Area Tempestas Research Articles

Noradrenaline and Seizures: A Perspective on the Role of Adrenergic Receptors in Limbic Seizures.

Noradrenergic fibers originating from the locus coeruleus densely innervate limbic structures, including the piriform cortex, which is the limbic structure with the lowest seizure threshold. Noradrenaline (NA) modulates limbic seizures while stimulating autophagy through β2- adrenergic receptors (AR). Since autophagy is related to seizure threshold, this perspective questions whether modulating β2-AR focally within the anterior piriform cortex affects limbic seizures. In this perspective, we analyzed a potential role for β2-AR as an anticonvulsant target within the anterior piriform cortex, area tempestas (AT). We developed this perspective based on current literature on the role of NA in limbic seizures and autophagy. The perspective is also grounded on preliminary data obtained by microinfusing within AT either a β2-AR agonist (salbutamol) or a β2-AR antagonist (butoxamine) 5 minutes before bicuculline. β2-AR stimulation fully prevents limbic seizures induced by bicuculline micro-infusion in AT. Conversely, antagonism at β2-AR worsens bicuculline-induced seizure severity and prolongs seizure duration, leading to self-sustaining status epilepticus. These data indicate a specific role for β2-AR as an anticonvulsant in AT. NA counteracts limbic seizures. This relies on various receptors in different brain areas. The anterior piriform cortex plays a key role in patients affected by limbic epilepsy. The anticonvulsant effects of NA through β2-AR may be related to the stimulation of the autophagy pathway. Recent literature and present data draw a perspective where β2-AR stimulation while stimulating autophagy mitigates limbic seizures, focally within AT. The mechanism linking β2-AR to autophagy and seizure modulation should be extensively investigated.

Practical microscopic neuroanatomy of the limbic system and basal forebrain identifiable on clinical 3T MRI.

Microscopic neuroanatomy of limbic system and basal forebrain on MRI is complex and is a terra incognita for many radiologists, clinicians, and neuroscientists. Interestingly, most of the important structures/at least anatomical regions containing these structures demonstrable on cadaveric and surgical dissections can be identified on clinical MRI, with 3T being much better than 1.5T. This article teaches the practical MRI identification of these structures which will greatly help in evaluating complex ailments like temporal lobe epilepsy, Alzheimer dementia, and other neuropsychiatric disorders. This knowledge will also aid in accurate reporting of tumor spread along the white matter fasciculi in the temporal stem/basal forebrain region. Limbic system includes the mesial temporal structures and their connections, piriform cortex including "area tempestas," and the septal area comprising of subcallosal area and paraterminal gyrus. Basal forebrain includes structures like substantia innominata with basal nucleus of Meynert, diagonal gyrus/diagonal band of Broca, and nucleus accumbens lying in between the anterior perforated substance inferiorly and the anterior commissure superiorly.

Perspective on mTOR-dependent Protection in Status Epilepticus.

The piriform cortex, known as area tempestas, has a high propensity to trigger limbic epileptic seizures. Recent studies on human patients indicate that a resection containing the piriform cortex produces a marked improvement in patients suffering from intractable limbic seizures. This calls for looking back at the pharmacological and anatomical data on area tempestas. Within the piriform cortex, status epilepticus can be induced by impairing the desensitization of AMPA receptors. The mechanistic target of rapamycin complex1 (mTORC1) is a promising candidate. The present perspective aims to link the novel role of the piriform cortex with recent evidence on the modulation of AMPA receptors under the influence of mTORC1. This is based on recent evidence and preliminary data, leading to the formulation of interaction between mTORC1 and AMPA receptors to mitigate the onset of long-lasting, self-sustaining, neurotoxic status epilepticus. The perspective grounds its method on recent literature along with the actual experimental procedure to elicit status epilepticus from the piriform cortex and the method to administer the mTORC1 inhibitor rapamycin to mitigate seizure expression and brain damage. The available and present perspectives converge to show that rapamycin may disrupt the seizure circuitry initiated in the piriform cortex to mitigate seizure duration, severity, and brain damage. The perspective provides a novel scenario to understand refractory epilepsy and selfsustaining status epilepticus. It is expected to provide a beneficial outcome in patients suffering from temporal lobe epilepsy.

Optogenetic activation of the reticular nucleus of the thalamus attenuates limbic seizures via inhibition of the midline thalamus.

The nucleus reticularis of the thalamus (nRT) is most studied in epilepsy for its role in the genesis of absence seizures; much less is known regarding its role in other seizure types, including those originating in limbic structures and the temporal lobe. As it is a major source of inhibitory input to higher order thalamic nuclei, stimulation of the nRT may be an effective strategy to disrupt seizure activity that requires thalamic engagement. We recorded single unit activity from the nRT prior to and after infusion of bicuculline into the area tempestas. We monitored single unit activity time-locked with interictal spikes. We optogenetically activated the nRT in both the area tempestas and amygdala kindling models. We tested a role for projections from the nRT to higher order midline thalamic nuclei through the use of retrogradely trafficked viral vector. Mean firing rate in the nRT was decreased after infusion of bicuculline into the area tempestas as compared to the preinfusion baseline. nRT unit firing in response to interictal spikes was heterogeneous, with an approximately equal proportion of neurons displaying (1) no change in firing, (2) increased firing, and (3) decreasing firing. Optogenetic activation of the nRT significantly suppressed seizure activity in both the area tempestas and amygdala kindling models. Optogenetic activation of contralaterally targeting projections but not ipsilaterally targeting projections from the nRT to the midline thalamus significantly suppressed seizures in the kindling model. Although the nRT is typically thought of in the context of absence seizures, our data show that it may be a viable target for other seizure types. In two models that recapitulate the seizure types seen in temporal lobe epilepsy, nRT activation suppressed both electrographic and behavioral seizures. These data suggest that the nRT should be considered more broadly in the context of epilepsy.

Αlpha 5 subunit-containing GABAA receptors in temporal lobe epilepsy with normal MRI.

GABAA receptors containing the α5 subunit mediate tonic inhibition and are widely expressed in the limbic system. In animals, activation of α5-containing receptors impairs hippocampus-dependent memory. Temporal lobe epilepsy is associated with memory impairments related to neuron loss and other changes. The less selective PET ligand [11C]flumazenil has revealed reductions in GABAA receptors. The hypothesis that α5 subunit receptor alterations are present in temporal lobe epilepsy and could contribute to impaired memory is untested. We compared α5 subunit availability between individuals with temporal lobe epilepsy and normal structural MRI (‘MRI-negative’) and healthy controls, and interrogated the relationship between α5 subunit availability and episodic memory performance, in a cross-sectional study. Twenty-three healthy male controls (median ± interquartile age 49 ± 13 years) and 11 individuals with MRI-negative temporal lobe epilepsy (seven males; 40 ± 8) had a 90-min PET scan after bolus injection of [11C]Ro15-4513, with arterial blood sampling and metabolite correction. All those with epilepsy and six controls completed the Adult Memory and Information Processing Battery on the scanning day. ‘Bandpass’ exponential spectral analyses were used to calculate volumes of distribution separately for the fast component [VF; dominated by signal from α1 (α2, α3)-containing receptors] and the slow component (VS; dominated by signal from α5-containing receptors). We made voxel-by-voxel comparisons between: the epilepsy and control groups; each individual case versus the controls. We obtained parametric maps of VF and VS measures from a single bolus injection of [11C]Ro15-4513. The epilepsy group had higher VS in anterior medial and lateral aspects of the temporal lobes, the anterior cingulate gyri, the presumed area tempestas (piriform cortex) and the insulae, in addition to increases of ∼24% and ∼26% in the ipsilateral and contralateral hippocampal areas (P < 0.004). This was associated with reduced VF:VS ratios within the same areas (P < 0.009). Comparisons of VS for each individual with epilepsy versus controls did not consistently lateralize the epileptogenic lobe. Memory scores were significantly lower in the epilepsy group than in controls (mean ± standard deviation −0.4 ± 1.0 versus 0.7 ± 0.3; P = 0.02). In individuals with epilepsy, hippocampal VS did not correlate with memory performance on the Adult Memory and Information Processing Battery. They had reduced VF in the hippocampal area, which was significant ipsilaterally (P = 0.03), as expected from [11C]flumazenil studies. We found increased tonic inhibitory neurotransmission in our cohort of MRI-negative temporal lobe epilepsy who also had co-morbid memory impairments. Our findings are consistent with a subunit shift from α1/2/3 to α5 in MRI-negative temporal lobe epilepsy.

Divergent Effects of Systemic and Intracollicular CB Receptor Activation Against Forebrain and Hindbrain-Evoked Seizures in Rats.

Cannabinoid (CB) receptor agonists are of growing interest as targets for anti-seizure therapies. Here we examined the effect of systemic administration of the CB receptor agonist WIN 55,212-2 (WIN) against audiogenic seizures (AGSs) in the Genetically Epilepsy Prone Rat (GEPR)-3 strain, and against seizures evoked focally from the Area Tempestas (AT). We compared these results to the effect of focal administration of the CB1/2 receptor agonist CP 55940 into the deep layers of the superior colliculus (DLSC), a brain site expressing CB1 receptors. While systemic administration of WIN dose-dependently decreased AGS in GEPR-3s, it was without effect in the AT model. By contrast, intra-DLSC infusion of CP 55940 decreased seizures in both models. To determine if the effects of systemic WIN were dependent upon activation of CB1 receptors in the DSLC, we next microinjected the CB1 receptor antagonist SR141716, before WIN systemic treatment, and tested animals for AGS susceptibility. The pretreatment of the DLSC with SR141716 was without effect on its own and did not alter the anti-convulsant action of WIN systemic administration. Thus, while CB receptors in the DLSC are a potential site of anticonvulsant action, they are not necessary for the effects of systemically administered CB agonists.

Degeneration of cholinergic basal forebrain nuclei after focally evoked status epilepticus

Status epilepticus (SE) of limbic onset might cause degenerative phenomena in different brain structures, and may be associated with chronic cognitive and EEG effects. In the present study SE was evoked focally by microinfusing picomolar doses of cyclothiazide+bicuculline into the anterior extent of the piriform cortex (APC) in rats, the so-called area tempestas, an approach which allows to evaluate selectively the effects of seizure spreading through the natural anatomical circuitries up to secondary generalization. In the brain of rats submitted to SE we analyzed neuronal density, occurrence of degenerative phenomena (by Fluoro-Jade B-FJB- staining) and expression of heat shock protein-70 (HSP-70) in the piriform cortex, the hippocampus and ventromedial thalamus. We further analyzed in detail, the loss of cholinergic neurons, and the presence of FJB- and HSP-70 positive neurons in basal forebrain cholinergic areas, i.e. the medial septal nucleus (MSN, Ch1), the diagonal band of Broca (DBB, Ch2 and Ch3) and the Nucleus basalis of Meynert (NBM, Ch4). In fact, these nuclei are strictly connected with limbic structures, and play a key pivotal role in different cognitive functions and vigilance. Although recent studies begun to investigate these nuclei in experimental epilepsy and in persons with epilepsy, conflicting results were obtained so far. We showed that after severe and long-lasting, focally induced limbic SE there is a significant cell loss within all of the abovementioned cholinergic nuclei ipsi- and contra-laterally to the infusion site. In parallel, these nuclei show also FJB and heat shock protein-70 expression. Those effects vary depending on the single nucleus assessed and on the severity of the SE seizure score. We also showed the occurrence of cell loss and degenerative phenomena in limbic cortex, hippocampus and limbic thalamic areas. These novel findings show direct evidence of SE-induced neuronal damage which is solely due to seizure activity ruling out potential confounding effects produced by systemic pro-convulsant neurotoxins. A damage to basal forebrain cholinergic nuclei, which may underlie cognitive alterations, is documented for the first time in a model of SE triggered focally.

Optogenetic activation of superior colliculus neurons suppresses seizures originating in diverse brain networks

Because sites of seizure origin may be unknown or multifocal, identifying targets from which activation can suppress seizures originating in diverse networks is essential. We evaluated the ability of optogenetic activation of the deep/intermediate layers of the superior colliculus (DLSC) to fill this role. Optogenetic activation of DLSC suppressed behavioral and electrographic seizures in the pentylenetetrazole (forebrain+brainstem seizures) and Area Tempestas (forebrain/complex partial seizures) models; this effect was specific to activation of DLSC, and not neighboring structures. DLSC activation likewise attenuated seizures evoked by gamma butyrolactone (thalamocortical/absence seizures), or acoustic stimulation of genetically epilepsy prone rates (brainstem seizures). Anticonvulsant effects were seen with stimulation frequencies as low as 5 Hz. Unlike previous applications of optogenetics for the control of seizures, activation of DLSC exerted broad-spectrum anticonvulsant actions, attenuating seizures originating in diverse and distal brain networks. These data indicate that DLSC is a promising target for optogenetic control of epilepsy.

Connectivity of the Area Tempestas with the Limbic System (S22.004)

Connectivity of the Area Tempestas with the Limbic System (S22.004)

EEG-fMRI findings in late seizure recurrence following temporal lobectomy: A possible contribution of area tempestas.

Late seizure relapses following temporal lobectomy for drug-resistant temporal lobe epilepsy occur in 18–30% of operated-on cases, and recent evidence suggests that a significant proportion of them are due to maturation and activation of proepileptic tissue having defied initial resection and located at the vicinity of or at a short distance from its borders, usually over the posterior medial, basal temporal-occipital, and lateral temporal regions. Experimental studies in animals and functional imaging studies in humans suggest that the area tempestas, a particular region of the basal-frontal piriform cortex, is critical for kindling and initiation and propagation of seizure activity arising from different cortical foci, especially limbic ones. This case report of a patient with late seizure relapse, three years following an initially successful right temporal lobectomy for ipsilateral medial temporal sclerosis, is the first one in the literature to demonstrate interictal EEG–fMRI evidence of significant BOLD signal changes over the inferior, basal and lateral temporal and temporooccipital cortices posterior to the resection margin, plus a significant BOLD signal change over the ipsilateral basal frontal region, closely corresponding to the piriform cortex/area tempestas. Our case study provides further functional imaging evidence in support of maturation/activation of proepileptic tissue located at the vicinity of the initial temporal lobe resection in cases of late seizure relapses and suggests, in addition, a possible role for the piriform cortex/area tempestas in the relapsing process.

Enhanced efficacy of anticonvulsants when combined with levetiracetam in soman-exposed rats

Results from studies based on microinfusions into seizure controlling brain sites (area tempestas, medial septum, perirhinal cortex, posterior piriform cortex) have shown that procyclidine, muscimol, caramiphen, and NBQX, but not ketamine, exert anticonvulsant effects against soman-induced seizures. The purpose of the present study was to examine whether levetiracetam (Keppra®) may enhance the anticonvulsant potency of the above drugs to become optimally effective when used systemically. Levetiracetam has a unique profile in preclinical models of epilepsy and has been shown to increase the potency of other antiepileptic drugs. The rats were pretreated with pyridostigmine (0.1mg/kg) to enhance survival and received anticonvulsants 20min after onset of seizures evoked by soman (1.15×LD50). The results showed that no single drug was able to terminate seizure activity. However, when levetiracetam (LEV; 50mg/kg) was combined with either procyclidine (PCD; 10mg/kg) or caramiphen (CMP; 10mg/kg) complete cessation of seizures was achieved, but the nicotinic antagonist mecamylamine was needed to induce full motor rest in some rats. In a subsequent experiment, rats were pretreated with HI-6 (125mg/kg) to enhance survival and treatment started 40min following seizure onset of a soman dose of 1.6×LD50. LEV (50mg/kg) combined with either PCD (20mg/kg) or CMP (20mg/kg) terminated seizure activity, but the survival rate was considerably higher for LEV+PCD than LEV+CMP. Both therapies could also save the lives of rats that were about to die 5–10min after seizure onset. Thus, the combination of LEV and PCD or CMP may make up a model of a future autoinjector being effective regardless of the time of application.

Protection Against Sarin-Induced Seizures in Rats by Direct Brain Microinjection of Scopolamine, Midazolam or MK-801

Control of seizure activity is critical to survival and neuroprotection following nerve agent exposure. Extensive research has shown that three classes of drugs, muscarinic antagonists, benzodiazepines, and N-methyl-D: -aspartate antagonists, are capable of moderating these seizures. This study began to map the neural areas in rat brain that respond to these three drug classes resulting in anticonvulsant effects. Drugs of each class (scopolamine, midazolam, MK-801) were evaluated for their ability to prevent sarin-induced seizures when injected into specific brain areas (lateral ventricle, anterior piriform cortex, basolateral amygdala, area tempestas). Animals were pretreated by microinjection with saline or a dose of drug from one of the three classes 30 min prior to receiving 150 microg/kg sarin, subcutaneously, followed by 2.0 mg/kg atropine methylnitrate, intramuscularly. Animals were then returned to their cages, where electroencephalographic activity was monitored for seizures. Anticonvulsant effective doses (ED(50)) were determined using an up-down dosing procedure over successive animals. Scopolamine provided anticonvulsant effects in each area tested, while midazolam was effective in each area except the lateral ventricle. MK-801 was only effective at preventing seizures when injected into the basolateral amygdala or area tempestas. The results show a unique neuroanatomical and pharmacological specificity for control of nerve agent-induced seizures.

Anticonvulsant Efficacy of Drugs with Cholinergic and/or Glutamatergic Antagonism Microinfused into Area Tempestas of Rats Exposed to Soman

A group of antiparkinson drugs (benactyzine, biperiden, caramiphen, procyclidine, and trihexyphenidyl) has been shown to possess both anticholinergic and antiglutamatergic properties, making these agents very well suited as anticonvulsants against nerve agents. The first purpose of this study was to make a comparative assessment of the anticonvulsant potencies of the antiparkinson agents when microinfused (1 microl) into the seizure controlling area tempestas (AT) of rats 20 min before subcutaneous injection of soman (100 microg/kg). The second purpose was to determine whether cholinergic and/or glutamatergic antagonism was the effective property. The results showed that only procyclidine (6 microg) and caramiphen (10 microg) antagonized soman-induced seizures. Cholinergic, and not glutamatergic, antagonism was likely the active property, since atropine (100 microg), and scopolamine (1 microg) caused anticonvulsant effects, whereas MK-801 (1 microg), and ketamine (50 microg) did not. Soman (11 nmol) injected into AT resulted more frequently in clonic convulsions than full tonic-clonic convulsions. AT may serve as both a trigger site for soman-evoked seizures and a site for screening anticonvulsant potencies of future countermeasures.

Anticonvulsant effects of damage to structures involved in seizure induction in rats exposed to soman

In nerve agent research, it is assumed that the regions from which seizure activity is triggered may offer clues for the designing of effective anticonvulsive therapy. In the present study, selective brain lesions were made to identify critical cholinergic pathways and seizure controlling areas involved in the induction of epileptiform activity in rats challenged with soman. The results showed that rats with bilateral aspiration lesion of the seizure controlling substrate, area tempestas (AT) in the piriform cortex, displayed marked anticonvulsant effects, whereas such effects were not seen when substantia nigra was destroyed. Aspiration lesion of the medial septal area (MS) including the vertical limb of the diagonal band nucleus (DBN) caused increased latency to the onset of convulsions, whereas damage to the nucleus basalis magnocellularis (NBM), nucleus accumbens, or both MS and NBM did not cause anticonvulsant effects. Saporin lesion of MS, DBN (horizontal limb), or MS + DBN had no anticonvulsant effects, suggesting that aspiration lesion of MS disrupted pathways beyond cholinergic ones. Severe aphagia/adipsia and reduced body weight occurred in rats with lesions in the septal area. In separate sham operated rats, a strong positive correlation was found between body weight and latency to onset of convulsions in response to soman. Thus, weight loss and a relatively high dose of soman (1.6 × LD 50) in this context may have masked potential anticonvulsant effects among some lesioned animals. It is inferred that MS and AT/piriform cortex occur as prime target areas for induction of seizures by soman.

Anticonvulsant and anxiolytic activity of FrPbAII, a novel GABA uptake inhibitor isolated from the venom of the social spider Parawixia bistriata (Araneidae: Araneae)

This study was aimed at determining the effects of FrPbAII (174 Da), a novel isolated component from Parawixia bistriata spider venom, in the CNS of Wistar rats. Considering that FrPbAII inhibits the high affinity GABAergic uptake in a dose-dependent manner, its anxiolytic and anticonvulsant effects were analyzed in well-established animal models. Injection of FrPbAII in the rat hippocampus induced a marked anxiolytic effect, increasing the occupancy in the open arms of the elevated plus maze (EC 50 = 0.09 μg/μl) and increasing the time spent in the lit area of the light–dark apparatus (EC 50 = 0.03 μg/μl). Anxiolytic effects were also observed considering the number of entries in the open arms of the EPM and in the lit compartment of the light–dark box. Interestingly, when microinjected bilaterally in the SNPr of freely moving rats, FrPbAII (0.6 μg/μl) effectively prevented seizures induced by the unilateral GABAergic blockade of Area tempestas (bicuculline, 0.75 μg/μl). This anticonvulsant effect was similar to that evoked by muscimol (0.1 μg/μl) and baclofen (0.6 μg/μl), but differed from that of the specific GAT1 inhibitor, nipecotic acid (0.7 μg/μl). This difference could be accounted either for the parallel action of FrPbAII over glycinergic transporters or to an inspecific activity on GABAergic transporters. Data from the present investigation might be pointing to a novel compound with interesting and yet unexplored pharmacological potential.

Pharmacological therapies against soman-induced seizures in rats 30 min following onset and anticonvulsant impact

Systemic administration does not allow a clear differentiation between the anticonvulsant properties of GABA A (γ-aminobutyric acid) modulators. For this reason, various GABA A modulators have previously been micro-infused into seizure controlling substrates (area tempestas, substantia nigra) in the rat brain as a screening method for potential systemic administration. The purpose of the present study was to examine the anticonvulsant impact of the GABAergic modulators muscimol, ethanol, and propofol (screened by micro-infusions) when each drug was combined with procyclidine and administered systemically. The results showed that all 3 combinations could effectively terminate soman-induced (100 μg/kg s.c.) seizures when administered 30–35 min after onset. Procyclidine and propofol were considered as the most relevant double regimen to replace a previous triple regimen (procyclidine, diazepam, pentobarbital) against soman-induced seizures. Additionally, it was shown that unilateral implantation of hippocampal electrodes resulted in increased resistance to aphagia/adipsia and neuropathology, but not to lethality following soman. Efficient pharmacological treatment of soman-induced seizures at an early stage (< 20 min) is crucial to avoid neuropathology and cognitive deficits.

Anticonvulsant effects of GABAA modulators microinfused into area tempestas or substantia nigra in rats exposed to soman

Enhancement of GABAergic neurotransmission has anticonvulsant effects against nerve agent-induced seizures. However, systemic administration of drugs with GABA(A) agonist-like effects does not differentiate well between their anticonvulsant impact. In the present study, GABA(A) modulating drugs (1 microl) were microinfused bilaterally into the seizure controlling substrates, substantia nigra (SN) or area tempestas (AT), of rats subjected to seizures induced systemically by soman (100 microg/kg). The results showed that infusion of ethanol (0.47 micromol) and propofol (20 microg) in both SN and AT icrohad anticonvulsant effects (prevention of seizures or increased latency to seizures). Anticonvulsant effects were also obtained when muscimol (120 ng) was infused into AT or when diazepam (5 microg) was infused into SN. Pentobarbital (50 microg) did not attenuate soman-elicited seizures in any of the injection sites. Results from control experiments showed that the effects from the microinfusions were site-specific, and that the absence of effects of pentobarbital was not a result of too low dose of the drug. The microinfusion technique may allow a more detailed examination of anticonvulsant properties of drugs than by the use of systemic administration.

AMPA receptor desensitization as a determinant of vulnerability to focally evoked status epilepticus

Within the area tempestas (AT) in the anterior piriform cortex, unilateral microinfusions of GABA receptor antagonists and glutamate receptor agonists trigger brief episodic limbic seizures. In the present study, we document a synergistic effect of coinfusing bicuculline (GABAA receptor antagonist) with either carbachol (muscarinic receptor agonist) or cyclothiazide (inhibitor of AMPA receptor desensitization) but not with glutamate receptor agonists (AMPA, NMDA or kainate) in the rat AT. In particular, coadministration of bicuculline (118 pmol) with either carbachol (328 pmol) or cyclothiazide (1.2 nmol) triggered continuous self-sustaining seizures (status epilepticus; SE). Cyclothiazide alone did not evoke seizures. Although blockade of NMDA receptors with AP-7 (100 or 500 pmol) prevented episodic seizures evoked by carbachol or bicuculline alone, it was without effect on the continuous seizures evoked by combined treatments. NMDA-insensitive self-sustaining seizures were also evoked by the combination of AMPA and cyclothiazide. Regardless of the mechanism by which SE was evoked, it was prevented only by an AMPA receptor antagonist, NBQX, thus reinforcing the crucial role of AMPA receptors in the transition to SE. Further evidence for AMPA receptor regulation of seizure severity came from the overexpression of the GluR1 AMPA receptor subunit in AT. This resulted in substantially increased severity of bicuculline-evoked seizures that was reversed by focal application of NBQX. Thus, desensitization of AMPA receptors appears to limit the duration and severity of seizure activity, and a failure of this mechanism, or an overabundance of slowly desensitizing AMPA receptors, predisposes to severe and prolonged seizures.

A anticonvulsive fraction from Scaptocosa raptoria (Araneae: Lycosidae) spider venom

Several spider neurotoxins are known to show highly selective effects on nervous tissues. Intracerebral injection into rats of spider venom from Scaptocosa raptoria, prevents seizures induced by convulsant agents. Injection of phenytoin (390 pmol/200 nl), muscimol (90 pmol/200 nl), baclofen (500 pmol/200 nl) into the substantia nigra (SN) pars reticulata, protected rats from convulsions evoked by unilateral focal injection of bicuculline into the area tempestas by 50, 80, and 100%, respectively. Denatured S. raptoria crude venom (4.6 μg, 2.3 μg, and 920 ng/200 nl), when administered into the SN, prevented seizures elicited by bicuculline in the area tempestas by 100, 100, and 87.5%, respectively. The injection into the SN of 160 ng/200 nl of fraction SrTx1 isolated from S. raptoria venom, reduced the magnitude of seizures. This fraction was rechromatographed affording fractions SrTx1.1, SrTx1.2 and SrTx1.3, and they were administered into the SN at doses of 100, 200, and 400 ng/200 nl respectively. Fraction SrTx1.3 protected 50, 85.7, and 100% of the animals against the seizures elicited by bicuculline injected into the area tempestas. This suggests that S. raptoria venom as well as its SrTx1.3 fraction, might be potential sources of new anticonvulsant drugs.

Comparative fos immunoreactivity in the brain after forebrain, brainstem, or combined seizures induced by electroshock, pentylenetetrazol, focally induced and audiogenic seizures in rats

To help discern sites of focal activation during seizures of different phenotype, the numbers of Fos immunoreactive (FI) neurons in specific brain regions were analyzed following “brainstem-evoked,” “forebrain-evoked” and forebrain/brainstem combination seizures induced by a variety of methods. First, pentylenetetrazol (PTZ, 50 mg/kg) induced forebrain-type seizures in some rats, or forebrain seizures that progressed to tonic/clonic brainstem-type seizures in other rats. Second, minimal electroshock induced forebrain seizures whereas maximal electroshock (MES) induced tonic brainstem-type seizures in rats. Third, forebrain seizures were induced in genetically epilepsy-prone rats (GEPRs) by microinfusion of bicuculline into the area tempestas (AT), while brainstem seizures in GEPRs were induced by audiogenic stimulation. A final set was included in which AT bicuculline-induced forebrain seizures in GEPRs were transiently interrupted by audiogenic seizures (AGS) in the same animals. These animals exhibited a sequence combination of forebrain clonic seizure, brainstem tonic seizure and back to forebrain clonic seizures. Irrespective of the methods of induction, clonic forebrain- and tonic/clonic brainstem-type seizures were associated with considerable Fos immunoreactivity in several forebrain structures. Tonic/clonic brainstem seizures, irrespective of the methods of induction, were also associated with FI in consistent brainstem regions. Thus, based on Fos numerical densities (FND, numbers of Fos-stained profiles), forebrain structures appear to be highly activated during both forebrain and brainstem seizures; however, facial and forelimb clonus characteristic of forebrain seizures are not observable during a brainstem seizure. This observation suggests that forebrain-seizure behaviors may be behaviorally masked during the more severe tonic brainstem seizures induced either by MES, PTZ or AGS in GEPRs. This suggestion was corroborated using the sequential seizure paradigm. Similar to findings using MES and PTZ, forebrain regions activated by AT bicuculline were similar to those activated by AGS in the GEPR. However, in the combination seizure group, those areas that showed increased FND in the forebrain showed even greater FND in the combination trial. Likewise, those areas of the brainstem showing FI in the AGS model, showed an even greater effect in the combination paradigm. Finally, the medial amygdala, ventral hypothalamus and cortices of the inferior colliculi showed markedly increased FND that appeared dependent upon activation of both forebrain and brainstem seizure activity in the same animal. These findings suggest these latter areas may be transitional areas between forebrain and brainstem seizure interactions. Collectively, these data illustrate a generally consistent pattern of forebrain Fos staining associated with forebrain-type seizures and a consistent pattern of brainstem Fos staining associated with brainstem-type seizures. Additionally, these data are consistent with a notion that separate seizure circuitries in the forebrain and brainstem mutually interact to facilitate one another, possibly through involvement of specific “transition mediating” nuclei.