Parenteral penicillin in rats: an experimental model of multifocal epilepsy

Neuronal Firing and Waveform Alterations through Ictal Recruitment in Humans

Analyzing neuronal activity during human seizures is pivotal to understanding mechanisms of seizure onset and propagation. These analyses, however, invariably using extracellular recordings, are greatly hindered by various phenomena that are well established in animal studies: changes in local ionic concentration, changes in ionic conductance, and intense, hypersynchronous firing. The first two alter the action potential waveform, whereas the third increases the "noise"; all three factors confound attempts to detect and classify single neurons. To address these analytical difficulties, we developed a novel template-matching-based spike sorting method, which enabled identification of 1239 single neurons in 27 patients (13 female) with intractable focal epilepsy, that were tracked throughout multiple seizures. These new analyses showed continued neuronal firing with widespread intense activation and stereotyped action potential alterations in tissue that was invaded by the seizure: neurons displayed increased waveform duration (p < 0.001) and reduced amplitude (p < 0.001), consistent with prior animal studies. By contrast, neurons in "penumbral" regions (those receiving intense local synaptic drive from the seizure but without neuronal evidence of local seizure invasion) showed stable waveforms. All neurons returned to their preictal waveforms after seizure termination. We conclude that the distinction between "core" territories invaded by the seizure versus "penumbral" territories is evident at the level of single neurons. Furthermore, the increased waveform duration and decreased waveform amplitude are neuron-intrinsic hallmarks of seizure invasion that impede traditional spike sorting and could be used as defining characteristics of local recruitment.SIGNIFICANCE STATEMENT Animal studies consistently show marked changes in action potential waveform during epileptic discharges, but acquiring similar evidence in humans has proven difficult. Assessing neuronal involvement in ictal events is pivotal to understanding seizure dynamics and in defining clinical localization of epileptic pathology. Using a novel method to track neuronal firing, we analyzed microelectrode array recordings of spontaneously occurring human seizures, and here report two dichotomous activity patterns. In cortex that is recruited to the seizure, neuronal firing rates increase and waveforms become longer in duration and shorter in amplitude as the neurons are recruited to the seizure, while penumbral tissue shows stable action potentials, in keeping with the "dual territory" model of seizure dynamics.

Glutamate-Mediated Blood-Brain Barrier Opening: Implications for Neuroprotection and Drug Delivery

The blood-brain barrier is a highly selective anatomical and functional interface allowing a unique environment for neuro-glia networks. Blood-brain barrier dysfunction is common in most brain disorders and is associated with disease course and delayed complications. However, the mechanisms underlying blood-brain barrier opening are poorly understood. Here we demonstrate the role of the neurotransmitter glutamate in modulating early barrier permeability in vivo Using intravital microscopy, we show that recurrent seizures and the associated excessive glutamate release lead to increased vascular permeability in the rat cerebral cortex, through activation of NMDA receptors. NMDA receptor antagonists reduce barrier permeability in the peri-ischemic brain, whereas neuronal activation using high-intensity magnetic stimulation increases barrier permeability and facilitates drug delivery. Finally, we conducted a double-blind clinical trial in patients with malignant glial tumors, using contrast-enhanced magnetic resonance imaging to quantitatively assess blood-brain barrier permeability. We demonstrate the safety of stimulation that efficiently increased blood-brain barrier permeability in 10 of 15 patients with malignant glial tumors. We suggest a novel mechanism for the bidirectional modulation of brain vascular permeability toward increased drug delivery and prevention of delayed complications in brain disorders.

SIGNIFICANCE STATEMENT

In this study, we reveal a new mechanism that governs blood-brain barrier (BBB) function in the rat cerebral cortex, and, by using the discovered mechanism, we demonstrate bidirectional control over brain endothelial permeability. Obviously, the clinical potential of manipulating BBB permeability for neuroprotection and drug delivery is immense, as we show in preclinical and proof-of-concept clinical studies. This study addresses an unmet need to induce transient BBB opening for drug delivery in patients with malignant brain tumors and effectively facilitate BBB closure in neurological disorders.

The DNT-EST: a predictive embryonic stem cell-based assay for developmental neurotoxicity testing in vitro

As the developing brain is exquisitely vulnerable to chemical disturbances, testing for developmental neurotoxicity of a substance is an important aspect of characterizing its tissue specific toxicity. Mouse embryonic stem cells (mESCs) can be differentiated toward a neural phenotype, and this can be used as a model for early brain development. We developed a new in vitro assay using mESCs to predict adverse effects of chemicals and other compounds on neural development - the so-called DNT-EST. After treatment of differentiating stem cells for 48h or 72h, at two key developmental stages endpoint for neural differentiation, viability, and proliferation were assessed. As a reference, we similarly treated undifferentiated stem cells 2 days after plating for 48h or 72h in parallel to the differentiating stem cells. Here, we show that chemical testing of a training set comprising nine substances (six substances of known developmental toxicity and three without specific developmental neurotoxicity) enabled a mathematical prediction model to be formulated that provided 100% predictivity and accuracy for the given substances, including in leave-one-out cross-validation. The described test method can be performed within two weeks, including data analysis, and provides a prediction of the developmental neurotoxicity potency of a substance.

The antiepileptic effect of low-dose amino-phosphono-valeric acid (APV) is not enhanced by phosphatidylserine association

We investigated the effects of the NMDA antagonist amino-phosphono-valeric acid (APV), alone or in combination with phosphatidylserine (PS) in the penicillin model of epilepsy. After penicillin injection, rats were treated i.p. with either APV alone (5 mg/Kg) or APV (5 mg/Kg) + PS (740 mg/Kg). EEG epileptic activity decreased significantly in the group treated with APV alone, even at the very low dose used. This effect was not further enhanced by PS, suggesting that the previously reported effects of PS on GABA activity may be related to a specific interaction between these compounds.

Seizure exacerbation related to beta-lactam antibiotics in a child with cerebral dysgenesis

A three-year-old boy with right hemimegalencephaly, developmental delay and left hemiparesis had well-controlled left focal seizures. Occasionally he had generalized seizures if he was given beta-lactam antibiotics. An EEG with simultaneous penicillin injection was performed to examine this apparent association: a secondarily generalized seizure was documented within two minutes of intravenous administration of the penicillin. This procedure confirmed the suspicion of a causal link between antibiotic treatment and seizures. The factors increasing susceptibility to beta-lactam antibiotic-induced seizures are reviewed.

Chemical models of epilepsy with some reference to their applicability in the development of anticonvulsants

This paper reviews chemical models of epilepsy and their relevance in the identification and characterization of anticonvulsants. For each convulsant we discuss possible modes of administration, clinical type(s) of seizures induced, proposed mechanism(s) of epileptogenesis and, where available, responsiveness of the induced seizures to anticonvulsants. The following compounds are reviewed: pentylenetetrazol, bicuculline, penicillin, picrotoxin, beta-carbolines, 3-mercaptopropionic acid, hydrazides, allylglycine; the glycine antagonist strychnine; gamma-hydroxybutyrate; excitatory amino acids (glutamate, aspartate, N-methyl-D-aspartate, quisqualate, kainate, quinolinic acid); monosubstituted guanidino compounds, metals (alumina, cobalt, zinc, iron); neuropeptides (opioid peptides, corticotropin releasing factor, somatostatin, vasopressin); cholinergic agents (acetylcholine, acetylcholinesterase inhibitors, pilocarpine); tetanus toxin; flurothyl; folates; homocysteine and colchicine. Although there are a multitude of chemical models of epilepsy, only a limited number are applied in the routine screening of potential anticonvulsants. Some chemical models have a predictive value with regard to the clinical profile of efficacy of the tested anticonvulsants. Some chemical models may contribute to a better understanding of possible mechanisms of epileptogenesis.

Aggravation of penicillin-induced epilepsy in rats with locus ceruleus lesions

The rate and pattern of development of seizures induced by penicillin injected intraperitoneally were determined in rats that had been depleted of brain norepinephrine (NE) by bilateral injections of the neurotoxin 6-OH dopamine into the locus ceruleus. Behavioral observations and scalp electrographic recordings were made after injection and the efficacy of NE depletion was determined by high performance liquid chromatography measurement of cortical levels of NE and its metabolites. We found that in comparison to sham-operated control rats, NE-depleted rats had a significantly shorter latency to first observable myoclonic jerk, the first epileptic discharge, the first convulsion with sustained epileptic discharges, and a longer duration of convulsions. We observed a similar electrographic pattern of multifocal spikes with bilateral synchrony in both groups. However, more of the control rats (six of 12) had convulsions as compared to the lesioned rats (four of 12). These findings are consistent with previous evidence that depletion of neocortical NE facilitates the development of epileptiform activity in the CNS; however, a convulsive state was not induced by NE depletion.

The excitatory amino acid antagonist amino-phosphono-valeric acid (APV) provides protection against penicillin-induced epileptic activity in the rat

The effects of intraperitoneal injection of 2-amino-5-phosphono-valeric acid (APV) on EEG-monitored penicillin-induced epileptic activity in rats were evaluated. A significant decrease in the frequency of spikes occurred with low APV dosages (10 and 20 mg/kg), while an almost complete disappearance of spike activity was observed at higher APV doses (40 and 160 mg/kg). Our data suggest that excitatory amino acids play a relevant role in penicillin-induced epileptic activity in rats.

Animal models of the epilepsies

The study of mechanisms of the epilepsies requires employment of animal models. Choice of a model system depends upon several factors, including the question to be studied, the type of epilepsy to be modelled, familiarity and convenience. Over 50 models are reviewed. Major categories of models are those for simple partial seizures: topical convulsants, acute electrical stimulation, cortically implanted metals, cryogenic injury; for complex partial seizures: kainic acid, tetanus toxin, injections into area tempesta, kindling, rodent hippocampal slice, isolated cell preparations, human neurosurgical tissue; for generalized tonic-clonic seizures: genetically seizure-prone strains of mouse, rat, gerbil, fruitfly and baboon, maximal electroshock seizures, systemic chemical convulsants, metabolic derangements; and for generalized absence seizures: thalamic stimulation, bilateral cortical foci, systemic penicillin, gamma-hydroxy-butyrate, intraventricular opiates, genetic rat models. The lithium-pilocarpine, homocysteine and rapid repetitive stimulation models are most useful in studies of status epilepticus. Key findings learned from each of the models, the model's strengths and weaknesses are detailed. Interpretation of findings from each of these models can be difficult. Do results pertain to the epilepsies or to the particular model under study? How important are species differences? Which clinical seizure type is really being modelled? In a model are behavior or EEG findings only similar superficially to epilepsy, or are the mechanisms comparable? The wealth of preparations available to model the epilepsies underscores the need for unifying themes, and for better understanding of basic mechanisms of the epilepsies.

The THIP-induced model of bilateral synchronous spike and wave in rodents

Intraperitoneal administration of 5-10 mg/kg of THIP (4,5,6,7 tetrahydroxyisoxazolo (4,5,c) pyridine 3-ol) induced a transient, reliable model of bilaterally synchronous spikes and waves in rats. The paroxysmal bursts elicited by THIP had similar topographical distribution to the spontaneously-occurring spike and wave discharges often observed in naive rats. The responsiveness to electrical stimulation of subcortical nuclei was different in the two models. Systemic administration of THIP provided a simple, reliable model of bilaterally synchronous spike and wave discharges that may involve the same cerebral structures as those involved in the generation of the spontaneous paroxysms. The administration of the drug increased the yield of paroxysmal responses and provided a stable and predictable 4 hr test situation that may easily be quantified.

Liposome-entrapped gamma-aminobutyric acid inhibits isoniazid-induced epileptogenic activity in rats

Sprague-Dawley rats with interictal and ictal spike activity induced by intraperitoneally injected isoniazid (INH) were treated, 5 min before or 30 min later, with liposome-entrapped gamma-aminobutyric acid (GABA) (LEG) or GABA or phosphatidylserine. Crossover injections were given in random sequence and INH alone ws also injected in every animal as a control. LEG inhibited either seizures or interictal spikes in both groups. No decrease of epileptogenic activity was seen after GABA or phosphatidylserine treatment alone. It is suggested that LEG could contribute to the reconstitution of the GABA pool decreased by INH.

Parenteral penicillin epilepsy: tolerance to subsequent treatments

We recorded spike activity induced by i.p. penicillin injections in eight rats, repeating the experiment five times per each rat, at 48-h intervals. Spike frequency was significantly reduced in all sessions subsequent to the first, without changes in penicillin blood concentrations. The penicillin model of epilepsy is therefore not advisable in studies on antiepileptic drugs with a crossover design.

Neuronal firing patterns during generalized penicillin epilepsy in the awake cat

Single unit activity was recorded in the neocortex of awake cats during spontaneous spike wave discharges. Ongoing activity was interrupted during paroxysmal events and was replaced by (1) bursts of action potentials coincident with the EEG spike and cessation of firing during the wave, or (2) cessation of firing for the duration of the paroxysm. Other units which displayed little or no background activity were recruited to fire during the EEG spike. Surface cerebellar stimulation at high frequencies led to decreased neuronal activity while single pulse shocks resulted in short latency activation. These results were compared to studies of spike wave activity elicited by electrical stimulation.