Differential and time-dependent changes in gene expression for type II calcium/calmodulin-dependent protein kinase, 67 kDa glutamic acid decarboxylase, and glutamate receptor subunits in tetanus toxin-induced focal epilepsy

Complex effects of eslicarbazepine on inhibitory micro networks in chronic experimental epilepsy

OBJECTIVE

Many antiseizure drugs (ASDs) act on voltage-dependent sodium channels, and the molecular basis of these effects is well established. In contrast, how ASDs act on the level of neuronal networks is much less understood.

METHODS

In the present study, we determined the effects of eslicarbazepine (S-Lic) on different types of inhibitory neurons, as well as inhibitory motifs. Experiments were performed in hippocampal slices from both sham-control and chronically epileptic pilocarpine-treated rats.

RESULTS

We found that S-Lic causes an unexpected reduction of feed-forward inhibition in the CA1 region at high concentrations (300 µM), but not at lower concentrations (100 µM). Concurrently, 300 but not 100 μM S-Lic significantly reduced maximal firing rates in putative feed-forward interneurons located in the CA1 stratum radiatum of sham-control and epileptic animals. In contrast, feedback inhibition was not inhibited by S-Lic. Instead, application of S-Lic, in contrast to previous data for other drugs like carbamazepine (CBZ), resulted in a lasting potentiation of feedback inhibitory post-synaptic currents (IPSCs) only in epileptic and not in sham-control animals, which persisted after washout of S-Lic. We hypothesized that this plasticity of inhibition might rely on anti-Hebbian potentiation of excitatory feedback inputs onto oriens-lacunosum moleculare (OLM) interneurons, which is dependent on Ca²⁺ -permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Indeed, we show that blocking Ca²⁺ -permeable AMPA receptors completely prevents upmodulation of feedback inhibition.

SIGNIFICANCE

These results suggest that S-Lic affects inhibitory circuits in the CA1 hippocampal region in unexpected ways. In addition, ASD actions may not be sufficiently explained by acute effects on their target channels, rather, it may be necessary to take plasticity of inhibitory circuits into account.

Voltage-Dependent Calcium Channels, Calcium Binding Proteins, and Their Interaction in the Pathological Process of Epilepsy

As an important second messenger, the calcium ion (Ca²⁺) plays a vital role in normal brain function and in the pathophysiological process of different neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), and epilepsy. Ca²⁺ takes part in the regulation of neuronal excitability, and the imbalance of intracellular Ca²⁺ is a trigger factor for the occurrence of epilepsy. Several anti-epileptic drugs target voltage-dependent calcium channels (VDCCs). Intracellular Ca²⁺ levels are mainly controlled by VDCCs located in the plasma membrane, the calcium-binding proteins (CBPs) inside the cytoplasm, calcium channels located on the intracellular calcium store (particular the endoplasmic reticulum/sarcoplasmic reticulum), and the Ca²⁺-pumps located in the plasma membrane and intracellular calcium store. So far, while many studies have established the relationship between calcium control factors and epilepsy, the mechanism of various Ca²⁺ regulatory factors in epileptogenesis is still unknown. In this paper, we reviewed the function, distribution, and alteration of VDCCs and CBPs in the central nervous system in the pathological process of epilepsy. The interaction of VDCCs with CBPs in the pathological process of epilepsy was also summarized. We hope this review can provide some clues for better understanding the mechanism of epileptogenesis, and for the development of new anti-epileptic drugs targeting on VDCCs and CBPs.

Effect of astrocyte-targeted production of IL-6 on traumatic brain injury and its impact on the cortical transcriptome

Interleukin-6 (IL-6) is one of the key players in the response of the brain cortex to injury. We have described previously that astrocyte-driven production of IL-6 (GFAP-IL6) in transgenic mice, although causing spontaneous neuroinflammation and long term damage, is beneficial after an acute (freeze) injury in the cortex, increasing healing and decreasing oxidative stress and apoptosis. To determine the transcriptional basis for these responses here we analyzed the global gene expression profile of the cortex, at 0 (unlesioned), 1 or 4 days post lesion (dpl), in both GFAP-IL6 mice and their control littermates. GFAP-IL6 mice showed an increase in genes associated with the inflammatory response both at 1 dpl (Iftm1, Endod1) and 4 dpl (Gfap, C4b), decreased expression of proapoptotic genes (i.e. Gadd45b, Clic4, p21) as well as reduced expression of genes involved in the control of oxidative stress (Atf4). Furthermore, the presence of IL-6 altered the expression of genes involved in hemostasis (Vwf), cell migration and proliferation (Cap2), and synaptic activity (Vamp2). All these changes in gene expression could underlie the phenotype of the GFAP-IL6 mice after injury, but many other possible factors were also identified in this study, highlighting the utility of this approach for deciphering new pathways orchestrated by IL-6.

Characterization of the tetanus toxin model of refractory focal neocortical epilepsy in the rat

PURPOSE

To characterize in detail a model of focal neocortical epilepsy.

METHODS

Chronic focal epilepsy was induced by injecting 25-50 ng of tetanus toxin or vehicle alone (controls) into the motor neocortex of rats. EEG activity was recorded from electrodes implanted at the injection site, along with facial muscle electromyographic (EMG) activity and behavioral monitoring intermittently for up to 5 months in some animals. Drug responsiveness was assessed by using the antiepileptic drugs (AEDs) diazepam (DZP) and phenytoin (PHT) delivered systemically, while 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), a competitive antagonist at AMPA receptors, was administered directly to the brain to investigate the potential benefits of focal drug delivery.

RESULTS

Tetanus toxin induced mild behavioral seizures that persisted indefinitely in all animals. EEG spiking activity, occurring up to 80% of the time, correlated with clinical seizures consisting of interrupted behavioral activity, rhythmic bilateral facial twitching, and periods of abrupt motor arrest. Seizures were refractory to systemic administration of DZP and PHT. However, focal delivery of NBQX to the seizure site reversibly reduced EEG and behavioral seizure activity without detectable side effects.

CONCLUSIONS

This study provides a long-term detailed characterisation of the tetanus toxin model. Spontaneous, almost continuous, well-tolerated seizures occur and persist, resembling those seen in neocortical epilepsy, including cortical myoclonus and epilepsia partialis continua. The seizures appear to be similarly resistant to conventional AEDs. The consistency, frequency, and clinical similarity of the seizures to refractory epilepsy in humans make this an ideal model for investigation of both mechanisms of seizure activity and new therapeutic approaches.

The tetanus toxin model of chronic epilepsy

In experimental models of epilepsy, single and recurrent seizures are often used in an attempt to determine the effects of the seizures themselves on mammalian brain function. These models attempt to emulate as many features as possible of their human disease counterparts without many of the confounding factors such as underlying disease processes and medication effects. Numerous models have been used in the past to address different questions. Nevertheless, the basic questions are often the same: 1. Do seizures cause long-term damage? 2. Do seizures predispose to chronic epilepsy (epileptogenesis), that is long-term spontaneous repetitive seizures? 3. Are these results developmentally regulated? 4. Are the underlying mechanisms of epileptogenesis and brain damage related? In pursuing these questions, the goal is to determine how seizures exert their effects and to minimize any side effects from the methods employed to induce the seizures themselves. This requires a detailed characterization of the methods used to induce seizures. In this chapter, we will review the literature regarding the tetanus toxin model of chronic epilepsy with regard to its mechanisms of action, clinical comparisons, how it is experimentally implemented and the results obtained thus far. These results will be compared to other models of chronic epilepsy in order to make generalizations about the effects of repetitive seizures in adult and early life. At this time, it appears that repetitive seizures cause long-term changes in learning ability and may cause a predisposition to chronic seizures at all ages. In younger animals, both features of learning impairment and epilepsy are not typically associated with cell loss as they are in adult animals. At all ages, some form of synaptic reorganization has been demonstrated to occur.

N-methyl-D-aspartate receptor dependent transcriptional regulation of two calcium/calmodulin-dependent protein kinase type II isoforms in rodent cerebral cortex

Alpha Calcium/calmodulin-dependent protein kinase type II (CaMKII-alpha) expression is regulated in an activity-dependent manner, but it is not known whether other CaMKII isoforms (beta, delta, and gamma) are similarly regulated. We examined the activity-dependent regulation of these CaMKII isoforms in vivo, using a model of generalized seizures caused by i.p. injection of kainic acid. Following seizure induction, CaMKII-alpha expression was downregulated and CaMKII-delta expression upregulated while CaMKII-beta and CaMKII-gamma expression was unaffected. A transient downregulation in CaMKII-alpha and a transient increase in CaMKII-delta occurred throughout neocortex in the same temporal order. Although CaMKII-alpha mRNA was decreased by seizure activity, the less abundant, alternatively spliced, CaMKII-alpha33 mRNA was unaffected. Organotypic cortical slice cultures treated with bicuculline and 4-aminopyridine to induce seizure activity also showed a downregulation of CaMKII-alpha mRNA and an upregulation of CaMKII-delta mRNA. Prior exposure to tetrodotoxin prevented the changes in CaMKII-alpha and CaMKII-delta mRNA regulation and this was mimicked by D-L-2-amino-5-phosphonovaleric acid, but not by 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline, suggesting that CaMKII-alpha and CaMKII-delta mRNA expression is regulated in an N-methyl-D-aspartate receptor-dependent manner. Regulation was also transcription dependent. Blocking transcription with actinomycin-D prevented activity-dependent changes in CaMKII-alpha and CaMKII-delta mRNA, but produced opposite effects on basal transcription, resulting in more stabilized CaMKII-alpha mRNA and less stabilized CaMKII-delta mRNA. These results reveal unique patterns of seizure-induced alterations in CaMKII mRNAs. Activity-dependent changes in subunit composition could, therefore, differentially influence the functional attributes of the CaMKII holoenzyme.

The neurobiology of Rett syndrome

Rett syndrome is a neuropsychiatric disorder with onset in early childhood. Loss-of-function mutations of the X-linked gene encoding methyl-CpG binding protein 2 (MECP2) are responsible for more than 80% of Rett cases. Despite these recent advances in molecular genetics, little is known about the neurobiology of Rett syndrome and the role of MeCP2 protein in the nervous system. The molecular functions of the MeCP2 protein were primarily studied in nonneuronal cell lines and in vitro systems; MeCP2 binds primarily, but not exclusively, to methylated DNA, and it is thought to regulate gene expression, chromatin composition, and chromosomal architecture. In the brain, MeCP2 appears to be expressed ubiquitously by neurons. There is increasing evidence that the protein is important for maintenance of neuronal chromatin during late development and in adulthood. Levels of MeCP2 expression increase during the course of neuronal differentiation and remain at high levels in the adult brain. Furthermore, genetic studies in mice demonstrated that the selective deletion of the Mecp2 gene in neurons results several weeks later in a Rett-like phenotype, including a reduction in brain weight and neuronal dystrophy. Rett syndrome is not accompanied by a neurodegenerative process and thus perhaps may be amenable to therapeutic intervention at the time of symptom expression.

Thr123 of rat G-substrate contributes to its action as a protein phosphatase inhibitor

Rat G-substrate cDNA was isolated from a cerebellar library and characterized. The deduced amino acid sequence of rat G-substrate contained two putative phosphorylation sites for PKG at Thr72 and Thr123; the amino acid sequences (KPRRKDT(p)PA) around these sites are conserved in human, mouse and rabbit. G-substrate phosphorylated by PKG inhibited the catalytic subunits of both protein phosphatase-1 (IC(50) 14.1 nM) and -2A (IC(50) 5.9 nM). Mutation of Thr123 (site 2) to Ala significantly reduced the inhibition of both PP-1 and PP-2A, while mutation of Thr72 (site 1) to Ala had little effect on inhibitory activity. In situ hybridization analysis revealed that G-substrate mRNA was localized exclusively in cerebellar Purkinje cells. Immunoperoxidase staining showed that in Purkinje cells, G-substrate was present in somata, dendrites and axons. In rat cerebellar slices, activation of PKG with a nitric oxide (NO) donor, NOR3, or 8-Br-cGMP, increased phosphorylation of G-substrate, as demonstrated with a phosphorylation-specific antibody. These results characterize further the inhibition of PP-1 and PP-2A by phospho-G-substrate, and demonstrate its physiological phosphorylation in rat Purkinje cells.

Alteration of GLUR2 expression in the rat brain following absence seizures induced by gamma-hydroxybutyric acid

We explored the involvement of the glutamate receptor subunit B (GluR2) in the mechanism of absence seizures induced by gamma-hydroxybutyric acid (GHB). The expression and distribution of GluR2 protein in rat brain were examined during and after GHB-induced absence seizures. The data indicate that GluR2 protein expression significantly decreases following the onset of absence seizures. The suppression of GluR2 expression was prolonged and it outlasted the duration of the continuous absence seizure activity. The alteration of GluR2 protein levels was accompanied by a re-distribution of GluR2 expression from laminae V to IV in cerebral cortex. We also analyzed the duration and latency of absence seizures induced by GHB 72 h following an initial GHB-induced absence seizure, a time when suppression of GluR2 protein was maximal. The second absence seizure was significantly more prolonged than the first. These data may indicate that the putative down-regulation of GluR2 following GHB-induced absence seizure could have contributed to the potentiation of subsequent seizures in animals. A related hypothesis posed by the data is that down-regulation of GluR2 is involved in the mechanisms of the maintenance of recurrent absence seizure activity once it is initiated and therefore, may contribute to the chronicity of seizures in absence epilepsy.

Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We find that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca(2+)-dependent and parallels the appearance of GABA. We show that xGAD 67 transcripts first appear in the embryonic spinal cord during the period in which these Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNase protection and competitive quantitative RT-PCR demonstrate that transcript levels are approximately threefold greater when neurons are cultured in the presence of extracellular Ca2+ that permits generation of transients than when cultured in its absence. The frequency of spontaneous Ca2+ spikes plays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues normal expression. We conclude that naturally occurring low frequencies of these Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neurons.

The suppression of testis-brain RNA binding protein and kinesin heavy chain disrupts mRNA sorting in dendrites

Ribonucleoprotein particles (RNPs) are thought to be key players in somato-dendritic sorting of mRNAs in CNS neurons and are implicated in activity-directed neuronal remodeling. Here, we use reporter constructs and gel mobility shift assays to show that the testis brain RNA-binding protein (TB-RBP) associates with mRNPs in a sequence (Y element) dependent manner. Using antisense oligonucleotides (anti-ODN), we demonstrate that blocking the TB-RBP Y element binding site disrupts and mis-localizes mRNPs containing (alpha)-calmodulin dependent kinase II (alpha)-CAMKII) and ligatin mRNAs. In addition, we show that suppression of kinesin heavy chain motor protein alters only the localization of (alpha)-CAMKII mRNA. Thus, differential sorting of mRNAs involves multiple mRNPs and selective motor proteins permitting localized mRNAs to utilize common mechanisms for shared steps.

Increased pyramidal excitability and NMDA conductance can explain posttraumatic epileptogenesis without disinhibition: a model

Partially isolated cortical islands prepared in vivo become epileptogenic within weeks of the injury. In this model of chronic epileptogenesis, recordings from cortical slices cut through the injured area and maintained in vitro often show evoked, long- and variable-latency multiphasic epileptiform field potentials that also can occur spontaneously. These events are initiated in layer V and are synchronous with polyphasic long-duration excitatory and inhibitory potentials (currents) in neurons that may last several hundred milliseconds. Stimuli that are significantly above threshold for triggering these epileptiform events evoke only a single large excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). We investigated the physiological basis of these events using simulations of a layer V network consisting of 500 compartmental model neurons, including 400 principal (excitatory) and 100 inhibitory cells. Epileptiform events occurred in response to a stimulus when sufficient N-methyl-D-aspartate (NMDA) conductance was activated by feedback excitatory activity among pyramidal cells. In control simulations, this activity was prevented by the rapid development of IPSPs. One manipulation that could give rise to epileptogenesis was an increase in the threshold of inhibitory interneurons. However, previous experimental data from layer V pyramidal neurons of these chronic epileptogenic lesions indicate: upregulation, rather than downregulation, of inhibition; alterations in the intrinsic properties of pyramidal cells that would tend to make them more excitable; and sprouting of their intracortical axons and increased numbers of presumed synaptic contacts, which would increase recurrent EPSPs from one cell onto another. Consistent with this, we found that increasing the excitability of pyramidal cells and the strength of NMDA conductances, in the face of either unaltered or increased inhibition, resulted in generation of epileptiform activity that had characteristics similar to those of the experimental data. Thus epileptogenesis such as occurs after chronic cortical injury can result from alterations of intrinsic membrane properties of pyramidal neurons together with enhanced NMDA synaptic conductances.

Expression of mRNAs encoding flip isoforms of GluR1 and GluR2 glutamate receptors is increased in rat hippocampus in epilepsy induced by tetanus toxin

The messenger RNAs encoding the flip and flop isoforms of the glutamate receptor subunits GluR1 and GluR2 were detected and quantified by in situ hybridization in the hippocampal formation of rats following intrahippocampal injection of tetanus on one side. The mRNAs encoding the flip isoforms of both GluR1 and GluR2 were significantly increased 4 weeks after injection. The GluR1 flip mRNA was significantly elevated only in the dentate gyrus, whereas significant increases in the GluR2 flip mRNA were seen in all hippocampal subfields examined. There were no significant changes in the mRNA encoding the flop isoforms of either GluR1 or GluR2. The significant changes in flip isoform mRNAs occurred on both sides.

Molecular neuropathology of human mesial temporal lobe epilepsy

With the recent progress in surgical treatment modalities, human brain tissue from patients with intractable focal epilepsies will increasingly become available for studies on the molecular pathology, electrophysiological changes and pathogenesis of human focal epilepsies. An inherent problem for studies on human temporal lobe epilepsy (TLE) is the lack of suitable controls. Strategies to alleviate this obstacle include the use of human post mortem samples, hippocampus from experimental animals and, in particular, the comparative analysis of surgical specimens from patients with Ammon's horn sclerosis (AHS) and with focal temporal lesions but anatomically preserved hippocampal structures. In this review we focus on selected aspects of the molecular neuropathology of TLE: (1) the potential impact of persisting calretinin-immunoreactive neurons with Cajal-Retzius cell morphology, (2) astrocytic tenascin-C induction and redistribution as potential regulator of aberrant axonal sprouting and (3) alterations of Ca2+ -mediated hippocampal signalling pathways. The diverse and complex changes described so far in human TLE specimens require a systematic interdisciplinary approach to distinguish primary, epileptogenic alterations and secondary, compensatory mechanisms in the pathogenesis of human temporal lobe epilepsies.

Withdrawal from repeated amphetamine administration reduces NMDAR1 expression in the rat substantia nigra, nucleus accumbens and medial prefrontal cortex

Glutamate plays a critical role in neuroadaptations induced by drugs of abuse. This study determined whether expression of the NMDAR1 subunit of the NMDA receptor is altered by repeated amphetamine administration. We quantified NMDAR1 mRNA (using in situ hybridization with 35S-labelled oligonucleotide probes) and immunolabelling (using immunocytochemistry with 35S-labelled secondary antibodies) in rat ventral midbrain, nucleus accumbens and prefrontal cortex after 3 or 14 days of withdrawal from five daily injections of saline or amphetamine sulphate (5 mg/kg/day). No changes in NMDAR1 expression were observed after 3 days of withdrawal, whereas significant decreases were observed in all regions after 14 days. NMDAR1 mRNA levels in midbrain were too low for reliable quantification, but immunolabelling was decreased significantly in intermediate and caudal portions of the substantia nigra. This may indicate a reduction in excitatory drive to substantia nigra dopaminergic neurons. In the nucleus accumbens, there were significant decreases in NMDAR1 mRNA levels (74.8 +/- 7. 7% of control, P < 0.05) and immunolabelling (76.7 +/- 4.4%, P < 0. 05). This may account for previously-reported decreases in the electrophysiological responsiveness of nucleus accumbens neurons to NMDA after chronic amphetamine treatment, and contribute to dysregulation of goal-directed behaviour. In prefrontal cortex, there was a significant decrease in NMDAR1 mRNA levels (76.1 +/- 7. 1%, P < 0.05) and a trend towards decreased immunolabelling (89.5 +/- 7.0%). This may indicate decreased neuronal excitability within prefrontal cortex. A resultant decrease in activity of excitatory prefrontal cortical projections to nucleus accumbens or midbrain could synergize with local decreases in NMDAR1 to further reduce neuronal excitability in these latter regions.

Tetrodotoxin prevents posttraumatic epileptogenesis in rats

Severe cortical trauma frequently causes epilepsy that develops after a long latency. We hypothesized that plastic changes in excitability during this latent period might be initiated or sustained by the level of neuronal activity in the injured cortex. We therefore studied effects of action potential blockade by application of tetrodotoxin (TTX) to areas of cortical injury in a model of chronic epileptogenesis. Partially isolated islands of sensorimotor cortex were made in 28- to 30-day-old male Sprague-Dawley rats and thin sheets of Elvax polymer containing TTX or control vehicle were implanted over lesions. Ten to 15 days later neocortical slices were obtained through isolates for electrophysiological studies. Slices from all animals (n = 12) with lesions contacted by control-Elvax (58% of 36 slices) exhibited evoked epileptiform field potentials, and those from 4 rats had spontaneous epileptiform events. Only 2 of 11 lesioned animals and 6% of slices from cortex exposed to TTX in vivo exhibited evoked epileptiform potentials, and no spontaneous epileptiform events were observed. There was no evidence of residual TTX during recordings. TTX-Elvax was ineffective in reversing epileptogenesis when implanted 11 days after cortical injury. These data suggest that development of antiepileptogenic drugs for humans may be possible.

Effects of tetanus toxin on functional inhibition after injection in separate cortical areas in rat

Tetanus Toxin is widely used as a model of chronic focal epilepsy and is assumed to act by blocking neurotransmitter release with high selectivity for inhibitory synapses. However, the exact mechanisms are not fully understood, since, e.g., GABA release is only temporarily decreased although epileptiform activity persists pointing towards a change in the interplay of excitation and inhibition. Furthermore there have been reports about different effects of tetanus toxin after injection in separate brain areas. Therefore, we investigated the functional inhibition after injecting tetanus toxin either in the motor or sensory cortex of adult rats by using a paired-pulse paradigm as a measure of excitatory and inhibitory drive. Tetanus toxin injection into the motor cortex (n=10) induced a marked, long-lasting reduction in inhibition which was highly significant in most parts of the injected cortical area. Injections into the sensory cortex, however, showed less marked changes in inhibition which were more widespread and significant only in 3 of 14 animals injected. These results give further evidence for a prominent effect of tetanus toxin on functional inhibition and strengthen the idea of a differential effect in separate cortical areas. They may be accounted for by the different cytoarchitecture of cortical areas with variable inhibitory and excitatory intracortical connections.

Brain hypometabolism in a model of chronic focal epilepsy in rat neocortex

PURPOSE

Metabolic mapping of the human brain has become a widely used method for identifying and localizing epileptic foci. A reduction of glucose consumption usually is found interictally in the area of the focus. By contrast, animal models of acute epilepsy show a hypermetabolism in the epileptic focus. Here we investigated how metabolism is altered in an animal model of chronic epilepsy caused by focal injection of tetanus toxin into rat neocortex.

METHODS

A total of 27 male Wistar rats were anesthetized and injected into the motor or sensory cortex either with dissolved tetanus toxin or with the solvent only. Animals recovered for 7, 14, or 30 days and then were anesthetized again for quantitative 14C-deoxyglucose autoradiography. Data were analyzed with an imaging program, and regional cerebral glucose metabolism (rCMRGlc) was determined.

RESULTS

Injection of tetanus toxin into the motor cortex caused a focal hypometabolism which was confined to the cytoarchitectonic boundaries of the injected area, whereas sensory cortex injection caused a more widespread hypometabolism in all sensory cortical and connected, areas. None of the animals displayed focal hypermetabolism and we observed no significant time-dependent alteration of brain metabolism.

CONCLUSIONS

Tetanus toxin injection into the cortex of the rat induces chronic epileptic activity accompanied by a focal hypometabolism. The data suggest that the spread of the metabolic alterations depends on the connectivity of the injected cortical area.

Cell- and lamina-specific expression and activity-dependent regulation of type II calcium/calmodulin-dependent protein kinase isoforms in monkey visual cortex

In situ hybridization histochemistry and immunocytochemistry were used to study localization and activity-dependent regulation of alpha, beta, gamma, and delta isoforms of type II calcium/calmodulin-dependent protein kinase (CaMKII) and their mRNAs in areas 17 and 18 of normal and monocularly deprived adult macaques. CaMKII-alpha is expressed overall at levels three to four times higher than that of CaMKII-beta and at least 15 times higher than that of CaMKII-gamma and -delta. All isoforms are expressed primarily in pyramidal cells of both areas, especially those of layers II-III, IVA (in area 17), and VI, but are also expressed in nonpyramidal, non-GABAergic cells of layer IV of both areas and in interstitial neurons of the white matter. CaMKII-alpha and -beta are colocalized, suggesting the formation of heteromers. There was no evidence of expression in neuroglial cells. Each isoform has a unique pattern of laminar and sublaminar distribution, but cortical layers or sublayers enriched for one isoform do not correlate with layers receiving inputs only from isoform-specific layers of the lateral geniculate nucleus. CaMKII-alpha and -beta mRNA and protein levels in layer IVC of area 17 are subject to activity-dependent regulation, with brief periods of monocular deprivation caused by intraocular injections of tetrodotoxin leading to a 30% increase in CaMKII-alpha mRNA and a comparable decrease in CaMKII-beta mRNA in deprived ocular dominance columns, especially of layer IVCbeta. Expression in other layers and expression of CaMKII-gamma and delta were unaffected. Changes occurring in layer IVC may influence the formation of heteromers and protect supragranular layers from CaMKII-dependent plasticity in the adult.

Activity-dependent scaling of quantal amplitude in neocortical neurons

Information is stored in neural circuits through long-lasting changes in synaptic strengths. Most studies of information storage have focused on mechanisms such as long-term potentiation and depression (LTP and LTD), in which synaptic strengths change in a synapse-specific manner. In contrast, little attention has been paid to mechanisms that regulate the total synaptic strength of a neuron. Here we describe a new form of synaptic plasticity that increases or decreases the strength of all of a neuron's synaptic inputs as a function of activity. Chronic blockade of cortical culture activity increased the amplitude of miniature excitatory postsynaptic currents (mEPSCs) without changing their kinetics. Conversely, blocking GABA (gamma-aminobutyric acid)-mediated inhibition initially raised firing rates, but over a 48-hour period mESPC amplitudes decreased and firing rates returned to close to control values. These changes were at least partly due to postsynaptic alterations in the response to glutamate, and apparently affected each synapse in proportion to its initial strength. Such 'synaptic scaling' may help to ensure that firing rates do not become saturated during developmental changes in the number and strength of synaptic inputs, as well as stabilizing synaptic strengths during Hebbian modification and facilitating competition between synapses.

Increased expression of GABA transporters, GAT-1 and GAT-3, in the deafferented superior colliculus of the rat

GABA transporters (GATs) play a critical role in the translemmal transport of GABA in neurons and glial cells. Two major brain GATs, GAT-1 and GAT-3, are found in astrocytes in the adult brain. Astroglia demonstrate morphological and molecular changes in response to brain injury and deafferentation. The present study was designed to determine whether the expression of GATs changes after nerve deafferentation using the rat superior colliculus (SC) as a model. The immunoreactivity for GAT-1 and GAT-3, as well as GABA and glutamic acid decarboxylase (GAD)-65 and GAD-67, was studied in the SC of control rats and rats with unilateral optic nerve transections. Immunolabeling for both GAT-1 and GAT-3 was increased in the neuropil of the denervated SC as compared to that for the SC of control rats or for the unaffected SC of experimental rats. In contrast, immunoreactivity for GABA, GAD-65 and GAD-67 was not altered. The change in the immunolabeling of GAT-1 and GAT-3 was detectable at 1 day postlesion and became more evident between 3-30 days postlesion. At the electron microscopic level, immunoreactivity for both GAT-1 and GAT-3 in the unaffected SC was localized to astrocytic processes, whereas GAT-1 immunolabeling was also present in synaptic terminals. In the deafferented SC, immunolabeling for both GATs was elevated in the somata and processes of hypertrophied astrocytes as compared to that in the unaffected SC, whereas GAT-1 labeling in neuronal profiles was largely unchanged. A substantial increase of GAT-1 and GAT-3 in astrocytes following optic nerve transection suggests that these cells play a role in modulating GABA's action in the deafferented SC.

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.