Anticonvulsant and neurotoxic effects of intracerebroventricular injection of phenytoin, phenobarbital and carbamazepine in an amygdala-kindling model of epilepsy in the rat

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

A novel kindling model of temporal lobe epilepsy in rhesus monkeys induced by Coriaria lactone

One of the major challenges in developing novel therapeutics for human epileptic disorders derives from the limitation of knowledge of the processes by which epilepsy is generated (epileptogenesis). Furthermore, the inability to obtain human samples at the early stage of epilepsy hinders studies designed to further understand epileptogenesis. Thus, an effective animal model is critical for studies investigating this process. The purpose of this study was to establish a new primate kindling model of temporal lobe epilepsy (TLE) as an animal model of epileptogenesis. Here, repeated injections of Coriaria lactone (CL) at a subthreshold dose elicited partial seizures that culminated in secondarily generalized tonic-clonic seizures. The sequence of events and features of the behaviors observed in this model simulated those observed in human TLE. Electroencephalogram monitoring revealed the temporal lobe origins of the epileptiform potentials, which were consistent with the behavioral changes observed. A total of 7 rhesus monkeys (78%) were kindled with a median of 48 (41 to 60) CL injections. Both the seizure-induction and mortality rates were dose-dependent. A CL injection at 1.50mg/kg showed the lowest animal mortality rate (0%) and the highest seizure-induction rate (100%). Extensive kindling by CL injections with a median of 97 injections (overkindling) subsequently resulted in the recurrence of spontaneous seizures in rhesus monkeys with frequency patterns that were similar to those observed in human TLE. In addition, rhesus monkeys subjected to large numbers of kindling stimuli displayed mitochondrial damage and astrocyte activation in a pattern that was similar to the neuropathological changes characteristic of human TLE. Thus, a kindling TLE model in rhesus monkeys representing a primate animal model of epileptogenesis was established for the first time using repeated intramuscular injections of Coriaria lactone. This model was easily and efficiently performed and resulted in behavioral, electrographical, and anatomical characteristics of human TLE. Thus, this model might be used in future investigations of the mechanisms involved in the epileptogenesis of TLE and in the development of new antiepileptic drugs.

Pharmacological and neuroethological studies of three antiepileptic drugs in the Genetic Audiogenic Seizure Hamster (GASH:Sal)

Epilepsy modeling is essential for understanding the basic mechanisms of the epileptic process. The Genetic Audiogenic Seizure Hamster (GASH:Sal) exhibits generalized tonic-clonic seizures of genetic origin in response to sound stimulation and is currently being validated as a reliable model of epilepsy. Here, we performed a pharmacological and neuroethological study using well-known and widely used antiepileptic drugs (AEDs), including phenobarbital (PB), valproic acid (VPA), and levetiracetam (LEV). The intraperitoneal administration of PB (5-20mg/kg) and VPA (100-300mg/kg) produced a dose-dependent decrease in GASH:Sal audiogenic seizure severity scores. The administration of LEV (30-100mg/kg) did not produce a clear effect. Phenobarbital showed a short plasmatic life and had a high antiepileptic effect starting at 10mg/kg that was accompanied by ataxia. Valproic acid acted only at high concentrations and was the AED with the most ataxic effects. Levetiracetam at all doses also produced sedation and ataxia side effects. We conclude that the GASH:Sal is a reliable genetic model of epilepsy suitable to evaluate AEDs.

Novel methods of antiepileptic drug delivery -- polymer-based implants

Epilepsy is a neurological disorder characterised by spontaneous seizures. Over one third of patients receive insufficient benefit from oral anti-epileptic drug (AED) therapy, and continue to experience seizures whilst on medication. Epilepsy researchers are consequently seeking new ways to deliver AEDs directly to the seizure focus in the brain in order to deliver higher, more effective doses to the seizure focus whilst bypassing the remainder of the brain and body to prevent side effects. The focus of this review will be polymer-based implants, which are polymeric devices loaded with AED that are designed for implantation at the seizure focus in order to achieve gradual, continuous release of AED direct into the region of the brain responsible for seizures. Polymer-based implants produced for epilepsy to date are based on a range of polymers, both biodegradable and non-biodegradable, and range from simple materials development studies through to investigations of implants in animal models of seizures and epilepsy, with varying degrees of success. This review describes the range of methods employed to manufacture polymer-based implants and compares their advantages and potential appeal to industry, and describes and compares the results and successes of polymer-based materials and devices produced to date for the treatment of epilepsy. We also discuss disadvantages and hurdles to be overcome in the field, and describe our predictions for advances to be made in the field in the coming decade.

Intraventricular and intracerebral delivery of anti-epileptic drugs in the kindling model

A means to avoid the pharmacokinetic problems affecting the anti-epileptic drugs may be their direct intracerebroventricular (ICV) or intracerebral delivery. This approach may achieve a greater drug concentration at the epileptogenic area while minimizing it in other brain or systemic areas, and thus it could be an interesting therapeutic alternative in drug-resistant epilepsies. The objective of this article is to review a series of experiments, ranging from actute ICV injection to continuous intracerebral infusion of anti-epileptic drugs or grafting of neurotransmitter producing cells, in experimental models, especially in the kindling model of epilepsy in the rat. Acute ICV injection of phenytoin, phenobarbital or carbamacepine is able to diminish the intensity of kindling seizures, but it is also associated with a high neurologic toxicity, especially phenobarbital. Continuous ICV infusion of anti-epileptic drugs can effectively control seizures, but neurologic toxicity is not improved compared with systemic delivery. However, systemic toxicity may be improved, as in the case of valproic acid, whose continuous ICV infusion results in very low plasmatic or hepatic drug concentrations. Continuous intracerebral infusion at the epileptogenic area was studied as an alternative to minimize neurologic toxicity. Thus, intra-amygdalar infusion of gamma-aminobutyric acid (GABA) controls seizures with minimal neurotoxicity in amygdala-kindled rats. Similarly, continuous infusion of GABA into the dorsomedian nucleus of the thalamus improves seizure spread, while not affecting the local epileptogenic activity at the amygdala. Grafting of GABA releasing cells may reduce kindling parameter severity without behavioral side effects. We may conclude that ICV or intracerebral delivery of anti-epileptic drugs or neurotransmitters may be a useful technique to modulate epilepsy.

Prolonged attenuation of amygdala-kindled seizure measures in rats by convection-enhanced delivery of the N-type calcium channel antagonists omega-conotoxin GVIA and omega-conotoxin MVIIA

Convection-enhanced delivery (CED) permits the homogeneous distribution of therapeutic agents throughout localized regions of the brain parenchyma without causing tissue damage as occurs with bolus injection. Here, we examined whether CED infusion of the N-type calcium channel antagonists omega-conotoxin GVIA (omega-CTX-G) and omega-conotoxin MVIIA (omega-CTX-M) can attenuate kindling measures in fully amygdala-kindled rats. Rats were implanted with a combination infusion cannula-stimulating electrode assembly into the right basolateral amygdala. Fully kindled animals received infusions of vehicle, omega-CTX-G (0.005, 0.05, and 0.5 nmol), omega-CTX-M (0.05, 0.15, and 0.5 nmol), proteolytically inactivated omega-CTX-M (0.5 nmol), or carbamazepine (500 nmol) into the stimulation site. CED of omega-CTX-G and omega-CTX-M over a 20-min period resulted in a dose-dependent increase in the afterdischarge threshold and a decrease in the afterdischarge duration and behavioral seizure score and duration during a period of 20 min to 1 week after the infusion, indicating an inhibitory effect on the triggering and expression of kindled seizures. The protective effects of omega-conotoxins reached a maximum at 48 h postinfusion, and then they gradually resolved over the next 5 days. In contrast, carbamazepine was active at 20 min but not at 24 h after the infusion, whereas CED of vehicle or inactivated omega-CTX-M had no effect. Except for transient tremor in some rats receiving the highest toxin doses, no adverse effects were observed. These results indicate that local CED of high-molecular-weight presynaptic N-type calcium channel blockers can produce long-lasting inhibition of brain excitability and that they may provide prolonged seizure protection in focal seizure disorders.

Intraventricular administration of gabapentin in the rat increases flurothyl seizure threshold

INTRODUCTION

We investigated efficacy of prolonged intraventricular gabapentin (GBP) infusion in the rat flurothyl epilepsy model.

METHODS

Sprague-Dawley rats, under anesthesia, were implanted with bilateral Alzet model 2001 osmotic pumps. The pumps infused GBP 80 microg/microL (3.8 mg/day) or isotonic saline control at 1.0 microL/h into each ventricle for 5 days. After 5 days of GBP infusion, seizures were induced by flurothyl dripped onto filter paper. Time to first myoclonic jerk, first partial seizure and first tonic-clonic seizure was recorded by an observer unaware of the treatment group. Determination of seizures was behavioral.

RESULTS

Data were obtained from 54 rats. First tonic-clonic seizure was at 295.8+/-58.8s (n=28) for control rats, versus 338.0+/-89.9 s (n=26) for rats with GBP in the pump (p=0.049). First myoclonic jerk occurred at 158.7+/-20.8 versus 164.6+/-33.5 s (p=0.44, n.s.). Regression of time to seizure versus weight was not significant. No animal had measurable serum levels (<1 microg/ml) of GBP. The distribution of GBP in brain was not studied, but qualitative observations of methylene blue dye installed in the pumps showed dye in periventricular white matter and also over cortex, especially ipsilaterally.

DISCUSSION

GBP instilled into the lateral ventricles by pump for 5 days delays onset of generalized tonic-clonic seizures produced by flurothyl in the rat. Time to first myoclonic or partial seizure was not influenced. Effects were not due to systemic absorption of GBP. This study provides a proof-in-principle for intraventricular therapy with AEDs.

Effects of intracerebroventricularly-injected morphine on anxiety, memory retrieval and locomotor activity in rats: involvement of vasopressinergic system and nitric oxide pathway

Morphine has been shown to alter several behavioural processes. We aimed to investigate the effects of intracerebroventricular (i.c.v.) morphine on anxiety, memory retrieval and locomotor activity in rats and to elucidate the possible involvement of the vasopressinergic system and the nitric oxide (NO) pathway in these effects. Rats were pretreated with morphine (0.5, 5, 50 microg/5 microl; i.c.v.) or saline (5 microl; i.c.v.) 30 min before the elevated plus maze test, the probe trial of the Morris water maze and the open field test. Morphine (5 microg/5 microl; i.c.v.) induced significant anxiolytic effects in the elevated plus maze. None of the doses of morphine produced any effects in the probe trial of the Morris water maze and the open field. Pretreatment with an arginine vasopressin (AVP) V(1) receptor antagonist (25, 125 ng/5 microl; i.c.v.), an AVP V(2) receptor antagonist (25, 125 ng/5 microl; i.c.v.), or L-NAME, an NO synthase inhibitor (5, 25 microg/5 microl; i.c.v.) 30 min before morphine significantly prevented the anxiolytic effects of morphine. These results suggest that i.c.v. morphine has significant anxiolytic effects, probably mediated by both vasopressinergic system and NO pathway, but has no effect on memory retrieval or locomotor activity, at least at the applied doses.

Effect of intracerebroventricular continuous infusion of valproic acid versus single i.p. and i.c.v. injections in the amygdala kindling epilepsy model

Two protocols were tested to assess anticonvulsant efficacy and drug concentrations after intracerebroventricular (i.c.v.) continuous valproic acid (VPA) infusion, as compared with acute injections in the kindling epilepsy model. Protocol 1: amygdala-kindled rats were injected via intraperitoneal (i.p.) and i.c.v. routes with varying doses of VPA and tested for seizure intensity, afterdischarge and seizure duration, ataxia and sedation. Concentrations of VPA were determined by immunofluorescence in the brain, plasma, cerebrospinal fluid (CSF) and liver in matching rats. Protocol 2: amygdala-kindled rats were implanted with osmotic minipumps containing a VPA solution in saline and connected to intraventricular catheters for 7 days. Seizure threshold, latency and duration, afterdischarge duration, ataxia and sedation were recorded daily before, during, and until 5 days after VPA infusion. In matching animals, CSF, brain, plasma and liver VPA concentration was determined. Acute i.c.v. VPA injection suppressed seizures with a remarkable ataxia and sedation. However, continuous i.c.v. infusion controlled generalised and even focal seizures without producing important side effects, high plasma levels or hepatic drug concentrations. In conclusion, continuous i.c.v. VPA infusion may protect against kindled seizures by minimising ataxia and sedation, and achieving suitable intracerebral, yet low plasma or hepatic drug concentrations, thus avoiding potential systemic toxicity.

Aggravation of Absence Seizures by Carbamazepine in a Genetic Rat Model Does Not Induce Neuronal c-Fos Activation

The mechanisms underlying carbamazepine aggravation of absence seizures are uncertain but are thought to involve enhancement of neuronal activity within the thalamocortical circuitry. We used c-Fos immunohistochemistry (cFos-ir) to examine patterns of neuronal activation and the relationship to seizure expression following administration of carbamazepine in a rat model of absence epilepsy (Genetic Absence Epilepsy Rats of Strasbourg, GAERS). Female ovariectomized GAERS implanted with extradural EEG electrodes received either 20 mg/kg carbamazepine or vehicle IP. Seizure expression was quantified by measuring the total number and duration of spike-wave discharges (SWD) and with the individual burst discharge lengths over a 90-minute EEG. This was correlated with cFos-ir in thalamocortical slices from rats killed 180 minutes after carbamazepine administration. Carbamazepine-treated rats (n = 5) had a significantly greater total duration of SWD than vehicle-treated rats (17.9% versus 8.8%, P = 0.04). Despite this aggravation of seizures, the level of cFos-ir did not differ between the treatment groups. A positive correlation was found between cFos-ir in the reticularis thalami (Rt) and the total seizure duration (R = 0.66, P = 0.04) and mean burst length (R = 0.68, P = 0.03) but not total number of seizures. The lack of difference in cFos activation patterns between carbamazepine and vehicle-treated animals suggests that the mechanism for carbamazepine aggravation of absence seizures may not involve neuronal activation but rather enhanced neuronal synchronization. The association between increased neuronal activation in the Rt and seizure burden in GAERS provides further support for the critical role of this structure in the maintenance, but not initiation, of absence seizure activity.

Observer-rated ataxia: rating scales for assessment of genetic differences in ethanol-induced intoxication in mice

Identification of genetic and physiological mechanisms underlying a drug's or mutation's effects on motor performance could be aided by the existence of a simple observation-based rating scale of ataxia for mice. Rating scales were developed to assess ataxia after ethanol (2.75, 3.0, and 3.25 g/kg) in nine inbred mouse strains. Each scale independently rates a single behavior. Raters, blinded to dose, scored four behaviors (splay of hind legs, wobbling, nose down, and belly drag) at each of four time points after injection. The severities of hind leg splaying and wobbling were quantifiable, whereas nose down and belly dragging were expressed in all-or-none fashion. Interrater reliabilities were substantial (0.75 <or= r <or= 0.99). Splay scores (rated 0-5) displayed significant effects of strain, dose, and time point. Wobbling (rated 0-4) was dependent on strain and time point. Ethanol affected wobbling (most strains scored >0 at some time), but all doses were equally effective. Incidence of nose down and belly dragging behaviors increased strain dependently after ethanol, but strains did not differentially respond to dose. Ethanol-induced splaying was modestly, and negatively, genetically correlated with wobbling. Nose down and belly dragging tended to be associated with splaying and wobbling at later times. Four distinct ataxia-related behaviors were sensitive to ethanol. Strains differed in ethanol sensitivity for all measures. Modest strain mean correlations among behaviors indicate that these behaviors are probably under control of largely different genes and that ataxia rating scales should rate separate behaviors on discrete scales.

Continuous intra-amygdalar infusion of GABA in the amygdala kindling model of epilepsy in rat

OBJECTIVE

To explore the effect of continuous intra-amygdalar infusion of GABA in the amygdala kindling model of epilepsy in rat.

METHODS

An electrode and cannula complex was implanted in adult male Wistar rats, the electrode being targeted to the left basolateral amygdala. The animals were subjected to a standard kindling procedure. Osmotic minipumps filled with either GABA or mannitol were connected to cannulas and allowed to infuse during 7 days. Kindling experiments measuring after-discharge and seizure thresholds, seizure severity and duration, and behavioral toxicity were performed before, during and after the drug infusion period.

RESULTS

Both after-discharge and seizure thresholds were significantly increased both during and after GABA infusion with respect to pre-infusion controls, while the group receiving mannitol showed no significant differences. Seizure duration was not affected by GABA infusion. Seizures were either completely blocked or fully developed.

CONCLUSIONS

Direct continuous GABA infusion within the epileptogenic focus raises the seizure thresholds without affecting the generalized components in the amygdala kindling model of epilepsy.

The benzodiazepine partial inverse agonist Ro15-4513 alters anticonvulsant and lethal effects of carbamazepine in amygdala-kindled rats

Ro15-4513 (ethyl-8-azido-5,6-dihydro-5methyl-6-oxo-4H-imidazo-[1,5-a]-1,4-benzodiazepine-3-carboxylate), a benzodiazepine partial inverse agonist of the GABA(A) receptor, is known to protect against alcohol toxicities. The present study was designed to determine the role of Ro15-4513 in preventing anticonvulsant, toxic, and lethal effects of carbamazepine (CBZ) in amygdala-kindled rats. Acute treatment with CBZ (25 mg/kg, i.p.) produced anticonvulsant effects in fully kindled rats characterized by a significant decrease in afterdischarge and seizure duration and stage. Repeated administration of this high dose of CBZ induced sedation and high (56%) lethality. The anticonvulsant and sedative effects of CBZ were strikingly suppressed by pretreatment with Ro15-4513 (2.5 and 5 mg/kg, i.p.), and there was no mortality in animals co-administrated with Ro15-4513 during the entire experimental period. These results indicate that Ro15-4513 protects against CBZ-induced sedation and lethality, while suppressing the anticonvulsant effects of CBZ, suggesting a role for the GABA(A) receptor in CBZ efficacy and side effects. The potential clinical implications for CBZ-induced toxicity and overdose remain to be explored.