Abstracts

Rationale: In an absence seizure, thalamic and cortical neurons are proposed to be recruited into a transient, highly synchronized state. An intact thalamocortical network is required to generate absence seizures. Although absence seizures appear to spread throughout the thalamocortical network in EEG and LFP recordings, single cell recordings suggest that individual thalamic neurons contribute sparsely, firing on only one or two cycles of each seizure. To resolve these contradictory findings, we sought to observe the pattern and degree of synchronization across thalamic neurons during seizures. We recorded calcium transients from a local group of thalamic neurons during seizures and compared it to calcium activity during spontaneous interictal and sensory-evoked activity in awake, head-fixed mice. Methods: To image thalamic activity during seizures, the calcium indicator GCaMP6f was expressed in the right somatosensory relay thalamus of a mouse model that has spontaneous absence-like seizures due to a mutation in a sodium channel gene (Scn8a-med). A gradient refractive index lens (GRIN lens) was implanted in thalamus over the injection site, 4 silver wires were implanted under the skull to record electrocorticogram (ECoG), and a head post was secured to the skull.  Animals were adapted to head fixation on a treadmill over 5 sessions before imaging. Neurons were imaged at 50Hz under epifluorescence. ECoG and animal running were recorded simultaneously. In some recordings, air puffs were delivered to the contralateral whiskers to drive sensory responses. Results: We report what we believe to be the first calcium imaging of deep brain, thalamic activity in unanesthetized animals during absence seizures. Seizure-related calcium transients were smaller in amplitude and more widely dispersed than those driven by sensory-evoked or spontaneous activity. Synchronized oscillatory signals were present in both somatic and neuropil regions of interest (ROIs). In all thalamic ROIs, calcium fluctuated in sync with each cycle of the seizure and disappeared upon seizure termination. In some cases, thalamic calcium oscillations preceded seizure onset by up to 500ms. When air puffs were delivered during seizures, they often coincided with seizure termination and cessation of calcium oscillations, suggesting that sensory stimuli may be capable of interrupting seizures. Conclusions: We find that calcium transients during seizures are surprisingly weak but strictly yoked to each cycle of a spike-and-wave seizure.  Such weak calcium responses are consistent with single spikes and perhaps even subthreshold activity, but their tight synchronization may be sufficient to cluster sparse outputs from individual neurons to generate strong, rhythmic thalamic output. Our finding that sensory input may interrupt seizures departs from the traditional view that absence seizures represent a state when the thalamus and cortex are essentially suppressed and unable to process incoming sensory input. Based on these results, well-timed sensory interventions could eventually be developed as a real-time treatment for absence seizures.  Funding: NIH/NINDS 5R01 NS034774-21A1NIH 5T32NS007280-32