Stanisław J. Czuczwar

Limbic seizures produced by pilocarpine in rats: Behavioural, electroencephalographic and neuropathological study

Behavioural, electroencephalographic and neuropathological responses to increasing doses of pilocarpine (100-400 mg/kg) administered intraperitoneally to rats were studied. At the dose of 400 mg/kg pilocarpine produced a sequence of behavioural alterations including staring spells, olfactory and gustatory automatisms and motor limbic seizures that developed over 1-2 h and built up progressively into limbic status epilepticus. Smaller doses showed different threshold for these behavioural phenomena but a similar time course of development. The earliest electrographic alterations occurred in the hippocampus and then epileptiform activity propagated to amygdala and cortex. Subsequently electrographic seizures appeared in both limbic and cortical leads. The ictal periods recurred each 5-15 min and were followed by variable periods of depression of the electrographic activity. The sequence of electrographic changes correlated well with the development of behavioural phenomena. Histological examination of frontal forebrain sections revealed disseminated, apparently seizure-mediated pattern of brain damage. Neuropathological alterations were observed in the olfactory cortex, amygdaloid complex, thalamus, neocortex, hippocampal formation and substantia nigra. Pretreatment of animals with scopolamine (20 mg/kg) and diazepam (10 mg/kg) prevented the development of convulsive activity and brain damage. These results show that systemic pilocarpine in rats selectively elaborates epileptiform activity in the limbic structures accompanied by motor limbic seizures, limbic status epilepticus and widespread brain damage. It is suggested that a causative relationship between excessive stimulation of cholinergic receptors in the brain and epileptic brain damage may exist.

Selection of antiepileptic drug polytherapy based on mechanisms of action: the evidence reviewed

Summary: Purpose: When monotherapy with antiepileptic drugs (AEDs) fails, combination therapy is tried in an attempt to improve effectiveness by improving efficacy, tolerability, or both. We reviewed the available studies (both animal and human) on AED polytherapy to determine whether AEDs can be selected for combination therapy based on their mechanisms of action, and if so, which combinations are associated with increased effectiveness. Because various designs and methods of analysis were used in these studies, it was also necessary to evaluate the appropriateness of these approaches.

Mechanisms of Action of Antiepileptic Drugs

Gamma-aminobutyric acid (GABA), one of the main inhibitory neurotransmitters in the brain, interacts with three types of receptors for GABA--GABA(A), GABA(B) and GABA(C). GABA(A) receptors, associated with binding sites for benzodiazepines and barbiturates in the form of a receptor complex, control opening of the chloride channel. When GABA binds to the receptor complex, the channel is opened and chloride anions enter the neuron, which is finally hyperpolarized. GABA(B) receptors are metabotropic, linked to a cascade of second messengers whilst the physiological meaning of ionotropic GABA(C) receptors, mainly located in the retina, is generally unknown. Novel antiepileptic drugs acting selectively through the GABA-ergic system are tiagabine and vigabatrin. The former inhibits neuronal and glial uptake of GABA whilst the latter increases the synaptic concentration of GABA by inhibition of GABA-aminotransferase. Gabapentin, designed as a precursor of GABA easily entering the brain, was shown to increase brain synaptic GABA. This antiepileptic drug also decreases influx of calcium ions into neurons via a specific subunit of voltage-dependent calcium channels. Conventional antiepileptics generally inhibit sodium currents (carbamazepine, phenobarbital, phenytoin, valproate) or enhance GABA-ergic inhibition (benzodiazepines, phenobarbital, valproate). Ethosuximide, mainly controlling absences, reduces calcium currents via T-type calcium channels. Novel antiepileptic drugs, mainly associated with an inhibition of voltage-dependent sodium channels are lamotrigine and oxcarbazepine. Since glutamate-mediated excitation is involved in the generation of seizure activity, some antiepileptics are targeting glutamatergic receptors--for instance, felbamate, phenobarbital, and topiramate. Besides, they also inhibit sodium currents. Zonisamide, apparently sharing this common mechanism, also reduces the concentration of free radicals. Novel antiepileptic drugs are better tolerated by epileptic patients and practically are devoid of important pharmacokinetic drug interactions.

Protection against chemically induced seizures by 2-amino-7-phosphonoheptanoic acid

The anticonvulsant activity of 2-amino-7-phosphonoheptanoic acid (2APH) (an antagonist of excitation induced by N-methyl-D-aspartic acid) was studied against N-methyl-DL-aspartic acid (NMDLA), kainic acid, 3-mercaptopropionic acid (3MPA), thiosemicarbazide (TSC), quinolinic acid, bicuculline, picrotoxin and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) in Swiss S mice. 2APH, 0.33 mM/kg i.p., antagonizes convulsions induced by NMDLA, 3MPA, TSC, DMCM and picrotoxin but not those produced by the other convulsants. It is proposed that an aspartergic component may contribute to the development of convulsions after 3MPA, TSC, DMCM and picrotoxin.

The new generation of GABA enhancers. Potential in the treatment of epilepsy.

Abstractγ-Aminobutyric acid (GABA) is considered to be the major inhibitory neurotransmitter in the brain and loss of GABA inhibition has been clearly implicated in epileptogenesis. GABA interacts with 3 types of receptor: GAB Aa, GAB Ab and GABAc. The GABAA receptor has provided an excellent target for the development of drugs with an anticonvulsant action. Some clinically useful anti-convulsants, such as the benzodiazepines and barbiturates and possibly valproic acid (sodium valproate), act at this receptor.In recent years 4 new anticonvulsants, namely vigabatrin, tiagabine, gaba-pentin and topiramate, with a mechanism of action considered to be primarily via an effect on GABA, have been licensed. Vigabatrin elevates brain GABA levels by inhibiting the enzyme GABA transaminase which is responsible for intracellular GABA catabolism. In contrast, tiagabine elevates synaptic GABA levels by inhibiting the GABA uptake transporter, GAT1, and preventing the uptake of GABA into neurons and glia. Gabapentin, a cyclic analogue of GABA, acts by enhancing GABA synthesis and also by decreasing neuronal calcium influx via a specific subunit of voltage-dependent calcium channels. Topiramate acts, in part, via an action on a novel site of the GABAA receptor. Although these drugs are useful in some patients, overall, they have proven to be disappointing as they have had little impact on the prognosis of patients with intractable epilepsy.Despite this, additional GABA enhancing anticonvulsants are presently under development. Ganaxolone, retigabine and pregabalin may prove to have a more advantageous therapeutic profile than the presently licensed GABA enhancing drugs. This anticipation is based on 2 characteristics. First, they act by hitherto unique mechanisms of action in enhancing GABA-induced neuronal inhibition. Secondly, they act on additional antiepileptogenic mechanisms. Finally, CGP 36742, a GABAb receptor antagonist, may prove to be particularly useful in the management of primary generalised absence seizures.The exact impact of these new GABA-enhancing drugs in the treatment of epilepsy will have to await their licensing and a period of postmarketing surveillance. As to clarification of their role in the management of epilepsy, this will have to await further clinical trials, particularly direct comparative trials with other anticonvulsants.

Interactions between oxcarbazepine and conventional antiepileptic drugs in the maximal electroshock test in mice: An isobolographic analysis

Summary:  Purpose: The aim of this study was to determine the types of interactions between oxcarbazepine (OCBZ) and conventional antiepileptic drugs (AEDs) against maximal electroshock‐induced seizures (MES test) in mice, by using a method of isobolographic analysis.

Seizures produced by pilocarpine: Neuropathological sequelae and activity of glutamate decarboxylase in the rat forebrain

Morphological analysis of brains from rats receiving a convulsant dose of the muscarinic cholinergic agonist, pilocarpine hydrochloride (380 mg/kg), revealed a widespread damage to the forebrain as assessed by light microscopy 5-7 days after seizures. The substantia nigra, olfactory cortex, amygdala, hippocampus, septum, temporal cortex and thalamus underwent prominent morphological injury and cell loss. A concurrent assessment of the activity of L-glutamate decarboxylase (GAD), the gamma-aminobutyrate (GABA) synthesizing enzyme, demonstrated marked deficits in GAD activity in the brain regions undergoing morphological insult. Diazepam, 10 mg/kg, and scopolamine hydrochloride, 10 mg/kg, administered 30 min prior to the injection of pilocarpine, 380 mg/kg, prevented acute behavioral and electrographic, and long-term morphological and biochemical sequelae of seizures. These findings suggest that the muscarinic antagonist, scopolamine, and the anticonvulsant benzodiazepine, diazepam, may aid in preventing extensive brain damage related to pathological muscarinic cholinergic overactivity. The similarity of the topography of the damage and deficits in the GAD activity in brains of rats treated with pilocarpine indicates that GABAergic neurons are lost in the subregions of the brain preferentially sensitive to the convulsant action of pilocarpine.

Anticonvulsant and acute neurotoxic effects of imperatorin, osthole and valproate in the maximal electroshock seizure and chimney tests in mice: A comparative study

The aim of this study was to determine and compare the anticonvulsant and acute adverse (neurotoxic) effects of imperatorin and osthole (two natural coumarin derivatives) with valproate (a classical antiepileptic drug) in the maximal electroshock seizure and chimney tests in mice. The anticonvulsant and acute adverse effects of imperatorin, osthole and valproate were determined at 15, 30, 60 and 120 min after their systemic (i.p.) administration. The evaluation of time-course and dose-response relationships for imperatorin, osthole and valproate in the maximal electroshock seizure test revealed that the compounds produced a clear-cut antielectroshock action in mice and the experimentally derived ED(50) values for imperatorin ranged between 167 and 290 mg/kg, those for osthole ranged from 253 to 639 mg/kg, whereas the ED(50) values for valproate ranged from 189 to 255 mg/kg. The evaluation of acute neurotoxic effects in the chimney test revealed that the TD(50) values for imperatorin ranged between 329 and 443 mg/kg, the TD(50) values for osthole ranged from 531 to 648 mg/kg, while the TD(50) values for valproate ranged from 363 to 512 mg/kg. The protective index (as a ratio of TD(50) and ED(50) values) for imperatorin ranged between 1.13 and 2.60, for osthole ranged from 0.83 to 2.44, and for valproate ranged between 1.72 and 2.00. In conclusion, both natural coumarin derivatives deserve more attention from a preclinical point of view as compounds possessing some potentially favorable activities in terms of suppression of seizures, quite similar to those reported for valproate.

Pharmacological and Behavioral Characteristics of Interactions between Vigabatrin and Conventional Antiepileptic Drugs in Pentylenetetrazole-Induced Seizures in Mice: An Isobolographic Analysis

To characterize the anticonvulsant effects and types of interactions exerted by mixtures of vigabatrin (VGB) and conventional antiepileptic drugs (valproate (VPA), ethosuximide (ESM), phenobarbital (PB), and clonazepam (CZP)) in pentylenetetrazole (PTZ)-induced seizures in mice, the isobolographic analysis for three fixed-ratio combinations of 1 : 3, 1 : 1, and 3 : 1 was used. The adverse-effect profile of the combinations tested, at the doses corresponding to their median effective doses (ED50) at the fixed-ratio of 1 : 1 against PTZ-induced seizures, was determined by the chimney (motor performance), step-through passive avoidance (long-term memory), pain threshold (pain sensitivity), and Y-maze (general explorative locomotor activity) tests in mice. Additionally, the observed isobolographic interactions were verified in terms of a pharmacokinetic interaction existence. VGB combined with PB or ESM exerted supra-additive (synergistic) interactions against the clonic phase of PTZ-induced seizures, which was associated with the increment of PB or ESM concentrations in the brains of examined animals. The remaining combinations tested (ie VGB+VPA and VGB+CZP) occurred additive in the PTZ test, which was associated with no significant changes in the brain concentrations of VPA and CZP. None of the examined combinations exerted motor impairment in the chimney test in mice. In the standard variant of passive avoidance task (current of 0.6 mA; 2 s of stimulus duration), the combinations of VGB+CZP and VGB+VPA significantly affected long-term memory in mice. Moreover, VGB in a dose-dependent manner lengthened the latency to the first pain reaction in the pain threshold test in mice. The modified variant of step-through passive avoidance task (current of 0.6 mA; stimulus duration based on the latency from the pain threshold test) revealed no significant changes in the long-term memory of animals for the combinations of VGB+VPA and VGB+CZP; so the observed effects in the standard variant of passive avoidance task were a result of the antinociceptive effects produced by VGB. In the Y-maze test, VGB also, in a dose-dependent manner, increased the general explorative locomotor activity of the animals tested. Similarly, the total number of arm entries in the Y-maze was significantly increased for the combinations of VGB+CZP and VGB+ESM, but not for VGB+PB and VGB+VPA. The application of VGB in combination with PB, ESM, CZP, and VPA suppressed the clonic phase of PTZ-induced seizures, having no harmful or deleterious effects on behavioral functioning of the animals tested, which might be advantageous in further clinical practice.

Statin-induced myopathies.

Statins are considered to be safe, well tolerated and the most efficient drugs for the treatment of hypercholesterolemia, one of the main risk factor for atherosclerosis, and therefore they are frequently prescribed medications. The most severe adverse effect of statins is myotoxicity, in the form of myopathy, myalgia, myositis or rhabdomyolysis. Clinical trials commonly define statin toxicity as myalgia or muscle weakness with creatine kinase (CK) levels greater than 10 times the normal upper limit. Rhabdomyolysis is the most severe adverse effect of statins, which may result in acute renal failure, disseminated intravascular coagulation and death. The exact pathophysiology of statin-induced myopathy is not fully known. Multiple pathophysiological mechanisms may contribute to statin myotoxicity. This review focuses on a number of them. The prevention of statin-related myopathy involves using the lowest statin dose required to achieve therapeutic goals and avoiding polytherapy with drugs known to increase systemic exposure and myopathy risk. Currently, the only effective treatment of statin-induced myopathy is the discontinuation of statin use in patients affected by muscle aches, pains and elevated CK levels.