Non convulsive spike-wave discharges do not induce Fos in cerebro-cortical neurons

Rhythm and blues: animal models of epilepsy and depression comorbidity

Clinical evidence shows a strong, bidirectional comorbidity between depression and epilepsy that is associated with decreased quality of life and responsivity to pharmacotherapies. At present, the neurobiological underpinnings of this comorbidity remain hazy. To complicate matters, anticonvulsant drugs can cause mood disturbances, while antidepressant drugs can lower seizure threshold, making it difficult to treat patients suffering from both depression and epilepsy. Animal models have been created to untangle the mechanisms behind the relationship between these disorders and to serve as screening tools for new therapies targeted to treat both simultaneously. These animal models are based on chemical interventions (e.g. pentylenetetrazol, kainic acid, pilocarpine), electrical stimulations (e.g. kindling, electroshock), and genetic/selective breeding paradigms (e.g. genetically epilepsy-prone rats (GEPRs), genetic absence epilepsy rat from Strasbourg (GAERS), WAG/Rij rats, swim lo-active rats (SwLo)). Studies on these animal models point to some potential mechanisms that could explain epilepsy and depression comorbidity, such as various components of the dopaminergic, noradrenergic, serotonergic, and GABAergic systems, as well as key brain regions, like the amygdala and hippocampus. These models have also been used to screen possible therapies. The purpose of the present review is to highlight the importance of animal models in research on comorbid epilepsy and depression and to explore the contributions of these models to our understanding of the mechanisms and potential treatments for these disorders.

Aggravation of Absence Seizures by Carbamazepine in a Genetic Rat Model Does Not Induce Neuronal c-Fos Activation

The mechanisms underlying carbamazepine aggravation of absence seizures are uncertain but are thought to involve enhancement of neuronal activity within the thalamocortical circuitry. We used c-Fos immunohistochemistry (cFos-ir) to examine patterns of neuronal activation and the relationship to seizure expression following administration of carbamazepine in a rat model of absence epilepsy (Genetic Absence Epilepsy Rats of Strasbourg, GAERS). Female ovariectomized GAERS implanted with extradural EEG electrodes received either 20 mg/kg carbamazepine or vehicle IP. Seizure expression was quantified by measuring the total number and duration of spike-wave discharges (SWD) and with the individual burst discharge lengths over a 90-minute EEG. This was correlated with cFos-ir in thalamocortical slices from rats killed 180 minutes after carbamazepine administration. Carbamazepine-treated rats (n = 5) had a significantly greater total duration of SWD than vehicle-treated rats (17.9% versus 8.8%, P = 0.04). Despite this aggravation of seizures, the level of cFos-ir did not differ between the treatment groups. A positive correlation was found between cFos-ir in the reticularis thalami (Rt) and the total seizure duration (R = 0.66, P = 0.04) and mean burst length (R = 0.68, P = 0.03) but not total number of seizures. The lack of difference in cFos activation patterns between carbamazepine and vehicle-treated animals suggests that the mechanism for carbamazepine aggravation of absence seizures may not involve neuronal activation but rather enhanced neuronal synchronization. The association between increased neuronal activation in the Rt and seizure burden in GAERS provides further support for the critical role of this structure in the maintenance, but not initiation, of absence seizure activity.

Depressive-like behavioral alterations and c-fos expression in the dopaminergic brain regions in WAG/Rij rats with genetic absence epilepsy

Wistar derived inbred line, the WAG/Rij rats, genetically absence epilepsy prone and their normal counterparts, outbred Wistar rats, were compared in respect to differences in behavior, in acute and chronic antidepressant imipramine treatment and in the immediate early gene c-fos expression in the brain regions induced by forced swimming test procedure. The WAG/Rij rats as compared with Wistar rats were found to exhibit decreased activity in the open field test, increased immobility in the forced swimming test and decreased sucrose intake (anhedonia). Interline differences indicating increased anxiety in the WAG/Rij rats were not revealed in the light-dark choice, social interaction and elevated plus-maze tests. The WAG/Rij rats in contrast to Wistar rats responded only to chronic antidepressant imipramine treatment with a reduction in their enhanced immobility in the forced swimming test. "Behavioral despair" induced by forced swimming led to c-fos expression in frontal cortex, nucleus accumbens and striatum, terminal regions of three dopaminergic brain systems (mesocortical, mesolimbic, nigrostriatal). The c-fos expression in the brain of WAG/Rij rats was substantially higher than that of Wistar rats. Moreover, the strains differed in the distribution of c-fos expression between brain regions. Results suggest that WAG/Rij rats are prone to adopt passive strategies of behavior in stressful situations, and so in this certain aspect this strain might be regarded as new experimental (genetic) model of depressive-like (passive) behavior accompanying absence epilepsy. Further testing this hypothesis is proceeding. This putative model could be used for the investigation of neurobiological basis and mechanisms of such "double pathology" and for the examination of new concepts of its therapy.

Picrotoxin-, kainic acid- and seizure-induced Fos in brainstem, with special reference to catecholamine cell groups

One of the original views about motor convulsions with bilateral synchronous EEG discharges was that ascending projections from deep midline brainstem neurons subserve discharge synchronisation. We examined this in kainic acid- and picrotoxin-induced seizures using the distribution of Fos protein expression in the brainstem as a possible marker of brainstem neuronal activation. Before seizure, Fos was present in parabrachial nucleus, central grey and A1/C1 cell groups and, to a lesser extent, in A2/C2 cell groups and A6 neurons. Seizure correlated with further induction of Fos in these cell groups as well as in A6 neurons. There was no Fos in other cell groups with bilateral cerebro-cortical projections nor in cell groups likely to participate in the expression of seizures, reflecting a limitation of the Fos method. This study provides evidence of locus coeruleus involvement in these two models of seizure.

Nonobligate role of early or sustained expression of immediate-early gene proteins c-fos, c-jun, and Zif/268 in hippocampal mossy fiber sprouting

Axon sprouting in dentate granule cells is an important model of structural plasticity in the hippocampus. Although the process can be triggered by deafferentation, intense activation of glutamate receptors, and other convulsant stimuli, the specific molecular steps required to initiate and sustain mossy fiber (MF) reorganization are unknown. The cellular immediate early genes (IEGs) c-fos, c-jun, and zif/268 are major candidates for the initial steps of this plasticity, because they encode transcription factors that may trigger cascades of activity-dependent neuronal gene expression and are strongly induced in all experimental models of MF sprouting. The mutant mouse stargazer offers an important opportunity to test the specific role of IEGs, because it displays generalized nonconvulsive epilepsy and intense MF sprouting in the absence of regional cell injury. Here we report that stargazer mice show no detectable elevations in c-Fos, c-Jun, or Zif/268 immediate early gene proteins (IEGPs) before or during MF growth. Experimental results in stargazer, including (1) a strong IEGP response to kainate-induced convulsive seizures, (2) no IEGP response after prolongation of spike-wave synchronization, (3) no IEGP increase at the developmental onset of seizures or after prolonged seizure suppression, and (4) unaltered levels of the intracellular Ca2+-buffering proteins calbindin-D28k or parvalbumin, exclude the possibility that absence of an IEGP response in stargazer is either gene-linked or suppressed by known refractory mechanisms. These data demonstrate that increased levels of these IEGPs are not an obligatory step in MF-reactive sprouting and differentiate the early downstream molecular cascades of two major seizure types.

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.

Cerebellar circuitry is activated during convulsive episodes in the tottering (tg/tg) mutant mouse

Tottering (tg) is an autosomal recessive mutation of the calcium channel alpha1A subunit in the mouse that results in epileptic spike and wave discharges, mild ataxia and paroxysmal episodes of involuntary spasms of the limbs, trunk and face. These convulsions have been especially difficult to characterize because of their unpredictable occurrence and lack of electroencephalographic correlates. However, it is, in fact, possible to induce these convulsions, making this facet of the tottering phenotype amenable to controlled experimentation for the first time. Here, the neuroanatomical basis of the convulsions in tottering mice has been identified using in situ hybridization for c-fos messenger RNA to chart abnormal neuronal activity. Convulsion-induced c-fos messenger RNA expression was most prominent in the cerebellum of convulsing tottering mice. Additionally, cerebral cortex and principal cerebellar relay nuclei were also activated during a convulsion. The c-fos activation in the cerebellum temporally preceded expression in cerebral cortex, suggesting that cerebral cortex is not driving the expression of convulsions. These results suggest that the cerebellum, a region not classically associated with paroxysmal events, is important in the generation and/or maintenance of the intermittent convulsions in tottering mutant mice.

Fos induction following systemic kainic acid: early expression in hippocampus and later widespread expression correlated with seizure

We determined the distribution of Fos protein expression in a model of generalised epilepsy caused by excessive neuronal excitation. Fos immunoreactivity was mapped in forebrain in unrestrained rats, previously prepared with an indwelling venous catheter, after the intravenous administration of kainic acid (10 mg/kg). We determined cerebral activation following various periods of exposure to kainic acid by using intravenous administration of pentobarbitone to prevent further activation. Within a few minutes, kainic acid caused episodes of staring, sniffing, wet dog shakes, nodding and chewing. Fos induction occurred initially and simultaneously in hippocampus, subiculum, septum and entorhinal cortex as early as 9.5 min after kainate injection. After up to 40 min of staring, sniffing, wet dog shakes, nodding and chewing, Fos induction was not further increased above levels present within the first 9.5 min. After 56 +/- 6 min a motor convulsion occurred, initially affecting the jaw, head and tail and variably extending to the forelimbs, trunk or hindlimbs. Following the convulsive event, additional Fos was expressed in hippocampus, thalamus, caudate-putamen and other subcortical structures and in the cerebral cortex. Fos induction was sometimes asymmetric in entorhinal, visual, piriform, cingulum, parietal and frontal cortices and in amygdala and dorsal endopiriform area. Electroencephalographic recordings after a few minutes exposure to kainic acid revealed an increased amplitude of fast frequencies in hippocampus which appeared to correlate with Fos induction in this structure. The findings are generally consistent with the reported distribution and slow development of kainic acid-induced seizure activity using electrophysiological and deoxyglucose methods. However, the Fos distribution suggests that (i) hippocampal, possibly dentate, activation precedes significant activation elsewhere, (ii) extensive involvement of other cerebral structures and cerebral cortex occurs simultaneously and correlates with motor seizures and (iii) brain structures can be recruited asymmetrically.

gamma-Butyrolactone-induced absence-like seizures increase nuclear CRE- and AP-1 DNA-binding activities in mouse brain

We examined the involvement of the GABAB receptor and the coordinated induction of nuclear transcriptional factors in experimental generalized absence seizures induced by gamma-butyrolactone (GBL) in mice. Although administration of GBL 50 mg/kg did not show any effects on behavior or ECoG pattern, higher doses of GBL (70 and 100 mg/kg, i.p.) induced behavioral changes associated with 3-6-Hz spike and wave discharges in the mice. CGP 35348, a GABAB receptor antagonist, suppressed both the GBL-induced absence seizures and the spike and wave discharges. The antiepileptic effects of CGP 35348 (200 mg/kg, i.p.) were stronger than those of ethosuximide (200 mg/kg, i.p.). Sodium valproate (100 mg/kg, i.p.) attenuated the early phase but not the late phase of the GBL-induced absence seizures. Gel-mobility assay demonstrated that administration of an effective dose of GBL for eliciting spike and wave discharges dose-dependently increased nuclear cyclic AMP-responsive element (CRE)- and activator protein 1 (AP-1) DNA-binding activities in mouse whole brain. The increases in nuclear CRE- and AP-1 DNA-binding were antagonized by CGP 35348 in a dose-dependent fashion. In addition, GABAB receptor binding assay revealed that GBL or antiepileptic drugs did not displace [3H]baclofen binding in cerebral cortical membranes. In contrast, gamma-hydroxybutyrate (GHB), an active metabolite of GBL, inhibited [3H]baclofen binding in a concentration-dependent manner. These results suggest that GABAB receptor-mediated synaptic responses are involved in GBL-induced generalized absence seizures and that the increases in nuclear CRE- and AP-1 DNA-binding activities are correlated with the GBL-induced generalized absence seizures.

Distribution of Fos-positive neurons in cortical and subcortical structures after picrotoxin-induced convulsions varies with seizure type

The distribution of Fos protein was mapped in rat brain following a single non-focal convulsive seizure. Single seizures were induced with intravenous picrotoxin in unhandled animals housed in isolation. Different convulsive behaviours occurred unpredictably. The least severe seizures were predominantly localised to the face, head and forelimbs, without loss of posture control (restricted seizures). The most extensive seizures affected all limbs and trunk, sometimes with falling (generalised seizures). There was a correlation between seizure behaviour and distribution of Fos induction. After restricted seizures, Fos was induced at highest levels in neocortex and piriform cortex and was prominent in entorhinal cortex, caudal-ventral caudate-putamen and amygdala. Regions of thalamus were consistently and lightly labelled, but Fos induction did not occur in hippocampus. After generalised seizures, there was Fos induction in cortex but less than after restricted seizures and, in three of four animals, also in dentate gyrus, hippocampus and subiculum. There was occasional or variable labelling of thalamus, basolateral amygdala and caudate-putamen. One animal with generalised seizures showed no hippocampal Fos induction. The findings indicate that picrotoxin induces seizures with at least two different patterns of neuronal involvement. The cortex, part of the caudate-putamen, amygdala and thalamus are involved in restricted seizures while the hippocampus, cortex and thalamus are involved in generalised seizures. The results do not support the view that generalised seizures are a progression from restricted forms. Cortical Fos involvement is entirely consistent with the participation of cortex in non-focal epilepsy. In these non-focal seizures, the dentate-hippocampus may be a source of excitation to cortex in the generalised group while the cortex appears to be the predominant site of excitation in the restricted group.

The substantia nigra pars reticulata, seizures and Fos expression

The induction of the proto-oncogene c-fos has been used extensively to identify spatially distributed neural systems activated by seizures. The substantia nigra pars reticulata (SNpr) has been implicated as a critical structure in neural networks involved in the modulation of seizure expression, yet the SNpr has not been reported to express Fos following seizures induced in a variety of seizure paradigms. In this study we determined whether (1) the temporal characteristics of Fos induction in the SNpr were different than those of other brain areas following kindled seizures, (2) neurons in the SNpr possess the cellular machinery to express Fos, (3) Fos can be induced in SNpr by direct electrical stimulation, and (4) Fos expression is induced in the SNpr following kainate or pilocarpine-induced status epilepticus. Results indicate that Fos is not induced in SNpr at any time point (1-12 h) after kindled seizures, and that serum response factor, a constitutively expressed nuclear protein necessary for Fos expression, is present in SNpr neurons. Results further indicate that Fos expression in the SNpr is induced following either direct electrical stimulation or pilocarpine status, but not status elicited by kainate. We conclude that, in so far as the SNpr represents a critical structure for modulating seizure expression, seizure activity does not represent a sufficient stimulus to induce Fos in SNpr neurons. Further, the neural networks defined by Fos expression following seizure may be incomplete, and should be interpreted conservatively.