Interaction between Thalamus and Hippocampus in Termination of Amygdala-Kindled Seizures in Mice

The thalamus and hippocampus have been found both involved in the initiation, propagation, and termination of temporal lobe epilepsy. However, the interaction of these regions during seizures is not clear. The present study is to explore whether some regular patterns exist in their interaction during the termination of seizures. Multichannel in vivo recording techniques were used to record the neural activities from the cornu ammonis 1 (CA1) of hippocampus and mediodorsal thalamus (MDT) in mice. The mice were kindled by electrically stimulating basolateral amygdala neurons, and Racine’s rank standard was employed to classify the stage of behavioral responses (stage 1~5). The coupling index and directionality index were used to investigate the synchronization and information flow direction between CA1 and MDT. Two main results were found in this study. (1) High levels of synchronization between the thalamus and hippocampus were observed before the termination of seizures at stage 4~5 but after the termination of seizures at stage 1~2. (2) In the end of seizures at stage 4~5, the information tended to flow from MDT to CA1. Those results indicate that the synchronization and information flow direction between the thalamus and the hippocampus may participate in the termination of seizures

The antiepileptic potential of nucleosides

Despite newly developed antiepileptic drugs to suppress epileptic symptoms, approximately one third of patients remain drug refractory. Consequently, there is an urgent need to develop more effective therapeutic approaches to treat epilepsy. A great deal of evidence suggests that endogenous nucleosides, such as adenosine (Ado), guanosine (Guo), inosine (Ino) and uridine (Urd), participate in the regulation of pathomechanisms of epilepsy. Adenosine and its analogues, together with non-adenosine (non-Ado) nucleosides (e.g., Guo, Ino and Urd), have shown antiseizure activity. Adenosine kinase (ADK) inhibitors, Ado uptake inhibitors and Ado-releasing implants also have beneficial effects on epileptic seizures. These results suggest that nucleosides and their analogues, in addition to other modulators of the nucleoside system, could provide a new opportunity for the treatment of different types of epilepsies. Therefore, the aim of this review article is to summarize our present knowledge about the nucleoside system as a promising target in the treatment of epilepsy

5'-nucleotidases, nucleosides and their distribution in the brain: pathological and therapeutic implications

Elements of the nucleoside system (nucleoside levels, 5'-nucleotidases (5'NTs) and other nucleoside metabolic enzymes, nucleoside transporters and nucleoside receptors) are unevenly distributed in the brain, suggesting that nucleosides have region-specific functions in the human brain. Indeed, adenosine (Ado) and non-Ado nucleosides, such as guanosine (Guo), inosine (Ino) and uridine (Urd), modulate both physiological and pathophysiological processes in the brain, such as sleep, pain, memory, depression, schizophrenia, epilepsy, Huntington's disease, Alzheimer's disease and Parkinson's disease. Interactions have been demonstrated in the nucleoside system between nucleoside levels and the activities of nucleoside metabolic enzymes, nucleoside transporters and Ado receptors in the human brain. Alterations in the nucleoside system may induce pathological changes, resulting in central nervous system (CNS) diseases. Moreover, several CNS diseases such as epilepsy may be treated by modulation of the nucleoside system, which is best achieved by modulating 5'NTs, as 'NTs exhibit numerous functions in the CNS, including intracellular and extracellular formation of nucleosides, termination of nucleoside triphosphate signaling, cell adhesion, synaptogenesis and cell proliferation. Thus, modulation of 5'NT activity may be a promising new therapeutic tool for treating several CNS diseases. The present article describes the regionally different activities of the nucleoside system, demonstrates the associations between these activities and 5'NT activity and discusses the therapeutic implications of these associations

Pharmacological and genetic modulation of the endocannabinoid system

Epilepsy is one of the most common chronic neurological diseases worldwide and the prevention of epileptogenesis is so far unmet. A major challenge in epilepsy research is the development of new therapeutic approaches for patients with therapy-resistant epilepsies, for epilepsy prevention and for disease modification. The endocannabinoid system serves as a retrograde negative feedback mechanism and one of its key functions is regulating neuronal activity within the central nervous system. Thus, the endocannabinoid system can be considered a putative target for central nervous system diseases including epilepsies. The purpose of this thesis was to evaluate the impact of the endocannabinoid and endovanilloid systems on both epileptogenesis and ictogenesis. Therefore, I modulated the systems pharmacologically and genetically and analyzed the impact on the generation of a hyperexcitable neuronal network as well as on ictogenesis in the kindling model of temporal lobe epilepsy. In addition, the impact of seizures on associated cellular alterations, like CB1-receptor (CB1R) expression and neurogenesis, was evaluated. I established that the endocannabinoid system affects seizure and afterdischarge duration dependent on the neuronal subpopulation being modulated. Genetic deletion of CB1Rs from GABAergic forebrain neurons caused shorter seizure duration. Deletion of CB1R from principal neurons of the forebrain and pharmacological antagonism with rimonabant (5 mg/kg) resulted in the opposite effect. Along with these findings, the CB1R density was increased in mice with recurrent induced seizures. However, neither genetic knockout nor pharmacological antagonism had any impact on the development of generalized seizures. In contrast to genetic deletion or pharmacological blockade of CB1Rs, modulation of transient receptor potential vanilloid receptor 1 (TRPV1) neither genetically nor pharmacologically with SB366791 (1 mg/kg) had an effect on the duration of behavioral or electrographic seizure activity. Pharmacological blockade of the 2-arachidonoylglycerol degrading enzyme, monoacylglycerol lipase (MAGL) with JZL184 (8 mg/kg), delayed the development of generalized seizures and decreased seizure and afterdischarge durations whereas in fully-kindled mice JZL184 (4, 8 and 16 mg/kg) had no relevant effects on associated seizure parameters. In addition, I confirmed by the use of conditional CB1R knockout mice that these effects are CB1R mediated. In conclusion, my findings support the concept that the endocannabinoid system may be a therapeutic target for decreasing seizure duration and that it is involved in terminating seizures as an endogenous mechanism. Moreover, targeting MAGL may be a promising strategy for an antiepileptogenic approach. Respective strategies are of particular interest for the management of long-lasting refractory status epilepticus and cluster seizures as well as for the prevention of the development of symptomatic epilepsies after an initial insult

Corticotropin releasing factor receptor type 1 signaling in epilepsy and traumatic brain injury

Stress increases the frequency by which epileptic seizures occur. Corticotropin-releasing factor (CRF) coordinates neuroendocrine, autonomic and behavioral response to stress. This thesis sought to study the cellular and molecular mechanisms by which CRF regulates the activity of neural circuits in the piriform cortex (PC) in normal and epileptic states. The PC is richly innervated by CRF and 5-HT containing axons arising from the central amygdala and raphe nucleus. CRFR1 and 5-HT2A/CRs have been shown to interact in a manner where CRFR activation subsequently potentiates the activity of 5-HT2A/CRs. The first purpose of this thesis was to determine how the activation of CRFR1 and/or 5-HT2Rs modulates PC activity at both the circuit and cellular level. Voltage-sensitive dye imaging showed that CRF acting through CRFR1 dampened activation of layer II in the PC and interneurons of the endopiriform nucleus. Application of the selective 5-HT2A/CRagonist 2,5-dimethoxy-4-iodoamphetamine (DOI) following CRFR1 activation potentiated this effect. Blocking the interaction between CRFR1 and 5-HT2R with a Tat-CRFR1-CT peptide abolished this potentiation. Application of forskolin did not mimic CRFR1 activity but instead blocked it, while a protein kinase A antagonist had no effect. However, activation and antagonism of protein kinase C (PKC) either mimicked or blocked CRF modulation respectively. DOI had no effect when applied alone indicating that the prior activation of CRFR1 receptors was critical for the DOI activity. This data shows that CRF and 5-HT, acting through 5-HT2A/CRs, reduce the activation of the PC. This modulation may be an important blunting mechanism of stressor behaviors mediated through the olfactory cortex. Anxiety and stress conditions induce neurons arising from the central amygdala and local interneurons to release CRF in PC, where it normally dampens excitability. The second aim of this thesis was to determine the role of CRF in stress associated epilepsy. We showed that CRF increased the excitability of PC in rats subjected to kindling, a model of temporal lobe epilepsy. In non-kindled rats, CRF activates its receptor, a G protein-coupled receptor (GPCR) and signals through a Gaq/11 mediated pathway as identified in the first aim of this thesis. After seizure induction, CRF signaling occurred through a pathway involving Gas. This change in signaling was associated with reduced abundance of regulator of G-protein signaling protein-2 (RGS2), which promotes the switch in CRFR1 signaling cascade to a Gas dependent mechanism. RGS2 knockout mice responded to CRF in a similar manner as epileptic rats. These observations indicate that seizures produce changes in neuronal signaling that can increase seizure occurrence by converting a beneficial stress response into an epileptic trigger. People with traumatic brain injury often develop epileptic seizures. The mechanisms underlying this are poorly understood. Considerable evidence suggests that association of stressful life experiences in brain injured patients lead them to develop post-traumatic stress disorder. CRF release in brain regions that are implicated in epileptogenesis make these situations worse. The third aim of this thesis was to understand the role of CRF in inducing excitability in PC after brain injury. We found that CRF has variable effects on the interneurons of ipsilateral and contralateral PC. Altogether, its actions lead to increased excitability of PC compared to healthy rat PC. The extent of excitability produced by CRF and the signaling mechanism of CRFR1 after brain injury were similar to CRF actions and CRFR1 signaling mechanism in kindling induced epilepsy. Overall, this thesis study provides the basic mechanisms by which certain forms of epilepsy, both stress and injury induced develops. It also points out the important discovery of this project that is, the capability of GPCRs to switch signaling cascades depending on the pathological condition of the brain

Cognitive Impairment and Aberrant Plasticity in the Kindling Model of Epilepsy

Epilepsy is a neurological disorder that affects approximately 1% of the population worldwide. Although motor seizures are the best known feature of epilepsy, many patients also experience severe interictal (between-seizure) behavioral and cognitive comorbidities that have a greater negative influence on quality of life than seizure control or frequency. To study the characteristics of these interictal comorbidities and the neural mechanisms that underlie them, I use the kindling model of epilepsy. Kindling refers to the brief electrical stimulation of a discrete brain site that produces a gradual and permanent increase in the severity of elicited seizure activity. The repeated seizures associated with kindling induce robust structural and functional plasticity that appears to be primarily aberrant. Importantly, the aberrant plasticity evoked by repeated seizures is thought to contribute to the pathophysiology of epilepsy and its associated behavioral and cognitive comorbidities. Unfortunately, the relationship between aberrant plasticity and cognition dysfunction following repeated seizures remains poorly understood. The aim of this dissertation is to gain a better understanding of the effects of repeated convulsions on aberrant neural plasticity and interictal behavior. In Chapter 2, I will examine the effect of short- and long-term amygdala kindling on amygdala- and hippocampal-dependent forms of operant fear conditioning. To evaluate whether kindling alters neural circuits important in memory, I will analyze post-mortem measures of neural activity following the retrieval of fearful memories. In Chapter 3, I will evaluate whether deficits in operant fear learning and memory are a general consequence of convulsions induced by kindling stimulations or whether these deficits occur following kindling of specific brain regions. To evaluate whether aberrant plasticity following kindling of different brain regions contributes to learning and memory deficits, I will make post-mortem examinations of the inhibitory neurotransmitter neuropeptide Y and its Y2 receptor. In Chapter 4, I will investigate the relationship between hippocampal neurogenesis and cognition. Specifically, I will determine whether kindling of different brain regions induces an aberrant form of hippocampal neurogenesis that contributes to cognitive dysfunction. In Chapter 5, I will investigate whether kindling of different brain regions alters different subpopulations of hippocampal GABAergic interneurons, in terms of number and morphological features. Finally, Chapter 6 will provide preliminary evidence that the cognitive impairments associated with kindling can be ameliorated through intrahippocampal infusions of recombinant reelin. The collection of studies in this dissertation improves our understanding of the relationship between aberrant plasticity and cognitive impairments associated with repeated convulsions

Underlying Mechanisms of Epilepsy

This book is a very provocative and interesting addition to the literature on Epilepsy. It offers a lot of appealing and stimulating work to offer food of thought to the readers from different disciplines. Around 5% of the total world population have seizures but only 0.9% is diagnosed with epilepsy, so it is very important to understand the differences between seizures and epilepsy, and also to identify the factors responsible for its etiology so as to have more effective therapeutic regime. In this book we have twenty chapters ranging from causes and underlying mechanisms to the treatment and side effects of epilepsy. This book contains a variety of chapters which will stimulate the readers to think about the complex interplay of epigenetics and epilepsy

Pharmacological and genetic modulation of the endocannabinoid system

Epilepsy is one of the most common chronic neurological diseases worldwide and the prevention of epileptogenesis is so far unmet. A major challenge in epilepsy research is the development of new therapeutic approaches for patients with therapy-resistant epilepsies, for epilepsy prevention and for disease modification. The endocannabinoid system serves as a retrograde negative feedback mechanism and one of its key functions is regulating neuronal activity within the central nervous system. Thus, the endocannabinoid system can be considered a putative target for central nervous system diseases including epilepsies. The purpose of this thesis was to evaluate the impact of the endocannabinoid and endovanilloid systems on both epileptogenesis and ictogenesis. Therefore, I modulated the systems pharmacologically and genetically and analyzed the impact on the generation of a hyperexcitable neuronal network as well as on ictogenesis in the kindling model of temporal lobe epilepsy. In addition, the impact of seizures on associated cellular alterations, like CB1-receptor (CB1R) expression and neurogenesis, was evaluated. I established that the endocannabinoid system affects seizure and afterdischarge duration dependent on the neuronal subpopulation being modulated. Genetic deletion of CB1Rs from GABAergic forebrain neurons caused shorter seizure duration. Deletion of CB1R from principal neurons of the forebrain and pharmacological antagonism with rimonabant (5 mg/kg) resulted in the opposite effect. Along with these findings, the CB1R density was increased in mice with recurrent induced seizures. However, neither genetic knockout nor pharmacological antagonism had any impact on the development of generalized seizures. In contrast to genetic deletion or pharmacological blockade of CB1Rs, modulation of transient receptor potential vanilloid receptor 1 (TRPV1) neither genetically nor pharmacologically with SB366791 (1 mg/kg) had an effect on the duration of behavioral or electrographic seizure activity. Pharmacological blockade of the 2-arachidonoylglycerol degrading enzyme, monoacylglycerol lipase (MAGL) with JZL184 (8 mg/kg), delayed the development of generalized seizures and decreased seizure and afterdischarge durations whereas in fully-kindled mice JZL184 (4, 8 and 16 mg/kg) had no relevant effects on associated seizure parameters. In addition, I confirmed by the use of conditional CB1R knockout mice that these effects are CB1R mediated. In conclusion, my findings support the concept that the endocannabinoid system may be a therapeutic target for decreasing seizure duration and that it is involved in terminating seizures as an endogenous mechanism. Moreover, targeting MAGL may be a promising strategy for an antiepileptogenic approach. Respective strategies are of particular interest for the management of long-lasting refractory status epilepticus and cluster seizures as well as for the prevention of the development of symptomatic epilepsies after an initial insult

Aberrant structural and functional plasticity in the adult hippocampus of amygdala kindled rats

Amygdala kindling is commonly used to study the neural mechanisms of temporal lobe epilepsy and its behavioral consequences. The repetitive seizure activity that occurs during kindling is thought to induce an extensive array of structural and functional modifications within the brain, particularly in the hippocampus and dentate gyrus regions. Some of these changes include the growth or sprouting of new axonal connections as well as the birth and integration of new neurons into hippocampal circuits. Previous work has shown that these changes in structural and functional plasticity are not necessarily beneficial events. For instance, the growth and reorganization of synaptic terminals in the hippocampus and other brain regions might serve as a substrate that enhances hyperexcitability and seizure generation. In addition, although seizures induce the birth of new neurons, many of these newly generated cells migrate and function improperly within the hippocampal networks. Considering the prominent role of the hippocampus in a variety of behaviours, including learning, memory, and mood regulation, it would appear that alterations involving the structural and functional properties of both mature and newly born neurons in this region could impact these hippocampal-dependent functions. However, to date, the role of kindling-induced changes in hippocampal structural plasticity and neurogenesis on behaviour is incomplete, and the molecular mechanisms that govern these pathological events are poorly understood. The aim of this dissertation is to gain a better understanding of the changes in synaptic plasticity and neurogenesis within the hippocampus that occur after amygdala kindling. In chapter 2, we will examine if kindling alters the expression of synapsin I, a molecular marker of synaptic growth and activity, in both the hippocampus and other brain regions. In addition, we will also set out to determine if changes in synapsin I are related to the development of behavioural impairments associated with kindling. In chapter 3, the effect of kindling on hippocampal neurogenesis will be examined. In addition, we will also evaluate the effect of kindling on the expression of Reelin and Disrupted-in-Schizophrenia 1 (DISC1), two proteins instrumental for mediating proper neuronal migrational and maturation during development. In chapter 4, the effect of altered DISC1 expression in the dentate gyrus after kindling will be examined more extensively. We will examine whether altered DISC1 expression in the dentate contributes to some of the pathological features associated with seizure-induced hippocampal neurogenesis, such as ectopic cell migration and dentate granule cell layer dispersion. Finally, in chapter 5, the impact of aberrant seizure-induced neurogenesis on behaviour will be examined by determining if seizure-generated neurons functionally integrate and participate in hippocampal circuits related to memory processing. The results of this dissertation enhances our understanding of the functional consequences that altered hippocampal synaptic plasticity and neurogenesis may have on the development of epilepsy and emergence of cognitive impairments associated with chronic seizures