Abstracts

Rationale: The most distinctive feature of absence seizures is a sudden arrest of movement, quite unlike the rapid shaking movements associated with convulsive seizures, yet the motor circuits that suppress rather than activate movements during absence seizures are not known. Answers to this long-standing question could provide key insights into the network dynamics that differentiate childhood absence epilepsy (CAE) from other forms of epilepsy and help us identify specific points of intervention in absence networks that can be targeted to develop more effective treatments. While motor control circuitry is complex, the motor cortex (M1) is an attractive target to probe in CAE-associated behavioral arrest given its well-established role in movement initiation coupled with increasing evidence for the prominent role of deep cortical layers in spike-wave dynamics. M1 layer 5 (L5) harbors pyramidal tract (PT) neurons that drive downstream lower motor neurons in the spinal cord while local and inter-laminar interneurons influence PT output. Thus, we set out to examine and manipulate cells in M1 L5 in the well-characterized tottering mouse model of CAE, to unravel their potential role in the unplanned immobility characteristic of absence seizures. Methods: In our effort to determine the neural correlates of ictal behavioral arrest, we record EEG, video, and extracellular activity from the caudal forelimb area of the motor cortex (CFA), in a head-fixed, awake tottering mouse using a multi-channel silicon probe that spans most cortical layers including L5. Next, we spike sort, cluster and manually curate extracellular recordings to identify putative units and time-synchronize them to EEG-video recordings. We classify putative units into ictal-low (decreased mean firing rate during seizures) and ictal-high neurons (increased mean firing rate during seizures). We determine synchrony measures including pairwise Pearson correlation coefficients and van Rossum distance to understand network dynamics during ictal and inter-ictal periods. Next, we attempt to alter motor circuit dynamics by selectively applying/injecting ethosuximide (ESM), a well-established anti-absence drug, locally in CFA and determine its effect on seizures and associated behavioral arrest. Results: Preliminary data suggests that most M1 L5 ictal-low neurons have a high degree of synchrony with each other during seizures and associated behavioral arrest when compared to inter-ictal periods (n=1,190 seizures,~90 min.). However, this degree of synchrony is not present in most ictal-high neurons. Ictal-low neurons are generally more active than ictal-high neurons. Unilateral or bilateral application of ESM on CFA cortical surface (n=4) did not abolish absence seizures or associated behavioral arrest. However, unilateral or bilateral (sequential) focal injection of ESM into deep CFA elicited myoclonic seizures in addition to continued absence seizures in tottering (n=1), not seen with local saline injection in tottering (n=1) or ESM injection in wild-type (n=1). Conclusions: We identified neural correlates of absence seizures in the motor cortex of tottering. Ictal-low neurons in M1’s output layer 5 synchronize their activity during absence-associated behavioral arrest. If borne out by additional recordings, this result suggests that behavioral arrest during spike wave is not due to silencing of all neurons in the output layer of M1 motor cortex, and is in fact correlated with synchronous firing between a yet to be defined subpopulation of M1 L5 cells. Restricted ESM injection in deep M1 does not suppress spike-wave seizures but elicits a novel myoclonic seizure phenotype in a tottering mutant. Funding: NIH NS29709 (JLN)