Evaluating the role of homeostatic plasticity in epileptogenesis
Lead Research Organisation:
University College London
Department Name: Institute of Neurology
Abstract
Epileptogenesis is the process by which mechanisms used by the brain to prevent seizures fail or are overcome by hyperactivity and hypersynchronicity. This results in a transition from "normal" brain function to the emergence of spontaneous, recurrent seizures - the key symptom that defines the epilepsies.
For genetic, congenital epilepsies this period may be considered as the period between birth and the first seizure. For acquired epilepsies, a precipitating event usually triggers this transition, e.g. status epilepticus1,2, traumatic brain injury3, stroke4, or infection. There is often a "latent period", defined as the period between the precipitating event and the first clinically detectable seizure. Although the notion of a latent period is now challenged by data indicating sub-clinical "seizures" may be occurring in focal regions during this phase 5,6, it is still clear that acquired epilepsy is a progressive disorder in which a gradual worsening of pathophysiology can be seen to occur post-injury.
It has previously been shown that during the latent period in a chemoconvulsant-induced model of temporal lobe epilepsy (TLE), there is an elevation of activity in the days following the induced status epilepticus (SE) that is subsequently followed by a drop in activity levels that is subsequently reversed by the emergence of seizures by around 2-weeks post-injection (pilot data, unpublished). We propose that these changes in activity reflect homeostatic changes that aim to suppress pathological elevations in activity. Moreover, we aim to assess if this process, which the emergence of generalized seizures indicates to be unsuccessful, contributes to the development of an epileptic network through the instigation of changes in response to activity which it is ill-suited to respond to. Homeostatic plasticity describes a variety of changes that can occur in response to both increased or decreased levels of activity7-9, and aims to restore activity to a set level. Commonly described mechanisms, summarized below, include alterations in the density of AMPA-type glutamate receptors at excitatory synapses10, changes in presynaptic properties11,12, structural changes such as extension/retraction of dendrites13,14, and plasticity of the axon initial segment length15.
The majority of work concerning homeostatic plasticity in vivo has focused on responses to chronic reduction in activity levels due to the relative ease of induction, e.g. sensory deprivation and identification of the area affected by this reduction, i.e. the appropriate sensory cortex16. Studying homeostatic plasticity during epileptogenesis will provide insights into in vivo mechanisms of plasticity in response to chronic elevation of activity.
To study homeostatic plasticity, we are using a model of intra-hippocampal kainic acid (KA)2 that re-capitulates many of the human pathophysiological changes seen in TLE. A (Cg)-Fostm1.1(cre/ERT2)/Luo mouse line will be crosssed with a Gt(ROSA)26Sortm9(CAG-tdTomato)Hze (Ai9) line. In the offspring, the expression of cFos, an immediate early gene (IEG) expressed by neurons during elevated activity17, induces transcription of the TdTomato (TdT) gene, permanently tagging these neurons with TdT. This tagging occurs in a tamoxifen-dependent manner, and the use of the tamoxifen derivative 4-hydroxy-tamoxifen (4-OHT) allows for tagging to be temporally confined to a 2 hour period18.
This system will initially be used to track areas of chronic activity during the latent phase, and the data derived from this experiment will provide pilot data for subsequent investigations, informing where and when the epileptic network spreads.
For genetic, congenital epilepsies this period may be considered as the period between birth and the first seizure. For acquired epilepsies, a precipitating event usually triggers this transition, e.g. status epilepticus1,2, traumatic brain injury3, stroke4, or infection. There is often a "latent period", defined as the period between the precipitating event and the first clinically detectable seizure. Although the notion of a latent period is now challenged by data indicating sub-clinical "seizures" may be occurring in focal regions during this phase 5,6, it is still clear that acquired epilepsy is a progressive disorder in which a gradual worsening of pathophysiology can be seen to occur post-injury.
It has previously been shown that during the latent period in a chemoconvulsant-induced model of temporal lobe epilepsy (TLE), there is an elevation of activity in the days following the induced status epilepticus (SE) that is subsequently followed by a drop in activity levels that is subsequently reversed by the emergence of seizures by around 2-weeks post-injection (pilot data, unpublished). We propose that these changes in activity reflect homeostatic changes that aim to suppress pathological elevations in activity. Moreover, we aim to assess if this process, which the emergence of generalized seizures indicates to be unsuccessful, contributes to the development of an epileptic network through the instigation of changes in response to activity which it is ill-suited to respond to. Homeostatic plasticity describes a variety of changes that can occur in response to both increased or decreased levels of activity7-9, and aims to restore activity to a set level. Commonly described mechanisms, summarized below, include alterations in the density of AMPA-type glutamate receptors at excitatory synapses10, changes in presynaptic properties11,12, structural changes such as extension/retraction of dendrites13,14, and plasticity of the axon initial segment length15.
The majority of work concerning homeostatic plasticity in vivo has focused on responses to chronic reduction in activity levels due to the relative ease of induction, e.g. sensory deprivation and identification of the area affected by this reduction, i.e. the appropriate sensory cortex16. Studying homeostatic plasticity during epileptogenesis will provide insights into in vivo mechanisms of plasticity in response to chronic elevation of activity.
To study homeostatic plasticity, we are using a model of intra-hippocampal kainic acid (KA)2 that re-capitulates many of the human pathophysiological changes seen in TLE. A (Cg)-Fostm1.1(cre/ERT2)/Luo mouse line will be crosssed with a Gt(ROSA)26Sortm9(CAG-tdTomato)Hze (Ai9) line. In the offspring, the expression of cFos, an immediate early gene (IEG) expressed by neurons during elevated activity17, induces transcription of the TdTomato (TdT) gene, permanently tagging these neurons with TdT. This tagging occurs in a tamoxifen-dependent manner, and the use of the tamoxifen derivative 4-hydroxy-tamoxifen (4-OHT) allows for tagging to be temporally confined to a 2 hour period18.
This system will initially be used to track areas of chronic activity during the latent phase, and the data derived from this experiment will provide pilot data for subsequent investigations, informing where and when the epileptic network spreads.
