Serotonin modulates fast ripple activity in rats with spontaneous recurrent seizures

High-Frequency Oscillations and Epileptogenic Network

Epilepsy is a network disease caused by aberrant neocortical large-scale connectivity spanning regions on the scale of several centimeters. High-frequency oscillations, characterized by the 80-600 Hz signals in electroencephalography, have been proven to be a promising biomarker of epilepsy that can be used in assessing the severity and susceptibility of epilepsy as well as the location of the epileptogenic zone. However, the presence of a high-frequency oscillation network remains a topic of debate as high-frequency oscillations have been previously thought to be incapable of propagation, and the relationship between high-frequency oscillations and the epileptogenic network has rarely been discussed. Some recent studies reported that high-frequency oscillations may behave like networks that are closely relevant to the epileptogenic network. Pathological highfrequency oscillations are network-driven phenomena and elucidate epileptogenic network development; high-frequency oscillations show different characteristics coincident with the epileptogenic network dynamics, and cross-frequency coupling between high-frequency oscillations and other signals may mediate the generation and propagation of abnormal discharges across the network.

Evaluation of the hippocampal immunoreactivity of the serotonin 5-HT1A, 5-HT2 and 5-HT7 receptors in a pilocarpine temporal lobe epilepsy rat model with fast ripples

Fast ripples (FRs) are found in the hippocampus of epileptic brains, and this fast electrical activity has been described as a biomarker of the epileptogenic process itself. Results from our laboratory, such as the observation of decreased seizure rates and FR incidence at a specific citalopram dose, have suggested that serotonin (5-HT) may play a key role in the FR generation process. Therefore, to gather more details about the state of the serotoninergic system in the hippocampus under an epileptogenic process, we studied the immunoreactivity of three 5-HT receptors (5-HT1A, 5-HT2 and 5-HT7) as well as the extracellular levels of 5-HT in the hippocampal tissue of epileptic rats with FR. Wistar rats (210-300 g) were injected with a single dose of pilocarpine hydrochloride (2.4 mg/2 µl) in the right lateral ventricle and video-monitored 24 h/d to detect spontaneous and recurrent seizures; microelectrodes were implanted in the dentate gyrus (DG) and CA3 and CA1 regions of these rats ipsilateral to the pilocarpine injection site 1 day after the first spontaneous seizure was observed, and only rats who suffered FR events were used in this work. Thirty-three days after the first spontaneous seizure, an immunostaining procedure and high performance liquid chromatography were performed to measure the 5-HT levels. A general depletion of the 5-HT and 5-HIIA levels in hippocampal tissue from epileptic animals compared with those in controls was observed; in addition, a general decrease in immunoreactivity for the three receptors was found, especially in the DG, which may support the establishment of an excitatory/inhibitory imbalance in the trisynaptic circuit that underlies the FR generation process.

The anticonvulsant effect of sparteine on pentylenetetrazole-induced seizures in rats: a behavioral, electroencephalographic, morphological and molecular study

Abnormal synchronous activity in neurons generates epileptic seizures. Antiepileptic drugs (AEDs) are effective in 70% of patients, but this percentage is drastically lower in developing countries. Sparteine is a quinolizidine alkaloid synthesized from most Lupine species and has a probable anticonvulsive effect. For this reason, the objective of the present work was to study the anticonvulsant effect of sparteine using a dose-effect curve and to determine its effectiveness against seizures using behavioral, electroencephalographic, morphological and molecular data. Wistar rats were grouped into control [saline solution (0.9%), pentylenetetrazole (90 mg/kg), and sparteine (13, 20 and 30 mg/kg), intraperitoneal (i.p.)] and experimental (sparteine + pentylenetetrazole) groups. The rats were implanted with surface electrodes to register electrical activity, and convulsive behavior was evaluated according to Velisek's scale. The rats were perfused to obtain brain slices for cresyl violet staining and cellular density quantification as well as for immunohistochemistry for NeuN and GFAP. Other animals were used to determine the hippocampal mRNA expression of the M2 and M4 acetylcholine receptors by qPCR. Sparteine exhibited a better anticonvulsant effect at a dose of 30 mg/kg (i.p.) than at the other doses used. This anticonvulsant effect was characterized by a decrease in the severity of convulsive behavior, 100% survival, an inhibitory effect on epileptiform activity 75 min after pentylenetetrazole administration, and the conservation of the cellular layers of CA1, CA3 and the dentate gyrus (DG); however, astrogliosis was observed after 30 mg/kg sparteine treatment. In addition, sparteine treatment increased the mRNA expression of the M4 receptor three hours after administration. According to our findings, the effective dose of sparteine as an anticonvulsant agent by i.p. injection is 30 mg/kg. The astrogliosis that was observed after sparteine administration may be a compensatory mechanism to diminish excitability and maintain neuronal homeostasis, possibly through redistributing potassium and glutamate. The increase in the mRNA expression of the M4 receptor may suggest the participation of the M4 receptor in the anticonvulsive effect of sparteine, as the activation of this receptor may inhibit acetylcholine release and facilitate the subsequent release of GABA. However, the precise mechanisms by which sparteine produces these effects are not known, and therefore, further experiments are necessary.

Changes in Physiological and Pathological Behaviours Produced by Deep Microelectrode Implantation Surgery in Rats: A Temporal Analysis

Physiological behaviours such as the sleep-wake cycle and exploratory behaviours are important parameters in intact and sham-operated animals and are usually thought to be unaffected by experimental protocols in which neurosurgery is performed. However, there is insufficient evidence in the literature on the behavioural and cognitive effects observed after deep microelectrode implantation surgery in animal models of neurological diseases. Similarly, in studies that utilize animal models of neurological diseases, the impact of surgery on the pathological phenomena being studied is often minimized. Based on these considerations, we performed a temporal analysis of the effects of deep microelectrode implantation surgery in the hippocampus of rats on quiet wakefulness, sleep, and exploratory activity and the pathological behaviours such as convulsive seizures according to the Racine scale. Male Wistar rats (210-300 g) were used and grouped in sham and epileptic animals. Single doses of pilocarpine hydrochloride (2.4 mg/2 μl; i.c.v.) were administered to the animals to generate spontaneous and recurrent seizures. Deep microelectrode implantation surgeries in both groups and analysis of Fast ripples were performed. Physiological and pathological behaviours were recorded through direct video monitoring of animals (24/7). Our principal findings showed that in epileptic animals, one of the main behaviours affected by surgery is sleep; as a consequence of this behavioural change, a decrease in exploratory activity was also found as well as the mean time spent daily in seizures of scale 4 and the number of seizure events of scales 4 and 5 was increased after surgery. No significant correlations between the occurrence of FR and seizure events of scale 4 (rho 0.63, p value 0.25) or 5 (rho -0.7, p value 0.18) were observed. In conclusion, microelectrode implantation surgeries modified some physiological and pathological behaviours; therefore, it is important to consider this fact when it is working with animal models.

High-frequency oscillations and focal seizures in epileptic rodents

High-pass filtering (> 80 Hz) of EEG signals has enabled neuroscientists to analyze high-frequency oscillations (HFOs; i.e., ripples: 80-200 Hz and fast ripples: 250-500 Hz) in epileptic patients presenting with focal seizures and in animal models mimicking this condition. Evidence obtained from these studies indicate that HFOs mirror pathological network activity that may initiate and sustain ictogenesis and epileptogenesis. HFOs are observed in temporal lobe regions of epileptic animals during interictal periods but they also occur before seizure onset and during the ictal period, suggesting that they can pinpoint to the mechanisms of seizure generation. Accordingly, ripples and fast ripples predominate during two specific seizure onset patterns termed low-voltage fast and hypersynchronous, respectively. In this review we will: (i) summarize these experimental studies; (ii) consider the evolution of HFOs over time during epileptogenesis; (iii) address data obtained with optogenetic stimulating procedures both in vitro and in vivo, and (iv) take into account the impact of anti-epileptic drugs on HFOs. We expect these findings to contribute to understanding the neuronal mechanisms leading to ictogenesis and epileptogenesis thus leading to the development of mechanistically targeted anti-epileptic strategies.

Bidirectional relations among common psychiatric and neurologic comorbidities and epilepsy: Do they have an impact on the course of the seizure disorder?

The treatment of epilepsy is not limited to the achievement of a seizure‐free state. It must also incorporate the management of common psychiatric and neurologic comorbidities, affecting on average between 30 and 50% of patients with epilepsy, which have a significant impact on their lives at various levels, including quality of life and the prognosis of the seizure disorder. Mood and anxiety disorders are the most frequent psychiatric comorbidities, whereas stroke and migraine are among the more common neurologic comorbidities, migraine among the younger patients and stroke among the older patients. Not only do these psychiatric and neurologic comorbidities each have a bidirectional relation with epilepsy, but primary mood disorders have a bidirectional relation with these 2 neurologic disorders. Furthermore, depression and migraine have been each associated with a more severe epilepsy course, whereas depression has been associated with a more severe course of stroke and migraines. The purpose of this article is to review the clinical implications of the complex relations among epilepsy and these 3 comorbid disorders, and to identify any clinical and/or experimental evidence that may suggest that having more than one of these comorbid disorders may increase the risk of and course of epilepsy.

The glutamate receptor antagonists CNQX and MPEP decrease fast ripple events in rats treated with kainic acid

Fast ripples (FR) are high frequency oscillations (250-600Hz) that have been associated with epilepsy. FR are assumed to be generated in small areas of the hippocampus (1mm³) that contain pathologically interconnected glutamate pyramidal cell clusters. Additionally, a relation between glutamate neurotransmission and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainite (AMPA/KA) and metabotropic mGluR5 receptors is well established. Therefore, we hypothesized that antagonism of these glutamate receptors would decrease FR activity. For this propose, we induced status epilepticus with a kainic acid injection in the posterior right hippocampus and performed intracranial EEG recordings to detect and evaluate the presence of FR 15days after the injection. The glutamate AMPA/KA receptor antagonist CNQX (10mg/kg) and the mGluR5 antagonist MPEP (20mg/kg) were administered intraperitoneally, and the effects of the drugs were evaluated for a period of three hours after their administration. The results show a decrease in the number of FR in the first hour after drug administration in both cases (CNQX, p=0.0125; MPEP, p=0.0132) and a return to basal values in the third hour of the experiment, but not significant differences in the number of oscillations per event of FR, and the frequency and duration of each event of FR. We therefore conclude that blockade of AMPA/KA and mGluR5 receptors transiently decreases the generation of FR; however, the mechanisms by which this effect is achieved are to be further analyzed in future experiments.

Decreased fast ripples in the hippocampus of rats with spontaneous recurrent seizures treated with carbenoxolone and quinine

Background. In models of temporal lobe epilepsy and in patients with this pathology, high frequency oscillations called fast ripples (FRs, 250–600 Hz) can be observed. FRs are considered potential biomarkers for epilepsy and, in the light of many in vitro and in silico studies, we thought that electrical synapses mediated by gap junctions might possibly modulate FRs in vivo. Methods. Animals with spontaneous recurrent seizures induced by pilocarpine administration were implanted with movable microelectrodes in the right anterior and posterior hippocampus to evaluate the effects of gap junction blockers administered in the entorhinal cortex. The effects of carbenoxolone (50 nmoles) and quinine (35 pmoles) on the mean number of spontaneous FR events (occurrence of FRs), as well as on the mean number of oscillation cycles per FR event and their frequency, were assessed using a specific algorithm to analyze FRs in intracranial EEG recordings. Results. We found that these gap junction blockers decreased the mean number of FRs and the mean number of oscillation cycles per FR event in the hippocampus, both during and at different times after carbenoxolone and quinine administration. Conclusion. These data suggest that FRs may be modulated by gap junctions, although additional experiments in vivo will be necessary to determine the precise role of gap junctions in this pathological activity associated with epileptogenesis.