Increased expression of GAD mRNA during the chronic epileptic syndrome due to intrahippocampal tetanus toxin

Dissociation Between the Epileptogenic Lesion and Primary Seizure Onset Zone in the Tetanus Toxin Model of Temporal Lobe Epilepsy

Despite extensive temporal lobe epilepsy (TLE) research, understanding the specific limbic structures' roles in seizures remains limited. This weakness can be attributed to the complex nature of TLE and the existence of various TLE subsyndromes, including non-lesional TLE. Conventional TLE models like kainate and pilocarpine hinder precise assessment of the role of individual limbic structures in TLE ictogenesis due to widespread limbic damage induced by the initial status epilepticus. In this study, we used a non-lesional TLE model characterized by the absence of initial status and cell damage to determine the spatiotemporal profile of seizure initiation and limbic structure recruitment in TLE. Epilepsy was induced by injecting a minute dose of tetanus toxin into the right dorsal hippocampus in seven animals. Following injection, animals were implanted with bipolar recording electrodes in the amygdala, dorsal hippocampus, ventral hippocampus, piriform, perirhinal, and entorhinal cortices of both hemispheres. The animals were video-EEG monitored for four weeks. In total, 140 seizures (20 seizures per animal) were analyzed. The average duration of each seizure was 53.2+/-3.9 s. Seizure could initiate in any limbic structure. Most seizures initiated in the ipsilateral (41 %) and contralateral (18 %) ventral hippocampi. These two structures displayed a significantly higher probability of seizure initiation than by chance. The involvement of limbic structures in seizure initiation varied between individual animals. Surprisingly, only 7 % of seizures initiated in the injected dorsal hippocampus. The limbic structure recruitment into the seizure activity wasn't random and displayed consistent patterns of early recruitment of hippocampi and entorhinal cortices. Although ventral hippocampus represented the primary seizure onset zone, the study demonstrated the involvement of multiple limbic structures in seizure initiation in a non-lesional TLE model. The study also revealed the dichotomy between the primary epileptogenic lesion and main seizure onset zones and points to the central role of ventral hippocampi in temporal lobe ictogenesis.

Increased expression of GABAA receptor subunits associated with tonic inhibition in patients with temporal lobe epilepsy

Epilepsy animal models indicate pronounced changes in the expression and rearrangement of GABA_A receptor subunits in the hippocampus and in para-hippocampal areas, including widespread downregulation of the subunits α5 and δ, and upregulation of α4, subunits that mediate tonic inhibition of GABA. In this case–control study, we investigated changes in the expression of subunits α4, α5 and δ in hippocampal specimens of drug resistant temporal lobe epilepsy patients who underwent epilepsy surgery. Using in situ hybridization, immunohistochemistry and α5-specific receptor autoradiography, we characterized expression of the receptor subunits in specimens from patients with and without Ammon’s horn sclerosis compared to post-mortem controls. Expression of the α5-subunit was abundant throughout all subfields of the hippocampus, including the dentate gyrus, sectors CA1 and CA3, the subiculum and pre- and parasubiculum. Significant but weaker expression was detected for subunits α4 and δ notably in the granule cell/molecular layer of control specimens, but was faint in the other parts of the hippocampus. Expression of all three subunits was similarly altered in sclerotic and non-sclerotic specimens. Respective mRNA levels were increased by about 50–80% in the granule cell layer compared with post-mortem controls. Subunit α5 mRNA levels and immunoreactivities were also increased in the sector CA3 and in the subiculum. Autoradiography for α5-containing receptors using [³H]L-655,708 as ligand showed significantly increased binding in the molecular layer of the dentate gyrus in non-sclerotic specimens. Increased expression of the α5 and δ subunits is in contrast to the previously observed downregulation of these subunits in different epilepsy models, whereas increased expression of α4 in temporal lobe epilepsy patients is consistent with that in the rodent models. Our findings indicate increased tonic inhibition likely representing an endogenous anticonvulsive mechanism in temporal lobe epilepsy.

Altered sensory processing and dendritic remodeling in hyperexcitable visual cortical networks

Epilepsy is characterized by impaired circuit function and a propensity for spontaneous seizures, but how plastic rearrangements within the epileptic focus trigger cortical dysfunction and hyperexcitability is only partly understood. Here we have examined alterations in sensory processing and the underlying biochemical and neuroanatomical changes in tetanus neurotoxin (TeNT)-induced focal epilepsy in mouse visual cortex. We documented persistent epileptiform electrographic discharges and upregulation of GABAergic markers at the completion of TeNT effects. We also found a significant remodeling of the dendritic arbors of pyramidal neurons, with increased dendritic length and branching, and overall reduction in spine density but significant preservation of mushroom, mature spines. Functionally, spontaneous neuronal discharge was increased, visual responses were less reliable, and electrophysiological and behavioural visual acuity was consistently impaired in TeNT-injected mice. These data demonstrate robust, long-term remodeling of both inhibitory and excitatory circuitry associated with specific disturbances of network function in neocortical epilepsy.

Structural and functional substrates of tetanus toxin in an animal model of temporal lobe epilepsy

The effects of tetanus toxin (TeNT) both in the spinal cord, in clinical tetanus, and in the brain, in experimental focal epilepsy, suggest disruption of inhibitory synapses. TeNT is a zinc protease with selectivity for Vesicle Associated Membrane Protein (VAMP; previously synaptobrevin), with a reported selectivity for VAMP2 in rats. We found spatially heterogeneous expression of VAMP1 and VAMP2 in the hippocampus. Inhibitory terminals in stratum pyramidale expressed significantly more VAMP1 than VAMP2, while glutamatergic terminals in stratum radiatum expressed significantly more VAMP2 than VAMP1. Intrahippocampal injection of TeNT at doses that induce epileptic foci cleaved both isoforms in tissue around the injection site. The cleavage was modest at 2 days after injection and more substantial and extensive at 8 and 16 days. Whole-cell recordings from CA1 pyramidal cells close to the injection site, made 8-16 days after injection, showed that TeNT decreases spontaneous EPSC frequency to 38 % of control and VAMP2 immunoreactive axon terminals to 37 %. In contrast, TeNT almost completely abolished both spontaneous and evoked IPSCs while decreasing VAMP1 axon terminals to 45 %. We conclude that due to the functional selectivity of the toxin to the relative sparing of excitatory synaptic transmission shifts the network to pathogenically excitable state causing epilepsy.

Role of the NR2A/2B subunits of the N-methyl-D-aspartate receptor in glutamate-induced glutamic acid decarboxylase alteration in cortical GABAergic neurons in vitro

The vulnerability of brain neuronal cell subpopulations to neurologic insults varies greatly. Among cells that survive a pathological insult, for example ischemia or brain trauma, some may undergo morphological and/or biochemical changes that may compromise brain function. The present study is a follow-up of our previous studies that investigated the effect of glutamate-induced excitotoxicity on the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67)'s expression in surviving DIV 11 cortical GABAergic neurons in vitro [Monnerie and Le Roux, (2007) Exp Neurol 205:367-382, (2008) Exp Neurol 213:145-153]. An N-methyl-D-aspartate receptor (NMDAR)-mediated decrease in GAD expression was found following glutamate exposure. Here we examined which NMDAR subtype(s) mediated the glutamate-induced change in GAD protein levels. Western blotting techniques on cortical neuron cultures showed that glutamate's effect on GAD proteins was not altered by NR2B-containing diheteromeric (NR1/NR2B) receptor blockade. By contrast, blockade of triheteromeric (NR1/NR2A/NR2B) receptors fully protected against a decrease in GAD protein levels following glutamate exposure. When receptor location on the postsynaptic membrane was examined, extrasynaptic NMDAR stimulation was observed to be sufficient to decrease GAD protein levels similar to that observed after glutamate bath application. Blocking diheteromeric receptors prevented glutamate's effect on GAD proteins after extrasynaptic NMDAR stimulation. Finally, NR2B subunit examination with site-specific antibodies demonstrated a glutamate-induced, calpain-mediated alteration in NR2B expression. These results suggest that glutamate-induced excitotoxic NMDAR stimulation in cultured GABAergic cortical neurons depends upon subunit composition and receptor location (synaptic vs. extrasynaptic) on the neuronal membrane. Biochemical alterations in surviving cortical GABAergic neurons in various disease states may contribute to the altered balance between excitation and inhibition that is often observed after injury.

Loss of GABAergic neurons in the subiculum and its functional implications in temporal lobe epilepsy

Clinical and experimental evidence suggest that the subiculum plays an important role in the maintenance of temporal lobe seizures. Using the pilocarpine-model of temporal lobe epilepsy (TLE), the present study examines the vulnerability of GABAergic subicular interneurons to recurrent seizures and determines its functional implications. In the subiculum of pilocarpine-treated animals, the density of glutamic acid decarboxylase (GAD) mRNA-positive cells was reduced in all layers. Our data indicate a substantial loss of parvalbumin-immunoreactive neurons in the pyramidal cell and molecular layer whereas calretinin-immunoreactive cells were predominantly reduced in the molecular layer. Though the subiculum of pilocarpine-treated rats showed an increased intensity of GAD65 immunoreactivity, the density of GAD65 containing synaptic terminals in the pyramidal cell layer was decreased indicating an increase in the GAD65 intensity of surviving synaptic terminals. We observed a decrease in evoked inhibitory post-synaptic currents that mediate dendritic inhibition as well as a decline in the frequency of miniature inhibitory post-synaptic currents (mIPSCs) that are restricted to the perisomatic region. The decrease in mIPSC frequency (-30%) matched with the reduced number of perisomatic GAD-positive terminals (-28%) suggesting a decrease of pre-synaptic GABAergic input onto pyramidal cells in epileptic animals. Though cell loss in the subiculum has not been considered as a pathogenic factor in human and experimental TLE, our data suggest that the vulnerability of subicular GABAergic interneurons causes an input-specific disturbance of the subicular inhibitory system.

Reduced dendrite growth and altered glutamic acid decarboxylase (GAD) 65- and 67-kDa isoform protein expression from mouse cortical GABAergic neurons following excitotoxic injury in vitro

The vulnerability of brain cells to neurologic insults varies greatly, depending on their neuronal subpopulation. However, cells surviving pathological insults such as ischemia or brain trauma may undergo structural changes, e.g., altered process growth, that could compromise brain function. In this study, we examined the effect of glutamate excitotoxicity on dendrite growth from surviving cortical GABAergic neurons in vitro. Glutamate exposure did not affect GABAergic neuron viability, however, it significantly reduced dendrite growth from GABAergic neurons. This effect was blocked by the AMPA receptor antagonists NBQX and CFM-2, and mimicked by AMPA, but not NMDA. Glutamate excitotoxicity also caused an NMDA receptor-mediated decrease in the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67) immunoreactivity from GABAergic neurons, measured using immunocytochemical and Western blot techniques. GAD is necessary for GABA synthesis; however, reduction of GABA by 3-mercaptopropionic acid (3-MPA), which inhibits GABA synthesis, did not alter dendrite growth. These results suggest that GABAergic cortical neurons are relatively resistant to excitotoxic-induced cell death, but they can display morphological and biochemical alterations which may impair their function.

Axonal sprouting of GABAergic interneurons in temporal lobe epilepsy

Temporal lobe epilepsy is one of the most common forms of epilepsy. Numerous contributing factors and compensatory mechanisms have been associated with temporal lobe epilepsy. One feature found in both humans and animal models is sprouting of hippocampal principal cell axons, which suggests that axonal sprouting may be a general phenomenon associated with temporal lobe epilepsy. This article highlights the evidence showing that hippocampal GABAergic interneurons also undergo axonal sprouting in temporal lobe epilepsy. The caveats and unanswered questions associated with the current data and the potential physiological consequences of reorganizations in GABAergic circuits are discussed.

GABA neurons survive focal ischemic injury

Focal cerebral lesions in rat brain induced by photothrombosis lead to an impaired inhibitory neurotransmission. A reduced gamma-aminobutyric acid (GABA)-mediated inhibition has been revealed by electrophysiological recordings associated with a diminished immunostaining of GABA handling proteins. Changes were found in ipsi- as well as in contralateral brain areas. Inhibition is mediated by interneurons using GABA as neurotransmitter. These cells use GAD (glutamate decarboxylase) to synthesize GABA. To analyze the vulnerability of GABAergic neurons in rats with a lesioned hindlimb area, cells expressing GAD65/67 mRNA were labeled using in situ hybridization. Positive somata were counted 7 and 30 days after focal ischemia in different cortical (hindlimb cortex, frontal cortex, primary and secondary somatosensory cortex) and hippocampal subsectors (pyramidal cell layer, stratum oriens and stratum radiatum/lacunosum-moleculare). The lesioned hemispheres were compared with the intact brain sides and with control brains. GABAergic interneurons survived the injury for up to 30 days in all investigated brain regions. Therefore it is unlikely that a loss of GABAergic neurons contributes to the reduced inhibition.

Gastrodin decreases immunoreactivities of gamma-aminobutyric acid shunt enzymes in the hippocampus of seizure-sensitive gerbils

Gastrodin is one of the natural compound isolated from Gastrodia elata and has known anticonvulsant effects, although the exact pharmacological principles of this natural compound and its effects on other aspects of gamma-aminobutyric acid (GABA) metabolism in vivo have not been explored. Therefore, in the present study, the effects of gastrodin on GABA metabolism in the gerbil hippocampus were examined, in an effort to identify the antiepileptic characteristics of this substance. Gastrodin reduced the seizure score in the treated group, although the immunoreactivities of GABA synthetic enzymes and GABA transporters were unaltered in gastrodin-treated animals. Interestingly, in the gastrodin-treated group, GABA transaminase (GABA-T) immunoreactivity in the hippocampus, particularly in neurons, was significantly decreased. In the gastrodin-treated group, both succinic semialdehyde dehydrogenase (SSADH) and succinic semialdehyde reductase (SSAR) immunoreactivities in the hippocampus was also decreased significantly, which stood in contrast to the nontreated group, in which strong SSADH and SSAR immunoreactivities were detected. From the neuroanatomical viewpoint, these findings suggest that gastrodin may cause the elevation of GABA concentration by inhibiting the GABA shunt.

The altered expression of GABA shunt enzymes in the gerbil hippocampus before and after seizure generation

In the present study, the distribution of succinic semialdehyde dehydrogenase (SSADH) and succinic semialdehyde reductase (SSAR) in the hippocampus of the Mongolian gerbil and its association with various sequelae of spontaneous seizure were investigated in order to identify the roles of GABA shunt in the epileptogenesis and the recovery mechanisms in these animals. Both SSADH and SSAR immunoreactivities in the GABAergic neurons were significantly higher in the pre-seizure groups of seizure sensitive (SS) gerbil as compared to those seen in the seizure resistant (SR) gerbils. The distributions of both SSADH and SSAR immunoreactivities in the hippocampus showed significant differences after the on-set of seizure. At 3 h postictal, when compared to the pre-seizure group of SS gerbils, a decline in the immunoreactivities in the perikarya was observed. At 12 h after seizure on-set, the densities of both SSADH and SSAR immunoreactivities were begun to recover to the pre-seizure level of SS gerbils. These results suggest that the GABAergic neurons in the hippocampal complex of the SS gerbil may be highly activated. In addition, the imbalance of GABA shunt expressions in the GABAergic neurons may imply a malfunction of the metabolism of GABAergic neurons in the SS gerbils, and this defect may trigger seizure on-set. Therefore, the initiation of seizure, at least in gerbils, may be the result of a malfunction in GABA shunt in the GABAergic neurons.

Tetanus toxin induces long-term changes in excitation and inhibition in the rat hippocampal CA1 area

Intrahippocampal tetanus toxin induces a period of chronic recurrent limbic seizures in adult rats, associated with a failure of inhibition in the hippocampus. The rats normally gain remission from their seizures after 6-8 weeks, but show persistent cognitive impairment. In this study we assessed which changes in cellular and network properties could account for the enduring changes in this model, using intracellular and extracellular field recordings in hippocampal slices from rats injected with tetanus toxin or vehicle, 5 months previously. In CA1 pyramidal neurones from toxin-injected rats, the slope of the action potential upstroke was reduced by 32%, the fast afterhyperpolarisation by 32% and the slow afterhyperpolarisation by 54%, suggesting changes in voltage-dependent conductances. The excitatory postsynaptic potential slope was reduced by 60% and the population synaptic potential slope was reduced at all stimulus intensities, suggesting a reduced afferent input in CA1. Paired-pulse stimulation showed an increase of the excitability ratio and an increase of cellular excitability only for the second pulse, suggesting a reduced inhibition. The polysynaptic inhibitory postsynaptic potential was reduced by 34%, whereas neither the inhibitory postsynaptic potential at subthreshold stimulus intensities,nor the pharmacologically isolated monosynaptic inhibitory postsynaptic potential were different in toxin-injected rats, suggesting a reduced synaptic excitation of interneurones. Stratum radiatum stimuli in toxin-injected rats, and not in controls, evoked antidromic activation of CA1 neurones, demonstrating axonal sprouting into areas normally devoid of CA1 pyramidal cell axons.We conclude that this combination of enduring changes in cellular and network properties, both pro-epileptic (increased recurrent excitatory connectivity, reduced recurrent inhibition and reduced afterhyperpolarisations) and anti-epileptic (impaired firing and reduced excitation), reaches a balance that allows remission of seizures, perhaps at the price of persistent cognitive impairment.

The temporal alteration of GAD67/GAD65 ratio in the gerbil hippocampal complex following seizure

In the present study, the distribution of glutamic acid decarboxylase (GAD) isoforms in the hippocampus of the Mongolian gerbil and its association with different sequelae of spontaneous seizure were investigated to identify the roles of balance of GAD isoforms in the epileptogenesis and the recovery mechanisms in these animals. The GAD67/GAD65 ratio in the hippocampus of pre-seizure seizure sensitive (SS) gerbil was approximately 3.5-fold higher as compared to seizure resistant (SR) gerbil. Following seizure, this ratio shifted to the level of SR gerbils up to 12 h postical. Therefore, the mismatched GAD67/GAD65 ratio (imbalance of GAD isoform expressions) in the hippocampus of SS gerbil implies that GABAergic neurons may be highly activated in order to regulate the increased neuronal excitability. In addition, the alteration in this ratio after seizure may be the compensatory response for reduction of epileptic activity in this animal.

Kindling induces transient fast inhibition in the dentate gyrus--CA3 projection

The granule cells of the dentate gyrus (DG) send a strong glutamatergic projection, the mossy fibre tract, toward the hippocampal CA3 field, where it excites pyramidal cells and neighbouring inhibitory interneurons. Despite their excitatory nature, granule cells contain small amounts of GAD (glutamate decarboxylase), the main synthetic enzyme for the inhibitory transmitter GABA. Chronic temporal lobe epilepsy results in transient upregulation of GAD and GABA in granule cells, giving rise to the speculation that following overexcitation, mossy fibres exert an inhibitory effect by release of GABA. We therefore stimulated the DG and recorded synaptic potentials from CA3 pyramidal cells in brain slices from kindled and control rats. In both preparations, DG stimulation caused excitatory postsynaptic potential (EPSP)/inhibitory postsynaptic potential (IPSP) sequences. These potentials could be completely blocked by glutamate receptor antagonists in control rats, while in the kindled rats, a bicuculline-sensitive fast IPSP remained, with an onset latency similar to that of the control EPSP. Interestingly, this IPSP disappeared 1 month after the last seizure. When synaptic responses were evoked by high-frequency stimulation, EPSPs in normal rats readily summate to evoke action potentials. In slices from kindled rats, a summation of IPSPs overrides that of the EPSPs and reduces the probability of evoking action potentials. Our data show for the first time that kindling induces functionally relevant activity-dependent expression of fast inhibition onto pyramidal cells, coming from the DG, that can limit CA3 excitation in a frequency-dependent manner.

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.

'Dormant' inhibitory neurons: do they exist and what is their functional impact?

The concept of dormant interneurons is proving to be hard to define precisely. We argue here that the term is best used as an operational description of interneurons which are not lost from the epileptic brain, but which fail to perform adequately. We present evidence for the existence of functionally dormant interneurons in the tetanus toxin model of chronic epilepsy, and we explore the roles of a partial dormancy (and also of charge-screening) in the acute low magnesium model of epilepsy.

Parvalbumin- and calbindin-containing neurons express c-fos protein in primary and secondary (mirror) epileptic foci of the rat neocortex

The present experiments aimed at the description and further immunocytochemical characterization of activated neocortical neurons expressing the c-fos gene. Focal seizures were induced by the topical application of isotonic, isohydric 4-aminopyridine solution to the frontal neocortex of adult anesthetized Wistar rats. The EEG of both hemispheres was recorded from the surface of the skull. The animals were perfused with fixative, coronal plane vibratome sections were cut and stained with cocktails containing polyclonal c-fos and monoclonal calbindin or parvalbumin antibodies. The polyclonal c-fos antibody was tested with Western blotting and the diffusion of 4-aminopyridine investigated with autoradiography of [3H]4-aminopyridine. The c-fos protein was detected in every layer of the neocortex (primary focus) and in some allocortical areas of the treated hemisphere. Scattered immunostained nuclei were observed in layers II, III, IV and VI of the contralateral neocortex (mirror focus). Several parvalbumin- and calbindin-positive neurons contained the c-fos protein in both foci. The medium-sized non-pyramidal parvalbumin neurons were found in layers II-IV and VI of the neocortex and in stratum multiforme of the prepiriform cortex. The c-fos protein was colocalized with calbindin mainly in layers II and III in small and medium-sized non-pyramidal neurons. The results prove that focal epileptiform activity of the neocortex activates diverse inhibitory neuronal populations. As concluded, the inhibitory control is probably more effective in the contralateral hemisphere (mirror focus) than on the side of 4-APY treatment (primary focus).

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

To study potential molecular mechanisms of epileptogenesis in the neocortex, the motor cortex of rats was injected with tetanus toxin (TT), and gene expression for 67 kDa glutamic acid decarboxylase (GAD-67), type II calcium/calmodulin-dependent protein kinase (CaMKII), NMDA receptor subunit 1 (NR1), and AMPA receptor subunit 2 (GluR2) was investigated by in situ hybridization histochemistry. Injections of 20-35 ng TT induced recurrent seizures after a postoperative period ranging from 4 to 13 d. A majority of rats perfused 5-7 d after TT injection showed altered gene expression, but the changes varied in their areal extent, ranging from most neocortical areas on the injected side in some rats to mainly the frontoparietal cortex or the motor cortex in others. Epileptic rats perfused 14 d after TT injection showed a focus of increased GAD-67 and NR1, and of decreased alpha-CaMKII and GluR2 mRNA levels at the injection site. A zone of cortex surrounding the focus showed changes in alpha-CaMKII, GAD-67, and NR1 mRNA levels that were reciprocal to those in the focus. The results suggest that TT-induced seizure activity initially spread to a variable extent but was gradually restricted 2-3 d after seizure onset. The focus and the surround showing reciprocal changes in gene expression are thought to correspond to the electrophysiologically identified epileptic focus and inhibitory surround, respectively. The findings suggest that lateral inhibition between neighboring cortical regions will be affected and contribute to a neurochemical segregation of an epileptic focus from surrounding cortex.

Changes in hippocampal circuitry after pilocarpine-induced seizures as revealed by opioid receptor distribution and activation

The pilocarpine model of temporal lobe epilepsy was used to study the time-dependent changes in dentate gyrus circuitry after seizures. Seizures caused a decrease in mu- and delta-opioid receptor immunoreactive (MOR-IR and DOR-IR, respectively) neurons in the hilus and MOR-IR neurons in the granule cell layer. Additionally, diffuse DOR-IR, MOR-IR, and GABA immunoreactivities (GABA-IR) were increased in the inner molecular layer. Using the in vitro hippocampal slice preparation to study the physiological consequences of the anatomical changes, we found that the disinhibitory effects of the mu-opioid receptor agonist [D-Ala2, MePhe4,Gly-(ol)5]-enkephalin (DAMGO) and the GABAA receptor antagonist bicuculline were greatly depressed 5-13 d after pilocarpine injection but returned to control levels within 6 weeks. The amplitudes of monosynaptic evoked IPSCs and the effects of DAMGO on this parameter were also slightly decreased 5-13 d after pilocarpine injection but significantly increased at 6 weeks. DAMGO significantly decreased the mean amplitude of spontaneous IPSCs (sIPSCs) at 6 weeks after pilocarpine injection but not in controls. The delta-opioid receptor agonist [D-Pen2,5]-enkephalin (DPDPE) principally inhibited excitatory transmission in saline-treated animals without affecting either sIPSCs or evoked IPSCs. The DPDPE-induced inhibition of excitatory transmission became more pronounced at 6 weeks after pilocarpine injection. These results illustrate the anatomical reorganization and functional changes in dentate gyrus circuitry evident in an animal model of temporal lobe epilepsy and provide evidence of compensatory changes after trauma to the hippocampal formation.

Review of the role of inhibitory neurons in chronic epileptic foci induced by intracerebral tetanus toxin

Blocking inhibition provides one of the most common experimental means of triggering epileptic activity in hippocampus and neocortex. However, it has proved much more difficult to show that chronic models of epilepsies are due to disinhibition. One problem is knowing how much inhibition needs to be blocked to provide a sufficient mechanism for epileptic activity. We have found that inhibitory (GABAA) transmission, estimated from evoked monosynaptic IPSCs, must be reduced to 17% of their control amplitude (by 4-7 microM bicuculline) before hippocampal slices generate all-or-none epileptic discharges. Similar estimates of inhibition in chronic epileptic foci induced by intrahippocampal injection of tetanus toxin showed that monosynaptic IPSCs dropped to 10% of control in the injected hippocampus during the first 2 weeks after injection. At all other stages of the active epileptic foci in the two hippocampi the reduction in IPSCs was not alone sufficient for epileptic activity; at 4-6 weeks IPSCs were normal despite continued epileptic activity. One likely mechanism for the late epileptic activity is a reduction of either the intrinsic excitability, or the synaptic excitation, of inhibitory interneurons so they fail to be recruited normally. Alternative mechanisms include the formation of new excitatory connections, as found at modest levels in the dentate gyrus. Several mechanisms may play a part in chronic foci such as those induced by tetanus toxin, either acting together, or sequentially during the progression of the epileptic focus.

Loss of hilar somatostatin neurons following tetanus toxin-induced seizures

A loss of inhibitory interneurons has been reported in the hippocampus following seizure activity in various animal models of epilepsy and in human epileptic tissue. The question of whether particular populations of inhibitory neurons are similarly affected by the chronic block of inhibition that results after tetanus toxin injections directly into the brain has not previously been addressed. In the present study a unilateral intrahippocampal injection of tetanus toxin into the ventral hippocampus was used to produce a chronic epileptic syndrome characterised by brief seizures that recurred intermittently for 6-8 weeks. The results reveal, for the first time, the morphological changes in somatostatin interneurons following tetanus toxin-induced seizures in the rat. A bilateral short-term increase in immunoreactivity of somatostatin neurons is present 1 week after injection. This is accompanied by an increased intensity of somatostatin-immunoreactive axon terminals in the outer molecular layer of the dentate gyrus, which is more marked on the contralateral side. A chronic and significant loss of somatostatin-immunoreactive neurons was noted in the hilus of the dentate gyrus 2 months later. The significance of the chronic loss of the hilar somatostatin neurons in the control of excitatory activity in the dentate gyrus and whether the acute morphological changes are due to a direct action of the toxin on release mechanisms or as a result of seizure activity are discussed.

Tetanus toxin-induced seizures in infant rats and their effects on hippocampal excitability in adulthood

A new experimental model of developmental epilepsy is reported. Behavioral and EEG features of seizures produced by unilateral intrahippocampal injection of tetanus toxin in postnatal day 9-11 rats, are described. Within 24-72 h of tetanus toxin injection, rat pups developed frequent and often prolonged seizures which included combinations of repetitive wet dog shakes, and wild running-jumping seizures. Intrahippocampal and cortical surface EEG recordings showed that coincident with these behaviors, electrographic seizures occurred not only in the injected hippocampus, but also in the contralateral hippocampus and bilaterally in the neocortex. Analysis of the interictal EEG revealed multiple independent spike foci. One week following tetanus toxin injection, the number of seizures markedly decreased; however, interictal spiking persisted. After injection rats were allowed to mature some were observed to have unprovoked behavioral seizures and/or epileptiform EEG activity. Mature animals were also studied using in vitro slice techniques. Recordings from hippocampal slices demonstrated spontaneous epileptiform burst discharges in the majority of rats which had tetanus toxin induced seizures as infants. These events occurred in area CA3 and consisted of interictal spikes and intracellularly recorded paroxysmal depolarization shifts (PDSs). On rarer occasions, electrographic seizures were recorded. The use of the tetanus toxin model in developing rats may facilitate a better understanding of the unique features of epileptogenesis in the developing brain and the consequences early-life seizures have on brain maturation and the genesis of epileptic conditions in later life.

Chronic focal epilepsy induced by intracerebral tetanus toxin

A single, minute dose of tetanus toxin injected into mammalian cerebral cortex induces a chronic epileptic syndrome. Seizures lasting up to 3 minutes occur spontaneously and intermittently for several weeks to months. The cellular mechanisms of this model have been studied in detail using brain slices in vitro. Initially the release of the inhibitory neurotransmitter, GABA, is blocked, but after 2-4 weeks, other mechanisms take over. Intrahippocampal tetanus toxin models human complex partial seizures (temporal lobe epilepsy). It results in consistent behavioural changes analogous with those seen clinically, in spite of the limited neuronal loss found in only 10-30% of rats. Treatment with carbamazepine ameliorates both the seizures and their behavioural consequences. Tetanus toxin provides a versatile and long-lasting model of focal epilepsies.

Epileptic activity outlasts disinhibition after intrahippocampal tetanus toxin in the rat

1. A single dose of tetanus toxin, injected under anaesthesia into one dorsal hippocampus of the rat, produces chronic epileptic foci involving both hippocampi. Generalized seizures occurred 1-6 weeks after injection and epileptic discharges were found in hippocampal slices in vitro. Here we measured the time course of decay of epileptic activity and the level of GABAA receptor-mediated inhibition in hippocampal slices 1-16 weeks after toxin injection in vivo. 2. Epileptic activity peaked in the dentate granule cell and CA3 pyramidal cell layers 2 weeks after toxin injection and at 4 weeks in CA1. Thresholds for evoking epileptic activity were lowest in the suprapyramidal blade of the dentate gyrus and area CA3c. Recovery from epileptic activity occurred more rapidly in the contralateral hippocampus. Polyspike activity ceased by 8 weeks and interictal activity by 16 weeks. Epileptic discharges could still be evoked from CA1 16 weeks after toxin injection. 3. The maximal monosynaptic fast inhibitory postsynaptic current (IPSC) conductance changes (gIPSC) decreased to < 10% of control values at the time of peak epileptic activity and remained lower than controls for 4 weeks ipsilaterally. In the contralateral hippocampus, gIPSC fell to ca 50% of control values for the first 2 weeks. Responses to exogenous GABA remained unchanged. 4. After 8 weeks dentate granule cells had gIPSC significantly larger than controls. No increase in gIPSC occurred in CA3. Epileptic activity persisted 8-10 weeks after recovery from disinhibition ipsilaterally and 4 weeks contralaterally. 5. Epileptic activity was seen when monosynaptic GABAA receptor-mediated IPSCs were normal or supranormal. At these times polysynaptic inhibition was still profoundly reduced. These observations provide strong evidence for long-term changes in the pattern of synaptic excitation contributing to a chronic epileptic syndrome syndrome following disinhibitory insult, and are consistent with weakened excitation of inhibitory neurones.

Quantitative immunofluorescence data suggest a permanently enhanced GAD67/GAD65 ratio in nerve endings in rat cerebral cortex damaged by early postnatal hypoxia-ischemia: a comparison between two computer-assisted procedures for quantification of confocal laser scanning microscopic immunofluorescence images

The aim of the present study was 2-fold: (1) to determine the ratio between the amount of GAD67 and GAD65 (two isoforms of the GABA synthetizing enzyme glutamic acid decarboxylase) in nerve endings in the mature rat cerebral cortex damaged by hypoxia-ischemia during early postnatal life; and (2) to compare two different computer-assisted procedures developed for quantitative analysis of immunofluorescence images obtained with a confocal laser scanning microscope (CLSM). One procedure was based on a program present in the standard Leica CLSM software packet for full-field analysis, the other on a specially written program for object-oriented analysis run on a Kontron IBAS-KAT image analysis system. To this end, rat pups were unilaterally exposed to hypoxic-ischemic conditions and, after a survival period of 6.5 months, sacrificed by perfusion fixation. After dissection of the brain and vibratome sectioning, three animals with substantial damage on one cortical side were selected. Sections of these animals were double-stained with primary antibodies against GAD67 and GAD65 and fluorophore-conjugated secondary antibodies and subsequently sampled with a CLSM. Analysis of the CLSM images with both computer-assisted procedures showed for all three animals a clear tendency to higher GAD67/GAD65 ratios in cortical GABAergic nerve endings on the hypoxia-damaged side than in matched areas on the contralateral side. This outcome led to the following conclusions. (1) The correspondence between the outcome of both analysis procedures indicates that both procedures are valid for quantification of immunofluorescence images of nerve endings obtained with a CLSM. (2) The outcome lends further support to our view that hypoxic-ischemic encephalopathy, sustained during early postnatal life, may result in an unstable cortical network generating abnormal synchronizations and oscillations which can be amplified and propagated as true epileptic discharges. In such a network both excitatory and inhibitory processes are tonically enhanced, the latter probably as a homeostatic reaction tending to keep abnormal excitation within physiological limits.

Seizure-induced expression of G proteins in the rat hippocampus

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

Permanent increase of immunocytochemical reactivity for gamma-aminobutyric acid (GABA), glutamic acid decarboxylase, mitochondrial enzymes, and glial fibrillary acidic protein in rat cerebral cortex damaged by early postnatal hypoxia-ischemia

A former study indicated that hypoxic-ischemic encephalopathy in rat sustained during early postnatal life may result in permanent epileptic activity in the baseline electroencephalogram. We, therefore, investigated whether the presumed higher firing frequency and metabolic activity of neurons in such hypoxia-damaged cortical areas would be reflected by an enhanced light microscopic immunoreactivity of gamma-aminobutyric acid (GABA), the two isoforms of glutamic acid decarboxylase (GAD67 and GAD65), the mitochondrial enzymes cytochrome c oxidase and ATP synthase, and/or glial fibrillary acidic, protein (GFAP). To that end rat pups, 12-13 days of age, were unilaterally exposed to hypoxic-ischemic conditions and, after a survival period of 2 and 6 1/2 months, respectively, killed by perfusion fixation. After dissection of the brain, coronal vibratome sections of animals showing cortical damage were immunostained for the presence of the above-mentioned antigens. Subsequent qualitative analysis revealed that the surroundings of cortical infarctions were unambiguously characterized by a disordered neural network containing numerous nerve cells, fibers and/or endings showing an enhanced immunoreactivity for GABA, both isoforms of glutamic acid decarboxylase, and cytochrome c oxidase and ATP synthase, while the astrocytes showed an enhanced immunoreactivity for GFAP. The diverse patterns of enhanced immunoreactivity suggested, furthermore, a wider low-to-high range of metabolic activities in both excitatory and inhibitory neurons.

Spontaneous recurrent seizures in rats: amino acid and monoamine determination in the hippocampus

Rats subjected to structural brain damage induced by sustained convulsions triggered by systemic administration of pilocarpine (PILO) are a useful model for investigation of the mechanisms essential for seizure generation and spread in rodents. After PILO administration, three distinct phases are observed: (a) an acute period of 1-2 days' duration corresponding to a pattern of repetitive limbic seizures and status epilepticus; (b) a seizure-free (silent) period characterized by a progressive return to normal EEG and behavior of 4-44 days' duration; and (c) a period of spontaneous recurrent seizures (SRS) starting 5-45 days after PILO administration and lasting throughout the animal's life. PILO (320-350 mg/kg intraperitoneally, i.p.) was administered to rats, and the content of hippocampal monoamines and amino acids was measured in the acute, silent, and SRS periods by liquid chromatography. Norepinephrine (NE) level was decreased during all periods whereas dopamine (DA) content was increased. Serotonin (5-hydroxytryptamine, 5-HT) was increased only in the acute period. Utilization rate measurement of monoamines showed increased NE consumption and decreased DA consumption during all phases. 5-HT utilization rate was increased only in the acute period. Amino acid content showed a decrease in aspartate (ASP) and glutamate (GLU) concentrations associated with increased gamma-aminobutyric acid (GABA) level during the acute period. The silent phase was characterized by a decrease in glycine (GLY) and GABA levels and an increase in GLU concentration. The SRS period showed an increase in all amino acid concentrations. These findings show important neurochemical changes in the course of establishment of an epileptic focus after brain damage induced by status epilepticus triggered by pilocarpine.

Injection of tetanus toxin into the neocortex elicits persistent epileptiform activity but only transient impairment of GABA release

Focal injection of a minute quantity of tetanus toxin into the rat neocortex induces chronic epileptogenesis. Within a day, spontaneous and stimulus-evoked paroxysmal discharges appear in widespread regions of both hemispheres and this lasts for at least nine months. Tetanus toxin blocks transmitter release, apparently by catalysing the breakdown of synaptobrevin, a synaptic protein. It specifically binds to neuronal membranes but its potent epileptogenic properties have been ascribed to a higher affinity for inhibitory neurons. Following focal injection of tetanus toxin into the hippocampus a long-lasting epileptic syndrome also develops. During the early part of the syndrome GABA release is depressed in slices from the injected side, but not in slices from the contralateral, secondary focus. In the present experiments on neocortex, release of radiolabelled GABA was measured from primary and secondary epileptic foci induced by unilateral focal injection of tetanus toxin into the parietal cortex. By four weeks after the injection, no differences were detected in GABA release from any neocortical site in control or toxin-injected animals, despite the persistence of profound epileptic activity in slices from the latter. At earlier times (1.5 days) after the toxin injection, however, release was significantly depressed in both hemispheres. The results indicate that at first, the toxin induces focal neocortical epileptogenesis by directly impeding GABAergic synaptic transmission but that with time there is a recovery from this initial effect. We propose, as has also been suggested for other models, that the initial epileptogenesis leaves in its wake a long-lasting change in the local functional connectivity, such that the neocortex is rendered permanently epileptic.

Rapid increases in cerebellar Purkinje cell glutamic acid decarboxylase (GAD67) mRNA after lesion-induced increases in cell firing

Loss of the inferior olive-climbing fiber input to the cerebellar cortex doubles the simple spike activity of the cerebellar Purkinje cell. There is a 3- to 4-fold increase in Purkinje cell messenger RNA for the 67 kDa form of glutamic acid decarboxylase (a synthetic enzyme for the neurotransmitter GABA) within 4-5 h of the increase in electrical activity, suggesting a rapid response of mechanisms influencing neurotransmitter synthesis or stability to altered electrophysiological activity.

Synaptic inhibition in primary and secondary chronic epileptic foci induced by intrahippocampal tetanus toxin in the rat

1. Injecting twelve mouse minimum lethal doses of tetanus toxin into one hippocampus of a rat leads to the development of chronic epileptic foci in both hippocampi. These generate intermittent epileptic discharges for 6-8 weeks. Here we compare GABAergic inhibition, 10-18 days after injection, in slices prepared from the injected and contralateral hippocampi (respectively the primary and the secondary or 'mirror' foci), using both neurochemical and electrophysiological methods. 2. Epileptic activity was recorded from slices of both hippocampi from all tetanus toxin-injected rats. Evoked epileptic discharges were similar on the two sides, but spontaneous epileptic discharges were more common contralaterally. 3. Ca(2+)-dependent, K(+)-stimulated (synaptic) release of radiolabelled GABA was depressed in slices from the injected hippocampus, compared with vehicle-injected controls. In contrast, slices from the contralateral hippocampus had normal levels of Ca(2+)-dependent, K(+)-stimulated GABA release, even though adjacent slices were epileptogenic. 4. Intracellular recordings revealed that both fast and slow stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were abolished in CA3 pyramidal cells in the primary focus. In the secondary focus, however, fast IPSPs were seen in seven of twenty-five cells, and slow IPSPs were seen in all cells if the stimulus was strong enough. 5. Monosynaptic IPSPs were isolated pharmacologically by blocking glutamatergic excitatory postsynaptic potentials (EPSPs) with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphopentanoic acid (AP-5). No monosynaptic IPSPs were uncovered in cells from the primary focus at any stimulus strength. Monosynaptic IPSPs were evoked in all cells from both the secondary focus and control slices. The estimated conductances of monosynaptic fast IPSPs were similar in cells from the secondary focus and from the controls, although the former required twice the stimulus strength. 6. Slow IPSPs were found in the secondary focus and in controls, but not in the primary focus. They were sensitive to 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH saclofen). The estimated conductances of slow IPSPs evoked by weak stimuli in the secondary focus were much smaller than in the controls. However, stimuli that could trigger epileptic discharges in the secondary focus, evoked 2-OH saclofen-sensitive slow IPSPs with estimated conductances approaching the controls. This marked increase in the slow IPSP did not occur when EPSPs, and epileptic bursts, were blocked with CNQX and AP-5, suggesting that a strong barrage of excitation is needed to generate full-sized slow IPSPs in the secondary focus.(ABSTRACT TRUNCATED AT 400 WORDS)

Abundant Gs alpha mRNA in basket cells of the dentate gyrus in adult rat hippocampus

In situ hybridization histochemistry (ISHH) has been used to study the differential distribution and relative abundance of mRNAs encoding a stimulatory alpha subunit of the G-protein (Gs alpha) and glutamic acid decarboxylase (GAD) in the dorsal hippocampus in adult rat brain. The present quantitative study shows that GABAergic neurons containing high levels of GAD mRNA, express considerably more Gs alpha message than excitatory principal neurons, the granule cells of the dentate gyrus and the pyramidal cells of CA1 subfield. A subpopulation of basket cells of the dentate gyrus exhibited a uniquely high level of Gs alpha mRNA, in addition to GAD. These findings may indicate a specific functional role for Gs alpha in these GABAergic neurons in the hippocampus.