Effects of phenobarbital and SC-13504 on partially kindled hippocampal seizures in rats

Sigma-1 receptor and seizures

Over the last decade, sigma-1 receptor (Sig1R) has been recognized as a valid target for the treatment of seizure disorders and seizure-related comorbidities. Clinical trials with Sig1R ligands are underway testing therapies for the treatment of drug-resistant seizures, developmental and epileptic encephalopathies, and photosensitive epilepsy. However, the direct molecular mechanism by which Sig1R modulates seizures and the balance between excitatory and inhibitory pathways has not been fully elucidated. This review article aims to summarize existing knowledge of Sig1R and its involvement in seizures by focusing on the evidence obtained from Sig1R knockout animals and the anti-seizure effects of Sig1R ligands. In addition, this review article includes a discussion of the advantages and disadvantages of the use of existing compounds and describes the challenges and future perspectives on the use of Sig1R as a target for the treatment of seizure disorders.

Allosteric Modulators of Sigma-1 Receptor: A Review

Allosteric modulators of sigma-1 receptor (Sig1R) are described as compounds that can increase the activity of some Sig1R ligands that compete with (+)-pentazocine, one of the classic prototypical ligands that binds to the orthosteric Sig1R binding site. Sig1R is an endoplasmic reticulum membrane protein that, in addition to its promiscuous high-affinity ligand binding, has been shown to have chaperone activity. Different experimental approaches have been used to describe and validate the activity of allosteric modulators of Sig1R. Sig1R-modulatory activity was first found for phenytoin, an anticonvulsant drug that primarily acts by blocking the voltage-gated sodium channels. Accumulating evidence suggests that allosteric Sig1R modulators affect processes involved in the pathophysiology of depression, memory and cognition disorders as well as convulsions. This review will focus on the description of selective and non-selective allosteric modulators of Sig1R, including molecular structure properties and pharmacological activity both in vitro and in vivo, with the aim of providing the latest overview from compound discovery approaches to eventual clinical applications. In this review, the possible mechanisms of action will be discussed, and future challenges in the development of novel compounds will be addressed.

Effects of classical antiepileptics on thresholds for phenomena induced by cortical stimulation in rats

Our aim was to study the effects of phenobarbital, phenytoin and ethosuximide on epileptic afterdischarges induced by cortical stimulation in rats. Fifteen-second series of low-frequency (8 Hz) rhythmic stimulation of the sensorimotor cortex were applied in rats with chronically implanted electrodes. Intervals between the stimulation series were at least 10 min and intensity was increased in a step-wise manner. Threshold current intensities were estimated for movements directly induced by stimulation, epileptic afterdischarges of the spike-and-wave type, clonic seizures accompanying this type of afterdischarge and transition into the limbic type of afterdischarge. Phenobarbital, phenytoin and ethosuximide were administered intraperitoneally before the first stimulation series. Phenobarbital (20, 40 and 80 mg kg(-1)) significantly increased the thresholds for the first three phenomena in a dose-dependent manner. Transition into the limbic afterdischarge was influenced only by the highest dose. Phenytoin (60 mg kg(-1)) only increased the thresholds insignificantly and ethosuximide (125 mg kg(-1)) was ineffective. We concluded that our model is useful for testing anticonvulsant effects. Results with three antiepileptic drugs correspond with their efficacy against myoclonic seizures in man.

Kindling alters the anticonvulsant efficacy of phenytoin in Wistar rats

We have previously shown that subgroups can be selected from large groups of amygdala kindled Wistar rats which either respond consistently or do not respond to the anticonvulsant effect of phenytoin. Phenytoin nonresponders were proposed as a model for pharmaco-resistant temporal lobe epilepsy. In the present study we examined whether the differences of individual rats in response to phenytoin are already present before kindling or are a consequence of kindling. For this purpose, 52 rats were once tested with phenytoin, then kindled, and then repeatedly tested with phenytoin for selection of subgroups. For subgroup selection after kindling, the phenytoin prodrug fosphenytoin was used because of its water solubility and its improved tolerability and absorption after i.p. administration in rats. Before kindling, phenytoin significantly increased the afterdischarge threshold (ADT), i.e. a sensitive measure of focal seizure activity, but there was large individual variation with only 32 of the 52 rats reacting with an ADT increase, while the remaining rats showed either no effect or ADT decreases. After kindling, the selection resulted in 16 rats with consistent ADT increases in response to phenytoin and ten nonresponders (the remaining 26 rats showed variable responses). Unexpectedly, in rats which were responders after kindling, phenytoin exerted no significant anticonvulsant effect before kindling, while kindled nonresponders were very sensitive to phenytoin before kindling, indicating that the kindling process was responsible for the loss of anticonvulsant efficacy in kindled nonresponders and the development of phenytoin's efficacy in kindled responders. The present results substantiate that kindled subgroups of Wistar rats with different response to phenytoin are a valuable source for studying the mechanisms underlying the development of pharmaco-resistant limbic seizures.

Limbic epileptogenesis alters the anticonvulsant efficacy of phenytoin in Sprague-Dawley rats

Studies on the anticonvulsant efficacy of the major antiepileptic drug phenytoin in kindled rats have often reported inconsistent effects. It has been proposed that technical and genetic factors or poor and variable absorption of phenytoin after i.p. or oral administration may be involved in the lack of consistent anticonvulsant activity of phenytoin in this model of temporal lobe epilepsy. We examined if kindling itself changes the anticonvulsant efficacy of phenytoin by testing this drug before and after amygdala kindling in male and female Sprague-Dawley rats. To exclude the possible bias of poor and variable absorption, blood was sampled in all experiments for drug analysis in plasma. The threshold for induction of focal seizures (afterdischarge threshold; ADT) was used for determining phenytoin's anticonvulsant activity. Before kindling, phenytoin, 75 mg/kg i.p., markedly increased ADT in both genders, although the effect was more pronounced in males. Following kindling, the anticonvulsant activity obtained with phenytoin, 75 mg/kg, before kindling was totally lost, and female rats even exhibited a proconvulsant effect upon administration of this dose, indicating that kindling had dramatically altered the anticonvulsant efficacy of phenytoin. Plasma levels of phenytoin were comparable before and after kindling, and were within or near to the 'therapeutic range' known from epileptic patients. When the dose of phenytoin was reduced to 50 or 25 mg/kg i.p., significant anticonvulsant effects on ADT were obtained. When phenytoin, 50 mg/kg, was administered i.p. or i.v. in the same group of fully kindled rats, both anticonvulsant activity and plasma drug levels were comparable with both routes, indicating that the i.p. route is suited for such studies. The data indicate that kindling alters the dose-response of phenytoin in that a high anticonvulsant dose becomes ineffective or proconvulsant after kindling, possibly by an increased sensitivity of the kindled brain to proconvulsant effects of phenytoin which normally only occur at much higher doses. If similar alterations evolve in humans during development of chronic epilepsy, this may be involved in the mechanisms leading to intractability of temporal lobe epilepsy.

Differential antiepileptic sensitivity between cortical sites in the rat

The relative efficacies of phenobarbital (PB), phenytoin (PHT), carbamazepine (CBZ), and valproic acid (VPA) in the suppression of focal and generalized seizures produced by electrical stimulation of two different cortical sites (areas 3 and 10) were evaluated in the rat. The two cortical sites were distinguished by significantly different dose-response curve slopes for the suppression of afterdischarge duration by PHT, CBZ and VPA, which suggests more than one mechanism of action for these drugs. The dose-response curve slopes for PB, on the contrary, were not significantly different, although its potency was significantly greater in area 10. For suppression of generalized convulsions, dose-response curve slopes were not significantly different for any of the drugs. Potencies of PHT, CBZ and VPA were equivalent in the two areas, but PB was significantly more potent in the suppression of generalized convulsions triggered from area 10. It is concluded that focal seizures elicited by the stimulation of different cortical sites are differentially refractory to antiepileptic drugs.

Does phenobarbital protect against trimethyltin-induced neuropathology of limbic structures?

Because of the similarity in the pattern of limbic sites damaged by both compounds, it has been suggested that trimethyltin (TMT) may be an excitotoxin like kainic acid (KA). KA produces seizures which eventually result in neuronal damage similar to that found in epilepsy. Anticonvulsants reduce both the seizures and pathology associated with KA. Because TMT may also produce seizures, we undertook to determine whether or not some of the TMT-induced limbic neuropathology could result from seizure activity. To do this, a single dose of TMT chloride (either 7.5 or 15 mg/kg) was given per os to rats, and then phenobarbital (30 mg/kg) was administered subcutaneously in repeated doses. Treatment with phenobarbital did not prevent pathologic changes in the hippocampus, dentate gyrus, and pyriform or prepyriform cortex. Since phenobarbital did not protect against TMT-induced neuronal damage, as it has been reported by others to protect against KA-induced damage, the present findings suggest that these two toxicants probably produce hippocampal pathology via different mechanisms and that the TMT-induced pathologic changes do not require sustained electrical seizure activity.

EEG activities during kindling in frog

The "kindling" phenomenon was investigated in relation to the EEG activities in frog, Rana nigromaculata. Repetitive electrical stimulation to the unilateral hippocampus produced the following results: (1) Prolongation in duration of after-discharges (AD) and shortening of AD intervals; (2) Appearance and enhancing of spontaneous epileptiform discharges (SEDs), which were suppressed by isolation of hippocampus; (3) Increase in power and duration of background EEG with a specific frequency (9 Hz in the nontreated group and 5 Hz in the hippocampal commissural bisected group), which coincided with the half length of the AO intervals during the steady phase; (4) Marked increase in the late components with peak latencies of 25 and 50 msec in the interhippocampal impulse responses as estimated from the bilateral background hippocampal EEG through an autoregressive model, of which the former only was suppressed by commissural section, while both were suppressed by hippocampal isolation. From the above evidences obtained in frogs, it may be inferred that the brain structures, e.g., thalamus, hypothalamus, reticular formation, etc., play an important role in kindling formation and that the alteration in background EEG generating system during kindling is closely associated with the sporadic epileptiform EEG activities.