Anticonvulsant effects of GABA(A) modulators microinfused into area tempestas or substantia nigra in rats exposed to soman

The piriform, perirhinal, and entorhinal cortex in seizure generation

Understanding neural network behavior is essential to shed light on epileptogenesis and seizure propagation. The interconnectivity and plasticity of mammalian limbic and neocortical brain regions provide the substrate for the hypersynchrony and hyperexcitability associated with seizure activity. Recurrent unprovoked seizures are the hallmark of epilepsy, and limbic epilepsy is the most common type of medically-intractable focal epilepsy in adolescents and adults that necessitates surgical evaluation. In this review, we describe the role and relationships among the piriform (PIRC), perirhinal (PRC), and entorhinal cortex (ERC) in seizure-generation and epilepsy. The inherent function, anatomy, and histological composition of these cortical regions are discussed. In addition, the neurotransmitters, intrinsic and extrinsic connections, and the interaction of these regions are described. Furthermore, we provide evidence based on clinical research and animal models that suggest that these cortical regions may act as key seizure-trigger zones and, even, epileptogenesis.

Miscellaneous central nervous system intoxicants

The topic of central nervous system intoxicants encompasses a multitude of agents. This article focuses on three classes of therapeutic drugs, with specific examples in which overdoses require admission to the intensive care unit. Included are some of the newer antidepressants, the atypical neuroleptic agents, and selected anticonvulsant drugs. The importance of understanding pertinent physiology and applicable supportive care is emphasized.

Therapeutic devices for epilepsy

Therapeutic devices provide new options for treating drug-resistant epilepsy. These devices act by a variety of mechanisms to modulate neuronal activity. Only vagus nerve stimulation (VNS), which continues to develop new technology, is approved for use in the United States. Deep brain stimulation of anterior thalamus for partial epilepsy recently was approved in Europe and several other countries. Responsive neurostimulation, which delivers stimuli to 1 or 2 seizure foci in response to a detected seizure, recently completed a successful multicenter trial. Several other trials of brain stimulation are in planning or underway. Transcutaneous magnetic stimulation (TMS) may provide a noninvasive method to stimulate cortex. Controlled studies of TMS are split on efficacy, which may depend on whether a seizure focus is near a possible region for stimulation. Seizure detection devices in the form of shake detectors via portable accelerometers can provide notification of an ongoing tonic-clonic seizure, or peace of mind in the absence of notification. Prediction of seizures from various aspects of electroencephalography (EEG) is in early stages. Prediction appears to be possible in a subpopulation of people with refractory seizures, and a clinical trial of an implantable prediction device is underway. Cooling of neocortex or hippocampus reversibly can attenuate epileptiform EEG activity and seizures, but engineering problems remain in its implementation. Optogenetics is a new technique that can control excitability of specific populations of neurons with light. Inhibition of epileptiform activity has been demonstrated in hippocampal slices, but use in humans will require more work. In general, devices provide useful palliation for otherwise uncontrollable seizures, but with a different risk profile than with most drugs. Optimizing the place of devices in therapy for epilepsy will require further development and clinical experience.

Combined diazepam and HDAC inhibitor treatment protects against seizures and neuronal damage caused by soman exposure

The occurrence of status epilepticus (SE) is considered the main cause of brain lesions and morphological alterations, such as hippocampal neuron loss, that result in chronic epilepsy. Previous work demonstrated the convulsive and widespread neuropathological effects of soman, an organophosphorus compound that causes SE and severe recurrent seizures as a result of exposure. Seizures begin rapidly after exposure, can continue for hours, and contribute to prolonged physical incapacitation of the victim. This study attempts to identify anticonvulsive and neuroprotective drugs against soman exposure. Male Sprague-Dawley rats were exposed to 1.0 LD(50) soman. EEGraphical and neuropathological (Fluoro-Jade B staining) effects were analyzed at 72 h post-exposure to soman and subsequent treatments with diazepam (DZP) alone or in combination with histone deacetylase inhibitors, suberoylanilide hydroxamic acid (SAHA) or valproic acid (VPA). The extent of brain damage was dependent on the length of SE and not on the number of recurrent seizures. DZP treatment alone decreased SE time and damage in hippocampus, amygdala, thalamus and cortex, but not in piriform nuclei. The combination of DZP and VPA 100 mg/kg showed more anticonvulsive effects, decreased SE time, and afforded more neuroprotection in the hippocampus, mainly the ventral portion. The combination DZP and SAHA 25 mg/kg was more neuroprotective, but not more anticonvulsant than DZP alone. The DZP combination with VPA HDAC inhibitor proved to be a good treatment for SE and neuronal damage caused by soman exposure.

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.

Dose-dependent anticonvulsive effect of ethanol on lindane-induced seizures in rats

Chronic ethanol consumption is a major risk factor for epilepsy, and seizures frequently occur during the withdrawal period. The aim of our study was to investigate effects of ethanol on lindane-induced seizures in rats. Male Wistar rats were injected i.p. with one of the following 5 treatments: (i) saline, (ii) dimethylsulfoxide, (iii) lindane (8 mg/kg) (L), (iv) ethanol in doses of 0.5 g/kg (E(0.5)), 1 g/kg (E(1)), and 2 g/kg (E(2)), and (v) groups that received ethanol 30 min before lindane (LE(0.5), LE(1), and LE(2)). Behavioral changes were described by using a descriptive scale as follows: 0, no response; 1, head nodding, lower jaw twitching; 2, myoclonic body jerks, bilateral forelimb clonus; 3, generalized tonic-clonic convulsions; 4, status epilepticus. The incidence of convulsions in the LE(2) group was significantly lower than the incidence in the L (p < 0.01) and LE(0.5) groups (p < 0.05). The median grade of convulsive behavior was significantly lower in the LE(2) (p < 0.01) and LE(1) groups (p < 0.05) compared with the L group. Latencies to the first seizure response were not significantly different among groups. ED50 of ethanol was 1.40 (1.19-1.65). Our findings suggest that ethanol decreased severity and incidence of lindane-induced seizures in a dose-dependent manner.

Neuronal structures involved in the induction and propagation of seizures caused by nerve agents: Implications for medical treatment

In epilepsy research, studies have been made to identify brain areas critical for triggering and/or controlling propagated seizure activity. The purpose of the present study was to focus on a similar approach in nerve agent research by reviewing relevant literature to map potential trigger sites and propagation pathways for seizures. The piriform cortex and medial septal area emerge as prime target areas for soman-induced seizures. The cholinergic hyperactivation in the latter structures seems to induce increased glutamatergic activity in the piriform, entorhinal, and perirhinal cortices along with the hippocampal region. For prophylactic or early treatment, mapping of muscarinic subreceptors in the piriform cortex and medial septum would be guiding for designing anticholinergic drugs with optimal properties. Sustained seizures governed by glutamatergic over-activity may primarily be terminated by drugs with optimal glutamatergic antagonism primarily in the piriform, entorhinal, and perirhinal cortices. Studies of radiolabeled ligands to map subreceptors may provide specification of wanted drug properties to guide the choice among existing agents or to synthesize novel ones.

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 x 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.

The combination of donepezil and procyclidine protects against soman-induced seizures in rats

Current treatment of nerve agent poisoning consists of prophylactic administration of pyridostigmine and therapy using atropine, an oxime and a benzodiazepine. Pyridostigmine does however not readily penetrate the blood-brain barrier giving ineffective protection of the brain against centrally mediated seizure activity. In this study, we have evaluated donepezil hydrochloride, a partial reversible inhibitor of acetylcholinesterase (AChE) clinically used for treating Alzheimer's disease, in combination with procyclidine, used in treatment of Parkinson's disease and schizophrenia, as prophylaxis against intoxication by the nerve agent soman. The results demonstrated significant protective efficacy of donepezil (2.5 mg/kg) combined with procyclidine (3 or 6 mg/kg) when given prophylactically against a lethal dose of soman (1.6 x LD(50)) in Wistar rats. No neuropathological changes were found in rats treated with this combination 48 h after soman intoxication. Six hours after soman exposure cerebral AChE activity and acetylcholine (ACh) concentration was 5% and 188% of control, respectively. The ACh concentration had returned to basal levels 24 h after soman intoxication, while AChE activity had recovered to 20% of control. Loss of functioning muscarinic ACh receptors (17%) but not nicotinic receptors was evident at this time point. The recovery in brain AChE activity seen in our study may be due to the reversible binding of donepezil to the enzyme. Donepezil is well tolerated in humans, and a combination of donepezil and procyclidine may prove useful as an alternative to the currently used prophylaxis against nerve agent intoxication.

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 GABAA (gamma-aminobutyric acid) modulators. For this reason, various GABAA 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 microg/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.