Differential expression of immediate early genes in the hippocampus in the kindling model of epilepsy

Altered adult neurogenesis and gliogenesis in patients with mesial temporal lobe epilepsy

The hippocampus is the most common seizure focus in people. In the hippocampus, aberrant neurogenesis plays a critical role in the initiation and progression of epilepsy in rodent models, but it is unknown whether this also holds true in humans. To address this question, we used immunofluorescence on control healthy hippocampus and surgical resections from mesial temporal lobe epilepsy (MTLE), plus neural stem-cell cultures and multi-electrode recordings of ex vivo hippocampal slices. We found that a longer duration of epilepsy is associated with a sharp decline in neuronal production and persistent numbers in astrogenesis. Further, immature neurons in MTLE are mostly inactive, and are not observed in cases with local epileptiform-like activity. However, immature astroglia are present in every MTLE case and their location and activity are dependent on epileptiform-like activity. Immature astroglia, rather than newborn neurons, therefore represent a potential target to continually modulate adult human neuronal hyperactivity.

Effect of repeated seizures on spatial exploration and immediate early gene expression in the hippocampus and dentate gyrus

Immediate early genes (IEGs) are coordinately activated in response to neuronal activity and can cause activation of secondary response genes that modulate synaptic plasticity and mediate long-lasting changes in behaviour. Excessive neuronal stimulation induced by epileptic seizures induce rapid and dramatic changes in IEG expression. Although the impact of acute seizure activity on IEG expression has been well studied, less is known about the long-term effects of chronic seizures on IEG induction during seizure free periods where behavioural and cognitive impairments are frequently observed in people with epilepsy and in animal models of epilepsy. The present study sought out to examine the impact of chronic pentylenetetrazole evoked seizures (PTZ kindling) on spatial exploration induced in IEG expression (c-Fos, ΔFosB, Homer1a, Egr1, Npas4, Nr4a1) in the hippocampus (CA1 and CA3 subfields) and dentate gyrus of rats. Male rats underwent two weeks of PTZ kindling (every 2 days) or received vehicle injections and were placed into a novel open field arena for 30 min either 24 hrs or 4 weeks after the last treatment. Although exploratory activity was similar between PTZ kindled and vehicle controls when examined 24 hrs after the last treatment, we observed a significant reduction in spatial exploration induced expression of c-Fos, Egr1, and ΔFosB in the hippocampus and dentate gyrus, and reduced expression of Nr4a1 in the dentate gyrus and Homer1a in the hippocampus only. When testing was conducted after a 4-week recovery period, only c-Fos continued to show reduced expression after exposure a novel environment in previously PTZ kindled animals. Interestingly, these animals also showed reduced activity in the center region of the open field suggestive of heightened anxiety-like behaviour. Collectively, these results suggest that repeated seizures may lead to longterm downregulation in hippocampal IEG expression that can extend into seizure free periods thereby providing a critical mechanism for the development of cognitive and behavioural deficits that arise during chronic epilepsy

An insight into crosstalk among multiple signaling pathways contributing to epileptogenesis

Despite the years of research, epilepsy remains uncontrolled in one-third of afflicted individuals and poses a health and economic burden on society. Currently available anti-epileptic drugs mainly target the excitatory-inhibitory imbalance despite targeting the underlying pathophysiology of the disease. Recent research focuses on understanding the pathophysiologic mechanisms that lead to seizure generation and on possible new treatment avenues for preventing epilepsy after a brain injury. Various signaling pathways, including the mechanistic target of rapamycin (mTOR) pathway, mitogen-activated protein kinase (MAP-ERK) pathway, JAK-STAT pathway, wnt/β-catenin signaling, cAMP pathway, and jun kinase pathway, have been suggested to play an essential role in this regard. Recent work suggests that the mTOR pathway intervenes epileptogenesis and proposes that mTOR inhibitors may have antiepileptogenic properties for epilepsy. In the same way, several animal studies have indicated the involvement of the Wnt signaling pathway in neurogenesis and neuronal death induced by seizures in different phases (acute and chronic) of seizure development. Various studies have also documented the activation of JAK-STAT signaling in epilepsy and cAMP involvement in epileptogenesis through CREB (cAMP response element-binding protein). Although studies are there, the mechanism for how components of these pathways mediate epileptogenesis requires further investigation. This review summarises the current role of various signaling pathways involved in epileptogenesis and the crosstalk among them. Furthermore, we will also discuss the mechanical base for the interaction between these pathways and how these interactions could be a new emerging promising target for future epilepsy therapies.

Searching basic units in memory traces: associative memory cells

The acquisition of associated signals is commonly seen in life. The integrative storage of these exogenous and endogenous signals is essential for cognition, emotion and behaviors. In terms of basic units of memory traces or engrams, associative memory cells are recruited in the brain during learning, cognition and emotional reactions. The recruitment and refinement of associative memory cells facilitate the retrieval of memory-relevant events and the learning of reorganized unitary signals that have been acquired. The recruitment of associative memory cells is fulfilled by generating mutual synapse innervations among them in coactivated brain regions. Their axons innervate downstream neurons convergently and divergently to recruit secondary associative memory cells. Mutual synapse innervations among associative memory cells confer the integrative storage and reciprocal retrieval of associated signals. Their convergent synapse innervations to secondary associative memory cells endorse integrative cognition. Their divergent innervations to secondary associative memory cells grant multiple applications of associated signals. Associative memory cells in memory traces are defined to be nerve cells that are able to encode multiple learned signals and receive synapse innervations carrying these signals. An impairment in the recruitment and refinement of associative memory cells will lead to the memory deficit associated with neurological diseases and psychological disorders. This review presents a comprehensive diagram for the recruitment and refinement of associative memory cells for memory-relevant events in a lifetime.

Retrieval-Induced Forgetting in a Pentylenetetrazole-Induced Epilepsy Model in the Rat

The selective retrieval of some information may lead to the forgetting of related, but non-retrieved information. This memory phenomenon is termed retrieval-induced forgetting (RIF). Active inhibition is thought to function to resolve interference from competing information during retrieval, which results in forgetting. Epilepsy is associated with impaired inhibitory control that contributes to executive dysfunction. The purpose of this study is to investigate whether rats in a kindling model of epilepsy demonstrate normal levels of RIF. Rats were divided into two groups: saline and kindling. Pentylenetetrazole was injected intraperitoneally until the rats kindled. RIF was tested using a modified version of the spontaneous object recognition test, consisting of a sample phase, retrieval or interference phase, and a test phase. Exploration time for each object was analyzed. RIF was demonstrated in the saline group when rats subjected to the retrieval phase failed to discriminate between the familiar object and the novel object later in the test phase. Kindled rats, on the other hand, did not suffer forgetting even when they were subjected to the retrieval phase, as they spent significantly longer times exploring the novel rather than the familiar object in the test phase. Therefore, RIF was not observed in the kindling group. These findings indicate impaired retrieval-induced forgetting in kindled rats, which may be suggestive of a deficit in the inhibitory control of memory.

Enhanced expression of immediate-early genes in mouse hippocampus after trimethyltin treatment

Immediate-early genes (IEGs) are transiently and rapidly activated in response to various cellular stimuli. IEGs mediate diverse functions during pathophysiologic events by regulating cellular signal transduction. We investigated the temporal expression of several IEGs, including c-fos, early growth response protein-1 (Egr-1), and activity-regulated cytoskeleton-associated protein (Arc), in trimethyltin (TMT)-induced hippocampal neurodegeneration. Mice (7 weeks old, C57BL/6) administered TMT (2.6mg/kg intraperitoneally) presented severe neurodegenerative lesions in the dentate gyrus (DG) and showed behavioral seizure activity on days 1-4 post-treatment, after which the lesions and behavior recovered spontaneously over time. c-fos, Egr-1, and Arc mRNA and protein levels significantly increased in the mouse hippocampus after TMT treatment. Immunohistochemical analysis showed that nuclear c-fos expression increased mainly in the DG, whereas nuclear Egr-1 expression was increased extensively in cornu ammonis (CA) 1, CA3, and the DG after TMT treatment. Increased Arc levels were detected in the cellular somata/dendrites of the hippocampal subregions after TMT treatment. Therefore, we suggest that increased IEGs are associated with TMT-induced pathological events in mouse hippocampus.

Persistent Hyperactivity of Hippocampal Dentate Interneurons After a Silent Period in the Rat Pilocarpine Model of Epilepsy

Profile of GABAergic interneuron activity after pilocarpine-induced status epilepticus (SE) was examined in the rat hippocampal dentate gyrus by analyzing immediate early gene expression and recording spontaneous firing at near resting membrane potential (REM). SE for exact 2 h or more than 2 h was induced in the male Sprague-Dawley rats by an intraperitoneal injection of pilocarpine. Expression of immediate early genes (IEGs) was examined at 1 h, 1 week, 2 weeks or more than 10 weeks after SE. For animals to be examined at 1 h after SE, SE lasted for exact 2 h was terminated by an intraperitoneal injection of diazepam. Spontaneous firing at near the REM was recorded in interneurons located along the border between the granule cell layer and the hilus more than 10 weeks after SE. Results showed that both c-fos and activity-regulated cytoskeleton associated protein (Arc) in hilar GABAergic interneurons were up-regulated after SE in a biphasic manner; they were increased at 1 h and more than 2 weeks, but not at 1 week after SE. Ten weeks after SE, nearly 60% of hilar GABAergic cells expressed c-fos. With the exception of calretinin (CR)-positive cells, percentages of hilar neuronal nitric oxide synthase (nNOS)-, neuropeptide Y (NPY)-, parvalbumin (PV)-, and somatostatin (SOM)-positive cells with c-fos expression are significantly higher than those of controls more than 10 weeks after SE. Without the REM to be more depolarizing and changed threshold potential level in SE-induced rats, cell-attached recording revealed that nearly 90% of hilar interneurons fired spontaneously at near the REM while only 22% of the same cell population did so in the controls. In conclusion, pilocarpine-induced SE eventually leads to a state in which surviving dentate GABAergic interneurons become hyperactive with a subtype-dependent manner; this implies that a fragile balance between excitation and inhibition exists in the dentate gyrus and in addition, the activity-dependent up-regulation of IEGs may underlie plastic changes seen in some types of GABAergic cells in the pilocarpine model of epilepsy.

The effect of STAT3 inhibition on status epilepticus and subsequent spontaneous seizures in the pilocarpine model of acquired epilepsy

Pilocarpine-induced status epilepticus (SE), which results in temporal lobe epilepsy (TLE) in rodents, activates the JAK/STAT pathway. In the current study, we evaluate whether brief exposure to a selective inhibitor of the JAK/STAT pathway (WP1066) early after the onset of SE affects the severity of SE or reduces later spontaneous seizure frequency via inhibition of STAT3-regulated gene transcription. Rats that received systemic WP1066 or vehicle at the onset of SE were continuously video-EEG monitored during SE and for one month to assess seizure frequency over time. Protein and/or mRNA levels for pSTAT3, and STAT3-regulated genes including: ICER, Gabra1, c-myc, mcl-1, cyclin D1, and bcl-xl were evaluated in WP1066 and vehicle-treated rats during stages of epileptogenesis to determine the acute effects of WP1066 administration on SE and chronic epilepsy. WP1066 (two 50mg/kg doses) administered within the first hour after onset of SE results in transient inhibition of pSTAT3 and long-term reduction in spontaneous seizure frequency. WP1066 alters the severity of chronic epilepsy without affecting SE or cell death. Early WP1066 administration reduces known downstream targets of STAT3 transcription 24h after SE including cyclin D1 and mcl-1 levels, known for their roles in cell-cycle progression and cell survival, respectively. These findings uncover a potential effect of the JAK/STAT pathway after brain injury that is physiologically important and may provide a new therapeutic target that can be harnessed for the prevention of epilepsy development and/or progression.

Convulsant doses of a dopamine D1 receptor agonist result in Erk-dependent increases in Zif268 and Arc/Arg3.1 expression in mouse dentate gyrus

Activation of dopamine D1 receptors (D1Rs) has been shown to induce epileptiform activity. We studied the molecular changes occurring in the hippocampus in response to the administration of the D1-type receptor agonist, SKF 81297. SKF 81297 at 2.5 and 5.0 mg/kg induced behavioural seizures. Electrophysiological recordings in the dentate gyrus revealed the presence of epileptiform discharges peaking at 30-45 min post-injection and declining by 60 min. Seizures were prevented by the D1-type receptor antagonist, SCH 23390, or the cannabinoid CB1 receptor agonist, CP 55,940. The effect of SKF 81297 was accompanied by increased phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK), in the granule cells of the dentate gyrus. This effect was also observed in response to administration of other D1-type receptor agonists, such as SKF83822 and SKF83959. In addition, SKF 81297 increased the phosphorylation of the ribosomal protein S6 and histone H3, two downstream targets of ERK. These effects were prevented by genetic inactivation of D1Rs, or by pharmacological inhibition of ERK. SKF 81297 was also able to enhance the levels of Zif268 and Arc/Arg3.1, two immediate early genes involved in transcriptional regulation and synaptic plasticity. These changes may be involved in forms of activity-dependent plasticity linked to the manifestation of seizures and to the ability of dopamine to affect learning and memory.

Semi-automated atlas-based analysis of brain histological sections

Quantifying the location and/or number of features in a histological section of the brain currently requires one to first, manually register a corresponding section from a tissue atlas onto the experimental section and second, count the features. No automated method exists for the first process (registering), and most automated methods for the second process (feature counting) operate reliably only in a high signal-to-noise regime. To reduce experimenter bias and inconsistencies and increase the speed of these analyses, we developed Atlas Fitter, a semi-automated, open-source MatLab-based software package that assists in rapidly registering atlas panels onto histological sections. We also developed CellCounter, a novel fully automated cell counting algorithm that is designed to operate on images with non-uniform background intensities and low signal-to-noise ratios.

The c-Jun N-terminal kinase inhibitor SP600125 is neuroprotective in amygdala kindled rats

c-Jun N-terminal kinase (JNK) is implicated in cell death in neurodegenerative disorders and has been taken as a critical point between the physiological and pathological status. JNK specific inhibitor SP600125 has been found to have a protective effect against transient brain ischemia/reperfusion-induced neuronal death in rat hippocampal CA1 region. Former studies have shown the relation between JNK phosphorylation and neuronal damage in models of brain ischemia or in chemically or acute electrically kindled animal in which the stimulus-induced JNK phosphorylation was temporary or transient. In this study, the effect of repeated activation of JNK was examined in the amygdala kindled rats, under or without intraventrical infusion of SP600125, compared with the sham control. JNK phosphorylation was detected by Western blot and immunofluorescent staining in hippocampal extracts and in slices respectively. Nissl staining was performed to detect the neuronal defect. We found that the level of JNK phosphorylation (46KD) in the hippocampus increased in the amygdala kindled rats, whereas such JNK phosphorylation could be inhibited by SP600125. Expression of total JNK in the hippocampus remained unchanged in kindled rats compared with the sham control, and was not affected by SP600125. Neuronal defect was marked in the kindling group and in the vehicle DMSO group, and was alleviated under the administration of SP600125. These findings suggest that JNK phosphorylation is involved in the process of hippocampal sclerosis in the mesial temporal lobe epilepsy (TLE). JNK signaling pathway may be a new target to interfere with the development of hippocampal sclerosis in the temporal lobe epilepsy. The JNK inhibitor SP600125 can show a protective effect on hippocampal neurons in TLE by inhibiting JNK activation.

CRE-mediated transcription and COX-2 expression in the pilocarpine model of status epilepticus

Status epilepticus (SE) triggers neuronal death, reactive gliosis and remodeling of synaptic circuitry, thus leading to profound pathological alterations in CNS physiology. These processes are, in part, regulated by the rapid upregulation of both cytotoxic and cytoprotective genes. One pathway that may couple SE to transcriptionally dependent alterations in CNS physiology is the CREB (cAMP response element-binding protein)/CRE (cAMP response element) cascade. Here, we utilized the pilocarpine model of SE on a mouse strain transgenic for a CRE-reporter construct (beta-galactosidase) to begin to characterize how seizure activity regulates the activation state of the CREB/CRE pathway in both glia and neurons of the hippocampus. SE triggered a rapid (4-8 h post-SE) but transient increase in CRE-mediated gene expression in the neuronal sublayers. In contrast to neurons, SE induced a lasting increase (up to 20 days) in CRE-mediated transcription in both reactive astrocytes and microglia. CRE-mediated gene expression correlated with expression of the pro-inflammatory enzyme cyclooxygenase-2 (COX-2). To examine the role of CREB in SE-induced COX-2 expression, we generated a transgenic mouse strain that expresses A-CREB, a potent repressor of CREB-dependent transcription. In these animals, the capacity of SE to stimulate COX-2 expression was markedly attenuated, indicating that CREB is a key intermediate in SE-induced COX-2 expression. Collectively these data show that SE triggers two waves of CREB-mediated gene expression, a transient wave in neurons and a long-lasting wave in reactive glial cells, and that CREB couples SE to COX-2 expression.

Lack of effect of secretin on kindling and seizures

Secretin infused into rats activates neurons located in brain areas controlling autonomic function and emotion. The brain activity of secretin is mediated, at least in part, through vagal pathways. It is known that afferent stimulation of the vagus nerve results in considerable antiepileptic effects. Whether or not secretin has an effect on seizures is unknown. In this study, we evaluated the efficacy and safety of secretin as an antiepileptogenic agent in electrical kindling and as an anticonvulsant in fully kindled seizures. To assess antiepileptogenic effects, we administered secretin (10, 30, or 100 microg/kg/dose) or normal saline intravenously 5 min before twice-daily kindling stimulation. To assess the anticonvulsant effect of secretin, we administered either normal saline or secretin (100 microg/kg/dose) 5 min before the electrical stimulation to fully kindled rats. We observed no effect on kindling rate or afterdischarge duration. In fully kindled rats, secretin administration had no effect on kindling stage or afterdischarge duration. Thus, in the dose range used in this preliminary acute treatment study, secretin had no discernible antiepileptogenic or anticonvulsant effects. Secretin was very well tolerated in this multidose protocol.

Effects of herbimycin A in the pilocarpine model of temporal lobe epilepsy

Pilocarpine-induced status epilepticus (SE) causes widespread tyrosine phosphorylation in the brain. It has been postulated that this intracellular signal may mediate potentially epileptogenic changes in the morphology and physiology of particular brain regions, including the hippocampus. The present study evaluated the effects of herbimycin A, a protein tyrosine kinase (PTK) inhibitor, over the acute (during which intense biochemical and electrophysiological activation occurs) and the chronic phase (characterized by spontaneous and recurrent epileptic seizures and the presence of synaptic reorganization, e.g., mossy fiber sprouting) of the pilocarpine model of epilepsy. The administration of a single dose of 1.74 nmol of herbimycin A (i.c.v., 5 microL) 5 min after the onset of SE did not change the acute behavioral manifestation of seizures despite significantly decreasing c-Fos immunoreactivity in different areas of the hippocampus and of the limbic cortex. Herbimycin-treated animals developed spontaneous recurrent seizures, as did control animals, with a similar latency for the appearance of the first seizure and similar seizure frequency. Neo-Timm staining revealed that all animals experiencing SE, regardless of whether or not injected with herbimycin, showed aberrant mossy fiber sprouting in the supragranular region of the dentate gyrus. Herbimycin did not obviously affect neuronal cell death as evaluated in Nissl-stained sections. These results indicate that the PTK blockade achieved with the current dose of herbimycin reduced the acute c-Fos expression but failed to alter the spontaneous seizure frequency or to attenuate the morphological modifications triggered by the SE.

Influence of low dietary histamine on seizure development of chemical kindling induced by pentylenetetrazol in rats

AIM

To determine the role of dietary low histamine on the seizure development of pentylenetetrazol (PTZ)-induced kindling in rats.

METHODS

After 14 d of feeding on a low histamine diet (LH, containing 0.145 mumol/g of histamine), the rats were chemically kindled by repeated intraperitoneal injection of a subconvulsant dose of PTZ (35 mg/kg) once every 48 h, and seizure activity of kindling was recorded for 30 min. Histamine in brain samples was analyzed using a high performance liquid chromatography system with a fluorescence spectrofluorometer.

RESULTS

The LH diet induced an increase in seizure response (seizure susceptibility) to the first trial of PTZ, and resulted in facilitation of subsequent PTZ kindling process (seizure development). The histamine levels in the cortex, hippocampus, and hypothalamus of LH-treated rats decreased significantly and these changes correlated well with seizure behavior (r = 0.875, 0.651, and 0.796, respectively). In addition, chronic kindled seizures resulted in a significant increase of the histamine content in the cortex and hypothalamus in the LH-fed groups.

CONCLUSION

These findings indicate that the histamine in daily food could influence the brain histaminergic function, and play an important role in regulating seizure susceptibility.

Mechanisms of action of CHF3381 in the forebrain

(1) Aim of this study was to gain insight into the mechanism of action of CHF3381, a novel putative antiepileptic and neuroprotective drug. (2) CHF3381 blocked NMDA currents in primary cultures of cortical neurons: maximal effect was nearly -80% of the NMDA-evoked current, with EC(50) of approximately 5 micro M. This effect was selective, reversible, use-dependent and elicited at the concentrations reached in the rodent brain after peripheral administration of therapeutic doses. (3) CHF3381 also inhibited voltage-gated Na(+) currents in an apparently voltage-dependent manner. However, this effect could be obtained only at relatively high concentrations (100 micro M). (4) Consistent with the mild effects on voltage-gated Na(+) channels, CHF3381 (100 micro M) failed to affect electrical stimulation-evoked glutamate overflow in hippocampal slices. In contrast, the anti-convulsant agent and Na(+) channel blocker lamotrigine (100 micro M) inhibited stimulation-evoked glutamate overflow by approximately 50%. (5) CHF3381 reduced kindled seizure-induced c-fos mRNA levels within the same brain regions, and to a similar level, as the selective NMDA receptor antagonist MK801, providing circumstantial evidence to the idea that CHF3381 blocks NMDA receptors in vivo. (6) The present mechanistic studies suggest that the primary mechanism of action of CHF3381 in the forebrain is blockade of NMDA receptors. On this basis, this compound may have a potential use in other diseases caused by or associated with a pathologically high level of NMDA receptor activation.

Mechanism of systemically injected interferon-alpha impeding monoamine biosynthesis in rats: role of nitric oxide as a signal crossing the blood-brain barrier

The serious and characteristic side effects of interferon-alpha (IFN-alpha) therapy on the central nervous system, resulting in such problems as affective disorders or parkinsonism, have led us to investigate the biochemical mechanism of the effects of IFN-alpha on the monoaminergic neurotransmitter system using an animal model (rats). We first examined the concentrations of tetrahydrobiopterin (BH(4)) and monoamines in several regions of the brain after the intramuscular injection of IFN-alpha into rats; the levels of BH(4) and dopamine significantly decreased in the amygdala and raphe areas as compared with those of the controls. Based on these results, we further examined the concentrations of BH(4) and nitrite (NO(2)(-)) plus nitrate (NO(3)(-)), metabolites of nitric oxide (NO), in the amygdala and raphe areas after the intramuscular injection of IFN-alpha; the concentrations of both BH(4) and NO(2)(-)+NO(3)(-) significantly decreased as compared with the control. Furthermore, the addition of N(G)-monomethyl L-arginine, an inhibitor of NO synthase, after the injection of IFN-alpha restored the decreased levels of both NO(2)(-)+NO(3)(-) and BH(4) to control levels. As a result, nitric oxide induced by the intramuscular injection of IFN-alpha was found to cross the blood-brain barrier and suppress both tetrahydrobiopterin biosynthesis and dopamine production in the amygdala and raphe areas.

Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.

Neural growth, neural damage and neurotrophins in the kindling model of epilepsy

Do seizures change the brain? Studies on the kindling model--a widely used animal model of epilepsy--suggest that they do. Dr. Racine, one of the pioneers in the kindling field, describes the basic phenomena of kindling, and discusses the possible roles of cell growth and cell death in this model.

Genes differentially expressed in the kindled mouse brain

Kindling involves long-term changes in brain excitability and is considered a model of epilepsy and neuroplasticity. Differentially expressed genes in the kindled mouse brain were screened using an reverse transcription-polymerase chain reaction (RT-PCR) differential display (DD) method. C3H male mice were kindled with 40 stimuli in the hippocampus at 5-min intervals. Hippocampal RNA was isolated for DD from mice at 0.5 h, 1 day, 1 week, and 1 month after kindling and from sham-operated controls. About 30,000 bands were screened and of these, 50 were displayed differentially. Northern blot analysis confirmed that 26 of the 50 bands were differentially expressed following rapid kindling. Further sequence analysis revealed that 14 of the genes were previously identified and 12 were novel. The novel genes are referred to as King (1-12) genes because of their association with kindling. According to their temporal and quantitative pattern of expression in forebrain, the 26 genes were grouped into five types. Expression of five of the DD genes, one from each expression type, was further analyzed in hippocampus, forebrain, brainstem, and cerebellum of the kindled mice. Differential expression of these genes was observed in hippocampus and forebrain, but not in brainstem or cerebellum. Only one gene, a regulator of G-protein signaling 4 (RGS4), showed prolonged changes in expression in response to kindling. Our results show that rapid kindling produces spatial and temporal changes in gene expression that may influence kindling-associated neuroplasticity.

Mechanisms of brain plasticity: from normal brain function to pathology

Since this list of mechanisms covers much of what we know about how brain cells operate, one might object to using such a broad brush in characterizing a purportedly special feature of brain function--"plasticity." But that is really just the point. If a significant aspect of brain function is "plasticity," as I believe to be the case, then all (or at least most) brain mechanisms are likely to be involved in "plastic" processes. Indeed, we have identified very few "special" mechanisms associated with plasticity. Certainly, the factors that appear to be involved in epileptic pathologies are almost all old friends from the plasticity literature. It is this critical interrelationship between plasticity and pathology that was so important in Frank Morrell's work, a concept he advanced at a time when our understanding of these mechanisms was far less sophisticated than it is now. The influence of this idea is now pervasive in the neuroscience field, so much so that it is hard to imagine why there was so much resistance to these hypotheses when first advanced by Morrell. It is this general concept of plasticity-pathology relationship that will survive as the most influential legacy of Frank Morrell.

Sleep and wakefulness in c-fos and fos B gene knockout mice

G-protein coupled receptor (GPCR) stimulation has been implicated in the regulation of sleep. Upon stimulation of a GPCR an intracellular cascade involving second and third messengers is initiated. The latter include the fos-family of immediate early genes (IEGs). Although there is considerable evidence indicating that IEGs are expressed in response to sleep, the effects of their deletion on sleep is not known. The present study examined sleep-wakefulness in mice lacking the c-fos or fos B genes. Null c-fos mice compared to their wildtype (WT) and heterozygote (het) siblings had more wakefulness and less slow wave sleep (SWS); REM sleep was not affected. The null c-fos mice also had increased delta activity (0.3-4 Hz). In contrast, the null and heterozygote fos B mice had less REM sleep, but the time spent in SWS or wakefulness was not different from their wild-type (WT) siblings. In the null c-fos mice, the increased wakefulness and the reduction in SWS could not be due to a systemic alteration in temperature since the core temperature was similar in all mice. By demonstrating that these IEGs are involved in sleep, we suggest that the deletion of specific genes, even within a family of genes, can have a specific effect on sleep.

Specific amygdaloid nuclei are involved in suppression or propagation of epileptiform activity during transition stage between oral automatisms and generalized clonic seizures

Kindling is a model of the neural plasticity that occurs following stimulation to the brain, which can result in epileptogenesis. The amygdala (Am), one of the most sensitive structures from which to induce electrical kindling, is comprised of distinct nuclei that possess differences in threshold for seizure initiation, unique cellular and molecular morphology, and specific neuroanatomical connections within the amygdala and, to other cortical and subcortical brain structures. The aim of this study was to map the spread of epileptiform activity throughout the ipsilateral and contralateral hemispheres during the transition stage between oral automatisms and generalized clonic seizures, by measuring changes in mRNA expression for c-fos, NGFI-A, and BDNF. The stimulating electrode was implanted in either the basolateral (BL) or the lateral (CeL) or medial (CeM) subdivisions of the central nucleus of the amygdala. The rats were kindled once daily using afterdischarge-threshold electrical stimulation until the first forelimb clonic seizure was induced. They were sacrificed 30 min later, and their brains were prepared for in situ hybridization to measure mRNA expression of c-fos, NGFI-A and BDNF. The results demonstrate that: (1) the threshold to elicit an afterdischarge from the BL was lower than that of either the medial (CeM) or lateral (CeL) subdivisions of the Ce, which did not differ from each other; (2) the patterns of mRNA expression for c-fos, NGFI-A and BDNF were highly similar to each other when the stimulation site was the BL or the CeL, and included mainly limbic cortical and subcortical areas ipsilateral to the electrode; (3) c-fos was the only probe to be expressed in the contralateral hemisphere following the first motor seizure, and the pattern of its expression reflected a subset of structures recruited in the ipsilateral hemisphere including the claustrum, insular and perirhinal cortices; (4) unexpectedly, stimulation of the CeM elicited seizures and afterdischarges of shorter duration than those evoked by stimulation of the BL or CeL, and failed to increase mRNA expression for any of the probes in the hippocampus or in the contralateral hemisphere. A neuroanatomical model of Am-induced seizure propagation is proposed suggesting that the Claust-Ins-PRh play a pivotal role during the transition between oral automatisms and generalized clonic convulsions.

Potentiation of excitotoxicity in transgenic mice overexpressing neuronal cyclooxygenase-2

In this study we describe the generation of a transgenic mouse model with neuronal overexpression of the human cyclooxygenase-2, h(COX)-2, to explore its role in excitotoxicity. We report that overexpression of neuronal hCOX-2 potentiates the intensity and lethality of kainic acid excitotoxicity in coincidence with potentiation of expression of the immediate early genes c-fos and zif-268. In vitro studies extended the in vivo findings and revealed that glutamate excitotoxicity is potentiated in primary cortico-hippocampal neurons derived from hCOX-2 transgenic mice, possibly through potentiation of mitochondrial impairment. This study is the first to demonstrate a cause-effect relationship between neuronal COX-2 expression and excitotoxicity. This model system will allow the systematic examination of the role of COX-2 in mechanisms of neurodegeneration that involve excitatory amino acid pathways.

Enhanced Fos expression in the hippocampus of El mice after short-term vestibular stimulation

The El mouse is an animal model for human epilepsy. The mouse manifest seizures in response to periodically repeated vestibular stimuli such as being tossing-up. Although this technique has been traditionally employed to accelerate this disease in the mouse, its meaning remained obscure. The present study was conducted to estimate the effects of tossing-up stimuli on expression of c-fos, a well-known marker of neuronal activation. Expression of c-fos was significantly increased even after single tossing-up specimen. It was blocked by pretreatment of the mouse with MK-801, a NMDA receptor antagonist. A marked expression of the oncoprotein was observed in the granule cell layer of dentate gyrus and CA1-3 pyramidal cell layer in the hippocampus of the mouse. These results suggest that the El mice are genetically hyper-sensitive to the vestibular stimuli.

Fos-like immunoreactivity in the auditory brainstem evoked by bipolar intracochlear electrical stimulation: effects of current level and pulse duration

Fos-like immunoreactivity was used to compare the auditory brain stem excitation elicited by bipolar electrical stimulation of the cochlea at various current levels relative to the electrically evoked auditory brain stem response threshold for a 50-micros/phase monophasic pulse. Fos-like immunoreactive cells were labeled in primary auditory brain stem regions. The distribution of labeled cells was restricted to regions known to be cochleotopically related to the stimulated region of the scala tympani. Some labeled cells were observed at 2x electrically evoked auditory brain stem response threshold. The number, density and spatial distribution of labeled cells were quantified in the dorsal cochlear nucleus and inferior colliculus, and found to increase with increasing level of stimulation. For 50-micros pulses, the location of labeled neurons remained reasonably restricted to narrow bands within each region until the 1Ox level of stimulation (20 dB above electrically evoked auditory brain stem response threshold) was reached. While a monotonic increase in Fos-like immunoreactivity with increasing stimulus level was observed in most nuclei, for cells of the superficial layer of the dorsal cochlear nucleus, a non-monotonic change with increasing stimulus level was seen. This dorsal cochlear nucleus non-monotonicity may indicate that, at higher levels of stimulation, a secondary indirect inhibitory input, probably associated with activation of deep layer dorsal cochlear nucleus cells, reduces excitatory responses at the superficial layer of the dorsal cochlear nucleus. Electrically evoked auditory brain stem response and Fos expression showed parallel changes as a function of stimulus level and pulse duration. The data indicate that discrete activation of cell populations within the central auditory pathways can occur with bipolar electrical stimulation to the highest levels of stimulation typically useful in humans. The data also indicate a close, but not identical, quantitative relationship between Fos-like immunoreactivity and electrophysiological response amplitude. These findings support the view that a study of Fos-like immunoreactivity can provide a powerful and quantitative tool for study of the dynamic response characteristics of cells of the central auditory system to electrical stimulation at suprathreshold levels. The data suggest that there is a monotonic increase in the number of neurons responsive to intracochlear electrical stimulation as a function of stimulus level, at least through the upper half of the dynamic range, but that this increase does not result in a complete loss of spatial selectivity. Coupled with previous psychophysical studies, these results suggest that the increase in the number of activated neurons is functionally beneficial, resulting in improved discrimination of changes in the electrical signals.

Repetitive electroconvulsive seizures induce activity of c-Jun N-terminal kinase and compartment-specific desensitization of c-Jun phosphorylation in the rat brain

Electroconvulsive seizures (ECS) are used for therapy of pharmacoresistent depression and are supposed to induce long-lasting neuronal alterations in morphology and gene expression. In this study, we have investigated the phosphorylation of the transcription factor protein c-Jun at its serine 73 residue by immunohistochemistry and the activity of the c-Jun N-terminal kinase 1 (JNK1) by immunocomplex assay following repetitive ECS in adult rats. In untreated controls, nuclear c-Jun immunoreactivity, but not N-terminal phosphorylation, was present in a variety of neuronal populations including the hippocampus, the temporobasal cortex and the amygdalar complex. Daily ECS for 1, 5 or 10 days (1x, 5x or 10x ECS) did not alter the expression of c-Jun but caused a substantial N-terminal phosphorylation of c-Jun (phospho-c-Jun). Nuclear phospho-c-Jun immunoreactivity was maximal within 15 min following ECS, and became absent after 30 min. The highest levels of phospho-c-Jun labeling were found after 1x ECS in the amygdalar complex, the dorsomedial hypothalamus and the piriform cortex. The inducibility of c-Jun N-terminal phosphorylation was preserved in the medial amygdala and piriform cortex, but significantly declined in the basal amygdala and medial hypothalamus with progressive ECS stimulation. One single ECS 3 or 5 days following 10x ECS yielded a pattern of phospho-c-Jun as seen following 10x ECS; thus, a lag of 5 days was not sufficient to provoke the initial level of N-terminal phosphorylation of c-Jun. In the rostral hippocampus, c-Jun was not phosphorylated at any investigated time inspite of its high constitutive expression. In some contrast with this compartment-specific phosphorylation of c-Jun, immunocomplex assays revealed that the JNK1 activity was strongly enhanced in both amygdala and hippocampus. Our findings demonstrate that rapid JNK activation and phosphorylation of c-Jun as stand-by transcription factor characterize the beginning of neuroplastic changes, e.g., following ECS, a classic treatment of mental disorders. The N-terminal phosphorylation is compartment specific and can habituate following repetitive stimulation suggesting that the differential activation of the JNK/c-Jun axis is part of the neuronal strategy to integrate transynaptic excitation.

A gene expression approach to mapping the functional maturation of the hippocampus

Previous studies have shown an association among seizures, neuronal death and the expression of cellular immediate-early genes (cIEG). To understand further the relationship between these processes, we investigated the ability of kainic acid (KAI) to induce behavioral responses and gene expression in the hippocampus of developing fos-lacZ transgenic mice. Despite the fact that KAI elicited seizure-like activity from P2 onwards, Fos-lacZ was first detected at P5 in CA3 pyramidal neurons. Thus, intense behavioral responses were not invariably associated with fos-lacZ expression. Furthermore, while adult CA3 neurons are highly susceptible to KAI toxicity, they are resistant at P5. Therefore, the presence of Fos-lacZ in CA3 neurons is not necessarily predictive of their fate. By P10, Fos-lacZ was induced in CA3 neurons and in the most mature granule neurons of the dentate gyrus (DG). Between P15 and P20, KAI induced fos-lacZ in all CA1 and CA3 pyramidal neurons and most granule neurons of the DG. This stereotypical pattern of fos-lacZ expression mirrors the ontogeny of hippocampal circuitry and glutamate signalling. Thus the fos-lacZ mice can be used to map the functional maturation of the nervous system with single cell resolution. The scope of this approach was extended by administration of additional chemoconvulsants to fos-lacZ mice and by analysis of fos-lacZ transgenic mice with mutations in their FAP site. These additional studies revealed anatomical and mechanistic differences in glutamate receptor-mediated transcriptional responses in the nervous system.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Paradoxical effects of lithium on serotonergic receptor function: an immunocytochemical, behavioural and autoradiographic study

Lithium is the preferred treatment for bipolar affective disorder, yet its mechanism of action is poorly understood. Our study was designed to investigate the effect of lithium on the 5-HT2A or 5-HT2C (5-HT2A/2C) receptor subtypes, by comparing the consequences of chronic pre-treatment of rats with lithium on 5-HT2A/2C receptor-mediated behavioural responses, Fos expression, and the density of these receptors in the brain. In addition, the time-course and persistence of the effect of chronic lithium on 5-HT2A/2C receptor-mediated Fos expression was examined. Furthermore, the acute action of lithium on Fos expression was also examined. In an investigation of the dose response of Fos to the 5-HT2A/2C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), rats received saline or 1, 2, 4, 8, 12, 16, 24 or 32 mg/kg DOI, then were sacrificed 3 h later for immunocytochemical localisation of Fos. In a chronic lithium study, rats received either control or lithium-containing (0.1% LiCO3) chow for 3 weeks prior to challenge with 8 mg/kg DOI. DOI-induced locomotor activity was measured for 30 min immediately following the drug challenge, then 150 min later, the animals were sacrificed for Fos immunocytochemistry. The brains of another group of rats, also receiving either control or lithium-containing diet for 3 weeks, were analysed for the distribution and density of 5-HT2A receptor binding sites by quantitative [3H]ketanserin autoradiography. One group of chronic lithium treated rats received ritanserin (0.4 mg/kg), a 5-HT2A/2C receptor antagonist, 40 min before DOI challenge and were sacrificed 3 h later for Fos localisation. In the time-course experiment, rats received lithium-containing diet for 3 weeks followed by normal, control diet for 48 h, 1, 2 or 4 weeks prior to DOI or saline challenge. A further group of animals received an injection of LiCl (3 mM/kg) before being challenged with DOI or saline 12, 24, 36 or 48 h later. The dose-response experiment revealed that little Fos-like immunoreactivity was evident above basal levels following administration of 1 mg/kg DOI. However, at all other doses examined, Fos-like immunoreactivity was elevated in a number of brain areas, particularly in cerebral cortex, olfactory tubercle and amygdala. Following 24 mg/kg DOI, the number of Fos-positive nuclei appeared to have reached a plateau level. Treatment of rats with chronic lithium significantly enhanced DOI-induced locomotor activity and Fos-like immunoreactivity throughout the cerebral cortex. This elevation in Fos-like immunoreactivity was completely abolished by prior treatment with ritanserin. In contrast, chronic lithium treatment had no effect on the density of [3H]ketanserin binding to 5-HT2A receptors in any brain region examined. The results of the time-course experiment demonstrated that the enhancing effect of lithium on 5-HT2A/2C receptor-mediated Fos expression was short-lived such that Fos-like immunoreactivity returned to untreated levels within 48 h. In the acute lithium experiment, administration of lithium to rats 12 or 24 h before DOI resulted in a similar elevation of Fos-like immunoreactivity to that seen in chronically treated animals. Administration of acute lithium 36 or 48 h before DOI had no effect. The effects of lithium on 5-HT2A/2C receptor function thus appear to be complex. In particular, the results of this study indicate that the enhancing effects of lithium on DOI-induced locomotor activity and Fos-like immunoreactivity are not accompanied by any alteration in the density of 5-HT2A receptor binding sites. If changes in receptor numbers therefore do not account for the physiological effect of chronic lithium, other explanations must be sought. The study also suggests that the inositol depletion hypothesis of lithium's therapeutic action does not adequately explain the mechanism of action of lithium in man.

Differential expression of c-fos mRNA and Fos protein in the rat brain after restraint stress or pentylenetetrazol-induced seizures

1. c-fos mRNA expression and Fos protein expression were investigated by in situ hybridization and immunohistochemistry after 30 min of forced restraint stress or pentylenetetrazol (PTZ; 64 mg/kg, i.p.)-induced seizures. 2. Forced restraint stress and PTZ-induced seizures generated c-fos mRNA expression of distinct intensities, but in similar brain regions, including the hippocampus, the amygdala, the piriform cortex, the paraventricular hypothalamic nucleus, the habenula, and parts of the cerebral cortex. 3. The distribution of Fos-like immunoreactivity induced by stress or seizures only partially overlap. No Fos-like expression was found in the hippocampus or the habenula after restraint stress. Nevertheless, both areas presented Fos-like expression after PTZ-induced seizures. 4. Our results support the suggestion that immediate early gene expression in vivo may exhibit both region- and stimulus-specific expression.

Different patterns of induction of FGF-2, FGF-1 and BDNF mRNAs during kindling epileptogenesis in the rat

Neurotrophic factors (NTF) play important roles in the developing and in the adult brain. NTF involvement in neuronal plasticity is suggested by the modulation of NTF expression patterns in different physiological and pathological situations and by the effects they produce in the adult brain (e.g. axonal sprouting induction and neuroprotection). We used the RNAase protection assay to investigate the expression patterns of some NTFs during amygdala kindling, an animal model of epilepsy in which 'pathological' neuronal plasticity appears to occur. After a single kindling stimulation, fibroblast growth factor-2 (FGF-2) mRNA levels were increased in the hippocampus, the cortex and the hypothalamus, whereas they were not significantly altered in the thalamus and the striatum. A single stimulation did not alter fibroblast growth factor-1 (FGF-1) and brain-derived neurotrophic factor (BDNF) gene expression. Fully kindled animals, left unstimulated for a week, did not exhibit any alteration in the mRNA levels for any of the NTFs examined. However, in contrast with the effect of a single stimulation, amygdala stimulation of kindled animals (evoking a generalized tonic-clonic seizure) produced a great increase in hippocampal and cortical BDNF mRNA levels, but FGF-1 mRNA levels were not altered, and FGF-2 mRNA levels were significantly increased only in the cortex. These results suggest that different NTFs can be recruited at different stages of kindling epileptogenesis and, accordingly, may play different parts in the adaptive changes taking place in this experimental paradigm.

Kindling and associated mossy fibre sprouting are not affected in mice deficient of NGFI-A/NGFI-B genes

Kindling is an animal model of epileptogenesis, whereby repeated administration of an initially subconvulsive electrical stimulation eventually leads to the development of generalized motor seizures. Once established, the kindling effect is permanent. Although the molecular basis of kindling remains incompletely understood, emerging lines of evidence suggest that the induction of immediate-early genes could represent a link between periodic short-term stimuli and the long-lasting functional and structural alterations in the brain associated with the development of seizures. A recent study showed that null mutation of the immediate-early gene c-fos impairs the structural and functional plasticities in kindling. In the present study, we examined whether two other seizure-inducible immediate-early genes--NGFI-A (also termed EGR-1, zifl268, and Krox-24) and NGFI-B (also termed Nur77)--play requisite roles in kindling. We found that neither the rate of kindling nor seizure-induced granule cell axonal sprouting was affected in mice carrying a null mutation of NGFI-A. Furthermore, double knock-out of NGFI-A and NGFI-B genes does not result in detectable changes in kindling development and associated mossy fibre sprouting. Taken together, our observations indicate that neither constitutive nor seizure-induced expression of NGFI-A or NGFI-B is uniquely required for the establishment of kindling. These findings underscore the specificity of the immediate-early genes whose transcriptional activation contributes to kindling epileptogenesis.

Differential regulation by MK801 of immediate-early genes, brain-derived neurotrophic factor and trk receptor mRNA induced by a kindling after-discharge

Transient changes in immediate-early genes and neurotrophin expression produced by kindling stimulation may mediate secondary downstream events involved in kindling development. Recent experiments have demonstrated conclusively that both kindling progression and mossy fibre sprouting are significantly impaired by administration of the N-methyl-D-aspartate (NMDA) receptor antagonist MK801. To further examine the link between kindling, changes in gene expression and the NMDA receptor, we examined the effects of MK801 on neuronal induction of immediate-early genes, brain-derived neurotrophic factor (BDNF) and trk receptor mRNA expression produced by a single electrically induced hippocampal after-discharge in rats. The after-discharge produced a rapid (after 1 h) increase in Fos, Jun-B, c-Jun, Krox-24 mRNA and protein and Krox-20 protein in dentate granule neurons and a delayed, selective expression of Fos, Jun-D and Krox-24 in hilar interneurons. MK801 pretreatment produced a very strong inhibition of Fos, Jun-D and Krox-20 increases in dentate neurons but had a much smaller effect on Jun-B and c-Jun expression. MK801 did not inhibit Krox-24 expression in granule neurons or the delayed expression of Fos, Jun-D and Krox-24 in hilar interneurons. BDNF protein and trk B and trk C mRNA expression were also strongly induced in dentate granule cells 4 h following an after-discharge. MK801 abolished the increase in BDNF protein and trk B, but not trk C mRNA in granule cells at 4 h. These results demonstrate that MK801 differentially regulates the AD-increased expression of a group of genes previously identified as being likely candidates for an involvement in kindling. Because MK801 significantly retards the development of kindling and mossy fibre sprouting, it can be argued that those genes whose induction is not significantly attenuated by MK801 are unlikely to play an important role in the MK801-sensitive component of kindling and the changes in neural connectivity (mossy fibre sprouting) associated with kindling. Conversely, the role in kindling of those genes whose expression was significantly attenuated by MK801 (Fos, Jun-D, Krox-20, trkB and BDNF) requires further examination.

Zif268 and Fos-like immunoreactivity in tetanus toxin-induced epilepsy: reciprocal changes in the epileptic focus and the surround

Altered gene expression for a number of molecules has been suggested as one of the underlying mechanisms of epileptogenesis. Changes in expression of the immediate early genes, zif268 and c-fos, were investigated in chronic focal epilepsy induced by tetanus toxin (TT, 20-35 ng) injected in the rat motor cortex. Most rats injected with TT and perfused on postoperative day 5, 7 or 14 had recurrent focal seizures after a latent period of 4-13 days, and showed enhanced Zif268 immunoreactivity in a cluster of neurons at the injection site, as well as reduced Zif268 immunoreactivity in a distinct cortical zone around this cluster. C-fos or Fos-related immunoreactivity was decreased over widespread areas of frontoparietal and piriform cortex in epileptic rats, except for a focus at the injection site which, in most cases, showed increases in Fos-like immunoreactivity. Some epileptic rats showed increased Zif268 immunoreactivity in neurons of the ipsilateral ventral lateral and central lateral thalamic nuclei and increased Zif268 and Fos-like immunoreactivity in the pontine nuclei. Rats perfused before onset of seizures, showed no overt changes other than a slight decrease in Zif268 and Fos-like immunoreactivity at the injection site. The reciprocal changes in Zif268 immunoreactive neurons in the epileptic focus and the immediate surround parallel changes in gene expression for a number of molecules important in epileptogenesis and suggest a state of functional disconnection of the epileptic focus from other cortical areas that may contribute to the development and maintenance of focal epilepsy.

Egr transcription factors in the nervous system

The Egr proteins, Egr-1, Egr-2, Egr-3 and Egr-4, are closely related members of a subclass of immediate early gene-encoded, inducible transcription factors. They share a highly homologous DNA-binding domain which recognises an identical DNA response element. In addition, they have several less-well conserved structural features in common. As immediate early proteins, the Egr transcription factors are rapidly induced by diverse extracellular stimuli within the nervous system in a discretely controlled manner. The basal expression of the Egr proteins in the developing and adult rat brain and the induction of Egr proteins by neurotransmitter analogue stimulation, physiological mimetic and brain injury paradigms is reviewed. We review evidence indicating that Egr proteins are subject to tight differential control through diverse mechanisms at several levels of regulation. These include transcriptional, translational and post-translational (including glycosylation, phosphorylation and redox) mechanisms and protein-protein interaction. Ultimately the differentially co-ordinated Egr response may lead to discrete effects on target gene expression. Some of the known target genes of Egr proteins and functions of the Egr proteins in different cell types are also highlighted. Future directions for research into the control and function of the different Egr proteins are also explored.

Putative roles for the inducible transcription factor c-fos in the central nervous system: studies with antisense oligonucleotides

Although immediate-early genes such as c-fos are widely believed to play an important role in neuroplasticity, there is limited evidence to support involvement in the initiation of molecular events leading to medium- and long-term changes in brain function following a stimulus. Results using techniques such as transgenic knockout of the gene are often difficult to interpret. Antisense oligonucleotide technology offers an alternative. Infusion of antisense oligonucleotide to modify the expression of c-fos in the brain results in dramatic changes in rotation behaviour in animals challenged with psychostimulant drugs such as amphetamine. Similarly, the knockdown of c-fos expression using antisense oligonucleotides can also alter the rate of amygdala kindling in response to electrical stimulation of the brain. While studies using antisense oligonucleotides to knockdown c-fos expression provide evidence that the expression of c-fos plays an important role in regulating neuronal function, the use of antisense nucleotides has limitations and experiments must be very carefully controlled. Many details of antisense oligonucleotide actions remain unknown.

Early induction of mRNA for calbindin-D28k and BDNF but not NT-3 in rat hippocampus after kainic acid treatment

The influence of kainic acid (KA), which induces acute seizures, on expression of mRNA for the calcium-binding protein, calbindin-D28k, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and early-response genes [c-fos, zif268 (NGFI-A), nur77 (NGFI-B)] was examined in rat hippocampus by Northern blot analysis. A significant increase (3.2-fold) in BDNF mRNA was observed 1 h after KA injection (12 mg/kg i.p.) and peak expression (9.4-fold) occurred 3 h after KA. The induction of BDNF mRNA was preceded by the induction of c-fos, mRNA (30 min after KA) and was followed by the induction of calbindin-D28k mRNA (3.5-fold 3 h after KA; a maximal response was at 3-6 h after KA). Region-specific changes, analyzed by immunocytochemistry and in situ hybridization, indicated that the most dramatic increases in calbindin protein and mRNA after KA treatment were in the dentate gyrus. Although calbindin-D28k and BDNF mRNAs were induced, a 3.4-3.8-fold decrease in NT-3 mRNA was observed by Northern analysis 3-24 h after KA treatment. Calbindin-D28k gene expression was also examined in rats with a chronic epileptic state characterized by recurrent seizures established with an episode of electrical stimulation-induced status epilepticus (SE). When these animals were examined 30 days post-SE, no changes in hippocampal calbindin-D28k mRNA were observed. Our findings suggest that the induction of calbindin-D28k mRNA (which may be interrelated to the induction of BDNF mRNA) is an early response which may not be related to enhanced neuronal activity or seizures per se, but rather to maintaining neuronal viability.

Rat nurr1 is prominently expressed in perirhinal cortex, and differentially induced in the hippocampal dentate gyrus by electroconvulsive vs. kindled seizures

We isolated a rat orphan nuclear hormone receptor from a brain cortex cDNA library. The sequence of the cDNA insert was 2154 bp with an open reading frame of 1794 bp encoding a putative protein of 598 amino acids and predicted molecular mass of 65 kDa. The deduced amino acid sequence showed a strong homology to the mouse nurr1 and human NOT1 orphan nuclear hormone receptors of the NGFI-B/nur77/NAK1 gene subfamily. We refer to this rat clone as r-nurr1. Northern blot analysis showed that r-nurr1 mRNA was highly expressed in the brain and moderately in the lung as a 4.0 kb transcript. A smaller transcript of 2.5 kb was also detected in the testes. The level of r-nurr1 transcript in the heart, skeletal muscle, liver, kidney and spleen was marginal. In situ hybridization showed that r-nurr1 mRNA was constitutively expressed in various regions of the CNS, particularly in the deeper layers (IV to VI) of the perirhinal cortex and area 2 of parietal cortex. We further evaluated the modulation of r-nurr1 expression in CNS by an electroconvulsive seizure (ECS) and by an amgydala-kindled seizure. A single ECS administered via earclip electrodes induced a rapid and transient increase of r-nurr1 mRNA in the granule cells of the dentate gyrus, being significant at 15 min after the seizure, maximal approximately 1 h and back to baseline at 4 h. The amygdala kindled seizure revealed a less robust and restricted nurr-1 induction in the CNS, as only two of the four kindled animals showed a unilateral induction of nurr1 mRNA in the dentate gyrus. These results suggest that r-nurr1 is an immediate-early gene that is differentially induced by ECS vs. kindled seizures. In addition, as r-nurr1 is prominently expressed in the specific brain sites associated with memory acquisition and consolidation, it may play a role in memory processing.

The effect of cycloheximide on the regulation of proenkephalin and prodynorphin gene expressions induced by kainic acid in rat hippocampus

The effect of cycloheximide (CHX), a protein synthesis inhibitor, on the regulation of proenkephalin (proENK) and prodynorphin (proDYN) mRNA levels, proto-oncogenes, such as c-fos, 35-kDa fra and c-jun mRNA, and the levels of their products induced by kainic acid (KA) in rat hippocampus was studied. The proENK and proDYN mRNA levels were markedly increased 4 and 8 h after KA (10 mg/kg i.p.) administration. However, the intracellular proENK protein level was not affected by KA. The elevations of both proENK and proDYN mRNA levels induced by KA were inhibited by pre-administration of CHX (15 mg/kg i.p.). The increases of proENK and proDYN mRNA levels induced by KA were well-correlated with the increases of c-Fos, 35-kDa Fra and c-Jun protein levels. KA administration increased the hippocampal levels of c-Fos, 35-kDa Fra and c-Jun proteins with the time. The increases of c-Fos, 35-kDa Fra and c-Jun protein levels induced by KA administration were also inhibited by CHX pre-administration. KA administration markedly increased both c-fos and c-jun mRNA levels during 1 and 4 h and the increased levels of these proto-oncogene mRNA were further prolonged by the treatment with CHX. In addition, CHX alone increased both c-fos and c-jun mRNA levels although the onset times of induction were different. In electrophoretic mobility shift-assay, both AP-1 and ENKCRE-2 DNA-binding activities were increased by KA. KA-induced increases of AP-1 and ENKCRE-2 DNA-binding activities were also attenuated by CHX. In addition, KA-induced AP-1 and ENKCRE-2 DNA-binding activities were diminished by the antibodies against Fos and Jun family proteins. Furthermore, the cross-competition studies revealed that AP-1 proteins actively participated in ENKCRE-2 DNA domain. The results suggest that KA-induced proENK and proDYN mRNA expressions may require on-going synthesis of proteins, such as c-Fos, c-Jun and 35-kDa Fra, which may have a possible role in the up-regulation of proENK and proDYN gene expression through the binding with AP-1 and ENKCRE-2 DNA-binding motifs.

Glutamate receptors and gene induction: signalling from receptor to nucleus

Activation of glutamate receptors has been linked to a diversity of lasting physiologic and pathologic changes in the mammalian nervous system. The cellular and molecular mechanisms underlying permanent modifications of nervous system structure and function following brief episodes of neuronal activity are unknown. Immediate early genes (IEGs) have been implicated in the conversion of short-term stimuli to long-term changes in cellular phenotype by regulation of gene expression. Many of the long-term consequences of glutamate receptor activation correlate with increases in specific IEGs; the intracellular signalling pathways coupling activation of receptors at the cell surface with induction of IEGs in the nucleus are incompletely understood. Analysis of mechanisms of how extracellular factors control gene expression implicate activation of second messenger systems and protein kinases. Activation of glutamate receptors results in an initial increase in intracellular calcium; the route of calcium influx may differ depending on the specific receptor subtype activated. Intracellular calcium is often the first messenger in response to an extracellular stimulus and can be the trigger for activating numerous other signalling pathways. Results obtained over the past several years support a hypothesis where selective activation of distinct intracellular signalling pathways and IEG responses, following activation of different glutamate receptor subtypes, involve spatial restriction of key enzymes to sites of local calcium increases. The specificity in long-term neuronal responses following brief synaptic activation may depend on the specific intracellular signalling mechanisms triggered and the unique array of IEGs transcribed.

Prostaglandin F2alpha is required for NMDA receptor-mediated induction of c-fos mRNA in dentate gyrus neurons

Activation of NMDA receptors has been linked to a diversity of lasting physiological and pathological changes in the mammalian nervous system. The cellular and molecular mechanisms underlying permanent modifications of nervous system structure and function after brief episodes of neuronal activity are unknown. Immediate-early genes (IEGs) have been implicated in the conversion of short-term stimuli to long-term changes in cellular phenotype by regulation of gene expression. The intracellular signaling pathways coupling activation of receptors at the cell surface with induction of IEGs in the nucleus are incompletely understood. NMDA produces a striking increase in the IEG c-fos in dentate gyrus (DG) neurons in vitro; this induction is dependent, in part, on the arachidonic acid cascade. Here we show that NMDA receptor activation triggers the synthesis of the prostaglandins PGF2alpha and PGE2, but not PGD2, in rat cerebral cortical neurons in vitro. We further demonstrate that PGF2alpha, but not PGE2 or PGD2, is necessary but not sufficient for NMDA induction of c-fos mRNA in DG neurons. These findings provide insight into the molecular events coupling activation of the NMDA receptor with regulation of the IEG c-fos and identify the diffusable messenger PGF2alpha as obligatory for NMDA receptor-mediated transcription of a nuclear IEG.

Age-related changes in c-fos mRNA induction after open-field exposure in the rat brain

In the present study, functional activation of brain regions was measured by the induction of c-fos and c-jun mRNA following exposure to a novel open field. Fischer 344 rats at 5 months, 14 months, and 21 months were exposed to a square open field for 20 min. Rats were then immediately sacrificed and their brains were examined for c-fos and c-jun mRNA using in situ hybridization. Control rats were sacrificed directly from their home cage. Results showed no significant age-related changes in locomotor activity. Autoradiogram analyses showed that open-field exposure induced c-fos mRNA throughout the brain, while c-jun mRNA was induced in a few brain regions. Aged rats showed a lower elevation of c-fos mRNA in the prelimbic cortex compared to 5-month rats. In addition, grain analyses revealed age-related decreases in c-fos mRNA induction in the medial prefrontal cortex, caudate, and ventral lateral septum. These findings indicate age-related changes in the induction of c-fos mRNA in certain brain regions following exploration of a novel environment.

Altered expression of group I metabotropic glutamate receptors in the hippocampus of amygdala-kindled rats

Kindling is a well documented model of acquired focal epilepsy and synaptic plasticity in the nervous system. Previous biochemical studies have indicated an increase in mGluR-mediated phosphoinositide hydrolysis in the amygdala or hippocampus of fully kindled animals. In this study we have used in situ hybridisation techniques to examine the mRNA expression of group I metabotropic glutamate receptors (mGluR1 and mGluR5 both linked to phosphoinositide hydrolysis) in the hippocampus of amygdala-kindled animals sacrificed 24 h, 7 days or 28 days following the last electrically evoked stage 5 seizure, and in implanted non-stimulated control rats. Results indicate an initial up-regulation in mGluR1 mRNA (expressed as percentage of control) bilaterally in the DG (35-40%) and CA3 (16-48%), and unilaterally in CA4 (12%) in the 24 h post-kindled group. In kindled animals studied 7 days after the last seizure, these changes were either reduced or had returned to control levels. By 28 days mGluR1 mRNA levels had returned to control levels, with only a persistent increase in expression unilaterally in the DG (14%). In contrast, an initial down-regulation in mGluR5 mRNA was observed bilaterally in CA4 (-45 and -25%) and CA1 (-46 and -45%), and unilaterally in DG and CA3 (-27 and -42% respectively) 24 h after the last kindled seizure. In the 7 and 28 day kindled groups significant alterations in expression of mGluR5 mRNA were still apparent. These data show that the mRNAs for mGluR1 and mGluR5 are differentially regulated by kindling, indicating that the expression of each of these receptors is under independent regulatory control. These perturbations in mRNA expression may contribute to kindling epileptogenesis but are unlikely to account for the maintenance of the 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.

Cyclo-oxygenase-2 messenger RNA induction in focal cerebral ischemia

We have characterized the induction of the mitogen-inducible form of cyclo-oxygenase, COX-2, during focal cerebral ischemia following permanent middle cerebral artery occlusion (MCAO) in the rat. Marked unilateral induction of COX-2 mRNA was detected in ischemic regions ipsilateral to the occlusion. A significant increase in COX-2 mRNA was detected in "core" and "penumbra" regions of the cerebral cortex between 4 and 24 h after occlusion; this was most marked at 4 h in the penumbra region, in which a 19-fold increase above untreated control levels was detected. A smaller but significant induction was also detected at 4 h in the caudate. A correlation was demonstrated between the extent of COX-2 mRNA induction in cortical regions at 4 h and the severity of tissue damage subsequently detected at 24 h post MCAO. MK-801 significantly attenuated the induction of COX-2 mRNA in the penumbra region at 4 h. The demonstration of COX-2 induction following experimental ischemia highlights the importance of this reaction and its products and by-products, for example, free radicals, in the tissue response to this insult.