McCafferty, C. P. | Search bioRxiv

bioRxiv2021

A novel mouse model of focal limbic seizures that reproduces behavioral impairment associated with cortical slow wave activity

Patients with focal temporal lobe seizures often experience loss of consciousness. In humans, this loss of consciousness has been shown to be positively correlated with EEG neocortical slow waves, similar to those seen in non-REM sleep. Previous work in rat models of temporal lobe seizures suggests that decreased activity of subcortical arousal systems cause depressed cortical function during seizures. However, these studies were performed under light anesthesia, making it impossible to correlate behavior, and therefore consciousness, to electrophysiologic data. Further, the genetic and molecular toolkits allowing for precise study of the underlying neural circuitry are much more developed in mice than in rats. Here, we describe an awake-behaving, head-fixed mouse model of temporal lobe seizures with both spared and impaired behavior reflecting level of consciousness. Water-restricted mice were head-fixed on a running wheel and trained to associate an auditory stimulus to the delivery of a drop of water from a dispenser. To investigate the effect of seizures on behavior, seizures were electrically induced by stimulating either the left or right hippocampus via a chronically-implanted electrode, while mice were performing the task. Behavior was measured by monitoring lick responses to the auditory stimulus and running speed on the wheel. Further, local field potentials (LFP) signals were simultaneously recorded from hippocampus and orbitofrontal cortex (OFC). Induced focal seizures were 5-30s in duration, and repeatable for several weeks (n=20 animals). Behavioral responses showed a decrease in lick rate to auditory stimulus, and decreased running speed during seizures (p<0.01, n=20 animals). Interestingly, licking response to sound could vary from being impaired to normal during seizures. We found that behavioral impairment is correlated with large amplitude cortical slow-wave activity in frontal cortex, as seen in patients with temporal lobe seizures. These results suggest that induced focal limbic seizures in the mouse can impair consciousness and that the impaired consciousness is correlated with depressed cortical function resembling slow wave sleep. This novel mouse model has similar characteristics with previously studied rat models and human temporal lobe seizures. By leveraging the genetic and molecular techniques available in the mouse, this model can be used to further uncover fundamental mechanisms for loss of consciousness in focal seizures.

neuroscience↗

bioRxiv2021

Decreased overall neuronal activity in a rodent model of impaired consciousness during absence seizures

Absence seizures are characterized by a brief behavioural impairment including apparent loss of consciousness. Neuronal mechanisms determining the behavioural impairment of absence seizures remain unknown, and their elucidation might highlight therapeutic options for reducing seizure severity. However, recent studies have questioned the similarity of animal spike-wave-discharges (SWD) to human absence seizures both behaviourally and neuronally. Here, we report that Genetic Absence Epilepsy Rats from Strasbourg recapitulate the decreased neuroimaging signals and loss of consciousness characteristic of human absence seizures. Overall neuronal firing is decreased but rhythmic in the somatosensory cortex and thalamus during these seizures. Interestingly, individual neurons in both regions tend to consistently express one of four distinct patterns of seizure-associated activity. These patterns differ in firing rate dynamics and in rhythmicity during seizure. One group of neurons showed a transient initial peak in firing at SWD onset, accounting for the brief initial increase in overall neuronal firing seen in cortex and thalamus. The largest group of neurons in both cortex and thalamus showed sustained decreases in firing during SWD. Other neurons showed either sustained increases or no change in firing. These findings suggest that certain classes of cortical and thalamic neurons may be particularly responsible for the paroxysmal oscillations and consequent loss of consciousness in absence epilepsy.

neuroscience↗