pentylenetetrazole increased the level of proenkephalin messenger RNA, but slightly decreased that of prodynorphin messenger RNA in the dentate gyrus. Administration of kainate to rats caused a profound increase in messenger RNAs of the transcription factor genes c-fos and c-jun in the dentate gyrus, followed by an increase in the level of the transcriptional complex activator protein-1 in hippocampal neurons. Pentylenetetrazole also elevated the formation of activator protein-1, but the effect appeared earlier than that induced by kainate. Thus, recurrent limbic seizures activate both prodynorphin and proenkephalin genes, whereas generalized clonic-tonic seizures seem to decrease the prodynorphin and increase the proenkephalin gene expression in the dentate gyrus. Furthermore, our present results suggest that the transcription factors, c-fos, c-jun and activator protein-1 complex may be involved in the process of inducing the hippocampal proenkephalin gene, while these factors might be differently involved in regulation of prodynorphin gene expression.

Opioid peptide expression in models of chronic temporal lobe epilepsy

Expression of the opioid peptides dynorphin and enkephalin is altered within the first 24 h after acutely induced seizures in certain experimental models of epilepsy. Using in situ hybridization, we examined the expression of prodynorphin and preproenkephalin messenger RNA acutely following induction of kindling with recurrent seizures and in two models of chronic temporal lobe epilepsy: (i) rats fully kindled with rapidly recurring hippocampal seizures; and (ii) rats surviving after self-sustaining limbic status epilepticus induced with focal electrical stimulation of the hippocampus. In naive animals, a ventral-dorsal gradient was identified in the expression of both prodynorphin and preproenkephalin messenger RNA in the dentate gyrus and expression of prodynorphin message was demonstrated for the first time in the ventral portion of cornu Ammonis regio superior. After stimulation producing rapidly recurring hippocampal seizures, acute decreases in prodynorphin messenger RNA were seen in the dentate gyrus and cornu Ammonis regio superior at 24 h after the last seizure. In contrast, increases in preproenkephalin messenger RNA expression were seen acutely in the dentate gyrus, with a decrease seen in the entorhinal cortex. The change in prodynorphin message expression in cornu Ammonis regio superior persisted in kindled animals that were studied after one month seizure-free period. There were no changes in preproenkephalin message in kindled animals studied after the one month seizure-free interval. No statistically significant changes were found for either prodynorphin or preproenkephalin message in the post-self-sustaining limbic status epilepticus group at one month following induced seizures. Acute changes in peptide expression may contribute to increased excitation in the dentate gyrus during induction of kindling, while the chronic change identified in cornu Ammonis regio superior may contribute directly to persistently increased excitability in this region.

Kappa opioid receptor agonists inhibit the pilocarpine-induced seizures and toxicity in the mouse

Involvement of the kappa opioid receptor in regulation of the pilocarpine-induced seizures and neurodegeneration was studied in mice. Administration of pilocarpine (400 mg/kg i.p.) resulted in a sequence of behavioral alterations including motor limbic seizures. Pretreatment of mice with the selective kappa opioid receptor agonist U69,593 (2 and 20 mg/kg i.p.) or PD117,302 (0.1 and 1 mg/kg i.p.) increased the latency of motor seizures and decreased the seizure severity and mortality. Those effects were abolished in animals pretreated with the specific kappa opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 10 mg/kg i.p.). Examination of frontal forebrain sections by light microscopy revealed widespread damage, especially within the hippocampal formation, in pilocarpine-treated mice. Both U69,593 and PD117,503 protected the integrity of hippocampal neurons, especially in the CA1 region, that effect being reversed by Nor-BNI. The above data indicate that activation of the kappa opioid receptor exerts an inhibitory effect on the pilocarpine-induced limbic seizures and neurotoxicity.

Release of alpha-neoendorphin from the anterior pituitary gland of conscious rats

alpha-Neoendorphin (alpha-NEO) is a proenkephalin B-derived opioid peptide and kappa type opioid receptor agonist. In the present study, we used a combination of microdialysis and a highly sensitive radioimmunoassay to measure the extracellular levels of immunoreactive (ir)-alpha-NEO from the anterior pituitary gland of conscious free-moving rats. When rats were given water ad libitum under a 12:12 h light-dark cycle with lighting from 06.00 to 18.00 h, ir-alpha-NEO showed circadian rhythmicity that peaked at 00.00-03.00 h and reached a minimum at 12.00-15.00 h. Furthermore, we investigated whether naloxone (10(-6) to 10(-8) M) affected ir-alpha-NEO level. The alpha-NEO release induced by naloxone was optimum at 10(-7) M, and 10(-6) M naloxone seemed to induce alpha-NEO release to a lesser degree. These results suggested that alpha-NEO release by naloxone might be mediated via anterior pituitary cell auto-receptor (kappa type) inhibition.

Seizure-induced expression of G proteins in the rat hippocampus

The effects of kainic acid (15 mg/kg i.p.) on alpha subunits of the Gs and Go protein mRNA levels in the rat hippocampal formation were investigated. An in situ hybridization study showed an increase in the Gs alpha mRNA level in the dentate gyrus at 3 h (by ca. 17%), 24 h (by ca. 75%), 72 h (by ca. 89%) and 30 days (by ca. 59%) after kainic acid administration. An emulsion autoradiography revealed enhancement in the Gs alpha mRNA signal intensity over granular cells of the dentate gyrus and over some hilar cells adjacent to the granule cell layer, most likely in GABA interneurons. The Gs alpha mRNA showed a slight tendency to increase in the CA1 and CA3 pyramidal cell layers at 3 h after kainic acid administration, but it decreased after 24 h, 72 h and 30 days. The latter decrease correlated well with the pyramidal cells loss in those areas. Kainic acid differently influenced the Go alpha mRNA level in the dentate gyrus: it had no effect after 3 h, while after 24 h the mRNA level tended to decrease (by ca. 16%); then it increased after 72 h (by ca. 20%) and, to a lesser extent, after 30 days (by ca. 12%). The Go alpha mRNA level in CA1 and CA3 tended to decrease at 3 h after kainic acid administration; the signal completely disappeared after 24, 72 h as well as after 30 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentylenetetrazole-induced seizures stimulate transcription of early and late response genes

Cellular immediate early gene and neuropeptide gene expression have each been demonstrated to be modulated in hippocampus in response to a variety of seizure-inducing stimuli. In this study, gene transcription for three immediate early genes, c-fos, c-jun and NGFI-A, and three neuropeptide genes, enkephalin, dynorphin and neuropeptide Y, was investigated using nuclear run-on assays following a single injection of the convulsant pentylenetetrazole (PTZ). At 15 min following PTZ injection, only transcription for c-fos was increased. By 6 h following PTZ treatment, transcription for all immediate early genes and for dynorphin and neuropeptide Y was increased; however, this increase was transient in that transcription of all genes returned to control values by 48 h following PTZ treatment. Thus, regulation of immediate early and neuropeptide gene mRNA levels and immunoreactivity occurs, at least in part, at the level of transcription for the genes encoding neuropeptide Y, dynorphin, c-fos, c-jun, and NGFI-A. Moreover, the difference between increased transcription rates reported here and increased mRNA levels reported here and elsewhere suggests that additional post-transcriptional regulation of gene expression occurs in hippocampal neurons.