Siri Enger

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 20 min after onset of seizures evoked by soman (1.15 × LD(50)). 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 (125 mg/kg) to enhance survival and treatment started 40 min following seizure onset of a soman dose of 1.6 × LD(50). 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 5-10 min 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.

Roles of perirhinal and posterior piriform cortices in control and generation of seizures: A microinfusion study in rats exposed to soman

Identification of critical receptors in seizure controlling brain regions may facilitate the development of more efficacious pharmacological therapies against nerve agent intoxication. In the present study, a number of drugs with anticonvulsant potency were microinfused into the perirhinal cortex (PRC) or posterior piriform cortex (PPC) in rats. The drugs used exert cholinergic antagonism (scopolamine), glutamatergic antagonism (ketamine, NBQX), both cholinergic and glutamatergic antagonism (procyclidine, caramiphen), or GABAergic agonism (muscimol). The results showed that in the PRC anticonvulsant efficacy against soman-induced seizures (subcutaneously administered) was achieved by procyclidine or NBQX, but not by ketamine, scopolamine, caramiphen, or muscimol (Experiment 1). Hence, both muscarinic and glutamatergic NMDA receptors had to be antagonized simultaneously or AMPA receptors alone, suggesting increased glutamatergic activation in the PRC before onset of seizures. In the PPC, anticonvulsant effects were assured by scopolamine or muscimol, but not by procyclidine, caramiphen, NBQX, or ketamine (Experiment 2). Thus, muscarinic and GABA(A) receptors appear to be the critical ones in the PPC. Microinfusion of soman into the PRC or PPC resulted in sustained seizure activity in the majority of the rats of both infusion categories. The rhinal structures encompassed in this study apparently have critical functions as both control and trigger sites for nerve agent-evoked seizures.

Behavioral side effects in rats treated with acetylcholinesterase inhibitors suggested used as prophylactics against nerve agents

Acetylcholinesterase inhibitors in combination with an anticholinergic, particularly anticholinergics with antiglutamatergic properties, can effectively protect against nerve agent-induced seizures and lethality. The objective of the present study was to examine potential behavioral side effects of the anticholinesterases physostigmine (0.1mg/kg), galantamine (3mg/kg), huperzine (0.5mg/kg), and donepezil (2.5mg/kg) alone or each drug in combination with anticholinergic procyclidine (3mg/kg). The results showed that rats injected intraperitoneally with galantamine displayed a mild cognitive deficit in terms of reduced preference for novelty that was similarly found among animals treated with procyclidine combined with either galantamine or donepezil. Locomotor activity and rearing were radically depressed in all groups treated with anticholinesterases as well as in combination with procyclidine. Reductions in activity were most prominent for rats injected with galantamine alone. Equalizing effects of cholinesterase inhibitors and anticholinergics were absent in the present context. Findings from previous studies that both systemic and local (amygdala) application of physostigmine cause increased fear-motivated freezing response in rats, may explain the marked reductions in activity among the present rats. In view of these findings, use of anticholinesterases (crossing the blood-brain barrier) as prophylactics against nerve agents must be carefully examined to avoid severe side effects.

Anticonvulsant impact of lesions in the ventrolateral forebrain of rats challenged with soman

Mapping of trigger sites and/or propagation pathways for soman-induced seizures may provide clues for the designing of anticonvulsant drugs. In the present study, anticonvulsant efficacy against soman intoxication (1.3 x LD50) was examined in rats with either lesion of the perirhinal cortex, posterior piriform cortex, entorhinal cortex, hippocampal region, or amygdala. The results showed that prevention of convulsions or increased latency to onset of convulsions was ensured in rats with perirhinal or piriform cortical lesions, whereas anticonvulsant effects were not achieved in rats with damage to the entorhinal cortex, hippocampal region, or amygdala. The results from the present study suggest that critical structures for induction of seizures after soman exposure are located in the ventrolateral aspect of the forebrain. This suggestion is in compliance with convulsant reactions to microinfusions of soman or VX into ventrolateral brain structures and increased neuronal activity in corresponding structures revealed by c-fos staining in response to soman. Furthermore, results from studies of kindling, lesions, and microinfusion of chemoconvulsants in experimental epilepsy also imply that the perirhinal and piriform cortices are critically involved in seizure control.

Two medical therapies very effective shortly after high levels of soman poisoning in rats, but only one with universal utility

A treatment regimen consisting of HI-6, scopolamine, and physostigmine (termed the physostigmine regimen) has been based on the serendipitous discovery that it exerts powerful antidotal effects against high levels of soman poisoning if it is administered 1 min after exposure. A medical therapy with corresponding efficacy, but without the time limitation of the latter regimen, has been developed through studies of microinfusions of anticonvulsants into seizure controlling sites in the forebrain of rats. From these studies procyclidine emerged as the most potent anticonvulsant, and its potency was further enhanced when being combined with the antiepileptic levetiracetam during systemic administration. In the present study, the capacity of HI-6, levetiracetam, and procyclidine (termed the procyclidine regimen) was tested against that of the physostigmine regimen. The results showed that both regimens were very effective against supralethal doses of soman (3, 4, 5 × LD₅₀) when given 1 and 5 min after intoxication. When the treatments were administered 10 and 14 or 20 and 24 min after soman exposure, only the procyclidine regimen was able to terminate seizures and preserve lives. When used as prophylactic therapies, both regimens protected equally well against seizures, but only the procyclidine regimen provided neuroprotection. The procyclidine regimen has apparently capacities to serve as a universal therapy against soman intoxication in rats.

The perirhinal cortex of rats: an intricate area for microinfusion of anticonvulsants against soman-induced seizures.

Microinfusion of anticonvulsants into the perirhinal cortex through 1 guide cannula in each hemisphere only invades a small area of this seizure controlling site in rats exposed to soman. The purpose of the present study was to examine whether infusions made through 2 cannulas in each perirhinal cortex may produce more efficacious anticonvulsant action against soman intoxication than the use of 1 cannula only in rats infused with the ionotropic antagonists procyclidine and caramiphen or the metabotropic glutamate modulators DCG-IV and MPEP. The results showed that the mere presence of indwelling double cannulas caused proconvulsant effect in response to subsequent systemic administration of soman. Both the control and caramiphen groups with double cannulas had significantly shorter latencies to seizure onset than the corresponding groups with single cannula. Procyclidine resulted in anticonvulsant efficacy, even in rats with double cannulas. In rats that received twin infusions of DCG-IV or MPEP, the anticonvulsant impact was very high, inasmuch as a majority of the rats in each group was protected against seizure activity. Drugs possessing powerful anticonvulsant potency can apparently counteract the proconvulsant effect of double cannulas, and some can even gain enhanced anticonvulsant capacity when invading a larger area of the perirhinal cortex. Perirhinal EEG recordings (electrodes in indwelling cannulas) in a separate set of rats not exposed to soman or drugs showed no differences in basal electrical activity (total power 0.5-25Hz or the theta band 4-12Hz) between groups with single or double cannulas. The intrinsic excitability and synaptic connectivity of the perirhinal cortex may be associated with the proconvulsant impact observed in rats with double cannulas when exposed to soman.

Determination of anti-convulsant and life-preserving capacities of three types of auto-injector therapies against soman intoxication in rats.

More effective countermeasures against nerve-agent poisoning are needed, because current ones do not protect sufficiently, particularly the central nervous system (CNS). The purpose of the present study was to make a comparison of the antidotal capabilities of atropine/obidoxime/diazepam (termed the obidoxime regimen), atropine/HI-6 (1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium)/avizafone (termed the HI-6 regimen), and scopolamine/HI-6/physostigmine (termed the physostigmine regimen) against various doses of soman (2, 3, 4 x LD50 ). The results showed that each regimen administered twice (1 min and 5 min after exposure) effectively prevented or terminated epileptiform activity within 10 min. However, the regimens differed markedly in life-saving properties with the physostigmine regimen ranking highest followed in descending order by the HI-6 and obidoxime regimens. Pretreatment with pyridostigmine increased the potency of the HI-6 regimen, but not the obidoxime regimen. The latter regimen administered thrice (1 min, 5 min, and 9 min after exposure) did not compensate for the insufficiency. In half of the rats that lived for 7 days, neuropathology was unexpectedly observed predominantly in the left hemisphere unrelated to whether they seized or not. Local glutamatergic excitotoxic activity may occur even if manifest toxic signs are absent. The physostigmine regimen has excellent antidotal capacity, but the very narrow therapeutic window (< 10 min) makes it unsuitable for use in the field. The HI-6 regimen appears to constitute an efficacious therapy against lower doses of soman (2 and 3 x LD50).

Behavioral side effects of prophylactic therapies against soman-induced seizures and lethality in rats

Highlights • Behavioral side effects are inevitable for potent prophylactics against soman.• HI-6 or levetiracetam had no adverse effects on behavior in a novelty test.• Scopolamine or procyclidine had detrimental effects in the novelty test.• A powerful drug therapy after nerve agent exposure is essential.